Thermodynamic Performance Assessment of a Bio-gasification Based Small-scale Combined Cogeneration Plant Employing Indirectly Heated Gas Turbine

Abstract

Thermodynamic model development of biomass gasification based indirectly heated combined cogeneration plant and its simulated performance is reported in this present study. Saw dust is considered as biomass feed, which undergoes gasification in a downdraft gasifier and the producer gas is combusted in a combustor-heat exchanger duplex (CHX) unit. The CHX unit heats up air for a 100 kWe Gas Turbine (GT) and the exhaust heat of CHX unit is utilized in generating bottoming steam turbine work output and utility steam. The performance of the plant is assessed over a wide range pressure ratio (rp) and turbine inlet temperature (TIT) for the GT block, as well as, by varying the steam turbine inlet pressure and temperature along with the outlet gas side temperature of the economizer. For the base case configuration (rp= 4 and TIT=1000 deg C) the plant gives an overall electrical efficiency of about 41% and, at the same time, produces utility steam at a rate of about 180 kg/hr. Its cogeneration performance, expressed in terms of fuel energy saving ratio (FESR), is found to optimize at particular values of topping cycle pressure ratio for different TITs. The study also includes discussion on the sizing of the major plant components. Further, a Second law analysis of the plant concludes that maximum exergy destruction takes place at the gasifier, followed by the CHX unit, together accounting for nearly 40% of the fuel exergy input.