It All Comes Down To The Downcomer

Modern cement plants all use a kiln preheater, which is connected to the tertiary air, the precalciner, the bypass, and the rotary kiln. The hot oven gases which come out of the kiln inlet are subsequently used to heat up and precalcine the raw meal. In addition, fuel, as well as tertiary air, is added to the precalciner section in order to drive out the CO₂ from the limestone before it enters the kiln and is sintered into clinker. Any unwanted by-gases, such as chlorine, are branched off through the bypass.

Just this short description is enough to demonstrate that the gas that exits the preheater and flows into the downcomer, comes from one of the most essential and complex steps of the cement production process. It is not hard to see how controlling this gas flow is essential to several aspects of the cement production:

• The preheating of the raw meal.
• The correct calcination with as little NOx formation as possible.
• The prevention of larger agglomerations of material upstream of the kiln inlet.
• The correct gas flow through the kiln, reducing excess air to a minimum whilst preventing a reduced atmosphere.

Through these different processes, gas is subtracted from the main flow (bypass) or added to the main flow (tertiary air, leakage of the preheater cyclones). It is only possible to measure the total gas flow once this has happened, and that must occur in the downcomer. The downcomer carries all of the gas from this process and is connected to an ID fan. The ID fan operation has a major effect on all of the aforementioned parameters and reaches up to the clinker cooler where the draft into the kiln head, which is caused by this ID fan, has a direct impact on the cooling behaviour of the clinker cooler.

So how is the gas flow in the downcomer controlled? As previously mentioned, the most critical parameter in the kiln process is the O2 content. The atmosphere in the kiln should not be reducing as this will damage the clinker. With this in mind, an O2 measurement upstream of the kiln inlet where the gas flows out is essential. Only in cases where a minimum of excess O2 can be guaranteed will the clinkering process will be safe. Measuring the O2 on the downcomer however has the disadvantage of measuring a large amount of false air which enters via the tertiary air, as well the leakage of the cyclones. The O2 levels in the downcomer can be higher than 10% when actually a 2% margin is to be maintained on the kiln inlet.

In recent years, more and more cement plants have installed sampling systems in the kiln inlet which run a water-cooled lance into the kiln and take a gas sample which is subsequently analysed.

The major advantage of this probe is that it will only measure the O2 that is coming out of the kiln. So other leakages will not influence the measurement, which allows for far better control of the kiln.

However, the shortcomings of any method of extractive measurement like this are the time lag of the measurement itself as well as the fact that it is not totally representative, especially over the short term. If a measurement like this is used to control the ID fan and hence the kiln process, there will be changes and fluctuations in the ID fan control which might not be originally from the gas flow but due to the O2 distribution over the large cross-sectional area of the kiln inlet. This distribution will vary over time so that, over the long-term, an average O2 value which is representative of the stoichiometry in the kiln, can be found. However, short-term fluctuations measured by the O2 sampler will not necessarily reflect a change in gas flow or fuel flow. They may not even reflect a short-term change of the overall stoichiometry in the kiln. Hence the control of the ID fan using just O2 will have clear limitations in reducing process noise.

As a consequence of a control purely based on O2, the ID fan control will get noisier, the gas flow will change more than necessary and bring all of the other process parameters (bypass gas, stoichiometry of the precalciner, neutral zone in the clinker cooler etc.) into a transient mode. The process will start to generate unnecessary process noise and will no longer be as stable as possible.

It is much better to use a direct gas flow measurement in the downcomer as a short-term control signal which is cascaded by the O2 measurement of the lance in the long-term. By using this approach, the gas flow can be stabilised and process noise can be reduced, whilst making sure that the O2 corridor of the kiln exhaust gas is not left. The reason this has not already been carried out over the years is due to the unavailability of a reliable and drift-free gas flow measurement system that can survive in the harsh and dusty conditions of a cement plant.

With the Promecon McON air system utilising the tribo-electric measuring method, a precise flow measurement can be taken directly behind the last cyclone stage and directly at the scene of the gas analysis. Based on an independent expert’s estimation, a more stable operation of the rotary kiln could save up to 5% of fuel energy. The Promecon McON air has a maximum measurement error of 2% in volumetric mass flow, where such a precise instrument provides the necessary energy saving control of the kiln ID fan in order to stabilise the complete pyroprocess.

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