A respected colleague of mine once stated a rather solid control principle which I still support to this day, however conditionally so when it comes to combustion control with steam boilers. What he said was this: the most effective control is achieved with a direct (as opposed to an indirect) approach. Now let us look at and explain this principle in light of boiler control, specifically with coal burning boilers.
Of specific concern to users of steam boilers is the steam pressure, as it is an indicator of the difference between the steam demand and the current rate of steam generation. A dropping steam pressure, for example, indicates that steam demand exceeds steam production, and this is the signal used by the control system to regulate the combustion response to meet the increased steam demand. It does so by feeding more fuel to the furnace (stoker speed increases), which in turn requires more combustion air (FD fan speed increases), and also increase of the ID fan speed to ensure the furnace pressure stays below atmospheric pressure at all times. Yes, positively no smoke allowed inside the boiler house! It is toxic stuff.
Thus the control system has to perform a delicate balancing act: feeding more coal to produce more steam; supplying more combustion air to meet the oxygen demand of increased fuel feed; and increasing suction by the ID fan to keep the furnace pressure in check.
The big question for many years now boils down to this: upon a change in steam pressure (steam demand), which one of the combustion components should be the first to respond? If we assume a drop in steam pressure, then according to my colleague’s principle of direct control, it means that the stoker should be the first device to respond (known as stoker leading control), seeing that more steam is produced by feeding more fuel. The control sequence in this instance is as follows: when the steam pressure drops, the stoker speeds up (to fuel the fire to produce more steam); the FD fan speeds up (to increase combustion air to the fire, but this also pressurizes the furnace); the ID fan speeds up (to balance the increased furnace pressure).
But there are a number of negatives associated with the “direct control” approach in this instance:
- The ID fan, because of its greater inertia, may be slow to respond to changes in furnace pressure and furnace pressure may turn positive, even just momentarily, and cause some smoke discharge into the boiler house. To cater for the sluggishness of the ID fan the rate of response of the control system to changes in steam pressure may have to be set rather slow so as to avoid the furnace pressure from fluctuating too widely around the set point
- Controlling the furnace pressure by means of the ID fan wastes some electricity because of the inertia of the heavy fan impellor, which is continually adjusting its speed to maintain furnace pressure.
- The big fan speeding up and down all the time in its quest to control the furnace pressure is clearly audible in most instances and can become an annoying aspect of boiler operation.
- The biggest challenge with stoker leading control however is when it is used with a twin flue boiler equipped with a single ID fan, two stokers and two FD fans. With this configuration and for stoker leading control to function properly both stokers are required to operate at the same speed and to respond to changes in steam pressure in the same way. Consequently both FD fans have to operate at the same speed too to provide equal quantities of combustion air to the fire on each stoker (assuming FD fan curves are identical). The ID fan controls the furnace pressure from any one of the two furnaces, or at least from the one with the highest pressure. In theory thus the air-fuel ratio of combustion can be controlled fairly accurately in both furnaces.
- In the practical situation however we often encounter substantial differences in furnace pressure, even with FD fans of both flues operating at the same speeds and delivering equal quantities of combustion air. For some reason or the other the resistance to combustion air and flue gas flow per furnace is seldom identical under (perceived) identical combustion conditions. During a recent visit to a user of a steam boiler I found the left hand flue operating at -20 Pa furnace pressure, and the right hand flue at -56 Pa with FD fans and stokers operating at similar speeds, left and right. If it wasn’t for the high volatile coal being used they may have experienced problems with the right hand fire losing ignition because of cooling of the ignition arch. Ingress (tramp) air on the right hand side will also be higher, eroding combustion efficiency without the user knowing it.
An indirect control approach eliminates some of the drawbacks of the stoker leading concept. It is known as an ID fan leading approach. In this instance the ID fan is the first device to respond to changes in steam pressure. Although it does not feed fuel directly into the furnace, it initiates a sequence of events leading up to an increased fuel feed rate. The control sequence in this instance is as follows: steam pressure drops; ID fan speeds up (and furnace pressure drops); FD fans speed up (to balance furnace pressure and to increase combustion air flow); stokers speed up (to fuel the fire for increased steam production). The positives associated with this “indirect control” approach are as follows:
- Furnace pressure is controlled by the FD fan. Responses are generally faster, less audible and more energy efficient.
- With twin flue boilers furnace pressures are individually controlled. Although this implies that each set of FD fans and stokers will operate at a different speeds, it still offers the opportunity to control each furnace as an individual single flue boiler, except that they share a common ID fan, the speed of which is primarily determined by the steam demand.
Let us however not be fooled into thinking that combustion control is a simple matter, and so much the more with a twin flue boiler which fires pea coal on chain grate stokers, and a control system which is efficiency based. Although I personally favour the ID fan leading type of control, it is not a perfect control approach and presents its fair share of challenges which may be addressed in future Boiler Bits.
This post was compiled by René le Roux for Le Roux Combustion, all rights reserved. Do you want to know more about efficiency of combustion or combustion optimization? Please contact us for your professional boiler automation, steam system efficiency and coal characterization needs.
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