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Combustion characteristics of high ash coal in a pulverized coal combustion
Fuel 80 (2001) 1447±1455
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Combustion characteristics of high ash coal in a pulverized coal combustion R. Kurose*, M. Ikeda, H. Makino Chemical Energy Engineering Department, Yokosuka Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), 2-6-1 Nagasaka, Yokosuka-shi, Kanagawa-ken 240-0196, Japan Received 3 August 2000; accepted 27 December 2000
Abstract The in¯uences of ash content on pulverized coal combustion characteristics are experimentally and numerically studied under a staged combustion condition. The stage combustion ratio (the ratio of air volume of the staged combustion air to the total air) is 0 or 30%, and the coals tested are the three high ash coals with different ash contents of 36, 44 and 53 wt%, which were separated using the ¯oatation method. The results show that as the ash content increases, gas temperature decreases and O2 consumption and NOx formation becomes slow near the burner. Also, the increase of the ash content leads to the increase in NOx concentration and unburned carbon fraction at the furnace exit. The reasons being, for the high ash coal, the large heat capacity of the ash and the covering of combustible matter suppress combustibility with ash during the char oxidization. The numerical simulations designed to match the experimental setup for the staged combustion show that the numerical results are in general agreement with the measurements. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Pulverized coal combustion; High ash coal; NOx
1. Introduction Coal is an important energy resource to meet the future demand of electricity because its reserve is more abundant than those of the other fossil fuels. At present, the main use of coal is for pulverized coal combustion, and bituminous coal is generally used for it in Japan. However, from the viewpoints of fuel security and fuel cost, it will be necessary for electric power companies to use low-rank coal, which is rich in moisture or ash and has a calori®c value lower than bituminous coal. Although the coal usually ®red in thermal power stations in Japan contains ash under 30 wt%, coal which contains ash higher than 30 wt% is mined throughout the world. Therefore, it is of great importance to identify the effects of the ash content in the coal on the pulverized coal combustion characteristics. Recently, computational ¯uid dynamics (CFD) becomes prevalent because it can easily provide a detailed information on the ¯uid ¯ow that experimental data alone cannot provide. However, at the present stage, since the computation of the pulverized coal combustion comprises additional numerical models accounting for gaseous turbulent combustion, radiative energy transport, coal particle reaction * Corresponding author. Tel.: 181-468-56-2121; fax: 181-468-56-3346. E-mail address: [email protected] (R. Kurose).
(devolatilization and char oxidation) and so on, it appears to be hardly possible to select the favorable models and give reliable solutions for arbitrary conditions. The purpose of this study is to experimentally investigate the in¯uences of ash content on pulverized coal combustion characteristics by using Ikeshima coal with different ash contents of 36, 44 and 53 wt%, in a pulverized coal combustion test furnace under a staged combustion condition. The ratio of air volume of the staged combustion air to the total air (staged combustion ratio) is 0 or 30%. Hereafter, the conditions where the staged combustion ratios of 0 and 30% are referred to as `standard combustion condition' and `staged combustion condition', respectively. Threedimensional numerical simulations, which are designed to match the experimental setup for the staged combustion condition, are also performed, and the application of the numerical simulation of the pulverized coal combustion for the high ash coal is assessed. The rest of this paper is organized as follows. In Section 2, the experimental test facility and method, and the coal properties used are described. The numerical method is explained in Section 3. In Section 4, the combustion characteristics are presented both on the standard (staged combustion ratio is 0%) and staged (staged combustion ratio is 30%) combustion conditions, and the effect of the interaction between the ash and the combustible matter in
0016-2361/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0016-236 1(01)00020-5
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Fig. 1. Schematic diagram of pulverized coal combustion test furnace.
coal on combustibility is discussed. The numerical solutions obtained on the staged combustion condition are compared with the measurements in Section 5. Finally, the results are summarized in Section 6.
2. Experiments 2.1. Test facility Figs. 1 and 2 show the schematic diagrams of the test furnace and burner, respectively. The test furnace was a type of water-cooled horizontal and cylindrical furnace made of steel. The diameter of this furnace was 0.85 m and the length was 8 m. Refractory materials were placed on the inside wall of the furnace at a thickness of 0.075 m. Fourteen ports were mounted on the furnace sidewall for the staged combustion air injection and the measurements of temperature and gas concentration. The distance from the burner outlet to the injection port for staged combustion air was 3.0 m. The burner was designed to have a coal combustion capacity of about 100 kg/h. Combustion air, which was supplied from a forced draft fan, was injected into the furnace through the burner and the staged combustion air ports, and the air through the burner was divided into primary air, secondary air and tertiary air. The primary air
Fig. 2. Schematic diagram of pulverized coal burner.
carries coal from the table feeders to the burner through a coal exhauster. The secondary and tertiary combustion air was fed into the furnace via a wind box. The staged combustion air was divided before the wind box and injected at the side ports of the furnace. The primary air had a weak swirl, and the secondary and tertiary air had a strong swirl ¯ow. This type of burner is conventional and is used in some thermal power plants. The coal feed rate was 97.0 kg/h. The tested coal was pulverized until 80% passes through a 200 mesh screen and was stored in a couple of bins. During the combustion test, the table feeders settled under each storage bin supplied coal. The temperature of the combustion air was kept constant by air heaters, and combustion gas was cooled by passing through a gas cooler and an air heater. A multi-cyclone and a bag ®lter removed the particulate matter from the combustion gas. Combustion gas was exhausted from the stack after reducing SOx in the desulfurization equipment. 2.2. Measurement In the experiment, the furnace was warmed by kerosene combustion. After warming the furnace suf®ciently, the fuel was gradually shifted from kerosene to coal. The combustion condition is shown in Table 1. The value of thermal input in the coal combustion test furnace was 7.60 £ 10 2 kJ/ s. The air ratio was 1.24, and the excess O2 concentration was 4.00%. The mass ratio of the pulverized coal (dry and 30% ash base) to the sum of the air and moisture in the primary air is 1:2.2, and the mass ratio of the secondary air to the tertiary air is 1:6. When the temperature of the combustion air and exhaust gas became constant, measurement commenced. The measured gas components were NOx, O2 and CO. Gas temperature in the furnace and outside the furnace were measured with a Pt/Pt±Rh (13%) sheath thermocouple and with a CA-type thermocouple, respectively. Fly ash was collected in the gas cooler, the multi-cyclone and the bag ®lter. The weight of the unburned carbon in each facility was determined from the weight loss upon heating to 800 ^ 108C in air. The measurements of temperature and gas concentration were carried out under
R. Kurose et al. / Fuel 80 (2001) 1447±1455 Table 1 Combustion condition Standard Thermal input (kJ/s) Air ratio (±) Excess O2 concentration (%) Staged combustion ratio (±) Total air (N m 3/h) Primary air (N m 3/h) Secondary air (N m 3/h) Tertiary air (N m 3/h) Staged combustion air (N m 3/h)
7.60 £ 10 1.24 4.00 0 912 210 100 602 0
Staged 2
7.60 £ 10 2 1.24 4.00 0.3 912 210 61 367 274
the standard combustion condition (staged combustion ratio is 0%) and the staged combustion condition (staged combustion ratio is 30%). 2.3. Coal properties Table 2 shows the coal properties of Ikeshima coal (Japanese bituminous coal), which have almost the same fuel ratio and carbon, hydrogen, nitrogen and oxygen contents on a dry ash free basis, but different ash content of 36, 44 and 53 wt%. These samples were prepared by the ¯oatation separation. The ash contents of the tested coals are higher than that of usual coals, which are ®red in the thermal power stations in Japan. The mean diameter of the pulverized coal was 32 mm. 3. Numerical simulation 3.1. Computational domain and conditions The three-dimensional numerical simulations were designed to match the experimental setup for the staged combustion, and were performed for high ash Ikeshima coals, containing 36, 44 and 53 wt% ash. The computational domain and conditions used here are essentially the same
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as those in Kurose et al. [1]. It was impossible to completely emulate the computational domain and burner because of their complications. The domain sizes were 20.16 % x % 8 m, 0 % r % 0.42 m and 2p/6 % u % p/ 6 rad in axial (x), radial (r) and azimuthal (u ) directions, respectively. A periodic condition was applied in the azimuthal (u ) direction. The combustion air was injected into the furnace through the burner and the staged combustion air port, and the air through the burner was divided into primary air, secondary air and tertiary air. The secondary and tertiary air had swirls. At the inlet x 20:16 m; the air velocity in the radial (r) direction was zero, and that in the azimuthal (u ) direction was set to the same value in the axial (x) direction Uf r 0; Uf u Uf x : The pulverized coal was carried with the primary air. It was assumed that the pulverized coal particles consisted of many particles with diameters of 10, 20, 40, 80 and 100 mm, and the mass fractions were 10, 20, 30, 30, 10 wt%, respectively. The motions of the pulverized coal particles were calculated using 20 representative particles for each size. The temperature and velocity of the particles at the inlet x 20:16 m were equal to those of the primary air. The temperatures of the combustion air and the wall were set to be at 573 and 873 K, respectively. 3.2. Computational details The basic conservation equations for gas phase [mass (continuity), momentum, energy and masses of chemical species] based on the k± e two-equation turbulence model [2] were solved using the SIMPLER algorithm [3] in cylindrical coordinates. The interaction of the conserved properties between the gas phase and coal particles was calculated by using the Particle-Source-In Cell (PSI-Cell) technique [4]. Coal devolatilisation was simulated by a ®rst-order single reaction model [5], and the gaseous combustion was calculated using Mixed-is-burnt (MIB) model [6]. On the other
Table 2 Properties of tested coal
Proximate analysis
Ultimate analysis c
Heating value (low) b (MJ/kg) a b c
Moisture a (%) Ash b (%) Volatile matter b (%) Fixed carbon b (%) Fuel ratio (%) Carbon (%) Hydrogen (%) Nitrogen (%) Oxygen (%) Total sulfur (%) Combustible sulfur (%)
Ikeshima-A (ash 36%)
Ikeshima-B (ash 44%)
Ikeshima-C (ash 53%)
2.7 36.0 30.5 33.5 1.10 80.5 7.5 1.1 9.6 1.5 1.4 20.9
3.0 44.4 26.6 29.0 1.09 79.0 7.9 1.1 10.1 1.8 1.7 18.4
2.6 52.6 22.9 24.5 1.07 78.7 8.0 1.1 10.1 2.3 2.0 14.8
Equilibrium moisture basis of coal at 75% relative humidity and room temperature. Dry basis. Dry ash free basis.
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input were calculated as 7.47 £ 10 2, 7.43 £ 10 2 and 7.39 £ 10 2 kJ/s for the coal with the ash contents of 36, 44 and 53 wt%, respectively. These were 1.7, 2.2 and 2.8% less than the actual thermal input of 7.60 £ 10 2 kJ/s. The grid points, excluding the inlet section, were 61 (8.0 m) £ 58 (0.42 m) £ 11 (p/3 rad) in axial (x), radial (r) and azimuthal (u ) directions, and the x and r grids were compressed around the point of expansion (the number of the grid points was decided by comparing with the results for the grid points of 81 £ 68 £ 21). For the inlet section, 2 (0.16 m) £ 16 (0.08 m) £ 11 (p/3 rad) grid points were used in the x, r and u directions. The calculations were continued until the temperature distribution becomes steady (20 000 time steps). The CPU time required for each case was about 60 h on a DEC: Alpha Station 500/500 MHz work station. 4. Experimental results for standard and staged combustions 4.1. Standard combustion
Fig. 3. Gas temperature [temperature of a Pt/Pt±Rh (13%) sheath thermocouple], O2 and NOx concentrations at the center of furnace under the standard combustion condition: (a) gas temperature; (b) O2 concentration; (c) NOx concentration; W, 36 wt% ash; K, 44 wt% ash; A, 53 wt% ash.
hand, char oxidization rate was calculated using Field et al.'s model [7]. For the formation of NOx, three different mechanisms were employed, namely Zeldovich NOx, prompt NOx and fuel NOx. Here, Zeldovich NOx and prompt NOx are classi®ed into thermal NOx. The Zeldovich NOx was evaluated by applying a quasi-steady state approximation for N species to the extended Zeldovich mechanisms (N2 1 O , NO 1 N, N 1 O2 , NO 1 O, N 1 OH , NO 1 H) with the rate constants of Baulch et al. [8]. On the other hand, the prompt NOx and the fuel NOx were predicted using the models by De Soete [9,10]. The discrete transfer radiation method of Lockwood and Shah [11] was used to simulate the radiative heat transfer among the gas, particles and wall. The higher the ash content in coal, more is the combustibility suppressed by the large heat capacity of the ash. To take this effect into account, the value of the thermal input was roughly corrected by subtracting the heat capacity of the ash from the experimental value of 7.60 £ 10 2 kJ/s. The value of the heat capacity of the ash was estimated by a heat capacity database presented by Davis [12]. It was assumed that the ash consists of SiO2 (70%), Al2O3 (25%) and Fe2O3 (5%) and the temperature of the ash increases from 573 to 1800 K. According to this method, the values of the thermal
Fig. 3 shows the distributions of gas temperature measured by Pt/Pt±Rh (13%) thermocouple, O2 and NOx concentrations at the center of the furnace in the standard combustion of Ikeshima coal. As the ash content increases, the gas temperature decreases while the O2 consumption and the NOx formation and reduction are delayed near the burner. Although the gas temperature and the O2 concentration at the exit of the furnace tend to approach certain values independent of the ash content, NOx concentration is higher for a high ash coal. This is because of the shortage of the NOx reduction time because of the delay in NOx formation. Fig. 4 shows the relationship between the conversion of fuel bound nitrogen to NOx, CR, and the index of the fuel ratio divided by the fuel bound nitrogen, FR/FN, for Ikeshima coal, together with that for some typical bituminous coal with low ash content of 7.0±18% [13]. In pulverized coal combustion, most of NOx is thought to be formed from fuel bound nitrogen, and so CR (±) is de®ned by CR 0
CNOx
1 FN 22 2:24 £ 10 £ B 1:4 £ 1022 C B C @ A V
1
dry
where CNOx is the NOx concentration at the furnace exit (±), FN the fuel bound nitrogen (±), and Vdry the ¯ow rate of dry air per feeding rate of coal [(m 3/h)/(kg/h)]. In a previous study [13], CR for bituminous coals was found to increase linearly with FR/FN. On the other hand, although CR for Ikeshima coal with the ash content of 36 wt% is almost on the averaged line for bituminous coals, it tends to monotonously increase with the increasing ash content at a ®xed FR/FN.
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Fig. 4. Relation between CR and FR/FN under the standard combustion condition: W, bituminous coal with low ash content (previous study); X, Ikeshima coal (present study).
The relation between the unburned carbon fraction, Uc p, and the fuel ratio, FR, is shown in Fig. 5, together with those for some typical bituminous coals with low ash contents of 7.0±18% [13]. Uc p (±) is de®ned by Ucp 1 2 h
Uc CAsh £ 1 2 Uc 1 2 CAsh
2
where h is the combustion ef®ciency of carbon (±), Uc, the unburned carbon concentration in ¯y ash (±), and CAsh, the ash content in coal (±). It was shown that Uc p was
Fig. 6. Gas temperature [temperature of a Pt/Pt±Rh (13%) sheath thermocouple], O2 and NOx concentrations at the center of furnace under the staged combustion condition: (a) gas temperature; (b) O2 concentration; (c) NOx concentration; W, 36 wt% ash; K, 44 wt% ash; A, 53 wt% ash.
proportional to FR for bituminous coals with low ash contents [13]. Uc p of the high ash coal is found to be much higher than that of low ash coal. With increasing the ash content, the unburned carbon fraction increases and the combustion ef®ciency decreases. As mentioned above, the combustibility of the pulverized coal combustion is suppressed as the ash content in coal increases. One of the reasons of this is likely because the heat capacity of the ash increases with the increasing ash content. It is also considered that the ash impedes the char oxidization by covering the combustible matter [14]. The interaction between the ash and the combustible matter in coal will be discussed later. 4.2. Staged combustion
Fig. 5. Relation between Uc p and FR under the standard combustion condition: W, bituminous coal with low ash content (previous study); X, Ikeshima coal (present study).
Fig. 6 shows the gas temperature, O2 and NOx concentrations in the staged combustion of Ikeshima coal. The O2 concentration in the region before the staged air injection port is much less than that in the standard combustion (see Fig. 3), and the NOx concentration at the furnace exit is also
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Fig. 8. Effect of ash content on RNOx and RUc p: W, RNOx; K, RUc p.
NOx concentration and the unburned carbon fraction in the staged combustion increase with the ash content. Fig. 8 shows the effect of the ash content on the ratio of the NOx reduction and the unburned carbon fraction by the staged combustion, RNOx and RUc p. RNOx (±) and RUc p (±) are de®ned by RNOx
NOxstd 2 NOxstg
RUcp 2
Fig. 7. Effect of ash content on NOx concentration and unburned carbon fraction at the furnace exit under the standard and staged combustion conditions: (a) NOx concentration; (b) unburned carbon fraction; W, standard combustion; K, staged combustion.
less because the de®ciency of O2 promotes the NOx reduction. The trends of the gas temperature, the O2 and NOx concentrations with the ash content are similar to those in the standard combustion. As the ash content increases, the gas temperature decreases, and the O2 consumption as well as NOx formation are delayed near the burner, and then NOx concentration at the furnace exit increases. Fig. 7 shows the effect of the ash content on the NOx concentration and the unburned carbon fraction at the furnace exit, together with those in the standard combustion. It is clearly observed that the staged combustion decreases the NOx concentration and increases the unburned carbon fraction at the furnace exit. The decrease of the NOx concentration is resulted from the promotion of the NOx reduction, as mentioned above. On the other hand, the increase of the unburned carbon fraction is caused by the remarkable de®ciency of the O2 before the staged combustion air port. Similar to the results in the standard combustion, both the
NOxstd Ucpstd 2 Ucpstg Ucpstd
3
4
and these values are valid to clarify the effects of the staged combustion on the decrease in NOx concentration and the increase in the unburned carbon fraction, respectively. Here, NOxi (±) and Ucip (±) are the NOx concentration and the unburned carbon fraction at the furnace exit, and the subscripts `std' and `stg' indicate the values in the standard combustion and the staged combustion, respectively. It is found that RNOx decreases and RUc p increases in proportion to the ash content. This means that with the increasing the ash content, the NOx reduction because of the staged combustion weakens, while the increment of the unburned carbon fraction because of the staged combustion becomes remarkable. 4.3. Interaction between ash and combustible matter in coal The in¯uence of the interaction between ash and the combustible matter in coal on the combustion characteristics was investigated by a SEM (Scanning Electron Microscope) photograph and Si and Al image taken by an EPMA (Electron Probe Micro Analyzer). The mass fraction of SiO2 and Al2O3 in the ash of Ikeshima coal is about 90%, therefore the location of the particles can be checked by the SEM photograph and the ash component can be roughly judged by the image of Si and Al. An example of the SEM photograph and the Si and Al image is shown in Fig. 9. It was observed that the Ikeshima coal consists of
R. Kurose et al. / Fuel 80 (2001) 1447±1455
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Fig. 9. SEM photograph and Si and Al image by EPMA for Ikeshima coal containing 53 wt% ash: (a) SEM photograph; (b) Si and Al image by EPMA.
Fig. 10. Contour of gas temperature and NOx concentration in the furnace for Ikeshima coal containing 36 wt% ash. Red and blue colors indicate high and low values, respectively (300 # T # 16008C, 0 # YNOx # 500 £ 10 26): (a) gas temperature; (b) NOx concentration.
Fig. 11. Contour of gas temperature and NOx concentration in the furnace for Ikeshima coal containing 53 wt% ash. Colors are as explained in Fig. 10: (a) gas temperature; (b) NOx concentration.
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Fig. 12. Comparisons of measured and calculated data of gas temperature and NOx concentration at the center of furnace: (a) gas temperature; (b) NOx concentration; W, K, A, experiments for 36, 44 and 53 wt% ash; Ð, ± ± ±, - - - -, calculated values for 36, 44, 53 wt% ash, respectively.
three kinds of particles; a combustible particle, an ash particle and a mixed particle. The comparisons of the SEM photograph and the Si and Al image for the Ikeshima coal containing 36, 44 and 53 wt% showed that the ratio of the ash particle and that of the mixed particle increases for higher ash coal, although the ratio of the combustible particle decreases. This suggests that the combustibility of the pulverized coal combustion is suppressed not only by the increase in the heat capacity of the ash but also the interaction between the ash and the combustible matter in coal, since the mixed particle is considered to be dif®cult to burn because the ash covers the combustible matter and impedes the char oxidization [14]. 5. Numerical results for staged combustion In this section, the experimental results for the pulverized coal combustion characteristics in the staged combustion are compared with the numerical solutions. Figs. 10 and 11 show the contour of gas temperature and NOx concentration in the furnace for the coals containing 36 and 53 wt% ash. For both cases, the gas temperature is high near the center of the furnace. The region of the maximum gas temperature corresponds to the region of the maximum NOx concentration. The gas temperature
Fig. 13. Comparisons of measured and calculated data of NOx concentration and unburned carbon fraction at the furnace exit: (a) NOx concentration; (b) unburned carbon fraction; W, experiment; Ð, calculation.
and NOx concentration are higher for the 36 wt% ash coal than for the 53 wt% ash coal near the burner. The calculated gas temperature and the NOx concentration at the center of furnace are compared with the measurements in Fig. 12. Fig. 13 shows the comparisons of the NOx concentration and the unburned carbon fraction at the furnace exit. The numerical trends are found to be in general agreement with the measurements. As the ash content increases, the calculated gas temperature and NOx concentration at the center of the furnace decrease near the burner, and both the NOx concentration and the unburned carbon fraction at the furnace exit increase. The results suggest that the present numerical simulation for the pulverized coal combustion is applicable for the pulverized coal combustion at least to know the qualitative effects of the ash. It is considered that the discrepancies between calculations and measurements are probably due to that the shapes of the furnace and the burner modeled in this calculation are not completely the same as those of the measurements. Also,
R. Kurose et al. / Fuel 80 (2001) 1447±1455
compared to the measurements, the calculated unburned carbon fraction at the furnace exit are low especially for high ash content coals [Fig. 13(b)]. It is speculated that this is attributed to the fact that the interaction between ash and the combustible matter is not taken into account for the numerical model of char oxidization. To estimate these combustion characteristics for high ash content coal more precisely, the char oxidization model, which takes the presence of ash into account, should be proposed.
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4. The numerical solutions obtained for the staged combustion condition are in general agreement with the measurements. The differences between them are partially because the interaction between ash and combustible matter is not taken into account for in the numerical model.
Acknowledgements 6. Conclusions The in¯uences of ash content on pulverized coal combustion characteristics were experimentally and numerically investigated by using three high ash coals. The main result can be summarized as follows. 1. Under both standard and staged combustion conditions, as the ash content increases, the gas temperature decreases, while the O2 consumption, the NOx formation and reduction are delayed near the burner. Also, the conversion of fuel bound nitrogen to NOx and unburned carbon fraction at the furnace exit increase with the ash content. 2. In the staged combustion, the O2 concentration in the region before the staged combustion air injection port is found to be much less than that in the standard combustion, and therefore the NOx concentration at the furnace exit is less because the de®ciency of O2 suppresses the NOx formation. The unburned carbon fraction at the furnace exit is higher in the staged combustion than in the standard combustion. With increasing the ash content, the NOx reduction because of the staged combustion weakens, and the increment of the unburned carbon fraction due to the staged combustion becomes remarkable. 3. The low combustibility of high ash coal is considered to be caused partly by the large heat capacity of the ash and partly by the blocking of gas diffusion due to ash layer over char.
The authors would like to thank M. Kimoto and H. Tsuji of CRIEPI for helpful discussions. References [1] Kurose R, Tsuji H, Makino H. Submitted for publication. [2] Launder BE, Spalding DB. Comput Methods Appl Mech Engng 1974;3:269. [3] Patankar SV. Numerical heart transfer and ¯uid ¯ow. New York: Hemisphere, 1980. [4] Crowe CT, Sharma NP, Stock DE. Trans ASME J Fluids Engng 1977;99:325. [5] Mitchell JW, Tarbell JM. AIChE J 1982;28(2):302. [6] Manickam M, Schwarz MP, Perry J. Appl Math Model 1998;22:823. [7] Field MA, Gill DW, Morgan BB, Hawksley PGW. The combustion of pulverised coal. British Coal Utilisation Research Association, Leatherhead, Surrey, 1967. [8] Baulch DL, Drysdall DD, Horne DG, Lloyd AC. Evaluated kinetic data for high temperature reactions. London: Butterworth, 1973. [9] De Soete GG. 15th Symposium (International) on Combustion. The Combustion Institute, 1975. p. 1093. [10] De Soete GG. 23rd Symposium (International) on Combustion. The Combustion Institute, 1990. p. 1257. [11] Lockwood FC, Shah NG. 18th Symposium (International) on Combustion. The Combustion Institute, 1981. p. 1405. [12] Davis PR. A model to predict the physical characteristics of ®reside deposits in pulverized coal combustion chambers. MS thesis, Department of Chemical Engineering, Brigham Young University, Provo, UT, December 1988. [13] Makino H, Sato M, Kimoto M. J Jpn Inst Energy 1994;73:906 (in Japanese). [14] Saro®m AF, Howard JB, Padia AS. Combust Sci Tech 1977;16:187.
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Combustion characteristics of high ash coal in a pulverized coal combustion R. Kurose*, M. Ikeda, H. Makino Chemical Energy Engineering Department, Yokosuka Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), 2-6-1 Nagasaka, Yokosuka-shi, Kanagawa-ken 240-0196, Japan Received 3 August 2000; accepted 27 December 2000
Abstract The in¯uences of ash content on pulverized coal combustion characteristics are experimentally and numerically studied under a staged combustion condition. The stage combustion ratio (the ratio of air volume of the staged combustion air to the total air) is 0 or 30%, and the coals tested are the three high ash coals with different ash contents of 36, 44 and 53 wt%, which were separated using the ¯oatation method. The results show that as the ash content increases, gas temperature decreases and O2 consumption and NOx formation becomes slow near the burner. Also, the increase of the ash content leads to the increase in NOx concentration and unburned carbon fraction at the furnace exit. The reasons being, for the high ash coal, the large heat capacity of the ash and the covering of combustible matter suppress combustibility with ash during the char oxidization. The numerical simulations designed to match the experimental setup for the staged combustion show that the numerical results are in general agreement with the measurements. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Pulverized coal combustion; High ash coal; NOx
1. Introduction Coal is an important energy resource to meet the future demand of electricity because its reserve is more abundant than those of the other fossil fuels. At present, the main use of coal is for pulverized coal combustion, and bituminous coal is generally used for it in Japan. However, from the viewpoints of fuel security and fuel cost, it will be necessary for electric power companies to use low-rank coal, which is rich in moisture or ash and has a calori®c value lower than bituminous coal. Although the coal usually ®red in thermal power stations in Japan contains ash under 30 wt%, coal which contains ash higher than 30 wt% is mined throughout the world. Therefore, it is of great importance to identify the effects of the ash content in the coal on the pulverized coal combustion characteristics. Recently, computational ¯uid dynamics (CFD) becomes prevalent because it can easily provide a detailed information on the ¯uid ¯ow that experimental data alone cannot provide. However, at the present stage, since the computation of the pulverized coal combustion comprises additional numerical models accounting for gaseous turbulent combustion, radiative energy transport, coal particle reaction * Corresponding author. Tel.: 181-468-56-2121; fax: 181-468-56-3346. E-mail address: [email protected] (R. Kurose).
(devolatilization and char oxidation) and so on, it appears to be hardly possible to select the favorable models and give reliable solutions for arbitrary conditions. The purpose of this study is to experimentally investigate the in¯uences of ash content on pulverized coal combustion characteristics by using Ikeshima coal with different ash contents of 36, 44 and 53 wt%, in a pulverized coal combustion test furnace under a staged combustion condition. The ratio of air volume of the staged combustion air to the total air (staged combustion ratio) is 0 or 30%. Hereafter, the conditions where the staged combustion ratios of 0 and 30% are referred to as `standard combustion condition' and `staged combustion condition', respectively. Threedimensional numerical simulations, which are designed to match the experimental setup for the staged combustion condition, are also performed, and the application of the numerical simulation of the pulverized coal combustion for the high ash coal is assessed. The rest of this paper is organized as follows. In Section 2, the experimental test facility and method, and the coal properties used are described. The numerical method is explained in Section 3. In Section 4, the combustion characteristics are presented both on the standard (staged combustion ratio is 0%) and staged (staged combustion ratio is 30%) combustion conditions, and the effect of the interaction between the ash and the combustible matter in
0016-2361/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0016-236 1(01)00020-5
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Fig. 1. Schematic diagram of pulverized coal combustion test furnace.
coal on combustibility is discussed. The numerical solutions obtained on the staged combustion condition are compared with the measurements in Section 5. Finally, the results are summarized in Section 6.
2. Experiments 2.1. Test facility Figs. 1 and 2 show the schematic diagrams of the test furnace and burner, respectively. The test furnace was a type of water-cooled horizontal and cylindrical furnace made of steel. The diameter of this furnace was 0.85 m and the length was 8 m. Refractory materials were placed on the inside wall of the furnace at a thickness of 0.075 m. Fourteen ports were mounted on the furnace sidewall for the staged combustion air injection and the measurements of temperature and gas concentration. The distance from the burner outlet to the injection port for staged combustion air was 3.0 m. The burner was designed to have a coal combustion capacity of about 100 kg/h. Combustion air, which was supplied from a forced draft fan, was injected into the furnace through the burner and the staged combustion air ports, and the air through the burner was divided into primary air, secondary air and tertiary air. The primary air
Fig. 2. Schematic diagram of pulverized coal burner.
carries coal from the table feeders to the burner through a coal exhauster. The secondary and tertiary combustion air was fed into the furnace via a wind box. The staged combustion air was divided before the wind box and injected at the side ports of the furnace. The primary air had a weak swirl, and the secondary and tertiary air had a strong swirl ¯ow. This type of burner is conventional and is used in some thermal power plants. The coal feed rate was 97.0 kg/h. The tested coal was pulverized until 80% passes through a 200 mesh screen and was stored in a couple of bins. During the combustion test, the table feeders settled under each storage bin supplied coal. The temperature of the combustion air was kept constant by air heaters, and combustion gas was cooled by passing through a gas cooler and an air heater. A multi-cyclone and a bag ®lter removed the particulate matter from the combustion gas. Combustion gas was exhausted from the stack after reducing SOx in the desulfurization equipment. 2.2. Measurement In the experiment, the furnace was warmed by kerosene combustion. After warming the furnace suf®ciently, the fuel was gradually shifted from kerosene to coal. The combustion condition is shown in Table 1. The value of thermal input in the coal combustion test furnace was 7.60 £ 10 2 kJ/ s. The air ratio was 1.24, and the excess O2 concentration was 4.00%. The mass ratio of the pulverized coal (dry and 30% ash base) to the sum of the air and moisture in the primary air is 1:2.2, and the mass ratio of the secondary air to the tertiary air is 1:6. When the temperature of the combustion air and exhaust gas became constant, measurement commenced. The measured gas components were NOx, O2 and CO. Gas temperature in the furnace and outside the furnace were measured with a Pt/Pt±Rh (13%) sheath thermocouple and with a CA-type thermocouple, respectively. Fly ash was collected in the gas cooler, the multi-cyclone and the bag ®lter. The weight of the unburned carbon in each facility was determined from the weight loss upon heating to 800 ^ 108C in air. The measurements of temperature and gas concentration were carried out under
R. Kurose et al. / Fuel 80 (2001) 1447±1455 Table 1 Combustion condition Standard Thermal input (kJ/s) Air ratio (±) Excess O2 concentration (%) Staged combustion ratio (±) Total air (N m 3/h) Primary air (N m 3/h) Secondary air (N m 3/h) Tertiary air (N m 3/h) Staged combustion air (N m 3/h)
7.60 £ 10 1.24 4.00 0 912 210 100 602 0
Staged 2
7.60 £ 10 2 1.24 4.00 0.3 912 210 61 367 274
the standard combustion condition (staged combustion ratio is 0%) and the staged combustion condition (staged combustion ratio is 30%). 2.3. Coal properties Table 2 shows the coal properties of Ikeshima coal (Japanese bituminous coal), which have almost the same fuel ratio and carbon, hydrogen, nitrogen and oxygen contents on a dry ash free basis, but different ash content of 36, 44 and 53 wt%. These samples were prepared by the ¯oatation separation. The ash contents of the tested coals are higher than that of usual coals, which are ®red in the thermal power stations in Japan. The mean diameter of the pulverized coal was 32 mm. 3. Numerical simulation 3.1. Computational domain and conditions The three-dimensional numerical simulations were designed to match the experimental setup for the staged combustion, and were performed for high ash Ikeshima coals, containing 36, 44 and 53 wt% ash. The computational domain and conditions used here are essentially the same
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as those in Kurose et al. [1]. It was impossible to completely emulate the computational domain and burner because of their complications. The domain sizes were 20.16 % x % 8 m, 0 % r % 0.42 m and 2p/6 % u % p/ 6 rad in axial (x), radial (r) and azimuthal (u ) directions, respectively. A periodic condition was applied in the azimuthal (u ) direction. The combustion air was injected into the furnace through the burner and the staged combustion air port, and the air through the burner was divided into primary air, secondary air and tertiary air. The secondary and tertiary air had swirls. At the inlet x 20:16 m; the air velocity in the radial (r) direction was zero, and that in the azimuthal (u ) direction was set to the same value in the axial (x) direction Uf r 0; Uf u Uf x : The pulverized coal was carried with the primary air. It was assumed that the pulverized coal particles consisted of many particles with diameters of 10, 20, 40, 80 and 100 mm, and the mass fractions were 10, 20, 30, 30, 10 wt%, respectively. The motions of the pulverized coal particles were calculated using 20 representative particles for each size. The temperature and velocity of the particles at the inlet x 20:16 m were equal to those of the primary air. The temperatures of the combustion air and the wall were set to be at 573 and 873 K, respectively. 3.2. Computational details The basic conservation equations for gas phase [mass (continuity), momentum, energy and masses of chemical species] based on the k± e two-equation turbulence model [2] were solved using the SIMPLER algorithm [3] in cylindrical coordinates. The interaction of the conserved properties between the gas phase and coal particles was calculated by using the Particle-Source-In Cell (PSI-Cell) technique [4]. Coal devolatilisation was simulated by a ®rst-order single reaction model [5], and the gaseous combustion was calculated using Mixed-is-burnt (MIB) model [6]. On the other
Table 2 Properties of tested coal
Proximate analysis
Ultimate analysis c
Heating value (low) b (MJ/kg) a b c
Moisture a (%) Ash b (%) Volatile matter b (%) Fixed carbon b (%) Fuel ratio (%) Carbon (%) Hydrogen (%) Nitrogen (%) Oxygen (%) Total sulfur (%) Combustible sulfur (%)
Ikeshima-A (ash 36%)
Ikeshima-B (ash 44%)
Ikeshima-C (ash 53%)
2.7 36.0 30.5 33.5 1.10 80.5 7.5 1.1 9.6 1.5 1.4 20.9
3.0 44.4 26.6 29.0 1.09 79.0 7.9 1.1 10.1 1.8 1.7 18.4
2.6 52.6 22.9 24.5 1.07 78.7 8.0 1.1 10.1 2.3 2.0 14.8
Equilibrium moisture basis of coal at 75% relative humidity and room temperature. Dry basis. Dry ash free basis.
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input were calculated as 7.47 £ 10 2, 7.43 £ 10 2 and 7.39 £ 10 2 kJ/s for the coal with the ash contents of 36, 44 and 53 wt%, respectively. These were 1.7, 2.2 and 2.8% less than the actual thermal input of 7.60 £ 10 2 kJ/s. The grid points, excluding the inlet section, were 61 (8.0 m) £ 58 (0.42 m) £ 11 (p/3 rad) in axial (x), radial (r) and azimuthal (u ) directions, and the x and r grids were compressed around the point of expansion (the number of the grid points was decided by comparing with the results for the grid points of 81 £ 68 £ 21). For the inlet section, 2 (0.16 m) £ 16 (0.08 m) £ 11 (p/3 rad) grid points were used in the x, r and u directions. The calculations were continued until the temperature distribution becomes steady (20 000 time steps). The CPU time required for each case was about 60 h on a DEC: Alpha Station 500/500 MHz work station. 4. Experimental results for standard and staged combustions 4.1. Standard combustion
Fig. 3. Gas temperature [temperature of a Pt/Pt±Rh (13%) sheath thermocouple], O2 and NOx concentrations at the center of furnace under the standard combustion condition: (a) gas temperature; (b) O2 concentration; (c) NOx concentration; W, 36 wt% ash; K, 44 wt% ash; A, 53 wt% ash.
hand, char oxidization rate was calculated using Field et al.'s model [7]. For the formation of NOx, three different mechanisms were employed, namely Zeldovich NOx, prompt NOx and fuel NOx. Here, Zeldovich NOx and prompt NOx are classi®ed into thermal NOx. The Zeldovich NOx was evaluated by applying a quasi-steady state approximation for N species to the extended Zeldovich mechanisms (N2 1 O , NO 1 N, N 1 O2 , NO 1 O, N 1 OH , NO 1 H) with the rate constants of Baulch et al. [8]. On the other hand, the prompt NOx and the fuel NOx were predicted using the models by De Soete [9,10]. The discrete transfer radiation method of Lockwood and Shah [11] was used to simulate the radiative heat transfer among the gas, particles and wall. The higher the ash content in coal, more is the combustibility suppressed by the large heat capacity of the ash. To take this effect into account, the value of the thermal input was roughly corrected by subtracting the heat capacity of the ash from the experimental value of 7.60 £ 10 2 kJ/s. The value of the heat capacity of the ash was estimated by a heat capacity database presented by Davis [12]. It was assumed that the ash consists of SiO2 (70%), Al2O3 (25%) and Fe2O3 (5%) and the temperature of the ash increases from 573 to 1800 K. According to this method, the values of the thermal
Fig. 3 shows the distributions of gas temperature measured by Pt/Pt±Rh (13%) thermocouple, O2 and NOx concentrations at the center of the furnace in the standard combustion of Ikeshima coal. As the ash content increases, the gas temperature decreases while the O2 consumption and the NOx formation and reduction are delayed near the burner. Although the gas temperature and the O2 concentration at the exit of the furnace tend to approach certain values independent of the ash content, NOx concentration is higher for a high ash coal. This is because of the shortage of the NOx reduction time because of the delay in NOx formation. Fig. 4 shows the relationship between the conversion of fuel bound nitrogen to NOx, CR, and the index of the fuel ratio divided by the fuel bound nitrogen, FR/FN, for Ikeshima coal, together with that for some typical bituminous coal with low ash content of 7.0±18% [13]. In pulverized coal combustion, most of NOx is thought to be formed from fuel bound nitrogen, and so CR (±) is de®ned by CR 0
CNOx
1 FN 22 2:24 £ 10 £ B 1:4 £ 1022 C B C @ A V
1
dry
where CNOx is the NOx concentration at the furnace exit (±), FN the fuel bound nitrogen (±), and Vdry the ¯ow rate of dry air per feeding rate of coal [(m 3/h)/(kg/h)]. In a previous study [13], CR for bituminous coals was found to increase linearly with FR/FN. On the other hand, although CR for Ikeshima coal with the ash content of 36 wt% is almost on the averaged line for bituminous coals, it tends to monotonously increase with the increasing ash content at a ®xed FR/FN.
R. Kurose et al. / Fuel 80 (2001) 1447±1455
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Fig. 4. Relation between CR and FR/FN under the standard combustion condition: W, bituminous coal with low ash content (previous study); X, Ikeshima coal (present study).
The relation between the unburned carbon fraction, Uc p, and the fuel ratio, FR, is shown in Fig. 5, together with those for some typical bituminous coals with low ash contents of 7.0±18% [13]. Uc p (±) is de®ned by Ucp 1 2 h
Uc CAsh £ 1 2 Uc 1 2 CAsh
2
where h is the combustion ef®ciency of carbon (±), Uc, the unburned carbon concentration in ¯y ash (±), and CAsh, the ash content in coal (±). It was shown that Uc p was
Fig. 6. Gas temperature [temperature of a Pt/Pt±Rh (13%) sheath thermocouple], O2 and NOx concentrations at the center of furnace under the staged combustion condition: (a) gas temperature; (b) O2 concentration; (c) NOx concentration; W, 36 wt% ash; K, 44 wt% ash; A, 53 wt% ash.
proportional to FR for bituminous coals with low ash contents [13]. Uc p of the high ash coal is found to be much higher than that of low ash coal. With increasing the ash content, the unburned carbon fraction increases and the combustion ef®ciency decreases. As mentioned above, the combustibility of the pulverized coal combustion is suppressed as the ash content in coal increases. One of the reasons of this is likely because the heat capacity of the ash increases with the increasing ash content. It is also considered that the ash impedes the char oxidization by covering the combustible matter [14]. The interaction between the ash and the combustible matter in coal will be discussed later. 4.2. Staged combustion
Fig. 5. Relation between Uc p and FR under the standard combustion condition: W, bituminous coal with low ash content (previous study); X, Ikeshima coal (present study).
Fig. 6 shows the gas temperature, O2 and NOx concentrations in the staged combustion of Ikeshima coal. The O2 concentration in the region before the staged air injection port is much less than that in the standard combustion (see Fig. 3), and the NOx concentration at the furnace exit is also
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Fig. 8. Effect of ash content on RNOx and RUc p: W, RNOx; K, RUc p.
NOx concentration and the unburned carbon fraction in the staged combustion increase with the ash content. Fig. 8 shows the effect of the ash content on the ratio of the NOx reduction and the unburned carbon fraction by the staged combustion, RNOx and RUc p. RNOx (±) and RUc p (±) are de®ned by RNOx
NOxstd 2 NOxstg
RUcp 2
Fig. 7. Effect of ash content on NOx concentration and unburned carbon fraction at the furnace exit under the standard and staged combustion conditions: (a) NOx concentration; (b) unburned carbon fraction; W, standard combustion; K, staged combustion.
less because the de®ciency of O2 promotes the NOx reduction. The trends of the gas temperature, the O2 and NOx concentrations with the ash content are similar to those in the standard combustion. As the ash content increases, the gas temperature decreases, and the O2 consumption as well as NOx formation are delayed near the burner, and then NOx concentration at the furnace exit increases. Fig. 7 shows the effect of the ash content on the NOx concentration and the unburned carbon fraction at the furnace exit, together with those in the standard combustion. It is clearly observed that the staged combustion decreases the NOx concentration and increases the unburned carbon fraction at the furnace exit. The decrease of the NOx concentration is resulted from the promotion of the NOx reduction, as mentioned above. On the other hand, the increase of the unburned carbon fraction is caused by the remarkable de®ciency of the O2 before the staged combustion air port. Similar to the results in the standard combustion, both the
NOxstd Ucpstd 2 Ucpstg Ucpstd
3
4
and these values are valid to clarify the effects of the staged combustion on the decrease in NOx concentration and the increase in the unburned carbon fraction, respectively. Here, NOxi (±) and Ucip (±) are the NOx concentration and the unburned carbon fraction at the furnace exit, and the subscripts `std' and `stg' indicate the values in the standard combustion and the staged combustion, respectively. It is found that RNOx decreases and RUc p increases in proportion to the ash content. This means that with the increasing the ash content, the NOx reduction because of the staged combustion weakens, while the increment of the unburned carbon fraction because of the staged combustion becomes remarkable. 4.3. Interaction between ash and combustible matter in coal The in¯uence of the interaction between ash and the combustible matter in coal on the combustion characteristics was investigated by a SEM (Scanning Electron Microscope) photograph and Si and Al image taken by an EPMA (Electron Probe Micro Analyzer). The mass fraction of SiO2 and Al2O3 in the ash of Ikeshima coal is about 90%, therefore the location of the particles can be checked by the SEM photograph and the ash component can be roughly judged by the image of Si and Al. An example of the SEM photograph and the Si and Al image is shown in Fig. 9. It was observed that the Ikeshima coal consists of
R. Kurose et al. / Fuel 80 (2001) 1447±1455
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Fig. 9. SEM photograph and Si and Al image by EPMA for Ikeshima coal containing 53 wt% ash: (a) SEM photograph; (b) Si and Al image by EPMA.
Fig. 10. Contour of gas temperature and NOx concentration in the furnace for Ikeshima coal containing 36 wt% ash. Red and blue colors indicate high and low values, respectively (300 # T # 16008C, 0 # YNOx # 500 £ 10 26): (a) gas temperature; (b) NOx concentration.
Fig. 11. Contour of gas temperature and NOx concentration in the furnace for Ikeshima coal containing 53 wt% ash. Colors are as explained in Fig. 10: (a) gas temperature; (b) NOx concentration.
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R. Kurose et al. / Fuel 80 (2001) 1447±1455
Fig. 12. Comparisons of measured and calculated data of gas temperature and NOx concentration at the center of furnace: (a) gas temperature; (b) NOx concentration; W, K, A, experiments for 36, 44 and 53 wt% ash; Ð, ± ± ±, - - - -, calculated values for 36, 44, 53 wt% ash, respectively.
three kinds of particles; a combustible particle, an ash particle and a mixed particle. The comparisons of the SEM photograph and the Si and Al image for the Ikeshima coal containing 36, 44 and 53 wt% showed that the ratio of the ash particle and that of the mixed particle increases for higher ash coal, although the ratio of the combustible particle decreases. This suggests that the combustibility of the pulverized coal combustion is suppressed not only by the increase in the heat capacity of the ash but also the interaction between the ash and the combustible matter in coal, since the mixed particle is considered to be dif®cult to burn because the ash covers the combustible matter and impedes the char oxidization [14]. 5. Numerical results for staged combustion In this section, the experimental results for the pulverized coal combustion characteristics in the staged combustion are compared with the numerical solutions. Figs. 10 and 11 show the contour of gas temperature and NOx concentration in the furnace for the coals containing 36 and 53 wt% ash. For both cases, the gas temperature is high near the center of the furnace. The region of the maximum gas temperature corresponds to the region of the maximum NOx concentration. The gas temperature
Fig. 13. Comparisons of measured and calculated data of NOx concentration and unburned carbon fraction at the furnace exit: (a) NOx concentration; (b) unburned carbon fraction; W, experiment; Ð, calculation.
and NOx concentration are higher for the 36 wt% ash coal than for the 53 wt% ash coal near the burner. The calculated gas temperature and the NOx concentration at the center of furnace are compared with the measurements in Fig. 12. Fig. 13 shows the comparisons of the NOx concentration and the unburned carbon fraction at the furnace exit. The numerical trends are found to be in general agreement with the measurements. As the ash content increases, the calculated gas temperature and NOx concentration at the center of the furnace decrease near the burner, and both the NOx concentration and the unburned carbon fraction at the furnace exit increase. The results suggest that the present numerical simulation for the pulverized coal combustion is applicable for the pulverized coal combustion at least to know the qualitative effects of the ash. It is considered that the discrepancies between calculations and measurements are probably due to that the shapes of the furnace and the burner modeled in this calculation are not completely the same as those of the measurements. Also,
R. Kurose et al. / Fuel 80 (2001) 1447±1455
compared to the measurements, the calculated unburned carbon fraction at the furnace exit are low especially for high ash content coals [Fig. 13(b)]. It is speculated that this is attributed to the fact that the interaction between ash and the combustible matter is not taken into account for the numerical model of char oxidization. To estimate these combustion characteristics for high ash content coal more precisely, the char oxidization model, which takes the presence of ash into account, should be proposed.
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4. The numerical solutions obtained for the staged combustion condition are in general agreement with the measurements. The differences between them are partially because the interaction between ash and combustible matter is not taken into account for in the numerical model.
Acknowledgements 6. Conclusions The in¯uences of ash content on pulverized coal combustion characteristics were experimentally and numerically investigated by using three high ash coals. The main result can be summarized as follows. 1. Under both standard and staged combustion conditions, as the ash content increases, the gas temperature decreases, while the O2 consumption, the NOx formation and reduction are delayed near the burner. Also, the conversion of fuel bound nitrogen to NOx and unburned carbon fraction at the furnace exit increase with the ash content. 2. In the staged combustion, the O2 concentration in the region before the staged combustion air injection port is found to be much less than that in the standard combustion, and therefore the NOx concentration at the furnace exit is less because the de®ciency of O2 suppresses the NOx formation. The unburned carbon fraction at the furnace exit is higher in the staged combustion than in the standard combustion. With increasing the ash content, the NOx reduction because of the staged combustion weakens, and the increment of the unburned carbon fraction due to the staged combustion becomes remarkable. 3. The low combustibility of high ash coal is considered to be caused partly by the large heat capacity of the ash and partly by the blocking of gas diffusion due to ash layer over char.
The authors would like to thank M. Kimoto and H. Tsuji of CRIEPI for helpful discussions. References [1] Kurose R, Tsuji H, Makino H. Submitted for publication. [2] Launder BE, Spalding DB. Comput Methods Appl Mech Engng 1974;3:269. [3] Patankar SV. Numerical heart transfer and ¯uid ¯ow. New York: Hemisphere, 1980. [4] Crowe CT, Sharma NP, Stock DE. Trans ASME J Fluids Engng 1977;99:325. [5] Mitchell JW, Tarbell JM. AIChE J 1982;28(2):302. [6] Manickam M, Schwarz MP, Perry J. Appl Math Model 1998;22:823. [7] Field MA, Gill DW, Morgan BB, Hawksley PGW. The combustion of pulverised coal. British Coal Utilisation Research Association, Leatherhead, Surrey, 1967. [8] Baulch DL, Drysdall DD, Horne DG, Lloyd AC. Evaluated kinetic data for high temperature reactions. London: Butterworth, 1973. [9] De Soete GG. 15th Symposium (International) on Combustion. The Combustion Institute, 1975. p. 1093. [10] De Soete GG. 23rd Symposium (International) on Combustion. The Combustion Institute, 1990. p. 1257. [11] Lockwood FC, Shah NG. 18th Symposium (International) on Combustion. The Combustion Institute, 1981. p. 1405. [12] Davis PR. A model to predict the physical characteristics of ®reside deposits in pulverized coal combustion chambers. MS thesis, Department of Chemical Engineering, Brigham Young University, Provo, UT, December 1988. [13] Makino H, Sato M, Kimoto M. J Jpn Inst Energy 1994;73:906 (in Japanese). [14] Saro®m AF, Howard JB, Padia AS. Combust Sci Tech 1977;16:187.