Catalyst and process for oxidation and removal of nitrogen oxides (NOx) from combustion gases

This is a continuation-in-part application of Ser. No. 09/225,082, filed Jan. 4, 1999 now abandoned.

This invention pertains to catalytic oxidation and removal of nitrogen oxides (NO x ) from exhaust gases derived from combustion of hydrocarbon fuels. It pertains particularly to a bi-functional oxidation catalyst and methods for making same for advantageous use in a treatment process for such oxidation of NO x contained in combustion exhaust gases, The process utilizes an initial catalytic oxidation step and can be followed by further oxidation by a chemical oxidant such as ozone (O 3 ) for providing essentially complete NO x oxidation and removal from the combustion gases.

Conventional burning of fossil fuels such as coal, heavy oils and fuel gases to generate heat and energy results in formation of undesired concentrations of nitrogen compounds such as NO x contained in the resulting combustion exhaust and flue gas streams, and contributes to undesirable air pollution in the atmosphere. At least about 95% of such NO x in combustion gases is in the form of nitric oxide (NO). Because nitric oxide (NO) is relatively inactive chemically, its removal from combustion flue gas streams by scrubbing with suitable liquids is difficult and inefficient.

Some prior efforts for oxidation of NO x contained in combustion gases have utilized chemical oxidants such as ozone. For example, U.S. Pat. No. 4,011,298 to Fukui et al discloses oxidation and removal of sulfur oxide and nitrogen oxide (NO) from combustion gases by mixing with an ozone-containing gas. U.S. Pat. No. 4,024,219 to Takahaski et al discloses removing nitrogen oxides in waste gases by oxidizing with nitric acid in presence of a porous adsorbent agent such as silica gel or molecular sieves. U.S. Pat. No. 4,035,470 to Senjo et al discloses removal of sulfur oxides and/or nitrogen oxides from waste gases by adding chlorine dioxide or ozone and scrubbing with an aqueous solution. U.S. Pat. No. 4,351,811 to Matsuda et al discloses removing NO and NO 2 contained in exhaust gases by contact with a metallic oxide catalyst together with ammonia. U.S. Pat. No. 4,564,510 to Bechthold et al discloses removing nitrogen oxides from waste gases by oxidizing with an agent such as ozone to form NO 2 , then adsorbing the NO 2 . U.S. Pat. No. 4,971,777 to Firnhaber et al discloses removing SO x and NO x from industrial furnace exhaust gases containing 0.5 vol % O 2 by thermally oxidizing NO at 300-900 C followed by an adsorption step. Also, U.S. Pat. No. 5,206,002 to Skelly et al discloses treating combustion exhaust gases containing oxides of nitrogen and sulfur from an electric power plant by mixing with a chemical oxidant such as ozone (O 3 ). U.S. Pat. No. 5,756,057 to Tsuchitani et al and U.S. Pat. No. 5,759,947 to Zhou disclose catalysts which can be used for removal of nitrogen oxides from combustion gases. However, it is apparent that known processes for oxidation and removal of nitrogen oxides from combustion exhaust or flue gases have generally been ineffective and undesirable, and that use of ozone alone for NO x oxidation involves undesirably high operating costs. Thus, further improvements in catalysts and processes for removal of NO x from combustion exhaust and flue gases are needed in industry for effectively reducing air pollution and smog and providing a clean air environment.

It is known that if the NO x contained in combustion flue gases could be efficiently oxidized to NO 2 or N 2 O 5 , substantial amounts of such oxidized nitrogen compounds could then be effectively removed by liquid scrubbing of the treated flue gases. However, because oxidation of nitrogen oxide NO by O 2 is very slow, improved methods for such oxidation are needed. Combustion flue gases containing NO x usually also contains some unreacted oxygen, such as 5-10% by volume. It is believed that such oxidation of NO x could be substantially enhanced by initial adsorption of the NO and the small concentration of O 2 contained in combustion flue gases on a suitable catalytic surface, which also provides simultaneous desorption of an oxidized form of NO x such as NO 2 from the catalytic surface. Such catalytic oxidation step for NO x could be followed by further chemical oxidation such as with ozone (O 3 ) to form N 2 O 5 . Because the oxidized NO x has much greater solubility in suitable scrubbing liquids, substantially all of the NO x contained in combustion exhaust or flue gases could be effectively removed in the form of oxyacids or salts.

This invention provides a bi-functional metal oxide catalyst which is effective for simultaneous catalytic adsorption and oxidation of nitrogen oxides (NO x ) contained in combustion gases derived from the combustion of hydrocarbon fuels. This bi-functional catalyst composition utilizes specific combinations of active metal oxides which are chemically bonded intimately together so as to form a metal oxide complex having crystalline form or structure which provides both an adsorption function and sites and closely adjacent oxidation function and sites for the NO x contained in the combustion exhaust or flue gases. Because of its unique composition and structure, this bi-functional catalyst is capable of effectively oxidizing the NO x to substantially NO 2 at desirable high reaction rates and at relatively low reaction temperatures in the range of 170-550° F. These performance characteristics make this bi-functional catalyst particularly useful for treating NO x containing flue gases from coal-fired or oil-fired industrial furnaces, or from steam generating boilers in electric power plants to at least partially remove the nitrogen oxides from the combustion flue gases.

Metal oxides which are useful for providing the bi-functional catalyst adsorption function and sites include oxides of metals selected from the group including barium (Ba), cesium (Cs), lanthanum (La), strontium (Sr), zirconium (Zr), and combinations thereof. Metals oxides useful for providing the bi-functional catalyst adjacent oxidation function and sites include oxides of transition metals including chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), manganese (Mn), platinum (Pt) and combinations thereof. The adsorption function metal oxides in the bi-functional catalyst are placed in close intimate contact with the oxidation function metal oxides by utilizing a tri-valent acid binding agent having at least two acid functional groups such as carboxylic acid during preparation of the catalyst. The catalyst also has a balanced molar ratio of the adsorption metals to the oxidation metals within a broad ratio range of 0.1:1 to 5:1, with a molar ratio range of 0.2:1 to 2:1 being preferred. For this bi-functional catalyst, the preferred adsorption metals are barium and lanthanum, and the preferred oxidation metals are copper and manganese with small percentage of platinum as a promotor metal. This invention also includes methods steps utilized for making the bi-functional catalyst, which is initially in powder form but can be deposited on a suitable porous support material or structure.

This invention also provides a process for using the bi-functional catalyst for initial catalytic oxidation of nitrogen oxides NO x contained in combustion exhaust or flue gases derived from the combustion of hydrocarbonaceous fuels such as coal, heavy oils, and natural gas. In such NO x initial oxidation process, the flue gases derived from the fuel combustion and containing up to about 0.5 vol. % NO x together with 5-10 vol. % O 2 are first contacted by the bi-functional oxidation catalyst of this invention, and for which process the NO x is adsorbed on the catalytic surface and simultaneously oxidized and desorbed from the catalytic surface at 170-550° F. temperature to provide an oxidized form of NO such as at least about 50 vol. % NO 2 , and preferably 60-98 vol. % NO 2 in the treated combustion gas.

In another embodiment of this invention, the partially oxidized NO x in the combustion exhaust gas downstream from the initial catalytic oxidation step is then preferably further treated by contact with a supplemental chemical oxidant such as hydrogen peroxide (H 2 O 2 ) or ozone (O 3 ) at reaction temperature of 100-250° F. and 0.5-20 psig pressure for providing additional oxidization reactions for converting the NO 2 to higher oxides of nitrogen such as N 2 O 5 . For this two-step NO x oxidation process including the initial catalytic oxidation of NO x to substantially NO 2 followed by chemical oxidation to substantially N 2 O 5 , the initial catalytic oxidation step will preferably utilize the bi-functional catalyst of this invention.

The resulting two-step treated combustion flue gas containing the higher oxidized form of NO x such as mainly N 2 O 5 is next scrubbed by intimate contact with a suitable aqueous scrubbing liquid such as water to remove the oxidized nitrogen compounds from the gas, after which the resulting treated combustion flue gas containing less than about 15 ppm NO x is discharged to the atmosphere. The scrubbing liquid is further processed as desired to remove the nitrogen compounds such as nitric acid. By utilizing the initial catalytic oxidation step for treating fuel combustion gases, followed by the subsequent chemical oxidation treatment step, significantly less of the chemical oxidant such as hydrogen peroxide (H 2 O 2 ) or ozone (O 3 ) is required to complete the oxidation of NO x in the combustion exhaust gases to essentially N 2 O 5 . Thus, by utilizing the bi-functional oxidation catalyst in an initial catalytic oxidation step of the two-step NO x oxidation process according to this invention, the nitrogen oxides (NO x ) contained in combustion gases derived from hydrocarbon fuel combustion are advantageously and effectively transferred from the exhaust or flue gases to a scrubbing liquid, from which the nitrogen oxides can be effectively removed by other known procedures, and the treated combustion gases containing minimal NO x less than about 15 ppm are discharged to the atmosphere.

This bi-functional oxidation catalyst and two-step process for oxidation and removal of NO x from combustion gases is useful for fossil fuel fired steam boilers such as used in electric power plants, and for fired industrial furnaces such as used in chemical, glass and petroleum refinery processes.

The bi-functional oxidation catalyst according to this invention utilizes a unique composition of active metal oxides in which one or more metal oxides selected for providing an adsorption function and sites are provided in close intimate contact with one or more metal oxides selected for effectively providing simultaneous oxidation function and sites on the catalyst. The intimate contact of the adjacent adsorption and oxidation function metal oxides is provided by utilizing a binding acid agent having at least two functional groups such as a carboxylic acid during preparation of the catalyst, so that after a calcining step the metal oxides are chemically bonded intimately together so as to form a metal oxide complex having a crystalline form or structure. The resulting bi-functional catalyst powder material may be coated/supported onto inert geometric shapes or structures having large surface area of at least about 50 m 2 /g and preferably 100-500 m 2 /g which provides low pressure drop for the combustion gases flowing through a catalytic oxidation reactor. The catalyst metal oxides are preferably suitably deposited or embedded onto a stable porous substrate support material such as a ceramic honeycomb structure which is capable of withstanding combustion gas temperatures up to at least about 600° F. or higher. Because the bi-functional catalyst of this invention is able to oxidize NO contained in a fuel combustion gas to mainly NO 2 or higher oxides within a desired relatively low temperature range of 170-550° F. and at substantially atmospheric pressure and high space velocity, the catalytic initial oxidation step can be advantageously and conveniently located in commercial electric power plant processes immediately downstream from a particulate solids removal step in the plant boiler section. Accordingly, this bi-functional catalyst and process for utilizing it thereby advantageously avoid any undesired re-heating of the combustion flue gas as would be needed for other oxidation catalysts requiring higher reaction temperatures such as above about 500° F.

The initially oxidized NO 2 contained in the combustion flue gas stream is preferably further oxidized in a second oxidation step by being mixed with a sufficient amount of a chemical oxidant such as hydrogen peroxide (H 2 O 2 ) or ozone (O 3 ) to form essentially N 2 O 5 at reaction conditions of 100-250° F. temperature, 0.5-5 psig pressure, and a molar ratio of the chemical oxidant to NO x of 0.5:1 to 1.2:1. Chemical oxidation conditions of 125-225° F. temperature and 0.6-2.0 psig. pressure are usually preferred. Depending upon the percentage catalytic conversion of NO to NO 2 achieved in the initial catalytic oxidation step, the molar ratio of hydrogen peroxide (H 2 O 2 ) to NO x should be in a range of 0.6:1 to 1.10:1 and the molar ratio of ozone (O 3 ) to NO x should be the range of 0.5:1 to about 1.0:1, with lower percentages of catalytic conversion requiring more of the chemical oxidant. Because of the initial catalytic preoxidation step of NO to form substantially NO 2 , only about one-third as much of the chemical oxidant such as ozone (O 3 ) is required to complete the oxidation of NO 2 to N 2 O 5 , as would be required if the chemical oxidant was used alone. Thus, this two-step NO x oxidation and removal process utilizing the combined catalytic and chemical oxidation steps requires significantly lower process costs for achieving substantially complete removal of NO x from combustion gases and to meet governmental acceptable levels, such as less than about 15 ppm NO x , and preferably less than about 10 ppm NO x remaining in the treated gases discharged to the atmosphere.

Catalyst Preparation Method

The bi-functional catalyst of this invention is prepared by providing the adsorption function metal components including barium, cesium, lanthanum, strontium, zirconium and combinations thereof, and the oxidation function metals components including chromium, cobalt, copper, iron, manganese and combinations thereof, so as to have a molar ratio of the adsorption function metals to the oxidation function metals in the range of 0.1:1 to 5:1, together with about 1 wt. % platinum provided as an oxidation promoter metal. Also for providing the two catalytic metal sites in closely adjacent positions in the bi-functional catalyst, a binding agent such as carboxylic acid having at least two acid functional groups and having a molar ratio of acid to the total metals between 0.5:1 and 2:1, and preferably 0.8:1 and 1.5:1, is utilized for chemically attaching and bonding the adsorptive and oxidative metals intimately together so as to form a metal oxide complex having a crystalline form or structure with the metals arranged in an orderly repetitive pattern between the adsorption metal oxide component and the oxidation metal oxide component. After an aqueous solution of one or more of the adsorption metal ions is combined with one or more of the oxidation metal ions within the desired molar ratio range, and the binding acid agent added, the resulting metals compound is dried and calcined in air at 500-800° C. (930-1470° F.) temperature for 0.4-5 hours to form a fine powder metal oxide precursor material. Such precursor material is then impregnated with another solution of the oxidation function metals sufficient to provide the desired molar ratio of the adsorption metals to the oxidation metals, and the resulting slurry dried and calcined at 500-800° C. for 0.5-2.0 hours so as to form a metal oxide complex having a crystalline form or structure in which the metal oxides have an orderly repetitive pattern. Suitable carboxylic acid binding agents and their salts are citric acid and sodium citrate.

A typical bi-functional catalyst of this invention can be prepared utilizing specific procedures as follows:

For preparing the precursor solution including adsorption and oxidation metals, 46.9 grams of BaCuO 2 , 52.3 g of Ba (NO 3 ) 2 and 46.5 g of Cu(NO 3 ) 2 ·2.5 H 2 O are dissolved in 300 ml distilled water, and 76.9 g citric acid is dissolved in 200 ml distilled water. The mole ratio between the total metal ions and the citric acid is 1:1. The two solutions are mixed together and water is evaporated at about 95° C. (203° F.) until a sol consistency is obtained. The sol is further dried at 70° C. (158° F.) in a vacuum oven, then calcined at about 750° C. (1380° F.) for 0.5 hours in air, and cooled to provide a Ba/Cu precursor material.

This obtained Ba/Cu precursor material is further impregnated by a Mn and Pt solution, for which 12.1 grams of Mn (NO 3 ) 2 ·6H 2 O is dissolved in 100 ml distilled water and 1.2 grams 5% Pt solution (ammonia platinum nitrate) is added to the Mn solution. Then 3 grams of the precursor material is impregnated by the Mn and Pt solution to provide the desired molar ratio of the adsorbent metals to the oxidation metals. The resulting slurry is heated during rotation until dried to a solid powder. The solid powder is further heated and calcined at 750° C. for 0.5 hours in air, then cooled to room temperature in air to provide a bi-functional catalyst in which the adsorption and oxidation metals are chemically bonded together so as to form a metal oxide complex having a crystalline form.

All other bi-functional catalyst samples were prepared using similar procedures for combining the specific adsorption and oxidation metals closely together so to form a metal oxide complex having a crystalline form, as are listed in Table 1 and 2 of the Examples below.

This invention will be described further with reference to the following drawings, in which:

FIG. 1 is a schematic flow diagram of a process for catalytic oxidation and removal of NO x from fuel combustion gases by successive catalytic oxidation and chemical oxidation steps according to the invention;

FIGS. 2 and 3 are graphs showing catalytic conversion of NO to NO 2 and/or higher oxides utilizing various bi-functional catalyst compositions according to this invention; and

FIG. 4 is an x-ray diffraction (XRD) diagram showing the crystalline structure of a typical bi-functional catalyst made according to this invention.

As shown in FIG. 1 , a hydrocarbon fossil fuel such as particulate coal is provided at 10 , and is fed together with combustion air supplied at 11 and preheated at 12 to a burner 13 located in the lower portion of a boiler unit 14 . The fuel is combusted with the preheated air in the burner 13 for heating water pressurized to 500-1500 psig and generating pressurized steam. The pressurized water is provided at 16 and preheated at 17 , then is further heated and vaporized in heat exchanger 18 to produce the pressurized steam which is removed at 19 .

The resulting hot flue gases containing CO 2 , CO and some undesirable nitrogen and sulfur oxides are removed from the boiler unit 14 as stream 20 and passed through an electronic precipitator unit 22 , from which fine particulate ash solids are withdrawn at 21 . The resulting cleaned combustion gas stream at 23 containing up to about 0.2 vol. % NO x and 5-10 vol. % oxygen is removed by exhaust fan 24 and is passed as stream 25 at temperatures of 170-550° F. and pressure of 0.8-5.0 psig to an initial catalytic oxidation unit 26 . The catalytic oxidation unit 26 has a suitable oxidation catalyst material 27 provided on inner surfaces of the oxidation unit. The catalyst 27 is preferably the bi-functional oxidation catalyst as disclosed herein above. However, other known effective oxidative catalyst materials capable of NO x oxidation at temperatures below about 550° F. could be utilized in the catalytic oxidation unit 26 . The oxidation catalyst material 27 is coated/supported on a suitable geometric structure providing large surface area such as at least about 50 m 2 /gm and need not exceed about 500 m 2 /gm, provides low pressure drop less than about 5 psi and preferably less than 0.5 psi for the flowing gas stream, and is capable of withstanding temperatures up to about 700° F. without significant damage.

In the catalytic oxidation unit 26 , the treated flue gas stream 25 is partially oxidized by contact with the catalyst material 27 , so that the NO x component contained in the flue gas stream 25 together with 5-10 vol. % oxygen is partially oxidized to form at least about 50 vol. % NO 2 and preferably 60-98 vol. % NO 2 and higher oxides. The oxidation reaction conditions in unit 26 are maintained within the ranges 170-550° F. temperature, 0.5-20 psig. static pressure, and at gas space velocity of 5,000-100,000 hr −1 . Preferred reaction conditions are 200-400° F. temperature, 1-15 psig. pressure, and gas space velocity of 8,000-50,000 hr −1 .

The resulting catalytically treated combustion gas stream at 28 containing partially oxidized NO x is mixed in chemical oxidation reactor 30 with sufficient ozone (O 3 ) gas provided at 29 such as from an ozone generator to provide further oxidation of the NO 2 to essentially N 2 O 5 . In the oxidation reactor 30 , a molar ratio of O 3 to NO x is at least about 0.5:1 and need not exceed about 1:1. The gas residence time in reactor 30 is dependent on the initial concentration of the NO x and sulfur oxides and the reaction temperature, and need not exceed about 10 seconds at about 100-250° F. temperature. The NO x concentration in the chemically oxidized combustion flue gas at 31 is reduced to a desired low level as required by governmental emission control standards, such as less than about 15 ppm NO x and preferably less than 10 ppm NO x .

From the chemical oxidation reactor 30 , the resulting combustion gas at 31 containing essentially fully oxidized N 2 O 5 is next scrubbed in a scrubber unit 32 against a suitable scrubbing liquid provided at 33 , such as water or a lime solution. The scrubbing liquid is circulated from the scrubber unit 32 lower portion 32 a of the by recycle pump 34 through a suitable liquid distributor means 35 including multiple spray nozzles provided in the scrubber 32 upper portion. Make-up scrubbing liquid is provided at 33 to the scrubber unit 32 as needed. After scrubbing, the resulting cleaned flue gas stream containing essentially no NO x is discharged at 36 to the atmosphere. A portion of the scrubber liquid containing the dissolved nitrogen oxides is withdrawn at 38 from the scrubber 32 and passed to a liquid-solids separation step at 40 , from which liquids are removed at 41 and solids are withdrawn at 42 for further use or for suitable disposal.

The bi-functional catalyst of this invention will be described further by use of the following examples, which should not be construed as being limiting in scope.