Boiler steam temperature control

Accurate steam temperature control is necessary for avoiding the over stressing of superheater tubes and turbine front stages and to maintain overall efficiency as high as possible.
Heating the steam further from saturation temperature is called superheating.
Saturated steam from the boiler is passed through superheaters, where the heat energy from combustion gases is added to it to generate superheated steam.
Water side steam temperature control

Desuperheater
Attemperator
Diverting part of the feed water through attemperator for condensing partially saturated boiler steam

Water side steam temperature control

Desuperheater action is just reverse of superheater action.
The temperature and heat content of steam is reduced here unlike the superheater which increases the temperature and heat content of steam.
Desuperheater may be located either between two successive sections of the superheater or at superheater outlet.
Contact type or spray type one are called desuperheaters and the non contact shell type ones are sometimes called attemperators.

A separate line is drawn from the main feed water pipe before it enters economizer.
The water flowing through this separate line is sometimes called injection water
Injection water flow is manipulated variable for controlling the temperature, the process variable.
FCE are installed in this line to regulate the amount of spray water as per demand.
The injected water joins the main steam flow and evaporates.
By absorbing latent heat of steam the injection water cools the steam.
By varying the amount of injection water, the steam temperature control can be made
possible.

diverting part of the steam

Part of steam is diverted with the help of dividing plate with orifice plate and a control valve in the diverted line.
The diverted steam flow is passed through an attemperator, shell and tube heat exchanger, before mixing up with the rest of the main steam.
The diverted steam flow becomes manipulated variable.

diverting part of feed water

Part of the feed water is diverted to an attemperator (a shell and tube heat exchanger) to remove variable amount of the latent heat and control the final steam temperature.
The outlet feed water from the attemperator joins back the main feed water coming out of economizer and before entering the boiler drum.

One of the methods of fire side temperature control is the raising and lowering of the percentage of excess air in order to control steam temperature.
This method tends to improve the overall heat rate of power generation equipment though the thermal efficiency of boiler itself may be lowered.
This method may be used on a boiler that was not specifically designed for temperature control.
Although increasing excess air flow increases boiler stack heat losses, turbine thermal efficiency will be increased thus providing greater economic benefit.

The flow stream of the flue gas passing through the superheater can be split, so that the mass of flue gas in contact with superheater can be varied.
Such a mechanism is called a superheater bypass damper
The required position of damper as a function of load variation is well established for any boiler design.
Therefore it is a common practice to program their position as a function of load.
Controller output varies directly with load and this tends to cancel the inversely varying process gain.

Burners mounted in the furnace corners are arranged so that the flame can be tilted up/down from horizontal.
By this arrangement the temperature of the flue gas entering the superheater can be changed.
Burner flame is aimed at a tangent to an imaginary circle in center of furnace and the burners in four corners tilted at same angle results in a fire ball in centre of the furnace which can be lowered / raised in the furnace by tilting the burners
Lowering fire ball increases furnace heat absorption, which lowers the flue gas temperature as it enters the superheater.
Raising fireball decreases heat absorption, with opposite effect on the temperature of flue gases entering the superheater
Burners in fixed wall of the furnace can be elevated to higher or lower
By varying the ratio of the fuel in the upper row of burners to that of in the lower row of burners, the furnace heat absorption can be modified, thereby changing the temperature of the flue gases entering the superheater.