As with coal, but more so in the case of oil, the cement industry uses fuels at the lowest - and cheapest - end of the quality scale. The oil grade most commonly used is Bunker C (API No 6) oil, which is the main liquid residue from refining processes. The oil is a mixture of complex aliphatics and polycyclic aromatics (typically around 50:50) with negligible vapour pressure. Because sulfur compounds tend to be non-volatile, the sulfur content is usually relatively high. Heavy metals – notably vanadium, iron and nickel – are concentrated in the form of porphyrins. Use of this oil requires that it be heated, to render it pumpable, and nebulised at the burner to provide sufficient surface area to ignite.
In addition to heavy fuel oil, a small amount of light fuel oil – in the form of heating oil, gas oil or diesel grades – is consistently used to fire kiln “lighting up” burners, and to heat auxiliary furnaces for raw material drying and coal drying purposes, where kiln system waste heat is not available for these functions.
As has been mentioned elsewhere, the use of oil as a fuel played a critical role in the development of the rotary kiln. However, after the 1890s, even in the USA, the price per unit available energy of oil rose relative to that of coal, and in Britain oil was a "luxury" fuel during the first half of the 20th century. The only use of oil was in white cement manufacture where a zero-ash fuel was required. However, in the 1950s, the relative price of oil steadily fell. In the late 1950s, a large number of cement plants converted from coal to oil firing. The economic justification for this was marginal, but politics also played a major part in the decision-making. The UK industry was required by the government to source its coal from UK mines, and the use of oil – mainly Russian, as it happened – provided cement companies with a bargaining chip to keep coal prices down.
Prices favoured oil use most of the time during the period 1957-1968. Numbers of kilns began operation during that period using oil, benefitting additionally from the relatively low capital cost of oil-burning equipment, compared to coal milling and handling systems. This meant that when the oil price rose, some plants were faced with the dilemma whether to spend more capital on converting to coal, or to tolerate the higher price. Some respite was provided by the availability of North Sea gas in the early 1970s, and some plants converted from oil to gas, but eventually, with the rise in gas prices, all were forced to “bite the bullet” and put in coal mills.