Strategies for Reduced Energy Use and Cost in Pulp and Paper Mills by Energy Integration of the Paper Drying Process

Sammanfattning: The cost of energy for drying paper is substantial for the pulp and paper industry, and studies of various strategies for energy cost reductions such as increased energy efficiency, increased heat recovery from the exhaust drying air and reduced cost of the process heat are presented in this thesis. The energy use and cost studies were performed using a block-model based simulation tool developed for this specific purpose. For a multicylinder drying-section, the heat recovery potential was shown to vary from 25 to 60%, based on total energy available in the exhaust air stream, as the dew point of the drying air was varied in the range of 45 to 70 °C. For minimum energy costs, the dryer should be operated at the highest dew point possible. Dryer-limited paper machines should, however, always be operated at the lowest exhaust air humidity possible since the revenue from the extra produced paper is much higher than the additional cost of energy. The use of drying-sections including gas-fired paper dryers was shown to increase the heat recovery potential by up to 40%. However, due to the higher cost of natural gas and the reduced backpressure power generation with such drying-sections, the overall energy costs will be significantly higher. If black liquor gasification is implemented the produced syngas can be used as a substitute for the fossil natural gas. The process economics will improve significantly, especially if the gas is used to fire a combined cycle system and the exhaust gases from the gas turbine are used as make-up drying air in the impingement hood. At current energy price levels, in-house power generation based on biomass fuels is greatly favoured by the electricity certificate system, and the high power-to-heat ratio of the combined cycle system is thus favourable. Differentiated pricing of secondary energy was shown to be a useful strategy for promoting the recovery and reuse of such energy, thus increasing the overall energy efficiency and reducing the energy costs for the pulp and paper mills.