Power-to-fuels via solid-oxide electrolyzer: Operating window and techno-economics

Power-to-fuel systems via solid-oxide electrolysis are promising for storing excess renewable electricity by efficient electrolysis of steam (or co-electrolysis of steam and CO2) into hydrogen (or syngas), which can be further converted into synthetic fuels with plant-wise thermal integration. Electrolysis stack performance and durability determine the system design, performance, and long-term operating strategy; thus, solid-oxide electrolyzer based power-to-fuels were investigated from the stack to system levels. At the stack level, the data from a 6000-h stack testing under laboratory isothermal conditions were used to calibrate a quasi-2D model, which enables to predict practical, isothermal stack performance with reasonable accuracy. Feasible stack operating windows meeting various design specifications (e.g., specific syngas composition) were further generated to support the selection of operating points. At the system level, with the chosen similar stack operating points, various power-to-fuel systems, including power-to-hydrogen, power-to-methane, power-to-methanol (dimethyl ether) and power-to-gasoline, were compared techno-economically considering system-level heat integration. Several operating strategies of the stack were compared to address the increase in stack temperature due to degradation. The modeling results show that the system efficiency for producing H2, methane, methanol/dimethyl ether and gasoline decreases sequentially from 94% (power-to-H2) to 64% (power-to-gasoline), based on a higher heating value. Co-electrolysis, which allows better heat integration, can improve the efficiency of the systems with less exothermic fuel-synthesis processes (e.g., methanol/dimethyl ether) but offers limited advantages for power-to-methane and power-to-gasoline systems. In a likely future scenario, where the growing amount of electricity from renewable sources results in increasing periods of a negative electricity price, solid oxide electrolyser based power-to-fuel sys

Techno-economic optimization of biomass-to-methanol with solid-oxide electrolyzer

François Maréchal, Maria del Mar Perez Fortes, Jan Van Herle, Ligang Wang, Hanfei Zhang

The purpose of this paper is to assess techno-economically the integration of solid-oxide electrolysis in biomass-to-methanol processes: (1) The hydrogen produced by electrolysis can be used to adjust the composition of syngas from gasification to increase the conversion of carbon in biomass, (2) the oxygen as a byproduct of electrolysis can be used in the gasifier to avoid expensive air separation units, and (3) the overall process can be thermally integrated. Two integration concepts are proposed with different sizing methods of the electrolyzer: (1) the case of full conversion of carbon in biomass, in which a large electrolyzer is driven by the electricity purchased from the grid, and (2) the case of zero power exchange, in which only part of the carbon in biomass is converted reaching self-sufficiency of electricity. The three cases including the state-of-the-art biomass-to-methanol process are investigated to identify (1) possible trade-offs between efficiency and costs, and (2) under which conditions, these concepts become economically viable. With a reference methanol production of 100 kton/year, the results show that there is an optimal design for the state-of-the-art case, which offers an efficiency of 53.3% due to steam cycles and a payback time of 4.8 years. For the integrated concepts, there are sharp trade-offs between the system efficiency and methanol production cost rate. The case of full carbon conversion can reach an energy efficiency of 64.5-66.0% but results in a longer payback time of over 11 years. The case of zero-power exchange can achieve a similar efficiency as the state-of-the-art case with a slightly increased payback time of over 5.5 years. The payback time of the full carbon conversion case can be shorter than 5 years with a reduction in stack cost and electricity price, and an increase in stack lifetime.

ELSEVIER SCI LTD2020

Trade-off designs and comparative exergy evaluation of solid-oxide electrolyzer based power-to-methane plants

François Maréchal, Jan Van Herle, Ligang Wang

Solid-oxide electrolyzer (SOE) based power-to-methane (PtM) system can efficiently store surplus renewable power into synthesis natural gas by electrolysis and methanation. The system performance depends on the operating point of the electrolyzer and system design, particularly the heat exchanger network. In this paper, we investigate a SOE based PtM plant with a fixed-bed catalytic methanator and a membrane module for methane upgrading. A top-down approach is first employed to derive optimal system designs step by step from the system concept, to optimal conceptual designs with the trade-off between system efficiency and methane yield, to design-point selection and heat exchanger network design. Then, exergy evaluation with the exergy calculated into thermal - mechanical - non-reactive - reactive parts is applied to the derived four specific system designs to understand how exergy dissipation and performance of the overall system and each component vary from one to another. The results show that the system efficiency can reach between 80 and 85% (HHV) or 75-80% (LHV) when operating SOE with an inlet temperature of 700 degrees C and a utilization factor over 60%, above which electrical steam generation can be avoided and the steam can be generated by the heat from methanation reaction (around 80-85%) and anode outlet (15-18%). The system's exergy efficiency can achieve around 75-80% with the input exergy mainly destructed within the SOE (25-35%), methanator (25-35%) and heat exchangers (10-17%). However, exergy efficiencies of the SOE and methanator are high, over 90%. Depending on the temperature level of the cold stream and the temperature difference, heat exchangers generally have an exergy efficiency of over 50-80%. The electrical steam generator can only achieve an efficiency of around 20% and leads to a significant drop of system efficiency if employed; however, small electrical heating to reach the desired SOE inlet temperature, although bad, is acceptable. Therefore, one preliminary design guideline for such systems should be the avoidance of electrical steam generation. (C) 2018 The Authors. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.

PERGAMON-ELSEVIER SCIENCE LTD2019

Techno-economic evaluation of biomass-to-fuels with solid-oxide electrolyzer

François Maréchal, Jan Van Herle, Ligang Wang, Hanfei Zhang

Thermochemical biomass-to-fuel conversion requires an increased hydrogen concentration in the syngas derived from gasification, which is currently achieved by water–gas-shift reaction and CO2 removal. State-of-the-art biomass-to-fuels convert less than half of the biomass carbon with the remaining emitted as CO2. Full conversion of biomass carbon can be achieved by integrating solid-oxide electrolyzer with different concepts: (1) steam electrolysis with the hydrogen produced injected into syngas, and (2) co-electrolysis of CO2 and H2O to convert the CO2 captured from the syngas. This paper investigates techno-economically steam- or co-electrolysis-based biomass-to-fuel processes for producing synthetic natural gas, methanol, dimethyl ether and jet fuel, considering system-level heat integration and optimal placement of steam cycles for heat recovery. The results show that state-of-the-art biomass-to-fuels achieve similar energy efficiencies of 48–51% (based on a lower heating value) for the four different fuels. The integrated concept with steam electrolysis achieves the highest energy efficiency: 68% for synthetic natural gas, 64% for methanol, 63% for dimethyl ether, and 56% for jet fuel. The integrated concept with co electrolysis can enhance the state-of-the-art energy efficiency to 66% for synthetic natural gas, 61% for methanol, and 54% for jet fuel. The biomass-to-dimethyl ether with co-electrolysis only reaches an efficiency of 49%, due to additional heat demand. The levelized cost of the product of the integrated concepts highly depends on the price and availability of renewable electricity. The concept with co-electrolysis allows for additional operation flexibility without renewable electricity, resulting in high annual production. Thus, with limited annual available hours of renewable electricity, biomass-to-fuel with co-electrolysis is more economically convenient than that with steam electrolysis. For a plant scale of 60 MWth biomass input with the renewable electricity available for 1800 h annually, the levelized cost of product of biomass-to-synthesis-natural-gas with co-electrolysis is 35 $/GJ, 20% lower than that with steam-electrolysis.

2020