Producer gas from dry wood holds about 4.3-6.2 MJ per normal cubic metre, with a midpoint near 5.2, which is roughly one seventh of natural gas at about 36 MJ per normal cubic metre. One kilogram of dry wood gives about 2.5-2.7 normal cubic metres, and a gas engine turns about a quarter of that gas energy into shaft work, so 1 kg of dry wood yields roughly 0.62-1.0 kWh of electricity.
Part of Biomass Gasification
The heating value of producer gas worked out from its composition, how much of the wood’s energy survives conversion, and what that means per cubic metre and per kilogram.
Add Up The Burnable Components
A normal cubic metre (Nm³) is a cubic metre of gas at 0 °C and 1 atm, where one mole occupies 22.414 L. Only carbon monoxide, hydrogen and methane contribute meaningful energy; nitrogen and carbon dioxide do not burn. Lower heating values per mole (standard data): CO 283.0 kJ, H₂ 241.8 kJ, CH₄ 802.3 kJ.
- CO: 283.0 ÷ 0.022414 = 12.63 MJ/Nm³.
- H₂: 241.8 ÷ 0.022414 = 10.79 MJ/Nm³.
- CH₄: 802.3 ÷ 0.022414 = 35.80 MJ/Nm³.
Apply the parent’s composition ranges (Syngas Composition), leaving out the 0.5-1% of other hydrocarbons:
| Case | CO | H₂ | CH₄ | Heating value, MJ/Nm³ |
|---|---|---|---|---|
| Low ends | 18% | 12% | 2% | 0.18 × 12.63 + 0.12 × 10.79 + 0.02 × 35.80 = 4.28 |
| Midpoints | 20% | 15% | 3% | 2.53 + 1.62 + 1.07 = 5.22 |
| High ends | 22% | 18% | 4% | 2.78 + 1.94 + 1.43 = 6.15 |
The ends cannot all hold at once, so 4.3-6.2 brackets the figure and 5.2 is the working value. Methane at 3-4% supplies about 1.1-1.4 MJ of it, a fifth to a quarter of the total, which is why the methane percentage is worth confirming for a real unit.
Natural gas is about 36 MJ/Nm³ (typical value, recalled). The ratio is 36 ÷ 5.22 = 6.9, about one seventh at the midpoint, and 8.4 to 5.9 across the range. The parent’s “roughly 1/6” matches only the high end.
How Much Gas A Kilogram Of Wood Gives
The corpus uses 18.5 MJ/kg for oven-dry hardwood (BTU Content Compared to Wood). The parent says 70-75% of the wood’s energy ends up in the gas (the cold-gas efficiency).
- Gas energy per kg: 18.5 × 0.70 = 12.95 MJ and 18.5 × 0.75 = 13.9 MJ.
- Gas volume at 5.22 MJ/Nm³: 12.95 ÷ 5.22 = 2.48 and 13.9 ÷ 5.22 = 2.66 Nm³ per kg, about 2.5-2.7.
- Across the composition range the yield is 2.1 (12.95 ÷ 6.15) to 3.2 (13.9 ÷ 4.28) Nm³ per kg.
This is on dry wood. Wet wood carries water that must be driven off first, at about 2.44 MJ per kg of water, so a kilogram of wood as burned yields less than the dry figure.
The Parent’s 70-75% Is Not The Wood-To-Electricity Figure
The parent says 1 kg of dry wood runs a generator for 1 kWh “roughly 70-75% of the energy theoretically available”. Those two statements cannot describe the same quantity. A kilogram of dry wood holds 18.5 MJ = 5.14 kWh, so 1 kWh of electricity is 1 ÷ 5.14 = 19.5% of it. The 70-75% is the share of the wood’s energy that ends up as gas, and the engine and generator then convert only part of it:
- Gas energy per kg: 12.95-13.9 MJ = 3.6-3.86 kWh.
- 1 kWh from 3.6-3.86 kWh of gas is an engine-plus-generator efficiency of 26-28%.
- The parent’s own consumption of 5-8 kg/h for a 5 kW set is 1.0-1.6 kg per kWh, or 0.62-1.0 kWh per kg, so “1 kWh per kg” is the optimistic end.
Per Cubic Metre Of Intake, The Gap Is Smaller
A rich fuel and air mixture is what the engine burns. The same arithmetic applied to the mixture gives about 2.46 MJ per m³ of gas-and-air mixture, against about 3.8 for petrol vapour and air, so the mixture holds about 65% as much energy per volume. That figure is worked out in Power Output Derating and sets a ceiling on engine power.
Common Mistakes
- Quoting heating value without the volume basis. Nm³ at 0 °C and a cubic metre at 25 °C differ by about 9%.
- Using the gas yield for wet wood. The yield above is per kg of dry wood.
- Treating cold-gas efficiency as overall efficiency. The engine takes most of what is left.
Safety
Energy you can burn is energy you can breathe
The same carbon monoxide that supplies about 2.5 MJ of the 5.2 MJ midpoint (20% × 12.63) is the poison, at 18-22% by volume. Never run a gasifier or its engine indoors, in a garage or in an enclosed yard. Because the gas burns, any enclosed volume holding a mixture of gas and air can explode as well as poison. See Gas Hazards (Methane, CO).
What’s Next
- Learn next: Power Output Derating — what this energy density means for engine power
- Combine with: Gasifier Types — the designs that produce it