Adsorption based deep syngas purification

In order to limit our dependence on fossil carbon, gasification emerges as a versatile conversion platform of various organic feedstocks to syngas for subsequent sustainable fuels and chemicals production. One of the challenges, especially for biomass-to-liquids (BTL) concepts, however, is the economic smaller-scale production of high-purity syngas in proximity to the feedstock origin. Conventional deep contaminant removal often comprises absorption technologies, originally developed for large-scale plants. A simplified final gas cleaning process is therefore proposed, primarily based on adsorption, and applied to smaller BTL processes. Sulfur species are some of the most challenging syngas impurities, and therefore this dissertation experimentally investigated several H2S adsorption materials for the proposed process. Zinc oxide exhibited in long-term packed-bed breakthrough experiments deep H2S removal capabilities even in tar-rich syngas. Breakthrough tests using electric arc furnace (EAF) dust, a zinc-containing steel making side stream, demonstrated excellent high-temperature desulfurization capabilities, even exceeding primary ZnO performance, with up to 240 mg g-1 H2S capture capacity. Low temperature activated carbon (AC) tests revealed that cheaper non-impregnated AC had poor desulfurization performance compared to impregnated- or doped AC, which exhibited capture rates comparable to metal oxides. However, introducing ammonia to the syngas improved non-impregnated AC performance to over 270 mg g-1. The beneficial effect of ammonia was validated in real syngas from biomass, and consequently this method was evaluated as the most compelling low-cost H2S removal method of the tested materials.

To determine the final gas cleaning concept's feasibility as a realistic alternative to conventional solutions, comprehensive gas cleaning experiments in real syngas were performed. A bench-scale final gas cleaning process, "UC5", was operated in a full BTL plant configuration in multiple week-long campaigns in residual biomass syngas with downstream coupling to FT synthesis. Complete and sustained impurities removal was achieved in syngas in varying impurities concentrations and compositions, hence proving the practical feasibility of the cleaning concept. Furthermore, in the subsequent campaigns in dirtier syngas, the contribution of each unit in the process was determined. Additionally, it was demonstrated that activated carbons had affinity for removal of other syngas contaminants such as COS and HCN. Based on the experimental results, this dissertation proposes optimized final gas cleaning process configurations that for H2S removal use either ammonia-enhanced AC or EAF-based adsorbent.