All boilers used for both process and power generation plants have to maintain steam parameters within a particular range. The pressure and temperature of steam at superheater and reheater outlets are to be maintained to achieve an optimum plant efficiency. In large capacity boilers, injection type desuperheating is more popular. Reheaters in power boilers can also be technically classified as superheaters, however the type of control adopted is mainly varying the heat available from the flue gas side by bypassing the flue gas to the superheater side and the burner tilt to vary the radiant heat availability.
Superheaters in large capacity boilers are of both convective and radiant type. The convective type superheaters are placed in the second pass of the boiler, and these are also called low temperature super heaters. Generally about a 40 to 60 degree centigrade temperature pick up takes place on the steam side in this superheater. The steam from this moves to a radiant type superheater called the panel or platen superheater, and about a 70 to 100 degree centigrade pickup take place there. This is followed by the final superheater, and the remaining required temperature is picked up here. One has to understand that the designer can vary the pickup pattern depending upon his way of practice or requirement. But in all the methods adopted, the final temperature must be within a narrow range generally plus / minus five degrees centigrade. The superheaters are so surfaced that, for the design fuel of the boiler, there will not be any desuperheating requirement.
Three major systems are adopted the world over: the injection type, the flue gas flow diverting type, and the burner tilt. The last two are adopted for reheater steam temperature control. The first one is adopted for superheater temperature control. It also used in reheaters during emergencies when there is a need to quench the temperature even after reaching the maximum of control on the normal system adopted. The material selection for the superheater before the injection will be greatly influenced by the quantity of injection.
Injection type desuperheating
In almost all large capacity boilers there are two injection points provided. The first desuperheater is placed between the low temperature superheater and the platen/panel superheater. The second one is placed between the platen/panel and the final superheater. The control is done mostly by the desuperheater, and the second one is more for fine tuning, as the response time for the first is long. In both the cases the feed water is tapped at an appropriate point and injected as fine spray through nozzles in the steam pipe of the desuperheater. The mechanical design aspect includes liners so that the temperature differential is not directly felt on the pressure part side of the desuperheater.
Flue gas diverting design
Here the flue gas path in the second pass of the boiler is divided in two. One houses the superheater and part of economizer; the other houses the reheater and part of the economizer. The superheater here is the low temperature (also called primary) superheater. The reheater here is called the horizontal reheater and forms a considerable portion of the total reheater surface. There are dampers provided in the bottom of each pass which can vary the flow in the section including a bypass flow area in between the two flow paths. By varying the flue gas flow in the reheater section, the pickup can be varied to control the final reheat steam temperature. Here also the superheater temperature control is done by injection, however there will be an effect on the heat transferred in the superheater section.
Burner tilt method
This method of reheater steam temperature control is applicable to only tilting tangential firing system boilers. Here the burners arranged in the corner are tilted up or lowered down so that the flame envelope in the furnace can move up or down causing a good variation in the amount of radiation heat the reheater receives. Many designers provide an emergency injection system for reheater temperature control. Feed water is injected in the cold reheat steam side to control the final temperature.
There are other methods like gas recirculation, non-contact type desuperheating, etc., which are adopted in cases of combination fuel firing, lower capacity units, etc.