Forest biomass in the energy transition: Modelling industrial flexibility and inter-sectoral dependencies

Doctoral thesis, 2026

Sustainably sourced forest biomass is a renewable but limited resource that can substitute for fossil carbon in many applications. As the energy transition adds demands for transportation fuels, flexible electricity generation, and negative emissions, the challenge is to allocate the biomass across competing uses, so as to increase its economic value and its contribution to climate change mitigation. This thesis explores how technologies and process configurations within the forest biomass system (defined as the value chain from harvested wood to end products), where chemical pulp mills currently play a central role, can contribute to achieving these goals during the transition. Two optimisation models are applied, representing a single pulp mill and the Swedish forest biomass system within the wider energy system.

Assuming that fossil carbon is phased out and that the demand for biogenic carbon increases, the results show that the investments and operational strategies that become rational in the near future depend on policy and market developments in sectors outside the traditional forest industry. The main contribution of the pulp mill (beyond pulp) may, thereby, shift from base-load electricity to flexibility, and eventually to replacing fossil feedstocks through lignin extraction or bioenergy with carbon capture and utilisation (BECCU), or to negative emissions through bioenergy with carbon capture and storage (BECCS). For the individual mill, carbon capture becomes the most profitable option if negative emissions are priced high enough. At the system level, however, gasification plants producing renewable methanol, in which CO₂ is a by-product of the syngas, deliver negative emissions at a lower cost when there is also a demand for methanol. The outcome of a policy for negative emissions therefore depends on whether biogenic carbon is also in demand for fuels and chemicals.

As new demands are placed on forest biomass, the interplay between the forest biomass system and the wider energy system changes at both the mill and the system level. Flexible operation of processes and boilers in the pulp mill increases the value of the mill’s surplus electricity and reduces the cost of lignin extraction, which is dominated by the foregone electricity generation. BECCU, where captured CO₂ is combined with hydrogen generated by electrolysis, raises the product yield further, but at an even higher cost in electricity. At the system level, targets for methanol and negative emissions reallocate biomass away from combined heat and power (CHP) plants, whose electricity generation is lost, while the heat pumps that replace them in the heating sector increase further the electricity demand.

Under the studied conditions, the role of forest biomass shifts from that of a fuel supplying heat and electricity towards that of a source of carbon, whose refinement into products or more valuable energy carriers is paid for with heat and electricity. The value of the biomass then varies with time and location, depending on which alternative it replaces.