ENERGY ENGINEERING

4th Term, B.Sc. Chemical Engineering

Session 2013

Delivered by: Mr. Rizwan Ali

Department of Chemical EngineeringUniversity of Engineering & Technology, Lahore

MACRO- COMPONENTS OF COAL

Petrography 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 visiblepartings.

These differences are most marked in the case ofbituminous coals but can be traced in lignitous coals, incertain black lignites and, to a less extent, in carbonaceouscoals.

…Continued

The macroscopic constituents in bituminous coal which are identifiable by eye have been termed by

• vitrain• clarain• durain• fusain

Vitrain

• Vitrain 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 .

…Continued

• The 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.

Clarain Clarain 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, areinter banded with more opaque bands consistinglargely of fragmented plant remains among whichcan be identified cellular material, spore exinesand cuticle.

A typical clarain structure is shown in Figure

Durain

• Durain 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 macrospores

…Continued• In 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.

Fusain

Fusain is the soft black powdery materialwhich occurs in very thin layers in the coal andat which fracture tends to occur. It shows the original cell structure of wood,

though considerably deformed, and isconsidered to be woody fragments which haveundergone a different process of decay fromthe rest of the coal.

Sections of Bituminous Coal taken perpendicular to the Bedding Plane( X 20).

Assignment

• Calorific Value• Weathering Index of Coal• Grindability Index of Coal• Specific Gravity• Bulk Density• Angle of repose• Porosity, Surface Area, Heat of Wetting• Refractive Index and Reflectance• Solubility of Coal

Calorific Value

Dulong FormulaCG = 1 ( 8080 C + 34500 (O+N-1) + 2220 S )

100 8

Goutal FormulaCG = 82 F + a. V

Caking Properties of Coal• The appearance of residue coke button in the

volatile matter gives an idea about cakingproperties or caking capacity of coal.

• Caking coals: Bituminous coal• Others are non caking.• a measure of caking properties is necessary for

– coal classification– Selection of coal– Production of coke– Performance of coal in combustion and gasification

…Continued

• Caking index of coal• Swelling Index of coal• Roga index• Gray-king Assay (G-K Index)• Thickness of Plastic LayerCoke button indicates ( Sintered , Pulverulent ,

weakly caking , caking , strongly caking)

Caking Index

• Measure of binding and agglutinatingproperty of coking coal.

• It is maximum number ratio of sand and coalin a 25 gm mixture , which on heating at Stdcondition produces a residue coke capable ofwithstanding a weight of 500 gms withoutproducing more than 5% of loose grains ofcoke.

• Typical value : 20-25

Swelling Index• Measure 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 minand 820C in 2.5 min.

• The crucible is cooled and coke button is removed andshape and size of coke button is compared with stdcoke buttons numbered from 1 to 9 with interval of0.5.

• Higher the swelling index , better the caking andswelling properties.

…Continued

Roga Index

• It 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 850°C 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'.

…Continued• This material is placed in a rotating drum of

diameter 20 cm and is rotated for three periodsof 5 mins each. At the end of each period the -1mm fraction is removed by screening.

• If b, c and d are the weights of the +1 mmresidues at the end of first, second and thirdperiods respectively, the Roga Index is given by

• Roga Index = (a+d)/2 + b + c x 1003 Q

Typical 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 600°C.• 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 = powder– Stage B =non-caking coal – Stage C and D = weakly caking coal– Stage E,F and G = medium caking coal.– Stage G = hard coherent mass with the same volume as

initially of coal– Stage G1, G2, G3 ,…..,G10 =Strongly caking, coals swell

…Continued

• The 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 Properties

• The dilatometric test is done for the knowledgeof the swelling and contraction of a coal duringheating under load without the access of air.

• In this test the temperature range of plasticity ofa coal can also be determined.

• There are a number of dilatometers used indifferent countries. Some are:– Audibert-Arnu Dilatometer– Sheffield Coking Test Apparatus– Giesler Plastometer

…Continued• The 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 1-3°C/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