Abstract:
Flue gas is desulfurized by mixing into the combustion zone a finely divided sorbent such as calcium carbonate and, downstream of the mixing point, reactivating the sorbent by spraying the gaseous suspension with an aqueous solution of solubilizing agent such as a deliquescent compound or a strongly ionizing inorganic salt.

Description:
FIELD OF THE INVENTION 
     This invention is directed to an improved process for reducing the sulfur content of flue gas derived from combustion of a sulfur-containing fuel, in which process first a dry sorbent is injected into the combustion zone and subsequently an aqueous solution containing a solubilizing agent is sprayed into the flue gas. Ash and sorbent can be subsequently separated from the flue gas. 
     BACKGROUND OF THE INVENTION 
     Efforts to reduce sulfur emissions in the gaseous products from combustion of a sulfur-containing fuel have been made in varying directions. Some processes attempt to reduce or eliminate the sulfur in the fuel prior to its combustion. Other processes propose the addition of compounds to the combustion zone which will in some manner change the nature of the sulfur compounds such that they may be more readily removed from the combustion products. And yet other processes remove sulfur compounds from the gaseous combustion products by chemical reaction. 
     U.S. Pat. No. 4,185,080, issued Jan. 22, 1980 to Rechmeier discloses a combustion gas desulfurization process wherein a powdered sorbent such as limestone or dolomite is added to a combustion zone, and a portion of the solids collected from the flue gas is reactivated and can be returned for injection into or downstream of the combustion zone. An earlier to Domahidy, U.S. Pat. No. 3,320,906 issued May 23, 1967, teaches mixing limestone with the coal being fed to a boiler, with the flue gas then being passed to a wet scrubber for additional capture of sulfur compounds by the calcined limestone. 
     The approach of chemical treatment to effect flue gas desulfurization can be further subdivided into wet scrubbing wherein a solution or suspension of reagent both enters and leaves the flue gas contacting zone in liquid state, spray drying wherein a solution or suspension of reagent enters the flue gas contacting zone in liquid state but is dried to produce a powdered solid leaving the contacting zone, and dry treatment wherein the treating reagent is a solid state powder both entering and leaving the contacting zone. 
     Illustrative of the wet scrubber approach is U.S. Pat. No. 3,928,537, issued Dec. 23, 1975 to Saitoh et al, which discloses contacting the exhaust gas with an aqueous solution of an organic acid to form a soluble sulfite or sulfate. The sulfite or sulfate is removed, and the organic acid regenerated, by a second step comprising reaction with a calcium compound such as an inorganic or an organic acid salt, for example calcium hydroxide or calcium formate. 
     The spray dryer approach is illustrated for example by U.S. Pat. No. 4,279,873, issued July 21, 1981 to Felsvang et al, which discloses spraying a suspension of fresh slaked lime and recycled fly ash plus spent calcium compound into the hot flue gas in such a manner as to evaporate the slurry droplets to dryness; the resulting powdered solids are removed from the flue gas by a downstream electrostatic precipitator or bag filter. 
     U.S. Pat. No. 4,178,349, issued Dec. 11, 1979 to Wienert illustrates the dry treatment; it discloses mixing a dry, powdered lime-bearing material in a reactor, and subsequently separating the solids from the treated flue gas. Another patent, U.S. Pat. No. 4,442,079, issued Apr. 10, 1984, to Donelly et al, outlines a flue gas desulfurization process which is primarily adapted to the spray dryer procedure just discussed, but is also stated to be applicable to injection of dry sorbent at a point of relatively low flue gas temperature, with water being sprayed into the gas either upstream or downstream of the sorbent injection point. 
     Another &#34;mixed&#34; (wet and dry) flue gas desulfurization process is described in U.S. Pat. No. 4,388,281, issued June 14, 1983 to Holter et al. In this patent, dry sorbent can be mixed with the coal prior to combustion, but the main feature resides in splitting the flue gas into two parallel streams, with one stream being treated with fresh dry sorbent, and the other being wet scrubbed with sorbent solution containing e.g. piperazine. 
     Current thinking seems to be that no one of the above-discussed strategies is the unique answer to the sulfur emission problem, either for new installations or for retrofit on an existing installation. Rather, numerous site-specific factors such as proximity to reagent source, space availability, and extent of sulfur removal required, must enter into the selection at each plant. 
     SUMMARY OF THE INVENTION 
     It is an objective of the present invention to provide a novel and advantageous method of removing sulfur compounds, especially sulfur dioxide, from combustion exhaust gas. 
     According to the present invention, flue gas is desulfurized by first contacting it in the combustion zone with a finely divided dry sorbent, and then reactivating the resulting gaseous suspension of calcined solids by spraying into it at a particular location downstream of the heat recovery exchangers an aqueous solution of a solubilizing compound. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the drawing, the sole FIGURE represents a flow diagram, partially schematic, of a flue gas desulfurization process according to our invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Flue gas containing sulfur dioxide from combustion of a sulfur-containing fuel is typically passed from the combustion chamber through heat recovery exchangers and thence by way of duct work to a stack for disposal. Where the fuel is a solid, such as coal, or a heavy liquid, the combustion product gases will also contain ash solids such as fly ash, in which instance the flue gas will first be treated for solids removal by such as a bag filter or electrostatic precipitator (ESP) prior to being passed to the stack. 
     The invention will now be described by referring to the drawing. A combustor such as boiler 10 is provided with a burner 12 which is adapted to introduce a pulverized solid fuel such as coal from conduit 14 and a combustion-supporting gas such as air from conduit 16. Hot combustion products leave the upper region of boiler 10, travelling past and through various heat exchange surfaces which typically extract heat by radiant and convective transfer, until they are discharged from the boiler assembly at duct 18. The final stage of heat recovery from the flue gas is often an air preheater, shown here as 20, which serves to preheat combustion air to conduit 16. Past practice has been that flue gas leaving air preheater 20 by way of duct 22 is passed directly to solids removal to separate out fly ash, such as by a bag filter or electrostatic precipitator 24, and thence discharged by a stack 26. 
     According to our invention, the combustion process is treated for reduction of its sulfur dioxide and sulfur trioxide content by injection of a finely divided dry sorbent powder such as calcium carbonate, magnesium carbonate, or mixtures of these two, into boiler 10 by way of a conduit 56. The dry powder can be carried through conduit 56 by use of a transport gas such as air. Further according to our invention, a short distance downstream from preheater 20, duct 22 enters humidifier 32, wherein solubilizing solution from conduit 34 is sprayed into the flue gas. Humidified flue gas passes by way of duct 36 to solids collector 24 and thence to stack 26. Solids removed in collector 24 will normally comprise a mixture of fly ash and spent sorbent, and are discarded to waste by way of conduit 42. 
     Returning now to a discussion of the sorbent, it is added in its relatively inexpensive but relatively unreactive carbonate form, as limestone or dolomite, at conduit 56. This material can either be added to boiler 10 by way of conduit 58 into burner 12, or by way of conduit 60 through separate injection ports into an upper region of the combustion zone, depending upon the prevailing temperature profile and residence time. In either event, the limestone or dolomite is calcined by the elevated temperature. The limestone or dolomite useful in the present invention is finely divided, and preferably of a size range that at least about 70 weight percent passes a 200 mesh Tyler sieve screen. In order to effect calcining, the material must reach a temperature slightly in excess of about 1000° C. but in order to avoid deadburning, as is known in the calcination art, it should not be subjected to temperatures much in excess of 1250° C. for any appreciable time. The resulting suspension of calcium and/or magnesium oxide is thus afforded significant reaction time for flue gas desulfurization in duct 18 and air preheater 20. 
     The process of the present invention is especially adapted to remove sulfur dioxide from a flue gas wherein availability of relatively short residence time in the duct work upstream of the ESP (as in a retrofit installation) and/or a relatively high concentration of sulfur dioxide combine to present difficult removal. The humidification of the flue gas and reactivation of its entrained sorbent powder by spraying water containing solubilizing agent is an essential aspect of our invention. As solubilizing agent we presently prefer sodium hydroxide, although other useful materials include sodium carbonate, calcium chloride, adipic acid, and glycerol. Although we do not wish to be so bound, we theorize that compounds which form highly ionized solutions or which are deliquescent serve to form a thin liquid film of greatly increased reactivity toward sulfur dioxide on the surface of each of the particles of gas-suspended solid sorbent, and thus serve to activate calcined particles which have not reacted. More importantly, we believe that the thin liquid film tends to bring fresh unreacted calcine to the surface of those particles which have already reacted, thus increasing overall sorbent utilization. 
     It is necessary for several reasons to control the humidity, or approach to saturation, of the flue gas containing sorbent and sprayed solution. It is preferable to approach the dew point at the inlet to the solids collector as close as possible without causing operating problems, since we postulate that a more humid flue gas permits protracted existence of a liquid film on the sorbent, and the reaction of gaseous sulfur dioxide with liquid solution is much more rapid than with a solid particle. On the other hand, if the dew point is approached too closely, the spray droplets tend to foul and plug the surfaces of bag filters or ESP collector plates, and to cause excessive corrosion of duct work. We prefer to approach to within at least about 35° C. of, but no nearer than about 10° C. of, the saturation temperature. 
     The degree of desulfurization achieved is also controlled by the Ca/S ratio, i.e. the amount of calcium (and/or magnesium) compound in the sorbent as compared to the amount of sulfur dioxide in the flue gas being treated. This ratio is normally expressed in moles, and we prefer that it be in the range from about 0.5:1 to about 3:1. It is controlled by the flow rate in conduit 56. Solubility of the solubilizing salt, e.g. calcium chloride, sodium hydroxide, etc. in water determines the maximum concentration of spray solution. It is advantageous that the duct work provides a residence time for the humidified solids of at least about 1 second, and preferably more. 
     In the usual application of this invention, the sulfur-containing fuel being burned will also contain a significant proportion of ash-forming constituents, and thus the flue gas will contain fly ash. Addition of solubilizing (humidifying) solution and dry sorbent according to the present invention causes an increase in the solids loading of the flue gas, and thus on the duty to be served by the downstream solids collector bag house or ESP. However, it has been determined that the solids retained on the surfaces of such solids collector remain active for SO 2  absorption for a longer period when humidifed with a solubilizing agent according to the present invention, particularly when the desired close dew point approach is maintained. 
     The temperature of the flue gas at the point of injection of the solution via conduit 34 should be between about 120° and about 230°, and preferably between about 140° and about 175° C. 
     The invention will now be illustrated by the following examples. 
     EXAMPLE I 
     An electrical utility station burning 2.4 weight percent sulfur coal is treated for reduction of sulfur dioxide emission according to the method of the present invention. Into an upper region of the boiler is injected by way of conduit 60 an air suspension of finely divided limestone in amount sufficient to provide a resultant Ca:S molar ratio of about 2.5. Next, a 2 weight percent solution of sodium hydroxide in water is sprayed into the gas stream by way of conduit 34 in amount sufficient to result in a gas temperature about 10° C. above the dew point at the ESP inlet. The flue gas velocity in the ducts 22 and 36 is such that the sorbent particles have a residence time of about 2 seconds prior to passage into the ESP. Average residence time of collected solids in the ESP is about 22 minutes. The SO 2  content of the gas exit the ESP is about 440 ppmv, dry (parts per million by volume, dry basis). 
     EXAMPLE II 
     The test of Example I is repeated, except that neither limestone sorbent nor sodium hydroxide solution is added. The SO 2  content of the gas exit the ESP increases to about 1760 ppmv, dry. 
     EXAMPLE III 
     As a basis for comparison, Example I is repeated except that no solubilizing solution is added through conduit 34. The sulfur dioxide analysis exit the ESP increases to 700 ppmv, dry. 
     EXAMPLES IV-VI 
     Solutions of calcium chloride, sodium carbonate, and glycerol are sequentially substituted for the sodium hydroxide solution in Example I, with similar sulfur dioxide removal results. 
     EXAMPLE VII 
     Example I is repeated, except that the quantity of sodium hydroxide solution is decreased to the point that the dew point approach is only about 25° C. Analysis of SO 2  exit the ESP increases to about 800 ppmv, dry. 
     EXAMPLE VIII 
     Example I is repeated, except that the limestone rate in conduit 60 is decreased to provide a Ca:S molar ratio of about 1. The SO 2  analysis exit the ESP increases to about 970 ppmv, dry. 
     Reasonable variation and modification are possible within the scope of the foregoing disclosure and the appended claims to our invention, the essence of which is that we have provided an improved method for reducing the sulfur dioxide content of flue gas by reaction with sorbent of increased activity, and for preparing such sorbent.