1156 posts
Raw materials Question 11
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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1156 posts
Re: Raw materials
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.
1156 posts
1156 posts
Re: Raw materials
It is possible to use Ca(OH)2 as a corrective source of CaO. I do not believe you would want to use CO2 as this is one of the exhaust products from the cement manufacturing process rather than making a contribution to the final product. I have not heard of the use of molasses in cement manufacturing and can envisage two problems with this material: (i) it is likely to be sticky and present handling problems, and (ii) there will be some organic material in the molasses. This organic material will smoulder and burn in the upper stages of the preheater, or in the chain systems of long kilns and will give rise to CO and hydrocarbon emissions in the exhaust gases. To add this material safely to a cement kiln would require some means to add the molasses to the hotter sections of the process such as the precalciner or the kiln inlet of a preheater kiln or some mid-kiln firing system of a long kiln.