From life-cycle assessment towards life-cycle design of carbon dioxide capture and utilization

• Von der Ökobilanz zum Ökodesign der Kohlendioxid-Abscheidung und -Nutzung

von der Assen, Niklas; Bardow, André (Thesis advisor); Patel, Martin (Thesis advisor)

1. Auflage. - Aachen : Wissenschaftsverlag Mainz GmbH (2015, 2016)
Book, Dissertation / PhD Thesis

In: Aachener Beiträge zur Technischen Thermodynamik 6
Page(s)/Article-Nr.: XX, 209 Seiten : Diagramme, Karten

Dissertation, RWTH Aachen, 2015

Abstract

The increasing use of fossil resources will inevitably lead to CO2 emissions and an increasing atmospheric CO2 concentration. The increased CO2 concentration is one of the main reasons for the earth's global warming. To mitigate global warming and the depletion of fossil resources, CO2 can be captured and subsequently utilized as alternative carbon source for fuels, chemicals and materials. However, both CO2 capture and utilization (CCU) typically require energy whose provision is again associated with fossil resource use and CO2 emissions. Thus, the intuitively expected environmental benefits of CCU are not guaranteed. Therefore, each individual CCU case requires a reliable environmental assessment.For a reliable environmental assessment, comprehensive process data are required such as mass and energy balances. However, data availability is typically limited for many CCU process in early development stages. To overcome the limited data availability, easily accessible ad-hoc metrics are often used. However, they usually cannot guarantee a reliable environmental assessment. In contrast to these ad-hoc metrics, Life-Cycle Assessment (LCA) evaluates the entire life cycle of process systems with respect to multiple environmental impacts. The holistic approach of LCA avoids problem shifting between life cycle phases or environmental impact categories. Although the suitability of LCA is generally acknowledged, LCA is not yet standard practice for an environmental assessment of CCU. The goal of this thesis is twofold: The first goal is to facilitate and illustrate the application of LCA for established CCU processes. The second goal is to provide an environmental design approach for new CCU processes and products.To facilitate the application of LCA for CCU processes, this work provides a jargon-free introduction for LCA in the context of CCU. Despite the easily accessible concept of LCA, particularly important and severe pitfalls have been identified for the application of LCA to CCU: The first pitfall is the intuitive treatment of CO2 as negative greenhouse gas emission. An illustrative example shows that utilized CO2 is not a negative emission but instead a feedstock with its own production emissions. The second pitfall is the choice between alternative allocation methods to still obtain environmental impacts for the individual products while the CCU system generally provides more than one product. General implications of alternative allocation methods in CCU are discussed and illustrated by a simplified case study. The third pitfall is the overestimation of the effect of the CO2 storage duration on the global warming impact. Whereas today the CO2 storage duration and the global warming impact are used as two separate metrics, a method to incorporate the CO2 storage duration into the global warming impact is presented and discussed. Finally, a framework is derived to avoid the described pitfalls in LCA of CCU.The derived framework is then applied to an industrial case study for LCA of CO2-based polymer production. Here, CO2 is captured in a pilot plant of a lignite power plant and then synthesized with epoxides into polyols with up to 30 weight percent CO2 in another pilot plant. The results of this study show that capture and utilization of CO2 for polymers reduces not only greenhouse gas emissions and fossil resource use, but all investigated environmental impacts. Furthermore, the LCA results provide valuable insights into the origin of the environmental benefits.In contrast to the industrial case study, many CCU processes are at early development stages and thus offer many degrees of freedom for the environmental optimization of CO2 capture and utilization. Therefore, the second part of this thesis extends the classical, retrospective scope of LCA towards a prospective Life-Cycle Design (LCD) approach. Starting with CO2 capture, an approach is developed for the selection of CO2 sources that minimizes the environmental impacts associated with CO2 capture, compression and transport. The approach is applied for CO2 sources in Europe to identify so-called CO2 oases: locations where CO2 can be obtained with lowest environmental impact.For the environmental optimization of CO2 utilization, two approaches are presented: The first approach identifies environmentally optimal reaction schemes for the hydration of CO2 as a function of the environmental impacts of H2 and CO2 supply. Since the approach requires only basic thermodynamic data, it is particularly useful for early stages of CCU process design. While the hydration of CO2 typically yields basic commodity chemicals with identical properties, the direct and indirect utilization of CO2 for polymers provide novel degrees of freedom to design the polymer supply chain and to modify the polymer properties. To exploit these degrees of freedom for minimization of environmental impacts, the second approach identifies environmentally optimal polymer compositions as well as optimal production processes in the polymer supply chain. In particular, the approach considers alternative methods for allocation of by-products along the supply chain. The presented optimization approaches allow fully exploiting the limited potential of CCU to reduce fossil resource use and CO2 emissions.

Institutions

• Chair and Institute of Technical Thermodynamics [412110]
• Junior Professorship of Sustainable Life Cycles in Energy, Chemical and Process Engineering [422130]