A “well-to-wheel” life cycle assessment (LCA) by a team from synthetic fuels producer Greyrock (earlier post), and the National Renewable Energy Laboratory (NREL) has determined the potential reduction of greenhouse gases and criteria pollutant emissions from the use of synthetic fuels directly converted from flare gas. The results of this open-access study are published in the International Journal of Energy and Environmental Engineering.

A Greyrock Flare Gas-to-Fuels conversion process at an Ohio oil well was used as the base case for this LCA. The liquid fuel produced directly from methane and other wellhead gases (associated gas) that would otherwise be burned or flared gas primarily consists of premium synthetic diesel with a small amount of synthetic gasoline.

The Greyrock Ohio plant produces directly about 11,007 L/day of fuel, resulting in an average plant thermal efficiency of approximately 58% (when calculating efficiency using feedstock to the plant) or a conversion efficiency of approximately 44% when all gas inputs (including feedstock and gas for parasitic/burner loads) are used in the calculation. This fuel comprises 8,694 and 2,313 L/day of synthetic diesel and synthetic gasoline, respectively.

In this LCA scenario, the synthetic diesel and synthetic gasoline are blended at 20 and 10 vol% with petroleum diesel and gasoline, respectively.

Greyrock’s proprietary GreyCat catalyst, which is not a traditional Fischer-Tropsch catalyst, directly converts syngas into diesel fuel with high selectivity. The GreyCat catalyst eliminates the wax upgrading stage and therefore reduces complexity and cost associated with small-scale DFP systems.

While the Greyrock synthetic diesel fuel can be used neat, a blend of 20% Greyrock synthetic diesel and petroleum-derived diesel was found to significantly improve engine performance, increase fuel economy, and reduce emissions compared to the use of petroleum-derived diesel.

This blend was also found to reduce criteria air pollutant emissions by an average of 28%, 18%, 24% and 5.5% for hydrocarbons, particulate matter, carbon monoxide, and nitrogen oxides from 1996 to 2015 diesel vehicles.

There is a global potential to produce up to 71.1 billion liters (18.8 billion gallons US) of synthetic fuel per year from flare gas. The study further established that the beneficial use of globally available flare gas for the production and use of synthetic fuel could reduce worldwide emissions of carbon dioxide and methane by up to 356 and 5.96 million metric tons/year, respectively.

Criteria emissions could also be reduced by up to 23.3, 0.37, 42.4, and 61.3 million metric tons/year globally for carbon monoxide, particulates, nitrogen oxides, and hydrocarbons, respectively. These clean mobility solutions may be realized without any changes to existing automotive infrastructure.

Resources

• Tan, E.C.D., Schuetzle, D., Zhang, Y. et al. (2018) “Reduction of greenhouse gas and criteria pollutant emissions by direct conversion of associated flare gas to synthetic fuels at oil wellheads” Int J Energy Environ Eng doi: 10.1007/s40095-018-0273-9