Pressurized hydrogasification

The hydrogasification reactor operates at pressures of 1000-1500 psig and at temperatures of 760°-982°C in order to obtain the proper reaction rates and yields of methane required for process optimization. About 50% of the feed carbon is converted to gases in the hydrogasifier. 

Hydrogasification. Hydrogasification of coal involves reaction of hydrogen with coal carried out at elevated temperatures under high partial pressure of hydrogen. The objective is to add sufficient hydrogen to coal to produce methane as the major product. It has been found that many types of coal can be hydrogasi-fied if the coal is heated rapidly to reaction temperatures. Even under favorable conditions, however, conversion to methane is not complete and aromatics such as benzene are made as by-products. 

Figure 7. Flash hydrogasification of coal maximum percentage of carbon conversion to CHk vs. pressure (at 900°C). Solid residence time (ts) gas residence time (tg) North Dakota lignite (O) New Mexico subbituminous (O).

At 850 C and 30 bar, beside 100% formation of the pyrolysis products C2H4, CO, and CO2, more than 95% of ethane, and about 80% of methane were formed within the first 10 minutes. After that only methane, and in much less extent ethane, continued to be formed, through the hydrogasification of char. Figure 6 presents the mean concentration of the main carbon-containing components in the product gas as a function of pressure. It can be seen how the concentration of CO, CO2, and C2H4 decreases with pressure, while the concentration of CH4 and CjH increases with pressure. 

We mounted samples of feed coal in epoxy resin under hydraulic pressure as described by Cole and Berry (2). An apparatus and procedure for vacuum-mounting the fragile chars was developed ana is described elsewhere (8). Samples of pretreated coal and hydrogasification residue were sieved, and tests were made on the sieve fractions to avoid error from size segregation. 

Results and Discussion. In dilute-phase hydrogasification, the composition of the effluent gas is determined by the feed gas rate and composition and the gas yield. The feed gas rate and composition can be selected somewhat freely, but the gas yield depends on many variables, some of which interact. The most prominent variables are coal rate, hydrogen coal ratio, maximum temperature attained by the solids and the vapors, residence times of the solids and the vapors, total pressure, hydrogen partial pressure, particle size and density, gas viscosity, heat... 

Catalysts. The catalysts used were commercial cobalt molybdate and a laboratory-prepared depleted uranium catalyst. The cobalt molybdate consisted of cobalt and molybdenum oxides on 6- to 8-mesh alumina granules. The uranium catalyst consisted of 7.7% depleted uranium (uranium from which the U-235 has been removed) in the oxide form on 1/8-in. H-151 alumina balls. This catalyst had produced high gas yields in previous hydrogenation experiments with shale oil, and these results suggested its possible use as a hydrogasification catalyst. Both catalysts were maintained under a hydrogen atmosphere at approximate reaction temperature and pressure for about 12 hours before each experiment. 

Because it requires many possible variables, such as temperature, pressure, the nature of chemical reaction, and the character of the solid surface, and because it incorporates many constants which require experimental evaluation, the general mathematical model to estimate the product gas distribution for different levels of carbon conversion can become exceedingly complicated. Practical application of this model is particularly difficult when a choice has to be made between reaction mechanisms, each of which can generate complex functions with a sufficient number of arbitrary constants to fit any given experimental curve. The purpose of the work discussed in this paper was to study the influence of temperature and the partial pressure of hydrogen and steam on the rate of steam-hydrogen and coal char reactions based on the previous pilot plant data obtained at IGT (10, 11) and to develop a correlation to estimate the performance of a hydrogasification reactor in terms of its product gas distribution for different levels of carbon conversion. 

Pressurized Hydrogasification of Raw Coal In a Dilute-Phase Reactor... 

Results of our most recent FDP reactor operations are summarized in Table I and the analyses of the feed coals used are listed in Table II. The main objectives of these experiments were (1) to establish the feasibility of directly producing a high Btu gas by hydrogasifying raw coal in a continuous reactor at economical pressures, (2) to measure the yields and distribution of coal hydrogasification reaction products, and (3) to provide data for scaling up the FDP reactor. 

This kinetic study of catalyzed hydrogasification reactions utilizes a high temperature, high pressure recording balance. A thermobalance is particularly useful in gas-solid reactions because the weight of small solid samples can be measured continuously. Direct kinetic analysis of the weight loss curves are straightforward. 

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