Bio- gasification Based Distributed Power Generation System Employing Indirectly Heated GT and Supercritical ORC: Energetic and Exergetic Performance Assessment

Abstract

Biomass based distributed power generation has immense possibilities due to availability and CO₂ neutrality of biomass feeds. In community scale distributed generation systems, conventionally biomass gasifier - gas engine is employed. Such systems offer low overall efficiency (about 20-25%) and require elaborate gas cleaning and gas cooling arrangements. These shortcomings of conventional systems can be overcome by employing an indirectly heated gas turbine cycle along with a coupled Rankine cycle. This paper presents thermodynamic model of a novel biomass gasification based combined cycle plant consisting of an indirectly heated gas turbine (GT) block as topping cycle and a supercritical organic rankine cycle (ORC) block as bottoming cycle.  A typical Indian solid biomass viz. saw dust, considered as the fuel 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 30 kWe Gas Turbine (GT) and the exhaust of CHX unit is utilized by the bottoming ORC, where toluene is the working fluid. The simulated performance of the plant is assessed over a wide ranges pressure ratio (r_p=4 to 16) and turbine inlet temperature (900 to 1100 deg C) for the GT block. For the base case configuration (r_p= 4 and TIT=1000 deg C) the plant gives an overall electrical efficiency of above 45%. The efficiency is found to maximize at a particular value of topping cycle pressure ratio, depending on TIT, optimum r_p being higher at higher 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.