GCI TECH NOTES ©
by
David Gossman
There are a number of factors to understand in "high heating value" vs "net heating value" (also sometimes referred to as “lower heating value”). US industries mostly look at lab results which are almost always "high heating value". In laboratory data no adjustment has been made for moisture except that on a weight basis water does not contribute to the heating value. So a higher moisture fuel will have a lower "high heating value" simply because it has less fuel. The real issue is what happens to the water during the laboratory test vs actual combustion. In the bomb calorimeter used for the test the water is recondensed after it is vaporized so any heat that went into the vaporization comes back out and is "counted". In most real combustion processes the water vapor remains in the vapor state so the vaporization of the water uses some of the heat content of the fuel - thus the phrase "net heating value".
It is also important to understand that the water can come from two different sources. One is actual contained water - coal, waste derived fuels, and some other fuels can have considerable contained water - sometimes as much as 20-30%.
(Green wood is even higher.) 10% moisture hits the heating value with about a 105 BTUs/lb penalty - as an example. The other source of moisture is from the actual chemical conversion of the hydrogen in the fuel to water. In the bomb
calorimeter this moisture condenses and again produces a small amount of heat that is counted in the "high heating value". This can be much larger than contained moisture.
Ash is not a factor in any of this. Some of the components in ash can contribute to heating value if they are oxidized during combustion - metals would be an example - and in that case might not be counted in the bomb calorimeter test
since they might not be heated high enough in the calorimeter to combust. Most ash has no impact but does lower the "high heating value" by virtue of not being a contributor to the fuel value.
The formula for converting a “high heating value” to “net heating value” is found below:
NHV = HHV – 10.55(W + 9H)
Where: