Biobased Combined Heat and Power Production in Sweden – Opportunities for and Challenges to Sustainability from the Plant Operator Perspective

Although combined heat and power (CHP) production in Sweden is already largely decarbonised, this technology has further potential to alleviate pressing environmental concerns. Today, biobased CHP plants connected to district heating (DH) grids dominate the Swedish residential heating sector. The societal context in which CHP plants operate is, however, subject to continuous change. New aims and requirements concerning decarbonisation and resource efficiency are being discussed and implemented in the political arena to promote the shift towards a sustainable energy system. These include the cascading use of biomass, bioenergy carbon capture, support schemes for innovative investments, life-cycle-based climate performance regulations, and circularity demands. Such shifts affect the operation of CHP plants and can provide new opportunities and challenges that have not yet been investigated. Neither are the consequences fully understood in the scientific or political community. This thesis presents an investigation of the opportunities and challenges facing CHP plant operators within the context of a shift towards a sustainable energy system. Issues of environmental relevance were investigated through a case study of a Swedish wood-fuelled CHP plant. The recycling of wood ash to forest soils after logging residue outtake is recommended to close the loop for forest nutrients and ensure forest production. However, co-incineration of waste wood and forest fuels in the Swedish DH sector was found to inhibit wood ash recycling, due to pollutants in the ash from waste wood. It was also found to be an overlooked challenge in the transition to a circular bioeconomy, where waste wood is utilised to produce energy. Other important issues in a circular bioeconomy are the efficient use of biomass and the production of high-value biobased products. CHP plants are dependent on a stable heat demand to operate efficiently. The addition of a pyrolysis unit, a heat-demanding process, to produce liquid biofuels that could increase the uptime and open up an additional market for CHP plants as biorefineries was studied. Life-cycle analysis showed this to be technically feasible at the CHP plant studied, and to substantially improve the overall greenhouse gas (GHG) benefits. Negative GHG emissions have been deemed by the IPCC to be an important complement in achieving net zero GHG emissions. Biobased CHP plants can contribute to negative emissions through the implementation of carbon capture. However, these technologies are energy-demanding, and thus reduce the energy efficiency of the plant. It was shown that the carbon mitigation potential of installing equipment for carbon capture or liquid biofuel production was highly dependent on which energy sources compen-sate for the changes in the value chains, which is in turn dependent on the decarbonisation of the surrounding energy system. An important outcome of the work presented in this thesis is the identification of existing and emerging opportunities and challenges related to sustainability in biobased CHP plants, which can contribute to, or hamper, the fulfilment of environmental goals. The studies on these opportunities and challenges can prove valuable knowledge for other countries and regions that are planning to develop biobased CHP plants with DH grids. Successful navigation of these opportunities and challenges by policy makers and CHP plant stakeholders will be instrumental in ensuring a decarbonised and resource-efficient energy system