Thermo-chemical conversion of biomass

Biomass is organic material from plants and animals (wood, crop residues, dung, municipal and food waste, energy crops) that stores solar energy chemically and can be used as fuel. It is renewable and carbon-neutral if grown sustainably.

Thermo-chemical conversion

These processes use heat (and sometimes limited oxygen) to convert biomass into fuel gas, liquid or solid. The main ones are combustion, gasification, pyrolysis and liquefaction.

Gasification (explained in detail)

Gasification partially burns solid biomass at 800–1000 °C with a limited supply of air, oxygen or steam to give a combustible producer gas (syngas): carbon monoxide, hydrogen, methane, with CO₂ and N₂.

Zones of a downdraft gasifier:

   Biomass in
      |
  [Drying]        100-200 C
  [Pyrolysis]     200-600 C
  [Oxidation]     1000+ C   C + O2 -> CO2
  [Reduction]     800 C     C + CO2 -> 2CO
                            C + H2O -> CO + H2
      |
  Producer gas + ash out

• Main reactions: C + O₂ → CO₂ (heat); C + CO₂ → 2CO; C + H₂O → CO + H₂.
• Gas heating value is about 4–6 MJ/Nm³ (with air).
• Uses: running gas engines and generators, boilers, kilns, and as raw material for methanol or Fischer–Tropsch fuels after cleaning.
• Benefits: more efficient than direct burning for electricity, can use various wastes. Problems: tar in the gas and gas cleaning cost.
• In Nepal, small gasifiers have been used for rural cooking and power in the Tarai.

Pyrolysis (brief)

Heating biomass to 400–600 °C in the absence of oxygen gives bio-oil (liquid), charcoal and gases. Fast pyrolysis gives mostly bio-oil.