WO2000004983A1

WO2000004983A1 - Process and apparatus for recovery of acid gases from flue gas

Info

Publication number: WO2000004983A1
Application number: PCT/US1999/016518
Authority: WO
Inventors: Ronald W. Bayliss
Original assignee: SPRAY DRYING SYSTEMS Inc
Current assignee: SPRAY DRYING SYSTEMS Inc
Priority date: 1998-07-24
Filing date: 1999-07-22
Publication date: 2000-02-03
Anticipated expiration: 2001-01-24
Also published as: AU5221599A

Abstract

A process and an apparatus for the removal of one or more pollutants from flue gases is disclosed. The process comprises a partial cooling and humidification of the flue gases in conjunction with pulverizing, contacting and mixing an alkaline reagent therewith followed by a filtering step to remove solids including particulate matter entrained in the flue gas before releasing the cleaned and filtered flue gas into the ambient atmosphere. The apparatus comprises (1) a component for partially cooling and humidifying the flue gas connected to (2) a component for pulverizing an alkaline reagent and contacting, mixing and reacting the pulverized alkaline reagent with the partially cooled and humidified flue gas to reduce the level of the acid gas pollutants contained therein connected to both (3) a component for preparing the alkaline reagent and (4) a component for separating solids including particulate matter from the flue gas previously reacted with the alkaline reagent. In a preferred embodiment, at least a part of the separated solids including the particulate matter is recovered and recycled with the alkaline reagent. The unrecycled solids are disposed of as waste and the essentially solids-free treated/cleaned flue gas is released into the ambient atmosphere.

Description

PROCESS AND APPARATUS FOR RECOVERY OF ACID GASES FROM FLUE

GAS

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention is directed to a process and an apparatus for treating flue gases. More particularly, the present invention is directed to a process and an apparatus for the reduction and/or removal of one or more acid gas pollutants such as SO₂, HCl, HF, HBr, NOx, and/or solids including particulate matter from flue gases typically generated by the combustion of sulfur containing coal including high sulfur coal, incineration of hazardous medical and municipal wastes, glass and metal thermal treating processes, and the like.

Detailed Description of the Related Art

Concerns continue to be voiced over the ever increasing levels of gaseous and particulate pollutants emitted from industrial and/or municipal sources. Use of high sulfur coal, a plentiful and cheap energy source, has been limited because its combustion produces unwanted pollutants including high levels of sulfur containing pollutants, for example, as sulfur dioxide (SO₂) or the like together with particulate matter. The sulfur dioxide ultimately converts to atmospheric sulfuric acid commonly known as a major component of "acid rain." Acid rain has been linked to a number of well documented environmental problems ranging from the discoloration of paint on automobiles to creating stressful growth conditions for trees and plants. Thus, environmental regulations mandating reductions in the levels of various harmful materials including SO₂, HCl, HF, HBr, NOx and particulate matter, and the like have been promulgated.

To reduce acid rain formation and to satisfy regulatory standards, various apparatuses and processes have been developed for acid gas recovery from flue gases before the flue gases are released into the ambient atmosphere. For example, wet scrubbers, semi-dry scrubbers, spray drying systems, dry injection systems, and the like have been employed for removing various pollutants from flue gases. However, these scrubbers and systems suffer from a variety of problems and disadvantages that are peculiar to the particular cleaning process and apparatus employed.

Wet scrubbers are the most efficient of all known systems because the gases are fully water saturated at the wet bulb temperature. This wet bulb temperature condition is one of the best known for absorption and subsequent reaction of the acid gases with an alkaline reagent. Wet scrubbers, however, suffer from disadvantages which include, but are not limited to, the following: (1) the wet scrubber vessels that "contain" the process are designed based on low internal flue gas velocities of 200 to 500 feet per minute which requires the use of high cross-sectional area equipment which, in turn, contributes to high equipment costs; (2) large quantities of wet slurries are produced which require expensive pumping and disposal systems; (3) careful pH control is necessary to effect difficult recovery of the acid gases; (4) because the system is wet, expensive corrosion resistant parts (e.g., stainless steel, fiber glass and other alloy parts) must be used; and (5) periodic cleaning is required to combat the effects of solids buildup including scaling, plugging, fouling and/or corrosion of equipment. These and other disadvantages contribute to the high capital and operating costs associated with wet scrubbers.

Spray drying systems are a combination of wet and dry processes in which an alkaline reagent is injected as a wet water slurry (sometimes as an overly wet slurry) that is dried as part of the process, resulting in a dry solid for disposal. Therefore, spray drying is classified as a semi-dry process. Spray dryers generally use a water slurry of hydrated lime as a feed reactant stream. The spray drying process cools the flue gas from its initial temperature (e.g., boiler flue gas outlet temperature) to a controlled outlet temperature (i.e., temperature of the gas entering a solids recovery filter) typically about 50 degrees F above the flue gas saturation temperature. The saturation temperature is the flue gas temperature at which it contains the maximum amount of water vapor i.e., the wet bulb temperature. To achieve such temperature control, only a fixed amount of water can be introduced for a given gas flow. The fixed amount of water limits the concentration of hydrated lime that can be pumped and atomized. For applications that have high acid gas content, it is not possible to maintain the necessary water balance and inject the required amount of alkaline reagent. With proper operation, the effluent stream to disposal is a dry solid. Depending upon the application, the efficiency of a spray drying system may vary from about 70% up to as high as 90%. Spray dryers are designed to use low internal flue gas velocities, 75 to 250 feet per minute, which results in the use of high cross-sectional area vessels contributing to increased capital costs. Further, as with all systems, to obtain higher cleaning efficiencies, it is necessary to operate spray drying systems close to the saturation temperature. To increase the acid removal efficiency, the flue gas temperature is lowered to approach the saturation temperature. The closer the temperature of the flue gas to the saturation temperature (especially below the saturation temperature when water condenses out of the flue gas), the higher the water content of the flue gas together with that of the alkaline reagent. As a consequence of the increased water content, the solids buildup problems including scaling, plugging, fouling and/or corrosion on the inner walls of the spray drying apparatus become exacerbated. Sometimes, the free flow of the flue gas together with the free flow of the alkaline reagent is severely impeded requiring interruption of the spray drying process for costly equipment cleanup. Further, as the saturation temperature is approached, water will condense at points of air leakage or at points or areas of cool surface temperatures also resulting in further solids buildup.

Dry injection systems are promoted as the newest technique for recovery of acid gases from flue gases. As the name implies, dry reagent is injected directly into the boiler combustion zone, ducts or into a special vessel to react with the flue gases. However, the known dry injection systems require a high excess of dry reagent and are, therefore, inefficient and expensive to operate.

Thus, there remains a strong need for an alternative acid gas recovery process (together with an apparatus for implementing the process) which is efficient (at any level of acid gases entrained in the flue gas), cost effective and which mitigates one or more of the aforementioned solids buildup problems.

Summary of the Invention

It is, therefore, an object of the present invention to provide a process (together with an apparatus for implementing the process) for cleaning flue gases that reduces one or more of the aforementioned problems associated with wet scrubbers, semi-dry scrubbers and/or dry injection systems.

It is another object of the present invention to provide a process (together with an apparatus for implementing the process) for reducing and/or removing one or more of the acid gas pollutants from flue gases in an efficient and cost-effective manner.

It is yet another object of the present invention to provide a process (together with an apparatus for implementing the process) for reducing and/or removing one or more of SO₂, HCl, HF, HBr, NO_x , particulate matter and/or other pollutants from flue gases.

It is still another object of the present invention to provide a process (together with an apparatus for implementing the process) for reducing and/or substantially eliminating one or more acid gas pollutants from flue gases generated by the combustion of high sulfur coal, the incineration of hazardous medical and municipal wastes, glass and metal thermal treating processes or the like.

These and other objects are accomplished by the exemplary process of cleaning a flue gas containing an amount of one or more acid gas pollutants including particulate matter, the process comprising steps:

(a) partially cooling and humidifying the flue gas;

(b) preparing an alkaline reagent by wetting a source of lime or hydrated lime; (c) pulverizing the alkaline reagent in a mill and contacting and mixing the pulverized alkaline reagent with the partially cooled and humidified flue gas for a residence time sufficient to react with the pulverized alkaline reagent and to reduce the amount of one or more acid gas pollutants to yield a treated flue gas mixed with solids; and

(d) separating the solids including any of the particulate matter from the treated flue gas. According to one embodiment, a portion of the solids including the particulate matter is recovered and recycled with the source of lime or hydrated lime. Preferably, the flue gas is traveling at a high flue gas velocity (e.g., from about 2500 to about 4500 feet per minute) and the pulverizing, contacting and mixing step is carried out within the mill and optionally completed within the ductwork leading to a solids recovery filter used in the separating step.

According to another embodiment, the apparatus for cleaning a flue gas containing an amount of one or more acid gas pollutants including particulate matter comprises:

(a) a means for partially cooling and humidifying the flue gas;

(b) a means for preparing an alkaline reagent;

(c) a means for pulverizing the alkaline reagent in a mill, and contacting and mixing the pulverized alkaline reagent with the partially cooled and humidified flue gas for a residence time sufficient to react with the pulverized alkaline reagent and to reduce the amount of the one or more acid gas pollutants to yield a treated flue gas mixed with solids; and

(d) a means for separating the solids including any of the particulate matter from the treated flue gas.

Detailed Description of the Drawings

Figure 1 is a schematic of a preferred apparatus according to one embodiment of the invention.

Detailed Description of the Preferred Embodiments

While the specification refers to a singular "flue gas," it is to be understood that the term "flue gas" may encompass one or more flue gases and solids that may be present. Typically, the "flue gas" is a combination of one or more gases and entrained solids such as fly ash which is produced by the combustion of, for example, high sulfur coal in a boiler steam generator, or the like, normally associated with the generation of electricity.

"High sulfur coal" refers to coal containing more than about 3-4% sulfur, by weight (measured as elemental sulfur). A typical "flue gas" leaving a boiler, for example, contains from about 1 ,200 parts per million (ppm) to about 2,500 ppm SO₂, from about 0.01 to about 0.08 grains/standard cubic foot (scf) particulate matter, and from about 200 ppm to about 400 ppm of NO . Typical values may be 2,000 ppm SO₂, 0.05 grains/scf particulate matter and 300 ppm NO . Further, the flue gas typically leaves a boiler at a temperature T, from about 320 °F to about 700 °F, more typically at a temperature T, from about 325 °F to about 375 °F. "Alkaline reagent" refers to a source of lime, a source of hydrated lime, mixtures thereof or equivalents thereof which have been wetted with an appropriate amount of a fluid, typically liquid water, to permit further cooling of and reaction with a partially cooled and humidified flue gas when the alkaline reagent is pulverized, contacted and mixed with the flue gas. As described in greater detail below, the further cooling of the partially cooled and humidified flue gas is from a set point temperature of T₂ down to a lower temperature T₄ which is above the wet bulb temperature T₃ of the flue gas. The amount of fluid added to prepare the alkaline reagent is sufficient to maintain T₄ above the wet bulb temperature T₃ of the flue gas and which amount of fluid substantially avoids solids buildup including scaling, plugging, fouling, and/or corrosion of the process equipment.

The process of the present invention comprises four broadly categorized operations: (1) preliminary gas conditioning, i.e., partial gas cooling and humidification; (2) alkaline reagent preparation; (3) contacting the conditioned flue gases with the alkaline reagent; and (4) separating particulate matter from the treated flue gas. Each of these operations is described in greater detail below.

Section (1): Partial Flue Gas Cooling and Humidification

An initial step in the inventive process involves preliminary conditioning of the flue gas. In this conditioning step, the flue gas is partially cooled and humidified, preferably by spraying a fluid into the, typically hot, flue gas. In a preferred embodiment, this may be accomplished, for example, by spraying water into the flue gas. Other methods for introducing water droplets including atomized water into the flue gas may be used. Such methods are well known to those of ordinary skill. The flue gas is cooled to a temperature above the saturation temperature with an amount of water that avoids solids buildup including scaling, plugging, fouling and/or corrosion of the inventive apparatus. Avoidance of corrosion is especially critical during preliminary conditioning (i.e., partial cooling and humidifying) of the flue gas because of the high acid content entrained therein. This preliminary conditioning is accomplished by introducing a minimum amount of water that is necessary to cool the flue gas to the desired temperature while maintaining essentially dry the internal surfaces of the inventive apparatus. While the cooling and humidifying fluid is discussed in terms of water, water in combination with other liquids and/or solids may be used. However, water is generally preferred.

Atomization of water into the flue gas for cooling and humidifying the flue gas is preferably accomplished by the use of one or more pressure nozzles connected to one or more pumps including positive displacement pumps. This atomization method is preferred because (1) the energy consumption per unit weight of water sprayed, for a given droplet size, is the lowest possible; (2) a high mechanical mixing efficiency is achieved; and (3) the water spray pattern is easily and reliably controlled. However, in the broad practice of the invention, other atomization methods may be used. These include, but are not limited to utilizing one or more "two fluid" nozzles using steam or compressed air as the energy source for atomization.

Preferably, for cooling and humidifying, an exemplary flue gas volume of 1,074,000 actual cubic feet per minute (acfm) from an exemplary coal fired 300 megawatt boiler, three water pumps may be provided for spraying water from about 50 spray nozzles. Typically, two water pumps are operating, while one is on standby. Flow control is preferably accomplished by the use of variable speed drives.

For the aforementioned exemplary flue gas volume, the flue gas typically is cooled from an exemplary initial temperature Ti of about 325 °F down to an exemplary cooled temperature T₂ of about 170°F, a preferred set point for this process step. The T₂ set point is a primary variable that is preferably fixed based on the system water (or other fluid) saturation temperature T₃ which typically varies with the ambient combustion conditions including water content of solids combusted, hydrogen content of the solids combusted, and ambient temperature and humidity. This T₃ temperature is continuously monitored with feed back control to set or reset T₂ to maintain T₂ above T₃. Section (2): Alkaline Reagent Preparation and Criteria Therefor

In addition, subsequent to humidifying and cooling the flue gas to T₂, a sufficient amount of alkaline reagent (e.g., fresh alkaline reagent plus any recovered/recycled alkaline reagent and any recovered/recycled particulate matter together with their respective water content) is added to lower the flue gas temperature from T₂ to a temperature T₄ which approaches T₃ preferably as close as practically and economically possible with the proviso that T₄ ≥ T₃. As previously noted, T₃ is denoted as the wet bulb temperature. T₄ is selected by the value of T₃ subject to the aforementioned proviso that T₄ > T₃. The temperature differential between T₄ and T₃ is, typically from about 5 to about 20 °F above T₃.

To achieve a desired acid removal efficiency, the initial flue gas acid content and T₃ are quantified. Based on these quantified values, a desired T₄ is selected that will permit the acid removal efficiency desired as described below. The flue gas volume, its composition and selected T₄ will dictate the T₂ value necessary to achieve the desired acid removal efficiency. Subject to the aforementioned criteria and conditions, T₂ is appropriately preselected. Once T₂ is selected, the differential temperature T₂ - T₄ is readily calculated. The differential temperature dictates the quantity of water necessary to lower T₂ to T₄.

The aforementioned quantity of water necessary to lower T₂ to T₄ is combined with the source of lime (or hydrated lime) to yield an alkaline reagent. Further, the necessary quantity of the source of lime is dictated by the initial flue gas acid content. However, the water content (e.g., % by weight water based on the total weight of the water and the source of lime) of the alkaline reagent shall not exceed the point at which solids buildup including scaling, plugging, fouling and/or corrosion of equipment becomes problematic (i.e., the alkaline reagent does not exhibit substantially free-flowing properties) with regard to the inventive apparatus. Unless indicated otherwise, in this section (2), all % values are by weight based on the total weight of the alkaline reagent. Preferably, to prevent substantial solids buildup from occurring, the cooled and humidified flue gas temperature T₂ is maintained from about 10 to about 45 degrees above T₄. Thus, the water added during the cooling and humidifying step should be limited by the preferred requirement of maintaining T₂ from about 10 to about 45 degrees F above T₄.

To avoid the aforementioned and other solids buildup problems including scaling, plugging, fouling and/or corrosion, it is preferred that the water content of the alkaline reagent be limited typically from about 1% to about 15%, more typically from about 2% to about 12%, preferably from about 3% to about 1 1%, and more preferably from about 5% to about 10%. However, it is understood that while water is preferred, other liquids and/or solids may be used to lower T₂ to T₄ at varying levels of fluid content sufficient to permit substantial free-flow of the alkaline reagent and the flue gas together with its particulate matter throughout the inventive apparatus.

To introduce the necessary quantity of the source of lime, the total quantity of the alkaline reagent (having the appropriate water/fluid content) may be varied as necessary during the pulverizing, contacting and mixing step of the inventive process (see section (3) below). Alternatively, to maximize efficient absorption and/or adsorption of the source of lime and its reaction with the acid gases, it may be preferable to lower (i.e., set) T₂ closer to T₄ (i.e., smaller temperature differential of (T₂ - T₄)). Conversely, T₂ may be raised (i.e., set) closer to T, (i.e., larger temperature differential of (T₂ - T₄)) when it is preferred to increase the water content of the alkaline reagent while maintaining the free flowing properties (i.e., absence of solids dropout) of the flue gases and other material flowing within the apparatus of this invention.

The function of the water is to wet the surface of the alkaline solids to enhance reaction with the acid gas or gases. Ultimately, as the water evaporates, the flue gases are further cooled towards T₄ For example, assuming a T, of about 325 °F and a T₂ of about 170°F, the required amount of heat transfer (for 1,074,000 acfm of flue gas) is about 1 19,149,000 British thermal units per hour (Btu/hr), which requires about 108,195 pounds of water per hour (lb/hr) or about 222.6 gallons per minute (GPM). This volume of water requires the use of about 50 spray nozzles to achieve effective atomization of the water as well as for proper mixing with the gas to achieve sufficient cooling and humidification of the gas. The number of nozzles, the flow rate of the water, and/or the number of pumps used can be readily adjusted, upwards or downwards, depending on the quantity of the flue gas to be cleaned, its acid gas content, and the volume of water to be sprayed to obtain the desired cooling and humidity prior to mixing the flue gas with the alkaline reagent and any recovered/recycled solids. While the above noted calculations are discussed in terms of the number of spray nozzles, one or more spray nozzles, pressure or "two-fluid" nozzles or the like may be used.

To summarize, the initial temperature T, of the flue gas is a function of, for example, the boiler outlet temperature set point. The T₂ set point, as previously noted, is selected as a function of the flue gas water saturation temperature T₃ and the desired acid gas pollutant removal efficiency. For coal fired boilers, the saturation temperature T₃ is a function of the total combustion air used, ambient temperature and humidity, and the hydrogen and water content of the coal. For incineration plants, the saturation temperature T₃ is also a function of the combustion air used, ambient humidity and, in this case, the hydrogen content of the waste(s) and the free water content.

If the gases are cooled below the saturation temperature T₃, water condenses out of the flue gases. The condensed water combines with particulate matter and causes solids buildup problems including scaling, plugging, fouling and/or corrosion similar to those associated with wet-scrubbers. These problems require the flue gas acid removal operation to be interrupted for equipment cleaning. This adds to the cost of operation of any desulfurization process and increases overall process inefficiency. Thus, it is important to control the flue gas temperature during the inventive process to maintain the proper balance between the water added in the initial cooling and humidification step, the water content of the alkaline reagent, and the quantity of alkaline reagent added in the contacting step (i.e., to carefully control the mass/water balance).

Furthermore, for the exemplary flue gas volume of 1 ,074,000 acfrn, the transport ducts from the boiler, generator, incinerator or the like should preferably be large enough to permit direct spraying of a fluid, preferably water, into the flue gas without substantially reducing the flue gas velocity or without having to rely upon any special mixing techniques. Additionally, it is preferred that the flue gas velocity should be sufficiently high to prevent solids dropout, e.g., from about 2500 to about 4500 feet per minute. For the aforementioned flue gas volume, a vessel and/or duct having a diameter of about 20 feet and a length of about 90 feet is sufficient. However, other vessel and/or duct diameters and lengths may be used in combination which are sufficient to permit cooling and humidifying of the flue gas while maintaining a flue gas velocity sufficient to prevent substantial solids buildup or solids dropout. Additionally, the above-noted diameters and lengths should be of sufficient size to permit substantially total evaporation of the water content (or other fluid content) to occur.

Section (3): Acid Gas Recovery

During this operation, one or more of the acid gas pollutants in the flue gas are reduced to an acceptable level by intimately contacting and mixing the flue gas with the alkaline reagent, preferably pulverized alkaline reagent. The flue gas temperature immediately prior to this contacting and mixing step is at the set point T₂.

According to one embodiment, a suitable alkaline reagent (preferably lime or hydrated lime together with or without recovered/recycled solids) is pulverized and, preferably essentially simultaneously, intimately contacted and mixed with the partially cooled and humidified flue gas. By turbulently mixing the alkaline reagent with the flue gas, high levels of acid gas removal can be achieved. During the turbulent mixing, the alkaline reagent and any recovered/recycled solids come in contact with the flue gas at a high velocity for a period of time sufficient to react with and reduce the acid gas pollutants (including SO₂) in the flue gas to acceptable levels.

Typically, a reduction in sulfur pollutants by 85% or more (over the initial levels of sulfur containing pollutants present in the flue gas e.g., measured in ppm or the like) is acceptable. However, a reduction of 90% or more is preferred, a reduction of 90-94% is even more preferred, and a reduction of 95 to 99-100% is most preferred. Pursuant to the present invention, reductions of 95-100% are readily achievable. While a reduction of 95- 100% might appear negligibly larger than a 90% reduction, it is to be understood that as reduction efficiencies approach 100%, the incrementally better efficiencies are both increasingly more difficult and more expensive to achieve. However, reaching such removal efficiencies are well within the scope of the claimed invention at a cost that is not prohibitively expensive. For use with the present invention, a suitable alkaline reagent (e.g., desulfurizing agent to which an appropriate amount of water or other suitable fluid has been added in accordance with section (2) above) includes, but is not limited to, the following (1 ) a calcium alkali (optionally together with a calcium-reactive silica or alumina) in amounts sufficient to allow for reaction (e.g., chemical reaction, absorption and/or adsorption) with one or more of the acid gases present in the flue gas; (2) virtually any composition which includes a calcium alkali (CaO or Ca(OH)₂) and which does not interfere with one or more objects of the present invention; (3) calcium alkali in the form of ( ) lime, ( /^') slaked lime, (Hi) hydrated lime, (iv) calcidic lime, (v) dolomitic lime, (vi) calcium hydroxide, (vii) calcium oxide, and/or ( iii) fly ash. Items (/) and (/^' ) are preferred.

Fly ash is a natural-by-product of coal combustion. As such, the amount of the alkaline content of the fly ash inherently and naturally present in the flue gas reduces the amount of the alkaline reagent (e.g., made from wetting lime or hydrated lime) required to achieve an acceptable reduction of one or more acid gas pollutants (e.g., SO₂) from the flue gas. Without engaging in undue experimentation, one of ordinary skill in the art can readily determine the amount of hydrated lime necessary to achieve the desired reduction of one or more acid gas pollutants based on the following chemical reactions noted below. Without being bound by theory, the chemical reactions effected between hydrated lime and SOτ are believed to be as follows:

Ca(OH)₂ + S0₂ *^■ CaS0₃ + H₂0 T (I)

Ca(OH)₂ + 1/20₂ + S0₂ - CaS0₄ + H₂0 T (II)

The amount of excess air (for example, as provided by the boiler or the like) determines the degree to which reaction (II) occurs. It has been found that gypsum (CaSO₄» H₂O) is formed in the presence of substantial amounts of excess air and humidity in the flue gas. Below, the claimed invention is further described in greater detail with reference to Figure 1. Referring to the embodiment of Figure 1 , flue gas 10 from a boiler, generator, incinerator, or the like of a power plant is directed via conduit 20 past spray nozzles 30 spraying water 40 which partially cools and humidifies the flue gas 10 from an initial temperature T, to a set point temperature T₂. The cooled and humidified flue gas 10a at temperature T₂ is then directed through conduits 20 and 20a to a gas/solids mixing mill 50. A mill 50 suitable for use with the present invention includes, but is not limited to a cage mill, a mixing mill, a rotary mill, a non-rotary mixer, or a high velocity mixer (e.g., Venturi mixer). Two or more mills appropriately connected to conduits 20a, 20b and 59b in a series or parallel configuration may be optionally used.

An exemplary, preferred mill suitable for use in conjunction with this invention is a rotary cage mill. See, for example, U.S. Patent No. 4,378,911, incorporated herein by reference in its entirety, for a description of a suitable rotary mill. While a rotary mill may be used, any rotating machine or component that adequately pulverizes the alkaline reagent and provides the intimate mixing and contact between the flue gas 10a and the pulverized alkaline reagent together with any recovered/recycled solids (in the presence of the cooled and humidified flue gas 10a) may be used. Alternatively, for example, a cage mill that contains a rotating cage in a suitable housing which mixes, deagglomerates and/or optionally pulverizes or optionally further pulverizes the alkaline reagent together with any recovered/recycled solids in the presence of the cooled and humidified flue gas 10a may be used.

Mill 50 is connected to (1) wetting unit 70, (2) one or more supply containers 60, 60a and 60b containing, for example, dry lime or dry hydrated lime, and (3) recovery solids filter 80 via tubes (e.g., pipes) 25, 25a and screw conveyors 28. The dry reagent can be shipped in via exemplary rail cars 60b which are connected to a silo 60a via pipe 62 and blower fan 62a. In turn, the reagent is conveyed to silo 60 via pipe 61 and blower fan 61a.

From silo 60, optionally provided with rotary feeder 59, the dry solid reagent is transported via conduit 59a to wetting unit 70 supplied with spray nozzles 30a for spraying, for example, water 40 (1) to wet the dry solid reagent (e.g., the source of lime or an equivalent thereof) yielding the alkaline reagent and (2) optionally, to wet any recovered/recycled solids. From the wetting unit 70, conduit 59b directs the alkaline reagent together with any recovered/recycled solids to mill 50. At mill 50, the alkaline reagent and any recovered/recycled solids are pulverized and intimately contacted and mixed with the partially cooled and humidified flue gas 10a introduced into/to the mill through conduit 20a. Substantially simultaneous alkaline reagent pulverization, contact, mixing and/or particle deagglomeration of the alkaline reagent and any recovered/recycled solids with the flue gas 10a leads to high acid gas removal efficiencies. The mill 50 provides a means for the intimate, preferably turbulent, mixing and contact between the flue gas 10a and the pulverized alkaline reagent produced therein and any recovered/recycled solids. This turbulent mixing and contact together with alkaline reagent pulverization substantially increases the surface area of the alkaline reagent and that of any recovered/recycled solids to further facilitate their reaction with the acid gases entrained in flue gas 10a substantially simultaneously delivered (i.e., to the mill 50) via conduit 20a. It is critical to provide the intimate mixing of the cooled and humidified flue gas 10a with the pulverized and/or deagglomerated alkaline reagent and any recovered/recycled solids in order to maximize the use of high flue gas velocities and to shorten residence times in, for example, conduit 20b. Preferably, complete intimate mixing takes place from the mill 50 to solids recovery filter 80 over a contacting period (i.e., residence time) sufficient to reduce (i.e., to acceptable levels) the amount of one or more pollutants including sulfur from the flue gas 10a to yield a mixture 10b containing treated flue gas and solids (including particulate matter and reacted solids). The mixture 10b may contain residual amounts of the flue gas 10a. The flue gas 10a is cooled from T₂ to T₄ between mill 50 and solids recovery filter 80. Mixing of the cooled and humidified flue gas 10a with the pulverized and/or deagglomerated reactants (e.g., alkaline reagent, any recovered/recycled solids, other flue gas components and/or other particulate matter) is preferably very turbulent. Turbulent contact and mixing contributed to by mill 50 together with the high flue gas velocity is critical to achieving high removal efficiency at a high flue gas velocity and minimum residence time within mill 50 itself and conveyor duct 20b. As noted, the mixing is accomplished by the input of mechanical energy from the exemplary rotating mill 50 which not only provides intimate contact but also creates increased solids surface area for achieving increased reaction rates (e.g., between the alkaline reagent and the acid gases) and improved removal efficiencies (e.g., 95-100%). Preferably, the residence time from mill 50 to solids recovery filter 80 is sufficient to reduce the one or more acid gas pollutant (including sulfur) levels by about 85% or more, more preferably, by at least 90-95%, and, even more preferably, by about 96-100% based on the total weight of the acid gases contained in the flue gas 10a. An exemplary residence time is from about 0.1 second to about 3 seconds, preferably, from about 0.5 second to about 2.5 seconds, more preferably, from about 0.75 second to about 2.25 seconds, and, even more preferably, from about 1 to about 1.5 - 2 seconds. These aforementioned residence times can be adjusted upwards or downwards based upon the velocity of the flue gas, the volume of the flue gas to be processed, the level of acid gases contained in the flue gas, the surface area of the alkaline reagent and any recovered/recycled solids and the removal efficiency desired or required.

Typically, the alkaline reagent and any recovered/recycled solids should be deagglomerated to a particle size sufficient to permit an acid removal efficiency of at least 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100%, respectively. Accordingly, for example, the average particle size of lime or hydrated lime in mill 50 is typically less than about 150 micrometers, preferably less than about 100 micrometers, more preferably less than about 80 micrometers, and most preferably less than about 74- 75 micrometers. Other preferred particle size ranges include, but are not limited to, less than or equal to 10, 20, 30, 40, 50, 60, 60, 70, 80 and 90 micrometers, respectively. With further reference to Figure 1, from mill 50 and during transport through conduit 20b up to the solids recovery filter 80, the acid gas pollutant(s) entrained in the cooled and humidified flue gas 10a reacts with the exemplary alkaline reagent (e.g., lime or hydrated lime) and any recovered/recycled solids to yield a treated flue gas 10b mixed with solids. For example, the chemical reactions between the SO₂ and the hydrated lime are exemplified by reactions (I) and (II) previously noted herein. As indicated by these reactions, the SO₂ forms reaction products including CaSO₃ together with CaSO₄ and H₂O. Thereby, the amount of SO₂ pollutants can be reduced to acceptable levels. Furthermore, during, for example, the combustion of coal, including high sulfur coal, fly ash is produced. The fly ash, which is a natural-by-product of coal combustion, may also react with the SO₂ entrained in the flue gas. As such, the greater the amount of reactive alkaline fly ash present (e.g., inherently and naturally produced by coal combustion) in the flue gas, the smaller the amount of the alkaline reagent and any recovered/recycled solids (e.g., desulfurizing agent) that needs to be added to mill 50 to reduce the SO₂ concentration in the flue gas 10a to acceptable levels.

A sample calculation summary for a proposed gas cooling-humidification and acid gas recovery system for processing 1,079,914 acfm of flue gas is provided in Table I below:

TABLE 1

CALCULATION SUMMARY: PROCESS FOR ACID GAS RECOVERY FROM FLUE GASSES

OVERALL ENERGY BALANCE Cooling from 325 °F to 140 °F

SYSTEMS CONDITIONS AND FLOWS

Table I indicates that for desulfurizing a flue gas volume of 1,079,914 acfm, an exemplary 135,044/hr of water (275.2 GPM) sprayed from about 50 pressure nozzles over a time of about 2.1 seconds into conduit 20 having a straight length of about 100 feet and a diameter of about 20 feet will result in lowering the flue gas temperature from 325 °F (T,) to 170°F (T₂). In mill 50 and conduit 20b having a diameter of 5 feet 6 inches and a length of 100 feet, during a residence time of 1.6 seconds, the SO₂ concentration in the flue gas can be reduced from 2,798 ppm to 56 ppm, a 98% desulfurization efficiency (i.e., removal efficiency). Other relevant parameters are provided in Table I. For example, the amount of hydrated lime that should be used according to Table I is 0.00033 lb per acfm of flue gas to be processed when the inlet load of SO₂ is 2800 ppm from combustion of 4.5 weight % S coal at 40 % excess air. According to the embodiment of Figure 1, there is no upper limit to the amount of flue gas that can be processed or acid gas recovered. The weight of the alkaline reagent will range from 5 % of the total solids including recovered/recycled solids to 100 % of the total solids with no recovered/recycled solids, depending upon the weight of recovered/recycled solids used to satisfy the water/mass/chemical balance.

Section (4): Particulate Matter Separation. Recycle and Disposal

After the acid gas recovery step is completed, the mixture 10b including reaction products (e.g., Ca SO₄« H₂O), fly ash, unreacted alkaline reagent (e.g., lime, hydrated lime and/or any recovered/recycled solids) and any other pollutants and particulate matter entrained therein are induced via a blower 100 via conduit 20 b into an exemplary optional solids recovery .filter 80. See Figure 1. The solids recovery filter 80 is utilized to separate cleaned flue gas entrained in mixture 10b to remove solids including any particulate matter present therein such as fly ash, solid reaction products, excess solid alkaline reagent and any other particulate matter that may be entrained therein. Solids recovery filter 80 is optionally equipped with one or more bag filters 90, which are well known. In Figure 1, solids recovery filter 80 is equipped with rotary air locks 80a and 80b. Other solids recovery filters suitable for use with the invention, include but are not limited to one or more of a cyclone, an electrostatic precipitator, or other solids filtering equipment or means.

As the mixture 10b enters the solids recovery filter 80, the particulate matter entrained in mixture 10b is separated by one or more filters 90. According to one embodiment, the bag filter material 90 is equipped with pores small enough to permit cleaned flue gas to pass therethrough into conduit 20c towards blower 100, while preventing substantially all the solids including particulate matter entrained in the mixture 10b from passing into conduit 20c.

Accordingly, the cleaned flue gas mixture passes through the solids recovery filter 80 to yield filtered and cleaned flue gas 10c which is directed into conduit 20c by blower 100 and ultimately to stack 110. The cleaned and filtered flue gas 10c is released via stack 110 typically into the ambient atmosphere. The particulate matter trapped by solids recovery filter 80 is directed down screw conveyors 28, conduits 25a and 25 and optionally recycled and combined with dry alkaline reagent from silo 60. Typically, it is preferred to recover and recycle the fly ash and any excess alkaline reagent collected at the solids recovery filter 80 while maintaining the mass balance of the apparatus (i.e., maintain substantial free flowing circulation and avoid solids buildup and mass balance problems). Additionally, separation between solids including any particulate matter intended to be recycled to mill 50 and particulate matter to be collected and disposed of as waste 120 can be accomplished by the use of a diverter 25b connected to lines 25a and 26. With regard to any unwanted solids collected by solids recovery filter 80, these solids are preferably directed by diverter 25b down conduit 26 to an exemplary optional spraydeduster 27. Therein, additional water or other suitable fluid is sprayed via spray nozzles 30b onto the unwanted solids to yield waste ready for disposal. Waste 120 for disposal is collected in a suitable container and properly disposed.

Section (5): System Control

Preferably, all the above-described operations previously described in sections (1), (2), (3) and/or (4) are controlled by a control system (not shown in Figure 1) with appropriate control loops managed by a DCS or a PLC (i.e., a computer or the like). For example, each temperature T,, T₂, T₃, and T₄ is measured by one or more temperature sensors appropriately positioned (not shown) together with the apparatus of Figure 1. The measured temperatures are preferably indicated and recorded. Temperature sensors for T₂ and T₄ are set point controlled via PID feedback loops connected to the system control. The exemplary apparatus of Figure 1 is preferably also equipped with weight monitors 200 and 210 which are preferably also feedback loop controlled. Optionally, one or more humidity sensors, acid gas sensors, mass flow sensors and the like well known to those of ordinary skill can be appropriately placed to maximize automation and/or acid gas removal efficiency.

Having described the invention, the following claims are appended.

Claims

CLAIMS What is claimed is:

1. A process for cleaning a flue gas containing an amount of one or more acid gas pollutants, the process comprising the steps of: (a) partially cooling and humidifying said flue gas from a temperature of about T, to a set point temperature of about T₂;

(b) preparing an alkaline reagent by wetting a source of lime or a source of hydrated lime with a quantity of fluid sufficient to cool said partially cooled and humidified flue gas further from said temperature of about T₂ to a lower temperature of about T₄;

(c) pulverizing said alkaline reagent in a mill, and at least partially within said mill and optionally outside said mill, contacting and mixing said pulverized alkaline reagent with said partially cooled and humidified flue gas for a residence time sufficient to react with said pulverized alkaline reagent and to reduce said amount of one or more acid gas pollutants to yield a treated flue gas exhibiting a first acid gas removal efficiency, said treated flue gas being mixed with solids; and

(d) separating said solids including any of said particulate matter from said treated flue gas, wherein said temperature T₄ is above a wet bulb temperature T₃ of said flue gas.

2. The process of claim 1 further comprising selecting as said fluid a liquid comprising water.

3. The process of claim 2 further comprising selecting as said fluid liquid water.

4. The process of claim 1 further comprising selecting as said quantity of said fluid an amount of a liquid comprising water sufficient to substantially avoid problematic effects of scaling, plugging, fouling, corrosion or a combination thereof in an apparatus for carrying out said steps (a) - (d).

5. The process of claim 1 further comprising selecting said temperature T, in a first range from about 320°F to about 700°F, selecting said temperature T₂ in a second range from about 10°F to about 45 °F above said temperature T₄, selecting said temperature T₄ in a third range from about 5°F to about 20 °F above said temperature T₃ and selecting as said fluid a liquid comprising water.

6. The process of claim 5 further comprising selecting said first range from about 325 °F to about 375 °F, selecting said temperature T₂ to be about 170°F, and selecting as said fluid liquid water.

7. The process of claim 1 further comprising selecting as said source of lime or said source of hydrated lime a composition comprising a calcium alkali.

8. The process of claim 7 further comprising selecting as said calcium alkali a composition which is lime, hydrated lime or mixtures thereof.

9. The process of claim 8 further comprising selecting said lime from the . group consisting of slaked lime, calcidic lime, dolomitic lime, calcium oxide, fly ash, and mixtures thereof.

10. The process of claim 2 further comprising selecting said calcium alkali from the group consisting of CaO, CaOH₂, and mixtures thereof.

1 1. The process of claim 1 , wherein said step (a) of cooling and humidifying said flue gas comprises distributing a cooling and humidifying fluid into said flue gas.

12. The process of claim 1 1 , wherein said distributing of said cooling and humidifying fluid comprises spraying said cooling and humidifying fluid into said flue gas.

13. The process of claim 12, wherein said spraying comprises spraying water from a plurality of spray nozzles.

14. The process of claim 13 , wherein said plurality of spray nozzles comprises about 50 spray nozzles.

15. The process of claim 1 , wherein said separating comprises filtering said treated flue gas mixed with said solids using a solids recovery filter.

16. The process of claim 15 further comprising the step of recycling said solids including said particulate matter separated in said step (d) with said alkaline reagent prepared in said step (b).

17. The process of claim 1 further comprising selecting said pollutants from the group consisting of SO₂, HCl, HF, HBr, NO_x , particulate matter and mixtures thereof.

18. The process of claim 4 further comprising selecting as said quantity of said fluid an amount of said water sufficient to yield said alkaline reagent having a water content in a range from about 1% by weight to about 15% by weight based on a total weight of said alkaline reagent, selecting a removal efficiency from about 85% by weight to about 100% by weight based on a total weight of said pollutants selected from the group consisting of SO₂, HCl, HF, HBr, NO , particulate matter, and mixtures thereof.

19. The process of claim 18 further comprising introducing said flue gas in said step (a) at a flue gas velocity from about 2500 to about 4500 feet per minute, and selecting said residence time in said step (c) from about 1 to about 2 seconds.

20. The process of claim 4, wherein said step (c) further comprises evaporating substantially all of said water from said flue gas during said residence time and said step (c) further comprising simultaneously pulverizing, mixing and contacting said alkaline reagent with said partially cooled and humidified flue gas.

21. An apparatus for cleaning a flue gas containing an amount of one or more acid gas pollutants, said apparatus comprising:

(a) a means for partially cooling and humidifying said flue gas from a temperature of about T, to a set point temperature of about T₂;

(b) a means for preparing an alkaline reagent by wetting a source of lime or a source of hydrated lime with a quantity of fluid sufficient to further cool said partially cooled and humidified flue gas from said temperature of about T₂ to a lower temperature of about T₄;

(c) a means for pulverizing said alkaline reagent in a mill, and at least partially within said mill and optionally outside said mill, contacting and mixing said alkaline reagent with said partially cooled and humidified flue gas for a residence time to react with said alkaline reagent and to reduce said amount of one or more acid gas pollutants to yield a treated flue gas exhibiting a first acid gas removal efficiency, said treated flue gas being mixed with solids; and (d) a means for separating said solids including any of said particulate matter from said treated flue gas to yield an essentially solids free treated flue gas, wherein said temperature T₄ is above a wet bulb temperature T₃ of said flue gas.

22. The apparatus of claim 21, wherein said element (a) comprises at least one spray nozzle mounted in a chamber or duct and at least one pressure pump connected to said spray nozzle.

23. The apparatus of claim 22, wherein said element (a) comprises a plurality of spray nozzles and a plurality of pressure pumps connected to each of said spray nozzles, and wherein said element (c) comprises said mill and a duct between said mill and said element (d).

24. The apparatus of claim 23, wherein said mill is a rotary mill, a non-rotary mill, a Venturi mixer or a combination thereof.

25. The apparatus of claim 21, wherein said element (d) comprises at least one solids recovery filter.

26. The apparatus of claim 25, wherein said element (d) comprises an electrostatic precipitator, a cyclone or a combination thereof.

27. The apparatus of claim 21 , wherein said element (b) comprises a container holding a source of lime or hydrated lime and at least one spray nozzle mounted in a chamber or a duct and at least one pressure pump connected to said spray nozzle for wetting said source of lime or hydrated lime.

28. The apparatus of claim 27 further comprising a stack for releasing said essentially solids free treated flue gas into the ambient atmosphere and a means for recycling at least a part of said solids separated from said treated flue gas together with said alkaline reagent.

29. The apparatus of claim 21 further comprising a plurality of conduits connecting said element (a) to said element (b), said element (b) to said element (c), and said element (c) to said element (d), at least one fan, at least one spray nozzle, at least one pump connected to said spray nozzle, at least one temperature sensor, at least one humidity sensor, at least one acid gas sensor, at least one weight sensor, and at least one computer connected to said fan, said spray nozzle, said pump, said temperature sensor, said acid gas sensor, said weight sensor, and said humidity sensor sufficient to achieve a removal efficiency of at least 85% and to maintain a flue gas temperature T₄ above a saturation temperature T₃ flue gas by at least about 5 to about 20 degrees Fahrenheit.

30. The apparatus of claim 21, wherein said acid gas pollutants comprises SO₂, HCl, HF, HBr, NOx, particulate matter or mixtures thereof.

31. The apparatus of claim 20 wherein said acid gas pollutant comprises SO₂.