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Continuous analysis of flue gas in the rotary kilns of cement factories is essential for the quality of the generated clinker, the efficient use of fuel, and protection of the environment from toxic emissions:
- The analysis permits detailed assessment of the combustion processes, and is therefore a prerequisite for optimization of burner control, fuel requirements and product quality.
- Malfunctions can be detected at an early point in time, and prevented using appropriate countermeasures. At the same time, stable control of the kiln prevents the emission of toxic materials, thus supporting environmental protection.
In a rotary cement kiln, gas samples are usually taken from the intake area by means of a system such as the FLK gas sampling probe, and the concentrations of oxygen (O2), carbon monoxide (CO) and nitrogen oxide (NO) measured continuously.
Oxygen (O2) and carbon monoxide (CO)
During cement production, the largest share of production costs results from the amount of fuel used. On the one hand, complete combustion is important for reducing toxic materials in the exhaust gas, on the other hand an excess of oxygen is a waste of resources. Already an oxygen excess of 1 % means an increased energy consumption of 15 kcal per kg of generated clinker.
Measurement of the concentrations of O2 and CO permits the furnace operator to optimize the combustion in the rotary kiln with respect to the quality of the generated clinker, reduction in toxic emissions, and reduced use of fuels.
Nitrogen oxide (NO)
The NO concentration in the rotary kiln largely depends on the flame temperature. A temperature held as constant as possible in the clinkering zone is of great significance for a high quality of the generated clinker. Variations in the clinkering zone temperature result in significant changes in the NO concentration.
The NO analysis is therefore an appropriate means for achieving stable and uniform operation of the kiln. Use of an NO2 converter for measuring nitrogen oxides (NO and NO2) is not recommended since with this analysis the variation is more important than the absolute value of the nitrogen oxide concentration.
Sulfur dioxide (SO2)
Because of the increasing share of alternative fuels, some of which have very high sulfur concentrations, analysis of SO2 in the rotary kiln is becoming increasingly important. High concentrations of SO2 in the gas circuits result in increased corrosion and frequently to undesirable caking of material in the rotary kiln and in the cyclones of the heat exchanger. In addition, a fast rise in the SO2 concentration is an early warning of a combustion fault.
The difficult environmental conditions in rotary kilns place high demands on the sampling systems. Problematical are the high gas temperature up to 1400 °C, the high dust concentration of up to 2000 g/m3 and the high concentrations of alkali, sulfate and chloride in the gas circuits. In addition, the gas sampling probe is subject to high mechanical stress resulting from falling material or the inflowing raw meal.
In particular, high concentrations of sulfur and alkali very frequently result in blockages in the gas paths, necessitating over-proportionally high maintenance of the gas sampling equipment.
The FLK gas sampling probe uses a heat transfer liquid with a boiling point of above 300 °C as the coolant. The temperature of the sampled flue gas is up to 200 °C, and is above its acid dew point. This reliably prevents condensation of the flue gas, which, together with the existing dust, can rapidly result in blockages.