A brief review of CO2 utilization for alkali carbonate gasification and biomass/coal co-gasification: Reactivity, products and process

Carbon gasification in molten alkali carbonate salts over CO2 is a process to produce gaseous fuels with mitigating the greenhouse effect. Alkali carbonate salts can facilitate Boudouard reaction, which is difficult to occur even at high reaction temperature if the catalyst is absent. Alkali carbonate allows easily carbon-CO2 contact by changing the reaction interface from gas-solid to gas-liquid. This review focused on CO2 utilization for alkali carbonate gasification and biomass/coal co-gasification. CO2 gasification of biomass and coal with the alkali carbonate salts enhances the carbon conversion rate and CO yield. Furthermore, alkali carbonate salts gasification of biomass and coal could reduce reaction temperature, activation energy, and tar formation in the product gas. The factors that influence the carbon conversion rate were discussed, such as reaction temperature, CO2 concentration, alkali carbonate addition, and char feeding mode. On the other hand, blending coal with biomass is considered as potential fuel base for CO2 co-gasification and displays an advantage for improving the coal char reactivity. The alkali and alkaline earth metals (AAEMs) present in biomass ash acted as a natural catalyst to increase the coal reactivity and CO production. The catalytic effect in the co-gasification enhances by raising the percentage of biomass in the blend. In contrast, the inhibiting effect in co-gasification could be attributed to silica and alumina reactions with AAEMs to form aluminosilicates complexes.