Effects of Thermal Pretreatment and Catalyst on Biomass Gasification Efficiency and Syngas Composition

This work explores the combined effects of thermal pretreatment and using a catalyst in situ on gasification carbon conversion efficiency, as well as product gas and tar content and compositions. To compare the effects of thermal pretreatment, pelletized and ground oak with three different levels of thermal pretreatment were gasified in a fluidized bed reactor. The pretreatments applied to the oak were (1) pelletization, (2) drying at 180°C in air, and (3) torrefaction at 270°C in nitrogen. The oak dried at 180°C produced syngas of similar quality and approximately the same amount of char as untreated oak. Torrefaction at 270°C resulted in syngas with a higher hydrogen to CO ratio, lower methane, and less than half of the total tar - all of which are desirable properties in terms of product gas quality. However, the oak torrefied at 270°C also produced more than two times the amount of char as the untreated, pelletized oak. To determine the effect of catalyst, a series of experiments were conducted using olivine impregnated with nickel and cerium as the fluidized bed material in the gasifier. These tests showed that modified olivine can improve hydrogen production and reduce methane and tar levels in the syngas. The result was observed for both treated and untreated oak; although the effect was more substantial for untreated oak, for which the use of modified olivine reduced tar concentrations in the product gas by 60%, with a larger reduction in heavier tars than lighter tars. This result is important because reduction in heavier tar plays a more important role in benefitting downstream operations.