There is no simple, single number, useful for all situations, to calculate the amount of energy in a fuel.
When biomass (or any fuel) is combusted, the products are heat, carbon dioxide, ash and water vapor. Hot water vapor, formed as part of the combustion reaction, has quite a bit of energy (compared to cool liquid water). Some think this energy should be considered a part of the fuel, while others do not. Therefore, there are two approaches to calculating the amount of energy available in a fuel sample.
Higher Heating Value (HHV)
The Higher Heating Value (HHV) is the total amount of heat in a sample of fuel – including the energy in the water vapor that is created during the combustion process.
Lower Heating Value (LHV)
The Lower Heating Value (LHV) is the amount of heat in a sample of fuel minus the energy in the combustion water vapor. The Lower Heating Value is always less than the Higher Heating Value for a fuel.
|Fuel Type||Higher Heat Value (kJ/kg)||Lower Heat Value (kJ/kg)|
|Dairy Manure, Dry||20.5||19.3|
Which Value to Use?
Traditionally, combustion equipment was generally not able to utilize the heat in the water vapor of the exhaust gases. As a result, the Lower Heating Value was usually used. However, most commercial combustors (and many residential systems) are now able to “condense” the water vapor in the exhaust gases, thus extracting the energy from the water vapor. Therefore, it is more sensible nowadays to use the HHV of the fuel.
Stove and boiler manufacturers still often prefer to use the LHV when calculating the efficiency of their equipment, because the efficiency rating will be higher if you use the LHV value instead of the HHV. When you calculate efficiency that way, it is even possible to have stoves with a rated efficiency greater than 100%.
Whenever you are comparing combustion equipment, it is important to know whether the manufacturer’s efficiency is based on Higher Heating Value or on Lower Heating Value. Otherwise, you may not be making an accurate comparison between systems.
Converting Between the Two Values
The thermal efficiency of a combustion heating device is equal to the amount of useful heat delivered by the equipment divided by the total amount of heat in the fuel:
If the efficiency is calculated based on the lower heating value, it is possible to convert it to a “higher heating value basis” by using the following formula:
Another Important Complication
In addition to the water that is created during combustion, many biomass fuels have water that is physically “mixed in” with the fuel. For example, green, fresh-cut wood is often about 50% moisture by weight, and air dried wood rarely drops below 20% moisture. This water reduces the available heat in a fuel in two ways:
- First, the water takes up volume and mass that could have been taken up by fuel (a ton of 50% moist fuel actually consists of half a ton of dry matter and half a ton of water).
- Second, the water must be heated, vaporized, and driven off from the fuel before the fuel can combust. This requires a great deal of energy that reduces the available heat in the fuel – unless the combustion equipment is able to recondense the water vapor from the exhaust gases and reclaim that energy.
Often, moisture content has a bigger impact on the available energy in a biomass fuel than the type of fuel. Therefore, it is important to use proper harvest and storage techniques to minimize the water content in your fuel.
The following graph shows the Higher Heat Value and the Lower Heat Value for a wood fuel as the moisture content increases.
Fuel with higher moisture levels contains much less energy per kilogram. Also, keep in mind that drier fuel is generally much easier to combust. Commercial scale equipment is often able to burn fuels that are up to about 50% moisture, but residental combustion equipment does not operate well at all when moisture levels are high.
Anonymous, 2009. Bioenergy Feedstock Development Program Conversion Factors. Oak Ridge National Laboratory, Oak Ridge, TN.
Bossel, U. 2003. Well-to-Wheel Studies, Heating Values, and the Energy Conservation Principle. Proceedings of 2003 Fuel Cell Forum.
Carlin, N., Annamalai, K., Sweeten, J, and S. Mukhtar. 2007. Thermo-chemical conversion analysis on dairy manure-based biomass through direct combustion. International journal of green energy. 4(2):133-159. Taylor & Francis, Philadelphia, PA
For Additional Information
Other articles in this Combustion Series:
- Biomass Feedstocks for Combustion
- How Much Heat Does BioFuel Have?
- Introduction to Biomass Combustion
- Processing Biomass for Combustion
- Shell Corn as a Fuel for Greenhouse Heat
- Using Combustion Heat for Energy
- Wood Heat for Greenhouses