From Combined Heat and Power to Polygeneration

Abstract: In order to reach targets on reducing greenhouse gas emissions from fossil resources it is necessary to reduce energy losses in production processes. In polygeneration, several processes are combined to complement each other to avoid sub-optimization of the standalone processes. This thesis addresses polygeneration with focus on Combined Heat and Power (CHP) production integrated with other processes. Biomass-fired CHP plants are commonly dimensioned to have surplus heat production capacity during periods with lower heat demand. At the same time, production of biomass based vehicle fuels and fuel upgrading are heat demanding processes. The opportunity to combine CHP with ethanol production from lignocellulosic feedstock and torrefaction with the aim of replacing fossil fuels are used as cases during the evaluation of polygeneration. Simulation models are used to investigate the performance of CHP integrated with production of ethanol and torrefaction. Measured data from commercial CHP plants have been used to reflect the operation boundaries. The findings show that polygeneration can compete with stand-alone production in both energy and economic performance. Polygeneration offers a wider operating range where reduced minimum load gives increased annual operating time. Therefore, under limited heat demand more renewable electricity production is possible due to increased operating hours and steam extraction from the turbine during part-load operation. Resource availability and fluctuations in fuel price have the largest impact on the profit of polygeneration. Other aspects that have substantial effects on the economy in polygeneration are the electricity spot price and subsidies. Furthermore, it has been proven that the yield of each product in a multiproduct process plant, the size of the plant and the heat demand have a large impact on the economy. Polygeneration turns by-products into buy-products.