ENHANCEMENT OF AMMONIA CONTAINING FLY ASH AND COLLECTION OF AMMONIA BYPRODUCT
Background of the Invention This invention relates to the enhancement of ammonia contaminated fly ash product by removal of a substantial portion of the ammonia content and the formation and collection of a solid ammonia-containing byproduct useful in soil amendment, plant fertilization and the like. This invention also relates to the removal of ammonia from water coming in contact with ammonia-contaminated fly ash and the formation and collection of a solid ammonia containing byproduct from such water.
This invention preferably employs the ultrasonic conditioning and wet scrubbing concepts and processes of my U.S. Patent No. 5,840,179 issued November 24, 1999, the content of which is specifically incorporated herein by reference (i.e. Patent No. ' 179). This invention also applies to cases where ammonia contaminated ash is placed in ponds or landfills.
Ammonia, in the form of a wet spray or the vaporization of ammonium hydroxide, to produce gaseous NH3 is commonly injected in flue gases of plants burning carbonaceous products, such as pulverized coal, for the conditioning of the ash for electrostatic precipitation, and for NOx removal using selective catalytic reduction and selective non-catalytic reduction processes. In these processes, ammonia is deposited on the fly ash in the form of ammonium sulfate ((NH4)2 SO4) and ammonium bisulfate (NH4 HSO4). Ammonia concentrations in fly ash in excess of about 100 milligrams per kilogram of ash can negatively affect fly ash utilization as an admixture to concrete, due to the resulting off gassing of ammonia during the concrete mixing and setting process. Therefore, otherwise useful fly ash with high ammonia content has low or no economic value, and in some instances has a negative value or is a liability, as it must be safely disposed of such as in a landfill or the like. Furthermore, disposal of wetted ammonia contaminated fly ash in landfills or ponds results in the desorption of ammonia from fly ash causing ammonia contamination of landfill leachate/runoff water or pond sluice water. Such contamination of power plant waters presents potential environmental damage to
-2- ground and surface waters.
Ultrasonic conditioning followed by froth flotation has been successfully used to remove carbon from high carbon fly ash, and to provide an enhanced fly ash pozzolan including microspheres, as described in Patent No. ' 179. Ammonium sulfate and ammonium bisulfate decompose on heating in the range of temperatures of about 300° to 500 °C. Therefore, procedures have been proposed by which the ammonia content of fly ash is released by such heating and the ammonia has been collected as a gas and reused in the combustion scrubbing system. Also, these ammonium salts are highly soluble in water under proper conditions.
The froth flotation processes as set out in Patent No. ' 179 have the ability to solublize the ammonia salts in water so as to remove the ammonia from the fly ash. The ammonium compounds disassociate to form ammonium (NH4 +) and sulfate (SO4 ") ions. Some of the ammonium (NH4 +) converts to dissolved ammonia gas (NH3) depending largely on pH and, to a lesser extent, temperature and other solubility factors. The following is a chart of the relative distribution of NH4 + and NH3 dissolved gas in 20 °C water, according to the pH of the solution:
pH NH4 + NH3 less than 7 100% 0%
8 94% 6%
9 84% 16%
10 28% 72%
11 6% 94% more than 12 0% 100%
Under high pH conditions, such as in a setting concrete mix, the ammonium equilibrium shifts toward dissolved ammonia gas. The extent of dissolved ammonia evolution from water into the air will also depend somewhat on ammonia partial pressure in air, temperature, and turbulence.
As previously mentioned, ammonium is washed out of fly ash during wet processing for carbon flotation or sluicing for disposal in ponds. The resulting
ammonium build up in water loops must be purged out. Ammonia removal can be accomplished by raising the water pH and stripping the ammoma gas. Ammonia gas is then collected and scrubbed in an acid solution forming ammonium sulfate. This is a well-established technology used in industrial wastewater treatment. This process requires caustic to raise the pH and sulfuric acid or the like to scrub ammonia gas and readjust the pH level.
Except in cases where the ammonia is scavenged and recycled at the fly ash producing plant, the ammonia fraction is not commercially utilized. In addition, in a conditioning and flotation process, as described in Patent No. ' 179, the dissolved ammonia salts in the water system, washed out of the fly ash during wet processing, begins to contaminate the water loop and must be purged out in order to secure a dry ash product having less than 100 ppm ammonia.
Summary of the Invention The invention provides a novel process and system by which the ammonia content of the water loop in a flotation process or sluice system or the like is reduced and controlled by an ammonium extraction process, and a useful low solubility ammonium product is formed having economic value, such as for use as a fertilizer component. This process has particular utility in the wet processing of fly ash where the soluble ammonium salt will necessarily and inherently wash out into the water loop, and contaminate the enhanced fly ash product and the water loop.
An ammoma contaminated or ammonium rich mixture is withdrawn from the flotation tank or fly ash pond and subject to an ammonia precipitation process so that the resulting low ammonia content water may be reintroduced to the system, preferably at the inlet to the ultrasonic conditioning that precedes flotation or recirculated for sluicing fly ash.
When the process includes ultrasonic conditioning the effectiveness of the process is enhanced as the conditioning fractures and breaks down the agglomerated fly ash components, and, at the same time, releases the particles into the water including the ammonia salts thereby resulting in a thorough stripping of the salt components of the slurry as it proceeds to the froth flotation stage. The process precipitates a relatively low soluble ammonia salt out of solution, in a crystalline
form, that can be utilized economically.
As previously noted, most ammonia salts are quite soluble, with the exception of magnesium ammonium phosphate hexahydrate, also known as struvite. Struvite ((Mg (NH4) PO4 • 6H2O) is a colorless "coffin-like" shaped crystal found in canned seafood and some kidney stones. Also, solids built-up in belt press filtrate lines at wastewater treatment plants are attributed to this low solubility compound.
Struvite is formed and precipitated in the presence of ammonium (NH4 +), magnesium (Mg+), and phosphate (PO4 ") ions under neutral to alkaline conditions. The stoichiometric reaction is as follows: NH4 + + 2H+ + PO4 "2 + Mg+2 + 2(OH+) + 4H20 → Mg (NH4) PO4 • 6H20
The compounds needed to complex ammonia and precipitate struvite are: phosphate anions and magnesium cations, such as sodium or potassium phosphate and magnesium chloride, or phosphoric acid and magnesium hydroxide. The formation and precipitation of this ammonium complex is best accomplished within the pH range of 9 to 12. The theoretical minimum solubility of magnesium ammonium phosphate is achieved at a pH of 10.7.
The resulting crystalline struvite may be separated by centrifuge, or by settling and decanting, or by filtration, then dried, and utilized as a slow-release fertilizer additive or soil amendment product. An object of the invention to provide a wet ultrasonic conditioning and flotation process for enhancing a high ammonia content fly ash utilizing ultrasonic conditioning to ensure a maximum removal of soluble ammonium salts in the fly ash followed by precipitation and removal of at least a portion of the liquid content to prevent an ammonia build up in the system that would otherwise recontaminate the fly ash with an unacceptably large amount of ammonia salt.
A further object of the invention includes a process and system, as outlined above, in which ammonia is removed by treating the water containing solublized ammonium salts with magnesium and phosphate ions, preferably under neutral to alkaline conditions, to cause the precipitation of ammonia in the form of struvite crystals that may be physically removed from the system or from the water loop.
The invention further comprises the process of treating a sluice water
-5- loop used to convey the high ammonia ash from the electrostatic precipitator hoppers to the disposal ponds and other power plant wasters coming in contact with high ammonia fly ash, in which the ammonia is precipitated from the solution and removed. The process removes over 90% of the soluble ammonia using phosphate and magnesium reagents such as Na3PO4 and MgCl, added at dosages equal to stoichiometry plus about 20%. Ammonia is precipitated as magnesium ammonium phosphate hexahydrate.
The recovered precipitate has plant nutrient (fertilizer) value and contain nitrogen and up to 10% phosphorous as P on a dry solids basis. Other nutrients present may include magnesium, sulfur, potassium, calcium, and iron. The type of nutrients and their content levels would depend on the reagents used, type of coal burned, the rate of ammonia injection, and water chemistry including pH These and other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.
Brief Description of The Drawings Fig. 1 of the drawing is a block diagram showing one embodiment of the process and system; and
Fig. 2 is a block diagram of a nitrous oxide (NOJ removal system in a coal fired power plant with an ash sluicing to a holding tank or pond, and using an ammonia precipitation process according to this invention.
Detailed Description of Preferred Embodiments
Referring to Fig. 1 of the drawings, fly ash having an ammonia content is applied to an ultrasonic conditioner 10 of the kind disclosed and described in Patent No. ' 179. The fly ash water slurry is electrosonically conditioned, as described in the patent under the influence of a conditioning agent 13 and applied to the flotation cell 15. A carbon rich fraction 16 is taken from the cell 15 and applied to a second stage of ultrasonic conditioning 18 and to further flotation 20 in which an enhanced fly ash product 24 is formed as a bottom flow and a carbon rich fraction 25
-6- is formed as a top flow.
A portion of the liquid content of the first flotation cell 15 is applied to an ammonia removal module 30. The liquid within the cell 15 will quickly become highly elevated with ammoma salts in solution. In the module, the removed ammonium rich solution is contacted by phosphate and magnesium ions such as by adding sodium monophosphate and magnesium chloride or adding phosphoric acid and magnesium hydroxide to the solution, and pH therein is maintained at neutral or slightly base to produce optimum precipitation of the resulting magnesium ammonium phosphate hexahydrate, also known as struvite. Separation of the struvite crystals within the module 30 may be by settling, filtration, centrifuge, or other well known separation apparatus and techniques.
The resulting low ammonia water is preferably reapplied such as on a line 35 as part of a water loop to the ultrasonic conditioner 10, while the ammonia byproduct struvite is extracted and dried, for subsequent utilization.
The system as shown in Fig. 1, may be used on a batch basis or intermittent basis, when the ammonium content begins to approach a predetermined maximum in the flotation cell, or may be operated continuously, for the extraction of a low soluble ammonia byproduct. An example of ammonia removed and production of fertilizer from fly ash water via precipitation using phosphate and magnesium: water having an ammonia content equal to 756 ppm as N was treated using 12 grams per liter of sodium phosphate and 6 grams per liter of magnesium chloride. The precipitate started to form almost immediately upon magnesium chloride addition. The precipitate was allowed to form and concentrate to about 30% of the original volume for about one hour prior to decanting. The ammonia concentration in the treated water or filtrate was reduced to 64 ppm as N, a 91.5% reduction. The concentrate was filtered and oven dried to yield about 14 grams of solids consisting of magnesium ammonium phosphate hexahydrate and other insoluble compounds of phosphates, sulfates, and silicates.
Referring to Fig. 2 of the drawings, ammonia is injected in a coal fired boiler flue gas stream that contains fly ash and NOx. NOx is reduced by ammonia in
-7- the SCR/SNCR modules 40 to nitrogen gas. Some residual ammoma is deposited on fly ash particles, which is collected in the electrostatic precipitator 42 or bag house. The ammonia-contaminated fly ash is sluiced using water 43 to a settling medium such as a pond 45 as shown or reservoir or settling vessel for removal and disposal. Ammonium compounds dissolve in the sluice water making an ammonium rich solution 47. The ammonium rich solution 47 is contacted in an ammonia removal module 50 by phosphate and magnesium ions and pH adjustment if needed to precipitate struvite in the manner described above. Separation of struvite crystals within the module may be by settling, filtration, centrifuge, or other well-known separation apparatus and techniques. The resulting low ammonia water 52 is preferably reused for sluicing ash, while the ammonia byproduct struvite 60 is extracted and dried for subsequent utilization.
It is therefore seen that the invention provides a process for removing excess ammonia content from an effluent containing ammonia, particularly adapted for use in installations that utilize ammonium hydroxide to control nitrous oxide (NOx) emissions, and, at the same time, providing a useful by-product in the form of struvite crystals. It should further be noted that in such installations where a quality fly ash is obtained, such as by ultrasonic scrubbing to remove any excess carbon component, the resulting fly ash product is substantially free of ammonia contamination that would otherwise cause the fly ash to have low value or no value at all as an add mixture or aggregate in concrete mixes.