Energy and Resource Efficiency in Convective Drying Systems in the Process Industry

University dissertation from Luleå tekniska universitet

Abstract: Growing concern about environmental problems has increased the public’s interest in energy usage. The subsidies for biomass, together with the rising energy prices have made biomass a desirable product on the energy market. This has led to higher biomass prices and an increased interest in improving the resource and energy efficiency associated withbiomass production. Biofuel is an interesting substitute for fossil fuels to decrease the greenhouse gas emissions. One challenge with biofuels is to find sufficient amounts of biomass since the foresting is already close to its maximum sustainable capacity. Sawmills are important suppliers to the biomass market, since the sawmill industries produce a significant part of the available biomass. This Doctoral thesis focuses on strategies to decrease biomass usage in order to increase the biomass availability at the market. This is done through mapping and system analysis of energy and material streams for process industries using convective drying techniques. The energy analysis is mainly done through thermodynamics and psychrometry. Available state-of-the-art technologies on the market are studied to determine their potential for decreasing the total energy usage in sawmills. Integration possibilities between biomass consumers are also investigated through process integration with mathematical programming and pinch analysis. Energy efficiency of berry drying in a juice plant is also studied. The main conclusions are as follows. The heat demand of drying lumber in Swedish sawmills is about 4.9 TWh/year. Using available state-of-the-art technologies (heat pumps,heat exchangers and open absorption system) it is possible to reduce the energy usagesubstantially. If the recovered heat is used for heat sinks inside, or close to, the sawmill, the energy efficiency can be improved significantly. Using mechanical heat pumps nationally could save 4.9 TWh/year ofheatandgenerate0.62 TWh/year of surplus heat, at the cost of 1TWh/year of electricity. Using open absorption systems nationally, could save 3.4 TWh/year of heat, at the cost of only 0.05 TWh/year of electricity. Saving this heat means that an even larger amount of biomass will be saved, since there are heat losses during the combustion and distribution. Another way of saving energy is to displacethe starting time between batch kilns, and recycle evacuation air between the kilns. Nationally, this could save 0.44 TWh/year of heat. Industrial site integration between sawmills and the main biomass users (pelleting plants an d CHP plants) can decrease the use of biomass in the industrial site with 43% wt compared to a standalone site with a comparable production. Nationally, this could save up to 7.1 TWh/year of biomass. Despite the significant savings in terms of resources, it is not profitable due to the current price ratio between district heating and biomass. Finally, drying and separation of berry press cake in a juice plant is found to be possible using only energy from the exhaust gases of the steam boiler, if the dryingair is sufficiently recycled. Instead of composting the press cake, the dried and separated skins and seeds could then be sold.

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