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- 3 EVERYTHING YOU NEED TO KNOW ABOUT KILN OPERATING AND CONTROL METHODS
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EVERYTHING YOU NEED TO KNOW ABOUT KILN OPERATING AND CONTROL METHODS
Methods of kiln control vary from plant to plant, and even between different operators, because each operator is apt to have his own ideas as to how to proceed when confronted with any given situation. Kiln control, however, must be a continuous around-the-clock matter, hence it is necessary for all operators on all shifts to operate the kiln in the same manner. This in turn means that all operators should be trained in the same principles of kiln operation.
There is nothing more destructive to operating stability than the
changing of controller setpoints en masse during shift changes. Unless an emergency exists or ati obvious change must be made, it is not possible for an operator to assess the need for controller-setting changes in the first few minutes after coming on duty. A difference in settings from the
previous day is not necessarily an indication to change these settings back to where they were 24 h before. An operator should allow 20 min at the onset of the shift to first observe the process before deciding that a change is indeed warranted.
There are three common techniques for burning clinker in a rotary kiln:
a)Maintain a constant kiln speed, and vary the fuel rate to counteract temperature changes in the burning zone.
B)Maintain a constant fuel rate, and vary the kiln speed to hold the burning-zone temperature at the desired Level.
C) Vary the kiln speed, the fuel rate, or both, to maintain the desired burning-zone temperature.
These techniques have one error in common; they show concern only for the burning-zone temperature. Unfortunately, many kiln operators think that this is good enough, reasoning that, as long as good clinker is produced, what more is necessary? The fallacy of !his reasoning lies in the fact that ideal stable kiln conditions can be obtained faster and more economically when equal consideration is given to all zones in the kiln and not the burning zone alone. Drying and calcining of the feed must be considered before one can consider making clinker. The process of clinker burning, and therefore the process of rotary-kiln control, starts not at the place where the feed enters the burning zone, but at the point where the feed enters the kiln. In preheater and precalciner kilns this applies to the point where the feed is given to the top stage of the preheater cyclones.
The technique described in this book can be summarized as follows:
VARY THE KILN SPEED, THE FUEL RATE, AND ThE. INDUCED DRAFT-FAN SPEED IN ANY COMBINATION TO MAINTAIN THE PROPER BURNING-ZONE TEMPERATURE AND A CONSTANT KILN BACK-END TEMPERATURE FOR A GIVEN FEED RATE.
This is sometimes referred to as “burning a kiln from the rear,” the most reliable technique that fulfills the four fundamental rules of clinker burning which, in order of priority, are:
- Protection of the equipment and personnel at all times
- Production of well-burned clinker
- Continuous stable kiln Operation
- Maximum production with maximum fuel efficiency.
Protection of Equipment and Safe operation
must have first priority at all times; the operator should never allow himself to bypass this rule. Red grates in the cooler, red spots on the kiln shell, or overheated chains can do much more damage than the production of a few barrels of bad clinker, if bringing the equipment trouble under control would happen to result in the production of a small amount of poor clinker. It is usually during times of unusual and severe upset of the operating conditions that an operator will most likely forget this fun damental rule. This is because of the attention understandably being given in such instances to leading the kiln back to normal operating conditions. But, it is precisely during these “tight” moments that the shift in attention toward the safety of the equipment and fellow workers must be made. The majority of accidents happen when the kiln is in an unusual operating condition such as during kiln starts, stops, and upsets. Safety around a rotary kiln being an important and integral part of kiln operation is covered in greater details Later.
A well-burned clinker is a clinker that is neither underburned nor overburned, has been properly cooled, and possesses the correct free-lime content and the desired liter weight.
Continuous Stable Kiln Operation.
Continuous operation should always have priority over maximum More can be gained by having emphasis on continuous operation rather than by pushing the kiln to peak production at the expense of periodical kiln upsets. Stable kiln operation is the key to long refractory life, high fuel efficiency, and uniform quality clinker.
The term “stable kiln condition” means the condition in which only very small changes, or no changes at all, have to be made to the control variables to hold the kiln in a state of equilibrium. It is the kiln operator’s duty to obtain stable conditions and not be satisfied until this goal has been achieved. The capability of a kiln operator is measured not so much by the length of time a stable kiln condition is maintained but by how well kiln upsets are handled, and how much skill is used in leading the kiln back to stable conditions after an upset.
Stable operation is also identified by the recording charts, most note worthy being the kiln speed, and burning-zone and back-end temperatures (all deviating very little over a long period of time). Upset conditions are marked by large changes in kiln speed and, when these speed changes occur in frequent intervals, e.g., every 2 h, the kiln is in a cycling condition. Working a kiln out of a cycle, i.e., breaking a cycle, is the most challenging task a kiln operator faces (see Chapter 25). This is the time when kiln burning becomes more of an art than a routine. Such kiln upsets can happen not only on dry- and wet-process kilns but, although less severe and less frequent, also on preheater and precalciner kilns. Even kilns with sophisticated computer and automatic control are not immune to these cycling conditions.
Maximum Production with Maximum Fuel efficiency
Only after the three previous requirements have been met can one try to raise producstion and start to concentrate on the details of fuel efficiency. Any move made in this direction should be done very slowly in order to avoid upsetting the prevailing stable kiln condition. Large kilns usually operate better in the upper range of their rated production rate, hence production increases can help to stabilize the operation if the kiln has been operating somewhat below its rated capacity.
Controlling a kiln in the conventional manner, as was done on most kilns twenty years ago, consisted of observing the operation and making a manual adjustment on the control panel based upon that observation. One of the many possible rotary kiln control functions is looking into the burning zone and noticing a change in temperature which requires an adjustment in the fuel rate. Experience tells the kiln operator that he can neither turn the fuel valve wide open nor fully closed, but must base the required fuel change on how much the temperature has changed. A small variation in temperature requires only a small adjustment, while a large temperature change necessitates a correspondingly large modification in the amount of fuel admitted into the kiln.
The next factor to take into consideration is the limit to which the burning-zone temperature can drop or increase without requiring more drastic action than adjustment to the fuel rate itself. Too much fuel addition, although justified by the existing burning-zone temperature, could lead later on to dangerous overheating or incomplete combustion. Likewise, cutting the fuel rate back future than an established limit might lead the kiln into a severe upset condition. Before any adjustment is made, the operator must consider the limit to which the burning-zone temperature and the fuel rate can be permitted to go without leading the kiln into an upset.
After an adjustment has been made to the fuel rate, the next question is how long the fuel rate can be left at the new setting until it has to be adjusted again. A certain time will transpire from the moment the fuel rate is adjusted until the burning zone reacts to this change and returns to its target temperature again. However, when the temperature has reacted and starts to tum in the other direction, at a given point another fuel-rate adjustment must be made so that the temperature does not overshoct its target. In other words, the temperature should not move from one extreme to another but should return and level off at the target. This is the essence of manual process control and the thought process required of the operator.
By now it shoold be obvious that the skill and experience of the operator play an all-important part in this type of control. Hi’ judgment and decisions are the main factors governing the degree of stability the kiln operation will obtain. If one considers the fact that an operator often has to exercise control over not one but sometimes two and more kilns at the same time and that each kiln has a multitude of controls such as described above, it is understandable that many of these fine aspects of control can be overlooked. Every kiln operator, regardless of how long he has been on the job, knows that an error in judgment can sometimes be made or a wrong decision can be reached. Often a decision could not be made strictly according “to the book” at U1e time it had to be made because of some uncertainty in the process not apparent to the operator. The only thing known at the time could have been that some adjustment was necessary. This adjustment may be shown on the strip-chart recording a few hours later to be completely contrary to what should have been done. Nobody feels worse than the operator himself when such evidence shows up on the strip chart. Thus, it is easy to say afterwards that something different should have been done, but it is far more difficult to make the right decision at the time when some uncertainty existed and in an immediate decision had to be made.
Manual control undoubtedly demands a heavy work load from the
operator because most of these manual adjustments .are repetitious in nature. Fortunately, the cement industry has experienced a phenomenal revolution in automatic process control in the past 20 years. This has made the manually controlled kilns more of an exception and it is expected tho a few years hence, they will become a rarity.
A process as complicated as the operation of a rotary kiln necessitates a multitude of instruments and controllers. It would be impossible to control a kiln without the help of instruments. Not too long ago, kiln control was limited to measuring temperatures, pressures, and flow rates, transmitting the information to a recorder on the ftring floor, then leaving actual control of the variables in the hands of the kiln operator. It was not unusual to observe an operator going from burner hood porthole to control panel and back again, making almost continuous adjustments to the controls. Understandably, operators became uneasy when the reactions didn’t come out exactly as expected, as individual human error played a large part in this method of operating.
Today’s kilns are not only equipped with more and better controllers, but the computer is also slowly taking over the controls of entire kiln systems. Automation has become the standard in the cement industry, performing the work the kiln operator had to do himself in the past. Concurrent with this revolution, there also appeared new terminologies, better known as computer jargon that the kiln operator must become familiar wi!h. During the early stage, when automation was still in its infancy, there were numerous workers that showed apprehension and sometimes outright objection to these developments. Advances made in control technology over the past few years have mostly eliminated this initial apprehension. Computer systems have been “humanized,” made more simple to understand, and control capabilities have become so reliable and advanced that even the most experienced kiln operators accept them as a blessing. It has definite advantages over pacing the burner floor for eight hours especially during cold winter and hot summer days. Control-room operators also have an ally in the computer engineer who can be of assistance when operating problems have to be resolved.
It would defeat the purpose of this book to discuss in details the
numerous types of computer systems that are being used today in the cement industry. Little benefit could be derived by discussing the control concepts of any particular kiln, because each kiln has, to some extent, its own design and its own automatic control idiosyncracy.