RECENTLY SEVERAL publications have appeared [l, 2, 31 on the gasification of coal under pressure by the Lurgi process. These have concentrated on the purification and enrichment of the towns gas produced, the gasification process itself receiving less attention. It was felt desirable to calculate the thermodynamically expected gas analysis based on the process variables used in practice, and to compare these with the practical results. In this way an assessment could be made of the degree of attainment of equilibrium. It isnot the intention here to describe the plant itself, it being sufficient to regard the operation as one in which steam and oxygen under pressure are passed counter currently to a bed of suitably sized non-caking solid fuel (coal or briquettes). The fuel bed, described from the bottom upwards, consists of more or less well defined zones of ash, oxidation zone, reduction and hydrogenation zones and a distillation zone respectively.
Any thermodynamic evaluation depends on a knowledge of pressure and temperature. Here the pressure is taken to be the mean operating pressure (the pressure drop across the bed is small in comparison with the mean pressure) and the temperature, somewhat less well defined, as that corresponding to the top of the reduction zones, where the gaseous equilibria apart from the distillation of volatile matter, are probably frozen. The steam referred to is that admitted with the oxygen at the bottom of the producer, any steam produced by the evaporation of moisture in the fuel being disregarded since it does not pass through the reaction zones.
Higher pressures lead to higher rates of gasification. The space velocity of the gases up through the fuel bed is much less than that in low pressure producers, being arranged in practice to be approximately inversely proportional to the pressure. The much lower space velocity does not necessarily lead to a more complete attainment of equilibrium, because there is, proportionate to the pressure, more gas to react. Since the times for reduction of the initial pressure of reactants are respectively directly proportional to pressure for a zero order reaction, independent of pressure for a first order reaction and inversely proportional to pressure for a second order reaction, it is clear that increase of pressure would unambiguously increase the degree of attainment of equilibrium for the gasification process only if all the component reactions involved are of order greater than unity. Without detailed investigation (see [4] for a kinetic treatment of the gasification process at atmospheric pressure) no definite statement can be made as to the effect of pressure on the degree of attainment of equilibrium, other factors affecting it remaining constant.
Methane, a desirable constituent of towns gas in view of its high calorific value, is expected to be formed more readily under high pressure. It is of interest to study its expected degree of formation under varying conditions.