Method for controlling a concentration of slurry in wet flue gas desulfurization apparatus

A method for controlling the concentration of a slurry in an absorption tower in which a flue gas comprising SO.sub.2 is brought into contact with a slurry containing suspended Ca compounds therein to absorb the SO.sub.2 with the compounds. The method comprises withdrawing the slurry from the absorption tower in such a way that the slurry is divided into at least two groups with different concentrations, one group being a slurry having a higher concentration of the Ca compound, the other group being a slurry having a lower concentration of the Ca compound, and regulating rates of the slurries being withdrawn from the absorption tower whereby the concentration of the Ca compound in the slurry being contacted with the flue gas is controlled.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the art of flue gas desulfurization and 
more particularly, to a method for controlling the concentration of a 
slurry in a wet flue gas desulfurization apparatus in which SO.sub.2 in 
the exhaust gas is absorbed in a slurry of suspended solid matters and the 
solid sulfur compounds are recovered as byproducts. 
2. Description of the Prior Art 
At present, the mainstream of flue gas desulfurization systems is a flue 
gas desulfurization apparatus using a so-called wet lime process in which 
the flue gas is desulfurized using CaCO.sub.3 or Ca(OH).sub.2 as a 
absorbent in order to recover sulfur in the form of calcium sulfite or 
calcium sulfate (gypsum). For instance, this process is described in 
detail in Japanese Laid-open Patent Application 57-63117 and other 
numerous publications. 
Reference is now made to FIG. 1 illustrating a currently, industrially, 
widely adopted flue gas desulfurization apparatus using wet lime 
processes. 
Flue gas 1 comprising SO.sub.2 is passed into a body of an absorption tower 
2. At the lower portion of the absorption tower 2 is provided a tank 3 
containing a slurry in which a Ca compound is suspended. The slurry is 
agitated by means of an agitator 4 to prevent the solid matters from 
settling. The slurry suspending the Ca compound is fed to the top of the 
tower 2 by means of a circulation pump 5, by which it is sprayed 
throughout the tower and flows down through the tower while contacting the 
flue gas, before being returned to the tank 3. The flue gas from which 
SO.sub.2 has been removed by contact with the slurry is discharged through 
a mist eliminator 6 as a purified gas 7. On the other hand, to the tank 3 
is fed a slurry of CaCO.sub.3 or Ca(OH).sub.2 through line 8 in an amount 
which depends on the amount of SO.sub.2 being absorbed. The slurry which 
contains calcium sulfite produced by the absorption of SO.sub.2 with the 
absorbent is fed from line 9 to an oxidizing column 10. Air 12 is blown 
from a bubble generator 11 provided at the bottom of the oxidizing column 
and sulfuric acid is fed from line 13, whereby calcium sulfite as well as 
unreacted CaCO.sub.3 or Ca(OH).sub.2 is oxidized into gypsum. The gypsum 
slurry from the oxidizing column 10 is passed through line 14 into a 
thickener 15 and the resulting concentrated gypsum slurry is fed through 
line 16, a tank 17 and a pump 18 into a centrifugal separator 19 to obtain 
gypsum 20. The filtrate is fed to a tank 21 and then through a pump 22 and 
line 23 into the thickener 15. The supernatant liquid in the thickener 15 
is passed from line 24 to a tank 25 and may be used in adjustment of, for 
example, the absorbent in the flue gas desulfurization apparatus or may be 
discharged from a pump 26. 
In view of these circumstances, the present inventors made intensive 
studies to simplify the existing flue gas desulfurization apparatus for 
economy. As a result, it was found that by analysis of experimental data 
of the velocity of reaction between crystals of CaCO.sub.3 or Ca(OH).sub.2 
and SO.sub.2, the oxidation reaction velocity of calcium sulfite produced 
by absorption of SO.sub.2, the rate of settle of gypsum, the underlying 
concept of the known `one operation in one step` process in which the 
respective reactions were carried out in individual steps could be 
overcome. More particularly, operations including absorption and oxidation 
of SO.sub.2, settlement and concentration of gypsum and recovery of 
supernatant liquid could be collectively carried out in an absorption 
tower having a tank in a simple and collective manner by proper control of 
concentrations of slurries in the desulfurization system. 
Accordingly, an object of the invention is to provide a method for 
controlling a concentration of a Ca compound in slurry in a wet flue gas 
desulfurization apparatus. 
The above and other objects, features and advantages of the present 
invention will become apparent from the following detailed description 
with reference to the accompanying drawings.

Reference is now made to FIG. 2 illustrating the wet flue gas 
desulfurization apparatus used in the method of the invention. Flue gas 
101 comprising SO.sub.2 is introduced into an absorption tower 102. In 
FIG. 2, the flue gas and an absorption slurry are shown to contact each 
other by parallel current flow, but the countercurrrent flow gas-liquid 
contact system as shown in FIG. 1 may be also used. In the practice of the 
invention, the parallel current flow is preferably used because of the 
advantage that the efficiency of the oxidization reaction into gypsum is 
improved as described hereinafter. 
At the lower portion of the absorption tower 102 is provided a tank 
receiving a slurry in which a Ca compound is suspended. The slurry is 
agitated by means of an agitator 104 so that solid matters are prevented 
from settling. The slurry suspending the Ca compound is passed to the top 
of the tower by means of a circulation pump 105 and is sprayed throughout 
the tower, and flows down through the tower while contacting with the flue 
gas and returning to the tank 103. The flue gas from which SO.sub.2 has 
been removed by contact with the slurry, is discharged through a mist 
eliminator 106 as a purified gas 107. 
On the other hand, to the tank 103 is fed, through a powder transport line 
108, powder of CaCO.sub.3 or Ca(OH).sub.2 in an amount corresponding to 
the absorption of SO.sub.2. CaCO.sub.3 or Ca(OH).sub.2 serving as the 
absorbent may be fed to the tank 103 in the form of a slurry in water. 
Calcium sulfite which is produced by the absorption of SO.sub.2 in the 
absorbent is oxidized with oxygen contained in the flue gas into gypsum 
crystals because the slurry in the gas-liquid zone of the parallel flow 
system is kept acidic. 
However, in the case where the content of oxygen in the flue gas is low, a 
gas containing oxygen gas is fed from an air nozzle 109, by which the 
absorbed SO.sub.2 can be converted to gypsum. 
In the manner as described above, because the slurry of the Ca compound in 
which gypsum crystals are suspended is maintained in the tank 103, the 
slurry containing gypsum crystals is passed through a slurry discharge 
port 110 and a pump 119 to a separator 111 in which a gypsum cake 112 is 
obtained. The resulting filtrate is returned from line 113 to the tank 
103. 
In the tank 103 is provided a partition wall 114 extending from the liquid 
level to a lower portion of the slurry to establish a liquid chamber 115 
which is separated from the agitated slurry. The partition wall 114 has an 
open lower end, which permits the slurry agitated by the agitator 104 to 
be passed to the liquid chamber 115 partitioned by the partition wall 114 
through the open lower portion. As will be seen from FIG. 2, a baffle 
plate 116 is provided so that the supernatant liquid in the liquid chamber 
115 is not disturbed by the movement of the agitated slurry. The 
supernatant liquid in the liquid chamber 115 is discharged through a 
supernatant discharge port 117 and a pump 118. The filtrate from line 113 
is passed from the upper to the lower portion, at the lower portion of the 
liquid chamber 115 in order to prevent gypsum crystals from rising in the 
liquid chamber. The slurry discharge port 110 is so located that gypsum 
crystals are settled and condensed by means of an inclined plate which is 
provided at the end of the tank 103 positioned at the lower portion of the 
liquid chamber 115. 
In this manner, the concentration of gypsum crystals in the slurry which is 
withdrawn by means of a pump 119 becomes high, thus leading to saving of 
energy required for transportation of liquid. 
In the wet flue gas desulfurization apparatus, it is usual to use a large 
quantity of water, for example, by passing wash water from a washing 
nozzle 121 in order to prevent the crystals of the Ca compound in the mist 
collected in the mist eliminator 106 from being deposited and accumulated 
to render a gas flow path narrow, or by flowing sealing water for the 
pumps in the apparatus. Such water will disturb the concentration of the 
slurry contained in the tank 103. Variation in concentration of the slurry 
will will unstable the operation and control of the wet flue gas 
desulfurization apparatus and invite scaling troubles accompanied by a 
variation in concentration of seed crystals. These problems have not been 
hitherto solved. In the wet flue gas desulfurization apparatus using a 
slurry of, especially, Ca compounds, the prevention of scaling is one of 
important problems. According to our studies, the main cause of the 
scaling problems was found to be due to variation in the concentration of 
the slurry which is attributable to the introduction of water into the 
system. 
In the practice of the invention, the variation in concentration of the 
slurry is suitably prevented. More particularly, according to the 
invention, the concentration of the Ca compound in the slurry in the wet 
flue gas desulfurization apparatus can be stably controlled by carrying 
out, simultaneously and arbitrarily without a delay of response, two 
operations including an operation of discharging the slurry suspending 
crystals of the Ca compound from the tank of the wet flue gas 
desulfurization apparatus and an operation of discharging the supernatant 
liquid having a low concentration of the crystals of the Ca compound. The 
present invention is characterized in that the above good effects of the 
invention can be obtained while simplifying the arrangement of the 
invention compared to the known wet desulfurization systems. 
The present invention is described in more detail by way of example. 
EXAMPLE 
The apparatus shown in FIG. 2 was used. 
The tank 103 containing a slurry comprising gypsum crystals had a section 
of 1000 mm.times.2000 mm with a liquid depth of 2000 mm. By the use of the 
circulation pump 105, 50 m.sup.3 /hour of the slurry was sprayed from the 
top of the absorption tower 102. In the tower were filled grids, and 3,000 
Nm.sup.3 /hour of a flue gas was treated by a gas-liquid parallel current 
procedure and desulfurized until 1200 ppm of SO.sub.2 at the inlet was 
reduced to 60 ppm of SO.sub.2 at the outlet. 
To the tank 103 was fed CaCO.sub.3 powder as absorbent from the line 108 in 
an amount corresponding to an amount of the absorbed SO.sub.2. In the 
inside of the tank 103 was provided a circular partition plate 114 having 
an inner diameter of 400 mm and a height of 2500 mm and opened at the 
lower end thereof. The supernatant liquid was withdrawn from the liquid 
chamber 115 established by the partition wall 114. The supernatant liquid 
from the line 117 contained a slight amount of solid matters. However, by 
control of the flow rate of the supernatant liquid and the flow rate of 
the slurry at the discharge port 110 with a microcomputer, the 
concentration of the Ca compound in the slurry contained in the tank 103 
could be controlled within a desired concentration ranging from 1 to 35 wt 
%. 
In this test, the solid matters discharged from the separator 111 had a 
composition of 97 wt % of CaSO.sub.4.2H.sub.2 O, 0.5 wt % of CaCO.sub.3 
and 2.5 wt % of others and was thus made substantially of gypsum, with to 
calcium sulfite being detected. During the operation, no air was fed from 
the air nozzle 109 provided in the tank 103. When the gas-liquid contact 
of the parallel current system was effected in the grids-filled tower, 
SO.sub.2 which was absorbed in the top of the absorption tower in the case 
of flue gas desulfurization by the wet lime process was all oxidized with 
oxygen in the exhaust gas upon passing downward through the grids-filled 
portion. Accordingly, no oxidation with air in the tank 103 was necessary. 
During the operation of the test, although water was charged from the 
washing nozzle 121 and water for sealing the pumps was used, the 
concentration in the slurry could be controlled within a desired range 
according to the method of the invention. 
According to the present invention, the oxidizing column, thickener, tank 
for filtrate, tank for supernatant liquid and pumps, valves and measuring 
instruments required for these devices as will be necessary in known wet 
flue gas desulfurization apparatus are not necessary, giving evidence that 
the operation procedure can be remarkably simplified.