# 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.fi3 

# Preface



# T he air we breathe exposes us to a growing number of airborne contaminants, including particulatematter invisible to the eye, as a result of ever-growing traffic 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 2002known as the FINE Particles – Technology, Environment and Health Technology Programme – together with the Academy of Finland, the Ministry of Transportand 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, identifythe potential for new technological innovations, and encourage their commercialisation. The FINE programme involved over 50 individualprojects and close to 60 companies and 20 research institutions, and focused on emissions and technology in fi ve main areas: energy and industry, traffic 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 andsmall, together with an assessment of current technologies and the potential for improving on them. Electrostatic precipitators and fabric filters, for example, are already widely used and offer good performance, but can be employed even more efficiently. 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 fine 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 325 

# Introduction 1


# 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 mixtureof 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 fine particle fraction as a result (Lind 2003). Soot and organic carbon species also exist as fine 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 particulatesfrom 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 generationParticulate 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 fine 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 fly 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 processesused 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 SO 2 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 Na 2 SO 4 in the gas phase can be limited due to kinetic considerations. Recent results from laminar flow reactor experiments and SEM samples taken from upper furnace areas, however, indicate that Na 2 SO 4 fume particles form at temperatures above6 Trace elements Pb, Cu...

K, Na, Ca ash particles heterogeneous 

### T g 

### [°C] 900-1500 400-800 0-500 Characteristics before filte r

sulphates chlorides oxides



### PbCl 2 

### , CuCl 2 

### ..M x 

### O y



### KCl, K 2 

### SO 4 

### , CaSO 4 

### Fuel

particle pyrolysis/ burn-out/ flame/ volatilisation

sulphates chlorides oxides

1,100 °C. When there is not suffi cient SO 2 to bind allthe Na and K present, these species convert to carbonates through surface reactions. These surface reactionsmay 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 fine particle formation in black liquor combustion. The fine particle mode takes the form of larger particles (1-3 μm) in black liquor combustion, due to thelarger amount of volatilised material and mass concentration. ESP (electrostatic precipitator) effi ciency is good here, because of the lack of fi ne particles between0.1 – 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 difficulty 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 fired in larger boilers, but generally in combination with other fuels.

Wood In the case of wood combustion, the main volatilised species are K, Cl, SO 4 , and CO 3 . The fi rst particles formed after volatilisation as the flue gas cools are produced by the homogeneous or heterogeneous condensation of K 2 SO 4 . KCl and K 2 CO 3 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 2 O

M (c) NaOH (g) , NaCl( g) SO 2 Na 2 SO 4 ( g) , NaOH( g) , NaCl( g)

M x O y (c) M x O y ( c) Na 2 SO 4 ( c)

M x O y (c)

HCl Na 2 SO 4 (c)

M x O y ( c) Na 2 CO 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) gaseous7 and involved in fi ne particle formation are typically Na, Zn, and small amounts of Ca. The schematics of fi neparticle 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 compoundsK, 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 oilFuel 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 andmethod 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 fine 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 

### T g 

### [°C] 900-1500 600-1000 0-500 Characteristics before fil ter

Incomplete combustion - soot+organics

Good combustion - ash ( inorganic)

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

### droplet

V, Ni, ...ashparticle s sulphates oxides H 2 SO 4 +hydrocarbons 

### T g 

### [°C] 1200-2500 800-1500 0-200 Emissi on

V, Ni, Fe catalyzes SO 2 SO 3 pyrolysis/ burn-out/ flame/ volatilisation 

### Fuel soo t

Incomplete combustion - soot+organics+H 2 SO 4

Good combustion - ash + H 2 SO 48 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 areas yet no emission standards for these types of particles – only for total particle mass released from power plants and industrial processes. However, the tendencyin 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/m 3

. As this corresponds to the binding limit of 40 μg/m 3 for respirable particles (PM10) valid at present, this ceilingvalue 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/m 3 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 forEurope) 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. PM 10 (μg/m 3) EU US Hourly – – 24-hour mean 50 a 150 b
Annual mean 40 50 c

PM 2.5 (μg/m 3) EU US 24-hour average – 65 d Annual average – (CAFE: 25) 15 e

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 recentLCP 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 onPM2.5 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/m 3 ] (NTP, dry gas O 2 11%) Sulphur dioxide, SO 2 50 Nitrogen oxides, NO x(as NO 2 ) 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 researchprojects 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 fine 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 mustbe 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 filter 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.1011 

# Current technologies for controlling fi

ne particulate emissions 2


# A dust or ash removal system is usually needed tocontrol the quantity and, in some cases, the characteristics of particles going to the stack. The most important factor in determining the type of fly ash collection technology to be used is the permissible particulate outlet emission concentration in mg/Nm 3

.If an invisible plume is required, this requires an outlet emission of typically 50 mg/Nm 3 (O 2 , 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/Nm 3

, 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 fine 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 efficiency 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 fine particles have a minor impact on total penetration or fi ltration effi ciency. However, as these particlesare 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, flue 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 formodern heat and power generation systems is usually between electrostatic precipitators and fabric fi lters. Both ESPs and fabric fi lters are highly efficient, 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 fine 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, andthis is expected to continue for the next couple of decades. The choice between ESP and fabric filtration 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 atintervals 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 fixed or rigid electrodes and located after the air pre-heater, operating at between 130 °C and 180 °C, account for the bulk of thecurrent 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 fourmain 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 ofthe 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. Significant temperaturerelated effects have also been observed in some dust notlinked 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 reductionsof 30-80% have been documented for a variety of controller retrofits, 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 (SO 3 ). 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 SO 3 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 uegas 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 acleaning mechanism and deposited in collection hoppers. See Figure 6. There are three types of fabric filters. 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 filters and require considerably more space. Overall, the technology is relatively simple compared to ESP. Outlet emission is generally less than 50 mg/ Nm 3 for a well-designed system, resulting in an invisible plume. Fabric filters 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 filters, 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 largeragglomerates and less re-entrainment. Elemental sulphur, ammonia (NH 3 ), and sulphur trioxide (SO 3 ) 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 requirementcan 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 SO x , 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 coalquality, whereas ESP performance can be affected signifi

cantly. Another advantage of fabric fi lters is that aclear 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 filters 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 fine 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 fit cyclones for particulate removal. More recently, fabric filters have become increasingly popular in a number ofindustrial 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 finest particles from flue 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 scrubbersScrubbers – 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 comesto 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 generallydrops 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 SO 2 from fl ue gas, and many units are designed to control both SO 2 and particulates by utilising alkaline fl y ash as a sorbent. Lime is frequently used to boost SO 2 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 considerationsthat 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 metallicspecies. 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 bemaintained 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 fly 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 effluent 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.1819 0.00001 0.0001

0.001 0.01

0.1

1 10 100

0.01 0.1 1 10 100 Dp, μmPenetration, % 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 

# Findings from FINE projects 3


# 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/m 3 compared to a concentration of around 10 μg/m 3 in clean outdoor ambient air. Electrostatic precipitators are at their least effi cient whenhandling 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 fine 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 efficient 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 filters, 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 shouldthey 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 filter 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/Nm 3 ) 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 cientwhen 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/

m 3 , and dioxins 10% of the WID boundary value. All trace metal and other measured emissions (NO
x , SO 2 , 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 outlet21 The volume of particles in the fl ue gases before the filter 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 andoutlet 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 Pband Cu in large quantities. Cd, Tl, Cu, and Pb were enriched in the fine 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 ofthe 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 efficiency, and trace element emissions. The measurements were conducted at the inlet and outlet of ESPs fitted 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 flue gas temperature of 190 °C. ESP outlet measurements were made at the stack. CO concentration varied between 0 and 500 mg/Nm 3 at both boilers, but was generally higher at Boiler A. The total particle mass concentrations at the ESP inlet were 11-23 g/Nm 3 (dry gas) for Boiler A, and 17- 24 g/Nm 3 (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/Nm 3 at Boiler A, and 14-21 mg/Nm 3 at Boiler B (Figure 14). Emissions measured according to the SFS3866 standard were 30-40 mg/Nm 3 at Boiler A, and 12-15 mg/Nm 3 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/m 3 fl ue gas mg/m 3 (NTP, dry gas (NTP, dry gas O 2 11%) O 2 11%) MEASURED EU WID SO 2 0.16 – 25 50 NO x(as NO 2 ) 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 1022 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 kraft23 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 alsohigh. 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 processtemperature, and the Cl, Pb, Cd, Cu and As concentrations in the fuel used. A higher process temperature andhigher 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 V 2 O 5 acting as a catalyst for SO 3 formation. Heavy fuel oil generates a larger proportion of smaller fine 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 combustiondm/dlog(Dp) [mg/m^3] (NTP)

Cyclone cut-off D50=~4 μm25 Biomass
Biomass is often combusted in small units that do notemploy 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 efficient 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 insidethe 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 includefairly 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

SO 4 SO 4 SO 4 SO 4 SO 4 SO 4 K Other

Other Zn Ni V Cl Na K SO 426 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 byunburned carbon species: organic carbon (OC, including PAH) and elemental carbon (soot). Firing wood inlarger 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 O 2 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, SO 4 230 200 – 200 SO 4 , K, Na spherical 100-200 nm 200 SO 4 , 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, SO 4 , 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 H 2 SO 4 , V, Ni spherical 100 nm small size toxic 150 3 number, V, Ni, SO 4 500 na** Si, S, K, Hg... na** Coal Hg, As 1000 Na, SO 4 , 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 fine 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 

# R egulations covering the emissions from energygeneration 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 beenagreed, 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 fine particle emissions generated by fi ring wood do not diverge signifi cantly from those from fossil fuels or peat, thanks to the use of efficient 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 filter 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 fine 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-efficient 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 peatBaseKyotoT10-20%T20-20%UEOBaseKyotoT10-20%T20-20%UEOBaseKyotoT10-20%T20-20%UEOBaseKyotoT10-20%T20-20%UEOBaseKyotoT10-20%T20-20%UEOParticulate emissions, PM2.5, 1,000 t 

# Future trends 429the 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 neparticle 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 onlow-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 fine 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 filters, 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 €/MWh fuel mg/m 3 n 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 0.13031 

# 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, asfuel 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 onshort-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 clarified 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 fine 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 fine 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 fine particle component 20–40 mg/MJ. On the other hand, the fine particulate matter from heavy fuel oil combustion includeshigh 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 isa 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 fine 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 fine particulates in the immediate environment are considered potentially more harmful than those carried over longer distances. 

# Conclusions 532 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