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NOx abatement by staged combustion
in the calciner – new findings
Dr.-Ing. N. Bodendiek, Düsseldorf, Dr.-Ing. V. Hoenig,
Forschungsinstitut der Zementindustrie, Düsseldorf, Germany
The Research Institute of the Cement Industry in Düsseldorf, Germany, has examined the staged combustion in the calciners of six rotary kiln plants with the aim of determining the options and limits for NOx abatement in precalciner plants. Systematic changes were made to the operational settings to determine the effects on the formation and decomposition of NO in the calciner. The investigations have shown that the optimum excess air ratio in the reducing zone lies significantly below 0.8, and is therefore lower than previously assumed. The reason for this is thought to be the catalytic conversion of the HCN intermediate product to NH3 by limestone. In the trials the staged introduction of air and meal proved to be effective operational means of optimizing the staged combustion and hence the NOx abatement. However, both measures can be limited by the fact that extreme modes of operation cause increased coating formation, damage to the refractories, formation of balls of kiln feed and increased CO emissions. These limits have to be determined individually for each kiln plant. Other important influencing factors for NO decomposition in the calciner are the reactivity of the fuel, the gas residence time in the reducing zone, the NO concentration at the calciner inlet and the intermixing of the fuel when it is fed into the reducing zone. The reactions that govern NO decomposition were determined by measuring the NO concentration inside the calciner. These are the reactions of NO with NHi and hydrocarbon radicals. The findings obtained during the trials can be used for discovering new approaches for the structural design of calciners and for optimizing their modes of operation.
Summary CEMENT INTERNATIONAL 01/2006 pp 64-75
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