A review of the thermochemistries of biomass gasification and utilisation of gas products
Conventional biomass gasification involves a complex set of chemical reactions leading to the production of a gas mainly composed on carbon monoxide, hydrogen, carbon dioxide and some methane. Some C2–C5+ hydrocarbon gases are also formed in the gasifier. This review has uniquely focused on the thermochemistries of conventional biomass gasification with emphasis on gasification temperature, gasifying agents (pure oxygen, air, carbon dioxide, steam or combinations of these) and the types of gasifiers as the key parameters that determine the yields and compositions of gas products. With air as the gasifying agent, the product gas is highly diluted with nitrogen (>45 vol%) and is known as producer gas, which is often more suitable for direct energy application via combustion. With nitrogen-free gasifying agents, syngas with ≤5 vol% nitrogen content is produced and therefore suitable for various downstream uses including enhanced hydrogen production via the water–gas shift reaction and, especially, the synthesis of organic compounds such as methanol and dimethyl ether as well as hydrocarbons (liquids and waxes via Fischer–Tropsch synthesis). The contributions of kinetic and thermodynamic studies to the understanding and progress of biomass gasification have been explored. In addition, the review covers the challenges of tar formation during biomass gasification and various strategies to reduce/eliminate this major bottleneck via catalysis and reactor design or configuration. The historical perspective of biomass gasification and current trends are presented, highlighting the exponential growth in high-quality research publications around biomass gasification over the last decade, possibly driven by current Net Zero initiatives.