Washing tower for the desulfurization of flue gases

A scrubber is provided with a grate in its sump through which oxygen is introduced to separate the oxidation zone above this grate from a reaction zone below the grate, a calcium containing additive being introduced into this reaction zone. Flue gas to be desulfurized is passed through a washing zone above the sump and there contacted with normally circulating scrubbing liquid containing the additive to absorb sulfur dioxide and sulfur trioxide. The sulfur dioxide is oxidized to sulfur trioxide and the additive induces precipitation of calcium sulfate in the sump. After a prolonged standstill, settled solids in the sump are induced into movement by drawing a portion of the sump liquid from an upper part thereof and injecting it into a lower part of the sump.

CROSS REFERENCE TO RELATED APPLICATIONS 
This application is related to the commonly assigned copending application 
Ser. No. 493,984 filed May 12, 1983 entitled SCRUBBER FOR THE 
DESULFURIZATION OF EXHAUST GASES. 
FIELD OF THE INVENTION 
Our present invention relates to a washing tower or column for removing 
sulfur-containing compounds, especially sulfur dioxide and sulfur 
trioxide, from flue gases, especially flue gases from the combustion 
chamber of a power plant boiler. More particularly, the invention relates 
to a scrubber for the desulfurization of such flue gases before they are 
discharged into the atmosphere. 
BACKGROUND OF THE INVENTION 
As explained in the aforementioned copending application, it is known that 
the release of sulfur-containing combustion products from power plant 
boiler combustion chambers or like industrial combustion systems, 
primarily as a result of the combustion of the high sulfur fuels utilized 
with increasing frequency, is detrimental to the environment and to the 
health of inhabitants of areas surrounding the power plant and may even 
adversely affect the environment at locations quite distal therefrom. 
For this reason scrubbers have been developed for the desulfurization of 
such flue gases which can have a flue gas inlet, a flue gas outlet, means 
between the inlet and the outlet for treating the flue gas with a 
scrubbing solution which can contain one or more substances capable of 
binding the sulfur compounds, a sump below the washing zone for collecting 
the scrubbing liquid which can contain, in addition to the sulfur 
compounds, particulates scrubbed from the flue gas and other substances 
soluble in or absorbed by the scrubbing liquid and means for introducing 
oxygen into the sump and also for introducing additives to the sump. 
The purpose of introducing oxygen into the sump is to oxidize any sulfite 
in the scrubbing liquid to sulfate, thereby allowing the recovery of 
calcium sulfate as a solid product (gypsum) for use in the construction 
industry. 
The scrubbing liquid decanted from the precipitate solids may be 
recirculated and can include part of the soluble compounds formed as a 
product and which therefore may be prevented from sedimenting out. The 
additives which are fed to the scrubbing water can include lime, limestone 
and other calcium containing compounds as described in the aforementioned 
application which ensure that the sulfate when and if it is formed and 
when and if it is precipitated, will precipitate out as calcium sulfate. 
To avoid the deposition of gypsum on the scrubber walls and baffles or 
other formations within the scrubber, the scrubbing liquid is operated 
with a calcium sulfate content of about 20 to 150 grams per liter, i.e. a 
relatively high concentration. 
This high gypsum content ensures precipitation of the gypsum in the sump of 
the scrubber both during operation and when, for some reason, the system 
is brought to standstill, i.e. recirculation of the scrubbing liquid is 
terminated. 
Conventional scrubbers are provided with numerous mechanical stirrers or 
agitators designed to prevent precipitation of the solids at various 
locations at which such precipitation is not desired. 
This, of course, is disadvantageous since such stirrers and agitators 
require considerable maintenance, introduce significant downtime 
possibilities and create high capital, energy and other operating costs. 
In addition, they generally require passages through the walls of the 
scrubber which must be sealed and the seals have maintenance and like 
problems. 
Indeed, should one or more of the agitators somehow fail, the locations at 
which such agitators are provided generally are prone to relatively dense 
packing of precipitated solids which make it difficult, if not impossible, 
to restart the agitator, thereby aggravating the problem still further. 
Finally, the operation of such scrubbers requires a standby source of 
emergency power to operate the agitators even when the main power supply 
fails either to keep the agitators working or to remove the liquid with 
its high gypsum content so that undesired deposit of solids does not 
occur. 
The removal itself has the disadvantage that on restarting of the apparatus 
it is necessary to build up the concentration of gypsum again in the 
scrubbing liquid and hence the most desired mode of operation is one which 
retains the scrubbing liquid in the scrubber. 
OBJECTS OF THE INVENTION 
It is the principal object of the present invention to provide an improved 
scrubber for the purposes described which is free from the disadvantages 
enumerated above. 
Another object of this invention is to provide a scrubber for the 
desulfurization of flue gas which does not require multiple mechanical 
stirrers or agitators with the ensuing disadvantages, which allows a 
scrubbing liquid with a high concentration of gypsum to be retained in the 
scrubber and which permits restarting of the system without difficulty 
after a period of downtime resulting from power failure or some other 
cause. 
Yet another object of this invention is to provide an improved scrubber 
which extends the principles set forth in the copending application 
mentioned previously. 
SUMMARY OF THE INVENTION 
These objects and others will become apparent hereinafter are attained, in 
accordance with the present invention which provides a scrubber of the 
aforementioned type, i.e. having an upper washing zone traversed by the 
flue gases and into which the scrubbing liquid is fed, a sump for 
collecting this liquid below this zone, means for introducing oxygen into 
the liquid to effect an oxidation reaction converting sulfite to sulfate, 
for example, and means for introducing an additive to this zone in order 
to precipitate out calcium sulfate in a solid product, the slurry being 
withdrawn at the bottom of the sump. According to the invention, the means 
for recirculating scrubbing liquid from the sump to the washing zone 
includes backwashing means, i.e. means for introducing the scrubbing 
liquid into the sump so as to agitate the solids therein, reform the 
flowable mixture and reestablish scrubbing conditions in the event of a 
shutdown. 
More particularly, a backwashing system is provided which comprises at 
least one intake pipe opening into the sump at an upper portion thereof, 
i.e. a portion of the sump above that in which solids will settle and 
usually containing liquid free from solids, although at a high solids 
concentration in solution, a pump connected to this conduit, and at least 
one discharge conduit opening into a lower portion of the sump for 
introducing the liquid pumped from the upper portion thereof into this 
lower portion to create the stirring action previously described. 
Preferably the scrubbing column is provided with a grate arrangement which 
partitions the sump into an upper oxidation zone and a lower reaction 
zone, the grate being formed by downwardly open mutually parallel and 
transversely spaced hoods or channels into which the oxygen is introduced 
either as pure oxygen or as technical grade oxygen or a gas-containing 
oxygen such as air. 
The spacing between the grate members is sufficiently small to obstruct any 
backflow upwardly from the reaction zone into the reduction zone and the 
channels can be provided with upwardly directed perforations to induce 
oxygen upwardly into the oxidation zone. 
In the best mode operation, gas cushions are maintained in these channels 
with the oxygen passing upwardly only through the orifices, the calcium 
containing additive is fed through pipes at nozzles oriented downwardly at 
the gaps between the grate members, and the intake conduit for the 
backwashing system communicates with the oxidations with the liquids in 
the oxidation zone above the grate, but the discharge conduit opens into 
the sump below this grate. 
The details of the grate and its relationship to the means for introducing 
the additive and the oxidation and reduction zones are fully set out in 
the above mentioned application. 
Because of the interaction between the backwashing system and the grate, 
especially intensive agitation and stirring can be generated in the 
reaction region below the grate, ensuring full interaction of the additive 
with the phases below the grate so that as such interaction occurs 
precipitation of solids is more efficient and incorporation of the 
additive in the fluid is likewise rendered more effective. 
The backwashing means can advantageously be provided with at least one 
discharge duct which opens within the sump and is trained upon the apex 
thereof, this sump being formed with a conically converging bottom having 
its apex at the lowest point at which the slurry is withdrawn. 
Alternatively or in addition, a backwashing pipe can open into the sump 
generally tangentially close to the base of the cone formed by the bottom. 
When two such outlets are provided, each can be connected to the 
discharging side of a respective pump having its intake side connected to 
a conduit which communicates with the oxidation zone. 
The system of the invention has been found to be highly advantageous since 
it eliminates the need for mechanical stirrers and the problems discussed 
above which are associated with them. By simple backwashing, any 
precipitated solids can be set into motion to allow full operation without 
delay and, since the decantate from the sump can be recirculated with full 
solids concentration to the washing zone, immediate commencement of the 
scrubbing operation is ensured and there is no need to withdraw the 
contents of the sump from the column in the event of shutdown.

SPECIFIC DESCRIPTION 
In FIG. 1 I have shown a scrubbing tower 1 of an apparatus for the 
desulfurization of a flue gas which can be provided downstream of the 
combustion chamber of a boiler, especially a power plant boiler which 
requires firing with high sulfur fuel. 
The scrubbing column 1 has a flue gas inlet 2 opening into the column at 
the lower end of a washing zone 1a which is fitted with a multiplicity of 
spray heads 4a connected in groups, disposed at different levels and 
supplied by two scrubbing liquid manifolds 4b and 4c forming part of the 
washing means 4 for the zone 1a. 
At the upper end of the zone, i.e. at the top of the column, there is 
provided a flue gas outlet 3 which can open into a further cleaning plant, 
if necessary, or into a stack for release of the flue gases into the 
atmosphere. Droplet separators 3a of a conventional design may be provided 
to intercept entrained scrubbing liquid. 
The flue gas is thus passed through the zone 1a between the inlet 2 and the 
outlet 3. 
The tower has a sump 5 below the zone 1a, this sump 5 comprising a 
cylindrical portion 5a below the inlet 2 and a downwardly converging 
conical portion 5b running to an apex 19 from which a pipe 19a can draw 
the sump product, generally a slurry in which the solids are primarily 
gypsum. 
The apparatus includes, as well, a means represented at 7 for feeding 
oxygen to the liquid in the sump and a means represented at 8 for 
introducing additives to the sump liquid. 
The scrubbing column operates above the liquid level in the sump as an 
absorption unit in which the scrubbing liquid, which can consist 
essentially of lime water, is contacted with the flue gases and absorbs 
the sulfur dioxide and sulfur trioxide therefrom, the liquid collecting in 
the sump from which the sump product in the form of a slurry of calcium 
sulfide and calcium sulfate is withdrawn. 
It is known to maintain the sump product in suspension by providing a 
circulation which maintains the gypsum in suspension and further to 
provide a recycling system whereby, for example, pumps 15a and 15b supply 
the manifolds 4b and 4c with the scrubbing liquid which is withdrawn from 
the liquid sump. 
For the reasons set forth above and in greater detail in the above 
mentioned copending application, the oxygen supply means 7 is 
advantageously formed as a horizontal grate 9 of oxygen supply ducts 10 
such that the grate subdivides the sump into an oxidation zone O above the 
grate and a reduction zone R below the grate. 
Below the grate 9, the device 8 for introducing the additive, e.g. lime 
water, into the sump opens at nozzles 13 which are directed downwardly and 
are disposed directly beneath the gaps 14 between the oxygen supply ducts. 
The ducts 10 are downwardly open hoods or channels and are formed with 
upwardly directed orifices 11 enabling oxygen to pass into the liquid 
layer overlying the grate. 
The distance between the ducts 10 is dimensioned so that a backflow from 
the reaction zone R to the oxidation zone O is restricted. In the ducts 
10, oxidation cushions 12 are maintained so that the scrubbing liquid from 
the oxidation zone cannot trickle through the orifices 11 into the 
reaction zone R. 
In the scrubber most of the sulfur dioxide which is absorbed in the 
scrubbing liquid is oxidized in the oxidation zone to the sulfate. This 
oxidation is effected usually at acid pH, i.e. a pH below 7 and preferably 
around 4 to 6.5. The acidity is contributed by the acid gases absorbed 
from the flue gas. 
In the reaction zone, the pH is raised by the additive, e.g. lime water, so 
that calcium sulfate in particulate form is produced and can be removed as 
described. 
The grate 9 is so formed that it separates the oxidation zone from the 
reaction zone. For an effective separation, the ducts forming the grate 
must be large by comparison with the spaces between them. The downwardly 
flowing scrubbing liquid thus is accelerated through these gaps and 
backflow is largely avoided. 
The oxygen bubbles released by the orifices 11 also agitate the liquid so 
that there is little tendency for deposits to form on the ducts and 
because the ducts are downwardly open, solids do not cake up with the 
ducts. 
The air supply is effected through a central passage 14a which can be seen 
in FIG. 3. In a practical embodiment of the invention, the channels 10 
have a width of about 1 meter while the spacing between them is 0.5 meter 
or less. Usually the additive is diluted with makeup water which will 
eventually be involved in the scrubbing circulation. 
According to the invention, the pumps 15a and 15b can, in the event of the 
need to restart the scrubber after shutdown, be connected by valves 22a 
and 22b and line 16 at their intake sides to upper portions of the sump, 
i.e. portions which tend to remain clear as solids precipitate on 
shutdown. 
One of these pumps is then connected by the open valve 25 (valves 23 and 24 
being closed) to the backwashing pipe 17 which opens at the lowermost 
portion of the sump 5. 
While it is possible to provide an intake for one of the pumps below the 
grate, preferably both intakes are located above the grate, i.e. in the 
oxidation zone. 
The conical portion 5b has a converging bottom 18 terminating in the apex 
19 and the pipe 17 is directed at this apex. 
A further backwashing or recirculation pipe 20 is shown to open 
tangentially at a midpoint along the height of the wall 18 and this pipe 
or an equivalent pipe 20' (see FIG. 1a), located at the region of the base 
21 of the cone, can be connected to the outlet of the other pump 15a by a 
valve 22c, the valves 22d and 22e being closed. 
The pumps 15a and 15b can be utilized for the customary recirculation of 
the scrubbing liquid by opening valves 23, 24, 22c and 22d and by closing 
valves 22a, 22b 25 and 22c. Surprisingly, even after long standstills with 
gypsum deposits several meters in height, the backwashing system of the 
invention has been found to be successful in reestablishing proper 
operating conditions. Additional pumps and backwashing circulations can of 
course be used, once the system is in operation one or more of the pumps 
can be shutdown as long as at least one other pump remains functioning 
and, of course, the pumps can be dimensioned to have different flow rates 
depending upon requirements. 
For startup, pump 15b can be energized, valve 22 opened, valve 23 closed, 
valve 24 closed and valve 25 opened. After agitation for a period of 1 to 
5 minutes, valve 23 can be opened, valve 22 closed, valve 24 opened and 
valve 25 closed. The operation of 15a can be coordinated with that of pump 
15b or offset therefrom. The apex angle of the bottom 5b of the sump is 
preferably 60.degree.. When only one pump is used, the preferred backwash 
inlet is directed at the apex 19 while with a two-pump system, liquid is 
fed into the sump at the apex 19 and tangential at the base of the cone. 
For a three-pump system, two inlets can be directed at the apex and one 
tangential at the base of the cone or vice versa or an additional inlet 
can be provided at the intermediate location shown. The tangential inlet 
tends to set the sump contents in rotation which has been found to be 
especially effective for through mixing with the additive. In practice, 
the system has been found to be effective with standstill periods of a 
week and more.