Cogeneration based on gasified biomass - a comparison of concepts

In this work, integration of drying and gasification of biomass into cogeneration (electricity + heat to district heating) power plants, comprising gas turbines, is investigated. The thermodynamic cycles considered are the combined cycle and the humid air turbine cycle (or evaporative gas turbine cycle). These are combined with either pressurised or near atmospheric gasification, and steam or exhaust gas dryer, in a number of combinations. The resulting systems are modelled using the software package ASPEN PLUS (TM), and for each system both the electrical efficiency and the fuel utilisation are calculated. The investigation of integrated gasification combined cycles (IGCC) reveals that systems with pressurised gasification have a potential for electrical efficiencies approaching 45% (LHV). That is 4 - 5 percentage points higher than the corresponding systems with near atmospheric gasification. The type of dryer in the system mainly influences the fuel utilisation, with an advantage of approximately 8 percentage points (LHV) for the steam dryer. The resulting values of fuel utilisation for the IGCC systems are in the range of 78 - 94% (LHV). The results for the integrated gasification humid air turbine systems (IGHAT) indicate that electrical efficiencies close to the IGCC are achievable, provided combustion of the fuel gas in highly humidified air is feasible. In addition to the thermodynamic calculations, some critical components and development needs are reviewed.