PNNL team develops new low-cost method to convert captured CO2 to methane

03 September 2021

Researchers at the Department of Energy’s Pacific Northwest National Laboratory have developed a new method to convert captured CO2 into methane, the primary component of natural gas. By using a water-lean post-combustion capture solvent, (N-(2-ethoxyethyl)-3-morpholinopropan-1-amine) (2-EEMPA), they achieved a greater than 90% conversion of captured CO2 to hydrocarbons—mostly methane—in the presence of a heterogenous Ru catalyst under relatively mild reaction conditions (170 °C and <15 bar H2 pressure).

Technoeconomic analyses (TEA) showed that the proposed integrated process can potentially improve the thermal efficiency by 5% and reduce the total capital investment and minimum synthetic natural gas (SNG) selling price by 32% and 12% respectively compared to conventional Sabatier process, highlighting the energetic and economic benefits of integrated capture and conversion. A paper on the work is published in ChemSusChem.

Methane derived from CO2 and renewable H2 sources is an attractive fuel, and it has great potential as a renewable hydrogen carrier as an environmentally responsible carbon capture and utilization approach.

—Heldebrant et al.

Earlier this year, PNNL researchers revealed that using EEMPA in power plants could slash the price of carbon capture to 19% lower than standard industry costs—the lowest documented price of carbon capture.

Different methods for converting CO2 into methane have long been known. However, most processes rely on high temperatures and are often too expensive for widespread commercial use.

In addition to geologic production, methane can be produced from renewable or recycled CO2 sources, and can be used as fuel itself or as an H2 energy carrier. Though it is a greenhouse gas and requires careful supply chain management, methane has many applications, ranging from household use to industrial processes, said co-author and PNNL chemist Jotheeswari Kothandaraman.

To explore the use of EEMPA in converting CO2 to methane, Kothandaraman and her fellow authors studied the reaction’s molecular underpinnings, then assessed the cost of running the process at scale in a 550-megawatt power plant.

Conventionally, plant operators can capture CO2 by using special solvents that douse flue gas before it’s emitted from plant chimneys. But these traditional solvents have relatively high water content, making methane conversion difficult.

Using EEMPA instead reduces the energy needed to fuel such a reaction. The savings stem partly from EEMPA’s ability to make CO2 dissolve more easily, which means less pressure is needed to run the conversion.

The authors’ assessment identified further cost savings, in that CO2 captured by EEMPA can be converted to methane on site. Traditionally, CO2 is stripped from water-rich solvents and sent off site to be converted or stored underground. Under the new method, captured CO2 can be mixed with renewable hydrogen and a catalyst in a simple chamber, then heated to half the pressure used in conventional methods to make methane.

The reaction is efficient, the authors said, and EEMPA captures more than 95 percent of CO2 emitted in flue gas. The new process gives off excess heat, too, providing steam for power generation.

Resources

• Heldebrant, D., Kothandaraman, J., Lopez, J..S., Jiang, Y., Walter, E..D., Burton, S..D. and Dagle, R..A. (2021), “Integrated Capture and Conversion of CO2 to Methane using a Water-lean, Post-Combustion CO2 Capture Solvent.” ChemSusChem. doi: 10.1002/cssc.202101590