Thermal characterization and modeling of woody biomass gasification for small-scale combined heat and power application

Abstract

World over, small-scale CHP systems are undergoing rapid development, and are emerging on the market with promising prospects for the near future. The choice of a suitable CHP system is driven by the need and local conditions at the target end user, keeping in mind that the system integration should have a good balance of being most efficient, reliable, cost effective, socially beneficial, least polluting and sustainable in the long run. In developing countries, small-scale biomass-fuelled CHP systems have a particular strong relevance in improving the quality of life, especially among rural communities. This research aimed at developing a small scale biomass-fired combined heat and power system applicable to Uganda’s local conditions. The target generation capacity was 100 kW sufficient to meet electricity needs of a rural community of 250 households. The system involved steam gasification of woody biomass in a fixed bed downdraft gasifier and the producer gas obtained was then led to a combustor integrated with a heat exchanger. Compressed air and steam mixture was heated up in the high temperature heat exchanger to turbine inlet temperature. The exhaust gases from the turbine were led to a heat recovery steam generator. The steam generated was used for both injection into the gas turbine and as a gasifying medium. The researcher therefore analysed, experimented on and modelled the system behaviour for better performance. The design problem was thus experimenting on feedstock, modeling the gasification phenomena, extensive parametric design analysis of the combustor and heat exchanger and performing an exergoeconomic analysis of the system. This was realized through extensive parametric process study. The results revealed that the proposed CHP system integration is feasible with sufficient exergy recovery to satisfy the process goals with the potential of obtaining relatively high electrical efficiency. The research has revealed the opportunities for efficiency improvement through process intensification as in the proposed CHP system. The analytical data developed for the Ugandan wood species reveal unique features that make these species ideal for thermochemical conversion. The results from the thermodynamic modeling of allothermal steam gasification reveal that both the quality of the producer gas and the reactor efficiency are greatly improved. The recommended feedstock for this system model is Eucalyptus with gasification temperature in allothermal mode set within 820oC. The system cold-starting can be implemented using LPG supply line from typical domestic gas cylinder-gas burner integration until the system approaches steady-state operation estimated to be 10 minutes.