a) Fixed Carbon. Fixed carbon represents the portion of coal that
must be burned in solid state after volatile matter is driven off. It is not
the total carbon in the coal since the volatile matter contains hydrocarbons.
The fixed carbon portion usually burns in the fuel bed on a stoker or as solid
particles in a pulverized coal furnace.
b) Volatile Matter. Volatile matter is the portion of the coal
which is driven off as a vapor when it is heated according to ASTM method
D 3175, Test Method for Volatile Matter in the Analysis Sample of Coal and
Coke. The main constituents of volatile matter in all ranks of coal are water
vapor, carbon monoxide, hydrogen, methane, and other hydrocarbons. The
volatile matter varies greatly for different ranks of coal and has a profound
effect on ignition rate when burning in a furnace. High volatile coals in a
range of 35 percent and higher result in quick ignition. This quick release
of burning hydrocarbons affects furnace design, location of overfire air,
ignition arches, and furnace heat release.
c) Ash. Ash is the noncombustible residue after complete
combustion of coal. The content (percentage) and characteristics of ash in
coal have substantial effects on furnace design.
1. Ash Content. The percentage by weight of ash in coal is
determined by ASTM method D 3174, Test Method for Ash in the Analysis Sample
of Coal and Coke from Coal. It is usually more meaningful to define ash
content as it relates to energy input rather than on the basis of a weight
percentage of the coal alone. The pounds of ash per Btu fired can be
calculated by dividing ash content (lb of ash per lb of fuel) by the heating
value of the coal (Btu/lb of fuel). Relating fuel ash content to the heat
input gives a realistic estimate of the amount of ash that must be handled as
bottom ash and by the pollution control equipment as fly ash. It also gives
an insight to the potential amount of ash that must be handled on heat
transfer surface. Note that the actual amount of refuse will be much greater
than the calculated amount of ash due to the presence of unburned carbon
2. Ash Characteristics. When ash is heated, it becomes soft
and sticky and may form deposits (clinkers) on boiler tubes. The temperature
at which the ash has a tendency to stick, is called the ash softening
temperature. The build-up of ash acts as an insulation which reduces the
efficiency of heat transfer. The initial deposits are usually easy to remove
by regular soot-blowing. If the deposits are permitted to build up in a zone
of high gas temperature, its surface (due to the insulating properties of the
ash) can reach the melting point and form a deposit that is tightly bonded and
difficult to remove. For this reason, it is important to determine the
softening temperature of the coal ash. This temperature is determined by ASTM
method D 1857, Standard Test Method for Fusibility of Coal and Coke Ash.
Coals with an ash softening temperature of 2700 degrees F (1482.2 degrees C)
or above are usually trouble free as to fouling of tube surfaces.