Emission Controls for

Small Wood­Fired Boilers

Prepared for:

United States Forest Service,

Western Forestry Leadership

Coalition

February 2010

DATA      ANALYSIS        SOLUTIONS
REPORT PREPARER

John Hinckley, Q.E.P. 

Senior Project Consultant 

Resource Systems Group, Inc. 

55 Railroad Row 

White River Junction, VT 05001 

Email: jhinckley@rsginc.com 

Voice: 802/295‐4999 
Resource Systems Group, Inc. Emission Control Technologies for Small Wood‐Fired Boilers

19 October 2010 Page i

TABLE OF CONTENTS

1.0 INTRODUCTION .......................................................................................1

2.0 SCOPE OF STUDY......................................................................................3

3.0 EMISSIONS OVERVIEW..............................................................................3

3.1 Vermont APCD Emission Study ................................................................................................................ 8

3.2 Hazardous Air Pollutant Information ....................................................................................................... 9

3.2.1 HAP Stack Emission Test Information................................................................................................. 9

3.2.2 Mercury Emissions............................................................................................................................10

4.0 BEST MANAGEMENT PRACTICES (BMPS)...................................................10

4.1 Fuel Quality ............................................................................................................................................10

4.2 Operation and Maintenance Plan..........................................................................................................13

4.3 Boiler Operator Training ........................................................................................................................14

4.4 Equipment Sensors ................................................................................................................................14

4.5 Automatic Ash Removal.........................................................................................................................14

4.6 Raking Grates as Needed .......................................................................................................................14

4.7 Combustion Efficiency Testing...............................................................................................................14

4.8 Visual Plume Observations ....................................................................................................................15

4.9 Recordkeeping .......................................................................................................................................15

4.10 Annual Tune Up .....................................................................................................................................16

5.0 ADD‐ON POLLUTION CONTROLS...............................................................16

5.1 Mechanical Collectors............................................................................................................................16

5.1.1 Cyclones and Multicyclones..............................................................................................................17

5.1.2 High Efficiency Multicyclones ...........................................................................................................17

5.1.3 Core Separator .................................................................................................................................17

5.2 Dry Electrostatic Precipitators ...............................................................................................................18

5.3 Baghouses (Fabric Filters) ......................................................................................................................18

6.0 SUMMARY OF EUROPEAN EMISSION CONTROL PRACTICES ...........................19

7.0 CAPITAL COSTS OF EMISSION CONTROLS FOR PARTICULATE MATTER .............20

8.0 COST EFFECTIVENESS OF PARTICULATE MATTER EMISSION CONTROLS............22

9.0 OVERVIEW OF EMISSION CONTROLS FOR OTHER RELEVANT POLLUTANTS .......23

9.1 Nitrogen Oxides (NOx) ...........................................................................................................................23

9.2 Sulfur Dioxide (SO2)................................................................................................................................24

9.3 Carbon Monoxide (CO) ..........................................................................................................................24

9.4 Volatile Organic Compounds (VOCs)......................................................................................................24

9.5 Hazardous Air Pollutants (HAPs) ............................................................................................................24

10.0 CONCLUSIONS .......................................................................................24
Resource Systems Group, Inc. Project Name

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11.0 RECOMMENDATIONS FOR FUTURE RESEARCH.............................................25

12.0 SUMMARY............................................................................................26

TERMS AND ACRONYMS

Total suspended particulate matter (TSP) ­ all sizes of filterable and condensable particles.  

PM10 ‐ particles equal to or less than ten micrometers in aerodynamic diameter, also referred to as 

“coarse particles” or “PM coarse”. 

PM2.5 ­ particles less than or equal to 2.5 micrometers in aerodynamic diameter, also referred to as “fine 

particles” or “PM fine." 

Filterable particulate matter – solid particles of all sizes (TSP, PM10, PM2.5, etc.) which can be 

collected on a filter.  

Condensible particulate matter – particles which form as vapors cool and condense into liquid 

droplets.  

CCG – close coupled gasifier or a combustion system which utilizes two separate combustion chambers in 

series.  

SA – a stoker combustor where the fuel is fed to the combustor with an auger.  

SP – a stoker combustor where the fuel is fed to the combustor pneumatically. 

CS ­ core separator.

HEMC – high efficiency multicylone. 

MC – conventional multicyclone. 

FF – fabric filter or baghouse. 

ESP – electrostatic precipitator. 

Whole tree chips – wood chips created by chipping the entire tree (stem, top, leaves, branches). 

Bole tree chips – wood chips created by chipping the tree stem. 

Mill chips – wood chips from sawmill residue, contain no bark.
Resource Systems Group, Inc. Emission Control Technologies for Small Wood‐Fired Boilers

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1.0 INTRODUCTION

In 2001, Resource Systems Group, Inc. (RSG) produced an emission control report for a consortium of 

northeastern government agencies called “An Evaluation of Air Pollution Control Technologies for Small 

Wood‐Fired Boilers.”1 While the study identified many ways to reduce emissions, it did identify a single 

add‐on control technology (the Core Separator) as “BACT” or Best Available Control Technology for 

controlling particulate matter emissions. While, the conclusions of this report were accepted by the 

consortium of northeastern state agencies, they did not translate into a formal BACT determination at the 

federal level. 

According to the Environmental Protection Agency (EPA), “BACT is an emissions limitation which is 

based on the maximum degree of control that can be achieved. It is a case‐by‐case decision that considers 

energy, environmental, and economic impact. BACT can be add‐on control equipment or modification of 

the production processes or methods. This includes fuel cleaning or treatment and innovative fuel 

combustion techniques. BACT may be a design, equipment, work practice, or operational standard if 

imposition of an emissions standard is infeasible.”2

For the purposes of this report, the best available control technology (BACT) may simply be defined as 

the highest performing control technology for a specific pollutant that is available commercially for a 

general class and size of emission source. This is usually defined as resulting in the lowest emission rate 

although differences in available fuel specifications may complicate the issue. Other environmental, 

health, safety and energy consumption factors should be considered in making the BACT determination. 

The operation of a specific control technology applied to a comparable source anywhere in the US is 

usually considered sufficient evidence that the technology is BACT. In principle, the search for BACT 

should be worldwide, although local conditions make comparability complicated and in practice a control 

technology usually needs a US based customer support system to make it truly available. 

Costs are also a consideration in defining BACT for a specific application. Total cost per unit of pollutant 

removed decline with increasing size of the facility; therefore, a technology may be BACT for a large plant 

but not for a smaller one. Wood‐fired boilers in the size range of 3 to 10 MMBtu/hour have not been 

subject to formal federal BACT review for criteria pollutants given the comparatively high and therefore 

challenging cost of control technologies in this size range. However, state air pollution control permits 

are often required for this size range, which often require a number of technical analyses, including 

emissions estimation, air quality modeling and some degree of informal economic analysis for pollution 

control costs. 

The EPA BACT process follows a top down procedure. It begins with the most effective control 

technology available that will result in the lowest emission rate and then reviewing that technology to 

determine if there are technical, safety, health or other environmental factors which would make it 

impractical or undesirable. If the technology is not rejected because of any of these factors, then a cost 

analysis is conducted to determine the absolute costs and per unit costs of implementation. The cost 

analysis follows guidelines established by EPA. If it is relevant, the analysis may include special costs 

associated with retrofitting the technology in an existing plant. The cost analysis is then reviewed to 

determine if the technology is economically feasible in the specific case. If the first technology choice is 

rejected for technical, environmental, safety or costs reasons, then the analysis proceeds to the second 

best performing technology and so on until a feasible technology is accepted or all available options are 

                                                                  1 An Evaluation of Air Pollution Control Technologies for Small Wood‐Fired Boilers.” Prepared for Vermont Department of Public Service;

Vermont Department of Environmental Conservation, Air pollution Control Division; New Hampshire Governor’s Office of Energy

Resources and Community Services; and the Massachusetts Division of Energy Resources. Resource Systems Group, Inc., White River

Junction, VT. Revised, September 2001.

2 US EPA. http://www.epa.gov/nsr/psd.html
Resource Systems Group, Inc. Emission Control Technologies for Small Wood‐Fired Boilers

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exhausted. This process may include not only add‐on technology but combustion process modifications 

and changes in fuel specifications.  

The study described by this report builds upon the 2001 report, but differs in that its goal was not to 

identify a single Best Available Control Technology (BACT). Rather, its goal was to identify multiple 

emission controls in order to provide more flexibility in the design process for biomass combustion 

systems. This is because there are many factors affecting the degree of control due necessary to meet the 

National Ambient Air Quality Standards (NAAQS), which were promulgated by EPA to protect human 

health and welfare. 

Regarding the NAAQS, we note EPA significantly strengthened the NAAQS for PM2.5 (aka fine particulate 

matter or particles less than 2.5 micrometers in aerodynamic diameter) in 2006. PM2.5 consists of solid 

particles and liquid droplets less than 2.5 microns in aerodynamic diameter and is widely held to be the 

most critical pollutant resulting from biomass combustion. For comparison, the average period at the end 

of a sentence is approximately 500 microns in diameter.1 Concern for health impacts from PM2.5 

exposure coupled with the strengthened PM2.5 NAAQS has led to a much greater emphasis on emission 

control than in previous years.   

The need to develop environmentally beneficial uses of low grade timber, improve forest health, mitigate 

climate change, offset rising fossil fuel oil prices and reduce foreign oil dependence have increased 

demand for biomass energy systems. Given frequent budget limitations, biomass developers are pressed 

to find cost‐effective ways to reduce emissions of PM2.5 and other pollutants. In addition to being 

affordable, emission controls must be practical and able to be easily implementable, otherwise they will 

not be effective. This study was commissioned by the U.S. Forest Service (USFS) to identify and evaluate 

such emission controls. 

This report contains the following sections: 

 Scope of study 

 Emissions overview 

 Best Management Practices (BMPs) 

 Add‐on pollution controls 

 Summary of European emission control practices 

 Capital costs for particulate matter control 

 Cost effectiveness of add‐on emission controls for particulate matter 

 Overview of emission controls for other relevant pollutants 

 Summary 

 Conclusion 

 Recommendations 

The use of trade or firm names is for information only and does not imply endorsement by the authors or 

this study’s sponsor.

                                                                  1 “Health Effects of Wood Smoke.” Washington State Department of Ecology. http://www.ecy.wa.gov/biblio/92046.html
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2.0 SCOPE OF STUDY

The scope of this study was to update and expand the RSG 2001 report as follows: 

1) Identify and Evaluate Best Management Practices (BMPs).  BMPs, also called “work practice 

standards” and “inherently lower emitting processes and practices,” are used to control 

emissions upstream of add‐on control technologies such as mechanical collectors (cyclones, 

multicyclones), baghouses (fabric filters), electrostatic precipitators (ESPs), etc. Attention is 

typically directed toward add‐on emission control selection. These solutions are typically more 

costly. While not as effective as most add‐on controls BMPs can control substantial quantities of 

emissions, improve system efficiency and improve system performance; therefore, this report 

will focus on BMPs in addition to add‐on controls. This said, BMPs alone will not likely satisfy the 

requirements for “LAER” or lowest achievable emission rate, which is typically required in non‐

attainment areas. Non‐attainment areas are areas where one or more of the National Ambient 

Air Quality Standards (NAAQS) are not met.  

2) Expand the original size range evaluated from 3 MMBtu/hr ­ 10 MMBtu/hr to 3 MM Btu/hr

­ 30 MM Btu/hr (heat input).1 This report will still focus on small (less than 10 MMBtu/hr) 

wood boilers. However, there is new information available from recently constructed wood 

boilers smaller than 3MMBtu/hr and larger than 10 MMBtu/hr that can be extrapolated to the 3 

to 10 MMBtu/hr size range. This information was evaluated for the purposes of this report. 

3) Include emissions control information for PM2.5 and a number of hazardous air

pollutants (HAPs) including Mercury. This is primarily in response to EPA’s strengthening of 

the PM2.5 NAAQS and due to recent availability of stack emission test data for PM2.5 and HAPs. 

4) Include pellet boiler emission information. Many new pellet boilers and pellet production 

plants (pellet mills) have been constructed in recent years in response to demand for this fuel. In 

addition, a number of stack emission tests have been completed for pellet boilers in the United 

States. This report will discuss this new information. 

3.0 EMISSIONS OVERVIEW

When evaluating emission controls for biomass boilers, it is important to first develop an understanding 

of current actual emissions from biomass boilers. The most current and descriptive emissions 

information is obtained from exhaust stack emission tests performed according to EPA reference 

methods. These tests are typically performed to fulfill air pollution control permit requirements 

requiring a demonstration that emission limits are being met. In addition to compliance emission tests, 

many voluntary tests have been sponsored by interested parties given the level of interest in knowing 

actual emissions and effectiveness of emission controls. These tests were completed throughout the 

United States, with different fuel characteristics, different firing rates and different emission controls.  

This study focused on particulate matter emissions. There are many terms used to characterize 

particulate matter. For the purposes of this report, the term particulate matter includes all sizes of solid 

particles and liquid particles (droplets). Solid particles are referred to as “filterable” particulate matter 

because they can be removed from exhaust with a filter. Liquid particles are also called “condensable” 

                                                                  1 30 MMBtu/hr is the threshold level at which the federal PM emission limit drops to 0.030 lb/MMBtu. This emission limit cannot be

achieved with a mechanical collector. See 40 CFR Part 60, Subpart Dc. – Standards of Performance for Small Industrial‐Commercial‐

Institutional Steam Generating Units.  
Resource Systems Group, Inc. Emission Control Technologies for Small Wood‐Fired Boilers

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particulate matter because they are formed by vapors in exhaust which cool and condense into particles. 

These vapors can also condense onto the solid particles.  

Filterable and condensable particulate matter is grouped into three size classes. “Total suspended 

particulate matter” (TSP) includes all sizes of filterable and condensable particles. The next smallest size 

class is PM10 or filterable and condensable particles equal to or less than ten micrometers in 

aerodynamic diameter. PM10 particles are also called “coarse particles.” PM2.5 is the smallest particle 

size class currently regulated. PM2.5 particles, also called “fine particles,” include all filterable and 

condensable particles less than or equal to 2.5 micrometers in aerodynamic diameter. For the purposes 

of this report, it was assumed that all condensable particulate matter falls in the PM2.5 category.  

Condensable particulate matter is reported separately from filterable PM2.5 because it is controlled 

differently. Some of the “condensables” will condense on filterable particles. Hence, anything controlling 

filterable particulate matter will inherently control some portion of the total condensables. Good 

combustion practices are typically used on small wood boilers to control condensables.   

Unless stated otherwise, the terms PM10 and PM2.5 will refer to filterable particulate matter only. This is 

because the methods used to measure the PM10 and PM2.5 emissions listed in this report measure 

filterable particulate matter only.    

RSG reviewed 24 recent stack emission tests to develop an understanding of existing emissions (see 

Appendix A for the supporting stack test reports). All but one of these tests was completed after the 2001 

RSG report. These stack tests were performed in Idaho, Montana, New Hampshire, North Dakota, Rhode 

Island and Vermont. Twenty‐two of the tests were performed on wood chip boilers and two tests were 

performed on wood pellet boilers. All tests measured some form of particulate matter emissions 

(filterable PM10, filterable PM2.5 and condensable PM). Some of the tests included other pollutants such 

as carbon monoxide (CO) and a selected number of Hazardous Air Pollutants (HAPs). Fuels burned 

included sawmill residue chips, bole chips, whole tree chips, bark chips, sawdust and municipal 

vegetative waste combined with ground pallets. Add‐on emission controls included cyclones, 

multicyclones, high efficiency multicyclones (HEMCs), core separators and baghouses. 

This study did not identify any stack emission data for ESPs on small wood‐fired boilers in the United 

States. However, according to the EPA “RACT‐BACT‐LAER Clearinghouse”, the lowest emission limit listed 

for TSP and PM10 for large wood‐fired boilers controlled with ESPs is 0.02 lb/MMBtu (PM2.5 was not 

listed).1 Given this limit is based on demonstrated technology, it is technically possible that small wood‐

fired boilers could meet this limit. However, the economic analysis on which this limit is based is for 

significantly larger systems (100 MMBtu/hour or greater), where the total cost per ton of pollutant 

removed is significantly lower.  This is why the economic analysis performed for this report was based on 

outlet emissions not exceeding 0.045 lb/MMBtu of all filterable particulate matter. 

It should also be noted that facilities subject to a limit of 0.02 lb/MMBtu have the potential to emit more 

than an order magnitude more emissions and therefore must meet stringent emission limits in order to 

meet federal ambient air quality requirements.  

As shown below in Table 1, there is a large difference between the maximum and minimum values 

measured. The difference was as much as three orders of magnitude for PM10. The maximum PM10 

value of 0.506 lb/MMBtu resulted from a facility burning a low quality fuel ‐ whole tree chips produced 

by a grinder and having notable quantities of dirt and rock. The fuel was burned in a stoker combustor 

with no add‐on emission control. The lowest PM10 value of 0.06 lb/MMBtu resulted from burning a high 

quality mill chip (no bark and no soil/rock impurities) in a close‐coupled gasifier controlled with a high 

efficiency multicyclone (HEMC).  

                                                                  1 This emission limit applies to a utility sized boiler whose heat input exceeds 100 MMBtu/hour.
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Table 1 summarizes the 24 particulate matter stack test results. Emissions are grouped into the three 

categories: filterable PM10, filterable PM2.5 and condensable PM. Table 2 through Table 4 summarize 

emissions by the following heat input categories:  

 Less than 30 MMBtu/hour and greater than 10 MMBtu/hour (based on seven stack tests). 

 Less than or equal to 10 MMBtu/hour (based on 18 stack tests). 

 Less than or equal to 5 MMBtu/hour (based on nine stack tests).

A more detailed summary of all 24 tests is shown in Table 5. 

As shown below in Table 1, there is a large difference between the maximum and minimum values 

measured. The difference was as much as three orders of magnitude for PM10. The maximum PM10 

value of 0.506 lb/MMBtu resulted from a facility burning a low quality fuel ‐ whole tree chips produced 

by a grinder and having notable quantities of dirt and rock. The fuel was burned in a stoker combustor 

with no add‐on emission control. The lowest PM10 value of 0.06 lb/MMBtu resulted from burning a high 

quality mill chip (no bark and no soil/rock impurities) in a close‐coupled gasifier controlled with a high 

efficiency multicyclone (HEMC).1  

Table 1: Summary of all Stack Emission Test Results

Category

PM10 Emissions

(lb/MMBtu)

PM2.5 Emissions

(lb/MMBtu)

Condensable PM

(lb/MMBtu)

Average2 0.178 0.111 0.021

Median 0.140 0.122 0.014

Maximum 0.506 0.267 0.039

Minimum 0.016 0.014 0.006

Table 2: Stack Test Summary for Heat Input Less than 30 MMBtu/hour and Greater than 10 MMBtu/hour

Category

PM10 Emissions

(lb/MMBtu)

PM2.5

Emissions

(lb/MMBtu)

Condensable

PM Emissions

(lb/MMBtu)

Average 0.230 0.164 0.011

Median 0.101 0.188 0.014

Maximum 0.382 0.267 0.014

Minimum 0.019 0.062 0.006

Table 3: Stack Test Summary for Heat Input Less Than or Equal to 10 MMBtu/hr

Category

PM10 Emissions

(lb/MMBtu)

PM2.5

Emissions

(lb/MMBtu)

Condensable

PM Emissions

(lb/MMBtu)

Average 0.175 0.107 0.018

Median 0.156 0.104 0.014

Maximum 0.506 0.179 0.039

Minimum 0.016 0.014 0.007

                                                                  1 The HEMC was determined to be BACT for small wood‐fired boilers in Rhode Island in a 2006 BACT study completed by Resource

Systems Group. BACT for PM10 and PM2.5 were determined to be 0.20 lb/MMBtu and 0.18 lb/MMBtu respectively.

2 Average values represent the average of instances when both PM10 and PM2.5 were measured at the same site.
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Table 4: Stack Test Summary for Heat Input Less Than or Equal to 5 MMBtu/hour

Category

PM10 Emissions

(lb/MMBtu)

PM2.5

Emissions

(lb/MMBtu)

Condensable PM

Emissions

(lb/MMBtu)

Average 0.231 0.114 0.025

Median 0.161 0.110 0.026

Maximum 0.506 0.179 0.039

Minimum 0.016 0.014 0.009

Table 5 provides more information for each stack test than the tables above. Information is provided in 

ascending order of measured PM10 emissions. Table cells were left blank to represent instances when 

PM2.5 and condensable PM data were not available. Also shown is the type of fuel burned and type of 

emission control. The following combustion technologies are listed: close‐coupled gasifier), auger fed 

stoker (SA), pneumatically fed stoker (SP). 

Table 5: Stack Emission Test Summary for Particulate Matter

Location

Design Heat

Input

(MMBtu/hr)

Comb.

Type Fuel Burned

Emission

Control

PM10

Emissions

(lb/MMBtu)

PM2.5

Emissions

(lb/MMBtu)

Condens.

PM

Emissions

(lb/MMBtu)

Glocester, RI 4.6 CCG Mill chips HEMC 0.016 0.014   

Middlebury, VT 29.0 CCG Bole chips MC + FF 0.019       

Dillon, MT 19.0 CCG Bole chips MC 0.052      

North Scituate,

RI 9.1 CCG Mill chips HEMC 0.058 0.054 0.007

North Scituate,

RI 9.1 CCG Mill chips HEMC 0.066 0.066   

Thompson Falls,

MT 2.2 CCG Bole tree chips Cyclone 0.070      

Brattleboro, VT 6.9 CCG Wood chips Core Sep. 0.078      

Greenfield, NH   5.7 & 11.4 SA Bole chips MC + FF 0.078 0.062 0.014

Newport, VT 15.6 CCG Bole chips MC 0.101    0.006

Peterborough,

NH 2.8 SA Pellets MC 0.101      

Bennington, VT 16.8 SP

Whole tree

chips 2 MC 0.140       

Darby, MT 3.3 SA Bole chips None 0.156      

Victor, MT 2.6 SA Bole chips None 0.166 0.098 0.009

Springfield, NH 15.5 SA Sawdust None 0.168      

Hinesburg, VT 6.5 SA Bole chips Cyclone 0.171 0.147 0.012

Brattleboro, VT 10.0 SA Mill chips Core Sep. 0.172 0.162 0.012

Burlington, VT 10.0 SA Mill chips MC 0.187    0.015

Darby, MT 3.3 SA Bole chips None 0.192 0.110 0.015

Bismarck, ND 1.0 SA

Muni. veg. &

pallets None 0.199 0.151 0.077

Burlington, VT 10.0 SA Bole chips MC 0.257    0.017

Townsend, MT 0.75 SA Pellets None 0.305 0.133 0.036

Bennington, VT 16.8 SP

Whole tree

chips 2 MC 0.382 0.267 0.014

Council, ID 1.9 SA Whole tree None 0.506 0.179 0.039
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chips

Dillon, MT 19.0 CCG Bole chips MC    0.188   

The following observations of the tests were noted: 

 Close‐coupled gasifiers emitted the lowest levels of emissions.  

 There is limited PM2.5 emissions data. This is partially because PM2.5 is still not officially 

enforced by most state air quality agencies; therefore, state agencies are requiring compliance 

stack testing PM10 only. 

 The lowest PM10/PM2.5 emissions were produced by a close coupled gasifier burning a 

relatively high quality wood chip, with emissions controlled by a HEMC. 

 PM10 emissions from baghouses were surprisingly not the lowest for all tests. They were the 

third and eighth lowest emissions of all tests.  The lower than expected control efficiency is 

suspected to be due to a portion of the boiler exhaust gases circumventing the bag house via a 

leaking damper in the bypass duct. 

 PM10 emissions were equal to or less than 0.20 lb/MMBtu for 19 of the 24 tests (79%) and less 

than 0.10 lb/MMBtu for 10 of the 24 tests (41%).   

 All but one of the PM2.5 tests were less than 0.20 lb/MMBtu. Five of the 13 PM2.5 tests were less 

than 0.10 lb/MMBtu. 

 The results suggest bark increases PM emissions. In one example, the two results from 

Burlington, Vermont, which was tested with mill and bole chips, indicate bark can increase PM 

emissions. In this case, PM10 emissions were 0.187 lb/MMBtu and 0.257 lb/MMBtu respectively.  

 There are two stack test results for pellet fired systems. PM10 emissions from these systems 

ranged from 0.101 lb/MMBtu to 0.305 lb/MMBtu. However, the higher number corresponds to a 

much older system with no add‐on emission controls. Conversely, the lower number 

corresponds to a new system with a multicyclone. Average PM2.5 emissions from the older 

system were 0.133 lb/MMBtu. PM2.5 emissions were not measured for the newer system. 

The EPA has developed emission factors for wood boilers. These emission factors are included in a 

document called “AP 42”, which is a compilation of emission factors.1 Comparable emission factors from 

the AP 42 are summarized for reference below. Note these emission factors represent an average for a 

group of emission tests. These emission factors were published in September, 2003 and likely correspond 

to systems larger than most of those considered for this report. The majority of the stack test data used to 

develop these emission factors was collected in the early to mid 1990’s.  

Table 6: Comparable EPA AP 42 Emission Factors

Fuel Type Control Device

PM10 Emissions

(lb/MMBtu)

PM2.5

Emissions

(lb/MMBtu)

Condensable

PM Emissions

(lb/MMBtu)

Bark & wet wood None 0.50 0.43 0.017

Bark & wet wood Mechanical collector 0.32 0.19 0.017

Wet wood None 0.29 0.25 0.017

Wet wood   Mechanical collector 0.20 0.12 0.017

All fuel types   Fabric filter (baghouse) 0.074 0.065 0.017

All fuel types Electrostatic Precipitator (ESP) 0.04 0.035 0.017

                                                                  1 Can be accessed at http://www.epa.gov/ttn/chief/ap42/ch01/index.html
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3.1 Vermont APCD Emission Study

The Vermont Agency of Natural Resources (ANR) recently completed an emission study focusing on air 

pollutant control efficiency for a number of add‐on pollution controls. The study evaluated inlet and 

outlet particulate matter emissions from five wood chip fired boilers in the Northeast. These boilers are 

located at Crochet Mountain Rehabilitation Center (Greenfield, NH), Bennington College (Bennington, 

VT), Brattleboro Union High School (Brattleboro, VT), Ponaganset High School (North Scituate, RI) and 

Champlain Valley Union High School (Hinesburg, VT). Emissions data and design heat inputs for these 

boilers are provided previously in Table 5.  

Table 7 shows the approximate PM2.5, PM10 and TSP control efficiency for the five sites.1 The control 

efficiencies listed below refer to the percent of filterable particulate matter removed by the add‐on 

pollution control device. Control efficiencies for different add‐on pollution controls are further discussed 

later in this report.  

Table 7: Vermont ANR Emission Study Approximate Control Efficiencies

Location PM Control

PM2.5

Control

Efficiency

PM10  

Control

Efficiency

TSP Control

Efficiency

Greenfield, NH

Multicyclone +

baghouse 74% 74% 83%

Bennington, VT

Two multicyclones in

series 14% 22% 61%

Brattleboro, VT Core separator (24”) 24% 32% 60%

North Scituate, RI

High efficiency

multicyclone 15% 21% 23%

Hinesburg, VT Cyclone 3% 6% 4%

The following observations were made from these results: 

 Bennington and Brattleboro, VT.  The overall control efficiencies are significantly higher than

the PM2.5 and PM10 efficiencies because there were significant quantities of particles larger 

than PM10 particles (ten microns in aerodynamic diameter) emitted during the stack tests.

 Greenfield, NH.  The control efficiency is relatively low for a multicyclone followed by a 

baghouse. Baghouses are widely thought to achieve 99% control efficiency for PM2.5 and smaller 

filterable particles. As mentioned, the low control efficiency is suspected to be due to a portion of 

the boiler exhaust gases circumventing the bag house via a leaking damper in the bypass duct. 

 Bennington, VT. While this is the highest control efficiency value of all mechanical collectors 

tested, the actual emissions were higher than all other five sites (0.382 for PM10 and 0.267 

lb/MMBtu for PM2.5).  The higher emissions may be due to carry over of large particles from the 

combustion chamber caused by pneumatic feeding of fuel into the combustion chamber.  

It was discovered after this testing that some of the underfire air passages were obstructed by a 

buildup of boiler bottom ash.  This ash was subsequently cleaned out, the boiler combustion air 

was adjusted and an improved ash management procedure was implemented. The retesting 

which occurred after these measures were implemented indicated the total PM emission rate 

dropped to 0.14 lb/MMBtu (approximately a 63% reduction). 

                                                                  1 The PM2.5 and PM10 control efficiencies were calculated using the inlet and outlet emission factors (in lb/MMBtu) from the control

device. The TSP control efficiency was taken directly from the study report.  
Resource Systems Group, Inc. Emission Control Technologies for Small Wood‐Fired Boilers

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 Brattleboro, VT. The TSP collection efficiency was nearly the same as at Bennington, but the 

outlet emissions were lower (0.172 lb/MMBtu for PM10 and 0.162 for PM2.5). However, PM2.5 

and PM10 control efficiencies were higher at Bennington. This shows the core separator is more 

effective at removing fine particles than conventional multicyclones, even conventional 

multicyclones in series.  

The Core Separator operating at Brattleboro was designed for the exhaust volume from a 400 

horsepower (hp) boiler.  However, the boiler size was reduced to 332 hp during project 

development. So while a smaller “2‐core” unit would have worked, a “3 core” unit was operating.  

The result is that during emission testing, the Core Separator was operating at half it's design 

pressure drop. A higher collection efficiency would likely have resulted if it was operating at its 

design pressure drop. 

 North Scituate, RI. The outlet emissions were the lowest of all sites (0.0662 MMBtu/hr for 

PM10 and 0.0660 MMBtu/hr for PM2.5) despite one of the lowest collection efficiencies 

measured (23%). This is because the system was operating at low capacity (30% load) which led 

to a low pressure drop (approximately 0.7 inches of WC). The design pressure drop for this 

system is 4” of WC. At this pressure drop it is quite conceivable that the collection efficiency 

would have been 75% or greater. Vendor calculations suggested an TSP collection efficiency of 

80% or greater at design load/pressure drop. It should also be noted that the vast majority of the 

inlet loading was PM2.5 (88.7%). Therefore, the control efficiency is relatively high for a 

mechanical collector operating at low pressure drop.  

 Hinesburg, VT. Single cyclones have the lowest collection efficiency of all mechanical collectors. 

This is evidenced by the collection efficiency measured, which is due to a considerably higher 

portion of PM2.5 in the inlet exhaust. Despite the type of control device, the outlet emissions 

were relatively low (0.171 for PM10 and 0.147 lb/MMBtu for PM2.5). Similar to North Scituat, 

this boiler operated at relatively low load and consequently there was relatively low pressure 

drop across the cyclone.  

3.2 Hazardous Air Pollutant Information

3.2.1 HAP STACK EMISSION TEST INFORMATION

In addition to PM2.5, there is growing interest in Hazardous Air Pollutant (HAPs) emissions from wood 

combustion. The EPA publishes HAP emission factors in Section 1.6 of the AP 42, which was last updated 

in September, 2001 and, as mentioned, is based on emission tests conducted before that date. Since that 

time, there have been advances in combustion technology and practices, which suggest HAP emissions 

have and will likely continue to decline with time.  

This study compared a number of AP 42 HAP emission factor values with measured emissions from five 

test sites. Both gaseous and particulate HAPs were evaluated. A full list of the HAPs evaluated is provided 

in the appendix. HAP emissions were not weighted according to their respective toxicity level. 

The information provided in this report is intended to establish a starting point for understanding HAP 

emissions. Firm conclusions should not be drawn from the information provided as it is based on a 

limited number of stack emission tests.  

Not all 188 federal HAPs were measured at each of the five test sites. Each HAP measured at each site was 

compared with its AP 42 equivalent.  

The comparisons made showed actual emissions of individual HAPs were both higher and lower than AP 

42 equivalents. They also showed the total HAPs measured at each site were lower than the total AP 42 

HAP equivalent for all but one site. The average actual total HAP emission from all sites was 68% lower 

than the AP 42 total HAP equivalent. The comparisons are summarized below. Detailed information is 

provided in the appendix.  
Resource Systems Group, Inc. Emission Control Technologies for Small Wood‐Fired Boilers

23 February 2010 Page 10

Table 8: Summary of HAP Test Sites

Location

Design Heat

Input

(MMBtu/hr)

Combustion

Type

Emission

Control

Emission

Test Date

Number of

HAPs

Compared

Total Measured

HAP Percent of

AP 42 Total HAP

North Scituate, RI 9.1 CCG HEMC   2009 24 13%

Glocester, RI 4.6 CCG HEMC   2009 24 8%

Council, ID 1.9 SA Uncontrolled   2007 22 23%

Green Acres, VT 2.2 SA Uncontrolled   1996 24 123%

Hazen Union, VT 2.8 CCG MC   1996 24 26%

3.2.2 MERCURY EMISSIONS

Mercury emissions have been mainly considered with coal projects. Mercury emissions have been 

considered for wood combustion projects, but to a lesser extent. There is much information about 

mercury control from wood fired power plants via fuel and exhaust cleaning. Unlike NO2 emissions, 

Mercury emissions are a function of fuel mercury content. Therefore, a fuel analysis provides a good 

indication of potential mercury emissions.  

Mentz et al, describes work performed to measure mercury content of bark and stemwood in 30 

locations throughout the country.1  The average bark and stemwood concentrations at the 30 sites were 

1.42 lb/1012 Btu and 0.28 lb/1012 Btu for bark and stemwood respectively. The mean mercury content for 

each of the 30 sites ranged from 0.57 lb/1012 Btu to 3.14 lb/1012 Btu in bark and from 0.12 lb/1012 Btu to 

0.46 lb/1012 Btu in stemwood.  

These numbers are based on the assumption that the entire quantify of mercury is released from the fuel 

into a vapor form and not does not combine with any other constituents in the exhaust gas or be removed 

by some form of emission control. For context, there is an EPA emission limit of 9 lb/1012 Btu for existing 

sources and 3 lb/1012 Btu for new sources; therefore, actual mercury emissions are below an established 

limit.2 Assuming that bark is a small portion of the fuel burned (e.g. bole tree or whole tree chip), the 

weighted average of bark and stemwood would be less than the limit of 3 lb/1012 Btu for new sources. 

Pease et al, describes research performed to evaluate the potential for flue gas cooling, flue gas 

humidification, pulsed energization, and sorbent injection in wet and dry ESPs to reduce mercury 

emissions. This study found that all of these measures are effective. It also found that some mercury will 

attach to fly ash thereby increasing the potential for mercury removal. 

4.0 BEST MANAGEMENT PRACTICES (BMPS)

BMPs range from physical equipment such as oxygen sensors to operational practices such as visual 

observations of plume opacity. Properly implemented, BMPs will optimize combustion conditions 

thereby helping maximize energy efficiency and minimize emissions from any system.  

4.1 Fuel Quality

The first step in implementing BMPs is to obtain the highest quality fuel possible. There are many factors 

affecting fuel quality. Fuel quality is an important consideration as improved fuel quality improves 

combustion conditions, increases efficiency and reduce emissions. Fuel quality is a function of fuel 

                                                                  1 Karen Mentz, John Pinkerton, and Jeff Louch.“Potential Mercury and Hydrochloric Acid Emissions from Wood Fuels.” Forest Products

Journal, 55(2): 46‐50. Received for publication in August, 2004. Article No. 9919.

2 “National Emission Standards for Hazardous Air Pollutants for Industrial, Commercial and Institutional Boilers and Process Heaters; Final

Rule.” U.S. Environmental Protection Agency. Federal Register/Vol. 69, No. 176, September 13, 2004.
Resource Systems Group, Inc. Emission Control Technologies for Small Wood‐Fired Boilers

23 February 2010 Page 11

moisture content, bark content, uniformity, size, and purity. These factors are described in detail later in 

this report. 

There are no formally established grades of wood chips. However, there are four basic types of wood 

chips, whose quality and corresponding emissions are fairly well understood. For the purposes of this 

report, the term “high quality” describes a chip which has minimal ash content, is of uniform and proper 

size and results in the least amount of emissions.  

1. Sawmill residue chips (mill chips). This chip is thought to be the highest quality in that it 

contains no bark. The availability of this chip is limited due to the decline in U.S. sawmills 

coupled with the demand for higher value products from those chips such as pulp, wood 

composite products and wood pellets.  

2. Bole tree chips (bole chips). Are produced by chipping the tree trunk. This is a moderate to

high quality chip as it contains relatively minimal quantities of bark is relatively uniform in size.  

3. Whole tree chips. Are produced by chipping the entire tree and therefore include the tops, 

leaves and branches in addition to the trunk. This category would also include municipal 

vegetative waste in addition to trees removed in a given forest. These are a moderate to low 

quality chip as they are less uniform in size/shape and have higher ash content than bole tree 

chips.  

4. Bark chips. These chips consist primarily of bark and are the lowest quality chip given the high 

ash content of bark. In addition to its mineral content, bark can have higher ash content because 

it may have impurities adhered to it from harvesting and transport. 

Wood chips can be produced with chippers and grinders. Grinders can handle more dirt and rocks and 

therefore have greater potential for contaminating wood chips with those impurities. Grinders also 

produce chips with greater size variation than chippers. As is discussed later in this section, fuel 

homogeneity is important for effective fuel handling and combustion. 

In 2007, the Biomass Energy Resource Center (BERC) published a report for the South Dakota 

Department of Agriculture, Resource Conservation & Forestry Division entitled “Woodchip Fuel 

Specifications and Procurement Strategies for the Black Hills.”1 While this document is intended for a 

specific geographic area, the overall principles can be applied throughout the United States. This 

document characterizes four grades of wood chip quality and provides guidelines for obtaining each of 

those four grades.  

The information in the BERC report was combined with the author of this report’s working knowledge to 

develop the fuel quality BMPs summarized in the table below below. Any number of these BMPs can be 

applied to planned as well as existing facilities.   

Table 9: Summary of Fuel Quality BMPs for Wood Chips

BMP Category Description

Fuel Quality

Improvement

Combustion

Improvements / Emission

Reductions

Bark content

Minimize bark content.

Mill chips and sawdust

based pellets do not

contain bark. Reduces ash content.

Reduces clinker formation on

grates thereby maintaining proper

airflow through the grates.

Reduces emissions associated with

fly ash carry over. Increases

combustion efficiency.

                                                                  1 Biomass Energy Resource Center (BERC). “Woodchip Fuel Specifications and Procurement Strategies for the Black Hills.” Prepared for the

South Dakota Department of Agriculture, Resource Conservation & Forestry Division. May 15, 2007. www.biomasscenter.org
Resource Systems Group, Inc. Emission Control Technologies for Small Wood‐Fired Boilers

23 February 2010 Page 12

BMP Category Description

Fuel Quality

Improvement

Combustion

Improvements / Emission

Reductions

Moisture content

Moisture content must

be within range

meeting combustion

system design

requirements.

Ensures design fuel

heat content met.

Energy loss occurs when excess

moisture is vaporized. Fugitive

dust and excess PM emissions

occur when fuel is excessively dry.

Storage surface

Minimize/prevent  

storage on ground

surfaces. Store on

concrete or other type

of clean paved surface.

Prevents transfer of

soil, rocks, salts and

other impurities from

the ground surface to

the fuel.

Improves combustion efficiency by

reducing clinker and ash

formation. Minimizes particulate

matter emissions.

Storage time for

trees with needles.

When dry, needles fall

off tree when fallen

tree mechanically

shaken.

Reduces ash content.

Increases fuel

uniformity.

Lower ash content and reduced

potential for clinker formation

increases combustion efficiency.

Reduced PM emissions from

reduced fly ash carry over.

Storage Coverage Store in covered area.

Ensures design

moisture content met.

Maximizes heat content

of fuel.

Ensures design combustion

efficiency met by preventing

energy loss associated with excess

moisture vaporization. This

minimizes excess fuel

consumption.

Method of chip

production

Sharp and properly

adjusted equipment is

critical for grinders and

chippers. Chipping

typically produces

better chip than

grinder.

Maximizes wood chip

uniformity. "Stringer"

formation minimized.

Promotes uniform combustion

thereby maximizing combustion

efficiency. Prevents system

interruptions (upset conditions)

caused by stringers in metering

bins.

Chipping/Grinding

Equipment

Operation and

Maintenance

Manufacturer’s

operation and

maintenance

requirements should be

adhered to.

Ensures chip

uniformity.

Promotes uniform combustion

thereby maximizing combustion

efficiency.

Uniformity of fuel

input to chipper or

grinder

Uniform size material

fed to chipper or

grinder.

Increases chip

uniformity.

Promotes uniform combustion

thereby maximizing combustion

efficiency.

Chip screening

Mechanical screening

(sizing) of chips.

Increases chip

uniformity, removes

oversized material and

removes fines. Has

potential to separate

and remove some

portion of bark from

raw chips.

Promotes uniform combustion

thereby maximizing combustion

efficiency and reducing overall

emissions. PM emissions

potentially reduced as carry over

of fines into exhaust eliminated.

PM emissions reduced through

reduced bark content.

Long term fuel

supply contracts

Encourages investment

in wood chip

production equipment

producing higher

quality chip.

Optimal fuel

characteristics

developed.

Promotes optimal combustion

conditions which maximizes

energy efficiency and minimizes

emissions.

Fuel supply testing

Visually inspect fuel

geometry, uniformity,

moisture content prior

Ensures design fuel

specifications are met.

Ensures optimal combustion

conditions.
Resource Systems Group, Inc. Emission Control Technologies for Small Wood‐Fired Boilers

23 February 2010 Page 13

BMP Category Description

Fuel Quality

Improvement

Combustion

Improvements / Emission

Reductions

to fuel being dumped

into storage bin. Retain

grab samples if need

for future fuel

measurements

anticipated.

There are a number of grades of wood pellets defined by the Pellet Fuel Institute (PFI). They are super 

premium, premium, standard and utility. As shown in Table 10, grades are a function of bulk density, 

diameter, pellet durability index, percent of fines, inorganic ash content, length, moisture content, 

chloride content, ash fusion and heating value. These categories are further described on the PFI Internet 

Site. 1 There are no legal factors necessitating use of any particular grade of wood pellets. However, it is 

useful to know the pellet grade for purposes of meeting air quality requirements for a given area. 

Table 10: Values used for Classifying Residential Grades of Pellets according to the Pellet Fuels Institute

Fuel Property PFI Super Premium PFI Premium PFI Standard PFI Utility

Bulk Density, lb./cubic foot 40.0‐46.0 40.0‐46.0 38.0‐46.0 38.0‐46.0

Diameter, inches 0.250‐ 0.285 0.250‐ 0.285 0.250‐ 0.285 0.250‐ 0.285

Diameter, mm 6.35‐7.25 6.35‐7.25 6.35‐7.25 6.35‐7.25

Pellet Durability Index >97.5 >97.5 > 95.0 >95.0

Fines, % (at the mill gate) <0.50 <0.50 <0.50 <0.50

Inorganic Ash, % ‐ See Note 1 <0.50 < 1.0 <2.0 <6.0

Length, % greater than 1.50 inches < 1.0 < 1.0 < 1.0 < 1.0

Moisture, % <6.0 <8.0 <8.0 < 10.0

Chloride, ppm < 300 < 300 < 300 < 300

Ash Fusion NA NA NA NA

Heating Value As‐Rec. ± 2SD As‐Rec. ± 2SD As‐Rec. ± 2SD As‐Rec. ± 2SD

4.2 Operation and Maintenance Plan

An operation and maintenance plan (O&M Plan) is a document describing the equipment and work 

practices that will take place to ensure optimal combustion conditions and compliance with applicable 

emission limits. These plans also specify the frequency with which all work practices will be completed. 

Consequently, they may include daily, weekly, monthly and annual checklists to ensure all work practices 

(BMPs) are completed. Facilities are oftentimes required to record and maintain this information for a 

period of time as part of a permit condition. O&M plans are developed in concert by the boiler operator, 

wood boiler equipment vendor and state regulatory office. O&M plans should be flexible to allow for 

improved O&M measures if/when they are identified for a given facility. Ideally, all O&M plans are 

written and approved within a few months after start‐up.  

Here is a sample list of O&M components as specified in a Vermont air pollution control permit: 

 Descriptions of routine maintenance and inspection procedures. 

 Description of procedure for and frequency of ash removal from the boiler and the particulate 

matter emission control device. 

                                                                  1 http://www.pelletheat.org/3/institute/standards/PFI%20Standards.pdf
Resource Systems Group, Inc. Emission Control Technologies for Small Wood‐Fired Boilers

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 Provisions for maintaining records of maintenance and inspection procedures, including both 

routine activities and actions taken in response to observations of low combustion efficiency. 

 Provisions for calibration and maintenance of any testing instruments and/or equipment used to 

measure the concentrations of CO2 and CO in the boiler exhaust gases.  

4.3 Boiler Operator Training

Boiler operator training is essential to proper operation and maintenance. This is typically provided by 

the vendor prior to and shortly after start‐up. There are currently no standardized training programs for 

acquiring boiler operator certification.  

There are occasions when the boiler operator who was trained by the vendor upon start‐up leaves for a 

new job. In these situations, it is critical to ensure the replacement operator has sufficient training and 

experience. 

4.4 Equipment Sensors

Many wood boilers are equipped with internal sensors that provide real time information about some 

aspect of the combustion process to an automated control system. The information from these sensors 

helps the system self‐regulate with the intelligence they provide. Sensors are frequently used to measure 

pressure drop across a mechanical collector or baghouse, opacity in the exhaust stack (with smoke 

density meters), oxygen level in the combustion chamber and/or exhaust stack, and temperature in the 

combustion chamber and/or exhaust stack.  

4.5 Automatic Ash Removal

Bottom ash, or ash collected at the bottom of the combustion chamber, can become re‐suspended and 

carry over into the exhaust, thereby increasing particulate matter emissions. Automatic ash removal can 

ensure frequent bottom ash removal. This may be especially useful if burning a high ash content fuel.  

4.6 Raking Grates as Needed

In most direct burn combustion systems, biomass is combusted on grates. Ash will accumulate on these 

grates. If left to accumulate, this can lead to clinker formation and limit under‐fire airflow thereby 

reducing combustion efficiency. Raking the grates reduces this problem. 

4.7 Combustion Efficiency Testing

Combustion efficiency testing is a way to quantify the degree of combustion completeness, not the overall 

thermal efficiency (heat input divided by heat output). One method for measuring combustion efficiency 

is by measuring carbon monoxide (CO) and carbon dioxide (CO2) concentrations in the boiler exhaust 

with a hand‐held portable analyzer. 

CO is an indicator of the level of gaseous air toxics in boiler exhaust and therefore a good surrogate for 

gaseous air toxics. The relationship between CO and carbon dioxide (CO2) concentrations provides an 

indication of the degree combustion completeness and is therefore appropriate for the levels of all 

emissions in boiler exhaust.  

Vermont and Rhode Island implemented a permit condition requiring measurement of both CO and CO2

in the exhaust gas to determine combustion efficiency. Combustion efficiency is determined using this 

equation, taken from a Vermont air pollution control permit:   
Resource Systems Group, Inc. Emission Control Technologies for Small Wood‐Fired Boilers

23 February 2010 Page 15

Equation 1

( ) % 100

2

2 ×

+ = CO CO

CO CE

Where: 

       CE = Combustion efficiency, 

       CO2 = % by volume of carbon dioxide in the flue gas, and 

       CO = % by volume of carbon monoxide in the flue gas.  

Compliance is demonstrated when the combustion efficiency is calculated to be equal to or greater than 

99%. A representative number of measurements should be taken given the broad range of operating 

conditions that can occur in a given biomass boiler.  

4.8 Visual Plume Observations

Visually observing the exhaust plume is a way to confirm good combustion conditions are occurring. EPA 

publishes two methods for visually evaluating plume opacity. The first one, Method 22, is mostly 

qualitative and does not require formal training.1 The second, Method 9, is more quantitative and 

requires the observer be re‐certified every six months.2 If the plume characteristics pass a Method 22 

test, then the observation is complete. If the Method 22 test is not passed, then the observer can perform 

a Method 9 test to quantify the plume characteristics, if necessary. At minimum, steps should be taken to 

correct combustion conditions if a Method 22 test is not passed.  

4.9 Recordkeeping  

Consistent and thorough record keeping is another means to ensure optimal combustion conditions 

occur continually. Record keeping is therefore considered a means for demonstrating ongoing 

compliance with pollutant emission limits. Record keeping is required in the areas of fuel use, 

maintenance and monitoring. Record keeping has been required for the items below. These requirements 

listed are directly quoted or derived from recent permits issued in Vermont and New Hampshire. 

 Track fuel use on a monthly basis if heat input equal to or greater than 10 MMBtu/hr. Track fuel 

use on an annual basis if heat input less than 10 MMBtu/hr. 

 Measure and record oxygen in percent volume, in the exhaust gas and permanently record the 

output in a log book.   

 Maintain records of the results of the combustion efficiency testing conducted on the Facility’s 

boiler.  These records shall at least include the test date, identification of boiler tested, a 

measurement of the load on the boiler (such as fuel feed rate or steam production rate), the 

concentrations of oxygen, carbon monoxide and carbon dioxide in the exhaust gas as well as the 

calculated combustion efficiency. 

 Install and maintain a temperature sensor to measure the wood‐fired boiler’s exhaust exit 

temperature and permanently record the output in the log book.   

 Observe visible emissions (via EPA Method 22) once a day on normal business days. Record the 

date, time, duration of excursion, and corrective actions taken if visible emissions are not typical 

of good operation. 

 Inspect the differential pressure across the cyclone (once per shift). 

                                                                  1 More information can be accessed at http://www.epa.gov/ttn/emc/methods/method22.html

2 More information can be accessed at http://www.epa.gov/ttn/emc/methods/method9.html
Resource Systems Group, Inc. Emission Control Technologies for Small Wood‐Fired Boilers

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 Visually inspect the cyclone shell, piping, and ducts for leaks; inspect the ash collection 

equipment and check for abnormal noise or hot spots (once per shift). 

 Clean the boiler grates once a day on normal business days. 

 Inspect the cyclone/multicyclone at least once per year or if conditions indicate it may need 

maintenance. Clean the boot and vanes if possible on the annual inspection. 

 Empty the cyclone/multicyclone ash collection vessel as necessary, but not less than once per 

week, in accordance with the manufacturer’s recommendations.1

4.10 Annual Tune Up

Annual tune ups are typically performed on wood boilers. The annual tune up includes a comprehensive 

inspection of the combustor, boiler and pollution control system components.  

Adjustments/improvements to system components are performed as needed. Combustion efficiency is 

typically measured when the annual tune up is performed.  

5.0 ADD‐ON POLLUTION CONTROLS

Add‐on pollution controls are emission control devices which remove pollutants from the exhaust gas 

stream somewhere between the boiler combustion chamber and the exhaust stack. They are installed 

when the combustion equipment cannot reduce emissions to a desired level. In the absence of a fixed 

BACT requirement,2 emission controls are determined on a case by case basis and are a function of the 

following: level of uncontrolled emissions, applicable state/federal emission limits, existing ambient 

pollutant concentrations (background concentrations), stack height and stack proximity of stack to 

sensitive populations. Potential add‐on controls reviewed for this study include cyclones, multicyclones, 

high efficiency multicyclones (HEMCs), core separators, electrostatic precipitators (ESPs) and baghouses 

(fabric filters). The core separator was determined in the 2001 emission control report by Resource 

Systems Group as the Best Available Control Technology (BACT) for small wood‐fired boilers burning a 

wood chip type fuel and capable of limiting PM10 emissions to 0.1 lb/MMBtu.3 The core separator will be 

discussed in further detail later in this section. Again, the conclusions of this report were accepted by a 

consortium of New England states, they did not translate into a formal BACT determination at the federal 

level. 

This section will focus on add‐on controls for reducing filterable particulate matter because for the boiler 

size range considered in this study, combustion controls are used to limit emissions of other pollutants 

such as condensable particulate matter, nitrogen oxides (NOx), carbon monoxide (CO), volatile organic 

compounds (VOCs) and most hazardous air pollutants (HAPs or “air toxics”). Add‐on controls are not 

discussed for sulfur dioxide (SO2) given the low sulfur content of biomass. 

5.1 Mechanical Collectors

Mechanical collectors use centrifugal forces to separate particulate matter from an exhaust gas stream.  

Mechanical collectors include single cyclones, multicyclones, high efficiency multicyclones (HEMCs) and 

                                                                  1 This would also suffice for any other add‐on control device.

2 Massachusetts currently requires all wood boilers meeting the state permit applicability threshold meet a PM10 emission limit of 0.10

lb/MMBtu.

3 An Evaluation of Air Pollution Control Technologies for Small Wood‐Fired Boilers.” Prepared for Vermont Department of Public Service;

Vermont Department of Environmental Conservation, Air pollution Control Division; New Hampshire Governor’s Office of Energy

Resources and Community Services; and the Massachusetts Division of Energy Resources. Resource Systems Group, Inc., White River

Junction, VT. Revised, September 2001.
Resource Systems Group, Inc. Emission Control Technologies for Small Wood‐Fired Boilers

23 February 2010 Page 17

core separators. They are often used as exhaust gas pre‐cleaners for other control devices, such as 

baghouses or ESPs.  

The exhaust gas flow rate is directly proportional to the operating load of the boiler. Pressure drop, an 

indicator of centrifugal separation force which removes particles from exhaust, is directly proportional to 

the exhaust gas flow rate. Therefore, mechanical collectors work best when operating at their respective 

design (maximum) pressure drop. 

5.1.1 CYCLONES AND MULTICYCLONES

Single and multicyclones can remove a large percentage (approximately 90%) of large particles (PM10 

and larger) and remove a small percentage (less than 10%) of fine particles (PM2.5). HEMCs and core 

separators will collect higher percentages of PM10 and PM2.5. 

5.1.2 HIGH EFFICIENCY MULTICYCLONES

The high efficiency multicyclone (HEMC) is similar to a conventional multicyclone, but has a higher 

collection efficiency due to use of a higher pressure drop. Conversely, the additional pressure drop has a 

higher energy demand. The HEMC was found to be the Best Available Control Technology (BACT) in a 

2006 permit application to the Rhode Island Department of Environmental Management (RI DEM) for 

new institutional wood boilers with heat input less than 10 MMBtu/hour. The BACT study translated into 

emission limits of 0.20 lb/MMBtu and 0.18 lb/MMBtu for PM10 and PM2.5 respectively (these limits 

include both filterable and condensable particulate matter).  

Inlet‐outlet testing was performed for the Vermont APCD study at the HEMC installed at the Ponaganset 

Middle School in North Scituate, RI. Two, one‐hour tests were completed for a single wood boiler. The 

HEMC there is designed to operate most effectively at four inches of pressure drop. During testing, the 

wood boiler operated at low load (30% capacity) which created a pressure drop of approximately only 

0.7 inches of water. The HEMC collected 23% of the particulate matter in the boiler exhaust despite the 

low pressure drop and a very high percentage of fine particles in the inlet exhaust gas stream 

(approximately 90%).   

A collection efficiency of 23% is a relatively low number in comparison with other add‐on controls.  

However, it is approximately 10% higher than what a conventional multicyclone can achieve for PM2.5 

removal under design (maximum) pressure drop conditions. Furthermore, design calculations provided 

by the vendor indicate the PM2.5 collection efficiency would have been approximately four times higher 

at design pressure drop.  

HEMC’s can be designed to maintain a high pressure drop at low loads. This can be achieved by using 

valves to regulate the number of cyclones through which exhaust gas passes. For example, at high load, all 

valves would open thereby allowing exhaust gas to distribute among all the cyclones. As load decreased, 

valves would close causing exhaust gas to be distributed among a smaller number of cyclones.   

5.1.3 CORE SEPARATOR

The core separator was previously determined to be BACT for particulate mater, in a 2001 report written 

by Resource Systems Group.1 This technology became commercially unavailable after the report was 

issued, when LSR Technologies went bankrupt. Since that time, the rights to the patent were transferred 

to Easom Corporation, from whom core separators can be purchased.  

Unlike conventional cyclone/multicyclones and the HEMCs in Rhode Island, the core separator design 

will maintain a relatively high pressure drop at all operating loads. Therefore, this technology’s collection 

efficiency will not deteriorate with reduced operating loads. Stack test results indicate it has a TSP 

                                                                  1 An Evaluation of Air Pollution Control Technologies for Small Wood‐Fired Boilers. Resource Systems Group, 55 Railroad Row, White River

Junction, Vermont 05001. September 2001.
Resource Systems Group, Inc. Emission Control Technologies for Small Wood‐Fired Boilers

23 February 2010 Page 18

collection efficiency of approximately 60% and outlet PM2.5 emissions of less than 0.1 lb/MMBtu with 

close coupled gasifiers and approximately 0.15 lb/MMBtu with stoker combustors.  

5.2 Dry Electrostatic Precipitators

Dry Electrostatic precipitators (ESPs) work on the principle of electrostatic attraction. In this, particles in 

an exhaust gas stream are charged as they pass through the ESP and are pulled out of the exhaust gas 

stream by oppositely charged plates on the side of the ESP.  This technology is widely used in Europe to 

control particulate emissions from biomass systems. The control efficiency of PM10 and PM2.5 appears 

to be 99% or greater, making this control technology very compelling.1 There no demonstrated 

applications of ESPs on small wood boilers in the United States. The pervasiveness of ESPs in Europe is 

due to stricter emission limits and higher subsidies.2 There is one demonstrated ESP on a small coal fired 

boiler in North Carolina.  

Until recently, it has been commonly held that ESPs have significantly higher capital costs than 

baghouses. Given changes in ESP design and recent cost analyses, it is now thought that ESPs have 

comparable capital costs to baghouses as they require less ancillary equipment (such as insulated 

ductwork, multicyclone for exhaust pre‐cleaning) than baghouses. This combined with significantly lower 

operating costs and smaller spatial requirements than baghouses, make ESPs seem especially attractive 

when advanced emission control is necessary.   

Significant pressure drops do not occur in ESPs; therefore, they do not require the extra energy to run 

fans to overcome the pressure drop. This means ESPs potentially will have a lower energy demand than 

all other add‐on controls.  

5.3 Baghouses (Fabric Filters)

Baghouses utilize fabric filtration to remove particles from an exhaust gas stream. They are thought to 

provide the highest degree of control of all add‐on controls. They are thought to be capable of removing 

99%+ of filterable PM2.5 emissions.3 This is higher than the control efficiency measured in Greenfield, 

NH or 74%. Again, this lower value was likely due to tramp air flowing through a bypass during the stack 

test there 

Cyclone/multicyclones are used to pre‐clean exhaust gas upstream of baghouses to prevent partially 

burned fly ash particles to reduce fire hazard. As with mechanical collectors, there is pressure drop 

across this control device caused by the exhaust gas passing through fabric. Therefore, energy is required 

to draw exhaust through the fabric.  

The 2001 RSG report determined baghouses were technically infeasible due to threat of fire. A 2006 RSG 

BACT study also found them technically infeasible due to fire hazard and due to potential for filter bag 

clogging, a condition which occurs when the exhaust gas cools to the dew point causing moisture to 

condense on the particulate “cake” on the side of the bag walls. The end product is impermeable and can 

cause bags to rupture.  

Historically in New England, baghouses have not been selected for systems with design inputs less than 

10 MMBtu/hour because the facilities which they would serve did not have the financial or technical 

resources to purchase and service them. For example, this size boiler would serve a small school, which 

would not have a large and experienced facilities staff which could service the baghouse. In the absence 

of a significant subsidy, a small school would not have the financial resources to purchase a baghouse. 

                                                                  1 Compilation of Emission Factors, AP 42, Chapter 1.6. US EPA, revised September, 2003

2 Personal communication with Biomass Energy Resource Center. September, 2009.  

3 Compilation of Emission Factors. AP 42 Chapter 1.6. Revised September, 2003
Resource Systems Group, Inc. Emission Control Technologies for Small Wood‐Fired Boilers

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There are now three demonstrated applications of baghouses on relatively small wood fired boilers in 

New England. These installations are not experiencing filter bag clogging problem because the vendors 

developed a design to avoid this problem. However, there was one fire which occurred in one of the 

systems which required the bags to be replaced. A multicyclone was installed after the fire. No fires have 

been reported since that time. These systems are described below:  

 Mount Wachusett College, Gardiner, MA ‐ 10 MMBtu/hour wood chip boiler.  

 Crochet Mountain Rehabilitation Center, Greenfield, NH – 5.7 and 11.4 MMBtu/hour wood chip 

boilers.  

 Middlebury College, Middlebury, VT – 29 MMBtu/hour wood chip boiler.  

These applications indicate baghouses are technically feasible, provided they are designed to avoid fire 

and filter clogging.  

Baghouses have the highest operating cost of all add on controls discussed in this report. This is largely 

due to the cost of replacement bags and to the amount of time required to keep baghouses in proper 

working order. At this time, it is not known exactly how frequently bags need to be replaced on small 

wood boilers. The rule of thumb is to replace a given bag every three years, which means replacing one 

third of all the bags every year. However, this may not be applicable to wood boilers operating only 

during the heating season.  

6.0 SUMMARY OF EUROPEAN EMISSION CONTROL PRACTICES

The Biomass Energy Resource Center (BERC) recently completed a research trip to Europe to learn more 

about their biomass facilities. Here is a synopsis of their findings. 

 There are larger economic incentives for bioenergy in Europe as compared to the US. For 

example, electricity from biomass power plants can be sold by utilities for approximately 30 euro 

cents/kwh. Utilities in Austria are purchasing electricity for 11 euro cents/kwh. For comparison, 

in Vermont, biomass electricity is sold to utilities for approximately 12.5 cents/kwh and in turn 

sold by utilities for approximately 13 to 14 cents/kwh. 

 Most of the biomass systems deploy energy efficiency measures such as: 

- High combustion temperature 1013°C (1855 F);  

- Low excess air (approximately 50 to 75%);  

- Continuous oxygen content monitoring (%O2) to achieve target content in flue of 

approximately about 7 to 9%; 

- Setting target CO2 exhaust content.  The target of the CO2 in the flue gas is 13%. If less than 

10%, the secondary air is adjusted. Even for the small wood pellet boilers (residential scale) 

the O2 % in the flue gases is monitored continuously and the excess air is modulated based 

on the % O2 content; 

- Pre‐heating of both the primary and the secondary air using an economizer; 

- A heat recovery system using the hot air from the upper level of the boiler room is used to 

dry the woodchips;  

- Water preheated using the flue gas, cooling the flue gases from 900°C (1652 °F) to 180°C 

(356°F)1 before the ESP; and 

- Variable speed drives on the hot water distribution pumps. 

 Emissions control equipment normally used is a multicyclone and ESP in series. ESP’s are 

frequently installed outside of buildings.  

                                                                  1 This temperature is maintained to prevent condensation.  
Resource Systems Group, Inc. Emission Control Technologies for Small Wood‐Fired Boilers

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 Some projects have condensers to remove the moisture from the flue gases simultaneously 

recovering some additional thermal energy.  

 The average ash content is about 2‐5% of the biomass and reflects burning lower quality 

biomass (in some cases). The ash streams are collected separately and utilized where 

appropriate. The ash from the multi‐cyclone and the combustion chamber of the boiler may be 

used as fertilizer in farm operations.  The ash from ESP (fine ash) is land filled, as it may contain 

heavy metals. The plant pays for the removal of the ash from the ESP. In some cases, 

multicyclones are used upstream of ESPs to recover lower metal content ash which is therefore 

less toxic and can be land applied.  

 The capital cost of a typical ESP for a new power plant of 5 to 10 MW capacity range was 

typically about 12‐14% of the cost of the project. This percent may be more for smaller capacity 

plants producing only heat.   

 The quality of woodchips is not considered to be very critical. This is likely attributable to the 

widespread use of ESPs. 

7.0 CAPITAL COSTS OF EMISSION CONTROLS FOR PARTICULATE

MATTER

Capital and operating costs were estimated with quotes from and personal communication with 

equipment vendors as well as the equations and methods presented in the “EPA Cost Control Manual.”1

Information used to generate costs is detailed in the appendix. In addition to the size of the biomass 

combustor, there are a number of other factors which cause variability in the capital costs. Here is a 

selected list of factors affecting price variability:  

• Change in the price of steel. This change had a significant affect on the price of the Core 

Separator and other mechanical collectors. 

• Foreign exchange rates. For equipment purchased overseas, specifically Europe, the cost is 

significantly affected by the exchange rate, which now increases price. The two high efficiency 

multicyclones featured in this report were purchased from a European vendor.  

• Pollution control device design. Capital costs are also affected by the pollution control 

equipment design. For example, the price of electrostatic precipitators is sensitive to the size of 

the particle collection plates. Collection efficiency is related to collection plate size; therefore, 

projects requiring relatively high collection efficiency will result in larger collection plates and a 

more expenive electrostatic precipitator than otherwise. As another example, some baghouses 

are designed with additional cells, to guard against filter clogging which can significantly 

increase the price of a standard baghouse.  

• Space requirements. The amount of horizontal and vertical space required for a given control 

can affect the installation cost. For examples, baghouses can require more space than 

electrostatic precipitators. The additional space required can increase the footprint and/or 

height of the building housing the equipment, thereby increasing construction costs.  

• Ancillary equipment. For example, baghouses require more ancillary equipment, such as 

insulated ductwork and a mechanical collector (to reduce fire risk), than a baghouse. 

• Shipping costs. The proximity of the location to major transportation hubs as well as the 

equipment production location can affect costs.  

                                                                  1 EPA Cost Control Manual, Sixth Edition. U.S EPA report #EPA/452/B‐02‐001. January, 2002. Available at:

http://www.epa.gov/ttn/catc/dir1/c_allchs.pdf.
Resource Systems Group, Inc. Emission Control Technologies for Small Wood‐Fired Boilers

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Estimated capital costs for add‐on particulate matter controls are shown below for a single cyclone, 

multicyclone, high efficiency multicyclone (not valved), high efficiency multicyclone (valved),1 Core 

Separator (with 24 inch diameter separators), Core Separator (with 12 inch diameter separators), 

cyclone coupled with baghouse, and an electrostatic precipitator. Approximate costs are shown for 

systems from approximately 2 MMBtu/hour to 34 MMBtu/hour heat input in Figure 1. 

Figure 1: Particulate Matter Add‐On Emission Control Costs

All capital costs shown above except those for “cyclone + baghouse” were derived exclusively with 

vendor quotes. This category was calculated by inputting the cost of the bags into an equation listed in 

Table 1.9 of the EPA Cost Control Manual, which calculated remaining costs based on the price of the 

bags. They are intended to represent all costs leading up to and including installation. 

  

                                                                  1 As previously mentioned, the term “valved” refers to the use of valves to maintain pressure drop over a range of firing rates.
Resource Systems Group, Inc. Emission Control Technologies for Small Wood‐Fired Boilers

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8.0 COST EFFECTIVENESS OF PARTICULATE MATTER EMISSION

CONTROLS

Cost effectiveness was estimated for the same particulate matter add‐on controls. For the purposes of 

this report, the term “cost effectiveness” refers to the dollars spent to remove one ton of a given pollutant 

in a given year and are a function of the capital and operating costs. Costs were estimated using the 

methodology in the EPA Air Pollution Cost Control Manual and with price quotes and personal 

communication with equipment vendors and other technology experts.1 Cost effectiveness is linearly 

related to the: 

1) Design heat input of the system, 

2) Annual fuel consumption rate (annual capacity factor), 

3) Pollutant inlet loading of the pollution control device and  

4) Size of particle being controlled (PM10 and PM2.5).  

This means the cost effectiveness values listed later in this section can be scaled upward or downward, 

given the linear relationship of the aforementioned factors with cost effectiveness, 

Cost effectiveness was estimated for PM10 and PM2.5. Table 11 summarizes the assumed parameter 

values used to model cost effectiveness. The values used are intended to help portray a small institutional 

wood boiler operating approximately half the year to provide heat and hot water. The inlet loading values 

were taken from AP 42 and correspond to the “wet wood” category. These values were deemed as being 

generally representative for a stoker combustion system. Actual numbers may be lower for “stokers” and 

are likely lower for close coupled gasifiers.  

Table 11: Assumed Parameter Values for Cost Effectiveness Analysis

Wood boiler design heat input 5.0 MMBtu/hour

PM10 Inlet loading 0.31 lb/MMBtu

PM2.5 inlet loading 0.25 lb/MMBtu

Operating hours per year 4380 (half the year)

Average daily operating capacity 50%

Annual capacity factor 25%

Fuel heat content at 40% MC 5,013 btu/lb

Annual fuel consumption 1,095 tons/year

Table 12 and Table 13 show estimated cost effectiveness PM10 and PM2.5 removal respectively, from the 

system summarized in Table 11. A best estimate has been made to assign control efficiencies, capital 

costs and operating costs. Control efficiencies were estimated with the emission test information 

reviewed for this report, AP 42 uncontrolled and controlled emission factors and “Controlling Emissions 

from Wood Boilers,” the RSG 2001 BACT report, personal communication with equipment vendors, and a 

                                                                  1 EPA Air Pollution Control Cost Manual, Sixth Edition. EPA/452/B‐02‐001. United States Environmental Protection Agency, Office of Air

Quality Planning and Standards. Research Triangle Park, North Carolina. January, 2002
Resource Systems Group, Inc. Emission Control Technologies for Small Wood‐Fired Boilers

23 February 2010 Page 23

draft report written by the Northeast States for Coordinated Air Use Management (NESCAUM).1

“Assumed control efficiency values reflect optimal operating conditions are occurring for both the 

combustor and the control equipment. It should be noted that the HEMC’s ranking is based on vendor 

calculations, not actual performance. Furthermore, actual control efficiency will be lower for “un‐valved” 

HEMC’s whose wood boiler is operating below full load.  

As mentioned, there are many factors which cause variability in capital and operating costs. Therefore, 

actual costs could vary considerably than quoted below. Furthermore, these costs should not be regarded 

as exact costs.  

Table 12: Cost Effectiveness for Removing PM10

Pollution Control Device

Control

Efficiency

Capital Cost of

Equipment

Annual

Operating Costs

Total Annual

Costs

Total Cost per

Ton Removed

Cyclone 50% $2,243 $580 $791   $930

Multicyclone 75% $9,424 $580 $1,469   $1,151

HE Multicyclone 77% $62,878 $800 $6,980   $5,297

HE Multicyclone (valved) 77% $125,756 $800 $12,915   $9,802

Core Separator (12") 98% $63,337 $1,239 $7,784   $4,668

Core Separator (24") 98% $111,709 $1,459 $12,570   $7,537

Cyclone + Baghouse 99% $109,878 $3,920 $14,291   $8,483

ESP 95% $138,005 $1,867 $14,894   $9,213

Table 13: Cost Effectiveness for Removing PM2.5

Pollution Control Device

Control

Efficiency

Total Capital

Investment

Annual

Operating Costs

Total Annual

Costs

Total Cost per

Ton Removed

Cyclone 5%   $ 2,243 $580 $791   $11,534

Multicyclone 10%   $9,424 $580 $1,469   $10,707

HE Multicyclone 77%   $65,478 $800 $6,980   $6,569

HE Multicyclone (valved) 77%   $128,356 $800 $12,915   $12,154

Core Separator (12") 75%   $69,337 $1,239 $7,784   $7,563

Core Separator (24") 60%   $117,709 $1,459 $12,570   $15,266

Cyclone + Baghouse 99%   $109,878 $3,920 $14,291   $10,519

ESP 90%   $138,005 $1,867 $14,894   $         12,059

9.0 OVERVIEW OF EMISSION CONTROLS FOR OTHER RELEVANT

POLLUTANTS

9.1 Nitrogen Oxides (NOx)  

Minimizing excess air usage reduces thermal NOx emissions. Oxygen concentration is an indicator of the 

amount of excess air; therefore, monitoring oxygen concentration and linking oxygen measurements with 

automated controls establish the appropriate quantity of excess air on a continual basis and prevent 

excess thermal NOx emissions. 

                                                                  1 “Controlling Emissions from Wood Boilers.” Northeast States for Coordinated Air Use Management (NESCAUM). October, 2008.

Available at: http://www.nescaum.org/topics/commercial‐wood‐boilers.
Resource Systems Group, Inc. Emission Control Technologies for Small Wood‐Fired Boilers

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9.2 Sulfur Dioxide (SO2)  

SO2 emissions from wood combustion are negligible given very low sulfur content in biomass. 

9.3 Carbon Monoxide (CO)  

Carbon monoxide emissions are minimized by good combustion conditions, specifically, maintaining the 

proper air to fuel ratio. Plume opacity observation, proper operation and maintenance, periodic 

combustion efficiency testing, and in‐situ oxygen concentration monitoring are ways to ensure ongoing 

good combustion conditions. A combustion chamber designed with staged combustion increases the 

degree of combustion completeness and is therefore useful for minimizing CO emissions.  

9.4 Volatile Organic Compounds (VOCs)

The same measures for minimizing CO emissions will minimize VOC emissions.  

9.5 Hazardous Air Pollutants (HAPs)

Hazardous air pollutants include both gaseous and particulate based pollutants. The aforementioned 

controls for CO and VOCs are also effective at controlling HAPs, but most effective at controlling gaseous 

HAPs. Particulate HAPs are also controlled with an add‐on control device.   

10.0 CONCLUSIONS

 There is new stack emission test information which provides some additional insights into how 

fuel characteristics and add‐on controls affect levels of particulate emissions.  

 Limited information is available for pellet combustion emissions, high efficiency multicyclone 

(HEMC) control efficiency, and electrostatic precipitator (ESPs) control efficiency for small wood 

boilers only.  

 While the quantity of emissions information for wood boilers approximately 10 MMBtu/hour 

and smaller is growing to a level on which generally meaningful interpretations can be made, this 

quantity of information is not available to make meaningful interpretations for wood boilers 

smaller than 5.0 MMBtu/hour. 

 Fuel characteristics and fuel quality affect fuel handling and combustion conditions and 

therefore levels of pollutant emissions.    

 There are many measures which can be implemented to characterize, enhance, preserve and test 

fuel quality. Implementing these measures will translate into improved combustion conditions 

thereby increasing energy efficiency and reducing emissions.  

 There are many Best Management Practices (BMPs), also called work practice standards which, 

when implemented, promote combustion completeness, minimize energy loss and reduce 

emissions. 

 The increased combustion completeness created by BMPs not only helps control particulate 

matter emissions, but also helps control emissions of gaseous pollutants, including carbon 

monoxide, nitrogen oxides, volatile organic compounds (VOCs) and hazardous air pollutants 

(HAPs).  

 BMPs help prevent combustion upset conditions which will help eliminate nuisance episodes 

such as excessive plume opacity. 
Resource Systems Group, Inc. Emission Control Technologies for Small Wood‐Fired Boilers

23 February 2010 Page 25

 Particle size has a large affect on collection efficiency in conventional mechanical collectors 

(mechanical collectors with a relatively low pressure drop). Substantially smaller quantities of 

fine particles (PM2.5) than coarse particles (PM10) can be collected in conventional mechanical 

collectors.  

 Pressure drop has a large affect on mechanical collector collection efficiency. Core separators 

collect substantially larger quantities of fine particles than conventional mechanical collectors, at 

all firing rates, as they are designed to maintain a high pressure drop at all firing rates. 

 New baghouse designs are safer and technically feasible. These conclusions are demonstrated by 

three relatively new baghouse installations in the northeast. 

 Potential exists for implementing an HEMC design which maintains a relatively high pressure 

drop at a range of firing rates using valves.  

 The core separator was commercially unavailable, but now has the potential to emerge back into 

the marketplace.  

 ESPs and baghouses have comparable capital costs for some size systems. ESPs have lower 

operating costs and have lower energy demand.  

 Recent experience in Europe indicates greater prevalence of ESPs due to greater financial 

incentives and stricter emission limits.  

 Baghouses and ESPs are the add‐on controls providing the highest degree of control of PM2.5 for 

all firing rates. 

 For small systems less than 2.0 MMBtu/hr, the annual operating cost of all add‐on emission 

controls except cyclones, multicyclones and “non‐valved” high efficiency multicyclones may 

likely exceed the annual fuel savings. 

11.0 RECOMMENDATIONS FOR FUTURE RESEARCH

Based on the analysis completed for this project, it is clear there are many variables which affect emission 

rates (location of source, type of fuel burned, firing rate, type of combustion technology, etc.). 

Consequently, it would be beneficial for more testing to be completed. It would be helpful for future tests 

to be structured to further understanding of the following: 

 Particulate and gaseous emissions from pellet boilers (PM10, PM2.5, CO and NOx).  

 Inlet‐outlet testing of PM10 and PM2.5 for add‐on control devices controlling pellet boiler 

emissions. Particle size distribution should also be measured at the inlet and outlet, in addition 

to measuring mass emission rates. 

 Inlet‐outlet testing at a range of firing rates for particulate emissions (PM10, PM2.5) controlled 

by HEMCs. Particle size distribution should also be measured at the inlet and outlet, in addition 

to measuring mass emission rates. 

 Inlet‐outlet testing of PM10 and PM2.5 for ESPs on small wood‐fired boilers (there will likely be 

a number constructed in 2010). Particle size distribution should also be measured at the inlet 

and outlet, in addition to measuring mass emission rates. 

 HAP emissions in concert with CO and PM2.5. Inlet and outlet testing should be performed for 

particulate HAPs. PM2.5 should be tested because it is considered a surrogate for particulate 

HAPs. CO should be tested (outlet testing only) because it is considered a surrogate for gaseous 

HAPs.  

Stack testing for any given site should be expanded to include emissions from the following: 

 Low, medium and high firing rates 
Resource Systems Group, Inc. Emission Control Technologies for Small Wood‐Fired Boilers

23 February 2010 Page 26

 At least two fuels, such as, wood with bark (bole chips or whole tree chips) and wood without 

bark (mill chips). 

The following are also recommended: 

 Development of a voluntary universal boiler operator training program for obtaining boiler 

operator certification. 

 Further development of fuel quality specifications to further establish grades of wood fuels.  

12.0 SUMMARY

A number of emission controls for small wood‐fired boilers have been evaluated. This study evaluated a 

number of Best Management Practices (BMPs – also called work practice standards) and add‐on controls. 

While these controls are focused on particulate matter control, their implementation will control 

emissions of all types of pollutants, including carbon monoxide, sulfur dioxide, nitrogen oxides, volatile 

organic compounds and hazardous air pollutants. Maximizing fuel quality, optimizing combustion 

conditions and selecting a well designed add‐on pollution control are the three main categories for 

controlling emissions. Control efficiency and cost varies by boiler size and type of add‐on pollution 

control.  
APPENDIX A 

STACK TEST REPORTS 
APPENDIX B 

EMISSION CALCULATIONS 
Hazardous Air Pollutant Synopsis

PollutantCategory System Information / Pollutant

Ponaganset

MS Result

(lb/MMBtu)

Ponaganset HS

Result

(lb/MMBtu)

Council Result

(lb/MMBtu)

Green Acres

Result

(lb/MMBtu)

Hazen Union

Result

(lb/MMBtu)

Overall Average

Result

(lb/MMBtu)

AP 42

Emission

Factor

(lb/MMBtu) % of AP 4

Design Heat Input 9.1 4.6 1.9 2.2 2.8

Combustion type CCG CCG SA SA CCG

Emission control HEMC HEMC Uncontrolled Uncontrolled Multicyclone

Metals Arsenic 1.54E‐06 8.40E‐07 4.54E‐06 2.66E‐06 5.71E‐07 2.03E‐06 2.20E‐05 9% 

Cadmium 2.15E‐05 1.78E‐05 3.93E‐06 1.44E‐05 4.10E‐06 352%

Chromium 2.34E‐05 2.64E‐05 6.51E‐06 1.88E‐05 2.10E‐05 89%Chromium VI 4.83E‐06 1.31E‐06 1.31E‐06 8.37E‐07 2.07E‐06 3.50E‐06 59% 

Nickel 9.87E‐06 3.29E‐06 2.20E‐05 2.11E‐05 3.57E‐06 1.20E‐05 3.30E‐05 36% 

Manganese 9.58E‐05 8.66E‐05 9.12E‐05 1.60E‐03 6%

Phosporus 1.00E‐04 1.29E‐04 1.15E‐04 2.70E‐05 425%

Organics Formaldehyde 9.30E‐04 4.77E‐04 9.50E‐04 1.05E‐02 1.38E‐03 2.84E‐03 4.40E‐03 65% 

Benzene 1.50E‐04 4.11E‐06 4.49E‐05 5.80E‐05 6.43E‐05 4.20E‐03 2% 

PAHs Acenapthene 4.27E‐08 5.05E‐08 0.00E+00 7.53E‐07 4.65E‐07 2.62245E‐07 9.10E‐07 29% 

Acenapthylene 4.80E‐06 4.81E‐06 0.00E+00 3.33E‐05 2.04E‐04 4.93813E‐05 5.00E‐06 988% 

Anthracene 1.82E‐06 1.41E‐06 0.00E+00 2.44E‐06 1.42E‐05 3.97491E‐06 3.00E‐06 132% 

Benz(a)anthracene 2.38E‐05 2.84E‐05 0.00E+00 1.54E‐06 1.63E‐05 1.40124E‐05 6.50E‐08 21,558% 

Benzo(b)fluoranthene 2.18E‐05 1.83E‐05 0.00E+00 3.66E‐06 2.51E‐05 1.37725E‐05 1.00E‐07 13,772% 

Benzo(ghi)perylene 2.82E‐06 2.57E‐06 2.52E‐07 1.93E‐06 1.44E‐05 4.39315E‐06 9.30E‐08 4,724% Benzo(e)pyrene 1.50E‐05 9.65E‐06 0.00E+00 2.38E‐06 1.57E‐05 8.5388E‐06 2.60E‐09 328,416% 

Benzo(a)pyrene 4.56E‐06 5.50E‐06 0.00E+00 9.91E‐07 1.34E‐05 4.88928E‐06 2.60E‐06 188% 

Chrysene 4.91E‐05 4.46E‐05 0.00E+00 3.15E‐06 2.43E‐05 2.42272E‐05 3.80E‐08 63,756% 

Dibenz(a,h)anthracene 3.73E‐07 4.15E‐07 0.00E+00 1.33E‐07 6.97E‐06 1.57829E‐06 9.10E‐09 17,344% 

Fluoranthene 2.82E‐05 9.15E‐05 1.31E‐06 1.08E‐05 9.51E‐05 4.53915E‐05 1.60E‐06 2,837% Fluorene 1.17E‐07 1.39E‐07 1.43E‐06 6.22E‐07 2.90E‐06 1.04166E‐06 3.40E‐06 31% Ideno(1,2,3‐cd)pyrene 2.22E‐06 1.98E‐06 0.00E+00 1.28E‐06 2.77E‐06 1.65057E‐06 8.70E‐08 1,897% 

2‐methylnapthalene 1.02E‐06 9.51E‐07 0.00E+00 1.32E‐05 1.32E‐05 5.67378E‐06 1.60E‐07 3,546% 

Napthalene 2.85E‐05 2.57E‐05 6.55E‐06 1.26E‐04 7.47E‐05 5.2285E‐05 9.70E‐05 54% 

Phenanthrene 2.46E‐05 2.05E‐05 3.17E‐06 2.67E‐05 2.64E‐04 6.77896E‐05 7.00E‐06 968% 

Pyrene 3.05E‐05 3.22E‐05 4.69E‐06 9.38E‐06 8.12E‐05 3.15871E‐05 3.70E‐06 854% 

AP 42 Ponaganset Total HAPs 1.53E‐03 9.91E‐04 1.04E‐02

Comparison Ponaganset % of AP 42 15% 10%

Council Total HAPs 1.04E‐03 4.60E‐03

Council % of AP 42 23%

Green Acres Total HAPs 1.08E‐02 8.81E‐03

Green Acres % of AP 42 123%

Hazen Total HAPs 2.32E‐03 8.81E‐03

Hazen % of AP 42 26%Green Acres & Hazen Total HAPs 3.29E‐03 8.81E‐03

Green Acres & Hazen % of AP 42 37%
APPENDIX C 

OPERATING COST CALCULATIONS 
Operating Cost Calculation Values & References

GENERAL INPUTS

Category Value Notes

Interest rate (%) 0.07 Default value from EPA Cost Control Manual

Economic life (years) 20 Default value from EPA Cost Control Manual

Capital Recovery Factor (CRF) 9% Equation 2.8a from EPA Cost Control Manual

CYCLONE / MULTICYCLONE OPERATING COSTS

Category Value Notes

total ash disposal cost $0.00 all ash land applied for soil enhancement

electricity price ($/kwh) 0.1 av'g commercial elec. cost in 2009 (http://www.eia.doe.gov/cneaf/electricity/epm/table5_3.html)

waste gas flow rate (acfm) 2562 from review of recent stack test reports in Vermont

static pressure drop (in wc) 2 based on project experience

combined fan‐motor efficiency 60% from EPA Cost Control Manual

hours per year 2,190 operating half the year at average operating capacity of 50% load

incremental electricity cost $219.76 EPA Cost Control Manual, Equation 1.46.

labor hours per year 12 regular inspections, unexpected maintenance and annual inspection of the interior

labor rate $30.00 personal communication with wood boiler vendor

labor costs $360.00

total annual costs $579.76

HIGH EFF MULTICYCLONE

Category Value Notes

total ash disposal cost $0.00 all ash land applied for soil enhancement

electricity price ($/kwh) 0.1 av'g commercial elec. cost in 2009 (http://www.eia.doe.gov/cneaf/electricity/epm/table5_3.html)

waste gas flow rate (acfm) 2,562 from review of recent stack test reports in Vermont

static pressure drop (in wc) 4 from vendor quote

combined fan‐motor efficiency 60% from EPA Cost Control Manual

hours per year 2,190 operating half the year at average operating capacity of 50% load

incremental electricity cost $439.52 EPA Cost Control Manual, Equation 1.46.

labor hours per year 12 regular inspections, unexpected maintenance and annual inspection of the interior

labor rate $30.00 personal communication with wood boiler vendor

labor costs $360.00

total annual costs $799.52

CORE SEPARATOR (24 INCH)

Category Value Notes

total ash disposal cost $0.00 all ash land applied for soil enhancement

electricity price ($/kwh) 0.1 av'g commercial elec. cost in 2009 (http://www.eia.doe.gov/cneaf/electricity/epm/table5_3.html)

waste gas flow rate (acfm) 2,562 from review of recent stack test reports in Vermont

static pressure drop (in wc) 8 personal communication with vendor

combined fan‐motor efficiency 60% from EPA Cost Control Manual

hours per year 2,190 operating half the year at average operating capacity of 50% load

incremental electricity cost $879.03 EPA Cost Control Manual, Equation 1.46.

labor hours per year 12 regular inspections, unexpected maintenance and annual inspection of the interior

labor rate $30.00 personal communication with wood boiler vendor

labor costs $360.00

total annual costs $1,239.03
CORE SEPARATOR (12 INCH)

Category Value Notes

total ash disposal cost $0.00 all ash land applied for soil enhancement

electricity price ($/kwh) 0.1 av'g commercial elec. cost in 2009 (http://www.eia.doe.gov/cneaf/electricity/epm/table5_3.html)

waste gas flow rate (acfm) 2,562 from review of recent stack test reports in Vermont

static pressure drop (in wc) 10 personal communication with vendor

combined fan‐motor efficiency 60% from EPA Cost Control Manual

hours per year 2,190 operating half the year at average operating capacity of 50% load

incremental electricity cost $1,098.79 EPA Cost Control Manual, Equation 1.46.

labor hours per year 12 regular inspections, unexpected maintenance and annual inspection of the interior

labor rate $30.00 personal communication with wood boiler vendor

labor costs $360.00

total annual costs $1,458.79

BAGHOUSE OPERATING COSTS

Category Value Notes

total ash disposal cost $0.00 all ash land applied for soil enhancement

total bag cost $4,250.00 vendor quote for coated bags

% of bags replaced annually 8% 1/12 of bags replaced every year assuming 50% annual capacity factor

bag replacement cost ($/yr) $350.63

electricity price ($/kwh) 0.1 av'g commercial elec. cost in 2009 (http://www.eia.doe.gov/cneaf/electricity/epm/table5_3.html)

waste gas flow rate (acfm) 2,562 from review of recent stack test reports in Vermont

static pressure drop (in wc) 5 from permit application for new wood boiler in central Massachusetts

combined fan‐motor efficiency 60% from EPA Cost Control Manual

hours per year 2190 operating half the year at average operating capacity of 50% load

incremental electricity cost $549.40 EPA Cost Control Manual, Equation 1.46.

labor rate $30.00 personal communication with wood boiler vendor

labor hours per year 81 1 hr/wk general maint., 80 hours to replace all bags, 8 hours for black light testing at 2x per year

labor costs $2,440.00

total annual costs $3,340.02

ESP OPERATING COSTS

Category Value Notes

total ash disposal cost $              ‐ all ash land applied for soil enhancement

pressure drop (in. water) 0.04 page 3‐34 in EPA Cost Control Manual

operating time (h/yr) 2,190 operating half the year at average operating capacity of 50% load

system flow rate (acfm) 2,562 from review of recent stack test reports in Vermont

fan power req. (kWh/yr) 41 EPA Cost Control Manual, equation 3.46

electricity req. (kWh/yr) 2431 personal communication with vendor

electricity price ($/kwh) 0.1 av'g commercial elec. cost in 2009 (http://www.eia.doe.gov/cneaf/electricity/epm/table5_3.html)

total electricity demand 569.4 sum of fan and other electricity requirements

total power cost $        247.20

labor hours per year 8 personal communication with vendor. Open, inspect and clean ESP.

labor rate 30.00 $           personal communication with wood boiler vendor

labor costs $        240.00

maintenance costs $    1,380.05 page of EPA Cost Control Manual, equation 3‐45

total annual costs $    1,867.25