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WO1997017127A1 - A method of decomposing the oxides of nitrogen in flue gas - Google Patents

A method of decomposing the oxides of nitrogen in flue gas Download PDF

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Publication number
WO1997017127A1
WO1997017127A1 PCT/CA1996/000720 CA9600720W WO9717127A1 WO 1997017127 A1 WO1997017127 A1 WO 1997017127A1 CA 9600720 W CA9600720 W CA 9600720W WO 9717127 A1 WO9717127 A1 WO 9717127A1
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Prior art keywords
gas
oxide
metal
oxides
nitrogen
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Application number
PCT/CA1996/000720
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French (fr)
Inventor
Klaus Oehr
Original Assignee
Dynamotive Corporation
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Priority to AU72738/96A priority Critical patent/AU7273896A/en
Publication of WO1997017127A1 publication Critical patent/WO1997017127A1/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/02—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the alkali- or alkaline earth metals or beryllium
    • B01J23/04—Alkali metals
    • 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
    • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
    • B01D53/86—Catalytic processes
    • B01D53/8621—Removing nitrogen compounds
    • B01D53/8625—Nitrogen oxides
    • B01D53/8628—Processes characterised by a specific catalyst
    • 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
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/9404—Removing only nitrogen compounds
    • B01D53/9409—Nitrogen oxides
    • B01D53/9413—Processes characterised by a specific catalyst
    • B01D53/9418—Processes characterised by a specific catalyst for removing nitrogen oxides by selective catalytic reduction [SCR] using a reducing agent in a lean exhaust gas
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2251/00—Reactants
    • B01D2251/20—Reductants
    • B01D2251/208—Hydrocarbons
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/202—Alkali metals
    • B01D2255/2022—Potassium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/202—Alkali metals
    • B01D2255/2025—Lithium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/202—Alkali metals
    • B01D2255/2027—Sodium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/204—Alkaline earth metals
    • B01D2255/2045—Calcium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/204—Alkaline earth metals
    • B01D2255/2047—Magnesium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/40—Mixed oxides

Definitions

  • This invention relates to a method of reducing acid rain and ozone depletion precursors from combustion flue gas, to additives useful in the method and to a method of producing the additives.
  • Ozone depletion and acid rain are problems throughout the world.
  • the deterioration of the ozone layer is creating an epidemic in skin cancer and acid rain affects the environment by reducing air quality, rendering lakes acid and killing vegetation, particularly trees. It has been the subject of international dispute. Canada and the United States have argued over the production of acid rain. Britain and Scandinavia are other antagonists.
  • the sulphur dioxide typically originates from the combustion of a sulphur containing fuel, for example coal.
  • the sulphur dioxide is oxidized in the atmosphere to sulphur trioxide and the trioxide is dissolved to form sulphuric acid.
  • the rain is thus made acid.
  • the oxides of nitrogen are known to be precursors to acid rain and NO is a catalytic agent in the destruction of the ozone layer.
  • N 2 0 is both a greenhouse gas, 270 times more absorptive than carbon dioxide, and a precursor to NO formation in the ozone layer. It has been argued that N 2 0 photo-dissociation in the ozone layer is a greater source of NO than is the direct flux of NO from the earth's surface.
  • the emission of oxides of nitrogen in the United States and Canada is about one fifth sulphur dioxide emissions. But that still means that millions of tons of oxides of nitrogen are fed to the atmosphere each year. Although it is believed that the production of sulphur dioxide has stabilized, larger emissions of the oxides of nitrogen are anticipated because of the increased use of fossil fuels.
  • Gas desulphurization systems are known. The majority rely on simple basic compounds, for example, calcium carbonate, sodium carbonate and calcium hydroxide, to react with the acidic sulphur containing species to produce non-volatile products such as calcium sulphite, calcium sulphate and sodium sulphate. Urea and ammonia have been used to react with oxides of nitrogen generated during fossil fuel combustion to produce non ⁇ toxic nitrogen gas but urea and ammonia are expensive and are unsuitable for destruction of NOx produced in mobile applications, e.g., from vehicles. Conventional alkali adsorbents such as lime have not shown an ability to destroy nitric oxide (NO) .
  • NO nitric oxide
  • Applicant's United States patent No. 5,548,803 describes and claims a process for reducing acid emission from a flue gas produced by combustion of a sulphur containing fuel.
  • a pyrolysis liquor containing a thermolabile alkaline earth metal compound is introduced into a flue containing the flue gas.
  • the alkaline earth metal compound is able to decompose at flue gas temperature to produce an alkaline compound able to react with sulphur dioxide in the flue gas.
  • This method has achieved excellent results.
  • a particular advantage over the prior art is that the thermal decomposition of the organic salts, for example, calcium salts, produces high surface area calcium oxide, certainly of a surface area higher than has been achieved from non-organic metal salts such as calcium carbonate.
  • Patentees have postulated that this is due to the abrasive and turbulent action of gases such as carbon dioxide and water generated during the thermal decomposition. This effect is particularly useful for rupturing sulphite and sulphate films that foul the oxide adsorbents, such as calcium oxide, used in sulphur dioxide removal during fossil fuel combustion.
  • U.S. Patent 5,071,815, issued December 10, 1991 to Oehr et al describes and claims a method of forming a solid metal oxide crystal structure having a peroxide defect and doped with an alkali metal or alkaline earth metal.
  • the method comprises growing the doped metal oxide crystal by thermally decomposing, in an oxidizing atmosphere, a molten mixture of an alkaline metal salt or alkaline earth metal salt and the metal oxide, or a compound that can decompose to form the metal oxide under the thermal decomposition conditions, and then cooling the reaction mixture.
  • the metal oxide has a cubic crystal structure.
  • the metal oxide may, for example, be an oxide of a rare earth, for example, samarium oxide or it may be zirconium oxide.
  • Useful alkali metals include lithium, sodium or potassium.
  • a useful alkaline earth metal is calcium.
  • the crystal may be grown in the presence of a support.
  • the catalysts are especially useful in the production of ethane from methane. They assist in producing methyl free radicals from the methane. The free radicals combine to form ethane.
  • These catalysts have high concentration of peroxide defects in their crystal structure which means that the compounds are essentially non-stoichiometric with respect to oxygen; they possess an excess of oxygen.
  • M is an inorganic cation
  • p is the stoichiometric number of moles of the inorganic cation
  • M, p, O and x are as defined above;
  • D is a monovalent alkali metal cation dopant, for example lithium, sodium or potassium;
  • y is the number of extra peroxide defects created by the presence of the monovalent alkali or alkaline earth metal dopant cation in the metal oxide crystal structure.
  • certain metal oxide crystals can be doped with small alkali metal or alkaline earth metal cations.
  • the dopants replace the normal cation in the metal oxide crystal structure.
  • lithium and sodium with, respectively, ionic radii of about 0.6 and about 0.65 angstrom units, can replace calcium with an ionic radius of 0.99 angstrom units in the calcium oxide crystal structure.
  • the resulting doped crystals are believed to have the following empirical formulae: CaO x+y Li y and CaO x+y Na y
  • the general formula for a doped metal oxide is: M p O q+x+y D y/2 Ila
  • magnesium oxide the value of x in formula II above is zero.
  • the magnesium oxide can be made to contain peroxide defects as shown by y in formula II. The presence of peroxide defects in lithium doped magnesium oxide crystals has been verified experimentally by, for example, Driscoll et al, in the Journal of the American Chemical Society, 107:58-63.
  • the prior art teaches the doping by alkali metal oxides to generate peroxide defects and the use of these doped metal oxides to convert methane to ethane.
  • Prior art processes for producing maximum peroxide defects in metal oxides for catalysts particularly useful for methane conversion suffer from a number of disadvantages.
  • First many of the prior art processes use a solid dopant, for example, a carbonate.
  • the salt is mixed with a solid metal oxide in a technique known as dry impregnation. The mixing is achieved by kneading or physically massaging and grinding the salt with the metal oxide. This can only achieve slight penetration of the metal oxide crystal structure by the dopant .
  • aqueous catalyst dopant such as a nitrate
  • a solid metal oxide in a process known as wet impregnation, followed by heating to remove moisture and the dopant anion, for example, nitrate, metal oxide impregnation is only slightly improved over the dry impregnation method.
  • the present invention is based on a realization that catalysts having peroxide defects can be used to decompose the oxides of nitrogen, both directly and indirectly.
  • Nitrogen (I) oxide (N 2 0) can be destroyed directly by peroxide defects according to the following general electron transfer mechanism:
  • reaction (2) is a heterogeneous destruction of N 2 0 using peroxide defects and reaction (3) is the peroxide defect regeneration reaction.
  • the overall catalytic decomposition reaction for N 2 0 is then the summation of reactions (2) and (3) as follows: N 2 0 ⁇ N 2 + l/20 2 (4)
  • Applicant believes that the speed at which N 2 0 can be destroyed is a function of the peroxide defect concentration of the doped alkali metal oxide or alkaline earth metal oxide crystals, that is, it is a function of the values of x and y in mechanism (2) .
  • Nitrogen (I) oxide can also be destroyed indirectly in two steps by peroxide defects in the presence of organic compounds, for example, methane, according to the following general electron transfer mechanism: M p O q+x+y D y + (x+y)CH 4 ⁇ M p O ⁇ y H ⁇ y D y + (x+y) CH 3 (5) 2 (x+y)CH 3 +3 (x+y)N 2 0+2 (x+y)/20 2 ⁇
  • Mechanism (5) illustrates the formation of methyl free radical from methane via peroxide defect catalysis.
  • Mechanism (6) illustrates the gas phase destruction of N 2 0 via oxygen and methyl free radicals and mechanism (7) illustrates the regeneration of peroxide defects with oxygen.
  • Nitrogen (II) oxide (NO) can be destroyed indirectly in a similar fashion by peroxide defects in the presence of organic compounds, for example, methane, according to the following general electron transfer mechanism: 4(x+y)CH 3 + 6 (x+y)NO + 4(x+y)0 2 ⁇
  • alkali metal salts or alkali earth salts have been identified in the above U.S. patent 5,548,803 as having the ability to destroy the oxides of nitrogen the significance of doping these salts in a manner that produces organic free radicals and alkali doped alkali metal oxides or alkali doped alkaline earth metal oxides having high concentration of peroxide defects has not been previously observed.
  • the prior art has not described a technique for enhancing oxides of nitrogen destruction rates during oxide of sulphur adsorption by alkali metal oxides or alkaline earth oxides.
  • the absorption of the oxides of sulphur by alkali oxides capable of catalytically destroying the oxides of nitrogen will reduce the speed of oxide of nitrogen destruction due to fouling of the oxide surface by sulphite or sulphate salts.
  • Increasing the peroxide defect concentration of the oxide and its ability to generate free radicals will minimize the effect of sulphite or sulphate salt poisoning of the oxide.
  • the present invention builds from the prior art technique, as described and claimed in the United States patent 5,548,803 to maximize oxide surface area during decomposition of organic salts.
  • the present invention therefore seeks to provide a method of decomposing the oxides of nitrogen present in a gas.
  • the method is effective even in the presence of oxides of sulphur.
  • the present invention is a method of decomposing the oxides of nitrogen present in a gas that comprises contacting the gas with catalyst containing a peroxide defect and doped with a metal from Group 1 or Group 2 of the periodic table.
  • the metal oxide has a cubic crystal structure.
  • Preferred metal oxides are calcium oxide or magnesium oxide.
  • Preferred dopants are lithium, sodium and potassium.
  • the invention is particularly useful in providing a method of destruction of the oxides of nitrogen in the presence or absence of sulphur dioxide in gases derived from the combustion of fuels such as coal, oil, natural gas, alcohol, bitumen or fuel emulsions.
  • An alkaline earth salt liquor e.g. calcium oxide in water
  • another alkaline earth salt e.g. lithium hydroxide, sodium hydroxide or potassium hydroxide
  • an organic liquor such as biomass pyrolysis liquor, fermentation liquor or pyrolyzable organic waste in the presence or absence of air or other oxidant to produce a mixture of alkali earth salts containing at least two different alkali earth or alkaline earth metal elements capable of producing alkali doped oxides during their thermal decomposition in the absence or presence of NOx or SOx.
  • Alkali doped oxides produced in Example 1 are used as NOx destruction catalysts in mobile applications such as catalytic converters in automobiles burning alcohol, gasoline or natural gas fuels.
  • the unburnt hydrocarbon emissions are used as a source of organic free radicals to destroy NO and N 2 0.
  • the N 2 0 emissions are also destroyed in the absence of organic free radicals.
  • Alkali doped oxides produced in Example 1 are used as NOx destruction catalysts in stationary applications such as power production from fossil fuels. Both the organic content of the fuel and doped alkali earth or alkaline salt mixture during and after alkaline earth or alkali earth oxide production are used as a source of organic free radicals to destroy NOx species either alone or in combination with the doped alkaline earth or doped alkaline oxide catalyst.

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  • Environmental & Geological Engineering (AREA)
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Abstract

A method of decomposing the oxides of nitrogen present in a gas. The gas is in contact with a catalyst containing a peroxide defect and doped with a metal from Group 1 or Group 2 of the periodic table. The metal oxide preferred has a cubic crystal structure.

Description

A METHOD OF DECOMPOSING THE OXIDES OF NITROGEN IN FLUE GAS
This invention relates to a method of reducing acid rain and ozone depletion precursors from combustion flue gas, to additives useful in the method and to a method of producing the additives.
Ozone depletion and acid rain are problems throughout the world. The deterioration of the ozone layer is creating an epidemic in skin cancer and acid rain affects the environment by reducing air quality, rendering lakes acid and killing vegetation, particularly trees. It has been the subject of international dispute. Canada and the United States have argued over the production of acid rain. Britain and Scandinavia are other antagonists.
In the main acid rain stems from sulphur dioxide produced in smoke stacks. The sulphur dioxide typically originates from the combustion of a sulphur containing fuel, for example coal. The sulphur dioxide is oxidized in the atmosphere to sulphur trioxide and the trioxide is dissolved to form sulphuric acid. The rain is thus made acid. The oxides of nitrogen are known to be precursors to acid rain and NO is a catalytic agent in the destruction of the ozone layer. N20 is both a greenhouse gas, 270 times more absorptive than carbon dioxide, and a precursor to NO formation in the ozone layer. It has been argued that N20 photo-dissociation in the ozone layer is a greater source of NO than is the direct flux of NO from the earth's surface.
The emission of oxides of nitrogen in the United States and Canada is about one fifth sulphur dioxide emissions. But that still means that millions of tons of oxides of nitrogen are fed to the atmosphere each year. Although it is believed that the production of sulphur dioxide has stabilized, larger emissions of the oxides of nitrogen are anticipated because of the increased use of fossil fuels.
With the passage of the International Clean Air Act amendments, such as issued in the United States in 1990, the curbing of NOx and SOx emissions has become a priority. Planners for electrical utilities in particular are developing strategies for reducing emissions of sulphur dioxide and nitrogen oxides in the production of electrical and thermal power. The majority of fossil fuel used in power production contains sulphur and organically bound nitrogen. These fossil fuels produce sulphur dioxide and oxides of nitrogen during combustion.
Gas desulphurization systems are known. The majority rely on simple basic compounds, for example, calcium carbonate, sodium carbonate and calcium hydroxide, to react with the acidic sulphur containing species to produce non-volatile products such as calcium sulphite, calcium sulphate and sodium sulphate. Urea and ammonia have been used to react with oxides of nitrogen generated during fossil fuel combustion to produce non¬ toxic nitrogen gas but urea and ammonia are expensive and are unsuitable for destruction of NOx produced in mobile applications, e.g., from vehicles. Conventional alkali adsorbents such as lime have not shown an ability to destroy nitric oxide (NO) .
Applicant's United States patent No. 5,548,803 describes and claims a process for reducing acid emission from a flue gas produced by combustion of a sulphur containing fuel. A pyrolysis liquor containing a thermolabile alkaline earth metal compound is introduced into a flue containing the flue gas. The alkaline earth metal compound is able to decompose at flue gas temperature to produce an alkaline compound able to react with sulphur dioxide in the flue gas. This method has achieved excellent results. A particular advantage over the prior art is that the thermal decomposition of the organic salts, for example, calcium salts, produces high surface area calcium oxide, certainly of a surface area higher than has been achieved from non-organic metal salts such as calcium carbonate. Patentees have postulated that this is due to the abrasive and turbulent action of gases such as carbon dioxide and water generated during the thermal decomposition. This effect is particularly useful for rupturing sulphite and sulphate films that foul the oxide adsorbents, such as calcium oxide, used in sulphur dioxide removal during fossil fuel combustion.
U.S. Patent 5,071,815, issued December 10, 1991 to Oehr et al, describes and claims a method of forming a solid metal oxide crystal structure having a peroxide defect and doped with an alkali metal or alkaline earth metal. The method comprises growing the doped metal oxide crystal by thermally decomposing, in an oxidizing atmosphere, a molten mixture of an alkaline metal salt or alkaline earth metal salt and the metal oxide, or a compound that can decompose to form the metal oxide under the thermal decomposition conditions, and then cooling the reaction mixture. Preferably the metal oxide has a cubic crystal structure. The metal oxide may, for example, be an oxide of a rare earth, for example, samarium oxide or it may be zirconium oxide. Useful alkali metals include lithium, sodium or potassium. A useful alkaline earth metal is calcium. The crystal may be grown in the presence of a support.
The catalysts are especially useful in the production of ethane from methane. They assist in producing methyl free radicals from the methane. The free radicals combine to form ethane.
These catalysts have high concentration of peroxide defects in their crystal structure which means that the compounds are essentially non-stoichiometric with respect to oxygen; they possess an excess of oxygen.
The general formula of a metal oxide that contains peroxide defects is:
MpOq+x I in which;
M is an inorganic cation; p is the stoichiometric number of moles of the inorganic cation;
0 is the oxide anion; q is the stoichiometric number of moles of oxide anions, and x is the number of oxygen atoms in excess of q (i.e., peroxide defects) . The invention of the above U.S. patent 5,071,815 was a particularly effective method of producing catalysts of the formula:
MpOq+x+yDy II where
M, p, O and x are as defined above; D is a monovalent alkali metal cation dopant, for example lithium, sodium or potassium; and y is the number of extra peroxide defects created by the presence of the monovalent alkali or alkaline earth metal dopant cation in the metal oxide crystal structure.
Thus, certain metal oxide crystals, particularly those having a cubic crystal structure and containing relatively large cations, can be doped with small alkali metal or alkaline earth metal cations. The dopants replace the normal cation in the metal oxide crystal structure. For example, lithium and sodium with, respectively, ionic radii of about 0.6 and about 0.65 angstrom units, can replace calcium with an ionic radius of 0.99 angstrom units in the calcium oxide crystal structure.
The resulting doped crystals are believed to have the following empirical formulae: CaOx+yLiy and CaOx+yNay For divalent dopant cations, for example, magnesium, the general formula for a doped metal oxide is: MpOq+x+yDy/2 Ila
Some metal oxides, for example, magnesium oxide, the value of x in formula II above is zero. However, with a lithium dopant, the magnesium oxide can be made to contain peroxide defects as shown by y in formula II. The presence of peroxide defects in lithium doped magnesium oxide crystals has been verified experimentally by, for example, Driscoll et al, in the Journal of the American Chemical Society, 107:58-63.
Thus, the prior art teaches the doping by alkali metal oxides to generate peroxide defects and the use of these doped metal oxides to convert methane to ethane. Prior art processes for producing maximum peroxide defects in metal oxides for catalysts particularly useful for methane conversion suffer from a number of disadvantages. First many of the prior art processes use a solid dopant, for example, a carbonate. The salt is mixed with a solid metal oxide in a technique known as dry impregnation. The mixing is achieved by kneading or physically massaging and grinding the salt with the metal oxide. This can only achieve slight penetration of the metal oxide crystal structure by the dopant . Secondly, by mixing an aqueous catalyst dopant, such as a nitrate, with a solid metal oxide in a process known as wet impregnation, followed by heating to remove moisture and the dopant anion, for example, nitrate, metal oxide impregnation is only slightly improved over the dry impregnation method.
However, applicant's above U.S. patent 5,071,815, has demonstrated an improved method of producing doped catalysts with peroxide defects and the present invention is a development of that work and applicant's other work as described and claimed in U.S. patent 5,548,803.
The present invention is based on a realization that catalysts having peroxide defects can be used to decompose the oxides of nitrogen, both directly and indirectly. Nitrogen (I) oxide (N20) can be destroyed directly by peroxide defects according to the following general electron transfer mechanism:
MpOq+x+yDy + (x+y) N20 → (x+y) N2 + (x+y) 0" + MpOq+ (x+y) +Dy ( 2 ) (x+y) 0- + MpOq^yj +Dy → MpOq+x+yDy + (x+y) /202 ( 3 )
In the above scheme, reaction (2) is a heterogeneous destruction of N20 using peroxide defects and reaction (3) is the peroxide defect regeneration reaction. The overall catalytic decomposition reaction for N20 is then the summation of reactions (2) and (3) as follows: N20 → N2 + l/202 (4) Applicant believes that the speed at which N20 can be destroyed is a function of the peroxide defect concentration of the doped alkali metal oxide or alkaline earth metal oxide crystals, that is, it is a function of the values of x and y in mechanism (2) .
Nitrogen (I) oxide can also be destroyed indirectly in two steps by peroxide defects in the presence of organic compounds, for example, methane, according to the following general electron transfer mechanism: MpOq+x+yDy + (x+y)CH4 → MpO^yH^yDy + (x+y) CH3 (5) 2 (x+y)CH3+3 (x+y)N20+2 (x+y)/202 →
3 (x+y) H20+3 (x+y) N2+2 (x+y) C02 ( 6 ) pOq+x+yHx+yDy + (x+y) /202 → MpOq+x+yDy + (x+y) /2H20 ( 7 )
Mechanism (5) illustrates the formation of methyl free radical from methane via peroxide defect catalysis.
Mechanism (6) illustrates the gas phase destruction of N20 via oxygen and methyl free radicals and mechanism (7) illustrates the regeneration of peroxide defects with oxygen.
Similar mechanisms can be written for organic species other than methane, which can form free radicals other than methyl free radical. Again, the speed at which N20 can be destroyed is a function of the peroxide defect concentration of the doped alkali metal oxide or alkali earth metal oxide crystals, that is the values of x and y in mechanism (5) .
Nitrogen (II) oxide (NO) can be destroyed indirectly in a similar fashion by peroxide defects in the presence of organic compounds, for example, methane, according to the following general electron transfer mechanism: 4(x+y)CH3 + 6 (x+y)NO + 4(x+y)02 →
6(x+y)H20 + 3(x+y)N2 + 4 (x+y) C02 (8)
Again the speed at which the NO can be destroyed is a function of the peroxide defect concentration. That is, the values of x and y in mechanism (8) .
Although organic salts derived from organic compounds, alkali metal salts or alkali earth salts have been identified in the above U.S. patent 5,548,803 as having the ability to destroy the oxides of nitrogen the significance of doping these salts in a manner that produces organic free radicals and alkali doped alkali metal oxides or alkali doped alkaline earth metal oxides having high concentration of peroxide defects has not been previously observed.
The prior art has not described a technique for enhancing oxides of nitrogen destruction rates during oxide of sulphur adsorption by alkali metal oxides or alkaline earth oxides. The absorption of the oxides of sulphur by alkali oxides capable of catalytically destroying the oxides of nitrogen will reduce the speed of oxide of nitrogen destruction due to fouling of the oxide surface by sulphite or sulphate salts. Increasing the peroxide defect concentration of the oxide and its ability to generate free radicals will minimize the effect of sulphite or sulphate salt poisoning of the oxide. Thus, the present invention builds from the prior art technique, as described and claimed in the United States patent 5,548,803 to maximize oxide surface area during decomposition of organic salts.
The present invention therefore seeks to provide a method of decomposing the oxides of nitrogen present in a gas. The method is effective even in the presence of oxides of sulphur.
Accordingly, in its first aspect, the present invention is a method of decomposing the oxides of nitrogen present in a gas that comprises contacting the gas with catalyst containing a peroxide defect and doped with a metal from Group 1 or Group 2 of the periodic table.
In a preferred aspect the metal oxide has a cubic crystal structure. Preferred metal oxides are calcium oxide or magnesium oxide. Preferred dopants are lithium, sodium and potassium. The invention is particularly useful in providing a method of destruction of the oxides of nitrogen in the presence or absence of sulphur dioxide in gases derived from the combustion of fuels such as coal, oil, natural gas, alcohol, bitumen or fuel emulsions.
The invention is illustrated in the examples.
Example 1
An alkaline earth salt liquor (e.g. calcium oxide in water) is mixed with another alkaline earth salt (e.g. lithium hydroxide, sodium hydroxide or potassium hydroxide) and further mixed with an organic liquor such as biomass pyrolysis liquor, fermentation liquor or pyrolyzable organic waste in the presence or absence of air or other oxidant to produce a mixture of alkali earth salts containing at least two different alkali earth or alkaline earth metal elements capable of producing alkali doped oxides during their thermal decomposition in the absence or presence of NOx or SOx.
Example 2
Alkali doped oxides produced in Example 1 are used as NOx destruction catalysts in mobile applications such as catalytic converters in automobiles burning alcohol, gasoline or natural gas fuels. The unburnt hydrocarbon emissions are used as a source of organic free radicals to destroy NO and N20. The N20 emissions are also destroyed in the absence of organic free radicals.
Example 3
Alkali doped oxides produced in Example 1 are used as NOx destruction catalysts in stationary applications such as power production from fossil fuels. Both the organic content of the fuel and doped alkali earth or alkaline salt mixture during and after alkaline earth or alkali earth oxide production are used as a source of organic free radicals to destroy NOx species either alone or in combination with the doped alkaline earth or doped alkaline oxide catalyst.

Claims

I CLAIM :
1. A method of decomposing the oxides of nitrogen present in a gas that comprises contacting the gas with a catalyst containing a peroxide defect and doped with a metal from Group 1 or Group 2 of the periodic table.
2. A method as claimed in claim 1 in which the catalyst has the structure: pOq+x+yDy II wherein;
M is an inorganic cation; p is the stoichiometric number of moles of the inorganic cation;
0 is the oxide anion; q is the stoichiometric number of moles of oxide anions; x is the number of oxygen atoms in excess of q; D is a monovalent alkali metal cation dopant; y is the number of extra peroxide defects created by the presence of the cation D in the metal oxide crystal structure.
3. A method as claimed in claim 1 in which the metal oxide has a cubic crystal structure.
4. A method as claimed in 2 in which M is a metal from Group 2 of the Periodic Table.
5. A method as claimed in claim 4 in which M is calcium or magnesium.
6. A method as claimed in claim 1 in which the dopant is lithium, sodium, potassium or calcium.
7. A method as claimed in claim 1 in which the reaction is conducted in the present of an organic compound.
8. A method as claimed in claim 1 in which the catalyst is developed in situ in said gas containing an oxide of nitrogen.
9. A method as claimed in claim 1 in which the gas is a flue gas and the method includes introducing into said flue gas an alkaline earth salt liquor, an alkali metal compound and an organic liquor to produce a mixture of alkaline earth metal salts containing at least two different alkali or alkaline earth metals capable of producing alkali doped oxides during their thermal decomposition.
10. The method as claimed in claim 9 in which the organic liquor is a biomass pyrolysis liquor or a fermentation liquor.
11. A method as claimed in claim 7 in which the gas is an exhaust gas from an internal combustion engine and the catalyst is present in a catalytic convertor in an exhaust pipe, unburned hydrocarbons acting as a source of the organic compound.
PCT/CA1996/000720 1995-11-03 1996-10-30 A method of decomposing the oxides of nitrogen in flue gas WO1997017127A1 (en)

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US08/552,725 1995-11-03

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5071815A (en) * 1989-09-01 1991-12-10 British Columbia Research Corporation Method for producing catalysts
EP0532024A1 (en) * 1991-09-12 1993-03-17 Sakai Chemical Industry Co., Ltd., Catalyst for catalytic reduction of nitrogen oxide
US5458803A (en) * 1993-09-30 1995-10-17 Dynamotive Corporation Acid emission reduction
WO1996007468A1 (en) * 1994-09-09 1996-03-14 Solvay (Societe Anonyme) Reactive composition and method for the purification of a nitric oxide containing gas

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5071815A (en) * 1989-09-01 1991-12-10 British Columbia Research Corporation Method for producing catalysts
EP0532024A1 (en) * 1991-09-12 1993-03-17 Sakai Chemical Industry Co., Ltd., Catalyst for catalytic reduction of nitrogen oxide
US5458803A (en) * 1993-09-30 1995-10-17 Dynamotive Corporation Acid emission reduction
US5458803B1 (en) * 1993-09-30 1999-08-03 Dynamotive Corp Acid emission reduction
WO1996007468A1 (en) * 1994-09-09 1996-03-14 Solvay (Societe Anonyme) Reactive composition and method for the purification of a nitric oxide containing gas

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AU7273896A (en) 1997-05-29

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