A wide range of coals are used in cement plants. As is the case with many coal applications, the plant can be designed in part to suit the coal. Although coal the quality criteria are not very exacting, it is important that variability in quality be small, as the composition of the raw feed must normally be tailored to the coal ash chemistry. As the rotary kiln is by the most common cement plant in operation the following discussion focuses on this type of plant.
In direct firing systems the coal is milled on-line with the pulverized coal being directly fed to the kiln burner. The quantity of primary air used with direct firing systems is normally that required to dry the coal and sweep the mill, and is more than required for transporting the pulverised coal or producing a satisfactory flame.
Direct firing systems are not particularly suited to high moisture coals because:
The moisture which is evaporated during milling is fed into the kiln with the primary air. This has the effect of lowering the flame temperature, and process efficiency.
Additional mill air may be required so that the coal can be effectively dried without having excessive air preheat. This additional air also enters the kiln as primary air. Excessive primary air also has a similar effect to moisture.
In indirect firing systems the coal is milled off-line and is stored in a bin from which it is fed to the kiln burner. High moisture in coals can be associated with low rank and the propensity for spontaneous combustion. This could mean problems when storing the PF in badly designed bins of an indirect fired system. If coals which are subject to spontaneous combustion are to be used successfully, they may need to be sold selectively to plants with a proven track record in this area.
Milling behaviour of coals for cement kilns has special significance because kiln operators generally place some emphasis on tailoring the coal fineness to suit its reactivity. The accepted measure of coal particle size in the cement industry is the percentage greater than 90ìm. It is widely held that low volatile matter in the coal can be compensated by finer grinding. In order for this option to be open the mills must have the spare capacity to achieve finer grinding, or the coals must have a high Hardgrove Grindability Index (HGI). The trade-off between HGI and VM is therefore important. Fortunately, low VM bituminous coals tend to have high HGI values.
The main consideration for coal storage is the propensity for spontaneous combustion and explosion of stored PF. If fresh PF remains in one spot, particularly in the presence of hot moist air, self heating and ignition can occur rapidly. This causes any dispersion of coal dust in air, within certain concentration limits, to ignite and explode.
In order to heat the clinker to the required temperature of around 1500°C, it is necessary to have a flame temperature of around 1700°C. This is achieved by providing preheat to the secondary air and limiting the quantity of primary air. As noted above, for high moisture coals in direct fired systems there may be an excess of moist primary air and the required temperature may not be obtained. However for indirect firing systems using dry primary air, a satisfactory flame temperature can be achieved even with low energy coals.
The correct distribution of temperature along the kiln requires that the flame front be located close to the discharge end of the kiln, close to the burner. Coals which are unreactive and slow to ignite may therefore cause problems in some kilns, and in these cases medium to high volatile coals would normally be specified. The use of swirl burners with low volatile coals is not always successful, as the diverging jet may impinge on the kiln walls just as the coal is igniting. The reducing conditions that this causes in the clinker affects product quality, and damages the refractory lining of the kiln.
While it is normal to have some build up of deposits in the clinkering zone of a cement kiln, excessive deposits in this area, called "clinker ring", can hinder the movement of solids through the kiln and in extreme cases require plant stoppages. A second problem is the formation of deposits in the suspension preheaters. These problems are commonly associated with the presence of chlorine, sodium, potassium, and sulphur. When the levels of these elements are considered, the combined effects of coal ash and raw feed composition must be considered. The major source of sodium and potassium would normally be the raw feed, however coals with unusually high sulphur or chlorine could contribute to deposit formation and would not be favoured as cement kiln coals.