fine ash formation during combustion of pulverised coal–coal property impacts
TRANSCRIPT
Fine ash formation during combustion of pulverised
coal–coal property impacts
B.J.P. Buhre a,*, J.T. Hinkley a, R.P. Gupta a, P.F. Nelson b, T.F. Wall a
a Cooperative Research Centre for Coal in Sustainable Development, Department of Chemical Engineering,
University of Newcastle, Callaghan, NSW 2300, Australiab Cooperative Research Centre for Coal in Sustainable Development, Graduate School of the Environment,
Macquarie University, NSW 2109, Australia
Received 5 October 2004; received in revised form 24 February 2005; accepted 14 April 2005
Available online 15 September 2005
Abstract
In many countries, legislation has been enacted to set guidelines for ambient concentrations and to limit the emission of fine particulates with an
aerodynamic diameter less than 10 mm (PM10) and less than 2.5 mm (PM2.5). Ash particles are formed during the combustion of coal in pf boilers
and fine ash particulates may potentially pass collection devices. The ash size fractions of legislative interest formed during coal combustion are
the result of several ash formation mechanisms; however, the contribution of each of the mechanisms to the fine ash remains unclear. This study
provides insight into the mechanisms and coal characteristics responsible for the formation of fine ash. Five well characterized Australian
bituminous coals have been burned in a laminar flow drop tube furnace in two oxygen environments to determine the amount and composition of
the fine ash (PM10, PM2.5 and PM1) formed. Coal characteristics have been identified that correlate with the formation of fine ash during coal
combustion. The results indicate that coal selection based on (1) char characterization and (2) ash fusion temperature could play an important role
in the minimization of the fine ash formed. The implications of these findings for coal selection for use in pf-fired boilers are discussed.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: Coal combustion; Fine ash; Coal characterisation
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 e.g. [1]. Governments worldwide acknowledge these
studies 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]. 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
which commenced in January 2004 [3]. PM10 has been
included in the NEPM Ambient Air Quality measures since,
0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.fuel.2005.04.031
* Corresponding author. Tel.: C61 2 4921 6179; fax: C61 2 4921 6920.
E-mail address: [email protected] (B.J.P. Buhre).
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
monitor and characterise their emissions of fine particulate
matter.
Air pollution control devices (APCDs) are employed at power
stations to capture the ash particulates from the flue gas.
Increasing the capture efficiency of these APCDs to decrease
fine ash emissions can be a costly exercise. Selecting coals which
reduce the formation offine ash particulates in the first place could
be a cost-effective method of minimizing fine ash emissions.
This paper presents the results of a study for the coal
characteristics responsible for the formation of fine ash
particles. Five Australian black coals have been characterised
extensively using advanced analytical techniques such as
QEMSCAN, SIROQUANT, and ICP-AES on ash obtained
from radio-frequency ashing technique. The coals were then
burned in a drop tube furnace, which simulates the combustion
in a pf fired boiler. The ash was characterised and the coal
characteristics responsible for the formation of the ash size
fractions of legislative interest (PM10, PM2.5, and PM1) have
been determined.
Fuel 85 (2006) 185–193
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B.J.P. Buhre et al. / Fuel 85 (2006) 185–193186
2. Experimental
Five Australian black coals have been selected to represent
the range of ash chemistry and char swelling behaviour
occurring in Australian bituminous coals; the coals were
chosen on the basis of their sulphur content, other constituents
and their vitrinite content. The coals were milled and a size cut
between 63 and 90 mm was obtained, suitable for feeding into
the drop tube furnace. The coals were subjected to proximate
and ultimate analysis, reflectogram, and QEMSCAN analyses.
QEMSCAN is a novel version of CCSEM technique that
can directly measure the coal–mineral and mineral–mineral
associations. In the technique, pulverised coal samples are
mounted in carnauba wax. The sample block is sectioned and
the surface is polished and coated with a thin layer of carbon to
ensure electrical conductivity. The particles are thus, well
separated and in random orientations with random sections
exposed. Qem*SEM (quantitative evaluation of materials by
scanning electron microscopy) is an earlier version of
QEMSCAN and has been described by Creelman and Ward
[6]. The technique is an automated image analysis system that
uses Backscattered Electron (BSE) and Energy Dispersive
X-ray (EDX) signals from a scanning electron microscope to
create pixilated images in which each pixel represents the EDX
spectrum obtained from the centre of the pixel. The light
element X-ray detectors enable the detection of organic
material, enabling a direct measurement of the coal-mineral
associations. The pixel spacing is an operator setting and
determines the resolution of the QEMSCAN images. For the
analyses used in this study, a pixel spacing of 2 mm was used.
Ash was obtained from the coal samples using a low
temperature radio frequency ashing technique [7], after which
the ash was analysed for chemical composition using ICP-
AES, and quantitative mineralogy using SIROQUANT. The
procedure applied for the ash dissolution has been described in
detail elsewhere [8]. The combination of the analytical
techniques resulted in a thorough understanding of the organic
and inorganic characteristics of the coal samples.
After characterisation, the coals were burned in the Tetlow
Model HTF375A drop tube furnace situated at the Chemical
Engineering department at the University of Newcastle in
Table 1
Considered coal characteristics for each size fraction and the ash formation mecha
Fine Ash Coal characteristic
PM10 Ash Fusion Temperature
Basic to acidic oxide ratio
Average mineral grain size
Total ash content
Char Group I content / vitrinite content
PM2.5 Ash Fusion Temperature
Ash content
Char Group I content / Vitrinite content
Sulphur content
PM1 Ash content
Char Group I content / Vitrinite content
Sulphur content
Average mineral grain size
Australia. The ash was collected using a cyclone, a cascade
impactor and subsequently a filter. The cyclone has a
theoretical d50 of 3 mm at the operating conditions applied
during the experiments. The cascade impactor is a seven stage
MRI cascade impactor, model 1503. After correction for an
assumed particle density, the d50’s of the seven impaction
plates were 17.9, 8.2, 3.2, 1.4, 0.8, 0.4, and 0.3 mm,
respectively. The experimental set-up is described elsewhere
in more detail [9,10].
After combustion, the ash was characterised for its size
distribution, morphology, and chemical composition. The ash
collected in the cyclone was suspended in ethanol and the size
distribution of the ash was determined using a Malvern
Mastersizer S with a 300 mm lens. The amounts of ash
collected on the different plates of the cascade impactor were
combined with the size distribution obtained from the Malvern
Mastersizer to obtain a complete size distribution (Table 1).
The measurement of PM10 from the ash samples was based
on the analysis results of the Mastersizer, while the PM2.5 and
PM1 was determined from the masses collected in the cascade
impactor. The amount of PM2.5 is estimated as the combined
masses of the amounts collected on the filter and on the bottom
five stages. The PM1 is calculated from the mass collected on
the filter combined with the bottom three stages. Moisture
adsorption onto the filter can significantly affect the observed
mass of the material collected onto the filter [9]. The moisture
is an artefact of the experimental procedure and should not be
considered when determining the mass of PM. Previous
publications presented PM1 measurements as the balance
mass, which does not take the effect of moisture absorption into
account [10]. The chemical compositions of the material
collected on the filters have been determined quantitatively
using a combination of Proton Induced X-Ray Emission
(PIXE) and Proton Induced Gamma Ray Emission (PIGE)
analysis. The PIXE and PIGE analysis results have been used
to calculate the mass of the elements present (converted to
oxides). This method ensures that moisture adsorbed from the
combustion gas or the atmosphere does not affect the results.
The repeatability of the experimental set-up to determine
the amount of PM1 using this technique is described elsewhere
[9,10].
nisms they could affect
Ash formation mechanism affected
Included mineral coalescence
Included mineral coalescence
Included mineral coalescence, Vaporization and condensation
Included mineral coalescence, Char fragmentation
Char fragmentation
Included Mineral Coalescence
Char Fragmentation, Included mineral coalescence
Char fragmentation
Vaporisation and condensation
Included mineral coalescence, Char fragmentation
Char fragmentation
Vaporisation and condensation
Included mineral coalescence, Vaporisation and condensation
Table 2
Proximate analysis, rank and vitrinite content of the five coals used in this study (HV-Bit, high volatile bituminous; MV-Bit, medium volatile bituminous coal)
Sample ID CRC 240 CRC 272 CRC 296 CRC 297 CRC 306
Moisture (% ad) 2.14 2.47 2.09 1.27 1.64
Ash (% db) 12.96 9.18 13.81 11.93 18.77
Volatile matter (% db) 30.41 35.69 30.34 45.41 19.68
Fixed carbon (% db) (by difference) 56.63 55.13 55.85 42.66 61.55
Total sulphur content (% db) 0.60 0.96 0.61 5.07 1.72
Coal rank HV-Bit HV-Bit HV-Bit HV-Bit MV-Bit
Vitrinite content (% v/v mmf) 68.3 50.7 39.0 57.8 28.1
Ash fusion temperature (deformation
temperature, 8C)
O1600 1280 1490 1300 1240
Average mineral size (mm) 11.4 16.7 7.7 10.8 14.0
B.J.P. Buhre et al. / Fuel 85 (2006) 185–193 187
The composition of the oxidizing gas was varied during the
experiments to determine its effect on fine particle formation.
The coals were burned at 1400 8C in 21% O2 (air) and in 50%
O2. The elevated oxygen concentration results in higher char
combustion temperatures. Simulations and literature indicated
that by increasing the oxygen content from 21 to 50%, the
temperature of the burning char particles increase from
approximately 2030–2530 8C [11].
3. Results
The coal compositions are provided in Tables 2 and 3.
Table 2 provides the proximate analysis, coal rank, vitrinite
content as determined from the reflectogram [12], and the
deformation temperature as determined during the ash fusion
test in a reducing environment [13] for all five coals. For
mineral analysis, the coals have been ashed using a low
temperature radio frequency ashing technique [7]. Table 3
provides the ash elemental composition, together with total
sulfur and chlorine content.
Tables 4 and 5 provide the total ash recovered, the PM10,
PM2.5, and PM1 formed during the combustion experiments
Table 3
Elemental composition of the low-temperature ash obtained from the five coals
measured by ICP-AES, results presented in elemental wt%
LTA ash elemen-
tal composition %
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 Sulfur as determined by [33].b Chlorine as determined by [34].
together with the total ash collected during the experiments.
The amounts are reported as percentage of the ash generated
during combustion. The proportion of the ash larger than
10 mm was typically much larger in size than 10 mm and
displayed a mean size in the order of tens of micrometers.
4. Ash formation mechanisms
During coal combustion, ash particles are formed from the
inorganic matter present in coal. The mechanisms of ash
formation have been studied extensively in the literature, and
are affected by the combustion conditions and the coal
characteristics [14–17]. Two important coal characteristics
influencing the ash formation process are
† The mode of occurrence of the inorganic matter and
† The combustion behaviour of coal particles containing both
organic and inorganic matter.
The majority of inorganic matter present in black coals
occurs in the form of minerals of various types and sizes. These
minerals can be closely associated with the organic matter
(included minerals), or they occur excluded from the organic
matter (excluded minerals). The majority of ash particles are
formed from four formation mechanisms, shown in Fig. 1:
† Included mineral coalescence,
† Char fragmentation,
† Excluded mineral fragmentation, and
† Vaporization and subsequent condensation of inorganic
matter.
Table 4
Total ash collected, PM10, PM2.5, and PM1 generated during combustion in
21% O2 at 1400 8C
PM10, PM2.5, and PM1 as % of ash generated during combustion in 21% O2
21% O2 CRC 240 CRC 272 CRC 296 CRC 297 CRC 306
PM10 0.9 6.1 10.7 3.2 3.8
PM2.5 0.15 0.53 0.49 0.78 0.40
PM1 (dry) 0.08 0.30 0.20 0.58 0.11
Total ash
collected
81.9 82.8 91.6 72.4 73.3
PM10 determined from Malvern Mastersizer, PM2.5 and PM1 determined from
weight collected in cascade impactor. Results presented as % of ash generated.
Table 5
Total ash collected, PM10, PM2.5, and PM1 generated during combustion in
50% oxygen at 1400 8C
PM10, PM2.5, and PM1 as % of ash generated during combustion in 50% O2
50% O2 CRC 240 CRC 272 CRC 296 CRC 297 CRC 306
PM10 19.8 11.7 17.4 13.3 13.3
PM2.5 0.72 1.41 1.06 1.60 0.44
PM1 (dry) 0.34 0.44 0.39 0.85 0.19
Total ash
collected
66.7 86.1 90.0 77.5 84.0
PM10 determined from Malvern Mastersizer, PM2.5 and PM1 determined from
weight collected in cascade impactor. Results presented as % of ash generated.
Fig. 1. Schematic of ash formation mechanisms during pulverised coal
combustion.
B.J.P. Buhre et al. / Fuel 85 (2006) 185–193188
Typically, ash particles are characteristic in size for the
mechanism via which they are formed. The first mechanism is
believed to be responsible for the bulk of the supermicron ash
particles formed [16]. The formation of ash particles from the
fragmentation of thin-walled cenospherical char particles (i.e.
the second mechanism) contributes to ash particles of only a
few micrometer in size [18–20]. Excluded mineral fragmenta-
tion contributes mainly to supermicron-sized ash particles, and
the last formation mechanism contributes mainly to submicron
sized ash particles [14].
Coal characteristics play an important role in the extent to
which the three ash formation mechanisms contribute to the
total ash formed. A summary of the main characteristics
affecting the formation mechanisms is provided below.
4.1. Included mineral coalescence
When char particles do not fragment, the ash formed from
included mineral coalescence is affected by the type, size and
distribution of included minerals. The high temperatures
occurring inside burning char particles cause included minerals
to turn viscous and coalesce. The ash fusion temperature is a
standard test done on coal ashes to characterise the various
stages of melting [13]. Although the ash fusion test does not
differentiate between included and excluded minerals, the test
indicates the melting behaviour of the inorganic matter upon
heating. The results in this paper suggest that this melting
behaviour could indicate the affinity of minerals to coalesce
during combustion. The ratio of basic oxides to acidic oxides is
often used in the literature as an indicator of ash slag viscosity
at high temperatures (e.g. [21]). If the ash fusion temperature
could be used as an indicator, this ratio is likely have a similar
relationship with the extent of mineral coalescence during coal
combustion.
4.2. Char fragmentation
The extent of char fragmentation depends on the char
swelling behaviour during combustion [20]. If the char
fragments are large and contain large minerals, the minerals
inside these fragments will coalesce and form large ash
particles. However, if the char swells extensively, the char will
fragment into small pieces and minerals inside these small
fragments will result in small ash particles. The two
mechanisms shown in Fig. 1 are extreme cases and ash formed
from coal combustion is a combination of the two.
Monroe suggested a mathematical model to predict the
coalescence behaviour of minerals as a function of cenosphere
shell thickness, coal particle size, mineral grain size, and
mineral volume fraction [22]. Yan validated this model by
determining the char swelling behaviour of several Australian
coals and its effect on the ash formation [17]. He modelled the
char fragmentation based on a char classification scheme, in
which the char is classified in three classes based on their
swelling behaviour. Char Group I particles are defined as char
particles that are highly porous and have low densities.
The combustion of these thin-walled cenospherical char
particles can result in more extensive fragmentation during
combustion, resulting in fine ash formation. Benfell showed
that the amount of Char Group I particles of Australian coals
could be estimated from the coal vitrinite content and the
pressure at which the char was formed [23,24]:
Char Group I ðnumber%Þ
Z 0:994!Pressure ðatmÞC0:621 ðVitrinite; v=v% mmfÞ
C29:87
This correlation is an updated version of a correlation
published earlier by the same research group [25,26]. The
updated version is based on a more extensive coal database but
produces similar results as the earlier correlation [24].
4.3. Excluded mineral fragmentation
The amount, size, and type of excluded minerals determine
the extent of excluded mineral fragmentation. The main
fragmenting mineral types in Australian black coals are calcite
and pyrite [27]. Although extensive fragmentation of these
20.0
25.0
cted
)
LT
B.J.P. Buhre et al. / Fuel 85 (2006) 185–193 189
minerals has been noted, the bulk of the newly formed ash
particles are generally supermicron in size [27,28].
0.0
5.0
10.0
15.0
1000 1200 1400 1600 1800
Ash deformation temperature,°C
PM
10 (%
of a
sh c
olle HT
Fig. 2. PM10 correlation with ash fusion temperature for the five coals (LT, low
temperature; HT, high temperature experiments).
4.4. Vaporization and condensation
The extent of vaporization of inorganic matter during coal
combustion depends on (a) the temperature inside the burning
char particle, and (b) the occurrence of readily vaporised
material such as alkalis, sulphur, and phosphorus [14,29]. The
vaporization of refractory oxides from included minerals has
also been suggested to depend on the size distribution of
included minerals containing refractory oxides [14,30,31].
Summarizing, the size fractions of legislative interest are the
result of several ash formation mechanisms. Several coal
characteristics affect the extent to which the formation
mechanisms result in fine ash. This study aims at determining
the dominating coal characteristics that correlates with the
amounts of fine ash formed. Table 1 shows the coal
characteristics considered for three size fractions (PM10,
PM2.5 and PM1), together with the ash formation mechanism
they could affect.
Fig. 3. Typical SEM image of ash from CRC 240 generated in 21% oxygen.
5. Discussion
5.1. PM10
The amounts of PM10 have been compared with the coal
characteristics indicated in Table 1. The PM10 showed no
correlation with average mineral size, ash content, or Char
Group I content. The ratio of basic to acidic oxides showed
moderate correlation with the amounts of PM10, and a good
correlation was observed between PM10 and the ash fusion
temperature. In literature, the ratio of basic to acidic oxides is
frequently used to predict the slagging behaviour of ash [21].
The observed correlation between PM10 and ash fusion
temperature (and to a lesser extent the correlation with the
ratio of basic to acidic oxides) suggests that the heating
characteristics of the ash are indicative of the fine ash formed
from the coalescence of included minerals.
PM10 is formed from (a) the coalescence of very small
included minerals, which combined result in particles smaller
than 10 mm or (b) the shedding of small minerals (!10 mm)
from the burning char surface. If small included minerals are
not released from the surface by shedding, these minerals
adhere to the char surface and coalesce with other minerals
inside the burning char particle to form large ash particles. The
ash fusion temperature is a bulk ash characteristic and does not
directly relate to small included minerals inside the coal.
However, if we assume that the melting behaviour of the
minerals is independent of size, the ash fusion temperature
could indicate the likelihood of coalescence of the minerals
during combustion; a high ash fusion temperature indicates that
minerals originally present in the coal are less likely to
coalesce during combustion. If included minerals are less likely
to coalesce, they can form individual ash particles, smaller than
mineral agglomerates, resulting in elevated PM10 levels. Fig. 2
shows how the amounts of PM10 measured during the
experiments correlate with the ash deformation temperatures.
The PM10 of the different coals in 50% O2 correlate well
with the ash deformation temperatures of the five coals, as
indicated for the high temperature (HT) experiments in the
figure. The amounts of PM10 formed during the low
temperature experiments show one significant outlier: CRC
240. This outlier can be explained by the morphology of the ash
particles formed from this particular coal. The Malvern
Mastersizer calculates the size distribution of a sample in
suspension based on the diffraction of a laser light through the
sample. In its analysis, it assumes that all particles are dense
spheres. CRC 240 displays the highest ash deformation
temperature, which results in non-spherical ash particles,
indicated in Figs. 3 and 4.
The amount of PM10 formed in 50% O2 (high temperature
experiments) is significantly higher than the amount formed in
21% O2 (low temperature experiments). The gas temperature
during both temperatures was held constant, and the volatile
matter release (and thus, possible coal fragmentation during its
release) should not be affected significantly by the change in
oxygen concentration. However, at low temperatures, the burning
char surface recedes at a pace at which included minerals have
enough time to coalesce, while at high temperatures, it appears
that there may be more shedding from the fast receding surface.
This increased shedding could result in increased fine ash particle
formation. Literature has shown that coal combustion at higher
Fig. 4. Typical SEM image of ash from CRC 240 generated in 50% oxygen.
0.0
0.5
1.0
1.5
2.0
0 20 40 60 80 100
Group I char (number %)
PM
2.5
(% o
f ash
gene
rate
d)
LTHT
Fig. 5. Amount of PM2.5 as a function of Char Group I content of the five coals
(LT, low temperature; HT, high temperature experiments).
B.J.P. Buhre et al. / Fuel 85 (2006) 185–193190
temperatures results in finer ash and elevated PM10 levels [32]. In
that study, the higher combustion temperatures were achieved by
elevating the gas temperatures from 1100 to 1300 8C, which
could have affected the extent of coal fragmentation during
volatile matter release. In this study, the elevated levels of PM10
are thought to be resulting from the decrease in time for
coalescence of included minerals during the char combustion.
5.2. PM2.5
PM2.5 is formed from two mechanisms: vaporisation and
condensation (the homogeneously nucleated submicron ash
particles) and the release of included minerals due to char
fragmentation. Depending on the char combustion temperature
(oxygen concentration), the submicron material accounts for
the majority of PM2.5. Several studies have suggested that the
release of small included minerals by fragmentation of
cenospherical char particles contributes to ash particles in the
size range between one and a few micrometers [18–20]. The
formation of cenospherical char particles (Char Group I
particles) has been correlated with the pressure of char
formation and the coal vitrinite content by Benfell [24].
The PM2.5 has been correlated to the coal characteristics
indicated in Table 1. No correlation was observed between the
amounts of PM2.5 and the ash fusion temperatures, the ash and
sulphur contents. A poor correlation was observed between the
PM2.5 and the Char Group I content estimated from the vitrinite
content, shown in Fig. 5.
A correlation between PM2.5 and Char Group I content
could be expected, however, the experimental results shown in
Fig. 5 provide no conclusive evidence. There are two possible
explanations for the lack of a good correlation between the two:
† During the low temperature experiments, the contribution
of PM1 to PM2.5 is significantly higher (up to 74%) than that
during the high temperature experiments. As a result, the
PM2.5 formation is dominated by the submicron ash, and
little correlation with Char Group I content is expected.
† A correlation between PM2.5 and Char Group I content is
only expected when the included minerals of all coals have
the same size distribution and are uniformly distributed
through the maceral types.
Experiments using more coals and more detailed coal
analyses could provide more insight in the coal characteristics
that correlate with PM2.5. It must be noted that the Char Group I
content is estimated from the vitrinite content, and that a
similar correlation between PM2.5 and vitrinite content and
with the mean vitrinite reflectance can be observed.
The elevated PM2.5 emissions during the experiments in
50% O2 can be attributed to:
† Increased vaporisation of elements, which increase PM1
formation and
† At high temperatures, there may be more shedding from the
fast receding surface, similar to the mechanism resulting in
elevated PM10 levels at higher temperatures.
5.3. PM1
The chemical composition of PM1 is significantly different
from the bulk chemical composition, and depends on the
combustion temperature and the mode of occurrence of
the inorganic material in the coal. The main components in
the submicron ash are sulphur, silicon, sodium, and phos-
phorus, with sulphur being the most abundant element detected
on the filters [10]. Particles collected onto the filter were
typically around 20–30 nm, much smaller than the cut-off of
the last impactor stage. Although the amount of particles
collected on the last stages were much smaller, they accounted
for a significant proportion of the mass. This makes
identification of the dominating coal characteristic correlating
with the amount of PM1 a difficult task.
The amount of PM1 has been compared with the average
mineral size, the ash and sulphur content and the Char Group I
content. No correlation was observed between PM1 and the ash
content or the average mineral grain size. Fig. 6 shows how
PM1 and the total sulphur content in the coal correlate. Fig. 7
shows the correlation between PM1 and Char Group I content.
Of the two coal characteristics correlated with the amount of
PM1, the total sulphur content is the best indicator of the
amount of PM1 formed. Sulphur is the most abundant element
detected on the filters and a similar correlation is observed
between the amount of ash collected on the filter and the coal
0.0
0.2
0.4
0.6
0.8
1.0
0 20 40 60 80
Char Group I content (number %)
PM
1 (%
of a
sh c
olle
cted
)
LT
HT
Fig. 7. Amount of PM1 as a function of char group I content of the five coals
(LT, low temperature, HT, high temperature experiments).
0.0
0.2
0.4
0.6
0.8
1.0
4 620
Coal sulphur content (% db)
PM
1 (%
of a
sh c
olle
cted
)
LT
HT
Fig. 6. Amount of PM1 as a function of coal sulphur content of the five coals
(LT, low temperature, HT, high temperature experiments).
B.J.P. Buhre et al. / Fuel 85 (2006) 185–193 191
sulphur content. However, no correlation could be observed
between the Char Group I content and the amount of ash
collected on the filter. The ash collected on the filter is very fine
(around 20–30 nm) compared to the ash collected on the latter
stages of the impactor (a few hundred nanometers). The ash
collected on the last stages could be the result of char
fragmentation, while the material collected on the filter is the
result of vaporisation and condensation of inorganic material.
As a first pass, the coal sulphur content could be used as an
indicator of the amount of PM1 formed during coal
combustion; however, more experiments using higher sulphur
coals could confirm this conclusion.
The amounts of PM1 formed during the high temperature
experiments are consistently higher than the amounts formed
during the low temperature experiments. This observation is
well documented in the literature and can be attributed to
Table 6
Guidelines for black coal assessment for their potential to form fine ash
Fine ash Analysis technique Measured coal characteristic
PM10 Ash fusibility test in oxidizing
environment, [13]
Ash deformation temperature
PM2.5 Maceral analysis (e.g. reflecto-
gram [12])
Char Group I content (proporti
of coal that shows swelling beh
viour during char formation),
determined from vitrinite conte
PM1 Ultimate analysis, total sulphur
content [35]
Total amount of sulphur in the c
the enhanced vaporization of refractory oxides inside burning
char particles [14].
5.4. Coal selection guideline
This study has characterised coals using various methods
and analysis techniques and compared the results of these
techniques with the amount and characteristics of the fine ash
formed. Table 6 summarizes the analysis techniques and coal
characteristics that have been found best to assess coals for
their potential to form fine ash in this study.
The guidelines can be used to assess different coals for their
potential to form fine ash during combustion. The guideline for
PM1 is questionable, as the correlation is skewed by the PM1
formed from one coal with high sulphur content. PM1 is formed
from a wide variety of elements and the elemental mode of
occurrence in the coal, which is responsible for the occurrence
of these elements in the submicron ash is not always known.
For example, two coals show significant amount of sodium in
the submicron ash, which is not reflected in the previous
guideline [10].
6. Conclusion
The ash size fractions of legislative interest (PM10 and
PM2.5, and PM1) formed during coal combustion are the result
of several ash formation mechanisms. They are formed by a
combination of (1) included mineral coalescence (2) excluded
mineral fragmentation, (3) char fragmentation, and (4)
vaporization and subsequent condensation of inorganic matter.
Ash particles are characteristics in size for the mechanism via
which they are formed. Coal characteristics and combustion
conditions determine the extent to which the ash formation
processes contribute to the total ash formed.
Five well-characterized black coals have been selected to
represent the range of ash chemistry and char swelling
behaviour of Australian bituminous coals. Ash was generated
by combustion of the coals in a drop tube furnace, simulating
combustion in a pf boiler. The amount of fine ash was
determined and the coal characteristics responsible for its
formation have been established.
Trends observed Comments
The higher the ash fusion tem-
perature, the more PM10 (% of ash)
is formed during combustion
Combustion in 21 and 50% O2
showed similar trends, with one
outlier (Fig. 2)
on
a-
nt.
The higher the Char Group I
content, the more the char swells,
the more the PM2.5 is expected to
be formed
This correlation is expected, how-
ever, the experiments are incon-
clusive. (Fig. 5)
oal The more sulphur is detected, the
higher the amount of submicron
ash formed
Sulphur has been shown to be a
major contributor to PM1 and the
observed correlation is mainly the
result of the high sulphur presence
in CRC 297 (Fig. 6)
B.J.P. Buhre et al. / Fuel 85 (2006) 185–193192
The amount of PM10 formed during the experiments
correlates with the ash fusion temperature. A high ash fusion
temperature could indicate that the minerals originally present
in the coal are less likely to coalesce. If included minerals are
less likely to coalesce, they can transform into individual ash
particles, smaller than mineral agglomerates, resulting in
elevated PM10 levels.
The amounts of PM2.5 formed would be expected to
correlate with the amount of char particles displaying swelling
behaviour, expressed as Char Group I particles. Thin-walled
cenospherical char particles can fragment during combustion,
resulting in the release of fine included minerals originally
present in these char particles, which transform to ash particles
of a few micrometers in size. This expectation could not be
confirmed conclusively from the experiments. More exper-
iments using different coals and maceral types could confirm
this expectation.
The amounts of PM1 correlate slightly with the coal sulphur
content. More experiments using higher sulphur coals could
confirm this observation. The amount of PM10, PM2.5 and PM1
increased consistently when increasing the oxygen concen-
tration in the combustion gas from 21 to 50%. The enhanced
temperatures increase the char fragmentation and vaporization
of inorganic matter. Finally, it is shown that these results can be
used a guidance for coal selection for minimization of fine
particle formation.
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 Commonwealth
Government of Australia.
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