Classifications

• • 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
  • B01J8/00—Chemical or physical processes in general, conducted in the presence of fluids and solid particles; Apparatus for such processes
  • B01J8/18—Chemical or physical processes in general, conducted in the presence of fluids and solid particles; Apparatus for such processes with fluidised particles
  • B01J8/24—Chemical or physical processes in general, conducted in the presence of fluids and solid particles; Apparatus for such processes with fluidised particles according to "fluidised-bed" technique
  • B01J8/32—Chemical or physical processes in general, conducted in the presence of fluids and solid particles; Apparatus for such processes with fluidised particles according to "fluidised-bed" technique with introduction into the fluidised bed of more than one kind of moving particles
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/34—Chemical or biological purification of waste gases
  • B01D53/46—Removing components of defined structure
  • B01D53/54—Nitrogen compounds
  • B01D53/56—Nitrogen oxides
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23C—METHODS OR APPARATUS FOR COMBUSTION USING FLUID FUEL OR SOLID FUEL SUSPENDED IN  A CARRIER GAS OR AIR 
  • F23C10/00—Fluidised bed combustion apparatus
  • F23C10/005—Fluidised bed combustion apparatus comprising two or more beds
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23C—METHODS OR APPARATUS FOR COMBUSTION USING FLUID FUEL OR SOLID FUEL SUSPENDED IN  A CARRIER GAS OR AIR 
  • F23C10/00—Fluidised bed combustion apparatus
  • F23C10/02—Fluidised bed combustion apparatus with means specially adapted for achieving or promoting a circulating movement of particles within the bed or for a recirculation of particles entrained from the bed
  • F23C10/04—Fluidised bed combustion apparatus with means specially adapted for achieving or promoting a circulating movement of particles within the bed or for a recirculation of particles entrained from the bed the particles being circulated to a section, e.g. a heat-exchange section or a return duct, at least partially shielded from the combustion zone, before being reintroduced into the combustion zone
  • F23C10/08—Fluidised bed combustion apparatus with means specially adapted for achieving or promoting a circulating movement of particles within the bed or for a recirculation of particles entrained from the bed the particles being circulated to a section, e.g. a heat-exchange section or a return duct, at least partially shielded from the combustion zone, before being reintroduced into the combustion zone characterised by the arrangement of separation apparatus, e.g. cyclones, for separating particles from the flue gases
  • F23C10/10—Fluidised bed combustion apparatus with means specially adapted for achieving or promoting a circulating movement of particles within the bed or for a recirculation of particles entrained from the bed the particles being circulated to a section, e.g. a heat-exchange section or a return duct, at least partially shielded from the combustion zone, before being reintroduced into the combustion zone characterised by the arrangement of separation apparatus, e.g. cyclones, for separating particles from the flue gases the separation apparatus being located outside the combustion chamber
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23C—METHODS OR APPARATUS FOR COMBUSTION USING FLUID FUEL OR SOLID FUEL SUSPENDED IN  A CARRIER GAS OR AIR 
  • F23C2206/00—Fluidised bed combustion
  • F23C2206/10—Circulating fluidised bed
  • F23C2206/101—Entrained or fast fluidised bed
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
  • F23C—METHODS OR APPARATUS FOR COMBUSTION USING FLUID FUEL OR SOLID FUEL SUSPENDED IN  A CARRIER GAS OR AIR 
  • F23C2206/00—Fluidised bed combustion
  • F23C2206/10—Circulating fluidised bed
  • F23C2206/103—Cooling recirculating particles
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S423/00—Chemistry of inorganic compounds
  • Y10S423/09—Reaction techniques
  • Y10S423/16—Fluidization

Definitions

• Fluidized bed combustion has developed recently for burning sulfur-bearing, carbonaceous fuels. This is primarily due to the ability to contact the sulfur dioxide reaction product with a sulfur sorbent for a period of time sufficient to capture a high percentage of the sulfur dioxide. Work on nitrogen oxide removal has trailed the work on sulfur oxide capture because of less stringent legislation to this point.
• the invention is a method for lowering nitrogen oxides to a desired level and at the same- time minimizing sulfur dioxide in the reaction gases from the combustion of carbonaceous fuel in a multisolid fluidized bed.
• the multisolid fluidized bed combustor comprises a lower dense fluidized bed of relatively large particles, an upper, dispersed, entrained bed of relatively fine particles recirculating through the dense fluidized bed, and an entrained sulfur sorbent material in the dispersed, ent ' rained bed of fine particles.
• the inventive method comprises operating a lower region of the fluidized bed under fuel rich or substoichiometric conditions such that the NO x is reduced to the desired level, operating an upper region of the fluidized bed above the substoichiometric lower region under oxidizing conditions to complete the combustion of the fuel and recycling at least a portion of the cooled, relatively fine particles from the entrained bed through only the upper region which is operating under oxidizing conditions whereby to reduce the temperature of such oxidizing region to a level more conducive to sulfur capture by the sulfur sorbent material.
• the substoichiometric region is preferably limited to the dense fluidized bed or a substantial portion thereof.
• the region of reducing conditions is preferably maintained such that air is about 40-60% of stoichiometric requirements and the region of oxidizing conditions is preferably maintained at about 18% excess air.
• the temperature in such fluidized bed varies depending, among other things, on the specific fuel used, but is preferably maintained at a uniform level of about 1600-1700°F (870-925°C) for most coals and about 1700-1750°F (925-950°C) for most cokes.
• Air for the reducing region is preferably provided by the primary fluidizing gas while oxidizing conditions are preferably provided by introducing secondary air at one or more locations between a point in the dense bed near the upper boundary thereof to a point in the dispersed, entrained bed several feet upwards from the dense bed.
• the position of the secondary air affects the relative effectiveness of nitrogen oxide reduction and sulfur capture; the higher the ' entrance of secondary air, the more favorable the nitrogen reduction relative to sulfur capture.
• the residence time of the gas in the two regions is a significant factor in the completeness of reaction therein.
• a portion of the cooled fine particles of the entrained bed are conventionally recycled through the dense bed in order to provide mixing therein. The remainder of the fine particles are recycled through the oxidizing region in order to depress the temperature therein.
• substantially all the particles in the dense bed be at least about 4 times the size (diameter) of substantially all the particles in the entrained bed.
• Specific size ranges are (U.S. Standard sieve sizes) -4+16- (preferably -6+12) for the dense bed particles and -40 (preferably -50+200) for the entrained bed particles.
• Multisolid fluidized beds have demonstrated the ability to burn sulfur-bearing, carbonaceous fuel at low SO2 pollution levels when a sulfur sorbent such as limestone is contacted with the reaction gases.
• the method of operating a multisolid fluidized bed is described in detail in our prior U.S. Patent 4,084,545.
• the important features to be remembered are a dense, coarse-particle fluidized bed at the lower extreme of the apparatus and a larger ⁇ entrained bed of finer particles overlapping and recirculating through the dense bed.
• the fluidizing gas enters at the bottom to fluidize the coarse particles and to entrain the fines.
• the overlapping dense and fine beds provide thorough mixing for more complete reaction than available in one-component fluidized beds.
• the present invention is a further improvement in the operation of multisolid fluidized beds of the type shown in the prior two mentioned patents.
• the improved operation allows the combustion of sulfur-bearing carbonaceous fuels while
• a multisolid fluidized bed 1 is shown schematically.
• a fluidizing gas or primary air 2 enters the combustor at the bottom and passes through distributor plate 3.
• Fuel and possibly a sulfur sorbent enter at 4.
• the fuel is generally oil or particulate coal or coke.
• the sulfur sorbent is conventional and may for example be limestone, lime or dolomite.
• the sulfur sorbent could also enter at locations 8 or 9 with the secondary air to be later described.
• a coarse, particulate material is present in the combustor and is fluidized in the region I shown in the Figure.
• the size of the coarse component is generally in the range of -4+16 mesh (U.S. Standard) and preferably -6+12 U.S. mesh.
• the superficial gas velocity of the primary gas is generally in the range of about 10-60 feet per second (305-1830 cm/sec) .
• the coarse component should be stable and inert under the operating conditions.
• Metal oxides are desirable materials for the dense bed. Iron oxide such as contained in hematite, is preferred though aluminum oxide, silica, or nickel oxide among others may be utilized.
• the primary air 2 also entrains a relatively fine particle component in the combustor regions I and II.
• the entrained fine particles are captured above the combustor by cyclone 11 and are recycled back to the combustor through recycle leg 7 and (in the present invention) also through recycle leg 6.
• Heat is removed from the entrained fines either by heat exchange tubing within region II in the combustor or preferably through heat exchange tubing in an external heat exchanger 12.
• a fluidizing gas 5 causes the fines to be retained in the external heat exchanger for a period of time sufficient to give up their heat.
• the fine particles can be made of the same materials as the coarse particles, but silica has been found to be particularly useful.
• the particle size of the fine particles is chosen to be successfully entrained at the superficial velocity of the primary air 2 but also to prevent backmixing into the dense bed when recycled through recycle leg 6 in the Figure. As described ' later, a particle size of -40 U.S. mesh is desirable and a range of -50+200 U.S. mesh is preferred with a coarse particle size of about -6+12 U.S. mesh. Larger "fine” particles would backmix and finer "fines” may pass through the cyclones and not be recycled. However, larger fines could be used if larger coarse particles were used in the dense bed. The relative size is the important factor in preventing backmixing.
• the combustor described may be used to completely burn sulfur-bearing fuels in the dense bed region I and to capture sulfur dioxide with limestone in region II which is also known as the freeboard region.
• undesirable nitrogen oxides are formed in the dense bed during combustion and are emitted in the waste gas stream. It is known to limit the excess oxygen or to burn fuels under fuel rich or substoichiometric conditions in order to limit nitrogen oxide formation but several problems have resulted in prior methods utilizing this approach, not the least of which was a decrease in sulfur capture.
• the present invention proposes to reduce problems associated with co-removal of NO and x
• the method further includes the recycle of at least a portion of the fine entrained bed particles into the upper oxidizing region for depressing the temperature therein. Backmixing of such recycled fines into the lower fuel-rich region may be prevented by maintaining a substantial size difference between the coarse dense bed particles and the fine entrained bed particles, thus eliminating the need for the orifice plate disclosed in our previous U.S. Patent 4,154,581.
• the orifice plate can be used however if a lesser size difference between coarse and fine fractions is found necessary for other reasons.
• the primary air ratio and the gas residence time are important variables.
• the primary air ratio as used herein means the ratio of the air introduced in the bottom of the combustor to fluidize the particles (total of the primary air 2 and any air used to inject fuel and other solids at 4 in the substoichio- metric region) and the calculated stoichiometric air requirement for complete combustion of the fuel.
• the gas residence time as used herein shall mean the time interval for the gas to travel between a lower point and an upper point in the combustor. Mathematically, the gas residence time is equal to the distance between the two points divided by the superficial gas velocity.
• Oxidation in a conventional one-component bed may also be catastrophic in that unburned fuel (especially fines) and carbon monoxide from the fuel-rich zone will burn uncontrolled in the oxidizing zone thereby promoting wide temperature excursions.
• the high temperature can be both a threat to construction materials and an enemy of sulfur dioxide capture since any sulfates are decomposed back to sulfur dioxide at about 1900°F.
• the present invention therefore minimizes these problems in the following manner.
• a lower region of the combustor marked "Reduction Region" is operated at substoichiometric conditions such that nitrogen oxides released during burning therein are reduced to gaseous nitrogen by char and carbon monoxide according to the following reactions: xC + 2NO. xC0 2 + N. x -5>
• 2xCO + 2NO Secondary air is then injected at a preselected point, for example at 9, to bring the oxygen to the desired level (preferably about 10-50% excess air over stoichiometric) in the "Oxidation Region".
• Sulfur sorbent may also be injected at points 4, 8, 9 or 10 or may be recycled through recycle legs 6 and 7 and to react with the sulfur and oxygen in the oxidation region to form disposable sulfate products.
• fine limestone is injected at 9 with the secondary air and reacts with sulfur to form gypsum.
• Combustion of unburned fuel also takes place in the upper oxidation region but temperature increases due to such combustion are depressed by recycling cool, fine particles from the entrained bed through cyclone 11, external heat exchanger 12 and recycle leg 6.
• the lower reduction region may be operated at as low a temperature as will support combustion, typically about 1450 ⁇ F (790°C) for coal and about 1650°F (900°C) for coke. This allows the primary air ratio to be reduced to as low as about 0.35.
• the primary air ratio is controlled to about 0.4 to 0.6 (i.e 40-60% of theoretical stoichiometric air) .
• the upper oxidation region is preferably operated at a temperature of about 1600-1700°F (870-925°C) but at least below about 1900°F (1040°C ) where CaS0 4 decomposes.
• the height of the reduction region is selected to allow sufficient residence time to reduce the NO to the desired level.
• the higher the reduction zone the longer the residence time and the less NO will escape without reacting with char and carbon monoxide.
• the reduction zone is chosen to be the same height (above the distributor plate 3) as the dense fluidized bed I.
• Both the upper recycle leg 6 for the entrained bed particles and the secondary air inlet 9 could be at the same level just above the dense bed as shown in the drawing Figure.
• the secondary air could actually be used to inject the recycle sand through leg 6 and thereby provide better distribution of the recycled particles across the combustor.
• the recycle leg 6 and/or the secondary air inlet 8 could be located just below the upper boundary of the dense bed I such that the better mixing therein is used to distribute the recycled particles. If necessary for better reduction of NO , the secondary air inlet could be located substantially above the dense bed such as at location 10.
• the air inlets at 8, 9 and 10 may be used one at a time or in concert to control the height of the reduction region.
• the height of the oxidation region is chosen to afford sufficient residence time for capture of S0 2 by the sorbent and the oxidation thereof.
• a balance must be struck between the height of the reduction and oxidation regions by the placement of the secondary air inlet.
• two or more secondary air inlets can be used, such as at locations 8, 9, and 10 in order to better control the temperature and oxidizing conditions over a larger region.
• the entrained bed particles continue to be recycled through the dense bed via recycle leg 7 from the external heat exchanger.
• the cooled particles are beneficially used to depress the temperature excursions in the oxidation region resulting from the burning of carbon monoxide and unburned fines coming from the reduction zone. Enough of the fine particles are recycled to maintain the temperature at a satisfactory level to protect construction materials and to promote sulfur dioxide capture and oxidation, preferably in the range of about 1600°-1700°F (870-925°C) .
• Example 1 Staged Combustion of Petroleum Coke.
• a multisolid combustor test unit was used to demonstrate the invention.
• the combustor column is made of Type 304 stainless steel pipe with a wall thickness of 1/8 inch (3.2 mm).
• the bottom section is 5 feet (1.5 m) in height (above the distributor plate) and has a 6 3/8 inch (16.2 cm) I.D. which is expanded to 8 1/4 inch (21 cm) I.D. for the remaining 15 foot freeboard region of the combustor.
• a uniform diameter pipe has also been used but the expanded freeboard region provides longer gas retention time and better SO ? capture. Thermal insulation around the combustor limits heat loss.
• the entrained bed particle recirculation system comprises a 12 inch (30.5 cm) diameter cyclone, an external heat exchanger and lower and upper solid recycle legs such as shown in the drawing Figure.
• the recycle legs enter the combustor chamber at vertical locations 2 inches (5 cm) and 48 inches (1.22 ) above the distributor.
• Other cyclones are installed iii series for collecting fly ash. Fuel and the limestone sorbent are either premixed at a predetermined Ca/S ratio before feeding or are fed separately to the combustor at specified rates.
• a 1 inch (2.5 cm) diameter pneumatic injection line is used to introduce the fuel and limestone to the combustor at about 2 inches (5 cm) above the distributor plate.
• Petroleum coke having a higher heating value of 15,300 BTU/dry lb (8500 cal/Kg) was used as the fuel. It contained less than 3% moisture and was screened to -8+50 U.S. mesh before use.
• Chemical analysis was as follows: Component Weight Percent
• Minus 325 Piqua limestone was used as sulfur sorbent. African iron ore (- 6+16 ' ⁇ .s. ⁇ mesh. 312 lb/ft density) and silica sand (-20+70, 162
• Example 2 Staged Combustion of Coal.
• the apparatus was similar to that described in Example 1.
• the fuel was petroleum coke and was fed at a rate of 36.9 pounds per hour (16.75 Kg/hr) into the combustor dense bed.
• the coarse particle dense bed was made up of African ore with a size of -6+12 U.S. mesh.
• the entrained bed was made up of -40 U.S. mesh silica sand having a size distribution of: U.S. mesh size Weight percent
• the fine entrained bed particles were cooled and recycled into the combustor through either upper recycle leg 6 or lower recycle leg 7 in the Figure.
• the upper recycle leg enters the combustor about 48 inches (1.22 ) above the distributor plate in the oxidation region.
• the combustion was staged by operating a lower region under substoichiometric conditions and then adding secondary air into the combustor above the dense bed to complete combustion and aid in sulfur capture efficiency.
• Primary air was used at 36.9 SCFM (17.4 A/sec) to fluidize the bed and secondary air was added as follows:
• combustion temperature while the temperature of the freeboard or oxidation region may be controlled over a wide range by controlling the relative amount of cool fines which are recycled through the dense bed or directly through the oxidation region. By balancing these amounts we were able to reduce the temperature in the oxidation region about 150°F (83°C) while maintaining a constant temperature in the dense bed. This control can protect the construction materials in the freeboard region from thermal damage and can assure more efficient sulfur capture.

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• 1979
  • 1979-12-26 US US06/106,745 patent/US4704084A/en not_active Expired - Lifetime
• 1980
  • 1980-12-22 WO PCT/US1980/001737 patent/WO1981001873A1/en active IP Right Grant
  • 1980-12-22 JP JP81500438A patent/JPS56501811A/ja active Pending
  • 1980-12-22 EP EP81900256A patent/EP0042418A1/en not_active Withdrawn
  • 1980-12-22 BR BR8009003A patent/BR8009003A/pt unknown
  • 1980-12-29 ZA ZA00808096A patent/ZA808096B/xx unknown
  • 1980-12-29 AT AT80304736T patent/ATE12188T1/de not_active IP Right Cessation
  • 1980-12-29 EP EP80304736A patent/EP0033808B1/en not_active Expired
  • 1980-12-29 DE DE8080304736T patent/DE3070358D1/de not_active Expired
  • 1980-12-29 CA CA000367656A patent/CA1156516A/en not_active Expired
• 1981
  • 1981-08-25 DK DK375981A patent/DK150285C/da active
  • 1981-08-25 NO NO812872A patent/NO153746C/no unknown
  • 1981-08-26 FI FI812631A patent/FI69694C/fi not_active IP Right Cessation

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