Design and validation of reactant feeding control strategies for the solar-autothermal hybrid gasification of woody biomass

Abstract

Several hybridization strategies of a solar-autothermal biomass gasifier were examined for stable and continuous operation under variable solar irradiation. The ultimate objective was to demonstrate the feasibility of controlled syngas production, through the modification of oxygen, water, and biomass injection rates. Various hybridization strategies were probed by thermodynamic analysis and experimentally validated. Thermodynamic equilibrium calculations detailed the impact of both H2O and O2 injection rates on the produced syngas composition under constant wood feeding. Oxygen injection decreased the H2:CO molar ratio, while reducing the solar thermal power required to carry out the gasification reaction. Meanwhile, the total H2+CO production dropped by 1.36 mol of H2 and 0.64 mol of CO per mole of O2 added, independently of the quantity of water provided. Validation experiments were then carried out under real concentrated solar flux in a directly-irradiated conical spouted-bed reactor, following distinct hybridization paths. Maintaining constant the H2:CO ratio above 1 during hybridization required to provide high amounts of water steam with oxygen, which penalized the gasifier efficient heating. In contrast, minimizing the water injection rate throughout hybridization strongly altered the H2:CO ratio but decreased the CO2 production and the solar thermal power requirement. Finally, the successful control of the outlet H2+CO volume flow rate with simultaneous oxygen and wood injection was demonstrated (under constant water feeding rate). Solar-to-fuel efficiencies were kept around 20%, while hybridization decreased the cold-gas efficiency below 80%.