Measurement of flue gases in rotary kilns of cement plants

The Germany industry produces 80 million t of carbon dioxide each year, 37 % of this alone by cement and lime producers according to the European Oeko-Institut e.V. The aim of reducing these emissions in the long term puts these manufacturers under increasing pressure. Despite stricter emission laws, production has to be as efficient as possible. A common method to achieve this efficiency is the measurement of the flue gases. Thus, not only the amount of CO2 can be determined, but also the concentration of residual oxygen and unburned molecules. Continuous measurements have been so far possible only to a limited extent because of the probes being unable to withstand high temperatures and high dust concentrations. The probes even needed to be manually cleaned after each measurement.

Hot cement raw meal with temperatures of up to 950 °C, high speeds and high temperatures of the dust-laden flue gas of up to 1200 °C: The difficult conditions found directly at the measuring point at the furnace inlet of rotary kilns makes capturing of gas values difficult. Spenner Cement Co. near ­Dortmund has to deal with these difficult conditions: taking control measurement in the oven was always associated with many precautionary measures and much effort. “The four meter long probe had to be manually pushed into the furnace by two workers during the measuring process and had to be constantly observed”, explained Franz Haselhorst, Deputy Head of the Electrical Maintenance Department at the company. The probe could remain in the oven half an hour at most, and then both the probe and the inlet chamber were so heavily caked with hot meal deposits that the probe had to be taken out again and cleaned with compressed air. Moreover, the wear due to the high speeds of the dust-laden flue gas is usually very high. Spenner Cement Co. previously had a standard probe in use, which was equipped with a water cooling system and a downstream air cooler. The associated control had to be constantly monitored. “We actually measured only to determine the cause when the gas concentration in the furnace was not optimal,” said Haselhorst.


Continuous measurement

directly in the kiln

To avoid time and labor intensive processes in the future and particularly to identify the ­relevant process values, the Spenner Cement Co. has switched to the CEMTEC probe (Fig. 1), specifically developed for continuous measurements. It is based on a double walled and water cooled gas sampling probe and has been further developed by ENOTEC to suit the requirements of cement manufacturers. The flue gas is constantly sucked through the tip of the probe into the filter unit directly from the inlet chamber of the furnace and is then transported via a heated line into the heated sample chamber of the analyzer. The oxygen content and the amount of unburned molecules (COe) in the kiln are permanently determined here. These include carbon monoxide (CO), methane (CH4), hydrogen (H2), nitrogen oxide (NO) and sulfur dioxide (SO2). The sensors located in CEMTEC analyzers are positioned so that the prevailing gas concentrations can accurately be determined in real time. Thanks to the ­in-situ measurement, the smallest changes can be quickly detected and thus the consumption of energy can be precisely controlled. It is necessary to regulate to the smallest possible oxygen value in order to keep emissions low and at the same time ensure that the system runs efficiently and safely. Hereby a highly efficient combustion and also a higher cement output are guaranteed.

“By continuously measuring the gas concentration, we can always determine if the combustion process is optimized, and if not, adjust the airflow accordingly,” said Haselhorst. “In addition, the service is minimal, which saves us much time, effort and costs, and prevents disruption of the furnace operation.” The large combustion plant at Spenner Cement is in operation around the clock. Pulverized lignite and secondary fuels are used as fuel. Despite these conditions, the ENOTEC probe needs only be checked every nine months by ENOTEC employees.

Rotary device prevents the

probe from being “cemented”

in place

The CEMTEC system features a patented swivel drive, through which the probe shaft rotates continuously in a radial direction (Fig. 2). This prevents the probe from cementing stuck within the combustion chamber and prevents the probe tube of becoming thermally deformed. If for instance a power failure or coolant leakage occurs, the 200 kg probe is automatically driven out of the oven on rails by a pneumatic traversing device. The required compressed air for this is supplied by a reserve tank.

In addition, the system cleans itself every three minutes (Fig. 3). “In the cooling tube we have installed a large surface dust filter with a pneumatic impact plate which moves out at certain intervals and removes even the toughest of deposits,” explained Fred Gumprecht, Managing Director of ENOTEC. In addition, a patented turning and driving device provides a continuous radial and axial movement of the cooling tube. A control unit, a PLC and a heat exchanger system is included on-site.

By constantly monitoring the oxygen concentration in the kiln inlet chamber, it is possible for Spenner Cement to control the flue gas, which is directed through the calzinator by way of a bypass, to control the combustion air. Using a SNCR process, the nitric oxide levels (NOx) are reduced by injecting ammonia water (NH3) in the flue gases.


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