With just a few short term exceptions, British steam locomotives burned coal, but from the different varieties of coal that could be obtained, only a few types were suitable for locomotives and many railway companies designed their engines for a specific type of coal.
Coal consists of many constituents and the typical Welsh coal used by the Great Western contained elements such as;
Nitrogen 1% and
The heat produced by coal comes from its carbon and hydrogen content and coals with a high proportion of these elements, known as high-calorific coals, were always preferred. A high sulphur content is detrimental, however, especially when it is combined with the moisture content within the coal as it will form acids in the smokebox char and in the residual ash in the firebox. These acids then attack the metal surfaces of those areas.
If the firebox is at a temperature lower than its normal working temperature to sustain steam, clinker will form on the firebars. Clinker is a hard deposit that occurred when ash and sulphur fused together and removal usually meant chipping off with a long poker. The best locomotive coal has a low ash and sulphur content particularly since ash tends to block the boiler tubes and it can pit and score the tube surfaces as it is swept through to the smokebox.
Bituminous coals, which includes house coal, were widely used for locomotives, but only those types that were hard and with a low ash content. Welsh coal, especially that used by the Great Western, was semi-bituminous and had a high carbon content that burnt with a shorter flame.
In general, a smoky exhaust displays inefficient combustion. Although many photographers crave this pyrotechnic effect, it is purely a sign of waste. To achieve complete combustion, a high firebox temperature together with a good air flow is required by using the dampers, firehole door and maintaining an even firebed thickness. Heaping coal into the firebox at long intervals produces black smoke and wastes consumption - the better method was to fire "little and often".
Coal has a tendency to form a hot mass over the fire known as a cake. This tendency is an inherent property in the coal that cannot be removed, although it can be reduced by non-caking coals. If its chemical composition is satisfactory, coal that has caked needed only a little work by the fireman to break up the crust.
In addition to the composition of the coal, the size of coal also mattered. Small sized pieces of coal increase the total surface area of coal in the firebox and therefore produce excellent steaming rates. Poor quality coal, especially that supplied after 1939, contained a high proportion of coal dust and slack. Most coal dust is drawn, unburned, straight up the chimney while the slack tended to block the flow of gases through the boiler tubes. One good use of the coal dust was in the form of briquettes made from compressed coal dust.
Coal that was handled in mechanical coaling plants tended to break into small pieces, and for that reason hard coals were preferred by most railways. Due to the Great Western's
insistence of Welsh coal that was of a high calorific value with low volatility, it stayed with hand/tub coaling to reduce degradation even though it was a dirty and time-consuming method.