Effects of hydrogen pressure on hydrogasification of Taiheiyo (Japan) coal

Effects of hydrogen pressure on hydrogasification of Taiheiyo (Japan) coal Mitsunori

Makino

and Yuzo Toda

National Research institute for Pollution and Resources, Tsukuba, lbaraki, Japan (Received 29 August 1980; revised 5 December 1980)

The non-isothermal hydrogasification of Taiheiyo coal isstudied at hydrogen pressures up to 5 MPa and temperatures of 900°C using a high-pressure thermobalance and tubular reactor. Gaseous products are analysed and liquid products obtained from the mass balance. Rates of formation of methane increased with temperature to two maxima, at 550°C and at 750°C. Corrections to rate are necessary because of appreciable weight losses. In the temperature range 650-800X the activation energy of methane formation is w 115 kJ mol-‘. Below 55O”C, the pressure dependence of reaction is 0.3, becoming first order at higher temperatures. Rates of formation of methane and ethane indicate a similar mechanism of formation. Rates of formation of liquid hydrocarbons maximize at ~450°C and increase with hydrogen pressure.

Many investigators report studies on hydrogasilication of carbonaceous substances from industrial and scientific view-points to obtain information for producing high BTU gas and for an understanding of reaction mechanisms’. Many have studied hydrogasification reactions of carbonaceous substances, but not untreated raw coal The reactions of coal are not understood because of its highly complicated structure resulting in secondary reactions between tar and hydrogen. Proposed theories’ - ’ still require detailed information about coal gasification to be able to interpret comprehensive reaction mechanisms of hydrogasification of carbonaceous substances. The authors followed the hydrogasification reactions of an untreated raw coal under pressure by measuring the changes in both coal weight and formation of product gases with temperature using a high-pressure thermobalante and a tubular reactor under non-isothermal conditions which, as recommended by van Heek et d8, have advantages for gasification studies.

EXPERIMENTAL The weight change of Taiyeiyo coal during hydrogasification was measured by a flow-type, high-pressure thermobalance, as shown in Figure 2. Approximately 1 g of coal, sized: 32-60 Tyler mesh sieve, was placed in the balance and heated at 3.3”C min - ’ to 900°C under hydrogen flowing at ~2 1 min- l at experimental pressures of 0.5-5 MPa. The pressurized hydrogen flows downwards in the inner tube to the outlet accompanying evolved gases through the space between the inner and outer tube. Argon gas filled the outside of the outer tube at the same pressure as the hydrogen preventing the breakage of the outer tube. The weight change of sample is detected electrically. It was impossible to analyse the composition of effluent gases with reasonable accuracy because of the unavoidable large deadspace of the balance. Also, the time-lag in getting product gases to the outlet resulted in distorted values. The changes in rate of formation of gaseous products, there001~2361/81/040321-06$2.00 01981 IPC Business Press

fore, was measured by use of the flow-type tubular reactor’ connecting with a series of gas chromatograms under the same conditions as those of the thermobalance except for the flow rate of hydrogen. Excess hydrogen was supplied, flowing with constant linear velocity of * 20 cm m- ’ independent of pressure, to avoid the possible and to shorten the time lag. secondary reactions”” Weight changes in the high-pressure thermobalance were not affected by the flow of hydrogen”. The analyses of the coal have been described previously”. RESULTS AND DISCUSSION Figure 2 shows the weight loss of Taiheiyo coal with temperature at various pressures of hydrogen, and as a control, under 0.1 MPa pressure of helium. Differences between the curves are attributable to reaction with hydrogen’. Weight losses begin at ~320°C; they have the same values as for pyrolysis up to ~420°C indicating no hydrogenation of coal below %42O’C with hydrogenation reactions starting at 420°C. Hydrogasilication increases with pressure at a given temperature. At 5 MPa, a weight loss of 50% occurs at 500°C and of 100% at 900°C. At 0.6 MPa, 70% weight loss occurs at 900°C in the non-isothermal process. Rates of hydrogasilication are greatest 450-500°C and 750-850°C. Figure 3, 6 and 8 show rates of formation of gaseous products at increasing temperature using the flow-type tubular reactor. The solid line is for the experiment using helium. Figure 3 indicates little formation of methane up to 420°C by hydrogasification. Rate of methane formation by hydrogasilication reaches a first maximum at 2: 550°C independent of pressure. In pyrolysis, methane formation shows a maximum at 500°C. Rates of methane formation then decrease to a minimum at 60&650”C to reach a second maximum at 800_85O”C, followed by a decrease. Hence, methane may be formed by two distinct mechanisms at these lower and higher temperatures. At 550°C rates of hydrogasification are pressure dependent.

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K- ‘) is almost independent of pressure, whereas below 650°C k has larger values at lower pressures. Thus, above 650°C the rate of methane formation is proportional to both the hydrogen pressure and the weight of unreacted coal; below 650°C these assumptions in equation (1) are not applicable. This shows that the reaction mechanisms above and below 650°C are intrinsically different. The reaction below 650°C will be discussed later. The activation energy for methane formation, w 115 kJ mol-‘, was obtained from the straight line position of Figure 5 in the temperature range 65C~80O”C, and is in agreement

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Figure 2 Weight loss during hydrogasification. atmospheric pressure;. . . . , 0.6 MPa; - - - -, 3.0 MPa; -, 5.0 MPa

-, Pyrolysis at 1 .l MPa; -. -,

Flow-type, high-pressure thermobalance. 1, Permanent Figure 1 magnet; 2, electromagnet; 3, photocell to detect inclination; 4, hydrogen inlet; 5, gas outlet; 6, inner tube (quartz); 7, outer tube (quartz); 8, electric heater; 9, argon inlet; 10, thermocouple; 11, sample holder (net made from stainless steel)

At SO&850°C rates are independent of pressure. This may be due to the fact that no correction is made at 8OtS85o”C for differences in weight loss of char. A pressure dependence was observed when the rate of methane formation, per weight of existing unreacted coal, was used instead of initial weight of coal (Figure 4). To examine the quantitative dependence of rate of formation of methane with pressure and weight of unreacted coal, it can be assumed that, as a first approximation: R

kPW

w,-mW,

where: R, the rate of methane formation (cm3 “C-l); W0 weight of coal initially fed (g); k, rate constant (cm3 min- ’ gg’ MPa-‘); P, h yd rogen pressure (MPa); m, heating rate (“C min- ‘); and W weight of unreacted coal (g). The rate constant, k, at any required temperature can be calculated because R/W,, left side and W/W, right side, of equation (1) can be obtained from the ordinates of Figures 2 and Figure I, respectively. Calculated values of k at intervals of 25°C are plotted against T- ’ (K- ‘) in Figure 5 which shows that the rate constant k above 650°C (1.1 x 10e3

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0

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Figure3 Formation rate of methane versus temperature. -, Pyrolysis at atmospheric pressure; . . . ,0.6 MPa; - - - -, 1.1 MPa; -.,3.0 MPa; -, 5.0 MPa

Effects of hydrogen

pressure on hydrogasification

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TQmpQratUrQ (“C) Figure 4 Formation rate of methane per weight of existing unreacted coal versus temperature. Pyrolysis at atmospheric pressure;. . . , 0.6 MPa; - - - -, 1.1 kPa; . -, 3.0 MPa; -, 5.0 MPa

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with Zielke et aL3, 70-210 kJ mol-I, Moseley et al.‘, 4% 80 kJ mol-’ and Birth et al.“, 120 kJ mol-‘. The rate constant above 800°C falls below the extrapolated straight line between 65&8OO”C. This may be owing to an increase of the proportion of inactive carbon in residual char at high burn-off3, or to entry into a diffusion control of rate. Below 650°C the rate constant is higher with decreasing hydrogen pressure. Below 550°C the rate constants decrease rectilinearly at constant gradient (Figure 5). It may be that below 550°C the pressure dependence of rate for hydrogen is not first order. An order of 0.3 with respect to hydrogen pressure eliminated the pressure dependence of the rate constant, leaving unchanged the order with respect to weight of residual char. Complicated changes in the rate constant between 55s650°C probably result from the overlap of production of methane below 550°C and above 650°C via different mechanisms. Figure 6 shows rates of formation of hydrocarbons other than methane with reaction temperature. No acetylene was detected with only traces of propylene and butane found at ~450C. Rates of formation of the gaseous hydrocarbons in the presence of hydrogen are equal to those of pyrolysis to * 42O”C, but then deviate above the pyrolysis values. At *55O”C, rates of formation of ethane, ethylene and propane under higher pressure show maxima, whereas those of propane and ethylene under lower pressures do

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Figure 5 Arrhenius plot for methane 0, 1 .l MPa; n, 3.0 MPa; q,5.0 MPa

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Formation rate of geseous hydrocarbons versus temfigure 6 perature. (a) CzH4; (b) CsH,; (cl CzHg. Pyrolysis at atmospheric pressure;. . . , 0.6 MPa; - - - -, 1.1 I\r;Pa; - . -, 3.0 MPa. , -, 5.0 MPa

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ing to Figure 7, show similar behaviour to the data of Figure 7, although points were scattered because of large

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Figure 7 Relation between formation rate of ethane and methane 0, 0.6 MPa; 0, 1 .l MPa; n, 3.0 MPa; 0, 5.0 MPa by hydrogasification.

not show maxima. At w 64O”C, the rates of formation of ethane and propane under higher pressure show second maxima, whereas those of ethane, ethylene and propane under lower pressure do not show these maxima. For ethane formation under higher pressure, the decrease in rates from the maxima at =55O“C to the minima at ~580°C correspond to the formation mechanisms of methane. Figure 7 shows, for all experiments, that the rate of formation by hydrogasification of ethane varies rectilinearly with methane. This suggests that the mechanisms of formation of ethane and methane by hydrogasification are identical and unchanged from about 475 to 600°C. Figure 7 also shows that the gradient of the straight line, the ratio of formation rate of ethane to that of methane, increases with pressure. It is established that devolatilization is extensive in this temperature range. The devolatilization, therefore, must influence the formation of gaseous hydrocarbons by hydrogasilication in some way. The active sites caused by cleavage of chemical bonds on devolatilization may play an important role in the formation of gaseous hydrocarbons under pressurized, hydrogen. The gaseous hydrocarbons may be formed by the addition of hydrogen to the active sites and/or to double bonds within adjacent aromatic rings. With increase in hydrogen pressure, the addition of hydrogen becomes extensive within clusters that consist of several aromatic rings, resulting in the increase of the probability to form larger molecular gaseous hydrocarbons, e.g. ethane and propane. This may be a reason for the pressure effect on the increase in the gradient of the straight line of Figure 7. The increase in gradient in Figure 7 does not vary with hydrogen pressure in a direct way, suggesting that the effect of using higher pressure begins to lose its advantage. This may occur on account of the limited numbers of aromatic rings in a cluster within a coal molecule. Rates of formation of ethylene and propane, when plotted against the rate of formation of methane accord-

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analytical errors associated with low rates of formation. This seems to imply also that ethylene and propane are formed by hydrogasification by the same mechanism as that of methane and ethane. Figure 8 shows the rates of evolution of carbon monoxide and dioxide, with reaction temperature. Rates of evolution of carbon monoxide and carbon dioxide increase up to maxima at ~0(400~C,for both pyrolysis and gasification systems and decrease to zero at 57&7OO”C and * 600°C for carbon monoxide and carbon dioxide, respectively, in gasification reactions. In the presence of helium these oxides of carbon continue to be evolved. Clearly, the decrease to zero at ~6OO’C of evolution of oxides of carbon must be due to hydrogasification process’. Figure 9 shows the changes with temperature in rates of formation of liquid hydrocarbons as calculated from the carbon balance* using the measured weight of coal-feed, of gases evolved and of residual clear. The rates of formation of liquid hydrocarbons, (Figure 9) increase from zero at w300°C, independently of hydrogen pressure to a maxima at ~450“C, the rates now increasing with increasing pressure. The rate for the hydrogenation at 5 MPa is double that of the pyrolysis reaction, but decreases to zero at w 700°C and 800°C for * The rate in Figure 9 includes larger errors than those in Figures $6 and 8, as residual chars (daf basis) are assumed to consist of carbon alone in the whole range of temperature

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Formation rate of carbon oxides versus temperature. Figure 8 Pyrolysis at atmospheric pressure: . . . (a) CO; (b) COz. 0.6 MPa; - - - -, 1 .l GPa; . -, 3.0 MPa; -, 5.0 MPa

.,

Effects

of hydrogen

pressure

on hydrogasification

of coal:

M. Makino

and

Y. Toda

carbonaceous materials including carbon monoxide and carbon dioxide. The values were obtained from the integrations of curves in Figure 9, from the values in Figure 10 by eliminating the content of hydrogen in the gaseous hydrocarbons, and from the values in Figure 2 and ultimate analyses of residual char at 900°C respectively. The conversion to total carbonaceous materials and to

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Figure9 Formation rate of liquid hydrocarbons versus temperature. , Pyrolysis at atmospheric pressure; .,0.6 MPa; 0, 1.l MPa; 8, 3.0 MPa; 0,5.0 MPa

pyrolysis and hydrogenation reactions, respectively. The fact that the temperature, 450°C at which the rate shows a maximum for all the hydrogenations coincides with that for pyrolysis, and that the maximum value increases with increasing hydrogen pressure suggest that the depolymerization of coal material to the lower-molecular-weight materials, occurs substantially with similar mechanisms for pyrolysis and hydrogasification. Effects of pressure on the yield of gaseous hydrocarbons are shown in Figure 10. Yields were obtained from the area under the respective curves of formation rates below 900°C in Figures 3 and 6*. The yields of ethane, propane and ethylene increase with pressure rectilinearly, whereas the yield of methane increases to a maximum at w 1.5 MPa and then decreases above 1.5 MPa. The yield of methane is much higher than that of other hydrocarbons. As seen in Figure 3, the contribution of amount of methane evolved above =65O”C to the total yield of methane is very large. The rate of methane formation above 650°C is, as described previously, approximately proportional to the weight of unreacted coal and the hydrogen pressure. The gradual decrease in methane yield from the maximum at w 1.5 MPa, therefore, may be attributed more to a decrease in weight that to an elevation in hydrogen pressure. However, the increase in the yield up to the maximum may have resulted from the advantageous pressure effects. The yield of the other gaseous hydrocarbons, C,H,, C,H, and C,H,, which only had a small contribution to the amounts of gas evolved above 650°C continued, unlike methane, to increase with pressure. Figure I I shows the effects of pressure on the conversion, up to 900°C of carbon in the coal to liquid hydrocarbons, to gaseous hydrocarbons and to total * In all measured

the Figures in this paper, except Figures 10 and II, the values for 2 and 4 MPa were eliminated for the sake of clarity

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liquid hydrocarbons increases with pressure; the former shows a steep increase to a value more than 80 wt% at 1 MPa followed by a gradual approach to the saturation value. The latter increases almost rectilinearly. The decrease from the maximum at 1.5 MPa corresponds to the decrease of methane yield which is predominent compared with the yields of ethane, ethylene and propane. It must be noted that, under our limited experimental conditions, namely the non-isothermal method with the slow heating rate of 3.3”C min-’ and the excess flow of hydrogen, the conversion to the gaseous hydrocarbons may decrease with pressure above w 1.5 MPa comincrease to liquid with the continuous pared hydrocarbons. CONCLUSIONS

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REFERENCES

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Below w 420°C only coal pyrolysis occurs, the presence of hydrogen having no detectable effects. In the temperature range between 45@-600°C the formation of liquid hydrocarbons was accompanied with formation of gaseous hydrocarbons. The rate of formation of liquid hydrocarbons reached maxima at ~450C, the value of which became higher with increasing hydrogen pressure resulting in a rectilinear increase in yield of liquid hydrocarbons with pressure. The rectilinear relation of the formation

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rates between methane and ethane in the temperature range 45&6OO”C suggests that the formation of gaseous hydrocarbons occurs by the same reaction mechanism in this temperature range. Above 600°C methane was predominantly produced at a rate proportional both to the hydrogen pressure and to the weight of simultaneous unreacted coal with an activation energy of w 115 kJ mol-‘.

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Anthony, D. B. and Howard, J. B. AIChE J. 1976, 22, 625 Moseley, F. and Paterson, D. J. J. Inst. Fuel 1965, 38, 13 Zielke, C. W. and Gorin, E. Jnd. Eng. Cent. 1955, 47, 820 Feldkirchner, H. L. and Linden, H. R. Ind. Eng. Chem. Process Des. Deo. 1963, 2, 153 Blackwood, J. D. and McCarthy, D. J. Aust. J. Chem. 1966, 19, 797 Zahradnik, R. L. and Glenn, R. A. Fuel 1971, 50, 77 de Koranyi, A., Parkyns, N. D. and Peacock, S. J. Proceedings of 5th London International Carbon and Graphite Conf., London, 1978, 139 van Heek, K. H., Juntigen, H. and Peters, W. J. J. Inst. Fuel 1973, 249 Makino, M. and Toda, Y. Fuel 1979, 58,231 Makino. M. and Toda. Y. Fuel 1979.58. 573 Makino; M. et al. J. F;el Sot. Japan. 1980, 59, 18 Birch, T. J., Hall, K. R. and Urie, R. W. J. Inst. Fuel 1960,33,422