The effects of pressure on coal reactions during pulverised coal combustion and gasification

Progress in Energy and Combustion Science 28 (2002) 405–433 www.elsevier.com/locate/pecs

The effects of pressure on coal reactions during pulverised coal combustion and gasification Terry F. Walla,b,*, Gui-su Liua,b,1, Hong-wei Wua,b,2, Daniel G. Robertsa,c, Kathy E. Benfella,d, Sushil Guptaa,b,3, John A. Lucasa,b, David J. Harrisa,c a

Cooperative Research Centre for Black Coal Utilisation, Advanced Technology Centre, The University of Newcastle, Callaghan, NSW 2308, Australia b Department of Chemical Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia c CSIRO Energy Technology, P.O. Box 883, Kenmore, QLD 4069, Australia d Department of Geology, The University of Newcastle, Callaghan, NSW 2308, Australia Received 19 October 2001; accepted 28 May 2002

Abstract Advanced clean coal technologies, e.g. power generation from integrated gasification combined cycle (IGCC) and pressurised fluidised bed combustor, have attracted increased interest from the scientific and technological communities over the last few decades. Pressures up to 40 atm have been applied to these technologies, which inherently result in an increase in coal throughput, a reduction in pollutant emissions and an enhancement in the intensity of reaction. Therefore, fundamental understanding of the effect of operating pressure on coal reactions is essential to the development of these technologies. In this paper, the pressure effect on a variety of aspects of coal reactions reported in the open literature has been reviewed. Major emphasis of the paper is given to experimental observations, although some theoretical modelling is reviewed. The pressure has been found to significantly influence the volatiles yield and coal swelling during devolatilisation, hence the structure and morphology of the char generated. More char particles of high porosity are formed at higher pressures. Char structure appears to play a significant role in burnout of residual char and ash formation. In general, at higher pressures, coal particles burn quicker and form finer ash particles. Increasing reactant pressure enhances char combustion and gasification reaction rate, which can be understood by an adsorption – desorption mechanism. These factors have been applied to the understanding of a practical high-pressure gasifier. Most of the work published has been at the lower temperatures (typically ,1000 8C), which can be achieved in experiments involving captive particles or coal samples. Experiments in pressurised TGA and wire mesh systems at these low temperatures are the most commonly reported, with some experiments for entrained flow system reported at the higher temperatures typical IGCC conditions in entrained flow reactors. Although the difficulty and cost have restricted these experiments, the entrained flow work is the current research need. The structure of the char generated has recently been related to char reactivity and the ash formed, but the mechanisms leading to the effect of pressure on this structure are not understood. Progressing the understanding of the formation of char structure at pressure and its relation to coal properties is an obvious research need. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Pressure; Gasifier; Coal combustion; Coal gasification; Devolatilisation; Char structure; Char reactivity; Ash formation

* Corresponding author. Address: Department of Chemical Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia. Tel.: þ 61-2-4921-6179; fax: þ61-2-4921-6920. E-mail address: [email protected] (T.F. Wall). 1 Currently at Niksa Energy Associates, 1745 Terrace Drive, Belmont, CA 94002, USA. 2 Currently at Department of Chemical Engineering, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia. 3 Currently at School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia. 0360-1285/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 1 2 8 5 ( 0 2 ) 0 0 0 0 7 - 2

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Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Effect of pressure on coal devolatilisation and char formation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Volatile yield. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Experimental studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3. Mathematical modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Char formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Coal swelling upon heating. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Char morphological study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3. Effect of pyrolysis pressure on char surface area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Reactivity of char produced under high-pyrolysis pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Summary on pressure effects on volatiles yield and char structure . . . . . . . . . . . . . . . . . . . . . . . 3. Effect of reactant pressure on char combustion and gasification rates . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. High pressure char combustion kinetics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Experimental measurements on char combustion rates at pressure. . . . . . . . . . . . . . . . . . 3.1.2. Reaction mechanism on char combustion at pressures . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3. Modelling char burnout at high pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. High-pressure char gasification kinetics with CO2, H2O and H2 . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1. Experimental measurements on char gasification with CO2, H2O and H2. . . . . . . . . . . . . 3.2.2. Reaction mechanisms of char gasification at pressure. . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3. Modelling of char gasification rate at high pressure and high temperature . . . . . . . . . . . . 3.3. Summary on pressure effect on char reactivity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Effect of pressure on ash formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Effect of pressure on ash formation in relation to char burnout . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1. Fragmentation of porous char during combustion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2. Included mineral coalescence and ash liberation during char burnout . . . . . . . . . . . . . . . 4.1.3. Effect of pressure on ash characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Effect of pressure on other ash transformation processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1. Effect of pressure on cenosphere ash formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2. Effect of pressure on excluded minerals transformation . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3. Effect of pressure on vaporisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.4. Effect of pressure on chemical reaction equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.5. Effect of pressure on condensation and aggregation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Char, ash and slag characteristics in a practical gasifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Summary on pressure effect on ash formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction As we enter into the 21st century, there is an increasing need for energy due to global economic growth. Fossil fuels will continue to dominate the world energy supplies into the 21st century and coal as a fuel must play an increasing role [1]. However, there has also been increasing environmental concern directed towards utilising coal, for example, CO2 emissions, pollutant emissions and particulate disposal. Therefore, there is a need for clean coal burning technologies. Clean coal technologies are now becoming popular because of their high efficiencies and minimal environmental impacts [2]. Several technologies, such as integrated

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gasification combined cycle (IGCC), pressurised fluidised bed combustor (PFBC) and pulverised coal injection (PCI), have been identified as the most viable alternatives for the clean utilisations of coal due to the use of combined cycles [2– 5]. These advanced clean coal technologies have gained increased technological and scientific interest over the last few decades [2]. Higher operating pressures have been applied to these technologies, for instance, 10 – 15 atm for PFBC, 15 – 25 atm for IGCC and less than 5 atm for PCI. Higher pressure operations will inherently result in an increase in coal throughput, a reduction in pollutant emissions and an enhancement in the intensity of reaction [4]. There are three types of pressure effects, i.e. the effect of

References

Coal type(s)

Pressure range (atm)

Temperature (K)

Heating rate (K s21)

Particle size (mm)

Experimental apparatus

[7] [8] [10] [10] [17] [9]

1023 –100 1023 –100 0.1– 90 0.1– 100 1.33 £ 1024 –100 1023 –100

1273 1373 1273 1223 1273 373 –1273

650–104 100–104 200 0.05 100–104 100–104

70 74–1000 200– 315 800– 1000 50, 85, 125 97–180

Electrical grid Screen mesh reactor Wire mesh reactor Thermal balance Electrical grid reactor Electrical grid reactor

[14] [16] [26] [13] [15] [20] [25]

Pittsburgh bituminous coal Montana lignite, Pittsburgh bituminous Five German coals, VM: 5.9– 36.4% Five German coals, VM: 5.9– 36.4% Pittsburgh No. 9 Pittsburgh HVA, Beth-Elkhorn, Cambria, Van Cleaning British bituminous coal Illinois No. 6 Illinois No. 6, Timanstone Pittsburgh No. 8 Westerholt bituminous coal Han Qiao bituminous, Yan Quan anthracite Daw Mill coal

[24] [22] [23]

Five Australian bituminous coals Yallourn brown coal Australian HV bituminous

,10 1–37.3 1–70 1–100 1–40 1–3 1–30 2–30 1–20 1–10 10

923 –973 1189 973 753 –1223 1373 1173 1273 1273 1273 873 –1273 ,1273

1–1000 ,104 1000 1000 0.03–1 0.33 – 1000 1000 – 3500–9250

100– 150 62 100– 150 63–75 100– 2000 (5 size fraction) 400– 4000 (4 size fraction) 106– 150 106– 150 105– 150 37–53 75–106

Electrical wire mesh High-pressure entrained-flow reactor High-pressure wire mesh Screen heater reactor Pressurised TGA Pressurised dual-chamber TGA Fluidised bed Wire mesh reactor Wire mesh reactor PDTF Pressurised radiant coal flow reactor

T.F. Wall et al. / Progress in Energy and Combustion Science 28 (2002) 405–433

Table 1 Published experimental studies on coal pyrolysis at pressure

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total pressure with a fixed partial pressure of a certain reactive gas, the effect of total pressure with a fixed molar fraction of a certain reactive gas and the effect of partial pressure of a certain reactive gas at a fixed total pressure. Fundamental understanding of the effect of operating pressure on coal reactions is essential to the development of these technologies. Early reviews summarised the kinetics of coal devolatilisation [7] and the heterogeneous kinetics of coal char gasification and combustion [119]. The scope of this paper is to review the current understanding of the effect of operating pressure on a number of aspects of coal reactions, based on the open literature, as well as the recent advances at authors’ laboratory over the last few years. The present paper is in particular dedicated to recent experimental observations on pressure effects on a variety of respects of coal reactions, while limited theoretical modelling is also noted. In particular, the effect of pressure on (1) coal pyrolysis and char formation, (2) char combustion and gasification reactivity and (3) subsequent ash formation during char conversion, was reviewed. Careful characterisations of char and ash obtained from a drop tube furnace (DTF) and a pressurised drop tube furnace (PDTF) experiments on Australian black coals were reviewed. Implications have been given to the design and operation of practical high-pressure gasifiers.

2. Effect of pressure on coal devolatilisation and char formation Coal devolatilisation occurs at the early stages of combustion and gasification of coal particles, generating volatiles and resulting in porous residual chars which play a significant role in the subsequent conversion [6]. The following section reviews the effect of pressure on both volatile yield and physical structure of the resultant char. 2.1. Volatile yield 2.1.1. Experimental studies Coal pyrolysis at elevated pressures has been extensively investigated and reviewed over the last few decades [7 –26]. Several experimental techniques have been developed, and the effect of pressure on gas and tar yields has been observed for a variety of coals under a wide range of operating conditions. Table 1 summarises the published experimental studies of coal pyrolysis at high pressure. The following review falls into three categories in terms of experimental techniques. 2.1.1.1. Thermal balance. The thermal balance technique provides more precise measurements of mass loss of reacting materials. However, due to low heating rate, only a few studies on high-pressure coal pyrolysis have been published. Seebauer et al. [15] investigated the effects of

pressure, particle size and heating rate on coal pyrolysis using a thermogravimetric analysis. The pressure ranged from 1 to 40 atm, heating rate from 0.03 to 1 K/s, and particle size from 100 to 2000 mm with five size fractions. They found that the total volatile yield decreased with increasing pressure, whereas the tar yields showed even more reduction as the detected gases slightly increased with increasing pressure. They confirmed the mechanism that at higher pressures the tar evaporation is suppressed and thus more cross-linking reactions occur, although they concluded that vapour pressure equations cannot be used directly to describe the evolution of tar. The TGA experiments alone have been found insufficient to derive kinetic parameters for pyrolysis reactions, due to larger effects of the bed of particles. Recently, Sun et al. [20] studied the pyrolysis of two Chinese coals (0.4– 4 mm) under pressure (1– 13 atm) using a pressurised dual-chamber TGA with a heating rate as low as 0.33 K/s. Their results showed that at higher temperatures the total yield decreases with increasing pressure, while the total weight loss is almost independent of pressure at low temperatures (about , 873 K). The results also suggested that the volatile yield is almost independent of particle size and heating rate during pyrolysis at pressure. Arendt and van Heek [10] performed high-pressure pyrolysis for five German coals using wire mesh reactor and thermal balance, respectively, and found the similar trends with respect to pressure. 2.1.1.2. Electrical grid or wire mesh reactor. An electrical grid or wire mesh reactor has been widely applied to coal pyrolysis due to the heating rate of the facility being well controlled. Anthony and Howard [7] developed an electrical grid reactor for coal pyrolysis at pressure, and initiated the study of coal pyrolysis at pressure. Subsequent work by Suuberg et al. [8] modified the reactor and studied highpressure coal pyrolysis. The experiments were performed within a wide range of pressure, from almost vacuum to 100 atm, and with different heating rates. It has shown that increasing pressure from about 1024 to 69 atm reduced the total yield of volatiles by roughly 5 wt% for lignite and 15% for bituminous coal. Arendt and van Heek [10] performed high-pressure pyrolysis for five German coals using a wire mesh reactor and a thermobalance, respectively. They found that with increasing pressure, tar is repolymerised and cracked more significantly, resulting in increased yields of char and light hydrocarbon gases. Hydrogen has been found to influence devolatilisation significantly at increased pressures. Additional amounts of aromatic products are released by hydrogenation of the coal itself, particularly between 773 and 973 K, and the yield of light products, CH4 and C2H6, increases significantly. The studies by Griffin et al. [13] also showed that above 970 K, hydrocarbon gas yields increased with pressure from 1 to 10 atm. Particularly, the yields of the C2H4 and C3H6 increased by 60 – 75%. Niksa et al. [17] developed a wire mesh reactor, which

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allowed the operation at pressures of 1024 – 100 atm and wide heating rates of 100 – 104 K/s. The results for Pittsburgh No. 9 coal have shown a decrease in ultimate yield with increasing pressure but little or no effect on the yields during heating up or in the early isothermal reaction period. Above 20 atm, the ultimate yield became insensitive to pressure changes. Serio et al.’s work [19] showed that the relative reduction in tar yield as the pressure was increased from 3 to 13 atm was about 25%. They proposed that the effect of pressure on heat transfer must be considered, particularly for entrained flow reactors. Sundaram et al. [27] found that yields went through a maximum before declining with increasing pressure. It is likely that, at the short residence times of their experiments (0.6– 1.9 s at 1173 K), the enhanced heat transfer due to increased gas pressure was more beneficial than the detrimental effects on mass transfer. Bautista et al. [9] studied high-pressure pyrolysis for four different coals using a electric grid reactor. They found that the weight loss of Pittsburgh coal decreases rapidly with increasing pressures of helium and hydrogen to an apparent limiting value at 10 atm. The decrease in weight loss with increasing pressure arises from diminishing tar yields only slightly compensated by increasing gas yields. The tar yields are identical in the inert and reducing atmospheres, so consistently higher gaseous yields under hydrogen resulted in the increased weight loss. The yields of CH4, C2H6, and other saturated hydrocarbons are higher in hydrogen than in helium. In contrast, the yields of unsaturated hydrocarbons are only slightly higher in hydrogen, and those of CO and CO2 are virtually identical at corresponding pressures. Griffin et al. [13] studied effects of pressure (1 – 10 atm) and temperature (750 – 1230 K) on pyrolysis of pulverised Pittsburgh No. 8 bituminous coal under a helium atmosphere, using an electrical screen heater reactor. They found that volatile yields decreased slightly with increasing pressure, which was more pronounced at higher temperatures. Below 970 K, pressure had little effect on yields. This is similar to the results obtained by Anthony and Howard [7]. Their results also indicated that increased pressure reduced the average molecular weight of tar, and shown a similar observations for pressure effects on the molecular weight of tar with the work of Unger and Suuberg [28] and Oh et al. [18]. Their work showed similar observations for both temperature and pressure effects on the molecular weight of tar. The decrease in number-average molecular weight caused by increased pressure was most significant at high temperatures. Cai et al.’s recent work [11] in wire mesh pyrolysis reactor with a H2 pressure up to 70 atm showed that total volatile yields increased with increasing H2 pressure while tar yields decreased and the degree of reduction in tar yields became less with increasing pressure. The results also showed that the influence of pressure on volatiles yields and tar yields became more significant at higher temperature. It is worth mentioning that coal particle temperature is

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difficult to measure in a wire mesh reactor at high heating rates. Mill [29] recently developed a high-pressure wire mesh reactor for high heating rate coal pyrolysis, and has found that there was an upper heating rate at which coal particles could be uniformly heated using such a reactor. Care has also been taken when agglomeration of coal samples occurs in a wire mesh reactor [30]. 2.1.1.3. Entrained flow reactor. Entrained flow reactors are well suited to studies of pressure effects on devolatilisation and are designed to simulate important aspects of coal gasification system. However, one must consider the effect of pressure on heat transfer and consequently the particle temperature history as well [19]. Many of the previous studies on entrained-flow pyrolysis at elevated pressures have been conducted for low-rank coals [27]. Very little work has been reported on pyrolysis of caking coals at elevated pressures in entrained-flow reactors because of the experimental difficulties caused by coal particle agglomeration [16]. The effect of pressure on pyrolysis kinetics has recently been of considerable interest in several laboratories [8,14,16]. The trend of decreasing weight loss with increasing pyrolysis pressure has been observed for lignites [12] and subbituminous coals [19,27,31] pyrolysed in entrained-flow furnace. Lee et al. [16] investigated the pyrolysis behaviour of a bituminous coal under the rapid heating (103 –104 K/s) and elevated pressures (up to 3.8 atm) relevant to gasification, and described the effect of elevated applied pressure on devolatilisation mechanism and kinetics. They found that increasing pressure slowed the global release rates of volatiles, lowered the asymptotic volatiles yields and promoted secondary reactions of the volatiles which reduced the tar yield and changed the gas yields. The results obtained also indicated a complex variation on devolatilisation and swelling behaviour of a softening coal with applied pressure. The pressure effect on structure and reactivity of a rapidly pyrolysed caking bituminous coal has been investigated by Lee et al. [32]. At 6.8 atm or higher, the reduction in both devolatilisation and swelling rates suggests that bubble transport was the main mass-transfer mechanism. At atmospheric pressure the devolatilisation rate is higher but the swelling rate is lower than those observed at elevated pressures, indicating that a diffusional mechanism is predominant [33]. Fatemi et al. [12] investigated the pressure effect on devolatilisation of pulverised coal particles up to temperature and pressure of 1373 K and 68 atm, respectively, in an entrained flow reactor. The results showed that tar yield is affected by the pressure, decreasing significantly with increasing pressure up to 13.8 atm. Weight loss and gas yield decrease with increasing pressure up to 13.8 atm, and above this pressure there is no significant effect. Yeasmin et al. [22] studied the high-pressure devolatilisation of brown coal using a PDTF. The residence time of coal particles in the furnace was calculated based on the gas

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Fig. 1. Published results for total volatile yields as a function of pressure [34].

flow field. Simultaneous density and particle size change during devolatilisation were taken into account when the particle residence time was calculated. Partially devolatilised coal or char particles were collected using a collection tube which was able to move up and down to control the residence time of coal particles in the furnace. The weight loss then determined decreased with increasing pyrolysis pressure. A pressurised radiant coal flow reactor (P-RCFR) has recently been developed for studying high-pressure coal pyrolysis [23], where an inductively heated furnace allows coal particles being heated up to temperature of 1100 K, while gas remaining cool, due to different adsorptive properties of coal and gas. The residence time and temperature of coal particles can be accurately estimated using a FLUENT code, and the yields under rapid quenching can be quantitatively analysed. More studies using such a well-controlled facility are on the way. Some of the published data for total volatile yield as a function of pressure are shown in Fig. 1. 2.1.1.4. Summary. In brief, the effects of pressure on devolatilisation behaviour vary with the coal rank, gas environment and operating conditions. General trends observed from experiments can be summarised as follows: † The total volatile and tar yields decrease with increasing pressure, whereas tar yield is more distinctly dependent on the pressure [7 – 9,11 – 13, 15 – 20]. The reduction in tar and total volatile yields appears to be most significant for bituminous coals, but less pronounced for lignite [7,8]. † The effect of pressure on tar and total volatile yields appears to be more pronounced at higher temperatures [7,8,11,13,20]. † The pressure effect on the tar and total volatile

yields appears to be less pronounced at high pressures [11,12,17]. † With increasing pressure, tars shift toward lower molecular weights [13,18,28,35]. † Increasing pressure results in higher yields of gases, particularly under hydrogen atmosphere [9,10,13,15]. † Increasing pressure improves the fluidity of the coal melt and reduces char reactivity [11,16,19]. 2.1.2. Mechanisms To explain the above observations, several mechanisms have been proposed. It is generally agreed that the secondary reactions and mass transport limitations are the mechanistic bases for the influence of ambient pressure [36,37], and the major effect of pressure on the tar yields is found to be on the evaporation of tar precursors [8,19]. Suuberg [37] confirmed that evaporation is a significant mechanism for tar evolution by examining the molecular weight distributions of coal tars and coal char extracts. While the vapour pressure of tar precursors is inversely proportional to their molecular weight, higher pressure inhibits the escape of larger tar molecules that may evaporate at low pressures. This reasonably explains that the tar yield decreases and tars shift toward lower molecular weights with increasing pressure. On the other hand, the experimental observation [7,11,13,20] that the effect of pressure on the total volatiles and tar yields is pronounced at high temperature implies that secondary reactions are also contributors to the pressure influence on product yields. Moreover, increases in gas yields (particularly for CH4) have been attributed to the secondary repolymerisation of the tar and the auto-hydrogenation phenomenon at elevated pressure, where hydrogen evolved from coal back reacts to form CH4 [19]. Other possible effects of pressure may be on physical properties (such as diffusivity decreases with increasing pressure; high pressure can also improve fluidity of coal

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Fig. 2. Physical picture of coal pyrolysis proposed by Oh et al. [18].

melt and change the porous structure), which may contribute to the pressure-dependent behaviour of coal devolatilisation. However, there is little literature available on these matters. 2.1.3. Mathematical modelling Several attempts have been made to combine chemical kinetics and mass transfer to investigate the pressuredependent behaviour of devolatilisation. Such competition has been incorporated into the simple external film mass transport model by some authors [7]. Others [8,38,39] have treated the transport essentially as evaporation. Several devolatilisation models [18,37,40,41] have been incorporated more detailed mass transport with kinetics. However, due to the lack of reliable physical properties needed for characterising transport processes, most of previous models are based on empirical fitting coefficients or only give a qualitative prediction. Suuberg et al. [8] modelled the limiting transport process as surface evaporation and in later work [28] coupled evaporation with the production of a metaplast (described by a Gaussian distribution of molecular weights) and with repolymerisation. A semi-empirical tar formation correlation between maximum tar yield from rapid pyrolysis and coal type (from lignites to anthracites) and a wide range of pressure (1024 – 90 atm) was proposed by Ko et al. [21], based on 12 studies of rapid devolatilisation (100 – 1500 K/s) in screenheater reactors. Pressure effects are correlated via empirical parameters from best-fit analyses of experimental data. They proposed that the pressure has a negligible effect on tar yield above 25 atm. Oh’s model [18,42], which incorporated bubble transportation mechanisms, properly predicted the major qualitative trends in observed effects of pressure on devolatilisation yields. However, the predicted tar yields were somewhat higher than the measured values under vacuum conditions and were significantly lower than the experimental data at 69 atm [8]. Several network models [35,39,43 – 52] employ tar evaporation combined with cross-linking reaction as the major mechanism of pressure effect.

2.2. Char formation 2.2.1. Coal swelling upon heating Swelling is the most significant feature of softening coals during heating, which determines the particle size, porosity, density and reactivity of the residual char [6]. This issue becomes more complicated when considering the diversity of the behaviour of individual particles during heating due to the variation of the coal maceral constituents among the particles within the same coal [72]. Recently, the transient swelling of fuel particles had been observed in situ at both low and high heating rates [53]. Through the direct observations, it was seen to undergo significant changes of swelling and shrinking, followed by a rapid contraction, repeating until resolidification occurred [54]. These transient processes have shown that particles undergo dramatic changes in diameters, which cannot be explained by the measurements of residual chars. It has been known that the behaviour of bubbles inside the particle plays a significant role in variation of the particle size with reaction time [18]. Fig. 2 is the basic physical framework of the coal devolatilisation process proposed in Oh’s model [18]. Bubbles are formed during the early softening stage, followed by coalescence, moving towards the particle surface and rupture. These processes are determined directly by the viscosity of the liquid particle, which is a function of the mass proportion of an intermediate liquid material called metaplast, and is affected by the degree of devolatilisation, coal property and particle temperature. The bubble coalescence, movement and diameter are controlled by the balance of the vapour pressure inside the bubble and the ambient pressure. Either single bubble or multi-bubble (foam) based char particles may be formed, depending on heating rate, temperature, ambient pressure and coal property [18]. However, understandings of the influences of these factors are still poor due to the lack of in situ observing technique. A number of attempts have been made to correlate the coal swelling with coal rank [55]. It was shown that the swelling of coal exhibits a maximum for a coal of about 25– 28% VM, while the fluidity reached a maximum at about

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Fig. 3. Swelling ratio of chars generated from devolatilisation of different coals at a gas temperature of 1573 K and pressures indicated in N2, OM denotes data from optical microscopy, SEM denotes data from scanning electron microscopy.

28 – 32% VM content. Vitrinite mean reflectance can also serve as an appropriate parameter in predicting both maximum swelling parameter and characteristic temperatures when carbonised at elevated pressures. Plastic behaviour is extremely sensitive to prior modification of the overall H/C ratio of the coal. Microscopic observations and dilatometric measurements on separated macerals or maceral groups have shown that propensities for plastic behaviour reside almost exclusively in vitrinites and exinites, which have accordingly been labelled reactive constituents [56]. A few attempts have been made to understand the effect of ambient pressure on swelling of coal particle during pyrolysis [11,16,32,55,57– 59]. Lee et al. [16,33] investigated the rapid pyrolysis behaviour of a bituminous coal under the rapid heating (103 – 104 K/s) and elevated pressures (up to 38 atm) relevant to gasification, and described the effect of elevated applied pressure on devolatilisation mechanism and kinetics. A coal with almost no swelling at atmospheric pressures swelled significantly under elevated pressures. The physical properties of softening coals during pyrolysis at elevated pressures cannot be predicted from data taken at atmospheric pressure. At

elevated pressure, swelling behaviour and properties are not sensitive to heating rates [56]. However, a review by Solomon and Fletcher [6] summarised from several experimental and theoretical investigations and concluded that swelling ratio ðr=r0 Þ does not change monotonically with increasing applied pressure [6]. The swelling ratio increases when pressure increases, and reaches a maximum value, then drops again. The pressure at which this maximum occurs is independent of coal rank, and mostly between 10 and 20 atm. No data show the relationship between dilatation and pressure, and the maximum swelling ratio cannot reveal the whole transient swelling process. Similar results have been reported by Lee et al. [32] for Illinois No. 6 coal under two heating rates in the atmosphere of H2 and He. A recent experiment was conducted by examining coal swelling at high pressures [60]. The swelling index was obtained by careful image characterisation of chars obtained from a PDTF within a pressure range 1– 15 atm, as shown in Fig. 3. It is shown that pressure affects the swelling behaviour significantly even though the crucible swelling indices for the four coals (either 1 or 0) are low. Furthermore, this pressure effect varies with coal type. For coals A and B, the swelling indices increase with increasing pressure. For coals C and D, the swelling indices initially increase with pressure and then reach a peak value. Any further increase in pressure decreases the extent of swelling observed. For comparison, the conditions and swelling data of Lee et al.’s work [16] are also shown in Fig. 3. The observation of the swelling behaviour for coals C and D [61] indicate the same trend as seen for Lee et al.’s work. It is also shown that the pressure effect on swelling depends on other parameter such as coal type, which is consistent with Khan and Jenkins [55]. Table 2 shows the comparison of experimental conditions between Lee et al. [16] and Wu et al. [61] and Benfell’s [62] work. Modelling thermal-plastic behaviours of coal particles is thought to be extremely difficult because of the severe uncertainties of the physical properties of coals upon heating. A few attempts have been made to model the swelling of coal particles. Solomon and Hamblen [63] developed a first single bubble model, where the swelling

Table 2 Comparison of the two experimental investigations performed by Lee et al., Wu et al. and Benfell Lee et al. [16]

Wu et al. [61] and Benfell [62]

Coal

Illinois No. 6 bituminous coal

Particle size Apparatus Heating rate (K/s) Temperature (K) Pressure range (atm) Atmosphere

200 £ 270 mesh (62 mm mean particle diameter) HEF (high-pressure entrained flow furnace) ,104 1189 1–38 N2

Four Australian coals, ranging from subbituminous to semi-anthracite 63–90 mm PDTF ,104 1573 1–15 N2 (with the amount of air fed to burn 75% of the released volatile matter)

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413

Table 3 Char classification system [60,72] Char groups

Group I

Group II

Group III

.70 ,5 Spherical subspherical .1.3 0.1–0.5

Variable, 40–70 .5 Subspherical ,1.0 0.1–0.5

,40 .5 Angular ,0.9 1.0

Two-dimensional schematic representation

Porosity (%) Average wall thickness (mm) Shape Typical swelling ratio Typical residual mass ratio

was assumed to be due to the pressure of trapped evolved gases against viscosity forces. Following this single bubble, a multi-bubble model was proposed by Oh et al. [18], which most completely described the bubble behaviours, including the bubble growth, coalescence, and transport to the surface. A mostly recent model [64] has been developed for the transient swelling of coal particles on the basis of a single bubble and a porous shell assumptions, and volatile release was predicted using the advanced chemical percolation devolatilisation model [51]. The formation, growth and rupture of the bubble, and the swelling and shrinking of the particle were modelled in detail, however, these processes have not been justified due to the lack of experimental observations. The predictions were compared with the recent experimental data available in literature. This model is only suitable to those coals tending to form cenospherical chars. 2.2.2. Char morphological study Bailey et al. [65,66] have detailed a morphological study of char. Over the last decade, the work has been extended extensively [67– 70]. Very little work on char morphology at pressure has been published until recent comprehensive studies [60,71,72]. In these works, chars were collected from devolatilisation of several Australian bituminous coals from DTF and PDTF, and were examined under optical microscope. Char morphology was analysed using a computer-based semi-automated image analysis system

[72]. Classification of chars according to geometric parameters and porosity has been previously described [65,71]. Table 3 summarises the scheme used to assign chars into different groups in this work. The typical SEM crosssectional images for the three groups of char particles are shown in Fig. 4 [60]. Typically, Group I particles have a very porous structure, with large voids inside the particle and a thin wall, Group II particles have a medium porosity and wall thickness, while Group III char particles have low porosity. 2.2.2.1. Maceral influences on char morphology. Char morphology is generally a function of parent coal rank, parent coal petrography and process conditions [65,71,72]. Fig. 5 shows the percentage of Groups I, II and III chars from four bituminous coals arranged in order of increasing parent coal vitrinite content, at 5, 10 and 15 atm furnace pressure, respectively [72]. The morphology of the chars shows a strong relationship with increasing vitrinite content. 2.2.2.2. Influences of pressure on char morphology. In general, as furnace pressure increases, the overall proportion of Group I chars formed increases, while the proportions of Groups II and III chars decrease (Fig. 5). Fig. 6 highlights this trend for coal CI. Since this coal only has 27.8% vitrinite, but produces 38, 52 and 72% Group I chars at 5, 10 and 15 atm pressure, inertinite must also contribute to formation of Group I chars. Inertinite is clearly capable of

Fig. 4. SEM images of three typical char particles shown in cross-section [60]. The scale bar is 100 mm.

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Fig. 5. Percentages of Groups I –III types in A, B, CI and CV chars prepared at 1573 K and (a) 5 atm (b) 10 atm and (c) 15 atm, in order of increasing vitrinite content [72].

displaying high fusibility similar to vitrinite, under highpressure conditions. Mean diameter and porosity of coal CV increase from 5 to 10 atm, then decrease from 10 to 15 atm, probably due to char fragmentation. Microscopic observation of this char showed that the Group I particles had thinner walls and a more spherical structure than those of lower pressure chars, factors which make this sample more susceptible to fragmentation within the furnace and during handling. The following correlation has been generated from the char morphological study presented earlier, which has been applied to the mathematical modelling of a pressurised entrained flow gasifier [73] and ash formation [74] nGrpI ¼ ð0:6Pt Þ þ ð0:53vitrÞ þ 37

ð1Þ

where nGrpI is the number percentage of Group I char particles, Pt is the total pyrolysis pressure (atm), and vitr is the volume percentage of vitrinite content of coal. 2.2.3. Effect of pyrolysis pressure on char surface area The internal surface area of char is one of the significant parameters used for interpreting char reaction rates, particularly under gasification conditions. Little literature

Fig. 6. Percentages of Groups I –III for CI char prepared at 1573 K and 5, 10 and 15 atm furnace pressures [72].

data have been presented for the surface area of chars produced under high-pressure conditions. Lee et al. [32] investigated the development of CO2 surface area of char as functions of weight loss and pyrolysis pressure for Illinois No. 6 bituminous coal under a low heating rate condition. The surface area of char is generally lower under higher pressure pyrolysis conditions. The development of surface area is relevant to the thermal-plastic property of coal during heating. A recent study presented the CO2 surface area for chars produced from Australian bituminous coals in a PDTF at various pressures [75]. Generally, the surface areas for chars produced at higher pressures are lower. The effect of pressure on char surface area is believed to be related to the fluid behavior during devolatilisation [16,55]. 2.3. Reactivity of char produced under high-pyrolysis pressures The pressure at which the parent coal is devolatilised also plays an important role in the reactivity of the resulting char. Sha et al. [76] noted a significant decrease in the reactivity of the char as the pyrolysis pressure was increased, postulating effects on the pore structure as the reason. In the review by van Heek and Muhlen [77], it was noted that the steam reactivity for chars is not affected by pressure if the pyrolysis is performed under inert conditions. Under a hydrogen atmosphere, however, increased pressure resulted in a decrease in the steam reactivity of the resulting char. Two reasons for this are postulated, one being that compression of the pyrolysis products fails to open the structure of the char formed, and the other being an indirect temperature effect. Such a temperature effect could arise from the exothermic hydrogasification reaction that may be occurring on the surface of the charring particle, following or simultaneous to the hydropyrolysis step. Cai et al. [11] also found combustion reactivities (calculated as maximum rate observed during the conversion profile at 773 K) to decrease with hydropyrolysis pressures up to 40 atm and increase at pressures above this.

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415

from 1 to 15 atm. Comparisons of the initial apparent and intrinsic rates of vitrinite- and inertinite-rich char samples from the same coal were given [72]. The inertinite-rich sample has a faster apparent rate than the vitrinite-rich sample when made at 5 atm, but a slower apparent rate when made at 15 atm. However, there is less variation between intrinsic reaction rates across maceral-concentrated samples. This suggests that the pyrolysis pressure significantly influences the physical structure of coal chars, as discussed earlier, but has little effect on chemical structure of char which determines the intrinsic reactivity to a larger extent. The conclusion has a significant implication for interpreting char reaction rates and mathematical modelling of char burnout [72]. 2.4. Summary on pressure effects on volatiles yield and char structure

Fig. 7. The effect of pyrolysis pressure on reactivity of an Australian bituminous coal char. (a) Apparent reaction rate and (b) intrinsic reaction rate. The reaction rate was measured using a pressurised thermogravimetric analyser at 10 atm [78].

The eventual increase in reactivity was the result of some char conversion by H2 at the higher pressures exposing a fresh and enlarged carbon surface. Lee et al. [32] investigated the structure and reactivity of Illinois No. 6 coal char following pyrolysis at elevated pressures. They found that increasing the pyrolysis pressure slowed the rate of release of volatiles, increased the amount of char remaining after pyrolysis and altered the composition of the volatile products. Their data also demonstrated how pressure hinders the development of the mesopore system that develops after the coal passes through the plastic phase of pyrolysis. The increased fluidity that resulted from higher-pressure pyrolysis led to enhanced ordering of carbon layers and the subsequent loss of gasification reactivity of the char residue. A recent advance in this area has been reported by Roberts [78], who measured the apparent and intrinsic gasification rates of an Australian coal char made in a PDTF under various pressures, as shown in Fig. 7. Char apparent rates vary significantly with pyrolysis pressures, whereas the intrinsic rates, which were obtained by normalising the apparent rates by internal surface area, are almost independent of pyrolysis pressure over a pressure range

† In summary, the total volatile and tar yields decrease with increasing pyrolysis pressure, whereas the gas yield increases with pressure. This effect is pronounced at high temperatures, and decreases with increasing pressure. The mechanisms for this pressure effect are due to the enhanced secondary reactions and mass transportation at high pressure. Several models are being developed to interpret this effect. † The coal swelling increases with pressure from atmospheric to 5 – 10 atm; further increase may reduce the swelling, which is dependent on coal type. Coal swelling is strongly associated with complex bubble behaviour within a melting coal upon heating. † Pyrolysis pressure has significant effect on char morphology. More porous char particles are formed at elevated pressures. The number percentage of the highly porous char particles has been well correlated to the vitrinite content in parent coal and pyrolysis pressure. † Chars produced at different pyrolysis pressures have different apparent reaction rate, but have the similar intrinsic rates. It is suggested that the chemical structure in which the intrinsic reactivity is dependent is similar between this chars, while the different structures contribute to the variation between apparent rates, and also rates observed under practical conditions.

3. Effect of reactant pressure on char combustion and gasification rates 3.1. High pressure char combustion kinetics 3.1.1. Experimental measurements on char combustion rates at pressure A number of investigations have been published on the effect of oxidant pressure on the combustion reaction rate of coal over the last two decades, in which both oxygen partial

416

Table 4 Published experimental studies on coal char combustion and gasification reactivity at pressure Samples

Reactant gas(es)

Pressure (atm)

Temperature (K)

Particle diameter (mm)

Experimental methods

[90] [91,92] [93]

Spanish lignite Gottelborn coal Blind Canyon (UT) bituminous, Beulah-Zap (ND) lignite Coconut, pure graphite, Yallourn brown char Jarrah char Jarrah char North Dakota Lignite, Wyoming subbituminous, Illinois No. 6 HV bituminous Western Kentucky bituminous char Most, Sokolov-Czech brown char Brown, lignite, HV bituminous coals Lignite, HV bituminous, anthracite Westerholt coke, Pitch coke, Oxicoke, Active coke Pittsburgh and Illinois No. 6 bituminous coal chars, carbon black Holinhe, Fuxin, Guande char Char Daw Mill coal Blair Athol bituminous coal char Anthracite char Westholt char Daw Mill Daw Mill

CO2 O2 O2

225 0–8 1.01, 5.07

1073–1273 1500–3000 1200–1400

100– 500 5 5.5– 8 mm

High-pressure fluidised bed reactor Shock tube High-pressure drop tube reactor

H2

240

923–1143

–

High-pressure reactor

H2 O CO2 O2, H2O, CO2

240 240 5.57–10.1

1023–1103 1063–1143 –

1200–2400 1200–2400 45.4–50.5

High-pressure reactor High-pressure reactor Entrained coal gasifier

H2 O H2 O CO2, H2O

4–28 2–16 25 (CO2), 1 –25 (H2O)

1198–1311 1073–1223 1073, 1173

335– 850 – 2000–3000

High-pressure thermal balance reactor Differential flow reactor Pressurised fluidised bed

O2 (3–30%) H2O, CO2

2–10 1–40

1300–2800 1173

75– 180 1000–3000

Pressurised entrained flow reactor Pressurised TGA

O2

5.5–10

1700–2200

13, 24.8, 15.5 (mass)

Shock tube

H2 O CO2, H2O, H2 CO2 O2

1–24.5 19.6 1–30 ,20

1023–1273 1073–1223 1123, 1273 559–1273

180– 250 250– 420 105– 150 250– 500

Packed-bed balance reactor Packed-bed balance reactor Wire-mesh reactor, hot-rod reactor Pressurised fixed bed reactor

H2 O O2 CO2 CO2

215 2–14 1–30 1–30

1141–1369 773–923 1273 1273

450– 1000 90– 106 106– 150 106– 150

CO2, H2O CO2, H2O, H2, CO O2

1–30 215 1–15

1273 1023–1223 1400–2100

106– 150 – 40, 70

High-pressure tube reactor Pressurised fixed bed High-pressure fluidised bed reactor High-pressure fluidised bed, fixed bed, wire-mesh reactor High-pressure wire mesh reactor Pressurised TGA High-pressure drop tube reactor

CO2, H2O, H2 H2 CO2

270 1–40 0.2–25

1173 1273 1123

– – 500– 590

Pressurised TGA Pressurised TGA High-pressure tube reactor

O2

0.05–13

1700–4000

0.0045

Shock tube

[94] [95] [96] [84]

[97] [98] [99] [81] [100] [85]

[101] [102] [103] [104] [105] [83] [106] [106] [107] [108] [80] [109] [110] [111] [112]

Daw Mill non-caking Peat char Utah and Pittsburgh bituminous coal chars German bituminous char German bituminous, Westerholt Yallourn, Baiduri, Taiheiyo, Hongei char Soot

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References

Pressurised TGA Pressurised combustor 40 £ 100 mesh 300– 700 H2 O2

27 1.1–17

1253 1073–1173

– Fixed bed reactor – 420– 840 943–1203 1173 1–24 0.12–3.1 O2 CO2, H2O, H2

reactor TGA TGA entrained reactor Pressurised Pressurised Pressurised Pressurised 1–200 100– 150 600– 1000 140– 180 733–842 723–1023 773–1213 1600–2100 1–64 1–21 1–30 0–8

[115] [79]

[114] [76]

Crystalline graphite Gardanne, Westerholt, Arcadia Australian black coal chars Polish bituminous and Niederberg anthracite coal Bituminous, anthracite Xiao Long Tan lignite char Lignite, subbituminous, anthracite Lignite, coke, etc. [113] [88] [89] [82]

O2 O2 O2, CO2, H2O CO2, O2

Temperature (K) Samples References

Table 4 (continued)

Reactant gas(es)

Pressure (atm)

Particle diameter (mm)

Experimental methods

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pressure and total pressure have been studied [79 – 88]. These studies are summarised in Table 4. Turnbull et al. [79] measured the combustion rate of large carbon particles using a fluidised bed at pressures up to 17 atm and bed temperatures of 1023 –1173 K. Coke and chars derived from coals of different ranks were used in the experiments. The combustion rates were observed to increase with increasing pressure. At low pressures the chemical reaction rate controlled the overall reaction rate. With an increase in pressure, the reaction approached pore diffusion controlled conditions. Cope et al. [84] investigated the pressure and coal rank effects on char combustion. They concluded that the effects of operating pressure on carbon burnout depend on the coal rank. Coal burnout was observed to increase with increasing pressure and decreasing coal rank. They also indicated that coal burnout increases as the O2/coal ratio is increased. This effect was observed on coal rank due to the inherent oxygen content in char. Richard et al. [88] separated the effects of total pressure and partial pressure of oxygen on the combustion rates for three different chars using a thermobalance. The system was operated at total pressures up to 21 atm and temperatures of 723–1023 K. At a fixed total pressure, the combustion rate increased with the partial pressure of oxygen. However, at a constant partial pressure of oxygen, it dramatically decreased with an increase in total pressure from 1 to 5 atm. This decrease levelled off with further increases in the total pressure. Similar trends were observed for all three char types. Roberts and Harris [89] measured the apparent and intrinsic reaction rate of Australian bituminous coal chars with O2 at increased pressures up to 30 atm using a pressurised TGA. It was found that the reaction order for apparent rate is almost unity and remains unchanged over the pressure range investigated, and the nth equation is applicable to the intrinsic rates with a constant reaction order. Monson et al. [80] investigated high-pressure and hightemperature char oxidation using a high-pressure controlledtemperature profile drop-tube reactor. The measurements were performed for Utah and Pittsburgh coal chars at a reactor temperature between 1000 and 1500 K and total pressures of 1, 5, 10 and 15 atm. The results showed that at constant oxygen mole fractions (0.05, 0.10 and 0.21), increasing the total pressure from 1 to 5 atm led to a slight increase in the reaction rate, with the rate decreasing with further increases in pressure from 5 to 15 atm, as shown in Fig. 8. Fig. 9 shows the measured char particle temperature as a function of total pressure and O2 partial pressure. To explain the effect of oxygen partial pressure and total pressure on the combustion rate, Monson et al. [80] used a global char oxidation model. As a result the calculated apparent rate coefficients showed significant pressure dependence, since both the activation energy and frequency factor decreased with increasing pressure. It has been

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Fig. 8. The effect of total pressure and O2 mole fraction on char combustion rate at high temperatures [80].

concluded that low-pressure kinetic parameters cannot be accurately extrapolated to elevated pressures. This suggests that the empirical nth order rate equation is not valid over a wide pressure range. Lester et al. [85] also investigated the influence of oxygen partial pressure and total pressure on the surface oxidation rate of bituminous coal. Experiments were performed using a shock tube reactor over a particle temperature range of 1700– 2200 K, with a total pressure range of 5.5– 10 atm and oxygen mole fractions of 0.1 – 0.5. It was observed that as the total pressure increased from 5.5 to 10 atm, the oxidation rates for chars in air decreased, which is consistent with the conclusions drawn by Monson et al. [80]. Lester et al. [85] explained the reducing rate in terms of the decreasing pore area available for reaction with oxygen with increasing total pressure. It was found that the total surface area of larger mm-sized particles exposed to oxygen was approximately 10 times the external surface at a total pressure of 5.5 atm and about six times the external

surface area at a total pressure of 10 atm. A similar effect was observed by Bateman et al. [93] using a high-pressure reactor in a temperature range of 900–1200 K. It was observed that increasing the total pressure from 1 to 5 atm reduced the oxidation time by about one-quarter. Further increases in the pressure did not result in further reductions in burnout time. Recently, Joutsenoja et al. [81] and Saastamoinen et al. [82] measured the burnout (90%) time for different coals under pressurised combustion conditions. A common trend was observed for all coal chars studied. At a fixed PO2 the burnout time increased with total pressure and at a fixed total pressure, it decreased with increasing PO2 : At a fixed oxygen volume fraction of 10%, however, the burnout time was observed to decrease rapidly as the total pressure increased from 2 to 5 atm and then level off with further increases in total pressure for all the coals, except the Niederberg anthracite, which showed a slight increase in burnout time with further increases in total pressure. A

Fig. 9. The effect of total pressure and O2 partial pressure on temperatures of combusting char particles [80].

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similar study on measuring char oxidation kinetics using a pressurised entrained flow reactor has recently been reported [116], where the effect of pressure on particle temperature distribution has been presented. 3.1.2. Reaction mechanism on char combustion at pressures 3.1.2.1. Reaction chemistry. A review on char oxidation mechanisms has recently been presented [117]. Two reaction mechanisms for gas – solid reactions have appeared in the open literature: one being one-step reaction and the other being two-step mechanism. The one-step reaction mechanism, consequently the derivation of the well-known nth order equation, has been widely used in interpreting the reaction rates, and is not discussed here. This empirical mechanism has recently been found to be inadequate to understand the pressure effect [80,118]. Laurendeau reviewed heterogeneous kinetics of the coal char gasification and combustion, and the dearth of the high-pressure experimental data was noted [119]. The two-step reaction mechanism, i.e. adsorption and desorption processes [119,120], has recently been increasingly acceptable for heterogeneous reactions. For example, for carbon oxidation, the following mechanism has been proposed C þ O2 $ CðOÞ

ð2Þ

CðOÞ ! CO

ð3Þ

where C(O) denotes an oxygen surface complex. At low pressures, the concentration of C(O) is low, and the reaction is controlled by adsorption reaction (2), whereas at high pressures, the C(O) concentration approaches saturation, and the reaction is controlled by desorption process (3), which is independent of O2 pressure. A Langmuir type rate equation is therefore derived RO2 ¼

k1 k2 PO2 k1 PO2 þ k2

ð4Þ

where k1 and k2 representing reaction rate constants for reactions 2 and 3 have Arrhenius form. Essenhigh [118,121] developed an alternative format of this expression. Similar methods have been used in some other recent studies [83, 87]. The adsorption – desorption mechanism perfectly explains the pressure effect on reaction rate at low temperatures. Recent reviews on carbon oxidation [117,122] have addressed that the pressure order approaches zero at high temperatures, and have argued that the current two-step format (Eq. (4)) predicts the reverse trend. Hong et al. [123] also addressed this problem. A three-step reaction mechanism has recently been proposed by Hurt and Calo [117], based on the most recent oxygen complex study for char surface oxidation [124]. This mechanism is expressed by the following

419

elemental reactions: C þ O2 $ CðOÞ CðOÞ þ O2 $ CO2 þ CðOÞ

ð5Þ ð6Þ

CðOÞ ! CO

ð7Þ

A complex rate expression has then been derived RO2 ¼

k1 k2 P2O2 þ k1 k3 PO2 k1 PO2 þ k3 =2

ð8Þ

where k1, k2 and k3 represent the reaction constants for reactions 5, 6 and 7, respectively. Discussions on these rate constants have been detailed elsewhere [117]. The expression appears to be able to interpret the existing experimental data for both temperature and pressure effects [89,104,113,117,125,126]. 3.1.2.2. Reaction regime—controlling mechanisms. The controlling mechanism is an important factor when one investigates high-temperature pressurised char combustion. Saastamoinen et al. [82] broadly discussed the mechanisms for char combustion under both high temperature and pressure conditions. He states that both heterogeneous and homogeneous reactions are enhanced by pressure and that the combustion rate appears to be controlled by the diffusion of oxygen to the particle surface. For large particles, the effect of pressure is minor [82]. High pressure combustion of fine coal particles has also been investigated using a shock tube [91,92]. The heterogeneous combustion was found to be controlled by a combination of pore diffusion and chemical reaction. Both the intrinsic reaction rate and the process of pore diffusion need consideration when conversion rates measured at high pressure are interpreted. Recently, Roberts et al. [127] demonstrated lack of any effect of total pressure on the low temperature, chemical reaction rates of chars reacting with oxidising gases. It follows from such a result that effects of total pressure on the high-temperature conversion rates of chars that are observed are likely to be due to pressure effects on diffusional processes, rather than intrinsic char reactivity. In predicting global rates of char oxidation at high pressure and high temperature, allowance for mass transfer in both pore structure and boundary layer is necessary. Essenhigh and Mescher [86] employed a second effectiveness factor in their extended rate equation involving mass transfer mechanisms. The factor was extracted from Monson et al.’s experimental data [80], and decreases with increasing total pressure; the effectiveness factors obtained by existing mass transportation theory were illbehaved [128]. Hong et al. [123] developed a correction factor for the effectiveness factor for Langmuir type rate equation. The char combustion rates obtained from their correlation overestimated the burnout measured at 15 atm by Monson et al. [80].

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Fig. 10. Schematic representation of a cenospherical char particle used in the char structural submodel.

The theoretical relationship does not correlate well the observed decrease with pressure of the combustion rate found by Monson et al., Bateman et al., and Lester et al. [80, 85,93] for pressures above 10 atm. It is possible that CO oxidation in the boundary layer plays a significant role [81, 129], which may account for a larger loss of oxygen concentration at the char surface. 3.1.3. Modelling char burnout at high pressures Very little work has been reported on modelling of coal and char burnout at pressure. Most recently, Hong et al. [123] incorporated the Langmuir rate equation with an effectiveness factor into the existing carbon burnout kinetic (CBK) model [130]. The predicted burnout of char was compared with the experimental measurements by Monson et al. [80]. The incorporation of the Langmuir rate equation accounting for pressure effect has improved the prediction of burnout. Also most recently, a char structural submodel [72,131]

was developed to account for the effect of pressure on the char physical structure, hence the char burnout, and was incorporated into the CBK model. In this char structural submodel (the details of which are described elsewhere [18]), two groups of char particles named cenospherical and dense chars are considered. The number percentages for cenospherical chars formed at different pressures can be approximated using the correlation generated from the previous char morphological study [72]. Fig. 10 shows the schematic representation of a cenospherical char particle. Fig. 11 shows the experimental measurements of char burnout at a gas temperature of 1573 K and pressures of 1 and 15 atm, with a comparison of model prediction. The char formed at 15 atm has a higher percentage of reactive Group I particles (70%) than that formed at 1 atm (40%). The chars made at 15 atm burn more rapidly than those made at 1 atm during early and middle stages of combustion (burnout less than 80%). Cenospherical char burns more quickly than dense char, due to their smaller mass, high internal and external surface area, and char fragmentation. A reasonable agreement between experimental data and prediction, as shown in Fig. 11, has indicated that the integration of such a submodel developed in Ref. [72] improves the predictability of char burnout. 3.2. High-pressure char gasification kinetics with CO2, H2O and H2 3.2.1. Experimental measurements on char gasification with CO2, H2O and H2 High-pressure char gasification with CO2 and H2O has been studied for a wide range of coal char types at temperatures between 973 and 1373 K using various apparatuses [25,76,90,94 – 99,101 – 103,105 – 111]. These studies are also included in Table 4. Studies of pure gas (H2O, CO2) gasification using various carbon materials have been previously conducted

Fig. 11. Comparison of burnout of a bituminous coal burning in air at a gas temperature of 1573 K and pressures of 1 and 15 atm [72].

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by Blackwood and co-workers [95,96]. In these studies the pressure was increased up to 50 atm and the temperature ranged from 1023 to 1103 K. The reactivity of carbon was observed to increase with an increase in the pure gas pressure. The reaction rate for steam gasification was observed to be higher than that for CO2. The pressure dependence of the gasification reactions for CO2 and steam was also investigated by Mu¨hlen et al. [109] using a thermobalance at pressures up to 70 atm. The results showed a linear increase in reaction rate at low pressures for all gasifying reactant gases. This increase levelled off with further increases in pressure. The H2O gasification rate was observed to be several times higher than that for CO2 gasification. The inhibiting effect of the product gases, H2 and CO, was observed for both steam and CO2 gasification [95,96,108]. Sha et al. [76] measured the gasification rates of seven chars prepared from coals varying in rank from lignite to bituminous, using a pressurised thermobalance. The gasification rates were observed to increase with increasing pressure up to 10 atm and level off with further increases in pressure. A similar trend was observed for both CO2 and H2O gasification. The H2O gasification rate was inhibited more by CO than by H2, which is consistent with the results obtained by Mu¨hlen et al. [109]. Goyal et al. [97] studied the gasification kinetics of chars, prepared from Western Kentucky No. 9 bituminous coal, in steam, steam-hydrogen and synthesis gas mixtures in a temperature range of 1198–1311 K and a pressure range of 4 – 28 atm. The rate for the char – steam reaction was observed to be the highest. The inhibiting effect of H2 on char –steam reaction was observed. The total pressure had a negligible effect. Li and Sun [102] reported the gasification reactivity for a lignite char with CO2, H2O and H2 using a packed bed balance reactor at an elevated pressure of 19.6 atm. Carbon conversion was observed to increase with an increase in the partial pressure of CO2, H2O and H2. The rate equations were developed using a shrinking core model. Ma et al. [105] studied the char – steam gasification kinetics for Jincheng anthracite coal char using a fixed bed reactor over a temperature range of 1141– 1373 K and a pressure range 1 –14.2 atm were used in the investigation. The reaction rate was observed to increase with increases in steam partial pressure. In contrast to this observation, Moilanen and Mu¨hlen [108] reported that for peat char the gasification rates decreased with increasing pressure in both steam and carbon dioxide. It was concluded that the presence of H2 and CO inhibited the gasification of a peat char. The pressure effect on the gasification rate may vary with the coal char rank. Li and Xiao [101] measured the steam gasification rate for three Chinese coal chars and found that the reactivity decreased as the coal rank increased. Moilanen and Mu¨hlen [108] measured the steam and carbon dioxide gasification rates of a peat char using a pressurised

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thermobalance at temperatures between 1023 and 1233 K and pressures up to 15 atm. Both the gasification rates of H2O and CO2 dramatically decreased with increasing pressure. Hill and Fott [98] studied the steam gasification kinetics for Czech brown coals of different rank using a differential flow reactor at temperatures of 1073– 1223 K and pressures of 2 – 16 atm. At a fixed H2/H2O ratio, the reaction rate appeared to decrease with total pressure at the temperature investigated, which may be due to increased inhibiting effect of H2. Pressurised coal gasification has recently been examined by the Imperial College research group using different experimental facilities [106,107,132 –134]. A high-pressure fluidised bed reactor [25] and a high-pressure wire-mesh reactor [134] have been used for measuring the char gasification reactivity of a UK (Daw Mill) coal. The gasification reactivities for both the untreated Daw Mill coal and coal char prepared in situ in a fluidised bed reactor (high heating rate to 1000 K/s with a 60 s holding time in a CO2 atmosphere) were observed to be similar to those obtained using a wire-mesh reactor (heating rate 1000 K/s to 1273 K with a 60 s holding time in CO2 atmosphere) and were about one order of magnitude higher than those obtained using a hot-rod reactor (low heating rate 10 K/s to 1273 K with a 60 s holding time in CO2 atmosphere). The reaction rate was also observed to increase with an increase in pressure from 1 to 30 atm [25,106]. Megaritis et al. [106] explained that the low reactivity obtained using the hot-rod reactor was due to secondary char formation as well as poor gas contact with char surface. Figs. 12 and 13 summarise some of the published data for CO2 and H2O partial pressure effect on gasification rates for a variety of char types. It has shown that the apparent rates vary by approximately two orders of magnitude among different char types, and the effect of char type is more pronounced than that of oxidant partial pressure. The larger variation of apparent rates between char types is probably due to differences in char surface area [135], chemical structure and mineral catalytic effect [136]. The variation of intrinsic rates with coal rank, which were obtained by normalisation by the char surface area, is found to be reduced to one order of magnitude [137]. Very little has been reported on pressure effect of H2 gasification in the open literature because of its lower reaction rates compared to CO2 and H2O gasification [4]. Blackwood [94] studied the H2 gasification reaction for a number of different carbon materials at pressures up to 40 atm and in the temperature range 923– 1143 K. Results indicated that the rate of methane formation was proportional to the hydrogen pressure, hence a first order Arrhenius type rate equation was proposed. The reactivity of the carbon has been associated with the number of active sites in which the oxygen in the carbon plays a significant role. Tomita et al. [115] measured the reaction rate for hydrogen gasification for PSOC-91 char at pressures from 7 to 28 atm using TGA and found that the reaction rate was

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Fig. 12. Effect of CO2 partial pressure on apparent reaction rate (s21) of char gasification. Particle temperatures were adjusted to 1123 K using original rate expressions. (S) subbituminous char [111]; (A) subbituminous char [111]; (K) subbituminous char [111]; (B) anthracite char [111]; ( p ) lignite char [76]; (W) purified carbon [96]; (þ ) bituminous char [109]; ( –) lignite char [102]; (O) bituminous char [138]; ( £ ) lignite char [90].

directly proportional to the H2 pressure. Li and Sun [102] also measured the high-pressure hydrogen gasification rate for a char and used a volume reaction model to produce the rate parameters. The pressure order obtained for hydrogen gasification was 0.5.

3.2.2. Reaction mechanisms of char gasification at pressure Mechanisms for both steam and carbon dioxide gasification reactions have been proposed using sets of elemental reactions [95,96,111,120,139]. For example, the adsorption– desorption elemental reactions have been proposed for CO2

gasification at low pressures [120,139] C þ CO2 $ CðOÞ þ CO CðOÞ ! CO

ð9Þ ð10Þ

where C(O) is the oxygen surface complex. Similar to char oxidation at low pressures, increasing the pressure of CO2 results in higher concentration of surface complex on the carbon surface and a higher reaction rate is observed. At high pressures, the formation of carbon dioxide has been observed; the following reaction mechanism was

Fig. 13. Effect of H2O partial pressure on apparent reaction rate of char gasification. Particle temperatures were adjusted to 1123 K using original rate expressions, (S) lignite char [102]; (A) lignite char [76]; (K) char [96]; ( p ) brown char [98]; (W) anthracite, [105]; ( £ ) bituminous char [109]; ( –) brown char [98]; (þ) bituminous char [97].

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then proposed [96]: C þ CO2 $ CO þ CðOÞ

ð11Þ

C þ CðOÞ ! CO þ C

ð12Þ

CO þ C $ CðCOÞ

ð13Þ

CO2 þ CðCOÞ ! 2CO þ CðOÞ

ð14Þ

CO þ CðCOÞ ! CO2 þ 2C

ð15Þ

With increasing CO2 pressure, the concentrations of oxygen complexes C(O) and C(CO) on the carbon surface approach unity, i.e. C(O) and C(CO) saturation; then pressure effect becomes insignificant. Therefore, the complex Langmuir – Hinshelwood type expression was derived [96]: RCO2 ¼

k1 PCO2 þ k4 P2CO2 1 þ k2 PCO2 þ k3 PCO

ð16Þ

Similarly, the char – H2O gasification reaction at high pressures can be explained using following reactions [95,109] H2 O þ C $ CðOHÞ þ CðHÞ

ð17Þ

CðOHÞ þ CðHÞ ! CðOÞ þ CðH2 Þ

ð18Þ

CðOÞ ! CO

ð19Þ

CðH2 Þ þ H2 O þ C ! CH4 þ CðOÞ

ð20Þ

and the rate equation can be derived: RH2 O

k5 PH2 O þ k8 PH2 O PH2 þ k9 P2H2 O ¼ 1 þ k2 PH2 O þ k3 PH2

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Compared to the nth rate equation, the Langmuir– Hinshelwood type equation: † does not involve the pressure order which is uncertain for the nth order equation; † is derived from reaction mechanism, whereas the nth order equation is empirical; † accounts for the inhibiting effect of H2 and CO, which are considerably present at high pressures. The Langmuir – Hinshelwood type rate equation has been used in the modelling of char combustion kinetics at high pressures [117,118,123]. 3.2.3. Modelling of char gasification rate at high pressure and high temperature Very little has been published on modelling hightemperature gasification rates in an entrained flow system. Liu et al. [137] have developed a mathematical method for extrapolating low temperature rates to high temperatures. A Langmuir– Hinshelwood type rate equation was used to account for pressure effects, and used to derive an effectiveness factor to allow for pore diffusion at high temperatures. The complicated porous structure was taken into account in calculating the effective diffusivity. 3.3. Summary on pressure effect on char reactivity

ð21Þ

In these reactions, methane is formed as well as carbon monoxide. As can be seen, the derived equations for char oxidation (Eq. (8)) and char gasification with CO2 (Eq. (16)) and H2O (Eq. (21)) are very similar in format, suggesting a unified reaction mechanism.

Table 5 shows the summary of the effect of pressure on coal reactions. The following conclusions are drawn. † Generally, the char combustion and gasification reactivity increases with increasing reactant pressure. The magnitude of effect becomes independent of pressure at elevated pressures. Such an effect can be explained by an adsoprtion – desorption reaction mechanism, which is

Table 5 Summary of the effect of pressures on various aspects in relation to coal reactions Coal chemical or physical process

The effect of pressure on the process

References

Char combustion Char combustion CO/CO2 ratio Char temperature Char gasification Pyrolysis volatile yield Char reactivity Swelling property Average char porosity Initial char surface area Heat transfer Homogeneous reaction Bulk diffusivity Knudson diffusivity

Rate " with increasing O2 partial pressure at a fixed total pressure Rate first " and then # with increasing total pressure at a fixed O2 mole fraction Rate # with increasing O2 partial pressure " with increasing O2 partial pressure at a fixed total pressure Rate " with increasing reactant gas pressure # with increasing total pressure # with increasing pyrolysis pressure First " , then # with increasing pyrolysis pressure " with increasing total pyrolysis pressure # with increasing total pyrolysis pressure No effect of total pressure (,20 atm) on gas conductivity Rate " with increasing total pressure # with increasing total pressure No effect of total pressure

[80,85] [80,85] [140] [82] [95,96,109] [7,8,17] [11,32,78] [16,55,60] [59,69] [32,75] [141] [82] [142] [142]

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composed of several elementary reaction steps, and a saturation of the reacting surface at sufficiently high pressures. † Char oxidation rate at elevated temperatures increases with an increase in total pressure from atmospheric pressure to 10 atm. Futher increasing pressure reduces the rate. † Char gasification rate with H2O is higher than that of char – CO2 gasification. The inhibiting effect of H2 and CO are considerably higher at elevated pressures. † More work is needed on measuring char gasification rates at high pressures and temperatures, and on developing mathematical models to predict the rates.

4. Effect of pressure on ash formation 4.1. Effect of pressure on ash formation in relation to char burnout Ash formation is strongly associated with char fragmentation behaviour [143,144] and included mineral coalescence [145] during combustion. However, very little work has been published in relation to ash formation at pressures. The following review summaries the recently published advances at authors’ laboratory [147,148], through careful characterisation of char particles obtained from DTF and PDTF during conversion using a microscopic technique. 4.1.1. Fragmentation of porous char during combustion Char fragmentation has been found to play a significant role in ash formation [143,144]. Such a mechanism has been well verified in the literature [143,144]. Frequent fragmentation reveals a reduced coalescence of included minerals during combustion. The previous char fragmentation studies [143,146] have also shown that the fragmentation is strongly associated with the porous char structure. Highly porous char tends to fragment frequently. A more recent study [147] on char fragmentation of an Australian bituminous coal indicated that fragmentation of char begins below an overall coal burnout level of 54%, and becomes violent at burnout levels between 54 and 70%, due to significant number of cenospherical char particles in the bulk samples. Fig. 14 shows the SEM images of char samples generated at various burnout levels at a gas temperature of 1573 K in a DTF under atmospheric condition. Percolation theory [146] has been applied to the fragmentation study, and it has shown that the fragmentation of Group I char particles appears to be associated with the macropores in the particle shell, which provide weak points from which a fragment can detach. Macropores in the shell are formed during both devolatilisation and combustion. The conventional percolation model can explain Group I char fragmentation by

incorporating a non-uniform pore structure [147]. Violent char fragmentation plays an important role in char burnout and ash formation, as discussed in the following sections. It is suggested that at high pressures, char fragmentation will be more violent, as more cenospherical char particles are formed at the completion of the high-pressure pyrolysis [60,72]. 4.1.2. Included mineral coalescence and ash liberation during char burnout A recent experimental study has reported the relationship between char structure and ash formation [148]. Particle size distribution (PSD) for liberated ash in the combustion residues was obtained through detailed image analysis, and is presented in Fig. 15 [148]. In Fig. 15, the PSD for ash in the char particle is presented for the burnoff level of 35.5% (near completion of devolatilisation) as no ash particles were liberated at this stage. Ash in char particles was analysed to determine the included mineral matter in the char [148]. At a burnoff level of 54.3%, which corresponds to the early combustion stage, some free-ash particles, approximately 11%, were also observed. There was almost no change between the PSD of the liberated ash between burnoff levels of 54.3 and 35.5%. At the middle combustion stage where the burnout level is 70%, the percentage of liberated ash particles in the sample increased, and the PSD of liberated ash shows that char fragmentation was still the dominant mechanism for ash formation during combustion from 54.3 to 70.1% burnoff levels. However, the PSD shifts to a slightly larger size. The change in PSD of liberated ash implies that some coalescence of included mineral matter occurred at this burnoff level. At an 87.1% burnoff level the PSD for the liberated ash increased significantly compared to the 70.1% burnoff level. This suggests that included mineral matter experienced more significant coalescence during this stage. At the 95.6% burnoff level, the majority of the particles in the sample are presented as liberated ash (about 80%). The largest shift in the liberated ash PSD was observed indicating that the most significant extent of coalescence for included mineral matter occurred during this stage. Char particles with various structures have significantly different behaviour, including char fragmentation, reaction mode and burnout, as discussed earlier. Char particles of Group I type are highly porous with a thin wall, and have been found to burn quickly during the early combustion stage (Fig. 16), which leads to a significant decrease in the amount of the Group I type char particles observed during the combustion. This type of char particles also fragment significantly during combustion, reducing the extent of coalescence of included mineral matter. Coalescence of the included mineral particles is possible only for the small fragments, which are produced by the fragmentation of Group I particles, containing more than one mineral particle inclusion. Complete combustion of these small fragments results in the fine ash particles being liberated in early

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Fig. 14. SEM images of char samples generated at various burnout levels at a gas temperature of 1573 K in a DTF under atmospheric condition. The scale bar is 500 mm for (a)–(c), and 20 mm for (d).

stages. Fragmentation of Groups II and III particles occur at the late stages of combustion, and the burnout of these particles and ash liberation occur at much longer times than those of Group I particles, as indicated in Section 4.1.1. A mechanism for ash formation has been proposed as shown in Fig. 17 [61,148]. Group I type char particles

fragment extensively during the early and middle combustion stages and burn out early. The extent of coalescence for the included mineral particles is very low. One Group I type char particle may produce a number of small ash particles, resulting in a small PSD for liberated ash. Group II type char particles fragment less compared to the Group I type char

Fig. 15. PSDs of ash liberated from coal burning in a DTF at different burnout levels [148].

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Fig. 16. The predicted mass fraction of remaining char vs. residence time for Groups I and III char particles.

particles. Char fragmentation is still the dominant mechanism for ash formation but the included mineral particles undergo some coalescence. One Group II type char particle may produce several ash particles with a relatively larger size compared to the Group I type char particle, resulting in an increase in PSD of the liberated ash during the middle burnout stage. Group III type char particles exhibit low or no fragmentation. The included mineral particles undergo a large degree of coalescence. One Group III type particle may form only one or two ash particles of a larger size compared to Groups I and II type char particles, resulting in a significant shift to larger size of the liberated ash PSD during the late combustion stage. The ash formation

mechanisms proposed in this study provide a mechanistic explanation for the observations. A comprehensive ash formation model including this char structural mechanism has been recently presented [74]. 4.1.3. Effect of pressure on ash characteristics The PSDs for ashes produced during the combustion of a bituminous coal at the four pressures are presented in Fig. 18 [61]. It indicates that ash formed at high pressure has a much finer size. Sink/float analysis shows that over 70% of the mineral matter in coal is present as included species, which are the minerals within the carbonaceous particle fed as pf. Char structure has been found to be a significant linkage

Fig. 17. Proposed mechanisms for ash formation from different char types [61,148].

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Fig. 18. PSD of ash formed from combustion of coal B in air at a gas temperature of 1573 K and the pressures as indicated [61].

between pressure and ash formation. At 15 atm, the char sample contains mainly highly porous Group I particles. However, at 1 atm, it is dominated by the Groups II and III type particles. During combustion, Group I particles will undergo extensive fragmentation, reducing the coalescence of included mineral matter and therefore producing a large number of small ash particles. For Group III particles, the chance for char particle fragmentation is much less. There is a much higher probability for the coalescence of the included mineral particles to form large ash particles during combustion. The ash formed from Group II particles will therefore be of a size between that of Groups I and III derived ash particles. 4.2. Effect of pressure on other ash transformation processes 4.2.1. Effect of pressure on cenosphere ash formation The mechanism of cenosphere formation can be explained in terms of the surface-tension force, the nonwetting property of fused slag when in contact with coal or coke surface, and chemical reactions that produce gas bubbles inside slagged ash globules [145,149]. Internal pressure build-up is the physical mechanism for the formation of cenospheric ash. Thermodynamically, pressure increase also reduces the extent of reaction of iron oxide with carbon to form CO, the reaction shown by Raask [145] to be responsible for cenosphere formation. Less gas will be generated at high pressures. At the same time, the external pressure is increased. Obviously, increasing pressure will suppress the cenosphere formation due to the decrease in the pressure differences across the cenosphere shell. 4.2.2. Effect of pressure on excluded minerals transformation Excluded minerals are those that are fed as separate particles in the pf, and undergo their own individual transformation due to the poor interaction in the dilute combustion flow. Pressure may not affect the physical

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transformation of most of the minerals. However, pressure does affect the chemical transformation of minerals. A typical example is the decomposition of CaCO3. Decomposition of CaCO3 produces many small ash particles due to the internal pressure build-up when it is heated rapidly to its decomposition temperature [150]. Thermal shock also contributes to the fragmentation. Decomposition of carbonates depends on the partial pressure of CO2, at fixed temperature; if the CO2 partial pressure in the ambient gas is above the equilibrium decomposition pressure, the carbonate minerals do not decompose. Under the typical conditions in an PFBC, calcite does not decompose, which in turn indicates that (i) the calcite will produce much less fines under PFBC conditions and (ii) direct calcite sulfation occurs. However, during entrained flow gasification, the temperature is sufficiently high that the equilibrium CO2 pressure exceeds the ambient CO2 partial pressure and the calcite will decompose. 4.2.3. Effect of pressure on vaporisation When the pressure exerted on a system is changed, the chemical equilibrium of the system will shift. That means the vaporisation and subsequent condensation of species will be changed. In addition, changes in pressure will result in changes in the heat and mass transfer, which also have an effect on the ash coagulation. When the temperature of the liquid reaches the boiling point, the species begins to vaporise. The amount of the species that can vaporise into the gaseous phase is determined by the vapour pressure of the substance. When elevated pressure is applied above liquid, the vapour pressure will be changed. For a non-reacting system [151], when an increase in the total pressure applied to the liquid from PT;0 to PT;1 at temperature T, vapour pressure of species i in the gaseous phase (P0i and P1i ) obeys the following equation: P1i ¼ P0i expðVm DP=RTÞ

ð22Þ

This equation is suitable for small pressure changes. The equation implies that vapour pressure will increase with the increased applied pressure, however, the change is very small even though the pressure change is 100 atm. Hence, in practical coal utilisation system, pressure effect on vapour pressure of species can be neglected. In summary, the above correction for vapour pressure can be neglected. 4.2.4. Effect of pressure on chemical reaction equilibrium According to Sheehan [152], the chemical reaction equilibrium constant K of a reaction is a function of temperature, and is independent of pressure. However, pressure might shift the chemical composition at equilibrium. To investigate the pressure effect on ash vaporisation, a facility for the analysis of chemical thermodynamics (FACT) has been employed to calculate the pressure effect on the ash vaporisation [153]. The results show that the vaporisation of iron decreases dramatically in both reducing

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Fig. 19. SEM images of char –ash particles collected inside the gasifier at different locations. The scale bar in each figure is 100 mm [156].

and oxidising atmospheres as the pressure is increased. Less ash vaporisation will obviously result in less fume formation, which decreases the formation of fine ash particles by nucleation and growth. 4.2.5. Effect of pressure on condensation and aggregation Refs. [154,155] indicate that condensation of gaseous phase vapour mainly depends on the gas phase cooling rate and the surface area for condensation. According to the previous analysis, pressure effect on equilibrium vapour pressure is negligible. However, because increasing pressure causes a decrease in vaporisation, the total amount of vapour for condensation decreases greatly. The results are less condensation. Less fume formation in turn decreases the total surface area for condensation to some extent. In addition, pressure can affect the char physical structure and burnout behaviour [72]. In this way,

pressure will change the final ash size. Increasing pressure shifts the PSD of ash to smaller size [148] which somehow increases the surface area for condensation. However, compared to the great decrease in vapour generation, such a change should not play a significant role in the fume formation. A decrease in the total amount of fine particles also decreases further aggregation. Increasing pressure will lead to an increase in gas viscosity and density directly, which also affect the aggregation mechanism to form relative larger clusters. 4.3. Char, ash and slag characteristics in a practical gasifier Ash and slag characteristics from a practical entrained flow gasifier have been studied by the authors [156]. Char–ash

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Fig. 20. The PSD of char/ash particles from coal B at various locations in the gasifier [156].

samples and slag samples were obtained from a 2T/day oxygen blown Hitachi gasifier. The gasifier was running with a temperature as high as 2073 K. The description of the gasifier was detailed elsewhere [156]. Like other gasifiers of this type, a swirling flow tends to force particles of char and ash onto walls to create a gravity-driven slag flow, for its removal. CCSEM and SEM analyses of the gasifier slag and char – ash samples indicate that included minerals tend to be entrained off the gasifier due to the low apparent densities of char particles containing minerals, whereas excluded minerals, particularly pyrite, tend to be tapped in the slag due to their high densities relative to that of char. The gasifier slag can be characterised as an inhomogeneous mixture of undissolved quartz particles, voids and some precipitated crystalline phase. No carbon was found in the slag for this gasifier. The oxide analyses of char – ash deposits at different locations in the gasifier indicate a reduction in ash content by about 10% as the particles leave the gasifier duct which suggest that carbon conversion continues till the particles reach the heat exchanger section [156]. The analysis of char ash particles shows high proportion of unburnt char (up to 75%). Fig. 19 shows the SEM images of char – ash particles collected at different locations of the gasifier. Most of the liberated ash particles have diameters under 10 mm. The number percentage of liberated ash particles increases as the particles move from the interior of gasifier to the cyclone. The char particles have high porosity such that fine mineral inclusions are retained in the boundary of char. The mineral inclusions in the char and particles attached to the char surfaces were also smaller than 10 mm. The PSDs of the char at cyclone, heat recovery boiler and gasifier exit are also determined using a Malvern Particle Sizer and are presented in Fig. 20. About 60% of particles are below 20, 60, and 80 mm char at cyclone, heat recovery section and gasifier exit, respectively. The PSD at the gasifier exit is coarse and as particles move from the gasifier

duct to the heat recovery section, the particles size appears to decrease. Fig. 20 also shows that there is a significant difference between the PSD of the heat recovery char –ash sample and cyclone char – ash sample. Ash particles with sizes under 10 mm contributed to 50% of volume in char – ash sample. On a carbon-free basis, this volume percentage would be much higher than 50%. Although the coal type and mineral characteristics used in the gasification experiments are different from those in the combustion, we still can make a qualitative conclusion that ash formed in a practical gasifier tends to be finer than that formed during combustion. The practical impact of the size and density of the char and ash formed in such a gasifier is that † The slag flowing down the walls does not necessarily have the composition of the coal ash, i.e. that the slag is preferentially formed by the ash from the excluded minerals. The amount of fluxing agent (limestone) necessary for the slag to flow will therefore always not be estimated by the viscosity of the slag formed by the total coal ash. † The solids entering into the heat-exchanger section following the gasification comprise the porous char and the fine ash. The deposition of these particles onto surfaces will depend on the character of these particles. These effects are coal-specific and depend on the mineral character as well as the mineral distribution through the coal [156]. The design and operation will therefore depend on this character. Mathematical models of particle trajectories in the swirling flow appear to provide a technique for estimating this impact. 4.4. Summary on pressure effect on ash formation At high pressures, finer ash tends to be formed during

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combustion, due to the higher number of highly porous char particles formed. These char particles burn more rapidly, fragment more than dense chars, resulting in less coalescence of included minerals during combustion, hence formation of finer ash particles. An ash formation mechanism has been proposed which includes these mechanisms. Gasifier data have shown that excluded minerals tend to be tapped in the slag, with little carbon in it. The slag flowing down the walls does not necessarily have the composition of the coal ash. Included minerals tend to be entrained off the gasifier. A significant amount of char has left in the deposits in the gasifier and in the cyclone, and the deposition of the char and ash particles onto surfaces will depend on the character of these particles. Ash particles formed in the gasifier are likely to be finer than combustion ash. The design and operation of a gasifier will depend on the mineral character of coal.

5. Conclusions Clean coal technologies, all of which operate at high pressures, have gained increasing technological and scientific interest. Understanding of the effect of pressure on coal reactions is essential to the design and operation of these technologies. Studies on pressure effect on a variety of aspects of coal reactions have been reviewed and summarised in this paper. The following conclusions have been drawn from the review. Pyrolysis pressure has a significant role in coal devolatilisation, in particular, volatile yield and char formation. † The total volatile and tar yield decrease, whereas the gas yield increases, with an increase in pressure. The effect of pressure levels off at elevated pressure. Secondary reaction and mass transfer limitation are enhanced at high pressure, which is believed to be the mechanisms for the pressure effect. † Pyrolysis pressure also influences coal swelling upon heating. The coal swelling index increases with an increase in pyrolysis pressure at low pressures, but decreases at high pressure, depending on coal type. The coal plastic behaviour is strongly associated with the complex bubble formation and movement during the plastic stage of coal. † Vitrinite content in coal as well as pyrolysis pressure significantly influence the morphology of residual char. Chars can be classified into different groups. The number percentage of highly porous Group I char is favoured by high pressure and high vitrinite content in coal. A correlation based on these studies has been developed. A larger number of experimental studies showed that reactant pressure has been found to affect the char gasification reaction rates. Generally, the reaction rate is

increased with an increase in partial pressure of oxygen, carbon dioxide, steam and hydrogen. The dependence of reaction rate on pressure decreases with increasing pressure. This effect can be well explained by the adsorption– desorption mechanism which is composed of several elemental reaction steps. Langmuir – Hinshelwood rate expression, derived from these sophisticated mechanisms, tends to be commonly used in mathematical modelling. The char structure has a significant role in burnout of residual char and ash formation, with a significant effect of pressure. It has been shown that † increasing pressures decreases the burnout time of coals due to the higher number of reactive Group I chars formed at the completion of pyrolysis. The burning time of Group I char is about one-third that of Group III char, which is due to their smaller mass, high internal and external surface areas, and fragmentation of Group I char during combustion. † At high pressures, finer ash tends to be formed during combustion, again, due to higher number of Group I chars formed. Group I chars burn more rapidly, fragment more than Group III chars, resulting in less coalescence of included minerals during combustion, hence formation of finer ash particles. An ash formation mechanism has been proposed which includes these mechanisms. Ash particles formed in the gasifier are likely to be finer than combustion ash.

6. Implications This review presents the current knowledge of the mechanisms of coal reactions at the high pressure of emerging coal utilisation technologies, together with the significance of these mechanisms. In summary, most of the work to date has been at the lower temperatures (typically ,1000 8C), which can be achieved in experiments involving captive particles or coal samples. Experiments in pressurised TGA and wire mesh systems at these low temperatures are the most commonly reported, with some experiments for entrained flow system reported at the higher temperatures typical of IGCC conditions in entrained flow reactions. Although the difficulty and cost has restricted these experiments, the entrained flow work is the need. Table 6 presents a summary of the current knowledge on the mechanisms reviewed in this paper. Relevant devolatilisation and char reactivity studies have been undertaken for some time, with Tables 1 and 4 indicating that the state of knowledge and predictive ability, while being far from complete, is reasonably well developed. Char and ash formation mechanisms at pressure, however, are not well understood and, in fact, the recent work by the CRC for

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Table 6 Summary of the importance and knowledge of pressure effects on coal reactions, with suggested levels: L-low; M-medium; and H-high State of knowledge Coal reaction mechanisms

Significance to design and operation of equipment

Understanding of mechanisms

Predictive ability

1. Devolatilisation 2. Char formation 3. Char reactivity 4. Ash formation

M M-H–as it determines 3 and 4 M-H M-H

M L M L

M L L L

black coal utilisation appears to be the first to progress the scientific understanding of the mechanisms involved. This work has revealed that pressure does have an impact. The structure of the char generated has now been related to reactivity and ash formed, but the mechanisms leading to the effect of pressure on this structure are not understood. Progressing the understanding of the formation of char structure at pressure and its relation to coal properties is the obvious research need.

Acknowledgements The authors wish to acknowledge the financial 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. The authors also thank Dr Raj Gupta for the char and ash analysis of the gasifier sample. Thanks also go to Dr George Shan and Mr Jianglong Yu for sharing their review on coal devolatilisation and coal swelling, respectively, and Mr Yu for giving assistance in editing the manuscript.

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