(286a) Designing Chemical Value Chains for Net-Zero Emissions and Nature Positive Decisions

Adherence to the Paris Agreement necessitates the curbing of global average temperature rise to 1.5 degrees above pre-industrial levels. While direct contribution of the chemicals and materials industry (CMI) to global greenhouse gas emissions is relatively small, the growing importance of chemical energy carriers requires all chemical value chains to transition to a state of net-zero or net-negative emissions.¹ Biomass is an essential driver towards carbon-neutrality, especially in “hard to decarbonize” sectors such as the CMI which cannot be decoupled completely from carbon mobilization.^2,3 The availability of biomass and its fair allocation among polluting operations is crucial and still remains unaddressed in current relevant literature. Moreover, the impact of chemicals manufacturing on the natural environment is not limited to GHG emissions. Water use, critical mineral use and impact on bio-diversity are some considerations which are often overlooked in the scope of net-zero emission value chain design.⁴ These can be quantified by the absolute environmental sustainability (AES) metrics which calculate the transgression of environmental impact of a product or process on nature’s carrying capacity. Most existing literature in the area focusses on the net-zero objective and the economics associated. The few works that seek to quantify AES do not discuss the methodological gap in accounting for ecosystem services in LCA or incorporate spatially explicit datasets.^5,6 To bridge this gap, we design chemical and plastics value chains to study the implications of net-zero value chains on AES.

We postulate that while carbon sequestration by biomass is a driver of both carbon-negativity and AES, design for one objective might not necessarily satisfy the other. The biomass accounting for life-cycle and AES assessment methods operate on different principles. LCA treats biomass derived products as either inherently carbon neutral or expands the system boundary to incorporate carbon sequestration by biomass in the scope of emission calculations. For example, the inclusion of greater amounts of biomass derived feedstocks into chemical value chains may lower the overall greenhouse gas emissions of the value chain, when accounted for by the later principle. LCA thus implicitly assumes that the demand of biomass by the industry may be equitably fulfilled. However, AES assessment may allocate a different amount of biomass for sequestration of these emissions, depending upon the choice of sharing principle. Therefore, while a process network formulated to minimize emissions might seek to incorporate greatest permissible amount of biomass usage to leverage sequestration, a network formulated to maximize AES will instead lower the demand of ecosystem services in accordance with their available supply. Thus, the objectives of net-zero design and design for absolute environmental sustainability are not perfectly aligned.

To study the interplay of these objectives and determine technological solutions robust to the environmental objective, we formulate the steady state design of sustainable chemical value chains as a multi-objective optimization problem. The net emissions of the chemical process network are minimized using LCA practices for biomass accounting. We also seek to minimize the transgression level utilizing the best available regional information on chemical value chains, for the AES objective. We use our model of the global CMI developed earlier, as the baseline technological matrix.⁷ Additionally, emerging technologies operating at all stages of the life cycle are incorporated. These include input side solutions such as renewable power, green hydrogen, biomass, captured carbon-dioxide; process upgradations such as process electrification, transformative technologies; output side solutions such as carbon capture, incineration, pyrolysis; and recycle options ranging from primary to tertiary recycling. Regional information on chemical production, usage, sourcing of inputs, and background emissions is used to estimate emissions and sequestration. The multiscale techno-ecological synergy (TES) based AES assessment method is used to quantify the AES metric. Integrated with biophysical models, this method brings in high geographical resolutions in assessing nature’s carrying capacity. Private and public ownerships are considered while downscaling ecological thresholds at large scale which avoids subjectiveness and encourages stakeholders taking actions towards net-zero objectives⁸.

The results of the optimization problem yield multiple Pareto optimal value chain solutions. We study the trade-offs between the most net-negative and most sustainable solutions. The net-negative solutions are disproportionately dependent on carbon sequestration to offset their emissions. The non biogenic emissions from this network reflect the same, with some fossil-based options being selected to offer greater sequestration credits. The most sustainable solutions on the other hand minimize the non-biogenic emissions from the network, relying on circular solutions like plastics recycling and carbon capture and utilization for carbon circularity. This helps us determine the technological solutions most robust to the subjective choice of environmental metrics, like carbon capture and utilization and plastics recycling. Our results caution us against dependence on one class of solutions in the quest to a net-zero world. While we recognize the importance of bio-based solutions to offset the hardest-to-abate emissions and their criticality towards net-negative operations, the abuse of such solutions raises issues of damage to bio-diversity and food-fuel conflict. This work provides a stepping stone towards consideration of multiple environmental stakeholders in planning the net-zero transition.

References

1. Wevers, J. B., Shen, L., & Van der Spek, M. (2020). What does it take to go net-zero-CO2? A life cycle assessment on long-term storage of intermittent renewables with chemical energy carriers. Frontiers in Energy Research, 8, 104.

2. Haus, M. O., Winter, B., Fleitmann, L., Palkovits, R., & Bardow, A. (2022). Making more from bio-based platforms: life cycle assessment and techno-economic analysis of N-vinyl-2-pyrrolidone from succinic acid. Green Chemistry, 24(17), 6671-6684.

3. Gabrielli, P., Gazzani, M., & Mazzotti, M. (2020). The role of carbon capture and utilization, carbon capture and storage, and biomass to enable a net-zero-CO2 emissions chemical industry. Industrial & Engineering Chemistry Research, 59(15), 7033-7045.

4. Persson, L., Carney Almroth, B. M., Collins, C. D., Cornell, S., de Wit, C. A., Diamond, M. L., ... & Hauschild, M. Z. (2022). Outside the safe operating space of the planetary boundary for novel entities. Environmental science & technology, 56(3), 1510-1521.

5. Bachmann, M., Zibunas, C., Hartmann, J., Tulus, V., Suh, S., Guillén-Gosálbez, G., & Bardow, A. (2023). Towards circular plastics within planetary boundaries. Nature Sustainability, 1-12.

6. Meng, F., Wagner, A., Kremer, A. B., Kanazawa, D., Leung, J. J., Goult, P., ... & Cullen, J. M. (2023). Planet-compatible pathways for transitioning the chemical industry. Proceedings of the National Academy of Sciences, 120(8), e2218294120.

7. Sen, A., Stephanopoulos, G., & Bakshi, B. R. (2022). Mapping Anthropogenic Carbon Mobilization Through Chemical Process and Manufacturing Industries. In Computer Aided Chemical Engineering (Vol. 49, pp. 553-558). Elsevier.

8. Xue, Ying, and Bhavik R. Bakshi. "Metrics for a nature-positive world: A multiscale approach for absolute environmental sustainability assessment." Science of The Total Environment 846 (2022): 157373.