The implication of mineral coalescence behaviour on ash formation and ash deposition during pulverised coal combustion

Fuel 80 (2001) 1333±1340

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The implication of mineral coalescence behaviour on ash formation and ash deposition during pulverised coal combustion L. Yan*, R.P. Gupta, T.F. Wall Cooperative Research Centre for Black Coal Utilisation, Department of Chemical Engineering, The University of Newcastle, Callaghan, New South Wales 2308, Australia Received 19 November 1999; revised 19 July 2000; accepted 12 November 2000

Abstract In modelling the fate of minerals of pulverised coal in coal-®red boilers, ash formation is a primary step that decides ash particle size distribution and composition. Frequently, two extreme schemes have been used regarding mineral coalescence behaviour within individual char particles, i.e. no coalescence and full coalescence. However, their implications on ash deposition and thermal performance have not been investigated. In the present paper, the implications of coalescence of included minerals on ash character and subsequent ash deposition are demonstrated by means of an ash formation model and an ash deposition model. The simulated results in a cylindrical furnace for an Australian bituminous coal indicate that ash deposition and thermal performance are signi®cantly in¯uenced by the coalescence behaviour of included minerals. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Mineral coalescence; Ash formation and deposition; Modeling

1. Introduction Ash deposition is one of the major problems encountered in conventional power plants. Traditionally, ash is characterised in terms of its bulk properties (e.g. ash chemistry and fusion temperatures). Various empirical indices had been developed to characterise ash deposition tendencies, in terms of high temperature slagging and low temperature fouling [1,2], based on bulk ash properties. However, traditional indices did not prove to work adequately in predicting ash deposition tendencies. For instance, traditional indices generally failed to address the heterogeneous nature of ash properties from particle to particle [2]. There are also systematic effects in¯uencing ash deposition, e.g. boiler operating conditions, the aerodynamics of the gas and particulate transport, etc. Therefore mechanistic studies have been carried out to cover at least part of these effects. Generally three sequential processes are involved in ash deposition: ² transformation of mineral matter into ash particles; ² transport of ash particles to heat transfer/deposit surfaces (impaction); ² retention of ash particles on surfaces.

* Corresponding author. E-mail address: [email protected] (L. Yan).

These processes are primarily related to mineral distribution in pulverised coal. Recent advances in the CCSEM (Computer-Controlled Scanning Electron Microscope) technique paved the way for a more accurate insight into mineral distribution in pulverised coal and ash formation [8,22]. The CCSEM technique determines sizes and chemical compositions of individual mineral grains so that the abundance of various mineral species (mineral types), mineral size distribution and the included or excluded nature for several thousands of mineral grains are obtained [3]. Included minerals, which are embedded within coal particles, usually undergo coalescence to some extent depending on physical closeness within a singe char particle. On the other hand, excluded minerals evolve into ash particles individually with or without fragmentation, depending on thermal behaviour of the mineral species. Two extreme models for coalescence behaviour of included minerals were frequently used in the past to predict ash size and chemical composition [4±7] i.e. nocoalescence scheme and full-coalescence scheme. The nocoalescence scheme assumes that one mineral grain evolves into a single ash particle regardless of the included or excluded nature. The full-coalescence scheme, on the contrary, considers that all included mineral grains within a single char particle coalesce to form a single ash particle after complete burnout of the char particle. In other words, one ash particle is generated per char particle irrespective of

0016-2361/00/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0016-236 1(00)00194-0

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L. Yan et al. / Fuel 80 (2001) 1333±1340

Nomenclature I1, I2, I3 Inertial resistances for particulate diffusion in the turbulent core, the buffer layer and the boundary layer, respectively (s/m) B3 Diffusion resistance from Brownian motion in the boundary layer (s/m) T3 Thermophoretic resistance in the boundary layer (s/m) Rt Total resistance of ash transport from the turbulent core to deposit surface (s/m) Se Overall effective stickiness, i.e. stickiness ef®ciency (±) Sp, Ss Stickiness of particles and deposit surfaces (±) Greek symbols hc Critical viscosity of ash particles (Pa.s) hp Viscosity of ash particles (Pa.s) the number of mineral grains in the char particle. Different levels of coalescence would lead to different size distribution and different composition distribution (therefore stickiness) of resulting ash particles. The no-coalescence scheme is expected to result in relatively ®ner and less sticky ash particles, whereas the full-coalescence scheme could result in relatively larger and more sticky ash particles. For instance, pure siderite in Australian coals was found to generate ¯ame products of high viscosity, however, if siderite coalesces and reacts with alumino-silicate, the resulting products are sticky due to low viscosity [19]. A partial coalescence is more likely to occur in real situations [18] and is expected to be somewhere intermediate between these two extreme cases. Intuitively, the degree of mineral coalescence increases with higher mineral loading in individual coal particles. The coalescence behaviour is associated with char fragmentation during combustion. Extensive char fragmentation implies poor mineral coalescence resulting in ®ne ash particles whereas little or no char fragmentation results in extensive mineral coalescence in solid char particles and consequent large ash particles. The ash particle size distribution in¯uences the transport of ash from the ¯ames onto the heat transfer surfaces signi®cantly. The extent of retention of ash particles is related to the stickiness of ash particles, as these particles impact on the surfaces. Although these two simple schemes of mineral coalescence have been used in predicting ash character [4,7] and deposition performances [3,5,6], the implications of these schemes on ash deposition have not been studied yet. The present paper theoretically illustrates the implications of various coalescence schemes by using models of ash formation, transport and deposition in a furnace. Brief descriptions of these models are ®rst detailed, followed by simulation results and discussions of possible implications of these various coalescence schemes on ash deposition.

2. Ash formation model The ash formation model used in this study comprises two

sub-models: mineral distribution sub-model based on CCSEM data and mineral transformation sub-model. 2.1. Mineral distribution sub-model CCSEM data on minerals in pulverised coal is categorised into a number of mineral types as well as a number of size ranges. For statistical purpose, the total number of mineral grains is scaled up from several thousand to millions. Corresponding coal particles are also divided into a number of size ranges. The total number of coal particles should be suf®cient enough to ensure that at least several coal particles are present in the largest size bin. A Monte Carlo procedure is used to disperse included mineral grains, one-by-one, into randomly selected coal particles. This method is similar to that of Charon et al. [9] Detailed description of the model was presented elsewhere [10]. The mineral distribution model provides mineral variations on a particle-by-particle basis. 2.2. Mineral transformation model As mentioned earlier, there are two extreme schemes of mineral coalescence within a single coal particle. Nocoalescence scheme simply stipulates any mineral grain becomes a single ash particle after combustion. There is no interaction between two mineral grains regardless of included or excluded nature. The full-coalescence scheme, however, assumes all included minerals in a coal particle coalesce together to form a single ash particle, once the inventory of included minerals for individual coal particles is known. A partial coalescence scheme has been developed to treat included minerals based on cenospherical char structures. By assuming the char combustion follows a shrinking cenosphere mode in diffusion-controlled regime, Monroe's study [18] indicated that the extent of coalescence of included minerals is a function of char structure (the shell thickness of char cenosphere), the sizes of the parent coal particle and of mineral grains as well as the mineral volume fraction. There are quite different char structures generated in coal devolatilisation under typical combustion conditions.

L. Yan et al. / Fuel 80 (2001) 1333±1340

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Fig. 1. Schematic diagram of main mechanisms for ash transport considered.

Recent studies by Bailey et al. [11] and Benfell et al. [12] indicated that char structures could be classi®ed into three groups. Group I particles are characterised by thin-shelled cenospheres with a big void inside each particle; Group III particles are solid-like and dense particles with little vesicles inside a particle; and Group II particles are those intermediate between Group I and III. In the modelling effort, the shell thickness for char Group I, II and III are assumed to be 5, 25 and 45% of char outside diameter respectively (50% shell thickness infers a solid particle). The proportions of the three type char particles for a given coal were found to relate to coal properties (e.g. vitrinite content) and combustion conditions (e.g. pressure). The relationship between char morphology and coal properties and combustion conditions is being investigated currently [12]. The number proportions of these three group chars observed with optical microscope have been used in the partial coalescence model. Detailed description of the partial coalescence model was given elsewhere [13]. 3. Ash deposition model Ash particles formed are carried by turbulent gas ¯ow along with the main stream direction. Some of the ash particles may impact onto heat transfer surfaces where they may be captured or rebound depending on the overall effective stickiness. Sub-models have been developed to consider ash transport onto surfaces, subsequent surface deposition and heat transfer across ash deposit as described below. 3.1. Ash transport sub-model A number of mechanisms in¯uence the transport of ash particles to the surfaces, i.e. inertial impaction, Brownian diffusion, thermophoresis, photophoresis, electrostatic and gravitation forces. Three main mechanisms are considered in the present work, i.e. inertial impaction for larger particles, Brownian diffusion and thermophoresis for ®ner particles. The gas ¯ow ®eld is divided into three zones: a fully turbulent core, a buffer layer and a boundary layer. In the outer turbulent core and buffer layer, thermophoresis and Brownian motion may be negligible due to small

temperature gradients. However, in the boundary region, the contribution from Brownian motion becomes signi®cant as the inertial resistance increases rapidly in the region. Also, thermophoresis becomes much stronger than that in the buffer layer and the turbulent core due to higher temperature gradients. Thus, three mechanisms are taken into account in the boundary region. It is expected that these mechanisms work independently. Analogous to the well-known electric resistance, the total resistance of ash transport Rt is expressed as: Rt ˆ

1 1 I2 1 I1 1=I3 1 1=B3 1 1=T3

…1†

where I1, I2 and I3 are inertial resistances belonging to the turbulent core, buffer layer and boundary layer, respectively; T3 the thermophoretic resistance in the boundary layer; and B3 the resistance for Brownian motion in the boundary layer. Fig. 1 schematically shows these transport resistances. The details of these individual resistances were given elsewhere [14]. The arrival velocity of ash particles is simply reciprocal of the total transport resistance Rt. Ash arrival rate onto heat transfer surfaces (kg/m 2.s) is obtained by multiplying ash arrival velocity (m/s) with the particulate burden in the furnace (kg/m 3 of gas). 3.2. Ash deposition sub-model Ash deposition rate onto the wall surface is directly proportional to the arrival rate of ash particles. The retention of an impacting ash particle depends not only on its stickiness but also on the stickiness of the heat transfer surface. The determination of the particle stickiness is incompletely quanti®ed. Baxter et al. [20] used particle impaction velocity and the characteristics of the heat transfer surface to estimate a rebound velocity and force. The particle stickiness is then determined by comparing the rebound force to a force required to keep the particle from bouncing. Viscosity had been found to be the most common parameter used to describe the stickiness extensively after Walsh et al. [21] In the present work, the stickiness of ash particles is estimated using a method similar to that of Walsh et al. [21] A critical viscosity value (h c) is arbitrarily chosen below which ash

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L. Yan et al. / Fuel 80 (2001) 1333±1340

Fig. 2. Schematic three-layer structure of deposit and heat transfer mechanisms [16]

particles are considered to be sticky. The stickiness of an ash particle (Sp) is de®ned as the probability of its retention on a non-sticky surface and given by the ratio of the critical viscosity to the viscosity of the ash particle (h p) upon impaction: Sp ˆ hc =hp if hp . hc

…2†

Sp ˆ 1 if hp . hc

…3†

The ash deposition is also dependent on the stickiness of the

wall surface (Ss), which is estimated from the fractional area of particles on the surface that are sticky at the surface temperature. The overall effective stickiness (stickiness ef®ciency) is given by: Se ˆ 1 2 …1 2 Sp †…1 2 Ss †

…4†

The value of the critical viscosity was reported in a range from 10 4 to 10 6 Pa.s. A critical value of 10 4 Pa.s is used in the present work. The viscosity of ash particles is calculated using Urbain's model [15]. Although Urbain's viscosity

Table 1 Summary CCSEM data of Ulan coal (data in parentheses are included mineral wt% (on total mineral basis) for the mineral type) Mineral type a

Quartz Calcite Dolomite Kaolinite Montmorillonite K±Al±silicates Aluminosilicate Pyrite Pot Chloride Gyp/Al±Silicate Si Rich Unclassi®ed a

wt%

34.66 (18.10) 1.08 (0) 3.82 (0) 11.60 (5.12) 16.78 (12.69) 1.83 (0.80) 1.69 (0.94) 2.03 (1.11) 1.02 (0) 1.72 (1.53) 15.05 (11.67) 6.89 (4.79)

Size distribution (wt% on total mineral basis) , 2.5 mm

2.5±5.0 mm

5.0±10 mm

0.94 (0.75) 0

10.46 (6.99) 0.28 (0) 0

10.79 (4.28) 0

1.47 (0.84) 2.58 (2.51) 0.2 (0.2) 0.12 (0) 0

2.55 (2.05) 1.22 (0.51) 1.49 (0.46) 0.62 (0.62) 0

0.23 (0) 0.23 (0.23) 3.18 (3.01) 0.77 (0.46)

0.72 (0) 0.5 (0.5) 1.35 (1.35) 1.35 (1.03)

0 0.12 (0.12) 0.27 (0.27) 0 0.04 (0.04) 0 0 0 0.79 (0.79) 0

Only those mineral types with wt% .1% are listed.

0

10±20 mm 3.95 (2.14) 0 2.78 (0) 4.38 (1.61) 3.59 (3.59) 0 0 (0) 0 0 0 0 0.57 (0.57)

20±50 mm

. 50 mm

2.26 (1.64) 0.38 (0) 0.15 (0) 0.73 (0.25) 3.51 (2.13) 0.14 (0.14) 0.63 (0) 0.28 (0) 0.07 (0) 0.58 (0.39) 2.47 (2.23) 1.75 (1.25)

6.26 (2.30) 0.42 (0) 0.89 (0) 2.35 (0.24) 5.62 (3.67) 0 0.28 (0.28) 1.75 (1.11) 0 0.4 (0.4) 7.26 (4.29) 2.44 (1.48)

L. Yan et al. / Fuel 80 (2001) 1333±1340

Fig. 3. Schematic diagram of the furnace, with deposition occurring on the inner surface.

model (and other existing models) can not predict viscosity of ash particles precisely at low temperatures (,10008C), it may be used to rank stickiness levels of different ash particles at low temperatures. The retention of ®ne ash particles is more due to surface forces. Therefore, the stickiness of ®ne particles (,2 mm) is assumed to be unity. Thus, ash deposition rate is determined based on the ash arrival rate and the overall effective stickiness. The possible scouring effect of arriving particles on those already loosely adhered to the surface and the entrainment effect due to high gas velocity are neglected in the current model. These effects may in¯uence the ash deposition rates, particularly for large size particles. However, these may not signi®cantly in¯uence the relative ranking discussed in the following sections. Further work is needed to address these effects. 3.3. Heat transfer sub-model Fig. 2 is a schematic diagram showing the main mechanisms of heat transfer through the ash deposit. The incident heat radiation absorbed by the deposit surface is conducted

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across the deposit to the metal tube surface. Owing to the high ¯ame temperature, thermal radiation is also an important mechanism. Heat transfer within porous ash deposit is determined by conduction and radiation as well. The temperature of deposit surface increases as the ash deposit grows in thickness. Local temperature and composition determine the structure at various thickness levels within the deposit. The effective thermal conductivity, including radiation across the deposit, varies across the whole thickness. In the present model for heat transfer, ash deposit is simply characterised into three layers of distinct structures: a particulate layer, a sintered layer and a slagging layer (Fig. 2). The inner layer adjacent to the tube is usually particulate in nature with a continuous gas phase and discrete particles embedded in the continuous phase. This layer has relatively low thermal conductivity contributing to the overall thermal resistance signi®cantly. The outermost layer is completely fused slag at high temperatures thus has low porosity and high thermal conductivity. The middle layer is partially sintered with thermal properties between those of the inner particulate layer and the outside slag layer. The typical values of porosity and thermal conductivity for the three layers used in this model are given in Table 2. The ash deposit is assumed to stop growing when a suf®ciently low viscosity is present on its surface as the surface temperature rises high. Detailed description of deposit structure and heat transfer is available elsewhere [17]. 4. In¯uence of mineral coalescence on ash deposition This section discusses the coalescence behaviour of included minerals during coal combustion, whose in¯uence on ash deposition is not clear so far, even though the two extreme schemes of no-coalescence and full-coalescence have been often employed. One Australian bituminous coal is used in the current study. Table 1 summarises the CCSEM data of minerals in the coal. As mentioned earlier, the ash deposition in boilers is very complex in reality. For simplicity, numerical simulations are made for a furnace in a cylindrical geometry, as schematically shown in Fig. 3. Its operating parameters and properties of ash deposit assumed are given in Table 2. As seen from Fig. 4, ¯y ash particle size distribution predicted by the no-coalescence scheme is the ®nest

Table 2 Parameters used for the test furnace and deposit characteristics Furnace

Diameter (m) Flow velocity (m/s) Dust burden (g/m 3) Gas temperature (8C) Surface temperature (8C)

Deposit

1.0 10 3.82 1600 600

Thermal conductivity (W/m K) Porosity (±) Interfacial viscosity (Pa s)

Particulate layer

Slag layer

0.5 0.8 10 7

4.0 0.2 10 4

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L. Yan et al. / Fuel 80 (2001) 1333±1340

Fig. 4. Particle size distribution of ¯y ash, predicted with the three coalescence schemes, respectively.

one, and that from the full-coalescence scheme is the coarsest whereas the partial coalescence scheme gives intermediate size distribution. If ash particles with viscosity at 12508C above 10 7 Pa.s are treated as non-sticky, and the other ash particles are treated as partially sticky (10 5 Pa.s # h p # 10 7 Pa.s) or completely sticky (h p , 10 5 Pa.s), Fig. 5 shows various proportions of these ash groups with different levels of stickiness. Comparatively, the full-coalescence scheme predicts the least fraction of non-sticky ash particles, whereas the no-coalescence scheme predicts the highest fraction of non-sticky particles. Correspondingly, the full-coalescence scheme predicts the largest fraction of ash particles partially or completely sticky (h p # 10 7 Pa.s), and the no-coalescence scheme generates the smallest fraction of particles partially or completely sticky. The partial coalescence scheme generally predicts ash viscosity distribution between these two extreme cases. Therefore, ash characteristics predicted with these various coalescence schemes are clearly

Fig. 6. Effect of particle size on ash arrival velocity on heat-transfer surfaces.

different, which lead to dissimilarities in ash deposition characteristics as a result. Ash arrival velocity is a function of particle size, density and the aerodynamics of the gas. When the ash density and gas aerodynamics are ®xed for simplicity, the effect of particle size on ash arrival velocity is clearly shown in Fig. 6. Small ash particles (,10±20 mm) exhibit lower arrival velocity, since thermophoresis dominates transport of small ash particles. Large ash particles (.20 mm) show much higher values of arrival velocity primarily due to increased inertial effect. Considering the differences in particle size distribution predicted (Fig. 4), one can conclude that the ash arrival rate in the full-coalescence scheme is the highest compared to the other two. The stickiness ef®ciency at the initiation of ash deposition (at a deposit surface temperature of around 6008C) and at a later stage (deposit surface temperature 9008C) predicted by the full-coalescence scheme are consistently higher than

Fig. 5. Viscosity distribution of ¯y ash particles, predicted by the three coalescence schemes respectively.

L. Yan et al. / Fuel 80 (2001) 1333±1340

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Fig. 7. Variation of stickiness ef®ciency (a) and ash deposit rate (b) with deposit surface temperature, predicted by the three coalescence schemes respectively.

those predicted by the no-coalescence scheme, as compared in Fig. 7(a). Initially, the overall stickiness ef®ciency is overwhelmingly governed by the stickiness of impacting ash particles. Later the difference in overall stickiness ef®ciency between the two extreme schemes diminishes since the overall stickiness ef®ciency includes the stickiness of deposit surface as well, shown in Eq. (4). Fig. 7(b) compares the ash deposit rates predicted by these schemes as the combined results of the stickiness ef®ciency and ash arrival rate. The ash deposition rates predicted by the fullcoalescence scheme are, therefore, much higher than those predicted by the no-coalescence scheme, particularly in early stages of ash deposition. For illustrative purpose, the ash deposition rate is used to predict measurable phenomena. As the result of higher ash deposition rate, the ash deposit predicted by the fullcoalescence scheme grows much faster than that predicted by the no-coalescence scheme, as seen from Fig. 8(a). To reach the same deposit thickness of 3 mm, for instance, it only takes about 2/3rds of time duration for the full-

coalescence case compared to that in the no-coalescence case. The possible scouring effect of arriving particles on those previously adhered to the surface is not expected to in¯uence the ranking here. Ash deposition affects heat transfer from the ¯ame to water-cooling wall. Fig. 8(b) clearly shows that the fullcoalescence scheme predicts the fastest decline in heat ¯ux compared to the no-coalescence scheme and the partial coalescence scheme. For instance, decline of heat ¯ux to 70% of clean tube value takes 7, 12, and 18 relative times for the three coalescence cases, respectively. The above predictions under various coalescence schemes indicate that ash deposition is greatly in¯uenced by the coalescence behaviour of included minerals during coal combustion. In fact, coalescence of included minerals affects both ash particle size and chemistry. During initial stages of ash deposition, the effect of ash chemistry is greater than that of ash size, because the overall stickiness ef®ciency is dominantly controlled by impacting ash chemistry. In later stages, the effect of size becomes more

Fig. 8. Ash deposit growth (a) and heat ¯ux drop (b) with relative time, predicted by the three coalescence schemes respectively.

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dominant when deposit surface temperatures become higher, and stickiness ef®ciency (largely controlled by the stickiness of the surface) increases to high levels such that most of arriving ash particles are captured. Generally, the full-coalescence scheme predicts the coarsest ash particle size distribution, the highest ash arrival velocity and the highest ash deposit rate. Hence, the full-coalescence scheme, as favoured by some researchers [4,6,7], might overestimate ash deposition rates. On the contrary, the nocoalescence scheme predicts the ®nest ash particle size distribution, the least ash arrival velocity and the least ash deposit rate. The partial coalescence scheme generally predicts ash and consequent deposit characters somewhere between the two limiting cases. Therefore, a more accurate understanding and description of coalescence behaviour of included minerals during combustion is important, particularly for coals with large fractions of included minerals. 5. Conclusions Various extents of mineral coalescence within individual char particles could lead to various ash characters, and consequently in¯uence ash deposition performances. In this paper, the implications of three coalescence schemes (i.e. the no-coalescence, the full-coalescence and a partial coalescence) have been studied by combining two subsequent models of ash formation and ash deposition. Partial coalescence behaviour of minerals can be related to coal petrographic properties and combustion conditions, which is currently under investigation. Predictions in a cylindrical geometry with an Australian bituminous coal indicate that thermal performances predicted with these different coalescence schemes are different. Generally, the full-coalescence scheme predicts the fastest ash deposit growth and the fastest drop in heat transfer, which is attributed to the coarsest particle size distribution as well as the largest proportion of sticky ash particles predicted. The no-coalescence scheme predicts the slowest ash deposit growth and the least drop of heat transfer, whereas the partial coalescence scheme predicts results intermediate between the two extreme cases. This study demonstrates the signi®cance of coalescence behaviour of included minerals on ash deposition performances. Acknowledgements The authors wish to acknowledge the ®nancial support

provided by the Cooperative Research Centre for Black Coal Utilisation, which is funded in part by the Cooperative Research Centres Program of the Commonwealth Government of Australia.

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