Calorific Values of Woods and other Biofuels
Table of contents:
The table below gives the calorific values of various woods and other biofuels, burned in ideal conditions, so enough oxygen for full combustion and no kiln inefficiencies.
The heat available from a fully dried fuel is the gross calorific value. But in practice there will always be some water present, which consumes 2440 kJ/kg to get to boiling point and evaporate off. Subtracting the energy needed to eliminate the water results in the nett calorific value. The upper value in the table is of dry fuel, and the lower value is with a higher level of moisture.
The calorific value of oven dried woods (i.e. as dry as is reasonably possible - 0% in a lab, but in practice 5% at best) varies very little between species, at 18500 - 19000 kJ/kg. Softwoods are about 2% higher than hardwoods. This is primarily due to a higher lignin content (which releases 26000 - 27000 kJ/kg) compared to cellulose (17200 - 17500 kJ/kg) and hemicellulose (16000 kJ/kg). Secondary to this, conifers are also helped by the higher content of resin, wax and oil. Also some species have a higher hydrogen content, which releases more heat, and other have a higher ash content, which reduces the heat release per kg. Herbaceous plants are about 9% lower, and overall biofuels are in the range of 16500 - 19000 kJ/kg.
If we measure the moisture content of the wood, then we can calculate it's calorific value:
Given
M, the moisture content (%) of the wood
NCVO, the calorific value of the wood when fully oven dried (which we can assume to be 19000 kJ/kg if we don't have specific data for that species)
then the calorific value at moisture level M (NCVO) is given by:
NCVM = (NCVO x (100 - M) - 2.44xM)/100
So, for example, if we have wood at 40% moisture content the calorific value is:
(19000 x (100 - 40) - 2.44 x 40) / 100 = (19000 x 60 - 97.6) / 100 = 11399012.4 / 100 = 113990 kJ/kg - about 40% lower than fully dried wood.
When considering the moisture content of your wood, you have to remember that wood will absorb (and expel) moisture to work towards that of the surrounding air. So there's little point in oven drying wood and then stacking it outdoors in a humid environment.
| Material | Lower (kJ/kg) | Upper (kJ/kg) |
| Alfalfa | 17030 | 18500 |
| Bamboo | 19000 | 19800 |
| Birch wood | 18700 | 20100 |
| Cedar bark | 19800 | 21100 |
| Cherry wood | 17900 | 19100 |
| Coconut trunk wood | 16600 | 17800 |
| Corn stalks/stover | 16800 | 18500 |
| Cotton gin trash | 15200 | 16000 |
| Cotton stalks | 17000 | 18300 |
| Cypress wood | 21500 | 23000 |
| Douglas fir bark | 20800 | 22100 |
| Douglas fir wood | 19700 | 21000 |
| Elm wood | 19000 | 20500 |
| Eucalyptus bark | 17300 | 18500 |
| Eucalyptus wood | 18300 | 19600 |
| Flax | 17800 | 18800 |
| Grass | 17000 | 18100 |
| Hardwood bark | 18300 | 19600 |
| Hardwood | 18600 | 20700 |
| Hay | 16200 | 18300 |
| Hemp | 16500 | 17600 |
| Jute stick | 17800 | 19400 |
| Larch wood | 18700 | 20100 |
| Lucerne | 17700 | 19000 |
| Maple wood | 18700 | 20000 |
| Miscantus/Napier grass | 17800 | 19100 |
| Oak bark | 18300 | 19300 |
| Oak wood | 17400 | 18800 |
| Pine bark | 19700 | 21000 |
| Pine wood | 19500 | 20800 |
| Poplar wood | 19400 | 20800 |
| Rice husk | 14200 | 15400 |
| Rice straw | 13500 | 14800 |
| Softwood | 17500 | 21100 |
| Spruce bark | 19100 | 20300 |
| Spruce wood | 18500 | 19800 |
| Sugar cane bagasse | 17700 | 19400 |
| Switchgrass | 16800 | 19100 |
| Teak wood | 18900 | 20200 |
| Wheat straw | 15000 | 18900 |
| Willow wood | 17300 | 18600 |