Australian coal

Coal composition Coal is fossilised plant matter. After millions of years of compression, it forms various stages of a hard brown or black substance known as coal. As it ages over millions of years, it progresses from peat to lignite (brown coal), subbituminous black and bituminous black to anthracite. As it ages the water content decreases and the carbon content increases.

Coal at all stages also contains moisture, ash, sulphur volatiles and fixed carbon.

Water. Water is contained in pores within the coal. It diminishes the heating value and can make it more difficult to ignite. In the processing stage it can affect the handling and crushing properties. Particular problems can be encountered in cold climates where the water can freeze. A low moisture content is desirable, but is not critical.

Ash. Ash is non-combustable material which lowers the heating value and also presents problems for its disposal after burning. It can also cause corrosion and abrasion of boilers and clinkering or slagging of furnaces. The ash content of coal is usually formed by clay mnerals, carbonates (calcite, dolomite, siderite), sulphides (pyrite, sphalerite, galena) and silicates (quatrz). A low ash content is desirable.

Sulphur. A subset of ash, the sulphur can cause corrosion. A low sulphur content is desirable.

Volatile materials. These flammable gases are released at relatively low temperatures and assist in quick ignition of coal particles. They cause smoke, but are necessary for stable combustion. A high volatile matter content is desirable, but too high as this will result in increased fuel consumption.

Fixed carbon. This is the remainder of the combustable material after the volatiles have been released. It decomposes to volatiles at high temperatures and continues the combustion process. A high fixed carbon content is desirable for steady combustion and lower fuel consumption.

Specific energy. This is the amount of heat available per unit weight of coal. Two figures are often quoted, "wet" (gross as received) and "dry" (usually only relevant for pulverised fuel firing). Cobbled black coal for grate firing is "wet" as there is a small amount of water trapped in pores within the coal. A high specific energy is desirable for efficient combustion and lower fuel sconsumption. Coal with a low specific energy may not be suitable for locomotives with small grates.

Problems with coal firing

Corrosion. Corrosion occurs slowly when soot and ash deposits are in contact with metal for some time. Alkalis and sulphates can corrode boiler tube metal and pyrosulphates can corrode the furnace (firebox) wall. Sulphur oxides reacting with water vapour can form weak acids. Direct oxidation of iron (in steel) can occur, but this is more common on the "wet" side of boiler tubes or plates.

Pulverised fuel. This applies to both pulverised coal firing (in power stations), and burning small (0.03 - 0.1mm) particles of black coal. Small particles will burn to completion in less than one second. The ash from this is very fine, forming fly ash and very fine soot, and even vapourous ash and char. Some ash vapours can react again forming "fume". Fume is mainly very fine particles of silica which can react with vapourous salts to form silicate deposits and weak acids. At typical gas flows of 10-20 metres per second, this means such deposits will be formed in the front of tubes and flues, in the smokebox and on superheater headers.

Clikering. As coal burns, ash is supposed to fall through the grate into the ashpan. If the ash is not fine enough, or if the ash fusion temperature is low, it can fuse and form hard clinker in the grate. Fine particles of ash can clinker in other parts of the boiler. If a furnace is solely pulverised fuel fired, this is known as slagging.

Subbituminous and bituminous black coals

TM = Total Moisture (%)
Ash = Ash content (% dry basis)
VM = Volatile Materials (% dry basis)
TS = Total sulphur (% dry basis)
SE = Specific energy (MJ/kg, gross as received)

Brown coal

Sources:
Australia's Export Coal Industry, Coal Development Branch, Department of Primary Industries and Energy, 1996
Coal Audit 1, NSW Department of Mineral Resources, 1996.
The Science of Victorian Brown Coal, R.A. Purie (ed.), CSIRO, 1991

But what coals are used for locomotives today?

RAILCo, Qld. - Blair Athol (C17)
Pichi Richi Railway, SA - Clarence
Castlemaine & Maldon Railway, Vic. - South Bulli
3801 Limited, NSW - Hunter (SMR10), blend of Hunter and South Coast (3801)

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