Charcoal
From Unofficial Stationeers Wiki
Plants, decayed food, or seeds gives 1 reagents mix when going through the Recycler.
After that you take your reagents mix, and put it through the Centrifuge, Arc Furnace, or Furnace, which then gives you 1 Biomass.
After that, you put it into your Furnace and cook the Biomass at over 580K (or put it in the Arc Furnace, which consumes energy), which then gives you 1 Charcoal - the end product!
1 Charcoal gives you 120 kJ when used as fuel in the Solid Fuel Generator.
Charcoal can also be used instead of Coal in the Steel smelting recipe.