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ENERGY ENGINEERING4th Term, B.Sc. Chemical EngineeringSession 2013

Delivered by: Mr. Rizwan AliDepartment of Chemical EngineeringUniversity of Engineering & Technology, LahoreMACRO- COMPONENTS OF COALPetrography of coal.Visual examination of coal in the seam or in large pieces shows that it is not homogeneous throughout its mass. With most coals differences in texture are apparent, some bands having a dull and others a bright appearance. These bands may be separated from one another by ' dirt' bands or may merge into one another without visible partings. These differences are most marked in the case of bituminous coals but can be traced in lignitous coals, in certain black lignites and, to a less extent, in carbonaceous coals.

ContinuedThe macroscopic constituents in bituminous coal which are identifiable by eye have been termed by vitrain claraindurainfusain

VitrainVitrain is the bright black brittle coal which normally occurs in very thin bands. It breaks with a conchoidal fracture and, when viewed in very thin sections (0.0005 in), is generally translucent and amber-red in colour. It is almost free from plant structures but shows a faintly defined cellular structure. A typical thin section of vitrain is shown in Figure .

ContinuedThe cells of vitrain are generally filled with ulmins and often consist of complete pieces of bark. Bark tissues are more resistant to decay and form a larger proportion of coal than might be expected owing to the fact that bark formed a larger proportion of the tree then than it does now.

A brown amorphous substance = ulminsBark= outer tissue of stem5ClarainClarain is bright black but less bright than vitrain. It is often finely banded so that it tends to break irregularly. In thin sections it shows partly the same appearance as vitrain in thin bands, but these, are inter banded with more opaque bands consisting largely of fragmented plant remains among which can be identified cellular material, spore exines and cuticle. A typical clarain structure is shown in Figure DurainDurain is the dull greyish-black coal which is hard and tough and breaks irregularly. In thin sections it is fairly opaque and shows large and small spore exines and woody fragments in a matrix of opaque grains. Figure shows a typical durain structure with large flattened macrosporesThe outer layer of the wall of a spore or pollen grain = exines

7ContinuedIn the coal seam, durain bands are often thick, and can be followed throughout the area of the seam. The fragmentary nature of the plant remains in durains suggests that its origin was the vegetable mud carried into the coal basin by water flow; the presence of a higher proportion of clay supports this. With very high proportions of clay, this material becomes 'carbonaceous shale' and with very high proportions of spore material it becomes the variety of coal known as cannel.

Spore is minute one celled reproductive unit which give rise to new individual Cannel is dull, hard material mainly composed of only slightly changed plant remains which make it high in volatile matter.Shale is sedimentary rock subjected to carboneous when it is observed to contain organic matter8FusainFusain is the soft black powdery material which occurs in very thin layers in the coal and at which fracture tends to occur. It shows the original cell structure of wood, though considerably deformed, and is considered to be woody fragments which have undergone a different process of decay from the rest of the coal. Sections of Bituminous Coal taken perpendicular to the Bedding Plane( X 20).

bedding plane is the line separating one layer of compressed rock from the next layer of compressed rock.10AssignmentCalorific ValueWeathering Index of CoalGrindability Index of CoalSpecific GravityBulk DensityAngle of reposePorosity, Surface Area, Heat of WettingRefractive Index and ReflectanceSolubility of Coal

Calorific ValueDulong Formula CG = 1 ( 8080 C + 34500 (O+N-1) + 2220 S ) 100 8

Goutal FormulaCG = 82 F + a. V12Caking Properties of CoalThe appearance of residue coke button in the volatile matter gives an idea about caking properties or caking capacity of coal.Caking coals: Bituminous coalOthers are non caking.a measure of caking properties is necessary for coal classification Selection of coalProduction of cokePerformance of coal in combustion and gasification

ContinuedCaking index of coalSwelling Index of coalRoga indexGray-king Assay (G-K Index)Thickness of Plastic LayerCoke button indicates ( Sintered , Pulverulent , weakly caking , caking , strongly caking)Caking IndexMeasure of binding and agglutinating property of coking coal.It is maximum number ratio of sand and coal in a 25 gm mixture , which on heating at Std condition produces a residue coke capable of withstanding a weight of 500 gms without producing more than 5% of loose grains of coke.Typical value : 20-25The higher the caking index, the higher the amount of sand that can be bound by coal15Swelling IndexMeasure of caking capacity of coal.1 gm ( -72 mesh ) air dried coal is heated in covered crucible until volatile matter ceases to evolve.Rate of heating is controlled in such a way that the temperature of inner surface of base is 800C in 1.5 min and 820C in 2.5 min.The crucible is cooled and coke button is removed and shape and size of coke button is compared with std coke buttons numbered from 1 to 9 with interval of 0.5. Higher the swelling index , better the caking and swelling properties.

Continued

Roga IndexIt is variation of agglutinating index.Mixture of coal and anthracite in a fixed proportion is carbonized under Std conditions.1 gm of coal and 5 gm of anthracite (0.3-0.4 mm) is compacted in a porcelain crucible under a load of 6 kg applied for 30 sec. After being brought to a temperature of 850C in 5 mins, the coke button is weighed (Q) and screened through 1 mm sieve, the weight of the +1 mm fraction being designated as 'a'.

The strength of the residue is expressed as numerical figure termed as roga index.18ContinuedThis material is placed in a rotating drum of diameter 20 cm and is rotated for three periods of 5 mins each. At the end of each period the -1 mm fraction is removed by screening. If b, c and d are the weights of the +1 mm residues at the end of first, second and third periods respectively, the Roga Index is given byRoga Index = (a+d)/2 + b + c x 100 3 QTypical value : 0 - 70

Gray-king Assay (G-K index)Gray King Assay test is also carried out to observe the coking property of coal.20 gms of fine coal (-72 mesh size) is heated in a silica tube to 600C. After cooling, the coke formed is compared with standard profiles numbers A, B, C,...... G, G1, G2, G3 ,..,G10. If the residue remained is a Stage A = powderStage B =non-caking coal Stage C and D = weakly caking coalStage E,F and G = medium caking coal.Stage G = hard coherent mass with the same volume asinitially of coal Stage G1, G2, G3 ,..,G10 =Strongly caking, coals swell

ContinuedThe suffix 1, 2,..... 10 indicates the number of grains of inert carbon which must be added to 20 gms coal charge to give zero swelling or a standard G type coke.Roga index is related with Gray-king Assay as index, 0 - 5=A - B, 5 - 10 = B - D, 20 = G, 45 = G4.

Dilatometric PropertiesThe dilatometric test is done for the knowledge of the swelling and contraction of a coal during heating under load without the access of air. In this test the temperature range of plasticity of a coal can also be determined. There are a number of dilatometers used in different countries. Some are:Audibert-Arnu DilatometerSheffield Coking Test ApparatusGiesler PlastometerContinuedThe basic principles of all these dilatometers is to record the expansion or contraction or the fluidity of the coal sample which is heated at a constant rate, say 1-3C/min to a temperature until the plastic mass is resolidified.The per cent expansion or contraction is plotted against temperature for the first two apparatus,Whereas in the third one the number of dial divisions per minute is plotted against temperature.The nature of curve gives the range of plasticity of the coal sample. Sheffield Coking Test Set-up & Typical Dilatometric Curves A And B