The heating value of a fuel may be determined either by a calculation
from a chemical analysis or by burning a sample in a calorimeter.
In the former method the calculation should be based on an ultimate
analysis, which reduces the fuel to its elementary constituents of
carbon, hydrogen, oxygen, nitrogen, sulphur, ash and moisture, to secure
a reasonable degree of accuracy. A proximate analysis, which determines
only the percentage of moisture, fixed carbon, volatile matter and ash,
without determining the ultimate composition of the volatile matter,
cannot be used for computing the heat of combustion with the same degree
of accuracy as an ultimate analysis, but estimates may be based on the
ultimate analysis that are fairly correct.
An ultimate analysis requires the services of a competent chemist, and
the methods to be employed in such a determination will be found in any
standard book on engineering chemistry. An ultimate analysis, while
resolving the fuel into its elementary constituents, does not reveal how
these may have been combined in the fuel. The manner of their
combination undoubtedly has a direct effect upon their calorific value,
as fuels having almost identical ultimate analyses show a difference in
heating value when tested in a calorimeter. Such a difference, however,
is slight, and very close approximations may be computed from the
ultimate analysis.
Ultimate analyses are given on both a moist and a dry fuel basis.
Inasmuch as the latter is the basis generally accepted for the
comparison of data, it would appear that it is the best basis on which
to report such an analysis. When an analysis is given on a moist fuel
basis it may be readily converted to a dry basis by dividing the
percentages of the various constituents by one minus the percentage of
moisture, reporting the moisture content separately.
_Moist Fuel_ _Dry Fuel_
C 83.95 84.45
H 4.23 4.25
O 3.02 3.04
N 1.27 1.28
S .91 .91
Ash 6.03 6.07
------
100.00
Moisture .59 .59
------
100.00
Calculations from an Ultimate Analysis--The first formula for the
calculation of heating values from the composition of a fuel as
determined from an ultimate analysis is due to Dulong, and this formula,
slightly modified, is the most commonly used to-day. Other formulae have
been proposed, some of which are more accurate for certain specific
classes of fuel, but all have their basis in Dulong's formula, the
accepted modified form of which is: