WO1992001627A1 - Chaux sulfatee hydratee a grande superficie et procedes - Google Patents

Chaux sulfatee hydratee a grande superficie et procedes Download PDF

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Publication number
WO1992001627A1
WO1992001627A1 PCT/US1991/005226 US9105226W WO9201627A1 WO 1992001627 A1 WO1992001627 A1 WO 1992001627A1 US 9105226 W US9105226 W US 9105226W WO 9201627 A1 WO9201627 A1 WO 9201627A1
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Prior art keywords
lime
hydration
water
solution
organic solvent
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PCT/US1991/005226
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English (en)
Inventor
David L. Moran
Massoud Rostam-Abadi
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Research Corporation Technologies, Inc.
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Publication of WO1992001627A1 publication Critical patent/WO1992001627A1/fr

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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
    • B01D53/46—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
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01F—COMPOUNDS OF THE METALS BERYLLIUM, MAGNESIUM, ALUMINIUM, CALCIUM, STRONTIUM, BARIUM, RADIUM, THORIUM, OR OF THE RARE-EARTH METALS
    • C01F11/00—Compounds of calcium, strontium, or barium
    • C01F11/02—Oxides or hydroxides
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2/00—Lime, magnesia or dolomite
    • C04B2/02—Lime
    • C04B2/04—Slaking
    • C04B2/06—Slaking with addition of substances, e.g. hydrophobic agents ; Slaking in the presence of other compounds
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2004/00—Particle morphology
    • C01P2004/60—Particles characterised by their size
    • C01P2004/61—Micrometer sized, i.e. from 1-100 micrometer
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/12—Surface area
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
    • C01P2006/00—Physical properties of inorganic compounds
    • C01P2006/14—Pore volume

Definitions

  • This invention relates generally to hydrated lime and methods for preparation and use thereof and, more particularly, the invention is directed to an improved process for making hydrated lime having favorable physical properties such as high surface area, high porosity, and small particle size, which in turn provide favorable SO 2 capture properties.
  • Dry sorbent injection technologies offer many advantages over other systems, notably wet flue gas systems, for desulfurization in controlling the emissions of SO 2 produced during combustion of high sulfur coal.
  • Some dry sorbent injection technologies including furnace sorbent injection (FSI), boiler economizer injection, and post furnace ductinjection/humidification (Coolside) systems have been extensively researched.
  • FSI furnace sorbent injection
  • boiler economizer injection boiler economizer injection
  • Coolside post furnace ductinjection/humidification
  • a distinguishing factor of dry processes is the injection of a calcium-based sorbent such as hydrated lime (Ca(OH) 2 ) into different locations within a pulverized coal boiler unit.
  • a calcium-based sorbent such as hydrated lime (Ca(OH) 2 )
  • Ca(OH) 2 hydrated lime
  • CaO calcium oxide
  • SO 2 SO 2
  • high surface area hydrated lime is prepared in an efficient, practical process wherein lime, which has been prepared directly by calcining limestone, is hydrated by contact with an aqueous solution of a slaking rate retarding organic solvent.
  • lime is mixed at a temperature which is sufficiently low to produce a homogeneous slurry with an aqueous/organic solvent hydration solution containing a sufficient amount of water relative to the lime to result in a desired degree of hydration, at least partially
  • the organic solvent is a lower alcohol
  • the lime may be a high surface area lime prepared by calcining limestone under low temperature, low carbon dioxide conditions.
  • the hydrated lime may be washed in an aqueous/organic solvent in order to displace water prior to drying to further increase surface area and reduce particle size.
  • the invention comprehends the high surface area hydrated lime product, and methods of sorbing SO 2 from waste gas streams using the high surface area hydrated lime as a sorbent.
  • the invention allows the physical properties of hydrated lime to be controlled for optimum
  • Figure 1 is a schematic flow diagram of one embodiment of a process for preparing high surface area hydrated lime according to the invention.
  • Figure 2 is a schematic flow diagram of another embodiment of a process for preparing high surface area hydrated lime according to the invention.
  • Figure 3 is a schematic flow diagram of a modified form of the process of Figure 2.
  • Figure 4 is a graphical depiction
  • Figure 5 is a graphical depiction of temperature profiles obtained in processing several different limes into hydrates.
  • Figures 6A, 6B and 6C graphically depict the degree of SO 2 removed from waste gas streams as a function of surface area for several hydrated limes under furnace sorbent injection, boiler economizer, and Coolside conditions, respectively.
  • Figure 7 is a graph illustrating cumulative pore volume vs. pore diameter for high surface area and commercial hydrated limes.
  • lime which has been prepared directly by calcining limestone is hydrated by contact with an aqueous solution of an organic solvent which is effective in retarding the rate of hydration ("slaking") of the lime.
  • the product can be washed with another solution of an organic solvent in a post-hydration wash step in order to displace water before drying in order to further increase surface area and reduce particle size.
  • the inventive method is capable of producing a hydrate with an N 2 -BET surface area of greater than 35 m 2 /g, and preferably greater than 55 m 2 /g. Surface areas of up to 85 m 2 /g are readily obtainable.
  • Preferred organic solvents useful in the hydration and washing steps are lower alcohols
  • Lime which has been prepared either commercially in a kiln at a high temperature and/or under high carbon dioxide concentration conditions or in a kiln, fluidized bed reactor, entrained flow reactor, or other calciner under conditions of low temperature and low carbon dioxide concentration conditions may be used as a starting material.
  • Figure 1 illustrates a process for hydrating lime using so-called "commercial" limes which are defined as being produced by calcining limestone in kilns in an environment having a relatively high (e.g., greater than 25 vol. %) CO 2 level and/or at high temperatures (i.e., above 1100°C) at a residence time of about 1 to about 3 hours or higher, depending on the type of kiln.
  • a relatively high e.g., greater than 25 vol. %) CO 2 level and/or at high temperatures (i.e., above 1100°C) at a residence time of about 1 to about 3 hours or higher, depending on the type of kiln.
  • lime preferably minus 100 mesh in size
  • an alcohol-water solution in a mixing stage 1 maintained at a temperature, preferably below about 40°C, which is sufficiently low to achieve a homogeneous slurry.
  • the alcohol-water solution required for the overall process need contain only a sufficient amount of water relative to the lime to result in a desired degree of hydration, usually at least 0.9 and preferably at least about 1 and less than about 2 times (highly preferably about 1.0 to about 1.4 times) the stoichiometric amount of water relative to lime and about 0.5:1 to about 5:1 (preferably about 1:1 to about 3:1) volumetric ratios of alcohol to water.
  • a sufficient amount of water relative to the lime usually at least 0.9 and preferably at least about 1 and less than about 2 times (highly preferably about 1.0 to about 1.4 times) the stoichiometric amount of water relative to lime and about 0.5:1 to about 5:1 (preferably about 1:1 to about 3:1) volumetric ratios of alcohol to water.
  • For each kilogram of lime about 0.4 to about 4 liters (preferably about 0.8 to about 1.4 liters) of hydration solution is used.
  • Ratios of alcohol to water in excess of 5:1 may be used, especially with highly reactive limes, where hydration solutions may contain up to 75 to 85 vol.% alcohol, if desired.
  • hydration solutions may contain up to 75 to 85 vol.% alcohol, if desired.
  • the alcohol to water ratio is about 5.67:1.
  • Such high ratios of alcohol to water are especially suitable with limes having very high surface areas (e.g. 10 m 2 /g or more).
  • the alcohol or other solvent in the hydration solution retards the slaking rate, and maintains the temperature of the hydration mixture below the boiling point of water (thus preventing or minimizing the degree of gas phase hydration, which would inhibit surface area development).
  • the use of alcohol or other solvents lowers surface tension and helps prevent agglomeration, which leads to increases in surface area due to increased dispersion.
  • the solvent also lowers the solubility of the hydrate, thus promoting precipitation of hydrate from the solution of lime in water.
  • Hydroxyl carboxylic acids and/or surfactants or other additives may be used in combination with the alcohol in the hydration solution to further reduce the hydration rate, thus increasing hydrate surface area and decreasing hydrate particle size.
  • concentrations of 3 wt.% or less based on water are preferred, and concentrations of about 0.5 to 1 wt.% are highly preferred.
  • Surfactant concentrations of 2 wt.% or less are preferred, depending on molecular weight, and concentrations in the range of 0.2 to 1 wt.%, and preferably less than 0.5 wt.% are typical.
  • Useful acids include lactic acid, glyceric acid, citric acid, maleic acid, tartaric acid, gluconic acid, and others.
  • Useful surfactants include those of the Triton X series, sodium dodecyl sulfate, sodium abietate, Vinsol, etc. Acids are believed to function by inhibiting the growth of calcium hydroxide
  • Surfactants function by sorbing onto the surface of the calcium hydroxide nuclei, and cause the crystals to better disperse by repulsion due to ionic or steric stabilization.
  • phosphates borates, fluorides, sulfates, and silicates
  • sugars e.g. sucrose, glucose, etc.
  • lignosulfonates which contain sulfonate and carboxylate groups.
  • the slurry from the mixing vessel 1 enters a hydrator 2 which may be integral with the mixer 1 and which can include a preheating stage, a paste mixing stage, and preferably a drying stage.
  • the types of mixing elements (blades) used in each stage are selected to accomplish thorough mixing of the slurry, paste, and the powder phases present in the hydrator.
  • the mixing stage 1 and the hydrator 2 can be
  • each of the preheating and paste mixing stages can be incorporated in one mixer/hydrator or hydrator vessel, or in separate single or multistaged vessels, with or without internal walls.
  • the drying stage can be accomplished in a separate vessel, preferably in a vacuum drier, or may be incorporated into a single vessel which includes the hydrator 2 and the mixer 1, if desired.
  • the remaining portion of the hydration water, if any, can be added in the preheating stage or in the paste mixing stage of the hydrator 2.
  • the slurry is heated, autogenously or by external heating means, if necessary, to a
  • the temperature preferably at least about 40°C and up to about 70°C, which is sufficiently high to sustain the hydration reaction.
  • the temperature may vary depending on the type of organic solvent used.
  • the residence time in the preheating stage is typically between about 1 and about 10 minutes.
  • the partially hydrated paste from the preheating stage is heated autogenously to the boiling point of the
  • Hydration temperature is dependent on the boiling point of the hydration solution.
  • the residence time in this stage is typically between about 3 and about 10
  • the crude product exiting the paste mixing stage is in powdered form.
  • the crude product is heated, possibly autogenously (although heat may be applied from an external source, if necessary) to about 60 to about 110°C for a residence time of about 3 to about 30 minutes. Additional hydration occurs during the drying stage with the final product typically containing greater than about 90 wt.% calcium
  • the drying stage can be accomplished in a separate vessel (preferably a vacuum drier) to provide greater operating flexibility.
  • the drying stage can also be accomplished in two separate vessels consisting of an atmospheric drier and a vacuum drier to improve alcohol recovery. If all of the hydration solution is driven off in the hydration stages, no drying stage separate from the preheating and paste mixing stages is needed.
  • stoichiometric ratio typically results in less than about 90% hydration. Operation at more than 1.5 times stoichiometric ratio of water can result in the
  • Total residence time in the hydrator is determined by the time required to evaporate hydration solution, which is a function of the reactivity (and thus surface area) of the lime and the amount of liquid present. Sufficient hydration solution should be present to prevent hydration temperatures from
  • the hydrator 2 is preferably provided with a flow of nitrogen or other inert gas to purge oxygen-containing gas from the hydrator.
  • the nitrogen purge can also be used during operation to provide heat for the drying and/or preheating stages (if needed), maintain pressurization of the hydrator to about 1 to about 15 psig to prevent leakage of oxygen into the system, and remove the organic solvent vapor generated during operation.
  • the solvent vapor/nitrogen stream enters a condenser 4 to recover and cool the solvent which is then recycled to the mixing vessel 1.
  • the nitrogen stream exiting the condenser 4 is heated to about 80 to about 110°C in a heater 5 and recycled to the hydrator 2.
  • heating for the preheating and drying stages can be provided by external heating units, such as steam jackets.
  • the hydrate exiting the hydrator 2 can be subjected to an air classification and milling stage 3, if necessary, to separate any undesired grit particles formed during the hydration process.
  • hydrates with surface areas between about 35 and about 50 m 2 /g and mean particle diameters between about 1 and about 3 micrometers are produced, depending on the hydration conditions and type and properties of the lime used.
  • Figures 2 and 3 illustrate embodiments of the inventive process wherein high surface area hydrates are made using limes having relatively high surface areas, produced by calcining limestone under low temperature, low carbon dioxide conditions.
  • higher surface area hydrates can be made using limes that are produced by calcining limestone (preferably greater than about 95% calcium carbonate) in a calciner 6, preferably at atmospheric pressure and for residence times of about 1 to about 3 hours at temperatures between about 800 and about 1100°C (generally less than 950°C, preferably less than 900°C) under a gas
  • Low CO 2 partial pressure and low calcination temperature conditions are ideal for producing, depending on the properties of the limestone feed, limes with N 2 -BET surface areas at least about 2.5 m 2 /g, and preferably at least about 3.5 m 2 /g.
  • a fluid bed reactor such as a circulating fluid bed reactor
  • an entrained flow reactor are ideal for the calcination because of their excellent mass and heat transfer characteristics which allow calcination to be accomplished at low temperatures and at residence time comparable to those of commercial kilns.
  • a commercial kiln operated at low temperatures and purged with air or an inert gas to dilute the CO 2 concentration can also be used.
  • the limestone particle size preferably is minus 1 inch in size for a commercial kiln, minus 8 mesh in size for standard fluid bed reactors, and minus 50 mesh in size for circulating fluid bed and entrained flow reactors.
  • calcination conditions For high purity limestone (greater than 98% calcium carbonate and less than 0.05% total alkali) the influence of calcination conditions on lime surface area is not critical. Some calcination conditions may be varied (e.g. extremely high temperatures such as 1200°C may be used) if other conditions (e.g. short residence times such as 1 to 10 seconds) are favorable. Selection of appropriate conditions is within the skill of those familiar with the art, guided by the present
  • Hydrates may be prepared from the high surface area limes either without (Figure 2) or with ( Figure 3) a post-hydration wash step.
  • the mixing and hydrator stages of Figures 2 and 3 may be embodied in separate vessels or in a single vessel, and the preheating, paste mixing and drying stages of the hydrator can be formed in a single hydrator vessel (with or without internal walls to physically separate stages) which may also include the mixing stages, in particular when "commercial" lime is being processed. If preheating, paste mixing and drying stages are carried out in a such vessel without internal walls, a twin screw blade is preferred to transfer and blend the paste.
  • the lime (preferably minus 100 mesh in size) is mixed with an alcohol-water hydration solution in a mixing stage 7 maintained at a
  • the alcohol-water solution required for the overall process preferably should contain at least about 0.9 and preferably at least about l and less than about 2 times the stoichiometric amount of water
  • the slurry from the mixing vessel 7 enters a hydrator 8, which is similar to that of Figure 1.
  • the remaining portion of the hydration water, if any, can be added in the preheating stage or in the paste mixing stage of the hydrator.
  • the temperatures in the hydrator are between about 40 and about 70°C in the preheater stage, between about 60 and about 80°C in the paste stage, and between about 80 and about 110°C in the drying stage, depending on the type of solvent used. Residence times ranging between about 5 and about 30 minutes in the hydrator, depending on the reactivity of the lime and type and amount of solvent used, are typical.
  • the drying stage is similar to the embodiment of Figure 1. The drying stage is not required if the hydrate is to be processed using a post-hydration wash step.
  • the hydrator 8 is preferably provided with a flow of nitrogen or another inert gas to purge oxygen-containing gas from the hydrator, similar to the embodiment of Figure 1.
  • the nitrogen purge can also be used during operation to provide heat required for the drying and/or preheating stages, maintain pressurization of the hydrator to about 1 to about 15 psig to prevent ignition of the organic solvent, and remove the organic solvent vapor generated during operation.
  • the solvent vapor/nitrogen stream enters a condenser 10 to recover and cool the solvent which is then recycled to the mixing stage 7.
  • the nitrogen stream exiting the condenser 10 is heated to about 80 to about 110°C in a heater 11 and recycled to the hydrator 8.
  • heating for the preheating and drying stages, if necessary could be provided by external heating units, such as steam jackets.
  • hydrated lime with a surface area of about 50 to about 80 m 2 /g is prepared depending on the surface area of the lime feed and hydration conditions.
  • the crude product obtained can be classified and milled at 9 to separate undesirable grit particles formed during hydration.
  • the reactivity of the hydrate can be further enhanced by using a post-hydration wash step to
  • Retained water will inhibit surface area development due to agglomeration and recrystallization, and may result in pore structure collapse during drying.
  • wash step allows the use of greater amounts of water during hydration, if desired, in order to increase the degree of hydration, since the wash step displaces excess water before drying.
  • the wash step also provides rapid cooling of hydrated material, which is beneficial since high temperature soaking of hydrates in liquid is detrimental to surface area development.
  • Figure 3 depicts an alternative to the embodiment of Figure 2 using a post-hydration wash step, and uses common reference numerals to represent elements which are common to both embodiments.
  • the hydator 8 does not include a drying stage.
  • the nitrogen purge line and heater are not shown in Figure 3, but may be utilized, if desired, for the purposes used in the embodiments of Figures 1 and 2.
  • FIG. 2 is washed in a vessel 12 with about 0.7 to about 4.5 liters of about 70 to about 98% by volume (preferably 90-95 wt.%) alcohol (or other organic solvent) solution per kilogram of hydrate (about 1 to about 6 liters per kilogram of feed lime).
  • the alcohol may but need not be the same type of alcohol used in the hydration step. It is preferred, however, to use the same type of alcohol in both steps, in order to avoid an additional separation step.
  • a surfactant or other dispersing agent could also be added during the wash step to further reduce the hydrate particle size.
  • the wash step is carried out at atmospheric pressure or slightly above and at a temperature
  • wash liquid contain at least about 1-2 vol.% water, in order to avoid the undesirable reaction of calcium with alcohol to produce calcium alkoxides.
  • the crude product enters a single or multistage solid-liquid separator 13 (such as a
  • the crude product then enters an atmospheric or vacuum drier 14 operating at
  • the drier may be purged with nitrogen or other inert gas to enhance the transport of alcohol vapor from the drier to the condenser 10.
  • the hydrate exiting the drier can be air classified and milled at 9 to reduce its mean particle diameter.
  • the final hydrated lime product typically has a surface area of about 50 to about 85 m 2 /g depending on the surface area of the lime feed and conditions during hydration (8) and washing (12) .
  • the alcohol-water solution from the solid-liquid separator 13 is recycled to the mixing vessel 7, to the wash vessel 12, and to the condenser 10, which in Figure 3 is a scrubber. Vapors from the hydrator 8 and the drier 14 also enter the scrubber 10 for recovery of alcohol. Depending upon the flow rates and. temperatures of the vapor streams and alcohol-water stream entering the scrubber 10, some cooling will be necessary to ensure complete condensation of the
  • the liquid recycle stream entering the scrubber may have to be adjusted to maintain the proper
  • a liquid-liquid separator 15 is used to separate alcohol from the solution for a
  • the concentrated alcohol stream is added to the scrubber recycle stream to increase the alcohol
  • the water stream is recycled back to the mixing stage 7.
  • the liquid-liquid separator 15, if needed, may be a distillation unit (vacuum, flash, or atmospheric), or a reactor in which a solid material is used to preferentially adsorb or react with water.
  • a liquid-liquid separation step is not needed if the concentration of alcohol in the liquid stream exiting the solid-liquid separator 13 is sufficiently high.
  • Figure 3 illustrates the use of a post-hydration wash step wherein the feed lime has
  • the wash step can also be used in processing of "commercial" limes (as in Figure 1) , if desired, and may reduce the particle size and, possibly, increase the surface area of the resulting hydrate.
  • mixing and hydration can be carried out in the same reactor at varying temperatures along the length of the reactor, especially if the reactor has plug flow characteristics or if relatively low reactivity "commercial" limes are utilized.
  • the water/alcohol hydration solution may be introduced to the system separately from each other.
  • hydration water may be introduced in two (or more) separate streams to the mixing stage 1 or 7 and the hydrator 2 or 8 as shown in Figures 1-3 when either commercial lime or high surface area, relatively reactive lime is being processed, and this procedure may be particularly desirable when relatively reactive limes are used.
  • introduction of the second (downstream) water stream should preferably occur at a point in the system before the lime/hydration solution mixture reaches the boiling point of alcohol or other organic solvent.
  • lime may be mixed in the premixing stage with a hydration solution that contains all the necessary alcohol and about 5 to 35% of the hydration water with separate, preferably downstream addition of the remaining water to the hydrator.
  • dry lime and an alcohol/water mixture may be introduced separately to the mixing or preheating stages of the hydrator. This scheme is preferable when the mixer 7 and hydrator 8 are combined into a single vessel.
  • An advantage of this method of feeding dry lime is that particle size of feed lime is not limited to below about 100 mesh and lime particles as large as three inches can be routinely fed into the reactor.
  • the advantages of the present invention over the prior art include (1) the preparation of hydrates having surface areas of at least 50 m 2 /g, generally in the range of 55 to 85 m 2 /g compared to only 35 to 55 m 2 /g reported in some prior art; (2) the preparation of hydrates of comparable or improved physical properties (such as high surface area, small particle size and high porosity) compared to those of the prior art using fewer processing steps than prior art processes; and (3) the preparation of hydrates whose surface areas (80 m 2 /g) are comparable to those of prior art hydrates, which are prepared using
  • High surface area hydrates of the invention One important characteristic of the high surface area hydrates of the invention is the provision of both high surface area and small particle size, which is an important factor in SO 2 sorption and other applications where sorption efficiency is important.
  • the invention provides means for optimizing the surface area and other physical properties of hydrated lime for SO 2 capture in a simple, commercially practical process.
  • relatively reactive limes i.e., those having surface areas of at least 2.5 m 2 /g and up to about 30 m 2 /g, typically up to about 10 m 2 /g
  • the process may be conveniently carried out in a single reaction vessel with advantageous use of a post-hydration wash step, staged hydration (split water streams), high
  • concentrations of organic solvent e.g., 75-85 vol.% in the hydration solution, and the use of hydration solution additives to maximize product surface area, and the flexibility to use pebble lime in the hydration process.
  • the process integrates the use of highly reactive lime feed, organic solvent-water hydration in a single reactor vessel without the use of an external heat source to initiate or maintain the hydration reaction, and an organic solvent wash step.
  • Table 1 presents physical properties of eighteen hydrated limes (designated A-O, HSA1, HSA2 and HSA3) that were prepared from two commercial limes made by burning limestone in a kiln under high temperature and CO 2 conditions, from one lime.made by processing limestone in a rotary kiln under low temperature and CO 2 conditions, and from six limes made by processing limestone in fluidized bed reactors under low
  • Table 1 also presents surface area and crystallite size data for a hydrate (HSAG) made according to the procedure of Bestek U.S. Patent No. 4,636,379.
  • the HSA hydrates were examined by X-ray diffraction (XRD) and the data were used for
  • the commercial hydrates A and B were prepared by water hydration. Hydrate surface area ranged from 20 to 25 m 2 /g, and mean hydrate particle size ranged from 1.7 to 3.5 micrometers.
  • Inventive hydrates C and D were made from commercial limes by hydrating with a 1.2 stoichiometric ratio of water to lime and a volumetric ratio of ethanol to water of 2:1 with maximum temperatures below 115°C. More particularly, in each case 1.82 kg lime was hydrated with a solution comprising 0.70 liter water and 1.40 liter ethanol. No alcohol wash step was used. Hydrate surface area in each case was about 40 m 2 /g, with a mean particle diameter of 1.0-2.7
  • Hydrate E was made from lime produced in a continuous rotary tube kiln.
  • the kiln had an inner diameter of 4 inches and a length of 72 inches, and was heated by a three-zone furnace rated at 2000°F
  • the surface area of the resulting hydrate was 50-55 m 2 /g with a mean hydrate particle diameter of 0.7-1.0 micrometers.
  • Hydrates were also made from lime prepared by calcining limestone in a fluidized bed reactor. These hydrates were made without (hydrates F-J) and with (hydrates K-O) an ethanol wash step.
  • Samples K-O were prepared by hydrating limes at a stoichiometric water to lime ratio of 1.5 and an ethanol to water volumetric ratio of 2:1 at maximum temperatures below 115°C.
  • the hydration solution contained 2.4 ml water and 4.8 ml ethanol.
  • each moist hydrate was washed in 20 ml anhydrous ethanol (4 liters per kilogram of lime) at a temperature of 20 to 40°C. Hydrate surface areas ranging from 79-84 m 2 /g were obtained.
  • the surface areas of hydrates made from commercial Burlington lime were about 40 m 2 /g compared to 50-55 m 2 /g for rotary kiln lime and 70-79 m 2 /g for FBR limes. Hydrates prepared using a post-hydration wash step exhibited higher surface areas, depending on the hydration conditions, and in separate tests were determined to have higher sulfur capture capacities (60 minutes sulfation time at 850°C) than hydrates prepared without the wash step.
  • HSA1 and HSA2 which were prepared from extremely reactive limes
  • all of the alcohol and 50% of the water were added to the lime initially to allow 30 to 60 seconds for the slurry to be homogenized before adding the remaining 50% of the water.
  • the temperature typically rose 10 to 20°C, while after the second water addition, the temperature very quickly rose to the boiling point of the solution.
  • no external heating was required to start the reaction, although external heat was provided after the completion of hydration to insure that the final products were dry.
  • the tests performed with the commercial Salem lime
  • Typical properties of commercial hydrates, hydrates prepared by the inventive process and those reported in Bestek U.S. Patent No. 4,636,379 are summarized in Table 2.
  • the inventive hydrates have smaller mean particle diameters, higher pore volumes and smaller calcium hydroxide crystallite size than those of the prior art.
  • the enhanced properties of the HSA hydrate make this material a superior sorbent for controlling the emission of sulfur dioxide (SO 2 ) .
  • the parameters of hydration conditions, the choice of reactor type, and the properties of feed lime are important for production of hydrates with desired properties (surface area, particle size, and flow characteristics). These parameters influence the shape of the temperature profile (temperature history) which the reactants are subjected to during preheating and hydration stages.
  • thermo shock a sudden temperature rise of the reactant
  • the profile can be shifted to the right by using:
  • reaction inhibitors 4) gradual heating of reactants in a reactor, similar to the approach of U.S. 4,636,379 where a pre-heating stage is used before a hydration reactor; and, 5) controlled temperature rise of reactants by staged water addition.
  • the rate of sulfur capture depends on complex interactions between sulfation kinetics, sintering, and build up of a CaSO 4 product layer barrier.
  • sulfation proceeds, CaSO 4 builds up on the CaO surface, which requires that the SO 2 diffuses through the CaSO 4 to reach unreacted CaO. It is generally accepted that sulfation can be limited by any of the following processes:
  • the rate is limited by either internal or external diffusion processes.
  • diffusion of SO 2 across the product layer due to the increase in molar volume when converting from CaO to CaSO 4 , i.e., 16.9 vs. 48
  • the amount of SO 2 capture depends on three competing reactions:
  • reaction 4 competes with the reaction between the abundant CO 2 and Ca(OH) 2 to form CaCO 3 (reaction 4). Furthermore, dehydration (reaction 5) forms relatively unreactive CaO, reducing the amount of Ca(OH) 2 available for reaction.
  • reaction (3) is controlled by bulk diffusion of SO 2 for particles larger than 5 micrometers (diffusion rate for spherical particles is inversely related to particle size to the second power), whereas reaction (4) is controlled by intrinsic rate.
  • reaction (3) depends both on pore surface area and particle size of the sorbent. Increasing pore surface area would favor the carbonation reaction (4) if particle diameter is held constant. Decreasing particle size and holding pore surface area constant would favor reaction (3).
  • a sorbent with high pore surface area and small particle size would be expected to show high SO 2 removal efficiency under boiler economizer conditions.
  • the hydrate is injected either upstream or downstream of a water spray.
  • the sorbent particles are wetted by inertial impaction with droplets or bulk condensation.
  • SO 2 is removed by the entrained sorbent/liquid droplets in the ductwork, forming calcium sulfite:
  • the capture of sulfur by the slurry droplet is controlled by a number of processes, including: 1. The rate of transport of sulfur to the external surface of the droplet.
  • step l is believed to be the limiting resistance for sulfur capture for about one half of the droplet lifetime. During this period physical properties of the sorbent do not influence the rate of sulfur
  • the most probable limiting mechanism for the remaining life of the droplet is diffusion of reactant or product through a product layer of calcium sulfite on the surface of unreacted hydrated lime (step 5).
  • the rate of reaction at any conversion level should vary as the square of the initial surface area. However, if a large portion of surface area is
  • Furnace sorbent injection tests of the three inventive calcium hydroxide sorbents and those of U.S. 4,636,379 and commercial hydrates were performed in a 14 kw pilot-scale innovative furnace reactor at the
  • Boiler economizer tests of the sorbents were performed in a bench-scale flow reactor. The tests were performed under differential conditions with respect to the SO 2 concentration (i.e., very low sample feed rates were used, SO 2 concentration remained essentially constant at 3000 ppm, and Ca/S approaching zero). Reaction temperatures of 540°C and 2.0 second residence times were selected. Five to ten repeat tests were made to confirm the data.
  • Coolside tests of the sorbents were performed in the 100 kw Coolside pilot unit located in the R&D Department of the Consolidation Coal Company.
  • the test program involved testing the sorbents at Ca/S ratios of 0.5, 1.0, and 2.0 and approaches to adiabatic saturation temperature of 25 and 35°F.
  • One of the tests was repeated to confirm the data, and a test was performed using a commercial hydrated lime that had given the best performance during four years of examining the Coolside unit.
  • the common conditions were 300°F inlet flue gas temperature, 1500 ppm inlet SO 2 content (dry basis), and 125°F adiabatic saturation temperature.
  • the flue gas flow rate was set at 175 scfm, which provided a 2.0 second humidifier residence time. SO 2 removals reported include capture both in the
  • Pore volume analyses of raw sorbents indicate the volume of pores between 0.01 and 0.1 micrometers (10 and 100 nm) was substantially higher for the HSA1 and HSA2 hydrates than for commercial hydrate A in Table 1.
  • Pore volumes of hydrated limes are expected to correlate with pore volumes of the corresponding calcines. Due to the increase in molar volumes when converting from CaO to CaSO 4 (16.9 vs. 46.0 cm 3 /mole), pore plugging is known to limit the sulfation reaction. Therefore, sorbents with a high volume of larger pores are expected to capture more SO 2 .
  • Figure 5 shows temperature profiles obtained during production of HSA1, HSA2, and HSA3 hydrated limes (see Table 1) in a batch hydrator.
  • HSAl 50 m 2 /g
  • Profile B The conditions used to prepare HSAl (50 m 2 /g), Profile B, provided the smallest particle size. This hydrate was the most reactive hydrate for FSI and boiler economizer applications (see Figures 6A and 6B). Hydrate (HSA3) produced according to Profile C using a commercial lime had a comparable surface area to that of the product made according to Bestek process (U.S. 4,636,379). This hydrate showed the lowest SO 2 capture in the Coolside and boiler economizer processes.
  • Profile D in Figure 5 is the optimum profile for producing a hydrate for the boiler economizer process. From a processing point of view, the best conditions to achieve profile D is to hydrate an intermediate surface area lime, i.e., 3 m 2 /g lime and to use more alcohol than was used to achieve Profile B (an A:W ratio of 2:4:1 was used for Profile B). The use of more alcohol in the hydration solution will shift Profile B to the right (toward Profile D).
  • the key processing variable is to use just enough alcohol to manufacture a dry hydrate (with minimum use of external heat) with desired properties for boiler economizer applications.
  • Profile D can also be achieved by hydrating the 8 m 2 /g lime; but this alternative is not as desirable.
  • a very reactive lime i.e., one having a surface area greater than 6 m 2 /g
  • a large amount of alcohol is required to shift Profile A to D. This, however, results in formation of loose agglomerates with effective diameters greater than 5 micrometers.
  • the optimum hydrates for maximizing SO 2 removal under FSI or boiler economizer conditions appear to have surface areas in the range of 40 to 50 m 2 /g, mean particle diameters below about 2.5 microns, and pore volumes above about 0.25 cc/g. Hydrates with surface areas above 50 m 2 /g would likely be more effective for capturing SO 2 if their particle size could be significantly reduced (i.e., to less than 1 micron).
  • the best method for producing hydrates for optimizing SO 2 capture under FSI or boiler economizer conditions involves hydration of a 1.5 to 3.0 m 2 /g lime with 1.0 to 1.2 times stoichiometric water and 2.0:1 to 3.0:1 volumetric ratio of alcohol to water. Use of larger amounts of water and/or alcohol would appear to result in the production of undesirable large
  • hydrates having optimum properties for SO 2 removal in the Coolside process have surface areas of at least about 55 m 2 /g (e.g. about 60 m 2 /g), mean particle diameters of below about 5 micrometers, and pore volumes above about 0.3 cc/g. For this system, particle size of hydrates appears to be of secondary importance.
  • the optimum method for producing hydrates for the Coolside process involves principally the use of the most reactive starting lime available (2.5-10 m 2 /g or greater surface area).

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Organic Chemistry (AREA)
  • Ceramic Engineering (AREA)
  • Environmental & Geological Engineering (AREA)
  • Oil, Petroleum & Natural Gas (AREA)
  • Inorganic Chemistry (AREA)
  • Biomedical Technology (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Analytical Chemistry (AREA)
  • General Chemical & Material Sciences (AREA)
  • Health & Medical Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Geology (AREA)
  • Materials Engineering (AREA)
  • Structural Engineering (AREA)
  • Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)
  • Solid-Sorbent Or Filter-Aiding Compositions (AREA)

Abstract

On prépare de la chaux sulfatée hydratée présentant une grande superficie et une faible granulométrie par hydratation de chaux à l'aide d'une solution d'hydratation aqueuse d'un solvant organique, et de préférence par lavage de l'hydrate obtenu à l'aide d'une solution aqueuse d'un solvant organique avant de procéder au séchage. Les hydrates de grande superficie de l'invention sont d'excellents sorbants permettant l'élimination de SO2 de courants de gaz.
PCT/US1991/005226 1990-07-24 1991-07-24 Chaux sulfatee hydratee a grande superficie et procedes WO1992001627A1 (fr)

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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1997014650A1 (fr) * 1995-10-19 1997-04-24 Lhoist Recherche Et Developpement S.A. PARTICULES DE Ca(OH)¿2?
WO2007000433A3 (fr) * 2005-06-28 2007-03-15 Lhoist Rech & Dev Sa Composition de chaux pulverulente, son procede de fabrication et son utilisation
CZ305594B6 (cs) * 2013-12-20 2015-12-30 Ústav Teoretické A Aplikované Mechaniky Av Čr, V. V. I. Akademie Věd České Republiky, V.V.I. Způsob výroby disperze částic hydroxidu vápenatého a zařízení k provádění tohoto způsobu
US10822442B2 (en) 2017-07-17 2020-11-03 Ecolab Usa Inc. Rheology-modifying agents for slurries

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US948045A (en) * 1909-06-03 1910-02-01 George G Floyd Manufacture of lime and gas.
US3991172A (en) * 1970-05-20 1976-11-09 Rheinische Kalksteinwerke Gmbh Process for the production of reactive calcium oxide
SU633810A1 (ru) * 1977-05-24 1978-11-25 Предприятие П/Я А-3732 Способ получени гидроокиси кальци
US4226839A (en) * 1978-07-21 1980-10-07 The United States Of America As Represented By The Administrator Of The U.S. Environmental Protection Agency Activation of calcium oxide as a sorbent
US4636379A (en) * 1984-09-11 1987-01-13 Rheinische Kalksteinwerke Gmbh Process for producing calcium hydroxide

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US3120444A (en) * 1961-05-31 1964-02-04 Kaiser Aluminium Chem Corp Hydrated lime and method of making the same
DE3804180C1 (fr) * 1988-02-11 1989-08-24 Rheinische Kalksteinwerke Gmbh, 5603 Wuelfrath, De

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US948045A (en) * 1909-06-03 1910-02-01 George G Floyd Manufacture of lime and gas.
US3991172A (en) * 1970-05-20 1976-11-09 Rheinische Kalksteinwerke Gmbh Process for the production of reactive calcium oxide
SU633810A1 (ru) * 1977-05-24 1978-11-25 Предприятие П/Я А-3732 Способ получени гидроокиси кальци
US4226839A (en) * 1978-07-21 1980-10-07 The United States Of America As Represented By The Administrator Of The U.S. Environmental Protection Agency Activation of calcium oxide as a sorbent
US4636379A (en) * 1984-09-11 1987-01-13 Rheinische Kalksteinwerke Gmbh Process for producing calcium hydroxide

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Title
See also references of EP0559650A4 *

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1997014650A1 (fr) * 1995-10-19 1997-04-24 Lhoist Recherche Et Developpement S.A. PARTICULES DE Ca(OH)¿2?
US6322769B1 (en) 1995-10-19 2001-11-27 Lloist Recherche Et Developpement S.A. Ca(OH)2 particles
WO2007000433A3 (fr) * 2005-06-28 2007-03-15 Lhoist Rech & Dev Sa Composition de chaux pulverulente, son procede de fabrication et son utilisation
BE1016661A3 (fr) * 2005-06-28 2007-04-03 Lhoist Rech & Dev Sa Composition de chaux pulverulente, son procede de fabrication et son utilisation.
US7744678B2 (en) 2005-06-28 2010-06-29 S.A. Lhoist Recherche Et Developpement Powdered lime composition, method of preparing same and use thereof
CZ305594B6 (cs) * 2013-12-20 2015-12-30 Ústav Teoretické A Aplikované Mechaniky Av Čr, V. V. I. Akademie Věd České Republiky, V.V.I. Způsob výroby disperze částic hydroxidu vápenatého a zařízení k provádění tohoto způsobu
US10822442B2 (en) 2017-07-17 2020-11-03 Ecolab Usa Inc. Rheology-modifying agents for slurries

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CA2087173A1 (fr) 1992-01-25
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CA2087173C (fr) 2000-02-15
EP0559650A1 (fr) 1993-09-15

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