Carbon Dioxide-Based Coproducts for Commercially Viable Renewable Natural Gas Production

The California Energy Commission has identified renewable natural gas as an important alternative fuel that can contribute to California’s goals to diversify fuels and reduce emissions in the transportation sector. Removing carbon dioxide (CO2) allows renewable natural gas to be used as a transportation fuel, and this project studied converting the removed CO2 into a commercially viable coproduct to increase the competitiveness of renewable natural gas in the marketplace.
Researchers demonstrated a process for converting CO2 into dimethyl ether (another alternative fuel that can be used in a variety of sectors), along with a combined CO2 separation and conversion technology to convert CO2 into another commercially valuable coproduct, in a laboratory-scale flow reactor. This process can be applied directly to an existing renewable natural gas system. The results showed 17 percent carbon conversion efficiency from the CO2 into the product. The process was also studied using the Aspen Plus Process Simulation model, estimating 79.8 percent thermal efficiency (or thermal energy used) under optimum process conditions. Thermal efficiency is an important metric used when evaluating the commercial potential of the processes.
Researchers also demonstrated formation of potassium carbonate (a commercially valuable coproduct) from CO2 and found that when potassium hydroxide concentrations are 20 percent or higher, more than 80 percent CO2 absorption is possible. Potassium carbonate has commercial value if produced with adequate quality, and the additional revenue stream from producing high quality potassium carbonate can contribute to the economic viability of renewable natural gas production projects.
For dimethyl ether, economic analysis showed that reductions in renewable natural gas production costs depend strongly on the required hydrogen feed cost. According to the analysis, viable renewable natural gas production cost reduction requires hydrogen cost to be equal or below the U.S. Department of Energy’s long-term cost target of $1 per kilogram from renewable sources. Also, revenue from creating CO2 coproducts alone will not decrease the production cost of renewable natural gas.