Hearthstone Quality Home Heating Products, Inc.
Heritage Model 8090 Pellet Stove
Page 49 of 53
MOISTURE CONTENT
Fuel moisture content has a dramatic effect on efficiency: wood at 50% moisture has a heating value of 4,000
Btu/lb., at 20% the heating value is 6,200 Btu/lb., and oven dry wood delivers up to 8,600 Btu/lb. One of the
advantages of pellet fuel is its high Btu content (about 8,000 Btu/lb.) and consistently low moisture content,
usually between 6-10%, calculated on the wet basis.
Because there are two methods for determining wood moisture content (M.C.), the wet basis and the dry basis,
the concept can be confusing. The pellet fuel industry universally uses the wet basis when describing residential
pellet feedstock and fuel. The dry basis is used primarily in labs and technical situations. The following M.C.
calculations demonstrate the difference in the two methods.
In the dry basis, the wood is weighed wet, then dried to an oven dried condition. The oven-dried weight is
subtracted from the wet wood weight to determine the weight of the lost water. Moisture content is then
calculated by dividing the weight of the water by the weight of the oven dry wood. In the wet basis, the weight of
the water is divided by the weight of the wet wood.
Example:
A quantity of wood weighs 10 pounds. It is dried to oven-dry condition, and then it weighs 8 pounds.
What is its wet basis M.C.?
Weight of the wet wood (10 lbs.) - weight of the oven dried wood (8 lbs.) = weight of the water (2 lbs.)
Weight of water
(2)
--------------------------------------- = .20 M.C. (Wet Basis)
Weight of wet wood
(10)
The dry basis used in labs follows the same procedure but divides the weight of the water by the weight of the
dried wood.
Weight of wet wood minus weight of dried wood = weight of water
10 - 8 = 2 (pounds of water).
Weight of water
(2)
-------------------------------------- = .25 M.C. (Dry Basis)
Weight of dried wood
(8)
The dry basis moisture content is thus 25%, while the wet basis moisture content of this same quantity of wood
is 20%. Customers are more likely to understand and relate to wet basis percentages since dry basis figures
can exceed 100%. Pellet industry standards use the wet basis.
The dried materials are conveyed to the
conditioning chamber where steam may be
added to lubricate the materials and to help
soften the natural lignens that act as a
bonding agent to hold the pellets together.
The materials (feedstock) go next to the
pelleting chamber where they are extruded,
or pressed, through thousands of 1/4 to
5/16" diameter holes in a steel die 1.5 to 3.5
inches thick. This extrusion process heats
the newly formed pellets to temperatures
approaching 250° F. The hot, still soft pellets
are conveyed to a cooler to achieve room
temperatures and hardening. Dust and loose
fines are shaken off and recycled as the
pellets proceed to be bagged.