Natural gas, which consists primarily of methane, accounts for nearly one quarter of global energy production. Although the shale gas boom significantly increased the supply of natural gas, natural gas cannot be transported to processing plants using existing infrastructure for petroleum. Consequently, remote sources of natural gas are in effect “stranded.” Methods to use this “stranded” natural gas productively would be highly beneficial and would reduce unproductive flaring.
The conversion of natural gas to a liquid product like methanol offers economic advantages, as it would enable more facile and economical transportation using either existing or inexpensive infrastructure, unlike liquefied natural gas which requires specialized equipment, and it can avoid the controversial building of natural gas pipelines to isolated sites. The development of new environmentally friendly and scalable technologies is crucial to avoid unproductive greenhouse gas contributions from natural gas flaring.
Methane is a potent and deleterious greenhouse gas, with a global warming potential thirty-five times greater than that of CO2. The unproductive flaring of natural gas is estimated to contributed 300 million tons of CO2 to the atmosphere annually and has been estimated to represent the loss of approximately $2 billion/year. Selective partial oxidation, where methane is converted directly to a liquid product(s), is a potential solution.
In industry, methane is typically used in the syngas process, which converts steam and methane to a mixture of carbon monoxide and hydrogen, known as synthesis gas, at ~900°C. The synthesis gas can then be reacted over a catalyst in a second step, also at high pressures and temperatures, to produce methanol. Methanol can then be used directly as a fuel or as a precursor for high-value chemicals. The syngas route is energy-intensive as it requires high temperatures and pressures.
A multidisciplinary research effort funded by the US Department of Energy’s (DOE) Energy Efficiency and Renewable Energy Advanced Manufacturing Office, involving scientists from the University of Virginia, the California Institute of Technology, Princeton University, Oak Ridge National Laboratory, and the National Renewable Energy Laboratory, is developing gas-to-liquid technology to convert the components of natural gas to liquid products (e.g., methane to methanol) at lower pressures and temperatures than those utilized in current industrial processes.
Initial studies demonstrated that methane could be efficiently oxidized to a methanol derivative using inexpensive salts in a process termed oxyesterification (OxE) with minimal (<2 percent) production of CO2. With the OxE process, ~40 percent yield was reported with >97 percent selectivity for the desired product. Further investigation into the process indicated that the partial oxidation is efficient because the product is protected from unwanted over-oxidation. This cutting-edge process provides a potential route to use natural gas productively.
Current research efforts by the group are focused on the extension of the OxE process to overcome limitations that inhibit scale up. For example, researchers are analyzing the economic viability of the OxE process and are comparing it to existing systems, and initial findings show that the OxE process offers several potential advantages. In addition to operating at lower temperatures and pressures than existing technologies, the OxE process has demonstrated tolerance to sulfur impurities common in natural gas, which will reduce the cost of purification prior to the reaction. Further, facilities for the OxE process are projected to be less costly than current technologies, enabling natural gas conversion at the wellhead rather than requiring pipeline construction to bring the natural gas to large chemical plants. The ability to use stranded natural gas provides an additional economic advantage as it is less expensive than pipeline natural gas and can have a negative cost due to the industrial costs associated with flaring.
As the transition away from carbon-based fuels will be gradual, the development of more efficient ways to utilize existing infrastructure is crucial. Further, minimizing greenhouse gas emissions is another a key step toward decarbonization. Processes like OxE seek to contribute to the transition to renewable energy sources by enabling productive use of stranded natural gas.
Nichole Liebov is a researcher at the University of Virginia and holds a PhD in chemistry.