You cannot burn coal in air (nitrogen&oxygen) and not create NOx ,SOx and heavy metal vapors. No amount of post combustion filters or precipitation devices can remove those to zero/zero/zero levels. Especially the heavy metal in vapor form which is a product of combustion physics in an air fired process. You will never get the heavy metals down to part per billion or less levels. There is no safe level of biological exposure to heavy metals, science is clear on that.
So coal will never ever be clean it cannot physics limits it. You can make it cleaner but it will never be as clean as natural gas or even cleaner nuclear power. Nukes only release noble gasses to the air which are chemically and biologically inert.
You can clean coal before you burn it via gasification processes which do not use nitrogen atmospheres this eliminates NOx, with the lower temps of controlled gasification you also do not form heavy metal vapors they stay in the ash as solid oxides, while water soluble they are at least contained in the ash not up the stack inot our air and drinking waters. Then you can clean the syngas via a six stage gas clean up down to part per billion or less levels of SOx , and other VOCs such that all you are left with us CO,H2 and some CH4 all that get burnt in a gas turbine with SCR cats in the tail end to scrub out the NOx that still forms when you burn anything at high temps in air not pure oxygen.
You could go the oxy fired route and have a air separation plant next door feeding oxygen only to the turbines and gasification plant this would mean close cycle gas turbines using argon as the inert gas vs chemically active nitrogen in air. Co2 could also be used as the inert gas for closed cycle tirbines.
Syngas also works with solid oxide fuel cells eat CO or H2 as anode fuels with pure O2 on the cathode side you get CO2 and water as your only exhaust products. You have to have ppb or less sulfur levels or it will kill the fuel cells catalysts so only 6 stage clean syngas works there.
Coal burnt in air is best left in the dust bin of history it will always put out heavy metal vapors and NOx,soot and sulfur oxides no amount of post combustion filters can get it to zero or even to the same clean level of your average gas turbine. That’s just how physics works. Mitsubishi are the world leaders in coal gasification and they can get close to gas turbine clean but still not as clean as just burning methane without all the cheap that is in coal, the rock itself is loaded with impurities and contaminates there is no way around that fact.
Coal is the fuel that builds solar and wind turbines. How much pollution is being emitted, doing that, as compared to clean coal energy in the US? You can’t smelt iron with wind or solar, you nead coal to make coke, to melt steel.