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.