Reference data for common fuels in US and metric units. This comparative table brings together practical data for kiln planning and can support rigorous engineering calculations or practical estimates for fuel supply and key dimensions for chimney sizing, firebox and gas burner designs.
| Calorific value (energy released) | Stoich air/ fuel ratio | Volume of air per MJ | Ignition temp | Adiabatic flame temp | Practical flame temp | Density at 15oC and 1 atmos | Boiling point | |
|---|---|---|---|---|---|---|---|---|
| Units | MJ/kg | m3/MJ | oC | oC | oC | kg/m3 | K | |
| Gases | ||||||||
| Hydrogen H2 | 120.2 | 2.38 | 0.02 | 500 | 2210 | 2040 | 0.090 | 20 |
| Methane, CH4 | 48.7 | 9.50 | 0.20 | 580 | 1963 | 1950 | 0.714 | 112 |
| Natural gas | 48.7 | 9.50 | 0.20 | 580 | 1963 | 1950 | 0.714 | 112 |
| Propane C4H10 | 46.3 | 23.80 | 0.51 | 455 | 1995 | 1960 | 1.970 | 231 |
| LPG | 46.6 | 23.80 | 0.51 | 450 | 1995 | 1980 | 1.970 | 231 |
| Butane | 45.3 | 31.00 | 0.68 | 405 | 1975 | 1970 | 2.590 | 273 |
| Biogas | 21.1 | 5.80 | 0.27 | 700 | 1850 | 1200 | 1.170 | n/a |
| Carbon monoxide | 12.6 | 2.38 | 0.19 | 609 | 2121 | 2120 | 1.250 | 81 |
| Air | n/a | n/a | n/a | n/a | n/a | n/a | 1.225 | n/a |
| Liquids and solids | ||||||||
| Kerosene (Paraffin) | 45.0 | 12.1 | 0.22 | 210 | 2093 | 1970 | 800 | 500 |
| Charcoal | 32.0 | 11.5 | 0.29 | 349 | 1980 | 1150 | 300 | n/a |
| Coal | 26.1 | 6.9 | 0.32 | 454 | 2172 | 1100 | 1050 | n/a |
| Wood, oak | 18.0 | 4.4 | 0.30 | 480 | 1980 | 1200 | 750 | n/a |
| Wood, pine | 19.0 | 4.4 | 0.28 | 430 | 1810 | 1100 | 550 | n/a |
| Calorific value (Energy released) | Stoich Air/fuel ratio | Volume of air per 1000 BTU | Ignition temp | Adiabatic flame temp | Practical flame temp | Density at 32oF and 1 bar | Boiling point | |
|---|---|---|---|---|---|---|---|---|
| Units | BTU/lb | ft3 air/lb | ft3 | oF | oF | oF | lb/ft3 | K |
| GASES | ||||||||
| Hydrogen | 51,644 | 2.38 | 0.046 | 932 | 3,998 | 3,704 | 0.0023 | 20 |
| Methane | 20,944 | 9.5 | 9.8 | 1,076 | 3,581 | 3,542 | 0.0447 | 112 |
| Natural gas | 20,944 | 9.5 | 9.8 | 1,076 | 3,581 | 3,542 | 0.0447 | 112 |
| Propane | 19,911 | 23.8 | 25.9 | 851 | 3,623 | 3,572 | 0.1255 | 231 |
| LPG | 20,040 | 23.8 | 25.7 | 842 | 3,623 | 3,596 | 1.1255 | 231 |
| Butane | 19,482 | 31.0 | 35.0 | 761 | 3,533 | 3,518 | 0.1548 | 273 |
| Biogas | 9,072 | 5.8 | 13.6 | 1,292 | 3,362 | 2,192 | 0.0749 | n/a |
| Carbon monoxide | 5,411 | 2.4 | 7.3 | 1,128 | 3,860 | 3,848 | 0.0715 | 81 |
| Air | n/a | n/a | n/a | n/a | n/a | n/a | 0.0807 | n/a |
| LIQUIDS AND SOLIDS | ||||||||
| Kerosene (Paraffin) | 19,332 | 12.1 | 11.6 | 410 | 3,803 | 3,518 | 50.00 | n/a |
| Charcoal | 13,745 | 11.5 | 15.3 | 660 | 3,600 | 2,100 | 19.00 | n/a |
| Coal | 11,207 | 6.9 | 20.6 | 849 | 3,950 | 2,400 | 65.00 | n/a |
| Wood, oak | 7,730 | 4.4 | 19.3 | 900 | 3,600 | 2,200 | 46.00 | n/a |
| Wood, pine | 8,159 | 4.4 | 17.9 | 800 | 3,300 | 2,000 | 34.00 | n/a |
Explanation of headings
Calorific value: The heat energy available per pound or kilogram of fuel. Technically known as the lower heating value because the data assumes that the latent heat in water vapour is not recovered.
Stoich air-fuel ratio: The Stoichiometric air-fuel ratio is the theoretical proportion of air needed for complete combustion; real burners require excess air to achieve a neutral or oxidising atmosphere.
Volume of air per BTU or MJ. This is the theoretical volume of air at the standard temperature and pressure that is needed to produce 1000BTU or 1 MJ of energy.
Ignition temperature: The temperature needed to light the fuel and sustain the flame.
Adiabatic flame temperature: This is the theoretical upper limit which would be achieved if the exact stoichiometric air-fuel ratio is achieved and no energy is consumed in heating the burner or firebox. This may be used in calculations and comparisons for pure gases.
Practical flame temperature. This is an approximation that can be helpful in the design of fireboxes and burners. Note that flame temperature for solid fuels is very variable and is dependent on preheating fuel and air within the firebox.
Gas density: Thermal calculations have to be based on a stated standard temperature and pressure. These tables use the international IUPAC standard of 0oC and 1 bar. Other sources may use 0oC and 1 atmosphere or 25oC and 1 atmosphere.
Boiling point for gases and liquid: This is given in Kelvin (absolute temperature) for convenience in calculations. The Kelvin scale starts at absolute zero, which is -273oC or -460oF, the temperature at which all molecular movement ceases. One kelvin is the same amount of temperature change as one degree Celsius, so the freezing point of water is 273K, or 0 °C, 32 °F.
Accuracy and reliability
All figures in the tables may be rounded make them more readable and to avoid giving a false impression of accuracy.
Pure gas figures are rigorously based on molecular data and provide a chemically clean baseline.
Natural gas varies slightly, but is mainly methane. This means that figures for methane are accurate enough for practical applications like gas volume, jet size, and burner sizing.
LPG is mainly propane in the UK and USA, so propane figures are sufficiently accurate for most purposes. In some countries where freezing is unlikely, butane may be the main component of LPG.
Biogas is extremely variable due to its origin. Methane, the combustible component, is typically slightly more than 50% of the total gas volume. The table is based on 60% methane, 40% CO2.
All solid fuels are very variable because of their natural origin. The figures in the table are purely a guide to compare and contrast with the gases.
Applications of the data
- How much fuel is needed to produce the heat you need?
- Use for planning solid fuel storage or to determine the capacity of gas supply lines and piping.
- How much air is needed to burn the fuel?
- What size air intakes and flue exit are required?
- How must the firebox or burner be designed to mix the air with the fuel?
- Flame characteristics
- How easily can the flame be lit and sustained?
- Theoretical maximum temperature vs practical flame temperature.
- Flame temperature is only a guide for planning. It changes significantly with the position within the flame and depends on design of burner or hearth as well as draft and the fuel-air mix.
- Density of gas fuels
- Density is required for all practical gas calculations. In gas burners, the jet size determines the volume of gas being delivered, but the heat output is proportional to mass.
- At room temperature, butane is about twice as dense as air, and propane is 1.5 times the weight of the same volume of air. That means that leaked butane or propane sinks through the air and can spread along the ground, collect in basements or drains and form an explosion risk.
Our separate article Solid fuel, Gas or Oil? explores some of the important differences in the physical nature of fuel and the implications for kiln design.
A new article will soon be published to cover the practical design of gas burners.
