**2.4 Other gasification agents: H2 and CO2**

Not many studies on gasification by hydrogen and carbon dioxide were found although these two agents are reactants in methanation (4) and Boudouard (7) reactions.

Methanation reaction can be increased when more H2 exists in the reaction zone of a gasifier. Since methanation is exothermic, hydrogen can be mixed with air in air-based gasification or can be used as the only gasification agents without slagging problems in the gasifiers like conventional oxygen/air gasification. Pure hydrogen gasification is expected to be able to run at lower temperature and milder conditions because less heat is generated from methanation reaction (ΔH = �87.5 kJ/mol) than from combustion step in air-based gasification [23], which may lead to the absence of oils and tars [24]. However, catalysts are needed because the reaction rates are very low [25]. Otherwise, hydrogen gasification should be carried out in high H2 pressure, which rises several safety concerns.

CO2 is a Boudouard reactant, as well as it can react with H2 in the mixture via reverse water gas shift reaction. Hot flue gas is a popular product in industry, which includes steam, CO2, and heat from direct combustion of fuel, thus can be considered as a gasification agent [26]. This technique is available if a combustion process is combined with gasification because air-based gasification already has its combustion zone. CO2 utilization and enhancement of CO formation can be the purposes of CO2-gasification [27].
