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Reliable Kiln Inlet Gas Analysis: Part 2

World Cement,

Read part 1 here.

Hot/wet gas analysis solution

The best gas measurement solution at the kiln inlet consists of a heated gas sampling system, specially designed for harsh sampling conditions. The sample system is then coupled with the multi-component gas, hot/wet NDIR analyser.

The hot/wet system extracts a sample gas stream close to the centre of the kiln inlet through a water cooled/high temperature probe, a heated dust filter, heated gas sampling tubes and a heated gas pump. The gas sample is kept above 200 °C from the tip of the water cooled probe to the gas analyser. Here, the sample gas is introduced without any additional gas cleaning or conditioning steps. Inside the gas analyser, the sample passes into the measurement cell also heated to 200 °C. By avoiding any cold spot along the sample stream, one can ensure that the sample gas temperature is kept well above the water and acid dew points at all times. Therefore, any condensation of corrosive gas constituents, as well as any buildup of contaminations along the sample system or inside of the analyser, is eliminated.

The high content of solids and chloride/sulfur-containing compounds at the kiln inlet encourages the formation of deposits at the tip of the water-cooled probe. This can easily cause clogging of the sample probe if precautions are not taken. Clogging can be effectively avoided by the optimised geometry of the probe’s inlet opening and the regular back-flush purging of the probe with a high pressure shock blower. The probe tube is made of high temperature steel, which gives it a high flexural resistance. Additionally, the opening for the gas intake is mounted at a defined angle with respect to the gas stream, thus not affecting the composition of the gas/particle mixture to be analysed.

As a result, the particle load of the gas entering the probe tube is reduced. The extracted gas is further filtered by flowing through the heated dust filter installed at the probe head.

The filter, as well as the probe tube, can be back-flushed by pressurised air (automatically or manually, when needed) by means of a shock blower. A probe with a patented mechanism, allowing for the probe to be rotated +/- 90° and 10 cm back and forth, helps to remove raw material and other solids that may accumulate on the outside of the probe tube. Buildup of this raw material can prevent the probe from being drawn back through the opening in the kiln wall.

Additionally, the probe must be able to be retracted at any time in order to avoid damage caused by problems with the cooling system, as well as for the purpose of maintenance. Therefore, the probe should be equipped with an electro-mechanical drive. Furthermore, a backup system based on pressurised air allows for retraction of the probe tube even under the loss of electrical power.

The complete sampling system is connected to a control unit that allows for the automated operation of the device as well as for a manual operation via a touch panel.

The hot, multi-component gas analyser utilises a non-dispersive IR process photometer and is able to simultaneously measure up to six IR active gas components (e.g. CO2, CO, NO, HCl and SO2). Oxygen is measured by a zirconium oxide sensor. The individual measuring channels can cover measurement ranges from the ppm up to vol% levels.

Field installation

These hot/wet systems have been installed with great success in many kiln inlet applications, using both traditional and alternative fuels. An example of this is illustrated below.

Mounting details

Application details

  • Mounting location: kiln inlet.
  • Gas temperature: 1000 °C (2290 °F).
  • Solids: up to 2000 g/Nm3.
  • Measured gas components: O2, CO, NO, SO2, HCl, H2O.
  • Measuring range(s):
    • O2: 0 – 10; 0 – 21 Vol-%.
    • CO: 0 – 3000 mg/Nm3.
    • NO: 0 – 3500 mg/Nm3.
    • SO2: 0 – 40 000 mg/Nm3.
    • HCl: 0 – 500; 0 – 5000 mg/Nm3.
    • H2O: 0 – 10%.

Description of the mounting location

  • Kiln production: 2500 tpd.
  • Heat consumption: <3.5 MJ/kg clinker.
  • Kiln type: 4-stage preheater kiln.
  • Kiln dimensions: 65 m (length); 4.3 m (dia.).
  • Alternative fuels: >40%.

Here, the probe is mounted in the upper quarter of the kiln, opposite to the kiln’s turning direction. The sampling probe has a length of 3.5 m and is inserted through a protection tube to the inside of the kiln inlet made of refractory material. The penetration depth of the probe into the kiln is approximately 40 cm. The gas analyser system is mounted in a shelter at a distance of approximately 10 m.

Maintenance intervals

In general, the hot/wet system is a low maintenance solution due to implemented automatic maintenance cycles involving both the sampling probe and the gas analysers. Nevertheless, in order to achieve high availability (>95%) and reliability, plant maintenance tasks are required. These tasks involve both the sampling probe and the gas analyser.

Sampling probe maintenance

  • Frequency: once a week.
  • Required time: 5 – 10 minutes.
  • Required equipment: protection gloves, compressed air.

O2 calibration

This operation refers to both the zero and the span adjustment of the ZrO2 probe or oxygen.

  • Frequency: monthly.
  • Required time: 20 – 30 minutes.
  • Required equipment:
    • Zero adjustment: maximum 2% O2 gas cylinder equipped with pressure regulator with 3-bar pressure outlet min.
    • Span adjustment: instrument air, already connected to the instrument.

Measuring example at the kiln inlet measurement

O2 and CO

Incomplete combustion is typically indicated by excessive CO levels at the kiln inlet. The trend shows the inverse exponential correlation between O2 and CO normally observed at the kiln inlet. By studying the correlation function and by extrapolating it to a linear trend, the process engineer can figure out which O2 level should be used as the O2set point in order to obtain an optimum combustion. In this example, the set point for this plant would be 2.5%. This means that all the plant equipment having an influence on this parameter will automatically function in order to keep the O2 level at the kiln inlet to 2.5%.


Hot/wet gas sampling and analysis is an ideal solution for gas measurement at the kiln inlet of cement plants.

Automatic cleaning of the sampling probe with back flushing of the filter probe, aided by a shock blow for complete removal of kiln inlet powder from the system, allows for low maintenance demands by plant personnel. Thanks to the hot/wet sampling system, the unit provides a more reliable measurement of typical combustion elements, such as O2, CO and NO, thereby allowing for a more accurate combustion control. It also allows for a more effective use of petcoke and alternative fuels by providing insights on SO2 and HCl at the kiln inlet, helping kiln operators to better control the formation of buildup in the preheater tower.

Written by Dan Kietzer, SICK Process Automation, USA, and Siegfried Andraess, SICK MAIHAK GmbH, Germany. This article appears in the April 2014 issue of World Cement. Subscribers can view the full issue by logging in.

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