PROPERTIES OF COAL
Important Properties of Coal
The most important properties of coal to the combustion engineer are as follows:
· Proximate analysis - to determine the moisture, ash, volatiles matter and fixed carbon
· Ultimate or elementary analysis - to determine the elemental composition of the coal
· Calorific value
· Caking properties - for bituminous coals only
· Grindability - where the coal is to be pulverised
Proximate and Ultimate analysis
These tests are undertaken following standard procedures laid out in British Standard 1016 pt 3, 1973.
Proximate analysis is the simpler of the tests and is used to determine the moisture, ash, volatile and fixed carbon content. Ultimate analysis is used to determine the elemental composition in terms of Carbon, Hydrogen, Sulphur, Nitrogen and Oxygen by difference.
The percentages can be reported by weight in a variety of different ways:
As sampled (as received) - exactly as the sample came to the lab
Dry - based on the air dried sample (not completely dried)
Dry, Ash free - based on the air dried sample with ash removed
Dry, Mineral Matter Free (DMMF) - based on the air dried sample with all mineral (inorganic) matter removed
Mineral matter is not directly measured but may be obtained by one of a number of empirical formulae based on the ultimate and proximate analysis. Further empirical relationships are also possible between carbon, hydrogen, oxygen and CV. The most commonly used is Seyler's chart, fig 1. Any two of the above properties may be used to estimate the other two.
Caking Properties of Coals
This is a unique property of coals in the bituminous group of coals and is an essential property for coals which are required for coking. As a caking coal is heated it passes through a region where it becomes very plastic, softens, swells and then re solidifies. The residue is a cellular coke mass. Coals which do not cake are simply form a non coherent or weakly coherent char.
A number of tests have been devised to classify the caking properties of coals including the Roga test, Free Swelling Index and Gray - King test. The free swelling index test entails heating a standard powder of the coal in a crucible and comparing the resultant "button" with a standard profile, fig 2, an index is given between 0 and 9. A non coherent sample is given 0. generally 0 - 3 implies marginal caking behaviour. The Gray King index is essentially the same except the residue is compared with a number of previously made standard cakes.
The caking behaviour is critical to coke making. A successful coke must be strong and not powdery. Prime coking coals have GK indexes of G (and its subdivisions). Much less and a weak coke unstable coke is made.
This is particularly important if a coal is to be burnt in the pulverised state. In this case, significant work must be done in order to reduce the coal down to particles of sufficient size for combustion. The Hardgrove grindability index is calculated by applying a standard amount of work on a sample of coal and determining the increase in surface area. The value, G is based on the fraction of coal of initially sieved with a size 16 mesh, passing through mesh size 30 after a standard mill. The value ranges between 20 and 100 for most coals. The easiest to grind being the bright, bituminous coals. Fig .3 shows the relationship between G and percentage volatiles. NB: the higher the value of G, the easier it is to grind the coal
THE MANUFACTURE OF COKE
Coke is manufactured from the carbonisation of prime coking coals. Carboniastion is performed for three main reasons: To make a smokeless fuel for domestic/industrial applications: to provide a coke for some other process (most importantly blast furnaces); to produce a combustible gas. However other important products are formed including coal tar which in the past was a very important chemical feedstock.
Rasing the temperature of coking coals in the abscence of oxygen results in their devolatilisation and the formation of a solid fuel, coke, which has a porous structure. Two types of coke can be made, hard and soft. The difference is in the temperature of carbonisation. Soft coke is carbonised at lower temperatures 600-700C. This results in a product with a reduced volatile content of the order 9% and hence better combustion characteristics.
Hard coke is carbonised at higher temperatures and resulting in devolatilisation and loss of porosity. Combustion characteristics are reduced making these cokes only suitable for more specialist purposes such as manufacture of carbon electrodes or in blast furnaces.