A high fixed carbon content is desirable for steady combustion and lower fuel consumption. This is the amount of heat available per unit weight of 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.
Sulphur oxides reacting with water vapour can form weak acids. 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. 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.