There are two potential causes that I would investigate:
1. a change or variability in the MgO content of your kiln feed, and
2. a change or rise in the Mn2O3 content of your kiln feed.
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Re: Burning
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NOx control approaches applicable to the cement industry may be grouped in three categories: process modifications, combustion control, and NOx 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?
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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 NOx emissions per tonne of clinker produced. The mineralising effect may also lead to lower burning zone temperature, and less thermal NOx 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.
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Where can I find results and data about sulphur or alkali by-pass system. How does one determine the percentage of bypass considering the feed material and the fuels?