You should understand that tgere was a reason for CARB.
After WW2 the population exploded in CA. That gave way to more and more cars. Those cars burned leaded fuel. There were days when the smog was so bad kids were not allowed to play outside. Your lungs literally hurt.
It became imperative to do something. The first was unleaded gas. That helped. The second was the anti-smog device. That helped a lot.
So, the answer is how clean can the coal be made to burn? I can tell you that if it is not better than unleaded, CA won't go along. It would not make economic sense.