Towards a business case for CO2 mineralisation in the cement industry

The cement industry, a sector characterized by low margins, contributes significantly to global CO2 emissions, accounting for approximately 7% of anthropogenic CO2 equivalent (CO2e) emissions. This high carbon intensity, coupled with the industry's projected growth, necessitates urgent decarbonization strategies. The Paris Agreement underscores the global commitment to limiting temperature rise, making emission reductions in the cement sector crucial. While alternative building materials could replace cement, a rapid transition is unrealistic. Carbon capture and storage (CCS) technologies offer an alternative, but often incur additional production costs. Ideally, decarbonization strategies should generate additional revenue rather than increasing costs. CO2 mineralization, a process involving the reaction of captured CO2 with minerals to form stable carbonate minerals, presents a potential solution. Early research indicates that this process not only stores CO2 but also yields products with various applications, including as supplementary cementitious materials (SCMs) in cement blends. This valorization of the mineralization products could potentially create substantial revenue streams, offsetting the costs of CO2 capture and potentially creating a profitable business case. This study focuses on using natural rocks, like those containing forsterite, lizardite, and wollastonite, as feedstocks for CO2 mineralization due to their abundance and stable composition. While industrial wastes offer an alternative, their variable composition and costs present challenges. The study investigates the economic viability of CO2 mineralization, considering the added value from the sale of resulting products as SCMs and potential revenue generation through emission certificates.

Previous research on CO2 mineralization has shown potential for CO2e emission reductions in the range of 0.44 to 1.17 tonnes per tonne of CO2 stored. Studies have suggested that under specific conditions, CO2 mineralization could even transform the cement industry from carbon-positive to carbon-negative. However, these studies often overlook the potential revenue generation from the sale of the resulting products. Moreover, the environmental impacts of mineral mining (metal depletion, freshwater consumption) need careful management. While previous techno-economic assessments of CO2 mineralization have estimated CO2 storage costs (excluding CO2 capture) in the range of €65-€443/tCO2, they often fail to account for the revenue generated from the sale of the resulting products which may prove to be critical for the adoption of such technology in a cost-sensitive industry. Therefore, this study aims to evaluate the conditions under which a positive business case for using mineral carbonation products in the cement industry can be established, going beyond simply considering the storage aspect of CO2 mineralization and including the economic benefits of the resulting products.

The authors developed integrated techno-economic models of two carbonation processes (direct and indirect) to produce SCMs suitable for cement replacement. The models considered a large-volume market scenario, focusing on cement replacement with SCMs, as other markets do not match the scale of cement production's CO2 emissions. Up to 25% cement replacement with SCMs is considered feasible based on performance parameters. The models incorporated two different carbonation processes: a direct process (reacting minerals directly with gaseous CO2) and an indirect process (first extracting metal oxides and then reacting them with CO2). The processes incorporate separation by classification to ensure quality for cement replacement. Material unsuitable for SCMs is assumed to be stored in a nearby quarry. Three scenarios (pessimistic, mid, and optimistic) were defined based on factors influencing the economics of CO2 capture and utilization through mineralization (CCUM). These factors included the share of silica in the SCM, share of SCM in cement, ETS price, and cement price. The optimistic scenario uses assumptions favorable to CO2 mineralization, while the pessimistic scenario uses unfavorable assumptions. A key aspect of the scenarios considers the level of ETS (European Emission Trading System) price, which determines the value of CO2 emission reduction credits, representing an additional revenue stream for CCUM. The cement price determines revenue from replacing cement with the SCM. The costs and revenues of CCUM were analyzed. The models evaluated levelized costs of products, and CO2 emission reductions under various scenarios. Global uncertainty analysis was used to identify the key cost and benefit drivers. The results were then compared to other commonly proposed emission reduction strategies such as alternative fuels, other SCMs (calcined clay), MEA CCS, and oxy-fuel CCS. Techno-economic assessments were done by using MATLAB to solve mass and energy balances, size equipment, estimate capital and operational expenditures, and ultimately calculate levelized cost of product and revenue. Calculations were made for the costs of all different equipment and process steps, as well as calculating revenue based on market prices, carbon credits and expected material usage as a supplement in cement production. Finally a sensitivity and uncertainty analysis was done to identify the most crucial factors which need to be improved or better understood to enhance the overall economic performance. The models use a hybrid approach, combining bottom-up and top-down cost estimation methods, and consider learning effects in capital costs.

Under the optimistic scenario, both direct and indirect carbonation processes yielded positive business cases. The implementation of CCUM resulted in profits of approximately €129/tSCM−1 (direct) and €117/tSCM−1 (indirect), translating to an additional profit of €32 and €29 per tonne of cement sold. In the mid-range scenario, the processes broke even. The pessimistic scenario, without ETS eligibility, generated losses of €108/tSCM−1 (direct) and €80/tSCM−1 (indirect). These results show that ETS eligibility is critical to economic viability. Economically optimal plant capacities were determined for the various scenarios, indicating that revenue from ETS certificates alone does not fully cover the mineralization costs. CO2 emission reductions ranged from 15–33% (direct) and 8–23% (indirect) compared to ordinary Portland cement at economically optimal capacities. Decarbonization of the electricity and heating sectors would lead to slightly higher emission reductions. Sensitivity analyses investigated the influence of factors like ETS price, mineral transport distance, and SCM silica content. The direct process was found to be more robust against changes in these parameters compared to the indirect process. The analyses showed that the economics of the process is significantly affected by mineral transport distance, suggesting regional sourcing of materials. Lower silica content in the SCM was found to enhance economic viability, although there exists an trade-off between revenue from ETS and cost of production. The uncertainty analysis determined that electricity price and interest rate were the most influential factors affecting levelized cost of product. The indirect process was more sensitive to the price of the additive (ammonium sulfate) compared to the direct process. Learning effects were shown to have a substantial influence on capital cost, with initial investments requiring substantial support to be economically viable. Comparisons with alternative emission reduction strategies (alternative fuels, calcined clay, MEA CCS, oxy-fuel CCS) showed that CO2 mineralization, particularly under the optimistic scenario, was competitive, especially compared to calcined clay cement due to the higher share of SCM required by the latter to achieve similar emission reduction results. The study also considered combining CO2 mineralization with other CCS strategies, suggesting potential synergies in cost reduction.

This study demonstrates the potential of CO2 mineralization to significantly reduce cement industry emissions while generating profit. The economic success depends on the market acceptance of the resulting SCMs as a cement replacement and inclusion of CO2 mineralization in emission trading schemes. Regional sourcing of minerals is important to minimize transport costs, while the silica content in the SCM should be optimized to balance production costs and ETS revenue. Further research needs to focus on standardizing SCM blends to meet construction industry requirements and exploring the potential of alkaline industrial wastes as alternative feedstocks. The finding that CO2 mineralization can be competitive with other emission reduction strategies offers a promising avenue for decarbonizing the cement industry. However, the need for combined strategies to achieve complete decarbonization highlights the importance of policy support through emission trading schemes and potential government subsidies for the initial deployment of CO2 mineralization plants. The substantial sensitivity of the economic viability to factors like interest rate underscores the role that government policies might have in facilitating the transition to low-carbon cement production.

CO2 mineralization offers a promising pathway for reducing CO2 emissions from the cement industry while generating economic benefits. Successful implementation hinges on the dual criteria of market acceptance of the SCMs and inclusion in emission trading schemes. Regional sourcing of minerals and optimized silica content in SCMs are crucial for economic viability. Government policies play a significant role in supporting the initial investments and accelerating the widespread adoption of this technology. Future research should focus on standardizing SCM blends, exploring alternative feedstocks, and investigating synergies with other emission reduction strategies.

The study's techno-economic models rely on several assumptions and estimations, particularly regarding future market prices and policy frameworks. The models may not fully capture all potential economic and environmental implications of large-scale CO2 mineralization deployment. The optimistic scenario's assumptions, while increasing the likelihood of economic viability, are based on more favorable conditions and therefore may not be reflective of all possible future realities. Similarly, some of the cost curves utilized for equipment sizing and cost estimation are based on regressions of data points, and care should be taken when extrapolating to values outside the range of data points used in the regressions. Finally, while the study considered the environmental impacts of mineral mining, a more comprehensive life cycle assessment would be necessary to fully assess the environmental footprint of CO2 mineralization.