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An often repeated claim is that transitioning from coal to natural gas reduces air pollution... but what pollutants? and by how much? how is the potential reduction calculated?

While natural gas (left) is roughly the same anywhere in the world, coal (right) is a very complicated compound (with chemical composition varying by type of coal, and location of deposit):

              

Then, for each fuel source, there are different combustion technologies, and different pollution controls applied to the exhaust.

My question is, for natural gas, and each of the four main types of coal (lignite, anthracite, bituminous, sub-bituminous):

  • What types of pollutants are released in the combustion process?
  • How much does the combustion process affect level of pollution output?
  • Before pollution controls are applied, how much of each type of pollution is released per unit of energy output, on average?
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    One of the issues, particularly with coal, is that the composition of coal is not uniform across all deposits around the world, or within seams of a deposit. Each deposit (or seam) has is own distribution of impurities, such as sulfur & ash. The types of coal are also varied, from brown coal (lignite) to anthracite. Consequently, each deposit will have its own pollution profile. The other thing, a power plant in the US may have one supplier of coal. A plant in China may have a number of sources from Indonesia, Australia & elsewhere. Coal chemistry is highly varied.
    – Fred
    Mar 7, 2017 at 12:51
  • The same goes for natural gas. We have our own gas bubble here in The Netherlands, but if we were to replace it with e.g. Russian gas we would have to adjust a lot of industrial/household appliances because of its different composition.
    – user2451
    Mar 7, 2017 at 13:06
  • @Fred I had forgotten that point. A breakdown by type of coal, with average pollutant ranges, would also be very helpful.
    – LShaver ♦
    Mar 7, 2017 at 16:19
  • @JanDoggen are you sure? I know this is true at the source, but my understanding is that, since it's a gas, processing to achieve product uniformity is relatively cheap and easy.
    – LShaver ♦
    Mar 7, 2017 at 16:23
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    @LShaver You are correct, I was too quick. That's an easier way out.
    – user2451
    Mar 7, 2017 at 18:52

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I found this interesting research paper that answers some of your questions, at least for US power plants. For people who don't like to read research papers, there's an easier-to-read summary in this Science Daily article.

The research paper discusses carbon dioxide (CO2), nitrogen oxides (NOx) and sulfur dioxide (SO2) emissions from both coal and natural gas power plants by comparing data from continuous emission monitoring systems (CEMS). The researchers found that natural gas plants that use the "combined cycle" technology release far less CO2, NOx and SO2 than coal plants.

 Average CO2 emissions per kilowatt hour:

   Coal:              915.0 ±  0.8g
   Natural gas:       549.4 ±  1.1g
   NG combined cycle: 436.0 ±  1.4g

Sadly the paper does not mention the exact amounts of NOx and SO2, but it does say that

the increased use of natural gas has led to emission reductions of NOx (40%) and SO2 (44%)

Also the paper provides a few graphs so you can get a rough estimate of the amounts of NOx and SO2.

Average emission intensities (in units of g/kWh) of CO2, NOx, and SO2 from U.S. power plants between 1995 and 2012. Error bars give the 1-σ uncertainty in the averages.

Average emission intensities (in units of g/kWh) of CO2, NOx, and SO2 from U.S. power plants between 1995 and 2012. Error bars give the 1-σ uncertainty in the averages.

Caveats:

  • The data is based on US power plant output emissions, so after filtering (which is not what you asked for)
  • Other pollution types such as heavy metals, particulate matter, VOCs and methane leaks were not included in the study.
  • Emissions from fuel extraction and transport were also not included.

Methane leaks

Since methane is a very powerful greenhouse gas, leakages from gas production system could potentially negate the positive outcome of natural gas described above. In 2009 the EPA estimated that the leakage rate is 2.4% of the gross U.S. natural gas production (source). Other researchers think the real amount is much higher, but also say that:

system-wide leakage is unlikely to be large enough to negate climate benefits of coal-to-NG substitution (source).

Surprisingly the same article also mentions that natural gas used as a fuel in cars is most likely not cleaner than diesel.

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    This is great... a few Q's: Do you know how prevalent combined cycle is vs standard natural gas combustion, because NOx emissions are on par with coal for the latter. Also, any idea what happened in 2010 to make SO2 drop off so much for all fuels? New tech, or new regulations?
    – LShaver ♦
    Mar 22, 2017 at 16:47
  • The first article I linked to mentions that in 2012 the contribution of NG combined cycle plants had increased to 34% of the total gross load. Not sure how much it is now, but the EPA probably has that information somewhere. Coal dropped from 85% in 1997 to 59% in 2012. Furthermore "The decreases in the NOx and SO2 emissions from [all] power plants are due in part to the implementation of emission controls, as enacted under various clean-air programs of the U.S. EPA."
    – THelper
    Mar 24, 2017 at 9:41

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