This invention relates to a method for scrubbing pollutants from an exhaust gas stream whereby materials in a scrubbing solution react with the gas stream to render such materials benign or convert them into useful products.

BACKGROUND OF THE INVENTION

Scrubbing of exhaust gas pollutants is generally costly, but has significant environmental returns. Unless it is possible to convert the waste products from scrubbing into useful products, there is little, or no, economic return to scrubbing. Scrubbing equipment is expensive. Additionally, the materials for scrubbing, such as oxides, carbonates, or hydroxides are a continuing expense. Moreover, disposal of the reaction products derived from reaction of the scrubbing materials with the exhaust gas adds to the continuing cost of scrubbing, especially if the scrubbing reaction products contain toxic components. Such expenses in operating pollution control devices is a major disincentive to scrubbing exhaust gas. To counteract this disincentive it is necessary to develop less costly, but equally effective, scrubbing processes.

In alkaline (basic) scrubbing systems, the art has invested a significant effort into minimizing the amount of scrubbing reactant consumed to minimize the costs of raw material and spent reactant disposal.

Most alkaline (basic) scrubbing processes for sulfur dioxide use limestone as a reactant. Calcium carbonate (limestone) reacts with sulfur dioxide to form calcium sulfite or sulfate plus carbon dioxide. In addition to the cost of the limestone reactant and the reacted limestone (scrubber sludge) disposal cost, the carbon dioxide (a greenhouse gas) release by such processes may also be harmful to the environment. In effect, reduction of one atmospheric pollutant (sulfur

dioxide) results in the increase of another (carbon dioxide) plus generation of solid waste.

U.S. Patent No. 5,100,633 reports a process that minimizes the use of commercial alkaline materials by using boiler ash. Boiler ash contains a significant proportion of alkali and alkaline earth metal oxides, hydroxides, and/or carbonates. That patent also reports recovering valuable alkaline metal salts from the alkali solutions after neutralization to offset the operating cost. This patent also reports that the carbon dioxide content of the exhaust gas is reduced by forming recoverable carbonates . BRIEF DESCRIPTION OF THE INVENTION

We have now discovered that ammonium hydroxide can be used to scrub exhaust gas pollutants containing sulfur oxides and provide an economic benefit despite the continuing cost of the ammonium. The ammonium can be used alone, or in combination with other basic (high pH) materials such as an aqueous extract from ash of the type described in U.S. Patent No. 5,100,633, which is hereby incorporated by reference.

The ammonium hydroxide reacts with sulfur oxides to form a soluble compound which can be selectively separated from other products of the exhaust gas scrubbing. Thereafter, the product of this reaction can be recovered as a valuable by-product. Other products of scrubbing exhaust gas include materials initially insoluble as well as those that were rendered insoluble by the process. The reaction products from the scrubbing process may have a number of beneficial uses depending on the initial composition of the scrubbing solution used and on the selected mode of operation of the process. For example, the recovered ammonium salts may include potassium sulfate, ammonium sulfate, ammonium chloride and combinations thereof which can be separated and used

as either an agricultural fertilizer or chemical feedstock.

One of ordinary skill in the art will recognize that the scrubbing solution may be more of a slurry than an ideal "solution".

In one embodiment, ammonium hydroxide, which may, if desired, be prepared from ammonia and water, is combined with an aqueous ash extract or slurry. This embodiment produces several benefits including, for instance, reducing the volume of the ash waste which needs to be disposed. Another benefit of this embodiment is that alkaline earth metals from the ash react with (and thereby remove from the exhaust gas) carbon dioxide by forming a carbonate precipitate, which is an innocuous solid (primarily calcium carbonate) .

In another embodiment, ammonium hydroxide is the only reactant in the scrubbing solution. This embodiment maximizes the production of ammonium sulfate, a marketable commodity and allows the scrubbing process to be used in situations where the amount of ash available is insufficient to scrub the exhaust gas stream.

In practice, each user of the process will balance between maximizing income and minimizing waste to fit their conditions including ash availability and waste utilization-disposal options.

Spent ash may be used as raw material feed for a cement kiln in situations were transport costs are acceptable. In other situations, the spent material may be used to manufacture aggregate or may have to be disposed of in a landfill. Because of its aqueous extraction, the spent ash will no longer contain soluble salts or caustic. As a result, in most cases, the spent ash can be disposed of as benign common fill. .An aqueous scrubbing solution of ammonium hydroxide is prepared, for example by (a) diluting a concentrated solution or (b) adding ammonium gas to

water. The aqueous ammonium hydroxide is used alone, or in combination with another alkaline scrubbing medium such as an aqueous ash extract, as a scrubbing reactant for removing from the exhaust stream the acidic oxides of sulfur, nitrogen and other acidic gaseous pollutants such as the compounds of the halogens and/or their oxides which, when dissolved in, or reacted with, water produce an acid. More specifically the process reacts the scrubbing solution with acids produced from the oxides of sulfur, nitrogen and carbon and from the compounds of halogens and their oxides. The process scrubs a portion of the oxides of sulfur, nitrogen, and carbon, and the compounds of halogens and their oxides from the exhaust gas stream and makes them available for reaction with the ammonium hydroxide, produces ammonium salts composed of ammonia [(NH ₄ ) ⁺ ] and components derived from the exhaust gas stream, and utilizes heat in the hot exhaust gas stream as an energy source to concentrate by evaporation, and to crystalize ammonium salts to allow their recovery as the solid salt.

In accordance with the invention there is provided a novel method of treatment of a hot exhaust gas stream containing as pollutants at least one of the acidic oxides of sulfur, nitrogen, carbon, and halogen compounds, for producing scrubbed exhaust and useful or benign by-products.

The method comprises providing a basic aqueous scrubbing solution of ammonium hydroxide, which reacts the exhaust gas stream with the scrubbing solution to dispense the exhaust gas stream in water and to produce acids from pollutants in the exhaust gas, allowing the said acids to react with the scrubbing solution alone, or in combination with other basic materials, such as an ash extract, thereby to produce a solution with a precipitate of any insoluble salts comprising at least one of halogen compounds, carbonate, sulfate, sulfite, nitrate and

nitrite, recovering those salts from the solution and expelling the scrubbed exhaust gas stream.

The method comprises cooling and dehumidifying the gas stream in a manner to remove heat and collect water therefrom, reacting the cooled and dehumidified gas stream with an aqueous, alkaline scrubbing solution, separating the scrubbing solution after such reaction from any precipitates or insolubles, and heating the separated solution, using heat collected during cooling and dehumidifying, to remove water and crystallizing the dissolved salts.

In other embodiments, the used scrubbing solution is fractionated (e.g., by differential crystallization) to purify the several dissolved salts in the scrubbing solution. In some embodiments, chlorides are removed prior to purifying the ammonium salts dissolved in the used scrubbing solution, for example, by a membrane or by conventional electrolysis to produce chlorine gas and potassium or sodium hydroxide (both hydroxides are useful within the scrubbing process) . BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

Fig. 1 is a schematic representation of one apparatus for practicing the invention; and Fig. 2 is a flow diagram showing the mode of operation of the apparatus shown in Fig. 1. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT AND METHOD

In the following specification and claims, ash derived from burning biomass material, fossil fuels and industrial or municipal wastes or other by-products, usable in the process herein described and claimed, are collectively identified by the generic term, "ash" .

Percentages, unless otherwise specified, are by weight. Preferred methods of the present invention may include a number of additional processing steps. For example:

1) Passing (a) the exhaust gas stream through one part of a heat exchanger prior to reacting it with the scrubbing solution and (b) used scrubbing solution free of insoluble material through another part of the same heat exchanger to transfer heat from the exhaust gas stream to the used scrubbing solu ion;

2) condensing and reusing the liquid evaporated from the scrubbing solution in the scrubbing solution;

3) separating the undissolved components of the used scrubbing solution from the scrubbing solution by conventional methods; and

4) dehumidifying the exhaust gas stream and using the removed moisture as part of the make-up water for the scrubbing solution. A preferred method of separating insoluble material from the used scrubbing solution is to transfer the used scrubbing solution to a settling tank or filtration system or other separation system, to separate the used scrubbing solution from any precipitate or insoluble material.

Preferred methods of recovering the dissolved solids employ evaporation processes. The heat used to remove water from the salt solution can be derived from heat sources such as the hot exhaust gas, the latent heat of moisture in the said exhaust, the hydration reaction between ammonium hydroxide, ammonia or other material and water, and from compressing the exhaust gas prior to contacting the exhaust gas with the scrubbing solution.

The dissolved salts recovered by this evaporation include salts of ammonia such as ammonium sulfate. These salts are suitable for use as fertilizer or as raw material for extraction of chemicals.

In some embodiments of the present invention, the used scrubbing solution contains a substantial amount of chloride salts. To purify the other dissolved salts, the chlorides can be removed before proceeding to purify the other salts. For example, the used scrubbing

solution can be concentrated so that the chlorides constitute at least about 10 percent, by weight, reported as sodium chloride. A portion of the concentrated used scrubbing solution is transferred to, or circulated through, a conventional electrolysis cell. There, by electrolysis, the chloride is removed from the used scrubbing solution as chlorine gas, which gas is collected. Additionally, potassium or sodium hydroxide is also generated in the used scrubbing solution and is available for reaction with additional acidic compounds in the exhaust gas.

When ash is used in the scrubbing process, the process removes caustic components from the ash, rendering the residue neutral and suitable for further use, for example, as raw material in a cement kiln, as raw material for use in making aggregate, or it can be disposed of as non-hazardous solid waste.

Fig. 2 shows the operation of a preferred embodiment of the system in diagrammatic form. Fig. 1 shows in greater detail the treatment of the ammonium hydroxide solution by the exhaust gas containing one or more of the oxides of sulfur, nitrogen, carbon, the halogens, or acidic compounds of halogens and the settling of the treated product. In the accompanying figures, as described herein, like-numbered apparatus elements are the same.

Referring now to Fig. 2, concentrated ammonium hydroxide, either purchased as such, or prepared by dissolving ammonium gas in water, is stored in tank 60. As needed, the concentrated ammonium hydroxide is piped via pipe 61 to mixer 63 where it is blended with water from pipe 62 (outside water supply, not shown) to form a stock solution of ammonium hydroxide.

The stock solution of ammonium hydroxide is piped via pipe 64 to mixer 66 where it is blended with an alkaline aqueous solution, for example, an ash extract from pipe 2 to form the scrubbing solution. Typically,

the stock solution of ammonium hydroxide constitutes between about 0.01 and about 20 percent of the scrubbing solution. Preferably, the stock solution of ammonium hydroxide constitutes between about 0.05 and about 5 percent of the scrubbing solution. These percentages are strongly influenced by the sulfur dioxide content of the flue gas being scrubbed and by the percentage of scrubbing solution that is ash extract.

It is preferred that the scrubbing solution has a pH of between about 6 and about 8, and it is further preferred that the scrubbing solution has a pH of between about 6.5 and about 7.2. Typically, the scrubbing solution pH is controlled by adjusting the ammonium hydroxide concentration of the stock ammonium hydroxide solution.

It is also preferred that the scrubbing solution has an excess of species that react with the acidic species produced by dissolving components of the exhaust gas in water, it is more preferred that the scrubbing solution has a minimal excess of such species that react with such acid species in order to minimize release of unreacted ammonia or ammonium compounds to the atmosphere with scrubbed flue gas. In other words, the scrubbing solution typically has between a stoichiometric and a twice stoichiometric amount of the species that react with the acids formed by dissolving the exhaust gas in water.

Ash from a suitable source (not shown) and water from pipe 4 (outside water supply, not shown) and from pumps 27 (condensed exhaust gas moisture) , 26 and 34 are mixed by mixer 36 in mixing tank 35 to produce a slurry which is pumped through pipe 1 by pump 42 via pipe 2 to inlet pipes 41 where it is released into treatment tank 3 as a dilute slurry 5. Additional water from the outside water supply may be added through pipe 4 in treatment tank 3. Outside water may also be delivered by pipe 4 to dilution tank 28 and by connection to pipe 39

to coils 40 in heat exchanger-crystallization unit 6. The ash is obtained from a source such as a generating plant fired by fossil fuel or biomass or an incinerator fired by waste material. An exhaust gas stream from a cement kiln, incinerator or boiler (not shown) containing one or more of the oxides of sulfur, nitrogen, carbon, and the halogens or acidic compounds of halogens, enters heat exchanger crystallization unit 6 through inlet 7 from which it emerges, after cooling and dehumidifying, as cooled exhaust. Condensed exhaust gas moisture is collected in the heat exchanger crystallization unit 6 and conveyed by pump 27 to the mixing tank 35 or to the treatment tank 3 through pipe 8. The exhaust then travels to compressor 9 through pipe 10 and is delivered via pipe 11 to distribution pipes 12 in the bottom of the treatment tank 3. To prevent settling of the solids to the bottom of treatment tank 3, the slurry may be stirred or recirculated by suitable means, for example by recirculation pump or pumps 13. The exhaust gas contacts slurry 5 of ash and water and emerges from the tank top as scrubbed exhaust 14. Slurry 5, in the form of a mixture of treated solid, water and dissolved materials, is pumped by pump or pumps 15 via pipe 16 to the settling tank 17 where the settled solids 18 are in turn pumped out by pump 19 while the water 20, comprising a solution laden with dissolved salts, is pumped by pump 26 through pipe 38 back to the coils of heat exchanger crystallization unit 6 to provide cooling for the input exhaust gas. The water from salt solution 20 is evaporated to a vapor in heat exchanger crystallization unit 6 and released via pipe 21 into the atmosphere or recondensed to a liquid to recapture the latent heat and water for reuse and discharged into distilled water storage tank 23. The salts from the salt solution 20 are concentrated and/or precipitated and collected from the heat exchanger crystallization unit 6 via pipe 22. The cationic components of the collected

salts are principally ammonium, calcium, potassium, magnesium, and sodium. The anionic components of the salts are principally sulfate, carbonate and nitrate. The actual composition of the salts will depend on the initial composition of the ash to be treated, the amount of ammonium hydroxide added, and on the composition of the exhaust gas to be scrubbed.

Distilled water for use in the system is stored in a distilled water storage tank 23 from which is may be pumped as needed to other locations by pumps 24 and drained when desired to drain 25. Distilled water from the heat exchanger crystallization unit 6 is fed to the tank 23 through line 21. The salt solution 20 from settling tank 17 is pumped by pump 26 through pipe 38 to the heat exchanger crystallization unit 6 and through coils 40 as shown at the upper left-hand side of the drawing of Fig. 2. There it is concentrated, yielding precipitated salts or a concentrated salt solution in pipe 22, the vapor being either released to atmosphere or fed through pipe 21 back to the distilled water storage tank 23. Solids from the bottom of settling tank 17 may be withdrawn by pump 19 and fed to dilution tank 28 where they are diluted by water from tank 23 and stirred by stirrer 37 and thence removed by pump 29 to a second settling tank 30 from which the settled solids 31 are pumped by pump 32. Where dissolved solids are not necessarily separated from the settled solids, the dilution and second settling step may be omitted. In the case of use of the invention in a cement plant, these settled solids are usable as raw feed for the kiln; in the case of a boiler installation, wherein ash has been used as the neutralizing agent, the solids, which are now benign, may be used for raw feed for a cement plant, if the composition is suitable, may be used as raw material for production of aggregate, or may go to waste removal facilities.

Tank 30 at this point also contains solution 33, from which solids 31 have settled. Solution 33 may be pumped by pump 34 either to the primary mixing tank 35 (shown in Fig. 2 but not shown in Fig. 1) or directly to treatment tank 3 as part of the process water supply. In the primary mixing tank 35, it may be added to the cement dust or ash along with water to produce a slurry mixed by stirrer 36 of the desired consistency which is then fed to pump 42 through pipe 1 and thence through line 2 into treatment tank 3 to be reacted with the exhaust gas stream entering through pipe 11 and exiting, after neutralization, through outlet 14 to the stack. As previously described, the slurry and precipitating solids are continuously agitated in tank 3 by being recirculated by pumps 13. As can be seen from the schematic, additional water can be added to the slurry in tank 3 from source 4 as well a from other parts of the system through the piping shown. The exhaust gas stream is preferably reacted with the basic solution in the slurry (produced by mixing ammonium hydroxide, cement dust or ash with water) by being bubbled through it by release from pipes 12 in the bottom part of tank 3.

Ash derived from biomass burning systems may contain unburned carbon which, in come situations, will float in water. The process illustrated can be modified, if desired, to allow removal of the carbon. Water 20 carrying unburned carbon may be pumped from the surface of the settling tank 17 to be filtered or otherwise conventionally treated by means not shown to remove the carbon and is then returned to the process.

If the solution 20 containing dissolved alkali and alkaline earth metal salts has been fouled with unwanted particulate matter it may delivered to a filtration unit (not shown) or otherwise cleansed before being returned to the heat exchanger 6.

The heat exchanger 6 is a dual purpose heat exchanger-crystallization unit of a known type which will

extract heat from the exhaust gas and use that heat, including latent heat derived from condensation of the exhaust gas moisture, to evaporate water from salt solution 20. The salts crystallized from salt solution 20 are then recovered as solid material.

In some embodiments, some of the heat of solution released by dissolving ammonia gas in water is transferred by conventional means to the used scrubbing solution to concentrate the solution. Exhaust gas passing through the scrubbing solution in the treatment tank 3 is cleansed of a significant portion of the compounds of the halogens and oxides of sulfur, nitrogen and halogens by forming salts of these components.

The Apparatus

The whole system is created from well known parts combined by standard methods. For example, typically the treatment tank 3 may have a volume of one million gallons (3,800,000 liters) and be provided with gas distribution and stirring means; the settling tank 17 may have a volume of one hundred thousand gallons (380,000 liters) , both being constructed from stainless steel, or other suitable materials, such as rubber, which can tolerate highly alkaline or acidic solutions. Example

Exhaust gas from, e.g. a boiler, may be drawn or fed through duct 7 to heat exchanger 6 at a rate of 200,000 cubic feet (6000 m ³ ) per minute by fan or compressor 9. The exhaust gas is variable in composition, but may contain roughly 10% water, 15% carbon dioxide, 65% nitrogen, 10% oxygen and 500 to 1000 ppm nitrogen oxides and 100 to 1000 ppm sulfur dioxide. In heat exchanger 6 the exhaust gas is cooled and water is condensed, resulting in a decrease in flow volume. The exhaust gas is then fed through pipe 11 to distribution pipes 12 and allowed to react with slurry 5 where the halogens and the oxides of sulfur, nitrogen, carbon and halogens are reacted.

Scrubbing solution may be introduced to treatment tank 3, for example, at a rate of about 0.001 to 0.01 pounds per cubic foot of exhaust gas. Recirculation through pipes 13 may be at a rate of about 0.1 to 0.5 pounds per cubic foot of exhaust gas, or for use of ammonium hydroxide, a rate of about 0.7 to 1.0 pounds of ammonium hydroxide per pound of sulfur dioxide present in the exhaust gas .

After reaction with the exhaust gas, the scrubbing solution is pumped at a rate of 100 to 200 gallons per minute (760 liters) to settling tank 17. In this tank the solids settle to form a slurry 18 of

approximately 35 to 60% water and approximately 40 to 65% solids, beneath a solution 20 of water and soluble salts dissolved during treatment. This solution is pumped by pump 26 to heat exchanger 6 at approximately 50 gallons (190 liters) per minute to provide cooling for the exhaust gas and to evaporate the water therefrom to produce the by-product salts. Any floating carbon can be removed by conventional methods. The by-product salts are removed via pipe 22. The by-product salts will include other salts with cationic components such as ammonium, potassium, calcium, sodium and magnesium and anionic components including nitrate, sulfate and chloride. A portion of the nitrate should oxidize the sulfite to sulfate.