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Ind. Eng. Chem. Res. 1996, 35, 3824-3825
CORRESPONDENCE Comments on “Modeling of a Fluidized-Bed Coal Carbonizer” Adam Luckos Council for Mineral Technology (MINTEK), Pyrometallurgy Division, P.O. Box X3015, Randburg 2125, South Africa
Sir: The paper submitted by Goyal and Rehmat (Ind. Eng. Chem. Res. 1993, 32, 1396-1410) describes a computer model of a fluidized-bed coal carbonizer. This model is based entirely upon empirical correlations developed for yields of various species as functions of coal properties and carbonizer operating parameters. The presented study is of great practical interest as the yields and chemical composition of tar, gas, and char produced are important quantities required for process simulation, control, and equipment design purposes. However, a closer examination of the published correlations shows that some of them are incorrect. 1. Equation 28 yields negative values of the nitrogen to carbon ratio in the char for the entire range of applicable temperatures (i.e., 1300-1600 °F). According to Goyal (1994), eq 28 in the paper is in error and should be replaced by the following correlation:
RNC ) -4.5714 × 10-6T + 0.106 21 In Table 1 tar and char yields and compositions calculated in the absence of oxygen at 1472 °F and atmospheric pressure using data described as “Pittsburgh No. 8 bituminous coal” are presented. It is clearly seen from this table that the above correlation gives an enormously high content of nitrogen in the char. As a consequence, the yield of nitrogen gas determined by a nitrogen balance among the feed coal, tar, NH3, and char is negative (see Table 2). This fact indicates that even the corrected version of the correlation for RNC is also in error. 2. Equations 33 and 34 seriously overestimate the fraction of tar carbon cracked due to sorbent addition. The fractions of cracked tar carbon yielding methane and C2+, hydrocarbons, YCH4, and YC2+, calculated according to these equations, are 0.2184 and 0.4021, respectively. For these values of YCH4 and YC2+, the amount of hydrogen required for the conversion of cracked tar carbon to CH4 and C2+ hydrocarbons (C2H4 and C2H6) is greater than the total amount of hydrogen gas (H2) present in the system before sorbent addition. As a result, the value of the hydrogen gas yield in the presence of lime is negative (see Table 2) and, thus, the elemental balance of hydrogen within the system cannot be completed. This observation is also valid for lignites since eqs 33 and 34 are used in their case as well. 3. The model developed by Goyal and Rehmat also predicts substantially lower values for the hydrogen to carbon atomic ratio in the tar than those reported in the literature. Calculations performed at 1472 °F using the ultimate analysis of Pittsburgh No. 8 coal give the following atomic composition of tar produced:
CH0.5024O0.0240N0.0083S0.0100 S0888-5885(95)00021-2 CCC: $12.00
Table 1. Tar and Char Yields and Compositions (per 1 kg of Coal DAF) tar
char
element
kg
% wt
kg
% wt
carbon hydrogen oxygen nitrogen sulfur
0.188 628 0.007 897 0.006 045 0.001 832 0.005 057
90.05 3.77 2.89 0.87 2.41
0.480 970 0.021 182 0.011 202 0.055 649 0.035 841
79.52 3.50 1.85 9.20 5.93
total
0.209 459
99.99
0.604 844
100.00
Table 2. Yields of Gaseous Species with and without Sorbent Addition
CO CO2 H2O H2 CH4 C2H4 C2H6 C3H6 C3H8 C4H8 C6H6 C7H8 C8H10 C6H5OH NH3 H2S COS N2
without sorbent (kmol/kg of coal DAF)
with sorbent (kmol/kg of coal DAF)
0.001 645 0.000 430 0.002 180 0.000 903 0.002 160 0.001 115 0.000 298 0.000 224 0.000 060 0.000 036 0.000 035 0.000 034 0.000 024 0.000 040 0.000 204 0.000 587 0.000 009 -0.001 619
0.001 645 0.000 430 0.002 395 -0.006 463 0.004 141 0.002 573 0.000 662 0.000 224 0.000 060 0.000 036 0.000 035 0.000 034 0.000 024 0.000 040 0.000 204 0.000 670 0.000 009 -0.001 586
The carbonizer balance presented in Figure 7 of the paper shows an even lower value of the hydrogen to carbon atomic ratio in tar produced from the same coal. During devolatilization experiments with 13 different coals ranging in rank from a high volatile bituminous coal to a lignite, Solomon and Colket (1978) and Solomon et al. (1982) have noticed a striking similarity of the structure and atomic composition of the tar to that of the parent coal. The carbon, oxygen, and nitrogen concentrations in the tar were very close to those in the parent coal, whereas the hydrogen concentration was slightly higher, with the H/C atomic ratio being typically around 1.2. Lacey (1967), Kimmel et al. (1976), and Yoon et al. (1978) assume that the chemical composition of tar from bituminous coal may be represented by the formula CH1O0.05. For the tar produced from Montana lignite, Saxena (1990) assumes, following the experimental data of Suuberg et al. (1978), that its chemical composition may be described by the general formula (neglecting oxygen and nitrogen) CH1.5. According to experimental results reported by Tyler (1980), the composition of the tar is proportional to the pyrolysis temperature and the coal atomic H/C ratio. Experiments performed with 10 bituminous coals (including © 1996 American Chemical Society
Ind. Eng. Chem. Res., Vol. 35, No. 10, 1996 3825
Pittsburgh No. 8) on a small-scale fluidized-bed pyrolyzer revealed that the tar atomic H/C ratio decreases with the pyrolysis temperature from approximately 1.2 at 500 °C (932 °F) to 0.7 at 900 °C (1652 °F). For Pittsburgh No. 8 coal at 600 °C (1112 °F), Tyler reports the tar atomic H/C ratio to be as high as 0.97. It seems quite obvious that, in light of the experimental findings described above, the model proposed by Goyal and Rehmat fails to accurately predict the chemical composition of tars produced from bituminous coals under pyrolysis conditions. The proposed model is also physically inconsistent since the elemental balances of nitrogen and hydrogen are not satisfied due to errors in derived experimental correlations. Literature Cited Goyal, A. Private communication, Aug 1994. Kimmel, S.; Neben, E. W.; Pack, E. E. Economics of Current and Advanced Gasification Processes for Fuel Gas Production. EPRI AF-244, Project 239, Final Report, 1976.
Lacey, J. A. Gasification of Coal in a Slagging Pressure Gasifier. Adv. Chem. 1967, 69, 31. Saxena, S. C. Devolatilization and Combustion Characteristics of Coal Particles. Prog. Energy Combust. Sci. 1990, 16, 55. Solomon, P. R.; Colket, M. B. Coal Devolatilization. Proc. Symp. (Int.) Combust. 1978, 17, 131. Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Krause, J. L. Coal Thermal Decomposition in a Entrained Flow Reactor: Experiments and Theory. Proc. Symp. (Int.) Combust. 1982, 19, 1139. Suuberg, E. M.; Peters, W. A.; Howard, J. B. Product Composition and Kinetics of Lignite Pyrolysis. Ind. Eng. Chem. Process Des. Dev. 1978, 17, 37. Tyler, R. J. Flash Pyrolysis of Coals. Devolatilization of Bituminous Coals in a Small Fluidized-Bed Reactor. FUEL 1980, 59, 218. Yoon, H.; Wei, J.; Denn, M. M. A Model for Moving-Bed Coal Gasification Reactors. AIChE J. 1978, 24, 885.
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