Comparing Low- and High-Temperature Electrochemical Process Pathways for Converting H2o and CO2 into Synthetic Natural Ga
Authors: Braun Robert J. 
Published: February 2020

The conversion of water and carbon dioxide into useful products such as synthetic natural gas using renewably supplied electrical energy can offer several advantages for stabilizing the electric grid and increasing the security of energy supply chains. In addition to providing increased grid operational flexibility for dealing with intermittent renewable electrical energy supplies, infrastructure changes such as hydrogen pipelines could conceivably be avoided in situations where natural gas pipelines and CO2 sources coincide. The natural gas distribution infrastructure is well developed in many countries, enabling the fuel to be transported long distances and easily delivered throughout cities. Using the existing pipeline to transport renewably generated synthetic natural gas (SNG) can further enhance the value of the product via offering grid services such as load leveling. While the price of natural gas is near record lows in the United States, many other countries are working to develop SNG as an alternative fuel for transportation and residential/commercial heating markets, especially in Europe and for island nations. This study presents pathway studies of low- and high-temperature electrolysis integrated with an SNG plant design and evaluates their performance for producing SNG by reacting renewably generated hydrogen with carbon dioxide. The carbon dioxide feedstock is assumed to be captured and scrubbed from either an existing coal fired power plant at the city-gate or from sequestered CO2, where the SNG plant is co-located. In the first pathway, water is split using either low-temperature alkaline or proton exchange membrane technology. In a second path, the co-electrolysis of steam and CO2 is done in-situ using high temperature (600°C) solid oxide cell technology. Historically, methanation has been a common practice for eliminating carbon monoxide and carbon dioxide in various chemical processes such as ammonia production and natural gas purification; for these processes, only small amounts (1-3% molar basis) of carbon oxides need to be converted to methane. A “bulk” methanation process is unique due to the high concentration of carbon oxides and hydrogen. In addition, the carbon dioxide is the only carbon source, and the reaction characteristics of carbon dioxide are much different than carbon monoxide. In process paths involving low-temperature electrolyzers, a methanation plant is required. In this study, thermodynamic and kinetic considerations of the methanation reaction are explored to model and simulate a system of reactors for the conversion of hydrogen and carbon dioxide to SNG. Multiple reactor stages are used to increase temperature control of the reactor and drain water to promote the forward direction of the methanation reaction. Heat recuperation and recovery using organic Rankine cycle units for electricity generation utilizes the heat produced from the methanation reaction. Bulk recycle is used to increase the overall reactant conversion while allowing a satisfactorily high methane content SNG product. A hydrogen membrane separates hydrogen for recycle to increase the Wobbe index of the product SNG by increasing the methane content to nearly 93% by volume. In both pathways, the product SNG has a minimum Wobbe index of 47.5 MJ/m3 which is acceptable for natural gas pipeline transport and end-use appliances in existing infrastructures. The overall plant efficiencies of both pathways are compared as well as the expected price of the product SNG using the H2A tool.