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Österreich www.bioenergy2020.eu centre@bioenergy2020.eu UID-Nr.: ATU 56877044 

European Biomass Heating 

Technologies: An overview of 

combustion principles and 

performance characteristics of 

commercially available systems 

Written by Birgit Musil-Schlaeffer birgit.musil@bioenergy2020.eu

Date 28th January 2010 

Report Number 372 TR nK-I-1-65 01 
II 

European Biomass 

Heating 

Technologies 

commercial 

partner 

New York State Energy Research and Development Authority 

Project manager Birgit Musil-Schlaeffer birgit.musil@bioenergy2020.eu 

Collaborator Aislinn McCarry aislinn.mccarry@bioenergy2020.eu

 Christoph Schmidl christoph.schmidl@bioenergy2020.eu

 Walter Haslinger walter.haslinger@bioenergy2020.eu

Project number nK-1-I-65 

Project duration 01 August 2009 - 31 December 2009 
III 

Report 

Executive Summary 

In an effort to develop a high-efficiency biomass heating market of technologies with 

acceptable emissions performance, the New York State Energy Research and 

Development Authority’s (NYSERDA’s) Environmental R&D and Building R&D Programs 

have jointly developed the Biomass Heating R&D program. The objectives of this 

program are to: 

• Evaluate the energy-efficiency and emissions performance of a wide range of 

conventional and advanced biomass-fired heating technologies; 

• Evaluate the energy, moisture, and chemical composition of biomass fuel feed 

stocks; 

• Develop advanced boiler technologies by supporting R&D and commercialization 

efforts with New York manufacturers; 

• Demonstrate advanced technologies in representative applications; and 

• Provide objective scientific information for the development of high-efficiency and 

low emission biomass heating initiatives in New York State. 

Currently the United States Environmental Protection Agency (EPA) is updating the 

emissions performance required of all residential wood heating technologies. These New 

Source Performance Standards (NSPS) revisions will require that all newly manufactured 

heating systems meet emission levels that represent the current best demonstrated 

technology; i.e., all new units in the United States will have to meet the level of 

performance of today's best designs. 

This document, prepared by BIOENERGY 2020+ surveys the current state of biomassfired heating technology in use in Austria, Germany, and Scandinavian countries 

Denmark, Norway and Sweden. The report provides critical information to assist 

NYSERDA develop a high-efficiency biomass heating market and help EPA assess current 

best demonstrated technology on which to base revisions to the residential wood heater 

NSPS. 

A key aspect of this study is the description of the European methodology for 

determining thermal efficiency using the net calorific value (lower heating value) of the 

wood fuel and the conversion of this value to the gross calorific value (high heating 

value) used to determine thermal efficiencies by the U.S. heating industry. Throughout 

this report thermal efficiency values are converted to the American convention using 

gross calorific value. The emissions for “dust” or Total Suspended Particulates are 

converted to pounds per million British thermal unit (lb/MMBtu), the common unit used 

in the United States. Other emissions are normalized to values at standard conditions at 

12% O2 levels in the flue gas. 

Legislative regulations and incentives on national and European levels are used in the 

European Union to continuously improve the performance of biomass appliances. In the 

European Standard EN 303-5, which regulates performance requirements for solid fuel 

boilers, three classes (class 1 to 3) for efficiency requirements and emission limits are 

defined in order to embrace the diverse technology stages of the EU member states. In 
IV 

European Biomass 

Heating 

Technologies 

the revision of EN 303-5, which is currently under discussion, more stringent classes 

(class 3 to 5) will be introduced, thereby resulting in the steady increase of the 

performance of installed biomass appliances. In addition to the EN 303-5, national 

regulations and eco-labels may request even higher efficiencies and lower emissions. 

Regional and national financial incentives for the installation of biomass appliances are 

very often linked to eco-label requirements. 

45%

50%

55%

60%

65%

70%

75%

80%

85%

90%

0 300 600 900 1200 1500

nominal load in mBTU/h

class 3

class 2

class 1

class 4

class 5

efficiency based on GCV

Blue Angel (G)/Umweltzeichen (A)

Nordic Swan (Scandinavia)

Figure 0-1 efficiency 

requirements for boilers from 

EN 303-5 (class 1 to class 5) 

and efficiency requirements 

from Austrian, German and 

Scandinavian eco-labels; 

efficiencies based on GCV 

were converted for pellets 

fuel 

Austrian biomass boiler technology is a perfect example of how the improvements in 

combustion technology can be driven by legislative requirements and incentives. 

Figure 0-2 shows the technological improvement of boilers for solid wood, wood chips 

and pellets (starting in 1998) through test data from FJ-BLT. Over the last 30 years 

average efficiencies of biomass boilers have increased from approximately 55% to more 

than 90% based on the NCV and the average CO-emissions have decreased from 15,000 

to less than 50 mg/m3 at 13% O2 in Austria. 

Figure 0-2 Small scale wood combustion systems: Efficiencies and CO emissions 

measured at the test bench through certification tests since 1980 - data according 

(FJ-BLT 2005), compilation in (Voglauer 2005) 

Modern combustion technology for biomass is based on staged combustion in order to 

enable maximum burnout rates. In the primary zone located on the grate, drying, 

devolatilization and solid combustion take place. In the secondary combustion zone, 

which is situated in the combustion chamber, the volatile gases are burnt with secondary 

air. In order to minimize emissions, combustion must be as complete as possible in the 

secondary zone. In Figure 0-3, Figure 0-4 and Figure 0-5 the function principles of a solid 

wood boiler, an underfeed pellet boiler with condensing technology and a pellet boiler 

with top feed technology are illustrated. 
V 

Report 

Figure 0-3 Under-burning solid 

wood boiler (Froeling 2005) 

Figure 0-4 Condensing boiler for wood pellets 

with underfeed burner (Oekofen 2007) 

Figure 0-5 Top feed 

pellets boiler 

(Evotherm 2010) 

The development of solid bio-fuel standards in Europe is linked to the beginning of wood 

pellet production. In order to gain a common understanding between the manufacturers 

and the users, codes of good practice, quality guidelines and later national standards 

have been developed. For the development of high performing combustion appliances, 

harmonized fuel properties are advantageous because the optimization of operating 

conditions is only possible if the fuel properties are known. 

Although modern solid wood boilers are able to modulate their operation load from 100 

to 50%, the use of heat storage is strongly recommended in order to reach high 

efficiencies and keep emissions low. Even though pellet and wood chip boilers can 

modulate their operation load from 100 to 30%, some studies also recommend the 

integration of accumulator tanks in these heating systems in order to increase the annual 

efficiencies of the heating systems. In addition to improved performance, the design of 

heat storage systems also influences the need for maintanance and the durability of 

boilers. 

Due to their high availability, non-wood biomasses are very attractive fuels. They may 

however have certain general physical and chemical properties that may cause problems 

during combustion. In addition to the possible increase of NOX, HCl, SO2, and other 

emissions in comparison to wood biomass, higher ash contents combined with lower ash 

melting temperatures and corrosive effects negatively influence the failure rate and 

durability of the boilers. 

The summary of performance data from European biomass applications and of European 

regulation schemes for boilers themselves as well as for its periphery (bio-fuel, heat 

storage, emission control) shows that stringent regulations and financial incentives 
VI 

European Biomass 

Heating 

Technologies 

encourage technology development resulting in higher performance levels in the 

European Union sustainably. 

The TOP 25% performing biomass appliances that were identified within this study reach 

high efficiencies and low emissions. 

Table 0-1 summary of average efficiency and emission data from considered biomass 

technologies 

ηGCV1) 

(% )

CO 

(mg/m312% O

2)

TSP1) 

(lb/MM Btu

NOx 

(mg/m312%O

2)

< 55 87 43 0.022 140 12221

55 - 100 86 39 0.024 136 13659

100 - 170 87 38 0.023 132 16844

170 - 340 86 24 0.036 154 23537

> 340 86 17 0.031 133 -

< 85 83 174 0.039 139 10116

85 - 120 83 220 0.044 126 10319

120 - 170 83 152 0.066 115 12672

> 170 82 97 0.033 119 14089

< 100 76 60 0.046 157 14350

100 - 200 79 39 0.027 134 25504

200 - 340 78 30 0.038 138 31581

340 - 680 80 22 0.042 125 34056

> 680 78 34 0.092 131 65180

pellets 82 34 0.027 145

solid wood 79 252 0.027 120

stove with 

hot water pellets 25 - 50 85 117 0.019 143 9843

< 30 88 164 0.045 88 8113

> 30 87 180 0.043 110 4820

solid wood 20 - 130 73 1069 0.050 126 -

stove 

pellets

solid wood

wood chips

pellets

16870

data at nominal load

load range 

(M Btu/h)

avg. retail 

price (US $)

50 - 150

boiler

combined 

boiler technology

fuel

1) related to the Gross Calorific Value (upper heating value)

Figure 0-6 and Figure 0-7 exhibit clearly, that efficiencies of the TOP 25% performing 

biomass appliances are far beyond and emissions are far below the future requirements 

of EN 303-5, class 5. 

75%

80%

85%

90%

0 200 400 600 800 1000

nominal load in mBTU/h

class 3

class 4

class 5

efficiency based on GCV

TOP 25% pellet boilers

TOP 25% solid wood boilers*

TOP 25% wood 

chip boilers*

* conversion from efficiency based on NCV to efficiency based on GCV was performed based on pellets fuel, therefore a 

correction factor for wood chip boiler (+10.6%) and for wood chip boilers (+3.1%) was used to enable comparability, 

efficiencies for wood chip and solid wood boilers are not exact.

Figure 0-6 efficiency classes 

3 to 5 from EN 303-5 and 

efficiency performance of 

TOP 25% pellet, solid wood 

and wood chips boilers; 

efficiencies from solid wood 

and wood chip boilers were 

corrected because efficiency 

limits based on the GCV 

were converted for pellets 

fuel 
VII 

Report 

0.000

0.050

0.100

0.150

0.200

0 200 400 600 800 1000

nominal load in mBTU/h

class 3

class 4

class 5

TOP 25% pellet boilers

TOP 25% solid wood boilers*

TOP 25% wood chip boilers*

class 2

* conversion from dust emission limits from mg/m3 to lb/MMBTU was performed for pellets fuel, therefore a correction factor 

for wood chip boiler (0.754) and for wood chip boilers (0.911) was used to enable comparability dust emissions in lb/MMBtu

Figure 0-7 dust emission limit 

classes 2 to 5 from EN 303-5 

and dust emissions from TOP 

25% pellet, solid wood and 

wood chips boilers; emissions 

from solid wood and wood 

chip boilers were corrected 

because dust emission 

conversion from mg/m3 to 

lb/MMBtu was performed for 

pellets fuel 

Figure 0-8 shows the average efficiencies at nominal and partial load for the TOP 25% 

performing pellet, solid wood and wood chip appliances. For solid wood boilers partial 

load equals 50% of nominal load, for pellets and wood chip boilers the partial load is 

30% of the nominal load. 

Efficiencies for nominal and partial load are almost identical. Solid wood and wood chip 

boilers sometimes even reach higher efficiencies at partial load operation. 

75%

80%

85%

90%

< 55

55 - 100

100 - 170170 - 340

> 340

< 85

85 - 120

120 - 170

> 170< 100

100 - 200200 - 340340 - 680

> 680

pellet boilers solid wood boilers wood chip boilers

nominal load partial load

efficiency based on GCV

load category

(MBtu/h)

Figure 0-8 average 

efficiencies based on GCV 

at nominal and partial heat 

load for the TOP 25% 

performing boilers 

By making careful conversions of conventional methodology and units, the energy 

efficiency and emissions performance of the European heating systems can be more 

easily summarized in the context of the American nomenclature. After conversions, 

differences in evaluation duty-cycle and the emissions sampling and analysis protocols 

still exist in the regulatory test methods between the European Commission and the U.S. 

For example, the European Commission evaluation duty cycles do not include operation 

for the very low burn rates (low heat demand rates) that U.S. evaluation duty cycles 

typically include. The U.S. evaluation duty cycles include these low burn rates because 

many U.S. designs do not always automatically minimize or eliminate idle modes that 

have poor combustion characteristics. There is considerable interest in the U.S. in this 

opportunity for much improved performance. Also, the U.S. sampling and analysis 

typically include condensable particulate matter that is not included in the European tests 

because the high-efficiency designs combust virtually all of the organic precursors to 
VIII 

European Biomass 

Heating 

Technologies 

condensable aerosol particles. Thus one must be very careful in comparing results and 

recognize that not all results are directly comparable. 

Even considering the differences described above, it is clear that by using fuels with 

lower water content (only 22.5% for wood chips and 14.5% for solid wood), improved 

staged combustion design, requirements for minimum thermal efficiency, and 

sophisticated controls that optimize combustion and minimize or eliminate idle modes, 

the top 25% performing commercially available European technologies greatly exceed 

not only the current classes (1, 2, and 3) of boilers but also the proposed classes (4 and 

5) for thermal efficiency and emissions. Technology forcing policies, regulations, and Ecolabeling efforts have clearly resulted in greatly improved technology now available in 

Europe which is starting to make its way into the American wood heating market as well. 
IX 

Report 

Table of Content 

1 Introduction - Calorific Value and Efficiency Discussion 1

1.1 Chemical composition 1

1.2 Calorific Value 3

1.2.1 Measurement and conversion 3

1.2.2 Formula from Boie 5

1.3 Conversion factors 6

2 Biomass combustion technology 9

2.1 Overview – combustion principles 9

2.1.1 Staged combustion 9

2.1.2 Manual stoking 9

2.1.3 Automated stoking 10

2.2 Small scale combustion systems 13

2.2.1 Room heaters 13

2.2.1.1 Fireplaces 14

2.2.1.2 Pellet stoves 14

2.2.1.3 Pellet stoves with hot water production 15

2.2.1.4 Wood stoves 16

2.2.1.5 Masonry heaters 17

2.2.2 Boilers 19

2.2.2.1 Pellet boilers 19

2.2.2.2 Solid wood boilers 21

2.2.2.3 Wood chip boilers 21

2.2.2.4 Combined boiler technologies 22

2.3 Medium to large scale combustion 23

2.3.1 Fixed-bed combustion 23

2.3.1.1 Underfeed stokers 23

2.3.1.2 Grate furnaces 24

2.3.2 Fluidised bed combustion 26

2.3.2.1 Bubbling fluidised bed combustion (BFB) 27

2.3.2.2 Circulating fluidised bed (CFB) combustion 27

2.3.3 Dust combustion 28

3 Emissions 30

3.1 Conversion factors 30

3.2 Carbon Monoxide (CO) 31

3.2.1 General 31
X 

European Biomass 

Heating 

Technologies 

3.2.2 Measurement 32

3.2.3 Reduction 32

3.3 Total Organic Bound Carbon/Organic Gaseous Substances (OGC) 34

3.3.1 General 34

3.3.2 Measurement 34

3.3.3 Reduction 35

3.4 Dust · Particulate Matter (PM) · Totals Suspended Particulate Matter (TSP) 35

3.4.1 General 35

3.4.2 Measurement 38

3.4.3 Reduction 38

3.4.3.1 Primary measures 38

3.4.3.2 Secondary measures 39

3.5 Nitroxen Oxides (NOX) 43

3.5.1 Measurement 44

3.5.2 Reduction 45

3.5.2.1 Primary measures 45

3.5.2.2 Secondary measures 46

4 European Test Methods 48

4.1 Residential heating systems 49

4.1.1 EN 303-5: Heating boilers for solid fuels, hand and automatically 

stoked 49

4.1.2 EN 12809: Residential independent boilers fired by solid fuel 51

4.1.3 EN 12815: Residential cookers fired by solid fuel 51

4.1.4 EN 13229: Inset appliances including open fires fired by solid fuels 51

4.1.5 EN 13240: Room-heaters fired by solid fuel 53

4.1.6 EN 14785: Residential space heating appliances fired by wood pellets 55

4.1.7 EN 15250: Slow heat release appliances fired by solid fuel 56

4.2 Commercial heating systems 58

4.3 Summary 59

5 Regulations for biomass installations 60

5.1 European level 61

5.1.1 Regulations for boilers 61

5.1.2 Regulations for room heaters 63

5.2 National levels 63

5.2.1 Austria 64

5.2.2 Germany 67
XI 

Report 

5.2.3 Denmark 71

5.2.4 Finland 73

5.2.5 Sweden 74

6 Performance of biomass installations 76

6.1 Introduction 76

6.1.1 Data sources 76

6.1.2 Selection method for TOP 25% performing appliances 77

6.2 Stoves/room-heaters 77

6.2.1 Pellet stoves 77

6.2.2 Wood stoves 80

6.2.3 Masonry heaters 81

6.3 Boilers 81

6.3.1 Pellet boiler 82

6.3.2 Solid wood boiler 83

6.3.3 Wood chip boiler 85

6.3.4 Combined technologies 87

6.3.5 Summary 89

6.4 Commercial biomass installations 91

6.4.1 Data collection of Austrian medium scale biomass appliances 

(Musil et al. 2006) 92

6.4.2 Data collection of Swiss automated biomass appliances 

(Hasler et al. 2000) 93

6.4.3 Emission factors 96

6.4.4 Summary 97

7 Emission control technologies 99

7.1 Emission control technology for PM/dust 99

7.1.1 Small scale appliances 99

7.1.2 Medium scale appliances 101

8 European Biomass Fuel Requirements 102

8.1 General remarks 102

8.2 European Bio-fuel standards 104

8.2.1 Origin of Bio-fuels 104

8.2.2 Specification of solid bio-fuels based on traded forms and properties105

8.2.3 Revision of EN14961:2005 105

8.3 European countries with national pellet standards 106
XII 

European Biomass 

Heating 

Technologies 

8.3.1 Austria 106

8.3.1.1 OeNORM M 7135 106

8.3.1.2 OeNORM M 7136 107

8.3.1.3 OeNORM M 7137 107

8.3.2 Germany 107

8.3.2.1 DIN 51731 107

8.3.2.2 DIN plus Standard 107

8.3.2.3 Certification of pellet logistics 107

8.3.3 Sweden 108

8.3.4 Italy 108

8.3.5 United Kingdom 108

8.4 Additional environmental or quality labels 108

8.5 Summary of European and national property specifications for bio-fuels 110

9 Heat storage (accumulator tank) 113

9.1 Introduction 113

9.2 Dimensioning 114

9.2.1 General aspects 114

9.2.2 Sizing guidelines 115

9.2.3 EN 303-5 116

10 Energy from non-Woody Biomass Fuels 117

10.1 Emissions and efficiencies of solid non wood biomasses 117

10.1.1 Fuels & fuel properties 117

10.1.2 Emissions 118

10.1.3 Efficiencies 119

10.1.4 Slagging & Corrosion 120
XIII 

Report 

Introduction 

Austria is a leading country in biomass combustion technology due to its long tradition of 

using biomass for heating purposes as well as the high environmental awareness of its 

population. Approximately 30 years ago, the first efforts were started towards improving 

combustion technology for biomass appliances. At this time, the first measurements of 

efficiencies and CO emissions from boilers were performed by national authorities and a 

competition was arranged amongst Austrian producers of biomass boilers in order to 

encourage technology development. 

Figure 0-9 shows the technological improvement of boilers for solid wood, wood chips 

and pellets (starting in 1998) through test data from FJ-BLT. Over the last 30 years, the 

average efficiencies of biomass boilers have increased from approximately 55% to more 

than 90% based on the NCV and the average CO-emissions have decreased from 15,000 

to less than 50 mg/m3. 

Figure 0-9 Small scale wood combustion systems: Efficiencies and CO emissions 

measured at the test bench through certification tests since 1980 - data according 

(FJ-BLT 2005), compilation in (Voglauer 2005) 

In the European Union, efficiency requirements and emission limits (CO, OGC and dust) 

for solid fuel boilers are regulated by one standard: EN 303-5. In order to include the 

differing technology levels within the EU, different classes are defined for emission limits 

and efficiency requirements. In the new revision, which is currently under discussion, 

more stringed efficiency and emission performances will be introduced in order to raise 

the overall performance of biomass appliances in Europe. 

All EU member states are allowed to introduce additional and more stringent 

requirements than those given in the European standards for combustion appliances. This 

possibility is used for example by Austria, Germany and the Scandinavian Countries. In 

addition to legal requirements, environmental labels with very high requirments for 

efficiency and emissions are also introduced by some countries. These eco-labels 

encourage the population to purchase high performing biomass appliances by giving 

additional financial incentives. 
XIV 

European Biomass 

Heating 

Technologies 

This report provides a description of: 

• combustion principles of modern biomass heating systems and commonly 

employed designs; 

• combustion emission formations and reduction concepts; 

• European test methods for various appliances; 

• European regulations for biomass heating systems; 

• a survey of current technology performance for commercially available heating 

systems; 

• a summary of the best performing units based on thermal efficiency and CO 

emissions; 

• emission control technology; 

• fuel requirements; 

• heat storage; and 

• heating systems for non-woody biomass fuels. 

A key aspect of this study is that it describes the European methodology for determining 

thermal efficiency using the net calorific value (lower heating value) of the wood fuel and 

how this is converted to the gross calorific value (high heating value) used to determine 

thermal efficiencies by the U.S. heating industry. Throughout this report thermal 

efficiency values are converted to the American convention using gross calorific value. 

The emissions for “dust” or Total Suspended Particulates are converted to pounds per 

million British thermal unit (lb/MMBtu), the common unit used in the United States. 

Other emissions are normalized to values at standard conditions at 12% O2 levels in the 

flue gas. 

By making these careful conversions of conventional methodology and units, the energy 

efficiency and emissions performance of the European heating systems can be more 

easily summarized in the context of the American nomenclature. After conversions, 

differences in evaluation duty-cycle and the emissions sampling and analysis protocols 

still exist in the regulatory test methods between the European Commission and the U.S. 

For example, the European Commission evaluation duty cycles do not include operation 

for the very low burn rates (low heat demand rates) that U.S. evaluation duty cycles 

typically include. The U.S. evaluation duty cycles include these low burn rates because 

many U.S. designs do not always automatically minimize or eliminate idle modes that 

have poor combustion characteristics. There is considerable interest in the U.S. in this 

opportunity for much improved performance. Also, the U.S. sampling and analysis 

typically include condensable particulate matter that is not included in the European tests 

because the high-efficiency designs combust virtually all of the organic precursors of 

condensable aerosol particles. Thus one must be very careful in comparing results and 

recognize that not all results are directly comparable. 
XV 

Report 

List of Abbreviations 

a Ash in kg/kg 

Btu British Thermal Unit 

CHP Combined heat and power 

CO Carbon Monoxide 

d.b. dry basis 

FJ-BLT Biomass · Logistics · Technology Francisco Josephinum (test house in 

 Wieselburg) 

g gramm 

GCV gross calorific value (upper heating value) 

h hydrogen content of the fuel in kg/kg 

HC Hydro Carbons 

J Joule 

lb pound 

m meter 

NCV net calorific value (lower heating value) 

Nm3 m3 at normal conditions (1 atm, 32 °F) 

NO Nitrogen Monoxide 

NO2 Nitrogen Dioxide

NOX Nitrogen Oxides 

O2 Oxygen 

OGC Oganic Gaseous Substances 

PM Particulate Matter 

P nominal load in MBtu/h 

TSP Total Suspended Particulates 

w water content of the fuel related to the mass of wet biomass in kg/kg (w.b.) 

W Watt 

w.b. wet basis 

η Efficiency 

ηGCV Efficiency based on GCV 

ηNCV Efficiency based on NCV 
XVI 

European Biomass 

Heating 

Technologies 

Units and basic conversion factors 

Prefixes according to DIN 1301 

Abbreviation Name Value

M Mega 106

k Kilo 103 

d Deci 10-1

c Centi 10-2

m Milli 10-3

Example: 1 MW (1 Megawatt) = 103 kW (1,000 Kilowatt) = 106

 W (1,000,000 Watt) 

Custom prefixes for North America (in correlation with Btu)

Abbreviation Value 

MBtu 103·Btu 

MMBtu 106·Btu 

Calorific value [CV] 

[CV] = 1 J/g = 0.43 Btu/lb 

Density [ρ] 

[ρ] = 1 g/m3 = 6.5·10-8 lb/oz 

Energy [E] 

[E] = 1 J = 9.4782·10-4 Btu 

Length [l] 

[l] = 1 m = 39.2701 in = 3.28084 ft 

Mass [m] 

[m] = 1 g = 2.2046·10-3 lb 

Power [P] 

[P] = 1 W = 3.4121 Btu/h 

Volume [V] 

[V] = 1 m3 = 1·103 l = 2.641·103

 fl gal = 3.381·104 fl oz 

Exchange rate Euro – Dollar

1 € = 1.478 US$ 
1 

Report 

1 Introduction - Calorific Value and Efficiency 

Discussion 

1.1 Chemical composition 

Biomass is an inhomogeneous raw material. Properties and therefore the composition of 

biomass influence its combustion behavior. A complete analysis of the biomass fuel would 

be time extensive and very expensive. For the characterization of the biomass with 

regard to its combustion behavior, proximate analysis is common. It comprised the 

determination of the content of water, ash and volatiles in the fuel. Additionally the 

heating value of the fuel is determined (see afterwards). 

Biomass fuel can be divided into burning and not burning substances (Figure 1-1). Non 

burning substances are water and substances which are converted to ash during 

combustion. 

BIOMASS - FUEL

NOT burning 

substances

burning

substances

water ash volatiles charcoal

Figure 1-1 classification of biomass fuel with regard to its combustion behaviour 

(Marutzky, Seeger 1999) 

Burning substances can be divided into volatiles and charcoal (fixed carbon). Volatiles 

equal the fraction of biomass, which is transferred to gaseous state when the biomass is 

heated up hermetically sealed. The amount of volatile matter in biomass fuels is higher 

than in coals and usually varies between 76 and 86 wt.% (d.b.). As a result of this high 

amount of volatiles, the major part of a biomass fuel is vaporized before homogeneous 

gas phase combustion reactions take place; the remaining char then undergoes 

heterogeneous combustion reactions. Therefore, the amount of volatile matter strongly 

influences the thermal decomposition and combustion behavior of solid fuels. 

Figure 1-2 shows the mass and energy ratio of ash, volatile matter and charcoal from 

biomass. From ash forming substances, no energy is gained. Charcoal contributes to the 

energy quantity in a high degree. 
2 

Introduction - 

Calorific Value and 

Efficiency 

i i

energy ratio

minerals = ash

charcoal

1 %

83 %

16 %

67 %

33 %

volatiles

mass ratio

Figure 1-2 Mass and energy 

ratio of ash, charcoal and 

volatiles from biomass 

(Ebert, 1998) 

Looking at the elemental composition of biomass, main elements, macro and micro 

nutrients and others can be identified (Figure 1-3). Figure 1-4 shows the average 

elemental compositions of plants. 

Carbon (C) 

Hydrogen (H)

Oxygen (O)

MAIN ELEMENTS

MACRO NUTRIENT

MICRO NUTRIENT

OTHERS

Sulphur (S)

Nitrogen (N) 

Potassium (K)

Phosphor (P) 

Magnesium (Mg) 

Calcium (Ca)

Ferrite* (Fe)

Boron (B)

Ferrite* (Fe)

Manganese (Mn) 

Molybdenum 

(Mo)

Copper (Cu) 

Chlorine (Cl)

Selenium (Se) Zink (Zn)

Cobalt (Co)

Silicon (Si)

Sodium (Na), Aluminium (Al)

micro elements and heavy metals

Carbon (C) 

Hydrogen (H)

Oxygen (O)

MAIN ELEMENTS

MACRO NUTRIENT

MICRO NUTRIENT

OTHERS

Sulphur (S)

Nitrogen (N) 

Potassium (K)

Phosphor (P) 

Magnesium (Mg) 

Calcium (Ca)

Ferrite* (Fe)

Boron (B)

Ferrite* (Fe)

Manganese (Mn) 

Molybdenum 

(Mo)

Copper (Cu) 

Chlorine (Cl)

Selenium (Se) Zink (Zn)

Cobalt (Co)

Silicon (Si)

Sodium (Na), Aluminium (Al)

micro elements and heavy metals

Figure 1-3 concentration and function of 

elements in biomass 

(Kaltschmitt, Hartmann 2001) 

composition 

of plants 

(wt% d.b.)

Figure 1-4 average elemental 

composition of plants 

(Kaltschmitt, Hartmann 2001) 

While the macro and micro nutrients are important for the growth of the plant, they can 

influence the combustion process negatively. Table 1-1 shows the guiding values and 

guiding ranges for elements in biomass fuels and biomass ashes for unproblematic 

thermal utilisation. 
3 

Report 

Table 1-1 Guiding values and guiding ranges for elements in biomass fuels and 

biomass ashes for unproblematic thermal utilisation 

Element 

Guiding concentration in fuel 

(wt% d.b.) 

Limiting parameter 

Outside guiding 

concentration ranges, 

problems can occur for 

< 0.6 Straw, cereals, grass N 

< 2.5 NOx emissions Waste wood, fibre boards 

< 0.1 Corrosion Straw, cereals, grass 

Cl < 0.1 HCl emissions Straw, cereals, grass 

< 0.3 PCDD/F emissions Straw, cereals, grass 

< 0.1 Corrosion Straw, cereals, grass S < 0.2 SOx emissions Grass, hay 

Ca 15 - 35 Ash-melting point Straw, cereals, grass 

< 7.0 Ash-melting point, 

depositions, corrosion Straw, cereals, grass K 

-- Aerosol formation Straw, cereals, grass 

< 0.08 Ash recycling Bark, wood chips, sawdust Zn -- Particulate emissions Bark, wood chips, sawdust 

< 0.0005 Ash recycling Bark, wood chips, sawdust Cd -- Particulate emissions Bark, wood chips, sawdust 

Guiding values for ashes related to the biomass fuel ashes according to ISO 1171-1981 at 1022 °F 

1.2 Calorific Value 

1.2.1 Measurement and conversion 

Gross calorific value (upper heating value) 

The wet based GCV is determined by measurement in bomb calorimeter: 

• Temperature of fuel before combustion and of combustion products: 77 °F. Before 

and after combustion all water (water in fuel and formed water by h) is liquid 

• complete oxidation of c and s in the fuel (CO2 and SO2) 

• no oxidation of n in the fuel 

( )

m

H GCV R

w b

Δ . . = − F 1 

GCVw.b. wet based gross calorific value Btu/lb 

 enthalpy of reaction Btu 

m fuel mass lb 

In order to calculate the dry based GCV, the moisture content w has to be considered: 

ΔHR
4 

Introduction - 

Calorific Value and 

Efficiency 

i i

w

GCVd b GCVw b − = ⋅

1

1 . . . . F 2 

GCVw.b. wet based gross calorific value Btu/lb 

GCVd.b. dry based gross calorific value Btu/lb 

w moisture content in the fuel lb/lb 

Net calorific value (lower heating value) 

In the NCV the water in the fuel and the formed water by h is gaseous after combustion, 

but still at a temperature of 77 °F (25 °C). Therefore the difference between NCV and 

GCV is the evaporation heat of the water amount after combustion at 77 °F. In order to 

determine the dry based NCV from the dry based GCV, the evaporation heat of the 

hydrogen in the fuel has to be subtracted: 

d.b. d.b. 9 hd.b. NCV = GCV − r ⋅ ⋅ F 3

NCVd.b. dry based net calorific value Btu/lb 

GCVd.b. dry based gross calorific value Btu/lb 

r evaporation heat of water at 77 °F (= 1,050) Btu/lb 

hd.b. dry based hydrogen in the fuel lb/lb 

w moisture in the fuel lb/lb 

For calculation of the wet based NCV, the mass and the evaporation heat of the moisture 

in the fuel has to be considered: 

NCVw.b. = NCVd.b. ⋅ (1 − w) − r ⋅ w F 4

NCVw.b. wet based net calorific value Btu/lb 

NCVd.b. dry based net calorific value Btu/lb 

r evaporation heat of water at 77 °F (= 1,050) Btu/lb 

h hydrogen in the fuel lb/lb 

w moisture in the fuel lb/lb 

Conversion from dry based GCV to wet based NCV 

If direct conversion of dry based GCV to wet based NCV is wished, combination of F 3 

and F 4 result in: 

NCVw.b. = (GCVd.b. − r ⋅ 9 ⋅ hd.b.) ⋅ (1 − w) − r ⋅ w F 5

NCVw.b. wet based net calorific value Btu/lb 

NCVd.b. dry based net calorific value Btu/lb 

r evaporation heat of water at 77 °F (= 1,050) Btu/lb 

hd.b. dry based hydrogen in the fuel lb/lb 

w moisture in the fuel lb/lb 
5 

Report 

In Figure 1-5 the relationship of the net calorific value as well as the gross calorific value 

and the dry based water content is given. Additionally typical water contents for pellets, 

log wood (seasoned wood) and fresh wood are drawn in. 

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

0% 20% 40% 60% 80% 100%

water content w in % w.b.

GCV BTU/lb

NCV BTU/lb

seasoned wood 

Calorific Value in BTU/lb

wood pellets 

fresh wood 

Figure 1-5 relationship of Net and Gross Calorific Value and water content 

1.2.2 Formula from Boie 

The heating value can only be determined exactly by measurement. If measurement is 

not possible or only estimation is requested, the experimentally developed Formula from 

Boie can be used to approximate the heating value of the biomass. Although this Formula 

considers the nitrogen in the fuel, which is hardly oxidized, it still furnishes reasonable 

results for bio-fuels: 

s n w

NCV c h o

w b w b

w b w b w b w b

+ ⋅ + ⋅ − ⋅

≈ ⋅ + ⋅ − ⋅

4,515 2,709 1,050

14,964 40,377 4,644

. . . .

. . . . . . . . F 6 

NCVw.b. wet based net calorific value Btu/lb 

c w.b. wet based carbon in the fuel lb/lb 

h w.b. wet based hydrogen in the fuel lb/lb 

o w.b. wet based oxygen in the fuel lb/lb 

s w.b. wet based sulfur in the fuel lb/lb 

n w.b. wet based nitrogen in the fuel lb/lb 

w water content lb/lb 
6 

Introduction - 

Calorific Value and 

Efficiency 

i i

1.3 Conversion factors 

For the conversion of efficiencies and emissions, it is necessary to assume the elemental 

composition, the calorific value and the water content of the considered biofuels. In Table 

1-2 the considered values are shown. The elemental composition and the gross calorific 

value of the biofuels correspond to the standard values that are used for emission 

calculations by BE2020+, Branch Wieselburg. The average water contents of the fuels 

from all input data (wNCV) are assumed to be equal to the average water content of each 

fuel from the FJ-BLT1

 test reports. The water contents of the biofuels for the calculation 

of the efficiencies based on the GCV (wGCV) were provided by NYSERDA. 

Table 1-2 assumed elemental composition, gross calorific value and water content for 

conversion 

GCV1)

(d.b.) c1,2) h1,2) a1,2) wNCV3) wGCV4) NCV3)(w.b.) 

GCV4)

(w.b.)

 Btu/lb % (d.b.) % Btu/lb 

pellets 7.5 5 7 300 8 200 

solid wood 14.5 20 6 700 6 900 

wood chips 

8 600 50.70 6.23 0.20 

22.5 40 6 000 5 200 

1) standard values for emission calculation of BE2020+, Wieselburg 

2) c: carbon in the fuel, h: hydrogen in the fuel, a: ash in the fuel 

3) averaged from FJ-BLT test reports 

4) given from NYSERDA

In the test reports that were used for this study, the efficiencies were based on the lower 

heating value (net calorific value NCV). These efficiencies were achieved with test fuels 

having the assumed averaged water content of the test fuels as shown inTable 1-2. 

The efficiencies must however be based on the GCV which is calculated with new water 

contents of the fuels given by NYSERDA (see Table 1-2). In order to convert the 

efficiencies based on the GCV values with the new water contents provided in the test 

reports by NYSERDA, the correlation between the water content of the fuel and the 

efficiency have to be considered. 

In order to calculate the conversion factors, the excess air ratio of the combustion 

conditions must be determined, whereby it is assumed to equal the average excess air 

ratio found in the data available from FJ-BLT: 

• pellets: λ = 1.50 

• solid wood: λ = 1.42 

• wood chips: λ = 1.43 

Figure 1-6 shows the efficiencies which were reached for water contents of 0, 22.5 and 

40% at an excess air ratio of 1.43 (these values correspond to wood chips). In Figure 1-6 

only losses by heat in the flue gas are considered. Losses by radiation, un-burnt gases in 

 

1

 FJ-BLT is the test house for wood boilers in Wieseburg 
7 

Report 

the flue gas or un-burnt material in the ash are not considered. Theses losses are 

however small in comparison to the flue gas losses. 

65%

70%

75%

80%

85%

90%

95%

100%

0 100 200 300 400 500 600 700

flue gas temperature in °F

water content 40.00%

water content 22.50%

water content 0.00%

efficiency based on GCV

Figure 1-6 correlation between the efficiency and corresponding flue gas temperature 

for different water contents of the fuel at an excess air ratio of 1.43 (only losses by 

heat in the flue gas considered) 

An efficiency of 85% with a fuel water content of 22.5% will be reduced to 80% if the 

fuel water content is increased to 40%, assuming that the fuel gas temperature stays 

constant (about 220 °F). 

In order to convert the efficiencies based on the NCV and the average water contents of 

the test fuels to the efficiencies based on the GCV and the water contents provided by 

NYSERDA, the graphs for each fuel corresponding to Figure 1-6 were determined. F 7 

shows the approximaed equation that was used for conversion and in Table 1-3 the 

factors for F 7 are given. 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ − ⋅ = − ⋅

2

1

1 2 X

X f

Y Y BLT

NYSERDA

CV NCV

GCV

η

η F 7 

ηGCV,NYSERDA efficiency based on GCV and water contents from 

NYSERDA 

- 

ηNCVBLT efficiency based on NCV and averaged water contents 

of the test fuel 

- 
8 

Introduction - 

Calorific Value and 

Efficiency 

i i

Table 1-3 factors for conversion from efficiency determined on basis of the NCV and 

averaged water contents of the test fuels to efficiency based on GCV and water 

content of NYSERDA 

fuel X1 (-) X2 (-) Y1 (-) Y2 (-) fCV (-) 

pellets 9.32·10-1 5.37·10-4 9.36·10-1 5.35·10-4 9.22·10-1

solid wood 9.21·10-1 5.21·10-4 9.12·10-1 5.28·10-4 9.11·10-1

wood chips 9.07·10-1 5.35·10-4 8.62·10-1 5.70·10-4 8.96·10-1

Figure 1-7 exhibits the resulting conversion functions. 

60%

65%

70%

75%

80%

85%

90%

95%

100%

75% 80% 85% 90% 95% 100%

efficiency based on NCV and averaged water contents of the fuels 

solid wood (14.5% -> 20%)

pellets (7.5% -> 5%)

wood chips (22.5% -> 40%)

efficiency based on GCV and water

 content from NYSERDA

Figure 1-7 conversion function for the conversion of the efficiencies based on the NCV 

and average water contents of test fuel to efficiencies based on the GCV and average 

water contents provided by NYSERDA 

When converting from NCV with a 7.5% water content pellet to the American convention 

of GCV with a pellet having 5% water content, a loss of approximately 5% is observed. 

This is due to the water formed from h in the wood fuel. In the case of changing from a 

wood chip fuel with 22.5% water content to one with 40% water content, not only is the 

5% loss of energy from the fuel h conversion to water accounted for but the loss in 

energy efficiency of an additional 10% due to the loss of energy required to heat the 

water in the fuel is also apparent. 
9 

Report 

2 Biomass combustion technology 

2.1 Overview – combustion principles 

2.1.1 Staged combustion 

Combustion technology for biomass appliances is based on staged combustion in order to 

enable maximum burnout rates. Figure 2-1 illustrates the stages of biomass combustion 

that take place in the primary and secondary combustion zones. 

combustion 

of biomass

reaction in firebed reaction in comb. chamber

gas

combustion

drying

solid

combustion

devolatilization

combustion

air

primary combustion zone secondary combustion zone

combustion

air

combustion 

of biomass

reaction in firebed reaction in comb. chamber

gas

combustion

drying

solid

combustion

devolatilization

combustion

air

primary combustion zone secondary combustion zone

combustion

air

Figure 2-1 primary and secondary combustion zone in biomass combustion 

In the primary combustion zone, the bio-fuel is first dried from the heat within the 

combustion chamber followed by devolatilization, whereby the volatile substances are set 

free from the solid fuel and ascend to the secondary combustion zone. Finally, the 

remaining solid charcoal is burnt in the firebed. Primary air is needed for devolatilization 

and solid combustion. 

In the secondary combustion zone, the volatile gases from devolatilization and gases 

from solid combustion are burnt with secondary air. In order to achieve low emissions, 

the reaction in the secondary combustion zone must be as complete as possible.

2.1.2 Manual stoking 

Manually stoked furnaces can burn logs (from 10 in up to 40 in in length) as well as 

briquettes. Some log wood furnaces also work with coarse wood chips. There are three 

common, basic classifications which are determined by the airflow of the primary air: top 

burning, through burning or under burning furnace. These designs are illustrated in 

Figure 2-2. Numerous combinations and modifications of these three principles are 

available. 
10 

Biomass 

combustion 

technology 

Figure 2-2 Classification of log wood combustion principles (FNR I 2007) 

In through-burning systems, the primary air enters the combustion chamber through a 

grate. The wood logs are then ignited at the bottom of the batch, thus heating the entire 

stack and commencing the burning. This combustion system is suitable for stoves 

frequently fed with small batches of log wood. For log wood boilers, this combustion 

principle is no longer a state-of-the-art technology. 

In top burning systems, the primary air enters the combustion zone sideways. The stack 

is ignited at the top or in the middle and as such, the combustion reactions are slower 

than those in through-burning systems. The combustion process is nevertheless 

discontinuous. This combustion system is suitable for small or medium batches of log 

wood. 

Both combustion systems, through and top-burning, are usually operated with natural 

draught, meaning that no auxiliary ventilation devices are needed. 

Under-burning systems are the latest development in log wood combustion technologies. 

This principle is a state-of-the-art technology for log wood boilers. The fuel is ignited at 

the bottom and the combustion gases are drawn downwards or sideways, resulting in a 

relatively continuous combustion process. The continuous combustion process and 

separation of the primary and secondary combustion zones result in very low emissions; 

a fan is however necessary to ensure proper operation. This fan is usually placed at the 

flue gas exit of the furnace. 

2.1.3 Automated stoking 

Automatically stoking furnaces can be divided into furnaces with fixed bed combustion, 

fluidised bed combustion and dust combustion. The basic principles of these three 

technologies are shown in Figure 2-3 (Marutzky, Seeger 1999). 
11 

Report 

Fuel Ash

Air

Fixed bed furnace

(grate furnace)

Fuel

Ash

Air

bubbling fluidised

bed furnace circulating fluidised

bed furnace

Air Air

Fuel

Fuel

dust firing

Figure 2-3 Diagram of principle combustion technologies for biomass 

(Marutzky, Seeger 1999) 

Fixed-bed combustion systems include grate furnaces and underfeed stokers. Primary air 

passes through a fixed bed, in which drying, gasification, and charcoal combustion takes 

place. The combustible gases produced are burnt after secondary air addition has taken 

place, usually in a combustion zone separated from the fuel bed. 

Within a fluidised bed furnace, biomass fuel is burnt in a self-mixing suspension of gas 

and solid-bed material into which combustion air enters from below. The type of 

combustion can be distinguished depending on the fluidisation velocity, bubbling fluidised 

bed and circulating fluidised bed 

Dust combustion is suitable for fuels available as small particles (average diameter 

smaller than 0.1 in). For this type of combustion, a mixture of fuel and primary 

combustion air is injected into the combustion chamber, whereby the combustion takes 

place while the fuel is in suspension. The gas burnout is achieved after the addition of 

secondary air. 

For small scale appliances, fixed bed combustion technology is used in medium to large 

scale appliances, inclusive for all technologies. 

Small scale appliances - pellets 

Existing combustion technologies for wood chips have been adopted for pellet 

combustion, and a new principle has been developed. The development of small-scale 

pellet burners has taken place to a great extent in Sweden and Austria. Pellet burners are 

divided according to the feed direction of the pellets into three main types: top feed, 

underfeed and horizontal feed (Alakamgas, Paju 2002). 
12 

Biomass 

combustion 

technology 

Figure 2-4: Classification of pellet combustion principles (Alakamgas, Paju 2002) 

In underfeed burners (underfeed stoker or underfeed retort burners), the fuel is fed into 

the bottom of the combustion chamber or combustion retort. These burners are best 

suited for fuels with low ash content (wood pellets, wood chips). They are built for a 

nominal power between 35 MBtu/h and 8 MMBtu/h or greater.

In horizontal feed burners, the combustion chamber is either fitted with a grate or a 

pusher plate. The fuel is introduced horizontally into the combustion chamber. During 

combustion, the fuel is moved or pushed horizontally from the feeding zone to the other 

side of the pusher plate or the grate. While the fuel is drying, gasification and solid 

combustion take place; following this the remaining ash drops into an ash container. 

Horizontal feed burners can burn wood chips and pellets and are built for nominal heat 

flows from 50 MBtu/h to more than 50 MMBtu/h. Purpose-built horizontal feed systems 

(with moving grate, with water cooling) are fitted for the use of bark, straw or grain as 

well. 

Top feed burners have been specifically developed for pellet combustion in small-scale 

units. The pellets fall through a shaft onto the fire bed, consisting of either a grate or a 

retort. The separation of the feeding system and the fire bed ensures the effective 

protection against back-burn into the fuel storage. The proper distance between the 

combustion retort and the feeding system prevents early ignition of pellets in the feeding 

system (Obernberger, Thek 2002). The ash is removed manually or mechanically by a 

dumping grate. This feeding system allows very accurate feeding of pellets according to 

the current power demand and is thus often used in furnaces with very small nominal 

heat flows. 
13 

Report 

Pellet burner systems are always operated with a forced draught. The fan can be placed 

at the combustion air inlet or at the flue gas exit of the furnace. 

2.2 Small scale combustion systems 

Common small scale combustion systems for woody biomass are listed in Table 2-1 and 

are categorized in two groups: Room heaters and boilers. Small scale combustions 

systems are used in the residential and commercial sectors as well as in district heating 

systems. 

Table 2-1 Types and features of small-scale wood combustions systems, modified from 

(FNR I 2007) 

type 

nominal 

heat load in 

MBtu/h 

combustion principle feature 

ROOM HEATERS

fireplaces 0 - 15 through- / top burning inappropriate for permanent 

heating 

pellet stoves 10 – 35 top feed 

automatically operated, regulated 

fuel and combustion air input 

(fan), with or without hot water 

wood stoves 10 – 40 through- / top burning fired from living room 

no water installations 

Masonry 

heaters 5 – 60 through- / top burning

diverse heat storing/release 

characteristics , with or without 

hot water, combined technologies 

with pellets 

BOILERS

pellet boilers ≥ 10 

top feed / horizontally 

feed / underfeed 

burner 

fully automated operation, 

induced draught 

solid wood 

boilers ≥ 35 through/ under burning 

up to 40 in log wood length, 

natural or induced draught, buffer 

accumulator is needed 

wood chip 

boilers ≥ 35 

through burning / 

horizontally feed / 

underfeed burner 

fully automated operation 

possible, induced draught, buffer 

accumulator advantageous 

2.2.1 Room heaters 

Room heaters are intended to heat a space directly, unlike central heating furnaces or 

boilers, which supply heat to the house through a system of ducts or pipes. Varying 

concepts for space heaters have been developed and are still in use. Common space 

heaters are: 
14 

Biomass 

combustion 

technology 

• Open and closed fireplaces 

• Log wood stoves 

• Masonry heaters (Heat storing (tiled) stoves) 

• Pellet stoves 

2.2.1.1 Fireplaces 

Conventional fireplaces have a long history in home heating. They are built from masonry 

materials such as brick, block and stone. More recently, factory-built models are 

constructed mainly of steel. Conventional open fireplaces (Figure 2-5, left) do not heat a 

home effectively and their use for permanent heating is thus not recommended. In many 

settlement areas and especially urban areas, the use of open fireplaces is even forbidden. 

Factory-built closed fireplaces combine the beauty of an open fire with modern wood 

stove features, such as controlled and staged combustion air supply, baffle plates, 

properly dimensioned combustion chambers and fireclay lining. Burnout rate and 

efficiency clearly exceed those of open fireplaces. Many closed fireplaces provide heat by 

direct flame radiation and by convection (Figure 2-5, right). 

Figure 2-5 Open fireplace (left) and closed fireplace with convection casing 

(right)(Kachelofenbauerinnung 2007) 

Fireplaces will not be considered in this study because they are not an appropriate 

technology for heating purposes. 

2.2.1.2 Pellet stoves 

Pellet stoves are suitable for continuous operation; their minimum heat output is usually 

30 % of the nominal heat flow. Underfeeding and top feeding are the most common 

combustion technologies for pellet stoves. 
15 

Report 

Figure 2-6 shows the principle of a pellet stove with a top feed burner. The pellets are 

conveyed by the fuel screw into the shaft from where they fall into the combustion retort. 

The pellets are usually ignited automatically with a glow-plug. The supply of primary and 

secondary combustion air is controlled by a low-noise fan. External air supply is also 

possible for some pellet stoves, which is important for houses with controlled ventilation. 

The storage compartment is placed in the rear of the stove and can hold up to 110 lb of 

pellets. 

Figure 2-6 Functional principle 

of a pellet stove with top feed 

burner (Calimax 2007) 

In latest technological developments, the quality of combustion is regulated by a carbon 

monoxide sensor, which ensures optimal combustion air supply at all times. Excess air 

ratio sensors are used less often for the control of ventilators in pellet stoves as they are 

quite expensive. They are however frequently used for control in boiler furnaces. 

2.2.1.3 Pellet stoves with hot water production 

The heat output of pellet stoves can usually be varied from 30 to 100 %. This enables 

pellet stoves to produce exactly the heat and, with integrated heat exchangers, also the 

hot water required. In these pellet stoves up to 80 % of the heat produced can be stored 

in the water (FNR I 2007). 

Figure 2-7 shows an example of a pellet stove with integrated hot water production. The 

combination of a pellet stove and solar hot water production allows the complete heat 

and hot water demand of a (low-energy) house to be covered without the need of an 

additional boiler. 
16 

Biomass 

combustion 

technology 

Figure 2-7 Pellet stove with 

integrated hot water production 

(Calimax 2007) 

2.2.1.4 Wood stoves 

Figure 2-8 Functional principle of a log 

wood stove (Westfeuer 2005) 

Figure 2-9 Example of a log wood stove 

(Heinz Aberle 2007) 

In contrast to fireplaces, log wood stoves are floor mounted (free-standing) in the living 

room. In general, they are built of cast iron and designed for logs with a length of 10 and 

15 in. Wide variations in design, such as doors with or without viewing glass or casings of 

tiles or soapstone are available. Wood stoves are usually based on the through-burning 
17 

Report 

principle. Conventional wood stoves are equipped with up to three doors; the upper door 

serves for loading the logs, the middle door is used for manual grate cleaning and the 

third for ash removal. Combustion air supply can be regulated manually by dampers or 

push handles. In simple designs, there is no separation of primary and secondary 

combustion air. Figure 2-8 and Figure 2-9 show the functional principle as well as an 

example of modern log wood stoves based on the through-burning principle. 

Solid wood stoves are still sold without test certification. 

2.2.1.5 Masonry heaters 

Two different basic types of masonry heaters, the fully handcrafted model and the 

prefabricated model, can be distinguished. The traditional type of masonry heater, the 

tiled base stove, is a fully handcrafted heating device. With the exception of the fire door 

and possibly some parts of the combustion chamber, it is completely built with ceramic 

materials such as chamotte, soapstone, clay and/or tiles. Due to its large heat storing 

mass, it is a slow reacting heating system; Figure 2-10 shows the functional principle of 

tiled base stoves and Figure 2-11 shows a design example of a tiled base stove. This 

stove is fed from another room (mostly from the anteroom). 

Figure 2-10 Functional principle of a tiled 

base stove (Kachelofenbauerinnung 2007) 

Figure 2-11 Example of a tiled base 

stove (Kachelofenverband 2007) 

The second type of masonry heaters consists of a prefabricated combustion chamber; the 

combustion gas channel (heat exchangers) can be built by hand or be prefabricated as 

well. A hot-air tiled stove has only a small heat storage mass. This type of stove consists 

of a prefabricated heater and optionally, a steel or cast-iron heat exchanger. Most of the 

heat is transferred by convection. Hot-air tiled stoves can react quickly to changing heat 

demands, but are not designed for long-lasting heat transfer. Depending on their 

dimensions, hot-air tiled stoves have similar heating characteristics as conventional wood 

stoves with convection casings. Combined tiled stoves are a combination of tiled base 

stoves and hot-air tiled stoves, consisting of a prefabricated heater and a ceramic heat 

storage unit (Figure 2-12 and Figure 2-13). 
18 

Biomass 

combustion 

technology 

Figure 2-12 Functional principle of a 

combined tiled stove 

(Kachelofenbauerinnung 2007) 

Figure 2-13 Example of a combined tiled 

stove (Kachelofenverband 2007) 

Masonry heaters are based on the top and through-burning combustion principles. 

Combustion air is supplied in stages as primary and secondary combustion air. 

Combined masonry heaters 

Different technologies have been developed for the combustion of pellets in tiled stoves 

over the past few years. Prefabricated heaters enable the optional combustion of log 

wood and pellets or have an integrated boiler function. Figure 2-14 shows a prefabricated 

heater for pellet combustion in a combined tiled stove. No auxiliary devices such as a fan 

or motor are needed for operation. Figure 2-15 shows a fully automated prefabricated 

heater with an underfeed pellets burner. Both prefabricated heaters can burn log wood as 

well. 

Figure 2-14 Prefabricated heater Pellets 

Power (Wienerberger Ofenkachel 2007) 

Figure 2-15 Prefabricated heater Pellet 

module (Brunner 2007) 
19 

Report 

2.2.2 Boilers 

In contrast to space heaters, boilers are intended to produce hot water for heating 

remote rooms. Usually, they are placed in special heating rooms in the cellar, in the 

garage or even in heating containers outside the house. Modern boilers achieve 

efficiencies beyond 80 % (based on the upper heating value). 

2.2.2.1 Pellet boilers 

Pellet boilers operate fully automatically with top feed, underfeed and horizontal feed 

burners. Manufacturers have typically specialized in a certain technology. Devices 

exclusively designed for wood pellets are typically used for a heat output range from 

25 MBtu/h. For higher heat flows, wood chip furnaces are usually used, which are often 

approved for wood pellets as well (see 2.2.2.3). Figure 2-16 and Figure 2-17 and show 

two different top feed boilers for wood pellets. This combustion principle allows for 

accurate feeding and is suitable for small heat demands as well as on/off-operation 

modes. In order to ensure proper operation, the grate must be automatically cleaned. 

Slagging can occur if low-quality pellets are used (Regionalenergie Steiermark 2005). 

Figure 2-16 Top feed boiler 

(Evotherm 2010) 

Figure 2-17 Top feed boiler 

(Windhager 2005) 

Underfeed boilers are used for small to large heat flows. Underfeed boilers are less 

sensitive to low-quality pellets than top feed burners (Regionalenergie Steiermark 2005). 

An example of an underfeed boiler is shown in Figure 2-18. A horizontal feed boiler is 

demonstrated in Figure 2-19. 
20 

Biomass 

combustion 

technology 

Figure 2-18 Underfeed boiler 

(Guntamatic 2007) 

Figure 2-19 Horizontal feed boiler 

(Hargassner 2005) 

Flue gas condensing 

With proper flue gas condensing technology the moisture can be removed from the flue 

gas to further reduce the exhaust gas temperature, thus achieve higher efficiencies. 

Figure 2-20 shows the functional principle of the first condensing boiler for wood pellets 

available on the European market. Its efficiency of over 100 % is of course only due to 

the chosen definition for the determination of the efficiency, which is based on the net 

calorific value of the used fuel. The measured total dust emissions are very low, most 

likely due to the effect of the flue gas condensation. 

Figure 2-20 Functional principle of a condensing boiler for wood pellets 

(Oekofen 2007) 
21 

Report 

2.2.2.2 Solid wood boilers 

Log wood boilers with natural draught based on the through-burning combustion principle 

were a state-of-the-art technology in the 1970s. Modern log wood boilers are based on 

the under-burning combustion principle and induced draught. Usually a fan, which is 

placed at the flue gas exit of the boiler, is used to suck primary and secondary 

combustion air into the corresponding combustion zones. The wood storage compartment 

may be filled during combustion without the risk of combustion gases leaving the boiler. 

The high degree of turbulence needed for the mixing of the fuel gas and the combustion 

air is ensured by the proper design of the combustion air inlet zone and the nozzles. 

Figure 2-21 and Figure 2-22 show two different log wood boilers with induced draught 

airflow and flue gas exits on the side and at the top of the boiler. 

Log wood boilers are usually designed for a heat output range from 50 to 135 MBtu/h. 

There are some specialty companies that make systems up to 3 MMBtu/h such as the 

Lopper company.

Figure 2-21 Under-burning boiler with 

induced draught, flue gas exit 

sideways(Buderus 2005) 

Figure 2-22 Under-burning boiler with 

induced draught, vertical flue gas exit 

(Froeling 2005) 

2.2.2.3 Wood chip boilers 

For wood chip furnaces, underfeed burners and horizontal feed burners with or without a 

grate are fairly common in various designs. Wood chip furnaces are therefore used in a 

wide nominal power capacity range from 50 MBtu/h up to over 70 MMBtu/h. 
22 

Biomass 

combustion 

technology 

Concerning small scale systems, the advantages of wood chip boilers over log wood 

boilers include automatic operation and low emissions due to continuous combustion. On 

the other hand, the process of producing, storing and, if necessary, drying wood chips 

requires more investment in machinery and storage space than is the case for log wood. 

In comparison to pellets, wood chips are less expensive and can be produced on site. 

Wood chip furnaces are often approved for wood pellets as well; specially designed pellet 

boilers can however not be fired with wood chips. 

Figure 2-23 shows an example of an underfeed burner for wood chips and pellets. The 

principle of a horizontal stoker burner is shown in Figure 2-24. 

Figure 2-23 Underfeed burner for wood 

chips and pellets (KWB 2007) 

Figure 2-24 Horizontal feed burner for 

wood chips, pellets and grain (BAXI 2007) 

2.2.2.4 Combined boiler technologies 

Special boilers have been developed to combine the comfortable combustion system for 

wood pellets with the combustion system for log wood. In some boilers manual 

rearrangements are necessary, while others offer a fully automatic switch. Figure 2-25 

shows a combined boiler with a fully automatic switch. When heat is demanded, the 

pellet burner is started automatically. Any log wood fed into the boiler will be ignited by 

the pellet flame. As soon as the log wood is ignited, the pellet burner stops and the log 

wood stack is burnt (under-burning principle). If there is no log wood in the fuel storage 

compartment, the pellet burner will continue combustion and the log wood compartment 

is used as secondary combustion zone for pellet combustion. 
23 

Report 

Figure 2-25 Combined boiler technology for pellet and log wood combustion (left: log 

wood mode, right: pellet mode) (sht 2007) 

2.3 Medium to large scale combustion 

In Table 2-2 the characteristics of common medium to large combustion technologies for 

woody biomass are listed. 

Table 2-2 Types and characteristics of medium to large combustions systems 

suitable for woody biomass (FNR II 2007) 

Combustion 

technology Stoking Fuel Nominal power Moisture content (% w.b.) 

Underfeed stoker Mechanic 

Pellets and wood 

chips (ash 

content ≤1%) 

35 MBtu/h – 

9 MMBtu/h 5-40 

Moving grate Mechanic 

All wood fuels 

(ash content up 

to 50%) 

510 MBtu/h – 

170 MMBtu/h 5-60 

Bubbling fluidised 

bed combustion Mechanic Particle size <3.6 in 

20 MMBtu/h – 

120 MMBtu/h 5-60 

Circulating fluidised 

bed combustion Mechanic Particle size <1.5 in 

50 MMBtu/h850 Btu/h 5-60 

Dust firing Pneumatic Particle size 

<0.20 in 

1700 MBtu/h170 MMBtu/h Usually <20 

2.3.1 Fixed-bed combustion 

2.3.1.1 Underfeed stokers 

In addition to small scale combustion systems where underfeed stokers are a common 

technology (see 2.2.2.1 and 2.2.2.3), they are also an inexpensive and operationally safe 
24 

Biomass 

combustion 

technology 

technology for medium-scale systems up to 20 MMBtu/h. They are suitable for biomass 

fuels with low ash content (wood chips, sawdust, pellets) and small particle sizes 

(particle dimension up to 2.0 in). Ash-rich biomass fuels such as bark, straw and grain 

need more efficient ash removal systems. Moreover, sintered or melted ash particles 

covering the upper surface of the fuel bed can cause problems in underfeed stokers due 

to unstable combustion conditions when the fuel and the air are breaking through the 

ash-covered surface. An advantage of underfeed stokers is their good partial-load 

behaviour and their simple load control. Load changes can be achieved more easily and 

quickly than in grate combustion plants because the fuel supply can be controlled more 

easily (Loo, Koppejan, 2002). 

Figure 2-26 Combustion chamber 

of an underfeed stoker furnace 

(BINDER 2007) 

2.3.1.2 Grate furnaces 

There are different grate furnace technologies available: fixed grates, travelling grates, 

moving grates, rotating grates, vibrating grates etc. All of these technologies have 

specific advantages and disadvantages depending on the fuel properties; as such careful 

selection and planning is necessary. Based on the flow directions of fuel and the flue gas, 

there are different systems for grate combustion plants (Figure 2-27): 

• counter-current flow (flame in the opposite direction as the fuel) 

• co-current flow (flame in the same direction as the fuel) 

• cross-flow (flue gas removal in the middle of the furnace) 

Counter-current combustion is most suitable for fuels with low heating values (wet bark, 

wood chips or sawdust). Co-current combustion is applied for dry fuels such as waste 

wood or straw, or in systems where pre-heated primary air is used. Cross-flow systems 

are a combination of co-current and counter-current units and are also especially applied 

in combustion plants with vertical secondary combustion chambers (Loo, Koppejan, 

2002). 
25 

Report 

Co-current Counter-current Cross-current

Figure 2-27 Classification of grate combustion technologies: co-current, cross-current 

and counter-current (Nussbaumer 1995) 

Moving grate systems 

Moving grate furnaces usually have an inclined grate consisting of fixed and moveable 

rows of grate bars (see Figure 2-27 and Figure 2-28). They are made of heat-resistant 

steel alloys and are equipped with small channels in their side-walls for primary air 

supply. By alternating horizontal forward and backward movements of the moveable 

sections, the fuel is transported along the grate, whereby un-burnt and burnt fuel 

particles are mixed and the surface of the fuel bed is steadily renewed. The fuel is evenly 

distributed over the grate surface (Loo, Koppejan, 2002). 

Figure 2-28 Combustion 

chamber of a moving grate 

furnace (Polytechnik 2007) 

In moving grate furnaces a wide variety of biofuels can be burnt. Air-cooled moving grate 

furnaces use primary air for cooling the grate and are suitable for wet bark, sawdust and 

wood chips. For dry biofuels or biofuels with low ash-sintering temperatures, watercooled moving grate systems are recommended (Loo, Koppejan, 2002). 

Travelling grate 

Travelling grate furnaces are built of grate bars forming an endless band (like a moving 

staircase) moving through the combustion chamber. Fuel is supplied at one end of the 

combustion chamber onto the grate, by screw conveyors for example or is distributed 
26 

Biomass 

combustion 

technology 

over the grate by spreader-stokers injecting the fuel into the combustion chamber (see 

Figure 2-29). The fuel bed itself does not move, but is transported through the 

combustion chamber by the grate, contrary to moving grate furnaces where the fuel bed 

is moved over the grate. 

The advantages of travelling grate systems are that they provide uniform combustion 

conditions and low dust emissions due to the stable and almost unmoving bed of embers. 

Furthermore, the maintenance or replacement of grate bars is easy to handle. In 

comparison to moving grate furnaces however, the fact that the bed of embers is not 

stoked results in a longer burn-out time (Loo, Koppejan, 2002). 

Figure 2-29 Diagram of a 

travelling grate furnace fed by 

spreader-stokers 

(Detroit Stoker Company 1998)

2.3.2 Fluidised bed combustion 

Fluidised bed (FB) combustion systems have been applied since 1960 for the combustion 

of municipal and industrial wastes. Regarding technological applications, bubbling 

fluidised beds (BFB) and circulating fluidised beds (CFB) have to be distinguished. 

Due to the good mixing achieved, FB combustion plants can provide flexibly with different 

fuel mixtures but are limited in terms of fuel particle size and impurities contained in the 

fuel. Therefore, an appropriate fuel pre-treatment system covering particle size reduction 

and separation of metals is necessary for fail-safe operation. Usually a particle size less 

than 1.6 in is recommended for CFB units and less than 3.1 in for BFB units. Moreover, 

partial load operation of FB combustion plants is limited due to the need of bed 

fluidisation. 

The low excess air quantities necessary increase combustion efficiency and reduce the 

flue gas volume flow. This makes FB combustion plants especially interesting for largescale applications. One disadvantage of FB combustion plants is posed by the high dust 
27 

Report 

loads carried along with the flue gas, which make efficient dust precipitators and boiler 

cleaning systems necessary (Loo, Koppejan, 2002). 

2.3.2.1 Bubbling fluidised bed combustion (BFB) 

In BFB furnaces (see Figure 2-30) a bed material is located in the bottom part of the 

furnace. The primary air is supplied over a nozzle distributor plate and fluidises the bed. 

The bed material is usually silica sand. The advantage of BFB furnaces is their flexibility 

concerning particle size and moisture content of the biomass fuels. Furthermore, it is also 

possible to use mixtures of different kinds of biomass or to co-fire them with other fuels. 

A large disadvantage of BFB furnaces is the difficulties they have at partial load 

operation, a problem which is solved in modern furnaces by splitting or staging the bed. 

BFB furnaces are typically used for nominal outputs of more than 17-34 MMBtu/h 

(Loo, Koppejan, 2002). 

bed partitioning

section A-A

1

2 3

1

A A

Fuel

Fluidised bed

Flue gas ash

tertiary air

primary air

bed 1

secundary air

bed 2,3

2 3

Figure 2-30 Diagram of a BFB furnace (Hein, Schmitz 1995) 

2.3.2.2 Circulating fluidised bed (CFB) combustion 

By increasing the fluidising velocity to 16 to 33 ft/s and using smaller sand particles 

(0.008 in to 0.02 in diameter), a CFB system is achieved. The sand particles will be 

carried with the flue gas, separated in a hot cyclone or a U-beam separator, and fed back 

into the combustion chamber (see Figure 2-31). The higher turbulence in CFB furnaces 

leads to a better heat transfer and a very homogeneous temperature distribution in the 

bed. The disadvantages of CFB furnaces are their larger size and therefore higher price, 

the even greater dust load in the flue gas leaving the sand particle separator than in BFB 

systems, the higher loss of bed material in the ash, and the small fuel particle size 
28 

Biomass 

combustion 

technology 

required (between 0.004 and 1.6 in in diameter). This often causes higher investments in 

fuel pre-treatment. Moreover, their operation at partial load is problematic. CFB furnaces 

are usually applied in a power range of more than 100 MMBtu/h (Loo, Koppejan, 2002). 

to baghouse

Economizer

Generator banks

Primary superheaters

Secondary superheater

Tertiary superheater

Gas flow

790-890°C

870°C

Cyclone

Furnace

Air

Fuel

910°C

Figure 2-31 Diagram of a 

CFB furnace (NREL 1996) 

2.3.3 Dust combustion 

Fuel quality in dust combustion systems has to be quite constant. Fuels such as sawdust 

and fine shavings are pneumatically injected into the furnace. The transportation air is 

used as primary air. Start-up of the furnace is achieved by an auxiliary burner. Due to 

the explosion-like gasification of the fine and small biomass particles, the fuel feeding 

needs to be controlled very carefully and forms a key technological unit within the overall 

system. Fuel gasification and charcoal combustion take place at the same time because 

of the small particle size. Therefore, quick load changes and an efficient load control can 

be achieved. Figure 2-32 shows a muffle dust furnace in combination with a water tube 

steam boiler. This technology is available for thermal capacity between 6.8 and 27 

MMBtu/h (Loo, Koppejan, 2002). 
29 

Report 

1...primary air supply, 

2...fuel feed, 

3...gasification and 

partial combustion, 

4...flue gas 

recirculation, 

5...ash disposal, 

6...secondary air 

supply, 

7...tertiary air supply, 

8...water tube boiler. 

Figure 2-32 Diagram of a dust combustion plant (muffle furnace) in combination with 

a water-tube steam boiler (Marutzky, Seeger 1999) 
30 

Emissions 

3 Emissions 

In this section, a short summary of the emissions from biomass combustion to be 

considered in this report is given. Additionally, measurement and reduction methods are 

summariesed. With regard to the reduction methods, primary and seconary measures 

have to be distinguished: While primary measures are implemented in the combustion 

process and/or design of the combustion chamber, secondary measures are implemented 

after combustion process has ended, mostly adjacent to the boiler. 

More detailed information on formation mechanisms, impacts on health and the 

environment, measurement and reduction methods can for example be found in 

Nussbaumer 1989, Kaltschmitt, Hartmann 2001, Marutzky, Seeger 1999 or 

Loo, Koppejan, 2002. 

3.1 Conversion factors 

Emissions are usually given in a concentration related to a certain O2-concentration in the 

flue gas at normal conditions2

 (mg/m3 at 13% O2 or ppm/m3 at 13% O2); this relation of 

the emissions to certain O2-concentrations enables comparability of data. For biomass 

appliances, the normal O2-concentrations range from 10 to 13%. Sometimes emissions 

are related to the fuel input (mg/MJ). Since the measurement of the exact energy input 

is impossible, especially in medium to large scale appliances, this measurement method 

is limited to small scale appliances. 

The most recent emission limits from the European Commission are usually given in 

mg/m3 based on 10 % oxygen in the dry flue gas; older regulations refer to 13 % 

oxygen in the dry flue gas. In this report, gaseous emissions will be given in 

mg/m3 at XX%O2, which always means that emissions are related to the dry 

volume in m3 at normal conditions (Nm3) at a certain oxygen level in the flue 

gas (XX % O2). According to NYSERDA’s specifications, emissions are to be based on an 

oxygen content of 12 %. In Table 3-1 the factors for the conversion of the O2-based 

emissions in mg/Nm3 for various oxygen levels are shown. 

Table 3-1 conversion factors for the conversion of O2-based emissions in mg/m3 

mg/m3 at 10% O2 1 0.90& & 0.81& & 0.72& &

mg/m3 at 11% O2 1.1 1 0.9 0.8 

mg/m3 at 12% O2 1.2

& 1.1

& 1 0.8

&

mg/m3 at 13% O2 1.375 1.25 1.125 1 

 mg/m3 at 10% O2 mg/m3 at 11% O2 mg/m3 at 12% O2 mg/m3 at 13% O2

read the table from the first column to the last row: 

Emissions in mg/m3 at 10%O2 · 0.90& & = Emissions in mg/m3 at 11%O2

 

2

 pressure of 1 atm and temperature of 32 °F 
31 

Report 

The European Commission also provides emission limits for dust in mg/m3 at a certain O2

level. The factors for the conversion from mg/m3 at 12% O2 to mg/kgfuel and lb/MMBtu 

are shown in Table 3-2. For the conversion from mg/m2 at a certain O2 level to mg/kgfuel

and lb/MMBtu, the elemental composition and the calorific value of the considered fuels 

have to be assumed. These assumptions are given in Table 1-2 on page 6. 

Table 3-2 conversion factors for the conversion of dust emissions from mg/m3 to 

mg/kgfuel and lb/MMBtu 

fuel / 3 12% 2

/

at O

fuel

mg m

mg kg

/ 3 12% 2

/

mg m at O

lb mmBTU

pellets 10.08 1.23·10-3

solid wood 9.31 1.35·10-3

wood chips 8.44 1.64·10-3

3.2 Carbon Monoxide (CO) 

3.2.1 General 

Carbon monoxide emissions can be regarded as the indicator of the combustion quality. 

Conversion of fuel carbon to CO2 takes place through several elementary steps, and 

through several different reaction paths. CO is the most important final derivative. It is 

oxidised to CO2 if oxygen is available. 

The rate at which CO is oxidised to CO2 depends primarily on temperature. In addition, 

sufficient residence time is important to achieve low CO emission levels, mainly because 

CO is generally a later intermediate than hydrocarbons. Furthermore, enough Oxygen 

must be available and mixed with the fuel for the combustion process. 

CO emissions are caused by incomplete combustion which is mainly a result of either: 

• inadequate mixing of the combustion air and the fuel in the combustion chamber, 

producing local fuel-rich combustion zones; 

• an overall lack of available oxygen; 

• combustion temperatures too low; 

• residence times too short; 

These variables are all linked together through the reaction rate expressions for the 

elementary combustion reactions. However, in cases in which sufficient oxygen is 

available, temperature is the most important variable due to its exponential influence on 

the reaction rates. An optimisation of these variables will in general contribute to reduced 

levels of all emissions from incomplete combustion. 

Figure 3-1 shows the CO emission level as a function of excess air ratio for different 

biomass combustion applications. For each system an optimum excess air ratio exists: 

higher excess air ratios will result in a decreased combustion temperature while lower 

excess air ratios will result in inadequate mixing conditions. 
32 

Emissions 

CO

[mg/m3]

0

excess air ratio

10

100

1000

10,000

100,000

12345

a

b

d

c

a) simple, manually 

charged wood boiler; 

b) downdraught wood 

log boiler; 

c) automatic furnace 

with combustion 

technology as of 

1990; 

d) automatic furnace 

with enhanced 

combustion 

technology as of 

1995 

Figure 3-1 CO- emissions - CO vs. excess air ratio (Nussbaumer 1989) 

3.2.2 Measurement 

CO emissions are measured directly after the flue exit of the appliance in an insolated 

pipe. Results are given in ppm per dry flue volume (m3) at normal conditions. In order to 

reference the emissions to an oxygen content of 10, 12 or 13 %, the O2 and/or the CO2

concentration have to be measured as well. 

3.2.3 Reduction 

In order to keep CO and correspondingly OGC emissions low (see chapter 3.2 and 3.3), 

optimal conditions for a high burnout rate of CO must be ensured. These conditions are 

often summarized as the 3T-rule (Time, Temperature, Turbulence) for combustion 

quality. In addition, the correct amount of oxygen (air) must participate in the 

combustion. 

Residence TIME of fuel and oxygen in the combustion zone 

Figure 3-2 shows the correlation between CO burnout and residence time in an ideal 

stirring reactor. The higher the combustion temperature and the oxygen content, the 

lower residence times are necessary to reach high CO burnout rate. 

The residence time is a function of the combustion chamber volume and combustion gas 

flow. The combustion chamber has to be designed to guarantee high residence times also 

at high load operation and to avoid fluidic disadvantageous conditions (e.g. dead zones, 

insufficient mixing, ...) 

High TEMPERATURES of the gases in the combustion zone 

The reaction rate of CO to CO2 depends primarily on temperature if the mixture of fuel 

and oxygen as well as a certain residence time are given. The temperature of the 

combustion zone is amongst others influenced by the materials used for the combustion 
33 

Report 

chamber. If ceramics are used to line the combustion chamber, temperatures will be 

higher than if the combustion chamber was built of steel (must be cooled with water). 

In addition, the air fuel ratio relevancies of extreme importance for the combustion 

temperature, as shown in Figure 3-3. The higher the air fuel ratio, the lower combustion 

temperatures will be. 

Leuckel, Römer, 1979

0,1

1,0

10,0

100,0

0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6

residence time in [s]

share of remaining

CO in [%]

1300 °F, 18%O2

1300 °F, 2%O2

1200 °F, 18%O2

1200 °F, 2%O2

Figure 3-2 CO-rest as function of residence time, combustion temperature and 

oxygen-concentration in an ideal stirring reactor (Leuckel, Roemer 1979) 

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2

Excess air ratio

adiabatic comb. temp. [°C]

20 wt% w.b.

30 wt% w.b.

40 wt% w.b.

50 wt% w.b.

fuel moisture content

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2

Excess air ratio

adiabatic comb. temp. [°C]

20 wt% w.b.

30 wt% w.b.

40 wt% w.b.

50 wt% w.b.

fuel moisture content

Figure 3-3 relationship between the adiabatic combustion temperature and the excess 

air (air fuel) ratio (Nussbaumer 1989) 
34 

Emissions 

High TURBULENCE in combustion zone 

The mixing quality of fuel and oxygen is described by the turbulence in the combustion 

zone. The turbulence is primarily influenced by the flow velocity of the combustion air. 

With increasing flow velocities, the mixing of fuel and air becomes increasingly ideal. This 

again results in lower amounts of air needed for the combustion and thus higher 

combustion temperatures (see Figure 3-3). 

3.3 Total Organic Bound Carbon/Organic Gaseous 

Substances (OGC) 

3.3.1 General 

OGC emissions from biomass appliances are directly linked to high CO emissions. In 

Figure 3-4 the measurement results of the correlation between hydrocarbon and COemissions for wood appliances are shown. If CO-emissions are low, hydrocarbon 

emissions will be 1/10 to 1/100 of the CO-emissions. 

a) Wood stove, b) solid wood boiler with underburning technology, 

c) Wood chips boiler with horizontal feed (Nussbaumer)

1

10

100

1.000

10.000

10 100 1.000 10.000 100.000

Carbon monoxide in [mg/m n3]

Hydrocarbons

[m g/mn3]

a)

b)

c)

1

10

100

1.000

10.000

10 100 1.000 10.000 100.000

Carbon monoxide in [mg/m n3]

Hydrocarbons

[m g/mn3]

a)

b)

c)

Figure 3-4 Correlation of hydrocarbon and CO-emissions in wood appliances 

(Nussbaumer 1989) 

3.3.2 Measurement 

OGC emissions are measured directly after the flue exit of the appliance in an insolated 

pipe. Results are given in ppm per dry flue volume (m3) at normal conditions. In order to 

reference the emissions to an oxygen content of 10, 12 or 13 %, the O2 and/or the CO2

concentration have to be measured as well. 
35 

Report 

3.3.3 Reduction 

If CO emissions are low, also OGC emissions will be low for biomass combustion 

processes. Therefore, no separate reduction concepts for OGC emissions are necessary 

for biomass combustion. 

3.4 Dust · Particulate Matter (PM) · Totals Suspended 

Particulate Matter (TSP) 

3.4.1 General 

Particulates formed during biomass combustion can be divided into bottom ashes and fly 

ashes. The fly ash fraction typically consists of a coarse and a fine mode. While the 

coarse mode (particles larger than some μm) is simply due to the ash entrainment from 

the fuel bed, the formation pathways of the fine mode (so-called aerosols, with a particle 

diameter <1 μm) are much more complex. 

Bottom ash

K, Na, S, Cl, easily 

volatile heavy metals 

released from the fuel 

to the gas phase

Condensation on coarse 

fly ash particles

Coarse fly ash particles 

entrained from the fuel bed

Gas phase reactions

Cooling phase

Nucleation

Condensation

Figure 3-5 ash and aerosol formation processes during biomass combustion 

The basic mechanisms concerning aerosol formation in combustion processes are in 

general well known from former research work (Friedlander 1977). Volatile ash forming 

compounds specific to biomass combustion such as K, Na, S and Cl as well as easily 

volatile heavy metals (Zn and Cd), are released from the fuel into the gas phase and 

subsequently undergo gas phase reactions. As soon as the vapour pressure of a 

compound exceeds the saturation pressure, which can happen either due to a high 

formation ratio of this compound or by cooling of the flue gas, particle formation by 

nucleation or condensation of these vapours on existing surfaces takes place. Nucleation 
36 

Emissions 

and condensation are always competing processes, and if enough surface for 

condensation is available, nucleation can partly or even totally be suppressed. Otherwise, 

very small (some nm) aerosol particles are formed. As soon as these particles are 

formed, they start to coagulate with other aerosols or with coarse fly ashes. Figure 3-5 

gives an overview of the different ash and aerosol formation processes, for more details 

see (Brunner et al 2004). 

Next to the formation path of the fly ash fraction by low volatile minerals, heavy metals 

and salt, aerosols can also be formed due to incomplete combustion. In this case unburnt carbon, fine wood particles, charcoal, soot, tar, ... are emitted as dust. Therefore 

complete combustion (low CO emissions) are necessary to keep also dust emissions as 

low as possible. In Figure 3-6 the chemical compostions of dust samples from an 

appliance that was operated under optimal conditions (low CO emissions) and from a 

wood stove that was operated under poor conditions (high CO emissions) are shown. 

Automatic appliance, optimum

operation conditions

CO < 50 ppm

Wood stove, poor

operation conditions

CO > 10 000 ppm

Figure 3-6 chemical composition of dust samples from different biomass appliances 

and operation conditions (Klippel 2007) 

While the un-burnt fraction of an automatic appliance under optimum operation 

conditions is about 2 wt%, the un-burnt fraction of the wood stove that was operated 

under poor conditions is about 98 wt%. Considering that the amount of salts is 

approximately equal for the automatically and the manually stoked appliances, overall 

dust emissions from the wood stoves will be a multiple higher than those of the 

automatic system. 

Definition and characteristics of coarse fly ashes and aerosols: 

• Coarse fly ashes: 

o ashes entrained from the fuel bed 

o particle sizes < 100 μm 

o mainly consist of refractory species such as Ca, Mg, Si as well as smaller 

amounts of K, Na, Al 
37 

Report 

• Aerosols: 

o Latin: solvere = solute 

o colloidal solid (dust) or fluid (fog) particulate matter in gases 

o formed from condensed ash vapours released from the fuel to the gas 

phase during combustion 

o particle sizes between 1 nm and 1 μm 

o mainly consist of K, Na, S, Cl, Zn and Pb 

• Fly ashes = coarse fly ashes + aerosols 

0 

100 

200 

300 

400 

500 

600 

700 

800 

900 

0.01 0.10 1.00 10.00 100.00 1,000.00 

xm [μm ae.d.]

wood chips [mg/Nm3]

bark [mg/Nm3]

waste wood [mg/Nm3]

aerodynamic diameter dae [μm]

Δm/Δlg(dae) [mg/Nm3]

Figure 3-7 Particle size distributions of fly ashes in the flue gas at boiler outlet in dry 

flue gas at 13% O2 (Brunner et al 2004) 

Figure 3-8 SEM images of coarse fly-ash (left) and aerosol particles (right) 

Description: Samples from wood combustion in a grate furnace; biomass fuel: wood 

(beech). Equipment used: Scanning Electron Microscopy (SEM) (Brunner et al 2004) 
38 

Emissions 

3.4.2 Measurement 

The dust content is determined discontinuously (usually over a period of 30 Minutes) 

using a gravimetric or electrostatic method. Suitable test equipment is shown in 

Figure 3-9. 

1. removable sampling probe 

2. union nut 

3. seal 

4. filtering bush 

5. quartz wool 

6. filter housing 

7. union nut 

8. clamping ring 

9. flat filter support 

10. flat filter 

11. end case 

Figure 3-9 tubular filter device for dust measurement 

With this method the total coarse and a portion of the fine dust fraction in the flue gas 

can be measured. Depending on the plugging method of the quartz wool into the filter 

housing, particulates down to 1 μm are collected in the filter and therefore considered in 

this measurement. In order to avoid the condensing of semi-volatiles, a minimum flue 

gas temperature is required. More information about measurement methods for dust can 

be found in VDI 2066, EN 13284-1 or Nussbaumer 2008. 

Results are given in mg per dry flue volume (m3) at normal conditions. In order to 

reference the emissions to an oxygen content of 10, 12 or 13 %, the O2 and/or the CO2

concentration have to be measured as well. 

3.4.3 Reduction 

3.4.3.1 Primary measures 

The study of the primary reduction of dust emissions is quite a young field of research. 

Oser, Nussbaumer 2004 developed a low-particle concept for biomass appliances: 

• operation at very low air-fuel ratio in the firebed (Excess air ratio in firebed: 0.2-

0.4) 

• gas tightness of combustion and fuel stoking unit 

• operation at low overall air-fuel ratio (Excess air ratio 1.3-1.6) 

• clear staging of secondary air and prevention of backflow of secondary air into the 

firebed. 

• adequate large dimensioned combustion chamber for a high firebed 

• high burnout rate of OGC 

• sufficient high temperatures in primary combustion zone 
39 

Report 

3.4.3.2 Secondary measures 

To reduce dust emissions diverse technologies with different efficiencies and function 

principles were developed. Since costs for investment, operation and maintenance are, 

depending on the technology, quite high and limits are given to operation conditions, 

these technologies were primarily used in large and mediums scale appliances. 

Due to strict dust emission limits in small scale appliances, dust control technologies 

were recently adopted for the utilisation in small appliances as well. 

Table 3-3 gives an overview of particle sizes removed and efficiencies reached by 

medium to large scale dust control technologies. 

Table 3-3 summary of typical sizes of particles removed by various particle control 

technologies 

Particle control 

technology 

Particle size 

(μm) 

Efficiency 

(%) 

Settling chambers > 50 < 50 

Cyclones > 5 < 80 

Multicyclones > 5 < 90 

Electrostatic precipitator < 1 > 99 

Baghouse filters < 1 > 99 

Spray chambers > 10 < 80 

Impingement scrubbers > 3 < 80 

Cyclone spray chambers > 3 < 80 

Venturi scrubbers > 0.5 < 99 

3.4.3.2.1 Cyclones 

Particle separation in a cyclone is based on the principle of gravity in combination with 

centrifugal forces. Gas and solid particles are exposed to centrifugal forces, whereby 

heavy particles experience higher centrifugal forces and therefore hit the wall and slide 

down into a container. The gas flow into the cyclone can be tangential (Figure 3-10) or 

axial. In the second case a fan is necessary to bring the gas into rotation. 

The separation efficiency of a cyclone can be improved by increasing the centrifugal force 

through reduction of the cyclone diameter. In order to prevent the loss of capacity, 

several cyclones can be used in parallel; this is called a multicyclone and is illustrated in 

Figure 3-11. There are however disadvantages of multicyclones in that they are more 

complicated, requiring a more complicated construction, and are as a result more 

expensive. They also require an increased pressure drop, and thus have a higher energy 

consumption. 

Cyclones represent a simple technology which are high reliable and require little 

maintenance. The requirements regarding the flue gas quality are low. Cyclones are used 

at pressure levels from 0.01 to 100 atm and can withstand temperatures of up to 

1650 °F. The efficiency from 80 – 90% is quite low and corresponds to dust loads of 
40 

Emissions 

0.150 to 0.075 lb/MMBtu when biomass is used. This technology is therefore quite often 

used as pre-precipitator. 

Figure 3-10 tangential cyclone 

(Jirkowsky et Al, 2002) 

Figure 3-11 axial multicyclone 

(Jirkowsky et Al, 2002) 

3.4.3.2.2 Electrostatic precipitator 

In an electrostatic filter, the particles are first electrically charged and then exposed to an 

electrical field whereby they are attracted to an electrode. Periodically, this electrode is 

cleaned through vibration, where the dust falls off the electrode into a collection unit. 

This operation principle is shown in Figure 3-12. 

Figure 3-12 operation principle of electrostatic precipitator (Jirkowsky et Al, 2002) 

Different variations of electrostatic precipitators are available: 

The dry electrostatic precipitator works as illustrated in Figure 3-12. In order to 

collect the precipitated particles, discontinuous operating rappers are used. 
41 

Report 

In wet electrostatic precipitators, the flue gas is completely saturated before entering 

the precipitator. This saturation reduces the electrical charging of the particles and 

increases the efficiency. 

Condensing wet electrostatic precipitators are cooled by ambient air, which leads to 

the cooling of the flue gas and the condensation of water on the inner surface of the 

precipitator. The condensed water flows down the electrode and cleans it at the same 

time. 

Electrostatic precipitators reach efficiencies greater than 99%. 

3.4.3.2.3 Baghouse filters 

Baghouse filters consist of a filter or cloth tightly woven from special fibres and hung in a 

closed construction through which flue gas passes. The separation efficiency of bag filters 

is quite high, even with high flue gas flow rates and high particle content. The first layer 

of particles actually improves the filtration efficiency. However, as more particles settle 

on the cloth, the pressure drop increases. The cloth is thus periodically cleaned by 

vibration or pressurised air (Figure 3-13). 

Figure 3-13 pulse jet 

baghouse filter 

(Jirkowsky et Al, 2002) 

The operating temperature range is limited to approximately 480 °F; there is a significant 

risk of fire if this temperature is exceeded or when un-burnt carbon is present in the fly 

ash. In order to limit the amount of particles settling on the filter and to reduce the 

chance of fire through sparks, a cyclone can be used. When the operating temperatures 

are too low, tars present in the flue gas may condense and clog the cloth. In addition, 

the moisture in the flue gas is limited to a maximum of 20 %. 

Baghouse filters reach efficiencies of greater than 99% and therefore reduce the 

emissions of wood appliances below 0.010 lb/MMBtu. 

3.4.3.2.4 Scrubbers 

In scrubbers, particles are scrubbed out from the flue gas by water droplets of various 

sizes depending on the type of scrubber used. The particles are removed by the collision 

and interception between droplets and particles. Upon impact, the particles are wetted 

and carried by the water droplet, thus effecting removal. The more droplets formed, the 
42 

Emissions 

more efficient the unit will be. The droplets must therefore be small. Smaller diameter 

spray nozzles will produce smaller droplets, but will also result in higher pressure drops, 

consuming more energy. Since efficiency increases as the droplet size decreases, the 

efficiency also increases with increasing pressure drop. 

Several types of scrubbers exist. In a countercurrent scrubber for example, the flue 

gas is introduced at the bottom side of the unit and flows upward against the current 

with respect to the settling of the atomised liquid droplets. In a cross-flow scrubber, 

the flue gas flows perpendicular with respect to the settling of the atomised spray water 

droplets. Although two sets of sprays atomise the water in horizontal directions, the 

settling of the resulting droplets is still downward, perpendicular to the direction of the 

flue gas. A cyclone spray chamber (Figure 3-14) is a combination of an ordinary spray 

and a cyclone. A venturi scrubber (Figure 3-14) works with a high pressure nozzle in 

which the flue gas and the liquid are atomised. 

Figure 3-14 venturi scrubber (left), cyclone spray chamber (right) (Corbitt 1999) 

Scrubbers reach efficiencies of up to 99%. They are usually only used if acid components 

in the flue gas (HCl, SO2) have to be precipitated as well (FNR 2005). This is necessary 

for contaminated biomass or for non wood biomass with high chlorine and/or sulphur 

contents. For medium and large scale wood biomasses appliances this technology is not 

relevant, but some scrubber systems – always combined with flue gas condensation – 

have recently been developed for small scale appliances. 

3.4.3.2.5 Flue gas condensation 

The primary aim of flue gas condensation is the heat recovery from fuels with a high 

water content. An additional effect of flue gas condensation is that the dust load is 

precipitated by the condensing water and dust loads can be reduced down to 

0.060 lb/MMBtu (FNR 2005). In medium to large scale appliances a cyclone should be 

used before flue gas condensation. In order to enable flue gas condensation, a return 

temperature of about 85 °F should be possible. 
43 

Report 

3.5 Nitroxen Oxides (NOX) 

NOX emissions from biomass combustion applications are mainly a result of complete 

oxidation of nitrogen in the fuel. Additional NOX may be formed from nitrogen in the air 

given certain conditions. However, these reaction mechanisms are not considered to be 

of significant importance in most biomass combustion applications. The main nitric oxide 

emitted is NO, which is converted to NO2 in the atmosphere. In Figure 3-15 the diverse 

reaction paths of the nitrogen in the fuel are exhibited. It shows that most of the 

nitrogen in the fuel is converted to N2, and some N is bound in the ash, but there is also 

a fraction of nitrogen in the fuel that is converted to NO and N2O. 

N in ash

N2O

NO

N2

biomass

volatile

HCN

coke

N

NHi

i=0,1,2,3 NO

N2

N2O

O, OH O, OH

NO

CHi

(„NO-Recycle“) NCO

NH

NHi

CO, CHi

C, ash

H

OH

pyrolysis and gasification intermediate

products

oxidation products in

flue gas

Figure 3-15 formation paths of biomass fuel nitrogen 

Besides the fuel NOX mechanism described in Figure 3-15, two further mechanisms – 

the thermal NOX mechanism and the prompt NOX mechanism - result in increased 

NOX emissions. These two NOX mechanisms gain relevance at combustion temperatures 

above 2300 °F (1300 °C), as shown in Figure 3-16. Since these temperatures are hardly 

ever reached during biomass combustion, the thermal and prompt NOX mechanisms are 

not considered to be relevant for biomass combustion. Figure 3-16 also shows that 

typical NOX-emissions for wood combustion are in the range of approximately 90 to 

250 mg/m3. For herbaceous biofuels, the typical NOX emissions are between 250 and 

550 mg/m3, because the nitrogen in the fuel is generally higher in herbaceous fuels than 

in wood fuels. 

As Figure 3-16 shows, the nitrogen content of the fuel and the NOX-emissions are directly 

related for biomass combustion, meaning that if the nitrogen content of the fuel is high, 

the NOX-emissions will be high as well. This relationship is however not linear. 

Conversion rate of Nitrogen in the fuel to NOX decreases with increasing Nitrogen 

content. 
44 

Emissions 

1600

0

100

200

300

400

500

600

700

800

900

800 1200 1400

Prompt- NOx

1600

0

100

200

300

400

500

600

700

800

900

800 1200 1400

Prompt- NOx

time, O2

Thermal NOx for

t=0.5 s, O2 = 11 vol %

Fuel NOx, wood

Fuel NOx, herbacious biofuels

Fuel NOx, UF-Chip board

temperature [°C]

NOx as NO2 at 11% O2

[mg/m3] Typical biomass combustion Thermal NOx and prompt NOx

Figure 3-16 NOx-emissions - dependence on the fuel and the furnace temperature 

(Nussbaumer 1997) 

Figure 3-17 correlation 

of the nitrogen content 

of the fuel and NOXemissions (Wopienka 

et al. 2007) 

3.5.1 Measurement 

NOX emissions equal the sum of NO and NO2-emissions. They are measured directly after 

the flue exit of the appliance in an insolated pipe. Results are given in ppm per dry flue 

volume (m3) at normal conditions. In order to refer the emissions to an oxygen content 

of 10, 12 or 13 %, the O2 and/or the CO2 concentration have to be measured as well. 
45 

Report 

3.5.2 Reduction 

3.5.2.1 Primary measures 

To reduce NOX emissions by primary measures, three different concepts are available. 

Air staging 

Primary and secondary combustion air is injected in clearly separated zones or 

combustion chambers. Between these zones the NO is reduced in the reduction zone, 

where a excess air ratio of 0.7 – 0.8 and temperatures from 2000 to 2200 F are required. 

With this technology, a NOX reduction in the range of 30 – 60% can be reached. 

In Figure 3-18 the concept for air staging and flue gas recirculation is shown. 

Flue gas recirculation 

The recirculation of flue gas is primarily used to reduce or control combustion 

temperatures and therefore the formation of thermal NOX. Thermal NOX is however not 

considered to be a relevant formation mechanism for biomass combustion (see chapter 

3.5). Some reduction of fuel NOX is nevertheless achieved by flue gas recirculation. 

pyrolysis

λ << 1

1650 F

fuel

prim. air

reduction zone

λ 0.7 – 0.8

2000 – 2200 F

oxidation zone

λ > 2

1500 F

secondary

air

flue gas

ash ash

heat 

exchanger

flue gas recirculation

pyrolysis

λ << 1

1650 F

fuel

prim. air

reduction zone

λ 0.7 – 0.8

2000 – 2200 F

oxidation zone

λ > 2

1500 F

secondary

air

flue gas

ash ash

heat 

exchanger

flue gas recirculation

Figure 3-18 concept of air staging and flue gas recirculation (adopted from 

Zuberbuehler 2002) 

Fuel staging 

The concept of fuel staging aims at the same mechanisms as air staging by producing a 

reduction zone where NO is reduced. In the reduction zone, a excess air ratio from 0.6 – 

0.8 is required. Approximately 70% of the fuel is stoked as primary fuel. As secondary 

fuel, biomass producer gas, shavings, sawdust, ... are possible. With this reduction 

mechanism, a NOX reduction in the range of 50 to 70% can be achieved. In Figure 3-19 

the concept for fuel staging is shown. 

devolitalisation and 

oxidation

λ > 1

primary

fuel

prim. air reduction zone

λ < 1

secondary

fuel

oxidation zone

λ > 1

secondary

air

flue gas

ash ash

devolitalisation and 

oxidation

λ > 1

primary

fuel

prim. air reduction zone

λ < 1

secondary

fuel

oxidation zone

λ > 1

secondary

air

flue gas

ash ash

heat exchanger

Figure 3-19 concept of fuel staging for primary NOX reduction 
46 

Emissions 

3.5.2.2 Secondary measures 

To reduce NOX emissions by secondary measures, two different concepts were 

developed. Both concepts are applied in large scale appliances only because operation 

requirements and costs are too high for small and medium scale appliances. Figure 3-20 

shows typical NOX emissions for biomasses and the NOX reduction potential of primary 

and secondary measures. 

For wood biomass combustion, secondary cleaning measures for NOX are 

usually not necessary because sufficient NOX reduction can be achieved by 

primary measures (FNR 2005). 

Figure 3-20 NOx-emissions and reduction potential – overview 

(Hasler et al. 2000) 

Explanations: NOx calculated as NO2 and related to the dry flue gas at 11 vol.% O2 

SNCR: selective non catalytic reduction 

In the SNCR process a reducing agent (ammonia or urea) is injected into the hot flue gas 

(in the afterburner chamber). The reducing agent reacts with the NOX to create elemental 

Nitrogen. The temperature range for optimal reaction is between 1550 and 1750 °F. If 

the temperature is too low, the reaction will be slow and if temperature is too high, side 

reactions will become more prominent and NO and N2O will be formed. To reach the 

maximum NOX removal, the reducing agents have to be added hyperstoichiometrically 

(1.5 for ammonia and 2 for urea). This leads to the emission of ammonia or urea in the 

flue gas. To keep these emissions low, the reducing agents have to be metered exactly. 

A NOX reduction of 60 to 70% can be reached. 
47 

Report 

SCR: selective catalytic reduction 

The SCR process works at the same principle as the SNCR process, but catalysts are used 

to accelerate the reaction and to keep reaction temperatures lower. Platinum, tungsten, 

titanium or vanadium oxide based materials are used as catalysts. The optimum 

temperature level for ammonia is between 430 and 520 °F, and between 750 and 840 °F 

for urea. Due to the higher reaction rate, the reducing agents can be added nearly 

stoichiometric thus resulting in low emissions of ammonia or urea. A NOX reduction of 80 

– 95% can be achieved. 

A drawback of this technology is however the danger of the deactivation of the catalyst 

by alkaline compounds and heavy metals. 
48 

European Test 

Methods 

4 European Test Methods 

Table 4-1 gives an overview of the European standards for biomass (solid fuel) 

combustion technologies. These standards consider requirements with respect to 

materials, design and construction, safety, performance, appliance instructions, marking, 

evaluation of conformity and type testing of the appliance. The performance 

requirements indluce a minimum thermal efficiency as well as limits for CO and optionally 

for OGC and dust. 

Table 4-1 European standards applied for biomass combustion technologies 

type Standard comments 

BOILERS

pellets, solid wood, wood 

chips, combined 

technologies 

EN 303-5 

<1000 MBtu/h 

Solid fuels 

max. water temperature: 100 °C, 

max. operating pressure: 6 bar 

boiler also providing direct 

heat EN 12809 

< 170 MBtu/h 

solid fuels (except for pellets) 

max. operating pressure: 2 bar 

ROOM-HEATERS

cook stoves EN 12815 

solid fuels (except for pellets) 

manual stoking 

hot water production also possible 

fireplace insets with or 

without doors 

masonry heaters (insets) 

EN 13229 

solid fuels (except for pellets) 

manual stoking 

with or without combustion chamber doors, 

no fan 

hot water production also possible 

wood stoves (continuous 

or intermitted operation) EN 13240 

solid fuels (except for pellets) 

manual stoking 

with combustion chamber doors 

no fan 

hot water production also possible 

pellet stoves EN 14785 

< 170 MBtu/h 

pellets only 

with or without fan 

closed fire doors only 

hot water production also possible 

masonry heaters 

(handcrafted) EN 15250 

solid fuels (pellets stoked manually) 

intermittend buring 

no fan 

no boiler function 
49 

Report 

4.1 Residential heating systems 

4.1.1 EN 303-5: Heating boilers for solid fuels, hand and automatically 

stoked 

Full title: EN 303-5. Heating boilers for solid fuels, hand and automatically stocked, 

nominal heat output of up to 1025MBtu - Terminology, requirements, testing and 

marking 

Prepared by/approved/amends: Prepared by CEN TC 57 Central heating boilers. 

Approved on 12 November 1998 and published in April 1999. The standard is to be 

revised: a first proposal has been elaborated and the discussion at CEN-level started in 

spring 2008. 

Appliances: Boilers for hydronic heating systems up to nominal heat output of 

1025 MBtu/h with manual and automatic stoking. Minimum heat loss of boilers has to 

stay within the requirements of the standard; otherwise the standard does not apply. 

Fuels: Wood (logs, chops, briquettes, pellets, sawdust) and fossil fuels (coal, coke, 

anthracite) 

Test Methods: Specifies test methods for heat output, efficiency and emissions (CO, 

OGC, dust); measurement methods for emissions are not defined. 

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Table 4-2 summary of EN 303-5 with regard to testing procedure 

MANUALLY STOKED BOILERS AUTOMATICALLY STOKED BOILERS

TEST DURATION AND NUMBER OF TESTS

o Before the tests may begin, the boiler 

has to be brought to its normal 

working condition using a complete 

fuel load (up to the maximum filling 

height). Testing can start when the 

necessary basic firebed is established. 

o Test duration starts immediately after 

placing the fuel on the basic firebed and 

ends with the refill. 

o For the nominal and partial heat 

loads, two nominal combustion periods 

and one combustion period are required 

respectively. 

o Before the tests may begin, the boiler 

has to be heated up to its normal 

working condition. Testing can start 

when the necessary basic firebed is 

established. 

o For the nominal and partial heat 

loads, test duration has to be at least 6 

hours. 
50 

European Test 

Methods 

MANUALLY STOKED BOILERS AUTOMATICALLY STOKED BOILERS

HEAT FLOW - EFFICIENCY

o At nominal heat load the mean value of the flow temperature has to lie in the range 

of 155 to 195 °F and the average temperature difference of flow and return has to lie 

in the range of 50 and 75 °F. In addition, the difference value between the ambient 

air temperature and the mean value of flow and return temperature has to be not less 

than 105 °F. 

o At partial heat load the same conditions as for nominal heat load have to be 

fulfilled, except for the requested temperature difference of flow and return 

temperature. 

o The efficiency is determined on the basis of the net calorific value and has to be 

calculated with the direct method. The indirect calculation enables control of the 

measurement accuracy of the test rig and determination of the single losses. 

EMISSIONS

o At nominal heat load, the test period 

covers two successive combustion 

periods (including refill), CO2 or O2, CO 

and OGC (and NOX where appropriate) 

are measured during the complete test 

duration. 

o At partial heat load the test period 

covers one combustion period, CO2 or 

O2, CO and OGC are measured. 

o For the measurement of TSP at 

nominal heat load, every combustion 

period is split into two equal time 

segments. Measurements start in each 

case at the beginning of a time 

segment, with the first measurement 

beginning immediately after the fuel is 

placed and the door closed. The suction 

time per filter is limited to 30 min. The 

average TSP content is determined from 

the 4 minimum half-hour values taken 

at half our intervals. 

o At nominal heat, CO2 or O2, CO and 

OGC (and NOX where appropriate) are 

measured over the entire test duration. 

o At partial heat load CO2 or O2, CO and 

OGC are measured over the entire test 

duration. 

o For the measurement of TSP at 

nominal heat load, the test period is 

divided into at minimum 4 equal time 

segments. The measurements begin in 

each case at the start of the segment, 

with the first measurement taken when 

the test begins. The suction time per 

filter is limited to 30 min. The average 

dust content is determined from the 4 

minimum values taken at half hour 

intervals. 

o For the calculation of the emissions from a combustion period, the measured data 

sets are first averaged over the complete combustion period. Afterwards 

transformation to the desired units (mg/m3 at certain oxygen level or lb/MMBtu) is 

performed. 

The ambient air temperature during testing procedure hast to be between 60 and 85 °F. 

Partial load is 50% of nominal heat load for manually stoked boilers and 30% of nominal 

heat load for automatically stoked boilers. 
51 

Report 

4.1.2 EN 12809: Residential independent boilers fired by solid fuel 

Full title: EN 12809. Residential independent boilers fired by solid fuel – Nominal heat 

output up to 170 Btu/h – Requirements and test methods. 

Prepared by/approved/amends: Prepared by CEN TC 295 Residential solid fuel 

burning appliances, approved on 7 April 2001, amends: EN 12809:2001/A1:2004 

Appliances: Hand and automatically fired, manual fuelled solid fuel appliances up to a 

nominal heat output of 170 MBtu/h for hydronic heating systems also providing direct 

heat at the place of installation. 

Fuels: Solid mineral fuels, peat briquettes, natural or manufactured wood logs or multifuel in accordance with the operation instructions of the appliance 

Test Methods: Specifies test methods for total heat output, water heating output, space 

heating output, efficiency and emissions (CO, dust, OGC); measurement methods for 

dust and OGC are not defined 

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4.1.3 EN 12815: Residential cookers fired by solid fuel 

Full title: EN 12815. Residential cookers fired by solid fuel – Requirements and test 

methods 

Prepared by/approved/amends: Prepared by CEN TC 295 Residential solid fuel 

burning appliances, approved on 7 April 2001, amendments: EN 12815: 2001/A1: 2004 

Appliances: Hand fired appliances which primary function is to cook and secondary 

function is direct heating. In case of connecting these appliances to a boiler, they also 

provide domestic hot water and/or central heating. 

Fuels: Solid mineral fuels, peat briquettes, wood logs or multi-fuel in accordance with 

the operation instructions of the appliance. 

Test Methods: Specifies test methods for total heat output, water heating output, space 

heating output, efficiency and emissions (CO, dust, OGC); measurement methods for 

dust and OGC are not defined 

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4.1.4 EN 13229: Inset appliances including open fires fired by solid 

fuels 

Full title: EN 13229. Inset appliances including open fires fired by solid fuels – 

Requirements and testing methods 
52 

European Test 

Methods 

Prepared by/approved/amends: Prepared by CEN TC 295 Residential solid fuel 

burning appliances, approved on 7 April 2001, amendments: EN 13229: 2001/A1: 2003, 

EN 13229:2001/A2:2004 

Appliances: Freestanding or inset hand fired appliances which have functional 

modification or inset appliances for fireplaces: recessed and enclosed. Where connected 

to a boiler, also domestic hot water and/or central heating are provided. 

Fuels: Solid mineral fuels, peat briquettes, wood logs or multi-fuel in accordance with 

the operation instructions of the appliance. 

Test Methods: Specifies test methods for total heat output, water heating output, space 

heating output, efficiency and emissions (CO) 

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Table 4-3 shows the categorisations of appliances to EN 13229 and EN 13240. 

Table 4-3 Categorisation of appliances (EN 13229 versus EN 13240) 

 a) Freestanding or 

inset appliances 

without 

functional 

modification 

b) Freestanding or 

inset 

appliances with 

functional 

modification 

c) Inset 

appliances for 

fireplace 

recess and 

enclosure 

1. Appliances operating 

with firedoors closed EN 13240 EN 13229 EN 13229 

2. Appliances operating 

with firedoors closed 

or open 

EN 13240 EN 13229 EN 13229 

3. Open fires without 

firedoors EN 13229 EN 13229 EN 13229 

NOTE: Without functional modification means “modification of the surrounding of an 

appliance, which only changes the transmission of heat without affecting combustion”. 
53 

Report 

Table 4-4 summary of EN 13229 with regard to testing procedure 

NOMINAL HEAT LOAD PARTIAL LOAD

TEST DURATION

o Before tests may begin, the appliance 

has to be operated until steady state is 

reached; at least one combustion period 

at nominal heat load is necessary before 

testing. Prefabricated heaters have to 

be operated before testing until the 

ceramics has completely dried

(usually 10 hours or more). 

o Testing starts immediate after refilling 

the fuel onto the firebed and ends when 

all fuel except the ash is burnt. 

o At least two tests in two different 

combustion periods have to be carried 

out for nominal heat load. 

o minimum combustion times with closed 

doors for wood fuel: 

o 1 hour for continuous 

operating stoves 

o 0.75 hours for intermittent 

operating stoves 

o 90 (+10/-20) min. for 

prefabricated heaters 

o Before test may begin, the appliance 

has to be operated at nominal heat 

load. 

o Testing starts when operating conditions 

are stable for not less than 15 min. 

o For prefabricated heaters the test 

duration ends when an ignitable firebed 

is left, for continuous operating 

stoves the test is finished when the 

mass of the firebed equals the mass of 

the firebed before start of the test. 

o minimum combustion times with closed 

doors for wood fuel: 

o 3 hours for continuous 

operating stoves 

o (50 ± 10)% of time at 

nominal load 

HEAT FLOW – EFFICIENCY

o Efficiency is calculated as the average of 

the two test periods. 

o If appliance is equipped with water 

circulation, flow temperature during test 

must be 175 ± 40 F and flow rate of 

water has to vary less then ± 5 %. 

o If appliance is equipped with water 

circulation, flow temperature must be 

below 185 F and flow rate of water has 

to vary less then ± 5 %. 

o Efficiency is calculated by the indirect method, considering flue gas loss from heat, 

un-burnt matter and losses in the ash from un-burnt combustible matter. If appliance 

has water circulation, the hot water output is considered as well. 

EMISSIONS

o CO2 or O2 and CO are measured during the complete test durations. 

o For the calculation of the emissions from a combustion period, the measured data 

sets are first averaged over the complete combustion period. Afterwards 

transformation to the desired units (mg/m3 at certain oxygen level or lb/MMBtu) is 

performed. 

4.1.5 EN 13240: Room-heaters fired by solid fuel 

Full title: EN 13240. Room-heaters fired by solid fuel – Requirements and test methods 
54 

European Test 

Methods 

Prepared by/approved/amends: Prepared by CEN TC 295 Residential solid fuel 

burning appliances, approved on 7 April 2001, amendments: EN 13240: 2001/A2: 2004 

Appliances: Manually stoked appliances which are operated with firedoors closed or 

firedoors closed or open, providing direct space heat. Standard also applies to slow heat 

release appliances which are available as complete units or pre-fabricated parts designed 

to be built on site. 

Fuels: Solid mineral fuels, peat briquettes, wood logs or multi-fuel in accordance with 

the operation instructions of the appliance. 

Test Methods: Specifies test methods for total heat output, water heating output, space 

heating output, efficiency and emissions (CO, dust, OGC); measurement methods for 

dust and OGC are not defined 

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Table 4-5 summary of EN 13240 with regard to testing procedure 

NOMINAL HEAT LOAD PARTIAL LOAD

TEST DURATION

o Before the test may begin, the 

appliance has to be operated until 

steady state is reached; minimum 

combustion times and number of 

combustion periods for wood fuel are: 

o 1.5 hours / 2 periods for 

continuous operating stoves 

o 0.75 hours / 3 periods for 

intermitted operating stoves 

o Testing starts immediate after refilling 

the fuel onto the firebed and ends when 

all the fuel except the ash is burnt. 

o Before the test may begin, the 

appliance has to be operated at nominal 

heat load or the necessary firebed can 

be reached within one hour after 

ignition in the cold appliance. 

o Testing starts when operating conditions 

are stable for no less than 15 min. 

o Testing is finished for continuous 

operating stoves when the minimum 

combustion time of 10 hours (or longer 

time claimed by manufacturer) is 

reached. The firebed at the end of the 

test duration has to equal the mass of 

the firebed before start of the test. 

HEAT FLOW – EFFICIENCY

o Efficiency is calculated as the average of 

the two test periods. 

o If appliance is equipped with water 

circulation, flow temperature during test 

must be 175 ± 40 F and flow rate of 

water has to vary less then ± 5 %. 

o If appliance is equipped with water 

circulation, flow temperature must be 

below 185 F and flow rate of water has 

to vary less then ± 5 %. 

o Efficiency is calculated by the indirect method, considering flue gas loss from heat 

and un-burnt matter and losses in the ash from un-burnt combustible matter. If 

appliance has water circulation, the hot water output is considered as well. 
55 

Report 

NOMINAL HEAT LOAD PARTIAL LOAD

TEST DURATION

EMISSIONS

o CO2 or O2 and CO are measured during the complete test durations. 

o For the calculation of the emissions from a combustion period, the measured data 

sets are first averaged over the complete combustion period. Afterwards 

transformation to the desired units (mg/m3 at certain oxygen level or lb/MMBtu) is 

performed. 

4.1.6 EN 14785: Residential space heating appliances fired by wood 

pellets 

Full title: EN 14785. Residential space heating appliances fired by wood pellets – 

Requirements and test methods 

Prepared by/approved/amends: Prepared by CEN TC 295 Residential solid fuel 

burning appliances, approved on 3 May 2006. 

Appliances: Residential space heaters fired by wood pellets. Appliances may be 

freestanding or inset appliances and provide direct heat. They can be operated by either 

a natural draught or fan-assisted combustion air. Where combined with a boiler, they 

also provide domestic hot water and/or central heating. These appliances are only 

operated with fire-doors closed. 

Fuels: wood pellets 

Test Methods: Specifies test methods for total/nominal/reduced heat output, water 

heating output, space heating output, efficiency and emissions (CO, dust, OGC); 

measurement methods for dust and OGC are not defined 

Availability: Sharing is restricted, English version purchasable: https://www.onnorm.at/shopV5/search/Details.action?showDetails=&dokkey=221785
56 

European Test 

Methods 

Table 4-6 summary of EN 14785 with regard to testing procedure 

NOMINAL HEAT LOAD PARTIAL HEAT LOAD

TEST DURATION

o Before the tests may begin, the 

appliance has to be de-ashed and 

operated for at least 30 min. in its 

steady state (normal working 

conditions). Steady state is reached 

when flue temperature varies less than 

± 40 °F. 

o Test starts with the refill of the 

appliance and has to last for at least 3 

hours. If manufacturer claims a longer 

combustion period, the test has to be 

prolonged according to the 

manufacturer’s declaration. 

o At least two tests in two different 

combustion periods have to be carried 

out. 

o Before the tests may begin, the 

appliance has to be operated at nominal 

heat load. After de-ashing the appliance 

has to be operated at partial load for at 

least one hour in steady state. Refill of 

appliance is possible before start of test.

o Test duration has to last at least 6 

hours. 

o No partial heat load tests are carried 

out, if the appliance works in the on/off 

– mode only. 

o One test is required. 

HEAT FLOW - EFFICIENCY

o Efficiency is calculated as the average of 

the two test periods. 

o If appliance is equipped with water 

circulation, flow temperature during test 

must be 175 ± 40 °F and flow rate of 

water has to vary less then ± 5 %. 

o If appliance is equipped with water 

circulation, flow temperature must be 

below 185 °F and flow rate of water has 

to vary less then ± 5 %. 

o Efficiency is calculated by the indirect method, considering flue gas loss from heat 

and un-burnt matter and losses in the ash from un-burnt combustible matter. If 

appliance has water circulation, the hot water output is considered as well. 

EMISSIONS

o CO2 or O2 and CO are measured during the complete test durations. 

o For the calculation of the emissions from a combustion period, the measured data 

sets are first averaged over the complete combustion period. Afterwards 

transformation to the desired units (mg/m3 at certain oxygen level or lb/MMBtu) is 

performed. 

4.1.7 EN 15250: Slow heat release appliances fired by solid fuel 

Full title: EN 15250. Slow heat release appliances fired by solid fuel – Requirements and 

test methods 

Prepared by/approved/amends: Prepared by CEN TC 295 Residential solid fuel 

burning appliances, approved on 13 January 2007. 
57 

Report 

Appliances: Manually stoked, intermittent burning, slow heat release appliances with 

thermal storage capacity. Appliances are always built on site, either completely hand 

crafted or assembled with pre-fabricated parts. This standard specifies the minimum time 

period within which the maximum differential surface temperature may be reduced to 

half of its original value. 

Fuels: Solid mineral fuels, peat briquettes, wood logs or multi-fuel in accordance with 

the operation instructions of the appliance. Manually stoked wood pellets can also be 

burnt either on the existing grate on in special insets. 

Test Methods: Specifies test methods for total heat output, efficiency and emissions 

(CO, dust, OGC); measurement methods for dust and OGC are not defined 

Availability: Sharing is restricted, preview is attached, English version purchasable: 

https://www.on-norm.at/shopV5/search/Details.action?showDetails=&dokkey=278524

Table 4-7 summary of EN 15250 with regard to testing procedure 

NOMINAL HEAT LOAD PARTIAL HEAT LOAD

TEST DURATION

o The test is to be started cold without a 

pre-test period, whereby ignition is to 

be performed either according to the 

manufacturer’s instructions or using 

kindling of mass of 500g or 10% of the 

fuel load, whichever is greater. Any 

bottom air entry is to be open during 

ignition. 

o The adding of further batch charges 

should not exceed 3 hours 

o Any batch charges shall not be less than 

20% of the total fuel loading. 

o The test is over once 25% of the mean 

maximum differential surface 

temperature of the appliance’s external 

surfaces has been reached. 

o One test is required. 

o A Temperature Safety Test is also 

required 

o The test is to be started cold without a 

pre-test period, whereby ignition is to 

be performed either according to the 

manufacturer’s instructions or using 

kindling of mass of 500g or 10% of the 

fuel load, whichever is greater. Any 

bottom air entry is to be open during 

ignition. 

o Any batch charges shall not be less than 

20% of the total fuel loading. 

o The adding of further batch charges 

should not exceed 3 hours 

o Any batch charges shall not be less than 

20% of the total fuel loading. 

o One test is required. 

o A Temperature Safety Test is also 

required. 

HEAT FLOW - EFFICIENCY

o If the test fuel is wood logs, the 

combustible constituents of the residue 

need not be measured, but can be taken 

as 0,5 % points of efficiency. 

o If the test fuel is wood logs, the 

combustible constituents of the residue 

need not be measured, but can be taken 

as 0,5 % points of efficiency. 

o Efficiency is calculated by the indirect method, considering the heat losses 

determined from the mean value of the flue gas and room temperature, the flue gas 

composition and the combustible matter in the residue. 
58 

European Test 

Methods 

NOMINAL HEAT LOAD PARTIAL HEAT LOAD

EMISSIONS

o CO2 or O2 and CO are measured during the complete test duration. 

o For the calculation of the emissions from a combustion period, the measured data 

sets are first averaged over the complete combustion period. Afterwards 

transformation to the desired units (mg/m3 at certain oxygen level or lb/MMBtu) is 

performed. 

4.2 Commercial heating systems 

For commercial heating systems > 1025 MBtu, no European standards are defined. These 

plants undergo individual commissioning procedures. An example of a commissioning 

report is added in the annex. 
4.3 Summary 

Table 4-8 summary of standards for residential heating systems 

 303-5 12809 12815 13229 13240 full title Heating boilers for solid 

fuels, hand and automatically stocked, nominal heat output of up to 300 kW - Terminology, requirements, testing and marking 

Residential independent boilers fired by solid fuel – Nominal heat output up to 50 kW – Requirements and test methods 

Residential cookers fired by solid fuel – Requirements and test methods 

Inset appliances including open fires fired by solid fuels – Requirements and testing methods 

Room-heaters fired by solid fuel – Requirements and test methods 

Appliance Boilers for hydronic heating 

without direct space heating output 

Residential independent solid fuel fired boilers with heat direct to living room 

Cookers (with or without water circulate) 

Freestanding or inset appliances 

Room-heaters (with or without water circulate) 

Fuel wood and fossil fuels Solid mineral fuels, peat briquettes, natural or manufactured wood logs or multi-fuel 

with the operation instructions of the appliance 

Heat Output 

determined on the basis of water flow and temperatures 

total heat output: based on efficiency, fuel input and calorific value hot water output: based on water flow and temperatures space heating output: difference of total heat output and hot water output 

Efficiency Direct method: Quotient of 

heat output and heat input. Indirect method for control purposes only 

Indirect method: flue gas loss from heat, un-burnt matter and losses in the ash from 

Emissions CO2 or O2, CO, OGC, TSP 

(NOX where appropriate) 

CO2 or O2 and CO 
60 

Regulations for 

biomass 

installations 

5 Regulations for biomass installations 

Performance regulations for biomass installations (emission limits and thermal efficiency 

requirements) aim at increasing the number of newly installed, state-of-the-arte 

appliances, thus decreasing the number of old technologies in operation. The end result 

is thereby the continuous improvement in performance of biomass appliances in 

operation. 

These regulations have to consider the different performance levels of appliances in 

Europe and at the same time encourage all countries to improve their technologies, 

regardless if the country is leading or laging behind in combustion technology. In order to 

embrace all technology stages in the European Union, different emission and efficiency 

classes are defined for boilers in the EN 303-5. Additionally, national requirements and 

eco labeling programs also encourage technology development to increase performance 

levels. 

Figure 5-1 shows efficiency requirements for boilers in the EN 303-5 (classes 1 to 3 are 

currently in practice and classes 3 to 5 are under discussion) and efficiency requirements 

from Austrian, German and Scandinavian eco-labels. A continuous increase of efficiency 

requirements is one major driving force for technology developments. 

45%

50%

55%

60%

65%

70%

75%

80%

85%

90%

0 300 600 900 1200 1500

nominal load in mBTU/h

class 3

class 2

class 1

class 4

class 5

efficiency based on GCV

Blue Angel (G)/Umweltzeichen (A)

Nordic Swan (Scandinavia)

Figure 5-1 efficiency requirements for boilers from EN 303-5 (class 1 to class 5) and 

efficiency requirements from Austrian, German and Scandinavian eco-labels; 

efficiencies based on GCV were converted for pellets fuel 
61 

Report 

5.1 European level 

5.1.1 Regulations for boilers 

EN 303-5 sets the standards for solid fuel boilers with a nominal heat load up to 

1025 MBtu/h. In EN 303-5 emission limits for CO, OGC and dust as well as thermal 

efficiency requirements for solid fuel boilers are specified. In order to embrace the 

diverse combustion standards from EU member states, three different emission and 

efficiency classes were defined. Table 5-1 shows the emission limits for wood fuel and 

fossil fuel boilers in the valid standard. 

Table 5-1 emission limits for biogenic fuels in EN 303-5 

Emission limits at 12% O2

CO 

in mg/m3 

OGC 

 in mg/m3 

Dust3) 

 in mg/m3 

(and lb/MMBtu) 

class class class class class class class class class 

Stoking 

Nominal 

heat 

load in 

MBtu/h 

1 2 3 1 2 3 1 2 3 

≤ 170 20455 6545 4090 1635 245 120 165 

(0.22) 

145 

(0.20) 

125 

(0.17)

170 - 

510 10227 4090 2045 1230 165 80 165 (0.22) 

145 

(0.20) 

125 

(0.17)

Manual1)

510 - 

1025 10227 1635 980 1230 165 80 165 (0.22) 

145 

(0.20) 

125 

(0.17)

≤ 170 12270 4090 2455 1430 165 80 165 

(0.20) 

145 

(0.18) 

125 

(0.15)

170 - 

510 10230 3680 2045 1020 120 65 165 (0.20) 

145 

(0.18) 

125 

(0.15)

Automatic2)

510 - 

1025 10230 1635 980 1020 120 65 165 (0.20) 

145 

(0.18) 

125 

(0.15)

1) Based on the higher heat value (GCV) and the heat value for solid wood (dust values only) 

2) Based on the higher heat value (GCV) and the heat value for pellets (dust values only) 

3) Dust emission limits are identical for automatical and manual systems in EN 303-5, conversion from 

mg/m3 to lb/MMBtu results in higher emission limits for manually stoked appliances, though (see Table 3-2 

on page 31) 

The emission limits for fossil fuels will be identical to the emission limits for biogenic 

fuels, except for dust – emissions (brown coal fuel assumed): 

• class 1: 145 mg/m at 12% O2 and 0.185 lb/MMBtu 

• class 2: 125 mg/m at 12% O2 and 0.160 lb/MMBtu 

• class 3: 100 mg/m at 12% O2 and 0.130 lb/MMBtu 
62 

Regulations for 

biomass 

installations 

The efficiency requirements in the valid EN 303-5 are calculated as follows (based on the 

GCV): 

• 41 + 5.5·log Pn for class 1 

• 50 + 5.5 log Pn for class 2 

• 59 + 5.5 log Pn for class 3 

where Pn is the nominal heat load of the boiler in MBtu/h. 

A revision of the EN 303-5 is currently in progress and new emission classes (class 4 and 

5) are being discussed. Class 1 and 2 will be phased out as more stringent class 4 and 5 

are phased in. The actual status of the discussion regarding wood fuel boilers is shown in 

Table 5-2. In addition to lower emission levels, the future standard will be valid for 

higher loads (1025 – 1700 MBtu/h) as well. 

Table 5-2 proposed future emission limits for biogenic fuels in EN 303-5 

Emission at 12% O2

CO limits 

in mg/m3 

OGC limits 

 in mg/m3 

Dust3) 

 in mg/m3 

(and lb/MMBtu) 

class class class class class class class class class 

Stoking 

Nominal 

heat 

load in 

MBtu/h 

3 4 5 3 4 5 3 4 5 

≤ 170 4090 820 410 125 60 40 125 

(0.17) 

60 

(0.08) 

50 

(0.07)

170 - 

510 2045 615 245 80 40 25 125 (0.17) 

60 

(0.08) 

50 

(0.07)

510 - 

1025 980 410 205 80 10 10 125 (0.17) 

60 

(0.08) 

50 

Manual

(0.07)

1)

1025 – 

1710 980 410 205 80 10 10 125 (0.17) 

60 

(0.08) 

50 

(0.07)

≤ 170 2455 820 410 80 60 40 125 

(0.15) 

60 

(0.08) 

50 

(0.06)

170 - 

510 2045 615 245 65 60 40 125 (0.15) 

60 

(0.08) 

50 

(0.06)

510 - 

1025 980 410 205 65 40 20 125 (0.15) 

60 

(0.08) 

50 

(0.06)

Automatic2)

1025 – 

1710 980 410 205 65 40 20 125 (0.15) 

60 

(0.08) 

50 

(0.06)

1) Based on the higher heat value (GCV) and the heat value for solid wood (dust values only) 

2) Based on the higher heat value (GCV) and the heat value for pellets (dust values only) 

3) Dust emission limits are identical for automatical and manual systems in EN 303-5, conversion from 

mg/m3 to lb/MMBtu results in higher emission limits for manually stoked appliances, though (see Table 3-2 

on page 31) 
63 

Report 

The emission limits for fossil fuels will be identical to the emission limits for biogenic 

fuels, except for dust – emissions (brown coal fuel assumed): 

• class 4 

o manual stoking: 60 mg/m at 12% O2/0.075 lb/MMBtu 

o automatic stoking: 50 mg/m at 12% O2/0.060 lb/MMBtu 

• class 5: 40 mg/m at 12% O2/0.050 lb/MMBtu 

In the future EN 303-5, the following efficiency requirements are being discussed for 

classes 4 and 5 (based on the GCV): 

• 76 + log Pn for class 4 

• 80 + log Pn for class 5 

where Pn is the nominal heat load of the boiler in MBtu/h. 

5.1.2 Regulations for room heaters 

For room-heaters a number of standards apply, providing efficiency requirements and 

limits for CO-emission. In Table 5-3 a summary of efficiency and emission requirements 

of these standards are given. This summary shows the broad range of emission limits 

and efficiency requirements for diverse room-heating technologies. 

Table 5-3 Summary of efficiency and CO-requirements in standards for room-heating 

appliances 

type of appliance Standard minimum efficiency1) 

in % 

CO limits in mg/m3 

at 12% O2

cookers EN 12815 552) 14065 

slow heat release 

inset 652) 280 

fireplace insets 

(closed doors) 

EN 13229 

252) 1410 

room-heater EN 13240 

for closed doors 

class 1:

class 2:

class 3:

602)

552)

452)

class 1: 

class 2: 

420 

1410

pellet stove EN 14785 nominal load:

partial load:

70 

65 

nominal load: 

partial load: 

565 

845 

slow heat release 

appliances EN 15250 602) 4220 

1) related to higher heating value (GCV) 

2) based on heat value for solid wood

5.2 National levels 

All EU member states are allowed to introduce stricter and additional requirements for 

combustion appliances than they are given in the standards. In the following section, 
64 

Regulations for 

biomass 

installations 

national regulations for emissions and efficiencies of biomass appliances are shown for 

Austria, Germany and the Scandinavian countries. Regulations for small scale 

(residential) appliances and, if available, for medium to large scale (commercial) 

appliances are quoted. 

Additional legal requirements and environmental labels, which are often linked to 

additional financial incentives, are also being introduced by some countries. 

5.2.1 Austria 

Currently, two regulations apply to the emission limits and energy-efficiency 

requirements of residual small combustion installations: 

• The Vereinbarung gemaeß Art. 15a B-VG ueber Schutzmaßnahmen betreffend 

Kleinfeuerungen sets emission limits for combustion units < 1365 MBtu/h. 

• In the Vereinbarung gemaeß Art. 15a B-VG ueber die Einsparung von Energie

minimum efficiency requirements are defined for combustion installations < 1365 

MBtu/h. 

The regulation for combustion units of < 1365 MBtu/h makes a distinction between 

manually and automatically stoked appliances as well as fossil fuels and biofuels. The 

Austrian limits for these installations are in any case lower than the emissions levels in 

EN 303-5, class 3. There are also additional limits set for NOX. 

Emission limit values for appliances < 1365 MBtu/h are presented in Table 5-4. 

Table 5-4 Austrian emission limits for installations < 1365 MBtu/h (Vereinbarung 

Art. 15a B-VG) 

Emission limits in mg/m3 12 % O2

Stoking and fuel types 

CO NOX OGC dust 

Dust 

in 

lb/MMBtu 

Biogenic solid fuels2) 18401) 2510 135 100 0.135 Manual 

Fossil solid fuels3) 1840 185 135 100 0.125 

Biogenic solid fuels4) 8470 2540 70 100 0.125 Automatic 

Fossil solid fuels3) 8470 185 70 75 0.095 

1) At partial load (30 %) the limit may be exceeded by 50 % 

2) solid wood fuel assumed 

3) brown coal fuel assumed 

4) pellets fuel assumed

The adoption of these emission limits due to improved combustion technologies and an 

increasing variety of biomass fuels is currently under discussion. The present status of 

this discussion is shown in Table 5-5. 
65 

Report 

Table 5-5 Future Austrian emission limits for installations < 1365 MBtu – present 

status of discussion 

Emission limits in mg/m3 12 % O2

fuel types and appliances CO NOX OGC dust 

Dust 

in 

lb/MMBtu 

room-heaters1) 1840 250 145/85 100/60 0.135/0.080 wood fuels 

boilers2) 850 250/170 85/50 85/50 0.105/0.060

other < 170 mBUT/h 1860 505 85 100/60 0.125/0.075

standardised 

biog. fuels2) > 170 mBUT/h 850 500 50 100/60 0.125/0.075

< 170 mBUT/h 2020 185 145 90/65 0.115/0.080 fossil fuels3)

> 170 mBUT/h 920 185 55 90/65 0.115/0.080 

XX/YY: values valid from coming into force/values vallid from 2015 

1) solid wood fuel assumed 

2) pellet fuel assumed 

3) brown coal fuel assumed

Regulations for the minimum efficiency distinguish between room-heating appliances and 

boilers. For room heating appliances the minimum efficiency is 70 %, whereas it 

is 62 % for cookers. For boilers, the minimum efficiency depends on the type of 

stoking and the nominal heat load, as shown in Table 5-6. 

Table 5-6 minimum efficiency requirements for boilers < 1365 MBtu/h (Vereinbarung 

Art. 15a B-VG) 

Stoking Nominal heat load 

in MBtu/h 

Efficiency1) in % 

< 35 65 % 

35 – 680 (57 + 7.1 log Pn

2) Manual stoking ) % 3)

> 680 75 % 

< 35 70 % 

Automatic stoking 35 – 680 (60 + 7.1 log Pn) % 4)

> 680 80 % 

1) related to the higher heating value (GCV) 

2) nominal heat load in MBtu/h 

3) based on the heat value for solid wood 

4) Based on the heat value for pellets

Environmental label – Umweltzeichen 

The Umweltzeichen (environmental label) was introduced by the Austrian Government. 

For wood appliances (boilers and room heaters) the Environmental Label number 37 

specifies emission and efficiency requirements. 

In Table 5-7 the emission limits for boilers and room heaters according to the 

Environmental Label are shown. 
66 

Regulations for 

biomass 

installations 

Table 5-7 Austrian Environmental Label emission limits for boilers and room-heaters 

(Umweltzeichen) 

Emission limits in mg/m3 at 12 % O2

Appliance fuel CO/OGC 

(nominal load) 

CO/OGC 

(partial load2)) NOX dust 

Dust 

in 

lb/MMBtu 

pellets 100/5 230/5 170 25 0.030 

Boiler wood chips 250/10 495/15 200 50 0.080 

solid wood 420/50 1250 200 50 0.070 

Room- pellets 205/10 450/20 170 35 0.042 

heater solid wood 1170/85 -/- 200 50 0.070 

1) related to the lower heating value (NCV) 

2) for pellets and wood chips: 30% of nominal load, for solid wood: 50% of nominal load

Table 5-8 shows the efficiency requirements for boilers and room-heaters according to 

the Austrian Environmental Label. 

Table 5-8 Austrian Environmental Label efficiency requirements for room-heaters and 

boilers (Umweltzeichen) 

Efficiency in % related to upper heating value (GCV) stoking Boilers Roomheaters 

Manual2) 62 + 7.1 log Pn

1) 70 

Automatic3) 85 85 

1) nominal heat load in MBtu/h 

2) based on heat value for solid wood 

3) based on the heat value for pellets

In addition to these requirements, the losses due to radiation of the boilers are also 

limited: 

• up to 340 MBtu/h: maximum losses of 2.5 % by radiation 

• 340 – 1365 MBtu/h: maximum losses of 1.5 % by radiation 

Medium to large scale (commercial) applications 

The Feuerungsanlagen-Verordnung, BGBl. II Nr. 331/1997, which came into force on 

June 1st 1998, covers residential installations with a nominal heat capacity > 1365 

MBtu/h kW as well as commercial combustion units > 170 MBtu/h. Table 5-9 and 

Table 5-10 show the emission limits for CO, dust, NOX and HC for biomass appliances and 

emission limits for CO, dust, NOX and SO2 for coal and coke appliances according to the 

Feuerungsanlagenverordnung. 
67 

Report 

Table 5-9 Austrian emission limits for commercial biomass appliances according to the 

Feuerungsanlagenverordnung (FAV) 

Emission limits in mg/m3 at 12% O2

NOX

Installation heat 

load in MBtu/h CO HC fuel1 fuel2 fuel3 dust 

Dust 

lb/MMBtu 

170 – 340 9001) 55 340 280 565 170 0.2103)

340 – 1195 900 55 340 280 565 170 0.2103)

1,195 – 6,830 280 25 340 280 565 170 0.2754)

6,830 – 17,075 280 25 340 280 565 552) 0.0902), 4)

17,075 – 34,150 115 25 340 280 395 55 0.0904)

> 34,150 115 25 340 225 395 55 0.0904)

1) at 30 % load limit may be exceeded by 50 % 

2) from 1st January 2010 

fuel1: beech, oak, non-treated bark, brushwood, cones 

fuel1: other non-treated wood 

fuel1: derived timber products (no heavy metals or halogen organic compounds in binder, hardener und 

coating) 

3) Based on the higher heat value (GCV) of the fuel, pellets assumed 

4) Based on the higher heat value (GCV) of the fuel, wood chips assumed

Table 5-10 Austrian emission limits for commercial coal and coke appliances according 

to the Feuerungsanlagenverordnung (FAV) 

Emission limits in mg/m3 at 12% O2 Installation heat 

load in MBtu/h CO NOX SO2 dust 

Dust1) 

lb/MMBtu 

170 – 340 1125 - - 170 0.215 

340 – 1195 1125 450 - 170 0.215 

1,195 – 6,830 170 450 - 170 0.215 

6,830 – 17,075 170 450 - 55 0.070 

17,075 – 34,150 170 395 450 55 0.070 

> 34,150 170 115 225 55 0.070 

4) Based on the higher heat value (GCV) of the fuel, brown coal assumed

In the Feuerungsanlagenverordnung, a minimum efficiency of 75 % based on the GCV 

for automatically stoked solid fuel appliances used for room heating or domestic hot 

water production is required. 

5.2.2 Germany 

In Germany, the 1. BImSchV (Bundes Immissions Schutz Verordnung fuer 

Kleinfeuerungsanlagen) regulates the Emissions for small scale installations. It was 

implemented in 1988 and has since then been revised twice (in 1997 and 2003) and is 

currently undergoing a third revision. In Table 5-11 the valid emission limits for biomass 

combustion appliances with nominal heat loads > 50 MBtu/h is shown. 
68 

Regulations for 

biomass 

installations 

Table 5-11 German Emission limits for biomass combustion appliances according to 

the valid 1st BImSchV (1st BImSchV) 

Emissions in mg/m3 at 12 % O2 fuel nominal heat 

load in MBtu/h CO dust 

dust in 

lb/MMBtu 

50 – 170 4500 170 0.2301)

170 – 510 2250 170 0.2301)

510 – 1705 1125 170 0.2752)

non-treated wood in 

pieces, i.e. shavings, 

and straw or other 

plant products > 1705 565 170 0.2752)

50 – 340 900 170 0.2752)

340 – 1705 565 170 0.2752)

painted, coated or 

laminated wood, 

plywood and 

chipboards > 1705 340 170 0.2752)

1) Based on the higher heat value (GCV) of the fuel, solid wood assumed 

2) Based on the higher heat value (GCV) of the fuel, wood chips assumed

No specific efficiency requirements are given in the 1. BImSchV for biomass boilers. 

Table 5-12 future German emission limits for biomass boilers according to the 1st

BImSchV (1st BImSchV – Draft from July 2009) 

Installation built after 1st BImSchV 

came into force Installations built after 20141)

E-limits in 

mg/m3 at 

12% O2

E-limits in 

mg/m3 at 

12% O2

fuel nominal 

heat load 

(MBtu/h) CO dust 

dust in 

lb/ 

MMBtu 

nominal 

heat load 

(Btu/h) CO dust 

dust in lb/ 

MMBtu 

15 – 1705 1125 115 0.1857) non 

treated 

wood2) > 1705 565 115 0.1857)

15 – 1705 900 70 0.0858)

pellets3)

> 1705 565 70 0.0858)

15 – 1705 1125 100 0.1259)

coal 

> 1705 565 100 0.1259)

≥ 15 450 25 

0.0407) 

0.0308) 

0.0309)

170 – 340 900 115 0.1857)

340 – 1705 565 115 0.1857) 170 – 1705 450 25 0.040 pre 7)

treated 

wood4) > 1705 340 115 0.1857) > 1705 340 25 0.0407)

straw 15 - 340 1125 115 - 15 – 340 450 25 - 

1) Solid wood emission limits instated in 2017 

2) Non-treated wood (pieces, shavings), straw or other plant products – wood chips assumed for dust emissions 

3) Pressed untreated wood (briquettes, pellets, ...) 

4) painted, varnished or coated wood; plywood, chipboards, fibre boards (no reservatives used, no heavy metals 

of halogen organic compounds – solid wood assumed for dust emissions 

5) straw and similar plant materials 

6) All dust emissions based on the higher heating value (GCV) 

7) Based on the higher heat value (GCV) of the fuel, wood chips assumed 

8) Based on the higher heat value (GCV) of the fuel, pellets assumed 

9) Based on the higher heat value (GCV) of the fuel, brown coal assumed
69 

Report 

Table 5-12 shows the recent status of the discussion for the new emission (and 

efficiency) requirements for biomass boilers. 

No specific efficiency requirements are given in the draft version of the 1st BImSchV for 

biomass boilers. 

Table 5-13 shows the recent discussion status for new emission (and efficiency) 

requirements for room-heaters. 

Room-heaters which were installed before the future 1st BImSchV was implemented are 

only allowed to be operated, if 

• dust emissions are < 0.228 lb/MBtu (150 mg/m3 at 13% O2) and 

• CO-emissions are < 6.090 lb/MBtu (4000 mg/m3 at 13% O2) 

Otherwise they must be upgraded with emission control technology or placed out of 

operation corresponding to a time schedule. 

Table 5-13 future German emission and efficiency requirements for room-heaters 

according to 1st BImSchV (1st BImSchV – Draft) 

Installation built after 1st BImSchV 

came into force Installations built after 2014 

Emission in 

mg/m3 limits 

at 12% O2

Emission in 

mg/m3 limits at 

12% O2

type of appliance Standard Efficiency1)

(%) CO dust 

dust in 

lb/ 

MMBtu CO dust 

dust in lb/

MMBtu 

room heater 

(intermittent 

operation) 

65 2250 115 0.1502) 1405 45 0.061 

room heater 

(continuous 

operation) 

EN 

13240 

60 2810 115 0.1502) 1405 45 0.061 

slow heat release 

appliances 

EN 

15250 65 2250 115 0.1502) 1405 45 0.061 

closed inset 65 2250 115 0.1502) 1405 45 0.061 

slow heat release 

inset (intermittent) 70 2250 115 0.1502) 1405 45 0.061 

slow heat release 

inset (continuous) 

EN 

13229 

70 2815 115 0.1502) 1405 45 0.061 

cookers 60 3375 115 0.1502) 45 0.061 

cookers (with 

room heater 

function) 

EN 

12815 65 3940 115 0.1502) 1690 45 0.061 

pellet stove 80 450 55 0.0703) 280 35 0.040 

pellet stove with 

boiler 

EN 

14785 85 450 35 0.0403) 280 25 0.030 

1) related to the higher heating value (GCV) 
70 

Regulations for 

biomass 

installations 

Environmental Label – The Blue Angel 

The Blue Angel label is sponsored and administered by the German Federal 

Environmental Agency and the Quality Assurance and Product Labeling Institute RAL 

Deutsches Institut fuer Guetesicherung und Kennzeichnung e.V. This label provides 

additional emission and efficiency requirements for pellet stoves and pellet boilers 

(Table 5-14). 

Table 5-14 Emission limits for the German Blue Angel label (Blue Angel) 

Emission limits in mg/m3 at 12% O2 for 

appliance nominal/partial load Efficiency 

(%)1)

CO NOX TOC dust 

dust in 

lb/mmBtu 

pellet stove 200/450 170/- 10/20 30/- 0.035/- 

pellet boiler ≥ 85 100/225 170/- 5/5 25/- 0.030/- 

1) related to the higher heating value (GCV) 

XXX/YYY: values for nominal load/values for partial load

Medium to large scale applications 

In the TA Luft, emission limits for CO, dust, NOX and OGC are given for medium to large 

scale appliances. The emission limits for non treated and pretreated biomass are given in 

Table 5-15. The installation heat load for installations which burn pretreated wood is 

limited to 170 MMBtu/h. 

Table 5-15 German emission limits for medium to large scale appliances according to 

the TA Luft 

Emission limits in mg/m3 at 12 % O2 fuel heat load 

in MMBtu/h CO OGC NOX dust 

Dust3) 

lb/MMBtu 

non treated biomass ≤ 8.5 1351) 10 225 90 0.150 

other biomass2) 1351) 10 360 45 0.075 

non treated biomass 8.5 – 17 135 10 225 45 0.075 

other biomass2) 135 10 360 45 0.075 

non treated biomass > 17 135 10 225 20 0.030 

other biomass2) 135 10 360 20 0.030 

1) emission limit only valid for nominal load 

2) painted, varnished or coated wood; plywood, chipboards, fibre boards (no preservatives used, no heavy 

metals or halogen organic compounds) 

3) based on heat value for wood chips 

For non treated biomass appliances, TA Luft requires continuous measurement of dust 

emissions: 

• from 17 to 85 MMBtu/h: qualitative measurement 

• from 85 MMBtu/h: measurement of mass concentration 

In addition, the continuous measurement of CO-emissions for heat loads > 9 MMBtu/h is 

required. No efficiency requirements are specified in the TA Luft. 
71 

Report 

For fossil solid fuels emission limits are identical to the emission limits for “other 

biomass” in Table 5-16. Additionally emission limits for SO2 are requested: 

• fluidised bed comubstion: 315 mg/m3 at 12% O2

• other combustion technology 

o mineral coal: 1170 mg/m3 at 12% O2

o other fossil fuels: 900 mg/m3 at 12% O2 (TA Luft) 

5.2.3 Denmark 

The Danish Guidelines for Air Emission Regulation (Environmental Guidelines no. 1, 

2002) apply to all emitting installations, while the Statutory Order, regulating air 

pollution from wood burners and boilers and certain other fixed energy producing 

installations applies to combustion installations with a total input of < 1025 MBtu/h. 

Handcrafted masonry heaters are excluded from this regulation. In Table 5-16 the Danish 

emission limits for biomass boilers are shown. 

Table 5-16 Danish emission limits for biomass boilers (according to EN 303-5, Class 3) 

(Statutory Order) 

Appliance and fuel Emission limits in mg/m3 at 12% O2

types 

nominal heat 

load MBtu/h CO OGC dust 

dust in 

lb/MMBtu 

< 170 4090 125 125 0.1653)

170 – 510 2045 80 125 0.1653) Biomass1)

510 – 1025 980 80 125 0.1653)

< 170 4090 125 100 0.1254)

170 – 510 2045 80 100 0.1254)

Manual 

stoking 

Fossil2)

510 – 1025 980 80 100 0.1254)

< 170 2455 80 125 0.1505)

170 – 510 2045 65 125 0.1255) Biomass1)

510 - 1025 980 65 125 0.1255)

< 170 2455 80 100 0.2104)

170 – 510 2045 65 100 0.2104)

Automatic 

stoking 

Fossil2)

510 – 1025 980 65 100 0.2104)

1) wood, plant seed and other residual products 

2) fixed carbon or coal 

3) based on the higher heat value (GCV) of the fuel, solid wood assumed 

4) based on the higher heat value (GCV) of the fuel, brown coal assumed 

5) based on the higher heat value (GCV) of the fuel, pellets assumed

In addition, the efficiency requirements according to EN 303-5 are requested (based on 

the GCV): 

• ηGCV ≥ 59 + 5.5 log Pn 

where Pn is the nominal heat load of the boiler in MBtu/h. 

For room-heaters, dust emissions are limited depending on the method of measurement: 
72 

Regulations for 

biomass 

installations 

• Measurement in dilution tunnel: 

0.16 oz/lb (10 g/kg) fuel; max. 0.32 oz/lb (20 g/kg) fuel in the individual 

testing intervals 

• Measurement in the flue gas pipe (according to EN 13240): 

0.115 lb/MMBtu (75 mg/m3 at 13 % O2) 

Efficiency and emissions requirements for room heaters according to EN 13240: 

• ηGCV ≥ 44 % (class 3) 

• CO < 1405 mg/m3

 at 12 % O2 (class 2) 

Environmental label – The Nordic Swan 

The Nordic Swan was introduced by the Nordic Council of Ministers and it is a common 

system for Sweden, Norway, Denmark and Finland. It provides criteria for closed 

fireplaces and solid biomass boilers. 

Table 5-17 presents efficiency and emission requirements for (closed) room-heaters. 

Table 5-17 Nordic Swan efficiency and emission requirements for closed room-heaters 

(Nordic Swan) 

emission limits in 

mg/m3 at 12 % O2

Appliance at nominal load Efficiency1)

(%) 

CO OGC 

dust in g/kgfuel and 

(lb/MMBtu) at nominal 

load/ 

partial load/ 

for each individual test 

slow heat release 

appliance2) ≥ 70 170 2250 1/-/- 

(0.020/-/-) 

inset (manual stoking)2) 170 2815 3/8/15 

(0.050/0.120/0.230) 

stove (manual stoking)2)

≥ 65 

170 2815 3/5/10 

(0.045/0.080/0.150) 

stove (automatic stoking)3) ≥ 70 55 1125 2/5/10 

(0.030/0.070/0.135) 

1) related to the lower heating value (NCV) 

2) based on heat value for solid wood 

3) based on heat value for pellets 

XXX/YYY/ZZZ: values for nominal load/values for partial load/values for each individual test

In Table 5-18 emission requirements for boilers are exhibited. 
73 

Report 

Table 5-18 Nordic Swan emission requirements for boilers (Nordic Swan) 

Emission limits in mg/m3 at 12 % O2 Stoking nominal heat 

load in kW CO NO2 OGC dust 

dust in 

lb/MMBtu 

Automatic ≤ 1025 330 280 20 35 0.0401)

≤ 340 1640 280 60 55 0.0802)

Manual 340 - 1025 820 280 40 55 0.0802)

1) based on heat values for pellets 

2) based on heat values for solid wood

Dust and NOX emissions are tested only at nominal load 

Testing procedure for OGC and CO-emissions: 

- nominal heat load for manually stoked boilers 

- nominal load and partial load for automatically stoked boilers

Efficiency requirements for boilers are shown in Table 5-19. 

Table 5-19 Nordic Swan efficiency requirements for boiler (Nordic Swan) 

Stoking Efficiency1) in % 

Manual3) at nominal load: 65 + 5.5 log Pn

2)

Automatic4)

at nominal load: 70 + 5.5 log Pn

2) 

and ηx ≥ 80 % where ηx = (η20 + η40 + η60)/3 

 with η20, η40, η60 are measured efficiencies 

 at 20, 40 and 60% load 

1) related to the higher heating value (GCV) 

2) nominal heat load in MBtu/h 

3) based on heat values for pellets 

4) based on heat values for solid wood

Medium to large scale appliances 

For medium to large scale appliances, efficiency and fuel quality requirements are 

defined, but there are no requirements for emissions. Medium to large scale appliances 

have to reach efficiencies larger than 68% based on the GCV (Brønnum 2009). 

5.2.4 Finland 

In Finland, a proposal for the regulation of emissions and efficiencies for heating 

appliances using wood fuels has been in the works, under discussion and review since 

2006. This regulation will apply to new biomass installations that provide heat, domestic 

hot water or other energy services with a nominal heat load up to 1025 MBtu/h. The 

proposal distinguishes primary and secondary heating systems. Primary heating systems 

provide all required heat/hot water on their own, while secondary heating systems have 

another (main) heat source. Table 5-20 shows the suggested emission and efficiency 

requirements for Finland. 
74 

Regulations for 

biomass 

installations 

Table 5-20 Proposed Finnish emission and efficiency requirements for biomass boilers 

< 1025 MBtu/h kW (EuP Lot 15, Task1) 

heating Emission limits in mg/m3 at 12% O2

system 

nominal heat 

load (MBtu/h) 

efficiency1) 

(%) CO OGC 

≤ 170 2455 80 

primary 170 – 510 2045 65 

> 510 

59+ 5.5 log 

Pn

2)

980 65 

≤ 170 4090 125 

secondary 170 – 510 2045 80 

> 510 

59 + 5.5 log 

Pn

2)

980 80 

1) related to the higher heating value (GCV), based on heat value for solid wood 

2) nominal heat load in MBtu/h

In Table 5-21 the Finnish emission and efficiency requirements for fireplaces (room 

heaters) is shown. Fireplaces which do not fulfill the requirements are still allowed, but 

they have to be marked as useable only for decorative purposes. 

Table 5-21 Proposed Finnish emission and efficiency requirements for fireplaces 

(room-heaters) (EuP Lot 15, Task1) 

heating system nominal heat load 

(MBtu/h) efficiency1) (%) CO (mg/m3) at 12% O2

primary 2455 

secondary ≤ 170 60 4220 

1) related to the higher heating value (GCV), based on heat value for solid wood 

Medium to large scale appliances 

For medium to large scale applications, national dust requirements are given: 

• 3.4 – 17 MMBtu/h: < 0.465 lb/MMBtu 

• 17 – 170 MMBtu/h: < 0.200 - 4

/3·(Pn-0.01) lb/MMBtu 

Additionally, emission limits can be requested by local authorities for heating plants > 

17 MMBtu/h. For appliances > 170 MMBtu/h approval is needed by even higher 

authorities and these limits are open to public (Oravainen 2009). 

Environmental label – The Nordic Swan 

5.2.5 Sweden 

In Sweden emission limits for small scale solid fuel appliances are included in the building 

regulation (BFS 2006:12). The regulation applies to solid fuel installations with nominal 

heat loads < 1025 MBtu/h. The emission limits for solid fuel appliances are shown in 

Table 5-22. The CO requirements do not apply to open fireplaces, tiled stoves (masonry 

heaters) and cook stoves. 
75 

Report 

Table 5-22 Swedish emission limits for solid fuel appliances < 1025 MBtu/h 

(EuP Lot 15, Task1) 

stoking nominal heat load 

(MBtu/h) 

OGC in mg/m3 at 12% O2

≤ 170 125 manual 170 – 1025 80 

≤ 170 80 automatic 170 – 1025 65 

For secondary heating systems (another main heat source is used), limits for COemission are given: 

• for stoves and fireplaces inserts: CO < 3300 mg/m3 at 12% O2

• for pellet stoves: CO < 440 mg/m3 at 12% O2

These CO requirements do not apply to open fireplaces, tiled stoves (masonry heaters) 

and cook stoves. 

For efficiency requirements only recommendations are given: 

• for stoves: η > 53 % (on GCV basis) 

• for fireplace inserts: η > 44 % (on GCV basis) 

• for pellet stoves: η > 65 % (on GCV basis) 

Environmental label – The Nordic Swan 
76 

Performance of 

biomass 

installations 

6 Performance of biomass installations 

Technology forcing efforts of regulations, technology cluster investment, competition 

among manufacturers and Eco-labeling efforts have resulted in commercially available 

technologies with performance far exceeding regulatory requirements. This chapter 

describes the top performing biomass technologies based on thermal efficiency, CO , NOX

and dust (fine particles) emissions. Additionally retail prices are given. 

For selection of the TOP 25% performing technologies in each size category, thermal 

efficiency and CO data at full load is used due to the availability of this information. 

Recall that for many technologies, especially boilers the thermal efficiency curve is linear 

down to very low part loads with these technologies (Figure 6-3). 

6.1 Introduction 

6.1.1 Data sources 

Before installation of biomass appliances is allowed, they have to pass the efficiency and 

emission requirements of the current European standards as well as the national/regional 

regulations. Small scale appliances are usually type-tested by certified testing houses. 

Results of these tests are property of biomass appliances’ manufacturer and are 

therefore not usually publicly available. However, some local and national organisations 

collect and publish test data which they whether collect from manufacturers or from test 

houses. In order to receive test data of biomass appliances in the considered countries, 

the following sources were used: 

• Website of FJ-BLT (http://www.josephinum.at), an Austrian testing laboratory 

• Publications of the Fachagentur Nachwachsende Rohstoffe (FNR) on biomass 

boilers and pellet stoves available on the German market 

• Webpage of Teknologisk Institut (http://www.teknologisk.dk), a Danish testing 

house 

• Webpage of the Danish Environmental Protection Agency (http://www.mst.dk) 

• Written information of the Teknikfoeretagens Branschgrupper AB, the Swedish 

Heating Boilers and Burners Association (contact person: Magnus Davidsson) 

• Website and written information of the HKI Industrieverband Haus-, Heiz und 

Kuechentechnik e.V., the German heat technology association (contact person: 

Frank Kienle) 

• Written information of following manufacturers of biomass appliances: 

o Veljekset Ala-Talkkari (Finland) 

o Nunnauuni Oy (Finland) 

o Rika (Austria) 

o Calimax (Austria) 

o ... 
77 

Report 

Information from these sources about test data varies from the simple statement that 

the appliances have passed the efficiency and emission requirements to detailed data on 

efficiency and emissions at nominal and partial load as well as investment costs. 

6.1.2 Selection method for TOP 25% performing appliances 

For the selection of the TOP 25% performing biomass appliances, the efficiency and 

CO-emissions at full load (nominal heating load) were considered: 

• The efficiency of an appliance provides information regarding the extend the fuel 

input is used to produce hot water and/or room heat. It is therefore one of the 

most important quality criteria. 

• CO-emissions are the prime indicators of the combustion quality of biomass 

appliances (chapter 3.2). If CO-emissions are low, the emissions of OGC as well as 

the organic fraction of the TSP (total suspended particulate matter) will also be 

low. NOx-emissions depend primarily on the nitrogen content of the fuel 

(chapter 3.5); therefore, they are not considered for the selection of the TOP 25% 

performing appliances. 

• The full load range is chosen for the selection method, because most data is 

available at this load. 

The appliances with the highest efficiencies and CO-emissions lower than the average 

CO-emissions of the considered technology and load range represent the TOP 25% 

performing appliances. 

6.2 Stoves/room-heaters 

6.2.1 Pellet stoves 

The European pellet stove market is quite manageable, because pellets combustion 

technology is a relatively young technology. Manufacturers of pellet stoves are used to 

having their products tested and are as such usually willing to publish their test data. As 

a result, a very broad database was able to be compiled for this study. Therefore, the 

data given in the list of the TOP 25% performing pellet stoves is considered to be 

representative of the current market. 

Table 6-1 shows a summary of the minimum, maximum and average efficiency and 

emission values at full load for all the considered pellet stoves and Table 6-5 shows a 

summary of the minimum, maximum and average efficiency and emissions values at full 

load for the TOP 25% pellet stoves with and without water circulation equipment. 

A total of 79 data sets were recorded, of which 53 were not identical. Identical data sets 

are a result of manufacturers producing two or more appliances, which are identical in 

terms of all technical aspects and combustion systems, but have different designs for the 

housing as well as different model names. However, due to the identical technical design, 

separate performance tests are not necessary. 
78 

Performance of 

biomass 

installations 

Table 6-1 summary of extreme and averaged values for all considered pellet stoves 

yes

< 30 > 30 all

34 18 27

17 16 19

445

min. 8 30 25

max. 29 132 51

avg. 23 66 38

min. 86 85 84

max. 89 87 85

avg. 87 86 84

min. 75 153 39

max. 844 1316 460

avg. 249 449 174

min. 0.011 0.027 0.014

max. 0.104 0.082 0.087

avg. 0.040 0.052 0.031

min. 15 92 84

max. 259 213 162

avg. 101 139 119

5262 3757 8820

load range (MBtu/h)

water circulate no

nominal load 

(MBtu/h)

ηGCV

1) at full load 

(%)

No. of all test data

No. of unequal test data

No. of top 25% appliances

CO at full load 

(mg/m3

12%O2)

TSP1) at full load 

(lb/MMBtu)

NOx at full load 

(mg/m312%O2)

avg. retail price (US$)

1) related to the Gross Calorific Value (upper heating value)

Pellet stoves with and without water circulation equipment are built in a load range from 

approximately 10 to 130 MBtu/h, whereby the efficiency of these appliances is 86% on 

average. Pellet stoves with hot water production attain slightly lower efficiencies. 

The measured CO-emissions exhibit a very broad range from 75 to 1350 mg/m3 at 

12% O2 . This broad variation is due to different combustion chamber materials and 

diverse control concepts used for the stoves: 

• If ceramics are used in the combustion chamber, the combustion temperature will 

be higher than if steel only is used – the combustion temperature is the most 

important indicator for low CO-emissions (chapter 3.2). 

• If the control concept of the stove involves CO or air ratio-measurement, CO 

emissions can be minimized more easily than if only the combustion gas 

temperature is measured. If no concepts for combustion condition optimisation are 

implemented, the control of the stove is not able to adopt to the operation mode. 

The measures for the reduction of CO described above will however result in an increase 

in cost of the stoves. This is clearly shown in a comparison of the CO emissions in 

relation to the costs of stoves without water installations having head loads of <30 and 

>30 Btu/h. The average pellet stove with a heat load of < 30 MBtu/h emits 

approximately 250 mg/m3 CO at 12% O2 and costs approximately 5250 US$. On the 

other hand, the average pellet stove with a heat load > 30 MBtu/H emits nearly twice as 

much CO (450 mg/m3 at 12% O2), but costs one quarter less (3750 US$). 
79 

Report 

Emissions of TSP vary from 0.015 to 0.100 lb/MMBtu. This broad range can be attributed 

to the same factors mentioned above. A low combustion temperature will result in a high 

organic fraction of TSP and the (control) concept for the air supply influences the amount 

of fly ash. 

NOX emissions range from 100 to 360 mg/m3 at 12% O2 and the average value is 

approximately 120 mg/m3 at 12% O2. These are typical values for wood combustion. 

Table 6-2 summary of extreme and averaged values for TOP 25% performing pellet 

stoves 

yes

< 30 > 30 all

34 18 27

17 16 19

445

min. 8 30 25

max. 29 34 51

avg. 21 32 38

min. 88 86 85

max. 89 87 85

avg. 88 87 85

min. 86 153 39

max. 236 234 169

avg. 164 180 117

min. 0.033 0.035 0.014

max. 0.072 0.050 0.025

avg. 0.045 0.043 0.019

min. 88 106 125

max. 88 114 162

avg. 88 110 143

8113 4820 9843

no

CO at full load 

(mg/m3

12%O2)

TSP1) at full load 

(lb/MMBtu)

NOx at full load 

(mg/m312%O2)

avg. retail price (US$)

water circulate

nominal load 

(MBtu/h)

ηGCV

1) at full load 

(%)

No. of all test data

No. of unequal test data

No. of top 25% appliances

load range (MBtu/h)

1) related to the Gross Calorific Value (upper heating value)

The TOP 25% performing pellet stoves reach average efficiencies from 85 to 87% based 

on the GCV. Their CO-emissions vary from 40 to 240 mg/m3 at 12% O2 and the average 

CO emissions are approximately 155 mg/m3 at 12% O2. The difference between the CO 

emissions from pellet stoves without water circulations having heat loads of < 30 MBtu/h 

and those with heat loads of > 30 MBtu/h still exists, but is definitely smaller. 

The average dust emissions range from 0.020 to 0.045 lb/MMBtu, a range which is 

slightly smaller than the average dust emissions calculated from all of the collected data. 

The NOX emissions from the TOP 25% performing pellet stoves are essentially no 

different than those from all the other pellet stoves collected for this study. 

For pellet stoves without water circulation having heat loads of < 30 MBtu/h in particular, 

a quite notable increase in price for the TOP 25% is observed in comparison to the 

average price of all the collected data. 

Detailed performance data of the TOP 25% performing pellet stoves and contact 

information of the manufacturers is given in the annex. 
80 

Performance of 

biomass 

installations 

6.2.2 Wood stoves 

The wood stove market is extremely broad and diverse as manufacturers come from 

many different backgrounds (masonry heater industry, steel industry, biomass boiler 

industry...) and as such define the main purposes for their products differetly (e.g., 

efficient heating, stove design, lifestyle product...). Moreover, many uncertified products, 

which do not conform to the according standard (EN 13240) are also offered on the 

market. 

The publishing of test data is not common for wood stoves. The fact that the emissions 

from wood stoves are difinitively higher those from pellet stoves or biomass boilers 

discourages manufacturers from publishing test data. 

For these reasons, only a small group of wood stoves data could be collected and 

evaluated. In addition to the detailed data collected for this study, a list of EN 13240 

certified wood stoves from German manufacturers is provided in the annex. 

Table 6-3 and Table 6-4 show the minimum, maximum and average efficiency and 

emissions values at full load for all analysed wood stoves and for the TOP 25% 

performing wood stoves. 

A total of 51 data sets were recorded, of which 40 were not identical. 

Table 6-3 summary of extreme and 

average values for selected wood 

stoves 

51

40

10

min. 25

max. 128

avg. 57

min. 64

max. 76

avg. 71

min. 776

max. 2535

avg. 1521

min. 0.009

max. 0.128

avg. 0.059

min. 59

max. 225

avg. 139

-

NOx at full load 

(mg/m312%O2)

avg. retail price (US$)

nominal load 

(MBtu/h)

ηGCV

1) at full load 

(%)

CO at full load 

(mg/m3

12%O2)

TSP1) at full load 

(lb/MMBtu)

No. of all test data

No. of unequal test data

No. of top 25% appliances

1) related to the GCV 

Table 6-4 summary of extreme and 

average values for TOP 25% performing 

wood stoves 

51

40

10

min. 25

max. 128

avg. 57

min. 71

max. 76

avg. 73

min. 776

max. 1340

avg. 1069

min. 0.020

max. 0.081

avg. 0.050

min. 59

max. 197

avg. 126

-

nominal load 

(MBtu/h)

ηGCV

1) at full load 

(%)

CO at full load 

(mg/m3

12%O2)

TSP1) at full load 

(lb/MMBtu)

NOx at full load 

(mg/m312%O2)

avg. retail price (US$)

No. of all test data

No. of unequal test data

No. of top 25% appliances
81 

Report 

The heat loads of the wood stoves vary from 25 to 130 MBtu/h and achieve an average 

efficiency of 71 % based on the GCV. 

CO emissions fall into a rather wide range between 750 to 2550 mg/m3 at 12% O2. Since 

dust emissions are related to CO emissions due to un-burnt particles in the fly ash 

fraction, dust emissions also widely vary from 0.010 to 0.130 lg/MMBtu. 

The average NOX emissions are approximately 140 mg/m3 at 12% O2, which is a typical 

value for wood combustion. 

The TOP 25% performing wood stoves reach efficiencies of approximately 73% and emit 

on average 1050 mg/m3 CO at 12% O2, a 33% reduction with respect to the entire data 

set. The dust emissions still show quite broad variations from 0.020 to 0.080 lb/MMBtu; 

however, the average data changes marginally in comparison to the entire data set. 

Altogether, the average data of the TOP 25% performing wood stoves does not exhibit 

significant improvements in comparison to the entire data set. 

Detailed performance data of all considered wood stoves and contact information of the 

manufacturers is given in the annex. This summary should give an idea of the average 

emissions as well as the performance of wood stoves. It does not imply that test data 

from other manufacturers is better or worse than the given data. 

6.2.3 Masonry heaters 

The masonry heater market is similar to that of wood stoves. Data from only two 

manufacturers (LEDA and Brunner) could be collected. For this reason, performance and 

emission data will not be summarized for masonry heaters. In the annex, a list of 

EN 13229 certified prefabricated masonry heaters from German manufacturers is 

provided. Additionally contact information of manufacturers is also given. 

6.3 Boilers 

The European biomass boiler market is quite large. A very competitive environment and 

local/national grant schemes for biomass appliances (see diverse environmental labels 

mentioned in chapter 5.2) lead to a high transparency of performance and emission 

data. Test data can be found on webpages from national authorities. It is estimated that 

approximately 90% of Austrian and German boilers were collected for this study. 

Therefore, the data given in the list of the TOP 25% performing pellet stoves is 

considered to be an accurate representation of the current market. 

A total of 954 data sets were recorded, of which 787 were not identical. These identical 

data sets are a result of manufacturers producing two or more appliances, which are 

identical in terms of all technical aspects and combustion systems, but have different 

designs for the housing as well as different model names. Due to the identical technical 

design, a separate performance test is not necessary. 
82 

Performance of 

biomass 

installations 

6.3.1 Pellet boiler 

Table 6-5 shows a summary of the minimum, maximum and average efficiency and 

emission values at full load for all the considered pellet boilers and Table 6-6 shows test 

data of the TOP25% performing appliances at the defined load ranges. 

A total of 433 data sets were recorded, of which 362 were not identical. 

Table 6-5 overview of extreme and averaged values for all considered pellet boilers 

< 55 55 - 100 100 - 170 170 - 340 > 340

124 142 83 52 32

102 130 66 45 29

26 33 17 11 7

min. 26 55 99 171 342

max. 54 98 169 338 997

avg. 45 78 134 254 568

min. 78 73 80 81 81

max. 89 88 89 88 88

avg. 85 84 85 85 85

min. 7 1 3 4 1

max. 371 603 281 338 543

avg. 91 116 76 66 62

min. 0.003 0.008 0.006 0.014 0.004

max. 0.069 0.348 0.377 0.377 0.058

avg. 0.023 0.028 0.030 0.047 0.031

min. 84 12 63 63 63

max. 398 365 378 293 247

avg. 154 168 144 151 133

11359 12149 16052 24795 30244

load range (MBtu/h)

nominal load 

(MBtu/h)

ηGCV

1) at full load 

(%)

No. of all test data

No. of unequal test data

No. of top 25% appliances

CO at full load 

(mg/m3

12%O2)

TSP1) at full load 

(lb/MMBtu)

NOx at full load 

(mg/m312%O2)

avg. retail price (US$)

1) related to the Gross Calorific Value (upper heating value)

Pellet boilers are built for heat loads starting at 25 MBtu/h and reach high efficiencies 

independent of their load, in the range of 73 to 89 %. The average efficiency for these 

boilers is 85 %. 

CO emissions vary from very small amounts to elevated values of 600 mg/m3 at 12% O2.

The average CO-emissions for all load rangs are however below 120 mg/m3 at 12% O2. 

Higher loads give rise to lower CO emissions, a fact that can be attributed to the higher 

combustion temperatures reached in larger appliances as a result of the favourable 

relationship between combustion chamber volume and combustion chamber surface as 

well as the better materials that are used. It can be stated that CO-emissions are 

generally low in pellet boilers. 

TSP-emissions vary from 0.003 to 0.377 lb/MMBtu; the average emissions are in the 

range of 0.025 to 0.045 lb/MMBtu, which are very low values. 

NOX-emissions fall within a range of 60 to 400 mg/m3 at 12% O2; the average value is 

approximately 150 mg/m3 at 12% O2, which can be considered normal NOX-emissions for 

wood fuel. 
83 

Report 

Table 6-6 overview of extreme and average values for TOP 25% performing pellet 

boilers 

< 55 55 - 100 100 - 170 170 - 340 > 340

124 142 83 52 32

102 130 66 45 29

26 33 17 11 7

min. 27 55 102 171 342

max. 53 92 169 331 885

avg. 44 78 141 258 524

min. 86 85 86 86 85

max. 89 88 89 88 88

avg. 87 86 87 86 86

min. 7 1 5 4 5

max. 77 105 73 59 41

avg. 43 39 38 24 17

min. 0.008 0.010 0.006 0.014 0.008

max. 0.043 0.056 0.057 0.053 0.056

avg. 0.022 0.024 0.023 0.036 0.031

min. 95 39 63 105 118

max. 230 260 183 198 171

avg. 140 136 132 154 133

12221 13659 16844 23537 0

nominal load 

(MBtu/h)

ηGCV

1) at full load 

(%)

CO at full load 

(mg/m3

12%O2)

TSP1) at full load 

(lb/MMBtu)

load range (MBtu/h)

No. of all test data

No. of unequal test data

No. of top 25% appliances

avg. retail price (US$)

NOx at full load 

(mg/m312%O2)

1) related to the Gross Calorific Value (upper heating value)

The TOP 25% performing pellet boilers exhibit average efficiencies 2% higher than that 

of the entire data set. The CO emissions are 50% less that of all considered appliances, 

and varying from nearly no CO emissions to 105 mg/m3 at 12% O2. 

The average dust emissions are slightly lower than the emission values of the entire data 

and range from 0.02 to 0.035 lb/MMBtu. 

Detailed performance data of the TOP 25% performing pellet boilers and contact 

information of the manufacturers is given in the annex. 

6.3.2 Solid wood boiler 

Table 6-7 shows a summary of the minimum, maximum and average efficiency and 

emission values at full load for all the considered solid wood boilers. Table 6-8 shows test 

data of the TOP25% performing appliances for the defined load ranges. 

A total of 279 data sets were recorded, of which 229 were not identical. 
84 

Performance of 

biomass 

installations 

Table 6-7 overview of extreme and average values from all considered solid wood 

boilers 

< 85 85 - 120 120 - 170 > 170

69 81 72 57

65 64 54 46

16 16 14 12

min. 40 85 120 171

max. 82 116 167 348

avg. 63 98 141 199

min. 62 73 73 73

max. 84 85 84 84

avg. 80 81 81 81

min. 64 20 18 44

max. 3912 4137 2878 2009

avg. 612 417 399 308

min. 0.008 0.009 0.008 0.012

max. 0.175 0.189 0.324 0.166

avg. 0.034 0.035 0.041 0.036

min. 20 57 34 75

max. 206 225 242 199

avg. 129 133 135 149

8436 9683 11965 14104

CO at full load 

(mg/m3

12%O2)

NOx at full load 

(mg/m312%O2)

avg. retail price (US$)

TSP1) at full load 

(lb/MMBtu)

load range (MBtu/h)

nominal load 

(MBtu/h)

ηGCV

1) at full load 

(%)

No. of all test data

No. of top 25% appliances

No. of unequal test data

1) related to the Gross Calorific Value (upper heating value)

Solid wood boilers are built for heat loads starting at 50 MBtu/h and reach efficiencies 

from 60 to 85 %. The average efficiencies are approximately 81%, which is 3% lower 

than the efficiencies reached by pellet boilers. 

CO-emissions vary from 20 to 4140 mg/m3 at 12% O2 which exhibits a very broad 

variation. This may be attributed to the broad range of control (or no control) concepts3

for solid wood boilers as well as the materials used in the combustion zone. The average 

values are approximately 400 mg/m3 at 12% O2 and decrease with higher load ranges. 

This can be attributed to the same phenomenon discussed for the pellet boilers. 

Dust emissions range from 0.010 to 0.325 lb/MMBtu, whereby the average emissions are 

approximately 0.035 lb/MMBtu, which is nearly as low as the dust emissions from pellet 

boilers. 

The average NOX-emissions are approximately 130 mg/m3 at 12% O2 and thus lie in the 

normal range for wood combustion. 

 

3

 The solid wood boiler is the oldest biomass boiler technology. There are thus a large number of traditional 

manufacturers who use very traditional design concepts without proper control concepts, thus resulting in a 

broad variation. 
85 

Report 

Table 6-8 overview of extreme and average values from TOP 25% performing solid 

wood boilers 

< 85 85 - 120 120 - 170 > 170

69 81 72 57

65 64 54 46

16 16 14 12

min. 40 85 123 171

max. 75 116 167 239

avg. 63 97 143 189

min. 82 82 82 81

max. 84 83 84 84

avg. 83 83 83 82

min. 64 40 50 44

max. 353 298 343 282

avg. 174 220 152 97

min. 0.014 0.014 0.021 0.015

max. 0.175 0.189 0.324 0.076

avg. 0.039 0.044 0.066 0.033

min. 90 100 34 75

max. 182 186 181 194

avg. 139 126 115 119

10116 10319 12672 14089

NOx at full load 

(mg/m312%O2)

avg. retail price (US$)

nominal load 

(MBtu/h)

ηGCV

1) at full load 

(%)

CO at full load 

(mg/m3

12%O2)

TSP1) at full load 

(lb/MMBtu)

load range (MBtu/h)

No. of all test data

No. of unequal test data

No. of top 25% appliances

1) related to the Gross Calorific Value (upper heating value)

The TOP 25% performing solid wood boilers reach average efficiencies which are 2% 

higher than the average values from the entire data set. 

The variation of CO emissions is significantly lower and varies from 40 to 350 mg/m3 at 

12% O2. The average CO emissions decrease by 50 to 75% and range from 100 to 220 

mg/m3 at 12% O2. A decrease of CO emissions with higher loads can also be observed 

for the TOP 25% performing appliances. 

Surprisingly and also inconsistent with the statement that low CO emissions result in low 

dust emissions, the dust emissions of the TOP 25% performing appliances are higher 

than the dust emissions for the entire data set. This could be attributed to quality 

variations of the dust measurement methods. 

Detailed performance data of the TOP 25% performing solid wood boilers and contact 

information of the manufacturers is given in the annex. 

6.3.3 Wood chip boiler 

Table 6-9 shows a summary of the minimum, maximum and average efficiency- and 

emission values at full load of all considered wood chip boilers and Table 6-10 shows test 

data for the TOP25% performing appliances at the defined load ranges. 

A total of 213 data sets were recorded, of which 174 were not identical. 
86 

Performance of 

biomass 

installations 

Table 6-9 overview of extreme and averaged values from all considered wood chip 

boilers 

< 100 100 - 200 200 - 340 340 - 680 > 680

35 65 44 39 30

26 49 42 33 26

7 12 11 8 7

min. 34 102 205 342 683

max. 96 188 340 666 5123

avg. 74 150 265 444 1459

min. 73 68 69 70 46

max. 86 86 86 86 86

avg. 77 77 76 77 75

min. 22 4 7 3 6

max. 489 649 641 368 369

avg. 128 117 106 74 86

min. 0.011 0.009 0.011 0.004 0.024

max. 0.134 0.258 0.237 0.127 0.460

avg. 0.058 0.051 0.052 0.048 0.106

min. 91 45 51 45 45

max. 438 394 411 371 235

avg. 180 145 146 150 122

15921 21887 26468 32373 54890

load range (MBtu/h)

NOx at full load 

(mg/m312%O2)

avg. retail price (US$)

nominal load 

(MBtu/h)

ηGCV

1) at full load 

(%)

CO at full load 

(mg/m3

12%O2)

TSP1) at full load 

(lb/MMBtu)

No. of all test data

No. of unequal test data

No. of top 25% appliances

1) related to the Gross Calorific Value (upper heating value)

Wood chip boilers are available from a nominal load of 35 MBtu/h. The efficiencies vary 

from 70 to 86% and the average is approximately 77%, meaning that their performance 

is 4% lower than that of the solid wood boilers. 

The CO-emissions range from very small amounts to 650 mg/m3 at 12% O2; the average 

value is approximately 100 mg/m3 at 12% O2. Due to the continuous operation 

conditions, these emission values are similar to the CO-emissions of pellet boilers. It can 

also be observed that the CO-emissions decrease as the load range increases. 

TSP-emissions vary from 0.004 to 0.460 lb/MMBtu; the average is approximately 0.052 

lb/MMBtu. Emissions in the largest load range (> 680 MBtu/h) are significantly higher, 

which can be attributed to the small amount of appliance data. Overall, the TSPemissions are slightly higher than the TSP-emissions for solid wood boilers, which may be 

due to the characteristics of the fuel as wood chips have more fine parts are more 

inhomogeneous than pellets. 

The NOX-emissions fall into the range of 50 to 440 mg/m3 at 12% O2; the average value 

is approximately 150 mg/m3 at 12% O2. This value is slightly higher than the NOXemissions from pellets and solid wood stoves, which can attributed to the characteristics 

of wood chips (bark). 
87 

Report 

Table 6-10 overview of extreme and averaged values from TOP 25% performing wood 

chip boilers 

< 100 100 - 200 200 - 340 340 - 680 > 680

35 65 44 39 30

26 49 42 33 26

7 12 11 8 7

min. 51 102 205 342 751

max. 96 171 340 666 2220

avg. 78 147 276 449 1322

min. 73 75 77 77 76

max. 78 81 80 86 79

avg. 76 79 78 80 78

min. 28 10 7 9 15

max. 108 93 87 40 68

avg. 60 39 30 22 34

min. 0.017 0.009 0.020 0.009 0.053

max. 0.110 0.053 0.075 0.072 0.156

avg. 0.046 0.027 0.038 0.042 0.092

min. 91 90 106 45 90

max. 185 179 180 180 178

avg. 157 134 138 125 131

14350 25504 31581 34056 65180

nominal load 

(MBtu/h)

ηGCV

1) at full load 

(%)

CO at full load 

(mg/m3

12%O2)

TSP1) at full load 

(lb/MMBtu)

NOx at full load 

(mg/m312%O2)

avg. retail price (US$)

load range (MBtu/h)

No. of all test data

No. of unequal test data

No. of top 25% appliances

1) related to the Gross Calorific Value (upper heating value)

The TOP 25% performing wood chip boilers reach efficiencies of approximately 78%, 

which is 1% higher than the average emissions of the entire data set. 

In comparison to the CO emissions of the entire data set, decreases of 50 – 70% can be 

observed in that the average CO emissions are very low at approximately 35 mg/m3 at 

12% O2. Dust emissions are approximately 20% lower than the emissions of the entire 

data set. The average dust emissions are approximately 0.040 lb/MMBtu/h. 

Detailed performance data of the TOP 25% performing wood chip boilers and contact 

information of the manufacturers is given in the annex. 

6.3.4 Combined technologies 

Table 6-11 shows a summary of the minimum, maximum and average efficiency- and 

emission values at full load of all considered combined technology boilers (pellets & solid 

wood) and Table 6-12 shows test data of the TOP25% performing appliances at the 

defined load ranges. 

A total of 29 data sets were recorded, of which 22 were not identical. 
88 

Performance of 

biomass 

installations 

Table 6-11 overview of extreme and average values from all considered combined 

technology boilers (pellets & solid wood) 

pellets solid wood

min. 51 51

max. 205 205

avg. 80 92

min. 79 72

max. 89 87

avg. 84 80

min. 1 158

max. 294 1384

avg. 109 367

min. 0.008 0.021

max. 0.139 0.079

avg. 0.036 0.035

min. 92 96

max. 218 174

avg. 131 135

fuel

No. of all test data

No. of unequal test data

No. of top 25% appliances

NOx at full load 

(mg/m312%O2)

avg. retail price (US$)

nominal load 

(MBtu/h)

ηGCV

1) at full load 

(%)

CO at full load 

(mg/m3

12%O2)

TSP1) at full load 

(lb/MMBtu)

15675

29

22

6

1) related to the Gross Calorific Value (upper heating value)

Combined boilers are built in load ranges from 50 to 200 MBtu/h. For pellets mode and 

solid wood mode, average efficiencies of 84% and 80% are respectively achieved. 

CO emissions in pellets mode are approximately 110 mg/m3 at 12% O2, meaning that CO 

emissions are similar to the emissions from pellet boilers. In solid wood mode, CO 

emissions are approximately 370 mg/m3 at 12% O2. These emissions are comparable to 

emissions from solid wood boilers. 

Dust emissions range from 0.010 to 0.140 lb/MMBtu and are approximately 0.036 

lb/MMBtu on average. These values also correspond closely with emission data from 

pellets and solid wood boilers. 

NOx emissions are approximately 130 mg/m3 at 12% O2, which can be considered normal 

NOX-emissions for wood fuel. 
89 

Report 

Table 6-12 overview of extreme and average values from TOP 25% performing 

combined technology boilers (pellets & solid wood) 

pellets solid wood

29

22

6

min. 51 51

max. 126 153

avg. 75 88

min. 79 72

max. 84 83

avg. 82 79

min. 6 158

max. 104 530

avg. 34 252

min. 0.017 0.023

max. 0.040 0.038

avg. 0.027 0.027

min. 92 96

max. 218 150

avg. 145 120

16870

nominal load 

(MBtu/h)

fuel

No. of all test data

No. of unequal test data

No. of top 25% appliances

ηGCV

1) at full load 

(%)

CO at full load 

(mg/m3

12%O2)

TSP1) at full load 

(lb/MMBtu)

NOx at full load 

(mg/m312%O2)

avg. retail price (US$)

1) related to the Gross Calorific Value (upper heating value)

The emissions and efficiencies of the TOP 25% performing appliances correspond to the 

increase of efficiency and decrease of CO and TSP emissions as discussed for the pellets 

and solid wood boilers. 

Detailed performance data of all combined technology boilers and contact information of 

the manufacturers is given in the annex. 

6.3.5 Summary 

In Figure 6-1 and Figure 6-2 the average efficiencies and dust emissions of the TOP 25% 

performing appliances are summarized. The efficiencies and emissions for classes 3 to 5 

in EN 303-5 are also illustrated. In order to enable the comparability of pellet, wood chip 

and solid wood appliances, a correction factor for wood chip and solid wood boilers was 

used because the efficiency requirements and dust emission limits from EN 303-5 were 

converted for pellets fuel. 

Figure 6-1 and Figure 6-2 clearly show that the average performance of the TOP 25% 

performing appliances identified in this report greatly exceed the efficiency requirements 

and are below the dust emission limits of class 5 from EN 303-5 (currently still under 

discussion and has not yet been introduced). At present, class 3 represents the most 

stringent requirements for biomass boilers. 
90 

Performance of 

biomass 

installations 

75%

80%

85%

90%

0 200 400 600 800 1000

nominal load in mBTU/h

class 3

class 4

class 5

efficiency based on GCV

TOP 25% pellet boilers

TOP 25% solid wood boilers*

TOP 25% wood 

chip boilers*

* conversion from efficiency based on NCV to efficiency based on GCV was performed based on pellets fuel, therefore a 

correction factor for wood chip boiler (+10.6%) and for wood chip boilers (+3.1%) was used to enable comparability, 

efficiencies for wood chip and solid wood boilers are not exact.

Figure 6-1 efficiency classes 3 to 5 from EN 303-5 and efficiency performance of TOP 

25% pellet, solid wood and wood chips boilers; efficiencies from solid wood and wood 

chip boilers were corrected because efficiency limits based on the GCV were converted 

for pellets fuel 

0.000

0.050

0.100

0.150

0.200

0 200 400 600 800 1000

nominal load in mBTU/h

class 3

class 4

class 5

TOP 25% pellet boilers

TOP 25% solid wood boilers*

TOP 25% wood chip boilers*

class 2

* conversion from dust emission limits from mg/m3 to lb/MMBTU was performed for pellets fuel, therefore a correction factor 

for wood chip boiler (0.754) and for wood chip boilers (0.911) was used to enable comparability dust emissions in lb/MMBtu

Figure 6-2 dust emission limit classes 2 to 5 from EN 303-5 and dust emissions from 

TOP 25% pellet, solid wood and wood chips boilers; emissions from solid wood and 

wood chip boilers were corrected because dust emission conversion from mb/m3 to 

lb/MMBtu was performed for pellets fuel 
91 

Report 

Figure 6-3 shows the efficiencies from the TOP 25% performing boilers for nominal and 

partial loads. For pellet and wood chips boilers partial load equals 30% of nominal load, 

for solid wood boilers partial load is 50% of nominal load. 

75%

80%

85%

90%

< 55

55 - 100

100 - 170170 - 340

> 340

< 85

85 - 120

120 - 170

> 170< 100

100 - 200200 - 340340 - 680

> 680

pellet boilers solid wood boilers wood chip boilers

nominal load partial load

efficiency based on GCV

load category

(MBtu/h)

Figure 6-3 average efficiencies based on GCV at nominal and partial heat load for the 

TOP 25% performing boilers 

Efficiencies that are reached at nominal and partial load are nearly identical, solid wood 

and wood chip appliances sometimes even reach higher efficiencies at partial load. 

6.4 Commercial biomass installations 

Medium to large scale commercial biomass installations undergo individual 

commissioning, meaning that these plants are dimensioned and built individually with 

respect to the technical requirements of the buyer. Commisioning is performed in order 

to guarantee that the emission and efficiency requirements (see chapter 5) are met. 

Depending on the local regulations, emissions and efficiencies have to be controlled 

periodically by consulting engineers. Results of these measurements are usually not 

published; therefore, no database on emissions and efficiencies of medium commercial 

biomass appliances is available. 

In order to give an idea of emissions and efficiencies of medium scale biomass plants, 

results from various studies on this topic will be cited in this section. In addition, national 

emission factors for biomass combustion units shall show which average emissions are 

assumed for the Austrian biomass plants. 
92 

Performance of 

biomass 

installations 

6.4.1 Data collection of Austrian medium scale biomass appliances 

(Musil et al. 2006) 

Emission data for Austrian biomass combustion and CHP plants was collected directly 

from the operators of medium scale biomass units using a standard questionnaire. In 

addition, the measurement results of one consulting engineer were used for the 

estimation of emissions from medium scale biomass appliances. Approximately 100 data 

sets in total were collected, all appliances were operated with wood chips, saw shavings 

or wood residues. 

In Figure 6-4 the average emission results for TSP and NOX assigned in three load ranges 

are shown. 

163

180

148

76 82

33

0

20

40

60

80

100

120

140

160

180

200

≤ 3.4 mmBTU/h 3.4 - 6.8 mmBTU/h 6.8 - 17.1 mmBTU/h

nominal heat load NOx TSP

sd.: 79

sd.: 40

sd.: 92

sd.: 31

sd.: 33

sd.: standard deviation

emissions in mg/m3 13%O2

sd.: 33

Figure 6-4 TSP and NOx emissions of medium scale biomass appliances in Austria 

While the NOX emissions were quite constant for these load categories (from 170 – 205 

mg/m3 at 12% O2), dust emissions decreased significant with increasing load because of 

stricter emission limits for loads from 6.9 MMBtu/h. Up to a load range of 6.8 MMBtu/h, 

TSP emissions were found to be around 80 mg/m3 at 13% O2 (0.150 lb/MMBtu), TSP 

emissions for higher loads are around 35 mg/m3 at 13% O2 (0.065 lb/MMBtu). 

Figure 6-5 shows the TSP emissions and the emission limits of the analysed biomass 

appliances in more detail including the utilised emission control technology. 
93 

Report 

0

20

40

60

80

100

120

140

160

1 2 35 1 35 2 3 45

≤ 3.4 mmBTU/h 3.4 - 6.8 mmBTU/h 6.8 - 17.1 mmBTU/h

TSP in mg/m3 @ 13% O2

technology:

heat load:

blue dots: emission limits for the appliance 

technology 1: cyclone 

technology 2: cyclone + electrostatic precipitator 

technology 3: cyclone + flue gas condensation 

technology 4: cyclone + flue gas condensation + electrostatic precipitator 

technology 5: electrostatic precipitator

Figure 6-5 dust emissions and emission limits of Austrian medium scale biomass 

appliances (Musil et al. 2006) 

For appliances of up to 6.8 MMBtu/h, cyclones are the most commonly used emission 

control technology. This is due to the relatively high emission limits of 150 mg/m at 13% 

O2 (0.275 lb/MMBtu) in comparison to emission limits for combustion units from 6.8 

MMBtu/h (50 mg/m3 at 13% O2 and 0.090 lb/MMBtu). Next to cyclones, electrostatic 

precipitators combined with cyclones are the second most common. No bag-house filters 

were used for appliances of this load range. 

6.4.2 Data collection of Swiss automated biomass appliances 

(Hasler et al. 2000) 

In a large data survey, approximately 1000 measurement results of 250 biomass 

appliances installed in Switzerland were collected and analyzed. Untreated wood chips 

(LRV Kat. b), wood residues (LRV Kat. c) and urban waste wood (Altholz) were used. For 

comparison, a small number of data from solid wood appliances (LRV Kat. a) were also 

considered. 

Figure 6-6 shows the number of the considered wood chip appliances by heat load and 

fuel category. 
94 

Performance of 

biomass 

installations 

Figure 6-6 number of considered wood chip boilers by heat load and fuel category 

from 100 to 2000 kW (340-6820 MBtu/h); fuels: untreated wood chips (LRV Kat. b), 

wood residues (LRV Kat. c) and urban waste wood (Altholz) 

how to read the graph: the first range of values (1st field of the X-axis) includes all 

appliances in the range from 0 to 100 kW (0-340 MBtu/h) 

(Hasler et al. 2000) 

Figure 6-6 shows the CO emissions and the excess air ratio of the considered boilers with 

untreated wood chips fuel (LRV Kat. b). 

Figure 6-7 CO emissions in mg/m3 at 11%O2 and the excess air ratio of the 

considered boilers with untreated wood chips fuel (LRV Kat. a) (Hasler et al. 2000) 

How to read the graph: the first range of values (1st field of the X-axis) includes all 

measurement points in the range from 0 to 100 mg/m3 at 11%O2 (0 to 90mg/m3

 at 

12%O2) respectively 0 to 1.2 

Two thirds of the considered appliances have CO emissions of < 250 mg/m3 at 11% O2

(225 mg/m3 at 12% O2); CO emissions > 1000 mg/m3 at 11% O2 (900 mg/m3 at 12% 

O2) are only measured in appliances with heat loads below 500 kW (1706 MBtu/h). The 

excess air ratio shows broad variations from 1.2 < λ < 5. Most of the appliances reach 

excess air ratios between 1.8 and 2.4 and about one third of the appliances are operated 

with excess air ratios > 2.4. 
95 

Report 

Figure 6-8 shows the NOX emissions of all considered wood chip appliances (untreated 

wood chips (LRV Kat. b), wood residues (LRV Kat. c) and urban waste wood (Altholz)). 

Figure 6-8 NOX -emissions from wood chip appliances (fuels: untreated wood chips 

(LRV Kat. b), wood residues (LRV Kat. c) and urban waste wood (Altholz)) 

(Hasler et al. 2000) 

How to read the graph: the first range of values (1st field of the X-axis) includes NOX

emissions in the range from 0 to 50 mg/m3 at 11% O2 (45 mg/m3 at 12% O2)

NOX emissions from untreated wood chips (LRV Kat. b) exhibit a nearly Gaussian 

distribution in the range from 100 to 400 mg/m3 at 11% O2 . For urban waste wood, NOX

emissions fall into a similar range. NOX emissions from wood residues (LRV Kat. c) fall 

into a broader range of up to 850 mg/m3 at 11% O2 due to higher N contents in some 

wood residues appliances.

Figure 6-9 shows the TSP/dust emissions of wood chip appliances (untreated wood chips 

(LRV Kat. b), wood residues (LRV Kat. c) and of solid wood appliances (LRV Kat. a)). 
96 

Performance of 

biomass 

installations 

Figure 6-9 TSP/dust emissions from wood chips and solid wood appliances (fuels: solid 

wood (LRV Kat. a) untreated wood chips (LRV Kat. b), wood residues (LRV Kat. c)) 

How to read the graph: the first range of values (1st field of the X-axis) includes dust 

emissions in the range from 0 to 25 mg/m3 at 11% O2 (0-0.030 lb/MMBtu for solid 

wood and 0-0.037 lb/MMBtu) 

(Hasler et al. 2000) 

For all solid wood appliances (LRV Kat. a) the dust emissions are below 150 mg/m3 at 

11% O2 (0.185 lb/MMBtu); most dust loads are in the range from 25 to 50 mg/m3 at 

11% O2 (0.030 – 0.060 lb/MMBtu). For untreated wood chips (LRV Kat. b) and wood 

residues (LRV Kat. c) the averaged dust emissions are between 100 and 150 mg/m3 at 

11% O2 (0.150- 0.220 lb/MMBtu). 

6.4.3 Emission factors 

While measurement results of emissions are related to the input load, emission factors 

are related to the annual energy use. Therefore a direct conversion from emissions to 

emission factors is not possible. 

Emission factors are used to calculate the national annual overall emissions of CO2, CO, 

NOX and other pollutants. 

Calculation methods for emission factors vary from country to country and depend 

strongly on the availabel data sources. Emission factors are supposed to represent the 

average annual emissions of the considered appliance type, including start- and stop 

emissions as well as summer and winter operation modes. Emission factors are derived 

as a function of plant size and fuel type and are combined in different classes which differ 

from country to country. To calculate the annual overall emissions, the shares of the 

diverse emission classes in the total energy consumption has to be determined. 

In order to estimate nationwide emissions of diverse facilities, emission factors are 

specified for diverse facilities and fuels. Each country identifies emission categories on its 

own, which means that emissions from biomass combustion units are often mixed with 

emissions from other combustion units. Table 7-4 shows emission factors from Austria, 

Germany and Switzerland. 
97 

Report 

Table 6-13 Austrian, German and Swiss emission factors 

CO 

(lb/MMBtu) 

TSP 

(lb/MMBtu) 

NOX

(lb/MMBtu)

AUSTRIA (SOURCE: BOEHMER ET AL. 2007) 

biomass district heating/CHP 

facilities < 170 MMBtu/h wood 

chips including bark and wood 

residues 

0.115 0.050 0.215 

GERMANY (SOURCE: STRUSCHKA ET AL. 2008) 

manual boilers > 170 MBtu/h 1.400 0.145 0.200 

pellet boilers > 170 MBtu/h 0.255 0.075 0.230 

wood chip boilers > 170 MBtu/h 2.305 0.150 0.370 

SWITZERLAND (SOURCE: HASLER ET AL. 2000) 

solid wood (LRV Kat. a) 0.105 0.235 

untreated wood chips (LRV Kat. b) 0.205 0.375 

wood residues (LRV Kat. c) 0.215 0.665 

urban waste wood (Altholz) 0.005 0.485 

Table 6-13 clearly shows that each country looks at different combustion systems and 

fuel categories in order to calculate the national emission factors. Therefore, the 

comparability of these emission factors is limited. 

CO emissions vary from 0.100 to 2.300 lb/MMBtu. While wood chip appliances are 

considered to reach low CO emissions in the Austrian study, very high emissions are 

considered in the German study. 

Except for the Swiss emission factors for urban waste wood (0.005 lb/MMBtu), TSP 

emissions vary from 0.050 to 0.220 lb/MMBtu. Since TSP emissions depend on the 

emission control system installed, this variation seems reasonable due to different TSP 

control concepts and/or limits. 

For medium scale biomass appliances, emission controls for NOX are usually not installed. 

Since NOX emissions result primarily from nitrogen in the fuel and the fuel quality is quite 

constant, NOX emission factors should be very similar. This is true for the emission 

factors shown in Table 6-13 where NOX emission factors are within the range of 0.200 to 

0.375 lb/MMBtu. Since wood residues and urban waste wood usually contain more 

nitrogen then the other biomass fuels, NOX emissions are thus higher for these 

appliances. 

6.4.4 Summary 

In two studies, emission data from medium scale biomass appliances were collected and 

analysed, whereby an emphasis was placed on NOX and dust emissions. In the Austrian 

study (Musil et al. 2006), biomass district heating/CHP facilities with heat loads of up to 

17 MMBtu/h were analysed, while the Swiss study (Hasler et al. 2000) considered all 

sizes of biomass appliances from residential stoves up to 17 MMBtu/h. In addition, 
98 

Performance of 

biomass 

installations 

emission factors are presented for Austria (Boehmer et al. 2007), Germany 

(Struschka et al. 2008) and Switzerland (Hasler et al. 2000). Since emission factors are 

based on the annual heat demand, they cannot be directly compared with measured 

average emission values. 

CO emissions were only anaysed in the study from Hasler et al. 2000. It was shown that 

two thirds of the appliances emit CO < 225 mg/m3 at 12% O2. Emission factors for CO 

show very broad variations from 0.100 to 2.300 lb/MMBtu (≈ 70 to 1400 mg/m3 at 12% 

O2). 

While the Austrian study shows averaged dust emissions (the following values are based 

on those for pellets) of about 0.097 lb/MMBtu for appliances < 6830 Btu/h and about 

0.060 lb/MMBtu for appliances > 6830 Btu/h, the Swiss study results in averaged dust 

emissions of 0.150 – 0.200 lb/MMBtu for all fuels. Emission factors for dust vary from 

0.050 to 0.220 lb/MMBtu. Since dust emissions depend greatly on the installed emission 

control technology, variations of these emissions may be attributed to different 

technologies. 

Average NOX emissions which were given in the studies vary from 150 to 220 mg/m3 for 

untreated wood. For wood residues the NOX emissions can be higher due to a higher 

nitrogen content of the wood fuel. Also the emission factors for NOX show high 

correlation. They are in a range from 0.200 to 0.375 lb/MMBtu (≈ 120 to 230 mg/m3 at 

12% O2) for untreated wood. For urban waste wood and wood residues, higher emission 

factors were fixed because of the higher nitrogen content of these fuels. 
99 

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7 Emission control technologies 

For biomass appliances, emission control technologies usually aim at the reduction of CO, 

OGC, dust and NOX emissions. Two different concepts for emission reduction, both 

primary and secondary measures have to be considered. While primary measures are 

implemented in the combustion process and/or design of the combustion chamber, 

secondary measures are implemented after combustion process has ended, mostly 

adjacent to the boiler. Table 7-1 gives a summary indicating which reductions measures 

are available for small and medium to large scale biomass appliances. 

Table 7-1 overview of primary and secondary emission control technologies for 

biomass appliances 

scale small medium to large 

measures primary secondary primary secondary 

CO/OGC X - X - 

PM/dust X X X X 

NOX - - X X1)

1) in biomass appliances NOX emissions are usually below the limit by primary measures 

7.1 Emission control technology for PM/dust 

Secondary measures for emission control in biomass appliances usually aim at the 

reduction of dust emissions. CO/OGC and NOX emissions are reduced below the limits by 

primary measures, as was shown above. 

7.1.1 Small scale appliances 

Due to stricter dust emission limits for small scale appliances, special solutions for dust 

reduction in this load range were recently developed. These developments were mostly 

performed by small companies (business start-ups) and manufacturers of small scale 

appliances, who were aware of this need. Small scale dust emission control is based on 

the technology of scrubbers and electrostatic precipitators. For heat loads > 650 MBtu/h, 

baghouse filter systems were also developed. Scrubber systems are always combined 

with flue gas condensation. 

Table 7-2 gives an overview of commercially available secondary dust reduction 

technology for small scale appliances; the contact data of the manufacturers is available 

in the annex. 
100 

Emission control 

technologies 

Table 7-2 commercial small scale dust control technologies (summarised from 

Bischof 2008) 

name 

heat load 

in 

MBtu/h 

efficiency 

in % 

life 

time in 

years 

market 

launch

sold 

systems

price in 

US$ producer 

SCRUBBER/CONDENSATION

Hydrocube 50 – 

3400 ≤ 70 15 - 20 2007 250 7400 Schraeder Abgastechnologie 

Power ≤ 85 2500 

Condenser ≤ 170 

751) 20 2008 20 

3700 

Powerkondens AG 

21002)

Carbonizer 75 - 

1700 ≤ 86 > 20 2003 500 

74003) Bschor GmbH 

34004)

Profitherm 50 - 500 50 > 20 - - 

66003)

BOMAT Heiztechnik 

GmbH 

ELECTROSTATIC PRECIPITATOR

Zumik®on ≤ 120 60 - 90 15 - 20 2006 > 140 1600 Rueegg Cheminée 

AG 

≤ 120 

Trion ≤ 170 

≤ 12000

90 15-20 2005 300 - 

400 

1500 - 

30005)

Trion 

Luftfiltersysteme 

GmbH 

OekoTube ≤ 240 90 15 2008 0 2000 OekoSolve AG 

Airbox ≤ 50 > 60 - 2008 10 2100 Spartherm 

BAGHOUSE FILTER

Keramikfilter > 680 99 15 - 20 - - - Herding GmbH 

Filtertechnik 

Koeb 

Winkel 

680 - 

2000 

< 10 

mg/m3 - - - - Koeb & Schaefer GmbH 

1) 50 without scrubber (condensing only) 

2) 75 MBtu/h 

3) 340 MBtu/h 

4) 70 MBtu/h 

5) depends on length 

As Table 7-2 shows, dust removal technologies for small scale appliances were not 

introduced to the market before 2003. Furthermore, the number of systems sold shows 

that this is a very young technology. A large number of prototypes are currently being 

tested. 

Scrubbers with flue gas condensing and electrostatic precipitators are available for very 

small heat loads. In general, electrostatic precipitators are less expensive and reach 

higher efficiencies. While for scrubber technology low return temperatures of about 85 °F 

are necessary for flue gas condensation, electrostatic precipitators have a maximum 

energy demand of 100 Btu/h (Bischof 2008). 
101 

Report 

7.1.2 Medium scale appliances 

For medium to large scale appliances, industrial solutions for emission control are used. 

These technologies are usually produced by plant or system manufacturers who are 

either specialists of flue gas cleaning technology or build complete (energy) plants. 

Emission control technologies for medium and large scale appliances are individually fit to 

the design data of the biomass appliance. 

For medium scale biomass appliances the cyclones for dust removal are usually built by 

the boiler manufacturers themselves. Table 7-3 shows important European 

manufacturers of dust emission control technologies for medium scale biomass 

appliances including their special fields. 

Table 7-3 European manufacturers for dust control technologies for medium scale 

biomass appliances( Brunner 2009, Lundgren 2009, Oravainen 2009) 

company cyclone scrubber electrostatic 

precipitator 

baghouse 

filter 

Beth X X 

Scheuch GmbH X X X X 

VAS - Verfahrenstechnik & 

Anlagensysteme X 

JM Stoftteknik AB X X 

Jaernforsen Energi System AB X 

Dust Control Systems Oy X X 

In Table 7-4 an overview of prices for the dust removal technologies is given. 

Table 7-4 prices for medium to large scale dust control technologies (FNR 2005) 

technology heat load in MMBtu/h costs in US$/MBtu/h 

0.3 – 1.7 7 – 11 multicyclone 1.7 – 6.8 4 – 7 

3.4 – 6.8 20 - 30 

6.8 - 17 15 - 22 

baghouse filters or 

electrostatic 

precipitators 17 - 68 9 - 18 

flue gas condensation 3.4 - 17 20 - 30 
102 

European Biomass 

Fuel Requirements 

8 European Biomass Fuel Requirements 

8.1 General remarks 

The development of solid bio-fuel standards in Europe is linked to the beginning of wood 

pellet production, which started in Sweden in the late 1980s. Other countries such as 

Finland, Denmark and Austria followed in the early 1990s. 

In order to gain a common understanding between the manufacturers and the users, 

codes of good practice, quality guidelines and later national standards have been 

developed. In Europe the first national standards on pellet quality were developed in 

Sweden and Austria in 1998 – other countries followed. 

In general these fuel quality standards should: 

• guarantee a common, official, national quality of a certain bio-fuel 

• ensure legal compliance and security for all involved market participants by 

defining responsibilities and duties 

• help to avoid problems at interface-points along the supply chain by defining 

quality standards and threshold values 

• inform the consumers about quality characteristics and therefore support the 

dissemination of a new biomass fuel to the market. 

In addition to the national efforts, the development of quality guidelines was also pushed 

on the European level. The Technical Committee 335 (TC335) of the European 

Committee for Standardization (CEN) developed the European standards CEN/TC 

14558:2003 Solid bio-fuels – Terminology, definitions and descriptions as well as CEN/TC 

14961:2005 Solid bio-fuels – Fuel specifications and classes which allow a differentiation 

of terminologically defined solid bio-fuels by means of defined parameters. 

Additionally, a series of technical standards dealing with determination methods of 

specific fuel properties or sampling methods have been worked out by the TC335. 

Table 8-1 lists all standards developed by this technical committee. 

Moreover, national standards for wood pellets logistics and storage have been developed. 

The definition of quality parameters for logistics and storage was also found to be 

important so as to avoid any problems along the supply chain. Furthermore, the end-user 

should have a clear guideline of how to ensure product quality and user safety during the 

storage of bio-fuels. 
103 

Report 

Table 8-1 List of standards developed by CEN/TC 335 

STANDARD REFERENCE TITLE

CEN/TS 14588:2004 Solid bio-fuels - Terminology, definitions and descriptions 

CEN/TS 14774-1:2004 Solid bio-fuels - Methods for determination of moisture content 

- Oven dry method - Part 1: Total moisture - Reference method 

CEN/TS 14774-2:2004 

Solid bio-fuels - Methods for the determination of moisture 

content - Oven dry method - Part 2: Total moisture - Simplified 

method 

CEN/TS 14774-3:2004 

Solid bio-fuels - Methods for the determination of moisture 

content - Oven dry method - Part 3: Moisture in general 

analysis sample 

CEN/TS 14775:2004 Solid bio-fuels - Method for the determination of ash content 

CEN/TS 14778-1:2005 Solid bio-fuels - Sampling - Part 1: Methods for sampling 

CEN/TS 14778-2:2005 Solid bio-fuels - Sampling - Part 2: Methods for sampling 

particulate material transported in lorries 

CEN/TS 14779:2005 Solid bio-fuels - Sampling - Methods for preparing sampling 

plans and sampling certificates 

CEN/TS 14780:2005 Solid bio-fuels - Methods for sample preparation 

CEN/TS 14918:2005 Solid Bio-fuels - Method for the determination of calorific value 

CEN/TS 14961:2005 Solid bio-fuels - Fuel specifications and classes 

CEN/TS 15103:2005 Solid bio-fuels - Methods for the determination of bulk density 

CEN/TS 15104:2005 Solid bio-fuels - Determination of total content of carbon, 

hydrogen and nitrogen - Instrumental methods 

CEN/TS 15105:2005 Solid bio-fuels - Methods for determination of the water soluble 

content of chloride, sodium and potassium 

CEN/TS 15148:2005 Solid bio-fuels - Method for the determination of the content of 

volatile matter 

CEN/TS 15149-1:2006 

Solid bio-fuels - Methods for the determination of particle size 

distribution - Part 1: Oscillating screen method using sieve 

apertures of 3,15 mm and above 

CEN/TS 15149-2:2006 

Solid bio-fuels - Methods for the determination of particle size 

distribution - Part 2: Vibrating screen method using sieve 

apertures of 3,15 mm and below 

CEN/TS 15149-3:2006 Solid bio-fuels - Methods for the determination of particle size 

distribution - Part 3: Rotary screen method 

CEN/TS 15150:2005 Solid bio-fuels - Methods for the determination of particle 

density 

CEN/TS 15210-1:2005 Solid bio-fuels - Methods for the determination of mechanical 

durability of pellets and briquettes - Part 1: Pellets 

CEN/TS 15210-2:2005 Solid bio-fuels - Methods for the determination of mechanical 

durability of pellets and briquettes - Part 2: Briquettes 

CEN/TS 15234:2006 Solid bio-fuels - Fuel quality assurance 

CEN/TS 15289:2006 Solid Bio-fuels - Determination of total content of sulphur and 

chlorine 
104 

European Biomass 

Fuel Requirements 

STANDARD REFERENCE TITLE

CEN/TS 15290:2006 Solid Bio-fuels - Determination of major elements 

CEN/TS 15296:2006 Solid Bio-fuels - Calculation of analyses to different bases 

CEN/TS 15297:2006 Solid Bio-fuels - Determination of minor elements 

CEN/TS 15370-1:2006 Solid Bio-fuels - Method for the determination of ash melting 

behavior - Part 1: Characteristic temperatures method 

8.2 European Bio-fuel standards 

The technical standard EN14961 from 2005 defines the fuel quality classes and 

specifications for solid bio-fuels originating from the following sources: 

• Products from agriculture and forestry, 

• Vegetable waste from agriculture and forestry 

• Vegetable waste from the food processing industry, 

• Wood waste, with the exception of wood waste which may contain halogenated 

organic compounds or heavy metals as a result of treatment with wood 

preservatives or coatings 

• waste originated from construction and demolition waste 

• Fibrous vegetable waste from virgin pulp production and from production of paper 

from pulp, if it is co-incinerated at the place of production and heat generated is 

recovered. 

• Cork waste 

NOTE: Demolition wood is not included in the scope of this standard. Demolition wood is 

defined as “used wood arising from demolition of buildings or civil engineering 

installations” (CEN/TS 14588). 

8.2.1 Origin of Bio-fuels 

The first step in the classification of solid bio-fuels according to En14961 is based on the 

bio-fuel origin and source. In the hierarchical classification system, the main origin-based 

solid bio-fuel groups are: 

• Woody biomass 

• Herbaceous biomass 

• Fruit biomass 

• Blends and mixtures 

Each group has several subdivisions to allow for a more detailed specification. 
105 

Report 

8.2.2 Specification of solid bio-fuels based on traded forms and 

properties 

Solid bio-fuels are traded in many different forms (sizes and shapes). The European 

standard EN14961:2005 distinguishes between solid bio-fuel trading forms, which are 

listed in Table 8-2. 

Table 8-2 Trading forms of solid bio-fuels and their preparation methods according to 

EN14961:2005. 

FUEL NAME TYPICAL PARTICLE SIZE COMMON PREPARATION METHOD 

Briquettes Ø>25 mm Mechanical compression 

Pellets Ø<25 mm Mechanical compression 

Fuel powder <1 mm Milling 

Sawdust 1 – 5 mm Cutting with sharp tools 

Wood chips 5 – 100 mm Cutting with sharp tools 

Hog fuel Varying Crushing with blunt tools 

Logs 100 – 1000 mm Cutting with sharp tools 

Whole wood >500 mm Cutting with sharp tools 

Small straw bales 

Big straw bales 

Round straw bales 

0.1 m3 

3.7 m3 

2.1 m3 

Compressed and bound to squares 

Compressed and bound to squares 

Compressed and bound to cylinders 

Bundle Varying Lengthways oriented & bound 

Bark Varying Debarking residue from trees Can be 

shredded or unshredded 

Chopped straw 10 – 200 mm Chopped during harvesting 

Grain or seed Varying No preparation or drying 

Shells and fruit stones 5 – 15 mm No preparation 

Fiber cake Varying Prepared from fibrous waste by 

dewatering 

The specifications of properties of these different forms are given in the annex. 

8.2.3 Revision of EN14961:2005 

Currently the EN14961 is under revision. The revised standard will not only define the 

specification of properties of solid bio-fuels, but also include definite properties such as 

threshold values for different quality classes. The main intention of this step is to simplify 

the acquisition of appropriate fuels for end users. To cover all kinds of solid biomass fuels 

as well as industrial fuels with lower quality requirements, a general section (part 1 of 

the revised EN14961) will be included in the revised standard. The structure of 

prEN 14961-part 1 is very similar to the first version of the standard from 2005 (see 

chapter 8.1): 
106 

European Biomass 

Fuel Requirements 

 EN14961-1: Scope and general requirements of solid bio-fuels 

Parts 2 – 6 of the new standard will define fuel quality classes and specifications explicitly 

for non-industrial bio-fuels: 

• EN14961-2: Fuel quality classes and specifications of wood pellets 

• EN14961-3: Fuel quality classes and specifications of wood briquettes 

• EN14961-4: Fuel quality classes and specifications of wood chips 

• EN14961-5: Fuel quality classes and specifications for firewood 

• EN14961-6: Fuel quality classes and specifications of non-woody pellets 

The current proposal prEN 14961 (status summer 2009) is shown in the annex. 

8.3 European countries with national pellet standards 

As previously mentioned, some European countries developed national quality standards 

for solid bio-fuels – mainly for wood pellets – long before a European standard was 

available. Some of these national standards do not only regulate the declaration of solid 

bio-fuels as the existing European standard EN14961:2005, but set definite specifications 

of properties (e.g. threshold values for contaminants) for high quality fuels. If the revised 

European Standard for solid bio-fuels is implemented, the national standards will be 

discarded. 

8.3.1 Austria 

In Austria, three standards referring to pellets product quality, logistic quality and 

storage quality are available: 

• OeNORM M 7135 - Compressed wood in natural state or bark in natural state – 

pellets and briquettes - requirements and test specifications 

• OeNORM M 7136 - Compressed wood in natural state - Wood pellets quality 

assurance in the field of logistics of transport and storage 

• OeNORM M 7137 - Compressed wood in natural state - Wood pellets – 

Requirements on pellets storage at the end-consumer 

8.3.1.1 OeNORM M 7135 

This product standard covers wood pellets as well as briquettes. Pellets or briquettes 

must be made of virgin wood; up to 2 % of natural binding agents are allowed. Physical 

and chemical property requirements for the highest quality class are listed in Table 8-3. 

The process of quality management includes internal and external audits. Manufacturers 

must perform internal quality assurance continuously, at least once a week by testing 

abrasion, water content, unit density and pressing aids. External audits include an initial 

inspection of the production plant and the sampling for laboratory tests, the control of 

the internal quality assurance system as well as the control of the labeling. Periodic, 

unannounced supervision is done once a year. 
107 

Report 

8.3.1.2 OeNORM M 7136 

In order to assure the quality of wood pellets produced according to OeNORM M 7135 

during transport and storage, certain requirements are specified under the OeNORM M 

7136. 

Specifications encompass transportation and temporary storage. These specifications are 

intended to assist pellet manufacturers, haulers and traders in avoiding mistakes to 

ensure customer satisfaction along the logistic chain. 

8.3.1.3 OeNORM M 7137 

This standard refers to the storage of wood pellets produced according to OeNORM M 

7135. It defines specifications and requirements of wood pellet storage for the end user. 

The standard addresses the end users of wood pellets as well as the manufacturers of 

pellets storage units. 

8.3.2 Germany 

In Germany there exist two standards, DIN 51731 for common pellets and briquettes and 

Din plus with higher requirements, especially for the use in automatically stoked pellet 

boilers. 

8.3.2.1 DIN 51731 

The German standard defines the physical and chemical property requirements such as 

shape, unit density and the chemical composition of wood pellets. Specifications of the 

highest pellet quality class according to DIN 51731 are listed in Table 8-3. 

8.3.2.2 DIN plus Standard 

The DIN plus certification programme is used for high quality pellets intended for use in 

small scale household appliances. The specifications are mainly based on the Austrian 

standard OeNORM M7135, extended by the threshold requirements for minor elements 

from the DIN 51731. External auditing is performed once a year and is unannounced. 

Specifications of the DIN plus pellet quality label are listed in Table 8-3. 

8.3.2.3 Certification of pellet logistics 

“DIN-Gepruefter Fachbetrieb – Pelletlogistik” (DIN tested qualified enterprise pellet 

logistics) monitors the transport to the end-costumer. The test criteria for suppliers 

include specifications for the storage depot, employees, internal and external monitoring 

and transport vehicles. The requirements are very similar to the Austrian standard 

OeNORM M 7136. 
108 

European Biomass 

Fuel Requirements 

DIN CERTCO developed a new standard for the storage of pellets for end consumers 

based on the Austrian standard OeNORM M7137: “DIN gepruefte Pelletlagerung” (“DIN 

certified pellet storage”). 

8.3.3 Sweden 

The Swedish standard SS187120: “Bio-fuels and peat—Fuel pellets—Classification” for 

wood pellets specifies properties and threshold values for three different wood pellet 

quality classes. Specifications are listed in Table 8-3. 

8.3.4 Italy 

The Italian standard for solid bio-fuel CTI - R 04/5 was published in March 2004 and 

specifies the requirements for biopellets for energetic purposes and is related to the 

technical specification defined by CEN/TC335 (EN14961:2005). 

This standard classifies 4 categories of pellets and includes the origin of the raw material. 

Categories A1 and A2 (with / without additives) determine the fuel quality classes and 

specifications for pellets originating from virgin wood, by-products and residues from the 

wood processing industry as well as chemically untreated used wood. Categories B and C 

encompass non-woody pellets produced from herbaceous biomass, fruit biomass and 

biomass blends and mixtures. Specifications of the CTI - R 04/5 are listed in Table 8-3. 

8.3.5 United Kingdom 

The Department of Trade and Industry of the UK published “Codes of Good Practice” for 

Bio-fuel pellets. These codes were adopted by British BioGen members as a standard for 

products and services. The Codes of good practice will be superseded by the European 

standards for solid bio-fuels once they are published. Specifications of the British code 

are listed in Table 8-3. 

8.4 Additional environmental or quality labels 

In addition to the official national standards, additional environmental or quality labels for 

pellets have been published. In some countries these quality labels act as a substitute for 

the lack of a national standard, while in others they function as a supplement to the 

national standard indicating the best available product quality or special ecological 

products. Figure 8-1 shows logos of environmental and quality labels for solid bio-fuels. 
109 

Report 

Figure 8-1: Environmental labels for solid bio-fuels 

Austria – “Umweltzeichen (Environmental Label)”: The Federal Ministry for the 

Environment has devised a special environment label for biomass fuels, where only raw 

materials from natural wood are allowed. 

Nordic Ecolabelling – “Nordic Swan”: The Swan-labelling of pellets includes requirements 

on manufacturing methods, transportation and storage. The aim is to identify the topgrade quality from an environmental perspective. The quality of the pellets shall mean 

that they are easy to use and thus meet the end-users’ wishes when converting to a 

renewable energy source that reduces the emission of greenhouse gases. In addition, the 

energy required to manufacture the pellets is limited to ensure the energy efficiency. 

Finally, the combustion shall not entail a health or environmental risk. Specifications of 

“Swan labeled” pellets are summarized in Table 8-3. 

Italy – “Pellet Gold” (Pelletgold 2009): is the highest Italian wood fuel quality standard. 

The certification system is based on clear operation rules to determine that the product 

satisfies the requirement indicated in the reference documents. 

The requirements of Pellet Gold are based on CEN/TS 14961, DINplus, OeNORM M 7135 

and the limits introduced by the American Pellet Fuels Institute (PFI). 

An additional element in the Pellet Gold standard which is not present in other European 

certification systems was introduced by AIEL (Associazione Italiana Energie 

Agroforestali): the Formaldehyde content (HCHO) which is fundamental to verify the 

presence of health hazardous materials such as glue and varnish (see Table 8-3). 
110 

European Biomass 

Fuel Requirements 

8.5 Summary of European and national property specifications for bio-fu

Table 8-3 Comparison of European and national wood pellet/briquette standards and environmental labe

 AUSTRIA EU GERMANY GREAT BRITAIN 

Property class 

Unit 

OeNORM M 

7135 

EN 

14961:2005 

Best pellet 

categories 

prEN 14961-2 

A1-class 

DIN 51731 

DIN plus 

BioGen - 

Code of good 

practice 

CTI – R 04/5 

Diameter, D and Length, L in 0.16 ≤ D ≤ 0.39 D06 0.24 ± 0.02 D06 0.24 ± 0.04 0.16 ≤ D ≤ 0.39 0.16 ≤ D ≤ 0.39 D 0.16 – 0.79 D 0.24 – 0.31 Moisture, M as received, w-% ≤ 10 M10 ≤10 M10 ≤10 ≤ 12 ≤ 10 ≤ 10 ≤ 10 Ash, a w-% dry ≤ 0.5 A0.7 ≤ 0.7 

A0.5 ≤

0.5 

A0.7 ≤

0.7 

≤ 1.5 ≤ 0.5 ≤ 1.0 ≤ 0.7 

Mechanical durability, DU or abrasion (100 – DU) 

as received, 

w-% ≥ 97.7 DU97.5 ≥ 97.5 DU97.5 ≥ 97.5 - ≥ 97.7 - ≥ 97.7 

Fines at factory gate in bulk transport 

w-% - F1.0 ≤ 1,0 F1.0 ≤ 1,0 - - ≤ 0,5 - 
 AUSTRIA EU GERMANY GREAT BRITAIN 

Fines in small bags at factory gate (at packing) Fines in small bags (at delivery to end-user) 

F0.5 ≤

0,5 

F1.0 ≤

1,0 

Additives w-% ≤ 2 

Type and 

amount 

to be 

stated 

Type and 

amount 

to be 

stated 

none ≤ 2 - none 

Net calorific value, Q 

as received, 

MBtu/lb ≥ 7.74 - Q16.5 ≥ 7.10 7.53 – 8.39 ≥ 7.74 ≥ 7.27 ≥ 7.27 

Gross density lb/oz ≥ 0.073 - - 0.065 – 0.091 ≥ 0.073 - - Bulk density, BD 103·lb/oz - - BD600 ≥ 39.0 - - ≥ 39.0 40.3- 746.8 Nitrogen, N w-% dry ≤ 0.3 N0.3 ≤ 0.3 N0.3 ≤ 0.3 ≤ 0.3 ≤ 0.3 - ≤ 0.3 Sulphur , S w-% dry ≤ 0.04 - S0.05 ≤ 0.05 ≤ 0.08 ≤ 0.04 ≤ 0.03 ≤ 0.05 Chlorine, Cl w-% dry ≤ 0.02 Cl0.03 ≤ 0.03 Cl0.02 ≤ 0.02 ≤ 0.03 ≤ 0.02 ≤ 0.08 ≤ 0.03 
112 

European Biomass 

Fuel Requirements 

 AUSTRIA EU GERMANY GREAT BRITAIN 

Arsenic, As mg/kg dry - - ≤ 1 ≤ 0.8 ≤ 0.8 - - Cadmium, Cd mg/kg dry - - ≤ 0.5 ≤ 0.5 ≤ 0.5 - - Chromium, Cr mg/kg dry - - ≤ 10 ≤ 8 ≤ 8 - - Copper, Cu mg/kg dry - - ≤ 10 ≤ 5 ≤ 5 - - Lead, Pb mg/kg dry - - ≤ 10 ≤ 10 ≤ 10 - - Mercury, Hg mg/kg dry - - ≤ 0.1 ≤ 0.05 ≤ 0.05 - - Nickel, Ni mg/kg dry - - ≤ 10 - - - - Zinc, Zn mg/kg dry - - ≤ 100 ≤ 100 ≤ 100 - - Sodium; Na w-% dry - - - - - - - EOX (extract. org. halogens) mg/kg dry - - - ≤ 3 ≤ 3 - ≤ 1 Formaldehyde mg/100g dry - - - - - - - Ash melting behavior, DT 

oF - - DT1200 

> 2190 - - - 
113 

Report 

9 Heat storage (accumulator tank) 

9.1 Introduction 

Function 

An accumulator tank is a heat-insulated steel tank in which a heat carrier (mostly water) 

circulates when it is loaded. Temperature stratification results from the density difference 

between cold and hot water (cold water has a higher density than hot water). Hot water 

from the boiler is introduced into the buffer accumulator at the top. The design of the 

inlet ensures that turbulence is low and that the temperature stratification is not 

destroyed. High flow temperatures result in good temperature stratification and high heat 

storage capacity. 

Types 

Different types of buffer accumulators are available depending on the use of the buffer 

accumulators as heat storage devices with or without integrated hot service water 

storage and/or integration of additional heating sources, e. g. solar energy (Figure 9-1). 

Figure 9-1 Different types of buffer (adapted from FNR I 2007) 

The capacity of the hydraulic installations can be reduced when using a combined storage 

unit for heating and hot water. Major drawbacks are however the reduction of the volume 

of the buffer accumulator and energy losses in summer, when only hot water is needed. 

Application area 

Although modern solid wood boilers are able to modulate their operation load from 100 

to 50%, the use of heat storage is strongly recommended by manufacturers of the 

boilers, by R&D institutions and authorities. 
114 

Heat storage 

(accumulator tank) 

Pellet and wood chip boilers can even modulate their operation load from 100 to 30%. 

Nonetheless, some manufacturers recommend the integration of accumulator tanks in 

these heating systems as well in order to increase the annual efficiencies of the heating 

systems. 

The research field on annual efficiencies in residential homes is quite young in Europe; 

there are however some ongoing research activities. BE 2020+ for example is currently 

working on the development of a test stand method for the determination of system 

efficiency and emission factors of small-scale biomass combustion systems. 

9.2 Dimensioning 

9.2.1 General aspects 

The necessary volume of the accumulator tank is influenced by several factors: 

• range of capacity of the boiler (only full load of variable heat load) 

• volume of the fuel storage in the boiler (for manually stoked boilers) 

• nominal heat load of the boiler 

• Calorific value and energy density of fuel 

• effective temperature difference in the heat storage 

• demand for convenience 

In Table 9-1 the benefits and disadvantages of large and small heat storage vessels are 

summarised. 

Table 9-1 benefits and drawbacks of large and small heat storages (adapted from 

FNR III 2007) 

 benefits disadvantage large heat storage (one fuel charge can be accumulated) 

o boiler can always be operated at 

full load (low emission 

operation) 

o reduction of combustion 

intervals 

o one combustion period with 

maximum fuel charge covers 

heat demand for a couple of 

days in spring/autumn 

o high investments costs for large 

heat storage 

o heat losses by accumulator tank 

(even if tank is well insulated) 

small heat 

storage 

o low investment costs o boiler will be operated in part load 

(which means higher emissions) 

o emergency shut-downs may occur 

if heat load of boiler is larger than 

heat demand (which results in very 

high emissions for a short time) 

o reduction of boiler durability 
115 

Report 

In general, the dimensioning of the accumulator tank is an optimisation function of 

investment costs and boiler performance. 

9.2.2 Sizing guidelines 

For the sizing of heat storage vessel for solid wood boilers, many guidelines can be found 

in literature. These guidelines are related to the nominal heat load of the boiler and vary 

from approximately 10 to 30 l/MBtu/h and 340 to 1010 fl oz/MBtu/h respectively 

(FNR II 2007, Struschka et al. 2004...). 

Struschka et al. 2004 analysed the emissions of solid wood boilers with heat loads of 100 

MBtu/h which were operated with and without accumulator tanks in field tests. Table 9-2 

and Figure 9-2 show the major results of this study. 

Table 9-2 average annual CO emission factors for solid wood boilers (adapted from 

Struschka et al. 2004) 

control concept: modulated nominal load only 

volume of heat storage average annual CO emissions in mg/MBtu 

0 l 0 fl gal 4 384 4 384 

750 l 200 fl gal 1 431 1 444 

1 500 l 400 fl gal 1 110 1 003 

CO with modulation

CO without modulation

OGC with modulation

OGC without modulation

volume of heat storage in l

Figure 9-2 development of CO and OGC emissions depending on the volume of the 

heat storage 

Heat storage vessels with larger dimensions give rise to lower annual emission factors 

and solid wood boilers, which are operated solely at nominal load, emit less than 

modulated boilers. The characteristics of the curve in Figure 9-2 shows that significant 

reductions of emissions will be reached for storage volumes between 0 to 2,500 l 

(650 fl gal). 
116 

Heat storage 

(accumulator tank) 

The authors of this study identify 2,250 l (600 fl gal) to be the optimum volume for the 

heat storage vessel, which corresponds to 20 l/MBtu/h and 750 fl oz/MBtu/h respectively. 

In spite of these common guidelines, FNR III 2007 recommends to dimension the heat 

storage vessel according to the volume of the fuel storage of the boiler and not according 

to the heat load of the boiler due to the fact that the volume of the fuel storage vessel 

determines the amount of heat which is produced during a combustion cycle, and not the 

nominal heat load of the boiler. 

9.2.3 EN 303-5 

The EN 303-5 requests manufacturers for boilers with manual stoking to specify how the 

amount of heat generated can be dissipated. The most probable solution is to install a 

heat storage vessel. 

For the sizing of a heat storage vessel for manual fed boilers, the following equation is 

given: 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = ⋅ ⋅ ⋅ −

min

4.5 1 0.3

Q

Q V T Q H

AT B N

VAT Accumulator tank capacity l 

QN Nominal heat output MBtu/h 

TB Burning period h 

QH Heating load of the premises MBtu/h 

Qmin Minimum heat output MBtu/h 

In order to convert the volume of the heat storage vessel from liter to gallons, a factor of 

0.264 must be multiplied with VAT. 
117 

Report 

10 Energy from non-Woody Biomass Fuels 

10.1 Emissions and efficiencies of solid non wood biomasses 

Carvalho et al. 2007 summarized the main challenges of using agricultural fuels in 

residential boilers as follows: 

“... agricultural fuels have, in general, certain physical and chemical properties that may 

induce problems during combustion. In addition to their low heating value, on average 

5% to 10% lower than wood, they have higher ash contents, lower melting points of the 

ash and higher concentrations of nitrogen, sulphur and chlorine than typical wooden fuels 

(Werther et al. 2000). Large ash quantities in connection with low ash melting points can 

cause disturbances by the formation of deposits or slags in combustion systems not 

prepared for agricultural fuels. High concentrations of sulphur, nitrogen and chlorine, 

intensify the emissions of SO2, NOx and chlorine compounds respectively 

(Werther et al. 2000). Chlorine and sulphur, besides promoting the formation of deposits, 

are also involved in corrosion reactions of metals (Bryers 1996, 

van Loo, Koppejan 2002).” 

In extensive tests, combustion behavior of diverse agricultural pelletized biomasses was 

analyzed by the BE 2020+. Results of these tests are discussed afterwards. 

10.1.1 Fuels & fuel properties 

For the analysis of the combustion behavior of agricultural pellets, most relevant 

biomasses for the Austrian market were chosen. Altogether, 6 different fuels – hay, 

maize, miscanthus, straw, vineyard pruning and wheat bran – were considered. In 

Table 10-1 the fuels and their most relevant properties with regard to combustion are 

shown as well as average properties of wood fuels. 

Table 10-1 fuel properties of the investigated pellets and average values for wood fuel 

(Wopienka et al. 2007) fuel diameter 

(in) 

GCV d.b 

(MBtu/lb) 

water 

(%) 

ash 

(%) d.b. 

volatile 

matter 

(%) d.b. 

nitrogen 

(%) d.b. 

sulfur 

(%) d.b. 

softening 

temperature 

(°F) 

wood (spruce) 8.6 0.20 0.10 0.04 2480 

hay 0.236 7.5 7.5 7.40 73.1 1.08 0.13 2085 

maize (whole crop) 0.236 8.2 8.7 3.39 80.2 0.89 0.09 1670 

miscanthus 0.236 8.2 8.7 3.26 82.9 0.24 0.03 1850 

straw 0.236 8.0 7.9 5.71 74.8 0.66 0.10 1470 

vineyard pruning 0.236 9.0 8.4 2.70 - 0.39 - 1465 

wheat bran 0.315 8.3 13.7 5.70 76.8 3.04 0.24 1780 
118 

Energy from nonWoody Biomass 

Fuels 

The ash content of agricultural pellets is 10 to 20 times higher than the ash content of 

wood pellets, the nitrogen content is 5 to 10 times and the sulfur content is even 10 to 

30 times higher. In comparison to average wood fuels, the softening temperature of 

agricultural fuels is 550 to 1100 °F lower. 

These differences clearly influence the combustion process and therefore emissions and 

efficiencies. 

10.1.2 Emissions 

In Figure 10-1 the average CO emissions and their deviations are shown for the analyzed 

bio-fuels and Figure 10-2 exhibits the average dust emissions including deviations for 

these fuels. 

0

50

100

150

200

250

300

350

123456

CO-emissions in mg/MJ

Maize Miscanthus Vineyard 

pruning

Wheat 

bran

Straw Hay

Figure 10-1 average CO emissions including their deviation of the investigated fuels 

(1 mg/MJ ≈ 1.5 mg/m2 at 12% O2)(Wopienka et al. 2007) 

0

20

40

60

80

100

120

140

160

Hay Maize 

(whole crop)

Miscanthus Straw Vineyard

pruning

Wheat bran

Particle emissions in mg/MJ

Figure 10-2 average dust emissions including their deviation of the investigated fuels; 

1 mg/MJ = 0.0023 lb/MMBtu (Wopienka et al. 2007) 
119 

Report 

The emission values of Figure 10-1 and Figure 10-2 show a week correlation of CO and 

dust emissions. If CO emissions are low, dust emissions will also be rather low. 

Miscanthus and vineyard pruning, which have a quite low ash content (Table 10-1), reach 

the lowest dust and CO emissions. Hay exhibits the highest ash content. It reaches the 

highest CO emissions, but its dust emissions are relatively low. A simple and reliable 

correlation of fuel properties and CO- and dust emissions could not be found. 

Figure 10-3 exhibits the NOX and SO2-Emissions of the considered bio-fuels depending on 

the nitrogen respectively sulfur content of the fuels. 

Figure 10-3 NOX and SO2 emissions depending on the nitrogen respective sulphur 

content of the investigated biomasses (Wopienka et al. 2007) 

In this case, quite clear correlations can be found: if the nitrogen respectively the sulphur 

content is low, NOX and SO2 emissions will be low as well and the other way round. 

10.1.3 Efficiencies 

In Figure 10-4 the CO-excess air ratio characteristic of the investigated fuels for the test 

boiler is shown. The CO-excess air ratio characteristic shows at which range of excess air 

the minimum CO emissions are reached in the test boiler. 

maize miscanthus

Wheat bran

h

ay

Vin

eyd. pr.

straw

Figure 10-4 correlation of CO emissions and excess air ratio for the considered fuels 

((Wopienka et al. 2007) 
120 

Energy from nonWoody Biomass 

Fuels 

The optimum excess air ratio varies from approximately 1.5 for maize to approximately 

1.9 for wheat bran. Straw shows a very broad characteristic. 

If the fuels are burnt at optimum conditions (low CO), different excess air ratios are 

needed. The higher the excess air ratio, the lower the efficiency of the boiler, because 

more combustion air is sucked through the boiler and heat losses in the flue gas are 

increased therefore. Hence, the boiler will reach higher efficiencies with maize than with 

wheat bran, if it is operated at optimum combustion conditions. 

10.1.4 Slagging & Corrosion 

If non wood bio-fuels are used for combustion, the slagging behavior of these bio-fuels 

has to be considered. Otherwise molten ash or slag can lead to an emergency shutdown 

of the combustion unit and cause severe damage in the primary combustion chamber 

(grate, lining...). 

Figure 10-5 ash (left) and ash lumps (right) from combustion of agricultural fuels 

In Figure 10-6 the slagging tendency of the considered fuels (except for hay) is shown. 

In order to evaluate the slagging tendency, a mash analysis of the ash was carried out to 

determine the fraction of all particles in the ash with a diameter > 5.6 mm (0.22 in). 

44,3

25,4

0,3 0,2

34,8

0

10

20

30

40

50

60

1

Fraction of slag particles (> 5,6 mm) in %

Maize Miscanthus

(whole crop)

Vineyard 

pruning

Straw Wheat bran

1470°F

1670°F

1850°F 1465°F

1240°F

5,7%

3,4%

3,3% 2,7%

5,7%

Softening temp.

Ash content

Figure 10-6 Slagging tendency of the investigated fuels (Wopienka et al. 2007) 
121 

Report 

Straw, wheat bran and maize showed a higher slag forming tendency than miscanthus or 

vineyard pruning pellets. From the combustion of hay almost no slag was found in the 

ash. Basically, fuels with higher ash content combined with lower softening temperature 

tend to from a bigger fraction of molten lumps. 

Next to the slagging phenomena also corrosion of the boiler (heat exchanger, ceramics, 

grate, combustion chamber) has to be considered when burning non wood bio-fuels. 

Chlorine and sulphur are involved in corrosion reactions, therefore the combustion 

conditions (temperature, atmosphere) have to be observed. Figure 10-7 shows two 

examples of corrosion attacks while burning agricultural fuels. 

Figure 10-7 corrosive attack of refractory materials after operation with grain (left) 

and corrosion of metal after straw combustion 

10.2 Summary 

Due to their high availability, non-wood biomasses are very attractive fuels. They may 

however have certain general physical and chemical properties that may cause problems 

during combustion. In addition to the possible increase of NOX, HCl, SO2, and other 

emissions in comparison to wood biomass, higher ash contents combined with lower ash 

melting temperatures and corrosive effects negatively influence the failure rate and 

durability of the boilers. 
122 

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128 

Annex 

Annex 

1. European standards – preview 

1.1.EN 12815 

1.2.EN 13229 

1.3.EN 13240 

1.4.EN 15250 

2. Test report for small scale appliance 

2.1.Solid wood appliance 

2.2. Pellet appliance 

3. Commissioning report 

4. Test details of biomass appliances 

4.1. TOP 25% models – solid wood boilers 

4.2. TOP 25% models – pellet boilers 

4.3. TOP 25% models – wood chip boilers 

4.4.All models – combined technology boilers 

4.5.All models – pellet stoves 

4.6.All models – solid wood stoves 

5. Model list of German masonry- and room-heaters 

5.1.EN 13229 certified prefabricated heaters 

5.2.EN 13240 certified wood stoves 

6. Manufacturers of biomass appliances 

6.1. Manufacturers considered in creating TOP 25% list 

6.2.Other manufacturers from Austria, Germany and Scandinavian countries 

6.3. Manufacturers of medium and large scale biomass appliances from Austria, 

Germany and Scandinavian countries 

6.4. Manufacturers considered in 5.1 and 5.2 

7. Manufacturers of dust emission control technology for small scale appliances 

8. Specifications of properties of biofuels 

9. Revision of EN 14961