The Implication of the Choice between Sour and Sweet Shift on Process Design and Operation of an IGCC Power Plant Integrated with a Dual-Stage Selexol Process

Abstract

In this study, it was sought to design and evaluate two different process configurations of an IGCC integrated with a dual stage Selexol unit where they differ in that one is based on sour shift and the other is sweet shift (Fig. 1). Depending on the choice of the two water-gas shift reactors, the process configuration undergoes significant changes in steam cycle, syngas cooling and Selexol process, apart from the location of the H2S removal step around the shift reactor. Integrating an IGCC with a pre-combustion capture involves a significant energy penalty. The energy penalty associated with carbon capture integration was mainly due to the power plant demanding a greater heat input for running the same gas cycle. Also the syngas must undergo water-gas shift reaction (WGSR) in order to convert CO to CO2, as the ensuing separation process can capture only CO2 efficiently from the syngas. The WGSR needs a huge amount of steam. The syngas leaving the scrubber already contains some steam, as it leaves the scrubber saturated with water. However more steam has to be added to the syngas to have the water-to-CO ratio around 2, to make sure good CO conversion. The extra steam has to be sourced from the steam cycle. Adding new units of the pre-combustion capture process and CO2 compressors augments the total capital cost and the operation of the CO2 capture and compression units increases the auxiliary power consumption. In turn, the net plant efficiency of the IGCC drops significantly with the cost of electricity rising accordingly. In this study, the whole IGCC power plant was simulated by using Honeywell UniSim based on the DOE study. The IGCC simulation also include a dual-stage Selexol process for pre-combustion capture to estimate the energy consumption involved, similarly to the past researches carried out by the group. It turned out that the sweet shift would give rise to greater reduction of the net power efficiency than the sour shift (Table 1), mainly due to it requiring approximately 4.6 times more shift steam, most of which has to be sourced from the steam cycle. This substantial increase of the shift steam usage involves significant alternation to the steam cycle. From the perspective of the energy penalty, the adverse effect resulting from the need for vast shift steam outweighs the positive aspect of the AGR (Acid Gas Removal) unit of the sweet shift case spending less power. On the other hand, the sour shift catalyst (CoMo) was estimated to have a reaction rate much lower than the sweet shift catalyst (Cu-ZnO) by two orders of magnitude at the condition of the low temperature shift reactor. It was certain that the sweet shift case had to undergo significant energy penalty due to it having to source vast steam from the steam cycle. This is not only because the syngas loses almost all the steam while being cooled for H2S removal, but also it requires a steam-to-CO ratio of water-gas shift reaction greater than it would do in sour shift case. To alleviate these adverse effects, the sweet shift case was designed to generate additional LP steam by recovering the heats from the raw and shifted syngas. Also the auxiliary power expensed in the AGR unit was found to be slightly less, due to the H2S absorber also functioning as a H2S concentrator. The resultant net plant efficiencies of the two cases exhibited that the sour shift case would be more energy-efficient than the sweet shift case by 3.7% points as shown in Table 1. However, the advantage would be compromised to some extent, given that the low temperature sour shift reactor had to be sized larger than the equivalent sweet shift reactor, due to the sour shift catalysts exhibiting lower reaction rates than the sweet catalysts at the condition.