Performance analysis of downdraft gasifier-reciprocating engine biomass fired small-scale cogeneration system

Energy and environmental performances of a small-scale cogeneration system composed of downdraft wood gasifier and reciprocating engine is theoretically examined in the paper. The gasifier is modelled using a non-stoichiometric thermodynamic quasi equilibrium approach. Achievable power and efficiency of the gas engine is estimated using a method based on analysis of the engine thermodynamic fuel-air theoretical cycle. It is assumed that the relative changes of operating parameters of a selected machine are proportional to the relative changes of the cycle parameters after the change of fuel. It is taken into account that the gasification agent can be composed of ambient air and recirculated exhaust gas from the engine. The proposed configuration of the system is examined in the aspect of electricity generation efficiency, total fuel energy utilization, potential for primary energy savings and global CO₂ emission reduction. It was found that the proposed modification of a typical downdraft reactor-gas engine system, that runs at a given reactor temperature, reduces the heating value of the producer gas. Consequently the electric power output of the engine module is derated. However, the recirculation of exhaust gas from the engine improves the cold gas efficiency and the performance of the whole system.