How to Measure Kiln Performance? by Matthias Mersmann Managing Director & Senior Consultant of aixergee GmbH
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Increasing kiln performance and the optimization of cement plants is everyday subject of thousands of engineers in the cement industry. With a subject that much basic and so widespread one should think that there are clear objectives and procedures to follow along with in order to increase the kiln performance. Surely there is a certain consensus in the community about which different aspects describing the kiln plant’s performance, but actually this is what they are: different…
So what is really describing the performance of a clinker production line?
Classically – and by far the easiest way to deal with it – global absolute numbers like clinker production or heat consumption are taken to describe the plants performance. This approach however is biased by the inherent reserves which might have been implemented in the design of the plant and consequently prevents using this number for cross-plant comparisons. It is like comparing just the horsepower of engines of cars: it does not tell you how fast this car can actually go on a race track.
In a more sophisticated approach, KPIs have been developed which relate the major performance numbers to characteristic process parameters such as the specific energy consumption or the specific exhaust gas volume – KPIs which predominantly are being used by process engineers to be able to rate several plants against each other. However, still these relative numbers do not overcome their dependence from inherent specific challenges which may be unique to a certain plant. Again, cross comparison cannot be done without knowing how to factor-in plant specific boundary conditions such as raw material moisture or types of alternative fuels used. This is comparable to rating the potential of a race car by taking into account it’s engine power, its weight, suspension and break performance. But it dos not tell you how fast your team is driving this car on the racetrack.
So how can one judge the performance of the operators and the plant without falsifying the assessment with hidden reserves or burden it with undue assumptions? How can we judge to what extend the operating team is really getting most out of their kiln plant? To take up the analogy of the race car: how can we find out to what extend the team (driver and mechanics) is squeezing out the maximum out of the potential of the car?
The indicator to look at for this purpose is the oxygen content in the kiln inlet chamber and after calciner. The lower the oxygen level after the combustion is, the more oxygen from the combustion air is used for capacity-delivering combustion. The lower the oxygen content, the lower the specific exhaust gas volume flow, the lower the required fan power per kg clinker produced. Of course the operatable limit of the oxygen content is in itself also dependent of the fuel used (some lumpy secondary fuels need a higher oxygen-“safety margin”) but this in itself tells, that the combustion process is not optimally driven. To explain this consider the following situation: A precalciner plant is fired with lumpy secondary fuels in the calciner. The fuel is injected at a position and in a way which does not lead to sufficient suspension of the fuel particles and these particles drop out and finally fall into the kiln inlet chamber. Nobody will recognize this process nuisance because one cannot see that, nor hear that happening. The dropped out fuel will however start generating problems in the inlet chamber or in the inlet section of the kiln: coatings and reducing atmosphere are signs of mis-functioning fuel suspension. The typical reaction is to increase the ID-fan and provide more oxygen in the kiln inlet. Plants usually know their specific threshold value under which the kiln starts making trouble.
Now this knowledge leads to several optimization options:
- Reducing the oxygen as much as possible greatly saves energy and allows “filling up the saved gas volume” through additional production. Note that with every percentage of oxygen reduced, about 5 times that amount of nitrogen is reduced for the fan and the filter to work on.
- If the lower threshold value for oxygen is approached, stabilization of the pneumatic transport of meal and fuels can unlock remarkable further potential. Controlling the flow of meal and fuel keeps the inlet shelf clean and helps reducing ring and ball formation in the rotary kiln.
The uncontrolled drop-out of fuel and meal can be minimized or even avoided completely by finding the best injection point and -method. Sometimes even small changes in position and injection impulse can make a huge contribution. If all those potentials are effectively raised, an oxygen level of below 1,5 % in the kiln inlet chamber should indicate a good performance and total control of the AF-fired kiln plants. Modern CFD-tools can provide a look inside the kiln plant which is not possible with conventional measurements. According to aixergee’s experience more than 50% of the cement kilns suffer from drop-out material in the riser duct. Often, this drop out can be prevented by slight modifications, sometimes only by adaptation of the brick lining. With this, the treshold value of minimum oxygen content can be lowered and the kiln plant can increase its performance.