Do you have an idea of each point decreased in the standard deviation of the kiln feed and how it affects to the specific heat consumption? (for example: 40kJ/kg of clinker for every additional 0.1 by which the standard deviation is raised in the usual range).

The effect of kin feed standard deviation on unit energy consumption is complex and is likely to vary from kiln to kiln. For a precalciner kiln each one per cent increase in the standard deviation of the kiln feed lime saturation factor you might expect an increase in the energy consumption by 150kJ/kg clinker. However, this relationship is unlikely to be linear and will vary around that figure from kiln to kiln.

NO_x control approaches applicable to the cement industry may be grouped in three categories: process modifications, combustion control, and NO_x reduction controls. One process modification that can be quantified is the CemStar process, which is a small addition of steel slag to the raw kiln feed. Can you provide more information on CemStar?

TXI have published various papers on their patented Cemstar process. The addition of steel slag or air cooled blast furnace slag boosts the output of the kiln with virtually no increase if fuel consumption as the slags have already effectively been clinkered and contains trace elements that mineralise the combination of the kiln feed into the clinker in the kiln. More product with the same amount of fuel burnt inevitably dilutes the NO_x emissions per tonne of clinker produced. The mineralising effect may also lead to lower burning zone temperature, and less thermal NO_x formation in the flame. The real crux of the technology is adjusting the chemistry of the kiln feed to maintain the desired clinker mineralogy when the slags are being added t the kiln inlet. With steel slag the iron oxide input is increased and has to be compensated by increasing the alumina and silca modulii. With air cooled blast furnace slag the material is deficient in lime therefore the LSF of the kiln feed has be increased. This inevitably increases the demand for high CaCO₃ content in limestone and is often the limiting factor.