Pitching gas against coal against renewables

The greenhouse gas benefits of natural gas is greatest in electricity generation when it displaces coal and oil, but not renewables, while the benefits are negligible in vehicle fuel use.

Using natural gas instead of coal or oil in electricity generation could have a significant effect on net carbon emissions into the atmosphere. By contrast, the benefits of using natural gas instead of petroleum products to drive vehicles are negligible, according to research published in the International Journal of Global Warming.

Daniel Cohan and Shayak Sengupta of the Department of Civil and Environmental Engineering, at Rice University, Houston, Texas, USA, have investigated the greenhouse gas savings that might be made by using natural gas, methane, instead of other fossil fuels such as coal or oil. Their work considered the main applications – electricity generation, heating, vehicle fuels; specifically light-duty vehicles, transit buses, residential heating, electricity generation, and export for electricity generation overseas. The team has evaluated each use on a fuel cycle basis, from production and transport of each fuel through end use combustion, based on recent conditions in the USA.

The researchers point out that the myriad policies affecting natural gas deployment across different sectors has provided the impetus for assessing how different uses of natural gas may impact greenhouse gas emissions. They have now demonstrated that the greatest emission reductions can be achieved by replacing existing coal-fired power plants (78 grams of carbon dioxide equivalent per megajoule of natural gas consumed) or fuel oil furnaces (66 grams of carbon dioxide). There was no significant difference between emissions when they calculated the whole cycle for compressed natural gas used as a vehicle fuel. Moreover, policies intent on promoting compressed natural gas vehicles will, the research suggests, be very unlikely to have the great benefits to which advocates and lobbyists have alluded.

The team suggests that their study “demonstrates the critical role of deployment choice on the net climate impact of natural gas.” The findings could affect the prospects for alternative natural gas sourcing, such as hydraulic fracturing and horizontal drilling if the use to which that gas were to be put is only as vehicle fuel.

In addition, the researchers conclude that should natural gas usage focus entirely on electricity generation then the net reduction in carbon emissions when compared to oil and coal-fired power stations would quickly be offset if renewable power supplies, such as wind and solar, were introduced instead. In other words any displacement of renewable electricity by natural gas could quickly offset the benefits of replacing coal.

Cohan, D.S. and Sengupta, S. (2016) ‘Net greenhouse gas emissions savings from natural gas substitutions in vehicles, furnaces, and power plants’, Int. J. Global Warming, Vol. 9, No. 2, pp.254–273

Author: David Bradley

Award-winning, freelance science writer based in Cambridge, England.