Coal has to be dried in the mill order to grind it properly and not for any reason connected with combustion efficiency. The moisture evaporated from the coal in the mill is carried through the. burner pipes into the boiler and causes the normal loss associated with moisture in fuel. It is only necessary to remove surface moisture from the fuel in order to be able to grind it; the -inherent moisture does not affect grinding. In fact coal containing quite a high percentage of inherent
moisture can be ground as though it were completely coal, and coal containing a few percent of free moisture could probably be dried sufficiently by the heat generated by the grinding energy. High moisture contents cause no real milling difficulties provided there is sufficient heat available in the primary air to evaporate the moisture.
If the moisture content of the coal is so high that the drying capacity of the mill is exceeded then there will be a marked falling off in mill output because the ground particles stick together and to the various internals of the mill. Plants specifically designed for very wet coal have extra primary air heaters to give exceptionally high air temperature and so increase the drying capacity of the mills.
Mill drying performance varies between different types of mill. From an efficiency aspect those which use small quantities of primary air at high temperature are preferable to those which require the same amount of heat to be supplied by a larger quantity of air. The reason is two fold firstly more primary air requires more fan power, and secondly if the primary air quantity has to be increased by tempering air which has not passed through the air heater then the final gas temperature will be higher than it would otherwise have been. The temperature of the fuel and primary air in the fuel pipes is controlled within the range 55° to 85°C although satisfactory drying of coals containing a large percentage of inherent nmisture can be achieved down to 55°C. In going below this temperature range there is some danger of traces of condensation in the fuel pipes. Temperatures above 85°C may cause coal deposits in the burner nozzles.
Effect of Ash
There is no direct relationship between the ash content of the coal and the energy required to grind it, but as the inert ash contributes nothing to combustion to calorific value of a high ash content coal will be relatively low. This means firing a greater tonnage of high ash coal for the same boiler heat input and consequently more milling power and more mills wear. The combustible in dust loss depends upon the percentage of unburnt carbon in the dust and also upon the quantity of dust produced. Thus, if the carbon in dust is 2 percent with a coal containing 10
percent of ash and ground to a fineness of 70 percent through a 200 mesh sieve then the carbon loss would be about 0.2 percent. If the ash content of a second coal were twice as much (20 percent) and the fineness of grinding maintained at the same figure then the carbon in dust would still be about 2 percent, but as the make of dust would be about twice as great the carbon loss would also be twice as much because the lower calorific value of the high ash content coal would necessitate firing a greater tonnage of fuel. It may therefore pay to grind the coal to a greater fineness in order to reduce the carbon in ash loss. If the carbon in ash loss could be reduced from more than 0.4 percent to 0.3 percent by increasing fineness from 70 percent to say 75 percent through a 200 mesh sieve, this should be done, provided the increased milling power does not offset the higher boiler efficiency. Excessive mill wear may also result but nevertheless as appreciable change in ash content should lead to a re-appraisal of the fineness of pulverized fuel required.
The physical properties of ash may have repercussion on boiler efficiency in addition to the effect of ash quantity given in the previous paragraph. The ash particles must have solidified sufficiently to lose their stickiness before leaving the furnace and entering the convective passes. The furnace exit gas temperature should therefore be lower than the softening temperature of the ash to prevent deposits on the convective passes. Should the ash fusion temperature be lower than normal, and the ash particles are still fluid at the furnace exit, then there seems to be no alternative to lowering the furnace exit temperature by admitting more excess air, and by so doing having to accept a higher dry gas loss and lower boiler efficiency.
Effect of Coal Hardness
The hardness or grind ability of a coal is found by grinding a sample in the Hard grove machine and is expressed as the Hard grove Index. A low index indicates a hard coal and a high index &. Soft coal. The range is usually between 40 and 100 in Great Britain. As would be expected, a mill cannot grind as much hard coal as soft coal. a mill capable of an output of 10 tons per hour when milling soft coal
with an index of 100 would probably only produce between 5 and 6 tons per hour with very hard coal with a Hard grove Index of 40. Harder coals, therefore, require more milling power and some sacrifice in pulverized > fuel fineness may be required to get the economic balance between the expenditure of power in milling and the carbon in dust loss. Mills are usually specified to give the required output with fairly hard coal, say, an index of 50 or 55. This ensures that boiler load can be maintained even with very hard coal. If softer coal becomes available then mill capacity will be more than adequate to grind the output required.
Effect of Volatile Matter
Lower volatile coals are slower burning and coals with a particularly low volatile content require special long flame burners like the U or downs hot burner. Low volatile coals are much more difficult to burn than high volatile coals from both the aspect of keeping down the carbon in dust loss and in maintaining stable ignition. More turbulence, more surface area of coal particles and more time in the combustion zone are required for low volatile coals. This means an increase in the fineness of grinding and a reduction in the quantity of primary air. The greater fineness of the coal particles exposes more surfaces for the combustion reaction. Reduction in-1 primary a;r leaves more of the total air required to be supplied as secondary or tertiary air to promote turbulence and complete burn out. Theoretically, hardly, sufficient primary air to burn the volatile matter should be supplied but the quantity should not be reduced so much that there is too low a velocity in the fuel pipes and separation of coarse and fine particles occur.
Number of Mills to Operate for Optimum Efficiency
Boilers are normally provided with at least one more mill than the number required for full load so that mill maintenance can be carried out without reducing boiler load. After a certain length of time the wear of the grinding elements will be such that the output and fineness of product will fall away and the quantity of rejects increase the power used to produce the pulverized fuel will also increase. The standby mill should then be brought into service and the worn mill taken out of service for maintenance. Normally, therefore, a boiler steaming at full load
would be operated with the number of mills the makers intended to be used for full load but as these mills will have a margin of capacity they will not all be working at full output. The most economical way of loading the mills is to have good mills loaded to their normal capacity and worn mills working at a smaller output. The electrical energy required by a mill group per ton of coal ground is lowest when the mill is working at its normal output so that only the minimum numbers of mills necessary to provide the required output should be in service. In conditions are favorable, say because of soft coal, then it may be possible to steam the boiler with one less than the normal number of mills, if this can be done it will reduce the total milling power. Quite often considerations other than minimizing milling power predominate and determine which mills should be run. It may be that steam temperature is affected by mill operation because in some installations each row of burners is associated with one mill to give some measure of steam temperature control. In other cases certain combination of mills and their associated burners will give an unbalanced furnace. Generally speaking, however, mills should be well loaded to give their most economical performance.