Hot syngas cleanup for pilot two-stage fluidized bed steam-oxygen biomass gasification plant

Abstract

Biomass gasification as a renewable energy technology has been a widely explored research and development area. The efficient and economic removal of harmful components, particularly tars, in raw syngas from the biomass gasifier is still a major challenge. In this study, a novel two-stage fluidized bed pilot-scale gasifier has been developed to enhance the steam-oxygen biomass gasification to generate low-tar syngas; while, a prototype hot syngas cleanup system has been designed, built and tested to further reduce the tar content and purify the syngas from the biomass gasifier for downstream applications. The results showed that the tar removal efficiency by a catalytic tar cracker using an iron-based bauxite residue derived catalyst prepared in-house can reach 82.8-98.0% at reaction temperatures of 678-801°C, and 90.6-98.0% at 784-801°C, respectively. Furthermore, the tar content of the cleaned syngas can be as low as 0.10-0.65 g/Nm³ when the raw syngas tar content is 2.59-27.71 g/Nm³. In the case of syngas composition, H₂ content ranged from 32.7% to 48.0%, CH₄ from 2.8% to 4.8%, CO from 26.3% to 35.7%, and CO₂ from 18.4% to 33.9%. The H₂/CO molar ratio varies from 1.0 to 1.8, requiring the application of the water-gas shift reaction to increase the H₂/CO ratio to 3 for downstream methanation to produce renewable natural gas.