Claims
- 1. A process for oxidizing gaseous pollutants in a flue gas stream consisting essentially of flue gases, water vapor and one or more gaseous pollutants selected from the group consisting of SO.sub.2, NO, NO.sub.2, NO.sub.x and H.sub.2 S, said oxidized form of the pollutants being more readily removable from the flue gas stream by water absorption than the non-oxidized form thereof, comprising injecting sufficient chlorine in a gaseous form, a liquid form, or as a water solution thereof into the said flue gas stream while the flue gas stream is at a temperature greater than 100.degree. C. to react with the said pollutants and permitting the flue gas stream/chlorine mixture to react for a time sufficient to enable a significant amount of oxidation of the pollutants to occur, whereby an oxidized flue gas stream consisting essentially of flue gases, water vapor and one or more gaseous oxidized said pollutants is formed.
- 2. A process as defined in claim 1 wherein the temperature of the gas stream is between 100.degree. C. and 650.degree. C.
- 3. A process as defined in claim 1 wherein the temperature of the gas stream is between 200.degree. C. and 650.degree. C.
- 4. A process as defined in claim 1 wherein the temperature of the gas stream is between 400.degree. C. and 650.degree. C.
- 5. A process as defined in claim 1 wherein the gas stream following treatment with the chlorine is subjected to a scrubbing step with a water or water solution of pH less than or equal to 7.
- 6. A process as defined in claim 2 wherein the gas stream following treatment with the chlorine is subjected to a scrubbing step with a water or water solution of pH less than or equal to 7.
- 7. A process as defined in claim 3 wherein the gas stream following treatment with the chlorine is subjected to a scrubbing step with a water or water solution of pH less than or equal to 7.
- 8. A process as defined in claim 4 wherein the gas stream following treatment with the chlorine is subjected to a scrubbing step with a water or water solution of pH less than or equal to 7.
- 9. A process as defined in claim 1 wherein the pollutant is SO.sub.2 and the Cl.sub.2 /SO.sub.2 molar ratio is between about 0.5 and about 5.0.
- 10. A process as defined in claim 1 wherein the pollutant is NO and the Cl.sub.2 /NO molar ratio is between about 0.5 and about 5.0.
- 11. A process as defined in claim 2 wherein the pollutant is SO.sub.2 and the Cl.sub.2 /SO.sub.2 molar ratio is between about 0.5 and about 5.0.
- 12. A process as defined in claim 2 wherein the pollutant is NO and the Cl.sub.2 /NO molar ratio is between about 0.5 and about 5.0.
- 13. A process as defined in claim 1 wherein the pollutant is SO.sub.2 and the Cl.sub.2 /SO.sub.2 molar ratio is between about 1.0 and about 2.0.
- 14. A process as defined in claim 1 wherein the pollutant is NO and the Cl.sub.2 /NO molar ratio is between about 1.0 and about 2.0.
- 15. A process as defined in claim 2 wherein the pollutant is SO.sub.2 and the Cl.sub.2 /SO.sub.2 molar ratio is between about 1.0 and about 2.0.
- 16. A process as defined in claim 2 wherein the pollutant is NO and the Cl.sub.2 /NO molar ratio is between about 1.0 and about 2.0.
- 17. A process as defined in claim 5 wherein the pollutant is SO.sub.2 and the Cl.sub.2 /SO.sub.2 molar ratio is between about 1.0 and about 2.0.
- 18. A process as defined in claim 5 wherein the pollutant is NO and the Cl.sub.2 /NO molar ratio is between about 1.0 and about 2.0.
- 19. A process as defined in claim 6 wherein the pollutant is SO.sub.2 and the Cl.sub.2 /SO.sub.2 molar ratio is between about 1.0 and about 2.0.
- 20. A process as defined in claim 6 wherein the pollutant is NO and the Cl.sub.2 /NO molar ratio is between about 1.0 and about 2.0.
- 21. A process as defined in claim 2 wherein the gas stream following water scrubbing is subjected to a chlorine removal process.
- 22. A process as defined in claim 5 wherein the gas stream following water scrubbing is subjected to a chlorine removal process.
- 23. A process as defined in claim 6 wherein the gas stream following water scrubbing is subjected to a chlorine removal process.
FIELD OF THE INVENTION
This application is a continuation-in-part of application Ser. No. 610,505, filed May 15, 1984 abandoned.
The present invention relates to the removal of objectionable components such as SO.sub.2 and NO.sub.x from a hot moving gas stream. More particularly, the invention permits the removal of sulfur dioxide (SO.sub.2) and nitrogen oxides (NO.sub.x) from the flue gases of industrial and utility sources.
Sulfur dioxide and nitrogen oxides are produced in very large quantities daily from the burning of fossil fuels and after oxidation in the atmosphere, are returned to the ground in what is called "acid rain". The higher levels of acidity in the rainfall downwind of large SO.sub.2 and NO.sub.x sources in the form of major industrial areas has a serious environmental impact in many parts of the world. The fish of many lakes and streams in these areas have vanished, either as a direct kill, or from a break in their food chain. Flora is also dstroyed. Many other long term and short term effects are still under investigation and all effects are causing much concern.
The major sources of SO.sub.2 and NO.sub.x produced by man derive from the burning of coal and petroleum products. All fossil fuels contain sulfur, and the most popular grades of fuel are the low sulfur varieties, since in the burning of sulfur containing fuels, the sulfur present forms SO.sub.2 with the associated environmental problems. Nitrogen oxides are produced in the burning process itself principally from the nitrogen in the air, and may be controlled to an extent by the manipulation of the burning conditions. Some NO.sub.x is nonetheless produced even in the best practical conditions.
Due to the present uncertainties affecting the world petroleum supplies, and the limited supply in any case, other energy sources are being sought to fuel the world's energy needs, which are continually increasing. One readily available energy source that is not being fully utilized at present is coal. Much of the coal that is not being exploited is unused due to environmental concerns as coal usually has an unacceptably high sulfur content. This is a problem as it has been estimated that 95 percent of the sulfur in coal forms SO.sub.2 on burning to be released to the environment.
Over the last few decades, much interest has been shown in the removal of SO.sub.2 from flue gases and many systems for SO.sub.2 removal from flue gas have been developed. Many large power plants have been fitted with some form of SO.sub.2 removal system.
The efficiency of these desulfurisation attempts is in general insufficient to prevent large scale environmental damage. In a report to the Air Pollution Control Association in 1978, William H. Megonnel of the National Association of Electric Companies, in an article entitled "Efficiency and Reliability of Sulfur Dioxide Scrubbers", examines the systems in operation in utilities at that time. Of thirty-two utilities classed as operational at that time, virtually all used some variation on a carbonate or alkaline scrubbing process, involving collection of SO.sub.2 as calcium sulphite utlimately. In the remaining example, the Wellman-Lord Allied Chemical system was used involving SO.sub.2 recovery and reduction to sulfur at 90 percent efficiency.
The primary deficiency of these systems is the unacceptably low removal efficiency obtained in operation, i.e., an average of around 75 percent in the examples quoted. As North America is forced, due to economic and political reasons, to switch to more and more coal, including more high sulfur coal, as an energy source, this removal efficiency demonstrated is insufficient to prevent large scale environmental damage.
In the years since Megonnell's report, the efficiencies in some systems have improved, but the use of coal has also increased. Thus the total emission load on the atmosphere has increased. The only figure that is truly meaningful is the overall sulfur oxides and nitrogen oxides emission tonnage. If this does not drop significantly, the present pollution situation will not change, regardless of quoted efficiencies.
In the large majority of SO.sub.2 removal systems in use, the acidic nature of a water solution of SO.sub.2 is used to trap the SO.sub.2 in the form of a sulfite or bisulfite by reaction with a basic material such as calcium oxide or hydroxide, or by reaction with a carbonate such as limestone or dolomite. In some cases the sulfite product is oxidized with atmospheric oxygen to an insoluble sulphate.
Other research has been carried out on various dry processes, catalytic processes, and processes involving additions to the fuel, but to date little practical use has been made of these other processes, although the disadvantages of the carbonate or hydroxide scrubbing processes described earlier are causing great interest in alternatie cleanup processes.
Nitrogen oxides have been removed with varying success by a very large number of methods on a laboratory scale, often in very uneconomical ways using expensive reagents. Methods used include dry catalytic reduction or oxidation, wet scrubbing with basic solutions or amines, and aqueous scrubbing with oxidizing solutions. The methods used for nitrogen oxides removal are very diverse, and it appears that relatively few processes have been put into practical use, with the emphasis having been on the removal of SO.sub.2, which is generally present in larger quantities and is more easily removed by conventional methods.
Generally, the wet scrubbing methods, especially the methods using aqueous oxidizing solutions, are the only systems readily useable for the simultaneous removal of NO.sub.x and SO.sub.2.
Several U.S. patents and one Canadian patent disclose various systems for treating gas streams.
U.S. Pat. No. 4,294,928, issued Oct. 13, 1981, Tamony et al., describes a process for removing nitrogen oxides from a cool gas stream by injecting excess chlorine in the presence of water in the liquid phase into that gas stream and subsequently scrubbing the reacted gas stream with an aqueous scrubbing solution for nitrogen dioxide. Specifically, Tamony et al. provide that the reaction between the gas stream and the chlorine takes place at a temperature preferably between 10.degree. C. and about 50.degree. C. Even in the broadest disclosed operable range, it is stated that the reaction takes place at less than the boiling point of the water or aqueous solution present during the oxidization (column 1, lines 42 through 45). The chlorine reaction with the gas stream must take place in the presence of water in the liquid phase (column 1, lines 30 through 32). The reason for this limitation is stated at column 2, lines 34 through 40, where it is pointed out that the rate of oxidization has been found to be significantly increased when the chlorine is added to the gas stream in the presence of water in the liquid phase. This is also emphasized at column 1, line 63, where it is stated, "The water present must be in the liquid phase". The theory which follows in columns 1 and 2 emphasizes the reason why Tamony et al. believe that the water must be in the liquid phase.
The process of Tamony et al. suffers from several decided disadvantages. First, as can be seen from the reaction disclosed at column 2, it is applicable only to nitrogen oxides and if the gas stream contains other than nitrogen oxides, there is no assurance that those other components will be oxidized. Second, the process of Tamony et al. must be carried out in conjunction with a quench column. Some residence time in the quench column is also required to ensure good oxidization (see column 2, lines 44 through 51). Third, since the Tamony et al. process must be carried out with water in the liquid phase, it would be inapplicable to a wide range of industrial uses.
U.S. Pat. No. 2,481,241, issued Sept. 6, 1949, Rapson et al., discloses basically that sulfur dioxide added to mixtures of chlorine and chlorine dioxide will react with the mixture. Rapson et al. also disclose the reaction of chlorine with sulfur dioxide. However, none of this takes place at elevated temperatures. Rapson et al. were essentially concerned with the removal of chlorine from a gaseous mixture containing chlorine dioxide, chlorine and water vapour, which is not relevant to the claimed process.
U.S. Pat. No. 3,803,290, issued Apr. 9, 1974, Gooch, specifically discloses a process which is a surface reaction, that is, reacting the molecular gas with the surface molecules of the particulate material to produce areas of altered surface characteristics. Surface chemistry mechanisms, from a chemical point of view, are not relevant to hot gas phase and solution phase reactions.
Canadian Pat. No. 643,389, issued June 19, 1962, Murray, teaches that chlorine, hypochlorous acid and calcium hypochlorite in solution can react with hydrogen sulfide, methyl mercaptan, methyl sulfide and methyl disulfide (page 1, lines 27 to 28). The Murray patent discloses only a liquid phase reaction (page 3, lines 20 to 24). Also, the Canadian patent relates to deodorization rather than treatment of pollutants in a hot flue gas stream and does not refer to SO.sub.2 or NO.sub.x.
The process involves oxidizing non-particulate chlorine oxidizable components selected from the group consisting of SO.sub.2, NO, NO.sub.2, NO.sub.x and H.sub.2 S, in a hot flue gas stream containing water molecules, said oxidized form of the components being more readily removable from the hot gas stream than the non-oxidized form thereof, comprising injecting sufficient chlorine in a gaseous form, a liquid form, or as a water solution thereof into the gas stream while the gas stream is at a temperature greater than 100.degree. C. to react with the chlorine oxidizable components and permitting the gas stream/chlorine mixture to react for a time sufficient to enable a significant amount of oxidation of the components to occur. In particular, gas streams of temperatures between 100.degree. C. and 650.degree. C. may be treated.
The process includes following the chlorine oxidation treatment with a water scrubbing treatment of pH less than or equal to 7 to increase the degree of component removal. Cl.sub.2 /NO or Cl.sub.2 /SO.sub.4 molar ratios of about 0.5 to about 5.0 may be used in oxidizing gas stream components such as NO and CO.sub.2. Preferably, the molar ratios are between about 1.0 to about 2.0.
US Referenced Citations (1)
| Number |
Name |
Date |
Kind |
|
3803290 |
Gooch |
Apr 1974 |
|
Continuation in Parts (1)
|
Number |
Date |
Country |
| Parent |
610505 |
May 1984 |
|
Patent Grant
4619608
