Higher and Lower Heating Values for Wood Fuel
By: Rick Curkeet, PE – Chief Engineer

Higher Heating Value (HHV) – This is the total energy produced by complete

combustion of fuel. It is measured in a “Bomb Calorimeter” (ASTM E711) by burning a dry sample in pure oxygen in a container immersed in a known quantity of water. The temperat ure increase of the water once the container has completely cooled is precisely measured to determine the heat produced.

For nearly all wood species the HHV is between 8500 and 9000 Btu/lb. The value of 8600 currently specified for Oak in ASTM E2618 and EPA M28 WHH is

therefore reasonable.

Lower Heating Value (LHV) – This is a measure of the heat energy available from combustion without recovery of lat ent heat from water vapor. In NA we

base LHV on a reduction from HHV based on the amount of water generated from combustion of hydrogen contain in t he fuel molecule. We do not deduct for latent heat of water that comes along with t he fuel as “moisture content”. In the EU procedures, latent heat of water from moisture content is also deducted when

LHV is calculated.

LHV = HHV – 10.3 (%H x 9)

Latent Heat of water is the energy requi red to convert water from a liquid to gaseous state. It is approximately 1030 Btu/pound of water. When water vapor condenses, this same amount of heat is recovered.

EXAMPLE

One Pound of dry wood – HHV = 8600

Water from combustion – most wood cont ains about 6.5% hydrogen by weight. 1 pound of hydrogen in the fuel produces 9 pounds of water.

Therefore burning on pound of dry wood produces 0.065 x 9 = 0.585 lb H 2 O

Latent heat of 0.585 lb/H 2 O = 0.585 x 1030 = 602.5 Btu.

LHV = 8600 -602.5 = 7997 Btu/lb(dry)

At 20% MC(dry), 0.20 lbs of H 2 O is added for each pound of dry fuel. This requires another 0.20 x 1030 = 206 Btu to covert from liquid to gas form.

Thus LHV eu = 7997-206 = 7791 Btu/lb(dry)Water from MC should not be deducted when calculating LHV.

Suppose we burn 1 lb of dry wood. We w ould liberate 8600 Btu of energy. If we then cool the products of combustion to t heir initial temperature, we will recover the 8600 Btu’s produced and the most of th e water produced will condense. But if we only cool the combustion products to a temperature t hat is just high enough to prevent condensation of t he water (about 160 F) we wil l lose both the latent heat (602 Btu) and the sensible heat it take s to maintain the combustion products at 160 F – about 250 Btus. Thus, the mo st we can recover in a non-condensing process is 8600-602-250 = 7748 Bt u. It should be noted t hat the sensible heat loss depends on the amount of excess air as well which must be heated to the exhaust temperature and r epresents additional heat loss. So the theoretical maximum efficiencies are never actually achieved in practice.

Since free water in the form of moisture content is also involved there is also a

loss due to the latent heat of this water. At 20% MC it’s equivalent to burning a pound dry wood and placing 0.20 lbs of water in the combustion ch amber. It will

take another 206 Btu’s to convert this water to vapor and 8 more to maintain it at 160 F. So, the most heat we can re cover from a non-condensing system burning

20% MC wood is about 7534 Btu/lb dry wood (1.2 lb wet wood).

Thus, the theoretical maximum Highe r Heating Value efficiency of a noncondensing wood burning appliance is (7534/8600) x 100 = 87.6%

Based on LHV as defined in NA the ma ximum theoretical efficiency for a noncondensing system is (7534/7997) x 100 = 94.2%

The ratio of Higher Heating Value to Low er Heating Value is 8600/7997 = 1.075. Therefore, applianc e heating efficiencies based on Higher Heating Value can be converted to Lower Heating Value efficiencies as follows:



## h LHV = 

## h HHV x 1.075