Reviewing life cycle assessments of carbon capture and utilisation - unclear goals lead to unclear results

Conference poster, 2023

Carbon dioxide capture and utilisation (CCU) is the process of capturing carbon dioxide and using it to produce a product. It is a potential strategy for mitigating greenhouse gas emissions and replacing fossil feedstock in chemical production. Life cycle assessment (LCA) is important to assess the carbon reduction capability and is often used to evaluate the environmental impacts of CCU processes. This study aims to analyse the methodological choices made in life cycle assessments of carbon capture and utilisation systems, and identify and evaluate the logics of the modelling in the studies.

LCA studies of CCU processes were found through a systematic search and reviewed regarding LCA methods. The collected articles were coded on different aspects (e.g. goal, system boundaries, impact assessment) and a framework was developed to describe the different scopes the CCU systems are modelled from in the assessments.

106 articles were reviewed, published between the years 2002 and 2021. 88 of them evaluate products produced through a CCU route and make a comparison to the existing conventional way. Thus many aim to do the same kind of assessment, but results from the review show that the scope differs, and the majority do not clearly state their goal with the LCA. There is likely an aim of the study which could include a reason for using LCA, but the goal of the LCA (as in goal and scope definition) is often not found in the article.

It was also found that the system boundaries stated in the body of literature are often "cradle-to-gate". The cradle can however be set to different points in the system, and the scope of the studies varies a lot depending on where the cradle starts and what is included in the assessment. In the case of CCU, it is found that the cradle can be at the process the flue gases are captured from (38 cases), the capture process (44) or at the CO2 conversion process (24).  

The justification for not including the whole life cycle of the product (only 19 are to the "grave") can be that the product has the same use and end of life as the product it is compared to, often the conventional alternative. However, only including part of the system in the analysis can give misleading results when the emissions can be presented as negative in the shorter perspective due to the temporary storage of carbon in the product. A longer time perspective and different system boundaries are needed to see if the carbon in the product is emitted or not shortly after leaving the factory gate.

Given that CCU processes are often emerging technologies, the purpose and context of the study matter for how the results can be used, but the goal of existing life cycle assessments seldom handles these aspects. The LCA results are often used for comparison with conventional technologies or for comparing the CCU product to an existing product, although not always reflected in the goal. With underdefined goals, different system boundaries and varying methods for accounting, understanding assessments of CCU becomes confusing. This highlights the need for methodological guidelines and clearer goal definitions in life cycle assessments of CCU to ensure meaningful and consistent evaluation of the environmental impacts and potential of these emerging technologies.