PROPERTIES OF COAL USED IN POWER PLANTS BASIC INFORMATION AND TUTORIALS


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.

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