Utilizing Electrolysis To Reduce Greenhouse Gas Emissions and Improve Product Yields in Gas-To-Liquids Processes

This paper seeks to investigate and benchmark the performance of two integrated flow schemes developed with the intent of reducing the process scope carbon emissions intensity and improving overall gas conversion efficiencies of conventional gas-to-liquids (GTL) Fischer-Tropsch (FT) synthesis processes. The presented reference case represents a conventional FT synthesis process utilizing a natural gas feedstock and autothermal reforming (ATR) unit for syngas generation. A carbon capture and sequestration (CCS) system is deployed within the process off-gas stream with the aim of abating process scope carbon emissions, representing a minimally invasive step change over more conventional FT synthesis flowsheets. In contrast, the primary study case that was developed leverages a hydrogen import generated through electrolysis, with the electrolysis power requirement supplied by the exothermic FT-synthesis process and supplemented with a renewable power source. The oxygen by-product from electrolysis is fed with natural gas to a partial oxidation (POx) unit to generate syngas for conversion within the FT synthesis loop. Furthermore, the co-production of hydrogen allows for changes in the GTL flow scheme, significantly decreasing the amount of CO₂ generated. Specifically, the deployment of a POx unit reduces steam, oxygen, and fuel gas consumption when compared to more conventional reforming technologies such as ATR at the sacrifice of reduced H₂:CO ratios. For the purposes of this study, the FT synthesis process was paired with a mild hydrocracker and product work-up unit to generate a mixture of naphtha, kerosene, and diesel end products. The balance of the plant systems, including steam, power, and fuel gas balances, were developed to maximize process efficiency based on the unique operating characteristics and parameters of the identified processes. Ultimately a direct comparison between the identified flow schemes is drawn wherein the presented study case exhibits an estimated gas conversion efficiency of 7,460 Standard Cubic Feed (SCF) per bbl of liquid product with a carbon conversion of 93.7%, representing a marked improvement over not only conventional FT synthesis processes but also over the presented reference case. This work ultimately demonstrates the potential benefits that can be achieved by customizing GTL designs to optimally accommodate a decarbonized hydrogen import stream. The results of this work have broader implications for not only the decarbonization of conventional GTL pathways but also less conventional waste and/or biomass to fuels processes that are similarly deficient in hydrogen, wherein analogous principles may potentially be applied to maximize product yields.