WO2011143517A1 - Catalytic sulfur dioxide mediation - Google Patents

Catalytic sulfur dioxide mediation Download PDF

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Publication number
WO2011143517A1
WO2011143517A1 PCT/US2011/036372 US2011036372W WO2011143517A1 WO 2011143517 A1 WO2011143517 A1 WO 2011143517A1 US 2011036372 W US2011036372 W US 2011036372W WO 2011143517 A1 WO2011143517 A1 WO 2011143517A1
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Prior art keywords
sorbent
catalyst
process according
sulfur
microns
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PCT/US2011/036372
Other languages
French (fr)
Inventor
Christopher R. Smyrniotis
Emelito P. Rivera
Lawrence L. Murrell
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Fuel Tech, Inc.
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Publication date
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Publication of WO2011143517A1 publication Critical patent/WO2011143517A1/en

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    • 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
    • B01D53/8603—Removing sulfur compounds
    • B01D53/8609—Sulfur oxides
    • 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
    • B01J29/00—Catalysts comprising molecular sieves
    • B01J29/04—Catalysts comprising molecular sieves having base-exchange properties, e.g. crystalline zeolites
    • B01J29/06—Crystalline aluminosilicate zeolites; Isomorphous compounds thereof
    • B01J29/064—Crystalline aluminosilicate zeolites; Isomorphous compounds thereof containing iron group metals, noble metals or copper
    • B01J29/072—Iron group metals or copper
    • 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
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/30—Catalysts, in general, characterised by their form or physical properties characterised by their physical properties
    • 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
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/02—Impregnation, coating or precipitation
    • B01J37/0201—Impregnation
    • 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
    • B01J38/00—Regeneration or reactivation of catalysts, in general
    • B01J38/48—Liquid treating or treating in liquid phase, e.g. dissolved or suspended
    • B01J38/68—Liquid treating or treating in liquid phase, e.g. dissolved or suspended including substantial dissolution or chemical precipitation of a catalyst component in the ultimate reconstitution of the catalyst
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2251/00—Reactants
    • B01D2251/40—Alkaline earth metal or magnesium compounds
    • B01D2251/404—Alkaline earth metal or magnesium compounds of 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
    • B01D2255/207—Transition metals
    • B01D2255/20761—Copper
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/50—Zeolites
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2257/00—Components to be removed
    • B01D2257/30—Sulfur compounds
    • B01D2257/302—Sulfur oxides
    • 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
    • B01D53/48—Sulfur compounds
    • B01D53/50—Sulfur oxides
    • B01D53/501—Sulfur oxides by treating the gases with a solution or a suspension of an alkali or earth-alkali or ammonium compound
    • B01D53/502—Sulfur oxides by treating the gases with a solution or a suspension of an alkali or earth-alkali or ammonium compound characterised by a specific solution or suspension

Definitions

  • the invention disclosed herein relates generally to reducing emissions of sulfur oxides, and sulfur dioxide in particular a process utilizing a catalyst and a sorbent.
  • the art has also provided a variety of dry scrubbing processes, which introduce a SO x - reducing reagent, such as a slurry of lime (CaO in slurry is Ca(OH) 2 ) trona (sodium sesquicarbonate), sodium bicarbonate, calcium carbonate, or blends of these materials, into a flue gas stream in a duct or separate reactor, wherein the SO x is captured to some extent and can be disposed of in dry particulate form.
  • a SO x - reducing reagent such as a slurry of lime (CaO in slurry is Ca(OH) 2 ) trona (sodium sesquicarbonate), sodium bicarbonate, calcium carbonate, or blends of these materials
  • the present invention provides processes, apparatus, compositions and systems that have as their advantage that their use will have a very positive effect on air q uality at a very reasonable cost.
  • the invention can be employed as a retrofit solution to existing plants and can be used in design of new plants.
  • the invention provides a process comprising: identifying locations within a combustor for feeding a sorbent and a sufur-active catalyst; determining the physical form and injection parameters for the sorbent and the catalyst; and injecting both the sorbent and the catalyst under conditions effective to dissocitate S0 2 and capture sulfur on the sorbent.
  • Preferred conditions will call for introducing the sorbent at a temperature within the range of from about 1800 ° to about 2800 °F, e.g., from 1900 ° to about 2300 °F, as a slurry in droplets having a mean diameter of from about 10 to about 1000 microns, e.g., from about 75 to about 300 microns, so that the sorbent is present as fine particles of Ca(OH 2 ) with a particle size of from about 2 to about 8 microns, e.g., about 3 to about 5 microns, in the zone where the catalyst is effective to dissociate the S0 2 , e.g., from about 1300 0 to about 1900 °F.
  • the catalyst wil typically be introduced downstream of the sorbent at an effective particle size, with sizes of from about 1 to about 5 microns being of sufficiently small size to facilitate introduction and distribution.
  • the sorbent will be introduced as a slurry upstream of the catalyst and it will dehydrate and be caused to shatter into fine particles (observed to be within the size range of from about 0.01 to about 1.0 microns) which are dispersed over the cross section of the furnace section, duct or other appartus where the catalyst is effective in dissociating the S0 2 .
  • the feed rate of the catalyst and the lime will depend on the amount of fuel and its sulfur content. For coal having about 2 to about 3% sulfur, a feed rate of about 5 pounds of sorbent per pound of fuel will be an adequate starting point, with the exact feed rate to be determined based on experimentation.
  • the invention provides several advantages compared with competitive processes, prominent among which are: reducing material usage due to the ability to present the sulfur in a very absorbable form for the sorbent; causing a more efficient utilization of sorbent; enabling very high sulfur removal rates; enabling the use of simple equipment; adsorbing sulfur in the lime and as metal sulfides, due to efficiencies brought on by half the weight of the S0 2 being liberated as oxygen, make much less material, do not clog or tie up solids capture equipment and are easier to use; and enabling the addition of supplemental materials that can increase boiler efficieny.
  • FIG. 1 is a schematic view of one embodiment of the invention.
  • Fig. 2 is a TEM image shows an image obtained from a zeolite catalyst substrate after impregnation with copper according to Example 1, showing particles as darkest spots in the size range of about 2-7nm.
  • Fig. 3 is a X-ray diffraction (XRD) analyses of one sample prepared according to Example 2.
  • Fig. 4 is a scanning electron microscopy map of a prepard catalyst composition showing copper particles (light) , about 6 nanometers in size, distributed in 12 nanometer mesopores throughout the structure of the substrate.
  • Fig. 5 is a photograph taken of a transmitting electron microscope image of the material shown in Fig. 4, with copper particles shown in black, distributed in the substrate. Magnification is 120,000X.
  • Fig. 6 is a graph obtained by GC/Mass Spec (Gas Chromatography/Mass Spectroscopy) of a synthetic boiler combustion gas containing about 1000 ppmv of S0 2 used for the glass tube furnace testing as described in Example 3.
  • Fig. 7 is a graph similar to Fig. 6, measuring micro-pressures (pTorr) of that same gas sample after it has passed through a plug of high temperature glass wool impregnated with the experimental reagent.
  • Gas pressure in pTorr is approximately 4.0 pTorr, demonstrating approximately 95% reduction in S0 2 pressure (concentration) in the treated sample gas.
  • the invention effectively deals with the problem of sulfur dioxide in a manner and by a mechanism that is very different from commercial SO x -reducing processes and apparatus.
  • Testing in laboratory-scale equipment demonstrates that sulfur is dissociated from S0 2 (mass 64) into smaller fragments in a defined reaction zone. Evidence suggests that the catalyst causes the presence of free S and possibly free O.
  • Various analyses show a significant increase in the amount of gas components at mass 32 (S, 0 2 ) with a major reduction of those at mass 64, which would identify S0 2 . Indeed, S0 2 reductions of 90 to 95% have been shown.
  • a test sorbent picked up 6% sulfur by weight (equivalent to 12% by S0 2 ,).
  • FIG. 1 is a schematic view of one embodiment of the invention.
  • Fig. 1 shows a large combustor 10 of the type used for producing steam for electrical power generation, process steam, heating or incineration. It will be understood that other types of combustors can be employed to utilize the advantages of the invention. Unless otherwise indicated, all parts and percentages in this description are based on the weight of the materials at the particular point in processing.
  • Coal is fed by burners 20 and 20a and burned with air in a combustion zone 21. It is an advantage of the invention that coal that is high in sulfur can be combusted and the resulting sulfur dioxides reduced. It will be understood that the principals of the invention can be applied to other carbonaceous fuels and fuel mixtures.
  • Air for combustion supplied by fan 22 and ductwork 24, is preferably preheated by gas- to-gas heat exchangers (not shown) which transfer heat from ductwork (not shown) at the exit end of the combustor.
  • Hot combustion gases rise and flow past heat exchangers 26, which transfer heat from the combustion gases to water or steam for the generation of steam or superheated steam.
  • Other heat exchangers including an economizer (downstream and not shown) may also be provided according to the design of the particular boiler.
  • modeling techniques such as computational fluid dynamics, are employed to initially direct treatment chemicals to the optimum locations within the boiler.
  • Vessel 30 is provided to hold sorbent slurry, preferably a form of lime hydrate with high surface area.
  • the sorbent slurry is typically characterized as containing from about 25 to about 45% calcium hydroxide (Ca(OH) 2 ) in water.
  • Suitable stablizers are used to avoid the need for constantly stirring the tanks, but stirring is preferably provided.
  • the material is further characterized by having a mass average paticle size of from about 2 to about 8 microns ( ⁇ ), e.g., nominally about 3 to 5 microns.
  • Calcium hydroxide which has been found effective according to the invention capturing sulfur dissociated from S0 2 , is mixed with water for introduction from tank 30 through associated lines with or without chemical stabilizers, to concentrations suitable for storage and handling, e.g., at least about 25%, and preferably at least about 40%, solids by weight.
  • a series of suitable, preferably air assisted atomizing, nozzles in each of nozzle banks 34 and 34a is provided for introducing lime hydrate alone or with another treatment chemical from vessel 30.
  • Supply lines e.g., 32
  • Valves e.g., 33 and 33a
  • the locations for the nozzles are preferably determined by computational fluid dynamics, as taught for example in U. S. Patent No. 5,740,745 and U. S. Patent No. 5,894,806.
  • the lime in the hydrated form i.e., Ca(OH) 2
  • Vessel 40 is provided to hold catalyst slurry, preferably a finely-divided form with high surface area, e.g., mass average particle sizes of from about 1 to about 5 microns, e.g., from about 2 to about 3 microns, being of sufficiently small size to facilitate stabliziation in a slurry, introduction into the combustion gases and distribution throughout the combustion gases to provide effective heterogeneous contact with the S0 2 .
  • catalyst slurry preferably a finely-divided form with high surface area, e.g., mass average particle sizes of from about 1 to about 5 microns, e.g., from about 2 to about 3 microns, being of sufficiently small size to facilitate stabliziation in a slurry, introduction into the combustion gases and distribution throughout the combustion gases to provide effective heterogeneous contact with the S0 2 .
  • the catalyst slurry is typically characterized as containing from about 30 to about 60% catalyst in water.
  • the catalyst material is further characterized as comprising a silica or alumino silicate support with an effective transition metal, e.g., copper, in catalytically effective form thereon.
  • the catalyst is comprised of a supported copper, preferably with a finely- divided zeolite support which is doped with copper.
  • catalyst herein we mean a composition of matter in suitable physical form to effect the dissociation of sulfur dioxide as described herein, into several of its component parts in a chemical form that enables its effective collection by the hydrated lime sorbent described.
  • the catalyst is preferably a catalytic cracking catalyst of the type used in hydrocarbon cracking (either a waste product or virgin manufactured material) that has been doped with copper, formed into a solid, dried, gound and slurried.
  • the examples provide preferred processing. Suitable stablizers can be used to avoid the need for constantly stirring the tanks, but stirring is preferably provided.
  • a second series of suitable, preferably air-assisted atomizing, nozzles in each of nozzle banks 44 and 44a is provided for introducing catalyst, preferably alone, from vessel 40.
  • Supply lines [e.g., 42) are shown as double lines in the drawing.
  • valves e.g., 43 and 43a
  • the locations for the nozzles are preferably determined by computation fluid dynamics,
  • Temperature sensors (e.g., 50, 50a and 50b) are represented by the symbol ( [i ). Valves and temperature sensors are shown to be connected to controller 52 via electrical leads shown in dotted lines. These valves, temperature sensors and leads are illustrative only, and the skilled worker using the principles outlined herein will place them strategically to provide appropriate control signals and responses.
  • the controller 52 can be a general purpose digital computer programmed in accord with a predetermined control regimen with both feed forward and feedback features.
  • the sorbent is fed to the furnace nozzle banks 34 and 34a ahead of the catalyst to allow for adequate distribution so that when it comes in contact with the reacted components of the catalytic S0 2 control reaction, absorbing the sulfur species directly or reacting with it to form calcium sulfide. It is important that the dissociated sulfur from the S0 2 be captured before it has opportunity to react to make capture more difficult.
  • the catalyst is preferably fed at approximately 1300° to about 1900°F, e.g., from about 1500° to about 1800°F, in the convection section of the furnace or boiler.
  • the catalyst when contacted by S0 2 molecules, causes the S0 2 to split up (crack) into molecular or gaseous oxygen (0 2 ) and sulfur radicals (S ⁇ ).
  • S " radicals are reactive and readily react with coal ash fines present in the combustion gas, and the sorbent to react with the basic oxides contained therein, forming metal sulfides or being absorbed directly by either or both the sorbent or the activator in the combustion gas.
  • the concentration and flow rates will be initially determined by modeling to assure that the proper amount of chemical is supplied to the correct location in the combustor in the correct physical form to achieve the desired results of reduced S0 2 .
  • the catalyst and the sorbent may be diluted as determined, e.g., by com putational fluid dynamics (CFD) to any effective concentration that will meet the objectives of the invention.
  • CFD com putational fluid dynamics
  • the feed rate of the catalyst and the lime will depend on the amount of fuel and its sulfur content. For coal having about 2 to about 3% sulfur, a feed rate of about 5 pounds of sorbent per pound of fuel will be an adequate starting point, with the exact feed rate to be determined based on experimentation.
  • the catalyst can be fed at similar rates initially and then adjusted.
  • This example describes the preparation of copper doped zeolite catalysts found effective according to the invention.
  • Spent zeolite catalytic cracking catalysts were employed as support matrices and impregnated with copper, dried and calcined. Following processing, they were found to have a loading of copper particles in the support matrices with sizes in the range of from 2-7 nm with a gross uniform dispersion.
  • Pore volume analysis data showed that the weight expected for the incipient wetness point would be 27-28% of zeolite weight. Based on this information, concentrations of copper nitrate in water for water content levels of 25% and 50% were calculated, two zeolite materials were impregnated with these solution levels by incipient wetness, and the impregnated samples were then dried at 110°C for two hours. [044] Samples (5 gram each) of the 25% incipient wetness impregnated materials were then calcined at temperatures of 300°C or 450°C to decompose the copper nitrate and leave copper metal particles in the structure of the support systems. Samples were calcined using a computer- controlled tube furnace to ramp the temperature to each selected value at a rate of 3°C per minute, held for 4 hours at the target temperature and then cooled.
  • the calcined materials were then ground in water using a ring mill to provide a slurry with 30% solids and a target particle size of about 2 ⁇ .
  • the majority of particle sizes observed by scanning electron microscopy (SEM) for these samples were in the 1-5 ⁇ range with few larger particles and very numerous smaller particles.
  • Energy dispersive X-ray spectroscopy (EDS) analysis of bulk samples of the powder produced by grinding shows only a slight change in copper content, indicating that the copper is relatively uniformly distributed through the support materials.
  • TEM Transmission electron microscopy
  • This example tests several catalysts prepared according to the procedures of Example 1, but with 1% copper and different moisture contents, namely 31.75%, 50% and 66.7% moisture to determine if they were successfully impregnated with copper.
  • Fig. 4 is a scanning electron microscopy map of a prepard catalyst composition showing copper particles (light) , about 6 nanometers in size, distributed in 12 nanometer mesopores throughout the structure of the substrate.
  • Fig. 5 is a photograph taken of a transmitting electron microscope image of the material shown in Fig. 4, with copper particles shown in black, distributed in the substrate. Magnification is 120,000X.
  • the sample was also analyzed by X-ray fluorescence spectroscopy (XRF) using a Bruker S2 X-Ray Fluorescence instrument with an energy dispersive x-ray detector and operated under helium. Data were calculated in standardless format. The relative concentration levels of copper appear higher than anticipated because much of the copper is deposited onto the surfaces of the catalyst particles rather than distributed into the porous internal structures of the catalyst supports.
  • XRF X-ray fluorescence spectroscopy
  • This example illustrates the ability to capture sulfur on a sorbent following dissociation of S0 2 by contact with a catalyst of the type prepared above.
  • a sample of portlandite Ca(OH) 2 was placed in quartz tube reactor downstream of a packing of quartz wool impregnated with the catalyst and was used as a downstream collector in an attempt to bind sulfur after exposure to the reaction zone.
  • the temperature of the reactor and its contents could be varied between about 300°C and 1000°C.
  • a synthetic combustion flue gas was prepared having approximate concentrations of 14% C02, 4% 02, 500ppm CO, 300ppm N0 X , lOOOppm S0 2 and 9.5% H 2 0.
  • Mass spectral data were collected at the inlet and outlet portions of the reactor.
  • the catalyst material and the portlandite were evaluated using XRF to determine if sulfur compounds were being formed in them. Reactions were conducted at various temperatures, with data recorded at 750°, 850° and 950°C. XRF indicates that the sulfur content of both materials has increased after exposure to the test reaction.
  • Fig. 6 and Fig. 7 are graphs obtained by GC/Mass Spec (Gas Chromatography/Mass Spectroscopy) of a synthetic boiler combustion gas containing about 1000 ppmv of S0 2 used for the glass tube furnace testing.
  • the y-axis on the right is the partial pressure in ⁇ , (micro Torr) a measure of microscopic gas pressures.
  • the S0 2 partial pressure in ⁇ is approximately 83 in this baseline sample.
  • Fig. 6 characterizes the gas prior to treatment and Fig. 7 does so after treatment.

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Abstract

Processes, apparatus, compositions and systems are provided that have a positive effect on air quality at a very reasonable cost. They can be employed as a retrofit solution to existing plants and can be used in design of new plants. In one aspect a process comprises: identifying locations within a combustor for feeding a sorbent and a sufur-active catalyst; determining the physical form and injection parameters for the sorbent and the catalyst; and injecting both the sorbent and the catalyst under conditions effective to dissocitate S02 and capture sulfur on the sorbent. Typically, the sorbent is introduced as a slurry upstream of the catalyst; the slurry dehydrates and shatters into fine particles which disperse over the cross section of the furnace section, duct or other apparatus where the catalyst dissociates the S02.

Description

Catalytic Sulfur Dioxide Mediation Field of the Invention
[001] The invention disclosed herein relates generally to reducing emissions of sulfur oxides, and sulfur dioxide in particular a process utilizing a catalyst and a sorbent.
Background of the Invention
[002] For years, the art has been dealing with varying degrees of success with the problem of sulfur oxides, referred to generally as SOx and comprising sulfur dioxide (S02) and sulfur trioxide (S03), are formed during the combustion of sulfur-containing carbonaceous fuels.
[003] The art has provided wide range of technologies including wet scrubbers, often called flue gas desulfurization scrubbers which typically include large towers which contact combustion flue gases with a slurry of calcium carbonate sprayed contercurrently to the flue gas flow. These scrubbers are expensive to install and operate and cannot be easily adapted to all plants.
[004] The art has also provided a variety of dry scrubbing processes, which introduce a SOx- reducing reagent, such as a slurry of lime (CaO in slurry is Ca(OH)2) trona (sodium sesquicarbonate), sodium bicarbonate, calcium carbonate, or blends of these materials, into a flue gas stream in a duct or separate reactor, wherein the SOx is captured to some extent and can be disposed of in dry particulate form.
[005] The current art that uses trona and/or sodium carbonate typically requires feeding very large quantities of these SOx-reducing reagents, typically to back end duct work of the boiler. Trona adds significant sodium to the ash, which can cause serious potential heavy metal leaching problems. It also adds so much solids to the ash capture equipment that it actually can degrade performance and cause operating and handling problems under certain conditions. Current performance limitations in the field are about 30-40% reductions of S02 with high solids addition to the ash. [006] The calcium based technologies, lime (calcium carbonate) and lime hydrate (calcium hydroxide) are fraught with problems: low performance (10-15% reductions); excessive material consumption; performance problems with calcium sulfate deposition; filling up ducts with excessive solids, etc. Additionally, lime and hydrate are difficult to handle, releasing considerable heat when slurried. Mechanical reliability of slaking equipment is a major issue.
[007] There is a present need for technology that can improve on the capture of S02 by capturing it at high percentages in an economical manner in terms of material, equipment and disposal costs.
Summary of the Invention
[008] The present invention provides processes, apparatus, compositions and systems that have as their advantage that their use will have a very positive effect on air q uality at a very reasonable cost. The invention can be employed as a retrofit solution to existing plants and can be used in design of new plants.
[009] I n one aspect, the invention provides a process comprising: identifying locations within a combustor for feeding a sorbent and a sufur-active catalyst; determining the physical form and injection parameters for the sorbent and the catalyst; and injecting both the sorbent and the catalyst under conditions effective to dissocitate S02 and capture sulfur on the sorbent.
[010] Preferred conditions will call for introducing the sorbent at a temperature within the range of from about 1800 ° to about 2800 °F, e.g., from 1900 ° to about 2300 °F, as a slurry in droplets having a mean diameter of from about 10 to about 1000 microns, e.g., from about 75 to about 300 microns, so that the sorbent is present as fine particles of Ca(OH2) with a particle size of from about 2 to about 8 microns, e.g., about 3 to about 5 microns, in the zone where the catalyst is effective to dissociate the S02, e.g., from about 1300 0 to about 1900 °F.
[011] The catalyst wil typically be introduced downstream of the sorbent at an effective particle size, with sizes of from about 1 to about 5 microns being of sufficiently small size to facilitate introduction and distribution. [012] Typically, the sorbent will be introduced as a slurry upstream of the catalyst and it will dehydrate and be caused to shatter into fine particles (observed to be within the size range of from about 0.01 to about 1.0 microns) which are dispersed over the cross section of the furnace section, duct or other appartus where the catalyst is effective in dissociating the S02. The feed rate of the catalyst and the lime will depend on the amount of fuel and its sulfur content. For coal having about 2 to about 3% sulfur, a feed rate of about 5 pounds of sorbent per pound of fuel will be an adequate starting point, with the exact feed rate to be determined based on experimentation.
[013] The invention provides several advantages compared with competitive processes, prominent among which are: reducing material usage due to the ability to present the sulfur in a very absorbable form for the sorbent; causing a more efficient utilization of sorbent; enabling very high sulfur removal rates; enabling the use of simple equipment; adsorbing sulfur in the lime and as metal sulfides, due to efficiencies brought on by half the weight of the S02 being liberated as oxygen, make much less material, do not clog or tie up solids capture equipment and are easier to use; and enabling the addition of supplemental materials that can increase boiler efficieny.
[014] Other preferred aspects and their advantages are set out in the description which follows. Brief Description of the Drawings
[015] The invention will be better understood and its advantages will become more apparent when the following detailed description is read in conjunction with the accompanying drawings, in which:
[016] Fig. 1 is a schematic view of one embodiment of the invention.
[017] Fig. 2 is a TEM image shows an image obtained from a zeolite catalyst substrate after impregnation with copper according to Example 1, showing particles as darkest spots in the size range of about 2-7nm.
[018] Fig. 3 is a X-ray diffraction (XRD) analyses of one sample prepared according to Example 2. [019] Fig. 4 is a scanning electron microscopy map of a prepard catalyst composition showing copper particles (light) , about 6 nanometers in size, distributed in 12 nanometer mesopores throughout the structure of the substrate.
[020] Fig. 5 is a photograph taken of a transmitting electron microscope image of the material shown in Fig. 4, with copper particles shown in black, distributed in the substrate. Magnification is 120,000X.
[021] Fig. 6 is a graph obtained by GC/Mass Spec (Gas Chromatography/Mass Spectroscopy) of a synthetic boiler combustion gas containing about 1000 ppmv of S02 used for the glass tube furnace testing as described in Example 3.
[022] Fig. 7 is a graph similar to Fig. 6, measuring micro-pressures (pTorr) of that same gas sample after it has passed through a plug of high temperature glass wool impregnated with the experimental reagent. Gas pressure in pTorr is approximately 4.0 pTorr, demonstrating approximately 95% reduction in S02 pressure (concentration) in the treated sample gas.
Detailed Description of the Invention
[023] The invention effectively deals with the problem of sulfur dioxide in a manner and by a mechanism that is very different from commercial SOx-reducing processes and apparatus. Testing in laboratory-scale equipment demonstrates that sulfur is dissociated from S02 (mass 64) into smaller fragments in a defined reaction zone. Evidence suggests that the catalyst causes the presence of free S and possibly free O. Various analyses show a significant increase in the amount of gas components at mass 32 (S, 02) with a major reduction of those at mass 64, which would identify S02. Indeed, S02 reductions of 90 to 95% have been shown. In laboratory tests, a test sorbent picked up 6% sulfur by weight (equivalent to 12% by S02,).
[024] Reference will first be made to Fig. 1, which is a schematic view of one embodiment of the invention. Fig. 1 shows a large combustor 10 of the type used for producing steam for electrical power generation, process steam, heating or incineration. It will be understood that other types of combustors can be employed to utilize the advantages of the invention. Unless otherwise indicated, all parts and percentages in this description are based on the weight of the materials at the particular point in processing.
[025] Coal is fed by burners 20 and 20a and burned with air in a combustion zone 21. It is an advantage of the invention that coal that is high in sulfur can be combusted and the resulting sulfur dioxides reduced. It will be understood that the principals of the invention can be applied to other carbonaceous fuels and fuel mixtures.
[026] Air for combustion, supplied by fan 22 and ductwork 24, is preferably preheated by gas- to-gas heat exchangers (not shown) which transfer heat from ductwork (not shown) at the exit end of the combustor. Hot combustion gases rise and flow past heat exchangers 26, which transfer heat from the combustion gases to water or steam for the generation of steam or superheated steam. Other heat exchangers, including an economizer (downstream and not shown) may also be provided according to the design of the particular boiler.
[027] It is an advantage of the present invention that modeling techniques, such as computational fluid dynamics, are employed to initially direct treatment chemicals to the optimum locations within the boiler.
[028] Vessel 30 is provided to hold sorbent slurry, preferably a form of lime hydrate with high surface area. The sorbent slurry is typically characterized as containing from about 25 to about 45% calcium hydroxide (Ca(OH)2) in water. Suitable stablizers are used to avoid the need for constantly stirring the tanks, but stirring is preferably provided. The material is further characterized by having a mass average paticle size of from about 2 to about 8 microns (μ), e.g., nominally about 3 to 5 microns. Calcium hydroxide, which has been found effective according to the invention capturing sulfur dissociated from S02, is mixed with water for introduction from tank 30 through associated lines with or without chemical stabilizers, to concentrations suitable for storage and handling, e.g., at least about 25%, and preferably at least about 40%, solids by weight.
[029] A series of suitable, preferably air assisted atomizing, nozzles in each of nozzle banks 34 and 34a is provided for introducing lime hydrate alone or with another treatment chemical from vessel 30. Supply lines [e.g., 32) are shown as double lines in the drawing. Valves (e.g., 33 and 33a) are represented by the common symbol ( ^: ). The locations for the nozzles are preferably determined by computational fluid dynamics, as taught for example in U. S. Patent No. 5,740,745 and U. S. Patent No. 5,894,806. Following injection into the combustor at a gas temperature of from about 1800 0 to about 2800 °F, the lime in the hydrated form, i.e., Ca(OH)2, is dehydrated by the hot combustion gases and shatters into fine pariticles with a mass average particle size on the order of from about 0.01 to about 1 μ.
[030] Vessel 40 is provided to hold catalyst slurry, preferably a finely-divided form with high surface area, e.g., mass average particle sizes of from about 1 to about 5 microns, e.g., from about 2 to about 3 microns, being of sufficiently small size to facilitate stabliziation in a slurry, introduction into the combustion gases and distribution throughout the combustion gases to provide effective heterogeneous contact with the S02.
[031] The catalyst slurry is typically characterized as containing from about 30 to about 60% catalyst in water. The catalyst material is further characterized as comprising a silica or alumino silicate support with an effective transition metal, e.g., copper, in catalytically effective form thereon. Typically, the catalyst is comprised of a supported copper, preferably with a finely- divided zeolite support which is doped with copper. By the term "catalyst" herein we mean a composition of matter in suitable physical form to effect the dissociation of sulfur dioxide as described herein, into several of its component parts in a chemical form that enables its effective collection by the hydrated lime sorbent described. The catalyst is preferably a catalytic cracking catalyst of the type used in hydrocarbon cracking (either a waste product or virgin manufactured material) that has been doped with copper, formed into a solid, dried, gound and slurried. The examples provide preferred processing. Suitable stablizers can be used to avoid the need for constantly stirring the tanks, but stirring is preferably provided. We note that while we believe the composition that we call a catalyst is probably acting as a catalyst, we cannot be sure at this stage of the mechanism involved; however, we have evidence that the sulfur dioxide is being broken down to its components. There may be mechanisms besides or in addition to catalysis responsible for this chemical breakdown. [032] A second series of suitable, preferably air-assisted atomizing, nozzles in each of nozzle banks 44 and 44a is provided for introducing catalyst, preferably alone, from vessel 40. Supply lines [e.g., 42) are shown as double lines in the drawing. Again, valves (e.g., 43 and 43a) are represented by the common symbol ( | ; ), and the locations for the nozzles are preferably determined by computation fluid dynamics,
[033] Temperature sensors (e.g., 50, 50a and 50b) are represented by the symbol ( [i ). Valves and temperature sensors are shown to be connected to controller 52 via electrical leads shown in dotted lines. These valves, temperature sensors and leads are illustrative only, and the skilled worker using the principles outlined herein will place them strategically to provide appropriate control signals and responses. The controller 52 can be a general purpose digital computer programmed in accord with a predetermined control regimen with both feed forward and feedback features.
[034] The sorbent is fed to the furnace nozzle banks 34 and 34a ahead of the catalyst to allow for adequate distribution so that when it comes in contact with the reacted components of the catalytic S02 control reaction, absorbing the sulfur species directly or reacting with it to form calcium sulfide. It is important that the dissociated sulfur from the S02 be captured before it has opportunity to react to make capture more difficult.
[035] The catalyst is preferably fed at approximately 1300° to about 1900°F, e.g., from about 1500° to about 1800°F, in the convection section of the furnace or boiler.
[036] The catalyst, when contacted by S02 molecules, causes the S02 to split up (crack) into molecular or gaseous oxygen (02) and sulfur radicals (S~). These S" radicals are reactive and readily react with coal ash fines present in the combustion gas, and the sorbent to react with the basic oxides contained therein, forming metal sulfides or being absorbed directly by either or both the sorbent or the activator in the combustion gas.
[037] As will be described, the concentration and flow rates will be initially determined by modeling to assure that the proper amount of chemical is supplied to the correct location in the combustor in the correct physical form to achieve the desired results of reduced S02. For use in the process, the catalyst and the sorbent may be diluted as determined, e.g., by com putational fluid dynamics (CFD) to any effective concentration that will meet the objectives of the invention.
[038] The feed rate of the catalyst and the lime will depend on the amount of fuel and its sulfur content. For coal having about 2 to about 3% sulfur, a feed rate of about 5 pounds of sorbent per pound of fuel will be an adequate starting point, with the exact feed rate to be determined based on experimentation. The catalyst can be fed at similar rates initially and then adjusted.
[039] The following examples are presented to further explain and illustrate the invention and are not to be taken as limiting in any regard. Unless otherwise indicated, all parts and percentages are by weight.
[040] Example 1
[041] This example describes the preparation of copper doped zeolite catalysts found effective according to the invention. Spent zeolite catalytic cracking catalysts were employed as support matrices and impregnated with copper, dried and calcined. Following processing, they were found to have a loading of copper particles in the support matrices with sizes in the range of from 2-7 nm with a gross uniform dispersion.
[042] The introduction of the copper material into the zeolite support matrices at the desired concentration, reduction of the copper compound to copper metal by thermal decomposition, grinding of the support materials to the desired final particle size, packing of a reaction vessel to study catalytic activity, supply of a suitable simulated flue gas environment and evaluation of the supply and output gases from the reaction zone were investigated.
[043] Pore volume analysis data showed that the weight expected for the incipient wetness point would be 27-28% of zeolite weight. Based on this information, concentrations of copper nitrate in water for water content levels of 25% and 50% were calculated, two zeolite materials were impregnated with these solution levels by incipient wetness, and the impregnated samples were then dried at 110°C for two hours. [044] Samples (5 gram each) of the 25% incipient wetness impregnated materials were then calcined at temperatures of 300°C or 450°C to decompose the copper nitrate and leave copper metal particles in the structure of the support systems. Samples were calcined using a computer- controlled tube furnace to ramp the temperature to each selected value at a rate of 3°C per minute, held for 4 hours at the target temperature and then cooled.
[045] The calcined materials were then ground in water using a ring mill to provide a slurry with 30% solids and a target particle size of about 2 μηι. The majority of particle sizes observed by scanning electron microscopy (SEM) for these samples were in the 1-5 μιη range with few larger particles and very numerous smaller particles. Energy dispersive X-ray spectroscopy (EDS) analysis of bulk samples of the powder produced by grinding shows only a slight change in copper content, indicating that the copper is relatively uniformly distributed through the support materials.
[046] Transmission electron microscopy (TEM) analysis was performed using Formvar coated copper grids for sample dispersion from solution and a model 4000EX transmission electron microscope marketed by JEOL, USA, Inc. of Peabody, MA. The images were collected onto Kodak EM 4489 electron microscopy film, developed and scanned into a Macintosh G3 workstation for processing. The TEM image of Fig. 2 shows an image obtained from one sample after impregnation with copper particles (darkest spots) noted in the size range of about 2-7nm.
[047] Example 2
[048] This example tests several catalysts prepared according to the procedures of Example 1, but with 1% copper and different moisture contents, namely 31.75%, 50% and 66.7% moisture to determine if they were successfully impregnated with copper.
[049] Very little copper was observed as having been impregnated into the bulk of the zeolite based catalyst supports using 50% and 66.7% moisture. In addition to the surface deposits, particles of copper were noted in the 31.75% moisture sample within the catalyst support structure with most in the 30-80nm and a few in the 5-10nm range. [050] X-ray diffraction (XRD) analyses were conducted using a Bruker D8 x-ray diffractometer. Diffraction patterns were obtained using monochromated copper K-alpha radiation and slits of 6mm at the x-ray tube. A VANTEC-l area detector was used for data collection. The spectra obtained were collected in the 10-90° 2Θ range. The XRD pattern of one sample is shown in Fig. 3. The XRD data verify that elemental copper has been deposited on the spent catalyst sample materials.
[051] Fig. 4 is a scanning electron microscopy map of a prepard catalyst composition showing copper particles (light) , about 6 nanometers in size, distributed in 12 nanometer mesopores throughout the structure of the substrate.
[052] Fig. 5 is a photograph taken of a transmitting electron microscope image of the material shown in Fig. 4, with copper particles shown in black, distributed in the substrate. Magnification is 120,000X.
[053] The sample was also analyzed by X-ray fluorescence spectroscopy (XRF) using a Bruker S2 X-Ray Fluorescence instrument with an energy dispersive x-ray detector and operated under helium. Data were calculated in standardless format. The relative concentration levels of copper appear higher than anticipated because much of the copper is deposited onto the surfaces of the catalyst particles rather than distributed into the porous internal structures of the catalyst supports.
[054] Example 3
[055] This example illustrates the ability to capture sulfur on a sorbent following dissociation of S02 by contact with a catalyst of the type prepared above.
[056] A sample of portlandite Ca(OH)2 was placed in quartz tube reactor downstream of a packing of quartz wool impregnated with the catalyst and was used as a downstream collector in an attempt to bind sulfur after exposure to the reaction zone. The temperature of the reactor and its contents could be varied between about 300°C and 1000°C. A synthetic combustion flue gas was prepared having approximate concentrations of 14% C02, 4% 02, 500ppm CO, 300ppm N0X, lOOOppm S02 and 9.5% H20. Mass spectral data were collected at the inlet and outlet portions of the reactor. The catalyst material and the portlandite were evaluated using XRF to determine if sulfur compounds were being formed in them. Reactions were conducted at various temperatures, with data recorded at 750°, 850° and 950°C. XRF indicates that the sulfur content of both materials has increased after exposure to the test reaction.
[057] The results indicate that sulfur is being dissociated from S02 (mass 64) into smaller fragments in the reaction zone. This is indicated by a significant increase in the size of the peak at mass 32 (S, 02) while the peak at 64 for S02 has been reduced by 90-95%. Increased amounts of S was found in the catalyst and in the Ca(OH)2 to suggest that these materials are collecting the sulfur released from the S02.
[058] The results are reported in Fig. 6 and Fig. 7, which are graphs obtained by GC/Mass Spec (Gas Chromatography/Mass Spectroscopy) of a synthetic boiler combustion gas containing about 1000 ppmv of S02 used for the glass tube furnace testing. In the graph, the y-axis on the right is the partial pressure in μΤθΓΓ, (micro Torr) a measure of microscopic gas pressures. The S02 partial pressure in μΊοπ is approximately 83 in this baseline sample. Fig. 6 characterizes the gas prior to treatment and Fig. 7 does so after treatment.
[059] The above description is for the purpose of teaching the person of ordinary skill in the art how to practice the invention. It is not intended to detail all of those obvious modifications and variations, which will become apparent to the skilled worker upon reading the description. It is intended, however, that all such obvious modifications and variations be included within the scope of the invention which is defined by the following claims. The claims are meant to cover the claimed components and steps in any sequence that is effective to meet the objectives there intended, unless the context specifically indicates the contrary.

Claims

Claims
1. A process for reducing the S02 content of a combustion gas comprising:
identifying locations within a combustor for feeding a sorbent and a sufur-active catalyst; determining the physical form and injection parameters for the sorbent and the catalyst; and injecting both the sorbent and the catalyst under conditions effective to dissocitate S02 and capture sulfur on the sorbent.
2. A process according to claim 1, wherein the S02 is dissociated catalytically.
3. A process according to claim 1, wherein the sorbent is introduced at a temperature within the range of from about 1800 0 to 2800 °F as a slurry in droplets having a mean diameter of from about 10 to about 1000 microns.
4. A process according to claim 3, wherein the sorbent is introduced at a temperature within the range of from 1900 0 to about 2300 °F.
5. A process according to claim 3, wherein the sorbent is introduced a as droplets having a mean diameter of from about 75 to about 300 microns.
6. A process according to claim 1, wherein the catalyst comprises a copper-doped zeolite.
7. A process according to claim 1, wherein the catalyst is effective to dissociate the S02 at a temperature of from about 1300 0 to 1900 °F.
8. A process according to claim 1, wherein the sorbent is introduced as a slurry upstream of the catalyst whereby it dehydrates and shatters into fine particles which are dispersed over the cross section of the furnace section, duct or other appartus where the catalyst is effective in dissociating the S02.
9. A process according to claim 1, wherein the sorbent is introduced as a slurry comprising fine particles of Ca(OH2) with a particle size of from about 2 to about 8 microns.
10. A process according to claim 9, wherein the catalyst is introduced downstream of the sorbent at particle sizes in the range of from about 1 to about 5 microns.
11. A process according to claim 1, wherein the sorbent is initially introduced at a rate of about 5 pounds of sorbent per pound of fuel having a sulfur content of 2 to 3%.
12. A system for reducing the S02 content of a combustion gas comprising: means for identifying locations within a combustor for feeding a sorbent and a sufur-active catalyst; means for determining the physical form and injection parameters for the sorbent and the catalyst; means for storing the sorbent and the catalyst; and means for injecting both the sorbent and the catalyst under conditions effective to dissocitate S02 and capture sulfur on the sorbent.
13. A system according to claim 12, wherein the storage means include means for agitating their contents.
14. A system according to claim 12, further including temperature sensors to measure temperatures within the combustion gas, valves responsive to control signals from a controller, and a controller to receive termparature inputs and calculate catalyst and sorbent feed rates.
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