(397c) Techno-Economic Analysis and Life Cycle Assessment of Bio-Based Hydrogen Production from Integrated Dark-Fermentation and Microbial Electrolysis Cells

Renewable Hydrogen (H₂) is a zero-carbon energy carrier that can contribute to the decarbonization of all energy sectors. The U.S. Department of Energy launched the Hydrogen Shot, which seeks to reduce the cost of low-carbon H₂ to $1 per 1 kilogram in 1 decade. Traditionally, H₂ is produced from steam methane reforming (SMR) of fossil natural gas (FNG), emitting 11.5 kg of greenhouse gases (GHG) per kilogram (kg) of H₂ produced on a well-to-wheels basis. To reduce the life cycle GHG emissions associated with the production of low-carbon H₂, renewable biomass resources, such as corn stover, are being considered as a model feedstock for H₂ production.

In this study, we conducted process simulation with Aspen Plus and performed techno-economic analysis (TEA) for bio-based H₂ production using the integrated dark fermentation (DF) - microbial electrolysis cell (MEC) system that converts corn stover to H₂. For dark fermentation, Clostridium thermocellum is engineered to decompose both cellulose (C6) and hemicellulose (C5) sugars simultaneously to acetic acid to improve H₂ yield per unit of biomass by weight. In the MEC process, acetic acid in the dark fermentation effluent is converted to H₂ with exoelectrogenic Geobacter-Shewanella co-culture. Compared to only 4 moles of H₂/mole sugar (i.e., glucose) from the standalone DF system, the MEC process produces another 8 moles of H₂/mole of starting glucose by utilizing DF by-product (i.e., acetic acid). The integrated DF-MEC system has a theoretical yield of 12 moles of H₂/mole hexose and 6 moles of CO₂. Since the by-product CO₂ is of high purity, we consider captured CO₂ as a potential revenue stream from sales to the beverage industry or as a tax credit through geological storage. Detailed mass and energy balances were obtained from Aspen Plus process model. The reaction parameters employed in the process model were obtained through the concerted efforts of a multi-national laboratory research team, including NREL, LBNL and PNNL, by using lab tested results as well as assumptions made in previous studies.

To perform the TEA analysis of the integrated DF-MEC system, we utilized the Hydrogen Analysis Production Models (H2A) model. Our results indicate that the capital cost of MEC as well as the current density of MEC strongly affect the H₂ production cost. Using recently demonstrated MEC current density of 20 A/m², the levelized H₂ production cost is 8.6/kg H₂, assuming 80% DF conversion rate (target) and 90% MEC conversion rate (validated), and assuming additional revenue of $50/metric ton CO₂ due to carbon capture and sequestration (CCS). Increasing the MEC current density to 100 A/m² would reduce the cost to $6.3/kg H₂. Further cost reduction opportunities include the continuous enhancement of current density, the use of zero-cost waste feedstock, the reduction of electricity use in MEC, and the reduction of NaOH use in biomass pretreatment.

We utilized the Greenhouse gases, Regulated Emissions, and Energy use in Technologies (GREET^®) model to perform the life cycle analysis of bio-H₂ production via biomass DF-MEC. Taking into account the carbon emission reduction from CCS, the life cycle GHG emissions of H₂ production via the integrated system is 5.7 kg/kg H₂, which is higher compared with the 4.8 kg/kg H₂ in the conventional FNG SMR with CCS pathway. This is mainly due to the large use of current fossil-based U.S. electricity generation mix by the integrated DF-MEC system (i.e., 28.3 kWh/kg H₂). If electricity from renewable sources such as wind/solar, is utilized, the integrated DF-MEC system can achieve a net negative life cycle GHG emissions (i.e., -6.8 kg/kg H₂) with CCS. In existing/emerging carbon markets, the reduction in life cycle GHG emissions can result in monetary incentives, which can further reduce the net H₂ production cost.