COAL AS AN ENERGY OPTION
British coals are classified according to 'rank' in numbers from 100 to 900. The high rank (low rank number) coals such as anthracite are rich in carbon and contain little volatile matter. The low rank (high rank number) coals are referred to as bituminous coals and may contain 40% or more volatiles.
Low rank coals are easy to ignite because of their volatile content, but complete combustion of these constituents has to be ensured to avoid smoke. High rank coals are less easy to ignite but burn smokelessly.
The tendency for coal particles to fuse together on heating, called caking property, is important when combustion takes place on grates because it affects the passage of air through the firebed and the burning rate. However, the combustion of non-caking coals (rank numbers at either end of the classification) requires attention in order to avoid grit emission and the loss of unburnt particles through the firebars.
Grade is a term used only in relation to the physical size of coal. A graded coal is one having a limitation upon both its maximum and minimum particle sizes.
Apart from graded coals there are 'smalls' which have a specified upper size limit, usually 50 mm, but no minimum size. 'Fines' are small coal with a top size of about 3 mm. There are also 'large' coals which have an agreed minimum size but no upper limit.
Apart from screening, coal may be 'treated' or 'washed', that is, submitted to cleaning processes which will leave it free of loose dirt. When a washed coal is burned there is naturally less ash residue, a desirable characteristic for many combustion appliances, although a minimum ash content is often specified to protect grate bars from heat, and ash also serves a useful purpose by retaining some of the sulphur content of the coal and thereby lessening corrosion and pollution