I believe it has been discussed that even the best quality coal is acidic and that causes both hopper corrosion and etching of the ceramic "glass".
This is just food for thought, would mixing a little powdered limestone with the coal before adding it to the hopper likely help or would it cause some unforeseen problem? Limestone acts as a buffer, reacting with acid, typically, if soaked in water a long time, the PH of the water would be mildly alkaline but "hard". It's sort of hard to overdue limestone (except for acid loving plants) since it is more a buffer, not constantly increasing alkalinity.
About 10 years ago my town started adding powdered limestone (not the caustic lime) to our water supply. Before that our hot water heaters lasted only 5-8 years and the copper plumbing in homes were constantly corroding and leaking after 15 years or so. Also, the copper piping would etch from the acid water and leave a green residue on showers and baths.
Just a thought - might limestone help, hurt or do nothing? I hate the hopper corrosion and "glass" etching.