Wood-Chip Heating Systems - Biomass Energy Resource Center

WOOD CHIP HEATING SYSTEMS
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content of 40%, is a good all-round fi gure to use for
the energy content of biomass fuel available in the
Northeast. For specifi c applications, the analyst can
select different GHV and MC fi gures. For example, a
more conservative approach (considering the variability
of fuel actually available) might be to use a GHV-DS of
8,200 Btus/lb. and a moisture content of 45%, giving a
GHV-AF of 4,510 Btus/lb. or 9.02 MMBtu/ton.
• NET BTU CONTENT AVAILABLE FOR HEAT
The amount of energy in a green fuel sample is
reduced because only a fraction of the sample is wood
(the remainder being water). The amount of useful heat
made available from combustion is further reduced by
other factors.
A certain amount of heat in the wood is required to
vaporize the water in the sample. There is also “latent”
energy that is not available for useful purposes, unless
it is released by condensing the water vapor in the
fl ue gases, which is not common practice in wood
combustion.
When the GHV is reduced by subtracting the heat of
vaporization and the latent energy of water vapor in the
fl ue gases, the resulting heating value is called the net
heating value (NHV) 4 or lower heating value (LHV). 5
II. Wood Combustion Effi ciency
Effi ciency can be defi ned by the amount of useful
heat output from combustion, divided by the heat input
of the fuel. For effi ciency calculations, the input fuel’s
energy content can be characterized either by its asfi
red gross heating value (GHV-AF) or by its net heating
value (NHV). For obvious reasons, there is a need to be
consistent in the way in which effi ciency is calculated.
In the United States, it is the standard to use the
gross heating value (GHV-AF) for calculating steady
state effi ciency. 6 This convention has also been adopted
by the Northeast Regional Biomass Program in its 1993
testing of biomass boilers in the Northeast. 7
Green wood combusts with relatively low effi ciency
because it contains a large amount of moisture. GHVbased
effi ciency calculations look at the total heating
potential of the wood, including the energy that is
“wasted” in vaporizing water and in not condensing
water vapor in the fl ue gases.
When effi ciency calculations are based on NHV,
which removes fuel moisture from the equation,
effi ciencies increase signifi cantly compared to
effi ciencies calculated on a GHV basis. Some European
manufacturers (or manufacturers with product lines
based on European technology) report their effi ciencies
based on NHV. Prospective buyers must be sure that
they know what heating value basis is being used when
they evaluate the effi ciency of different combustion
systems.
1 Peter J. Ince, “How to Estimate Recoverable Heat Energy in
Wood or Bark Fuels” (Washington, D.C.: Forest Products
Laboratory, USDA Forest Service), p. 3; Wood-fi red
Boiler Systems for Space Heating, Publication EM 7180-
2 (Washington, D.C.: Biomass Energy Program, USDA
Forest Service, 1982), vol. 1, p. 3-1.
2 Wood-fi red Boiler Systems, p. 3-4.
3 Wood-fi red Boiler Systems, p. 3-2.
4 Wood-fi red Boiler Systems, p. 3-2.
5 Georgia Institute of Technology, Technical Applications
Laboratory, Industrial Wood Energy Handbook (New
York: Van Nostrand Reinhold, 1984), p.9.
6 Ibid., p. 10.
7 Small and Medium-Sized Wood Energy Boiler Effi ciencies,
prepared by Commercial Testing and Engineering
Company for the Northeast Regional Biomass Program,
CONEG Policy Research Center, Washington, D.C.,
December 1993; ASME Power Test Code, PTC 4.1,
(Atlanta: American Society of Mechanical Engineers),
section 5.