Characteristics of CO2 gasification reaction and biochar structure evolution of rice straw in K2CO3-Na2CO3-Li2CO3 ternary molten salt

Abstract

The integration of solar molten salt thermal storage with biomass thermochemical conversion is a viable strategy for generating clean and renewable energy. Molten salt offers substantial benefits in catalysis and heat transfer, which are indispensable in gasification processes. However, achieving sufficient mixing of the gaseous atmosphere with the liquid-solid phase is challenging. In this study, a laboratory-scale slurry-bed reactor was utilized to ensure efficient mixing of the three phases. The evolution of product distribution and biochar structure during rice straw gasification in a K₂CO₃-Na₂CO₃-Li₂CO₃ molten salt mixture, under conditions emulating practical applications, was examined. High-temperature stage microscopy (HTSM) was used to observe the real-time morphological evolution of the biomass molten salt gasification process, providing valuable insights into its reaction properties. The findings suggest that the biomass molten salt gasification process is predominantly divided into three stages: encapsulation, erosion, and fragmentation. Compared to traditional gasification methods, molten salt gasification shows superior reactivity, leading to an increased gasification reaction rate. This enhancement is mainly due to the catalytic effects of alkali and alkaline earth metals within the molten salt, along with the exceptional heat transfer capabilities of the liquid medium. Moreover, this approach significantly increased syngas yield from 30.07 % to 50.62 % and reduced tar yield from 26.59 % to 23.72 % within a 15 min gasification period. Additionally, the structure of the biochar was improved, resulting in a larger specific surface area, which increased by 242.56 % at 5 min of gasification. The yields of H₂ and CO produced by gasification of biomass in molten salt for 15 min were 3.55 mmol/g biomass and 12.98 mmol/g biomass, respectively.