Coal-derived coke was the fuel used in most static kilns and so was the dominant fuel in the cement industry in the nineteenth century. Coal could not be used in static kilns in which the fuel is fed in with the raw material, mainly because, during the gradual heat-up of the charge, the fuel's volatiles are lost without ignition, so wasting up to half the fuel's calorific value. Burning coal also had less load-bearing strength than coke, causing the charge to collapse and "choke".
In recent years, petroleum coke has become a major fuel in the industry. Because they are produced by similar processes, they are treated together here. Both can have quite a wide composition range, depending on the process producing them, and their intended end-use.
Coke from Coal
The coke produced from coal was one of the major commodities of the Industrial Revolution, coming to prominence with the invention by Abraham Darby of the coke-fired blast furnace. Subsequently, the development of the coal gas supply industry in the nineteenth century greatly increased the amount of coke produced. The common feature of these materials is their production by heating coal in a "retort", in the absence of air. Coal consists of a wide variety of chemical structures, the amount of each depending on its rank. There are fused cyclic structures, mainly aromatic, but some alicyclic, from which branch aliphatic chains of varying lengths. On pyrolysis, the cyclic structures become progressively aromatized by dehydrogenation, and the side chains are cracked off. Due to inleakage of air, a certain amount of oxidation may take place. Deriving as it does from organic material, coal contains a significant amount of oxygen, and fairly constant small amounts of sulfur and nitrogen. (Sulfur may also be present as sulfate or pyrite in the mineral matter.) These "hetero-atoms" may be incorporated into the ring structures, or may be present in side chains in the form of hydroxy-, keto-, amino- or thiol-groups. On light pyrolysis, the side-chain atoms are removed, but the ring hetero-atoms may remain in place.
The latter effect brings out the distinction between "metallurgical" coke and the coke produced by the gas industry. Metallurgical coke needed to have low levels of sulfur and nitrogen, and so was subject to high temperatures for a long time to allow the hetero-atoms to re-arrange out of the structure, the result being close in composition to an amorphous graphite. In the gas industry, however, the coke was the by-product, and the economics of gas production depended on heating the coal only to the point where the diminishing yield of gas no longer justified further heat treatment. The result was a coke which still contained some volatile matter, and significant amounts of sulfur and nitrogen. This was the coke used by the cement industry, and there was a distinct symbiosis between the two industries. Britain's most concentrated centre of gas production in early times was London, first for street lighting, and then on a massive scale for domestic use. With no heavy industry to absorb the coke, it was virtually a waste product, and its ready availability at low price was a key influence upon the concentration of the cement industry in the south-east. Some larger cement manufacturers (e.g. White's and Bevans) protected coke supplies by setting up their own gas plants, using the waste heat from the retorts for slurry drying and selling on surplus gas in the locality.
The cement industry was further affected by the maturing of the gas industry. Gas production levelled off as competing forms of energy, particularly electricity, arrived on the scene, and producers became more cost conscious. The producer gas/water gas process was increasingly used to improve gas yields, and this consumed coke. Other industrial uses for coke arose, and it was no longer a waste material, resulting in dramatic rises in prices towards the end of the nineteenth century. Continental competitors for the trans-Atlantic cement export market were better able to cope with this because their industries had always been much less profligate with fuel. The move to the use of rotary kilns, despite the painful readjustments in working methods that they demanded, and despite their poor energy efficiency compared with shaft kilns, took place because they were fired with coal, which was still cheap in Britain. The result was a rapid diminution in the use of coke in the first two decades of the twentieth century, although a few static kilns continued to use it until the late 1960s.
Coke from petroleum ("petcoke") is entirely analogous. Its importance has risen as oil has supplanted coal as an energy source, and as the rising price of crude oil has demanded a higher recovery of high-value fractions from the raw material. The refining of oil in the early days by distillation and cracking proceeded until the residue was at the heavy fuel oil stage, and the latter could be sold on as fuel at a reasonable price. However, heavy fuel oil can be further treated, to the point where a solid product remains. This may take the form of a friable finely-porous lump material, or as nodular beads, depending on the process. The chemistry of production is not dissimilar to that from coal, and similarly, the degree of refinement towards pure carbon depends upon the economics of the end-use. Calcined petroleum coke, produced in a rotary kiln, is used to make anodes for electrochemical furnaces in the aluminium, titanium and steel industries. The normal "green" coke is used as a fuel, where it can be more convenient to use than fuel oil because its properties are sufficiently similar to those of coal that coal/petcoke mixtures can be used in power generation or cement manufacture in whatever proportions desired without major modifications to plant.
Petcoke differs from coal most obviously in its almost total lack of ash and its relatively high sulfur content. While the latter is a significant problem for power generators, it can be less so for cement manufacturers because the alkaline conditions in a preheater scrub SO2 out of the exhaust gas. However, there is a distinct trend towards the installation of separate SO2 scrubber units. Petcoke differs also in that it is tougher to grind, and there is a trend towards the use of more sophisticated fuel grinding circuits to cope with this. Despite these problems, cement kilns have been run on pure petcoke, and a 50:50 coal:petcoke mix is a fairly normal fuel mix nowadays. The increase in the use of petcoke has of course been due to the fact that "green" coke with metals contents too high for electrochemical use became a low-cost waste by-product. However, a fuel that is today universally used is no longer a waste, and prices, compounded with the volatility of the crude oil price, necessarily trend upwards, so useage will necessarily peak and fall.