(60p) A Novel Carbon Emission Optimization Method for Chemical Processes Based on Thermodynamic 1st and 2nd Law: Naphtha Cracking Center Application

The goal of the Paris Agreement is to prevent global warming from exceeding 2 and to strive for limiting it to 1.5.¹ Most industries urgently require precise carbon accounting in the context of achieving carbon neutrality.² Currently, life cycle assessment (LCA) is an efficient way to evaluate the environmental impact of most industrial systems.³ However, LCA is subject to limitations in its application to the domain of process-level design and optimization due to its macroscopic and approximate characteristics.⁴ This methodology employs an inventory approach that considers the entire cradle-to-grave boundaries of a product.⁴ To address these limitations, there is a requirement for a rigorous method to calculate carbon emissions at the process-level, as well as an approach to interpret and reduce the carbon emissions.

This study proposes a novel process-level carbon tracking (PLCT) method to deal with the above limitations. The target process is the naphtha cracking center process, which is one of the representative carbon-emitting processes. The PLCT method is a carbon analysis method that integrates the following two thermodynamic analyses: (i) energy analysis based on the first law of thermodynamics and (ii) exergy analysis based on the second law of thermodynamics. First, the type of utility and duty value consumed by each equipment can be determined by performing an energy analysis based on the first law of thermodynamics. Then, exergy analysis is carried out based on the second law of thermodynamics to identify the proportion of fuel exergy consumption attributed to naphtha cracking and each utility production process. The application of this proportion to the carbon emissions of the process allows the distinction between direct emissions from naphtha and indirect emissions from utilities. Moreover, the results of PLCT permit to identify the carbon contribution of each utility. The carbon analysis through exergy analysis enables consideration of process efficiency in naphtha cracking and utility production. This allows for more accurate carbon accounting of the target process than using conventional carbon emission inventory. As a result of the PLCT method, the unit carbon contribution factors (kg-CO₂/kW) for the heating, power, cooling, and refrigeration utilities were determined. The calculated unit carbon contribution factor for each utility was associated with the energy duty for each equipment, and the carbon contribution of each equipment was determined accordingly. This allows us to analyze the overall carbon flow of the process. Furthermore, a new objective function for carbon reduction was developed using the unit carbon contribution factors of each utility. As a result of optimization, total carbon emissions were reduced in the NCC process. The comparisons between the proposed carbon optimization and the conventional energy optimization in terms of carbon reduction are described as follows:

• Energy optimization results in a reduction in total energy, primarily by decreasing the consumption of heating utility with high total duty values, even though their carbon contribution factor is low.
• Carbon optimization results in a significant decrease in the consumption of power and cooling utilities with high carbon contribution factors, leading to an overall reduction in carbon emissions.
• Hence, the total energy consumption in the carbon optimization case is higher than that in the energy optimization case, but the total carbon emissions are lower

This study is expected to contribute to a carbon-neutral society in three ways. Firstly, the PLCT method can serve as an evaluation method for quantifying the actual reduction in carbon emissions achievable at the process-level. This method may be applied in the assessment of newly conceptualized carbon reduction processes, e.g., carbon capture, biomass resource utilization, and renewable energy-based processes. Secondly, by interpreting the carbon contribution of each equipment activity within the process, it can make it possible to identify the cause of carbon emissions and suggest a resolution for carbon reduction at the unit level. This approach is rigorous and enables the assessment of carbon emissions at a level that is comparable to the estimation method employed in IPCC Tier 3.⁵ Lastly, a new carbon reduction method is proposed through utility optimization using the unit carbon contribution factors. The conventional energy optimization approach does not consider identifying which utility reduction at specific equipment has a greater effect on carbon reduction, but this is possible with the proposed method. We believe that this study establishes a fundamental methodology for carbon analysis in process design and serves as a guideline for carbon reduction at the process-level.

REFERENCES

[1] United Nations Framework Convention on Climate Change (UNFCCC), “Adoption of the Paris Agreement, Paris,” (2015).

[2] H. Zhang., et al., “A carbon flow tracing and carbon accounting method for exploring CO2 emissions of the iron and steel industry: An integrated material–energy–carbon hub,” Appl. Energy. 309 (2022).

[3] W. Huang., et al., “Exergy-environment assessment for energy system: Distinguish the internal and total exergy loss, and modify the contribution of utility,” Energy Convers. Manag. 251 (2022)

[4] L. Meyer., et al., “Exergoenvironmental analysis for evaluation of the environmental impact of energy conversion systems,” Energy. 34 (2009)

[5] Eggleston, H. S., et al., "2006 IPCC guidelines for national greenhouse gas inventories." (2006).