I suggest that you get more information on the actual coal that will be used at this plant. How will the coal be handled and stored before it gets to the bunker? How long will coal reside in the bunker? What about frozen coal, maximum lump size, blending of different coals, etc.? What about liner panels or coatings?
Plugging and rat holing in coal bunkers can provide operational nightmares and coal properties can vary significantly. I'd want to get as much practical information as possible rather than relying on generic guesses.
I had the same problem some years ago , my company needed to supply a 55 cube round glassfibre reinforced polyester silo to a waste water treatment plant company , for their water regeneration process.
We didn't have any experience with their special coal material , and they specified us to take a silo wall inclination of 70° for our silo.
The silo was supplied and worked well. I must say that a polyester glasfibre reinforced silo has a very smooth internal surface shape , and that we foresaw a compressed air injection nozzles system inside the conical part of the silo , just in case , to be able to create a fluid bed internally if confronted with bridge forming and all the other crap that occurs with unknown powder bulk products.