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Minimising Emissions, Maximising Alternative Fuels

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World Cement,

Dr. Stefan Kern, A TEC Production and Services GmbH, details the conversion of the kiln at Lafarge Retznei and shows how an optimised calciner design allowed for 100% alternative fuel usage.

To minimise the environmental impact caused by the cement manufacturing process, central effort is given to the reduction of emissions alongside fuel flexibility, and fuel economics.

Alternative fuel utilisation plays a key role in the reduction of CO2 emissions caused by fossil fuels as well as reducing dependency on fossil fuels in general. The utilisation of solid alternative fuels, however, poses several challenges the clinker formation process, such as through the impact on the combustion process itself, but also on through trace components being introduced, such as chlorine. Due to the nature of solid alternative fuels, the impact on CO emissions can generally be observed by increasing the TSR (thermal substitution rate), which for the main burner often leads to a limitation in their usage or, for the calciner, in sufficiently large systems allow complete burnout of the fuels.

In addition to national regulations, many cement producers have also committed themselves to a reduction of NOx and CO emissions (among other emissions as well).

In the case of the Lafarge plant at Retznei, the aim was to reach NOx levels at the stack of less than 200 mg/Nm3 and CO levels of less than 1000 mg/Nm3. In-line calciners (ILC) are the preferred choice when it comes to the utilisation of solid alternative fuels. Unlike separate-line calciner (SLC) systems, where any heavy particles of the fuel would fall in the tertiary air bend, ILC systems can cope with larger fuel particles and any heavy fraction (if limited in its extent to being a minor component) can fall through the orifice and burn in the kiln directly without affecting the operation.

It is well known that NOx emissions are caused by the clinker burning process. Due to the required sinter zone temperature in the kiln (a minimum of 1450°C, depending on the raw material composition), the window of thermal NOx formation – where nitrogen in the combustion air is oxidised – is entered and a significant amount of NOx emissions are formed in the kiln. The options for treating this kiln-NOx are via primary measures in an in-line calciner, a reduction system like SNCR, a SCR system, or a combination of primary and secondary measures.

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