Properties of coal:

Properties of coal:

Physical and chemical properties of coal are very important factor to know before a particular coal sample is used. Coal as a fuel comes in various types or ranks, such as, (1) lignite or brown coal; (2) bituminous coal or black coal; and (3) anthracite. Several techniques have been developed for studying the physical and chemical properties of coal. The most commonly employed systems of classification are those based on analyses that can be performed relatively easily in the laboratory, e.g., determining the percentage of volatile matter lost upon heating to about 950 °C (about 1,750 °F), the amount of heat released during combustion of the coal under standard conditions etc. Each type of coal discussed above, has a certain set of physical parameters which are mostly controlled by moisture, volatile content and its carbon content.

(1) Moisture: It is an important property of coal. Moisture held within the coal itself is known as inherent moisture. Moisture in coal may be of (i) Surface moisture, i.e., water held on the surface of coal particles; (ii) Hydroscopic moisture, i.e., water held by capillary action within the micro-fissures of the coal; (iii) Decomposition moisture, i.e., water held within the coal's decomposed organic compounds; (iv) Mineral moisture, i.e., water which comprises part of the crystal structure of hydrous silicates such as clays. Total moisture is analyzed by loss of mass between an untreated sample and the sample once analyzed.

(2) Volatile matter: Volatile matter in coal is the presence of various components of coal (except for moisture), which are liberated at high temperature of about 950 °C (about 1,750 °F), in the absence of air. This is usually a mixture of short and long chain hydrocarbons, aromatic hydrocarbons and some sulphur. The volatile matter of coal is determined under rigidly controlled standards.

(3) Ash: Ash content of coal is the non-combustible residue left after coal is burnt. It is the bulk mineral residue left after carbon, oxygen, sulphur and water is driven off by combustion.

(4) Fixed carbon: The fixed carbon in a coal sample is the carbon found in the material which is left after volatile matters are driven off. This slightly differs from the ultimate carbon content of the coal because some carbon is lost in hydrocarbons with the volatiles. Fixed carbon in coal sample is a very useful parameter, as it is used as an estimate of the amount of coke that will be yielded from a sample of coal. Fixed carbon is determined by removing the moisture and the mass of volatile matters, above, from the original mass of the coal sample.

(5) Other physical and chemical criteria: Presence of oxygen, hydrogen, sulphur etc. is evaluated in a coal sample. Density, particle size distribution, abrasion etc., are some of the physical criteria of coal is also evaluated.

(6) Besides above physical or chemical tests and analysis made in order to determine the handling and pollutant profile of a coal, the energy output of a coal is the important factor, which also determined by using a bomb calorimeter. Energy output of a coal sample is the specific energy output of a coal during complete combustion. This is required particularly for coals used in boiler for in steam-raising in a coal-fired power plant.

(7) Spontaneous combustion of coal: Almost all types of coal ignite spontaneously in suitable environmental conditions. The spontaneous ignition of coal stockpiles is a serious economic and safety problem. Preventive measures may be adopted while coal stacking includes (i) periodic compaction, (ii) the use of a low angle slope, (iii) protection of the coal stockpiled with an artificial barrier and (iv) covering it with ‘ash–water’ slurry made with fly ash from the same power station.