Abstract
Thermo-economic performance analyses of an externally fired biomass gasification based combined cogeneration plant (EF-BGCCP) is reported in the present paper. The plant is capable of producing 700-800 kW electrical power, the gas turbine (GT) providing a fixed output of 500 kWe and the bottoming steam turbine (ST) providing the rest. Utility heat of 100 kW or more is also available from the plant. Saw dust is considered as fuel feed to the plant which undergoes gasification in a downdraft gasifier. A combustor-heat exchanger (CHX) duplex unit is used in the topping GT cycle to heat up the working fluid. Effects of plant design parameters, such as, topping cycle pressure ratio (rp=4-12), gas turbine inlet temperature (TIT=900-1100 oC), cold end temperature difference (CETD=230-280 oC) of the heat exchanger of CHX unit and outlet temperature of the economizer (EOUT=130-180 oC) on the energetic and techno-economic performance is reported in the study. Energetic performance of the EF-BGCCP reveals that, plant efficiency along with fuel energy savings ratio (FESR) is maximized at a particular value of rp, for a given GT TIT. Higher TIT results in higher efficiency and FESR value. However increased CETD value results in decreased overall area (UAHX) of the exchanger. Also, increased value of EOUT results in decrease in plant efficiency and increase in FESR of the plant. Techno-economic analysis of the plant suggest that levelized unit cost of electricity (LUCE) and levelized unit cost of heat (LUCH) change with changes in operating conditions of the plant. LUCE value is minimized at a particular value of pressure ratio, for a fixed GT TIT. Also, higher TIT results in higher LUCE and LUCH delivered to the consumers. Again, higher CETD value of the heat exchanger results in decreased LUCE value as the size of the heat exchanger decreases with increase in CETD. Delivered LUCH value as well as heat to power ratio of the plant is found to be decreased at higher economizer outlet temperature.
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