Why is natural gas as bad as (or worse than) coal?
Global Warming Potential is an index of the impact of various gases on planetary warming based on an impact per unit mass (Pound/Kilogram, etc) over a given time period following release. All other greenhouse gases are compared to carbon dioxide, which is defined as having a GWP of 1. The time period is arbitrary, but is usually assessed over 100 years, though is also reported for a 20 year period. The reason for the two values is that some gases have a much greater longevity in the atmosphere than others: carbon dioxide, for example, has a longevity before breakdown measured in the hundreds to thousands of years, while nitrous oxide exists for about a hundred. Methane meanwhile, has a longevity of about a decade and water vapor lasts a matter of days. The latter is particularly important for short-lived gases. Nitrous oxide, for example, has a 100-year GWP of 298 and a 20-year GWP of 268. Methane has a 100-year HWP of 34, and a 20-year GWP of 86. Meanwhile, chlorofluorocarbons and hydrofluorocarbons are much more potent, having 100-year and 20-year GWPs in the thousands of times greater than CO2.
What this means is that other gases, notably methane, contribute more (on a pound-for-pound basis) to global warming than does carbon dioxide. Fortunately, the other gases exist in much lower concentrations in the atmosphere (measured in parts per billion rather than parts per million) than carbon dioxide – which is why we tend to focus on carbon dioxide as the most important gas. But we must remember that others can be critical
Combustion Emissions are the greenhouse gases released into the atmosphere from the combustion of fossil fuel. This results in a focus on carbon dioxide since that is the most abundant product of combustion. Thus, when we burn fossil fuels, we find that coal produces about 216 tons of carbon dioxide per million BTUs of energy generated. Meanwhile, for diesel and gasoline, this value is about 160 tons, and for natural gas it’s about 117 tons (about 54% that of coal; 72% that of oil). On this basis, then, natural gas appears to be a huge improvement over coal as a source of energy in power plants. But this isn’t the whole story….
Full Life Cycle Assessment includes the full the cradle (extraction) to grave assessment of emissions thus comprising emissions during extraction, plus emissions resulting from processing and transmission/transportation of the fuel. Thus, it comprises all the emissions that might be occurring upstream from leakage (fugitive emissions). Up until the last five years or so, it was thought that upstream emissions were very small compared to combustion emissions and thus were essentially ignored. This is no longer the case.
Natural gas is about 90% methane. From this, and realization of the higher GWP of methane, we can see that not much leakage of natural gas is necessary before the combustion advantage of natural gas versus coal or oil is negated. In fact, compared to coal, and using the 20-year GWP basis for comparison, only 2.8% of the natural gas has to leak to negate the combustion advantage
Recent studies have suggested that the leakage of natural gas during extraction and transmission ranges from 1.7 – 6.8% (mean 3.8%) for conventional natural gas, and from 3.6% – 7.9% (mean 5.8%) for shale-fracked natural gas. This means that shale-fracked natural gas is probably never better than coal, while conventional gas may not be better – depending on the leakage.