If I had to guess, it uses wood gasifier technology to run an ICE engine. I am speculating, but gasification is faster than fermentation.
Edit: sorry I was wrong. It's a steam engine.
From Wikipedia:
"The robot was powered by a steam engine built by the Cyclone Power Technologies company called the Cyclone Waste Heat Engine, which produced power through external combustion of biomass (i.e. combustion outside of the engine), an example of a Rankine cycle engine. The engine would power the vehicle's movement as well as being used to recharge the batteries that run the sensors, arms and ancillary devices."
Edit: sorry I was wrong. It's a steam engine.
From Wikipedia: "The robot was powered by a steam engine built by the Cyclone Power Technologies company called the Cyclone Waste Heat Engine, which produced power through external combustion of biomass (i.e. combustion outside of the engine), an example of a Rankine cycle engine. The engine would power the vehicle's movement as well as being used to recharge the batteries that run the sensors, arms and ancillary devices."