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I sometimes hear people talking about how we should replace coal burning plants with natural gas ones, to alleviate the case of particulate matter pollution. What exactly is the difference between coal fuel and natural gas that makes the latter seem "cleaner"?

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  • At the same energy outcome, natural gas produces less carbon dioxide than coal. In a way, natural gas is half way between coal and hydrogen.
  • Coal produces smelly smoke, solid particles, sulfur dioxide and minor or trace heavy metal pollutants.
  • It is less known to common people, but power plants burning coal are more significant source of radioactive pollution than nuclear plants. This pollution is very diluted, but rather significant in absolute amount. Coal ash, used in past as a filler for some construction materials, has lead in some cases to significantly increased content of radium-226 in building walls. This radium is a product of long term decay of natural uranium. It further decays while producing radioactive gaseous radon-222, which is dangerous in long term inhalation because of lung cancer. As it stays in lungs as polonium-218 and its decay products.

See e.g. Uranium produced from coal ash

... the uranium concentration in the ash pile is about 150-180 parts per million, about 1/4th of the concentration often thought of as commercially viable for ISL[In Situ Leaching] mining. However, coal ash piles have some physical characteristics that might help overcome that disadvantage since they may be easier to drill and it might be easier to protect the local groundwater from contamination. ...

See Radon in building materials by Czech government agency for radiation protection.

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  • $\begingroup$ I think the second point answers my question about PM pollution. Also, is there a source for the third claim? Sounds interesting to me. $\endgroup$
    – Jono94
    Dec 10, 2022 at 11:25
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    $\begingroup$ @jonahuang Ahmed et al.. 2020; IEA 2020; Duke Surgery 2018 and of course Kurzgesagt: Why Don't We Shoot Nuclear Waste Into Space? ;) $\endgroup$ Dec 11, 2022 at 8:11
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Dealing specifically with particulates would depend on the coal and burning conditions. You could look up a specific coal you are interested in; However generally it will contain a few % of each sulfur, iron, silica, alkali earths, and smaller amounts of things like nickel. The burning conditions make a great difference whether the contaminant become fly ash or slag and clinker in the furnace. As a boy in Chicago in the 50s, our apartment was heated with coal as were the great majority. That meant someone had to dig about a cubic foot of ash and clinker from under the furnace firebox each month. Something missed with gas furnaces. I am tying to say there is hardly a comparison of relative particulates produced by gas versus coal.

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While the details depend strongly on the specific burning conditions, gas is almost always cleaner than coal and will produce far fewer particulates.

There are two main reasons for this. One is related to the level of non-carbon contaminants in the two fuels. Gas contains far fewer contaminants and most of them are gases many of which are not sources of pollution of any kind (gas is mostly methane but some gas has CO2, some has small levels of higher hydrocarbons like ethane). There is usually very little sulfur (if the original source has a lot it is usually stripped before distribution unless you count the very low levels of sulfur odorants added for safety reasons). Coal, even the best anthracite, can be thought of as mostly carbon but often contains several percent of a variety of contaminants. Some will be non-volatile minerals; some will be sulfur or nitrogen compounds and a variety of other non-carbon materials. These often burn to create particulate-forming compounds (sulfates often react to create particulates in the atmosphere).

But the second reason is the way even the pure carbon in the fuels burns. No burning reaction is "perfect" and side reactions leave stuff other than carbon dioxide (and water). Since burning the polymeric solid carbon in coal involves breaking up large chains or networks of carbon atoms, one of the common side reactions leaves incompletely burnt up lumps of carbon or partially reacted carbon. These form very small dust particles (in other words particulate pollution) and there is no perfect way to scrub them from the emissions. The combustion of methane is far cleaner as there are no long molecules of carbon that need to be broken up (though a very poorly controlled burning process could encourage some to form). So the expected level of imperfect reactions leading to particulate pollution will usually be far, far smaller when gas is the fuel.

In short, coal is a much dirtier fuel, even if all you are worried about is particulates. Add to this that, for a given amount of energy output, coal will produce a lot more CO2, and gas is always a better fuel for the environment.

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