CO2 conversion to CO by fluidized bed biomass gasification: Measuring CO2 utilization via stable carbon isotope ratios

Abstract

Thermochemical conversion of CO₂ in biomass gasification is a promising technology for utilizing CO₂ as a feedstock to produce a CO-rich gas. Simultaneous decomposition reactions of biomass and various gas-solid and gas-gas reactions form the product gas in this process. The overlap in sub-processes makes it challenging to assess the conversion of feedstock CO₂ with common methods like mass balancing. This work introduces stable carbon isotope ratio analysis (δ¹³C) to identify the sourcing of carbonaceous product gas components and determine the conversion of CO₂. This methodology is applied to evaluate experiments conducted for one hour of continuous operation in a lab-scale fluidized bed gasifier. Softwood pellets and wood char are used as fuel, with Olivine as a bed material, a target heating temperature of 1000 °C and atmospheric pressure. Product gas with more than 80 vol% CO was generated when wood char was used as fuel. Stable carbon isotope measurements show that CO₂ is converted at 48–93% in this process, underpinning the position of biomass CO₂ gasification as carbon capture and utilization technology. These results were up to 25% higher than suggested by mass balancing, with higher discrepancies at lower CO₂ conversions when using softwood as fuel. Therefore, stable carbon isotope ratio measurement can be a valuable tool for improving the process understanding of biomass CO₂ gasification. The results can be used for carbon accounting and the technical development of gasifiers with high CO₂ utilization efficiency.