I just scored this working propane-powered electric-start 4500W generator unit on craigslist, including a new starter battery and a 1/2-full tank of propane for $60! Apparently there is an issue with the generator unit that makes the breaker trip- seems like an internal short. It might be fixable, but the guy didn't want to have to deal with it, so he sold it to me cheap. This is a huge score for me, because I was going to remove the generator part anyway, at least to extend the driveshaft so I could also connect the gearbox and shredder. 

So if I can get generator part working, that's a bonus. But the reason I'm interested in this engine specifically is that it's already set up to run on gaseous fuels, which makes it much easier to run it on charcoal, syngas, or biogas without having to modify the carburetor or air intake.  It also has a 1- 1/2" NPT threaded pipe fitting already fitted onto the exhaust, which makes it easy to just screw on my own fittings and capture the waste heat from the exhaust stream. It also gives me the option of recirculating the exhaust gases into the biomass reactor, which allows for precision control of the air -fuel mixture within the reactor. 

Here's a photo of the engine I've been working with so far- notice the janky pipes I've stuck on there- none of them are very secure. This is a major upgrade for $60! Stay tuned!