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.