Other important properties of coal
include swelling, caking, and coking behavior; ash fusibility;
reactivity; and calorific value.
Calorific value measures the energy
available in a unit mass of coal sample. It is measured by ASTM
Standard Test Method D 201 5M, Gross Calorific Value of Solid Fuel by
the Adiabatic Bomb Calorimeter, or by ASTM Standard Test Method D
3286, Gross Calorific Value of Solid Fuel by the Isothermal-Jacket
Bomb Calorimeter.
In the absence of a directly measured
value, the gross calorific value, Q, of a coal (in Btu/lb) can be
estimated using the Dulong formula (Elliott and Yohe, 1981):
Q = 14,544C + 62,028[H - (O/8)]+ 4,050S
where C, H, O, and S are the mass
fractions of carbon, hydrogen, oxygen, and sulfur, respectively,
obtained from the ultimate analysis. Swelling, caking, and coking all
refer to the property of certain bituminous coals, when slowly heated
in an inert atmosphere to between 450 and 550 or 600 °F, to change
in size, composition, and, notably, strength.
Under such conditions, the coal sample
initially becomes soft and partially devolatilizes. With further
heating, the sample takes on a fluid characteristic. During this
fluid phase, further devolatilization causes the sample to swell.
Still further heating results in the formation of a stable, porous,
solid material with high strength.
There are several tests which have been
developed based on this property to measure the degree and
suitability of a coal for various processes. Some of the more popular
are the free swelling index (ASTM Test Method D 720), the Gray-King
assay test (initially developed and extensively used in Great
Britain), and the Gieseler plastometer test (ASTM Test Method D
2639), as well as a whole host of dilatometric methods (Habermehl et
al., 1981).
The results of these tests are often
correlated with the ability of a coal to form a coke suitable for
iron making. In the iron-making process, the high carbon content and
high surface area of the coke are utilized to reduce iron oxide to
elemental iron. The solid coke must also be strong enough to provide
the structural matrix upon which the reactions take place.
Bituminous coals which have good coking
properties are often referred to as metallurgical coals (Bituminous
coals which do not have this property are, alternatively, referred to
as steam coals because of their historically important use in raising
steam for motive power or electricity generation.)
Ash fusibility is another important
property of coals. This is a measure of the temperature range over
which the mineral matter in the coal begins to soften and eventually
to melt into a slag and to fuse together.
This phenomenon is important in
combustion processes; it determines if and at what point the
resultant ash becomes soft enough to stick to heat exchanger tubes
and other boiler surfaces or at what temperature it becomes molten so
that it flows (as slag), making removal as a liquid from the bottom
of a combustor possible.
Reactivity of a coal is a very
important property fundamental to all coal conversion processes (the
most important of which are combustion, gasification, and
liquefaction). In general, lower-rank coals are more reactive than
higher-rank coals.
This is due to several different
characteristics of coals, which vary with rank as well as with type.
The most important characteristics are the surface area of the coal,
its chemical composition, and the presence of certain minerals which
can act as catalysts in the conversion reactions. The larger surface
area present in lower-rank coals translates into a greater degree of
penetration of gaseous reactant molecules into the interior of a coal
particle.
Lower-rank coals have a less aromatic
structure than higher-rank coals, which, along with contributing to
larger surface area, also corresponds to a higher proportion of
lower-energy, more-reactive chemical bonds. Lower-rank coals also
tend to have higher proximate ash contents, and the associated
mineral matter is more distributed — down to the atomic level.
Any catalytically active mineral matter
is thus more highly dispersed, also. However, the reactivity of a
coal also varies depending upon what conversion is being attempted.
That is, the reactivity of a coal toward combustion (oxidation) is
not the same as its reactivity toward liquefaction, and the order of
reactivity established in a series of coals for one conversion
process will not necessarily be the same as for another process.
