Last reviewed 1 Oct 2026 · 7 min read
What is gasification?
Gasification is the thermochemical conversion of solid biomass (or coal) into a combustible gas (producer gas) by partial oxidation at high temperature (about 800–1100 °C) with a limited supply of air, oxygen or steam.
| Process | Air supplied | Product |
|---|---|---|
| Combustion | Excess air | Heat, CO₂, H₂O |
| Gasification | Limited (about 20–40 % of the stoichiometric need) | CO, H₂, CH₄ (combustible gas) |
| Pyrolysis | No air | Char, tar/bio-oil, gas |
The equivalence ratio (ER) is . Gasification works at ER ≈ 0.2–0.4; ER < 0.2 approaches pyrolysis and ER > 1 is combustion. Higher ER means a hotter reactor and more CO₂, but a lower heating value of gas.
Zones in a gasifier
FormulaFour thermochemical zones
- Drying zone (below ~150 °C) — moisture is evaporated.
- Pyrolysis (devolatilisation) zone (about 250–700 °C) — biomass decomposes into char, tar, volatile gases (CO, CO₂, CH₄, H₂), water vapour.
- Oxidation (combustion) zone (about 900–1200 °C) — part of the char and volatiles burns with the limited air, producing the heat for the other zones: (exothermic).
- Reduction zone (about 700–900 °C) — hot CO₂ and steam react with glowing char to form fuel gases (endothermic).
Main reactions in the reduction zone
| Reaction | Name | Heat |
|---|---|---|
| Boudouard | Endothermic | |
| Water-gas (steam–carbon) | Endothermic | |
| Water-gas shift | Mildly exothermic | |
| Methanation | Exothermic | |
| Partial oxidation | Exothermic |
Because the reduction reactions consume heat, the gas leaves cooler than the oxidation zone; gas quality depends on the bed temperature and residence time.