Educational model of a reduced smouldering-combustion physics. Never light real peat or burn vegetation; real peat fires are dangerous and can burn for months.
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Lab & science
Tap or click the cut face (or the surface) to light an ember there; drag to orbit, wheel / pinch to zoom. P = pause, R = reset.
Graphs (same as the panel)
Setup
Dry bulk density of the peat (kg of dry solids per m³)
60 loose 120 typical 200 compact
Mineral fraction of the dry solids (ash, sand, clay)
0 % 5 % 20 % 40 %
Time warp (model hours per real second)
0.25 h/s 1 h/s 3 h/s
Presets (they reset the burn)
Drought summer Normal bog Wet spring Drained peatland Mineral-rich soil Windy dry day
Rain: add 20 % moisture and re-wet the surface
Camera
Cut face From above Side view
Graphics (High on every device; Low = smaller shadows, lower pixel ratio)
High Low
Live readout (model values)
Experiments
1. Find the moisture threshold: 8 quick burns at the current density, mineral content, wind and water table, 6 model hours each. 2. Drain the bog: compare a water table at 15 cm with one at 50 cm for the current moisture.
Find the moisture threshold Water table 15 cm vs 50 cm
Missions
Mission 5: how much heat is needed to evaporate 1 kg of water, in MJ (one decimal)?
Check
The science
What you see Peat is a porous heap of partly decayed plants. Without a flame it can still burn: oxygen creeps into the pores, oxidises the peat on the hot side and the heat released warms the next layer until it ignites in turn. This glowing, flameless front (smouldering) is slow, usually a few centimetres per hour, but it can go deep, survive rain on the surface and release large amounts of carbon. Water works against it in three ways: it must be evaporated first (2.26 MJ per kg, plus warming it), it fills the pores so less oxygen reaches the front, and it raises the soil’s thermal mass. The cut face shows temperature as glow, moisture as darkness, char and ash where the fuel is gone.
Equations
Heat balance C dT/dt = div(k grad T) + q_ox - q_gas - lambda dm_w/dt (per m3)
C = rho_d (c_s + w c_w), k = 0.07 + 0.35 theta_w + 0.2 f_min W/m/K
Oxidation d rho_o/dt = - k_r(T) Y rho_o , k_r = A exp(-E/(R T)) , E = 90 kJ/mol
Heat release q_ox = 0.95 DeltaH (-d rho_o/dt) , DeltaH = 15 MJ/kg ; q_gas = 19 kJ/K per kg burned x (T - Ta)
Oxygen (relative Y) dY/dt = div(D_eff grad Y) - nu rho_o0/(0.28 a) k_r (rho_o/rho_o0) Y , D_eff = D0 a^1.5
Moisture water above 100 C evaporates: dw = (T - 373 K) C / (rho_d lambda) , lambda = 2.26 MJ/kg
Energy-balance limit MC_crit = ( eta (1 - f_min) DeltaH - c_s (T_ign - T0) ) / ( lambda + c_w (100 - T0) ) (eta = share of heat reaching the unburnt peat)
Surface h = 8 + 4 u W/m2/K, oxygen transfer 0.004 + 0.004 u m/s Assumptions & honest simplifications
Two-dimensional cross-section with 1 cm cells; one lumped oxidation step (peat to ash and gas); no pyrolysis chemistry, no char layer, no smoke chemistry, no flaming, no ground-surface vegetation fire.
Moisture only evaporates (no vapour transport or re-condensation ahead of the front, no capillary rise or rain infiltration), so the real drying-front effects are not captured. The 6 % to 90 % of the peat that is water is set by the water table profile only.
The oxygen diffusivity D0 = 1e-4 m²/s is an effective value that stands for buoyancy- and wind-driven air movement in the pores; the gas-cooling (19 kJ/K per kg burned) and heat-retention (0.95) constants were tuned so peak temperatures are 500 to 650 °C. They are not measurements, and the Arrhenius constants are illustrative.
Consequently the threshold of roughly 70 to 100 % moisture in this model (it depends on density, mineral content and wind) is lower than the 100 to 160 % often quoted for field peat; do not use it to predict a real fire. Spread rates of a few cm per hour agree in order of magnitude with experiments. Independent numerical checks
Also in node: node check.js (also checks mass conservation, the heat bookkeeping and the monotone trends).
Sources Rein, G. (2016) Smouldering fires and natural fuels; Frandsen, W. H. (1987, 1997) ignition probability of organic soils; Rein et al. (2008) smouldering combustion of Indonesian peat; Carslaw & Jaeger, Conduction of Heat in Solids (erfc solution used in the check). Typical values; spot-check before any teaching use.