Study on Influence of Semi-carbonization Treatment on Co-gasification of Biomass and Coal

The huge demand for energy has led to the massive consumption of coal, causing serious resource and environmental problems. Biomass energy resources are abundant, have a small impact on the environment, are renewable, and have great development potential. However, due to their inherent disadvantages, they are difficult to use on a large scale. Pre-treating biomass through pyrolysis can significantly improve its performance. At the same time, co-gasification of biomass and coal can fully exploit the advantages of high energy density of coal and high reactivity of biomass, making up for the shortcomings of gasification alone. This is of great significance for the industrial utilization of biomass energy on a large scale and the optimization of China’s energy structure. In this paper, corn stalks, rice straw, wheat straw and bituminous coal were used as experimental materials. Firstly, biomass was semi-carbonized at 200–600°C, and the changes in the structure and chemical properties of biomass were investigated. Then, thermal analysis technology was used to study the CO2 reaction characteristics of semi-carbonized biomass and biomass-coal mixture, and further explore the synergistic mechanism.

As the pyrolysis temperature increased, the fixed carbon and volatile content of semi-carbonized biomass increased, volatile matter decreased, O/C and H/C atomic ratios decreased, and the degree of coalification of biomass was transformed into anthracite. The chemical characteristics of biomass were studied using scanning electron microscopy (SEM), specific surface area, Fourier transform infrared absorption spectroscopy (FTIR), etc. The results showed that the polar functional groups in biomass decreased continuously, aromaticity increased continuously, carbon structure became more stable, and heat resistance increased. Higher fixed carbon content and more stable carbon structure increased the starting temperature, ending temperature and maximum weight loss rate of non-isothermal gasification process; isothermal gasification experiments determined that the gasification performance of biomass samples prepared at low temperature (200–300°C) would be slightly higher than that of raw samples, while the gasification performance of medium-high temperature (400–600°C) biomass carbon was lower than that of raw samples. The difference in gasification performance was the result of the combined effect of carbon structure stability and alkali metal content in biomass. Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) was used to study the physicochemical structure of co-pyrolysis coke and gasification residue. The results showed that the aromaticity of co-pyrolysis coke decreased and the stacking height of aromatic layers decreased. Alkali metals in semi-coke hindered the ordering process of co-pyrolysis coke, inhibited graphitization, and alkali metals accumulated on the surface of coal coke and reacted with carbon matrix during gasification process to become active centers for gasification reaction, promoting co-gasification reaction.