Coal – it’s Classification,Storage and Uses

Coal – it’s Classification,Storage and Uses

Coal is a black combustible compact, & stratified inhomogeneous mass of organic material originated from metamorphosed plant which is usually found interspersed with smaller amount of inorganic matter in its sedimentary rocks.  Coal formation has occurred over long period of time, often under high pressure of overburden at elevated temperature and also with the actions of some bacteria.

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Coal Classification

            There major classification systems currently used are the are the American society of testing Materials (ASTM), the British National coal Board (NCB), and the international classification of hard coals developed by the Economic commission of Europe (ECE).

Classifications have also been based on petrographic parameters.  For example, there is a correlation between chemical rank and maximum mean reflectance of incident light by samples classification of coal including;

  1. English National coal Board, based upon percentage of proximate volaties and Gray-king coking properties.
  2. International system for hard coals and known coals.
  3. Matt system is based upon volatile matter, heating values, and ultimate (O-H) analysis.

The standard organization of Nigeria (son) is yet to come up with a classification system for the country.  It is envisaged that will come with improvements on our coal, extraction and utilization.

Enugu coal is classified as sub-bituminous coal type.

Sub-Bituminous Coal

            Sub-bituminous coals have the characteristics between mature liquirtes and the hard bituminous coals.  They possess high mixture and volatile matter content when are-dried and, on the pure coals basis is 10-20 percent and carbon content 73-79 percent.  On carbonization they are quite non-caking but yield some 45% of volabite matter there hydrogen content is, relatively high and may reach 6 percent whereas the orgnic matter of mature lignite has substantially less than 5 percent hydrogen.

Coal Preparation

            Coal preparation refers to treatment of raw coal to enhance its stability for particular end – uses.  The extent and method of treatment depends on the properties of the coal and its intended purpose.  It may require only simple crushing or it may need to go though a complex treatment process to reduce the inorganic mineral impurities.

A modern coal preparation facility.

Around 80 percent of all coal mined and most of the black coal destined for export is washed to provide a twofold advantages.

  1. It upgrade the quality of the coal and
  2. It improves the economics of transportation by removing most of the non constructible materials.

In a case coal receive no treatment at the colliery beyond, perhaps, precautionary inspection in case large objects such as pit props or iron bars are included.  The treatment of coal at this stage between the pit and the consumer is known as “preparation for the market “ and is a specific profession.

Coal Storage

The chief object of storing coal is to provide reserves for power plants and factories.  Storage of coal is important for many reasons to ensure smooth operation of poer plant and industries utilizing coal, to enable a build up in summer against winter heavy consumption in wintry countries and to ensure more uniform fuel for a longer period.  For example at the Nigerian cement works, Nkalagu a stockpile of coal for at least two weeks is necessary to ensure undisputed production.  This is necessary because of strikes, shortages of fuel since coal is hauled from Enugu mines by road.  Reactions of coal when in bulk or compacted.

The Oxidation of Exposed Coal

            Coal stock may, by self ignition, render its untimely consumption necessary unless it is to be lost by being burnt completely.  On exposure to the atmosphere two processes are mitigated; the first, a chemical change is the oxidation process accompanied by the liberation of heat, the second, of a physical nature, is disintegration or degradation in size.

Oxidation adversely affects the useful properties of the coal, the caking power is impaired.

The Heating of stored coal

The internal heating or heat generated by exposed coal may be so slight, or may be dissipated so rapidly, as to be imperceptible but made suitable conditions the coal may be noticeably warm and then so hot that it thermally decomposer or actually ignites.  Enugu coal is so prone to self heating this way that it has been known to ignite in the vehicle conveying it.

Thermal Decomposition

a.         Destructive Distillation of Coal

When coal is thermally distilled by heating in the absence of air, it is converted into a variety of solid liquid and gaseous products.  The nature and amount of each products depends upon the temperature used in the pyolysis and the variety of coal used.  In ordinary practice, coke-oven temperatures are maintained above 900% but may range anywhere from 5000 – 10000c.  the principal product by weight of cokeable coal is coke.

b.        Devolatilization

            Devolatilization here refers to the destructive distillation of coal.  Volatiles are the primary gaseous decomposition products, some of which will be liquids even solids at ambient temperature and pressure coal devolatilization involves a number of distinct chemical and physical phenomena.  Here a qualitative model of coal volatilization and the part of addressed.

In the study of the thermal decomposition of coal, a fundamental understanding of the physical and chemical processes that take place is important in formatlating a mathematical model for the whole process of devolatisation.  The chemical processes inclual essentially the sequence of chemical reaction which involve the intermediate and final products.

c.         Carbonization

            Carbonization of coal is carried out in an oven or by-product co-product coking.  Believe coking is an old and slow process for producing coke.  Although it offers a low manufacturing capital cost, the disadvantage is that the co-chemical are wasted.  The feature of the process is that the necessary heat is produced by combustion with the oven itself.  In by-product coking, the heat required for combustion is in fuels adjacent to the chamber holding the coal.  The heat flows through the wall of the chamber.  No air is allowed to enter the by-product oven.  The heat flows towards the center of the coal and produces small pieces of coke.

Gasification of Coal

            In the late 1880’s ludwing Mond in England developed a process for the production of ammonia from bituminous coal.  Coal was gasiped with air and steam to give a mixture of gases, including ammonia derived from the nitrogen of the coal.  The ammonia was recovered as ammonium sulphate by subsequent treatment of the gas mixture with sulphuric acid.  The residual gas from this treatment of the was on low calorific value, but was suitable for industrial heating purposes.

Efforts to develop the gasification industry in Nigeria has been intensified but chamelled towards liquefied petroleum gas (LPG) and liquefied natural gas(LNG).  These developments have militated against the coal gas industry.  However it is hoped that more utilization of our coal the associated gas will logically given rise to its development.

Coal Quality and Rating

Facilities are available at a number of coal terminals to blend coals from different mines or produces.  State of the out handing equipment and sophisticated operating procedure ensure even blending of products before being lsoaded into ship.  Independent and highly expected superintending laboratories test and certify both weight and quality of all export consignment at loading ports.  They usually perform a range of standard test, but additional tests to suit the requirements of individual customers can be carried out.

Uses of Coal

There was a time when coal directly served as variety of important uses in our society; we burned it to heat our homes, cooking, mechanical power, transportation and heat for manufacturing processes.  In 1937 coal accounted for 74% of worldwide energy consumption.  But oil had replaced coal.

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 This article was extracted from a Project Research Work Topic “UTILIZATION STUDIES OF ENUGU COAL”

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