A downdraft gasifier converts solid waste into combustible syngas by feeding it through four stacked zones with only a fraction of the oxygen needed for full combustion:
- Drying — free moisture evaporates as the feed descends.
- Pyrolysis — heat cracks the dried waste into char, tar vapors and light gas, without oxygen.
- Oxidation — the limited air injected here burns part of the char/gas, supplying the heat that drives the whole process.
- Reduction — hot CO₂ and H₂O react with glowing char to make CO and H₂, the two main fuel gases.
The key control is the equivalence ratio ER = (actual air fed) / (air needed for complete combustion). Gasifiers run lean, ER ≈ 0.2–0.35: enough air to sustain the reactions, not enough to just burn everything to CO₂.
Boudouard: C + CO2 ⇌ 2 CO (favored at high T)
Water–gas: C + H2O ⇌ CO + H2 (favored by moisture)
Shift: CO + H2O ⇌ CO2 + H2 (favored at low T)
Methanation: C + 2H2 ⇌ CH4 (favored at low T)
This simulator uses a simplified engineering-scale model (not a full chemical-equilibrium solver) that reproduces the real qualitative trends: raising ER pushes more carbon to CO₂ and raises temperature; raising temperature favors Boudouard, so CO climbs while CH₄ (tar/methane) cracks away; raising moisture feeds the water–gas and shift reactions, lifting H₂ and CO₂ but costing extra heat to evaporate — lowering overall heating value.
Heating value is summed from each combustible species' own energy content:
LHV_syngas = 10.8·x(H2) + 12.6·x(CO) + 35.8·x(CH4) [MJ/Nm³]
Cold gas efficiency = energy leaving as syngas ÷ energy entering as waste feed
Air-blown gasifiers dilute the product with atmospheric N₂, which is why real syngas LHV (≈4–6 MJ/Nm³) is far below natural gas (≈36 MJ/Nm³) — it still burns, just at lower flame temperature and larger volume.