EXERGY ANALYSIS AND OPTIMISATION OF A DECENTRALISED WOODCHIP-FIRED COGENERATION PLANT

摘要

In this study, comprehensive exergy analysis of a woodchips fired cogeneration system based onrnorganic Rankine cycle (ORC) is presented. The analysis is conducted to an existing cogeneration plant located inrnScharnhauser Park, Stuttgart-Ostfildern, Germany that has been in operation since 2003. This study starts withrndiscussing the exergetic analysis methodology, and then defining the most important performance evaluationrnindicators based on the exergetic analysis (e.g. exergetic efficiency, exergy destruction ratio, relative exergyrndestruction ratio, and exergy loss ratio).In the first section of the study, the input parameters for a simulation-basedrnanalysis is defined. In following sections, a mathematic model of the energy generation process was established. Thernstructure of the model is explained, and the mass and exergy balances of the major system components are discussedrnin detail. The study shows that the Fixed Bed Combustion Boiler (FBCB) and the ORC evaporator are the mainrnsources of exergy destruction. The FBCB contributes to 58% of the total destructed exergy while the ORC evaporatorrncontributes to 30% of the total destructed exergy. Finally, some recommendations are suggested to optimize thernplant’s energy conversion process through reducing the exergy destruction within the FBCB.To achieve higherrnconversion efficiencies continuous development of the bioenergy systems is needed. The presented study, which isrnbased on experience from a full-scale system, aims to increase the knowledge about the influence of processrnparameters on the conversion efficiency. The primary and secondary air-flow rates are employed as major controllingrnparameters to maintain the stability of the combustion and achieve high conversion efficiencies. Simulation resultsrnshowed that the energy efficiency of the biomass furnace can be increased by more than 2% when an optimizedrncombustion air management system is applied. The study also examines the influence of the fuel moisture on thernexergetic efficiency of biomass conversion. Simulation results showed a considerable influence of the fuel moisturernon the process efficiency. The evaporation process of the moisture in the combustion chamber absorbs a considerablernfraction of the fuel energy, which had an important effect on decreasing the process temperature levels.Finally, clearrnrecommendations were suggested to optimize the plant’s energy conversion process through reducing the exergyrndestruction within the boiler. The main two factors that seem to improve the exergetic efficiency of the FBCB andrnconsequently of the overall exergetic efficiency of the plant are obtaining woodchips with lower moisture content andrndecreasing the excess air ratio.