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 to 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.

I wrote you before. Thanks for response. Our problem is high free lime. Here I am giving you the complete analysis of clinker and kiln feed: (data enclosed). We are using South African and Indonesian coal. What may be the cause of high free lime. Other than that kiln feed and clinker analysis, I want to tell you that we have two kilns and free lime increased on both kilns at the same time.

I presume the analysis should read K₂O and Na₂O in the clinker? If so we can discount the K₂O content as the reason for the high free CaO. The K₂O will preferentially combine with the SO3 in clinker and therefore there will be little surplus to combine in the clinker minerals and inhibit the formation of C₃S. The 90 micron residue of the kiln feed is also low at 12% per cent. However the critical factor is the mineral composition of this residue. If there is a high quartz content in these coarse particles that might be the cause of the problems. If not then it is some factor we haven't identified yet. Possibly the kiln operation or the combustion.The fact that the free CaO has risen simultaneously on both kilns suggests that this is a material or chemistry related problem rather than a process equipment problem. What has changed before the rise in free CaO? Raw material sources? A new level in the quarry? Introduction of a new fuel? You should also check the calibration of the equipment (XRF?) used in the quality control of the kiln feed.