Process integration to increase woody biomass utilization for energy purposes

Abstract: Woody biomass is an abundant renewable energy resource in Sweden, and the Swedish government has been promoting research and development programs for the exploitation of this resource as a mean to meet the targets on the reduction of the carbon dioxide emissions from the industrial, energy and transportation sectors. This thesis aims at expanding the knowledge on the efficient utilization of the available woody biomass, so that a larger amount of this renewable resource can be used for energy purposes. The thesis presents a collection of studies following the main two policies that have been identified for the reduction of carbon dioxide emission, i.e. the implementation of measures improving energy conservation and efficiency and a deep decarbonization of the energy sector. Process integration and optimization techniques are applied to forest industry sites in order to improve the resource and energy efficiency, the benefits of the integrated design configurations being evaluated by both technical and economic analyses. The integration of woody biomass with intermittent renewable energy sources is also studied in order to enable a large share of non-fossil sources in the energy mix.The results of the investigations show a significant potential for improving biomass resource utilization in the forest industry sites strictly from the energetic point of view. Optimizing the process integration in sites including Kraft pulp and paper mills and/or sawmills and a dedicated common CHP system can lead to a much greater power generation for the same input biomass and for the same production volume, or to large amounts of excess heat to be used in nearby processes or district heating, or even to the re-routing of part of the input biomass to other conversion processes (e.g. lignin separation and hemicellulose fermentation to produce biofuels). The operational profit of the site is consequently increased, but, when the investment costs are considered, some form of subsidies to the “green” byproducts are usually still required to make the integrated design configurations economically viable. The integration of woody biomass with intermittent renewable energy sources can result in an increased efficiency of hybrid power generation plants (e.g. with concentrated solar thermal collectors), and on a large scale it could facilitate the decarbonization of the energy sector with the fundamental contribution from power-to-X technologies in order to produce chemical fuels from the excess intermittent electricity. These technologies would be clearly incentivized by a carbon tax, but the benefit deriving from the large volumes of captured CO2 that are required for the synthesis of chemical fuels through co-electrolysis should also be taken into account. Keywords: Forest industry, process integration, pinch analysis, HEATSEP method, optimization, CHP system, techno-economic, biorefinery, intermittent renewables.