NEW YORK STATE

ENERGY RESEARCH AND

DEVELOPMENT AUTHORITY

FINAL REPORT 08-03

APRIL 2008

BIOMASS COMBUSTION IN EUROPE

OVERVIEW ON TECHNOLOGIES

AND REGULATIONS
The New York State Energy Research and Development Authority (NYSERDA) is a public benefit 

corporation created in 1975 by the New York State Legislature. NYSERDA’s responsibilities include:

• Conducting a multifaceted energy and environmental research and development program to meet 

New York State’s diverse economic needs. 

• Administering the New York Energy $martSM program, a Statewide public benefit R&D, energy 

efficiency, and environmental protection program.

• Making energy more affordable for residential and low-income households.

• Helping industries, schools, hospitals, municipalities, not-for-profits, and the residential sector, including low-income residents, implement energy-efficiency measures.

• Providing objective, credible, and useful energy analysis and planning to guide decisions made by

major energy stakeholders in the private and public sectors.

• Managing the Western New York Nuclear Service Center at West Valley, including: (1) overseeing the

State’s interests and share of costs at the West Valley Demonstration Project, a federal/State radioactive

waste clean-up effort, and (2) managing wastes and maintaining facilities at the shut-down StateLicensed Disposal Area.

• Coordinating the State’s activities on energy emergencies and nuclear regulatory matters, and 

monitoring low-level radioactive waste generation and management in the State.

• Financing energy-related projects, reducing costs for ratepayers.

NYSERDA administers the New York Energy $martSM program, which is designed to support certain public benefit programs during the transition to a more competitive electricity market. Some 2,700 

projects in 40 programs are funded by a charge on the electricity transmitted and distributed by the State’s

investor-owned utilities. The New York Energy $martSM program provides energy efficiency services,

including those directed at the low-income sector, research and development, and environmental protection

activities. 

NYSERDA derives its basic research revenues from an assessment on the intrastate sales of New York

State’s investor-owned electric and gas utilities, and voluntary annual contributions by the New York Power

Authority and the Long Island Power Authority. Additional research dollars come from limited corporate

funds. Some 400 NYSERDA research projects help the State’s businesses and municipalities with their

energy and environmental problems. Since 1990, NYSERDA has successfully developed and brought into

use more than 170 innovative, energy-efficient, and environmentally beneficial products, processes, and

services. These contributions to the State’s economic growth and environmental protection are made at a

cost of about $.70 per New York resident per year.

Federally funded, the Energy Efficiency Services program is working with more than 540 businesses,

schools, and municipalities to identify existing technologies and equipment to reduce their energy costs. 

For more information, contact the Communications unit, NYSERDA, 17 Columbia Circle, Albany, 

New York 12203-6399; toll-free 1-866-NYSERDA, locally (518) 862-1090, ext. 3250; or on the web 

at www.nyserda.org

STATE OF NEW YORK ENERGY RESEARCH AND DEVELOPMENT AUTHORITY

David A. Paterson Vincent A. DeIorio, Esq., Chairman

Governor
BIOMASS COMBUSTION IN EUROPE

OVERVIEW ON TECHNOLOGIES AND REGULATIONS

Final Report

Prepared for the

NEW YORK STATE

ENERGY RESEARCH AND

DEVELOPMENT AUTHORITY

Albany, NY

www.nyserda.org

Ellen Burkhard, Ph.D.

Project Manager

Ray Albrecht 

Project Manager

Prepared by:

VERENUM SWITZERLAND

Thomas Nussbaumer 

NYSERDA April 2008

Report 08-03

NYSERDA 9477
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3

Contents

1 Summary .................................................................................................................................................5

2 Introduction ............................................................................................................................................7

2.1 Motivation..............................................................................................................................................7

2.2 Objectives..............................................................................................................................................9

2.3 Conversion of emission factors ...........................................................................................................9

3 Wood combustion technology ..........................................................................................................11

3.1 Overview .............................................................................................................................................11

3.2 Biomass fuels......................................................................................................................................12

3.3 Environmental impact of biomass combustion .................................................................................13

3.4 Fundamentals .....................................................................................................................................14

3.5 Reduction of airborne pollutants........................................................................................................15

3.5.1 Staged combustion..........................................................................................................................15

3.5.2 Uncombusted pollutants .................................................................................................................17

3.5.3 NOX emissions .................................................................................................................................18

3.5.4 Particulate emissions ......................................................................................................................20

3.6 Combustion technologies...................................................................................................................21

3.7 Co-combustion....................................................................................................................................30

4 Particulate Matter (PM) .......................................................................................................................31

4.1 Characteristics of particulate matter (PM) ........................................................................................31

4.2 Health relevance of PM......................................................................................................................35

4.3 Ambient PM standards.......................................................................................................................37

4.4 Types of combustion particles ...........................................................................................................38

4.5 Influence of particle sampling methodology......................................................................................42

4.6 Measurement techniques...................................................................................................................44

4.7 PM emission factors ...........................................................................................................................47

4.7.1 Open fireplaces ...............................................................................................................................47

4.7.2 Closed inset appliances (closed fireplaces) ..................................................................................48

4.7.3 Wood stoves ....................................................................................................................................48

4.7.4 Log wood boilers .............................................................................................................................50

4.7.5 Pellet stoves and boilers .................................................................................................................52

4.7.6 Overview on residential wood combustion ....................................................................................55

4.7.7 Automatic biomass combustion plants ..........................................................................................56

4.8 Particle sampling: Solid particles, condensables, dilution tunnel....................................................59

5 Norms and emission limit values .....................................................................................................63

6 Research activities in Europe ...........................................................................................................80

6.1 Overview .............................................................................................................................................80

6.2 Research topics ..................................................................................................................................82

6.3 Research institutions ..........................................................................................................................83

7 Conclusions..........................................................................................................................................88

7.1 State-of-the-art of biomass combustion in Europe...........................................................................88

7.2 Relevance of PM emissions...............................................................................................................89

8 References............................................................................................................................................92

Section Page
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5

1 Summary

Biomass combustion in Europe is widely applied for residential heating in stoves and boilers, district 

heating in automatic combustion plants, and combined heat and power production in industrial scale 

boilers. This report gives an introduction to the combustion fundamentals, an overview of combustion 

technologies applied in Europe today, an assessment on particulate matter emissions smaller than 10 

microns (PM10), and an overview of regulations and norms on emissions from wood combustion 

devices. For small-scale applications, mainly log wood is used as fuel today with an increasing interest 

in wood pellets, while for medium-scale applications, wood chips are most common. The technologies 

of combustion systems are presented, and their relevance to pollutant emissions is discussed. For this 

purpose, the characteristics of wood as a fuel and the reactions during wood combustion are presented. It is shown that biomass combustion is related to relatively high pollutant formation due to the 

following mechanisms: 

1. An incomplete combustion can lead to high emissions of uncombusted pollutants, i.e., CO, VOC, 

PAH, soot, and condensable organic matter. A significant reduction of uncombusted pollutants can 

be achieved by two-stage combustion with gasification of the wood with primary air and a subsequent gas phase oxidation with secondary air. This technology is applied to modern type boilers

by application of the downdraft principle, while wood stoves are mostly based on conventional updraft combustion. In addition, manually-operated wood combustion strongly influences the emissions in practice. Non-ideal operation can lead to an increase of uncombusted pollutants, including 

PM, by more than one, or in some cases more than two orders of magnitude. Solid particles and 

condensables from incomplete wood combustion exhibit a high health risk. Investigations of such 

particles revealed a far higher health relevance (i.e., cell toxicity and chromosome aberration) than 

Diesel soot, while inorganic particles from almost complete combustion of wood exhibit a significantly lower health impact than Diesel soot. 

2. Inorganic particles and NOX are formed as a result of fuel constituents (N, K, Cl, Ca, Na, Mg, S 

and other elements). Hence biomass combustion exhibits relatively high emissions of NOX and 

particles (which under good combustion consist mainly of inorganic matter from ash constituents). 

Air staging and fuel staging offer a certain potential to reduce NOX, while staged combustion by 

low excess air and reduced temperatures in the primary zone enables a reduction of inorganic 

particles. Furthermore, particle separation and selective NOX reduction are available as secondary 

measures for pollutant reduction.

3. Contaminants such as heavy metals and chlorine can lead to additional airborne emissions. 

Based on a two-stage combustion principle, and with application of electronic combustion control devices, modern log wood boilers can achieve high combustion efficiencies (i.e., up to more than 90% 

based on lower heating value), high overall system efficiency (i.e., more than 80%), and low PM 

emissions (i.e., smaller than 30 mg/MJ or 0.070 lb/MMBTU). However, to ensure good operation 

conditions for residential heating applications, a heat accumulation tank is needed for log wood boilers

to avoid part-load operation. Properly operated modern log wood boilers allow significant fuel savings 

in comparison to old-type boilers, which do not exhibit a two-stage combustion design and electronic 
6

combustion control and often achieve efficiencies of less than 60% or even less than 50%. Furthermore, a reduction of PM emission of more than one order of magnitude can be achieved in 

comparison to poorly designed and operated log wood boilers. Therefore, replacing old-type log wood 

boilers operated without a heat storage (often in use in the United States for outdoor wood boilers 

(OWB)) by modern log wood boilers allows significant reduction of PM emissions and hence can 

strongly assist to avoid the excess of ambient air quality limits. For new wood heating appliances, the 

implementation of well designed combustion principles, as well as the application of heat accumulation 

tanks, is essential to avoid an increase of PM emissions. 

Keywords: Wood combustion, pollutant formation, pollutant reduction, particulate matter, regulations

 
7

2 Introduction

2.1 Motivation

Biomass is widely applied as a renewable energy source for heat and power production and will 

become more important in the next decades due to declining fossil fuels. While sustainable use of 

biomass can be regarded as partly CO2 free, biomass combustion is an important source of air emissions, especially inhalable particulate matter smaller than 10 microns (PM10) in the ambient air, polycyclic aromatic hydrocarbons (PAH), volatile organic compounds (VOC), nitric oxide emissions (NOX), 

and potentially polychlorinated dibenzo-p-dioxins and furans (PCDD/F). Since PM is regarded as one 

of the most relevant parameters in air pollution [Dockery et al. 1993, Donaldson et al. 2002], the PM 

emissions from biomass combustion can hinder the further propagation of bioenergy. To overcome 

this disadvantage, new technologies with reduced PM emissions are of interest: 

• Improved wood boilers include a two-stage combustion principle and an injection of secondary air 

prior to a hot combustion chamber. Furthermore, electronic combustion control should be applied 

to ensure optimum operation in practice, and forced ventilation should be applied to enable good 

mixing and less dependence on the ambient climate conditions such as air temperature and wind. 

In addition, wood boilers used for house heating should be equipped with a heat accumulation 

tank to avoid part-load operation with increased emissions. This type of boiler is available as stateof-the-art technology in Europe and can be widely implemented. 

• Similar to boilers, wood stoves might be developed, which exhibit an improved combustion principle with a hot combustion chamber and good mixing of combustion air with combustible gases. 

However, stove design for visible flames and operation without a ventilator exhibits limited possibilities of application of improved combustion principles. For stoves, design principles should be 

developed that avoid improper operation of the stove. Therefore, overfilling of the combustion 

chamber and the possibility to reduce the air inlet, resulting in smoldering conditions, should be 

avoided. Since wood stoves do not safely achieve these requirements, further development is 

needed. 

• In addition to the implementation of improved technologies for new heating appliances, it is essential to ensure ideal operation of existing wood stoves and boilers. This can be achieved to some 

extent by providing information and measures to the consumers. 

• Besides technical improvements for residential wood heating appliances, it is also essential to prohibit burning of wood and other biomass in open fires, as well as private waste incineration. These 

types of combustion are related to excessively high air pollution without energy use. 

• As an alternative for log wood, wood pellets can be used for residential heating. Thanks to constant fuel properties and small size, automatic feeding can be applied, which allows improved 

combustion conditions. However, for pellet combustion, optimum operation needs to be ensured 

by combustion control, which avoids smoldering conditions especially during start-up and shutdown. 
8

• As an additional measure, technologies for flue gas cleaning for small-scale applications are being 

developed and offers a certain potential for pollutant reduction for existing and new wood heating 

appliances. 

• Finally, wood fuel can be used in automatic biomass combustion plants, which can be equipped 

with flue gas cleaning such as electrostatic precipitators or fabric filters. If well designed and operated, automatic combustion plants for wood fuels achieve almost complete combustion, which 

safely avoid uncombusted particles. Inorganic particles can be precipitated by filters thus allowing 

low PM emissions. 

Improvements in different sectors should be aimed at achieving a significant reduction of PM 

emissions from wood combustion. Due to the steep increase in PM emissions and the high health risk 

of particles and condensables released during incomplete combustion, the main goal is to safely avoid 

the use of wood stoves and boilers operated at such conditions. Visible smoke and noticeable odors 

are clear indicators of high and toxic emissions, which need to be safely avoided, or be limited to short 

periods during start-up (i.e., certainly less than 15 minutes). Beside development of improved technologies, regulations are important, which allow a useful evaluation of PM emissions. 

In addition to the implementation of technical measures to reduce PM from wood combustion, there is 

a need to improve information on the contribution of biomass combustion to the total PM in the 

ambient air and its relevance for the environment and the health. This is true for Europe, where 

investigations on PM in the ambient air reveal a significant contribution of wood combustion. For 

example, more than 80% of the soot in the alpine valleys in Switzerland close to transit highways 

during winter originates from wood combustion [Prévot et al. 2006]. However, there is a lack of systematic information on the formation of PM under different conditions in Europe and in the United States. 

In the United States there is a need to improve the information on PM. The NARSTO Emission Inventory Assessment Team claims that the highest priority needs for improving emission inventories 

are [NARSTO 2005]: 

• Size-aggregated, speciated emissions of fine particles and their precursors, including black and 

organic carbon emissions 

• Speciated, spatially and temporally resolved organic emissions from biogenic sources 

• Emissions from open biomass burning, including agricultural and forest prescribed burning, 

wildfires, and residential backyard burning 

• Residential wood combustion in woodstoves and fireplaces 

In addition, the Northeast States for Coordinated Air Use Management identifies in their assessment of 

carbonaceous PM2.5 for New York and the region seven major sources for emissions [NESCAUM and 

MJB Associates 2008]: Five mobile source sectors (nonroad equipment, heavy-duty trucks, light-duty 

vehicles,airports, and marine ports) and two stationary source sectors (residential fuel combustion and 

commercial cooking). 

There is a specific need for improved information on emissions from residential wood combustion as 

well as from open biomass burning and its relevance to PM in the ambient air. Consequently, the 

conversion of emissions released from the stack to the ambient is an important issue, too, since 

secondary organic aerosols (SOA), induced by photochemical oxidation of volatile and semivolatile 
9

organic compounds, can significantly contribute to the total PM concentration in the ambient air (e.g. 

[Shrivastava et al. 2006, Lipsky & Robinson 2006, Robinson 2007, Robinson et al. 2007]). 

2.2 Objectives

The objective of the study is to give an introduction on combustion fundamentals for wood fuels and an 

overview of state-of-the-art technologies for wood combustion in Europe. The assessment of 

technologies and applications will focus on efficiency and on air pollution with special attention to

particulate matter in the ambient air resulting from wood fuel applications. Hence, testing procedures 

for PM emissions in Europe will be described and compared with testing according to US standards.

Ongoing R&D activities in Europe shall be discussed and recommendations for future developments in

the State of New York to reduce PM from wood combustion will be provided. 

2.3 Conversion of emission factors

Gaseous emissions are usually measured in volume concentrations such as Vol.-% or Vol.-ppm, while 

gravimetric measurements of particles (PM) result in mass concentrations such as mg/m3. Gravimetric 

PM emissions in mg/m3 can be compared to PM concentrations in the ambient air, which are also 

measured as mass concentrations and usually given in μg/m3. For specific gas compounds such as 

CO or NO2, measurements in volume concentrations can be directly converted to mass concentrations. To allow a comparison of emission factors achieved at different excess air ratios, a conversion to a reference oxygen concentration is performed. Consequently, European norms and regulations often use mass concentrations calculated at a reference O2 for the definition of emission limit 

values. In the present report, data in mg/m3 at 11 vol.-% O2 or 13 Vol.-% O2 (dry volume at 0°C and 

1013 mbar) are used (as valid examples for the Swiss Ordinance of Air Pollution Control). However, 

for the comparison of emission factors a calculation on energy basis is more favorable, as it allows for 

a comparison with other fuels. For this purpose, mass concentrations are converted to mg/MJ with reference to the lower heating value. In the USA, emission factors are also commonly used in g/MMBTU 

and lb/MMBTU, where MM stands for 1 million. Hence, the following conversion can be applied:

1 mg/MJ = 1.055 g/MMBTU = 0.00232 lb/MMBTU

Figures on conversion factors in the present report are given in mg/MJ and can easily be read for 

g/MMBTU by multiplying with 1.055. This is a conversion, which can be neglected for a generic interpretation. For data in lb/MMBTU, an indicative second axis is added in the figures and data are 

converted in the text.

11

3 Wood combustion technology

3.1 Overview

There are three basic technologies for an initial thermochemical conversion of biomass: pyrolysis, 

gasification, and combustion. Among these, combustion is the most proven technology for heat and 

power production. The technologies are described in [Nussbaumer 2003] and summarized in the 

present chapter. 

Pyrolysis is of interest as an initial process for liquid biofuel production. However, pyrolysis is in an 

early stage of development, and the potential advantages over combustion applications are uncertain. 

Gasification is of interest for future applications due to two advantages: higher electric efficiencies are 

possible by gasification, especially if the producer gas is used for combined cycle applications, and 

gasification allows an application with very low airborne emissions. PM emissions can be significantly 

reduced in comparison to conventional combustion, since highly efficient particle removal in the raw 

gas needs to be applied to protect the gas turbine and/or the turbo charger. Furthermore, an almost 

complete combustion of the producer gas in the gas turbine combustion chamber is reached, thus 

resulting in negligible organic emissions such as VOC and organic condensables. Hence, clean gas 

emissions from gasification plants can be very low. Although such applications should be considered 

in the future as promising alternatives, today, the majority of the wood fuel is used in combustion 

applications, which are available as proven technology. 

Biomass combustion systems are available from a few kW up to more than 100 MW. Typical 

applications in Europe cover the following size ranges: 

1. Manual wood stoves are most commonly used from 5 kW to 15 kW. However, due to the batchoperation, the instantaneouos combustion heat performance can also exceed 50 kW for a short 

period of time in applications as e.g. tile stoves which exhibit a heat accumulation in the high mass 

of the stove. 

2. Manual log wood boilers are most commonly used from 20 kW to 70 kW. For typical house 

heating, heat accumulation tanks with a few 1000 liter of water are used to increase comort and 

avoid part-load operation of the boilers at low efficiency and high emissions. 

3. Pellet stoves and boilers for house heating are most common from 15 kW to 70 kW (for boilers). 

However, larger applications of more than 1 MW for wood pellets are also used e.g. in cases, 

where the limited space does not allow the use of wood chips. 

4. Wood chips boilers are most often used from 200 kW to 10 MW heat output, although in some 

cases wood chips are also used from 20 kW and up to more than 100 MW. For applications 

greater than 1 MW, grate boilers are most common, while for smaller plants, under stoker boilers 

are in use in case of relatively high quality wood chips. Applications with heat output of more than 

10 MW to 20 MW are often combined with steam production for combined heat and power production. 

The efficiency for heat production is considerably high, and heat from biomass is economically 

feasible in many applications. Commercial power production is based on steam cycles. The specific 
12

cost and the efficiency of steam plants are only adequate at large-scale applications. Hence, co-combustion of biomass with coal is promising, as it combines high efficiency with reasonable transport 

distances for the biomass. 

However, biomass combustion is often related to significant pollutant formation and needs to be improved. To develop measures for emission reduction, the specific fuel properties need to be considered. Pollutant formation occurs due to three reasons: 

1. Incomplete combustion can lead to high emissions of uncombusted pollutants such as CO, soot,

organic condensables (“tar”), and polycyclic aromatic hydrocarbons (PAH).

2. Pollutants such as NOX and particles are formed as a result of natural fuel constituents such as N, 

K, Cl, Ca, Na, Mg, P, and S.

3. Biofuels can be carriers of additional contaminants such as heavy metals or chlorine, which can 

lead to high emissions of heavy metals, HCl, and potentially highly toxic polychlorinated dibenzop-dioxins and furans (PCDD/F). 

Biomass combustion exhibits relatively high emissions of NOX and submicron particles. Air staging 

and fuel staging can be applied as primary measures for NOX reduction that offer a potential of 50% to 

80% reduction. For further reduction, secondary measures such as selective catalytic and selective 

non-catalytic reduction (SCR, SNCR) can be applied. However, SNCR can lead to significant 

undesired side-products that need to be carefully avoided. 

For particle reduction, the main goal is to achieve an almost complete combustion that safely avoids 

soot, organic particles and droplets, and volatile organic compounds in the flue gas. Primary measures 

to reduce inorganic PM by staged combustion with an initial gasification stage offer a certain reduction 

potential. For this purpose, a new approach with extensively reduced primary air is under investigation. 

Furthermore, optimized plant operation is needed to guarantee low emissions and high efficiency 

under real-world conditions. Nevertheless, secondary measures for PM removal are needed for 

medium and large-scale applications. In addition, there is also an interest in secondary measures for 

future applications in residential wood combustion.

3.2 Biomass fuels

Combustion can be applied to biomass feedstock with water contents up to 60%. Fuel constituents 

beside C, H, and O are undesired, since they are related to pollutant and deposit formation, corrosion, 

and ash. The most relevant constituents in native biomass are nitrogen as a source of NOX, and ash 

components (e.g. K and Cl as a source of KCl) that lead to particulate emissions. Native wood is 

usually the most favorable bio fuel for combustion due to its low content of ash and nitrogen. Herbaceous biomass such as straw, miscanthus, and switch grass have higher contents of N, S, K, and 

Cl, which lead to higher emissions of NOX and particulates, increased ash, corrosion, and deposits. While 

wood is well-suited for household heating as well as for larger plants, herbaceous biomass is reserved 

for larger plants. The same is true for urban waste wood and demolition wood. The combustion of 

such contaminated biomass should be strictly limited to combustion plants with efficient flue gas 

cleaning for the abatement of toxic pollutants such as heavy metals and chlorine compounds. 

The driving force for biomass combustion for energy purposes is either the CO2 neutrality of 

sustainable cultivated biomass or the use of biomass residues and waste. Large potentials of both 
13

native biomass and biomass wastes are still available and allow a relevant increase of sustainable bio 

energy use in the future. Combustion is the most important technology available today for biomass 

use. Improvements in efficiency, emissions, and cost are needed for further exploitation. In addition, 

alternatives such as gasification and combinations of different processes such as gasification as fuel 

pre-treatment for co-combustion need to be considered. 

3.3 Environmental impact of biomass combustion 

Biomass combustion exhibits relatively high emissions of NOX and particulates in comparison to the 

combustion of natural gas or light fuel oil. Hence, biomass combustion contributes significantly to 

particulate matter (PM), ozone, and NO2 in the ambient air. For wood combustion, a life cycle assessment (LCA) indicates that 38.6% of the environmental impact of a modern automatic wood furnace is 

attributed to NOX, 36.5% to PM 10, only 2% to CO2 and 22.9% to all other pollutants (Table 3.1). The 

LCA for wood, light fuel oil, and natural gas also show that the environmental impact of wood is higher 

than that for natural gas for a standard valuation of the greenhouse effect. Hence, improvements in the 

wood chain are necessary. However, it is also evident that the conclusions of the LCA strongly depend 

on the valuation of the greenhouse effect, since the ranking changes significantly as a result of the 

different CO2 impacts of the three fuels.

On the other hand, in case of poor combustion conditions in manually operated wood stoves or 

boilers, PM emissions can be higher than assumed in the cited LCA by a factor of 10 thus leading to a 

far higher environmental impact than described here [Klippel and Nussbaumer 2007 (a) and (b)]. In 

addition, at excessively high emissions of unburnt hydrocarbons including methane, even the greenhouse gas effect can be higher than from light fuel oil or natural gas due to the higher impact of 

methane [Johansson et al. 2004]. Hence wood combustion can only be assessed as being environmentally friendly in case of low airborne emissions. 

Table 3.1 Environmental Impact Points (EIP) according to the Ecological Scarcity Method for heating with 

wood chips (base case for greenhouse effect), data from [Kessler et al. 2000]

[EIP/GJ] [%]

NOX 13 030 38.6%

PM 10 12 600 36.5%

CO2 670 2.0%

SOX, NH3, CH4, NMVOC, primary 

energy, residues, and others

8 200 22.9%

Total 34 500 100%

 
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Figure 3.1 Environmental impact points (EIP) for different valuations of the greenhouse effect. Data from 

Kessler et al. 2000, except wood pellets, own calculations [2003]. 

3.4 Fundamentals 

Biomass combustion is a complex process that consists of consecutive heterogeneous and homogeneous reactions. The main process steps are drying, devolatilization, gasification, char combustion, 

and gas phase oxidation. The time used for each reaction depends on the fuel size, properties, temperature, and combustion conditions. Batch combustion of a small particle shows a distinct separation 

between a volatile and a char combustion phase with time (Fehler! Verweisquelle konnte nicht 

gefunden werden.). For the design of combustion appliances, the high content of volatiles (80% to 

85%) needs to be respected. For large particles, the phases overlap to a certain extent. Nevertheless, 

even for log wood furnaces, a certain separation of distinct combustion regimes with time can be 

found. 

Figure 3.2 Mass loss as function of time during combustion of wood 
15

Since automatic combustion systems are operated continuously, the consecutive reactions occur 

simultaneously at different places in the furnace (e.g., in different sections on and above a grate). 

Therefore, the zones for different steps during combustion can be optimized by furnace design. A 

distinct separation of different steps can be advantageous with respect to pollutant formation. 

The main combustion parameter is the excess air ratio (lambda; λ) that describes the ratio between 

the locally available and the stoichiometric amount of combustion air. For typical biomass, the combustion reaction can then be described by the following equation, if fuel constituents such as N, K, Cl 

etc. are neglected:

CH1.44O0.66 + λ 1.03 (O2 + 3.76 N2) → Intermediates (C, CO, H2, CO2, CmHn etc.) 

→ CO2 + 0.72 H2O +(λ–1) O2 + λ 3.87 N2 –439 kJ/kmol

where CH1.44O0.66 describes the average composition of typical biomass used for combustion, i.e., 

wood, straw, or similar. As a result of the combustion process, different types of pollutants can be 

distinguished:

1. Uncombusted pollutants such as CO, CXHY, PAH, tar, soot, carbon, H2, HCN, NH3, and N2O

2. Pollutants from complete combustion such as NOX (NO and NO2), CO2, and H2O 

3. Ash and contaminants such as ash particles (KCl, etc.), SO2, HCl, PCDD/F, Cu, Pb, Zn, Cd etc.

3.5 Reduction of airborne pollutants

3.5.1 Staged combustion 

If staged combustion is applied, the excess air can vary in different sections. Two-stage combustion is 

applied with primary air injection in the fuel bed and consecutive secondary air injection in the 

combustion chamber. This allows good mixing of combustion air with the combustible gases formed 

by devolatilization and gasification in the fuel bed. If good mixing occurs, an operation at low excess 

air is possible (i.e., excess air lambda < 1.5), thus enabling high efficiency and high temperature 

(Figure 3.4) with complete burnout (Figure 3.5). If good mixing is achieved, the concentrations of 

uncombusted pollutants can be reduced to levels close to zero (e.g. CO < 50 mg/m3 and CXHY < 5 

mg/m3 at 11 vol.-% O2). However, an accurate process control (e.g. CO/lambda-controller with use of 

sensors for CO and lambda) is needed to ensure optimum excess air in practice. For this purpose,

self-adjusting control systems with use of sensors for CO and lambda (CO/lambda-controller) or for

CO and temperature have been developed.
16

Air staging applies air injection at two levels. In addition to conventional two-stage combustion, 

primary air needs to be understoichiometric (lambda primary < 1). A relevant residence time (and a 

reduction zone in the furnace leading to an enlarged furnace volume) is needed between the fuel bed 

and the secondary air inlet.

In fuel staging, fuel is fed into the furnace at two different levels. The primary fuel is combusted with 

excess air > 1. A consecutive reduction zone is achieved by feeding secondary fuel and late inlet of 

final combustion air for the secondary fuel. Both air staging and fuel staging have been developed as 

primary measures for reduction of fuel NOX in biomass combustion and are described below.

(CHxOyNz) + (H2O)

 Biomass

Char C

Drying

T > 100°C

T > 300°C

Devolatilization

+ O2 (!1)

+ O2 (!tot)

CO, H2, CH4, C2+

 

H2O, N2, 

O2 ! 0 

NO

CO2

O2

CO2, H2O, O2

Evaporation heterogeneous reactions homogeneous reactions

!1 < 1 !tot > 1

C + CO2 <–> 2 CO

T > 800°C Gasification of C

C + H2O <–> CO + H2

C + 0,5 O2 <–> CO

H2O, N2,

 HCN, NHi, ...

T > 800°C

Oxidation

Air Air

Gas and Tar:

Figure 3.3 Reactions during biomass combustion by injection of primary air for gasification and secondary air 

for gas-phase burnout 

Figure 3.4 Adiabatic flame temperature for the combustion of wood with different humidity u (u is based on dry 

fuel, hence u = 100% corresponds to a moisture content w = 50%) 
17

Figure 3.5 Carbon monoxide emissions as function of excess air ratio for different furnace types. a) Wood 

stove, b) downdraft boiler, c) automatic wood boiler, d) advanced automatic wood boiler 

3.5.2 Uncombusted pollutants

The main needs for complete burnout are temperature, time, and turbulence (TTT). The mixing 

between combustible gases and air is identified as the factor that is most limiting to the burnout 

quality, while the demands for temperature (around 850°C) and residence time (around 0.5 s) can 

easily be achieved. Sufficient mixing quality can be achieved in fixed-bed combustion by the abovedescribed two-stage combustion. In a fluidized bed, good mixing is achieved in the bed and the freeboard and dust combustion easily allows good mixing. 

For future improvements in furnace design, computational fluid dynamics (CFD) can be applied to 

calculate flow distributions in boilers. Furthermore, the reaction chemistry in the gas phase is also well 

known and can be implemented in CFD codes. However, the heterogeneous reactions during drying, 

transport, devolatilization, and gasification of solid biomass before entering the gas phase combustion 

need to be considered and needs further improvement to allow the application of whole furnace 

modeling.
18

Figure 3.6 CFD modelling for optimization of furnace design - the example shows the velocity distribution in 

the combustion chamber over a grate with secondary air nozzles and post combustion chamber 

[Nussbaumer 2003].

3.5.3 NOX emissions

In combustion processes, NO and NO2 (summarized as NOx) can be formed in three different 

reactions. Thermal NOx and prompt NOx are formed from nitrogen in the air at high temperatures and 

in the case of prompt NOx in the presence of hydrocarbons. Fuel NOx can be formed from nitrogen 

containing fuels. For biomass combustion, fuel-bound nitrogen is the main source of NOx emissions, 

while thermal and prompt NOx are not relevant due to relatively low temperatures, as has been shown 

by theoretical and experimental investigations.

Fuel nitrogen is converted to intermediate components such as HCN and NHi with i=0, 1, 2, 3. These 

can be oxidized to NOx if oxygen is available, which is the case in conventional combustion. If no oxygen is present, intermediates can interact in the reduction zone and form N2 in reactions such as NO + 

NH2 = N2 + H2O (Figure 3.7). During the past 10 years, staged combustion technologies have been 

developed as a primary measure for internal NOx reduction based on this concept, thus leading to the 

above-described techniques of air staging and fuel staging (Figure 3.8). Both measures allow a NOx

reduction of up to 50% for wood with low nitrogen content and up to 80% for bio fuels with high 

nitrogen content. However, different specific conditions have to be met accurately to exhaust this 

reduction potential. 

O, OH O, OH

CHi 

 (prompt NOx)

NO NHi

CO, CHi

C, Ash

Biomass

 CHON

 Volatiles

 HCN

 Char

 N

NHi NO

N2

N2O

NCO

NH

N2

NO

N2O

N in Ash

H

OH

Products

 O

(thermal 

 NOx) het. Oxidation/Reduction

CHi (NO Recycle / Reburning)

Figure 3.7 Conversion of fuel nitrogen in biomass combustion [Nussbaumer 2003]
19

Figure 3.8 Principle of conventional two-stage combustion, air staging with reduction zone, and fuel staging 

with reduction zone [Nussbaumer 2003]

In the case of air staging, a primary air excess around 0.7, a temperature in the reduction zone of 

1150°C, and a residence time of 0.5 s are needed. The relatively high temperature can limit the application in practice due to undesired ash softening and deposit formation. For fuel staging, similar 

results are achieved at lower temperatures (i.e., at temperatures as low as 850°C). However, the 

furnace concept and operation is more complex due to the need of two independent fuel feeding 

systems. Nevertheless, a pilot plant based on this concept has been successfully designed with a 

combination of understoker furnace and entrained flow reactor. For both types of staged combustion, 

accurate process control is needed to ensure an operation at the excess air ratio needed in the 

different zones. 

Beside primary measures, secondary measures are available for NOx abatement. The most relevant 

techniques are selective noncatalytic reduction (SNCR) and selective catalytic reduction (SCR) using 

the same reaction as mentioned for staged combustion, i.e., NO + NH2 = N2 + H2O. However, urea or 

ammonia is injected as a reducing agent and as a source of NH2. SNCR has to be applied in a narrow 

temperature window of around 820°C to 940°C, thus allowing a NOx reduction up to 90%. SCR is 

typically applied in the flue gas in a temperature range of 250° to 450°C and allows a NOx reduction of 

more than 95%. However, relevant concentrations of undesired side products such as HNCO, N2O, 

NH3, HCN, and others can be formed in both types of secondary measures under unfavorable conditions. Primary measures are preferable if they can achieve sufficient emission reduction.
20

3.5.4 Particulate emissions

Biomass combustion leads to relatively high emissions of particulates. The majority of the particulates 

are smaller than 10 micron (μm) (i.e., particulate matter PM10) with a high share of submicron particles 

(PM1).

Submicron and supermicron particles in fluidized-bed combustion are composed of fine particles of K, 

Cl, S, Na and Ca and coarse particles of Ca, Si, K, S, Na, Al, P, Fe. In fixed-bed combustion, increasing mass concentrations of particulate emissions are typically related to increasing mean 

diameter. A dependency of the particle composition on size can also be found in fixed bed conditions, 

as K, S, Cl and Zn is mainly found in the submicron fraction, while the content of Ca is increasing with 

increasing particle size. If almost complete burnout is achieved by appropriate furnace design, the 

resulting particulates are almost exclusively formed from ash components in the fuel with salts such as 

KCl as main components. The main fuel constituents, with respect to aerosol formation, are typically 

K, Cl, S, Ca, Na, Si, P, Fe, and Al. Primary measures that can safely meet a high reduction potential 

by at least a factor of 10 in this category of aerosols are not known so far. 

Recently a new approach for primary particle reduction has been presented. It was shown that particles from wood combustion are mainly formed by nucleation, coagulation, and condensation in the 

boiler during a decreasing temperature. Further, these particles are mainly salts and consist mainly of 

K. K is present as a salt in the fuel with a high melting point and devolatilization temperature. If oxygen 

is available at a high temperature, a high share of K can be oxidized. K oxides have significantly lower 

devolatilization temperatures than the K salts, and they are almost completely vaporized into the gas 

phase and lead to particle formation from the gas phase. If no oxygen is present in the fuel bed, the 

conversion of K to volatiles may be reduced, since the majority of K salts can be converted into the 

grate ash. Since a similar behavior for other ash components in the fuel is assumed, the oxygen 

content during the solid fuel conversion is regarded as a key parameter for aerosol formation. 

According to this hypothesis, an experimental setup was designed based on an understoker furnace 

that allows a wood combustion with extremely low primary air in the fuel bed (or glow bed) on the 

grate. At such operational conditions, the glow bed height increases significantly, and the furnace 

design must be adapted. Furthermore, combustion becomes unstable below a certain primary air 

excess, and an accurate process control is needed. Results from a laboratory furnace show that a 

reduction of inorganic particle emissions by a factor of 5 is achievable. The results are in accordance 

with the theory, although other effects may influence the particle formation.

Nevertheless, since fuel composition is the main parameter for aerosol formation, secondary measures are necessary for further reduction in the future. Among those measures, fabric filters are 

promising due to their high separation efficiency, while electrostatic precipitation is more robust for 

practical applications. 
21

3.6 Combustion technologies

Biomass combustion is mainly used for heat production in small and medium scale units such as wood 

stoves, log wood boilers, pellet burners, automatic wood chip furnaces and straw-fired furnaces. 

District heating systems range in size from 0.5 MWth to 5 MWth with some applications up to 50 MWth. 

Combined heat and power production (CHP) with biomass is applied by steam cycles (Rankine cycle) 

with steam turbines, steam engines, and organic Rankine cycles (ORC) that have typical power 

outputs between 0.5 MWe and 10 MWe. Stirling engines (10 kWe to 100 kWe) and closed gas turbines 

are in development or demonstration mode. Co-firing in fossil fired power stations allows the 

advantages of large size plants (> 100 MWe), which are not applicable for dedicated biomass combustion due to limited local biomass availability. 

Table 3.2 gives an overview of the most frequently used furnace types for biomass combustion. The 

various types can be distinguished by the flow conditions in the furnace. There are three different flow 

conditions: fixed-bed, fluidized-bed, and entrained flow or dust combustion (Figure 3.9). To achieve 

complete burnout and high efficiencies in small-scale combustion, downdraft boilers with inverse flow 

have been introduced, which apply the two-stage combustion principle described above (Figure 3.10). 

An operation of log wood furnaces at very low load should be avoided, as it can lead to high emissions 

of uncombusted pollutants. It is recommended to couple log wood boilers to heat storage tanks. 

Since wood pellets are well suited for automatic heating at small heat outputs as needed for today’s 

buildings, pellet furnaces are an adequate application with growing propagation. Thanks to the welldefined fuel at low water content, pellet furnaces can easily achieve high-combustion quality. They are 

applied both as stoves and as boilers. 

Understoker furnaces are mostly used for wood chips and similar fuel with relatively low ash content 

(Figure 3.11), while grate furnaces can also be used for high ash and water content (Figure 3.13). 

Figure 3.12 shows the principle of a the “low particle concept”, which was introduced by Verenum and 

Mueller AG and consists of a primary zone acting as a gasification section at very low primary excess 

[Oser & Nussbaumer 2006]. This principle allows the reduction of particle emissions by 50% to 80%. 

The following mechanisms are relevant for particle reduction: 

• Reduced temperature in the glow bed reduces evaporation of ash constituents and formation of 

salts such as KCl in the flue gas.

• Reduction of gas velocity through the fuel bed reduces entrainment of fuel particles that enter the 

hot section in the flame, where released vapours from ash constituents remain in the flue gas. 

This leads to salts in the stack instead of grate ash.

• A reduction of the oxygen content in the fuel bed reduces the tendency to form oxides from the 

ash constituents, which reduces evaporation because oxides exhibit lower evaporation 

temperatures.

• Finally, the high fuel bed may act as a filter for initially released particles. 

The low particle concept is being investigated in research projects in Switzerland. It exhibits a potential 

for improvement. However, particle separation is still needed to achieve very low particle emissions for 

large-scale application. For medium-scale combustion (100 kW to 500 kW), the low particle principle 

may achieve emission limits of smaller than 50 mg/m3

 without secondary measures. Furthermore, it 

allows a reduction of the filter ash if applied for large-scale combustion equipped with particle removal. 
22

This allows a cost reduction for the particle separation and a reduction of filter ash, which needs to be 

desposed as hazardous waste. 

Special types of furnaces have been developed for straw that has very low density and is usually 

stored in bales. Besides conventional grate furnaces operated with whole bales, cigar burners and 

other specific furnaces are in operation. 

Similar technologies are applied to residential pellet boilers (Figure 3.14). However, due to the higher 

fuel density and the homogeneous fuel properties, smaller auxiliary equipment such as stokers and 

electric motors can be used. Pellet boilers are smaller in size and exhibit lower cost than wood chip 

boilers of the same nominal heat output. 

Stationary or bubbling fluidized bed boilers (SFB) as well as circulating fluidized bed boilers (CFB) are 

applied for large-scale applications and often used for waste wood or mixtures of wood and industrial 

wastes from the pulp and paper industry (Figure 3.15, Figure 3.16). In CFB boilers, nearly homogeneous conditions of temperature and concentrations can be achieved, thus allowing high burnout 

quality at low excess air. The choice of different bed materials in CFB offers additional opportunities 

for catalytic effects. In addition, the option of heat removal from the bed allows control of the combustion temperature and allows an operation at low excess air without excessive ash sintering. Since 

similar conditions for nitrogen conversion with air and fuel staging are attained, relatively low NOX

emissions are achieved. 
23

Table 3.2 Types of biomass furnaces with typical applications and fuels-*Biomass covers typically less than 

10% of the total fuel input. a = ash content (wt.-%), d = typical fuel size (diameter) 

Application Type Typical size range Fuels Ash Moisture

content

Manual Wood stoves 2 kW – 10 kW dry wood logs < 2% 5% – 20%

Log wood boilers 5 kW – 50 kW log wood, sticky wood residues < 2% 5% – 30%

Pellets Pellet stoves and boilers 2 kW – 25 kW wood pellets < 2% 8% – 10%

Automatic Unterstoker furnaces 20 kW – 2.5 MW wood chips, wood residues < 2% 5% – 50%

Moving grate furnaces 150 kW – 15 MW all wood fuels and most 

biomass

< 50% 5% – 60%

Pre oven with grate 20 kW – 1.5 MW dry wood (residues) < 5% 5% – 35%

Understoker with rotating 

grate

2 MW – 5 MW wood chips, high water content < 50% 40% – 65%

Cigar burner 3 MW – 5 MW straw bales < 5% 20%

Whole bale furnaces 3 MW – 5 MW whole bales < 5% 20%

Straw furnaces 100 kW – 5 MW straw bales with bale cutter < 5% 20%

Stationary fluidized bed 5 MW – 15 MW various biomass, d < 10 mm < 50% 5% – 60%

Circulating fluidized bed 15 MW – 100 MW various biomass, d < 10 mm < 50% 5% – 60%

Dust combustor, entrained 

flow

5 MW – 10 MW various biomass, d < 5 mm < 5% < 20%

Co-firing* Stationary fluidized bed total 50 MW – 150 MW various biomass, d < 10 mm < 50% 5% – 60%

Circulating fluidized bed total 100 – 300 MW various biomass, d < 10 mm < 50% 5% – 60%

Cigar burner straw 5 MW – 20 MW straw bales < 5% 20%

Dust combustor in coal 

boilers

total 100 MW – 1 GW various biomass, d < 2 – 5 mm < 5% < 20%

Conversion factors:

1 MMBtu = 1,000,000 Btu = 1055 MJ = 293.06 kWh

1 MMBtu/h = 1,000,000 Btu/h = 293.06 kW

1 kW = 3,412 Btu/h

1 MW = 3,412,000 Btu/h = 3,412 MMBtu/h
24

Figure 3.9 Furnace types and flow conditions: Fixed bed, fluidized bed, and entrained flow reactor
25

Figure 3.10 Downdraft boilers with inverse combustion of log wood and with enforced air flow and air supply 

with primary and secondary air. Top: 1 Primary air, 2 fuel hopper, 3 glow bed, 4 secondary air, 5 

post combustion chamber, 6 heat exchanger, 7 chimney. Bottom: 1 Fuel inlet, 2 fuel hopper, 3 glow 

bed, 4 primary air, 5 secondary air, 6 ash bin, 7 mixing zone, 8 post combustion chamber, 9 heat 

exchanger, 10 chimney.
26

Figure 3.11 Understoker boiler with primary and secondary air, mixing zone and post combustion chamber. 1 

Screw feeder, 2 understoker zone with glow bed, 3 primary air, 4 secondary air, 5 post combustion 

chamber, 6 heat exchanger, 7 cyclone, 8 ash removal.

s

g

glow

bed

T

ash

pellets

feeding

primary

air

secondary

air

glow

bed

T

constriction

0

10

20

30

40

50

60

0 20 40 60 80 100

Heat Output [%]

P

a

r

t

i

c

l

e C

o

n

c

e

n

t

r

a

t

i

o

n i

n t

h

e F

l

u

e 

G

a

s [

m

g

/

m

3

] (

1

3 v

o

l

.

-

% O

2

)

Regular Conditions, Type B

Low-Particle Conditions, Type B

0

10

20

30

40

50

60

0.0 0.2 0.4 0.6 0.8 1.0

Excess Air Ratio in the Glow Bed [–]

Particle Concentration in the Flue 

Gas [mg/m3] (13 vol.-% 

O

2

)

Regular Conditions, Type B

Low-Particle Conditions, Type B

Figure 3.12 Schematic of an understoker boiler with “low particle concept” - The figure shows a 100 kW prototype pellet boiler. The cross-section of the combustion chamber is reduced from both sides in the 

section “constriction”, to improve the gas mixing by acceleration and subsequent velocity reduction. 

The influence of low-particle conditions on the particle mass concentration is shown in the graphs. 

The reduction of the particle emissions at very low primary excess air (comparable to gasification 

conditions) amounts to more than 50%. The particle emissions remain constant at varying load, 

which is an important advantage for practical applications. [Oser & Nussbaumer 2006]. 
27

6

7

8

4

3

1

8

5

2

Figure 3.13 Moving grate boiler with primary air in two stages in the grate and secondary air. 

Top: 1 Screw feeder, 2 moving grate, 3 primary air, 4 secondary air, 5 post-combustion chamber, 6 

heat exchanger, 7 cyclone, 8 ash removal 

Bottom: Example of a district heating plant with two boilers and electrostatic precipitator in 

Switzerland (app. 6 MW, Schmid) 
28

 

Figure 3.14 Examples of pellet boilers 

Top: Understoker boiler with small grate and primary air inlet (1) and subsequent secondary air inlet 

in the combustion chamber (2) (Oekofen) 

Bottom: Screw feeder on top of the grate with primary air inlet (6) and subsequent secondary air 

inlet (4) prior to the combustion chamber (10) (Hargassner)
29

 Fuel

 Flue Gas Ash

Primary Air 

Level 1

Secondary Air 

Levels 2,3

Tertiary Air

Fluidised Bed

Figure 3.15 Stationary fluidized bed combustor with staged combustion. (Lurgi, 5 MW).

Figure 3.16 CFB plant with steam boiler for coal and biomass (Lurgi) 
30

3.7 Co-combustion

A co-usage of biomass with other fuels can be advantageous with regard to cost, efficiency, and 

emissions. Lower specific cost and higher efficiencies of large plants can be used for biomass, and 

emissions of SOX and NOX can be reduced by co-firing. However, attention must be paid to increased 

deposit formation in the boiler and limitations in ash use due to constituents in biomass, especially 

alkali metals, which may disable the use of ash in building materials. Due to undesired changes of ash 

compositions, the share of biomass is usually limited to approximately 10% of the fuel input. Other 

opportunities are of interest, and the following three options for co-usage of biomass with coal are 

applied: 

1. Co-combustion or direct co-firing: The biomass is directly fed to the boiler furnace (fluidized bed, 

grate or pulverized combustion), if needed after physical pre-processing of the biomass such as 

drying, grinding, or metal removal. 

2. Indirect co-firing: The biomass is gasified and the product gas is fed to a boiler furnace (a 

combination of gasification and combustion).

3. Parallel combustion: The biomass is burned in a separate boiler for steam generation. The steam 

is used in a power plant with the main fuel.

Co-combustion of biomass leads to a substitution of fossil fuels and to a net reduction of CO2

emissions. In many countries co-firing is the most economical technology to achieve the target of CO2

reduction. Biomass co-firing can therefore be motivated by savings of CO2 taxes. 
31

4 Particulate Matter (PM)

4.1 Characteristics of particulate matter (PM)

Particulate matter describes the sum of airborne solid particles and droplets. Particle size is an important parameter for the characterization of particulate matter. According to different size ranges, coarse 

particles > 10 μm, fine particles (0.1–1 μm or 0.1–2.5 μm) and ultrafine particles < 0.1 μm are distinguished. PM10 defines particles as having an aerodynamic diameter < 10 μm. This includes both coarse 

and fine particles. The size and density affects the retention period and travel distances in the atmosphere. Coarse particles tend to settle to the ground within hours, while fine particles can remain in the 

atmosphere for several weeks. Furthermore, limit values for both particle emissions and ambient PM 

standards are indicated as mass concentrations. This does not account for the large surface area of 

fine particles, which can act as a potential carrier for toxic substances. Besides mass concentrations, 

additional parameters are relevant for the potential impact of particles on environment and human 

health. In particular, the particle size, the particle shape, the morphology, and the chemical composition are important parameters that need to be considered. For typical conditions, approximately 90% 

of PM consists of fine and ultrafine particles [UBA 2005]. Since PM indicates only mass concentrations, there are considerations for further parameters such as particle number and toxicity, which might 

be considered for future limit values. 

Particulate matter in the ambient air is a mixture of directly emitted primary aerosols and secondary 

aerosols formed in the atmosphere. PM is partially of natural origin, while anthropogenic emissions 

lead to additional PM in the ambient air. Coarse particles from primary aerosols originate mainly from 

mechanical processes (construction activities, road dust, re-suspension, and wind), whereas fine 

particles are particularly produced through combustion [WHO 2006]. Main sources of primary particulate matter are Diesel engines and biomass combustion. Secondary aerosols are formed in the 

atmosphere through conversion of gaseous precursors such as sulphur oxides (SO2, SO3), nitrogen 

oxides (NO, NO2), ammonia (NH3), and Non-Methane Volatile Organic Compounds (NMVOC). Reaction products are ammonium sulphates and ammonium nitrate. Aldehydes and ketones are oxidation products of NMVOC. The greatest producer of secondary aerosols are combustion processes 

(thermal power stations, industry, domestic heating: SO2, SO3 and NOX), agriculture (NH3), Diesel 

engines (NOX and SO2), use of solvents, chemical industry and petro chemistry (NMVOC) [UBA 2005]. 

Beside secondary aerosols that originate from NOX and SOX, secondary organic aerosols (SOA) can 

also contribute significantly to PM in the ambient air. Among other sources, incomplete combustion of 

wood is a potential source of SOA. 

PM in the ambient air can be influenced by the following secondary reactions: 

1. High molecular organic compounds (organic condensables, such as tar components) can be 

available in the vapour phase at flue gas temperature in the stack but can subsequently form 

additional particles or droplets by nucleation and condensation. This leads to an increase in 

particle number and mass or an increase in the particle mass concentration by adsorption on 

existing particles in the stack or in the ambient air. 
32

2. Low molecular organic compounds (VOC) can form SOA by photochemical oxidation in the atmosphere and/or can increase the particle mass by adsorption on existing particles. Due to the added 

mass of oxygen, the total particle mass is additionally increasing. 

3. PM in the ambient air can be reduced by evaporation of particles and droplets and by desorption 

of semi-volatile compounds adsorbed on the particles due to dilution in the ambient air, thus resulting in a reduced partial pressure of the respective compounds in the surrounding atmosphere. 

Consequently, organic PM from residential wood heating in the ambient air can be underestimated if 

only solid particles detected on hot filters in the stack at 160°C (as by VDI norms) are taken into 

account. Investigations from diesel and wood combustion show a distinctive increase in PM concentration by dilution with ambient air at low dilution ratios. For wood stoves, an increase of 2.5 to 9 

was found in a dilution tunnel (Figure 4.28) [Rhyde & Johansson 2007] where dilution ratios up to 10 

are commonly applied. Investigations from diesel exhaust also show a significant initial increase of 

detected particles by dilution with ambient air with a dilution ratio of 20 (Figure 4.1) [Lipsky & Robinson 

2006, Robinson 2007]. 

 

Figure 4.1 Primary organic aerosol (POA) in diesel exhaust as function of dilution ratio [Robinson 2007] 

1: Measurement in undiluted exhaust (tail pipe) 

2: Measurement with dilution with cold air at dilution ratio = 20 

3, 4, 5: Measurement with further increased dilution 

Cooling with subsequent condensation and adsorption is essential in the first dilution step and leads 

to a significant increase in particle mass. The further dilution of the flue gas, evaporation, and 

desorption of volatile and semi-volatile compounds due to a reduction of the partial pressure in the 

surrounding gas phase becomes relevant. This leads to a reduction of the particle mass. Consequently, the partitioning of semivolatile organics is a crucial factor with respect to the conversion of 

stack emissions from wood combustion or from diesel engines to the ambient air. Figure 4.2 illustrates 

the conversion mechanisms from emissions in the stack to ambient conditions by an initial rapid 

dilution and consecutive secondary reactions. This leads to additional secondary organic aerosols. 

The graph describes a fictitious example. However, the illustrated increase of detected solid particles 

in the hot flue gas to the particle mass sampled at a dilution ratio of approximately 10, corresponding 

to typical conditions in a dilution tunnel, is assumed to be 3.3 in the given example, which is in the 

range of results reported from wood stoves. The example describes a resulting PM10 concentration in 
33

the ambient, which is 2.8 times the mass of primary solid particles detected in the stack (i.e. in 

undiluted flue gas at a flue gas temperature of 160°C). The difference is due to secondary organic 

aerosols that result from organic compounds available as condensables at ambient conditions and 

additional particle mass resulting from secondary oxidation of volatile and semivolatile compounds. 

The sum of PM10 and VOC can exceed the mass of particles and condensables detected in the stack 

or immediately after an initial dilution, which is due to the added mass of oxygen available by 

secondary oxidation. To illustrate the potential effect of added oxygen, Figure 4.3 shows a fictitious 

example, where the resulting mass of PM10 exceeds the mass of solid particles and condensables in 

the stack. 

Figure 4.2 Conversion of stack emissions to PM10 in the ambient - The graph shows a virtual example with total 

SOA = 1.8 TSP as measured in the stack and consequently PM10 = 2.8 TSP 
34

Figure 4.3 Conversion of stack emissions of PM and organics to PM10 in the ambient - The graph shows a 

virtual example with total SOA = 2.8 TSP as measured in the stack and consequently PM10 = 3.8 

TSP. Here, the mass of PM10 in the ambient exceeds the total mass of solid particles and 

condensables as sampled in the stack due to the additional mass of oxygen added by secondary 

oxidation. 
35

4.2 Health relevance of PM

Long exposures to particulate matter can cause serious health problems such as higher morbidity, 

effects on lungs, and a shorter life expectancy, mainly in subjects with pre-existing heart and lung 

diseases. Particle size is a main determinant of the effect on health. Coarse particles are generally 

filtered in the nose and throat while PM10 can settle in the lungs and may reach the alveolar region. 

Many regulatory agencies have agreed on 10 microns, as a measure between respirable and nonrespirable particles, for monitoring ambient air quality. PM2.5 and PM1 are also used as indicators for 

air quality standards. The World Health Organization has published its Air Quality Guideline based on 

health effects with limit values for PM10 and PM2.5, for short-term and long-term exposures [ WHO 2006].

The aim of the Air Quality Guideline (AQG) of the World Health Organization (WHO) is to support 

actions to achieve air quality that protect human health. Countries are encouraged by WHO to adopt 

an increasingly stringent set of standards, and track progress through the monitoring of emission 

reductions and declining concentrations of particulate matter. To assist this process, the guideline and 

interim target values of the AQG reflect the concentrations at which increased mortality responses due 

to particulate matter air pollution are expected based on current scientific findings [WHO 2006]. 

Besides the guideline value, three interim targets were defined, since countries may find these interim 

targets particularly helpful in gauging progress over time in the process of steadily reducing population 

exposures to PM. Due to WHO’s Air Quality Guideline both the European Commission and the United 

States Environmental Protection Agency have used the approach to revise their air quality standards 

for particulate matter. Table 4.1 and Table 4.2 summarize the current data on PM reported by WHO 

and proposed for annual and daily mean concentrations. 
36

Table 4.1 WHO air quality guidelines and interim targets for particulate matter: annual mean concentrations. 

The use of PM2.5 guideline value is preferred.

PM10

[μg/m3]

PM2.5 

[μg/m3]

Basis for the selected level

Interim target-1 70 35 These levels are associated with about a 15% higher long-term 

mortality risk relative to the AQG level.

Interim target-2 50 25 In addition to other health benefits, these levels lower the risk of 

premature mortality by approximately 6% [2–11%] relative to theIT-1 

level.

Interim target-3 30 15 In addition to other health benefits, these levels reduce the mortality 

risk by approximately 6% [2-11%] relative to the -IT-2 level.

Air Quality 

Guideline 

(AQG)

20 10 These are the lowest levels at which total cardiopulmonary and lung 

cancer mortality have been shown to increase with more than 95% 

confidence in response to long-term exposure to PM2.5. 

Table 4.2 WHO air quality guidelines and interim targets for particulate matter: 24 hour concentrations (99th 

percentile (days/year) 

PM10

[μg/m3]

PM2.5 

[μg/m3]

Basis for the selected level

Interim target-1 150 75 Based on published risk coefficients from multi-centre studies and 

meta-analyses (about 5% increase of short-term mortality over the 

AQG value)

Interim target-2 100 50 Based on published risk coefficients from multi-centre studies and 

meta-analyses (about 2.5% increase of short-term mortality over the 

AQG value)

Interim target-3 75 37.5 Based on published risk coefficients from multi-centre studies and 

meta-analyses (about 1.2% increase in short-term mortality over the 

AQG value)

Air Quality 

Guideline 

(AQG)

50 25 Based on relationship between 24-hour and annual PM levels 
37

4.3 Ambient PM standards

Table 4.3 shows a comparison of ambient PM standards in different countries. The European Community has set limits for PM10 in the air in the directives 1999/30/EC and 96/62/EC. On Jan. 1, 2005 

these standards became effective. In a first phase, the limit of the yearly average is 40 μg/m3

 and the 

daily average (24-hour mean) 50 μ g/m3 . The daily average is not allowed to be exceeded more than 35 

times per year. In a second phase starting June 1, 2010, the yearly average standard will be 20 μg/m3 

and the daily average limit will be 50 μg/m3. The daily average will be allowed to exceed this 

amount only 7 days per year. 

Table 4.3 Ambient PM standards for PM10 (except USA: PM10 and PM2.5). *indicative value 

Country

Annual 

limit value

[μg/m3]

24-hour 

limit value 

[μg/m3

] Allowance for 24-h limit value 

EC to 1.6.10 40 50 35 exceedances p.a. (Austria: 30)

EC from 1.6.10 20 50 7 exceedances p.a. (Austria: 25)

Switzerland 20 50 1 exceedance p.a.

Norway 35 50

WHO (AQG) 20 50

WHO (AQG) 10 25

USA PM10 150 1 exceedance p.a. on average over 3 a

USA PM2.5 15 35 98% for an average over 3 a
38

4.4 Types of combustion particles

Investigations on particle emissions showed that well-designed and properly-operated advancedtechnology automatic wood combustion plants (mostly used in the size range > 200 kW) achieve a 

high combustion quality, which results in low emissions of uncombusted hydrocarbons and soot 

[Nussbaumer 2005]. However, automatic wood boilers exhibit fairly high mass concentrations of 

particle emissions in raw gas (typically a magnitude of 100 mg/m3

 at 13 Vol.-% O2 for natural wood 

chips and up to several 100 mg/m3

 for other fuels such as bark or non-woody biomass). Particles from 

advanced-technology and well-operated automatic wood combustion consist mainly of inorganic 

matter such as salts, while particles from wood stoves operated under poor conditions consist mainly 

of soot and organic substances. 

Typical inorganic particles in the flue gas of advanced-technology automatic combustion devices are 

shown in Figure 4.4. From a poorly operated wood stove, soot particles, as well as tar droplets, can be 

identified as shown in Figure 4.5. In conditions with lack of oxygen in the combustion chamber, the 

particle mass concentration increases dramatically as shown in Figure 4.6. This is due to uncombusted 

soot and organic primary aerosols. As a result of agglomeration, the particle size distribution is shifted 

to larger particles. In addition, high concentrations of condensable organic matter and volatile organic 

compounds (VOC) are released with the flue gas, which are potential pre-cursors of secondary 

organic aerosols (SOA). 

Soot from diesel engines has been identified as being carcinogenic in many investigations. However, 

particulate matter from incomplete combustion of wood consists of solid soot particles and condensable organic substances. Significantly higher concentrations of polycyclic aromatic hydrocarbons 

(PAH) (Table 4.4) as well as far higher cell toxicity (Figure 4.7) and carcinogenic potential indicated by 

chromosome abberation (Figure 4.8) were found in particles and condensables from incomplete 

combustion of wood than in diesel soot [Klippel & Nussbaumer 2007 a]. Condensable organic substances such as tar components are a product of wood pyrolysis, which may occur under smoldering 

conditions such as a wood stove operated with reduced air inlet at low-burning rate and low temperature. Highly toxic PAH may also be formed as secondary tar in subsequent gas phase reactions in 

the combustion chamber under conditions with lack of oxygen [Morf et al. 2002]. 
39

 

Figure 4.4 SEM-pictures of particles in the flue gas of an automatic wood combustion system. Left is the 

combustion of wood resulting in submicron particles. Right is the combustion of herbage grass 

resulting in larger particles at higher mass concentrations [Kaufmann et al. 2000]. 

 

Figure 4.5 TEM-pictures of soot agglomerates (left) and tar droplets (right) from the flue gas of a poorly operated wood stove - Scale of white line in the left picture: 100 nm, right: 500 nm - Pictures from Heuberger, EMPA Dübendorf, published in [Klippel & Nussbaumer 2007 b]. 
40 1.0E+0

1.0E+1

1.0E+2

1.0E+3

1.0E+4

1.0E+5

1.0E+6

1.0E+7

1.0E+8

1.0E+9

0.01 0.1 1 10 D [!m]

d

N/dlo

g

D [c

m -3

]

optimal operation

typical operation

very bad conditions

20 mg/m3 500 mg/m3

5000 mg/m 3

Figure 4.6 Number size distribution of particles from wood stoves under different operation conditions monitored in the size range from 20 nm to 10 μm by Scanning Mobility Particle Sizer (SMPS) and 

Optical Particle Counter (OPC) - Mass concentrations measured with gravimetric method according 

to VDI and indicating only solid particles without condensables at 13 Vol.-% O2 [Klippel & Nussbaumer 2007 b] 

Table 4.4 PAH concentrations in particles from badly operated wood stove and in Diesel soot [Klippel & 

Nussbaumer 2007 a].

mg/kg Particles 

from wood 

stove

Diesel 

soot

Naphthalene 13 42

Acenaphthylene 129 7.1

Acenaphthene 17 < 3

Fluorene 173 < 3

Phenanthrene 231 3.7

Anthracene 65 < 3

Fluoranthene 154 < 3

Pyrene 170 < 3

Chrysene 54 < 3

Benzo(a)anthracen e 44 < 3

Benzo(b )fluoranthene 30 < 3

Benzo(k)fluoranthene 11 < 3

Benzo(a)pyrene 25 < 3

Indeno(1,2,3-cd)pyrene 9 < 3

Dibenzo(a,h)anthracene < 8 < 3

Benzo(g,h,i)perylene < 8 < 3

Total PAH 1 120 53
41

0

10

20

30

40

50

60

70

80

90

100

0 2000 4000 6000 8000 10000 12000

Particle concentration in cell medium [!g/ml]

C

ell s

u

rviv

al [%]

Wood stove,

bad operation

Diesel

soot

Inorganic salts from

automatic furnace,

optimal operation

Figure 4.7 Cell survival as a function of particle concentration for Diesel soot, mainly inorganic particles from 

automatic wood combustion, and wood particles from incomplete combustion in a badly operated 

stove [Klippel & Nussbaumer 2007 b] 

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200 250 300 350 400 450 500

Particle concentration in cell medium [!g/ml]

C

ell s

urviv

al [%]

Chromosome

abberation

onset

(Diesel soot)

at 310 !g/ml

Chromosome

abberation

onset

(wood stove)

at " 21 !g/ml

Diesel soot

Particles from

wood stove under

bad combustion

conditions

Figure 4.8 Cell survival as a function of particle concentration for Diesel soot and wood particles from incomplete wood combustion (data as in figure above but in magnified scale) - In addition, the level of 

chromosome aberration onset is indicated [Klippel & Nussbaumer 2007 b]. 
42

4.5 Influence of particle sampling methodology

Combustion particles are a mixture of solid particles and condensables, containing both organic and 

inorganic fractions. The organic emission is strongly influenced by the combustion conditions. High 

organic emissions are correlated with low combustion efficiency and high potential for toxic organic 

substances such as benzo(a)pyrene (BaP) or fluorine. This can be avoided by high combustion temperature, sufficient oxygen availability in the flame enhanced by good mixing, and sufficiently long residence time in the combustion zone. 

Figure 4.9 Comparison of PM sampling with PM in the ambient 

SP: Filter (Method a) resulting in solid particles SP (total suspended particles TSP) 

SPC: Filter + Impinger (Method b) resulting in solid particles and condensables SPC 

DT: Dilution Tunnel (Method c) with typical dilution ratio (DR) in the order of 10 resulting in a PM 

measurement including SPC and most or all C. DT is identical or slightly smaller than SPC + C due 

to potentially incomplete condensation, depending on dilution ratio and sampling temperature (since 

dilution reduces not only the temperature but also the partial pressure of contaminants)

DS: Dilution Sampling with high dilution ratio (DR > 100) 

PM10: Total Particulate Matter < 10 microns in the ambient including SP and SOA 

SOA: Secondary organic aerosols, consisting of condensables C at ambient and SOA formed by 

secondary reactions such as photochemical oxidation 

 

*SO2 and other soluable gaseous compounds in the flue gas may be dissolved in the impingers 

**In case of determination of TOC in impingers, the mass of O, H, N, S and other elements 

contained in the organic condensables needs to be accounted for separately 

***Organic compounds that are liquid or solid at partial pressure in the flue gas and ambient 

temperature but volatile at sampling due to reduced partial pressure by dilution and temperature 

above ambient 
43

The emission of inorganic particles can be slightly influenced by the combustion conditions. However, 

this depends on different formation mechanisms than organic particles. High temperature enhances 

the conversion of ash material to the gas phase and consequently, the emission of inorganic particles. 

Under typical conditions, organic particles are relevant in poorly operated manual wood combustion 

devices, while inorganic particles are dominant in automatic wood combustion plants operated at high 

temperatures. Depending on the aim of the measurement, different sampling strategies are commonly 

used for product testing and health studies. In the present survey, four types of particle sampling are 

distinguished, as illustrated in Figure 4.9: 

A Sampling of particles on a heated filter, through a probe, from undiluted flue gas in the chimney at 

gas temperatures of 160°C (German norms by Verein Deutscher Ingenieure, VDI) or 120°C (US 

EPA) - These particles are defined as solid particles (SP) in the study and consist of filterable 

particles and droplets at the indicated temperature. If not otherwise indicated, data on PM in the 

present report are given as SP. 

Solid or filterable particles consist of inorganic particles and organic particles. The inorganic particles constituted a large part of K2SO4 and KCl (depending on the fuel composition) at favorable 

combustion conditions, while soot (elemental carbon, EC) and organic particles are also emitted 

as solid particles if the combustion conditions are poor. 

B Sampling of particles as described in method A, and subsequent sampling of condensable 

organic matter (C) in impinger traps at temperatures less than 20°C - Typically condensable particulate emissions are sampled in a series of impinger flasks containing water to collect inorganic 

compounds and containing methylene chloride to capture organic compounds. The sum of solid 

particles and condensables are defined as (SPC) in the present report. Inorganic matter, excluding water, found in the impingers can originate from gaseous products such as SO2. In the case 

of a considerable amount of such compounds, the allocation of the respective mass to either 

gaseous or particulate emissions is uncertain and needs further information. Under typical wood 

combustion conditions, the inorganic PM is sampled in the filter, while condensable inorganic 

matter in the impinger bottles is assumed to be negligible. 

C Sampling of filterable particles in a dilution tunnel with a filter holder gas temperature less than 

35°C (Norwegian standard NS 3058-2). Due to the cooling, condensable organic material in the 

hot flue gas condenses on the filter. Condensable inorganic PM is assumed to be insignificant for 

residential combustion of wood. The particle concentration found by method C is assumed to be 

similar or equal to the mass detected by method B. The particles detected by method C are 

defined as particles in dilution tunnel (DT) in the present study and correspond to particles 

detected at small dilution ratios (DR) (typically DR has a magnitude of 10). For a comparison and 

interpretation of results from dilution tunnel measurements, it needs to be considered that the dilution ratio may vary during one measurement and from one measurement to another. This can lead 

to differences of the detected particle mass in regimes, where effects of condensation and reevaporation are sensitive to the dilution ratio, as the initial dilution with cold air may lead to an 

increase of particle mass due to condensation. Further dilution may lead to a subsequent loss

in particle mass due to re-evaporation of volatile compounds, as described in chapter 4.1. 

D Dilution can also be performed at high dilution ratios (100 or more), which is defined as dilution 

sampling (DS) in the present study. To mimic conditions in the ambient air, sampling can be 
44

performed in cold, highly diluted flue gas. Dilution with cold air at high dilution ratios is applied by 

analyzing the plume by following a vehicle under real-world conditions in a certain distance on the 

road. As in dilution tunnels, the dilution ratio can vary during sampling. 

For on-line analysis of the particle size and particle number concentrations, a hot dilution stage 

followed by a cold dilution stage is usually applied resulting in a total dilution ratio of 100. The hot 

dilution avoids condensation and allows to identify the particles similar as found in the stack (i.e. 

SP) but gathers information on the particle size and number instead of the particle mass. 

For residential combustion devices, all methods can be applied, while for large combustion devices, 

sampling in a dilution tunnel is not applicable. 

Sampling on hot filters by method A is useful for testing of combustion devices, but can significantly 

underestimate the total organic PM in ambient air resulting from biomass combustion for applications 

with high concentrations of organic substances. Since condensable organic matter has been identified 

as highly toxic, method A may not only underestimate the mass concentration of PM, but the health 

impact as well. 

Measurements in dilution tunnel by method C may over-estimate the particle mass in the ambient air, 

since evaporation by subsequent dilution in the ambient air may lead to a reduction in particle mass 

which, however, is then resulting in volatile organic matter instead of particulate matter as illustrated in 

Figure 4.2. 

4.6 Measurement techniques

Table 4.5 shows a comparison of the most common methods for discontinuous measurements of particulate emissions, while Table 4.6 shows an overview of the covered size ranges and the advantages 

and disadvantages of continuous measurement methods. 

Discontinuous gravimetric sampling of total particle mass concentration 

The basic method used to measure mass concentrations of PM off-line in flue gases is gravimetric 

sampling of quartz or glass fibre filters, which have been in a desiccator or a room with controlled 

humidity prior to sampling. This method gives total mass concentration. As an option, mass 

concentration of a specific size fraction can be gathered in combination with a pre-cyclone with a cutoff of 10 μm or 2.5 μm thus resulting in PM10 or PM2.5. The sampling of particles on filters allows typical 

time resolution of 15 minutes and up and does not allow identification of fast dynamic processes. 

However, particles available on the filter are potentially available for additional chemical analysis. 

Continuous measurements of particulate matter 

When there is a need for real-time measurements of PM10 and PM2.5,Tapered Element Oscillating 

Micro-balance (TEOM) is a well established instrument. TEOM is often used for measurement of 

PM10 and PM2.5 in ambient air. In combustion applications, the ambient air model of the instrument is 

used with a dilution system. For larger combustion plants, there is a specific model of TEOM available 

that operates without dilution.
45

Continuous measurements of particle size distributions 

Since the particle size is an important parameter that influences the lifetime of the particles in the 

ambient air and the separation efficiency in the human respiratory tract, there is an increasing interest 

in the particle size of particulate emissions, as well as the particle size of particulate matter in the 

ambient air. For this purpose, particle number or particle mass distributions are determined depending 

on the analytical method. In addition, on-line measurements with high-time resolution are of interest to 

detect transient conditions such as the start-up phase of combustion devices. 

a) Mass size distribution 

For measurements of mass size distributions, low-pressure cascade impactors (e.g. Andersen Impactor, Dekati Low Pressure Impactor DLPI, Berner Low Pressure Impactor BLPI) are frequently used, 

typically ranging from 30 nm to 10 – 20 μm. They are sometimes combined with a filter that extends 

the measurement range to a few nanometres. Due to the sampling method, cascade impactors are 

also operated off line, and particle samples can potentially be used for additional analyses. 

b) Number size distribution 

In terms of health concerns, there is also an interest in particle number concentrations and particle 

number size distributions, since a high number of ultrafine particles are assumed to be relevant to 

certain health issues. There are several instruments available for on-line measurements of particle 

number mostly in need of dilution systems, as they do not cope with high temperatures. The main 

instruments are: 

• Scanning mobility particle sizer (SMPS) is commonly used for measurement of concentration 

and corresponding size distribution. There are a number of instrument versions, working in the 

size range of a few nanometres to 1 μm. 

• Electric low-pressure impactor (ELPI) is another instrument used for characterization of 

number concentration and distribution, which ranges from 7 nm to 10 μm. 

• Aerodynamic particle sizer (APS) measures particles from ~0.5 μm to ~10 μm. 

• Fast mobility particle sizer (FMPS) is a rather new instrument for number concentrations and 

size distributions ranging from 5.6 nm to 0.56 μm. 

• Optical particle counter (OPC) is another option for measurement of the number of particles, 

typically ranging from ~0.1 μm to a few micrometers. 

The instruments mentioned use different particle diameters for characterization of number size distributions. It is important to respect which diameter is considered to perform conversions from number to 

mass concentration.

Particle properties

In addition to mass and number concentrations, surface and volume concentrations are examples of 

other parameters that might be interesting to consider in certain applications. Finally, it should be 

emphasized that characterization of chemical content is essential, as it can give information on 

combustion conditions and mirrors the ash content in the fuel. 
46

Table 4.5 Comparison of methods for discontinuous particle measurements [Jokiniemi 2007]. LPI = Low 

pressure impactor, SEM/TEM = Scanning electron microscopy/tunnel electron microscopy 

Table 4.6 Comparison of methods for continuous particle measurements [Jokiniemi 2007] 
47

4.7 PM emission factors 

4.7.1 Open fireplaces

Figure 4.10 shows the PM emission factors for open fireplaces as described in an overview on 

emission factors from different IEA countries [Nussbaumer et al. 2007]. The range for typical measured emission factors on solid particles (SP) varies between 23 and 265 mg/MJ (0.053 and 

0.615 lb/MMBTU). National emission factors range from 160 (GER) to 447 mg/MJ (UKM) (0.371 and 

1.04 lb/MMBTU). The emission factors for Finland and Norway are 860 mg/MJ and 910 mg/MJ (2.0 

and 2.1 lb/MMBTU), respectively indicated for measurements with dilution tunnel (DT). 

Figure 4.10 Emission factors for PM in mg/MJ for solid particles (red) and for particles measured in dilution 

tunnel (green) 
48

4.7.2 Closed inset appliances (closed fireplaces)

For closed inset appliances (closed fireplaces according to EN 13229:2001), only few data were 

available. Worst results for emission factors range from 180 – 204 mg/MJ, typical results from 47 –

83 mg/MJ and best results from 14 – 26 mg/MJ (worst results from 0.42 – 0.47 lb/MMBTU, typical 

from 0.11 – 0.19 lb/MMBTU and best from 0.032 – 0.060 lb/MMBTU). 

Figure 4.11 Emission factors for closed inset appliances 

4.7.3 Wood stoves

Within the survey, most data was delivered for the category of wood stoves, therefore, this rubric represents a broad overview. For typical measured solid particles with start phase, the scope ranges from 64 

to 87 mg/MJ (0.15 to 0.20 lb/MMBTU). Measurements with condensables and in dilution tunnel show persistently higher results, which range from 340 to 544 mg/MJ (0.79 to 1.26 lb/MMBTU). 

National emission factors are the basis for calculations of emission inventory reports. One of the most 

important applications is the reporting of national greenhouse gas inventories under the United Nations Framework Convention on Climate Change. The comparison of default PM emission factors for 

wood stoves reveals a wide order of magnitude. The emission factor for solid particles ranges from 

94 mg/MJ (Germany) to 650 mg/MJ (Sweden) (0.22 lb/MMBTU to 1.51 lb/MMBTU). In countries with 

compulsory measurements with dilution tunnel, the factors are higher compared to solid particles. 

Factors vary from 750 mg/MJ (Finland) to 1932 mg/MJ (Norway) (0.22 lb/MMBTU to 1.51 lb/MMBTU). 

The Finnish value is a revised number, which will apply soon, while currently the old value of 

400 mg/MJ (0.93 lb/MMBTU) is still in use. The Norwegian emission estimate is based on an aggregated emission factor for traditional/conventional stoves, new stoves, and open fireplaces [Nordic 

Council of Ministers 2004]. An explanation of the high Norwegian emission data is that the Norwegian 

standard for wood stoves includes measurements at low thermal output with throttled air supply. 

Thus, the data should represent real use of wood stoves in Norway. 
49

Figure 4.12 shows the emission factors for wood stoves with an indication of ranges from best, typical 

to worst type of operation or combustion type. Figure 4.13 summarizes the same data with indication 

of the typical data only. 

Figure 4.12 Range of emission factors from worst to best (where available) for wood stoves depending of type 

of operation and/or equipment 

Figure 4.13 Average emission factors for wood stoves 
50

 

Figure 4.14 National emission factors for wood stoves 

4.7.4 Log wood boilers

For log wood boilers, the technology of two-stage combustion has been implemented for more than 20 

years. Most modern log wood boilers used in Switzerland, Austria, and Germany are based on a twostage combustion with primary air for the solid fuel conversion, a subsequent mixing zone with 

injection of secondary air followed by the post combustion chamber, and the heat exchanger in the 

boiler section [Nussbaumer 2003]. Such boilers are usually designed by the downdraft principle and 

most commonly operated with forced draft, by application of ventilators for the air inlet and/or for the 

flue gas exhaust. In the present survey, data from downdraft boilers with forced draft and data from 

updraft boilers were collected. 

Figure 4.15 shows the range of emission factors for updraft boilers, although available from a limited 

number of institutions only. If compared to data from the same countries, updraft boilers exhibit comparable ranges of emissions as wood stoves. 

Figure 4.16 shows emission data from log wood boilers with forced draft. Under typical operation conditions, boilers with downdraft principle allow significantly lower emissions than updraft boilers. 

In addition to the combustion principle, the hydraulic implementation of the boiler in the heating 

system, and the application and management of a heat storage tank can significantly influence the 

emissions in practical operation. Figure 4.17 shows a comparison of emission factors of log wood boilers with and without heat storage tank. The combination of a log wood boiler with a heat storage tank 

allows an operation at full load or at steady-state part-load with low emissions. On the other hand, 

boilers operated for house heating without heat storage tanks may be used with throttled air inlet, thus 

leading to excessively high emissions of uncombusted carbon and particles. 

Figure 4.18 shows an overview of data from log wood boilers. No information about the combustion 

principle was available. 
51

Figure 4.15 Range of emission factors from worst to best (where available) for log wood boilers with updraft 

combustion without ventilator 

Figure 4.16 Range of emission factors from worst to best (where available) for log wood boilers with forced 

downdraft combustion 
52

Figure 4.17 Emission factors for log wood boilers with and without heat storage tank 

Figure 4.18 Emission factors for log wood boilers, where combustion type is not specified 

4.7.5 Pellet stoves and boilers

Figure 4.19 and Figure 4.20 show the emission factors of pellet stoves and pellet boilers, respectively. 

For both applications, only results from the operation with pellets made from wood with low bark 

content are considered. The emission factors for pellet combustion are, in most cases, between 10 –

50 mg/MJ (0.023 – 0.116 lb/MMBTU) for typical operation and are comparable to the best results 

achieved by log wood combustion. If the worst results under poor operation conditions are compared, 

a relatively moderate increase in emissions is expected for pellet combustion. In comparison to log 

wood combustion, emission factors at non-ideal operation are at least a magnitude lower, which is regarded as the main advantage of pellet combustion. However, many investigations have shown that 

the use of biomass other than wood, with low amounts of bark for pellet combustion, in residential 

heating appliances results in significantly increased emissions of particulate matter (typically by a 

factor of more than 3), as well as in emission of NOX. The reported data are valid for wood pellets with 

low ash content only. 
53

Figure 4.19 Range of emission factors from worst to best (where available) for pellet stoves with natural draft 

Figure 4.20 Range of emission factors from worst to best (where available) for pellet stoves with forced draft 

and electronic combustion control 
54

Figure 4.21 Range of emission factors from worst to best (where available) for pellet boilers 
55

4.7.6 Overview on residential wood combustion

Figure 4.22 gives a summary of the average emission factors for pellet combustion, log wood boilers, 

and wood stoves. 

Figure 4.22 Average emission factors (best, typical, worst) for pellet boilers, pellet stoves, log wood boilers, and 

wood stoves - All data indicated as solid particles SP except for wood stoves, where data including 

condensables (SPC) or measured in dilution tunnel are given in the last column. 

• Pellet combustion and log wood boilers with forced draft achieve relatively low particle emissions, 

not only under ideal operation conditions, but also under operation conditions that are assumed to 

be typical and should reflect the emissions under practical conditions of most installed heating 

devices of the respective categories. The reported data lead to an estimated average emission 

factor of approximately 30 mg/MJ (0.070 lb/MMBTU). This is indicated as filterable or solid particles 

(SP) for these categories under typical operation conditions. However, these emission factors are 

valid for natural, uncontaminated dry wood only, while significantly higher emissions are found for 

other fuels such as bark, straw pellets, wet log wood, and other inappropriate fuels. 

• Under poor operation conditions, an increase of roughly a factor of 2 is expected for pellet combustion, although this value is uncertain due to limited investigations and might be underestimated. For 

downdraft boilers, an increase by a factor of 10 is expected under poor operation conditions. The 

main advantage of pellet combustion is that very poor operation is assumed to be very rare because of the homogeneous fuel and the continuous fuel feeding. Especially for log wood combustion, the effect of the start-up may significantly increase the average emissions. However, information on this issue is scarce and uncertain for pellet combustion. 

• Wood stoves may achieve similar emissions as pellet combustion or downdraft boilers, if operated 

ideally. However, ideal operation for wood stoves is expected to be rarely found in practice, as it 

demands small batches of small and dry logs, which lead to a constant and low heat output by 

continuous manual feeding in short periods and assumes an optimized start-up phase, which is 

often not the case. Since these conditions are often not fulfilled in practice, an increase of almost a 

factor of 10 is expected for wood stoves under typical operation conditions, thus leading to an 

estimated value of roughly 150 mg/MJ (0.35 lb/MMBTU) for solid particles. 
56

• Similar values are expected for old-type wood boilers with natural updraft combustion. Two-stage 

combustion with forced draft is regarded as a significant improvement. However, the type of operation of log wood boilers in practice strongly depends on the hydraulic integration in the heating 

system. For conventional house heating, a combination with a heat storage tank is usually needed 

to avoid sharply throttled air inlet and excessively high emissions. 

• For wood stoves, measurements including condensables found in impinger bottles and measurements of particles in diluted flue gases in a dilution tunnel are available. A comparison of these 

data with measurements of solid particles reveals significantly higher average emission factors. 

Since the data shown in Figure 4.22 are derived from different investigations and different combustion devices, they do not allow a direct comparison between the different measurement methods. 

However, the significantly higher (by a factor of roughly 4) average emission factor reported from

measurements in dilution tunnels, or data including condensables, is in line with the potential of 

additional particle mass from condensation of highly volatile organic condensables found during 

poor combustion conditions. The influence of the sampling method is discussed separately, since 

the data in Figure 4.22 do not allow a direct comparison of the sampling types. 

4.7.7 Automatic biomass combustion plants

Figure 4.23 and Figure 4.24 show the reported emission factors for automatic combustion plants from 

70 kW to 500 kW for under stoker and grate boilers, respectively. Average emission factors of this category range from 50 – 100 mg/MJ (0.12 – 0.23 lb/MMBTU), which is in line with the typical emission 

limit values valid today (e.g. 150 mg/m3

 at 11 or 13 Vol.-% O2). 

However, an investigation of field measurements in Switzerland for a large number of installations revealed a relatively broad range of emissions from 30 to 350 mg/MJ (0.070 to 0.81 lb/MMBTU), which 

demonstrates the type of operation is also important for automatic wood combustion plants, and that 

periodic monitoring of such plants is crucial. The same is true for plants from 500 kW to 10 MW, as 

shown in Figure 4.25 and Figure 4.26. However, emission factors of installations in this category 

strongly depend on the type of flue gas cleaning and on the imposed emission limit values. In 

combustion plants greater than 500 kW in Switzerland1

, significant changes are expected. New 

emission limit values in this category will make the application of fine particle removal systems 

necessary in future and lead to a significant reduction of PM emissions. 

For the comparison of different particle sources, it needs to be recognized that the presented emission 

factors are indicated as particle mass concentrations and do not respect the health relevance of 

different particle types. Particles and condensables found from wood burned under very poor combustion conditions, such as in a wood stove with throttled air inlet, contain high concentrations of 

polycyclic aromatic hydrocarbons and exhibit high cell toxicity and high carcinogenic potential, while 

particles found from automatic wood combustion staged design at good operation conditions consist 

mainly of salts. Consequently these particles exhibit far lower toxicity and carcinogenity [Klippel & 

Nussbaumer 2007]. 

 

1

 20 mg/m3 at 11 Vol.-% O2 are valid for plants > 1 MW from 9.1.07 instead of formerly 150 mg/m3. 

20 mg/m3

 at 13 Vol.-% O2 are introduced for plants > 500 kW in 2008 instead of formerly 150 mg/m3. 
57

Figure 4.23 Emission factors for under stoker boilers from 70 – 500 kW 

Figure 4.24 Emission factors for grate boilers from 70 – 500 kW 
58

Figure 4.25 Emission factors for under stoker boilers from 500 kW – 10 MW 

Figure 4.26 Emission factors for grate boilers from 500 kW – 10 MW 
59

4.8 Particle sampling: Solid particles, condensables, 

dilution tunnel

For the emission factors from wood stoves, a limited number of data is available from Sweden. [Ryde 

and Johansson, 2007] Parallel measurements were performed by sampling method A, resulting in 

solid particles (SP) and sampling method C, resulting in particles found on the filter in the dilution 

tunnel (DT), as described in chapter 4.5. In addition, data is available from measurements on wood 

stoves in Switzerland with separate sampling of solid particles and subsequent condensable matter, 

thus resulting in SP according to method A and SPC according to method B. 

Figure 4.27 shows that the PM emission factor found in the dilution tunnel is significantly higher than 

the solid particle emissions detected in the chimney. The same is true for the data found including condensables in impingers. 

Figure 4.28 illustrates the ratio of DT/SP, when measured in the dilution tunnel, and of SPC/SP when 

measured with condensables found in impingers. In one case (Verenum stove 3), which corresponds 

to a high combustion quality, resulting in low total PM emissions, the ratio SPC/SP is only slightly 

above one (≤ 1.1), while in all other cases with poor combustion conditions (throttled air supply), the 

reported ratios DT/SP are between 2.5 and close to 10 (data from Sweden), and the ratios of SPC/SP 

are between three and six (Switzerland). 
60

Figure 4.27 Comparison of PM emission factors on solid particles (SP), particles in dilution tunnel (DT), and 

solid particles plus condensables from impinger (SPC) for wood stoves

Figure 4.28 Ratio DT/SP and SPC/SP for wood stoves acc. to Figure 4.27
61

 

 

Figure 4.29 Ratio DT/SP and SPC/SP for wood stoves as function of SP 

 Figure 4.30 Ratio DT/SP and SPC/SP for wood stoves as function of SPC and DT, respectively 
62
63

5 Norms and emission limit values

Europe

Standard No. Application

CEN-303-5 Heating boilers for solid fuels, hand and automatically stocked, nominal 

heat output of up to 300 kW

Boiler

CEN-12809 Residential independent boilers fired by solid fuel, nominal heat output 

up to 50kW

CEN-14785 Residential space heating appliances fired by wood pellets

CEN-12815 Residential cookers fired by solid fuel

CEN-13229 Insert appliances including open fires fired by solid fuels

CEN-13240 Room heaters fired by solid fuel

CEN-14785 Residential space heating appliances fired by wood pellets

Stove

CEN-15250 Slow heat release appliances fired by solid fuel

Norm Titel / Inhalt

EN 12809

Heizkessel für feste Brennstoffe - Nennwärmeleistung bis 50 kW - Anforderungen 

und Prüfung

EN 12815 Herde für feste Brennstoffe - Anforderungen und Prüfung

EN 13229

Kamineinsätze einschliesslich offene Kamine für feste Brennstoffe -

Anforderungen und Prüfung

EN 13240 Raumheizer für feste Brennstoffe - Anforderungen und Prüfungen

EN 13284

Emissionen aus stationären Quellen - Ermittlung der Staubmassenkonzentration 

bei geringen Staubkonzentrationen

EN 13384 Abgasanlagen - Wärme- und strömungstechnische Berechnungsverfahren

EN 13502 Abgasanlagen - Anforderungen und Prüfverfahren für Keramik-Aufsätze

EN 1443 Abgasanlagen - Allgemeine Anforderungen

EN 14785 Raumheizer zur Verfeuerung von Holzpellets - Anforderungen und Prüfverfahren

EN 18160 Abgasanlagen, teil 1-x

EN 1856 Abgasanlagen - Anforderungen an Metall-Abgasanlagen
64

EN 267 Ölbrenner mit Gebläse - Begriffe, Anforderungen, Prüfung, Kennzeichnung

EN 303 Heizkessel - Teil 1-x

EN 304 Heizkessel - Prüfregeln für Heizkessel mit Ölzerstäubungsbrennern

CEN TS 14588

Feste Biobrennstoffe - Terminologie, Definitionen und Beschreibungen; Deutsche 

Fassung CEN/TS 14588:2003

CEN TS 14774

Feste Biobrennstoffe - Verfahren zur Bestimmung des Wassergehaltes -

Verfahren der Ofentrocknung

CEN TS 14775 

Feste Biobrennstoffe - Verfahren zur Bestimmung des Aschegehaltes; Deutsche 

Fassung CEN/TS 14775:2004

CEN TS 14918 

Feste Biobrennstoffe - Verfahren zur Bestimmung des Heizwertes; Deutsche 

Fassung CEN/TS 14918:2005

CEN TS 14961 

Feste Biobrennstoffe - Brennstoffspezifikationen und -klassen; Deutsche Fassung 

CEN/TS 14961:2005

CEN TS 15103 

Feste Biobrennstoffe - Verfahren zur Bestimmung der Schüttdichte; Deutsche 

Fassung CEN/TS 15103:2005

CEN TS 15104 

Feste Biobrennstoffe - Verfahren zur Bestimmung des Gehaltes an 

Gesamtkohlenstoff, Wasserstoff und Stickstoff - Instrumentelle Verfahren; 

Deutsche Fassung CEN/TS 15104:2005

CEN TS 15105 

Feste Biobrennstoffe - Verfahren zur Bestimmung des wasserlöslichen Gehaltes 

an Chlorid, Natrium und Kalium; Deutsche Fassung CEN/TS 15105:2005

CEN TS 15148 

Feste Biobrennstoffe - Verfahren zur Bestimmung des Gehaltes an flüchtigen 

Substanzen; Deutsche Fassung CEN/TS 15148:2005

CEN TS 15234 

Feste Biobrennstoffe - Qualitätssicherung von Brennstoffen; Deutsche Fassung 

CEN/TS 15234:2006

CEN TS 15289 

Feste Biobrennstoffe - Bestimmung des Gesamtgehaltes an Schwefel und Chlor; 

Deutsche Fassung CEN/TS 15289:2006

CEN TS 15290 

Feste Biobrennstoffe - Bestimmung von Hauptelementen; Deutsche Fassung 

CEN/TS 15290:2006

CEN TS 15296 

Feste Biobrennstoffe - Analysenberechnung auf unterschiedliche Bezugsbasen; 

Deutsche Fassung CEN/TS 15296:2006

CEN TS 15297 

Feste Biobrennstoffe - Bestimmung von Spurenelementen; Deutsche Fassung 

CEN/TS 15297:2006

prCEN TS 14918 

Feste Biobrennstoffe - Verfahren zur Bestimmung des Heizwertes; Deutsche 

Fassung CEN/TS 14918:2005

VDI 2006 Messen von Partikeln - Staubmessungen in strömenden Gasen
65

Comparison of the standard structure in Europe and US

Europe USA

Heater Boiler Heater Boiler

Description of the 

requirements

Subpart AAA(2) -

Description of the 

test conditions

Method 28

Method 28 

OWHH (project)

Description of the 

measurement 

method

CEN 13240

CEN 12815

CEN-13229

CEN-14785

CEN-15250

CEN-15544(2)

CEN 303-5

CEN-12809

Method 5H or 5G 

for particles(1)

Method 5H or 5G 

for particles(1)

OWHH = Outdoor wood fired hydronic heating appliances 

(1): There are methods to measure other emissions, like CO, NOx, etc., but there are no limits fixed 

(2): Depending on the type of heater - The difference is on the security requirements. 

(3): EPA: 40 CFR Part 60 - Standard of Performance for New Stationary Sources (NSPS) 

http://www.tceq.state.tx.us/permitting/air/rules/federal/60/60hmpg.html 

http://www.epa.gov/epacfr40/chapt-I.info/ 

Different structure of standards and legislation for emission measurements in Europe and the 

United States: 

 European standards refer to types of firing appliances. Standards generally include 

appliance requirements and test methods for emission measurements. Emission 

measurements include solide particles, CO, and partly VOC. 

 Situation in the USA: Subpart AAA – (Standards of Performance for New Residential 

Wood Heaters) summarizes all conditions a wood heater has to satisfy, and the methods that 

have to be used for the certification (see §60.534, page 407). Method 28 determines how the 

wood stove has to be operated during the certification. How to conduct the measurements is 

defined in Method 5H (stack location) or 5G (dilution tunnel). Only particulate matter measurements are mandatory for a wood heater certification. Methods for NOX, CO, SO2 measurements exist but are not part of the certification process. 
66

Test fuel requirements (wood)

CEN 13240 CEN 303-5 EPA method 28

EPA method 

28 OWHH

Wood humidity min 12% 12% 16% 16%

(wet basis) max 20% 20% 20% 20%

Length Manuf.(1) Manuf.(1) 5/6*L(2) 0.8*L(2)

width Manuf.(1) Manuf.(1) Wood size 4 in. 4 in.

height Manuf.(1) Manuf.(1) 4 or 2 in. 4 in.

Wood type Beech, Birch Beech, Birch, 

oak, spruce Douglas fir

Red or white 

oak

Wood amount pro 

charge f(P)(3) f1(V)(4) f2(V)(5) f2(V)(5)

(1) As specified by the manufacturer

(2) L: usable length of the firebox

(3) f(P) the amount of wood is a function of the output power of the wood heater

(4) f1(V) the amount of wood is the maximum which can be filled

(5) f2(V) the amount of wood is a function of the fire box volume

Sources: Wood humidity:

• CEN 13240 table B1, page 71

• CEN 303-5 table 8, page 17

• EPA method 28, section 7.1.2 page 1408

• EPA method 28 OWHH, section 12.2 page 10

Wood size

• CEN 13240: table B1, page 71

• CEN 303-5: table 8, page 17

• EPA method 28: section 7.1.4, page 1409 + fig 28.2 and 28.3, page 14.45

• EPA method 28 OWHH: section 12.2, page 10 + fig 1 page 25

Wood type

• CEN 13240: table B1, page 71

• CEN 303-5: section 5.3, page 16

• EPA method 28: section 7.1.1, page 1408

• EPA method 28 OWHH: section 12.2 page 10

Wood amount pro charge

• CEN 13240: A4.2, page 39

• CEN 303-5: section 5.7.4.1, page 19

• EPA method 28: section 8.8.4, page 1417

• EPA method 28 OWHH: section 12.2.3, page 12
67

Chimney

Europe USA

Heater Boiler Heater Boiler

CEN 13240 CEN 303-5 EPA method 28

EPA method 28 

OWHH

Chimney length - - 4.6m(1) 4.6m(1)

Chimney isolation Insulated Insulated Half insulated Half insulated

Chimney draught Forced(2) Forced(2) Natural(3) Natural(4)

(1) Measured from the ground - Has to be isolated from 2.6 m to the top 

(2) Either to value specified by manufacturer, or to values specified in the standard 

(3) Natural, but a ventilator can be installed. The draft created shall not be greater than 

1.25 Pa (the temperature is specified) 

(4) No details found 

Sources: Chimney length: 

• CEN 13240: 

• CEN 303-5: 

• EPA method 28: section 6.11.1, page 1406 

• EPA method 28 OWHH: section 9.1.2, page 7 

Chimney draught: 

• CEN 13240: section 6.4, figure 1 page 24 

• CEN 303-5: section 4.2.3, page 16, and figure 2, page 17 

• EPA method 28: section 6.12.3, page 1407 

• EPA method 28 OWHH: section 9.1.2, page 7 
68

Heat extraction

Europe USA

Heater Boiler Heater Boiler

CEN 13240 CEN 303-5

EPA 

method 28

EPA method 28 

OWHH

Temperature 

input

? Tin = Tout - Tdiff

10°K < Tdiff <25°K

- ?

Temperature 

output

75°C < Tout < 85°C 70°C < Tout < 90°C - ?

Water discharge constant constant - constant

Sources: 

• CEN 13240: section A.2.5, page 38

• CEN 303-5: section 5.8.2, page 19

• EPA method 28: -

• EPA method 28 OWHH: section 12.5.1, page 13
69

Operation mode

Europe USA

Heater Boiler Heater Boiler

CEN 13240 CEN 303-5 EPA 

method 28

EPA method 28 

OWHH

1 min. charge Qmin=0.25*Pn

(1) Qmin=0.3*Pn(2) Qmin < 4 kW

(*) Qmin=0.15*Pn

(1)

2 - -

P2 min = 4 

kW

P2 max = 6.25 

kW (**)

P2 min =0.16*Pn

(1)

P2 max =0.24*Pn

(1)

3 - -

P3 min = 6.25 

kW

P3 max = 9.5 

kW(***)

P3 min

=0.25*Pn

(1)

P3 max

=0.50*Pn

(1)

Operating charges

4 max. charge Pn

(1) Pn(1) Pmax Pn(1)

Batch

f(P)(3) f1(V)(4) f2(V)(5) f2(V)(5)

Pretest Yes Yes Yes Yes

Start On charcoal bed

50g

On charcoal bed, 

when operating 

temperature is 

reached,

at least 6 hrs

On 

charcoal 

bed

20-25% of 

M(6)

On charcoal bed

10-20% of M(6)

Charge 

adjustment ? No (only during the pretest)

Max once 

per batch(8)

Max once per 

batch

Operating mode 4

(8)

End M(6)=0 M(6)=0 M(6)=0 M(6)=0

Pretest Yes Yes Yes Yes

Start On charcoal bed

On charcoal bed, 

when operating 

temperature is 

reached

at least 6 hrs

On 

charcoal 

bed

20-25% of 

M(6)

On charcoal bed

10-20% of M(6)

Charge 

adjustment ? No (only during the pretest)

Max once 

per batch(8)

Max once per 

batch(8)

Batch length

Operating modes 1-3

End tmin

(7) M(6)=0 M(6)=0 M(6)=0

(1) Qn is the nominal heat output, as specified by the manufacturer 

(2) If this heat output can not be reached, it has to be specified by the manufacturer

(3) f(Qn) the amount of wood is a function of the heat output of the wood heater 

Bfl = 360 000 * Qn * tb / (Hu*η) 

Bfl = fuel mass (Brennstoffaufgabe) in kg) 

tb = minimum time of one batch or time of one batch as specified by manufacturer 

f1(V) the maximum amount of wood that can be filled 

(4) f2(V) the amount of wood is a function of the fire box volume. The shape of the test 

fuel crib shall be geometrically similar to the shape of the firebox volume. The test fuel 

density shall be 112+- 11.2 kg/m3

(w.w.) of usable firebox volume. 

(5) The mass of the wood charge 

(6) Minimum time, as specified by manufacturer, or as prescribed in standard 

(7) Once per Batch, after 60% of the wood has been consumed 
70

 (*) Original is: 0.8 kg/h converted to power using a heating value of 18000 kJ/kg. If a wood heater 

cannot be operated at a burn rate below 0.8 kg/hr, two test runs shall be conducted with burn 

rates within Cat. 2

(**) Original is: 0.8 – 1.25 kg/h converted to power using a heating value of 18000 kJ/kg

(***)Original is: 1.25 – 1.9 kg/h converted to power using a heating value of 18000 kJ/kg

Sources: Operating charges

• CEN 13240: 

o Min charge: section A4.8.2 page 43

o Max charge : section A4.7.2, page 42

• CEN 303-5:

o Min charge: section 4.2.5 page 14

o Max charge : section 5.8.2, page 19

• EPA method 28: section 8.1.1, page 1410

• EPA method 28 OWHH: section 4.3, page 5

Batch length

• CEN 13240: 

o Pretest A4.7.2, page 41 and A.4.8.2, page 43

o Start A.4.7.3, page 42 and A.4.8.2, page 43

o End A.4.7.3, page 42 and A.4.8.2, page 43

• CEN 303-5:

o Pretest: section 5.7.4.1, page 19

o Start: section 5.7.4.1, page 19

o Charge adjustment: section 5.7.1, page 18

o End: section 5.7.4.1, page 19

• EPA method 28: 

o Pretest: section 8.11.1, page 1420

o Start: section 8.12.1, page 1420

o Charge adjustment: section 8.12.3, page 1425

o End: section 8.13, page 1426

• EPA method 28 OWHH: 

o Pretest: section 12.4.1, page 12

o Start: section 12.4.2, page 13

o Charge adjustment: section 12.5.2, page 14

o End: section 12.5.3, page 14
71

Test duration 

EN 303-5: 

1) Boiler with manual fuel feeding 

• PM only at full load: 2 charges (including re-filling second batch), discontinuous 

• CO, OGC, CO2/O2 also at minimum heat output, one batch, continuous (20 sec.) 

• Boiler with automatic fuel feeding 

• PM only at full load during whole testing period, discontinuous 

• CO, OGC, CO2/O2 also at minimum heat output, one batch, continuous 

CEN 13240: 

2) Zeitbrand-Feuerstätte = Room heater for short heating periods, i.e., minimum 0.75 h at 

full load if operated with wood: 

• CO, CO2/O2 at full load, 0.75 h, 3 batches, continuous (1 min) (A.4.7.1, s.41) 

3) Dauerbrand-Feuerstätte = Room heater for short heating periods, i.e., minimum 1.5 h 

at full if operated with wood: 

• CO, CO2/O2 at full load, 1.5 h, 2 batches, continuous (1 min) (A.4.7.1, s.41) 

Method 28: 

Duration of emission sampling is two hours. End the test run when the scale has indicated a test fuel 

charge weight of 0.00 kg or less for 30 seconds (s. 1427). 

Test Method 28 OWHH: 

Data are recorded at intervals of 10 min or less (12.5.1, s.13). The test run is ended when the scale 

has indicated a test fuel charge weight of 0.00 kg or less for 30 seconds.
72

Measurements

Summary of the measures that have to be performed during standardization

Europe USA

Heater Boiler Heater Boiler Heater Boiler

EPA 28

EPA 28 

OWHH

EPA 28

EPA 28 

CEN 13240 CEN 303-5 OWHH

5-G 5-H 5-G 5-H

Efficiency yes yes - yes - yes

O2 yes yes - - - -

CO yes yes - - - -

NOX - optional - - - -

VOC - yes - - - -

Particle 

sampling

chimney chimney dilution 

tunnel

chimney dilution 

tunnel

chimney

Particles Solid – at 180°C at < 32°C at < 120°C at < 32°C at < 120°C

Cond. – – – at < 20°C – at < 20°C

Sources: 

• CEN 13240: §6.2, page 23, and §A3, page 38

• CEN 303-5 §5.9.1, page 19

• For EPA: §6.534, page 407 of Subpart AAA 

• EPA method 28 OWHH §9.1.1.5.1, page 8
73

Measurements methods 

1. The efficiency

The wood heater efficiency can principally be measured in two ways: 

• A direct method, where the energy output of the wood heater is directly measured. In the 

case of wood stoves, this is done with the help of a calorimeter room, and in the case of a 

wood boiler by measuring the energy output in the water. 

• An indirect method, where the gaseous heat losses, both thermal and chemical, are 

measured. 

Europe USA

Heater Boiler Heater Boiler

CEN 13240 CEN 303-5

EPA method 

28

EPA method 

28 OWHH

Efficiency indirect direct - direct

2. The emissions

Two types of measurments can be used:

• Directly in the stack

• In a dilution tunnel

Both Europe and United States use the method of measuring directly in the stack, with the exception 

of method 5G. The ISO project is to measure all the emissions in a dilution tunnel.
74

Emission limit values

Europe

European emission limit values according to EN 303-5 

[mg/m3] at 10% O2

Qn Fuel feeding CO

class 1 class 2 class 3

Qn < 50 kW manual 25000 8000 5000

50 < Qn < 150 kW manual 12500 5000 2500

150 < Qn < 300 manual 12500 2000 1200

Qn < 50 kW automatic 15000 5000 3000

50 < Qn < 150 kW automatic 12500 4500 1250

150 < Qn < 300 automatic 12500 2000 1250

Qn Fuel feeding PM10

class 1 class 2 class 3

Qn < 50 kW manual 2000 300 150

50 < Qn < 150 kW manual 1500 200 100

150 < Qn < 300 manual 1500 200 100

Qn < 50 kW automatic 1750 200 100

50 < Qn < 150 kW automatic 1250 150 80

150 < Qn < 300 automatic 1250 150 80

Qn Fuel feeding OGC

class 1 class 2 class 3

Qn < 50 kW manual 200 180 150

50 < Qn < 150 kW manual 200 180 150

150 < Qn < 300 manual 200 180 150

Qn < 50 kW automatic 200 180 150

50 < Qn < 150 kW automatic 200 180 150

150 < Qn < 300 automatic 200 180 150

Note that some European countries have individual standards:

Switzerland, Austria, Germany. The standards are mostly more strict than class 3.
75

European emission limit values for heating boilers according to CEN 303-5 (A. 4.2.6., p.14)

Europe CO PM OGC

class 1 class 2 class 3 class 1 class 2 class 3 class 1 class 2 class 3

Manual feeding mg/MJ mg/MJ mg/MJ mg/MJ mg/MJ mg/MJ mg/MJ mg/MJ mg/MJ

< 50 kW 12250 3920 2450 98 88 74 980 147 74

50 – 150 kW 6125 2450 1225 98 88 74 735 98 49

150 – 300 kW 6125 980 588 98 88 74 735 98 49

Manual feeding mg/MJ mg/MJ mg/MJ mg/MJ mg/MJ mg/MJ mg/MJ mg/MJ mg/MJ

Qn – 50 kW 7350 2450 1470 98 88 74 858 98 49

150 – 300 kW 6125 2205 613 98 88 74 613 74 39

150 – 300 kW 6125 980 613 98 88 74 613 74 39

1 mg/MJ = 1.055 g/MMBTU = 0.00232 lb/MMBTU

Switzerland, Germany, Austria, Norway

1 mg/MJ = 1.055 g/MMBTU = 0.00232 lb/MMBTU

Switzerland OAPC (LRV), from 1.1.2008 CO PM OGC

Appendix 3 Wood combustion mg/MJ mg/MJ mg/MJ

< 70 kW 2720

70 – 500 kW 680 102

0.5 – 1 MW 340 14

1 – 10 MW 135 11

> 10 MW 81 5 27

Appendix 4 Type test for wood heating devices

manual Log wood boiler 544 41

automatic Wood chip boiler 272 61

automatic Pellet boiler 204 41

manual Room heater 1020 68

automatic Pellet room heater 340 34

manual Hearth 2040 75

manual Hearth for central heating 2040 102

manual Chimney 1020 68

orignial values in [mg/m3, 13 Vol.-% O2]
76

Switzerland LRV, from 1.1.2012 CO PM OGC

Appendix 3 Wood combustion mg/MJ mg/MJ mg/MJ

< 70 kW 2720

70 – 120 kW 340 68

120 – 500 kW 340 34

0.5 – 1 MW 340 14

1 – 10 MW 135 11

10 MW 81 5

orignial values in [mg/m3, 13 Vol.-% O2]

Switzerland LRV, from 1.1.2012 CO PM OGC

Appendix 4 Type test for wood heating devices mg/MJ mg/MJ mg/MJ

manual Log wood boiler 544 34

automatic Wood chip boiler 272 41

automatic Pellet boiler 204 27

manual Room heater 1020 51

automatic Pellet room heater 340 27

manual Hearth 2040 61

manual Hearth for central heating 2040 82

manual Chimney 1020 51

orignial values in [mg/m3, 13 Vol.-% O2]

Austria CO PM OGC NOx

Quelle: EN 303-5 Anhang A1 mg/MJ mg/MJ mg/MJ mg/MJ

manual Boilers for biofuels 1100 60 80 150

automatic Boilers for biofuels 500 60 40 150

Germany CO PM

EN 303-5 Appendix A2 mg/MJ mg/MJ

< 50 kW Wood boiler 2720

50 – 150 kW 1360 102

150 – 500 kW 680 102

> 500 kW 340 102

orignial values in [mg/m3, 13 Vol.-% O2]
77

EPA

Germany CO PM PM

1. BImSchV, 2003 mg/MJ mg/MJ mg/MJ

Step 1: 3 mths aft. publ. Room heater flat 1080 54

> 8 kW Room heater full 1350 54

Room heat w. heat storage 1080 54

Chimney inlets 1080 54

Room heater flat 1080 54 300

Room heater full 1350 54

Stove 1080 54

Hearth 1620 54

Hearth with central h. 1890 54

Pellet stove 135 27

Step 2: after 31.12.14 Room heater flat 540 11

> 8 kW Room heater full 540 11

Room heat w. heat storage 540 11

Chimney inlets 540 11

Room heater flat 540 11 300

Room heater full 540 11

Stove 540 11

Hearth 540 11

Hearth with central h. 540 11

Pellet stove 81 5

Step 1: 3 mths aft. publ. Wood boiler > 4 - 500 kW 540 32

Wood boiler > 500 kW 324 32

Step 2: after 31.12.14 Wood boiler > 4 kW 216 11
78

Comparison of PM standards in Europe and USA

For log wood boilers, there are no emission limit values by EPA, while EPA method 28 is valid for 

room heaters and describes limit values on PM. However, a direct comparison between limit values 

according to EPA method 28 with standards in Germany, Switzerland, and Austria is not possible due 

to different sampling procedures and complex averaged weighing of emission data. To enable an indicative comparison, a fictitious example for EPA method 28 is given below. However, according to 

EPA, the difference caused by different sampling that may lead to higher PM data due to condensables, is not considered.

Quelle: EPA Subpart AAA EPA Method 28 e x a m p l e

USA PM MJ/kg kg/h kg/h kg/h kg/h for comp.

Quelle: EPA Subpart AAA Average 18.3 0.8 1.25 1.9 3 Average Average

g/h mg/MJ mg/MJ mg/MJ mg/MJ mg/MJ mg/MJ

Catalytic stove 4.1 280 179 118 75 163 160

Non-catalytic stove 7.5 512 328 216 137 298 300

Non-catalytic stove at Qmin 15 1025 656 431 273 596 600

Min Max

cat 1 < 0.8

cat 2 < 0.8 1.25

cat 3 < 1.25 1.9

cat 4 < max. 

The following figures illustrate the emission limit values for CO and PM.
79

Comparison of CO and PM emission limits for boilers

Fig. 1a Emission limit value on CO - There is no 

limit value for CO for boilers in the United States. 

Fig. 1b Emission limit value on PM - There is no 

limit value for PM for boilers in the United States. 

Comparison of CO and PM emission limits for room heaters

Fig. 2a Emission limit value on CO - There is no 

limit value for CO for room heaters in the United 

States. 

Fig. 2b Emission limit value on PM for room 

heaters, according to the fictitious example 

described above, which does not consider the 

different sampling in the United States.
80

6 Research activities in Europe

6.1 Overview

A large number of institutions are involved in research activates in biomass conversion, biofuel production, and related issues. Many topics such as pollutant formation are being investigated by 

research institutes with interest in combustion processes, where biomass is often one fuel among 

others. For example, combustion modeling is widely applied as a design tool for large combustion 

plants such as coal-fired power stations. Institutions involved in such activates have also focused on 

biomass, due to an increasing interest in renewable energies. 

An overview on research institutions and industry involved in bioenergy in Europe can be found in the 

proceedings of the “European Biomass Conference & Exhibition”, which is held every one and a half to 

two years. The latest conference was held in Berlin in 2007, while the next one will be held in 

Valencia, Spain in June 2008. Further information is available on the homepage: 

http://www.conference-biomass.com 

The conference attracts approximately 1500 participants from more than 70 countries, with a 

significant participation of 5% from the United States and Canada (Figure 6.1). Germany is by far the 

most important country with respect to total delegate number, while the Netherlands, Austria, Sweden, 

and Finland have an over proportional participation in comparison to their population. This reflects the 

important role of biomass in the nordic countries (pulp and paper industry) and in Austria (wood 

industry), while the Netherlands show an increasing interest in biomass to substitute imported coal. 

Roughly 50% of the conference topics cover contributions from policies and markets to biomass 

resources, while roughly 50% are related to research (Figure 6.2). Main research topics include: 

thermochemical conversion, biological conversion, and conversion to liquids. Biofuels have gained a 

great interest in the past 10 years due to significant funding and promoting by the European Commission. Due to funding of biofuels, many research institutions have focused their activities on fuel 

production with a decreasing interest in electricity production. Among thermochemical conversion, 

pyrolysis and gasification are of great importance in research, since these technologies are not yet 

widely applied. In combustion research, focus is given to pollutant formation, application of non-woody 

biomass, and implementation of combined heat and power production. 
81

Figure 6.1 Distribution of participants in the 15th European Biomass Conference 

Figure 6.2 Distribution of topics in the 15th European Biomass Conference 
82

6.2 Research topics

The main targets of research and development in biomass combustion are:

• Increase of efficiency:

– Combustion efficiency 

– System efficiency for heat production

– Electric efficiency for combined heat and power production (CHP) and for dedicated power 

production

• Reduction of airborne pollutants, especially:

– Uncombusted pollutants by primary measures, both gases (CO and HC), as well as particulate 

matter 

– PM by secondary measures (both from uncombusted pollutants and from mineral matter) 

– NOX by primary and secondary measures

– Additional pollutants such as PCDD/F for contaminated biofuels

• Use of non-woody biomass such as straw, grass, agricultural residues

• Co-use of biomass with fossil fuels, especially co-firing biomass in coal fired power stations

• New applications of combined heat and power production with different targets: 

Smaller size, lower cost, higher efficiencies than steam cycles. For this purpose, organic Rankine 

cycles (ORC), Stirling engines, and indirectly fired gas turbines (hot air turbine) are being 

investigated. 
83

6.3 Research institutions

Switzerland

Verenum

Contact: Thomas Nussbaumer

Langmauerstrasse 109, CH – 8006 Zürich 

++41 (0)44 377 70 70 

http://www.verenum.ch 

Activities: Biomass combustion 10 kW – 10 MW, pollutant formation, primary and secondary measures 

for pollutant reduction, legislation, consultancy, quality assurance, plant optimisation 

Hochschule Luzern – Technik & Architektur (University of Applied Sciences Lucerne) 

Bioenergy and Sustainability Research Group 

Technikumsstrasse 21, CH – 6048 Horw 

Contact: Thomas Nussbaumer 

http://www.hslu.ch 

Activities: Biomass combustion 10 kW – 10 MW, pollutant formation, emission monitoring, primary and 

secondary measures for pollutant reduction, CFD modeling, development of improved combustion 

technologies 

Ökozentrum Langenbruck (Centre of Appropriate Technology and Social Ecology) 

Contact: Christian Gaegauf 

Schwengiweg 12, 4438 Langenbruck/Schweiz 

http://www.oekozentrum.ch 

Activities: Biomass combustion 10 kW – 70 kW, emission monitoring 

Germany 

Technologie und Förderzentrum (TFZ) 

Hans Hartmann 

Schulgasse 18, 94315 Straubing 

http://www.tfz.bayern.de/

Activities: Biomass combustion 10 kW – 70 kW, emission monitoring, logistics of agricultural biomass 

harvesting, testing of boilers and stoves 

University of Stuttgart 

Institute of Process Engineering and Power Plant Technology 

Pfaffenwaldring 23, D-70569 Stuttgar 

Günther Baumbach: Air Quality (residential wood combustion) 

Activities: Emission monitoring of residentail wood combustion 

Uwe Schnell: Combustion Technology (power plants) 

Activities: CFD modeling of large utility boilers 

http://www.ivd.uni-stuttgart.de/ 
84

Technical University Hamburg-Harburg 

Eißendorfer Straße 40, D – 21073 Hamburg 

Martin Kaltschmitt, Wolfgang Calmano, Joachim Gerth 

http://www.tu-harburg.de/iue/ 

Activities: Techno-economic assessment and life cycle assessment of bioenergy, emission monitoring 

Fachagentur nachwachsende Rohstoffe e.V. 

http://www.fnr.de/

Activites: Coordination of German research studies concerning renewable resources (no own 

research) 

Fraunhofer Wilhelm-Klauditz-Institut Holzforschung (WKI) 

Bienroder Weg 54 E, D – 38108 Braunschweig 

Prof. Dr. Rainer Marutzky 

www.wki.fraunhofer.de 

Activities: Emission monitoring (mainly in the 80s and 90s, now focus on wood technology and indoor 

air quality) 

Austria 

In Austria, five research organizations, located in Vienna, Wieselburg, and Graz, are basically independent, but united in the „Austrian Bioenergy Centre (ABC)“. ABC is a GmbH (limited liability 

company) and controlled by the share holders. 

Due to the coordination by ABC, there is a theoretical apportionment of the tasks for the involved 

institutes. However, two of the five institutions in ABC (Uni Graz and Bios GmbH Graz) are controlled 

by one person (Ingwald Obernberger), who initially controlled more than 50% of the shares of ABC 

and controlled the activities of the total centre. This resulted in limited possibilities for cooperation with 

research institutes abroad due to strongly protected activities of ABC. 

Austrian Bioenergy Centre 

BLT 

Wieselburg 

Technical 

University 

Graz 

Bios GmbH 

Technical 

University 

Vienna 

Joanneum Research 

Forschungsgesellschaft 

mbH 

Austrian Bioenergy Centre 

Inffeldgasse 21b (same address as Bios GmbH) 

A-8010 Graz 

http://www.abc-energy.at/ 

Bios GmbH 

Inffeldgasse 21b – 8010 Graz 

Ingwald Obernberger 

http://bios-bioenergy.at/

Activites: Biomass combustion, plant planning 
85

BLT - Biomass · Logistics · Technology Francisco Josephinum

A-3250 Wieselburg, Rottenhauser Straße 1

Manfred Wörgetter, Leopold Lasselsperger, Günther Friedl

http://blt.josephinum.at/index.php?id=239

Activites: Testing of boilers and stoves, small scale biomass combustion (< 300 kW)

Technical University Graz

Institute for resource efficient and sustainable systems

Inffeldgasse 21 a – 8010 Graz

Ingwald Obernberger

http://rns.tugraz.at/index.php?&lang=_en

Activites: Biomass combustion, CFD modeling

Joanneum Research Forschungsgesellschaft mbH

Institute of energy research

Steyrergasse 17-19 – 8010 Graz

Josef Spitzer, Reinhard Padinger

http://www.joanneum.at

Activites: Biomass combustion, boiler development, process control

Sweden

SP Technical Research Institute of Sweden

Energy Technology

Claes Tullin, Linda Johansson

http://www.sp.se/en/units/energy/sidor/default.aspx

Activites: Emission monitoring

UMEÅ University

Enrgy Technologa and Thermal Process Chemistry

Christoffer Boman

http://www.umu.se/umu/eng/research/index.html

Activites: Combustion modeling

Finnland

University of Kuopio

Department of Environmental Science

Jorma Jokiniemi, Aerosol technology, P.O.Box 1627, FIN-70211 Kuopio

http://www.uku.fi/laitokset/ifk 

Activities: Particle formation and measurement

Finland

Pasi Vainikka

VTT-Energy, PO Box 1603, FIN-40101 Jyväskylä, Finland 

Activities: Biomass combustion, modeling, small scale and large scale (also fluidized bed) 

http://www.vtt.fi/ 
86

Denmark

Department of Atmospheric Environment, National Environmental Research Institute

Fredriksborgvej 399

DK-4000 Roskilde

Department of Chemistry, University of Copenhagen

Universitetsparken 5

DK-2100 Copenhagen

Force Technology

Anders Evald

Hjortekærsvej 99 DK-2800 Lyngby 

Activites: Boiler development (large scale)

Norway

SINTEF NBL (Norwegian Fire Research Laboratory)

Energy Research AS Sem Sælands vei 11, NO 7465 Trondheim

http://www.sintef.no/content/page3____2274.aspx

Activites: Testing of boilers and stoves 

NTNU (Norwegian University of Science and Technology)

Department of Energy and Process Engineering

Johan E. Hustad, Øyvind Skreiberg

http://www.ntnu.no

Activites: Combustion modeling 

Netherlands

Procede Biomass BV

PO Box 328, NL – 7500 AH Enschede

Sjaak van Loo, Jaap Koppejan 

http://www.procede.nl/

Activites: Combustion control, coordination of IEA Bioenergy Combustion task (current task 

leader of International Energy Agency Bioenergy Task 32 „Biomass Combustion an Co-firing)
87

7 Conclusions

7.1 State-of-the-art of biomass combustion in Europe

In many countries in Europe, biomass combustion is widely applied on small and medium scales. 

Small-scale applications are mainly for residential heating. For this purpose, manually operated stoves 

and boilers are commonly applied, while pellet combustion is available as an alternative with automatic 

feeding but does not allow the use of log wood available from forests. 

For log wood boilers, the technical standard was significantly improved in the past 30 years thanks to 

the application of the two-stage combustion principle with primary and secondary air, which is injected 

with mechanical ventilation and followed by a hot combustion chamber. This allows improvements in 

efficiency and a reduction of pollutant emissions, such as CO, which is often used as an indicator for 

the carbon burnout, as well as VOC and PM, which originate from soot and organic condensables. 

This exhibits a serious environmental impact. However, to ensure good combustion conditions in 

practice, operation at low load needs to be strictly avoided. For this purpose, the application of a heat 

storage tank is needed and mandatory in Switzerland. The use of log wood boilers without heat storage tanks and/or with simple combustion technology can lead to excessive pollutant formation. Due to 

a relevant number of existing wood boilers, which do not fulfill these requirements, log wood boilers 

are a relevant source of PM in the ambient air in some countries in Europe, which can be reduced by 

improved technology. For typical applications of log wood boilers in the United States (e.g. “outdoor 

wood boilers”), potential improvement is expected, since these boilers exhibit a fairly poor combustion 

technology, and they are typically operated without a heat storage tank, which is essential for simple 

wood boilers. 

For wood stoves, minor improvements were achieved in Europe, since conventional combustion 

principles without fans are usually applied. Here, ideal operation is essential to avoid high-pollutant 

emissions in practice, such as ignition of the wood from the top instead of ignition from the bottom and 

use of small batches of small dry logs. As for boilers, throttling the combustion air to achieve operation 

at low load needs to be strictly avoided. Since the ideal type of operation is related to frequent adding 

of small logs, wood stoves are often operated non-ideally and also a relevant source of PM in the 

ambient air. Consequently, investigations of PM in the ambient air in alpine valleys during the winter 

season revealed that approximately 80% of the soot is originating from wood combustion. 

To reduce PM emissions from residential wood combustion, small-scale electrostatic precipitators are 

under development and in the market implementation phase. However, the reduction potential is often 

relatively poor (less than 80% and often less than 50%) and is uncertain in practice. PM from residential wood combustion remains an important issue for the future and primary measures are of high 

priority, while precipitation is regarded as an additional measure.

As an alternative to manual operated devices, pellet boilers and stoves are available. Thanks to 

automatic feeding and small fuel size, improved combustion conditions with lower PM emissions can 

be achieved. Pellet combustion instead of manual stoves or badly operated boilers allows a reduction 

of PM emissions. The technology of pellet stoves and boilers in Europe is on a high technical level 

with usually very low emissions under stationary operation. However, boilers and stoves in practice 

are often operated with high excess air, leading to avoidable heat losses. In addition, automatic 
88

ignition and/or part load operation for glow bed maintenance can lead to periods of increased emissions. There is a potential of improvement, which is, however, of lower priority than the improvements 

needed to reduce PM from manual stoves and boilers. 

For medium scale applications from 100 kW to 10 MW, automatic boilers for wood chips are available 

and widely applied. Thanks to high combustion temperature, such plants exhibit usually low emissions 

of uncombusted gaseous and solid carbon emitted as PM but exhibit relevant emissions of inhalable 

particles resulting from ash constituents in the fuel. For plants greater than 500 kW or a few MW 

depending on national emission limits, particle removal mostly by ESP and in some cases with fabric 

filters is applied. However, boilers for heating purposes are often operated at part-load or with periodic 

on/off operation. During such operation modes, the particle removal is often ineffective due to reduced 

flue gas temperatures. Therefore system integration, boiler management, and combustion control 

need to be improved in the future. In addition, there is an increasing need for particle removal for applications smaller than 1 MW, which need to be developed in the next years due to emission limit values 

resuming fine particle removal even for applications smaller than 1 MW, and in Switzerland from 

500 kW. Finally, there is an interest in smaller particle removal equipment, especially for retrofitting 

existing plants that have limited space. 

7.2 Relevance of PM emissions

Data on emission factors of residential wood combustion have been collected from 16 institutions in 

seven IEA countries. Wide ranges of emission factors are reported for residential wood combustion, 

while emission factors of medium and large- scale applications mainly depend on particle removal 

equipment, related to national or local emission limit values. Consequently, conclusions from the 

present study mainly focus on residential applications. 

• The reported emission factors from manual wood combustion devices exhibit wide ranges from less 

than 20 mg/MJ (0.046 lb/MMBTU) under ideal conditions, to more than 5 000 mg/MJ 

(11.6 lb/MMBTU) under poor conditions (data refer to end energy indicated as lower heating value). 

Even national emission factors vary from less than 100 mg/MJ (0.23 lb/MMBTU) (measured as 

solid particles in the chimney) to almost 2 000 mg/MJ (4.6 lb/MMBTU) (measured in a dilution 

tunnel). 

• For wood stoves, wide ranges are found due to different operation conditions. Consequently, high 

priority should be given to avoiding inappropriate operation of manual wood combustion appliances. Excessive PM emissions are found during smoldering conditions at reduced load and 

throttled air supply. This type of operation needs to be strictly avoided but nevertheless seems to 

be relevant in practical operation in many countries. 

• For wood boilers, excessive PM emissions are reported for boilers operated without heat storage 

tank. This is in line with the observation found for stoves, since boiler operation for house heating 

without heat storage tanks often leads to part load combustion. In Switzerland, heat storage tanks 

for log wood boilers are virtually mandatory (i.e., an operation without heat storage tank is not 

permitted except if the boiler fulfills the emission limits at 30% load, which is not achievable with 

nowadays technology). 
89 

• For residential wood boilers, the type of combustion also significantly influences the PM emission. 

Modern boilers with forced downdraft combustion and electronic combustion control devices allow 

low-particle emissions under appropriate combustion conditions, while old-type boilers with updraft 

combustion exhibit higher emissions under similar conditions. However, the influence of operation 

mode cannot be evaluated in detail based on the reported data. 

• For pellet boilers and stoves, typical particle emissions of approx. 30 mg/MJ (0.07 lb/MMBTU) are 

reported with a relatively narrow variation from < 20 to < 60 mg/MJ (< 0.046 to < 0.07 lb/MMBTU). 

The total PM emissions under typical operation conditions are expected to be far lower than for 

manual wood stoves. The problem of variations between ideal operation and inappropriate operation are certainly less emphasized than in manual boilers, although data on poorly operated pellet 

combustion is scarce and the upper-range from pellet combustion is uncertain. It should be noted 

that the market for pellets is growing and the production of second grade quality pellets with higher 

ash content will increase. The use of these pellets in small-scale equipment without particle re–

moval will result in higher emissions (i.e. an expected increase of a factor of 3 to 5 in PM and NOX

for agricultural pellets instead of wood pellets). Domestic scale applications should be restricted for 

good quality wood pellets, while other pellets should be reserved for applications in larger plants 

which are equipped with flue gas cleaning.

• For automatic combustion plants, the emission factors without electrostatic precipitator (ESP) or 

fabric filter (FF) are relatively high, typically between 50 to 100 mg/MJ (0.115 to 0.23 lb/MMBTU), 

which, under good combustion conditions, is attributed to high emissions of inorganic particles. 

Clean gas emissions strongly depend on the type of flue gas cleaning applied, which depends on 

national or local emission standards. In many European countries, emission limit values for such 

applications have recently been made stricter. The situation of typical PM emissions will change in 

many countries in the near future, since particle removal allows clean gas emissions of less than 

30 mg/MJ (0.07 lb/MMBTU) (simple ESP) or less than 10 mg/MJ (0.023 lb/MMBTU) (improved ESP 

or FF). 

• For the comparison of different data, the sampling and measurement procedure needs to be considered. In the present study, three types of data are distinguished: Filterable, solid particles (SP) 

collected on heated filters (at 160° acc. to VDI or at 120°C acc. to EPA), solid particles plus 

condensables (SPC) found by liquid quenching at < 20° or < 5°C, and particulate matter sampled in 

cold, diluted flue gas in a dilution tunnel (DT). A comparison between SP and SPC shows that the 

mass of condensables may significantly exceed the mass of solid particles during poor combustion 

conditions in wood stoves. A comparison between results from a dilution tunnel and sampling in the 

chimney reveals significantly higher concentrations in the diluted flue gas and shows that the 

condensable matter is found partially or quantitatively as filterable material after a moderate dilution 

at a dilution ratio in the order of 10 of the flue gas with cold air. This results in a fast temperature 

reduction at a moderate decrease of the partial pressure of volatile organic compounds. Consequently, PM emission inventories based on emission factors of solid particles may significantly 

underestimate the contribution of biomass to PM in the ambient air. This is in line with results from 

PM measurements in the ambient air, where a higher contribution from wood combustion is found 

than expected from emission factors used currently in Switzerland [Prévot et al. 2006]. On the other 

hand, evaporation of volatiles may occur during further dilution to a dilution ratio of 10 000 in the 
90

ambient air. This can happen because of a significant reduction of the partial pressure of volatile 

organic compounds. The primary aerosols expected in the ambient air might be over estimated by 

measurements in the dilution tunnel. However, for the final PM concentration in the ambient air, the 

formation of secondary aerosols needs to be considered, leading to an additional increase of total 

PM resulting from wood combustion. This is induced by photochemical oxidation to secondary 

organic aerosols (SOA). 

• Due to different samplings of PM, a comparison of data reported from countries with regulations on 

solid particles, such as Germany, Austria, and Switzerland, cannot be directly compared with data 

reported from countries such as Norway using dilution tunnels. Consequently, the type of measurement is indicated in all results presented in this survey, and it is recommended to clearly indicate 

the type of measurement for future emission inventories. For measurements according to US EPA, 

sampling in the stack or in the dilution tunnel can be applied, which may also result in different 

emission factors and which also needs to be considered for comparison with European emission 

factors.

• Condensables from wood combustion have been identified as highly toxic substances that can 

significantly contribute to total PM in the ambient air. For improving emission inventories to calculate ambient air quality levels, it is essential to consider solid particles and condensables found in 

the stack. Furthermore, the potential formation of secondary organic aerosols resulting from volatile 

organic compounds, which are not trapped in the condensables, need to be considered as well. 

Since organic condensables are highly toxic, while solid particle can exhibit low toxicity (in case of 

salts) or high toxicity (if resulting from incomplete combustion or from contaminants such as heavy 

metals), a separate measurement of both parameters is advantageous. However, measurement of 

solid particles only requires less equipment, s less time consuming and still gives a performance

value for PM emissions for comparison of different combustion appliances. In the case of nearly 

complete combustion, as found in modern automatic wood boilers under good operating conditions,

the concentration of condensables is usually insignificant. 

• A survey of equipment used for particle size measurement reveals that cascade impactors are 

used in almost all laboratories dealing with particle measurement. In addition, half of the participating institutions use scanning mobility particle sizers (SMPS) and/or electric low-pressure 

impactors (ELPI). Optical particle counters (OPC), aerodynamic particle sizers (APS), and tapered 

element oscillating micro-balances (TEOM) are currently used. 
91

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BIOMASS COMBUSTION IN EUROPE OVERVIEW ON TECHNOLOGIES AND REGULATIONS

FINAL REPORT 08-03

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BIOMASS COMBUSTION IN EUROPE OVERVIEW ON TECHNOLOGIES AND REGULATIONS NYSERDA Final Report 08-03