High temperature production and desulphurisation of syngas.

Abstract

Synthetic gas (syngas) produced by the gasification of coal provides a cheap fuel alternative for the production of electricity. However advanced utilization of syngas is limited due to the contaminants which can seriously deactivate the catalysts used in the downstream reactions as well as downstream equipment such as gas turbines. Among the contaminants, sulphur compounds produced in the gasification process, which are mainly H2S with small amounts of COS must be removed. The method presented here is downstream sulphur capture by a metal oxide at high temperatures. In this study, a laboratory scale unit was used to produce and clean synthetic gas (syngas) containing 1.0-1.15 mole % H2S from a liquid hydrocarbon fuel consisting of 86 % methanol, 14 % propanethiol by mass, and 18 mole % oxygen gas as the oxidant. The gasifier operates at 830 0C at atmospheric pressure. Desulphurisation occurs in a fixed bed reactor packed with zinc oxide spherical pellets as the sorbent. Experiments were performed to determine whether the use of the liquid hydrocarbon mixture as fuel in the laboratory could actually produce H2S containing syngas, and eventually compare the composition of the gas produced experimentally to the one predicted by a model. Desulphurisation experiments were performed by varying reaction temperatures (350 0C and 550 0C) and sorbent particle sizes (1.63-2.03 mm) at atmospheric pressure using sorbent with varying surface areas (average of 5 m2/g and 25 m2/g) at high and low gas velocities (average of 3430 h- 1and 610 h-1). These variations were performed in a 2*2 factorial design to determine the effect of these factors on the desulphurisation process and observe whether there is any interaction between them. Statistical analysis was used to determine the significance of each factor on the sorbent sulphur sorption capacity. A packed bed model using shrinking core model was used to describe the desulphurisation process. GC analysis specific to sulphur compound detection showed consistent production of H2S during gasification. In addition to this the composition of the predicted syngas was validated through a GC equipped with a TCD detector and there was a good agreement giving the expected ratio of 2:1 of H2 and CO respectively. Small particle size sorbent with 5.3 m2/g surface area and an average space velocity of 610 h-1 at 350 0C reaction temperature had the highest sulphur sorption capacity of 3.71 Wt. % which was a 20% conversion of zinc oxide to zinc sulphide. There was no effect of increase in temperature on the process. The low surface area sorbents were more effective than the increased surface area sorbents. These results were verified by the statistical analysis performed on the sulphur sorption capacity obtained during experimentation. The packed bed model results were not in agreement with the experimental results.