Experimental and modeling analysis of Air and CO2 biomass gasification in a reverse lab scale downdraft gasifier

This research work aims at investigating the effect of carbon dioxide feed in biomass gasification as a possible way to directly exploit the exhaust gas from the engine of combined heat and power systems to convert carbon dioxide to carbon monoxide via Boudouard reaction and consequently increase the carbon conversion and reduce char yield. The effects on biomass gasification using air, and air diluted with carbon dioxide were assessed in a reverse downdraft lab-scale gasifier utilizing 2 kg of pelletized biomass. This reactor is mounted on a digital weighing balance which enables the recording of mass loss during the gasification process. Furthermore, the mixture of air and CO2 is obtained from two mass flow controllers which enable constant and desired flux of gasifying agents across the gasifier. At the same time, an in-house developed thermodynamic equilibrium model was applied to predict the gas composition and char output. Unlike the classical equilibrium strategy that calculates the gasification products using the Gibbs energy minimization method at fixed temperature and pressure, the current approach is based on the enthalpy of the reactants, analogous to the adiabatic combustion temperature. Also, a correction factor accounting for the heat losses, was implemented. The model outcome shows a good agreement with the experimental results, especially in terms of predicted char yields and trends of the dominant producer gas species. The same strategy was used to describe the behavior of the gasification system and estimate the quality of producer gas and the cold gas efficiency of the system.