CN1013642B - Method of scrubbing pollutants from exhaust gas streams - Google Patents

Abstract

A method of scrubbing a boiler exhaust gas stream containing pollutants while producing useful or health-friendly products, the method comprising contacting the gas stream with a solution of soluble components of ash dissolved in water, the ash containing one or more solid alkali and alkaline earth salts, to form a basic (pH greater than 7) solution. The contaminants react with water to form acids. The acid reacts with the alkali and alkaline earth metal oxides, hydroxides, and carbonates from the ash to produce a solution containing alkali and alkaline earth metal salts, anionic salt components, and halogen compounds.

Description

The present invention relates to a method and a device for scrubbing pollutants in an exhaust gas stream, by means of which substances reacted with the exhaust gas stream are converted into substances which are at least not harmful to health and in most cases are chemically converted to Transformation of matter into useful products.

Scrubbing of exhaust gas pollutants is usually very expensive, and it can bring obvious improvements to the environment, but it can bring little or no economic benefit unless different wastes can be converted into useful products through scrubbing. The initial investment of equipment is very high. Furthermore, substances used for washing, such as oxides, carbonates, hydroxides of alkali metals and/or alkaline earth metals, are continuously consumed. Furthermore, the expense required to dispose of the reaction product resulting from the reaction of the scrubbing material containing off-gas also adds to the cost, especially when the product contains toxic components.

There is an increasing number of boiler installations fueled by biological materials (hereafter collectively referred to as "biomas"), such as wood, peat or crop residues, which produce little or no sulfur oxides and therefore do not Exhaust gas components need to be scrubbed. On the other hand, the ash produced during these operations contains large amounts of salts of alkali metals or alkaline earth metals, usually in the form of oxides, if they are moist and/or react with carbon dioxide, in the form of Hydroxides or carbonates, or may also occur as hydrated salts of these substances.

The present inventors have found that ash from boiler plants with a large proportion of oxides, hydroxides and/or carbonates of alkali metals and alkaline earth metals can be used in the process of scrubbing exhaust gases in place of the above-mentioned commonly used substances, In this way, various waste materials are utilized instead of expensive substances that have to be purchased.

Still other substances can be used for this purpose. When dissolving industrial or municipal waste, incinerator ashes or by-products in alkaline (high pH) water and recycling these waste or by-products Certain economic benefits can be obtained when potassium, sodium or other soluble salts contained in the product are removed.

In the following description and claims, the ash, municipal or industrial waste, or other by-products converted from burning biomass, which may be used in the processes described in this description and claims, are grouped under one general term, namely "Ash".

Unfortunately, the insoluble portion of the reacted ash usually still has to be disposed of as waste. For example, in most boiler applications, these insoluble ash will find no other use. However, the waste will no longer be a caustic material and can be used as non-healthy general fill in most cases and can also be used as landfill cover material in some pit fills. These wastes can also be used as raw materials for cement kilns if the composition of the residues and transportation costs allow. In some cases now, the insoluble part of the washing material can be used to produce calcium sulfate or gypsum, or as a mineral filler.

When washing is required, if the ash-generating equipment itself does not have a boiler unit that produces waste gas with high sulfur content, the ash can be transported to other boiler units that have this problem, or to a unit that uses fuel with high sulfur content, In this way, economic benefits can be obtained. In addition, since the ash converted from biological materials generally contains salts of potassium and other alkali metals or alkaline earth metals, and these salts can be recovered using exhaust gas heat or other waste heat sources, the resulting alkali metal or alkaline earth metal Salts can be valuable by-products of the production process.

The present invention will provide a new method for scrubbing boiler exhaust streams containing pollutants, including one or more acidic oxides of sulfur, nitrogen and carbon, and any one of the halogen compounds, while scrubbing produces useful or products not detrimental to health; the method consists of mixing with water ash containing one or more solid salts of alkali metals and alkaline earth metals to produce an alkaline solution containing insoluble solids in aqueous solution (pH greater than 7) slurry; the waste gas stream is then contacted with this slurry so that the pollutants therein react with water to form an acid which in turn reacts with any oxides, hydroxides and Carbonates react to form a solution containing one or more of the following substances, namely alkali metal Generic and alkaline earth metal salts, mainly including cationic alkaline earth metal components calcium and magnesium, and alkali metal components namely potassium and sodium compounds, and anionic salt components namely carbonates, sulfates, sulfites, nitrates and Nitrite, as well as halogen compounds, also form alkaline earth metal and alkali metal salt precipitates with any undissolved components in the ash; finally after the waste gas stream contacts the slurry, as a washed Exhaust gas is discharged.

The presently preferred method comprises one or more of the following steps: contacting the waste gas stream with the slurry, i.e. passing the waste gas stream through the slurry; mixing the solution of the alkali metal and alkaline earth metal salts with the Separation of the precipitate and insoluble components; the solution of alkali metal and alkaline earth metal salts and the precipitate and insoluble components are sent to a separation system, in which the salt solution is separated from the precipitate and insoluble components Phase separation; pass the separated salt solution through a heat exchanger to extract heat from the exhaust gas stream before it contacts the slurry; cool the exhaust gas stream to dehumidify it before the contact process; use the salt solution to dehumidify the exhaust gas Cooling and dehumidification; the heat from the exhaust stream is used to remove water from the brine. and obtain said portion of the heat from one or more of the following sources: (A) hot exhaust of said gas stream, (B) latent heat of vaporization of water contained in said exhaust gas stream, (C) ash and water between the heat of hydration reaction, and (D) compressing the gas prior to the contacting process.

Further objects, features and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments with reference to the accompanying drawings.

The single figure schematically shows a device for carrying out the invention.

The slurry consists of ash and water from a gray water mixing tank (not shown), the slurry is pumped through line 10 into treatment tank 12, and water is introduced through inlet 14 from a suitable water source (not shown) and mixed with the slurry to produce a dilute slurry 16. Ash is transported here by car from ash sources such as power plants that burn biomass or incinerators that burn waste. Exhaust gases from cement kilns, incinerators, or boilers (not shown) contain one or more of the following substances: Sulfur oxides, nitrogen oxides, carbon oxides and/or halogen compounds and their oxides, the exhaust gas enters the heat exchanger 22 through the inlet 18, and is discharged from the heat exchanger after being cooled. The condensed waste gas moisture is collected in the heat exchanger 22 and sent to the treatment tank 12 through the pipeline 44 . The waste is then sent via line 23 to compressor 20 and via line 24 to distribution pipe 26 at the bottom of treatment tank 12 . In order to prevent solids from depositing at the bottom of the treatment tank 12, the slurry may be agitated or recirculated by suitable means such as a recirculation pump 27.

The waste gas bubbles through the ash and water slurry 16 and exits the top of the tank as scrubbed waste gas 28 . Slurry 16 is pumped as a mixture of treated solids, water and dissolved matter by pump 32 via line 30 to settling tank 34 where settled solids 36 are pumped out of the tank by pump 38 and filled with dissolved salts The water 37 is pumped to the heat exchanger 22 in order to use it to cool the incoming exhaust gas. The water in the brine solution 37 is evaporated into steam and released to the atmosphere via line 40, or the water is evaporated and then condensed into a liquid to capture reusable latent heat. The salts in the salt solution 37 are concentrated and/or precipitated, and flow out of the heat exchanger through the pipeline 42, and then collected. The cationic components of the collected salts mainly include calcium, potassium, magnesium and sodium. The anionic components of these salts mainly include sulfate, carbonate and nitrate. The actual composition of these salts will depend on the initial composition of the ash to be treated and the composition of the exhaust gas.

In some cases, ash from biomass combustion systems may contain unburned carbon that will float on water. The illustrated process can be modified, if desired, such as to allow removal of carbon. Water 37 containing unburned carbon is pumped from the surface of the sedimentation tank for filtration, or carbon removal, and then returned to the process. If necessary, the solution containing soluble alkali metal and alkaline earth metal salts can be drained through lines (not shown) and then filtered or cleaned of particulate matter in a particulate removal unit. This solution is then sent to heat exchanger 22 .

The heat exchanger 22 is a known dual-purpose heat exchange-crystallization device that extracts heat from the exhaust gas and uses this heat to evaporate water, the heat including the latent heat obtained when condensing the water content of the exhaust gas.

device

The entire system is constructed from known components assembled by standard methods. For example, a typical treatment tank may have a volume of one million gallons (3,800,000 liters) with gas distributors and agitation devices; a sedimentation tank may have a volume of one hundred thousand gallons (380,000 liters). Both the treatment tank and the sedimentation tank can be made of stainless steel or other suitable materials (such as rubber). These materials have high resistance to acid and alkaline solutions.

working principle

The basic working principle of the present invention is to remix and react the two wastes generated during the combustion process to neutralize each other. One is the exhaust stream, which includes gases and gaseous oxides that produce an acidic solution in water, and the other is particulate matter, which is ash from biomass, industrial or municipal waste combustion facilities, that produces an alkaline solution in water .

The above two wastes react after being partially dissolved in water and neutralize each other. For ash, the process reacts or removes caustic components. This makes the ash a neutral solid suitable for disposal as non-hazardous waste. At the same time, most of the halogen compounds, sulfur oxides, nitrogen oxides and halogen oxides are removed in the form of salt by the waste gas passing through the slurry in the treatment tank.

example

Exhaust gas from, for example, a boiler may be passed through line 18 by means of compressor 20 to heat exchanger 22 at a rate of 200,000 cubic feet per minute (6000 ^m3 ). The composition of the exhaust gas is variable but may contain approximately 10% water, 15% carbon dioxide, 65% nitrogen, 10% oxygen and 500 to 1000 PPM nitrogen oxides and 100 to 1000 PPM sulfur dioxide. The exhaust gas is cooled in the heat exchanger 22 and water condenses, which results in a reduction in flow. The exhaust gas is then drawn by compressor 20 through line 23 and sent through line 24 to distribution pipe 26 where it reacts with slurry 16 . When the exhaust gas passes through the slurry, the halogen sulfur oxides, nitrogen oxides, carbon oxides and halogen oxides react chemically.

Ash can be introduced into treatment tank 12 at a flow rate of 8 to 12 tons (7200 to 10,800 kg) dry weight per hour. Water is added to form a dilute slurry that contains up to 95% water. The water content of the slurry is determined by the initial concentration of alkali metal and alkaline earth metal salts or other metal salts in the ash, and also takes into account the desired degree of removal of salts in the residue.

After reacting with the flue gases, the treated ash slurry is pumped to settling tank 34 at a rate of approximately 200 gallons (760 liters) per minute. In this pond, solids settle below the solution of soluble salts and water that dissolve during treatment, forming a slurry of approximately 35% water and 65% solids. The aqueous solution is pumped through outlet 37 to heat exchanger 22 at a flow rate of approximately 200 gallons (760 liters) per minute to serve to cool the exhaust gases and evaporate the water to produce by-product salts. Any floating carbon can be removed as previously described. From the by-product salt withdrawn through line 42, approximately 5 to 20 tons (4500 to 18,000 kg) per day can be produced. By-product salts include potassium sulfate, calcium carbonate, salts containing potassium, calcium, magnesium and sodium, cationic components, and salts containing carbonate, sulfate and nitrate anionic components. Part of the nitrate oxidizes the sulfite to sulfate.

The best method and apparatus for carrying out the present invention are disclosed and explained herein, but nevertheless, it should be understood that this is only for illustration and not limitation, and the protection scope of the present invention is only limited by the correct interpretation of the claims.

Claims (10)

1. A method of treating an exhaust gas stream containing pollutants comprising at least one of halogens, acidic halogen compounds, and acidic oxides of sulfur, nitrogen, and carbon, and the temperature of the gas stream is higher than that of the gas The dew point of the dew point, after treatment to generate scrubbing exhaust gas and useful or non-healthy by-products, the method includes: providing an alkaline, aqueous slurry of ash containing oxides, hydroxides or carbonates of alkali and alkaline earth metals; contacting the exhaust gas stream with the slurry by scrubbing the exhaust gas stream and reacting pollutants in the exhaust gas with water in the slurry to form an acid; The acid described herein reacts with any alkali metal and alkaline earth metal oxides, hydroxides, carbonates in the slurry to form a solution containing soluble alkali metal salts and to form any insoluble alkali metal and alkaline earth metal Precipitation of salts comprising at least one of calcium, magnesium, potassium, sodium halides, carbonates, sulfates, sulfites, nitrates, nitrites in said slurry; recovering the precipitate from the solution; Evaporating said solution from which the precipitate has been removed; recovering any soluble alkali metal and alkaline earth metal salts present in the solution as a solid; The scrubbed off-gas stream is discharged. 2. The method according to claim 1, wherein potassium and sodium are the main reactants in said ash, and these reactants combine with sulfur-containing compounds in the flue gas to form a solution containing alkali metal sulfates, and from the flue gas The scrubbed carbon dioxide forms a precipitate containing alkaline earth metal carbonates. 3. The method of claim 1 wherein said acid is reacted to neutralize said oxides, hydroxides and carbonates. 4. The method of claim 1 wherein said soluble alkali metal and alkaline earth metal salts are recovered by crystallization. 5. A method according to claim 1, wherein water vapor produced by evaporation of said solution is condensed into distilled water and returned as required for use in said method. 6. The method according to claim 1, wherein the heat required for evaporation of said solution is partially derived from: the waste gas stream; the latent heat of moisture in said gas stream; the heat of hydration of ash and water; the gas stream and said slurry The heat of compression that compresses the gas stream prior to the reaction. 7. The method of claim 1 wherein said alkaline slurry is contacted with said waste gas stream by bubbling said gas stream through said slurry. 8. The method of claim 1, further comprising cooling said exhaust gas stream by removing and recovering heat from said exhaust gas stream prior to contacting said gas stream with said slurry; It also includes utilizing only said recovered heat to evaporate said solution from which the precipitate has been removed. 9. The method of claim 8 wherein said dissolved solids are recovered by crystallization. 10. The method of claim 8, further comprising evaporating the solution containing dissolved solids and recondensing the evaporated liquid.

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