Strategies for co-operated wood chip fired and municipal waste fired combined heat and power plants

Abstract: The Brista 1 plant is a wood chip-fired combined heat and power (CHP) plant located near Märsta, northwest of Stockholm, Sweden. The primary purpose of the plant is to supply heat to the northwest district heating grid. In order to meet increasing demand for district heating, Fortum Heat is constructing a second CHP plant next to Brista 1. The Brista 2 plant will use a mixture of municipal and industrial waste as fuel. Due to changes in the European Green Certificate program, the fuel subsidies for wood chips will be significantly reduced. This will cause the Brista 1 plant to incur significantly increased operating costs. The Brista 2 plant, however, will not be affected by these changes and will therefore be much cheaper to run than Brista 1. However, due to the large demand for district heating it will be necessary to run both plants in parallel at certain times in order to meet the heating demand and/or maximize revenue during periods of high electricity demand. A computer program has been constructed using MATLAB which simulates the Brista 1 and 2 plants and their combined operation in both backpressure and direct condensing mode. The results show that the optimum allocation of heat production does not seem to be affected by electricity price assuming both plants are operated in backpressure mode. The reason for this would seem to be that the production costs (fuel, emissions, O&M) are unaffected by the electricity price. Therefore, the allocation which maximizes electrical power production, and thus revenue from electricity sales, will always be favored. In certain cases, it is more profitable to run the Brista 1 plant in direct condensing mode. The reason for this would seem to be that the thermal efficiency is somewhat higher, and that at low electricity prices the revenues from electricity sales do not offset the cost of the reduced heat production.