(368z) Techno-Economic Analysis of Alcohol-Based Liquid Organic Hydrogen Carrier (LOHC) Systems

Research Interests - Power systems modeling, energy systems decarbonization, renewable energy technologies, process modeling, process design, process optimization and sustainability assessment

Abstract

To achieve an efficient and cost-effective energy system based on variable renewable energy sources such as wind and solar, effective energy storage and transport technologies are crucial. Deployment of electrical energy storage (EES) and transmission is one option; however, a hydrogen-mediated energy infrastructure provides flexibility in terms of energy source and end-use (electricity, transportation, and production of chemicals). To this end, liquid organic hydrogen carriers (LOHCs) are a potentially promising technology for energy storage and transport, given their higher energy density, relative ease of handling and low cost of transportation compared to compressed or cryogenic hydrogen (H₂) [1].

LOHC deployment involves : a) hydrogenation of H₂-lean carrier to H₂-rich carrier to store low-cost and low-carbon H₂, b) dehydrogenation of H₂-rich carrier to H₂-lean carrier to release H₂ to meet end-use demand and c) transport of H₂-rich carrier in the case of so-called “one-way” carriers (e.g. ammonia) and additional transport of “lean-carrier” in case of so-called “two-way” carriers (e.g. methylcyclohexane (MCH)/toluene). Key challenges for scaling known LOHCs include high capital cost of underlying processes (e.g. ammonia), substantial energy input for H₂ recovery via dehydrogenation, as well as limitations on hydrogen carrying capacity (e.g. MCH/toluene) [2], [3]. Here, we investigate the process design and techno-economic outlook for an alternative LOHC based on alcohols, specifically acid hydrogenation to alcohol for H₂ storage and alcohol dehydrogenation for H₂ release. Alcohols like ethanol are potentially appealing as “two-way” LOHCs, owing to their low reaction temperatures, higher H₂ carrying capacity as other “two-way” carriers, as well as potential to be sourced from biogenic sources [4].

We use process modeling and techno-economic analysis to evaluate the favorable process designs and supply chains for deployment of ethanol-based LOHC, where acetic acid serves as the H₂-lean carrier. For example, for hydrogenation of acetic acid to alcohol, we evaluate multiple process configurations defined by the differences in: a) reaction schemes and operating conditions, b) target purities for the ethanol product to be transported (e.g. transporting ethanol-water mixtures vs. pure ethanol), c) ethanol purification technologies used, including pressure-swing distillation and extractive distillation as well as d) energy integration with low-carbon H₂ production processes. For all these options, we evaluate process-level performance metrics, such as capital cost per MW H₂ throughput, alcohol/acid yield, carbon loss (as side products) per MWh H₂ throughput, and energy input per MWh H₂ throughput. These process-level metrics are then used as part of an overall supply chain analysis to identify the most favorable ethanol-based LOHC schemes and their relative merits vs. other traditional “one-way” and “two-way” LOHCs.

Preliminary results for acetic acid hydrogenation, based on single-reactor based schemes using Pt-based catalysts at 250°C [5], [6] highlight that >90% carbon recovery as ethanol is achievable with H₂ recycling at pressures above 20 bar. The resulting process has estimated capital costs near $200-300/kW H2 for 50 tonne H₂/day facility as well as low temperature heat input below 0.5 MW/MW H₂. Interestingly, we find that production of high purity ethanol (>90 mol%) as opposed to using the raw ethanol product (~50 mol% with rest mostly water) substantially increases capital cost and energy requirements for the hydrogenation process. Since water is a co-reactant in the dehydrogenation process, this suggests that there is a potential trade-off between purity of ethanol transported vs. carrier transportation costs that needs to be considered when designing alcohol-based LOHC supply chains. This includes an assessment of how different reaction conditions affect various system-level performance metrics, such as capital cost per MW H₂ throughput, alcohol/acid yield, carbon loss per MWh H₂ throughput, and energy input per MWh H₂ throughput. In addition, considering the energy-intensive nature of ethanol-water separation, we analyzed the impact of alternative separation methods and the relative advantages of using aqueous mixtures instead of pure components as carriers, which have the potential to balance processing costs and hydrogen capacity.

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