Submicron ash formation from coal combustion

B.J.P. Buhrea,*, J.T. Hinkleya, R.P. Guptaa, T.F. Walla, P.F. Nelsonb

aCooperative Research Centre for Coal in Sustainable Development, Department of Chemical Engineering,

University of Newcastle, Callaghan, NSW 2300, AustraliabCooperative Research Centre for Coal in Sustainable Development, Graduate School of the Environment, Macquarie University, NSW 2109, Australia

Received 13 November 2003; received in revised form 19 July 2004; accepted 10 August 2004

Available online 18 November 2004

Abstract

In recent years, fine particles have been found to be the cause of various harmful effects on health, and many countries have imposed

restrictions on emission of these particles. Fine ash particles are formed during coal combustion in power stations and, if not collected in the

air pollution control devices, are emitted into the atmosphere. The fine ash particles can remain airborne for long periods and can result in

deleterious health effects when inhaled and deposited in the lungs.

Previous studies have shown that combustion of coals of different rank can result in differences in the amount and chemistry of the

submicron ash particles. This study examines the variability occurring between the submicron ashes formed from coals of similar rank. Five

Australian bituminous coals were burned in a laminar flow drop tube furnace in two different oxygen environments to determine the amount

and composition of submicron ash formed. The experimental setup is described and the repeatability of the experiments is discussed. The

variability in the submicron ash yield as a percentage of the total ash collected and the submicron ash composition are presented and

discussed. This paper presents experimental results rather than a detailed discussion on its interpretation. However, the results indicate that

the condensation of evaporated species is responsible for the formation of ash particles smaller than 0.3 mm.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Submicron ash yield; Chemical composition; Coal combustion

1. Introduction

The health effects of ambient fine particulates have been

studied extensively and correlations have been observed

between ambient fine particulate matter and human

mortality rates [1]. Worldwide, governments acknowledge

these studies and the adverse health effects of ambient fine

particulates and as a result standards have been introduced

to assist in reducing ambient fine particulate concentrations.

In the United States, a National Ambient Air Quality

Standard (NAAQS) for both ‘coarse’ particulate matter with

an aerodynamic diameter less than 10 mm, PM10, as well as

an NAAQS for PM2.5 is currently in effect [2]. Very recently

in Australia, the National Environment Protection (Ambient

Air Quality) Measure (NEPM) has been modified to include

advisory reporting standards for PM2.5, the monitoring of

0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.fuel.2004.08.025

* Corresponding author.

E-mail address: bart.buhre@newcastle.edu.au (B.J.P. Buhre).

which is to commence in January 2004 [3]. PM10 has been

included in the NEPM Ambient Air Quality measures since

1998 [4]. Around 80% of the electricity generated in

Australia is generated by coal combustion [5], and coal-fired

power generators are increasingly being required to

characterise their emissions of fine particulate matter.

Studies have indicated that exposure to high concen-

trations of fine particulate matter may not be the sole contri-

butor to these adverse effects, but that particle toxicology

could also play an important role [6,7]. The emissions of fine

ash particles and trace (toxic) elements from coal combustion

are closely associated because of the relative enrichment in

trace elements of these fine particles [8,9].

Although the fine particles are enriched in trace

elements, their composition is not determined by these

elements. It has long been established that submicron sized

particles from coal combustion are primarily formed by

condensation of volatilised ash, soot and char particles

[10–12]. The major constituents of the submicron ash are

Fuel 84 (2005) 1206–1214

www.fuelfirst.com

B.J.P. Buhre et al. / Fuel 84 (2005) 1206–1214 1207

the same constituents that form the bulk of the ash, however,

their relative contribution varies significantly from that of

the bulk ash.

In a study of the relationship between various coals and

the amount and composition of the submicron ash formed,

Quann and Sarofim concluded that the combustion of

different coals did result in varying submicron ash yields

and compositions [13]. The submicron ash yield varied

between 0.54 and 9.6% of the ASTM ash, with absolute

yields measured between 0.00072 and 0.0065 g of fume per

gram of coal burned in a gas temperature of 1750 K and an

oxygen partial pressure of 0.20 atm. In their study, the

submicron ash was defined as the ash collected on the filter

after the cascade impactor. The last stage of their cascade

impactor had a theoretical effective cut-off diameter of

0.54 mm. In general, the submicron ash yield varied most

significantly with coal rank, the lower ranked coals

producing higher submicron ash yields than the higher

ranked coals. Quann and Sarofim reached a similar

conclusion in an earlier study [14]. In both studies it was

found that for bituminous coals, the major submicron ash

constituents were SiO2, FeO, and alkali metals (Na2O or

K2O), while the major constituents of the bulk of the ash

were SiO2, Al2O3, and CaO [13,14]. Both studies were

conducted using the same experimental set-up, described

elsewhere in more detail [15]. Flagan and Taylor have

characterized small ash particles generated from the

combustion of a bituminous coal in an 8–12 kW furnace

[16]. They found that the main components of the ash

particles less than 0.3 mm in diameter were silicon, sulphur,

and sodium, all assumed present as oxides. Only a limited

number of laboratory studies have sought to determine the

amount and composition of submicron ash formed from the

combustion of different coals. Many researchers have

therefore used the model and experimental results presented

by Quann as the basis for further modelling [17–19].

Combustion in enriched oxygen environments has been

shown to increase the submicron ash yield [14]. In a study of

the volatilisation of silica during gasification, Raask and

Wilkins concluded that the amount of silica vaporized in

coal-fired boilers could not be explained by direct vaporiza-

tion, but had to be the result of reduction of silica to a more

volatile silicon monoxide [20]. This reduction is thought to

be the result of the reaction of silica with carbon monoxide to

form the reduced silica monoxide and carbon dioxide [14]. A

similar mechanism has been successfully applied in models

to determine the extent of vaporisation of iron, aluminium,

calcium and magnesium [17,18]. The modelling results have

been compared with the submicron ash yield presented by

Quann and Sarofim [14]. These models use char combustion

models to estimate the reducing conditions and temperatures

occurring on the burning char particles.

Experiments have shown that the temperature and

reducing conditions in the burning char particle are

important parameters for the extent of mineral matter

vaporization [14]. This study examines the effect of coal

selection on the submicron ash yield and composition, and

the burning char particle temperature needs to be similar for

each coal. Temperature measurements of burning char

particles have shown that burning char particles can achieve

temperatures several hundreds of degrees higher than the

surrounding gas [21]. The dependence of particle tempera-

ture on char reactivity can be divided into regimes I, II, and

III [22]. In regime III, the reactant gas concentration is near

zero at the char surface, and the mass transfer of reactant

gases through the particle boundary layer determines the

char particle temperature. When char particles burn in

regime III, the particle temperature is virtually independent

of the char reactivity, and hence coal type [22].

In this study, a well-characterized selection of bitumi-

nous coals have been burned in air and elevated oxygen

partial pressure to determine the influence of the coal

characteristics and combustion environment on the sub-

micron ash yield and its composition. The coals have been

selected to represent the range of ash chemistry occurring in

Australian bituminous coals. The combustion conditions

have been chosen to ensure minimum variability between

the attained temperature and reducing conditions during

combustion of the different coals. This paper presents

experimental results rather than a detailed discussion on its

interpretation. Further interpretation will be provided in

subsequent papers (e.g. [23]).

2. Experimental

2.1. Laboratory combustion system

The coals were burned in the Tetlow Model HTF375A

drop tube furnace situated at the Chemical Engineering

department at the University of Newcastle in New South

Wales, Australia. The furnace is shown schematically in

Fig. 1 and contains a heated vertical alumina tube of

1200 mm in length, with an effective hot zone of 1000 mm.

The coal feeder is a syringe-type feeder that pushes the

coal particles into the primary gas stream. The primary gas

stream entrains the coal particles and feeds them into the

heated zone of the reactor via a water-cooled injector probe

onto the furnace axis at a feed rate of 8 g/h. The secondary

air is led through a flow straightener to provide a laminar

flow profile in the hot zone of the combustor. The ratio of

primary to secondary air has been chosen to ensure a

laminar gas profile. Temperature controllers maintain the

temperature of the hot zone of the reactor at constant

temperature. The combustion products are collected through

a water-cooled collection probe. To prevent condensation of

combustion products on the inner shell of the collector

probe, the probe is equipped with a sintered stainless steel

inner shell, through which quench gas (nitrogen) is pumped.

The cooled gas and combustion residue is sucked through an

ash collection train, consisting of a cyclone, a seven-stage

cascade impactor (MRI Model 1503), and a Teflon filter.

Fig. 1. Schematic diagram of drop tube furnace and ash collection train.

B.J.P. Buhre et al. / Fuel 84 (2005) 1206–12141208

Tygon tubing has been used to connect the instruments and

the tubing is replaced when deemed necessary. The

combustion products are sucked through the collection

train using a 1.5 kW vacuum pump.

The cyclone and cascade impactor performance are

sensitive to the gas flow rate, impaction plate loading, and

impaction substrate [24–26]. For this reason, the tempera-

ture, pressure, and flow rate of the gas at the pump inlet have

been monitored and kept constant during the experiments.

To prevent re-entrainment, the ash loadings on the

impaction plates have been kept lower than indicated in

the impactor manual. For each impaction plate, the cascade

impactor manual provides aerodynamic particle diameters

of which 50% of the particles should impact onto the

impaction plate, the so-called d50s [25]. These aerodynamic

particle diameters provided in the manual were verified by

images taken with Scanning Electron Microscopy (SEM) of

the ash collected on several impaction plates. After

correction for an assumed particle density, the d50s of the

seven impaction plates were 17.9, 8.2, 3.2, 1.4, 0.8, 0.4, and

0.3 mm, respectively.

Burning char particle temperatures affect the extent of

vaporization of inorganic species during coal combustion.

Table 1

Proximate analysis and rank of the five coals used in this study

Sample ID CRC 240 CRC 272

Moisture (% ad) 2.14 2.47

Ash (% db) 12.96 9.18

Volatile matter (% db) 30.41 35.69

Fixed carbon (% db) (by difference) 56.63 55.13

Total sulphur content (% db) 0.60 0.96

Coal rank HV-bit HV-bit

HV-bit, high volatile bituminous; MV-bit, medium volatile bituminous coal.

In this study, the combustion conditions were chosen as to

eliminate particle temperature differences between the

different coals by ensuring regime III combustion. This

resulted in the selection of the 63–90 mm size cut of the coal

at a gas temperature of 1673 K and oxygen partial pressures

of 0.21 and 0.50 atm.

2.2. Coal and ash analysis

The coal samples were crushed and wet-sieved to a

particle size between 63 and 90 mm. The chemical proper-

ties of the coal were determined by a combination of

analyses: Inductively Coupled Plasma Atomic Emission

Spectroscopy (ICP-AES) of ash generated by low tempera-

ture radio frequency ashing, proximate analysis and total

sulphur and chlorine determination [27–31]. The ash

samples collected on the cascade impactor plates have

been subjected to the same AES analysis for determination

of the chemical composition. Because of the small amounts

of sample collected on the last plate of the cascade impactor,

polycarbonate filters have been used as collection substrate.

These samples and the back-up filters have been subjected

to Proton Induced X-ray Emissions and Gamma Ray

Emission (PIXE and PIGE) analyses. PIXE and PIGE are

well established analysis techniques, widely used to

determine chemical composition of atmospheric dust

samples collected on filters [32]. PIXE is useful for the

analysis of elements from silicon to uranium, while PIGE is

useful for the analysis of light elements, such as Li, B, F, Na,

and Mg. SEM and TEM images of the submicron ash

particles were obtained to determine morphology of the

(submicron) ash particles.

3. Results

The coal compositions are given in Tables 1 and 2. Table 1

shows the proximate analysis and coal rank of the five coals,

which consists of four high volatile bituminous coals and one

medium volatile bituminous coal. For mineral analysis, the

coals have been ashed using low temperature radio frequency

ashing, and Table 2 shows the ash elemental composition,

together with the total sulphur and chlorine content.

Duplicate experiments were done to evaluate repeat-

ability. When two combustion experiments with one coal

CRC 296 CRC 297 CRC 306

2.09 1.27 1.64

13.81 11.93 18.77

30.34 45.41 19.68

55.85 42.66 61.55

0.61 5.07 1.72

HV-bit HV-bit MV-bit

Table 2

Elemental composition of the low-temperature ash obtained from the five

coals measured by ICP-AES, results presented in elemental wt%

LTA ash

elemental com-

position %

CRC

240

CRC

272

CRC

296

CRC

297

CRC

306

Si 20.8 18.3 32.2 10.7 15.6

Al 16.0 11.1 7.9 9.9 6.8

Fe 1.1 5.0 0.3 3.5 9.7

Ca 0.2 4.5 0.3 9.1 9.1

Mg 0.1 0.2 0.1 2.0 1.2

Na 0.1 0.1 0.02 0.2 0.1

K 0.8 0.2 1.3 0.1 0.6

Ti 0.80 1.08 0.63 0.70 0.48

Mn 0.005 0.05 0.004 0.12 0.13

Sa (db) 0.60 0.96 0.61 5.07 1.72

P 0.07 0.3 0.1 0.2 0.2

Ba 0.03 0.06 0.03 0.03 0.04

Sr 0.02 0.05 0.02 0.10 0.04

Clb (db) 0.03 0.07 0.01 !0.01 0.05

The remainder of the ash includes oxygen, water of constitution, and carbon

present as carbonates.a Sulphur as determined by [33].b Chlorine as determined by [29].

Table 3

Sub-micron ash yield, expressed as percentage of ash produced

Experiment CRC 240 CRC 272 CRC 296 CRC 297 CRC 306

21% O2 0.12 0.26 0.30 1.12 0.15

21% O2,

repeat

0.17 0.22 0.24 1.60 0.12

50% O2 0.61 0.50 0.55 1.39 0.31

50% O2,

repeat

0.36 0.80 0.48 1.45 0.37

Fig. 2. (a) and (b) TEM image (2a, top) and SEM image (2b, bottom) of submicr

CRC 240 in 0.50 atm oxygen partial pressure.

B.J.P. Buhre et al. / Fuel 84 (2005) 1206–1214 1209

showed too large variability in submicron ash yield or

colour, the experiment was repeated. This occurred a

number of times, and was mainly caused by irregularities

in the feeder, which in turn would cause a coal surge in the

furnace, resulting in char and soot formation. Experiments

during which soot formation did occur were easily detected

by its dark occurrence and were rejected. Subsamples of all

cyclone-ashes have been re-ashed at 815 8C until constant

mass to determine carbon content. In most cases the mass

loss was less than 0.5%. One coal, CRC 240, was more

difficult to burn in air and its ash showed a mass loss upon

reheating of 1.4 and 3.3% for the duplicate runs.

Table 3 shows the amounts of submicron ash collected

during the two experiments on the back-up filter and the

bottom three stages of the impactor (aerodynamic diameter

smaller than 0.79 mm). The amounts are presented as

percentages of total ash calculated from the amount of

coal fed and the ash content as determined by the proximate

analysis.

The ash collected on the Teflon filters from the different

experiments varied in colour. The variation between

the colours was similar to the variation between colours of

ash obtained from different coals when ashing coals at

815 8C. To obtain an idea of the submicron ash particle

morphology, some of the filters have been analysed using

SEM and Transmission Electron Microscopy (TEM). No

clear morphological distinctions between the ashes collected

on the filters were noticed between the different coals. Fig. 2a

shows a TEM image of typical ash particles collected on the

filter during the combustion of CRC 296, in 50% oxygen. Fig.

2b shows ash particles collected on the filter from CRC 240,

burned in a gas containing 50% oxygen.

on ash (aerodynamic diameter !0.30 mm) generated during combustion of

Table 5

Chemical composition of the ash collected on plate 5 of the cascade

impactor (d50 of 0.79 mm) during combustion of the coals in air and in 50%

oxygen at 1400 8C

Plate 5

(w0.79 mm)

O2 partial

pressure

(atm)

CRC

240

CRC

272

CRC

296

CRC

297

CRC

306

SiO2 0.21 36.9 36.0 88.8 13.3 17.1

0.50 42.0 33.7 69.7 19.2 34.1

Al2O3 0.21 24.1 15.1 2.7 12.9 7.0

0.50 28.4 19.2 12.8 18.5 10.9

Fe2O3 0.21 12.9 11.5 1.2 7.1 50.7

0.50 3.9 8.3 0.9 4.5 20.1

CaO 0.21 2.0 10.0 0.2 26.4 11.6

0.50 1.4 10.5 0.7 27.2 18.3

MgO 0.21 0.8 4.1 0.2 3.7 1.3

0.50 0.4 0.9 0.5 5.3 1.5

Na2O 0.21 0.6 2.5 0.4 bdl 0.2

0.50 0.7 1.4 0.5 0.1 0.2

K2O 0.21 0.4 0.7 0.9 bdl 0.1

0.50 1.0 0.4 1.9 bdl 0.3

TiO2 0.21 11.3 7.0 1.3 2.7 1.5

0.50 21.0 15.9 10.3 3.4 2.4

MnO 0.21 0.1 0.2 0.02 0.4 0.2

0.50 0.03 0.1 0.04 0.3 0.1

P2O5 0.21 2.5 6.9 0.8 11.3 8.5

0.50 0.5 5.9 0.6 6.8 8.4

SO3 0.21 8.5 5.9 3.4 22.3 1.9

0.50 0.7 3.5 2.1 14.8 3.6

bdl: below detection limit.

B.J.P. Buhre et al. / Fuel 84 (2005) 1206–12141210

The chemical composition of the ash collected on the

final filters indicates the variability of the submicron ash

from the combustion of different coals in different

combustion conditions. According to the cascade impactor

manual, the ash collected on these filters has an aero-

dynamic diameter below 0.30 mm [25]. PIXE and PIGE

analyses measure the elemental concentrations on the filter,

which makes comparison between experiments of different

duration and coal ash percentage difficult. To compare the

ash composition from the different combustion experiments,

the results have been converted to oxides, and have been

normalized to the total filter weight as measured during the

experiments. Variability between the two duplicate exper-

iments was much less than the variation in mass and the

average results are shown in Table 4.

The minimum detection limit (mdl) of PIXE and PIGE

analyses is dependent upon the detected element, the

chemical matrix and the mass collected on the filter. Due to

the low concentrations of the light elements (Na, Mg), no

data were obtained from the PIGE analysis from the air

combustion experiments from CRC 240, CRC 297, and CRC

306. The minimum detection limit for sodium is approxi-

mately 5 mg/cm2, resulting in the estimates in Table 4

specified as less than the values quoted.

Table 5 provides the chemical composition of the ash

collected on plate 5 of the cascade impactor. This impaction

plate has a cut-off of 0.79 mm. The table provides the results

of both the temperature in 21 and 50% oxygen environment.

The chemical composition was obtained from ICP-AES

analyses of the ash samples, and the experimental results of

the two repeat experiments were combined to obtain

sufficient sample for analyses.

Table 4

Chemical composition of the ash collected on the filter (!0.3 mm) during

combustion of the coals in air and in 50% oxygen at 1400 8C

Filter

(!0.3 mm)

O2 partial

pressure

(atm)

CRC

240

CRC

272

CRC

296

CRC

297

CRC

306

SiO2 0.21 17.3 10.6 15.1 9.0 21.4

0.50 72.5 30.5 62.8 22.0 25.9

Al2O3 0.21 3.5 !0.2 !0.2 2.1 2.8

0.50 9.3 3.2 6.6 1.3 5.1

Fe2O3 0.21 3.0 3.2 2.1 1.4 23.0

0.50 1.3 5.2 1.2 2.7 27.8

CaO 0.21 0.7 2.4 0.7 6.0 15.8

0.50 0.2 6.5 0.7 18.0 30.9

Na2O 0.21 !2.8 37.7 25.7 !4.5 !2.0

0.50 3.3 27.1 8.4 0.0 1.1

K2O 0.21 7.2 3.4 6.6 1.2 2.3

0.50 1.5 1.4 2.8 0.05 0.6

TiO2 0.21 1.3 1.6 0.9 0.4 0.8

0.50 5.4 5.3 4.7 1.2 1.0

MnO 0.21 0.03 0.1 0.0 0.2 0.1

0.50 0.1 0.2 0.1 0.6 0.1

P2O5 0.21 17.1 7.2 9.1 6.3 8.8

0.50 !0.1 2.3 0.4 2.2 1.3

SO3 0.21 49.9 33.7 39.9 73.4 25.0

0.50 6.3 18.4 12.2 51.9 6.2

4. Discussion

4.1. Submicron ash yield and morphology

Initial comparison to the results presented by Quann and

co-workers [13] might indicate that the submicron ash

yields in this experimental are somewhat lower. The

measured submicron ash yields in the study by Quann and

co-workers varied between 0.54 and 9.6% of the ASTM ash

for the total data for bituminous as well as lower rank coals.

For the six bituminous coals alone the value varied from 0.6

to 1.7 and is similar to the range of 0.1–1.4 found in this

study.

Few experimental studies of this nature have focused

on this topic, which could be because of the difficulty in

obtaining reliable and repeatable results for the sub-

micron yield and composition. Small fluctuations in coal

feed rate, gas flows, and temperature can disrupt the

experiment and result in a need to repeat the experiment.

Although one experiment showed a sub-micron yield

difference between two repeat experiments of a factor of

almost two, most of the experiments showed reasonable

repeatability in submicron ash yield.

Although limited by machine resolution, SEM and TEM

images of the ash collected on the final filter indicate that the

majority of ash collected on the final filter is approximately

spherical in shape. It is not clear whether these particles

have been deposited individually, or as aggregates.

Fig. 3. Major components of the submicron fume during combustion in

21% oxygen, expressed as oxides.

B.J.P. Buhre et al. / Fuel 84 (2005) 1206–1214 1211

The shape of these particles is characteristic of those formed

from the condensation of volatilized species [34]. During

combustion, species can vaporize as a result of the high

temperatures. The species can vaporize as a result of their

thermodynamic characteristics, or they can be reduced from

less volatile species to form more volatile sub-oxides. Upon

re-condensation or re-oxidation, new primary particles are

formed. Condensation of elements onto the primary

particles results in growth of these particles.

Neville and Sarofim investigated the structure of

submicron particles [35]. They measured primary particle

sizes around 160 A with a stratified structure, which is in

accordance with the mechanism of condensing material

onto primary particles. The resolution of the SEM and TEM

images did not allow an estimation of these primary particle

sizes. However, the images indicate that particles collected

on the filter had a diameter smaller than 0.3 mm, with a

number average of less than 50 nm (Z500 A). It is also

noted that the particles collected on the filter were

approximately spherical in shape and that no fragmented

minerals have been observed.

4.2. Submicron ash chemical composition

Coal composition is shown to be an important parameter

affecting the amount and composition of the submicron ash

particles. The coal with the highest sulphur content, CRC

297, produces the largest amount of fume even when burning

the coal particles at elevated oxygen partial pressures. The

abundance of sulphur in the submicron fume for all coals is

more easily observed in Fig. 3, which displays the chemical

composition represented in four categories:

(a)

Refractory oxides, which include SiO2, Al2O3, Fe2O3,

and CaO,

(b)

Alkali metal oxides which include K2O and Na2O,

(c)

Sulphur, presumable present as sulfates of the alkali and

alkaline earth metals, however, a proportion could be

present in the form of sulphuric acid,

(d)

Other (minor) elements such as TiO2, MgO, MnO, and

P2O5.

Table 6

Effect of elevating oxygen concentration on the contribution of the

categories to the ash collected on the filter (!0.3 mm)

Category Relative contribution to

ash collected on the filter

at elevated oxygen

concentration

Total amount of ash

collected on the filter at

elevated oxygen

concentration

Refractory oxides Increases Increases

Alkali metal

oxides

Decreases Similar

Sulphur Decreases Similar

Other elements Decreases Similar

The effect of increasing combustion temperature (elevat-

ing oxygen partial pressure) on the amount of the different

categories detected on the filter (!0.3 mm) can be

determined from Table 4. Table 6 summarizes these results.

The effect of increasing combustion temperature is

indicated by its relative contribution to the total amount

detected on the filter and by the absolute amount detected on

the filter. The table suggests that the effect of increasing

oxygen concentration in the combustion gas has a major

effect on the vaporization of the refractory oxides.

The variability in the chemical composition of the

submicron fume produced from the different coals is

significant. For example, the amount of sodium in the

submicron ash was below the detection limit for three

coals, while for the other two coals, sodium was the second

largest contributor to the submicron ash. The main

constituents of the sub-micron ash were sulphur, silicon,

sodium, and phosphorus, consistent with the results

presented by Flagan and Taylor [16]. Although the ultimate

analysis shows the presence of these elements, there is no

obvious correlation between the standard ash analysis and

the amounts of these elements present in the sub-micron

ash. More advanced coal analysis techniques could

contribute to the understanding of the variability between

the submicron ash composition and characteristics of the

coal from which they are formed.

The coals used in this study consisted of three coals

containing less than 1 wt% sulphur, one coal with 1.7 wt%

(CRC 306) and one with 5.1 wt% sulphur (CRC 297). Fig. 4

shows the submicron ash yield (the mass of the filter and the

bottom three stages) versus the weight percent of sulphur

detected in the coal. For comparison, the results of Quann

and co-workers have been added to this plot [13]. The coals

in that study were burned in a drop tube furnace at 1750 K in

a 0.20 atm oxygen environment. Fig. 4 shows that the

submicron ash yields obtained from this study are

comparable to the results obtained by Quann and co-

workers on similar coals. The figure also seems to indicate

Fig. 4. Submicron ash yield (combined mass of the filter and bottom three

stages) as a function of coal total sulphur content.

B.J.P. Buhre et al. / Fuel 84 (2005) 1206–12141212

an upward trend of the submicron ash yield as the coal

sulphur content increases.

Both the composition and yield of submicron ash change

with increasing oxygen partial pressure. The increase in

oxygen partial pressure in the combustion gas results in

higher particle temperatures during char combustion.

Timothy and co-workers have measured the temperatures

of burning char particles in a drop tube furnace with a gas

and wall temperature of 1700 K and varying oxygen partial

pressures [21]. Conditions similar to the ones used in this

study indicated burning particle temperatures of 2300 and

2800 K for combustion in 0.21 and 0.50 atm oxygen partial

pressure, respectively. Fig. 5 shows the submicron ash yield

at both low and high temperature combustion (i.e. 0.21 and

0.50 atm oxygen partial pressure).

For low sulphur coals, increasing the oxygen partial

pressure resulted in an increase in yield of sub-micron ash

by a factor of almost 3. In accordance with theory, all coals

showed an increase in the amount of refractory oxides in the

submicron ash; CRC 240 and CRC 296 showed an increase

in silicon of almost 10-fold compared to the amount

produced in air combustion. For the majority of coals,

increasing the oxygen partial pressure resulted in an

increase of the calcium content and the iron content of the

submicron ash. The contribution of sulphur to the submicron

ash composition decreased for all five coals.

Fig. 5. Submicron ash yield (combined mass of the filter and bottom three

stages) for all coals as a function of oxygen partial pressure during combustion.

4.3. Submicron ash formation mechanisms

Table 4 provides the chemical composition of the

submicron ash collected onto the filter during combustion

in air and in 50% oxygen. The chemical composition of this

ash shows characteristics consistent with ash formed from

the condensation of vaporized inorganic matter:

†

The ash formed during combustion in air is highly

enriched in the more volatile elements such as sulphur,

phosphorus, and sodium and is relatively depleted in less

volatile elements such as silicon and aluminium.

†

The chemical composition changes significantly when

increasing the oxygen concentration from 21 to 50%: the

contribution of refractory oxides such as silicon and

aluminium increases, with a resulting decrease of the

contributions of sulphur, sodium, and phosphorus.

Examination of the chemical composition of the ash

collected on plate 5 of the cascade impactor (Table 5)

suggests that this ash is formed from a different mechanism

than the ash collected on the filter. The chemical

composition is not significantly different to that from the

bulk of the ash, and the effect of increasing combustion

temperature on chemical composition is significantly

smaller than that observed for the ash collected on the

filter. The observed change in chemical composition is

expected as a fraction of the vaporized species will

condense onto existing (small) ash particles, which are

collected on the impaction plate.

The size-dependent chemical composition suggests that

ash particles smaller than 0.3 mm (the ash collected on the

filter) is formed from a different mechanism than ash

particles only slightly larger in size (the ash collected on

plate 5, which has an aerodynamic d50 of 0.79 mm). Taylor

and Flagan showed that the size distribution shifted abruptly

below 0.1 mm diameter, i.e. in the size range where the

freshly nucleated particles dominate the aerosol mass [36].

They also observed a change in chemical composition of the

fine sized ash particles, suggesting that the very fine ash

particles (!0.1 mm) are formed via a different mechanism

than larger sized particles. The size resolved chemical

composition of submicron ash particles presented in this

study supports this theory.

Both the samples collected on the filter and collected on

plate 5 form part of the submicron ash, or PM1. However,

closer examination of the chemical composition indicates

that these samples are formed from different mechanisms.

The ash collected on the filter is characteristics of that

formed from the condensation of vaporized species, while

the ash collected on plate 5 is similar to the bulk of the ash.

5. Conclusion

An experimental study has determined the amount and

characteristics of submicron fly ash generated from coal

B.J.P. Buhre et al. / Fuel 84 (2005) 1206–1214 1213

combustion. Few experimental studies have focused on this

topic, which could be because of the difficulty in obtaining

reliable and repeatable results. Duplicate experiments

showed very reasonable repeatability and reliability of the

experimental set-up used in this study. The largest variability

in submicron ash yield occurred as a result of coal selection.

The coal with high sulphur content showed the highest

amount of submicron ash yield. Although there is significant

variability between the submicron ash elemental compo-

sitions from the different coals, the main constituents are

sulphur, silicon, phosphorus, and sodium. Both the elemental

composition and yield of submicron ash were greatly affected

by increasing the oxygen partial pressure of the oxidizing gas

from 0.21 to 0.50 atm. All coals showed an increase in

submicron ash yield at the elevated oxygen partial pressure.

At the elevated oxygen partial pressure, the relative

contribution to the total submicron ash yield of silicon,

iron, and calcium increased, while the contribution of sulphur

decreased. This paper presents the result of the experiments

rather than a detailed discussion on its interpretation.

However, size dependent chemical composition of the

submicron ash indicates that the condensation of evaporated

species was responsible for the formation of ash particles

smaller than 0.3 mm, illustrated by the increase in silicon and

aluminium content of the ash collected on the filter at

elevated combustion temperature.

Acknowledgements

The authors wish to acknowledge the financial support

provided by the Cooperative Research Centre for Coal in

Sustainable Development, which is funded in part by the

Cooperative Research Centres Program of the Common-

wealth Government of Australia; and Dave Phelan at the

electron microscopy unit of the University of Newcastle for

his assistance with obtaining the SEM and TEM images.

References

[1] Dockery DW, Pope CA, Xu XP, Spengler JD, Ware JH, Fay ME, et al.

An association between air pollution and mortality in six U.S. cities. N

Engl J Med 1993;329(24):1753–9.

[2] Hopke PK. Advances in monitoring methods for airborne particles.

16th international clean air and environment conference, Christch-

urch, New Zealand; 2002.

[3] NEPC. Variation to the national environment protection (ambient air

quality) measure. Adelaide: National Environment Protection

Agency; 2003.

[4] NEPC. Ambient air quality.: National Environment Protection and

Heritage Council; 2003.

[5] WCI, World Coal Institute; www.wci-coal.com, Access date: June 17,

2003.

[6] Spurny KR. On the physics, chemistry and toxicology of ultrafine

anthropogenic, atmospheric aerosols (UAAA): new advances. Tox-

icol Lett 1998;96(97):253–61.

[7] Harrison RM, Yin J. Particulate matter in the atmosphere: which

particle properties are important for its effect on ealth? Sci Total

Environ 2000;249:85–101.

[8] Lighty JS, Veranth JM, Sarofim AF. Combustion aerosols: factors

governing their size and composition and implications to human

health. J Air Waste Manage Assoc 2000;50(9):1565–618.

[9] Buhre BJP, Helstroom R, Chase D, Prokopiuk T, Halliburton B,

Carras JN, et al. Full scale sampling and characterisation of fly-ash at

Australian coal fired power stations. Fifth international conference on

emission monitoring, Odense, Denmark; 2002.

[10] Flagan RC, Friedlander SK. Particle formation in pulverized coal

combustion—a review. In: Shaw DT, editor. Recent developments in

aerosol science. New York: Wiley; 1978. p. 25–59.

[11] Flagan RC. Submicron particles from coal combustion. 17th

symposium (international) on combustion; 1979.

[12] Senior CL, Flagan RC. Ash vaporisation and condensation of a

suspended coal particle. Aerosol Sci Technol 1982;1:371–83.

[13] Quann RJ, Neville M, Sarofim AF. A laboratory study of the effect of

coal selection on the amount and composition of combustion

generated submicron particles. Combust Sci Technol 1990;74:

245–65.

[14] Quann RJ, Sarofim AF. Vaporisation of refractory oxides during

pulverised coal combustion. 19th symposium (international) on

combustion; 1982.

[15] Quann RJ, Neville M, Janghorbani M, Mims CA, Sarofim AF.

Mineral matter and trace-element vaporization in a laboratory-

pulverized coal combustion system. Environ Sci Technol 1982;16:

776–81.

[16] Flagan RC, Taylor DD. Laboratory studies of submicron particles

from coal combustion. 18th symposium (international) on combus-

tion; 1981.

[17] Eddings EG, Sarofim AF, Lee CL, Davis KA, Valentine JR. Trends in

predicting and controlling ash vaporization in coal-fired utility boilers.

Fuel Process Technol 2001;71:39–51.

[18] Lee CM, Davis KA, Heap MP, Eddings E, Sarofim AF. Modeling the

vaporization of ash constituents in a coal-fired boiler. 28th symposium

(international) on combustion, Edinburgh, Scotland, UK; 2000.

[19] Senior CL, Panagiotou T, Sarofim AF, Helble JJ. Formation of ultra-

fine particulate matter from pulverized coal combustion. Preprints of

symposia, Division of fuel chemistry, American Chemical Society;

2000.

[20] Raask E, Wilkins DM. Volatilization of silica in gasification and

combustion processes. J Inst Energy 1965;38:255–62.

[21] Timothy LD, Sarofim AF, Beer JM. Characteristics of single particle

coal combustion. 19th symposium (international) on combustion;

1982.

[22] Laurendeau NM. Heterogenous kinetics of coal char gasification and

combustion. Prog Energy Combust Sci 1978;4:221–70.

[23] Buhre BJP, Hinkley JT, Gupta RP, Wall TF, Nelson PF. Coal quality

and fine ash formation during coal combustion. 21st annual Pittsburgh

coal conference, Osaka, Japan; 2004.

[24] Markowski GR. Reducing blowoff in cascade impactor measure-

ments. Aerosol Sci Technol 1984;431–9.

[25] MRI. Instruction manual. Inertial cascade impactor model 1502 and

1503. Meteorology Research Inc., Altadena, CA; 1976.

[26] Marple VA, Willeke K. Inertial impactors: theory, design and use.

In: Liu BYH, editor. Fine particles: aerosol generation, measure-

ment, sampling and analysis. New York: Academic Press; 1976. p.

411–46.

[27] Godbeer WC, Orban H, Riley KW, Steinberg K. Semi-micro analysis

of coal for major and trace constituents. J Coal Qual 1992;11(3–4):

43–5.

[28] AS1038.22. Coal and coke—analysis and testing, part 22: higher rank

coal-mineral matter and water of constitution. Strathfield, NSW,

Australia: Standards Australia International; 2000. p. 20.

B.J.P. Buhre et al. / Fuel 84 (2005) 1206–12141214

[29] AS1038.8.1. Coal and coke—analysis and testing, part 8.1: coal and

coke–chlorine–eschka method. Strathfield, NSW: Standards Australia

International; 1999.

[30] AS1038.6.3.1. Coal and coke—analysis and testing, part 6.3.1: higher

rank coal and coke—ultimate analysis—total sulfur—eschka method.

Homebush, NSW, Australia: Standards Association of Australia;

1997. p. 11.

[31] AS1038.3. Coal and coke—analysis and testing, part 3: proximate

analysis of higher rank coal. Strathfield, NSW, Australia: Standards

Australia International; 2000. p. 20.

[32] Cohen DD. Ion beam analysis techniques in aerosol analysis. Clean

Air 1992;26(3):113–21.

[33] AS1038.6.3.3. Coal and coke—analysis and testing, part 6.3.3:

ultimate analysis—infrared method. Homebush, NSW: Standards

Association of Australia; 1997.

[34] Helble JJ, Sarofim AF. Factors determining the primary particle size

of flame-generated inorganic aerosols. J Colloid Interface Sci 1989;

128(2):348–62.

[35] Neville M, Sarofim AF. The stratified composition of inorganic

submicron particles produced during coal combustion. 19th sym-

posium (international) on combustion; 1982.

[36] Taylor DD, Flagan RC. Aerosols from a laboratory pulverised coal

combustor. ACS symposium series 1981.