Forest-Based Biomass Basic Computations - Mathematics and Statistics for the Environment - page 328

TABLE 13.3
Effect of Fuel Moisture on Wood Heat Content

Moisture Content (MC) Wet Basis (%)

0 15 20 25 30 35 40 45 50 55 60

Higher heating value as
fired (HHV-AF) (Btu/lb)

8500 7275 6800 6375 5950 5525 5100 4575 4250 3825 3400

Sources: Borman, G.L. and Ragland, K.W., Combustion Engineering , McGraw-Hill, New York, 1998; Kluender, R.A., The
Forester's Wood Energy Handbook , Publ. No. 80-A-12, American Pulpwood Association, Washington, D.C., 1980;
Maker, T.M., Wood-Chip Heating Systems: A Guide for Institutional and Commercial Biomass Installations ,
Biomass Energy Resource Center, Montpelier, VT, 2004.

wet weight basis differs from the total dry weight basis method of expressing moisture content which
is more commonly used for describing moisture content of finished wood products. The dry weight
basis is the ratio of the weight of water in wood to the oven-dry weight of the wood. The formulas
used require that moisture content be expressed on the wet weight basis (Ince, 1979).

13.3.2.1 Moisture Contents (MC) Wet and Dry Weight Basis Calculations
Moisture content (MC) on a wet or dry weight basis is calculated as follows:

(Wet weight −

ry weight

dryweight)

MC (dry basis) =× 100 (13.1)

D

(Wet weight −

et weight

dryweight)

MC (wet basis) =× 100 (13.2)

W

To convert MC (wet basis) to MC (dry basis):

100 ×

−

MC (wet basis)

MC (dry basis) = (13.3)

100 MC (we ttbasis)

To convert MC (dry basis) to MC (wet basis):

100 ×

+

MC (dry basis)

MC (wet basis) = (13.4)

100 MC (dr yybasis)

Some sources report the heat contents of fuels “as-delivered” rather than at 0% moisture for practi-
cal reasons. Because most wood fuels have bone-dry (oven-dry) heat contents in the range of 7600
to 9600 Btu/lb (15,200,000 to 19,200,000 Btu/ton or 18 to 22 GJ/Mg), lower values will always
mean that some moisture is included in the delivered fuel.

13.3.3 F orestry v olume u nit to b iomass W eight C onsiderations
Biomass is frequently estimated from forestry inventory merchantable-volume data, particularly for
purposes of comparing regional and national estimates of aboveground biomass and carbon levels.
Making such estimations can be done several ways, but it always involves the use of conversion fac-
tors or biomass expansion factors (or both combined). Figure 13.1 defines what is included in each