Klimatpåverkan och energianvändning vid tillämpning av bio-CCS och bio-CCU från biogasanläggningar

Sammanfattning: To mitigate the human-induced climate change, Sweden has adopted a climate policy framework that states a net-zero emissions of greenhouse gases by the year of 2045 and thereafter negative emissions. To achieve this goal extensive emission mitigation alongside supplementary measures are required. Carbon Capture and Storage (CCS) is a concept that collects carbon dioxide with the purpose to store it permanently. When CCS is applied to streams of carbon dioxide (CO2) with biogenic origin, it is called bio-CCS. Bio-CCS is an example of such a supplementary measure. Another concept is bio-CCU, Carbon Capture and Utilization. It means that CO2 with biogenic origin is used in products. One example of bio-CCU is Power to Gas (P2G), where carbon dioxide and hydrogen through methanation produces biomethane, via biological or catalytic processes. By upgrading biogas, amounts of CO2 are separated which make it easily accessible for bio-CCS and bio-CCU. This study aims to determine the Global Warming Potential (GWP) and primary energy use for bio-CCS and bio-CCU (P2G) from biogas plants via Life Cycle Assessment (LCA) and compare it to the fossil fuel alternative. This will be in accordance with the LCA methods used in the Renewable Energy Directive (RED) and the International Organization for Standardization (ISO). Moreover, the study examines a total of six different scenarios, including a reference scenario. One scenario with bio-CCS for permanent storage of CO2 and four scenarios with P2G, either through biological methanation or catalytic methanation. The methanation takes place either at a centralized plant or at the biogas plant where it replaces the traditional upgrade of biogas. The biogas production assumes a 50 GWh/year Swedish co-digestion plant. Bio-CCS is the alternative with the biggest positive climate impact of the studied scenarios for produced biomethane. LCA shows that by storing CO2 through bio-CCS in saltwater aquifers, GWP is -39.2 g CO2eq/MJ biomethane according to ISO and -26.5 g CO2eq/MJ biomethane according to RED. It is proved that the transportation of CO2 by ship from Sweden to Norway for permanent storage affects the calculation of greenhouse emissions the most. By utilizing CO2 through P2G the GWP is between 4.0–6.4 g CO2eq/MJ biomethane according to ISO and 14.0–14.9 g CO2eq/MJ according to RED. Primary energy use for bio-CCS is 0.24 MJ/MJ biomethane according to ISO and 0.28 MJ/MJ biomethane according to RED. Primary energy use for bio-CCU is 1.45–1.57 MJ/MJ according to ISO and 1.57–1.67 according to RED. This can be compared to a reference scenario where CO2 is released into the atmosphere with the primary energy use is 0.21 MJ/MJ according to ISO and 0.26 MJ/MJ according to RED. In the scenarios that include bio-CCU, the differences are relatively small and the best solution is dependent on local conditions. Biomethane from bio-CCU leads to greenhouse gas savings of 84–85 % compared to the fossil fuel comparators (94 g CO2eq/MJ). Therefore bio-CCU meets the criteria for sustainability in RED. The sensitivity analysis shows that the electricity generation source has a large influence on the GWP for bio-CCU. Changing from the Swedish electricity mix to wind power decreases the GWP by 49–51 %. Apart from being used to phase out fossil fuels, biomethane can also be used for energy storage if produced when there is a surplus of electricity, which helps balancing the electric grid. In conclusion, there is big potential for future bio-CCS and bio-CCU from biogas plants due to the localized source of easily available biogenic CO2. Bio-CCS and bio-CCU can be seen as important tools to reach the climate goals by reducing greenhouse gas emissions and phasing out fossil fuels.