Removal of sulfur compounds from combustion product exhaust

A method and device are disclosed for removing sulfur containing contaminents from a combustion product exhaust. The removal process is carried out in two stages wherein the combustion product exhaust is dissolved in water, the water being then heated to drive off the sulfur containing contaminents. The sulfur containing gases are then resolublized in a cold water trap to form a concentrated solution which can then be used as a commercial product.

BACKGROUND OF THE INVENTION 
As governmental controls become stricter in the control of effluent gases 
from combustion product exhausts, new and more efficient means are needed 
to clean up exhaust gases from power plants and other devices. Virtually, 
all power plants employ fuels, either coal or liquid fuels which contain 
sulfur. If uncontrolled, effluent gases would deposit into the atmosphere 
large amounts of sulfur in the form of SO.sub.2 and H.sub.2 S, etc. This 
is particularly a problem when, in the future, there will be an expanded 
use of coal derived fuels which are traditionally high in sulfur content. 
If so, the commonly used stack gas scrubbers in use today will prove to be 
inadequate. 
Stack gas scrubbers serve to absorb sulfur compounds into a water base 
solution before vapors are exhausted into the atmosphere. The SO.sub.2 
combines with water to form H.sub.2 SO.sub.3 according to the following 
equation: 
EQU H.sub.2 O+SO.sub.2 .fwdarw.H.sub.2 SO.sub.3 
Hydrogen sulfide H.sub.2 S is also highly toxic by inhalation and is a 
strong irritant to eyes and mucus membranes. It is soluable in water and 
combines with water in a scrubber much like SO.sub.2. 
To prevent SO.sub.2 and H.sub.2 S from being carried into the atmosphere in 
water droplets, it is conventional to combine the scrubber effluent with 
chemicals such as calcium carbonate to precipitate out the sulfur 
compounds. SO.sub.2 combines with calcium carbonate according to the 
following equation: 
EQU H.sub.2 SO.sub.3 +CaCO.sub.3 .fwdarw.CaSO.sub.3 .dwnarw.+H.sub.2 CO.sub.3 
.fwdarw.H.sub.2 O+CO.sub.2 
Thus, under conventional systems, water and carbon dioxide are driven off 
while calcium sulfite, a white thick sludge is precipitated. The calcium 
sulfite sludge accumulates in large quantities and is difficult to dispose 
of. It furthermore has little intrinsic commercial value, thus presenting 
a disposal problem with little return benefits. 
SUMMARY OF THE INVENTION 
It is thus an object of the present invention to substantially eliminate 
sulfur containing contaminents in combustion product exhaust gases while 
eliminating the problems outlined herein. 
It is a further object of the present invention to substantially eliminate 
sulfur containing gases from combustion product exhausts while 
substantially decreasing the quantity of calcium sulfite sludge. 
It is yet another object of the present invention to substantially 
eliminate sulfur containing contaminents from combustion product exhausts 
while achieving a by-product which is commercially and economically of 
value.

Turning in more detail to FIG. 1, the present invention can be more readily 
visualized by following the block diagram presented therein. The apparatus 
of FIG. 1 is intended to achieve the objective of removing the exhaust 
products held in an aqueous solution without allowing the primary sulfur 
compounds (SO.sub.2 and H.sub.2 S) to escape into the atmosphere and to 
then concentrate them for more convenient disposal. Exhaust gases being 
expelled from a power plant or even a parallel-compound duel-fluid heat 
engine which is the subject of U.S. Pat. No. 3,978,661 can be scrubbed 
with water to substantially dissolve the contaminants found in the 
combustion product exhaust. Generally, in a stack operation, the exhaust 
is run in a counter-current flow relationship with the scrubbing water for 
more efficient solubilization. 
The first processing step comprises bringing the water combustion product 
exhaust solution to a high temperature wherein the sulfur containing 
impurities tend to bubble out of the solution for the solubility of 
SO.sub.2 and H.sub.2 S drops off dramatically at high temperatures. At 
this point, reference is made to FIGS. 2 and 3 which show the drastic drop 
off in solubility of SO.sub.2 and H.sub.2 S as the temperature of the 
solution is increased. 
The solution can be simply heated in a boiler or stack 2 by following path 
3 shown in FIG. 1. However, in order to achieve more efficient separation 
between the solution and the sulfur containing gases, pump 1 is preferably 
employed to increase the pressure typically between 2-10 psig and 
preferably approximately 5 psig. In this way the water can be heated to a 
higher temperature without boiling for boiling requires increased energy 
without any return benefit in SO.sub.2 and H.sub.2 S separation. With a 
pressure increase of approximately 5 psig, the solution can be heated to 
approximately 220.degree. F. without boiling. 
At this stage, the solution is fed into insulated tank 4 which can consist 
of a simple glass-lined tank. In tank 4, the sulfur containing 
contaminants bubble out of the water-combustion product solution and pass 
through regulator valve 6 into the next processing stage. It has been 
found that the sulfur containing vapor has a tendency to carry along 
droplets of liquid solution which, for optimum processing conditions, 
should be separated from the gas. This is accomplished by merely imposing 
a screen 5 through which the sulfur containing gas contaminants must pass. 
The liquid water droplets form on the screen and simply fall back into the 
solution sitting in tank 4. 
As a further optional expedient, it has been found that the sulfur 
containing gases can be more efficiently separated from the water solution 
found in tank 4 by bubbling non-reactive gas such as air or nitrogen 
through the base of tank 4. Schematically, this is shown in FIG. 1 wherein 
compressed air source 16 passes through regulator 17 and enters tank 4 at 
its base. The use of an inert gas acts to stir or mechanically drive the 
sulfur containing contaminants from the water solution. It further serves 
the function of reducing the partial pressure of the water vapor entering 
the next stage of the processing system and provides for a more efficient 
operation by lessening the heat exchange requirements of the next stage as 
will be described below. 
The gaseous mixture primarily composed of vaporous SO.sub.2 and H.sub.2 S 
pass through regulator valve 6 to lower the pressure of the gases to a 
fixed value which is a design parameter of the system. Generally, it has 
been found that the gaseous mixture should be in the range of 
approximately 0.5-1 psig, the exact value chosen being dependent upon the 
design of the next stage, aerator tank 7. The aerator tank comprises a 
cold water trap made up of internal cylinder 8 to which is introduced the 
sulfur containing gaseous mixture. The mixture is introduced into chamber 
8 as shown by arrow 19. The sulfur containing gases then bubble through 
the aerator tank which is filled to level 11 with cold water. Heat 
exchanger 9 is employed to stabilize the temperature of the water in 
areator tank 7 to be around the ambient temperature but above the freezing 
point of the solution for the temperature of the water is an important 
design feature of the present invention. A proper water temperature is 
chosen so the by-product at 12 can reach a desired sulfur compound 
concentration value. One can readily design a system wherein virtually all 
of the SO.sub.2 and H.sub.2 S is absorbed into the solution of aerator 
tank 7. The depth of tank 7 and the height of internal cylinder 8 dictate 
the pressure of the sulfur containing gases exiting regulator 6. As the 
tank depth increases, increased pressure is necessary for these feed 
gases. 
It is important that the design parameters be chosen so that concentrations 
of the sulfur containing contaminants be kept below the saturation point 
in tank 7. The design parameters consist of gas flow exiting regulator 
valve 6, water depth 11 and temperature of the water. For example, if the 
temperature in tank 7 is 10.degree. centigrade the water should be drained 
before SO.sub.2 concentration reaches 15 g/100 g etc. But at 20.degree. 
C., the water should be drained when SO.sub.2 concentration is below 8 
g/100 g H.sub.2 O. See FIGS. 2 and 3. 
The present invention is intended to eliminate sulfur containing 
contaminants from combustion exhaust products in a more efficient and 
economical manner than prior art systems. Combustion exhaust gases, 
however, contain components in addition to sulfur contaminants. For 
example, most combustion gases contain CO.sub.2, CO, various unburned 
hydrocarbons, O.sub.2, N.sub.2, and trace amounts of NO.sub.2. A number of 
these remaining gases are allowed to vent from the system as shown by 
arrow 10 exiting from aerator tank 7. Although these additional gases 
could be trapped and used as an extension of the present invention, it is 
the intent to limit the present invention to the disposal of the primary 
sulfur containing contaminants, SO.sub.2 and H.sub.2 S. 
Once the sulfur containing contaminants have solubilized within the water 
contained in aerator tank 7, it can be removed as shown schematically by 
arrow 12. What is achieved is a highly concentrated aqueous solution 
containing SO.sub.2 and H.sub.2 S which has a commercial value. The 
commercial value of this solution derives from its highly concentrated 
form and its relative purity compared to the initial aqueous solution of 
combustion exhaust products. Fresh makeup water is added to tank 7 in 
order to maintain level 11 at a constant height. The temperature of the 
water in tank 7 is maintained via heat exchanger 9. 
The aqueous solution stored in insulated tank 4 must also be drained in 
order to alleviate undo accumulation. As a means of accomplishing this, 
the aqueous solution contained in tank 4 is drained to chemical control 
tank 13. The liquid solution is acidic in nature for sulfur in the form of 
SO.sub.3 combines with water to form sulfuric acid and carbon dioxide 
combines to form carbonic acid (H.sub.2 CO.sub.3). Chemical control tank 
13 can be used as a holding tank wherein the pH of the trapped aqueous 
solution can be adjusted. This is commonly done by adding calcium 
carbonate which will react with sulfur containing residue to form the 
precipitate calcium sulfite while liberating water and carbon dioxide. As 
stated previously, this is commonly done in present day scrubbing systems. 
The present invention differs in that most of the sulfur impurities have 
been removed from the aqueous solution stored in insulated tank 4 before 
the addition of the calcium carbonate in tank 13. Thus, the amount of 
sludge residue is greatly reduced over prior art techniques. 
The water held in chemical control tank 13 can then be recycled to a boiler 
for further use as shown schematically by arrow 14. The precipitate mostly 
comprised of calcium sulfite can be removed as shown schematically by 
arrow 15. 
A system has thus been described where the primary sulfur pollutants, 
SO.sub.2 and H.sub.2 S, have been removed and condensed as a commercially 
valuable product in an aqueous solution with the additional benefit of 
reduced sludge output from the system. Furthermore, the amount of calcium 
carbonate needed to precipitate out the sulfur containing impurities is 
greatly reduced by practicing the present invention.