We have recently conducted some raw meal burnability testing on samples with varying residue and C₃S. Results showed little dependence on these two variables. Following further investigation I was told burnability can be highly dependent on 45 micron insoluble residue. What are your thoughts on this? I am still reluctant to believe the low dependence on C₃S because in the kiln I am still observing significant fuel variations with changing C₃S.

The classical burnability index takes into account both the chemical composition (i.e. LSF, C₃S etc.) and also the fineness and mineral composition of the kiln feed. The theory is that large quartz (SiO₂)grains and lime (CaO) grains are very difficult to combine in the kiln, and this is fairly well established. You can get the formula from the proceedings of the IEEE 2002 in Jacksonville.

I've been working with analysers for the cement plants for last nine years and have still a little doubt about the actual use of measurement of NO_x in the kiln inlet. If you can kindly send me some literature about the firing in the kiln and the advantages of measuring various gases at the kiln inlet.

The NO_x level at the kiln inlet is a very good indication of the burning zone temperature in the kiln. In turn this is one of the most important considerations in the control and optimisation of the kiln. NO_x arises from two sources,
(i) organic nitrogen in the fuel, and
(ii) oxidation of the atmospheric nitrogen in the combustion air drawn into the kiln. This oxidation of the atmospheric nitrogen rises in direct proportion to the temperature in the flame and the burning zone.