Clean coal

 Not quite as dirty  "Clean coal" is a Newspeak word and oxymoron used to encourage people to focus on coal as a source of energy, generally as a supplement to oil.

In theory, new production technologies burn the coal cleaner than before, and harness more energy per-pound than with prior technologies. CO2 is still released into the atmosphere, and thus "clean" does not mean carbon neutral, but sulfurous impurities can be dramatically cut using "clean" coal.

The term
The issue is more complex than both those against the use of coal and those who favor coal generally like to admit. "Clean coal" is strictly a marketing term used by the coal industry. In actuality, the idea behind clean coal is sequestering carbon emissions from coal plants, which will then be buried underground "for eternity." However, eternity is a long time and the technology is still in the development phase. In fact, burying the CO2 underground is a bigger problem than permanent disposal of nuclear waste: CO2 is a water-soluble gas rather than a chemically inert, insoluble solid and requires storage for an infinitely longer period of time as it does not chemically or physically decay.

New coal-burning power plants do not (yet) use this technology. On the other hand, new coal-burning plants are indeed "cleaner" than older plants, because of the addition of newer technologies which scrub sulfur and other emissions from the plants, and as coal currently (2018) generates 27.5% of U.S. electricity production - though with a rapidly falling trend. This has been used by coal advocates that new plants have to be built because they are — as a matter of fact — slightly less dirty. However, replacing the coal plants with renewable energies is cleaner than coal can ever get.

Cleaner, though, is not the same as clean. The idea, according to the there-is-no-clean-coal side of the debate, is to replace those old, dirtier coal plants with sources of energy which do not pollute. This would mean wind, solar, geothermal, hydroelectric, and nuclear. The coal industry, for its part, has muddled the debate too with billboards claiming coal is "now clean and green," which is just bullshit. It isn't, and they should just admit it. But then they would lose money, so you can guess where that goes.

The problem with Sulfur
As this article mentions sulfur quite a bit, it might be helpful to give a brief refresher, as to why sulfur is such a bad thing to have in coal. When sulfur burns, it produces either SO2 or SO3, depending among other things upon the availability of oxygen and the temperature at which it burns. While both these substances are harmful in and of themselves, the real problem is when they come into contact with water, which is a given when they are released into the atmosphere. SO3 reacts with H2O to form a substance known chemically as H2SO4, and commonly called sulfuric acid. You may know this as "battery acid" or "vitriol" or "that stuff people use to mutilate the faces of people they don't like". Sulfuric acid is the main driver of acid rain, which causes problems of soil acidification as well as a detrimental effect on monuments or statues exposed to it, especially those made out of limestone or marble. While there are certain methods to "wash" the exhaust of SO3 and SO2, those are expensive and have other downsides; the most common process produces huge amount of gypsum, that while high quality far exceeds demand. In short, coal with less sulfur in it in the first place is the vastly preferable option.

Anthracite
Complicating the issue is the fact that there is such a thing as clean coal &mdash; if by "clean" you mean "low sulfur" as opposed to "no carbon dioxide." It's called anthracite. Anthracite was once used to heat many homes, but since the dawn of electric and natural gas heating, homes having their own coal furnaces are a rarity, and anthracite is rarely used. Anthracite mainly comes from the in northeastern Pennsylvania where the coal industry (a few small family-owned underground mines) has been on life support for decades due to lack of demand. When the coal industry talks about "clean coal," you can be sure they aren't talking about clean-burning (and expensive) anthracite, but about the dirty sub-bituminous stuff from their non-union strip mines in Wyoming (the state is so named for the in Pennsylvania that once supplied anthracite). Anthracite is also clean in the sense of "less smoke", a fact that was exploited by blockade runners in the American Civil War wishing to avoid detection and later on several railroads advertised the fact that they used Anthracite, as it meant less soot and smoke for the passengers (ever seen an image of people on a steam train? They usually all wear black - There's a reason for that).

However, no matter how little sulfur or other chemicals anthracite possesses, it's pretty clear that burning it will produce substantial quantities of CO2 which will have to be dealt with somehow if it is to be called "clean". Anthracite is also rather difficult to light, and once lit, it's notorious for being almost impossible to put out; mine fires, like the one in have been a fact of life in the  since the start of the anthracite industry there.

Coke
Some coals can be turned into Koch coke, which is almost pure carbon with no impurities. Doing so requires the coal to be baked in an airless coking furnace, which is not only expensive to build but also consumes energy in its own right — and you still have to do something with the impurities liberated by the coking process.

Coke is just about the cleanest coal there is, but it's not a viable alternative for generating electric power and would not likely result in less total air pollution overall. And it certainly isn't "clean" as far as greenhouse gas production (CO2) is concerned.

IGCC
Integrated Gasification Combined Cycle, more commonly known as IGCC, is a highly complex method of reducing coal emissions by turning coal into synthesis gas (aka: syngas) and then removing impurities within the gas before it is used for fuel. Furthermore, the "combined cycle" allows for a secondary to recover residual heat and steam and use it for additional energy. Overall, IGCC reduces mercury and sulfur dioxide emissions, when compared to pulverized coal burning, by around 35%-45% and carbon dioxide emissions by around 25%-30%. It also increases the fuel efficiency of the coal burned to around 75% of peak efficiency (whereas traditional coal turbines get around 55% of peak efficiency at best).

However, IGCC, while cleaner, is still not totally clean. IGCC generation plants also cost around 20% more to build than pulverized coal plants, making them less attractive to utility companies.

Sequestering
Sequestering, also known by the woo term "carbon capture" (or Carbon Capture & Storage, abbreviated CCS), is another method of reducing coal emissions promoted by energy producing companies, even though the science behind it is, more or less, absolute bull.

Basically, sequestering is where coal-burning power plants pump all their carbon emissions underground into the coarse rock miles underground, thus attempting to "sequester" it into somewhere where it cannot escape. This method, however, is more about the utility companies burying their heads in the sand than actually burying carbon emissions, for the reasons explained below:


 * Once carbon dioxide is pumped underground in large quantities (in this case, billions of tons), it displaces natural groundwater pools, causing groundwater to seep elsewhere underground, which greatly increases the risk of sink-holes and water seepage on the surface above.
 * Carbon dioxide reacts with water to form Carbonic acid (H2O + CO2 reacts to H2CO3), which lowers the pH of the water and dissolves limestone. (This is also a natural process that is responsible for and limestone caves - however, natural CO2 concentrations tend to be rather low.)
 * Carbon dioxide is naturally buoyant, and will thus spread around its initial injection point, and seep back out of the earth unless it is injected through one or more impermeable boundaries.

Sequestering of carbon deep underground, through multiple impermeable boundaries, can increase earthquakes, just as with waste water storage from fracking.

There are also highly experimental ideas to sequester carbon dioxide in other forms, including by dissolving it in water, which produces huge amounts of acidic water which can then be stored underground, with risks similar to those mentioned above. Or there are proposals to process into limestone or other solid forms that can be used in construction or just dumped somewhere.

Extracting coal
Of course, nothing is said about how you get the coal. Mining costs and transportation usually make up a very sizable chunk of pollutants when following a life cycle assessment of a product and these cannot be eliminated from the mining process unless you powered your mine equipment on wind and solar power. And if you live in Colorado, for example, and see the mountain side strip mines, or have lived in the Appalachian Mountains where they cap (blow) the tops of mountains off - leaving you with a hill instead of a mountain - you wonder how it can be "low impact" on the environment.

Just for the record, those boys in West Virginia hard at work underground so you ungrateful kiddies can power up your computers don't like mountaintop removal either, as it requires far fewer miners than underground mining and is a threat to their job security.

A further question arises as to how coal is transported. Globally the vast majority of coal is transported either by ship (barge or ocean vessel) or train as these are the cheapest modes to move large amounts of non perishable goods. Sadly though - in part due to lack of regulations and the difficult enforceability of regulations on the open sea - freight vessels have among the "dirtiest" fuel there is: Unrefined high sulfur heavy oil. As for trains, the situation varies a lot from country to country, with Switzerland having almost exclusively water powered electrical trains and the US relying on diesel powered heavy trains for almost their entire freight network. So even if by some magic no CO2 were to come out of the exhaust pipe of the power plant, the mining and the transport would still cause major headaches from an environmental standpoint.