Novel sour water gas shift catalyst (SWGS) for lean steam to gas ratio applications

Abstract

Dry powder coal gasification is emerging as one of the most energy efficient methods for coal conversion. However, the low steam content, high temperature and high content of CO in the raw syngas make it difficult for a conventional sour water gas shift catalyst to be directly used for syngas conditioning. Conventional sour water gas shift takes place at very high steam to carbon monoxide ratio (H2O/CO), often 2 or above, but the H2O/CO ratio from a dry powder coal gasifier is often less than 0.8. To develop a sour water gas shift catalyst suitable for the lean steam raw syngas, we have prepared a series of MgAl2O4 spinel modified alumina supported CoMoOx catalysts by changing the content of K2O in the promoter, and tested them under lean steam to carbon monoxide (H2O/CO) ratio conditions for sour water gas shift process. Our results show that the addition of potassium into the catalyst increases the catalyst water gas shift activity at a lean steam to gas ratio, and that catalyst activity increases with the K2O content increase in 0–10wt.% range; the potassium additive helps to increase the dispersion of MoO3 and improves catalyst strength and surface area. The increase of K2O content leads to higher catalyst activity for the CO shift reaction with little methane yield, reducing the hot spot formation in the catalyst bed. This may be due to the high K2CO3 content in the catalyst enhancing the surface affinity to steam in the syngas, and the basicity of K2CO3 depresses methane formation over the MoS2 active site. The 10.0wt.% of K2O-containing SWGS catalyst showed the highest stability even in the absence of H2S in the feed gas for up to 90min, and little H2S is released from the catalyst (reverse sulfurization) under such conditions. The optimized K2O-containing SWGS catalyst, e.g., QDB-5-10 has been used in an industrial plant for 2years in a coal to methanol plant and showed stable and unique performance under the lean steam conditions, allowing the 1st stage SWGS reactor to be well within control. The potassium carbonate included in the catalyst is stable and little leachate occurred even after 2-years of use time on stream.