Characteristics and Applications of Fuel Types
Coal. Coals are generally ranked in one of four broad categories:
anthracite, bituminous, sub-bituminous, and lignite. The ASTM bases these
rankings on several properties which include heating value (Btu),
agglomerating (caking) characteristics, fixed carbon, and volatile matter.
The rankings are further divided on the basis of ash, moisture, and sulfur
content. It is essential to understand the effect of these properties when
selecting a coal for various types of fuel burning equipment and/or methods of
firing. For typical values, refer to Table 7.
by burning a weighed sample of the fuel in a controlled environment.
Expressed in Btu/lb of fuel, this property can be determined by either ASTM
method D 2015, Test Method for Gross Calorimeter, or D 3286, Test Method for
Gross Calorific Value of Solid Fuel by the Isothermal-Jacket Bomb Calorimeter.
As a measurement of the energy entering the boiler in the fuel, this property
is one of the main characteristics used to calculate boiler efficiency.
Agglomerating (Caking) Characteristics. The free swelling index
(F.S.I.) gives a measure of the extent of swelling of a coal and its tendency
to agglomerate when heated rapidly. This test is covered by ASTM method
D 720, Test Method for Free-Swelling Index of Coal, which rates the coal on a
scale of 1 to 9. Coals with a high free swelling index are referred to as
coking (caking) coals, whereas those with a low index are referred to as non-
coking (free-burning) coals.
a) Caking Type. When heated in a furnace, caking coals pass
through a plastic state during which the individual coal pieces will fuse
together into large masses of semi-coke. The semi-coke is impervious to
uniform flow of supply air. This characteristic, when applied to stokers
employing thick fuel beds in the burning area can cause problems with
maintaining uniform combustion and air flow through the coal mass.
b) Free Burning Coals. Free-burning coals with free swelling
index range of 1 to 3-1/2 do not fuse individual coal pieces together. The
pieces will burn separately. Any pieces which stick in a group when heated
will quickly break into fragments by vibration or movement of the fuel bed.
Items of Proximate Analysis of Coal. The next four properties
effecting the selection of coal have been combined by the ASTM into a test
called the proximate analysis. A proximate analysis ASTM method D 3172,
Proximate Analysis of Coal and Coke, is a determination of the percentage by
weights of moisture, volatile matter, and ash with the difference being
defined as fixed carbon.