Strategies for CO2 Reduction through Proactive Removal and Utilization

Abstract

Limiting the global temperature rise within 1.5-2°C above the preindustrial era would require mass-scale CO2 removal from the atmosphere. This dissertation explores novel strategies to lower the cost and accelerate the pace of removal by pioneering an alternative process for direct air capture (DAC) and innovative strategies to utilize CO2 in the concrete industry with a combination of experiments and systems-level analyses. First, the cost of using DAC as an industrial climate backstop by coupling it with geological sequestration is studied in the context of reducing CO2 emissions from the U.S. electric sector. The least-cost optimization framework presents a clear picture that immediate and sustained mitigation needs to be prioritized; delaying undertaking mitigation measures beyond 2030 and relying on the backstop would cost an additional 580-2015 billion USD through 2050 compared to starting mitigation in 2020 and avoid using the backstop. However, still increasing global greenhouse gas emissions necessitates decreasing the heavy energy demand and cost of DAC. Sorbent regeneration experiments using microwaves revealed that meeting such requirements is possible. A substantial reduction of the regeneration time from over an hour to a matter of minutes was confirmed with the application of microwaves which can be used to decrease the cost of DAC through system downsizing. The lower temperature of the desorbed CO2 gas (45-55°C) compared to that of the sorbents (>100°C) suggests a low-temperature desorption mechanism can be used to design a low-energy DAC system in the future. The recovered CO2 can be strategically utilized during the fabrication of cement and concrete products rather than being sequestered in the geosphere. The strength development of concrete induced by the added CO2 enables an opportunity to magnify the overall CO2 reduction while saving implementation costs through decreasing binder content. An assessment of the industry-wide application of such joint strategy reveals the significance of exploiting CO2-induced property changes, not maximizing absorbed CO2; over 10% of the emissions from the U.S. concrete manufacturing can be reduced through CO2-enabled binder reduction, compared to 1% possible through sequestration only. The saved material cost could fully mitigate the implementation costs of this strategy. Alternatively, CO2 can be applied to an ultra-ductile material such as engineered cementitious composite (ECC) to mass-produce durable CO2-embodying products using existing infrastructure. The environmental and economic benefits of CO2-cured ECC-based railroad ties are evaluated against conventional concrete ties with a combined lifecycle framework and stochastic tie failure model that considers a wide range of tie failure patterns, replacement strategies, and resultant impacts on train operation. Despite the increased cost and carbon footprint (>50%) of an ECC tie, using ECC ties reduces the overall system cost by 25% and carbon footprint by 19% by requiring nearly 50% fewer ties over 100 years. The increased product longevity is the primary driver of the improvements, rather than the quantity of the CO2 sequestered. Altogether, this dissertation highlights the urgency for minimizing the reliance on carbon backstop and demonstrates how novel removal and utilization strategies can reduce the overall cost of CO2 reduction; the virtue of proactive CO2 utilization is complementing fossil mitigation now to minimize the gross scale of sequestration throughout the century, not maximizing sequestration in products. The findings of this work can serve as a groundwork to explore further proactive CO2 removal and utilization opportunities to accelerate CO2 reduction.