CA1293359C - Process for removing oxides of nitrogen and sulfur from waste gases - Google Patents
Process for removing oxides of nitrogen and sulfur from waste gasesInfo
- Publication number
- CA1293359C CA1293359C CA000488446A CA488446A CA1293359C CA 1293359 C CA1293359 C CA 1293359C CA 000488446 A CA000488446 A CA 000488446A CA 488446 A CA488446 A CA 488446A CA 1293359 C CA1293359 C CA 1293359C
- Authority
- CA
- Canada
- Prior art keywords
- nitrogen
- gas
- acid
- nitrogen oxide
- denitration
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
- 238000000034 method Methods 0.000 title claims abstract description 38
- 239000002912 waste gas Substances 0.000 title claims abstract description 25
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 title claims abstract description 18
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 title claims abstract description 9
- 229910052757 nitrogen Inorganic materials 0.000 title claims abstract description 9
- 239000011593 sulfur Substances 0.000 title claims abstract description 8
- 229910052717 sulfur Inorganic materials 0.000 title claims abstract description 8
- MWUXSHHQAYIFBG-UHFFFAOYSA-N Nitric oxide Chemical compound O=[N] MWUXSHHQAYIFBG-UHFFFAOYSA-N 0.000 claims abstract description 111
- 239000007789 gas Substances 0.000 claims abstract description 49
- 238000010521 absorption reaction Methods 0.000 claims abstract description 29
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 claims abstract description 17
- GRYLNZFGIOXLOG-UHFFFAOYSA-N Nitric acid Chemical compound O[N+]([O-])=O GRYLNZFGIOXLOG-UHFFFAOYSA-N 0.000 claims abstract description 13
- 229910017604 nitric acid Inorganic materials 0.000 claims abstract description 13
- JCXJVPUVTGWSNB-UHFFFAOYSA-N nitrogen dioxide Inorganic materials O=[N]=O JCXJVPUVTGWSNB-UHFFFAOYSA-N 0.000 claims abstract description 12
- 239000007788 liquid Substances 0.000 claims abstract description 11
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 claims abstract description 9
- 229910052760 oxygen Inorganic materials 0.000 claims abstract description 9
- 239000001301 oxygen Substances 0.000 claims abstract description 9
- AAJFWZPKEVPIMB-UHFFFAOYSA-N [N]=O.OS(O)(=O)=O Chemical class [N]=O.OS(O)(=O)=O AAJFWZPKEVPIMB-UHFFFAOYSA-N 0.000 claims abstract description 8
- 238000003795 desorption Methods 0.000 claims abstract description 8
- 239000003546 flue gas Substances 0.000 claims abstract description 8
- MGWGWNFMUOTEHG-UHFFFAOYSA-N 4-(3,5-dimethylphenyl)-1,3-thiazol-2-amine Chemical compound CC1=CC(C)=CC(C=2N=C(N)SC=2)=C1 MGWGWNFMUOTEHG-UHFFFAOYSA-N 0.000 claims abstract description 7
- 238000004519 manufacturing process Methods 0.000 claims abstract 2
- 150000003464 sulfur compounds Chemical class 0.000 claims abstract 2
- 239000002253 acid Substances 0.000 claims description 50
- RAHZWNYVWXNFOC-UHFFFAOYSA-N Sulphur dioxide Chemical compound O=S=O RAHZWNYVWXNFOC-UHFFFAOYSA-N 0.000 claims description 24
- 238000001816 cooling Methods 0.000 claims description 9
- 238000010438 heat treatment Methods 0.000 claims description 7
- 230000003647 oxidation Effects 0.000 claims description 6
- 238000007254 oxidation reaction Methods 0.000 claims description 6
- 239000006096 absorbing agent Substances 0.000 claims description 4
- OLBVUFHMDRJKTK-UHFFFAOYSA-N [N].[O] Chemical class [N].[O] OLBVUFHMDRJKTK-UHFFFAOYSA-N 0.000 claims description 2
- 238000005406 washing Methods 0.000 claims 1
- DOTMOQHOJINYBL-UHFFFAOYSA-N molecular nitrogen;molecular oxygen Chemical class N#N.O=O DOTMOQHOJINYBL-UHFFFAOYSA-N 0.000 abstract 1
- QGZKDVFQNNGYKY-UHFFFAOYSA-N Ammonia Chemical compound N QGZKDVFQNNGYKY-UHFFFAOYSA-N 0.000 description 6
- 239000000443 aerosol Substances 0.000 description 5
- 239000003054 catalyst Substances 0.000 description 4
- VQTGUFBGYOIUFS-UHFFFAOYSA-N nitrosylsulfuric acid Chemical compound OS(=O)(=O)ON=O VQTGUFBGYOIUFS-UHFFFAOYSA-N 0.000 description 4
- 229910001868 water Inorganic materials 0.000 description 4
- UGFAIRIUMAVXCW-UHFFFAOYSA-N Carbon monoxide Chemical compound [O+]#[C-] UGFAIRIUMAVXCW-UHFFFAOYSA-N 0.000 description 3
- HEMHJVSKTPXQMS-UHFFFAOYSA-M Sodium hydroxide Chemical compound [OH-].[Na+] HEMHJVSKTPXQMS-UHFFFAOYSA-M 0.000 description 3
- 229910021529 ammonia Inorganic materials 0.000 description 3
- 230000015572 biosynthetic process Effects 0.000 description 3
- 229910002089 NOx Inorganic materials 0.000 description 2
- CBENFWSGALASAD-UHFFFAOYSA-N Ozone Chemical compound [O-][O+]=O CBENFWSGALASAD-UHFFFAOYSA-N 0.000 description 2
- 238000010306 acid treatment Methods 0.000 description 2
- 238000006243 chemical reaction Methods 0.000 description 2
- 239000003245 coal Substances 0.000 description 2
- 239000003344 environmental pollutant Substances 0.000 description 2
- 238000012856 packing Methods 0.000 description 2
- 231100000719 pollutant Toxicity 0.000 description 2
- 150000003839 salts Chemical class 0.000 description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 2
- 235000008733 Citrus aurantifolia Nutrition 0.000 description 1
- 239000005864 Sulphur Substances 0.000 description 1
- 235000011941 Tilia x europaea Nutrition 0.000 description 1
- 238000006887 Ullmann reaction Methods 0.000 description 1
- 150000007513 acids Chemical class 0.000 description 1
- BFNBIHQBYMNNAN-UHFFFAOYSA-N ammonium sulfate Chemical compound N.N.OS(O)(=O)=O BFNBIHQBYMNNAN-UHFFFAOYSA-N 0.000 description 1
- 229910052921 ammonium sulfate Inorganic materials 0.000 description 1
- 235000011130 ammonium sulphate Nutrition 0.000 description 1
- 239000012267 brine Substances 0.000 description 1
- 150000001768 cations Chemical class 0.000 description 1
- 239000012141 concentrate Substances 0.000 description 1
- 230000002844 continuous effect Effects 0.000 description 1
- 239000000498 cooling water Substances 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 239000003337 fertilizer Substances 0.000 description 1
- 239000003077 lignite Substances 0.000 description 1
- 239000004571 lime Substances 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 229910017464 nitrogen compound Inorganic materials 0.000 description 1
- 150000002830 nitrogen compounds Chemical class 0.000 description 1
- 238000000746 purification Methods 0.000 description 1
- 230000005855 radiation Effects 0.000 description 1
- 238000009751 slip forming Methods 0.000 description 1
- HPALAKNZSZLMCH-UHFFFAOYSA-M sodium;chloride;hydrate Chemical compound O.[Na+].[Cl-] HPALAKNZSZLMCH-UHFFFAOYSA-M 0.000 description 1
- 239000007921 spray Substances 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
Classifications
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B17/00—Sulfur; Compounds thereof
- C01B17/69—Sulfur trioxide; Sulfuric acid
- C01B17/74—Preparation
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/34—Chemical or biological purification of waste gases
- B01D53/46—Removing components of defined structure
- B01D53/60—Simultaneously removing sulfur oxides and nitrogen oxides
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B21/00—Nitrogen; Compounds thereof
- C01B21/20—Nitrogen oxides; Oxyacids of nitrogen; Salts thereof
- C01B21/38—Nitric acid
- C01B21/40—Preparation by absorption of oxides of nitrogen
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
- Y02A50/00—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE in human health protection, e.g. against extreme weather
- Y02A50/20—Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters
Landscapes
- Chemical & Material Sciences (AREA)
- Organic Chemistry (AREA)
- Engineering & Computer Science (AREA)
- Inorganic Chemistry (AREA)
- Health & Medical Sciences (AREA)
- Biomedical Technology (AREA)
- Environmental & Geological Engineering (AREA)
- Analytical Chemistry (AREA)
- General Chemical & Material Sciences (AREA)
- Oil, Petroleum & Natural Gas (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Treating Waste Gases (AREA)
Abstract
Process for removing oxides of nitrogen and sulfur from waste gases Abstract of the Disclosure A process for removing oxides of nitrogen and sulfur from waste gases, in particular from flue gases emitted by power plants, by means of a modified nitrogen oxide-sulfuric acid process for the production of sulfuric acid to obtain marketable nitrogen and sulfur compounds by absorption of more than half the concentration of nitrogen oxide present in the waste gas introduced into said process and desorption of the liquid that contains nitrogen oxide, in which process a) the nitrogen oxide set free is mixed with an oxygen-containing gas to give a gas which is relatively enriched with nitrogen oxide and oxygen and in which nitrogen monoxide oxidises to nitrogen dioxide, and the nitrogen dioxide so obtained and/or nitric acid produced therefrom, as well as a residual content of nitrogen monoxide, is introduced into the main stream of waste gas to be treated, the molar amounts of quadrivalent and pentavalent nitro-gen-oxygen compounds introduced into the system being greater than the molar amount of nitrogen monoxide also introduced, and b) the liquid from which nitrogen oxides have been at least partially removed, or the bulk thereof, is fed back, after passing through the desorption plant, into one or more of the gas treatment towers of the main stream of gas.
Description
~z~33359 71-150381+
Process for removing oxides of nitrogen and sulfur from wa~te ga~es The present invention relates to a proces~ which makes it po~sibla to ramQve oxides of nitrogen snd sulfur from wsste gases si~ulta-neously without the u~e of lime, sodium hydroxide, ammonia or other chemicals.
The separated n~trogen oxides are used as catalyst for oxidising sulfur dioxidæ with oxygen-containing gas, preferably atmopsheric oxygen. Ultimately, ~arketabl~ nitric acid and sulfur~c acid are obtained from the pollutants.
Tha advantages of the process of this invention will be explained by ~taking;as an example the purif~cation of flua gase~ from a power station ~uelled by ~alt-containing brown coal having a calorlfic valoe greater thsn 4500 kcallkg.
lt will be assumed that the power station produces the following wa-t~ gas, where~x in N0x i8 1 and 2~
amount o~ flue gas `1 mio. Nm3 per hour ~ ;
eoncentration of N0 (ca1cu1at~d~as N02)1.43 g/Nm3 (c. 700 ppm) ~
concantration of S02 ~~12.6 g/Nm3 ~c. 4300 ppm).~ :
Throughout~`thls specification, ppm concentrations will alway~ be~
understood as meaning parto by voIume per m1-l10n units of vo1ume.
: : ~ :
: :: ' :1~9335~
The process of tbis invention makes it possible to reduce the pollutant gases to valus~ lower than S02 0.1 g/Nm3 (c. 34 ppm) N0 0.2 g/Nm~ (c. lO0 ppm).
Provided cooling water wi~h an average annual temperature of l5C ~ 8 available, the operating requirements for the waste gas purification are:
electrical energy : le~s than 3 % of the energy produced in the power station, heat consumption for concentrating the acid in the course of the process: corresponds to a consumption of lsss than 2 % of the coal burned in the power plant.
When the nitric acid (concentration higher than 30 %~ Rnd sulfuric acid (concentration high~r than 75 %~ produced by ths proce6s of this invention can be utilised, for example, in a fertiliser factory, the value of the acids cover~ the operating costs referred to above. The capital expenditure for a procsss according to ths pr~sent invention is le6s than a quarter of that required for the powsr station. :
It is known to remove flue gases by adding ammonla to the still hot flue ga~es, before complete heat utilisation, and passing them over a catalyst, thereby reducing the nitrogen oxides to nitrogen. A
drawback of this process i8 the cost of the ammonia and the limited life of the catalyst. When using salt-containing coal, salt6 vaporise in the combu~tion chamber and form aerosols that contamin-~ ate the catalyst. A further drawback of the dry and also of the wet ; ~ use of ammonia is tha formation of ammonium sulfate aerosols and other salt aerosols which cannot bs separated completely despite the elaborate procedures employed. Ths aerosols released into the : :
, : ~
3~Z93359 atmosphere along with the treated flue gas promote vapour formationin the atmosph0re and prevent natural solar radiation by forming s~og.
In contradistinction to the known prior art, in the process of this invention the operating cost~ fall wlth increasing coDcentrations of N0 in the flue gases. Th~s any combu~tion measures for reducing the formation of N0x are rendered unnecessary.
The nltrogen oxides are essentially present in flua gase~ in the form of N0, which i8 Yery 3paringly absorable. It i5 known to convert N0 into N02 by addition of ozone and to absorb it, for example in ammoniacal liquids. In addition to the high costs of producing ozone, a drawback i8 the previously mentioned unavoidable formation of aerosols.
In recent year~, a~ a result of efforts to control atmospheric pollution, the nitrogen oxide-sulfuric acid process ha~ been proposed for separating SOz and N0x from waste gases containing about 1 % by volume of S02. The work carried out in this field forms the basis of the present in~ention and, in this connection, reference i~ made to the followlng publications:
- Fattinger, V., Proc. Brit. Sulphur Corp., 3rd Int. Conf. Fert., London, November 1979, Paper XXYI
~ Blanken3tein, K., Neumann, G.E., U.B.A., F and E Rsport ~o. 10403311, June 1980 - Sander, ~., Fattinger, V., Chem. Ing. Techn., 55 (1983) No. 8, S. 601l607 - Fattinger, V., CIBA-GEIGY CorporatioD US Patent 4 148 868 (1979) - Fattinger, V., CIBA-GEIGY Corporation US Patent 4 242 321 (1989) Ullmanns Enzyklopadie der technischen Chemie, 4. Deubearbeitete und erweiterte Auflage, Verlag Chemie, Weinheim, Vol. 21 (lg82), p. 148 " :
~93359~
~lue Kases emitted by power stations contain mostly only 0.02 to 0.5 % by volume of S02; ~nd up to now, an S02 enrichment has been considered necessary before processing in a nitrogen oxide-sulfuric acid system becomes po6sible. FurthermorP, expert opinion is that a nitrogen oxide-sulfuric acid process would cau~e atmospheric pollution through N0 losses.
A principal reason for the inapplicability of known nitrogen oxide-sulfuric acid processes for processing waste gases from power stations is the lengthy ~ojourn time for the oxidation of the N0 set free in the denitration tower. A prerequisite for nitrogen oxide absorption is a suitable ratio oE ~O:N02.
It has now been found that nitrogen oxide-sulfuric acid systems with a reaction requirement of less than 25 m3 per Nm3tsec of flue gas can be operated if more than half the concentration of nitrogen oxide present in the waste gas introduced into the process is absorbed, and the liquid which contains nitrogen oxide iB gubge-quently sub~ected to desorption, whereupon a) the nitrogen oxide set free is mixed with an oxygen-containing gas to give a gas which is relatively enriched with nitrogen oxide and oxygen, and in which nitrogen monoxide oxidises to nitrogen dioxide, and the nitrogen dioxide so obtalned and/or nitric acid produced therefrom, as well as a residual content of nitrogen monoxide, is introduced into the main stream of waste gas to be treated, the molar amounts of quadrivalent and pentavalent nitrogen-oxygen compounds introduced into the system being greater than the molar amount of nitrogen monoxide also introduced, and b) the liquid from which nitrogen oxides have been at least parti-ally removed, or the bulk thereof, is fed ~ack, after passing through the desorption plant, to one or more of the gas treatment towers of the main stream of gas. The acid which has been partially or completely freed from nitrogen compounds can be fed into the first towers of the sy~tem or else in its entirety or in part into ~ 933~
one of the N0x absorption towers (or also into a number of N0x absorption towers) at the end of the system. Both kinds of liquid feedback can be combined.
Because the acid for the purpo~e of N0x desorption has been heated andlor diluted by water or by a weaker acid, it is exp~dient to concentrate or cool the acid before the denitrated or partly denitrated acid enters an N0 absorption tower.
When treating the stream of waste gas, nitric acid is introduced, before the S02 absorptioD, into the denitration cycle or into the S2 absorption cycle in an amount such that an optimum ratio of NO:N02 for the N0x absorption is established. This step is not novel and is described e.g. in US patent specification 4 242 32l.
It has further been found that surprisingly good absorption results are obtained and that relatively high concentrations of nitrose can be employed in the absorption acid by carrying out the absorption at temperatures below 30C. It 18 advantageous to use heat pumps (cooling systems) to produce a cooling brine which, by indirect heat exchange, lowers the temperature of the acid in the cycle to below 20, 15 or even 10C. This measure has proved much less expensive than an excess1ve enlargement of the absorption ~one.
.
An important feature that distinguishe~ the process of this inven-tion from known processes resides in performing the denitration using Iow concentrations of scid. Sulfuric acid in a concentration of less than 73 % or even 70 % makes it possible to effect a sufficient acid denitration even at the low concentration of S02 in the flue gases emitted by power stations. The denitration tower employed in the main stream of gas is conveniently operated in direct~current between gas and acid.
, ~ :
The acid~discharged from the~pre-denitration ln the main current of gas can be sub~ected to indirect heating and to a fine denltration in~a tower through which flows a partial stream of sulfur dioxide.
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~LZ9335~
It is also possible to carry out the entire acid denitration out3ide the main stream of gas and to pass a current of air and/or a partial stream of gas which contains S02 through the denitration apparatus.
If necessary, the current of air or t~e stream of gaA which contains S02 can be heated in order to accelerate the expulsion of the nitrogen oxides.
It is known that the vapour pressure of the nitrogen oxides dis-solved in the sulfuric acid a8 nitrosyl-salfuric acid is greatly increased by addition of nitric acid. Thus, if necessary, the acid denitration can be considerably accelerated by addition of nitric acid. The requi3ite nitric acid is formed in the process of this invention from nitrogen oxides and returns from the denitration apparatus to the process in the form of nitrogen oxides.
Before the denitrated acid enters the N0x absorption zone, it is brought, in a concentrating tower, to the concentration required for a good absorption (73 to 76 % HzS04). The hot acid originating from the concentrating tower can conveniently be used for heating the cold exit gas from the N0x absorption zone.
The heated gases of very low relative humidity can be transported to a bricked flue without the rl.k of tbeir becoming moist.
The advantages of the process of the present invention compared with known nitrogen oxide-sulfuric acid processes become all the more signlficant the lower the concentrations of S02 and the higher the concentrations of N0x in the waste gas to be purified. Preferably the concentration o~ SOz should be below 0.8 % by volume and the concentration of N0 should be above 100 ppm.
x Pigures 1, 2 and 3 and a working Example will serve to illustrate ~ the invention in more detall.
::
~93359 Figure 1 illustrates the treatment of the stream of waste gas in the gas washiDg towers which are connected in series and have the following functions: gas cooling, acid denitration, SO2 absorption, NOx absorption 1, NO absorption 2 and heating treated gas.
The following Table indicates the concentrations of SO2, NOx, H20 and the gas temperature before and after the treatment steps:
Conc0ntrations and tempera- SO2 N3 H20 O
tuFes of the 9tream of ga3 g/Nm3 as 2 glNm3 C
before gas cooling 12.6 1.43 125 130 after gas coollng 12.5 - 1.43 30 26 after denitration 6 22 8 38 after SOz-absorption 1 20 5 25 after NOx-absorption 1 0.5 4.5 0.1 10 after NOx-absorption 2 0.1 0.2 0.1 7 after hssting treated gas 0.1 0.2 3 60 Each treatment ÆOne is provided with an acid cycle which is kept in circulation by pumps 4. The reference numbers 1 and 9 indicate respectively the waste gas inlet and the gas feed valve. The reference number 2 indicates connecting lines between gas treatment zones and 3 denotes the treated ga~ outlet. All gas treatment tower contain packing layers 5. As shown in figure 1, individual acid treatment stages are equipped with acid coollng means 6 for lowering the temperstures of the acid in order to reach the gas temperatures indicated in the above Table. The reference number 7 indicates the acid spray nozÆles in the gss treatment zones. The reference number 8 indicates the acid sumps in the bottom part of the towers. The packings in the layers 5 hare a surface area of more than 300 m2im3.
The reaction 3pace requirem2nt per 1 Nm3/sec of gas is smaller than 3 m3 for the denltrat1on, s=aller than 4 =3 ior th~ SOz absorpti~n, : `
~L~933~i9 smaller than 4 m3 for the N0 absorptlon 1, smaller than 12 m3 for the N0 absorption 2, and ~maller than 1.5 m3 for heating the treated gas.
As in every nltrogen oxlde-sulfuric acid system, an acid exchange takes place between the acid denitration and the N0 absorption. The acid freed from nitrose and which flows into the N0 absorption zone i8 hereinafter raferred to as "return acid", whilst the acid containing N0 ia referred to as "primary acid". The amount of sulfuric acid continuously formed in the S0z absorption zone is mixed ~ith the primary acid.
High concentrations of nitrose in the acid are achievad as a consequence of the strong cooling in the N0 sbsorption zone.
Expressed in HN03 equivalents, the concentration of nitrose in the primary acid is higher than 200 g of HN03 per litre. This high concentration makes it possible to work with small amounts of primary and return acid. In the working Example, amounts of less than 500 g/Nm3 of waste gas to be purified suffice.
The low temperatures in the entire treatment zone of the main stream of gas simplify the problem of chooaing suitable materials. In the entire area of the large treatment zones between gas cooling and heatlng treated gas, there are no temperatures hlgher than 40C.
Figure 2 illustrates the treatment of the primary acid. The primary acid is heated, in the heatlng means 20, to a temperature of about 60C and fed to the N0x stripper 22. Spent air ls drawn off from the primary acid treatment zone through this N0x stripper via the llne 4l~ and the ventilator 21. This spent air is essentially slightly impure air w~hlch becomes enriched with N0x in the stri,oper 22. The bulk of the N0 ls converted into N02 in the oxidation zone 24a~. Th~ gases inally enter the HN03 absorber 25, which ia equipped with a circulating pump 26 and an acid cooling means 27. The line 28a serves to introduce HN03 into the denitration zone.
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' ~L2933S9 _ 9 _ Excess HN03 produced is drawn off through the line 28. The exit gases containing N0 from the HN03 absorber pass into the oxidation zone 24b. The stream of gas containing more NOz than N0 anters the acid deDitration zone via the line 29.
Figure 3 illustrates the treatment of the return acid.
The return acid leaving the denitration zone snters the heatlng means 30 in which it i8 heated to 80C and then flows through the fine denitration tower 32. Lines 31a and 31b serve to charge this tower with gas which contains S02. It ~uffices to pass about 5 % of the total amount of waste gas through this tower.
The return acid then enters the S02 stripper 33 and subsequently paases into the cycle of the concentrating tower 34c The reference number 39 indicates the circulatlng pump of the concentrating towor, and 35 denotes the acid heatlng means. The H2S04 brought to a concentration of 75 % passes through the line 36 into the purified gas heating cycle. The water vapour containing air from the concentrating tower 34 pau~es through the line 37 into the condenser 38. Thiu condenser has an acid cycle with the coollng means 40.
Continuou ly forming condensate i9 drawn off through the line 42.
The dried air i~ used as stripping air in the S02 stripper before it i9 passed through the line 41 to the suction slde of the ~ventilator 21 (see figure 2).
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.:
.
Process for removing oxides of nitrogen and sulfur from wa~te ga~es The present invention relates to a proces~ which makes it po~sibla to ramQve oxides of nitrogen snd sulfur from wsste gases si~ulta-neously without the u~e of lime, sodium hydroxide, ammonia or other chemicals.
The separated n~trogen oxides are used as catalyst for oxidising sulfur dioxidæ with oxygen-containing gas, preferably atmopsheric oxygen. Ultimately, ~arketabl~ nitric acid and sulfur~c acid are obtained from the pollutants.
Tha advantages of the process of this invention will be explained by ~taking;as an example the purif~cation of flua gase~ from a power station ~uelled by ~alt-containing brown coal having a calorlfic valoe greater thsn 4500 kcallkg.
lt will be assumed that the power station produces the following wa-t~ gas, where~x in N0x i8 1 and 2~
amount o~ flue gas `1 mio. Nm3 per hour ~ ;
eoncentration of N0 (ca1cu1at~d~as N02)1.43 g/Nm3 (c. 700 ppm) ~
concantration of S02 ~~12.6 g/Nm3 ~c. 4300 ppm).~ :
Throughout~`thls specification, ppm concentrations will alway~ be~
understood as meaning parto by voIume per m1-l10n units of vo1ume.
: : ~ :
: :: ' :1~9335~
The process of tbis invention makes it possible to reduce the pollutant gases to valus~ lower than S02 0.1 g/Nm3 (c. 34 ppm) N0 0.2 g/Nm~ (c. lO0 ppm).
Provided cooling water wi~h an average annual temperature of l5C ~ 8 available, the operating requirements for the waste gas purification are:
electrical energy : le~s than 3 % of the energy produced in the power station, heat consumption for concentrating the acid in the course of the process: corresponds to a consumption of lsss than 2 % of the coal burned in the power plant.
When the nitric acid (concentration higher than 30 %~ Rnd sulfuric acid (concentration high~r than 75 %~ produced by ths proce6s of this invention can be utilised, for example, in a fertiliser factory, the value of the acids cover~ the operating costs referred to above. The capital expenditure for a procsss according to ths pr~sent invention is le6s than a quarter of that required for the powsr station. :
It is known to remove flue gases by adding ammonla to the still hot flue ga~es, before complete heat utilisation, and passing them over a catalyst, thereby reducing the nitrogen oxides to nitrogen. A
drawback of this process i8 the cost of the ammonia and the limited life of the catalyst. When using salt-containing coal, salt6 vaporise in the combu~tion chamber and form aerosols that contamin-~ ate the catalyst. A further drawback of the dry and also of the wet ; ~ use of ammonia is tha formation of ammonium sulfate aerosols and other salt aerosols which cannot bs separated completely despite the elaborate procedures employed. Ths aerosols released into the : :
, : ~
3~Z93359 atmosphere along with the treated flue gas promote vapour formationin the atmosph0re and prevent natural solar radiation by forming s~og.
In contradistinction to the known prior art, in the process of this invention the operating cost~ fall wlth increasing coDcentrations of N0 in the flue gases. Th~s any combu~tion measures for reducing the formation of N0x are rendered unnecessary.
The nltrogen oxides are essentially present in flua gase~ in the form of N0, which i8 Yery 3paringly absorable. It i5 known to convert N0 into N02 by addition of ozone and to absorb it, for example in ammoniacal liquids. In addition to the high costs of producing ozone, a drawback i8 the previously mentioned unavoidable formation of aerosols.
In recent year~, a~ a result of efforts to control atmospheric pollution, the nitrogen oxide-sulfuric acid process ha~ been proposed for separating SOz and N0x from waste gases containing about 1 % by volume of S02. The work carried out in this field forms the basis of the present in~ention and, in this connection, reference i~ made to the followlng publications:
- Fattinger, V., Proc. Brit. Sulphur Corp., 3rd Int. Conf. Fert., London, November 1979, Paper XXYI
~ Blanken3tein, K., Neumann, G.E., U.B.A., F and E Rsport ~o. 10403311, June 1980 - Sander, ~., Fattinger, V., Chem. Ing. Techn., 55 (1983) No. 8, S. 601l607 - Fattinger, V., CIBA-GEIGY CorporatioD US Patent 4 148 868 (1979) - Fattinger, V., CIBA-GEIGY Corporation US Patent 4 242 321 (1989) Ullmanns Enzyklopadie der technischen Chemie, 4. Deubearbeitete und erweiterte Auflage, Verlag Chemie, Weinheim, Vol. 21 (lg82), p. 148 " :
~93359~
~lue Kases emitted by power stations contain mostly only 0.02 to 0.5 % by volume of S02; ~nd up to now, an S02 enrichment has been considered necessary before processing in a nitrogen oxide-sulfuric acid system becomes po6sible. FurthermorP, expert opinion is that a nitrogen oxide-sulfuric acid process would cau~e atmospheric pollution through N0 losses.
A principal reason for the inapplicability of known nitrogen oxide-sulfuric acid processes for processing waste gases from power stations is the lengthy ~ojourn time for the oxidation of the N0 set free in the denitration tower. A prerequisite for nitrogen oxide absorption is a suitable ratio oE ~O:N02.
It has now been found that nitrogen oxide-sulfuric acid systems with a reaction requirement of less than 25 m3 per Nm3tsec of flue gas can be operated if more than half the concentration of nitrogen oxide present in the waste gas introduced into the process is absorbed, and the liquid which contains nitrogen oxide iB gubge-quently sub~ected to desorption, whereupon a) the nitrogen oxide set free is mixed with an oxygen-containing gas to give a gas which is relatively enriched with nitrogen oxide and oxygen, and in which nitrogen monoxide oxidises to nitrogen dioxide, and the nitrogen dioxide so obtalned and/or nitric acid produced therefrom, as well as a residual content of nitrogen monoxide, is introduced into the main stream of waste gas to be treated, the molar amounts of quadrivalent and pentavalent nitrogen-oxygen compounds introduced into the system being greater than the molar amount of nitrogen monoxide also introduced, and b) the liquid from which nitrogen oxides have been at least parti-ally removed, or the bulk thereof, is fed ~ack, after passing through the desorption plant, to one or more of the gas treatment towers of the main stream of gas. The acid which has been partially or completely freed from nitrogen compounds can be fed into the first towers of the sy~tem or else in its entirety or in part into ~ 933~
one of the N0x absorption towers (or also into a number of N0x absorption towers) at the end of the system. Both kinds of liquid feedback can be combined.
Because the acid for the purpo~e of N0x desorption has been heated andlor diluted by water or by a weaker acid, it is exp~dient to concentrate or cool the acid before the denitrated or partly denitrated acid enters an N0 absorption tower.
When treating the stream of waste gas, nitric acid is introduced, before the S02 absorptioD, into the denitration cycle or into the S2 absorption cycle in an amount such that an optimum ratio of NO:N02 for the N0x absorption is established. This step is not novel and is described e.g. in US patent specification 4 242 32l.
It has further been found that surprisingly good absorption results are obtained and that relatively high concentrations of nitrose can be employed in the absorption acid by carrying out the absorption at temperatures below 30C. It 18 advantageous to use heat pumps (cooling systems) to produce a cooling brine which, by indirect heat exchange, lowers the temperature of the acid in the cycle to below 20, 15 or even 10C. This measure has proved much less expensive than an excess1ve enlargement of the absorption ~one.
.
An important feature that distinguishe~ the process of this inven-tion from known processes resides in performing the denitration using Iow concentrations of scid. Sulfuric acid in a concentration of less than 73 % or even 70 % makes it possible to effect a sufficient acid denitration even at the low concentration of S02 in the flue gases emitted by power stations. The denitration tower employed in the main stream of gas is conveniently operated in direct~current between gas and acid.
, ~ :
The acid~discharged from the~pre-denitration ln the main current of gas can be sub~ected to indirect heating and to a fine denltration in~a tower through which flows a partial stream of sulfur dioxide.
: :
, :~
`:
~LZ9335~
It is also possible to carry out the entire acid denitration out3ide the main stream of gas and to pass a current of air and/or a partial stream of gas which contains S02 through the denitration apparatus.
If necessary, the current of air or t~e stream of gaA which contains S02 can be heated in order to accelerate the expulsion of the nitrogen oxides.
It is known that the vapour pressure of the nitrogen oxides dis-solved in the sulfuric acid a8 nitrosyl-salfuric acid is greatly increased by addition of nitric acid. Thus, if necessary, the acid denitration can be considerably accelerated by addition of nitric acid. The requi3ite nitric acid is formed in the process of this invention from nitrogen oxides and returns from the denitration apparatus to the process in the form of nitrogen oxides.
Before the denitrated acid enters the N0x absorption zone, it is brought, in a concentrating tower, to the concentration required for a good absorption (73 to 76 % HzS04). The hot acid originating from the concentrating tower can conveniently be used for heating the cold exit gas from the N0x absorption zone.
The heated gases of very low relative humidity can be transported to a bricked flue without the rl.k of tbeir becoming moist.
The advantages of the process of the present invention compared with known nitrogen oxide-sulfuric acid processes become all the more signlficant the lower the concentrations of S02 and the higher the concentrations of N0x in the waste gas to be purified. Preferably the concentration o~ SOz should be below 0.8 % by volume and the concentration of N0 should be above 100 ppm.
x Pigures 1, 2 and 3 and a working Example will serve to illustrate ~ the invention in more detall.
::
~93359 Figure 1 illustrates the treatment of the stream of waste gas in the gas washiDg towers which are connected in series and have the following functions: gas cooling, acid denitration, SO2 absorption, NOx absorption 1, NO absorption 2 and heating treated gas.
The following Table indicates the concentrations of SO2, NOx, H20 and the gas temperature before and after the treatment steps:
Conc0ntrations and tempera- SO2 N3 H20 O
tuFes of the 9tream of ga3 g/Nm3 as 2 glNm3 C
before gas cooling 12.6 1.43 125 130 after gas coollng 12.5 - 1.43 30 26 after denitration 6 22 8 38 after SOz-absorption 1 20 5 25 after NOx-absorption 1 0.5 4.5 0.1 10 after NOx-absorption 2 0.1 0.2 0.1 7 after hssting treated gas 0.1 0.2 3 60 Each treatment ÆOne is provided with an acid cycle which is kept in circulation by pumps 4. The reference numbers 1 and 9 indicate respectively the waste gas inlet and the gas feed valve. The reference number 2 indicates connecting lines between gas treatment zones and 3 denotes the treated ga~ outlet. All gas treatment tower contain packing layers 5. As shown in figure 1, individual acid treatment stages are equipped with acid coollng means 6 for lowering the temperstures of the acid in order to reach the gas temperatures indicated in the above Table. The reference number 7 indicates the acid spray nozÆles in the gss treatment zones. The reference number 8 indicates the acid sumps in the bottom part of the towers. The packings in the layers 5 hare a surface area of more than 300 m2im3.
The reaction 3pace requirem2nt per 1 Nm3/sec of gas is smaller than 3 m3 for the denltrat1on, s=aller than 4 =3 ior th~ SOz absorpti~n, : `
~L~933~i9 smaller than 4 m3 for the N0 absorptlon 1, smaller than 12 m3 for the N0 absorption 2, and ~maller than 1.5 m3 for heating the treated gas.
As in every nltrogen oxlde-sulfuric acid system, an acid exchange takes place between the acid denitration and the N0 absorption. The acid freed from nitrose and which flows into the N0 absorption zone i8 hereinafter raferred to as "return acid", whilst the acid containing N0 ia referred to as "primary acid". The amount of sulfuric acid continuously formed in the S0z absorption zone is mixed ~ith the primary acid.
High concentrations of nitrose in the acid are achievad as a consequence of the strong cooling in the N0 sbsorption zone.
Expressed in HN03 equivalents, the concentration of nitrose in the primary acid is higher than 200 g of HN03 per litre. This high concentration makes it possible to work with small amounts of primary and return acid. In the working Example, amounts of less than 500 g/Nm3 of waste gas to be purified suffice.
The low temperatures in the entire treatment zone of the main stream of gas simplify the problem of chooaing suitable materials. In the entire area of the large treatment zones between gas cooling and heatlng treated gas, there are no temperatures hlgher than 40C.
Figure 2 illustrates the treatment of the primary acid. The primary acid is heated, in the heatlng means 20, to a temperature of about 60C and fed to the N0x stripper 22. Spent air ls drawn off from the primary acid treatment zone through this N0x stripper via the llne 4l~ and the ventilator 21. This spent air is essentially slightly impure air w~hlch becomes enriched with N0x in the stri,oper 22. The bulk of the N0 ls converted into N02 in the oxidation zone 24a~. Th~ gases inally enter the HN03 absorber 25, which ia equipped with a circulating pump 26 and an acid cooling means 27. The line 28a serves to introduce HN03 into the denitration zone.
:: : :
: : : :
:: :
' ~L2933S9 _ 9 _ Excess HN03 produced is drawn off through the line 28. The exit gases containing N0 from the HN03 absorber pass into the oxidation zone 24b. The stream of gas containing more NOz than N0 anters the acid deDitration zone via the line 29.
Figure 3 illustrates the treatment of the return acid.
The return acid leaving the denitration zone snters the heatlng means 30 in which it i8 heated to 80C and then flows through the fine denitration tower 32. Lines 31a and 31b serve to charge this tower with gas which contains S02. It ~uffices to pass about 5 % of the total amount of waste gas through this tower.
The return acid then enters the S02 stripper 33 and subsequently paases into the cycle of the concentrating tower 34c The reference number 39 indicates the circulatlng pump of the concentrating towor, and 35 denotes the acid heatlng means. The H2S04 brought to a concentration of 75 % passes through the line 36 into the purified gas heating cycle. The water vapour containing air from the concentrating tower 34 pau~es through the line 37 into the condenser 38. Thiu condenser has an acid cycle with the coollng means 40.
Continuou ly forming condensate i9 drawn off through the line 42.
The dried air i~ used as stripping air in the S02 stripper before it i9 passed through the line 41 to the suction slde of the ~ventilator 21 (see figure 2).
:: : :
::: ~: :
:: :
.:
.
Claims (10)
1. A process for removing oxides of nitrogen and sulfur from waste gases, in particular from flue gases emitted by power plants, by means of a modified nitrogen oxide-sulfuric acid process for the production of sulfuric acid to obtain marketable nitrogen and sulfur compounds, which process comprises absorbing more than half the concentration of nitrogen oxide present in the waste gas introduced into said process and then subjecting the liquid that contains nitrogen oxide to desorption, whereupon a) the nitrogen oxide set free is mixed with an oxygen-containing gas to give a gas which is relatively enriched with nitrogen oxide and oxygen and in which nitrogen monoxide oxidises to nitrogen dioxide, and the nitrogen dioxide so obtained and/or nitric acid produced therefrom, as well as a residual content of nitrogen monoxide, is introduced into the main stream of waste gas to be treated, the molar amounts of quadrivalent and pentavalent nitrogen-oxygen compounds introduced into the system being greater than the molar amount of nitrogen monoxide also introduced, and b) the liquid from which nitrogen oxides have been at least par-tially removed, or the bulk thereof, is fed back, after passing through the desorption plant, into one or more of the gas treatment towers of the main stream of gas.
2. A process according to claim 1, wherein the nitrogen oxides present in the waste gas are absorbed in sulfuric acid.
3. A process according to either of claims 1 or 2, wherein the temperature of the absorption liquid is lower than 30°C.
4. A process according to claim 1, wherein the concentration of sulfuric acid in the acid denitration is less than 73 % by volume.
5. A process according to claim 19 wherein the waste gas and the acid to be denitrated pass through the denitration stage of the nitrogen oxide process in direct current.
6. A process according to either of claims 1 or 5, wherein the acid which is pre-denitratad in the denitration stage of the nitrogen oxide process is subjected to a fine denitration and/or is concen-trated.
7. A process according to claim 1, wherein the acid leaving the fine denitration and/or concentrating zone is used to heat the cold exit gases of the nitrogen oxide absorption zone.
8. A process according to claim 1 for treating waste gases contain-ing less than 0.8 % & by volume of sulfur dioxide.
9. A process according to either of claims 1 or 8 for treating waste gases containing more than 100 ppm of nitrogen oxide.
10. Apparatus for carrying out the process according to claim 1, having a plurality of gas washing towers, connected in series, for gas cooling, denitration, sulfur dioxide absorption, nitrogen oxide absorption 1 and 2 and heating treated gas, said apparatus compri-sing a system for the desorption of the liquid containing nitrogen oxides and introduced into the nitrogen oxide absorption, said system containing heating means (20) for said liquid containing nitrogen oxides, a nitrogen oxide stripper (22) with a line (41) for an oxygen-containing gas and provided with a ventilator (21), an oxidation zone (24a) connected to the stripper (22) via a line, from which oxidation zone a line leads to a nitric acid absorber (25) which is equipped with a circulating pump (26) and an acid cooling means (2), a line (28a) for feeding nitric acid into the acid denitration zone of the waste gas treatment, a line (28b) for drawing off excess nitric acid produced, a second oxidation zone (24b) connected to the nitric acid absorber (25) via a line, and a line (29) for feeding nitrogen dioxide into the acid denitration zone of the waste gas treatment.
FO 7.1 SI/eg*
FO 7.1 SI/eg*
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CH3871/84-4 | 1984-08-13 | ||
| CH387184 | 1984-08-13 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1293359C true CA1293359C (en) | 1991-12-24 |
Family
ID=4265274
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000488446A Expired - Lifetime CA1293359C (en) | 1984-08-13 | 1985-08-09 | Process for removing oxides of nitrogen and sulfur from waste gases |
Country Status (4)
| Country | Link |
|---|---|
| EP (1) | EP0174907A3 (en) |
| JP (1) | JPS6154224A (en) |
| CA (1) | CA1293359C (en) |
| DD (1) | DD237117A5 (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE3717835A1 (en) * | 1987-05-27 | 1988-12-08 | Bayer Antwerpen Nv | METHOD FOR NO-RECOVERY FROM THE EXHAUST GAS PRODUCED IN THE AMMONIUM NITRITHER PRODUCTION |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4148868A (en) * | 1975-03-10 | 1979-04-10 | Ciba-Geigy Corporation | Process for separating SO2 from a current of gas containing the same |
| DE2830214A1 (en) * | 1977-07-21 | 1979-02-08 | Ciba Geigy Ag | METHOD FOR SEPARATING SO LOW 2 FROM A GAS FLOW AND PLANT FOR CARRYING OUT THE METHOD |
-
1985
- 1985-08-07 EP EP85810365A patent/EP0174907A3/en not_active Withdrawn
- 1985-08-09 CA CA000488446A patent/CA1293359C/en not_active Expired - Lifetime
- 1985-08-12 DD DD85279598A patent/DD237117A5/en unknown
- 1985-08-13 JP JP60177059A patent/JPS6154224A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| DD237117A5 (en) | 1986-07-02 |
| JPS6154224A (en) | 1986-03-18 |
| EP0174907A2 (en) | 1986-03-19 |
| EP0174907A3 (en) | 1989-10-25 |
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