CN1131718C - Process for reducing NOx, SOx, chlorine and particulates while maintaining reliable operation of an electrostatic precipitator - Google Patents

Abstract

While maintaining the reliable function of the electrostatic precipitator, it can also effectively reduce SO _x , HCL and NO _x , and there will be no related problems and losses due to the use of ammonia. The process comprises the steps of: introducing a stabilized slurry containing a basic _SOx reducing compound and a nitrogenous composition effective for reducing _NOx (except ammonia, especially urea or urea hydrolyzate or components thereof) introducing into the exhaust gas and producing residual ammonia in the presence of the exhaust gas; and passing the ammonia-containing exhaust gas through the electrostatic precipitator.

Description

Method for Improving Collection Efficiency of Electrostatic Precipitators

The present invention relates to a method for effectively reducing the emission of various gaseous chemical pollutants and particles resulting from the combustion of organic fuels and wastes, while maintaining reliable operation of electrostatic precipitators while achieving reductions in chemical pollutants.

Combustion of carbonaceous fuels in domestic boilers and other power plants produces nitrogen oxides (NO _x ), sulfur oxides (SO _x ), and acidic chlorine (HCl), as well as solid particles, commonly known as fly ash.

One effective method of removing combustion exhaust particles is a device known as an "electrostatic precipitator". In an electrostatic precipitator, combustion exhaust gases pass between electrodes that generate corona discharges. This causes the particles to become charged, and when charged, the particles migrate towards and deposit on the collector. After exiting the electrostatic precipitator, the substantially particle-free exhaust gas can be discharged to the atmosphere.

Known electrostatic precipitator technologies are competent in _SOx reduction in some cases. For example, _SO3 is known to enhance the operation of electrostatic precipitators. However, _SOx reduction techniques can complicate _NOx reduction due to the need for stable slurries and different temperature windows for optimal operation.

One factor that has been found to be particularly important for the operation of electrostatic precipitators is the resistivity of the particles entrained in the exhaust gas. Particle resistivity must be high enough to facilitate collection. However, in many electrostatic precipitators, the resistivity of the particles is too high, on the order of about ¹⁰¹³ ohm-cm or higher. In this case, the particle collection efficiency can be significantly reduced. The reduction in collection efficiency can seriously compromise the pollution control gains provided by electrostatic precipitators. In general, the desired resistivity range is between about 10 ⁸ and about 10 ¹⁰ ohm-cm.

Certain agents are used in order to "condition" the particles in the exhaust gas to maintain their resistivity within a desired range. Including sulfuric acid, sulfur trioxide (SO ₃ ) and phosphoric acid, with sulfur trioxide being the best, these conditioners are the most versatile. In addition, another conditioner that is often used to improve the collection efficiency of electrostatic precipitators is ammonia, which has also been found to improve collection efficiency for a long time.

Unfortunately, ammonia is toxic and difficult to handle. In addition, the ammonia must be kept under high pressure or as a dilute aqueous solution ( _NH4OH ) adding to the expense. The use of ammonia during storage, handling and transportation creates serious problems that prevent its use as an electrostatic precipitator conditioner.

Alkaline agents (such as alkaline earth and alkali metal oxides, hydroxides, and carbonates) are known to reduce acidic chlorides and SO _x , however, these materials can further generate particles that are loaded into the exhaust gas and have a negative impact on static electricity. Dust collectors create difficulties. The low conductivity of metal oxides can lead to increased fly ash resistivity, making particle removal more difficult by electrostatic precipitators. As far as their SO _x reduction action is concerned, they also reduce the presence of SO ₃ , which in turn improves the function of the electrostatic precipitator.

For this reason, what is desired is a method suitable for the reduction of gaseous pollutants and solid particles of combustion exhaust gases through simple injection of chemicals and use of electrostatic precipitators - which has reliability and is not affected by the use of ammonia resulting in related problems and losses.

It is an object of the present invention to achieve effective _SOx reduction along with _NOx reduction without the problems associated with electrostatic precipitators removing particles from exhaust gases.

Another object of the present invention is to achieve effective reduction of _SOx and _NOx without the problems associated with electrostatic precipitator removal of particulates from exhaust gases and without the associated problems and losses associated with the use of ammonia.

Yet another object of the present invention is to achieve reliable particle removal by electrostatic precipitators while simultaneously achieving effective reductions in _SOx , HCl and _NOx without the problems and losses associated with the use of ammonia.

The present invention accomplishes these and other objects. The method provided by the present invention comprises the steps of: preparing a slurry, preferably stable, which comprises an alkaline SO _x reducing compound and, within a specified temperature range, effective for reducing NO _x and A nitrogen-containing composition (other than ammonia, especially urea or urea hydrolyzate or components thereof) that produces residual ammonia; at least one treatment zone having a temperature within said specified temperature range is selected; said slurry is introduced into said treatment zone to reduce NO _x , SO _x , and chlorides and generate ammonia; and pass the ammonia-containing exhaust gas through an electrostatic precipitator. Thereby, the reliable performance of the electrostatic precipitator can be maintained, and at the same time, the effective reduction of _SOx , HCl and _NOx can be achieved without the problems and losses related to the use of ammonia.

The invention will be better understood and its advantages will become more apparent from the following detailed description, especially when read with reference to the accompanying drawing, which is a schematic diagram of a boiler embodying the invention.

industrial application

Typical apparatus of the present invention is illustrated in the accompanying drawings, in which boiler 100 includes a flame zone 10 in which fuel 12 is combusted to form exhaust gas 14 . Exhaust gas 14 passes from flame zone 10 through exhaust gas duct 20 . The electrostatic precipitator 30 is arranged in the exhaust gas line 20 so that the exhaust gas 14 flows through the electrostatic precipitator at a suitable temperature, for example about 149°C (300°F).

A device 40 for introducing a stable slurry of a nitrogen-containing compound such as urea or urea hydrolyzate and a _SOx reducer such as Ca(OH) ₂ is placed on the exhaust gas line 20 so that the injection device 50 can be effective in _NOx reduction and ammonia The resulting exhaust gas 14 protrudes into the exhaust gas 14 at a temperature lower than that of the exhaust gas 14 . The ammonia stream is sent to electrostatic precipitator 30 which, in the presence of ammonia that has been produced, is capable of inducing precipitation of particles in the exhaust gas due to the selection of a treatment zone having a temperature range suitable for the purpose. Preferably, the temperature is below the optimum _NOx reduction temperature, for example below about 1010°C (1850°F). However, higher temperatures can also be used, depending on the chemicals used and the degree of dilution.

The purpose of the present invention is achieved by using the present invention which reduces or eliminates the necessity of transporting, storing and handling large quantities of ammonia or ammonia water. Even when the process of the present invention does not provide all the ammonia necessary to condition the exhaust gas to facilitate the function of the electrostatic precipitator, the reduction in the amount required still provides significant advantages in terms of both safety and cost.

The process of the present invention preferably employs the treatment in the form of a single aqueous slurry comprising a _NOx reducing agent of a nitrogen-containing composition other than ammonia and a suspension of a finely divided, preferably stable, basic _SOx reducing agent. The term "non-ammonia nitrogen-containing composition" is meant to comprise a plurality of components, at least one of which contains elemental nitrogen other than ammonia, although _NOx reducing agents can contain form of ammonia. The suspension of powdery alkaline SO _x reducing agent preferably contains at least one selected from the oxides, hydroxides and carbonates of alkali metals or alkaline earth metals (preferably magnesium or calcium hydroxides or carbonates). materials. Of particular interest for the respective _NOx and _SOx reducers are mixtures.

For slurry stabilization for simplified and efficient operation, suitable stabilizers must be used. A stabilizer system generally having a hydrophobic surfactant with an HLB of less than about 10 and a more hydrophilic surfactant with an HLB of at least about 10 is a preferred stabilizer system, wherein the HLB values of the hydrophobic and hydrophilic surfactants differ by at least About 3, preferably at least about 5. The preferred range of the weight ratio of hydrophobic dispersant to hydrophilic dispersant is from about 25:1 to about 1:1, for example from about 12:1 to about 2:1. Typically the concentration of the dispersant system used is from about 1 to about 20,000 parts per million (ppm), and preferably from about 250 to about 1000 ppm.

Preferred hydrophobic dispersants (HLB values below 8) are selected from the group consisting of ethoxylated phenol alcohols, block polymers of ethylene oxide and propylene oxide, petroleum sulfonates, organic phosphates, fatty acids such as Materials of oleic acid, castor oil, alkyl sulfonates, fatty acid alkanolamides and mixtures of any two or more thereof. Preferred hydrophilic dispersants are selected from alkanolamides, lauryl sulfates, dodecylbenzenesulfonates, amidesulfonates, phosphoric acid esters and mixtures of any two or more thereof. _NOx reducers can be an effective agent for the non-catalytic reduction of nitrogen oxides. The present method involves a selective, radical-mediated process, often referred to as selective non-catalytic reduction (SNCR).

Various nitrogen-containing compositions, in their pure and generally commercially available forms, will produce effective gas-phase _NOx reducers (eg amidozine-based, NH·) when introduced in aqueous solution. Among the prominent nitrogen-containing compositions are urea, precursors of urea, one or more hydrolysis products of urea (such as ammonium carbamate, ammonium carbonate, ammonium bicarbonate), other ammonium salts, and various urea complexes. Compounds, including urea hydrolysis products, which are composed of ammonium carbonate and ammonium bicarbonate in a unique structure complexed with ammonium carbamate, products of the reaction of urea with itself or other components, related components, or these compounds mixture. Also included among these compounds are ammonium carbonate, ammonium formate, ammonium citrate, ammonium acetate, ammonium oxalate, other ammonium salts (both organic and inorganic, but especially organic acids), ammonium hydroxide, various stable Amine, guanidine, guanidine carbonate, biguanide, miduride sulfate, melamine, dicyanamide, calcium cyanamide, biuret, 1,1' azodicarbonamide, methylol urea, methylol urea- Urea, dimethylurea, hexamethylenetetramine (HMTA), or a mixture of the above. Preferred _NOx reducing agents include urea, ammonium carbonate, ammonium bicarbonate, ammonium carbamate, ammonium hydroxide, or mixtures of any of these compounds in aqueous solution. A preferred mixture is urea hydrolyzate.

In terms of the effective temperature window of the _NOx -reductant, its utilization efficiency, etc., an enhancer may be used to improve the efficiency of the NOx _- reductant. Among the reinforcing agents that can be included with the treatment of the present invention are the materials described above when used in appropriate combinations, as well as other compositions such as those disclosed in USP 4,719,092, 4,844,878 and 4,877,591, incorporated herein by reference .

Various hydrolysis products of urea and a single urea hydrolysis product for nitrogen oxide reduction have been disclosed in the international patent application entitled "Proccss for the In-Line Hydrolysis of Urea" by Von Harpe and Pachaly , Publication No. WO 92/02291, filed August 1, 1991; and von Harpe and Pachaly, Lin, Diep, and Wegrzyn in International Patent Application entitled "Reduction of Nitrogen Oxides with Urea Hydrolysates," Publication No. WO 92/02450, filed August 1, 1991, the disclosures of each of which are incorporated herein by reference.

The products of the hydrolysis of urea or urea hydrolysis products as the nitrogen-containing composition in the treatment agent of the present invention will have significant advantages, because they can reduce nitrogen oxides in a relatively wide exhaust gas temperature range, and at the same time can make nitrous oxide (N ₂ O), another pollutant is minimized.

At least about 25% of the urea is required to be hydrolyzed in order to effectively supply sufficient urea hydrolyzate to reduce nitrogen oxides and condition the exhaust gas flow through the electrostatic precipitator. More preferably at least about 60% of the urea is hydrolyzed, most preferably at least about 80% of the urea is hydrolyzed.

The degree of hydrolysis can be determined by measuring the conductivity as well as the increase in specific gravity and alkalinity. A 10% urea solution has a conductivity of about 1.2 milliohms, so a fully hydrolyzed 10% urea solution has a conductivity of about 120 milliohms. For comparison, a 25% solution of urea has a conductivity of about 1.2 milliohms, whereby a 25% solution of the hydrolyzate has a conductivity of about 140 milliohms. The hydrolyzate will show higher conductivity and specific gravity.

The stable suspension of powdered alkaline _SOx reducer preferably contains at least one selected from the group consisting of alkali metal or earth metal oxides, hydroxides and carbonates (preferably magnesium or calcium hydroxides or carbonates) salt) material. A number of calcium and magnesium based sorbents for _SOx reduction are known including lime, calcium carbonate, magnesium carbonate, calcium hydroxide, magnesium hydroxide and mixtures thereof e.g. in the common ore form of limestone, dolomite and other calcium or magnesium carbonate forms, including shells, aragonite, calcite, chalk, marble, marl, and travertine, can be mined or manufactured. Calcium hydroxide (specified to include slaked lime) has now been found to form a synergistic combination with nitrogen-containing treating agents.

The sorbent preferably comprises at least about 10% calcium hydroxide, preferably at least about 40%, and most preferably all calcium hydroxide. The amount of calcium sorbent in the composition of the present invention may vary depending on the amount of _SO2 in the exhaust gas and the amount of nitrogen-containing treatment agent in the composition. Preferably, the calcium hydroxide is contained in the composition in a molar ratio of calcium to initial (i.e. pre-treatment level) SO of about 1: ₂ to about 4:1, most preferably about 1:1 to about 3:1 1. Generally, calcium hydroxide comprises at least about 10%, preferably between about 15% and about 50%, of the slurry of the present invention. When the slurry ingredients are considered dry, ie, without diluent, calcium hydroxide constitutes up to about 97%, more preferably from about 65% to about 97%, of the composition excluding diluent.

The nitrogen-containing treatment agent is preferably present in the form of a ratio of nitrogen in the treatment agent to the initial (ie, pre-treatment) amount of nitrogen oxides in the exhaust gas, which ratio can vary from about 0.5 to about 3.5. This ratio is called the "standard stoichiometric ratio" or "NSR". "Standard stoichiometric ratio" refers to the concentration of reducing groups such as _NHx groups ( _NHx groups, x is an integer, considered to be the part provided by the treatment agent, which is conducive to the series of reactions leading to the reduction of _NOx to _N2 ) vs. The ratio of the concentration of nitrogen oxides in the exhaust gas can be expressed by [NH _x ]/[NO _x ]. In other words, the molar ratio of treatment agent to _NOx concentration can be used instead of NSR when the chemistry of reduction is not well defined; the term NSR as used herein is also understood to mean the appropriate molar ratio.

Advantageously, the nitrogen-containing treating agent is included in an amount of from about 3% to about 35% by weight of the total composition excluding diluent (ie water) and from about 1% to about 15% of the final slurry. When adding urea as an aqueous solution, the solution water should not be included in the calculation of the weight of urea to be added.

Typically, the weight ratio of sorbent to nitrogen-containing agent of the slurry of the present invention is from about 1.5:1 to about 33:1, more preferably from about 2.7:1 to about 22:1. In addition to the nitrogenous agent and sorbent, the remainder of the composition includes water or other diluent sufficient to form a slurry. Typically, the slurry contains from about 20% to about 70% solids by weight, preferably from about 26% to about 50% solids by weight.

As an example, assume an exhaust gas volume of 2.83 x ¹⁰⁴ standard cubic meters per hour (1,000,000 standard cubic feet per hour), _SO2 concentration of 1000 parts per million (ppm) and _NOx concentration of 300ppm (as generally encountered), then a slurry of Ca(OH) ₂ and urea in water can be obtained with a sorbent to SO ₂ molar ratio of 2 (or 174.5 kg/h (384.4 lb/h)) and an NSR of 2 (21.2 kg /hour (46.75 lbs/hour)). This resulted in a sorbent to urea ratio of 8.22.

The slurry is then introduced into the exhaust gas under conditions effective to reduce the concentration of pollutants therein. Typically, the slurry is introduced into the exhaust as the exhaust temperature varies between about 815.6°C (1500°F) and about 1204.4°C (2200°F).

Preferably, the slurry is prepared by mixing calcium hydroxide slurry with a concentrated solution of a nitrogen-containing treatment agent in required proportions. If the slurry is not sent to the off-gas immediately after preparation, it may be required to contain suitable stabilizers, depending on the required resting time before introduction.

The slurry is then introduced, for example into the exhaust gas to be treated. In this way, uniform mixing and sufficient penetration of the slurry are ensured in view of effectiveness. The injection is designed to be carried out from the nozzles existing prior to the inlet, so as to achieve a uniform distribution of the slurry over the cross-section of each concentration of the exhaust gas.

Since the slurry needs to be evenly distributed throughout the required cross-sections of the exhaust gas to effectively reduce pollutants, the nozzles must be selected to be effective for this purpose. A skilled tradesman will recognize that certain conventional nozzles will be effective for this purpose. Suitable special nozzles for injecting aqueous slurries include Turbotak ^(R) nozzles. Other suitable nozzles are described in International Patent Application entitled "Method and apparatus for reducing the concentration of pollutants in combustion gases", application number PCT/EP91/00952, in the name of Chawla, by Bermann and Pachaly on May 21, 1991 Application and West German patent DE-2627880C2 published on November 11, 1982, the content of each patent disclosure is incorporated herein as a reference.

In some cases, the amount of water or other diluent in the slurry may have a "weakening" effect on the sorbent and nitrogen-containing formulation. That is, the time it takes to evaporate the diluent can delay the action of the treatment chemicals until the treatment is performed at a lower temperature than initially desired. This phenomenon may (though not necessarily) occur when the solids concentration of the slurry is below about 38%, more preferably below about 28% by weight.

Sufficient ammonia, at least in part, can be ensured by varying the exhaust gas temperature at which the nitrogen-containing treatment agent is introduced (eg, by adjusting where the introduction takes place), the specific composition, and the ratio of introduction of the treatment agent. Adjust these parameters to achieve the desired amount of ammonia in the exhaust. This can be accomplished using the methods and techniques described in US 4,780,289, US 4,830,839 and/or US 5,017,347, the disclosures of which are incorporated herein by reference, so that nitrogen oxides can be used to reduce the impact on treatment regimes. The relationship curve of exhaust gas temperature and the plateau of the curve to control the presence of secondary pollutants, such as ammonia in the reduction of _NOx .

The so-called "relationship curve of nitrogen oxide reduction versus exhaust gas temperature" means that when the treatment system is implemented, the treatment agent is introduced into the exhaust gas within the range of exhaust gas temperature and the reduction value of nitrogen oxide at each introduction temperature is measured (usually by the initial amount expressed as a percentage of ) from the curve of the resulting data points. "Plateau of the curve" refers to the region of the curve of reduction of nitrogen oxides versus exhaust gas temperature where the reduction of _NOx is substantially maximum in the range of temperatures, preferably at least two data values (of course skilled The skilled person will recognize that the plateau of the curve is not necessarily flat due to "data smearing" and other effects from actual data). "Treatment system" means the introduction (such as injection) of the treatment agent into the exhaust gas and the conditions for the introduction of the treatment agent, such as the composition of the treatment agent (the opinion of the composition is the composition of the chemical formula of the treatment agent), the dilution of the treatment agent (the dilution means When the treatment agent used is a solution, the concentration of the treatment agent components), the presence of the relevant treatment agent components (which means the relative weight ratio or share of the components forming the chemical formula for preparing the treatment agent), etc.

NH ₃ and NH ₄ -compounds can exist in two forms in the exhaust gas - on the fly ash and in the gas phase. NH ₄ -compounds on fly ash can be used as a pretreatment of fly ash to improve electrical conductivity and agglomeration or cohesion.

A selective catalytic reduction (SCR) step can be used to achieve further reduction of _NOx using ammonia, provided that the ammonia is produced by excess nitrogen-containing compounds required to ensure efficient operation of the electrostatic precipitator. The catalyst used is one that reduces the concentration of nitrogen oxides in the exhaust gas in the presence of ammonia. Such catalysts include, for example, activated carbon, charcoal or coke, barium oxide, tungsten oxide, titanium oxide, iron oxide, copper oxide, manganese oxide, chromium oxide, noble metals such as platinum group metals like platinum, palladium, rhodium and iridium, or mixtures thereof . Other catalyst materials commonly used in the art and known to those of ordinary skill may also be used. These catalyst materials are usually immobilized on supports such as ceramic substances or zeolites, although other known supports may also be used. Preferably, the ammonia-containing exhaust gas is passed over the catalyst when the exhaust gas is at a temperature between about 110°C (230°F) and about 510°C (950°F).

Furthermore, the introduction of nitrogen-containing compounds is not constrained by the SNCR or SCR pathways. In this case, urea, hydrolyzed urea or other compounds according to the invention (preferably in the form of a solution, especially an aqueous solution) are introduced into the exhaust gas at the exhaust gas temperature, where the compound is readily converted to ammonia in order to regulate the electrostatic precipitator, at substantially Under the same conditions as above, except without SNCR or SCR the reduction of _NOx occurs before the electrostatic precipitator.

Advantageously, the amount of nitrogen-containing treatment agent introduced to the exhaust gas is sufficient to provide the volumetric amount of ammonia produced in the exhaust gas (whether introduced after SNCR, SCR, or the compound without SNCR or SCR), which is sufficient The resistivity of the particles is reduced to below about 10 ¹³ ohm-cm, more preferably between 10 ⁸ and about 10 ¹⁰ ohm-cm. Generally, the amount of ammonia should be at least about 3.0 parts per million (ppm) by volume. Preferably, the treating agent is introduced into the exhaust gas to achieve an ammonia concentration of from about 3.0 ppm to about 100 ppm, more preferably from about 10.0 ppm to about 50.0 ppm.

Preferably when the exhaust gas temperature is between about 21°C (70°F) and about 732°C (1350°F), more preferably between about 121°C (250°F) and about 454.4°C (850°F), the The exhaust gas flows through the electrostatic precipitator. In some experimental electrostatic precipitators, operating temperatures can reach as high as about 815.6°C (1500°F). In this way, the presence of ammonia in the exhaust gas by the introduction of the treatment agent most effectively promotes the precipitation of the particles.

The above description is used to teach those skilled in the art how to implement the present invention, rather than attempting to detail all the obvious improvements and changes of the present invention. When reading the description, the present invention will become more obvious to those skilled in the art . However, all such obvious modifications and alterations are intended to be included within the scope of this invention as defined in the following claims. The claims cover the claimed elements and steps in any arrangement or order effective for the stated purposes unless the context clearly dictates otherwise.

Claims (6)

1. A method for improving the collection efficiency of an electrostatic precipitator, the method comprising: Preparation of a _slurry comprising a basic _SOx reducing compound comprising at least one oxide selected from the group consisting of an alkali metal or an alkaline earth metal, hydrogen, and a _NOx reducing nitrogen-containing composition. Oxide and carbonate substances, said _NOx reducing nitrogen-containing composition comprising urea, ammonium carbonate, ammonium bicarbonate, ammonium carbamate, ammonium hydroxide or a mixture of these substances, said _NOx reducing composition containing The composition of nitrogen effectively reduces NO _x and produces residual ammonia at a concentration of 3-100ppm within a defined temperature range; selecting at least one treatment zone whose temperature is within said defined temperature range; introducing the slurry into the treatment zone to reduce _NOx , _SOx , and chlorides and generate ammonia; and Ammonia-containing waste gas is passed through an electrostatic precipitator. 2. The method of claim 1, wherein said nitrogen-containing composition comprises urea hydrolyzate obtained by heating and pressurizing an aqueous urea solution for a period sufficient to at least partially hydrolyze said urea to form urea hydrolyzate Formed, the urea hydrolyzate comprises a complex of ammonium carbamate and ammonium bicarbonate/ammonium carbonate, wherein the ratio of ammonium carbamate to ammonium bicarbonate/ammonium carbonate is 10:1 to 1:200. 3. The method of claim 1, wherein the amount of said nitrogen-containing composition introduced into the exhaust gas is sufficient to provide an ammonia concentration effective to maintain the resistivity of the particles below ¹⁰¹³ ohm-cm. 4. The method of claim 1, wherein the nitrogen-containing composition is introduced into the exhaust gas in an amount sufficient to provide 10-50 ppm ammonia. 5. The method of claim 1, wherein the composition comprises a stabilizer system comprising a hydrophobic surfactant with an HLB value below 8 and a more hydrophilic surfactant with an HLB value of at least 8, wherein the hydrophobic The difference between the HLB value and the hydrophilic surfactant is at least 3. 6. The method of claim 5, wherein the difference between the HLB values of the hydrophobic and hydrophilic surfactants is at least 5.

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