District heating with negative emissions – direct air carbon capture and storage combined with small modular reactors

• District heating with direct air capture (DACCS) can reach negative emissions. • DACCS was modelled in a district heating case study with small modular reactors. • Techno-economic modelling shows high utilization rates for both SMR and DAC. • Economic feasibility depends on carbon market pricing and subsidies. • Carbon-negative district heating is possible if capture capacity and price suffice. Achieving Paris Agreement targets for climate change mitigation requires an urgent shift away from fossil fuels. In addition, negative emissions by permanently removing carbon dioxide from the atmosphere are required. Both targets require substantial amounts of carbon neutral electricity and heat production. While electricity can be produced and transferred over long distances, the heat production needs to be local. This study investigates an energy system integrating both carbon neutral heat production and carbon dioxide removal from the atmosphere. The system is modelled using the Backbone energy system modelling framework. The carbon neutral heat production in the study is based on small modular nuclear reactors (SMRs), large-scale thermal energy storages (TES), heat pumps (HPs) and electric boilers (EBs), and the carbon removal is implemented by direct air capture (DAC) combined with permanent geological storage. The studied technologies are integrated into a specific large-scale district heating system located in Northern Europe. The impact of outdoor temperature for the efficiency of the DAC process is considered, and the system integration potential with the district heating system is evaluated. The results show that high 70–90 % utilisation rates for both SMR and DAC units can be reached but depending on the case year and corresponding profiles for demand, outdoor temperature, electricity and carbon prices, a large variation in utilisation rates is observed. The variable CO 2 capture costs were between 115–126 €/t CO 2 in the modelled scenarios, and with higher OPEX values at the range 152–163€/tCO 2 , and the limit price for economic viability considering the investment was calculated to be in the range of 209–223 €/tCO 2, with lower, and 233–246 €/tCO 2 with higher adsorbent costs. When not accounting the biogenic CO 2 emissions, the carbon negativity can be reached in the system in all the scenarios where the CO 2 price is over 150€/t and the number of DAC modules is at least 400. When accounting the biogenic CO 2 emissions, the carbon negativity can be reached only in scenarios with DAC capacity at 900 modules and CO 2 price at 180–200€/t.