Methanation of Carbon Dioxide Using Hydrogen Obtained Through Water Electrolysis

Carbon dioxide emissions are in large part responsible for the climate crisis we currently find ourselves in. As the world begins to shift towards more sustainable practices, significant attention is being given to novel processes and innovative technologies that either reduce or eliminate carbon dioxide emissions. As our project sponsors, FortisBC asked us to look into a potential pathway to help them reduce their environmental impacts without significantly decreasing their natural gas output. Most homes in British Columbia rely on natural gas for heating, and most likely will for years to come. Its energy density, easy storage, and efficient transportation are almost unparalleled, and, if responsibly sourced and used, it can play a key role in transitioning away from non-renewable energy sources.

The Sabatier reaction is a much-studied but rarely implemented process to turn carbon dioxide into methane. It takes in hydrogen and CO₂ and produces water and methane. The reaction achieves over 99% conversion at high pressures over a nickel catalyst.

CO₂ + 4H₂ → CH₄ + H₂O ΔH = -165 kJ/mol

Our task was to find out why nobody was applying it at large scale. It’s only really being used in the International Space Station as part of their life support system. Surely it can be scaled up? The answer, according Swiss cement maufacturers the answer is “maybe”! In the only plant of its kind, Swiss engineers temporarily installed a Sabatier reactor on a CO₂-rich effluent stream to reduce their carbon emissions. This worked very well in reducing the carbon emissions, however an economic analysis suggested an investment of over 30 billion Swiss francs would be required to do this for all the cement plants in Switzerland. Needless to say it hasn’t really caught on. However, we are not working with CO2 from a cement plant with no nearby gas infrastructure. Our case study is a biogas plant currently operated by our sponsors. It is located in Abbotsford, BC and has two large anaerobic digesters are already sending natural gas into the city grid, and producing a significant amount of carbon dioxide. The pre-existance of gas infrastructure, higher carbon dioxide volume, and wider availability of renewable energy significantly reduce the costs of the process.

Using data provided by FortisBC and a variety of lab-scale studies, the process was scaled up to take in about 1400kg of biogas per hour. The process begins when biogas obtained from anaerobic digesters enters the front end of the plant and undergoes a dehydration process involving a triethylene glycol (TEG) contactor. The moisture content of the incoming biogas is greater than the product specification requirements provided by FortisBC hence the stream is dried prior to the biogas upgrade stage, so the methane can be sent directly to the pipeline after separation from the carbon dioxide. Furthermore, the biogas also contains hydrogen sulfide in trace amounts which must be removed according to product specifications. The technology chosen for this process is physical adsorption using activated carbon (AC), which can be affected by the presence of water. It is therefore preferrable to dry the gas before sweetening (removing hydrogen sulfide). The dried biogas stream then passes through an AC absorber for desulfurization where H2S, water residue, and oxygen (if present) are removed. The sweetened biogas with methane (RNG) and carbon dioxide (CO₂) being the major constituents is passed through a pressure-swing-adsorption (PSA) unit for separation. The RNG is sent directly to the pipeline as it meets product specifications, and the CO₂ stream is sent to the methanation reactor, where it is converted to RNG via the Sabatier reaction. The required hydrogen is generated using a PEM electrolyzer which splits regular tap water. This can be regarded as ‘green’ hydrogen production as electricity in British Columbia is obtained almost entirely from hydro power. Finally the exit stream from the methanation reactor undergoes post-treatment where water is removed such that product specifications are met prior to injection into the gas distribution pipelines.

The process has a Capital Expenditure (CAPEX) of $36.7 million. This is calculated using Aspen Plus, Lang, location and scaling factors, and literature. The project is eligible for up to 12.8 million of Total Capital Expenditure from the CleanBC Fund. The Operating Expenditure (OPEX) of the plant is $6 million per year, which consists of operations, maintenance, replacements, utilities, insurance, and raw material cost. The total loss of the plant is estimated to be $77 million over 20 years of operation. However, three breakeven scenarios were identified. The project can still be feasible over 20 years if:

1. A biogas feed of more than 3,000 kg per hour is available. This is a 2.1 times increase in the plant’s current capacity.
2. The Renewable Natural Gas cost rises to $0.91 per kilogram. The current Renewable Natural Gas cost is $0.66 per kilogram.
3. Oxygen obtained from PEM electrolyzer is upgraded to medical-grade, which currently sells for a much higher price than the process grade. The design and cost estimation of such a process is not performed due to project scope limitations, but some napkin math done suggests it would would