Wow, I hope you Brother-In-Law is not too badly hurt.

Nitro Blast is not an "air spray to blow off the dust". It's a serious chemical cleaner very much like automobile brake cleaner. How did it explode? Who knows, only your B-I-L was there so, unless it comes from him, it's entirely speculation. And since we know nothing about him or what, exactly, he was doing anything we say would only be a guess. He was using a very powerful chemical aresol so nearly any source of ignition (smoking, static electricity, a spark from a short circut, a bad battery, etc) could cause an explosion. I agree with you in that the explosion was probably caused by the cleaner but what set it off, who knows. I seriously doubt that the fuel had anything at all to do with the original explosion although it could have splattered and caused further burns/damage.