Classifications

• • 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
  • C01B21/02—Preparation of nitrogen
• • 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
  • B01D53/54—Nitrogen compounds
  • B01D53/56—Nitrogen oxides

Definitions

• the present invention relates to a process for converting gaseous nitric oxide (NO) , such as nitric oxide present in an exhaust gas, to molecular nitrogen and water at a relatively low temperature without the use of a separate catalyst.
• NO gaseous nitric oxide
• combustion processes occurring in electrical and/or thermal power stations or heating plants fueled by coal, coke, fuel-oil, gas, wood or the like;
• combustion processes occurring in internal combustion engines or other forms of drive engine such as gas turbines fueled by gasoline, kerosene, diesel oil, alcohols, butane/propane gas or the like.
• processes other than combustion processes e.g. certain petroleum refining processes and processes for the manufacture of chemicals, may also give rise to exhaust or flue gases containing appreciable levels of. nitric oxide. It is clear that unless measures are taken to remove nitrogen oxides from the mixture of gaseous products in an exhaust gas, they will thereafter be released into the atmosphere.
• N0 2 nitrogen dioxide
• RH represents an alkane (e.g. methane, CH 4 ) ; OH denotes a free hydroxyl radical (formed in the atmosphere via photochemical processes); R denotes a free alkyl radical; OOR denotes a free alkylperoxy radical; and OR denotes a free alkoxy radical] .
• Nitrogen dioxide reacts readily with various atmospheric species to form, inter alia, nitric acid, for example as fol- lows:
• catalyzed processes i.e. processes employing contact catalysts (normally solid-phase catalysts)
• contact catalysts normally solid-phase catalysts
• particular effort has nevertheless been directed towards the development of uncatalyzed processes.
• catalysts (a) are generally expensive to purchase, install and maintain, (b) can give rise, inter alia, to problems associated with the pressure drop resulting from passage of the combustion exhaust gas through a catalyst bed, or with clogging or fouling of the catalyst bed by gas- borne particulate matter, and (c) are often easily "poisoned" by various substances which may be present in combustion exhaust gas, e.g. in flue gas from coal-fired installations.
• US patent No. 3,900,544 relates to a process for reducing the concentration of nitric oxide present in combustion effluents, the process comprising contacting the combustion effluent stream with ammonia or an ammonia precursor (such as ammonium formate, ammonium oxalate or ammonium carbonate) in the presence of oxygen and at a temperature in the range of 1300-2000°F (ca. 704-1093°C) .
• ammonia or an ammonia precursor such as ammonium formate, ammonium oxalate or ammonium carbonate
• WO 85/02130 relates to an improvement in the "classical" type of DeNO x process employing ammonia [such as the process of US 3,900,544 (vide supra) 1 and discloses a process wherein ammonia is injected into a combustion effluent (containing NO and at least 0.1 volume percent of oxygen) at a point where the effluent is at a temperature of 975-1600 K (ca. 702-
• the amount of ammonia to be introduced into the combustion effluent and the point at which it is to be introduced are determined by solution of a set of simultaneous equations derived from a kinetic model described therein.
• US patent No. 4,213,944 relates to a process for converting NO x present in a NO x -containing gas to nitrogen and water, the process involving adding a reducing agent (in the form of ammonia, an ammonium salt, an amine or an amide) and hydrogen peroxide to the gas; the hydrogen peroxide is added simultaneously with or subsequent to the addition of the reducing agent to the hot (400-1200°C) gas.
• a reducing agent in the form of ammonia, an ammonium salt, an amine or an amide
• Japanese unexamined patent publication (kokai) No. 53-146968 discloses a method for decomposing nitrogen oxides (N0 X ) contained in a combustion gas to nitrogen and water, the method involving adding ammonia to the combustion gas in a high-temperature region (such as a region having a temperature of 850-1100°C) , and subsequently adding hydrogen peroxide in a low-temperature region. It is stated that the temperature in the low-temperature region should not be lower than 400°C.
• Japanese unexamined patent publication (kokai) No. 54-72764 discloses a method for controlling the supply of hydrogen peroxide used in a "denitrification" process employing the principles disclosed, for example, in Japanese kokai No. 53-146968 (vide supra) , i.e. in a process where a reducing agent (e.g. ammonia) is supplied to a high-temperature region in a N0 X -containing combustion exhaust gas, and hydrogen peroxide is subsequently added in a low-temperature region.
• the method in question involves supplying hydrogen peroxide to the low-temperature region in such a manner that the molar ratio of hydrogen peroxide to nitrogen oxides (i.e. nitrogen oxides remaining after the initial "upstream" reaction of the reducing agent with NO ⁇ present in the combustion exhaust gas) is substantially 1.5.
• the temperature in the low-temperature region should suitably be from about 500 to about 600°C.
• Japanese unexamined patent publication (kokai) No. 57-12819 discloses a method for reducing N0 X present in a combustion exhaust gas, the method comprising adding at least one member selected from (sic) "an oxygen-containing hydrocarbon, its precursor and hydrogen peroxide" to a combustion exhaust gas so as to convert the major part of the NO contained in the gas to N0 2 , and then carrying out catalyst-free
• reaction temperatures for the reaction with hydrogen peroxide and the subsequent reaction with ammonia are preferably within the range of 300-750°C.
• the present invention provides a DeNO x process which does not require the use of a catalyst and which exploits the transient gas-phase formation of ammonium nitrite (the formula of which is written herein either as NH 4 N0 2 or NH 4 ONO) and the hitherto unacknowledged gas-phase decomposition thereof to nitrogen (N 2 ) and water (H 2 0) at relatively low temperatures, such as temperatures ⁇ 400°C, suitably in the range of 250-350°C, e.g. in the vicinity of 300°C.
• ammonium nitrite in the gas phase takes place by reaction of ammonia (added to the NO-con- taining gas as such, or formed from another compound which upon thermal dissociation or decomposition thereof gives rise to ammonia) with gaseous nitrous acid (HONO) , the latter being formed from NO via a reaction, or a sequence of reactions, involving an oxidizing reagent, e.g. hydrogen peroxide.
• HONO gaseous nitrous acid
• the DeNO x process of the invention is well suited to on-line control.
• the chemistry involved in the process is relatively simple, and levels of the key reactants HONO and NH 4 0N0, as well as of NO and - if appropriate - other process-relevant species [e.g. H 2 0 2 or N0 2 (vide infra) 1. may be monitored continuously using, for example, spectroscopic techniques as described in the experimental section herein (vide infra) .
• the DeNO x process of the present invention appears to be capable of a high degree of conversion ( ⁇ 90%) of NO to N 2 and H 2 0.
• the invention thus relates to a process for converting nitric oxide (NO) present in an exhaust gas, such as an exhaust gas deriving from one of the above-mentioned sources, to nitrogen and water, the process comprising: A) contacting the exhaust gas with one or more reagents which, via reaction with the nitric oxide present in the exhaust gas, lead(s) to the formation of nitrous acid (HONO) in the gas phase, and
• HONO nitrous acid
• step B contacting the gaseous mixture from step A (which contains gaseous HONO) with ammonia, or an ammonia precursor (i.e. a compound which under the prevailing conditions liberates ammonia) .
• ammonia precursor i.e. a compound which under the prevailing conditions liberates ammonia
• step A The temperature at which step A is most advantageously carried out will, among other factors, depend on which sequence of reactions [e.g. one of the sequences according to Schemes II or III below (vide infra) 1 is employed to generate HONO from NO. It will generally be desirable to operate step A of the process of the invention at temperatures in excess of 300°C, such as temperatures of about, or in excess of,
• an important criterion is that the temperature must be sufficiently high to ensure that no deposition of solid ammonium nitrite can occur as a result of accumulation of gaseous NH 4 N0 2 and subsequent condensation thereof in solid form on relatively cool surfaces, e.g. within conduits or other structural parts of the plant in question with which the exhaust gas comes into contact after the gaseous mixture from step A has been contacted with ammonia or an ammonia precursor.
• solid ammonium nitrite decomposes explosively at temperatures of 60-70°C (see, for example, T. Urbanski, Chemistry and Technology of Explosives, Vol. II, Pergamon Press and PWN - Polish Scientific Publishers, pp. 491-493), and it is therefore clear that any accumulation of solid NH 4 N0 2 would constitute an explosion hazard.
• step B of the process should at least be such that the formation of ammonium nitrite by reaction of HONO with NH 3 (an equilibrium process) cannot lead to deposition and accumulation of solid- phase ammonium nitrite within conduits or in other parts of the plant in which the process of the invention is carried out, the gas-phase ammonium nitrite formed then undergoing non-detonative, irreversible decomposition to yield nitrogen and water.
• This sequence of reactions is summarized in Scheme I below:
• the reaction temperature in step B should, at the least, not be lower than about 70°C, i.e. it should preferably be above 70°C.
• step B of the present process Under the temperature conditions prevailing in step B of the present process, the rate-determining process in the entire sequence of reactions leading to formation of N 2 and H 0 appears to be the formation of ammonium nitrite from HONO and NH 3 in the gas phase in accordance with the equilibrium [reaction (1)] shown in Scheme I above.
• reaction temperature in step B should be above ca. 200°C. It appears that temperatures in the range of 200-400°C, such as 250-350°C, e.g. 300-350°C, will be an appropriate choice for step B of the process, and that temperatures ⁇ 300°C are often suitable.
• step A of the process of the invention it will in general be desirable to initiate step A of the process of the invention at an early stage after the exhaust or flue gas has emerged from the combustion source in question (incinerator, oven or the like) .
• the appropriate time-lapse between the performance of step A and the performance of step B will to some extent be determined by the temperatures at which step A and step B, respectively, take place.
• operation of step A at a higher temperature than step B will generally require provision of a cooling stage between the region of the treatment plant in which step A is performed and the region in which step B is performed, and the regions in question may be therefore be physically separated from one another by an appreciable distance.
• steps A and B are operated at essentially the same temperature, the time-lapse in question will largely be determined by the time required for substantial completion of the overall sequence of reactions involved in step A; in this case only a relatively short time-lapse (and a correspondingly shorter physical separation between the regions of the plant in question) may be necessary.
• the formation of gaseous nitrous acid (HONO) from NO in step A of the DeNO x process according to the present invention takes place via a reaction or sequence of reactions involving a reagent or reagents, at least one of which is an oxidizing reagent.
• HONO gaseous nitrous acid
• reagents suitable for use in step A of the process of the invention are:
• Hydrogen peroxide (H 2 0 2) is a particularly well suited reagent, and for the purposes of step A of the process of the invention it may conveniently be injected into (or otherwise brought into contact with) the exhaust gas in the form of an aqueous solution of hydrogen peroxide, for example as an aerosol, mist or spray.
• Aqueous solutions of hydrogen per ⁇ oxide having concentrations of about 3%, 6%, 27.5%, 30%, 35% and 50% by weight (w/w) are often readily available on a technical/industrial scale and will be applicable in the context of the present invention; however, for most purposes, aqueous solutions of concentration in the range of about 25- 50% w/w, such as ca. 30% w/w, will generally be preferable.
• more highly concentrated aqueous solutions e.g. ca. 90% w/w solutions, are also commercially available, they exhibit highly aggressive oxidizing properties and are generally rather unstable, rendering them less useful in connection with the present invention.
• the necessary ozone may conveniently be produced using, for example, an electrically powered ozone generator (ozonizer) , a number of types of which are commercially available.
• ozonizer electrically powered ozone generator
• the organic species in question is formaldehyde
• it is suitably contacted with the hot exhaust gas while in the form of an aqueous solution, such as an aqueous solution of concentration in the range of about 35-50% by weight of formaldehyde (such solutions often being known as "for ⁇ malin" .
• formaldehyde may be generated in situ by introducing solid paraformaldehyde, preferably in powder form, into the hot exhaust gas, whereupon formaldehyde will be released by thermal decomposition (depolymerization) of paraformaldehyde.
• ozone and an organic species, such as formaldehyde, from which a hydrogen atom can be abstracted are employed in step A of the process of the invention, it will - as a consequence of the reaction mechanism involved [see, e.g., the mechanism given below (vide infra) in the case of formaldehyde] - normally be appropriate to first introduce ozone into the exhaust gas and then introduce the organic species in question.
• step A of the DeNO x process of the present invention gives - by way of example - a brief schematic summary of reaction mechanisms which are believed to be operative in step A of the DeNO x process of the present invention when using (i) hydrogen peroxide, and (ii) ozone and formaldehyde, respectively, as the reagent (s) leading to the formation of HONO:
• step A If hydrogen peroxide is employed as reagent in step A, it will normally be most suitable to bring hydrogen peroxide into contact with the exhaust gas in amounts consistent with the reaction stoichiometry which follows from Scheme II (vide supra) , i.e. such that hydrogen peroxide is continuously introduced so as to provide substantially one molecule of H 2 0 2 per two molecules of NO present in the exhaust gas.
• ozone and formaldehyde are employed as reagents in step A, it will normally be most suitable to bring ozone into contact with the exhaust gas in amounts consistent with the reaction stoichiometry which follows from Scheme III (vide supra) . i.e. such that ozone is continuously introduced so as to provide substantially one molecule of 0 3 per molecule of NO present in the exhaust gas.
• the introduction of formaldehyde into the system, which - as already mentioned above - will normally take place subsequent to introduction of ozone, will for similar reasons normally take place such that formaldehyde is continuously introduced so as to provide substantially one molecule of HCHO per two molecules of NO which are present (i.e. present prior to the reaction thereof with ozone) in the exhaust gas.
• ammonia per se it is generally preferable and most convenient to use ammonia per se in step B of the process according to the present invention, for example by injecting ammonia gas into the gaseous product stream after performing step A.
• the stoichiometry of the reaction between ammonia and HONO is 1:1, and the ammonia should be introduced in such a manner that the added amount is continuously matched (ideally in a 1:1 ratio) to the level of HONO formed in step A of the process of the invention.
• the chemistry of the process of the invention is well suited to on-line regulation or control of, inter alia, the rate of addition of ammonia so as to ensure optimal conversion of HONO to NH 4 N0 2 and at the same time avoid the presence of an excess of ammonia in the treated exhaust gas.
• ammonia per se is generally to be preferred, it is also possible - as already indicated - to generate ammonia in situ using one or more ammonia precursor compounds which under the prevailing conditions liberate (s) ammonia.
• Ammonia precursors suitable for use in step B of the process of the present invention are, for example, substances such as ammonium carbonate, i.e. (NH 4 ) 2 C0 3 , ammonium hydrogen car ⁇ bonate, i.e. NH 4 HC0 3 , and ammonium carbamate, i.e. NH 2 C00NH 4 , all of which liberate ammonia readily and rapidly at temperatures of about 60°C or more, i.e. at temperatures relevant in connection with step B.
• ammonium carbonate often consists predominantly of a mixture of ammonium hydrogen carbonate and ammonium carbamate, and such products will clearly also be suited for the purposes of the present invention.
• the conditions employed will preferably be adjusted so that a residence time not exceeding about 30 seconds, more preferably ⁇ 20 seconds, is achieved for steps A and B; very desirable residence times will be ⁇ IO seconds.
• the operation of the process of the invention requires no use of a separate catalyst (i.e. a deliberately introduced catalytic material) in either of the steps A or B of the process.
• a catalyst or catalysts suitable for use in one or both of steps A and B e.g. a solid-phase or liquid-phase contact catalyst
• the process of the present invention may be operated satisfactorily as part of an overall process (sometimes referred to as a SNOX process) for the removal of both sulfur oxides (e.g. S0 2 ) and nitrogen oxides (primarily NO) from exhaust or flue gases arising from combustion processes.
• SNOX process sulfur oxides
• NO nitrogen oxides
• absorbance (A) is defined as log (I 0 /I) , where I 0 is incident light intensity and I is transmitted light intensity.
• HITRAN91 Molecular Data Base; for the spectrum of HONO, see, e.g., A. J. Maki. J. Mol. Spectrosc. 127 (1988) pp. 104-111] . It should be noted here that gaseous HONO occurs as an equi ⁇ librium mixture of cis and trans isomers, the thermodynami- cally more stable trans isomer being overwhelmingly predominant.
• the spectrum of gaseous H 2 0 2 exhibits three par ⁇ ticularly prominent rotational lines at 1235.5850, 1235.6080 and 1235.6476 cm -1 , respectively.
• the spectrum of gaseous HONO likewise exhibits prominent rotational lines at 1235.6206, 1235.6423 and 1235.6542 cm “1 , respectively
• the measuring cell was evacuated (using a rotary pump in combination with a condensing trap cooled with liquid nitrogen) and the vapour from 30% v/v aqueous hydrogen peroxide solution was then introduced into the evacuated cell to give a total partial pressure of hydrogen peroxide and water (vapour) at 22°C of 0.8 mbar.
• Nitric oxide was then added to the measuring cell (from a commercially available "lecture bottle", i.e. a small gas cylinder) to give an appropriate partial pressure of NO at 22°C (thus, for example, in one set of measurements a partial pressure of NO of 2 mbar was employed) .
• the decrease in the absorbance due to H 2 0 2 and the concomitant increase in the absorbance due to HONO were then monitored as a function of time, starting from the addition of NO to the measuring cell.
• gaseous HONO was generated by reacting crystalline potassium nitrite with dilute (10% v/v) sulfuric acid. After evacuating the measuring cell (using a rotary pump in combination with a condensing trap cooled with liquid nitrogen) , gaseous HONO was introduced into the cell to give a partial pressure of HONO of about 0.01 mbar.
• solid ammonium nitrite which is a recommended reagent for the generation of gaseous nitrous acid; see, e.g., Environ. Sci. Technol.
• HONO source 25 (1991) 255-260] was employed as HONO source; in this case, the vapour phase (consisting of HONO and NH 3 ) formed over a sample of solid ammonium nitrite in an initially evacuated container was introduced into the measuring cell in the same manner as described above to give a partial pressure of HONO of about 0.01 mbar. At this partial pressure of HONO, the absorbance for the most intense HONO rotational line was greater than 0.3, corresponding to greater than 50% absorption of the incident infrared radiation.
• reaction (1) already discussed (vide supra) , a thermal equilibrium is - to a first approximation which ignores subsequent slow decomposition of NH 4 ONO at ambient temperature to give N 2 and H 2 0 - attained in which the equilibrium concentration of gaseous NH 4 ONO depends on the prevailing concentration of gaseous NH 3 .
• the equilibrium constant for reaction (1) expressed in terms of partial pressures (p) , i.e.:
• K p p(NH 4 ONO)/ ⁇ p(HONO) -p(NH 3 ) ⁇ ,
• a 0 and A eq are the absorbance values for HONO before addition of ammonia and after establishment of equilibrium, respectively.
• K_ (8.2 ⁇ 0.3) -10 2 atm "1 was obtained at a temperature (T) of 295.6 K.
• the rate constant (k lf ) for the forward process from left to right in reaction (1) i.e. HONO + NH 3 ⁇ NH 4 ONO
• k lf (2.1 ⁇ 0.2) -10 3 M ' -'-s "1 was obtained.
• a value of ca. 2-10 2 was also determined for K p at 318.8 K.
• a gas or gas mixture is irradiated with a short pulse (typically of the order of 10 nsec) of high- energy (typically about 600 keV) electrons. If sulfur hexafluoride, SF 6 , is irradiated in this manner, a high yield of fluorine atoms is obtained as follows:
• M denotes a "third body” gaseous species which is present in a relatively high concentration and whose function is to absorb excess vibrational energy; in the present case, M would be SF 6 )
• reaction (9) If a large excess of H 2 0 relative to NO is employed, sub- stantially all the F atoms are consumed in reaction (9) , whereby reaction (11) is suppressed.
• the hydroxyl radicals produced by reaction (9) can be converted quantitatively to HONO via reaction (10) , and by further including NH 3 in the reaction mixture all of the reactions (8) -(11) - as well as reactions (1) and (2) which are the reactions of interest in the present context - are initiated upon irradiation and can be studied at high time resolution, since the transient in ⁇ frared absorption signals can be detected on a microsecond timescale.
• the kinetic/energetic data for the reactions involved in the process of the invention may be used in computer modelling or simulation with a view to predicting the optimum operating conditions for the process of the invention, e.g. the optimum conditions of temperature and/or the optimum conditions of introduction of necessary reagents or substances so as to provide a sufficiently short residence time within steps A and B of the process of the invention.

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• 1993
  • 1993-03-26 DK DK36193A patent/DK36193D0/da not_active Application Discontinuation
• 1994
  • 1994-03-25 AU AU63749/94A patent/AU6374994A/en not_active Abandoned
  • 1994-03-25 WO PCT/DK1994/000129 patent/WO1994022562A1/en active Application Filing

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