That "biomass" and "other organically-based energy sources" wouldn't necessarily be limited to plant material — animal and human corpses contain plenty of energy, and they'd be plentiful in a war zone.
EATR will be powered by the Waste Heat Engine developed by Cyclone Power Technology of Pompano Beach, Fla., which uses an "external combustion chamber" burning up fuel to heat up water in a closed loop, generating electricity.
The advantages to the military are that the robot would be extremely flexible in fuel sources and could roam on its own for months, even years, without having to be refueled or serviced.
Link to story. Link to website. -Thanks, Brother Bill!