Combating Particulate Emissions

in Energy Generation and Industry

Views and Conclusions from the FINE Particles – 

Technology, Environment and Health Technology Programme

Mikael Ohlström • Jorma Jokiniemi • Jouni Hokkinen • Pasi Makkonen • Jarkko Tissari

Edited by Peter Herring

Combating Particulate Emissions

in Energy Generation and Industry

Views and Conclusions from the FINE Particles 

– Technology, Environment and Health Technology Programme

Part of a series of fi ve reports that cover energy and industry, traffi c and transport, 

measurement technology, indoor air, and health and the environment.
Layout by Quickmac Oy

Printed by Libris Oy, Helsinki

ISBN 952-457-246-X

Copyright Tekes 2006. All rights reserved.

This publication includes material protected under copyright law, the copyright for which is held by Tekes or a third 

party. Material contained here may not be used for commercial purposes. The contents are the opinion of the writers 

concerned and do not represent the offi cial Tekes position. Tekes bears no responsibility for any possible damage arising 

from the use of this material. The original source must be mentioned when quoting from this publication.

Contact information:

VTT Technical Research Centre of Finland

P.O. Box 1000

FI-02044 VTT, Finland

www.vtt.fi 

email: fi rstname.lastname@vtt.fi 
3

Preface

T

he air we breathe exposes us to a growing number 

of airborne contaminants, including particulate 

matter invisible to the eye, as a result of ever-growing traffi c volumes and ever-higher levels of energy generation, as well as other factors. Fine particulates are 

known to have major health implications, and result in 

the premature death of over 300,000 people in the EU 

annually alone. Advances in technology are allowing us 

to measure smaller and smaller sizes of particles and 

gain a better understanding of how they impact health 

and the environment, however. 

The more we learn about the very real harm caused 

by these types of emissions, the more the benefi ts 

of introducing tougher air quality standards become 

clearer. Typical of the initiatives being introduced in this 

area is the European Commission’s Clean Air for 

Europe (CAFE) programme, the results of which will 

be used to enhance European air quality from 2010 

onwards.

Tekes launched a four-year programme in 2002 

known as the FINE Particles – Technology, Environment and Health Technology Programme – together 

with the Academy of Finland, the Ministry of Transport 

and Communications, and the Ministry of the Environment – to address some of these challenges. In particular, to generate new research data, leverage the existing 

broad range of Finnish expertise in fi ne particles and 

develop it further, and catalyse business opportunities 

in the fi eld. The programme’s ultimate goal has been to 

improve our understanding of the impact emissions of 

this type have on health and the environment, identify 

the potential for new technological innovations, and encourage their commercialisation.

The FINE programme involved over 50 individual 

projects and close to 60 companies and 20 research institutions, and focused on emissions and technology in 

fi ve main areas: energy and industry, traffi c and transport, measurement, indoor air, and health and the environment. A summary report has been produced on 

each of these areas. 

Drawing on the work of 14 projects carried out by 

teams from research institutions and the corporate sector, 

this report concentrates on particulate emissions related 

to the energy and industrial sector – and is designed to 

provide an overview of the programme’s measurement 

fi ndings in respect of different types of plants, large and 

small, together with an assessment of current technologies and the potential for improving on them. 

Electrostatic precipitators and fabric fi lters, for example, are already widely used and offer good performance, 

but can be employed even more effi ciently. No one technology offers a complete answer, however, and selecting 

the best technology or combinations of technologies for 

different applications is essential.

While much has been learnt from the FINE programme, it will be important to maintain the momentum 

that has been built up and continue research on fi ne particle emissions and emission reduction. This is particularly true in the case of industrial processes, where much 

less is known about the nature and extent of particulate 

emissions compared to the energy sector.

We would like to thank all the researchers and authors that have contributed to the FINE Programme, 

and whose work has made the publication of this report 

and the others in the series possible.
4

Contents

 Preface 3

1 Introduction 5

 Energy generation 5

 Waste incineration 5

 Industrial processes 5

 Small-scale generation 6

 The legislative framework 8

 The focus of the FINE Programme 9

2 Current technologies for controlling fi ne particulate emissions 11

 Electrostatic precipitators 11

 Upgrading ESP performance 12

 Fabric fi lters 13

 Economic comparison of ESPs and fabric fi lters 13

 Mechanical/inertial collectors 14

 Wet scrubbers 14

 Optimal combustion and additives 15

3 Findings from FINE projects 19

 Waste incineration 19

 Kraft recovery boilers 21

 Sintering plants 23

 Heavy fuel oil 23

 Biomass 25

 Summary 26

4 Future trends 28

 Ultra-low emissions technology 29

5 Conclusions 31

 References 32
5

1 Introduction

T

he main sources of combustion-related particulates 

are cars and trucks, residential wood combustion, 

and utility and commercial boilers. Primary emissions mainly comprise unburned fuel (hydrocarbons), elemental carbon (soot), sulphates, and mineral salts, often 

with traces of toxic metals. Secondary emissions are a 

combination of ammonia with either sulphuric acid or 

nitric acid and water, together with a complex mixture 

of VOC-related organic oxidation products. Alkali metals (Na and K) and some trace metal compounds (e.g. 

Hg, Se, Pb, Cd, Cu, Tl, Zn, As, Cr, Ni, V) occur at sizes 

less than 1 μm during combustion. 

Concentrations of fi ne particles (< 2.5 μm) depend 

on the fuel concerned, the combustion process used, and 

combustion conditions. Higher combustion temperatures 

typically increase the amount of fi ne particles released. 

Biomass combustion also generally produces more fi ne 

particles than coal combustion, for example, due to the 

different ash-forming compounds involved. 

Many trace elements are relatively volatile during 

combustion, and are enriched in the fi ne particle fraction as a result (Lind 2003). Soot and organic carbon 

species also exist as fi ne particles as a result of incomplete combustion, linked to issues such as poor mixing 

and short residence times at high temperatures.

This report will focus on emissions of fi ne particulates 

from wood and heavy fuel oil combustion, waste incineration, and industrial processes, as emissions of this type 

resulting from fi ring coal and peat were not measured as 

part of the FINE Programme, and have been reported 

in detail elsewhere (see e.g. Ohlström 1998). Small-scale 

combustion is detailed separately.

Energy generation

Particulate emissions generated during solid fuel combustion typically consists of two types of particles: fi ne 

particles approximately 0.1-1 μm in diameter, formed 

from the ash-forming species that are volatilised during 

combustion, and residual ash particles larger than 1 μm 

in diameter that are formed from the mineral impurities 

contained in fuels. 

A third type of particle is generated in fl uidised bed 

combustion, consisting of fragmented bed material. This 

is produced by the interaction that takes place between 

volatilised substances and combustion-related fi ne particles with the bed material and coarse residual ash particles. Combined with the fact that the maximum temperatures in fl uidised bed units are lower than those in 

grate boilers, this results in the fi ne particle emissions 

from fl uidised bed boilers typically being much lower 

than those from grate boilers.

Waste incineration

The emission levels covering waste incineration are the 

toughest of any imposed, and the stack emissions of 

fi ne particulates are typically so low as to be under the 

measurement threshold.

Waste fuels often contain high amounts of volatile 

chlorine, potassium, sodium, and toxic trace elements. 

Large amounts of fl y ash-bound Cd, Cu, Pb, and Tl have 

been found in fi ne particle fractions (Lind et al. 2005). 

During combustion, large fractions of these elements 

are released into the gas phase, and condense on fi ne 

fl y ash particles when the fl ue gases cool. Fuel chlorine 

content has a signifi cant effect on enrichment. Organic 

chlorine (PVC), in particular, is known to infl uence trace 

element partitioning. 

Other elements are not released from the fuel during combustion, or react with the coarse fl y ash and bed 

material particles, ending up in the coarse fl y ash fraction. The schematics of fi ne particle formation in waste 

incineration are illustrated in Figure 1.

Industrial processes

Black liquor, a waste sludge from the pulping processes 

used in papermaking, is burned in kraft recovery boilers to recover pulping chemicals and generate energy. 

Although black liquor does not contain mineral inclusions, it does contain a large amount of sodium, potassium, sulphur, and chlorine, which partially volatilise 

and condense to form small aerosol particles, commonly 

referred to as fume particles. 

At high temperatures (1400-1600 °C), all volatilised 

Na and K exists fi rst as hydroxides and chlorides in the 

gas phase. Alkali hydroxides and chlorides convert to 

sulphates by reactions with SO2 as the temperature decreases in a recovery boiler, according to equilibrium 

calculations. If the maximum temperature is below 

1,400 °C, sulphates form immediately after the released 

alkali hydroxides and chlorides mix with the main gas 

components. 

The formation of Na2SO4 in the gas phase can be 

limited due to kinetic considerations. Recent results 

from laminar fl ow reactor experiments and SEM samples taken from upper furnace areas, however, indicate 

that Na2SO4 fume particles form at temperatures above 
6

Trace elements

Pb, Cu...

K, Na, Ca ash 

particles

heterogeneous

Tg[°C] 900-1500 400-800 0-500 Characteristics before filter

sulphates

chlorides

oxides

PbCl2, CuCl2..MxOy

KCl, K2SO4, CaSO4

Fuel

particle

pyrolysis/

burn-out/

flame/

volatilisation

sulphates

chlorides

oxides

1,100 °C. When there is not suffi cient SO2 to bind all 

the Na and K present, these species convert to carbonates through surface reactions. These surface reactions 

may also be kinetically limited. Some chlorides condense directly as the temperature decreases further. See 

Jokiniemi et al., 1996 for more details and Figure 2 for 

the schematics of fi ne particle formation in black liquor combustion.

The fi ne particle mode takes the form of larger particles (1-3 μm) in black liquor combustion, due to the 

larger amount of volatilised material and mass concentration. ESP (electrostatic precipitator) effi ciency is 

good here, because of the lack of fi ne particles between 

0.1 – 1 μm, in respect of which ESPs offer minimal removal effi ciency.

Relatively little is known as yet about fi ne particle 

emissions from steelworks and similar facilities. One 

reason for this is the diffi culty of setting up measurement systems due to the dozens of emission sources 

involved, varying from stacks to open roofs. The high 

metallic content of particles creates electrical charges 

and measurement problems. 

Small-scale generation

Both wood and oil are largely used in small-scale units 

in Finland. Wood is also fi red in larger boilers, but generally in combination with other fuels.

Wood

In the case of wood combustion, the main volatilised 

species are K, Cl, SO4, and CO3. The fi rst particles 

formed after volatilisation as the fl ue gas cools are produced by the homogeneous or heterogeneous condensation of K2SO4. KCl and K2CO3 are also associated with 

particles at lower temperatures. Other species volatilised 

Figure 1. Fine particle formation mechanisms in waste incineration.

Figure 2. Fine particle formation mechanisms in 

recovery boilers (black liquor combustion).

Na(g)

NaOH (g)

H 2O

M (c)

NaOH (g) , NaCl( g)

SO 2

Na 2SO4 ( g) , NaOH( g) , NaCl( g)

M xO y (c)

M xO y ( c)

Na2SO 4 ( c)

M xOy (c)

HCl

Na2SO 4 (c)

M xO y( c)

Na2CO 3 (c)

NaCl ( c)

boundary layer

SO 2

CO 2

HCl

fume particles

Black liquor droplet,

char particle or

Agglomeration

char bed

NaCl (g)

(c) condensed

(g) gaseous
7

and involved in fi ne particle formation are typically Na, 

Zn, and small amounts of Ca. The schematics of fi ne 

particle formation in biomass combustion are illustrated in Figure 3. 

Emissions from small-scale wood combustion, especially of the batch type, vary considerably from those 

of MW-scale boilers. The amount and composition of 

fi ne particles varies, according to combustion technique, 

fuel quality (moisture and ash content), equipment performance, and possible reduction technologies. The combustion process can be better controlled in continuous 

combustion, such as in a stoker burner, which results in 

lower emissions. Emissions also vary during start-ups 

and shutdowns and low load situations. 

The particulates generated in small-scale wood combustion are composed of minerals (alkali compounds 

K, Ca) in cases where combustion equipment and conditions are good. Where they are poor, particles can be 

composed mainly of incomplete combustion constituents, such as soot and tar compounds.

Heavy fuel oil

Fuel oil and diesel oil contain sulphur, which forms sulphuric acid during combustion in the presence of vanadium and other metals. Vanadium catalyses sulphur 

dioxide conversion to sulphur trioxide, which readily 

forms sulphuric acid in the presence of water vapour. 

Sulphuric acid starts to condense onto particles below 

approximately 160 oC. 

This means that the measurement temperature and 

method used to assess the combustion of fuel oil containing sulphur must be selected carefully to achieve 

accurate results, as the lower the temperature, the more 

sulphuric acid will condense into fi ne particles (Lyyränen 

et al., 2002). See Figure 4 for an overview of fi ne particle formation in heavy fuel oil combustion.

Figure 3. Fine particle formation mechanisms in biomass combustion.

Fuel

particle

pyrolysis/

burn-out/

flame/

volatilisation

soot

K, Na, Ca inorganic 

species

Zn...

Heterogeneous 

sulphates

chlorides

oxides

carbonates

hydrocarbons

Tg[°C] 900-1500 600-1000 0-500 Characteristics before filter

Incomplete combustion

- soot+organics

Good combustion

- ash ( inorganic)

Figure 4. Fine particle formation mechanisms in heavy fuel oil combustion.

droplet

V, Ni, ...ashparticles sulphates

oxides

H2SO4+hydrocarbons

Tg[°C] 1200-2500 800-1500 0-200 Emission

V, Ni, Fe catalyzes

SO2 SO3

pyrolysis/

burn-out/

flame/

volatilisation

Fuel

soot

Incomplete combustion

- soot+organics+H2SO4

Good

combustion

- ash + H2SO4
8

The legislative framework

Although a lot of research on fi ne particulate (PM2.5, 

PM1) emissions from energy generation and industrial 

processes has been carried out in recent years, there are 

as yet no emission standards for these types of particles – only for total particle mass released from power 

plants and industrial processes. However, the tendency 

in contemporary air quality legislation is towards regulating fi ne particles (PM2.5), while the focus earlier 

was on total suspended particles (TSP) and respirable 

particles (PM10). 

The EU Commission announced its air protection 

strategy (COM(2005)446) and a proposal for a new air 

quality directive (COM(2005)447) in September 2005. 

The main target of the new strategy is to reduce human 

exposure both to direct (primary) and indirectly formed 

(secondary) fi ne particles, as these emissions have been 

identifi ed as reducing life expectancy by 8–9 months in 

the EU generally. The fi gure in some countries, such as 

Finland, is less (2–3 months).

The air quality directive has two new elements aimed 

at reducing the health impact of fi ne particles. The fi rst 

of these is a proposed annual ceiling limit of 25 μg/m3. 

As this corresponds to the binding limit of 40 μg/m3 for 

respirable particles (PM10) valid at present, this ceiling 

value will reduce long-range fallout from highly polluted areas for the most part. The second new element is 

the target of reducing human exposure caused by fi ne 

particle concentrations by 20% between 2010 and 2020 

everywhere outside cities where concentrations are 7 

μg/m3 or more. This target will be based on fi ne particle 

concentration measurements beginning in 2008.

These new legislative initiatives are based on work 

done as part of the three-year CAFE (Clean Air for 

Europe) Programme on the adequacy of present legislation to fulfi l the air quality targets of the EU’s Sixth 

Framework Programme for 2020. As US regulations on 

fi ne particles are even stricter, it is possible that the EU 

could move towards tougher legislation. 

PM10 (μg/m3) EU US

Hourly – –

24-hour mean 50a 150b

Annual mean 40 50c

PM2.5 (μg/m3) EU US

24-hour average – 65d

Annual average – (CAFE: 25) 15e

a) not to be exceeded more than 35 times a calendar year; b) not 

to be exceeded more than once a year c) expected annual arithmetic mean PM10 concentration at each monitor within an area; 

d) the 3-year average of the 98th percentile of 24-hour concentrations at each population-oriented monitor within an area; e) the 3-

year average of the annual arithmetic mean PM2.5 concentrations 

from single or multiple community-oriented monitors

Table 1. Air quality limit values in the EU and 

the US.

In respect of the energy sector, the CAFE strategy 

indicates that there will not be any changes to the recent 

LCP directive on plants generating over 50 MW (fuel capacity). Plants in the 20–50 MW range, however, could 

be covered under future iterations of the IPPC directive 

where they address air quality. 

For small-scale heating needs, the technical regulations covering domestic installations and fuels are to be 

harmonised and will prioritise low-emission solutions. 

The scope of the energy effi ciency directive could also 

be extended to smaller buildings than at present.

Although there are not any direct regulations on 

PM2.5 emissions at present, the directive on waste incineration, together with BAT (Best Available Technology) practise, are having an impact. The EU waste incineration threshold limits act as a guide for emissions 

from other plants, as it is common for the limits set for 

waste incineration to be placed on other plants as well. 

The EU limit values covering waste incineration-related 

airborne emissions are listed in Table 2. 
9

Emission component Concentration in 

fl ue gas [mg/m3] 

(NTP, dry gas O2 11%)

Sulphur dioxide, SO2 50

Nitrogen oxides, NOx (as NO2) 200

Particulates, TSP 10

Hydrogen chloride, HCl 10

Hydrogen fl uoride, HF 1

Dioxins and furans 0.1 x 10-6

Cadmium+thallium, Cd+Tl 0.05

Mercury, Hg 0.05

Heavy metals, 

Sb+As+Pb+Cr+Co+Cu+Mn+Ni+V

0.5

Gaseous and vaporous organic 

matter as total organic carbon, 

TOC

10

Table 2. Limit values for airborne emissions from 

waste incineration in the EU.

The focus of the FINE Programme 

The FINE Programme included a total of 14 research 

projects covering energy generation and industrial processes, listed in Table 3. Seven of these were led by companies and eight by research institutes. 

One of the main purposes of the FINE programme 

was to chart fi ne particle emissions resulting from energy generation in Finland, and a new and extensive summary table is provided on Pages 26-27 for all measured 

fi ne particle emissions from energy boilers. 

Despite the scope of these measurements, it must 

be remembered that these are normally one-off measurements made during optimal boiler operation. The 

PM2.5 and PM1 emission factors given here, therefore, 

could easily double on an annual basis due to fi lter disturbances and partial-load combustion.

Several industry projects also focused on the development of new methods and equipment to reduce 

these emissions. 

Project name Coordinated by

Development of aerosol models for nuclear power plant 

applications (AMY)

Fortum Nuclear Services Oy

Fine particle emissions from combustion of biomass and 

heavy oil without effi cient fi ltration (BIOPOR)

VTT Technical Research Centre of Finland

Fine particle and heavy metal discharges from CFB boilers 

and their control (CFB)

Foster Wheeler Energia Oy

Particles in the wood and energy peat fuel chain (HIUKE) Vapo Oy Energy

Fine particle emissions of waste incineration – development 

of waste quality and fl ue gas cleaning technique (JÄPPI)

VTT Technical Research Centre of Finland

Elimination of combustion emissions from ceramics ovens Kerapro Oy 

Large engine emission reduction Wärtsilä Oyj Abp 

Characterisation of fi ne particle and trace element 

emissions (PIHI-RAME)

VTT Technical Research Centre of Finland

Total emissions of fi ne particles and their reduction potential 

in Finland (PIHI-KHK)

VTT Technical Research Centre of Finland

Fine particle emissions from wood burning (PIPO) University of Kuopio

Pre-reservation system of the electricity fi lter (PreEsp) Tampere University of Technology

Converting accumulator sections to a comb structure Elixair Oy 

Fine particle emissions from the fl uidised bed combustion 

of biomass – a prediction method (PIHI-MALLI)

VTT Technical Research Centre of Finland

Improving power plant combustion air fi ltration and 

studying fi lter performance in different operating conditions 

VTT Technical Research Centre of Finland

Table 3. Energy- and industry-related projects included in the FINE Programme. Corporate projects are 

shown in bold.
10
11

Current technologies for controlling 

fi ne particulate emissions 2

A dust or ash removal system is usually needed to 

control the quantity and, in some cases, the characteristics of particles going to the stack. The most 

important factor in determining the type of fl y ash collection technology to be used is the permissible particulate outlet emission concentration in mg/Nm3. 

If an invisible plume is required, this requires an outlet emission of typically 50 mg/Nm3 (O2, dry) or less. For 

a high dust-formation fuel, however, the inlet burden to 

the fl ue gas cleaning plant can be as high as 30 g/Nm3, 

which means that the collection/fi ltration effi ciency of 

a plant must be as high as 99.83% to give an invisible 

outlet plume. 

The penetration of fi ne particles through a particulate removal system is also an important attribute. In the 

case of electrostatic precipitators (ESPs), the penetration 

maximum (or the minimum level of fi ltration effi ciency) 

is lowest for fi ne particles between 0.1 μm and 1.0 μm 

(aerodynamic diameter). Even where aggregate collection effi ciency is over 99% of the total particle mass, effi ciency in respect of ultra-small particles is often only 

between 90% and 95%. 

Due to their small size and the fact that they represent 

only a small proportion of the total particulate mass, 

these fi ne particles have a minor impact on total penetration or fi ltration effi ciency. However, as these particles 

are believed to be the most dangerous in terms of human health, it is important to achieve as high collection 

effi ciencies as possible for ultra-small size classes. 

Other important factors are the particulate inlet 

concentration before the removal system, fl ue gas volumetric fl ow rate, the fl ue gas temperature, and fl y ash 

constituents and properties. Once these basic design 

parameters are known, an estimated entire life cost 

(capital investment, operating costs, and maintenance 

costs) can be calculated for each available technology, 

and the technology with the lowest costs and the best 

benefi t can be selected. 

The choice of fl y ash collection technology for 

modern heat and power generation systems is usually between electrostatic precipitators and fabric fi lters. 

Both ESPs and fabric fi lters are highly effi cient, with design effi ciencies in excess of 99.5%. The effi ciency of both 

types can be further improved by fl ue gas conditioning. 

Other alternatives include mechanical or inertial collectors, and wet systems. 

The fi ne particle emissions from most of these systems were evaluated in the projects of the FINE Programme.

Electrostatic precipitators 

Electrostatic precipitators are widely used as the main 

technology for controlling particulate emissions, and 

this is expected to continue for the next couple of decades. The choice between ESP and fabric fi ltration generally depends on fuel, plant size, and boiler type and 

confi guration. 

An ESP captures dust particles by ionising the fl ue 

gas in high-voltage coronas that charge the particles. An 

electrical fi eld is used to move the particles onto large, 

earthed, fl at collecting electrodes, which are rapped at 

intervals to dislodge the dust, which falls into collection hoppers and is subsequently removed by a dust 

handling plant. 

Cold-side (dry) ESPs fi tted with fi xed or rigid electrodes and located after the air pre-heater, operating at 

between 130 °C and 180 °C, account for the bulk of the 

current market. Units with moving electrodes are becoming more widely used, however. See Figure 5.

Hot-side (dry) ESPs, located before the air pre-heater operating in a temperature range of 300-450 °C, are 

used mainly in the US and Japan. In these units, a liquid 

fi lm is maintained on the collection plates using spray 

nozzles, which eliminates the need for rapping, as the 

liquid fi lm removes any deposited fl y ash particles. This 

also eliminates problems with re-entrainment, fl y ash 

resistivity, and the capture of fi ne particles. However, 

wet ESPs require saturation of the fl ue gas stream with 

water and generate wastewater and sludge.

Removal effi ciency is usually 99->99.99% for particles in the 0.01 >100 μm size range.

Figure 5. A schematic view of a dry electrostatic 

precipitator (ESP). Courtesy http://www.zet.com/.
12

Upgrading ESP performance 

The performance of ESP units can be enhanced in four 

main areas: process conditions, controllers, gas conditioning, and size of installation.

Substantial improvements can sometimes be obtained 

by altering the process gas conditions, primarily in terms 

of volumetric fl ow rate and temperature. A reduction 

in gas volume will improve ESP performance in nearly 

all cases. The only exceptions are where extremely low 

velocities lead to excessive dust build-up on internal 

components, such as access walkways, leading to dust 

bridging to an ESP’s electrodes. 

The sensitivity of an ESP to changes in gas volume 

rises with increasing design collection effi ciency. Theory 

can demonstrate that emissions from an ESP designed 

to operate at 99% effi ciency will be reduced to half if 

gas fl ow is reduced by 28%. However, emissions from an 

ESP designed to operate at 99.9% effi ciency will halve 

with a fl ow reduction of only 19%. 

Changes in process gas temperature can either improve 

or degrade performance, depending on the nature of 

the particulates and the temperatures involved. Changes in performance at varying temperatures can be quite 

dramatic with some particulates. In an extreme case, 

a variation in emissions by a factor of 10 can sometimes occur with a change of only 15-20 °C in gas temperature. 

Most cases of extreme temperature sensitivity relate 

to high-resistivity dust, such as some types of fl y ash, 

operating on the threshold of back-corona or close to 

the acid dew-point of the gas. Signifi cant temperaturerelated effects have also been observed in some dust not 

linked to high resistivity problems. These appear to involve loss of dust layer cohesiveness at low temperatures, 

leading to signifi cant re-entrainment loss. 

Retrofi tting modern voltage controllers capable of 

intermittent energisation can also lead to quite signifi -

cant performance improvements. Emission reductions 

of 30-80% have been documented for a variety of controller retrofi ts, mainly in the power generation and cement industries. 

Signifi cant performance improvements can sometimes 

be obtained by injecting conditioning gases, liquids, or 

solids into the process gas stream upstream of an ESP. 

Improvements again relate mainly to the collection of 

high-resistivity dust prone to back-corona. 

Although many conditioning agents are used, the 

most common are water and sulphur trioxide (SO3). 

Other conditioning agents include anhydrous ammonia, 

triethylamine, and various sodium compounds. 

While gas conditioning can be an attractive option 

for upgrading performance, it is necessary in virtually 

all cases to carry out local trials to determine the degree 

of enhancement possible. 

The capital and operating costs of an SO3 plant are 

not insignifi cant, but a unit can often be justifi ed as an 

alternative to a much more costly ESP upgrade or fabric 

fi lter conversion because of its effectiveness. 

In cases where process adjustments, voltage controller 

upgrades, or gas conditioning are insuffi cient to achieve 

the required degree of performance enhancement, it may 

be necessary to enlarge the collecting surface of an ESP. 

Where process gas conditions are suitable, fabric fi lter 

conversion can also be considered, usually in conjunction 

with an upgrade of the main fan plant. A total re-design 

of a gas cleaning plant must be undertaken sometimes, 

particularly if the enhancement required in total particuFigure 6. Despite being highly effi cient, fabric fi lter technology is relatively simple. Two solutions are 

shown here. 
13

late collection is accompanied by additional gas cleaning 

obligations, such as sulphur dioxide removal. 

Fabric fi lters

Fabric fi lters (FFs), which generally operate at between 

120 °C and 180 °C, have become more widely used since 

the 1970s, especially in industry. Installed in a baghouse, 

a fabric fi lter captures dust particles by forcing the fl ue 

gas to pass through a permeable fabric, causing the particles to be collected on the surface of the fabric. The 

dust is then removed from the fi lter bag by means of a 

cleaning mechanism and deposited in collection hoppers. See Figure 6.

There are three types of fabric fi lters. The most commonly used type is the pulse jet fi lter, in which a pulse of 

compressed air is introduced down the inside of the bag 

to dislodge the dust accumulated on the outside of each 

bag. Other types of fi lters using shaker mechanisms or 

reverse gas fl ow have been installed in the past, but are 

more expensive than pulse jet fi lters and require considerably more space. 

Overall, the technology is relatively simple compared 

to ESP. Outlet emission is generally less than 50 mg/

Nm3 for a well-designed system, resulting in an invisible plume. 

Fabric fi lters are increasing their market share, mainly in the process industry. New developments in chemical addition (activated carbon, lime) to form a chemically reactive fi lter cake have been commercialised, and 

fi lter elements with catalytic properties are also now 

available. 

Removal effi ciency is >99- >99.99% for particles in 

the 0.01- >100 μm size range.

Flue gas conditioning can also be used with fabric 

fi lters, to achieve lower emissions at higher bag air-tocloth ratios, reduce pressure drop, and improve fl y ash 

cake cohesivity, leading to better dislodgement of larger 

agglomerates and less re-entrainment. Elemental sulphur, ammonia (NH3), and sulphur trioxide (SO3) are 

the main conditioning agents currently used. 

Economic comparison of ESPs and fabric fi lters 

The required outlet particulate emission level must be 

defi ned prior to any economic comparison of ESP and 

fabric fi lter technology, as a low emission requirement 

can increase the capital cost of an ESP plant markedly. The entire life cost, including capital, operating 

and maintenance costs, needs to be calculated on a net 

present value basis. 

For ESPs, this comprises: 

• Initial capital cost for the main unit and associated 

plant and equipment, such as ash and dust plant, induced draught (ID) fans, etc.

• Auxiliary power cost for energisation

• Auxiliary power cost for draught loss

• Maintenance cost, including an allowance for a halflife refi t if considered appropriate.

For fabric fi lters, this comprises: 

• Initial capital cost for the main unit and associated 

plant and equipment. This will always include a larger 

ID fan and motor than needed with an ESP, since the 

draught loss is much larger.

• Auxiliary power cost for draught loss

• Auxiliary power cost for the bag cleaning system, e.g. 

compressors or blowers for pulse jet fi lters

• Maintenance cost, consisting mainly of complete bag 

replacement after approximately 30,000 service hours 

or greater. Depending on the fuels used, it is common 

in some power plants, however, for bags to be replaced 

after two or three years of use, increasing costs. 

ESP FF

Investment cost - +

Operation and maintenance costs + -

Pressure loss + -

Collection effi ciency

- TSP + +

- Fine particles - +

- High-resistive dust - +

Moisture + -

Corrosion - +

High temperature + -

Glowing particulates + -

Acids and ammonia + -

Exploding gases wet ESP -

Removal of SOx, HCl, HF, etc. - +

Table 4. A comparison of ESP and FF properties 

(PIHI-KHK 2005).

The difference between the two technologies usually 

becomes much more apparent when very low emissions 

are required, and is normally refl ected in a lower entire 

life cost for fabric fi lters. In addition, fabric fi lter plant 

performance is not usually affected by variations in coal 

quality, whereas ESP performance can be affected signifi cantly. Another advantage of fabric fi lters is that a 

clear stack can be obtained under all operating conditions, including start-ups, whereas ESPs do not usually 

capture oil soot from start-up oil burners, resulting in a 

temporarily dirty stack. 

Retrofi tting fabric fi lter bags into existing ESP casings 

has been carried out successfully at several Australian 

power stations to achieve very low outlet emissions. This 

necessitates the removal of all ESP equipment, installing 

fabric fi lter equipment (including bag cleaning system), 

and upgrading or replacing ID fans and motors. 
14

Another alternative is to install fabric fi lter plant 

downstream of existing ESP equipment. This has been 

carried out successfully in the US and at Swanbank A 

Power Station near Ipswich in Australia, where fabric 

fi lters from an older de-commissioned station were installed downstream of existing ESPs, resulting in ‘clean 

stack’ outlet emissions. 

Helsinki Energy in Finland uses fabric fi lters at its 

Salmisaari power plant to remove the particulates formed 

in its semidry fl ue gas desulphurisation (FGD) process. 

See Figure 7. Flue gas passes through an ESP prior to 

the FGD unit. This approach has resulted in very low 

particulate emissions, as low as 1 mg/MJ. Using ESP and 

fabric fi lter technology in this way makes it possible to 

optimise both systems for specifi c sizes of particles, and 

reduces the requirement for fabric fi lter effi ciency.

Finnish measurements, including those carried out 

as part of the FINE Programme, show that collection 

effi ciencies of 98–99.9 % from total particle mass can be 

achieved using both ESP and FF technologies. 

Measurement data indicates that fabric fi lters are best 

at removing the smallest particles. Fabric fi lters offer an 

effi ciency of over 99% (by mass) for fi ne particles, while 

ESPs offer somewhat lower performance in respect of 

fi ne particles between 0.1–1 μm (by aerodynamic diameter), at between 90% and 99%.

Mechanical/inertial collectors

In the past, industrial plant operators tended to fi t cyclones for particulate removal. More recently, fabric fi lters have become increasingly popular in a number of 

industrial processes. Cyclones represent a robust technology capable of handling the cyclical operation and 

load changes typical of these types of plants. However, 

they offer only moderate effi ciency compared to ESPs 

or fabric fi ltration, and cannot remove the fi nest particles from fl ue gas. Their use is more common in smallscale applications today. See Figure 8.

The effi ciency of mechanical or inertial collectors is 

typically 50-99% for particles in the size range of 1.0-

100 μm. An effi ciency of 90% is typical for particles 

over 10 μm.

Wet scrubbers 

Scrubbers – such as venturi units, in which water is injected into the fl ue gas stream to form droplets on to 

which fl y ash particles impact, forming a wet by-product 

– are essentially a secondary technology when it comes 

to particulates. They tend to be used more in sulphur recovery applications in large plants and as heat exchangers in smaller facilities. See Figure 9.

The effi ciency of wet scrubbing systems generally 

drops off at smaller particle sizes. These type of sysFigure 7. Helsinki Energy uses an ESP unit upstream of its fl ue gas desulphurisation process followed by a 

downstream FF unit to achieve particulate emissions as low as 1 mg/MJ. Courtesy Helsinki Energy.
15

tems can also have a high level of energy consumption, 

due to the use of sorbent slurry pumps and fans. Wet 

scrubbers remove SO2 from fl ue gas, and many units are 

designed to control both SO2 and particulates by utilising alkaline fl y ash as a sorbent. Lime is frequently 

used to boost SO2 removal effi ciency. Other chemicals 

can also be used. 

Removal effi ciency in respect of fi ne particulates is 

typically between 90-99.9% for particles in the 0.5 - >100 

μm size range. 

Optimal combustion and additives 

Various fuel additives and chemical sorbents can be used 

to reduce fi ne particle formation and the incidence of toxic 

trace elements in fi ne particles. The latter are encountered 

in combustion environments such as waste incinerators, 

coal combustors, and smelters, and are enriched in the 

sub-micrometer size range in combustion fl ue gases. The 

problem here is that particulate control devices perform 

at their weakest in precisely these size ranges. 

Sorbents can control emissions of these metallic species emissions, and several studies on the subject have 

been conducted. Uberoi and Shadman have reported the 

use of minerals such as alumina, bauxite, and kaolinite 

for capturing alkali metal, lead, and cadmium species. 

Scotto et al. have reported the use of silica-based sorbents for capturing lead, while Ho et al. have reported the 

results of lead capture in a fl uidised bed of sand. 

Extensive equilibrium calculations have been carried 

out, and the results indicate that several silica, alumina, 

and titania compounds have the potential to form metallic complexes with lead, selenium, mercury, arsenic, 

and chromium. 

While sorbent injection into the combustion chamber provides an effective way of chemisorbing metallic 

species, there are several physicochemical considerations 

that may hinder the usage of bulk solid sorbents. Injection can result in the formation of outer layer metalsorbent complexes, for example, that block the inner 

sorbent material from further chemisorbing metallic 

species. Where kaolinite is used to capture lead, the resulting complex has a lower melting point and leads to 

the formation of a glassy surface that inhibits further 

chemisorption. 

Packed beds of sorbent materials also have to be 

maintained at an optimal temperature, which may result in signifi cant additional costs. A key factor to be 

noted is that coal combustors and incinerators have low 

concentrations of metallic species, and excess sorbent 

materials should not be used if sorbent systems are to 

be cost-effective. 

Studies also indicate that, when large sorbent particles 

are used, the inner material becomes ineffective due to 

pore pluggage. Alternate gas-phase sorbent precursors 

for capturing trace metal species have been proposed 

by Owens and Biswas. This approach provides an in 

Figure 8. A schematic view of a mechanical fl y ash 

separator. Courtesy VTT.

Cleaned gas (outlet)

 Guide vane

Particles to receiver

Figure 8. A schematic view of a wet fl y ash separator. Courtesy Sweetech Engineers.
16

situ means of capturing the toxic metal in the combustor and converting it into a benign form, with low resultant leachability, and suppressing the homogeneous 

nucleation of the volatile metal species downstream of 

the combustion chamber. 

Although detailed size distributions have been reported, the mechanisms of the metal-sorbent reactions 

involved are somewhat unclear (Biswas and Zachariah 

1997). This subject should be studied further, as it may 

provide a solution for reducing fi ne particle emissions 

and immobilising harmful trace elements.

Sulphur species have also been used to reduce the 

formation of alkali chlorides and prevent corrosion and 

fouling problems.

Combustion conditions can be optimised to reduce 

fi ne particle formation, by optimising air staging, mixing, 

and the geometry of the combustion chamber to eliminate the soot and organic carbon resulting from incomDevice Minimum particle size 

[μm]

Collection effi ciency [%*] 

(Flagan & Seinfeld 1988)

Separation chamber

(generally used as pre-separator)

>20 <50

Cyclone

(generally used as pre-separator)

5–25 50–90

Multicyclone 5

Wet scrubbers:

Spray tower >10 <80

Dynamic scrubber >2.5 <80

Collision scrubber >2.5 <80

Venturi scrubber >0.5 <99

Electrostatic precipitator <1 95–99+

Fabric fi lter >0 >99

* from total particle mass

Table 5. Advantages and disadvantages of various particulate control technologies.
17

Measured collection 

effi ciencies in Finland [%*]

Advantages Disadvantages

not available low pressure drop, relative simplicity 

and few maintenance problems, dry 

collection and disposal

low collection effi ciency, does not 

separate fi ne particles, inability to 

handle sticky materials

60–85 simple construction, no moving parts, 

little maintenance, low investment 

and operating costs, constant 

pressure drop, saves space, beyond 

temperatures control, dry relief during 

wet cyclone 

low output for low particle diameter, 

poor performance at charge, emission 

of effl uent at wet cyclone, erosionsensitive, possible noise nuisance

75–98 simple construction, constant pressure 

drop, continuous particulate removal, 

separates large particles quite 

effi ciently

low collection effi ciency for low 

particle diameter, danger of 

pluggage at high dust and moisture 

concentrations, suitable for low 

temperatures and sticky materials

not available simultaneous removal of gaseous 

and particle emissions, relatively little 

maintenance, rather high effi ciency, 

insensitive to fl uctuating gas fl ows, 

capable of handling cooling and 

cleaning hot and wet fl ue gases 

neutralising corrosive gases/mists, 

reduced dust explosion risk, effi ciency 

control

corrosion and erosion problems, 

increased disposal costs, potentially 

high pressure drop, low collection 

effi ciency for fi ne particles (<1 μm), 

possibly even resulting in higher 

levels of the smallest particles, 

freezing problems in cold climates

99.2–99.9 

(<2.5 μm particles: 96.9–99.1

 and <1 μm particles: 95–99)

>99% overall effi ciency, also collects 

fi ne particles, wet and dry separation, 

low pressure drops and relatively 

low energy demands compared to 

other effi cient methods, continuous 

operation with minimum maintenance, 

relatively low operating costs, capable 

of operating at high temperatures (up 

to 700 °C)

high capital costs, sensitive to 

fl uctuating dust loads and gas fl ows, 

diffi culties with collecting particles 

with extremely high or low resistivity, 

relatively large space requirements, 

special precautionary requirements 

due to high voltage during 

maintenance, collection effi ciency can 

decline gradually and imperceptibly

<2.5 μm: 99.9 <1 μm: 99.7 very high collection effi ciencies (99.9%) 

for both coarse and fi ne particles, 

dry recovery of collected material 

for subsequent processing and 

disposal, relatively simple operation, 

no corrosion problems, high collection 

effi ciency of submicron smoke and 

gaseous contaminants through the use 

of selected fi brous or granular fi lter 

aids, declining performance is easily 

detectable

rather high maintenance requirements 

and operatiing costs (fabric changes 

every 2-4 years), shortened fabric 

life at elevated temperatures and 

in the presence of acid or alkaline 

particulate or gas constituents, 

sensitivity to fi ltration velocity, hot 

gases must be cooled, glowing, sticky 

or large particles need pre-separation 

(explosion and fi re hazard), the effect 

of relative humidity (condensation), 

the effect of chemicals, bypassing 

during failure situations

plete combustion. Ash volatilisation can be minimised 

by reducing the combustion temperature.

Table 5 provides an overview of the advantages and 

disadvantages of the particulate control technologies 

reviewed in this section.
18
19

0.00001

0.0001

0.001

0.01

0.1

1

10

100

0.01 0.1 1 10 100

Dp, μm

Penetration, %

Plant A, SA coal, impactor

Plant A, SA coal, DMPS

FF Biomass

FF Coal

ESP Biomass (3-stage)

ESP Biomass (2-stage)

FF Biomass, pilot

ESP Biomass

FF waste 1

3 Findings from FINE projects

T

he level and type of fi ne particulate emissions are 

primarily determined by the fi lter used, and to a 

lesser extent by the combustion process and fuel. 

Fabric fi lters remove fi ne particles extremely effectively, 

and offer a removal effi ciency of better than 99.99%. 

FINE studies have shown levels of less than 30 μg/m3 

compared to a concentration of around 10 μg/m3 in clean 

outdoor ambient air. 

Electrostatic precipitators are at their least effi cient when 

handling particle sizes between 0.1 μm to 1 μm, and typically remove only 90% to 95% of these sizes of particles 

despite having an overall effi ciency of more than 99%.

Wet (tower) scrubbers remove coarse particles quite 

effi ciently, but are very poor at handling fi ne particles. 

FINE fi gures indicate a typical unit only offers 30-50% 

effi ciency in respect of fi ne particle removal. A venturi 

scrubber is also more effi cient in respect of fi ne particles.

Figure 10 presents some measured particle penetrations through ESPs and FFs as a function of particle 

size. Fabric fi lters designed for waste combustion (pilot 

and Waste 1) proved extremely effective, and penetration 

was less than 0.2% at all particle sizes. ESP effi ciency is 

at its worst in respect of submicron fi ne particles. Two 

other FFs in a biomass and a coal plant were not quite 

as effi cient as those for waste combustion, but were signifi cantly better than ESPs.

Fine particle emissions from fi ring coal and peat are 

covered in earlier studies, such as Ohlström 1998.

Waste incineration

Given the tough limits imposed on waste incineration 

plants, they are often equipped with both ESPs and fabric 

fi lters, as well as semi-dry or wet scrubbers located between these two types of primary control. Facilities are 

also required to use continuous measurement equipment 

to monitor emissions, and must be shut down should 

they exceed the limit values of the EU’s Waste Incineration Directive (WID).

Figure 10. Measured fi lter effi ciencies at different types of power plants.
20

The type of boiler employed also naturally has an impact on emissions formation, and Figure 11 compares 

particle mass size distribution at the fi lter inlet when incinerating waste in a grate boiler and a CFB unit. Fine 

particle concentration in a grate boiler is higher due to 

higher volatilisation (high temperature) and the absence 

of scavenging bed and other coarse mode particles. Fine 

particle concentrations at the fi lter outlet are extremely 

low (< 30 μg/Nm3) for both types of boilers due to the 

high removal effi ciency of fabric fi lters (Figure 12).

Measurements carried out under the FINE programme showed that a baghouse fi lter, combined with 

the use of lime and activated carbon, is highly effi cient 

when used at waste incineration plants employing a variety of combustion technologies. 

Collection effi ciency of fi ne particles was over 99.9%. 

Total particulate emissions were between 0.5 and 1 mg/

m3, and dioxins 10% of the WID boundary value. All 

trace metal and other measured emissions (NOx, SO2, 

HCl, HF, TOC) were below WID limits. 

Figure 12. Particle mass size distribution from waste incineration at the fabric fi lter outlet. 

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0.01 0.1 1 10 100

Dp [μm]

dm/dlog(Dp) [mg/m^3] (NTP)

Waste, grate boiler

Waste, CFB

Cyclone cut-off

D50=~5 μm

Figure 11. Particle mass size distribution from waste incineration at the fi lter inlet. 

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.01 0.1 1 10 100

Dp[μm]

dm/dlog(Dp) [mg/m^3 (NTP)]

Cyclone cut-off

D50=~2.5 μm

Waste, fabric filter outlet
21

The volume of particles in the fl ue gases before the 

fi lter was fairly large, and high heavy metal concentrations were identifi ed in the fi ne particles. Particle size 

distribution was bimodal at the fabric fi lter inlet and 

outlet at both the facilities studied. Fine particle concentrations were high at the fabric fi lter inlet. 

Waste quality had a signifi cant effect on fi ne particle 

concentrations according to pilot-scale investigations. In 

addition to Na, K, Cl, and S, fi ne particles contained Pb 

and Cu in large quantities. Cd, Tl, Cu, and Pb were enriched in the fi ne particle fraction (PM1). Higher chlorine content in the fuel increased its release to the gas 

phase, and enrichment in fi ne particles. Hg was mainly 

identifi ed in the gas phase at the fabric fi lter inlet, and 

approximately 90% of Hg was captured in the fi lter at 

the full-scale plant. 

See Table 6 for a comparison of FINE measurement 

results and EU limits for waste incineration.

Kraft recovery boilers

Kraft recovery boilers represent an important part of 

the energy economy and resource utilisation of the forest products industry, one of the traditional cornerstones of the Finnish economy. The industry has been 

a pioneer in extending its fuel use into areas that have 

traditionally been considered low-value or marginal, 

and in introducing new technology, such as fl uidised 

bed combustion.

As part of the FINE Programme, measurements 

were carried out at two recovery boilers to determine 

the mass concentration and size distribution of fi ne 

particulates, ESP collection effi ciency, and trace element emissions. The measurements were conducted at 

the inlet and outlet of ESPs fi tted to two modern, relatively new boilers.

Boiler A has a capacity of 2,000 tonnes of dry solids/

day, and fi res non-condensable gases (NCGs) together 

with black liquor. The measurements were carried out 

on three days during normal operation at varying loads: 

approximately 70% load (Day 1), 70% load without 

NCGs (Day 2), and 100% load (Day 3). 

Boiler B has a capacity of 1,800 tonnes of dry solids/day, and does not fi re NCGs. The measurements 

were carried out during normal full load (Day 1) and 

70% load (Day 2). As the ESP inlet was not accessible 

for measurement, particle measurements were carried 

out at the boiler’s Economizer 1 at a fl ue gas temperature of 190 °C. ESP outlet measurements were made 

at the stack. 

CO concentration varied between 0 and 500 mg/Nm3 

at both boilers, but was generally higher at Boiler A. 

The total particle mass concentrations at the ESP 

inlet were 11-23 g/Nm3 (dry gas) for Boiler A, and 17-

24 g/Nm3 (dry gas) for Boiler B (Figure 13). There was 

some variation in total particle concentrations between 

measurements at both boilers – presumably due in part 

to the different operating loads and unstable combustion 

conditions, and in part to experimental uncertainties.

Particle mass concentrations at the ESP outlet were 

36-44 mg/Nm3 at Boiler A, and 14-21 mg/Nm3 at Boiler B (Figure 14). Emissions measured according to the 

SFS3866 standard were 30-40 mg/Nm3 at Boiler A, and 

12-15 mg/Nm3 at Boiler B. Emissions were higher at 100% 

load than at 70% load for both boilers. 

Switching off NCG feed at Boiler A did not have any 

signifi cant effect on emissions; and emissions at Boiler B 

were lower overall, even considering the relatively large 

uncertainties in the mass concentrations. Size distribution 

at the ESP outlets was unimodal at both boilers in the 

impactor size range of 0.03-4 μm. (Lind et al. 2006)

Table 6. Comparison of measurement results and EU limits for waste incineration.

Emission component Concentration in Concentration in

fl ue gas mg/m3 fl ue gas mg/m3

(NTP, dry gas (NTP, dry gas 

O2 11%) O2 11%)

MEASURED EU WID

SO2 0.16 – 25 50

NOx (as NO2) 50 – 100 200

TSP 0.2 – 1 10

HCl 4 – 6 10

HF 0.02 – 0.2 1

Dioxins and furans 0.0034 x 10-6 – 0.01 x 10-6 0.1 x 10-6

Cd+Tl < 0.002 0.05

Hg 0.005 – 0.007 0.05

Sb+As+Pb+Cr+Co+Cu+Mn+Ni+V < 0.02 0.5

TOC < 1 10
22

Figure 13. Fine particle mass size distribution at a kraft recovery boiler, fi lter inlet.

Figure 14. Fine particle mass size distribution at a kraft recovery kraft boiler, ESP outlet.

0

5

10

15

20

25

30

35

0.01 0.1 1 10 100

Dp [μm]

dm/dlog(Dp) [mg/m^3 (NTP)]

Recovery kraft, ESP outlet

0

5000

10000

15000

20000

25000

0.01 0.1 1 10 100

Dp [μm]

dm/dlog(Dp) [mg/m^3 (NTP)]

Recovery kraft
23

Sintering plants

The process temperature in a sintering plant is signifi -

cantly higher than in an energy boiler, particularly a CFB 

unit, and the chlorine content of the environment is also 

high. As a result, As, Cd, Cu, Pb and Tl are highly enriched in the fi ne particle mode. Organic carbon content 

in the fi ne particle mode was measured at 2-3% of the 

total fi ne mode mass in the measurements carried out 

at a plant of this type as part of the FINE programme. 

The mass size distribution prior to the plant’s fi lter is 

shown in Figure 15.

The FINE measurements showed that there is a relation 

between the fi ne particle mass concentration, the process 

temperature, and the Cl, Pb, Cd, Cu and As concentrations in the fuel used. A higher process temperature and 

higher chlorine content in the fuel increases the concentration of trace elements in sub micron-size fi ne particles. 

When these kinds of chlorine-rich residues, such as the 

waste residues used in energy generation, are combusted, the concentration of fi ne particles increases, and an 

effective particulate removal device, such as an ESP or a 

fabric fi lter, is essential to keep emissions low. 

Heavy fuel oil 

Heavy fuel oil emissions were characterised from burners 

and diesel engines as part of FINE work. The typical size 

distribution here is bimodal (Figure 16). The major fi ne 

particle components from heavy fuel oil combustion are 

sulphuric acid 70-80%, Ni ~5%, and V ~5%. The coarse 

particle mode is mainly unburned fuel in the form of 

oil droplet residue. The large amount of sulphuric acid 

is formed due to V2O5 acting as a catalyst for SO3 formation. Heavy fuel oil generates a larger proportion of 

smaller fi ne mode particles compared to other stationary combustion processes.

Figure 15. Mass size distribution (after multi-cyclone) of fi ne particle emissions at a sintering plant. 

0

20

40

60

80

100

120

140

0.01 0.1 1 10 100

Dg [m ]

dm/dlog(Dp) [mg/m^3 (NTP)]
24

Figure 16. Particle mass size distribution from heavy fuel oil combustion.

Figure 17. Particle mass size distribution from wood combustion in a grate boiler fi tted with a wet 

scrubber.

0

20

40

60

80

100

120

140

160

180

200

0.01 0.1 1 10 100

Dp [μm]

dm/dlog(Dp) [mg/m^3] (NTP)

Wood, grate boiler, scrubber inlet

Wood, grate boiler, scrubber outlet

Cyclone cut-off

D50=~3 μm

0

5

10

15

20

25

30

35

40

45

50

0.01 0.1 1 10 100

Dp [μm]

Heavy fuel oil combustion

dm/dlog(Dp) [mg/m^3] (NTP)

Cyclone cut-off

D50=~4 μm
25

Biomass 

Biomass is often combusted in small units that do not 

employ any other technologies than cyclones for removing particulates from fl ue gas. The latter units are only 

capable of removing a small proportion of particulates, 

however, and no fi ne particulates. The scrubbers that 

are often fi tted in the role of heat exchangers remove 

larger particulates, but are relatively poor at handling 

fi ne particulates.

This is highlighted in Figure 17, which shows particle 

mass size distribution from wood combustion in a grate 

boiler fi tted with a wet scrubber. While effi cient at removing coarse particles, tower scrubbers are clearly not very 

effi cient when dealing with fi ne particulates. 

It should be noticed that fi ne particles grow inside 

the scrubber due to their hygroscopic properties, as potassium sulphate absorbs water below the water vapour 

dew point. Fine particles generated when burning wood 

consist mainly of potassium sulphate, but may include 

fairly large amounts of Zn, Cl, and Na as well, depending on the composition of the fuel feed. Much better fi ne 

particle removal effi ciency can be achieved using venturi 

scrubbers, electrostatic precipitators, and fabric fi lters 

compared to tower scrubber with fi ller units.

Figure 18. Elemental concentrations of fi ne particles (particle diameter Dp < 1μm). 

0

10

20

30

40

50

60

70

80

90

100

% of PM1 concentration

C l

Other

HFO Wood Peat, BFB,

ESP

Wood, Grate,

ESP

Wood, Grate,

Scrubber

WoodPeat SRF,

BFB, ESP

Recovery Kraft,

ESP

Other Other Other

Zn

Cl

Na

Zn

Cl

Na

Cl

Na

Ni

V

K

K

K

K

Na

Na

Cl

SO4 SO4 SO4 SO4 SO4 SO4

K

Other

Other Zn Ni V Cl Na K SO4
26

Summary 

Fuel Boiler/

Burner

Output kW Particle 

separator

Particle 

emissions

TSP 

mg/MJ

PM1 

mg/MJ

Biomass/Peat

batch burning

sauna stoves, fi replace inserts, 

masonry heaters, wood log 

boilers

–

–

80...230*

50...160*

continuous burning

wood chips grate, stoker burner < 500 – 20...80

wood pellets pellet burner, stoker burner < 500 – 10...60

wood grate 5000 wet scrubber 50 30

woodchips grate (circulating) 10000 – 10 3

wood grate 15000 ESP 11 5

wood, peat Bubbling fl uidised bed (BFB) 60000 ESP 2-3 1

wood, sludge Circulation fl uidised bed (CFB) 150000 ESP 1-2 0.1

wood, peat, sludge BFB 90000 FF 5 0.05

Waste

SRF grate 18000 FF 0.2 < 0.03

SRF CFB 75000 FF 0.4 0.03

mixed fuel BFB 60000 ESP 10 3

Oil

light fuel oil Oil burner (domestic) 20 – 2 2

heavy fuel oil Oil boiler 4000-7000 – 15-40 3-9

Diesel engine –

Coal Pulverised coal combustion 320000

246000

ESP

FF

10

 6

 2

–0

Black liquor Recovery boiler 300000 ESP 5-16 3-11

 * large differences between combustion devices and operating conditions (fuel quality etc.)

** below the limit of identifi cation or not analysed

The measurement results collected during the FINE 

programme, together with other data and information 

on fi ne particle chemical composition and morphology, 

are summarised in Table 7. This is the fi rst of its type to 

present such a broad overview of fi ne particle emission 

factors and characteristics to be published in Finland. 

It should be kept in mind, however, that all the values 

in Table 7 are individual measurements typically taken 

under optimal combustion conditions, and are not, as 

such, representative of actual annual emission factors. 

The latter are inevitably higher, due to load variations 

and incidents such as plant shutdowns and start-ups, 

the temporary bypassing of fi ltration equipment, and 

other factors.

It can be seen that the properties of fi ne particles 

from small-scale batch combustion are dominated by 

unburned carbon species: organic carbon (OC, including PAH) and elemental carbon (soot). Firing wood in 

larger plants, in contrast, generates mainly water-soluble alkali salts (K, S, Cl). As the harmful effects of these 

Table 7. Measured fi ne particle emissions (PM1) and their characteristics (incl. chemical composition, 

morphology, and harmful properties) for energy generation, small-scale wood combustion, 

and industrial processes.
27

Flue gas

characteristics

Medium size 

(nm)

Chemical composition

Main elements/compounds

Morphology 

spherical/agglomerate

Harmful properties

particle size/toxicity

T

°C

O2 aver.

%

PM1-AMMD Physical size Biomass

100-500

org. 80-30, soot 20, 

inorg 5-50

agglom., 

primary 50 nm

batch burning

 small size, PAH,

 some soot

350

210

9

13

continuous burning

small units

 small size

 some PAH

intermed & large units

 relatively small size

180

190

140

140

140

140

140

8

7

6

6

6

6

6

na**

100 org. 10-15, soot 1-5, 

inorg. 80-90

spherical 

50 nm

200 K, SO4

230

200 –

200 SO4, K, Na spherical 100-200 nm

200 SO4, K, Cl, Na K salts

500 na**

Waste

na** na** inlet inlet 140 11

na** na** compact agglomerates rel. small size, toxic 140 11

200 K, Cl, SO4, Pb, Cd... primary 40-100 nm 

(K, Cl and Pb, Cd...)

outlet

almost zero emission

140 6

Oil

50 mainly soot agglom. primary 20-50 nm small size soot 160 4

100 H2SO4, V, Ni spherical 100 nm small size toxic 150 3

number, V, Ni, SO4

500

na**

Si, S, K, Hg...

na**

Coal

Hg, As

1000 Na, SO4, K, Cl salts compact agglomerates

100-500 nm

Black liquor

none

140 6

two types of fi ne particles are rather different, they need 

to be evaluated differently as well. The same is true for 

other fuels and combustion systems.

It also needs to be remembered that fi ne particles 

from sources close to the air that people breathe are 

considered potentially more harmful and dangerous 

than those transported from more distant locations 

(sulphates, nitrates).

The elemental concentrations of fi ne particles resulting from various sources are shown in Figure 18. In the 

case of heavy fuel oil, a number of heavy metals such as 

vanadium and nickel are concentrated in the fi ne particle 

mode. Chlorine is a problem in waste combustion, as it 

causes corrosion problems. Wood combustion typically 

shows high concentrations of calcium and other salts, as 

well as zinc. Co-combustion of wood and peat usually 

results in a lower number of fi ne particles and lower mass 

concentrations compared to separate combustion. 
28

Regulations covering the emissions from energy 

generation and industry in Finland and elsewhere in Europe are set to become considerably 

tougher within a relatively short timeframe as a result 

of EU legislation covering existing and new facilities. 

While much of the focus of attention has been on large 

plants, as the new standards for these have already been 

agreed, small-scale combustion is likely to face proportionally the greatest tightening of limits, as and when 

the relevant decisions are taken.

Looking ahead, we can see that if the increasing use 

of biomass is focused on larger-scale heat and power 

plants rated at 20 MW and above – where the fi ne particle emissions generated by fi ring wood do not diverge 

signifi cantly from those from fossil fuels or peat, thanks 

to the use of effi cient devices, such as electrostatic precipitators and fabric fi lters – PM2.5 fi ne particle emissions 

in Finland will be reduced. A reduction of 15–21% is 

indicated between 2002 and 2010, and 30–38% up until 

2020 in the different scenarios evaluated as part of the 

FINE programme’s PIHI-KHK project, regardless of 

the fact that primary energy consumption is expected 

to rise in all the scenarios considered. 

It should be noted that the annual emissions given in 

Figure 19 have been calculated from the measured PM2.5 

and PM1 emission factors given earlier, and could easily 

be 1.5-2 times higher when fi lter disturbances and partial load combustion are taken into account. Extended 

measurement periods would be needed to verify annual 

emission levels. 

Efforts to reduce greenhouse gas emissions will speed 

up the reduction in emissions of fi ne particulates, generally speaking. If increased biomass usage is concentrated mainly in mini-CHP (combined heat and power) 

boilers, however, these types of emissions will probably 

increase due to the lack of cost-effi cient particulate control devices for units of this size. This scenario could 

well become reality because of the pressure to increase 

Figure 19. Fine particulate emissions from energy supply and use and from industrial processes in various 

scenarios in Finland (Ohlström et al. 2005).

2005 2010 2015 2020 2025

0

5

10

15

20

25

30

Traffic & 

transport

Machinery

Small-scale

combustion

Industry

District heat 

and electricity

Coal and peat 

Base

Kyoto

T10-20%T20-20%

UEO

Base

Kyoto

T10-20%T20-20%

UEO

Base

Kyoto

T10-20%T20-20%

UEO

Base

Kyoto

T10-20%T20-20%

UEO

Base

Kyoto

T10-20%T20-20%

UEO

Particulate emissions, PM2.5, 1,000 t

Future trends 4
29

the use of smaller, distributed facilities when combating climate change.

As small-scale wood combustion – and traffi c and 

transport – offers the largest potential for reducing fi ne 

particle emissions, developing the regulatory environment in these areas could have a signifi cant impact on 

air quality. 

The challenge at the moment is that cost-effi cient fi ne 

particle control technology for plants rated at below 10 

MW is not yet available. Work also needs to be done on 

low-emission biomass fuels and combustion technologies, as well as the use of combinations of technologies, 

such as ESP and FF, and sorbents. 

Ultra-low emissions technology

As the regulations covering emissions of fi ne particulates tighten, there could well be a need, at least locally, 

for ‘ultra-low emissions’ concepts. This can be expected 

to boost the use of ESPs and fabric fi lters, as a considerable proportion of new future electricity-generating 

capacity, estimated at 7,000 MW by 2020, is likely to 

be implemented using solid fuels, requiring effi cient fi ne 

particle fi ltration systems. 

Table 8 provides an overview of the approximate 

costs of operating various emission control systems, and 

shows that the capital costs for both ESP and FF in a 

typical power plant is usually between €1 million and 

€3 million, as of 2006. Operating costs are dependent 

on a number of variables, and range between €30,000/a 

and €500,000/a for ESPs, and between €30,000/a and 

€1.5 million for fabric fi lters, again as of 2006. 

The costs of the latter vary widely due to the many 

different fi lter materials used. The fact that they need 

to be changed at two-to-three year intervals increases 

the operating cost of this technology, but it is the most 

effective for dealing with fi ne particles.

Table 8. Approximate estimated costs for different particle control devices at 2005/2006 levels 

(PIHI-KHK 2005). 

Control device Fuel Output Investment Operating 

cost

Emission 

level

Pressure 

loss

MWfuel €/MWfuel €/MWhfuel mg/m3n mbar

ESP all below 5 40 000 n.a. 15–50 2–3

all 5–50 20 000 0.1

all 50–150 15 000 n.a.

solid and lye > 150 10 000 n.a.

liquid > 150 7 000 n.a.

Fabric fi lter all 5–50 18 000 0.3 5–25 10–20

solid > 150 13 000 0.2

liquid > 150 10 000 n.a.

Scrubber + 

Heat recovery

all 5–50 35 000 0.3 50–500 10–15

Scrubber all 5–300 60 000 0.5 50–500 10–15

all 300–1 000 80 000 0.3

all > 1 000 40 000 n.a.

Cyclone / multicyclone all below 5 6 000 n.a. 20–1500 10–20

all 5–50 1 600 0.1
30
31

I

n line with earlier research, the FINE programme 

confi rmed that the specifi c emissions of PM2.5 or 

PM1 fi ne particulates from large boilers are signifi -

cantly lower than those from small boilers in the 10–50 

kW range, and are typically 0.5–50 mg/MJ compared to 

10–300 mg/MJ. The specifi c emissions of fi ne particles 

depend primarily on the control systems employed, as 

fuel and combustion technique have a much smaller impact where effi cient control systems are used. 

Fabric fi lters are the most effective technology in 

this respect. Measurements taken during the FINE 

programme show that the proportion of PM1 in fl ue 

gas after passing through a fabric fi lter is only 1–10% 

of the total particle mass, which is mainly composed of 

coarse particles/material released from the fi lter or from 

exhaust gas ducts. 

Fluidised bed boilers equipped with ESPs or FFs can 

have specifi c emission factors as low as 3–5...10 mg/MJ 

for fi ne particles under optimal operating conditions. 

The emission factors for grate boilers are typically a little higher. With effi cient dust removal, they can achieve 

specifi c emission levels of 5–20 mg/MJ. 

In addition, earlier studies (Ohlström 1998) have 

shown that, in the case of pulverised coal combustion, 

particulate emissions are composed mainly of particles 

smaller than 5 μm in aerodynamic diameter. Roughly 

half of the total mass of these emissions is composed 

of fi ne particles (PM2.5). For large boilers equipped 

with an ESP and/or a fabric fi lter, the specifi c emission 

factor for fi ne particles can be as low as 0.5–5 mg/MJ. 

When peat is fi red in a pulverised fuel design, 20-25% 

of the total mass of particulate emissions comprises fi ne 

particles, and the specifi c emission factor is between 5 

and 8 mg/MJ. 

While summarising some of the latest measurement 

data on fi ne particle emissions from boilers used for 

energy generation collected under the auspices of the 

FINE programme, Table 7 on Pages 26-27 is based on 

short-term data and leaves a number of questions related to annual emissions partially unanswered. The frequency and impact of fi lter disturbances and full/partial 

load operation on fi ne particle emissions, in particular, 

needs to be clarifi ed by long-term or continuous measurements. The heavy metals bound to fi ne particles, and 

their chemical behaviour, also need to be looked at in 

greater detail.

Fine particles account for 50-60% (by mass) of the 

total particulate emissions from recovery boilers, and 

the specifi c emission factor for fi ne particles varies considerably according to the boiler size category, ranging 

between 10 and 80 mg/MJ. As it is not economically 

feasible to use ESPs or FFs with small boilers rated at 

less than 15 MW, these types of plants generate higher 

levels of fi ne particle emissions than large boilers during optimal operation. 

Our knowledge about fi ne particle emissions from 

industrial processes is much more sketchy compared to 

energy generation. One reason for this has been the fact 

that taking measurements from several emission sources, 

such as roof openings, can be diffi cult and/or expensive. 

These sources need to be investigated, however, if we are 

to better understand their impact on health and if we 

want to reduce these types of emissions from the plant 

design phase onwards.

The total suspended particulate matter (TSP) emissions of boilers fi red on heavy fuel oil is typically around 

25–50 mg/MJ, while fi ne particles account for only 2.5–5 

mg/MJ. This compares favourably to biomass combustion 

systems equipped with cyclones and scrubbers, where the 

TSP is around 30–50 mg/MJ and the fi ne particle component 20–40 mg/MJ. On the other hand, the fi ne particulate matter from heavy fuel oil combustion includes 

high levels of vanadium and nickel. In the case of biomass combustion, zinc is the main heavy metal component. All of these elements are harmful trace metal compounds, and are connected with direct toxicity.

Overall, the FINE Programme confi rms that there is 

a clear window of opportunity for innovation in combating particulate emissions in energy generation and 

industry, given the ongoing requirement for more effi -

cient boilers and better fi lter technology, resulting both 

from the introduction of ever-tougher emission limits 

and the limitations of existing solutions.

This is particularly clear in respect of small plants 

rated below 10 MW, for which no cost-effi cient fi ne particulate control technology exists as yet – something that 

is highlighted by the fact that fi ne particulate emissions 

from larger energy-generating facilities are low compared 

to those from small-scale wood combustion. It should 

also be noted that fi ne particulates in the immediate environment are considered potentially more harmful than 

those carried over longer distances.

5 Conclusions
32

Biswas, P. J. and Zachariah, M. R. Environ. Sci. & 

Technol. pp. 2455-2463.

Ho, T. C.; Chen, C.; Hopper, J. R.; Oberacker, D. A. 

Combust. Sci. Technol. 1992, 85, 1001.

Hokkinen, J.; Jokiniemi, J.; Aurela, M.; Hillamo, R. 

2004. Fine Particle and Trace Element Emission 

Characterisation from Energy Production and 

Process Industry [In Finnish: Energiantuotannon 

ja prosessiteollisuuden pienhiukkas- ja 

raskasmetallipäästöjen karakterisointi.] VTT Processes, 

Espoo. 39 p. + App. 18 p. VTT Research Notes : 2258. 

http://virtual.vtt.fi /inf/pdf/tiedotteet/2004/T2258.pdf 

(RAME project)

Jokiniemi, J., Pyykönen, J., Mikkanen, P. and Kauppinen, 

E. 1996. Modelling fume formation and deposition in kraft 

recovery boilers. Tappi Journal Vol. 79, No. 7. pp. 171 

- 180.

Lind, T., Hokkinen, J., Jokiniemi, J.K., Moilanen, A., 

Zevenhoven, M., Skrifvars, B-J., and Hupa, M. 2005. 

The Effect of fuel quality on Fine particle formation and 

trace metals from combustion of industrial and municipal 

waste. Submitted to Journal of the Air and Waste 

Management Association.

Lind T., Hokkinen J., Jokiniemi J., Saarikoski S., Hillamo 

R. 2003. Electrostatic Precipitator Collection Effi ciency 

and Trace Element Emissions from Co-Combustion 

of Biomass and Recovered Fuel in Fluidized-Bed 

Combustion, Environ. Sci. Technol., 37 (12), 2842 -2846, 

2003

Lyyränen, J., Jokiniemi, J. K., and Kauppinen E. I. 

2002. The effect of Mg-based additive on aerosol 

characteristics in medium-speed diesel engines operating 

with residual fuel oils, J. Aerosol Sci. 33 (7), 967-981.

Ohlström, M.; Tsupari, E.; Lehtilä, A.; Raunemaa, T. 

2005. Fine Particle Emissions and Their Reduction 

Potentials in Finland. The Effects of Greenhouse Gas 

Emission Reduction [In Finnish: Pienhiukkaspäästöt 

ja niiden vähentämismahdollisuudet Suomessa. 

Kasvihuonekaasupäästöjen rajoittamisen vaikutukset.] 

VTT Research Notes 2300, Espoo. 91 s. + app. 1 p. 

http://virtual.vtt.fi /inf/pdf/tiedotteet/2005/T2300.pdf (PIHIKHK project)

Ohlström, M. 1998. Fine particle emissions from energy 

production in Finland. [In Finnish: Energiatuotannon 

pienhiukkaspäästöt Suomessa]. VTT Research Notes 

1934, Espoo. 114 p. + app. http://virtual.vtt.fi /inf/pdf/

tiedotteet/1998/T1934.pdf.

Owens, T. M.; Biswas, P. J. Air Waste Manage. Assoc. 

1996, 46, 530.

Scotto, M. V.; Peterson, T. W.; Wendt, J. O. L. 2nd 

International Congress on Toxic Combustion ByProducts; 

Salt Lake City, UT, 1992.

Tissari, J. (edit.), Raunemaa, T., Jokiniemi, J., Sippula, 

O., Hytönen, K., Linna, V., Oravainen, H., Pyykönen, J., 

Tuomi, S., Vesterinen, R., Taipale, R., Kolsi, A., Nuutinen, 

I., Kouki, J., Vuorio, K. 2005. Fine Particle Emissions 

from Wood Combustion. End Report [In Finnish: Puun 

polton pienhiukkaspäästöt. Loppuraportti.] Kuopion 

yliopiston ympäristötieteiden laitosten monistesarja, 

2/2005. University of Kuopio, Fine Particle and Aerosol 

Technology Laboratory. http://www.uku.fi /laitokset/ifk/

PIPOLoppuraportti2005.pdf (PIPO project)

Web links

FINE Programme: http://www.tekes.fi /fi ne

EU’s CAFE Programme: http://ec.europa.eu/comm/

environment/air/cafe/index.htm

Air quality standards in Finland: http://www.ymparisto.

fi /default.asp?node=580&lan=fi

World Health Organisation (WHO): http://www.who.int.

References
Tekes, the Finnish Funding Agency for Technology and 

Innovation is the main publicly funded organisation for fi nancing 

applied and industrial R&D in Finland. 

Tekes’ primary objective is to promote the competitiveness 

of industry and the service sector in Finland by enhancing 

the country’s technological potential – through such areas as 

diversifying production structures, increasing production and 

exports, and creating a more solid foundation for prosperity 

today and into the future. 

Tekes’ technology programmes are a key part of the Finnish 

innovation system, and have proved a highly effi cient means 

of encouraging and stimulating cooperation and networking 

between companies, universities, and research institutes for 

developing innovative products, processes, and services. 

Programmes focus on specifi c sectors of technology or industry, 

and are designed to give business access to the latest research 

results. Together with the Tekes network in Finland and overseas, 

they also provide an excellent framework for international R&D 

cooperation. 

Tekes – Your contact for Finnish technology
As a result of ever-growing traffi c volumes and ever-higher 

levels of energy generation, the air we breathe exposes us to 

a growing number of airborne contaminants, including minute 

particulate matter. These particulates are known to have major 

health implications, and result in the premature death of over 300,000 

people in the EU annually alone. Advances in technology are allowing 

us to measure smaller and smaller sizes of particles and gain a better 

understanding of how they impact our health and the environment, however. 

Tekes launched a four-year technology programme in 2002, known as 

FINE, together with the Academy of Finland, the Ministry of Transport 

and Communications, and the Ministry of the Environment, to focus on 

particulate emissions. This concentrated on fi ve main areas: energy and 

industry, traffi c and transport, measurement technology, indoor air, and 

health and the environment. All in all, the FINE Programme involved over 50 

projects and close to 60 companies and more than 20 research institutions. 

A summary report has been produced on each of these areas. This report 

concentrates on particulate emissions related to the energy sector and 

industry. Drawing on the work of 14 projects, it provides an overview of the 

programme’s measurement fi ndings in respect of different types of plants, 

large and small, together with an assessment of current technologies and 

the potential for improving on them. 

Tekes, the Finnish Funding Agency for Technology and Innovation is the 

main publicly funded organisation for fi nancing applied and industrial R&D 

in Finland. For more information, see www.tekes.fi

ISBN 952-457-246-X