Hi Richard,
wikipedia has an article about wood gasification, that should clear things up:
http://en.wikipedia.org/wiki/Wood_gas_generator
There is a german word "Patsch"?
It's hard to translate a colloquial term but I'll try:
Blech = sheet metal
patschen = to affix something in the rough way (just slap it on there and let's get going)
so, it's just a slang word for a panel beater, used mainly in the south of germany
I hope that clears a few things up, feel free to ask more...
Jens