Retrofitting CHP Plant and Optimization of Regional Energy System

Abstract: The use of biomass-based combined heat and power (CHP) plants is considered by the EU administration to be an effective way to increase the use of renewables in the energy system, to reduce greenhouse gas emissions and to alleviate the dependency on imported fossil fuels. At present in Sweden, most of the CHP plants are operated in part-load mode because of variations in heat demand. Further use of the potential heat capacity from CHP plants is an opportunity for integration with other heat-demanding processes. Retrofitting the conventional CHP plants by integration with bioethanol and pellet production processes is considered a feasible and efficient way to improve the plants’ performances. Modeling and simulation of the CHP plant integrated with feedstock upgrading, bioethanol production and pellet production is performed to analyze the technical and economic feasibility. When integrating with bioethanol production, the exhaust flue gas from the CHP plant is used to dry the hydrolysis solid residues (HSR) instead of direct condensation in the flue gas condenser (FGC). This drying process not only increases the overall energy efficiency (OEE) of the CHP plant but also increases the power output relative to the system using only a FGC. Furthermore, if steam is extracted from the turbine of the CHP plant and if it is used to dry the HSR together with the exhaust flue gas, pellets can be produced and the bioethanol production costs can be reduced by 30% compared with ethanol cogeneration plants. Three optional pellet production processes integrated with an existing biomass-based CHP plant using different raw materials are studied to determine their annual performance. The option of pellet production integrated with the existing CHP plant using exhaust flue gas and superheated steam for drying allows for a low specific pellet production cost, short payback time and significant CO2 reduction. A common advantage of the three options is a dramatic increase in the total annual power production and a significant CO2 reduction, in spite of a decrease in power efficiency. The retrofitted biomass-based CHP plants play a crucial role in the present and future regional energy system. The total costs are minimized for the studied energy system by using wastes as energy sources. Analyses of scenarios for the coming decades are performed to describe how to achieve a regional fossil fuel-free energy system. It is possible to achieve the target by upgrading and retrofitting the present energy plants and constructing new ones. The conditions and obstacles have also been presented and discussed through optimizing the locations for proposed new energy plants and planting energy crops.