In situ adsorption of CO2 to enhance biomass gasification for hydrogen production using Ca/Ni based composites

Abstract

The concentration of greenhouse gases in the atmosphere continues to rise, leading to serious climate problems, such as global warming and the increase in the frequency of extreme weather. CO2 is the main contributor of greenhouse gases, accounting for 70% of greenhouse gas composition. The combustion of fossil fuels is the main source of CO2 emission in the atmosphere. Biomass energy has always been an important renewable clean energy for human survival. It is the fourth largest energy consumption in the world after coal, oil and natural gas, and plays an important role in the whole energy system. This study aims to develop a novel composite catalyst (Ni–Ce–Ca-ceramic membrane), and to evaluate its catalytic activity and CO2 adsorption capability during biomass gasification. Pine sawdust was used as feedstock for biomass gasification, and a two-stage fixed-bed reactor was used as gasification equipment. MCM-41-ceramic membrane support (MC support) was prepared by coating method, and the composite catalyst (Ni–Ce–Ca-ceramic membrane) was prepared by equal volume impregnation method. The catalyst has the functions of adsorption and catalysis at the same time. While adsorbing CO2, the catalyst can promote biomass gasification to produce hydrogen, and improve the syngas composition of biomass gasification. Not only can it reduce CO2 emission in the atmosphere, but it can also promote the use of biomass energy and reduce the use of fossil fuels. Furthermore, optimal operating conditions (i.e. Ca loading amount, reaction temperature and the ratio of Ni to Ce) of the Ni–Ce–Ca-ceramic membrane catalysts were also investigated. The results show that when the Ca loading amount is 20 wt%, the reaction temperature is 800 °C and the ratio of Ni to Ce is 2:1, the CO2 content in the syngas composition of biomass gasification is the lowest, which is 8.18 vol%. The minimum CO2 yield is 2.1 mmol/g, the maximum hydrogen yield is 14.19 mmol/g, and the lower heating value is greater than 10 MJ/Nm3. Through BET, XRD and SEM analyses, it was found that the prepared catalysts had good structure performance and good adsorption performance. The X-ray diffraction of the used catalyst shows that CaO in the catalysts adsorbs CO2 and produces CaCO3. The supported metals can be well dispersed on the surface of the catalysts. The research results show that the composite catalyst (Ni–Ce–Ca-ceramic membrane) prepared by using ceramic membrane as support can effectively capture CO2 produced from biomass gasification and promote hydrogen production in gasification.