A slightly more direct approach would be to wire up a blulectric engine to a magma crucible, then feed the lava into a (geo)thermal generator. It wouldn't require you to get biomass involved, just an igneous extruder making free cobble - it's a more direct conversion. As a bonus, you can centrifuge lava into electrum, copper and tin, or just store it for massive power storage.
Edit: although someone else already made that suggestion before. Darn.