As I noted on the Common Science® Twitter feed, NPR is in the middle of a week-long series on hydraulic fracturing (“fracking”). It’s good to see them catching up with us. Then in the midst of NPR’s usual high-quality science reporting the reporter said, “When producing electricity, natural gas is considered to be a cleaner burning fuel than coal and releases fewer greenhouse gases.” Then she moved on. This inspired me to look over and shout at the poor radio “tell us why, tell us why, it’s only two more sentences!”
This little story of my morning radio experience is a perfect example of my inspiration for writing Common Science. In order for “we the people” to make the best possible judgments about complex environmental and energy choices, we need to delve a bit deeper into the science than where NPR left off. So let me tell you what was missing from the story and how I would have managed the explanation.
As I outlined in Electricity Production 101, traditional power plants burn fuels to make steam. The steam is directed through a turbine causing it to spin. The turbine then spins a coil of wire within a magnetic field which generates the voltage which is supplied to the electric power grid.
So if you are going to compare the environmental impact of two fuel choices, natural gas versus coal in this case, you need to look at how much carbon dioxide (the greenhouse gas in the NPR story) and other pollutants you release as a function of how much heat you make by burning the fuel to make the steam. The amount of heat generated when burning a fuel is a function of the types of chemical bonds which are broken when you burn the fuel. For hydrocarbon fuels like natural gas, oil or coal this means breaking primarily carbon-carbon (C-C) and carbon-hydrogen (C-H) bonds.
The graphic at the top of the page shows the chemical structure for several simple hydrocarbons as reference for the discussion below. Methane (natural gas), the simplest of all hydrocarbons, only has carbon-hydrogen bonds. The next simplest hydrocarbon, ethane, has one C-C bond and 6 C-H bonds. As you move to larger and larger hydrocarbons the ratio of C-H bonds to C-C bonds continues to decrease. See the table below for more examples. Coal is a mixture of very large hydrocarbon molecules with an approximate composition as shown in the table.
|Compound||Formula||C-H bonds||C-C bonds||Ratio C-H/C-C|
* Approximate composition
I am reviewing these ratios because more heat is released from breaking a C-H bond than when breaking a C-C bond. Therefore, the higher the ratio of C-H to C-C bonds, the higher heating value for the fuel. Burning a pound of methane (natural gas) will give off 23,900 BTUs of heat. Coal, with its much lower ratio of C-H to C-C bonds, will only give off 8,000 BTUs per pound. Therefore, in order to make the same amount of steam, you would need to burn three pounds of coal for every one pound of natural gas. The higher heating value of natural gas compared to coal is the primary, but not the only reason, for it being a cleaner burning fuel compared to coal. There are two others that matter.
When you burn a hydrocarbon you convert all of the carbon atoms to carbon dioxide and all of the hydrogen atoms to water. The table below shows the carbon dioxide content of the emissions when burning the same hydrocarbons we discussed above.
|Compound||Formula||CO2 Formed||H2O Formed||CO2 Content|
When you burn methane it generates three gas molecules, one carbon dioxide and two waters. Therefore its gas emissions are 33% carbon dioxide (1 out of 3). As you move down the table from methane to coal, the percentage of carbon dioxide in the emissions grows to 74%. So not only do you get a lot less heat when you burn coal compared to methane, but the release of carbon dioxide to the atmosphere per BTU is approximately twice as high.
The effects of higher heating value and reduced carbon dioxide formation for natural gas compound with one another. Since you only need to burn 1/3 as much methane compare to coal to make the same amount of steam and burning natural gas only makes 1/2 as much carbon dioxide per BTU as coal, overall carbon dioxide emissions when using methane are 1/3 x 1/2 = 1/6th as much as coal.
The last disadvantage of coal is the easiest to explain. Since coal has a lot of impurities, such as sulfur and mercury, burning it also emits oxidation products of these problematic impurities to the atmosphere. Natural gas is generally free of impurities so, for the most part it only generates water and carbon dioxide.
So here is how I wish NPR would have addressed the coal versus natural gas comparison. “Environmentally, natural gas is an attractive fuel source since it generates three times as much heat per pound when burned compared to coal while releasing only 1/6th of the carbon dioxide per unit of electricity produced. Furthermore, since coal contains impurities like sulfur and mercury, burning it releases oxidation products of these impurities into the atmosphere resulting in acid rain and increased rates of respiratory disease.” This versions would only take up 10 more seconds of radio time and leave the listeners much better prepared to grapple with the trade-offs involved in determining whether fracking should be used to enhance natural gas recovery or not.
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