Method of Conditioning Flue Gas to Electrostatic Precipitator

This invention relates generally to the separation of particulate material from a gas stream and particularly to a method of chemically conditioning a particle-laden gas stream so that the particles may be efficiently removed in an electric field.

One conventional way of collecting dust particles from a gas stream in which the particles are entrained is by using an electrostatic precipitator. This apparatus utilizes a corona discharge to charge the particles passing through an electrical field established by a plurality of discharge electrode wires suspended by insulators in a plane parallel to a grouned collecting electrode plate. The charged particles are attracted to the collector plate from which they may then be removed by vibrating or rapping the plate. Examples of this type of precipitator are found in U.S. Pat. Nos. 3,109,720 and 3,030,753.

Dust particles have different collection characteristics depending somewhat upon their source. One such characteristic is resistivity which is measured in ohm-centimeters. For example, where the source of particles is a coal-fired boiler, there is usually a predictable relationship between the type of coal burned and the resistivity of the particles in the flue gas. Typically, lowsulphur coal, i.e., less than 1 percent sulphur, produces particles having high resistivity, e.g., 10.sup.13 ohm-centimeters resistance; coal with 3-5 percent sulphur produces particles having 10.sup.8 -10.sup.10 ohm-cm resistance; and, poor combustion of coal produces particles having 10.sup.4 -10.sup.5 ohm-cm resistance.

It has been found heretofore that the most efficient collection or precipitation of particles occurs when their resistivity is about 10.sup.8 -10.sup.10 ohm-centimeters. When the resistivity is lower than this, e.g., in the collection of highly conductive dusts, the dust particle loses its charge immediately upon reaching the collecting electrode. Once the charge is lost, the particle re-entrains back into the gas stream and has to be charged again. This results in a considerable loss of efficiency. Conversely, when the resistivity is higher than this, e.g., in the collection of highly resistive dusts, the dust particles act as electrical insulators and cannot conduct charges on the collected dust layer to the grounded electrode. As this condition progresses, the voltage drop across the dust layer increases, causing a drop in the applied voltage between the high voltage emitting wire and grounded electrode. Since high applied voltage is required to maintain corona current, the current also drops, causing the precipitator performance to deteriorate. As the voltage across the dust layer increases, eventually the dielectric strength of the dust layer is exceeded, back ionization occurs and the precipitator becomes no better than a settling chamber. However, when the particles are of the preferred resistivity, a balance is achieved between the tendency to have either overcharged or undercharged particles and optimum precipitation efficiency results.

The bulk resistivity of the particles to be conditioned can be determined, if desired, by measuring the bulk resistivity of a sample of such particles in accordance with the American Society of Mechanical Engineers Power Test Code No. 28 (ASME PTC 28) entitled "Determining the Properties of Fine Particulate Matter" (paragraph 4.05 describes the "Measurement of Resistivity" and Appendix FIGS. 7-10 describe the apparatus for measuring the resistivity). Attempts to control the resistivity of the particles have been made with only limited success. For example, to this end there have been injected into the gas stream various chemicals such as water, anhydrous ammonia, water and ammonia, sulfuric acid, sulfur trioxide, and phosphoric acid. These chemicals have usually been injected for reaction in situ with other chemicals naturally present in the gas stream with the hope that a conditioner would be formed in the gas stream. As a result, the resistivity of the particles in the gas has been of a random and uncontrolled nature and entirely dependent on the chemical composition of the gas and/or particles in the gas. Examples of and reference to chemicals injected into the gas stream and the conditioner formed thereby may be found in the following patents: water -- U.S. Pat. No. 2,746,563, Great Britain Pat. No. 932,895; ammonia -- U.S. Pat. No. 1,291,745, U.S. Pat. No. 2,356,717; water and ammonia -- U.S. Pat. No. 2,501,435, U.S. Pat. No. 3,523,407; sulfuric acid -- U.S. Pat. No. 2,746,563, Great Britain Pat. No. 932,895, U.S. Pat. No. 2,602,734; sulfur trioxide -- U.S. Pat. No. 2,746,563, Great Britain Pat. No. 932,895, Great Britain Pat. No. 933,286; and phosphoric acid -- U.S. Pat. No. 3,284,990.

An article by J. Dalmon et al, "A Comparison of Chemical Additives as Aids to the Electrostatic Precipitation of Fly-Ash" in Atmospheric Environment (Pergamon Press 1972, Vol. 6, pp. 721-734, esp. pp. 727-728, printed in Great Britain) describes a power station test by Lloyd indicating that the injection of an aqueous spray of ammonium sulfate into a gas stream at 500.degree. C prior to the air heater resulted in reduced dust emissions. The presence or absence of clogging of the air heater by the ammonium sulfate is not reported by Dalmon et al. for either this test or a confirmatory test wherein the dust emission was reduced even further by injecting prior to the air heater a higher level of ammonium sulfate at an even lower temperature, indeed a temperature well below the ammonium sulfate decomposition temperature.

U.S. Pat. No. 3,664,676 decribes a conditioner solution comprising an aqueous solution of ammonium sulfate or ammonium bisulfate, but specifically teaches that the conditioner solution must be injected into the gas stream only after the air preheater to avoid the tendency of the chemical therein (e.g., ammonium sulfate) to deposit in and clog the air preheater when the conditioner solution is injected upstream from the air preheater. Such clogging is completely unacceptable because the entire unit must then be shut down to wash the air preheater. By injecting the conditioner solution after the air heater, this problem is overcome; however, the engineering problem of insuring adequate distribution and mixing of the additive with the flue gas prior to the precipitator becomes much more difficult. The flue in a large coal fired boiler can have a cross-sectional area after the air preheater of as large as 1,000 sq. ft.; at best, only a few seconds mixing time are available prior to the precipitator, and often there is severe stratification of gas flow in this region. Obviously, the further upstream from the precipitator that the solution can be injected, the better chance of complete mixing; however, the requirement of the patent that injection be downstream of the preheater limits the opportunity for improved mixing. In addition, the turbulence caused by passing the flue gas and conditioner solution through the constructions of an air preheater would also contribute significantly to mixing; however, the same patent requirement also limits this opportunity for improved mixing.

Yet another drawback of the method described in U.S. Pat. No. 3,655,676 is the relatively large amounts of conditioner solution needed to condition the flue gas. Whether this requirement is a result of inadequate mixing or low activity of the conditioning agent in the 240.degree.-800.degree. F (116.degree.-427.degree. C) temperature range specified for the gas is not clear; however, the disadvantages of having to buy, store and pump large quantities of conditioner solution are clear.

Accordingly, an object of the present invention is to provide an improved method of conditioning a particle-laden gas stream to improve the collection characteristics of the particles entrained therein.

A second object is to provide such a method which insures adequate mixing of the conditioning agent with the flue gas by injecting it upstream from the preheater.

A third object is to provide such a method which injects the conditioning agent in such a manner as to insure that it does not deposit upon and plug the preheater.

A fourth object is to provide a method which conditions the particle-laden gas stream using a much smaller quantity of conditioning agent than hitherto thought possible.

It has now been found that the above and related objects of the present invention are obtained by a method of conditioning a particle-laden gas comprising forming a mixture of the particle-laden gas and finely divided ammonium sulfate, where the gas is at a temperature of 590.degree.-900.degree. C. and the mixture contains 75-1250 grams of ammonium sulfate per metric ton of coal burned to form the gas. Preferably the gas is at a temperature of 590.degree.-750.degree. C. at the time of mixing, and the mixture contains 150-600 grams of ammonium sulfate per metric ton of coal burned to form the gas. The ammonium sulfate may be added to the gas in the form of either a dry powder or an aqueous solution (preferably at a 20-50% ammonium sulfate concentration by weight).

In a preferred embodiment, the collection characteristics of particles entrained in a particle-laden gas stream are improved for collection by an electrostatic precipitator by injecting finely divided ammonium sulfate into a stream of particle-laden gas formed by the burning of coal while the gas has a temperature of 590.degree.-900.degree. C. Sufficient ammonium sulfate is injected to provide 75-1,250 grams of ammonium sulfate per metric ton of coal burned to form the gas. After injection, the gas stream is directed through a heat exchange means and finally into the precipitator to collect the particles therein.

The conditioner useful in the present invention is finely divided ammonium sulfate (NH.sub.4).sub.2 SO.sub.4. The conditioner may be utilized either in dry form (for example, as a powder of finely divided particles) or, preferably, as a solution (for example, as an aqueous solution readily prepared from ordinary tap water and a commercial grade of ammonium sulfate or by other conventional means for preparing an aqueous solution).

The amount of conditioner to be injected into the gas stream at the specified temperature varies according to the amount of solids entrained in the gas stream and the degree of improvement needed in the electrostatic precipitator efficiency, for example, in order to meet a maximum allowable emissions requirement of a local, state or federal regulatory body. Generally for conditioning the fly ash in a coal-burning utility boiler, sufficient conditioner is injected into the gas stream to provide 75-1,250, and preferably 150-600 grams, of the conditioner agent (i.e., ammonium sulfate) per metric ton of coal burned to form the gas. Since the flue gas volume is dependent on the temperature of the flue gas, another way of expressing this value is 2.46-41.1, and preferably 4.93-19.71, kilograms of conditioner per million cubic meters of flue gas measured at 694.degree. C. Generally conditioner levels below this range do not appreciably improve the collection characteristics of the particles, while any conditioner levels in excess of the specified range not only increase the cost of conditioning unnecessarily, but also increase the possibility of blockage of the preheater or other heat exchanger downstream of the point of injection.

The quantity of ammonium sulfate determined according to the foregoing criteria is preferably added in the form of an atomized aqueous solution, preferably a 20-50% by weight salt solution. Higher or lower concentration may be used, however, as the function of the water is merely to facilitate injection of the ammonium sulfate in atomized form into the gas stream, and the water itself is not believed to play a significant part in the process of the present invention.

The gas stream at the point of injection must have a temperature of 590.degree.-900.degree. C. It is recommended that the temperature not be in excess of 750.degree. C. for maximum economy. Injection of the conditioner into the gas stream within the specified temperature ranges does not result in subsequent salt deposits in and/or blockage of the air preheater means downstream for two specific reasons. First, the ammonium sulfate has a decomposition temperature of 399.degree. C. and is thus decomposed by the hot gas stream before it has an opportunity to reach the air preheater means. Second, the ammonium sulfate is of such high efficiency within the specified temperature range that only a small quantity of the ammonium sulfate need be injected. Indeed, when the gas stream is within the preferred range, the amount of conditioner required to effectively condition flue gas is typically only about a tenth of the amount needed when injecting the same conditioner into flue gas at temperatures well outside of the preferred range.

The mechanism by which the conditioner of the present invention changes the resistivity of the particles in the gas stream is not fully understood. The theory, advanced in U.S. Pat. No. 3,665,676, of a film of ammonium sulfate coating the particles to change their surface resistivity has some credibility when injecting the ammonium sulfate at temperatures below the decomposition temperature of the salt, as there recommended. However, in the present invention some other mechanism must be operative since the injection temperature are well above the decomposition temperature of ammonium sulfate.

Regardless of the operative mechanism of the present invention, an important advantage of the present invention arises out of the fact that the conditioner is effective irrespective of the chemical content of the gas being conditioned; that is, its effectiveness does not depend on dust particles or the gas inducing any particular initial chemical composition (such as an oxide of sulphur) to provide a chemical reaction which would then combined with the conditioner in situ to condition the particles. Such dependency upon an in situ chemical reaction was one shortcoming of the heretofore known practices which required the presence of definite amounts of other chemical constituents in the gas stream, such a dependency being especially significant in view of the current trend to low sulphur fuels.

The process of the present invention is effective to modify the resistivity of the particles entrained in the flue gas to 10.sup.8 -10.sup.10 ohm-centimeters regardless of the initial resistivity characteristics of the particles. Interestingly, the use of the conditioner is beneficial even when the bulk resistivity of the entrained particles is already at the 10.sup.8 -10.sup.10 ohm-centimeters believed to be the optimum resistivity for collection of particles. While the reason for this phenomen is not completely understood, it is believed to be based on the fact that while the bulk resistivity is an average of the resistivity of all the particles in the dust layer (which may, in fact, contain particles with both much higher and lower resistivities than the average), the collection efficiency is believed to be a function of the resistivity of the individual particles. Thus, the ability of the conditioner to raise the resistivity of low resistivity particles and to lower the resistivity of high resistivity particles, both to approximately the optimum resistivity, is believed to be extremely important for collection efficiency.

It will be recognized that a critical feature of the present invention is the injection of the conditioner into a gas stream having the proper temperature range. The gas temperature at the point of injection must be sufficiently high to insure substantial decomposition of the ammonium sulfate prior to contact of the ammonium sulfate with the air preheater means or any other heat exchange unit which the ammonium sulfate might deposit upon and/or clog. When the gas stream at the point of injection is at least 590.degree. C., the specified quantities of ammonium sulfate decompose with sufficient speed for this purpose. Of course, if there are no heat exchange units intermediate the point of injection and the precipitator, somewhat lower injection temperatures may be tolerated provided they are effective to decompose the ammonium sulfate prior to its contact with the precipitator. However, the presence of an air preheater means or other heat exchange unit intermediate the point of injection and the precipitator is preferred to insure complete and thorough mixing of the ammonium sulfate and its decomposition products with the particles entrained in the gas stream. In this instance, it is recommended that the injection amount and injection temperature be appropriately coordinated (within the ranges specified for the practice of the present invention) to insure the absence of deposits in and clogging of the heat exchange unit, higher injection amounts requiring higher injection temperatures according to the principles of the present invention.

In a typical power station, the flue gas produced by a coal-fired boiler passes successively from the boiler through a secondary superheater, a reheater-superheater, a "ball-room," a primary superheater, an economizer, an air preheater, a precipitator, a stack, and ultimately passes into the atmosphere. The temperature of the gas stream entering the ball-room is typically slightly under 900.degree. C., and the temperature of the gas stream leaving the primary superheater is typically slightly below 550.degree. C. In this situation, the preferred location for the injection ports for the conditioner would be somewhere between the ballroom entry duct and a point within the primary superheater. However, it is to be understood that this is only an illustrative example and that boilers vary widely in design and operating conditions. The criteria for selection of the injection ports is primarily the temperature of the gas stream at such points. Other criteria include the selection of a location permitting good mixing of the conditioner (preferably atomized) with the gas stream and the absence of direct impingement of the conditioner on the boiler tubing as that might result in severe damage by thermally shocking the boiler tubing. Preferably, the injection ports are disposed so that the gas stream (containing the conditioner) subsequently passes through the air preheater or some other heat exchange unit to insure thorough mixing of the conditioner and the particles of the gas stream before the gas stream contacts the precipitator.

The apparatus for injecting the conditioner into the gas duct may be conventional in design. Apparatus for injecting the conditioner typically includes a supply of the conditioner, nozzle means communicating with the interior of the gas duct, and means connecting the conditioner supply to the nozzle means, such connecting means typically including means for forcing the conditioner through the nozzle, preferably as an atomized spray, and means for metering the amount of conditioner injected, typically in proportion to either the quantity of gas being conditioned or the quantity of coal being burned.

Preferably the conditioner is injected on a continuous basis during operation of the furnace, but clearly, it may alternatively be injected on an intermmittent or periodic basis.

The efficiency of an electrostatic precipitator can be determined by comparing the amount of particles or dust load released to the atmosphere from the precipitator outlet to the amount of particles or dust load entering the precipitator. The following equation gives precipitator efficiency as a percentage:

where P.sub.1 is dust load at the precipitator inlet and P.sub.0 is dust load at the precipitator outlet. The dust loads, usually expressed as grains per cubic foot of gas, are conveniently measured by the procedure given in EPA Method No. 5 as described in the Federal Register, Vol. 36, No. 247, Part II, pp. 24,888-24,890 (December 23, 1971).

The efficiency of the present invention is illustrated in the examples below, wherein all parts are by weight unless otherwise noted.

A 135 Megawatt design capacity forced draft boiler with 2 Ljungstrom air heaters had been equipped with an American Standard electrostatic precipitator designed for 98% efficiency while burning 59,100-63,000 Kgs/hour of a 4.6% sulfur, 15% ash coal. Because of environmental restrictions on SO.sub.2 emissions, this boiler was switched to a 0.6% sulfur, 14% ash coal. While burning the high sulfur coal, the precipitator efficiency had been quite good, but with the low sulfur coal the emissions reached an unacceptable 1.03 Kgs/million BTU. To lower the emission level, a 40% aqueous solution of ammonium sulfate was injected into the superheat section of the boiler (where at 125 megawatt load the temperature was about 638.degree. C.).

As indicated by the 125 megawatt load data recorded in Table I for various treatment rates (grams ammonium sulfate per metric ton of coal burned), the particulate emissions were reduced at least by about a factor of 10.

A 315 Megawatt forced draft boiler with 2 Ljungstrom air heaters was equipped with a Research Cottrell electrostatic precipitator designed for approximately 99.5% efficiency. When firing a coal with a variable sulfur content of 0.6% to 2.6% and an ash content of 5 to 18%, particulate emissions generally varied from 0.11 to 0.16 Kg/million BTU's. To comply with state regulations restricting emissions to 0.045 Kgs/million BTU's, a 40% aqueous solution of ammonium sulfate was injected to condition the fly ash. The only ports available for injection were in the ballroom area where the gas temperature ranges from 829.degree. to 899.degree. C. at full load.

As indicated by the full load data recorded in Table II, the treatment is effective even at these high injection temperatures, although the magnitude of the change in emissions is less than found at lower temperatures and higher treatment rates are required.

A 500 Megawatt design capacity forced draft boiler with 2 Ljungstrom air heaters had been equipped with an American Standard electrostatic precipitator designed for 99.6% efficiency while burning 136,000 Kgs/hour of 3.0% sulfur, 12% ash coal. In order to comply with regulations calling for lower emissions of oxides of sulfur, this boiler was switched to a 1.0% sulfur, 14% ash coal. While burning the high sulfur coal, the precipitator efficiency was quite good, but with the low sulfur coal the unit was out of compliance due to excessive fly ash emissions. The criterion for compliance in this instance was the opacity of the stack gases, with the maximum allowed being 30%.

In order to lower the emissions (and therefore reduce the opacity), a 40% aqueous solution of ammonium sulfate was injected into the flue gas. Several injection locations were available and were used to determine the effect of gas temperatures at the injection points on the efficiency of the fly ash conditoning.

As indicated by the 450 Megawatt load data recorded in Table III, injection of ammonium sulfate into the reheat area (where the flue gas is about 700.degree. C.) brings the unit into compliance, while injection into the secondary superheater area (where the flue gas is over 950.degree. C.) had virtually no effect even though higher treatment rates were used, and injection into the ballroom area (where the flue gas is about 500.degree. C.) resulted in a small decrease in opacity but only at the cost of severe pluggage of the air preheater, probably from incomplete decomposition of ammonium sulfate during its brief exposure to this temperature before being swept downstream into the preheater.

A 500 Megawatt design capacity forced draft boiler with 2 Ljungstrom air heaters was equipped with a Buell electrostatic precipitator which had a design efficiency of 99% at 350 Megawatts when burning 146,000 Kgs/hour of a 3.1% sulfur, 15.0% ash coal. When the coal supply was changed to a 1.6% sulfur, 16.9% ash coal, emissions of fly ash increased to unacceptable levels. To comply with government regulations restricting emissions to 259 Kgs/hour, a 40% aqueous solution of ammonium sulfate was injected into the primary superheat region of the boiler (where the flue gas temperature was 596.degree. C.) to condition the fly ash emissions.

As indicated by the 315 Megawatt load data recorded in Table IV, a 40% reduction in emissions was obtained, thus bringing the unit into compliance.

A 180 Megawatt design capacity forced draft boiler with two Ljungstrom air heaters was equipped with a Research Cottrell electrostatic precipitator designed for 96.0% efficiency while burning 87,000 Kgs. of 1.2% sulfur, 8.1% ash coal per hour (172 Megawatts). When the coal supply was changed to a 0.8% sulfur, 9.2% ash coal, particulate emissions increased to unacceptable levels. To comply with government regulations restricting emissions to 0.118 Kgs/million BTU, a 40% aqueous solution of ammonium sulfate was injected into the primary superheat region (where the flue gas temperature was 693.degree. C.).

As indicated by the data recorded in Table V, the injection of even a very low amount of the conditioner sufficed to bring the unit into compliance.

A 350 Megawatt design capacity forced draft boiler with two Ljungstrom air heaters was equipped with a Research Cottrell electrostatic precipitator designed for 99.8% efficiency at 350 Megawatts when burning, 131,000 Kgs/hour of a 3.0% sulfur, 10.4% ash coal. While burning this coal and a coal of 2.41% sulfur, and 9.05% ash, the station was able to keep particulate emissions below the compliance level of 0.045 Kgs/Million BTU. However, as the utility was forced to change to lower and lower sulfur coals the emissions increased and flue gas conditioning became essential in order to keep in compliance. To meet compliance levels, a 40% aqueous solution of ammonium sulfate was injected into the primary superheat region (where the flue gas temperature was 654.degree. C.).

As indicated by the data recorded in Table VI, the treatment was effective to bring the unit into compliance.

A 150 Megawatt design capacity balanced draft boiler with a Ljungstrom air heater was equipped with a Research Cottrell electrostatic precipitator designed for 95% efficiency while burning 45,500 Kgs/hour of a 2.5 sulfur, 8% ash coal. When the coal supply was changed to a 0.7% sulfur, 15% ash coal, emissions of fly ash increased to unacceptable levels. The criterion for compliance in this case was the opacity of the stack gases, with the maximum allowed being 20%. In order to lower the emissions (and therefore reduce the opacity), ammonium sulfate was injected into the flue gas in the primary superheat region (where the flue gas temperature was 595.degree. C.). The ammonium sulfate was injected as a dry, finely divided powder by blowing it into the flue gas with pressurized air.

As indicated by the 150 Megawatt load data recorded in Table VII, the treatment resulted in a drop of opacity of the stack gases from 70 to 30%. This was not enough to meet compliance, but was nonetheless a very favourable result considering the fact that the ash level of the coal was nearly twice the level that the precipitator had been designed to handle.

Now that the preferred embodiments of the present invention have been shown and described, various modifications and improvements thereon will become readily apparent to those skilled in the art. For example, the principles of the present invention are easily adapted for similarly conditioning the entrained dust or ash particles produced by oil-fired furnaces (petroleum by-product particles), catalytic crackers (SiO.sub.2 -Al.sub.2 O.sub.3 particles), cement or lime kilns (calcium salt particles), electric furnaces (reduced copper particles), etc. Accordingly, the spirit and scope of the present invention is to be limited only by the appended claims, and not by the foregoing disclosure.