Sustainable Production of Isopropyl Alcohol via Basic Oxygen Furnace Gas Fermentation: A Life Cycle Assessment Perspective

Abstract

The CO2 emissions started to peak during the start of this century and the steel manufacturing sector accounts for 25% of the global CO2 emissions. The majority of steel production is based on the route of the basic oxygen furnace (BOF) route, which is energy efficient. This production route results in the emission of BOF gas. So a new method to produce Isopropyl alcohol via BOF gas fermentation was developed for the pilot scale plant and the LCA was performed by Liew et al., (2022). The carbon-negative emissions were calculated and estimated in the pilot scale study. A few inconsistencies were identified in the calculation method, in which the components utilized for the production process contributed less than 5% to the Global Warming Potential value, and the replacements provided to the steel mill for the BOF gas redirection were cut off. The LCA value for the pilot-scale plant was incomplete and inconsistent, and only the GWP was chosen as the prominent midpoint indicator. Based on this pilot-scale plant, an industrial-scale (46 kton/ yr) base case process model was developed for the gas-fermentation process to produce IPA. The fermentation is the major step involved in the production of IPA, which involves the acetogenic bacteria Clostridium Autoethanogenum. The initial process involves the capture of the emission of BOF gas for compression and cooling performed because the temperature of BOF gas is around 1100°C. This compressed gas is fermented using of microbes and filtered out to obtain the filtered broth. Then the IPA is separated from the filtered broth using extractive distillation, using glycerol. The major product of IPA is processed out of the system. This research aims to analyze and estimate the environmental impacts of this process model of BOF gas fermentation to produce Isopropyl alcohol. The assessments were performed for the base case and the 11 process parameters of CO conversion, Volumetric mass transfer rate of CO, Product selectivity, Dilution rate, Extractive distillation glycerol mole fraction, Temperature offgas condenser, Anaerobic waste conversion, Extractive distillation molar reflux ratio, Biomass liq-liq mole fraction and the broth and glycerol purges. The impact assessment results help to identify the potential contributors in the process that affect the environment, so the process model can be optimized to yield lower impact values concerning the environmental perspectives.

The 7 midpoint indicators namely Global Warming Potential (GWP), Stratospheric Ozone Depletion (SOD), Fine Particulate Matter Formation (FPMF), Freshwater Eutrophication (FE), Marine Eutrophication (ME), Human Carcinogenic Toxicity (HCT), and Land Use (LU) were chosen to obtain a detailed view on the ecosystem, human health, and the environmental effects. The replacement calculations estimated for 1 kg of IPA production is 4.324 MJ of heat, and 0.877 kWh of electricity, has to be replaced for the steel mill. The impact assessment for the base case model was performed and compared with the conventional IPA production method (GWP: 2.026 kg CO2 eq.), the global warming potential for the BOF gas fermentation method (GWP: 27.656 kg CO2 eq.) estimated to be a 1265% increase compared with the conventional IPA method. Similarly, all seven impact categories are estimated to have a huge increase in values. An elaborate study compared and identified the most influential process parameters. The process parameter of dilution rate in which the dilution rate is lowered by 30% from the base case value appears to be the process parameter with a lower impact value of 20.464 kg CO2 eq. for the Global Warming Potential (26% lower than base case), 1.72E-05 kg CFC11 eq. for the Stratospheric Ozone Depletion (31% lower than base case), 9.618E-03 kg PM2.5 eq. for the Fine Particulate Matter Formation (38% lower than base case), for the Freshwater Eutrophication the value is 9.853E-04 kg P eq. (53% lower than base case), 5.531E-03 kg N eq. for Marine Eutrophication (33% lower than base case), 1.733E-01 kg 1,4-DCB for the Human Carcinogenic Toxicity (53% lower than base case), and the 3.265 m2a crop eq. (32% lower than base case) for the Land use impact categories. The major contributors are the high-pressure and low-pressure steam utility contributing greater than 56% for the impact category values of the GWP, FPMF, FE, and HCT particularly. The glycerol used for the extractive distillation process contributed greater than 92% for the impact categories of SOD, ME, and LU. The impact assessments across different indicators were interpreted and the major process parameters that are more influential in reducing the impact values are identified. The results indicate that the major contribution to the impact value is reduced by the emission credit ±40% for preventing the BOF gas from flaring. The carbon dioxide emission from the process, glycerol component, and steam utility collectively contribute majorly which account for more than 90% of the total impact values in the impact categories. Lowering the dilution rate, glycerol purge fraction, and glycerol mole fraction by -30%, and increasing the volumetric mass transfer rate by +30% of the process model could result in lower impact values. The CO2 emitted from the process is estimated to be 9.34 kg CO2 eq. This emission is higher than the feedstock (BOF gas) used for the gas-fermentation process. The entertainer glycerol is anaerobically digested and combusted leading to the 10% of the total CO2 emission of the process model. The process model should be updated with the process parameters listed above and a similar life cycle impact assessment has to be performed to compare the impact values. This updated process model might have comparatively better results. The sensitivity study has been performed to estimate the percentage effects of the combined glycerol and steam components on the midpoint indicator impact value. Cutting off the components of glycerol and steam from the process model still yields the GWP value of 6.16 kg CO2 eq. for industrial scale process which is a 204 % increase than the conventional IPA process. This implies that the updated process model with similar process steps cannot obtain impact values lower than the conventional IPA method. To lower the impact value of GWP, the CO2 emission from the process should be sequestrated (carbon capture) to lower the GWP value by 42% from the base case.