Air is sucked from the boiler house through the forced draft fan and enters the

Recovery boiler at three or four levels. The portion of the air going to the primary

And secondary air ports is pre-heated with steam. The oxygen provided to the furnace

With primary and secondary air creates a reducing environment in the lowest

Section of the furnace, which is necessary to provoke the formation of sodium sulfide.

The oxygen supplied with tertiary air completes the oxidation of gaseous

Reaction products. The hot flue gas then enters the superheater section after passing

The bull nose, which protects the superheaters from the radiation heat of the

Recovery

Hearth. As the flue gas flows through superheaters, boiler bank and economizers,

its temperature is continuously falling to about 180 °C. After the superheaters,

Heat exchanger surfaces are located only in drafts with downward flow in order to

Minimize disadvantageous ash caking. After leaving the boiler, the flue gas still

Carries a considerable dust load. An electrostatic precipitator ensures dust separation

Before the induced draft fan blows the flue gas into the stack.

Ash continuously settles on the heat exchanger surfaces and so reduces the

Heat transfer. The most common means of keeping the surfaces clean is by periodical

sootblowing – that is, cleaning with steam of 20–30 bar pressure.

Feed water enters the boiler at the economizer, where it is heated countercurrently

By flue gas up to a temperature close to the boiling point. It enters the boiler

Drum and flows by gravity into downcomers supplying the furnace membrane

Walls and the boiler bank. Note that most of the evaporation of water takes place

in the furnace walls, and only 10–20% in the boiler bank. As water turns into

steam, the density of the mixture is reduced and the water/steam mixture is

Pushed back into the steam drum, where the two phases are separated. The saturated

Steam from the drum enters the superheaters, where it is finally heated to a

temperature of 480–500 °C at a pressure of 70–100 bar. The temperature of the

Superheated steam leaving the boiler is controlled by attemperation with water

Before final superheating. The high-pressure steam proceeds to a steam turbine

For the generation of electrical power and process steam at medium- and low-pressure

Levels. Excess steam not needed in the process continues to the condensing

Part of the turbine.

Material Balance

A summary of a simplified calculation of smelt and flue gas constituents from

Black liquor solids is provided in Tab. 9.3. An analysis of the black liquor sampled

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