Natural gas vs coal: It’s all in the leak rate

When it comes to generating electricity, coal is a dirty fuel. It’s dirty to mine, dirty to burn and dirty to dispose of the ash. But until recently coal was the cheapest fuel for electric power, so it has been widely used across the U.S. Last year about 32 percent of our nation’s CO2 emissions came from coal-fired power plants.

Now, two factors are pushing for a switch away from coal: President Obama’s commitment to cut carbon dioxide emissions to reduce global climate change and the low price of natural gas, which is now competitive with coal in electric power plants. There is widespread interest in natural gas across the electric utility industry, because various studies indicate that burning natural gas rather than coal can reduce CO2 emissions by about 40 percent for the equivalent amount of power generation. Utilities, such as Duke Energy, are proposing new natural gas-powered power plants, with the goal of reducing their CO2 emissions.

Environmental advocates face a dilemma with respect to natural gas. It is cleaner, but about half of our natural gas is now derived from hydraulic fracture (fracking) methodology, which many environmentalists oppose. And, natural gas is a fossil fuel that releases CO2 to the atmosphere when it is burned.

From the mine to the power plant, there is little loss of coal to the environment. Unfortunately, that is not the case with natural gas, where there are leaks in the oil field, leaks from the pipeline distribution system and leaks from storage.

Leakage of natural gas, which is predominately methane, has another drawback. Methane is a powerful greenhouse gas that exerts about 86 times the global warming potential in the atmosphere compared with CO2 during the first 20 years after emission. Leakage from oil and gas fields makes a significant contribution to the annual emissions of methane to the atmosphere. The concentration of methane in Earth’s atmosphere is increasing every year, although it is not at all certain whether this is due to greater leakage of natural gas or increases from other sources, such as wetlands.

In any case, we need to include the emissions of methane from leaks in the natural gas distribution system in any comparison with coal. Consider the following: In burning coal about 25 grams of carbon are emitted to the atmosphere as carbon dioxide per MJ of energy produced. With natural gas, only about 15 grams of carbon are released. At first glance, gas looks great.

The leak rate of natural gas averages about 2 percent to 4 percent of production. Over a 20-year-period, a leakage rate of 1.5 percent would give natural gas nearly twice the impact on global climate change versus coal. I pick a 20-year period for the calculation because that is taken by many atmospheric scientists to be the window-of-opportunity we have to prevent harmful levels of climate change. By that account, switching to natural gas is a really bad idea.

Indeed, any leakage rate above about 1 percent of gross production negates the advantages of natural gas with respect to mitigating climate change over the next 20 years. No surprise that the natural gas industry reports that the average leak rate in the U.S. is only about 0.5 percent of gas production, but individual studies report larger values, and the leak rate varies widely among U.S. cities.

As the arguments for coal versus natural gas play out in the coming months, the choice will depend upon the leak rate for natural gas. If you want to hedge your bets, encourage your power company to invest in wind, solar, tidal or geothermal power. These are now cost-competitive with coal and offer a no-regrets future.

This story was originally published August 18, 2016 at 2:59 PM with the headline "Natural gas vs coal: It’s all in the leak rate."