New insights into the mechanism of biomass char steam gasification process by oxygen-containing functional group as aromatic carbon boundaries: Experimental and DFT study

Abstract

Reducing carbon-based fossil fuel dependence and increasing environmentally friendly energy resources are crucial in combating global warming. Steam gasification technology is the most suitable for hydrogen production from biomass fuels. In addition to the main element “carbon,” the oxygen in biomass char exists in the form of oxygen-containing functional groups attached to the char surface. It could also impact the reactivity of biomass char during steam gasification. In this study, the biomass char model compounds are constructed with oxygen-containing functional groups as aromatic carbon boundaries at 800-1200℃. Firstly, the steam gasification process of different oxygen-containing aromatic boundaries is investigated using a high-temperature steam gasification experimental system. Results show that the presence of oxygen-containing functional groups increases the yield of combustible gas by nearly 20% and facilitates the gasification reaction. The oxygen-containing functional groups substituted at different positions had different effects on the gasification syngas fraction. The proportion of hydrogen produced when each oxygen-containing functional group reacted alone in the composite boundary is 16% higher than their combined level. Subsequently, a theoretical analysis is performed using a density functional theory (DFT). The oxygen-containing functional groups are preferentially generated and released as CO, and the remaining oxygen is transferred to small-molecule gaseous products. The formation of an H2 molecule normally consists of one hydrogen atom each from the H2O molecule and biomass char. The biomass char is expected to preferentially provide the oxygen-containing functional group's H rather than the aromatic boundary's H. It is demonstrated that the existence of oxygen-containing functional groups promotes hydrogen production. The optimal reaction paths and the rate-determining steps for each type of boundary are also determined. It provides new insights and a theoretical basis for understanding the biomass steam gasification process in detail.