Effects of CeO2 addition ratio on hydrogen production during cellulose gasification catalyzed by Fe-based catalysts

Abstract: Biomass energy is currently the fourth largest energy source in the world. Biomass steam gasification at high temperature is also one of the most promising methods of hydrogen production. However, there are some problems, such as high release content of alkali and alkaline earth metal, high tar content, deactivation of catalysts, and low hydrogen content in the conversion process of biomass. These shortcomings have prevented the application of biomass gasification in modern energy and chemical industries. In the process of gasification, the catalyst usually plays an important role in tar conversion and hydrogen production. The iron-based catalysts well perform in the process of thermochemical conversion, where the CeO2 is a very effective catalyst additive, indicating significantly improve the thermal stability of iron-based catalysts. When the proportion of CeO2 in CeO2/Fe2O3 bimetallic Catalyst is high, CeFeO3 with perovskite structure can be synthesized. It has also been proved that CeFeO3 has a high oxygen-carrying capacity, strong oxygen transferability, fast reaction rate, and high photocatalytic activity. This study focused on the effect of Ce addition on the catalytic performance of iron-based catalysts and the syngas in the process of biomass gasification. A co-precipitation method was used to prepare the CeO2/Fe2O3 bimetallic catalysts with different ratios. The cellulose was selected as biomass sample, in order to simplify the impact of high content of alkali and alkaline earth metals in biomass. The specific experiments were carried out in a two-stage fixed-bed gasification reactor. The distribution of gas products, gas yield, structure characteristics, the stability of catalyst were analyzed with variant approaches at different Ce:Fe molar ratios, temperatures and cycles. The optimal molar ratio of CeO2/Fe2O3 bimetallic catalysts was found, further to reveal the effect mechanism of Ce addition on biomass gasification with the iron-based catalyst. The results showed that the catalytic performance of CeO2/Fe2O3 catalyst was much better than that of pure CeO2 or Fe2O3. It infers that the addition of Ce and Fe had a synergistic effect on the volatile conversion and hydrogen production. When the molar ratio of CeO2/Fe2O3 catalyst was 3:7, the maximum yield of the H2 was 21.63 mmol/g cellulose at 800°C, while the total gas yield reached 92.21%. The CeFeO3 product can be generated at 800°C or higher temperature after the redox reactions without forming CeO2/Fe2O3 clathrate. The existence of CeFeO3 enhanced the process of biomass gasification with steam. After 3 cycles, the yield of each gas tended to be stable, while the catalytic activity of CeO2/Fe2O3 bimetallic catalyst did not decrease significantly. Due to the high capacity and oxygen mobility, the introduction of CeO2 can be used to improve the oxidation performance of iron-based catalysts, while to promote the oxidation of possible carbon deposits on the catalyst surface. Therefore, an enhancement effect can be achieved in an inner-looping chemical gasification and the stability of CeO2/Fe2O3 catalyst. This finding can provide a promising guidance for the in-depth understanding of biomass gasification mechanism. Some experiments are under the way to analyze the characteristics of catalyst reduction, surface micro-topography, and carbon deposition. A composite catalyst was recommended in the further research to combine perovskite-type oxygen carriers and carbon dioxide adsorption. It is also necessary to explore its catalytic characteristics in the internal cycle chemical chain gasification and influence on the biomass gasification process.