Steam boilers and mushrooms

JP_4

I have been asked to come up with a system to sterilize substrate on a local mushroom farm. These are plastic bags with about 2 to 3 pounds of sawdust, chips, etc. Right now they use a horse trough with 12” of water in it, and boil it from underneath with propane burners. The stuff needs to be steamed for about 12 hours, so only their mold will grow in the bags. The chamber is 5W x 8L x 4H. Their “consultant” tells me it is going to be around a 250,000 BTU unit. The steam is to be evenly jetted throughout the chamber then collected and exhausted through an outside wall. Anyone ever tried something like this? Any help would be greatly appreciated.