EXERGY ANALYSIS OF A BIOMASS COGENERATION SYSTEM

Abstract

This paper is dedicated to energy and exergy analysis of a usual biomass cogeneration system with a thermal power in the range from 100 to 300 kW. The analyzed system consists of a downdraft biomass gasifier, a producer gas cleaning system and a gas engine. The heat exchangers in the system use the sensible heat of producer and flue gases and waste heat from the gas engine to heat water in a district heating system 90/70 0C and for a domestic hot water supply 25/60 0C. The system has the overall electrical efficiency of 22.04%, whereas the total efficiency is 96.72% due to the omission of the system heat loss. The exergy efficiency of the system is barely 35.83%. The reason for this are the irreversibilities created in the gas engine, gasifier and in the heat exchanger network. The system efficiency can be improved by the use of waste heat to preheat gasifying air, to produce gasifying steam, or to perform biomass pyrolysis prior to its gasification. These measures enable production of a more valuable producer gas with a higher calorific value and a smaller amount of nitrogen. The use of a better gas produces higher temperature in the gas engine, which result in a higher gas engine efficiency.