Pretreatment and Posttreatment Approaches for Reducing Biomass Inorganic Impurities during Gasification

During gasification, biomass can be thermally decomposed into a mixture of combustible gases known collectively as syngas to produce heat, power, and liquid fuels in downstream processes. However, inorganic elements, present in biomass and released into syngas as impurities, are considered one of the main deterrent to the commercial deployment of biomass gasification technologies and have recently contributed to the failure of a thermochemical biorefinery. In conventional gasification installations, a syngas cleanup process area, which comprises a combination of processes, is used to reduce the concentrations of these inorganic impurities below tolerable limits.

In recent years, several treatment approaches have been explored to reduce the concentrations of inorganic impurities in syngas and, ultimately, decrease the cost associated with syngas cleanup. This project is aligned with these recent advancements and seeks to develop gasification treatments to mitigate the impact of inorganic species. Specifically, the project objectives are to i) develop a hot water extraction (HWE) based pretreatment approach to reduce select inorganic elements (N, S, Na, K, Ca, and Mg) relevant to gasification; ii) develop and evaluate new sorbents for posttreatment and; iii) evaluate the impact of HWE pretreatment and sorbent posttreatment on the overall economics of the gasification plant.

In the first objective, HWE was carried out at five temperatures (60, 80, 100, 120 and 140 °C) and three soaking times (15, 30, 45 min) and the effect of these two parameters on the responses (N, S, Na, K, Ca, and Mg reduction) were investigated by a two factor analysis of variance of an unbalanced, complete factorial design at the 0.05 significance level. It was observed that the acidity of the extraction liquor increased with both temperature and time with lower pH resulting in higher total inorganic reduction.

For posttreatment of gas phase inorganics in syngas, layered double hydroxide based mixed metal sorbent was synthesized, characterized and evaluated against commercial sorbents on a model syngas mixture. The sorbents were thermally stable in the hot gas cleanup temperature range (300 - 700 °C). Further, fixed bed experimental evaluation of hydrogen chloride (HCl) gas removal was conducted on the sorbents.