Cement Kilns

Coal Data


Firing Systems Coke Data Oil Data Gas Data Waste Fuel Data

Coal is the fuel that undergoes least human intervention before use: it is burned in essentially the same condition in which it is dug from the ground. Because of this, the term "coal" has a breadth of meaning which reflects the variety of geological conditions under which the fuel is formed. Coal forms only where there is rapidly-growing tropical vegetation, and in a sedimentary basin in which decaying vegetation is periodically covered over with a thick impervious layer of mud, with successive layers becoming deeply buried. This ensures that the organic material is protected from oxidation, and subject to high pressure and, to some degree, elevated temperature. The continued application of pressure causes a sequence of reactions referred to as “coalification”, and the degree to which this process takes place defines the nature of the coal. The degree of alteration is referred to as “rank”, high-rank coals being those most altered.

The main starting material for the process is wood, consisting of a mixture of cellulose [(C6H10O5)n], lignin [roughly (C10H12O3)n] and a certain amount of inherent moisture. Taking these in a typical ratio 6 : 2 : 1, the dry basis analysis is C 50.00%, H 6.34% and O 43.66%. Coalification takes place by the progressive removal of the elements of carbon dioxide and water into the surrounding rock, lowering the oxygen content, and reducing the amount of volatile matter (VM). The cyclic structures become increasingly polycyclic aromatic. Sulfur and nitrogen from the surroundings infiltrate the structure. The earliest members of the sequence recognisable as coal are the brown coals and lignites. These are soft and woody in texture. Successive stages are sub-bituminous, bituminous, semi-anthracite (“steam coal” in British parlance) and anthracite, with pure carbon in the form of graphite as the theoretical end-member. Broadly speaking, hydrogen content enhances the calorific value of the coal, while oxygen content diminishes it. Because they retain some hydrogen although the oxygen is largely gone, the steam coals have the highest calorific value.

The lower-rank coals are not much represented in Britain. Bituminous coals make the best coke and are in any case the most common, and these are most often used in the cement industry, although steam coals are occasionally used. When burning a pulverised fuel, higher volatiles are preferred because they speed up ignition and allow a coarser grind to be used. Anthracite is too expensive and has insufficient volatiles. Usually, the lowest-priced forms of coal are the finer sizes, and these are most often used, although they tend to have higher free moisture and ash contents.



Britain's coal fields, extensive and with coal seams reaching the surface in many places, have been worked probably from the Neolithic onwards. Coal - probably from the Tyne - was being traded during the Roman occupation. The coal trade rapidly extended due to the depletion of timber during the "Little Ice Age" (1500-1650), and its extention to an international market was the basis - and sole cause - of the Industrial Revolution. It was purely because of this well-developed cheap-energy economy that industrialisation first took place in Britain before it happened anywhere else.

The supply of Tyne coal to London dates from the latter's foundation in the mid first century, and a coastal trade developed with massive fleets of dedicated "collier" ships. Defoe describes a night's storm off Norfolk in 1692 in which more than 200 colliers were sunk. These ships also sailed to the Netherlands, north Germany and Scandinavia. The "chaldron of Tyne coal landed at London" became the standard benchmark of energy price, just as the "barrel of Brent crude" is used today.

Thus there was a well-established coal trade into London at the start of the cement industry. As explained elsewhere, the high-value produce of London had no market on Tyneside, so collier ships ballasted for the return journey with the cheapest available material - chalk from the Thames-side quarries, which were well developed before the cement industry began (see Northfleet history).

Clearly, the development of the cement industry in the London area depended upon this cheap energy supply. London coal gas production began in 1813 and by 1842 was producing 300,000 tons of coke a year, so coke was also readily available, and, being essentially a waste product, was cheaper than coal. Early cement plants used coal for drying slurry and for power generation, and coke for kiln burning. Per tonne of clinker produced, consumption was around 0.5 tonnes of coke and 0.1 tonnes of coal.

Coal could not be used in most static kilns because the fuel was loaded into the kiln with the rawmix, and in the process of gradually raising the temperature, coal would lose its volatiles - and half its calorific value - without ignition. Coal also tended to cause the kiln charge to collapse. With the rise in the relative price of coke towards the end of the nineteenth century, the use of coal was explored. One solution was the Dietzsch kiln, available from the late-1880s. This was a labour-intensive process, and had limited take-up, but the arrival of the rotary kiln in the late 1890s provided a highly productive coal-burning system, and the use of coal escalated rapidly. 50% of heat for drying and burning was provided by coal by 1913, and it was 90% in 1926. It reached 100% in 1943 with the demise of the last static kilns.

Oil displaced coal only for white clinker production until the late 1950s when a proportion of capacity - and most of that of Thames-side - was converted to oil burning, partially in response to a lower oil price, but also as a monopoly-breaking ploy. After this, and brief flirtations with natural gas, coal became emphatically the cheapest kiln fuel from the mid-1970s, and newer kilns had to be retro-fitted with coal grinding equipment. Since then, coal has remained the main fuel, although partially - and at some plants totally - displaced by Petcoke. The increased cost of primary fossil fuels has led to progressive increase, from its beginnings in the late-1970s, of the use of "alternative fuels" such as domestic refuse, landfill gas, tyres, waste lubricants and solvents and so on, although there seems little prospect of these consistently providing the majority of kiln energy.


The "typical" value is the average value for those coals having Nett CV values between the 10-percentile and the 90-percentile. The range is the 10-percentile and 90-percentile value for each parameter. The combustion air used has 50% humidity at 20°C (see composition). "Gross" calorific value is otherwise known as Upper Heating Value. "Nett" calorific value is otherwise known as Lower Heating Value.

Anthracite Steam Coal Bituminous Coal Sub-bituminous Coal Lignite
typical range typical range typical range typical range typical range
Air-dry basis analysis:
Inherent Moisture 1.27 0.9-1.7 0.79 0.6-1.2 4.35 1.2-8.8 11.16 6.7-14.8 19.07 15.0-22.5
Ash 3.91 1.8-6.7 5.81 1.5-8.5 6.81 1.1-20.4 4.65 3.4-7.4 4.82 1-9
Volatile Matter 4.45 2.8-8.0 16.50 11.2-24.0 33.65 26-41 37.75 34.9-39.6 35.95 20-55
"Fixed Carbon" 89.63 84.4-91.7 76.00 70.9-82.0 54.01 45-64 45.27 42-49 38.88 31-46
Gross CV MJ/kg 33.197 31.1-34.4 34.021 32.6-36.0 30.455 26-35 26.394 24-28 21.894 16-27
Nett CV MJ/kg 32.662 30.7-33.8 33.116 31.8-35.0 29.340 25-34 25.161 22.6-26.9 20.583 15-25
Mineral Matter 4.65 2.1-8.1 6.72 1.82-9.7 7.99 1.3-23 5.82 4.5-9.4 6.10 1-15
C (organic) 90.12 85.7-91.7 85.04 82.2-88.7 73.53 62-84 63.51 59-68 53.17 50-57
H (organic) 2.31 1.6-3.0 4.06 3.6-4.8 4.62 3.6-5.5 4.40 3.9-4.9 3.87 2.0-5.5
S (organic) 0.41 0.2-0.6 0.61 0.2-0.7 0.79 0.3-1.3 0.90 0.5-1.4 0.88 0-2
S (in sulfates) 0.02 0.01-0.03 0.02 0.01-0.03 0.09 0-1.1 0.08 0-0.3 0.02 0-0.3
S (in pyrite) 0.15 0.01-0.47 0.14 0.01-0.54 0.47 0-3.5 0.29 0.1-1.1 0.05 0-0.2
N 0.74 0.4-1.3 1.34 0.6-1.8 1.47 0.2-2.3 0.72 0-2.7 0.08 0-1
O (organic) 0.49 0.2-1.5 1.45 0.3-3.0 7.20 3.5-12.0 13.39 9-15 16.76 3-34
Cl 0.06 0.03-0.15 0.04 0.02-0.08 0.33 0-1 0.15 0-0.5 0.12 0-1.3
CO2 0.21 0.07-0.38 0.24 0.01-0.48 0.40 0-1.5 0.33 0-1 0.62 0-1.7
Air-dry mineral-free basis analysis:
Inherent Moisture 1.33 1.0-1.8 0.84 0.7-1.3 4.73 1.3-9.2 11.85 7-16 20.31 14-26
Volatile Matter 4.67 2.9-8.3 17.69 12.3-24.7 36.58 30-43 40.08 38.5-45.4 38.28 25-55
"Fixed Carbon" 95.33 90.7-95.6 82.31 74.3-87.0 63.42 52-68 59.92 45-52 61.72 30-51
Gross CV MJ/kg 34.816 33.9-35.3 36.472 35.9-37.0 33.100 30.8-35.9 28.027 25.2-29.5 23.315 15-30
Nett CV MJ/kg 34.254 33.4-34.7 35.502 34.9-36.0 31.887 29.5-34.7 26.717 23.9-28.3 21.919 15-28
C (organic) 94.52 93.0-95.7 91.17 89.4-92.6 79.92 74-87 67.43 62-72 56.62 49-64
H (organic) 2.42 1.7-3.1 4.35 3.9-4.9 5.02 4.3-5.9 4.68 4.2-5.1 4.12 2-6
S (organic) 0.43 0.2-0.6 0.65 0.2-0.8 0.86 0.3-1.5 0.96 0.5-1.5 0.94 0-2.3
N 0.78 0.4-1.3 1.44 0.6-1.9 1.60 0.2-2.4 0.76 0-3 0.09 0-1
O (organic) 0.51 0.2-1.6 1.56 0.3-3.1 7.83 4-12 14.22 10-16 17.85 4-34
Nett/Gross ratio 0.9839 0.980-0.988 0.9734 0.970-0.976 0.9634 0.956-0.969 0.9533 0.948-0.959 0.9401 0.934-0.947
"Dry ash-free" basis analysis:
Gross CV GJ/kg 35.011 34.0-35.6 36.424 36.0-37.0 34.279 32.8-36.4 31.349 29.4-32.4 28.766 19-37
Volatile Matter 4.70 2.9-8.3 17.66 12-25 37.88 30-45 44.83 43.6-46.5 47.23 31-67
Combustion air:
kg/kg air-dry coal 11.2350 10.6-11.6 11.2156 10.8-11.8 9.8541 8.2-11.4 8.3438 7.7-8.9 6.8023 5.1-8.4
kg/GJ 343.979 342-346 338.675 332-341 335.865 325-348 331.621 327-340 330.484 319-342
CO2 produced:
kg/kg air-dry coal 3.3022 3.13-3.36 3.1159 3.01-3.25 2.6943 2.26-3.07 2.3269 2.16-2.50 1.9482 1.83-2.09
kg/GJ 101.104 98.6-104.2 94.091 92.5-95.7 91.834 89.5-95.2 92.483 90.3-95.8 94.650 78-116


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© Dylan Moore 2011