A case study of the potential for CCS in Swedish combined heat and power plants

Carbon capture and storage (CCS) from fossil and biogenic (BECCS) emission sources has been identified as an important part of achieving long-term CO2 emission targets. CCS offers a strong CO2 mitigation option at energy intensive industries or power plants, where emissions are abundant and concentrated to stacks, but the cost of applying CCS is still higher than the cost to emit CO2 under current climate policies such as the EU ETS. Ways to reduce the capture cost and help incentivize CCS are being discussed and include, for example, heat integration. The aim of this work is to evaluate the heat integration of CCS and BECCS in combined heat and power (CHP) plants operating in local district heating systems; with respect to how such integration influences electricity and district heating (DH) generation. Process and system level perspectives are combined, to (1) analyze how heat integration of absorption-based CO2 capture can be implemented efficiently in existing CHP plant steam cycles that supply heat to district heating systems; (2) estimate the impact of such heat integration on a DH system level, in terms of the potential for CO2 emission reduction when incorporating CCS/BECCS into operational strategies for DH system dispatch; and (3) estimate the total national potential for carbon capture from CHP plants in Sweden. A challenge for the practical implementation of absorption-based CCS is the reboiler that requires a substantial amount of heat, leading to increased operational costs and/or interference with production of the prioritized plant products; district heating and electricity in the case of CHP plants. In Nordic CHP plants, where the plant operation is dictated by the DH demand, there may on the other hand be room for increased heat generation for CCS purposes, at times when the heat demand is low. Additionally, opportunities to recover excess heat from the capture process to generate district heating may be present, that further facilitates implementation of CCS in plants that deliver DH. Integration of CCS in DH networks can also be seen as an opportunity to achieve negative emissions, using BECCS, as many cogeneration plants in Sweden are fueled by biomass or waste (waste containing a certain share of biogenic material). These plants are traditionally dispatched based on a heat demand that varies seasonally. However, in future energy systems with a large share of non-dispatchable energy sources, for example wind power, provision of flexible electricity generation and grid services may be a valuable system service that CHP plants can offer the energy system and, when combined with CO2 capture, they can also offer negative emissions. The future demand for electricity and grid services may not necessarily coincide with the demand for district heating; consequently, if a plant is operated to provide power at times when the heat demand is low, there will be excess heat available. Finding new uses for heat, such as for powering the capture process, might help to keep plants running, as well as increase the utilization of investments by extending the annual operating time. The work consists of three parts. Firstly, process modeling and simulation computes the impact of heat integration of absorptionbased CCS on CHP steam cycle performance, in terms of electricity and district heating generation, for 110 Swedish CHP plants PCCC5 Beiron et al. 2 with different live steam conditions and power-to-heat ratios. The CSS reboiler is powered by steam extracted from the steam turbine. Secondly, the heat integration results from the process simulations are applied to a district heating unit commitment model. The unit commitment model analyzes the DH system level impact of integration of CCS in DH-supplying CHP plants, with respect to how the operation of CHP plants equipped with CCS units affect the operation of other DH producers in the local system. The unit commitment model is applied to all local DH systems in Sweden that are supplied by CHP plants. Thirdly, the total potential for CO2 capture from all the CHP plants and local DH systems studied are aggregated to estimate the total national level reduction in CO2 emissions if CCS/BECCS is implemented in the regular operation of CHP plants, with respect to DH demand profiles and future demand for grid services and flexible electricity production.