Influence of the fineness of coal and lignite on the flame formation in rotary cement kilns

Until now the fineness of grinding of medium-and high-volatiles pulverized coals for the cement burning process has usually followed the rule 'the lower the volatiles in the coal the finer it must be ground'. On average this has led to residues of 10-13% on 90 μm. Extension of the range of fuels to cover extremely ash-rich coals, non-bituminous coals and soft lignites means that the requirements must be more precise. Based on combustion trials with a large number of fuels, a method of evaluating fuels has been developed with the aid of a mathematical model which is tailor-made to the particular requirements of the clinker-burning process. It takes account of the effects of the burner (air percentage, outlet velocity, blending intensity) of the combustion space (excess air ratio, secondary air temperature, combustion space loading, wall loss) and of the fuel (composition, calorific value, particle size). Trials and simulation calculations have shown that the main factors affected by the coal grinding are flame temperature, the flame shape, and heat transfer to the kiln feed. Specifying the fineness means that inferences can be drawn about the anticipated kiln operating state, particularly the specific fuel energy consumption.