New roles for chemical pulp mills in the future energy system

Licentiatavhandling, 2024

Major transformations of the energy system are expected in the coming decades, in response to climate policy and reduced costs of renewable electricity sources. In this future, Swedish chemical pulp mills, as large processors of biogenic carbon and net suppliers of electricity, may have several roles to play. This thesis explores the conditions for chemical pulp mills to alter their position in the electricity system from being base-load providers to acting as flexibility providers, to introduce new biorefinery concepts that enable the production of advanced biofuels, materials and chemicals, and to provide negative emissions via bio-energy carbon capture and storage.

The analysis is carried out using a novel techno-economic model representing a typical pulp mill. The model is a linear optimisation model, optimising the operation of the mill, i.e., the load of all processes, boilers and turbines for each hour of the year, and, for a set of future scenarios, the investments in new technologies that can be added to the existing mill. The objective is to maximise the net revenue for the mill while maintaining the hourly balances of materials, steam and electricity throughout the mill. The outputs from the optimisation model are further processed to derive economic indicators, such as the product value generated from the mill and the price tipping points for profitability when comparing different options. The flows of carbon throughout the mill are also compared across the different scenarios.

The results show a potential for the mill to act as a flexibility provider for the electricity system, without any impact on pulp production. The flexibility is enabled by a combination of inter-dependent measures on the supply and demand sides. The main technical limitations to flexibility are the capacity limits of the flexible processes (minimum and maximum load levels), while storage units for intermediate products are typically large enough to handle variations on time-scales from hours to days. If, for an existing mill, supply-side flexibility is not already available in the recovery boiler, it can be unlocked by introducing a lignin extraction plant. This synergy increases the incentives for the mill to produce lignin as a new product, as it reduces the penalty on the electricity trade that is otherwise associated with lignin extraction.

When different options for the chemical pulp mill are compared, it is seen that the role that the mill plays in the energy system may evolve over the coming decades. In the short term, the proposed flexibility measures may facilitate the introduction of wind and solar power into the electricity sector and, thereby, support the electrification of the transport and industry sectors. In the longer perspective, an increasing demand for biomass may create incentives for more transformative changes, including the large-scale implementation of carbon capture, either for the production of e-fuels or to provide negative emissions.