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Article

Electrostatic Precipitator Collection Efficiency and Trace

Element Emissions from Co-Combustion of Biomass

and Recovered Fuel in Fluidized-Bed Combustion

Terttaliisa Lind, Jouni Hokkinen, Jorma K. Jokiniemi, Sanna Saarikoski, and Risto Hillamo

Environ. Sci. Technol., 2003, 37 (12), 2842-2846• DOI: 10.1021/es026314z • Publication Date (Web): 08 May 2003

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Electrostatic Precipitator Collection

Efficiency and Trace Element

Emissions from Co-Combustion of

Biomass and Recovered Fuel in

Fluidized-Bed Combustion

TERTTALIISA LIND,

JOUNI HOKKINEN, AND

JORMA K. JOKINIEMI*

Aerosol Technology Group, VTT Processes, P.O. Box 1602,

Biologinkuja 7, FIN-02044 VTT Espoo, Finland

SANNA SAARIKOSKI AND RISTO HILLAMO

Finnish Meteorological Institute, Sahaajankatu 20 E,

FIN-00880 Helsinki, Finland

Particle and trace element emissions from energy

production have continuously been subject to tightening

regulations. At the same time, not enough is known on the

effect of different combustion processes and different

fuels and fuel mixtures on the particle characteristics and

particle removal device operation. In this investigation,

electrostatic precipitator fractional collection efficiency and

trace metal emissions were determined experimentally

at a 66 MW biomass-fueled bubbling fluidized-bed combustion

plant. The measurements were carried out at the inlet

and outlet of the two-field electrostatic precipitator (ESP)

at the flue gas temperature of 130-150 °C. Two fuel

mixtures were investigated: biomass fuel containing 70%

wood residue and 30% peat and biomass with recovered

fuel containing 70% wood residue, 18% peat, and 12%

recovered fuel. The particle mass concentration at the ESP

inlet was 510-1400 mg/Nm3

. Particle emission at the

ESP outlet was 2.3-6.4 mg/Nm3

. Total ESP collection

efficiency was 99.2-99.8%. Collection efficiency had a

minimum in particle size range of 0.1-2 μm. In this size

range, collection efficiency was 96-97%. The emission of

the trace metals As, Cd, Co, Cr, Cu, Mn, Ni, Pb, Sb, Tl,

and V was well below the regulation values set by EU

directive for waste incineration and co-incineration.

Introduction

Fine particles emitted to the atmosphere have gained

increasing attention lately because of the adverse health

effects associated to fine particles in recent studies (1). Energy

production by combustion is one of the major stationary

sources of fine particles, and it has therefore been the subject

of tightening regulations as regard to emissions. Trace

elements are mainly found in the particulate phase at the

particle removal device operating temperatures, with the

exception of Hg and possibly Se (2, 3). However, many of

them are volatile during combustion, and consequently, they

may be enriched in the fine particle fraction that penetrates

easily through the particle removal devices.

The European Union has set Directive 2000/76/EC to

regulate the harmful emissions, including particles and 12

toxic trace elements, from waste incineration. The directive

also applies to co-incineration of waste materials with other

fuels. At the same time, the amount of waste is steadily

increasing, and waste incineration including co-incineration

is a viable option to reduce the amount of waste. Therefore,

the issue of particle and trace element emissions from waste

co-combustion needs to be addressed.

The particle formation and trace element behavior during

combustion of solid fuels have been extensively investigated,

and a fairly good description of these issues is available in

the literature (4-8). The particle emission to the atmosphere

is determined by the combination of the particle formation

and transformations in a specific combustion process and

fuel and the particulate removal system that is used. The

solid fuel combustion-generated particle emissions typically

consist of two types of particles: fine particles approximately

0.1-1 μm in diameter that are formed from the ash-forming

species that are volatilized during combustion and residual

ash particles larger than 1 μm in diameter that are formed

from mineral impurities in the fuels (4). In fluidized-bed

combustion (FBC), also a third particle type is found that

consists of fragmented bed material (9). The relative abundance of the different particle types depends on the fuel and

combustion system as well as combustion conditions.

Typically, for example, higher combustion temperature

increases the amount of fine particles. Also, biomass

combustion generally produces more fine particles than, for

example, coal combustion due to the different mode of

occurrence of the ash-forming compounds in the fuels. Many

trace elements are relatively volatile during combustion and

are consequently enriched in the fine particle fraction.

Electrostatic precipitators (ESPs) and fabric filters are

commonly used to remove particles from the flue gases of

power plants and waste incinerators when relatively low

emission values are needed. The ESP is generally accepted

as a reliable and efficient particulate control device with low

operating and maintenance costs. It has also proved to be

able to achieve the required emissions levels for most major

applications. Fabric filters offer high collection efficiency for

particles and can be combined with absorbents for gaseous

emission control but have relatively high maintenance costs.

Most of the particulate removal devices capture coarse

particles efficiently, whereas fine particle collection efficiency

depends on the device and the characteristics of the particles

to be captured as well as gas conditions and other factors.

Typically, ESPs have a penetration window in the particle

size range of 0.1-1 μm. In pulverized coal combustion, even

10% of the particles in this size range may penetrate the ESP

(10-12). Fabric filter collection efficiency also decreases with

decreasing particle size, but with a well-maintained fabric

filter, the maximum penetration is typically significantly lower

than with ESPs. Insignificant literature data are available on

the fractional collection efficiency of ESPs or fabric filters in

biomass combustion or on waste and biomass co-combustion. However, particle characteristics and other conditions

are different from, for example, pulverized coal combustion;

therefore, it can be expected that particle removal device

operation may differ significantly. This, in turn, is critical in

respect to the particle and trace element emissions to the

atmosphere.

In this investigation, we determined the fractional collection efficiency of a two-field ESP during biomass combustion and biomass-waste co-combustion in a bubbling

fluidized bed. In addition, we determined the emissions of

* Corresponding author e-mail: Jorma.Jokiniemi@vtt.fi; telephone: +358 9 456 6158; fax: +358 9 456 7021.

Environ. Sci. Technol. 2003, 37, 2842-2846

2842 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 12, 2003 10.1021/es026314z CCC: $25.00  2003 American Chemical Society

Published on Web 05/08/2003
the trace elements that are regulated by EU directive on waste

incineration. Mercury was not included in this investigation

despite its importance while assessing trace element emissions from combustion processes because, at the temperatures of the typical particulate removal devices, mercury

may occur both in the vapor phase and in the particulate

form. This investigation concentrated only in the emissions

in the particulate matter; therefore, mercury was not included

in the study.

Methods

Plant Description and Fuels. Fine particle measurements

were carried out at a 66 MW Forssa biomass-fueled CHP

bubbling fluidized-bed plant between February 13 and

February 16, 2001. The measurements were carried out

upstream and downstream of the two-field ESP at the flue

gas temperature of 130-150 °C. The particle mass size

distribution was determined with Berner-type low-pressure

impactor (BLPI), mass concentration was determined with

a tapered element microbalance (TEOM) and BLPI, and

number size distribution was determined with an electrical

low-pressure impactor (ELPI). The values of process parameters and gas composition were collected continuously.

Two different combinations of biomass fuels were studied.

During the first 2 d of measurements, 30% peat and 70%

wood residue were burned. Then, the effect of high-chlorine

fuel was tested by adding approximately 12% recovered fuel

(REF) to 18% peat and 70% wood residue for the next 2 d.

Numbers are the percentages from total fuel power. Sand

was used as bed material. Even though the aim was to keep

the power plant conditions as stable as possible during the

measurements with the two fuel combinations, the load of

the power plant varied slightly from day to day and even

during the measurements. However, the gas concentrations

were fairly stable during the measurements with O2 concentration before the economizer between 4.5 and 5.5%, CO

concentration at 8-10 ppm, and SO2 concentration very low

at about 1 ppm. HCl concentration was below 1 ppm without

REF and with REF was higher, up to about 15-20 ppm. Bed

temperature was typically about 770-830 °C without REF

feed and with REF was slightly higher at about 810-840 °C.

Samples of fly ash from ESP hoppers, bottom ash, sand,

and fuels were collected periodically for analysis. Fuel samples

were taken from the feeder line on the day of the measurements. For the analysis, the fuels were ashed at 500 °C. The

ashes of wood residue, peat, and REF were analyzed by XRF,

IC, and ICP-MS/AES. The analyses were carried out at the

Geological Survey of Finland, which has been accredited

according to Standard EN 45001 and ISO IEC Guide to use

the aforementioned methods.

The ash contents and the concentrations of the major

ash elements in the fuel ash are shown in Table 1. Chlorine

content in REF (2.3%) is approximately 10 times higher than

in wood residue and peat (Table 2). Also, as expected, many

of the trace element concentrations are higher in REF than

in the biomass fuels. The REF-containing fuel mixture

contains significantly more Cd, Cr, Cu, Pb, and Sb than the

fuel without REF.

The trace element concentrations of the fuel ash, the ESPcollected ash, and the bottom ash are shown in Table 2. ESP

1 was collected from the first ESP field, and ESP 2 was collected

from the second field. The specific surface area of ESP ash

from the first field was 25.3 m2/g and from the second field

was 14.0 m2/g. Typically, the coarsest particles and most of

the mass are collected in the first ESP field. Scanning electron

microscope (SEM) analysis of the fly ash particles showed

that the coarse mode particles (Dp > 1 μm) were porous

agglomerates whereas the fine mode particles (Dp < 1 μm)

were mostly single, close to spherical particles, or small

agglomerates with only a few primary particles. This explains

the somewhat surprising result of a larger specific surface

area of the particles collected in the first ESP field as compared

to the particles collected in the second ESP field. This result

is consistent with earlier studies from fluidized-bed combustion of biomass where the coarse fly ash particles were

found to have a very porous structure (13).

As, Cd, Mn, Pb, Sb, Tl, and V were more concentrated in

the second ESP field than in the first ESP field with both fuel

mixtures. Other trace metals were enriched either in the first

or second field, depending on the fuel. Some of the trace

element concentrations were significantly higher in the fuel

ash than in the ESP-collected ash. This was presumably due

to the heterogeneous nature of the waste fuel.

Aerosol Measurements. The fly ash particle mass size

distributions at the ESP inlet and outlet were determined by

collecting size-classified fly ash samples with an 11-stage,

multijet compressible flow BLPI (6). The collections were

carried out by inserting the impactor inside the flue gas

channel. To avoid overloading of the impactor, a precutter

cyclone was used prior to the impactor at the ESP inlet to

collect particles larger than 8 μm. Thin, greased aluminum

(Al) and polycarbonate (poreless Nuclepore) films were used

as impaction substrates at both measurement locations. BLPI

samples were taken at approximately the same time at the

inlet and outlet of the ESP, although the measurement before

ESP lasted 0.5-1.5 min and after ESP lasted 15-60 min. The

measurement system configuration is presented in Figure 1.

Aerosol samples were analyzed for inorganic ions (SO4,

Cl, NO3, Na, NH4, K, Mg, and Ca) using two Dionex DX500

ion chromatographs. Samples were first wet with 0.5 mL of

methanol and extracted with 4.5 mL of deionized water

(Millipore Alpha-Q) as described by Teinila ̈ et al. (14). The

resulting solutions were mixed by rotating the test tubes for

10 min. For anions, a sample solution was manually injected

through a 1000-μL loop to an AG 11 guard and an AS 11

analytical column. The eluent for anions was 0.4-25 mM

NaOH (gradient). A 300-μL loop, a CG 12A guard column,

and a CS 12A analytical column were used for cations. The

eluent for cations was 20 mM methanesulfonic acid. The

ions in both methods were detected by conductivity detector.

Elements in aerosol samples were analyzed using an ELAN

6100 ICP-MS instrument (PerkinElmer SCIEX). For ICP-MS

analyses, samples were extracted in a mixture of concentrated

HNO3 and concentrated HF, following the method of Jalkanen

and Ha ̈sa ̈nen (15). Analyzed elements were As, Cd, Co, Cr,

Cu, Mn, Ni, Pb, Sb, Ti, and V. Rhodium was used as an internal

standard. Laboratory filter blanks were made both for IC and

ICP-MS to determine the backgrounds of analytes for sample

handling and storage.

Number size distributions of the fly ash particles in the

size range of 0.01-3 μm were determined at the ESP outlet

with an ELPI (16). Prior to the ELPI, the flue gas was diluted

with a porous tube diluter to quench the sample gas flow.

The total particle mass concentration at the ESP outlet

was measured with a tapered element oscillating microbalTABLE 1. Ash Contents of Fuels and Chemical Analysis of

Major Elements in Fuel Ash

element

wood

residue

(%)

refuse

(%)

peat

(%)

total in

fuel no

REF (%)

total in

fuel with

REF (%)

ash 1.5 10 3.0 1.9 2.8

Ca 27.5 12.9 12.6 20.6 18.2

Cl 0.217 2.31 0.355 0.282 1.154

K 5.89 2.00 0.531 3.38 3.15

Na 0.883 2.97 0.156 0.543 1.65

S 0.299 1.62 2.23 1.20 1.25

Si 2.89 16.5 7.11 4.86 9.62

VOL. 37, NO. 12, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2843
ance (TEOM) (17). TEOM samples were undiluted, and the

measurements lasted less than 30 min each.

Results

Fly Ash Particle Size Distributions. Fly ash particle mass

concentration at the ESP inlet was 510-830 mg/Nm3 without

REF feeding and 800-1400 mg/Nm3 with REF (Table 3). The

outlet concentrations as determined with low-pressure

impactors were 3.4-5.6 and 2.3-6.4 mg/Nm3 without and

with REF feeding, respectively. The continuous mass monitor

TEOM gave outlet concentrations of 4.5-9 mg/Nm3 with

both fuels The lowest value for the outlet concentration with

REF feeding was measured when the boiler load was

significantly lower than typically during this test campaign.

The typical boiler load during the measurements was 36-44

MWfuel, whereas during the outlet sampling, which gave the

low mass concentration of 2.3 mg/Nm3, the boiler load was

only 27 MWfuel.

Fly ash consisted of two distinctly different particle modes

at both the ESP inlet and the ESP outlet (Figures 2 and 3).

A fine particle mode appeared at the particle size of 0.1-0.4

μm with both fuels, and a coarse particle mode appeared at

the particle size greater than 1 μm. Mass concentration of

the fine mode was significantly lower without REF than with

REF, with concentrations of 25-40 and 62-85 mg/Nm3,

respectively, at the ESP inlet. The fine particles contained on

average 4.5% and 7.0% of the total particle mass at the ESP

inlet, without and with REF, respectively. At the ESP outlet,

the fine mode contained 23% and 44% of the total particle

mass, without and with REF feed, respectively.

The enhanced fine mode fraction at the ESP outlet was

caused by the less efficient particle collection of the ESP at

the fine mode particle size range than in the coarse particle

TABLE 2. Trace Element Contents in Fuel Ash, ESP-Collected Ash from Fields 1 and 2, and Bottom Ash during Combustion with

and without REF Addition

element

wood

residue

(ppm)

refuse

(ppm)

peat

(ppm)

total in fuel

no REF

(ppm)

total in fuel

with REF

(ppm)

ESP 1

no REF

(ppm)

ESP 2

no REF

(ppm)

ESP 1

with REF

(ppm)

ESP 2

with REF

(ppm)

bottom ash

no REF

(ppm)

bottom ash

with REF

(ppm)

As 3 37 108 52 38 13 43 12 34 6.9 5.9

Cd 6.0 24 1.9 4.1 13 1.5 3.6 2.4 6.7 0.017 0.033

Co 15 67 78 44 50 4.2 3.6 2.0 4.5 1.6 6.9

Cr 90 1 020 143 115 505 24 40 67 55 14 16

Cu 111 2 800 245 174 1 310 390 320 300 480 140 na

Mn 14 100 1 650 6 760 10 700 7 240 1 300 3 200 1 800 3 400 730 700

Ni 95 209 138 115 153 8.8 27 53 25 4.2 4.9

Pb 128 1 100 40 87 533 35 61 60 160 14 3.2

Sb 0 79 0 0 34 3.5 8.8 11 40 2.0 4.2

Tl 0 0 0 0 0 0.45 0.70 0.47 1.4 0.062 0.032

V 15 52 350 172 97 27 60 24 40 9.1 8.1

FIGURE 1. Schematic diagram of the experimental setup. A precutter

cyclone (C) was only used at the ESP inlet in combination with

BLPI.

TABLE 3. Particle Mass Concentrations at the ESP Inlet and

Oulet As Determined by Low-Pressure Impactors with and

without REF Feed

particles <0.5 μm

fuel (mg/Nm3

) (%)

total

(mg/Nm3)

inlet no REF 26 3.1 830

no REF 25 4.9 510

no REF 40 5.4 740

with REF 85 6.1 1 400

with REF 62 6.2 1 000

with REF 72 9.0 800

outlet no REF 1.1 22 5.1

no REF 0.79 23 3.4

no REF 1.4 24 5.6

with REF 3.1 49 6.4

with REF 0.92 40 2.3

FIGURE 2. Particle mass size distributions at the ESP inlet as

determined with low-pressure impactor. The particles collected in

the precutter cyclone are not included in the figure.

FIGURE 3. Particle mass size distributions at the ESP outlet.

2844 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 12, 2003
sizes. It can also be seen from Table 3 that with certain fuelss

especially ones with high chlorine contentssa significant

fraction of the particle emission comes from the fine mode

particles. In this case, almost half of the particle emission

was caused by fine mode particles during REF firing.

The main components in the fine mode particles, as

determined with the IC and ICP-MS analysis, were Na, K,

and SO4 without REF feed and Cl, K, Na, and SO4 with REF

feed. The increased fine mode particle concentration with

REF was mainly due to the higher chlorine content of the

fuel. Chlorine is known to increase the amount of alkalis in

the vapor phase in post-combustion conditions. The fine

particle composition agrees well with earlier studies where

the fine mode particles in biomass combustion have been

found to contain mainly alkali salts that have condensed

from the gas phase (8, 9). The coarse residual ash mode

showed little variation with the two different fuels.

ESP Collection Efficiency. ESP collection efficiency as a

function of particle size was determined with simultaneous

low-pressure impactor measurements in the particle size

range of 0.1-6 μm. Particle penetration through the ESP is

defined as

ESP collection efficiency is then

The total collection efficiency of the ESP was 99.2-99.4%

without REF feed and 99.6-99.8% with REF feed. The

collection efficiency was 99.9% or greater for particles larger

than 5 μm (Figure 4). However, the collection efficiency

decreased (penetration increased) with decreasing particle

size. The collection efficiency was lowestsand penetration

highestsat the particle size of about 0.2-0.5 μm. At this size

range, the penetration through the ESP was approximately

3-4%. At particle sizes smaller than 0.2 μm, penetration

seemed to decrease again with decreasing particle size. This

is consistent with earlier results from ESP collection efficiency

at pulverized coal-fired boilers, where a clear penetration

window has been detected between 0.1 and 1 μm (12).

However, in this study, the mass collected in the smallest

impactor stages was so low that no reliable penetration curve

could be determined for particle sizes smaller than 0.1 μm.

More detailed measurements using, for example, number

size distribution determinations would be required to confirm

the shape of the penetration curve below 0.1 μm. ESP

penetration of ions (Cl, SO4, Na and K) followed pretty well

the penetration of the total mass of the particles.

The effect of ESP cleaning and soot-blowing in the boiler

on the particle emissions was studied with the continuous

mass monitor TEOM and with ELPI. ESP cleaning (rapping)

was found to have limited effect on the emission. In the

measurements without REF, the mass concentration increased from 5 to 8 mg/Nm3 and with REF increased only

from 9 to 10 mg/Nm3. There was a slight increase in the

concentration of particles larger than 0.4 μm seen in the

ELPI measurements (Figure 5).

Soot-blowing in the boiler had a more pronounced effect

on the emission. The soot-blowing is usually carried out once

a day in the boiler; each soot-blowing sequence lasting

approximately 1 h. A TEOM measurement was started 10

min after the beginning of the soot-blowing when REF was

included in the fuel mixture (Figure 6). “Start” in the Figure

6 denotes the starting time of the TEOM measurement. The

mass concentration at the ESP outlet at the start of the

measurement was almost 80 mg/Nm3, about 10-fold the

normal emission. The mass concentration decreased rapidly

during the soot-blowing, and after less than 10 min measurement (20 min from the start of the soot-blowing), the

concentration had dropped to 25 mg/Nm3. However, during

the whole soot-blowing period the mass concentration

remained well above the normal, “non-soot-blowing” value

of less than 10 mg/Nm3.

ELPI measurement (Figure 5) was started about 30 min

after the beginning of the soot-blowing. It shows substantially

increased particle number concentration for particles above

the size of 0.4 μm.

Emission of the Trace Elements. The emissions of trace

elements as determined by impactor measurements were

very low for both fuels (Table 4). The particle emission limit

values set by the EU directive for waste incineration are 50

FIGURE 4. ESP penetration as a function of particle size as

determined with simultaneous low-pressure impactor measurements.

penetration ) [100 × concn(outlet)/concn(inlet)]% (1)

collection efficiency ) (100 - penetration)% (2)

FIGURE 5. Particle number size distributions at the ESP outlet as

determined with ELPI during addition of REF. The effect of ESP

cleaning (rapping) and soot-blowing in the boiler.

FIGURE 6. Mass concentration at the ESP outlet during soot-blowing

in the boiler as determined with the continuous mass monitor TEOM.

“Start” and “Stop” in the figure denote the starting and stopping

time of the TEOM measurement.

VOL. 37, NO. 12, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2845
μg/Nm3 for the total of Cd + Tl and 500 μg/Nm3 for the total

of As + Co + Cr + Cu + Mn + Ni + Pb + Sb + V. The

measured values of 0.072 and 0.15 μg/Nm3 for Cd + Tl and

of 72 and 46 μg/Nm3 for the other metals were well below

the limit values. Mercury emissions were not included in

this investigation.

The measured mass concentration in the sample that was

chemically analyzed at ESP outlet with REF was quite low as

compared to other mass concentrations from ESP outlet (2.3

mg/Nm3 as compared to 6.4 mg/Nm3). This was presumably

due to the low and decreasing load during this measurement.

However, even considering the typical three times higher

mass concentration and, consequently, three times higher

emission of trace metals, the emission values would be well

below the limit values set by the EU directive for waste

incineration and co-incineration. Also, in Table 4, the

emission values for Cr and Ni are slightly overestimated as

there was some contamination of these elements in the

samples because of the use of stainless steel impactor

material. The real emission for these two elements would be

even lower than measured in these samples. Despite the low

total emission value measured during REF feeding, presumably due to the low boiler load, the emission of Cd, Cr, Cu,

Ni, Pb, and Sb clearly increased with REF addition to the

fuel.

Acknowledgments

The authors thank the Finnish National Technology Agency

TEKES, Foster Wheeler Energia Oy, and Forssan Energia Oy

for funding this study. We also thank Mr. Raoul Ja ̈rvinen and

the personnel at Forssan Energia Power Plant for the help

during the experiments.

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Received for review November 9, 2002. Revised manuscript

received February 26, 2003. Accepted April 10, 2003.

ES026314Z

TABLE 4. Trace Metal Concentrations (NTP) in the Particles at

the ESP Outlet

element

emission,

no REF

(μg/Nm3)

emission,

with REF

(μg/Nm3)

As 0.79 0.31

Cd 0.040 0.11

Co 0.39 0.23

Cr 7.2 17

Cu 2.8 4.2

Mn 57 14

Ni 2.1 6.1

Pb 1.1 3.2

Sb 0.035 0.61

Tl 0.032 0.037

V 0.78 0.22

Cd + Tl 0.072 0.15

As + Co + Cr + Cu + Mn + 72 46

Ni + Pb + Sb + V

2846 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 12, 2003