## N EW 

## Y ORK 

## S TATE 

## E NERGY 

## R ESEARCH AND 

## D EVELOPMENT 

## A UTHORITY 

## F INAL 

## R EPORT 

## 08-03 A PRIL 

## 2008 

# B IOMASS 

# C OMBUSTION IN 

# E UROPE 

# O VERVIEW ON 

# T ECHNOLOGIES
AND 

# R EGULATIONS

### 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 $mart SM 

### 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 radioactivewaste 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 $mart SM 

### 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 $mart SM 

### 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

S TATE OF 

### N EW 

### Y ORK 

### E NERGY 

### R ESEARCH AND 

### D EVELOPMENT 

### A UTHORITY 

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

# B IOMASS 

# C OMBUSTION IN 

# E UROPE 

# O VERVIEW ON 

# T ECHNOLOGIES AND 

# R EGULATIONS

Final Report

Prepared for the 

## N EW 

## Y ORK 

## S TATE 

## E NERGY 

## R ESEARCH AND 

## D EVELOPMENT 

## A UTHORITY Albany, NY www.nyserda.org

Ellen Burkhard, Ph.D. Project Manager

Ray Albrecht Project Manager

Prepared by: 

## VERENUM SWITZERLAND
Thomas Nussbaumer

April 2008 NYSERDA Report 08-03 NYSERDA 947723 

# Content s

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 impac t 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 pollutan ts

................................ ................................ ................................ ................. 17 3.5.3 NO X 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 Page45 

# 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 com bustion
techno logies applied in Europe today , an assess ment on particulate m atter emissions smaller than 10 microns (PM 10 ) , and an overview of regulations and norms on emissions from wood combusti on 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 com bustion are presen

ted. 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 sub

sequent gas phase oxidation with secondary air. This technology i s applied to modern type boi lers by application of the downdraft principle , while wood stoves are mostly based on conventional up

draft combustion . In addition, manually

- operated wood combustion stron gly influences the emis

sions 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 magni tude. Solid particles and condensables from incomplete wood com bustion 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 com bustion of wood exhibit a signi fi

cantly lower health impact than Diesel soot.
2. Inorganic particles and NO X 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 NO X 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 NO X , 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 NO X reduction are available as secondary measures for pollutant reduction.
3. Contaminants such as heavy metals and chlorine can lead to additional airborne emissions.

Based o n a two

- stage combustion principle
, and with application of electronic combustion control de

vices, 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 3 0 mg/ MJ or 0.070 lb/MMBTU ). However, to ensure good operation
conditions for residen tial heating applications, a heat accumulation tank is needed for log wood boilers to avoid part

- load ope ra tion

. 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 electronic6combustion 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
boil e rs 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.

Keywo r ds: Wood combustion, pollutant formation, pollutant reduction, particulate matter, regulations7 

# 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 CO 2 free, biomass combustion is an important source of air emissions, especially inhalable particulate matter smaller than 10 microns (PM 10 ) in the ambient air, polycyclic aromatic hydrocarbons (PAH), volatile organic compounds (VOC), nitric oxide emissions (NO X ), 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 accumulationtank 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 w ith a hot combustion chamber and good mixing of combustion air with combustible gases.
Howe v er, 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
exten t by providing information and measures to the consumers.

• Beside s technical improvements for residential wood heating appliances, it is also essential to prohibi t burning of wood and other biomass in open fires, as well as private waste incineration. These type s 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 ensuredby 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 bein g developed and offers a certain potential for pollutant reduction for existing and ne w wood heating appliances.

• Finally, wood fuel can be used in automatic biomass combustion plants, which can be equippedwith 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 P M emissions.

Improv e ments 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 shortperiods 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
duri n g winter originates from wood c o mbustion [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,
wild f ires, and residential backyard burning

• Residential wood combustion in woodstoves and fireplaces

In add ition

, the Northeast States for Coordinated Air Use Management identifies in their assessment of
carb o naceous PM 2.5 for New York and the region seven major sources for emissions [NESCAUM and MJB Associates 2008]: Five mobile source sectors (non r oad equipment, heavy-duty trucks, light-duty vehicles, airports

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

com mercial cooking). There is a specific need for improved information on emissions from residential w ood 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 semivolatile9 organic compounds, can significantly contribute to the total PM con centration 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 o f state

- of

- the

- art technologies for wood combustion in Europe. The assess ment of technologies and applications will focus on e fficiency and on air pollution with special attention to particulate matter in the ambient air resulting from wood fuel applications

. Hence, testing proce dures
for PM emissions in Europe wi ll be described and compared with testing according to US stan dar ds

. O ngoing R&D activit i es in Europe shall be discussed and recommendations for future develop ments in
the State of New York to reduce PM from wood combustion wi ll be provided

. 

# 2.3 Conversion of emission factors

Gaseous emissions are usually measured in volu me concentrations such as Vol.

- % or Vol.

- ppm, while gravimetric measurements of particles (PM) result in mass concentrations such as mg/m 3 . Gravimetric PM emissions in mg/m 3 can be compared to PM concentrations in the ambient air, which are also
measured a s mass concentrations and usually given in μ g/m 3 . For specific gas compounds such as
CO or NO 2 , measurements in volume concentrations can be directly converted to mass con cen
trations. To allow a comparison of emission factors achieved at different excess air ratios, a con

version to a reference oxygen concentration is performed. Consequently, European norms and re

gulations often use mass concentrations calculated at a reference O 2 for the definition of emission limit values. In the present report, data i n mg/m 3 at 11 vol.

- % O 2 or 13 Vol.

- % O 2 (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 re

fe rence 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 ca n be neglected for a generic inter

pretation. 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 alternative s , tod a y , the majority of the wood fuel is used in combustion applications, which are available as proven technology.
Bioma s s 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 than10 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 specific12 cost and the efficiency of steam plants are only adequate at large

- scale applications. Hence
, co
- com
bustion of biomass with coal is promising, as it combines high efficiency with reaso nable transport
distances for the biomass.
However
, biomass combustion is oft en related to significant pollutant formation and needs to be im

proved. To develop measures for emission reduction, the specific fuel properties need to be con

sidered. P ollutant formation occurs due to three reasons:

1. Incomplete combustion can lead to h igh emissions of uncombusted pollutants such as CO, soot, organic condensables (“tar”), and polycyclic aromatic hydrocarbons ( PAH )

. 2. Pollutants such as NO X and particles are formed as a result of natural fuel constituents such as N, K, Cl, Ca, Na, Mg, P, a nd 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 di ben zo
p

- dioxins and furans (PCDD/F).

B iomass combustion ex hibit s relatively high emissions of NO X and submicron particles. Air staging and fuel staging can be applied as primary measures for NO X reduction that offer a potential of 50% to
80% reduction . For further reduction, secondary measures such as selective c atalytic 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 t hat 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 gasif ication 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 nee ded for medium and large

- scale applications. In addition, there is also an interest in secondary measures for future applications in residential wood com bustion.



# 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 NO X , and ash components (e.g. K and Cl as a source of KCl) that lead to particulate emissions. Native wood isusually 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 NO X 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 l arger 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 CO 2 neutrality of sustainable cultivated biomass or the use of biomass residues and waste. Large potentials of both13 native biomass a nd biomass wastes are still available and allow a relev ant increase of sustainable bio
energy use in the future. Combustion is the most important technology available to day for biomass

u se . Improvements in efficiency, emissions, and cost are needed for further exploitation. In addition
, alternatives such as ga sification and combinations of different processes such as gasification as fuel
pre
- treatment for co

- combustion need to be con sidered.



# 3.3 Environmental impact of biomass combustion

Biomass combustion exhibit s relatively high emis si ons of NO X and particul ates in compa rison to the
combustion of natural gas or light fuel oil. Hence, biomass combustion con tri bute s signi ficantly to particulate matter (PM), ozone, and NO 2 in the ambient air. For wood com bus tion, a life cycle assess

ment (LCA) indicates th at 38.6% of the environ mental impact of a modern auto matic wood furnace is
attributed to NO X , 36.5% to PM 10, only 2% to CO 2 and 22.9% to all other pollutants ( Table 3

. 1 ) . The LCA for wood, light fuel oil, and natural gas also show that the environ mental impact of wood is higher
than that for natu ral gas for a standard valuation of the greenhouse effect. Hence, im provements in the
wood chain are neces sary. How ever , it is also evi dent that the con clusions of the LCA strongly depend on the valuation of the greenhouse effect
, since the ranking changes significantly as a result of the
different CO 2 impacts of the three fuels. On the other hand, in case of poor combustion conditions in manually operated wood stoves o r 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 hyd rocarbons including methane, even the green

house 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 environ

mentally 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] [%]

NO X 13 030 38.6%

PM 10 12 600 36.5%

CO 2 670 2.0%

SO X , NH 3 , CH 4 , NMVOC, pri ma ry energy, residues, and others 8 200 22.9%

Total 34 500 100%14 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
bet w een a volatile and a char combustion phase with time (Fehl e r! 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 wood15 Since auto matic com bus tion systems are ope ra ted continuously, the con se cutive reactions occur
simul taneously at different places in the furna ce (e.g., in different sections on and above a grate). Therefore, the zones for different steps during com bus tion can be optimized by fur nace design. A distinct sepa ration of different steps can be advantageous with res pect to pollutant formation.
Th e main com bus tion parameter is the excess air ratio (lambda; λ ) that describes the ratio between
the lo cally available and the stoichi ometric amount of com bustion air. For typical biomass, the com

bustion reaction can then be descri bed by the followi ng equation
, if fuel con sti tuents such as N, K, Cl etc. are neglected:

CH 1.44 O

0.66 + λ 1.03 (O 2 + 3.76 N 2 ) → Intermediates (C, CO, H 2 , CO 2 , C m H n etc.)

→

CO 2 + 0.72 H 2 O +( λ

–

1) O 2 + λ 3.87 N 2 – 439 kJ/kmol

where CH 1.44 O 0.66 describes the averag e 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. Un combusted pollutants such as CO, C X H Y , PAH, tar, soot, carbon, H 2 , HCN, NH 3 , and N 2 O

2. Pollutants from complete combustion such as NO X (NO and NO 2 ), CO 2 , and H 2 O

3. Ash and contaminants such as ash particles (KCl, etc.), SO 2 , HCl, PCDD/F, Cu, Pb, Zn, Cd etc.



# 3.5 Reduction of airborne pollutants

3.5.1 Staged combustion

If staged com bustio n

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 co mbus tible gases formed by devolatilization and gasi fi cation in the fuel bed. If good mixing occurs , an opera tion at low excess air is possible (i.e., excess air lambda < 1.5)
, thus enabling high efficiency and high tempe rature ( Figure 3

. 4 ) with complete burnout ( Figure 3

. 5 ). If good mixing is achieved, the concentrations of
uncombusted pollutants can be re duced to levels close to zero (e.g. CO < 50 mg/m 3 and C X H Y < 5 mg/m 3 at 11 vol.

- % O 2 ). 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 com bus tion, primary air needs to be understoichio metric (lambda primary < 1). A relevant resid ence 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

ex cess 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 NO X in biomass c ombustion and are described below.

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 tempe ra ture, 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 de mands for temperature (around 850°C) and residence time (around 0.5 s) can easily be achie ved. Suffi cient mixing quality can be achieved in fixed

- bed com bustion by the above

described two

- stage com bus tion. In a fluidized bed, good mixing is achieved in the bed and the free

board and dust combustion easily allows good mixing.
For future im provements 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 heterogene ous re actions during dry ing, transport, devolatilization, and gasi fication 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 - t he example shows the velocity distribution in the combustion chamber over a grate with secondary air nozzles and post combustion chamber [Nus s baumer 2003].



## 3.5.3 NO X 

## emissions

In combustion processes, NO and NO 2 (summarized as NO x ) can be formed in three diffe rent reactions. Thermal NO x and prompt NO x are formed from nitrogen in the air at high tempe ratures and in the case of prompt NO x in the presence of hydrocarb ons. Fuel NO x can be for med from nitrogen

con taining fuels. For biomass com bustion, fuel

- bound nitro gen is the main source of NO x emis sions, while thermal and prompt NO x are not relevant due to relatively low temperatures
, as has been shown by theore tical and experimental investigations. Fuel nitrogen is converted to intermediate com ponents such as HCN and NH i with i=0, 1, 2, 3. These
can be oxidized to NO x if oxygen is available, which is the case in conven tional combustion. If no oxy

gen is presen t, intermediates can interact in the re duction zone and form N 2 in reactions such as NO +
NH 2 = N 2 + H 2 O ( Figure 3

. 7 ). During the past 10 years, staged com bustion techno lo gies have been developed as a primary meas ure for internal NO x reduction based on this concept
, thus leading to the
above

- described tech niques of air staging and fuel staging ( Figure 3

. 8 ). Both measures allow a NO x reduction of up to 50% for wood with low nit rogen content and up to 80% for bio fuels with high nitrogen content. However, diffe rent specific conditions have to be met accurately to exhaust this reduction potential.

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 th e 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 appli

cation in practice due to undesired ash softening and de po sit formation. For fuel staging, similar
results are achie ved at lower temperature s ( i.e., at tem pe rature s as low as 850°C ) . However, the
furnace con cept and ope ration is more complex due to the need of two inde pen dent fuel feeding systems. Neverthe less, 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 opera tion at the excess air ratio needed in the
different zones. Beside primary measures, secondary measures are available for NO x abatement. The most relevant
techni ques are selective noncatalytic reduction (SNCR) and selective catalytic reduction (SCR) using
the same re action a s mentioned for staged combustion, i.e., NO + NH 2 = N 2 + H 2 O. However, urea or ammonia is injected as a reducing agent and as a source of NH 2 . SNCR has to be applied in a narrow temperature window of around 820°C to 940°C

, thus al l ow ing a NO x reduction up to 90%. SCR is typically applied in the flue gas in a temperature range of 250° to 450°C and allows a NO x reduction of more than 95%. However, relevant con centrations of undesired side products such as HNCO, N 2 O,
NH 3 , HCN

, and others can be formed in both types of secondary measures under unfavorable con
ditions. P rimary measures are preferable if they can achieve sufficient emission reduction.20 

## 3.5.4 Particulate emissions

Biomass combustion leads to relatively high emis sions of particulates

. The majority of th e particulates are smaller than 10 micron ( μ m) (i.e., particulate matter PM 10 ) with a high share of sub micron particles (PM 1 ).
S ubmicron and supermicron par ticles in fluidized

- bed combustion are composed of fine par ticles of K, Cl, S, Na and Ca and coarse particles of Ca, Si, K, S, Na, Al, P, F e. In fixed

- bed com bustion, in

creasing mass con centrations of particulate emis sions are typically re lated to increasing mean
diameter. A de pendency of the particle com position on size can also be found in fixed bed con ditions, as K, S, Cl and Zn i s mainly found in the submicron fraction, while the content of Ca is in creasing with
in creasing particle size. If almost complete burnout is achieved by appropriate furnace design, the
result ing parti cu lates are almost exclu sive ly formed from ash com ponents in the fuel with salts such as KCl as main components. The main fuel consti tu ents
, with res pect to aerosol for mation
, are typically K, Cl, S, Ca, Na, Si, P, Fe, and Al. Pri mary measures that can safely meet a high re duction potential by at le ast a factor of 10 in this category of aerosols are not known so far.
R ecently a new approach for primary particle reduction has been presented. It was shown that par

ticles from wood combustion are mainly formed by nucleation, coagulation, and condensati on in the
boiler during a decreas ing 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 tem perature. If oxygen
is available at a high temperatur e, a high share of K can be oxidized. K oxi des have significantly lower devolatilization temperatures than the K salts , and they are almost com pletely va porized into the gas phase and lead to particle formation from the gas phase. If no oxygen is presen t 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 conver sion 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 operation al conditions, the glow bed height increases significantly
, and the furnace design must be adapted. Furthermore, combustion becomes un stable below a certain primary air

excess , and an accurate process control is needed. R esults from a laboratory furnace show that a reduction of inorganic particle emissions by a factor of 5 is achievable. T he results are in accordance
with the theory, although other effects may influence the particle formation. Nevertheless, since fuel com po sition i s the main para meter for aerosol for mation, se condary me

asures are necessary for further re duction in the future. Among those measures , fabric fil ters are promising due to their high separation efficiency, while electrostatic precipitation is more ro bust 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 -f ired furnaces.
Dis t rict heating systems range in s i ze from 0.5 MW th to 5 MW th with some applications up to 50 MW th . 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 MW e and 10 MW e . Stirling engines (10 kW e to 100 kW e ) 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 MW e ), 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.
Sin c e wood pellets are well suited for automati c heating at small heat outputs as needed for today ’ sbuildings, 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
app l ied 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/m 3 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. Beside s conventional grate furnaces operated with whole bales, cigar burners and
othe r 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 industrialwastes 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 opportunitiesfor 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 NO X emissions are achieved.23 Tab le 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%

Un derstoker 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 combus tor 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/h24 Figure 3

. 9 Furnace types and flow conditions: Fixed bed, fluidized bed, and entrained flow reactor25 Figure 3

. 10 Downdraft boilers with inv erse 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 hoppe r, 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 z one and post com bus tion 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.

Figure 3.12 Schematic of an understoker boiler with “low particle concept” - The figure shows a 100 kW prototy pe pellet boiler. The cross-sect i on 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 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 Bott o m: 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) Bott o m: 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
emis sions. Lower specific cost and higher efficiencies of large plants can be used for biomass, and
emi s sions of SO X and NO X 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
app l ied:

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 physi cal pre

- processing of the biomass such as drying, grinding, or metal removal.
2. Indirect co

- firing : The biomass is ga sified and the product gas is fed to a boiler fur nace (a combination of gasification and combustion).
3. Parallel combustion : The bio mass is burn ed in a separate boiler for steam generation. The steam is used in a power plant with the main fuel.

Co

- combust ion of biomass leads to a sub stitution of fossil fuels and to a net reduction of CO 2 emissions. In many countries co

- firing is the most eco nomic al tech no logy to achieve the target of CO 2
re duction

. B io mass co
- firing can therefore be motivated by savi ngs of CO 2 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 μ m, fine particles (0.1–1 μ m or 0.1–2.5 μ m) and ultrafine particles < 0.1 μ m are distinguished. PM 10 defines particles as having an aerodynamic diameter < 10 μ m. This includes both coarseand 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. Beside s mass concentrations,
add itional parameters are relevant for the potential impact of particles on environment and human
hea l th. 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 (SO 2 , SO 3 ), nitrogen oxides (NO, NO 2 ), ammonia (NH 3 ), 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: SO 2 , SO 3 and NO X ), agriculture (NH 3 ), Diesel engines (NO X and SO 2 ), use of solvents, chemical industry and petro chemistry (NMVOC) [ UBA 2005]. Beside secondary aerosols that originate from NO X and SO X , 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 at mo
sphere 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 re

sulting in a reduced partial pressure of the respective compounds in the surrounding atmosph ere.

Consequently, o rganic PM from residential wood heating in the ambient air can be under estimated 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 comb ustion show a distinctive increase in PM con

centration 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: M easurement 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 s ignificant increase in particle mass. The further dilution of the flue gas, evaporation, and desorption of volatile and semi-vol a tile compounds due to a reduction of the partial pressure in thesurrounding 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 PM 10 concentration in33 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 16 0°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 PM 10 and VOC can exceed the mass of particles and condensables detected in the stack
or imme diately after an initial dilution, which is due to the added mass of oxygen available by secondary oxi dation. To illustrate the potential effect of added oxygen, Figure 4

. 3 shows a fictitious example, where the resulting mass of PM 10 exceeds the mass of solid particles and condensables in
the stack.

Figure 4

. 2 Conversion of stack emissions to PM 10 in the ambient - The graph shows a virtual example with total SOA = 1.8 TSP as measured in the stack and consequently PM 10 = 2.8 TSP34 Figure 4

. 3 Conversion of stack emissions of PM and organics to PM 10 in the ambient - The graph shows a vir tual example with total SOA = 2.8 TSP as measured in the stack and consequently PM 10 = 3.8 TSP. Here, the mass of PM 10 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 PM 10 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. PM 2.5 and PM 1 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 PM 10 and PM 2.5, 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

act i ons to achieve air quality that protect human health. Cou n tries are encouraged by WHO to adopt an increasingly stringent set of standards, and track progress through the monitor i ng 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 ]

. Beside s the guideline value, three interim targets were defined, since countries may find these interim

ta r gets particularly helpful in gauging progress over time in the process of steadily reducing population exposures to PM. Due to WHO ’ s Ai r 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.

PM 10 [ μ g/m 3 ] PM 2. 5 [ μ g/m 3 ] 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 level s 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.

A ir Quality Guideline (AQG) 20 10 These are the lowest levels at which total cardiopul monary and lung cancer mortality have been shown to increase with more than 95% confidence in response to long

- term exposure to PM 2.5.

Table 4

. 2 WHO air quality guidelines and interim targets for particulate matter: 24 hour concentrations (99th percentile (days/year)

PM 10 [ μ g/m 3 ] PM 2.5 [ μ g/m 3 ] Basis for the selected level

Int erim 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 levels37 

# 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 PM 10 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 t h e yearly average is 40 μ g/m 3 and the daily average (24

- hour mean) 50 μ g/ m 3

. 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 μ g/ m 3 and the daily average limit will be 50 μ g/m 3

. T he daily average will be allowed to exceed this amount only 7 days per year.

Table 4.3 Ambient PM standards for PM 10 (except USA: PM 10 and PM 2.5 ). *indicative value

Country Annual limit value

[μg/m 3 ] 24

- hour li mit value

[μ g /m 3 ] 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 exc eedance p.a.
Norway 35 50
WHO (AQG) 20 50
WHO (AQG) 10 25

USA PM 10 150 1 exceedance p.a. on average over 3 a

USA PM 2.5 15 35 98% for an average over 3 a38 

# 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/m 3 at 13 Vol.-% O 2 for natural wood chips and up to several 100 mg/m 3 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 incompletecombustion 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 temper a ture. Highly toxic PAH may also be formed as secondary tar in subsequent gas phase reactions in

t h e 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 partic les 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 - P ictures from Heuberger, EMPA Dübendorf, published in [Klippel & Nussbaumer 2007 b].40 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 t o VDI and indi c ating only solid particles without condensables at 13 Vol.-% O 2 [Klippel & Nussbaumer 2007 b]

Table 4

. 4 PAH concentrations in particles from badly operated wood stove and in Die sel 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 Chrysen e 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 5341 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]

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

c h romosome aberration onset is indicated [ K lippel & 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 residen c e 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) wi t h 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 wit h high dilution ratio (DR > 100) PM 10 : 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

*SO 2 and other soluable gaseous compounds in the flue gas may be dissolved in the impingers **In case of determin a tion 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 ambient43 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 s o lid particles ( SP ) in the study and consist of filterable particles and droplets at the indicated tempe r ature. 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 K 2 SO 4 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 partic u late emissions are sampled in a series of impinger flasks containing water to collect inorga n ic 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 SO 2 . 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 dilut i on 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

. F urther dilution may lead to a subsequent loss
i n 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 d i lution sampling (DS) in the present study. To mimic conditions in the ambient air, sampling can be44 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 gather s information on the particle size and number instead of the particle mass.

Fo r 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.
Me a surements 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, massconcentration 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 PM 10 or PM 2.5 . 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 PM 10 and PM 2.5 , Tapered Element Oscillating Micro-balance (TEOM) i s a well established instrument. TEOM is often used for measurement of
PM 10 and PM 2.5 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
d e tect transient conditions such as the start-up phase of combustion devices.

a) Mass size distribution For measurements of mass size distributions, low

- pressure ca scade impactors (e.g. Andersen Impactor, Dekati Low Pressure Impa c tor 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 u ltrafine 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) i s 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 distri

butions. It is important to respect which diameter is considered to perform conversions from num ber to
mass concentration.

P article 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 emph asized 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 cou ntries [ Nussbaumer et al. 2007 ] . The range for typical measu

red emis sion 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/MMBT U). The emission factor s 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 result s 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 repre

sents a b road 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 per

sis tently higher results, which range from 340 to 544 mg/MJ (0.79 to 1.26 lb/MMBTU).
National emission fac t ors are the basis for calculations of emission inventory reports. One of the mostimportant 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 of400 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 Figu re 4

. 12 shows t he 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 stoves50 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 two

stage combustion with primary air for the solid fuel conversion, a subsequent mixing zone with injection of secondary air followed by the p ost 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 th e 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 avail able from a limited number of institutions only. If compared to data from the same countries, updraft boilers exhibit com

parable ranges of emissions as wood stoves. Figure 4

. 16 shows emission data from log wood boiler s with forced draft. Under typical operation con

ditions, 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 in fluence the
emis sions in practical operation. Figure 4

. 17 shows a comparison of emission factors of log wood boi

lers with and without hea t 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 com bustion princi ple 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 combustion52 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 relativ ely mo derate increase in emissions is expected for pellet combustion. In comparison to log
wood com bustion, emission factors at non

- ideal operation are at least a magnitude lower, which is re

garded as the main advantage of pellet combustion. However, ma ny 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 incre ased emissions of particulate matter (typically by a
factor of more than 3), as well as in emission of NO X . 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 control54 Figure 4.21 Range of emission factors from worst to best (where available) for pellet boilers55 

## 4.7.6 Overview on residential wood combustion

Figure 4

. 22 gives a summary of the average emission factors for pell et 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. Themain 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 outp ut by continuous manual feeding in short periods and assumes an optimized start-u p 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 wi th natural updraft combustion. Two

- stage combustion with forced draft is regarded as a significant improvement. However, the type of ope

ration of log wood boilers in practice strongly depends on the hydraulic integration in the heating system. For convent ional house heating, a combination with a heat storage tank is usu ally needed to avoid sharply throttled air inlet and excessively high emissions.

• For wood stoves, measurements including condensables found in impinger bottles and measure

ments of particl es in diluted flue gases in a dilution tunnel are avai lable. A comparison of these data with measurements of solid particles reveals significantly higher average emission factors.
Since the data shown in Figure 4

. 22 ar e derived from different investigations and different com bus

tion devices, they do not allow a direct comparison between the different measurement me thods. However, the significantly higher (by a factor of roughly 4) average emission factor reported from

measure ments in dilution tun nels, 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 condi tions. 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 from70 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/m 3 at 11 or 13 Vol.-% O 2 ).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
p eriodic monitoring of such plants is crucial. The same is true for plants from 500 kW to 10 MW, as

s h own 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 Switzerland 1 , 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
f actors are ind i cated as particle mass concentrations and do not respect the health relevance ofdifferent 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/m 3 at 11 Vol.-% O 2 are valid for plants > 1 MW from 9.1.07 instead of formerly 150 mg/m 3

. 20 mg/m 3 at 13 Vol.-% O 2 are introduced for plants > 500 kW in 2008 instead of formerly 150 mg/m 3

.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 kW58 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 MW59 

# 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 addi tion, data is available from measurements on wood stoves in Switzerland with separate sampling of solid particles and subsequent condensable matter, thus resulting in SP ac cor ding 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 con

den sables in impingers. Figure 4

. 28 illustrates the ratio of DT/SP, when measured in the dilution tunnel, and of SPC/SP when
measured with conden sables found in impingers. In one case (Verenum stove 3), which corresponds to a high com bustion quality, resulting in low tot al 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

ar e 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 4.2761 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, respectively6263 

# 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 Resi dential space heating appliances fired by wood pellets Stov e

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 f ür 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 stati onären 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 A bgasanlagen

- Allgemeine Anforderungen

EN 14785 Raumheizer zur Verfeuerung von Holzpellets

- Anforderungen und Prüfverfahren

EN 18160 Abgasanlagen, teil 1

- x

EN 1856 Abgasanlagen

- Anforderungen an Metall

- Abgasanlagen64 EN 267 Ölbrenner mit Gebläse

- Begr iffe, 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 Fe ste 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

- Verfahre n 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 15104:200 5

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
Subst anzen; 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 Fass ung 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

- Staubm essungen in strömenden Gasen65 

# 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 isdefined in Method 5H (stack location) or 5G (dilution tunnel). Only particulate matter measurements are mandatory for a wood heater certification. Methods for NO X , CO, SO 2 measurements exist but are not part of the certification process.66 

# Test fuel requirements (wood)

CEN 13240 CEN 303

- 5 EPA metho d 28 EPA method
28 OWHH
min 12% 12% 16% 16% Woo d h u midity (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) 4 in. 4 in. Wood size height Manuf. (1) Manuf. (1) 4 or 2 in. 4 in.

Wood type Beech, Bi rch 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 h eater (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 met hod 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: tabl e 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 1267 

# 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) E ither 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 768 

# Heat extraction

Europe USA
Heater Boiler Heater Boiler

CEN 13240 CEN 303

- 5 EPA
method 28 EPA method 28
OWHH Temperature input ? T in = T out - T diff 10°K < T diff <25°K - ?

Temperature output 75°C < T out < 85°C 70°C < T out < 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 1369 

# Operation mode Europe USA Heater Boiler Heater Boiler
CEN 13240 CEN 303

- 5 EPA method 28 EPA method 28 OWHH
1 min. charge Q min =0.25*P n (1) Q min =0.3*P n (2) Q min < 4 kW (*) Q min =0.15*P n (1)

2

- - P 2 min = 4 kW P 2 max = 6.25

kW (**) P 2 min =0.16*P n (1) P 2 max =0.24*P n (1)

3

- - P 3 min = 6.25 kW P 3 max = 9.5

kW(***) P 3 min =0.25*P n (1) P 3 max =0.50*P n (1)Operating charges

4 max. charge P n (1) P n (1) P max P n (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 (8)Operating mode 4

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 lengthOperating modes 1-3

End t min (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(Q n ) the amount of wood is a function of the heat output of the wood heater
B fl = 360 000 * Q n * t b / (H u * η )
B fl = fuel mass (Brennstoffaufgabe) in kg)

t b = 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/m 3 (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 consumed70 (*) 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 heati ng 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: sect ion 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 8.1 2.1, 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 1471 Test duration

EN 303-5: 1) Boiler with manual fuel feeding

• PM only at full load: 2 charges (including re-filling second batch), discont in u ous

• C O, 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, discont in u ous

• C O, 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 Boil er Heater Boiler Heater Boiler

EPA 28 EPA 28
OWHH EPA 28 EPA 28
OWHH CEN 13240 CEN 303

- 5
5

- G 5

- H 5

- G 5

- H Efficiency yes yes
- yes
- yes O 2 yes yes
- - - - CO yes yes
- - - - NO X - optional

- - - - VOC

- yes
- - - - Particle sampling chim ney chimney dilution
tunnel chimney dilution
tunnel chimney

Solid

– at 180°C at < 32°C at < 120°C at < 32°C at < 120°C Particles
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 E PA: §6.534, page 407 of Subpart AAA

• EPA method 28 OWHH §9.1.1.5.1, page 873 

# Measurements methods

1. The efficiency

T he 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
mea sured.

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 Euro pe 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 3 03

- 5 [mg/m 3 ] at 10% O 2 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 12 500 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 limi t 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 39 20 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

S witzerland 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

manu al 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 1 020 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 M W 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 boile r 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 heate r 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 1178 

# Comparison of PM standards in Europe and US A
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 proce dure s and complex averaged weighing of emission data

. To en able an in di

cative comparison, a fictitious example for EPA method 28 is given below. However, according to
EPA
, the difference c aused by different sampling that may lead to higher PM data due to con

densables
, 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 stov e 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. F ig. 1b Emission limit value on PM - T here is no limit value for PM fo r 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 S tates. 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 l arge 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

r esearch 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 activ ities 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 15 th European Biomass Conference

Figure 6

. 2 Distribution of topics in the 15 th European Biomass Conference82 

# 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 gase s (CO and HC), as well as particulate

matter

– PM by secondary measures (both from uncombusted pollutants and from mineral matter)

– NO X 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, h igher 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 La ngmauerstrasse 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 comb u stion technologies

Ö kozentrum 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 planning85 BLT

- Biomass · Logistics · Technology Francisco Josephinum A

- 3250 Wieselburg, Rottenhauser Straße 1 Manfred Wörgetter, Leopold Lasselsperger, Günther Friedl http://blt.j osephinum.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, modeli ng, small scale and large scale (also fluidized bed) http://www.vtt.fi/86 Denmark

Department of Atmospheric Environment, National Environmental Research Institute Fredr iksborgvej 399 DK

- 4000 Roskilde

Department of Chemistry, University of Copenhagen Universitetsparken 5 DK

- 2100 Copenhagen

Force Technology Anders Evald Hjortekærsvej 99 DK

- 2800 Lyngby Activi tes: 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: Co mbustion 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 ta sk (current task leader of International Energy Agency Bioenergy Task 32 „Biomass Combustion an Co

- firing)8 7 

# 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 opera ted 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 secon dary air, which is injected
with mechanica l ventilation and followed by a hot combustion chamber. This allows im provements 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 m andatory in Switzerland. The use of log wood boilers without heat sto

rage 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
im proved technology. For typical applications of log wood boilers in the United States (e.g. “outdoor wood boilers”), potential improveme nt is expected, since these boilers exhibit a fairly poor combus tion 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 c onventional com bustion 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 rele vant 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 res idential wood combustion, small

- scale electrostatic preci pitators 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 resi

dential 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, aut omatic8 8ignition and/or part load operation for glow bed maintenance can lead to periods of increased emiss ions. 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 f uture. 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

r esuming 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 emission s

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

- s cale applications mainly depend on particle removal equipm ent, 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 combustio n 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 f rom 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 pri ority should be given to avoiding inappropriate operation of manual wood combustion appli

ances. 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 sto rage tank. This is in line with the observation found for stoves, since boiler operati on for house he ating 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 fulfil ls the emission limits at 30% load, which is not achievable with nowadays technology).8 9 • For residential wood boilers, the type of combustion also significantly influences the PM emis sion.
Modern boilers with forced downdraft combustion and electronic comb ustion control devi ces allow
low

- particle emissions under appropriate combustion conditions, while old

- type boilers with updraft

com bustion 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 ope

ration are certainly less emphasized than in manual boilers, although data on poorly operat ed 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 pel lets 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 NO X for agricultural pellets instead of wood pellets). Domestic scale applications should be restricted for
g ood quality wood pellets, while other pellets should be reserved for appli cations 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 appli ed, 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, sin ce 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 con

sid ered. 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 bet ween SP and SPC shows that the mass of condensables may significantly ex ceed the mass of solid particles during poor combustion
conditions in wood sto ves. A compa rison between results from a dilution tunnel and sampling in the chimney re veals significantly higher concen trations in the diluted flue gas and shows that the condensable matter is found par tially or quantitatively as filterable material a fter 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. Conse

quently, PM emission inventories based on emission factors of solid particles may significantly
under estimate the contribution of biomass to PM in the ambient air. This is in line with results from
PM measure ments in the ambient air , where a higher contribution from wood com bustion 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 the9 0 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 c omparison of data reported from countries with regulations on solid particles, such as Germany, Austria, and Switzerland, cannot be directly com pared with data
repor ted from countries such as Norway using dilution tunnels. Con sequently, the type of meas ure
ment is indicated in all results presented in this sur vey, and it is re commended to clearly indicate the type of measurement for future emission in ventories. For measurements according to US EPA, sampling in the stack or in the dilution tunnel can b e 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 cansignificantly 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 per formance 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 measuremen t reveals that cascade im pactors are used in almost all laboratories dealing with particle measurement. In addition, half of the par tici

pating institutions use scanning mobility particle sizers (SMPS) and/or electric low

- pres sure impactors (ELPI). Opti cal particle counters (OPC), aerodynamic particle sizers (APS), and tapered element oscillating micro

- balances (TEOM) are currently used.9 1 

# 8 References

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mass com bustion systems, 3 rd International Workshop on Aerosols from Biom ass Combustion, International Energy Agency (IEA) Bioenergy Task 32, Jyväskylä (Finland), 3 September 2007, www.ieabcc.nl Kaufmann, H.; Nussbaumer, T.; Baxter, L.; Yang, N.: Deposit formation on a single cylinder durin g combustion of herbaceous biomass. Fuel 2000, 79, 141

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# B IOMASS 

# C OMBUSTION IN 

# E UROPE 

# O VERVIEW ON 

# T ECHNOLOGIES AND 

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### ., C HAIRMANBIOMASSCOMBUSTION INEUROPEOVERVIEW ONTECHNOLOGIES ANDREGULATIONSNYSERDA Final Report 08-03