Some considerations relating to the accuracy of measuring the ash content of coal by X-ray backscattering

Measurement of the intensity of backscattered X-rays now forms the basis of continuous on-line coal ash monitoring equipment in routine application. The ultimate accuracy of this method is conditioned by the choice of incident radiation energy, and it is shown that 15keV, consistent with the energy of X-rays from a plutonium-238 source, give near-optimum sensitivity. However, at this energy variations in the concentration of iron in the ash become important, but the choice of 15keV is also near-optimum for the successful operation of a method of compensating for variations in iron concentration. A number of other characteristics of the coal can also affect the ultimate accuracy of ash measurement. Most important are particle size distribution, bulk density and moisture content. The effect of each of these variables has been examined, and it is shown that an overall performance well within most operational requirements can be achieved. (author)