GB1592844A - Method and apparatus for conditioning flue gas with a mist of acid - Google Patents

Description

(54) METHOD AND APPARATUS FOR CONDITIONING FLUE GAS WITH A MIST OF ACID (71) We, UOP INC, a corporation organized under the laws of the State of Delaware, United States of America, of Ten UOP Plaza, Algonquin & Mt. Prospect Roads, Des Plaines, Illinois, 60016, United States of America, do hereby declare the invention, for which we pray that a patent may be granted thus, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a conditioning system for preparing and introducing sulfuric acid (H2SO4) into a particle-laden flue gas stream so as to substantially reduce the resistivity of the fine fly ash particulates to in turn enhance their removal from the gas stream by electrical precipitator means.

It has been found, and is quite well known, that fly ash containing streams from the burning of coal, or from the burning of any fossil fuels, will have a certain amount of electrical resistance or "resistivity" developed in the particles such that an inefficient electrical precipitation thereby results. It is also well known that flue gas streams will have varying quantities of sulfur trioxide (SO3) present naturally and that when a sufficient quantity of SO3 or H2SO4 is present in the gas stream or with the fly ash the resistivity of the particles to giving up their electrostatic charges will be low enough that good precipitation results can be obtained.

With ever increasing Governmental pressure on industrial and utility companies to improve their emissions from coal fired boilers and comply with emission standards, many have switched to the use of low sulfur coal to reduce the amount of SO2 present in the flue gases. Unfortunately, while the flue gas from high sulfur coal contains sufficient SO3 to provide the proper resistivity, low sulfur coal lacks sufficient SO3 in the flue gas to provide the proper resistivity to the resulting fly ash to permit its effective precipitation. Thus, these users are more than ever seeking immediate and low cost solutions to poor fly ash collection efficiencies of their existing electrostatic precipitators. Their possible options are to expand or rebuild their existing equipment to handle the type of flue being fired or by going to gas conditioning of the boiler combustion gases. Gas conditioning is used to bring the exhaust fly ash within a more desirable resistivity range for precipitator collection.

Gas conditioning is economically more attractive to industry due to its relatively low purchase price when compared to the purchase price of an enlarged or new precipitator. Availability is a second advantage to this approach in that the system can be installed fairly quickly and with minimal load disturbance.

Various methods of gas conditioning are presently available. The more effective conditioning agents are H2SO4 and NH3. A system which is presently marketed and which is described in U.S. Patent 3,704,569 uses vaporized H2SO4 as its conditioning agent. With this system, large volumes of dry air are heated to a temperature of approximately 260"C. to be above the vaporizing temperature of about 235"C.

and then mixed with the acid in a glass lined vaporizing chamber. The hot vaporized acid is then conveyed to injection lances by means of glass lined pipe and uniformly dispersed in the flue gas. Although such a system provides excellent conditioning of the flue gases, it is quite expensive to produce due to the fact that the acid is transported in a hot vaporized state and is extremely corrosive, with the result that expensive, corrosion-resistant materials are required to be used. Furthermore, the system is expensive to operate since an excessive amount of energy must be used to heat the air to a point where it can vaporize the acid.

A second method of gas conditioning is that of utilizing SO3 directly. This system functions much the same as the aforementioned vaporizer except that heat is applied to liquid SO3 in an evaporator chamber resulting in the SO3 vapor.

A third method is disclosed in U.S.

Patent 1,441,713 where acid is proposed to be introduced in a gas stream in the form of very fine particles and specifically, in the form of a fume which is formed by boiling fuming sulfuric acid. Although the patentee broadly contemplates that the acid be introduced by some suitable form of atomizing device, no apparatus is disclosed other than the boiling pan and burners. In view of the extremely corrosive and dangerous nature of fuming sulfuric acid, it is doubtful that the aforesaid method would have ever been used. Certainly if it was it would have been expensive to provide corrosion-resistant materials and to provide the necessary heat for boiling the acid.

A fourth and more complex method of gas conditioning is that of burning liquid sulfur. The SO2 generated by the sulfur burner is passed through a catalyst that converts the SO2 to SO3. The final objective of all four methods is to disperse H2SO4 in the precipitator flue gases and condition particles therein to a more desirable resistivity for precipitator collection. The dispersion must be very fine since an electrical precipitator is an effective collector of sulfuric acid mist. As noted above, conditioning usually involves the injection fo H2SO4 or SO3 in the flue gas stream in vaporized form, and the injection of acid in liquid form apparently has not been done commercially, probably for the reason that one would expect that liquid injection would not condition beyond the first field of a precipitator since the acid particles would be collected, leaving the remaining fields current suppressed due to the presence of unconditioned fly ash accumulated on the electrodes.

Furthermore, until the rather recent development of a sonic atomizing nozzle, the available mechanically atomized spray nozzles were not able to produce a fine enough spray to be considered as a substitute for vapor injection. Mechanical nozzles typically are poor in their ability to operate satisfactorily when turned down to low flow rates. Also, the required high liquid pressures and small orifices used in a mechanical nozzle would increase the likelihood of erosion and plugging problems.

It can be readily appreciated that although all of the aforementioned prior art system for gas conditioning by injection of SO3 or H2SO4 provide satisfactory results, they achieve these results at considerable expense in terms of capital equipment requirement and in terms of the excessive amounts of energy which they utilize.

Obviously, it would be desirable to have a system which can be produced and operated at a lower cost and the present invention seeks to provide such a system.

By the apparatus and method of the present invention it is possible to achieve H2SO4 gas conditioning performance levels equivalent to the prior art levels of vaporizing systems, but at much lower costs in terms of equipment requirements and in terms of day-to-day operating expense requirements, particularly for energy.

According to one aspect of the invention a method of injecting an acid-conditioning agent into a flue gas stream containing fly ash to be conditioned to enhance the efficiency with which the fly ash can be electrostatically precipitated comprises the steps of passing a liquid acid-conditioning agent into the flue gas stream by feeding the agent under pressure through a first line into a lance incorporating a sonic atomizing nozzle producing sonic vibrations capable of breaking up said liquid acid into a mist having a mean droplet size no greater than 10 microns; heating a portion of said first line adjacent said lance so that said acid will be heated and will enter said lance at a temperature higher than ambient but lower than its vaporizing temperature: and passing a gas under pressure through a second line into said nozzle, said gas pressure being at least 10 psig higher than the liquid acid pressure.

Acoustic standing wave energy set up by the gas supply at the nozzle tip provides the energy necessary to dissociate the larger H2SO4 molecules into smaller droplets.

Employing the method of the invention has the distinct advantage of minimizing equipment and conveying line corrosion in that the H2SO4 is not conveyed to the or each injection lance in a vaporized form.

H2SO4 is in its most corrosive state under vaporized conditions. Conveying the acid in liquid form to the or each lance enables the equipment to be constructed or more available and inexpensive materials. Only the lance(s), the nozzle(s) and the section of heated line adjacent the lance(s) must be constructed of corrosion-resistive material capable of resisting the high temperature environment in the flue and the atomizing aeration of the acid at the nozzle. Since the gas entering the lance(s) is not heated, and the acid need only be warmed in a short section of line adjacent the or each lance to reduce its viscosity, the operating cost as well as construction cost will be substantially less than other conventional gas conditioning systems.

According to a further aspect of the invention there is provided an apparatus for injecting a mist of liquid acid into a flue gas stream to condition fly ash in the stream and enhance the efficiency with which the fly ash can be electrostatically precipitated downstream of the apparatus comprising: a reservoir for liquid acid; means to supply pressurised air; at least one sonic atomizing nozzle, able to break up the liquid acid by sonic vibrations into a mist having a mean droplet size no greater than 10 microns, a lance mounting the nozzle and adapted, in use of the apparatus to position the nozzle in the flue gas stream; a pump for delivering acid to said lance and nozzle under pressure; said nozzle and lance being connected to said reservoir for acid and said means to supply pressurised air by flow lines, the flow line for the acid being surrounded by heating means for a portion of its length immediately adjacent said lance which heating means is adapted in use of the apparatus to raise the temperature of the acid in the line to a value less than the vaporization temperature.

In operation of apparatus according to the invention in a pilot plant unit at ambient temperature, acid from a day storage tank is filtered prior to entering an acid pump. Depending upon a system feed control signal, a volume of H2SO4 corresponding with the desired injection rate is delivered to a plurality of nozzles in the flue gas stream. From the metering equipment the acid then passes through a set of trim valves and rotameters. This equipment enables the operator to bias the flow slightly between injection lances and monitor the flow to each. Pressure gauges are provided to insure proper liquid and air pressure to the nozzles. Prior to passing through the lance nozzles, electrically heated means surrounding the conveying lines heated to a temperature of about 120--137"C., warm the- acid to a temperature of between about 93--121"C.

prior to injection. The advantage of running warmed acid to the nozzles is to reduce the viscosity and surface tension of the individual acid droplets, thus enabling the acoustic shock wave to more effectively shatter the acid droplets. The warmed acid is then passed through the nozzles where it mixes with the dry atomizing air and is converted to a fine acid mist with mean droplet sizes of not more than 10 microns, suitably no greater than 5 microns, and preferably from 1--3 microns. A controlled air pressure of at least 10 psig higher than the acid pressure is maintained to the nozzles to insure proper atomizing energy.

Liquid and air pressure gauges are installed at the lances to insure the proper ratio.

A chemical feedback signal downstream of injection may be used to determine the H2SO4 content of the flue gas and signal the metering equipment to maintain the desired concentration. For example, a Land Dewpoint meter would be suitable for this purpose. The maximum injection concentration is desirably limited to the dew point temperature of the flue gas and suitably controlled at a safe margin from dew point. Alternatively, and especially where the coal used in a boiler has a uniform S03 content, the injection rate of acid can be controlled in response to changes in the plant load to provide 15-30 ppm (by weight) acid in the flue gas.

Tests of an acid mist injection system according to the present invention have indicated that substantial improvement in precipitator performance can be realized using the above system.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is an isometric view of a skid showing the various structural elements of the gas conditioning system which are connected to injection lances in a precipitator inlet duct; Fig. 2 is an axial cross-section of an injection lance; Fig. 3 is an axial cross-section of an atomizing injection nozzle; Fig. 4 is an axial cross-section of a heater hose; Fig. 5 is a schematic flow diagram of the "pilot plant" type of gas conditioning apparatus shown in Fig. 1; and Fig. 6 is a schematic flow diagram of an alternative "commercial" type of gas conditioning apparatus.

Referring to Fig. 1, the improved gasconditioning apparatus shown generally at 10 includes an acid day tank 12 and an air tank 14. Acid lines 16 and an air line 18 are connected to a plurality of lance assemblies indicated generally at 20 which have acid mist injection nozzles 22 at their outer ends.

The lance assemblies 20 are adapted to pass through the top or side of a duct member 24 at a position in a flue gas stream which is upstream from an electrostatic precipitator (not shown). The particular position upstream should be such that the acid mist will be uniformly dispersed by the time it reaches the precipitator and the number and position of the lances should be selected to provide uniform dispersion.

The embodiment of Fig. 1 shows a plant" unit wherein the major portion of the gas conditioning apparatus is mounted on skid member 26 which may be conveniently located near an existing electrostatic precipitator (not shown).

Mounted on the skid member 26 is a control panel 28 which encloses various structural elements to be described hereinafter.

Referring to Fig. 2, one of the lance assemblies 20 is shown partially in cross section. The lance assembly includes an air supply pipe 30 and a pipe bend 32 which may be made of a corrosion-resistant material such as Carpenter 20 steel capable of withstanding the flue gas environment.

The bend 32 is adapted to be threaded into an opening 34 in the end of a nozzle 22, preferably made of tantalum, for supplying air to the nozzle. Similarly, an opening 36 in the side of the nozzle is adapted to receive an adapter member 38 which is also preferably made of tantalum. The adapter 38 is connected to a coupling member 40 of Carpenter 20 steel which is in turn attached to a pipe member 42 of Carpenter 20 steel for supporting the lance 20 within a sleeve member 44 which is attached, such as by welding, to the wall of a duct member 24.

An inner tube 46 made of a corrosionresistant material such as polytetrafluoroethylene (hereinafter PFE) is attached at its lower end to the corrosionresistant tantalum adapter 38 and at its upper end to an adapter 48 formed of PFE which is threaded into a PFE elbow member 50.

The nozzle member 22 is shown in detail in Fig. 3. The nozzle includes a main body portion 56 preferably formed of tantalum and having positioned within it an orifice member 57 containing an orifice 58 and including an inlet cone portion 60 and an outlet cone portion 62. The orifice 58 acts as a venturi to increase the velocity of the air and to assist in drawing liquid acid through two pairs of opposed holes 64 which communicate with the annular acidcontaining reservoir 66 positioned between the orifice member 57 and the body 56 and the acid inlet opening 36. Positioned at the outer end of the nozzle 22 are a pair of support arms 68 which carry and support a resonator cup member 70 having a cavity 72. The nozzle 22 produces an intense field of sonic energy which breaks the acid particles up into an extremely fine mist having a mean droplet size no greater than 10 microns and preferably 1--3 microns.

The nozzle is preferably made in a shape similar to the Model 052 nozzle sold by Sonic Development Corporation of Upper Saddle River, New Jersey. The theory of operation of such nozzles is generally explained in U.S. Patent No. 3,240,254. To resist corrosion, the nozzle 22 must be made of tantalum or other material capable of withstanding the corrosive environment produced by the warmed liquid acid within the nozzle and the hot acid mist which can contact the exterior portions of the nozzle.

Materials such as stainless steel and Hastelloy normally offered by the nozzle manufacturer for corrosion resistance are of little use in a warm acid environment since they would corrode very quickly.

Fig. 4 shows an enlarged cross-section of a portion of a heating means indicated generally at 78 in Fig. 1. The heating means includes a PFE core tube portion 80 which is preferably threadably attached at one end to the PFE elbow member 50 (Fig. 2) and at its other end to one of the acid supply lines 16 which would normally be made of either stainless steel tubing or PFE tubing with a protective steel braid over it.

The PFE core tube 80 in the heating means 78 is shown as being covered by a stainless steel braid 82 which is in turn covered by resistance heating wires 84 embedded in flexible insulating material 86. The outside layer of the heating means 78 comprises a tough resilient jacket member 88. The aforementioned construction provides a heater which is quite bendable and thus facilitates the connection of the acid lines 16 which are attached to the apparatus mounted on the skid member 26 and the lances 20 which are mounted in the duct 24.

The heater wires 84 are, of course, attached to a suitable source of electric power, preferably through temperature controllers 126. The purpose of the heating means 78 is to heat the acid to lower its viscosity before it enters the lance 20 and the nozzle 22.

Although the "pilot plant" skid mounted structure shown in Fig. 1 provides a completely self-contained means (except for electrical power) to quickly and simply mount a gas conditioning apparatus 10 to a duct 24 leading to a precipitator, it would probably be preferable in most cases to mount the various elements of the apparatus in a more permanent fashion.

The principal use of the skid mounted apparatus is as a "pilot" unit to quickly demonstrate the usefulness of the system in various flue gas environments and determine the optimum injection rate therefor. The schematic flow diagram illustrated in Fig. 5 relates to the skidmounted unit shown in Fig. 1. The system is designed to be very flexible in that an extremely large range of flow rates of acid, from about 1/2 to about 25 gallons per hour, can be provided from a single pump. This very large range of adjustment, a turn down ratio of 50:1, permits the unit to supply from 1--14 nozzles, but provides some penalty in that it is somewhat difficult to maintain an accurate flow to each nozzle without actively monitoring and adjusting the controls. Where a "commercial" unit is designed to be used in a specific plant, the degree of turn down would not have to be over about 10:1, corresponding to a flow rate of perhaps 2.5 to 25 gallons per hour.

As compared to the 50:1 turn down ratio of the aforementioned "pilot" system, a "commercial" system with a lower turn down ratio and an individual metering pump for each, or each pair of, nozzles could operate unattended for extended periods with no adjustments.

Referring to the flow diagram of Fig. 5 which corresponds to the pilot plant system shown in Fig. 1, a compressor 96 takes incoming air and compresses it into the air tank 14 where its pressure is maintained between a predetermined set of values. The air is fed to the nozzles 22 through the common line 18 and has its pressure maintained at a constant value, such as 70 psig, by a pressure regular 98. For purging acid from the system when shutting down the unit, a side pipe 100 having a valve 102 is provided between the air line and acid line.

The acid system includes the day tank 12, having a sight gauge 103 (Fig. 1), and high and low level alarm systems connected to indicators 104 and a horn (not shown). A gear type pump 105 is connected to the outlet of the tank 12. The pump 105 produces a variable amount of flow depending upon the resistance downstream. A pipe line 106 downstream of the pump 105 incorporates a backpressure valve 107 which lets part or all of the acid being pumped circulate to the day tank 12.

This valve 107 maintains a set backpressure of 30 psi, for example A line 108 to the nozzle 22 is taken off the line 106 between the pump and the backpressure valve.

Thus, the line 108 always sees the same upstream pressure (the set point of the backpressure valve) and for a given line resistance, will have a set flow through it.

This line resistance is set by an air-operated valve 109. The acid then flows through a shut-off solenoid valve 110 which closes the acid line if any abnormal condition should develop such as a loss in air pressure or a drop in the duct temperature. The flow is measured by a main rotameter i 11 that reads total acid flow to the nozzles 22 and by smaller auxiliary rotameters 112 in the individual lines 16 which communicate with each nozzle. The smaller rotameters 112 have trim valves 114 which may be used to finely adjust the acid flow to the individual nozzles.

The metered amount of acid flows through the individual acid lines 16 to the lances 20. These lines preferably comprise small diameter stainless steel pipe, tubing or armored PFE hose. Just before entering the lances 20, which hold the nozzles 22 in the duct 24, the acid is heated in the short (25 feet is satisfactory) length of electrically heated PFE tubing 80 whose temperature is controlled by a controller 115. The warmed acid atomizes more easily into very small particles than cool acid. Finally, the acid flows to the lance pipes 46 into the nozzles 22 and is atomized in a fine spray which is dispersed in the flue gas.

The air-operated valve 109 provides the acid flow control. The position of the valve controls the flow by varying the resistance to flow in the acid line. The position can be adjusted manually by sending the valve a signal (air pressure) from a pressure regulator 116 set at the desired value between about 6-30 psig on the control panel 28. When a selector valve 117 is put on "automatic" control, a signal from the plant is sent to the valve. This signal should be proportional to the amount of flue gas being produced. The acid flow rate will then be proportional to the amount of flue gas being produced by the plant. It should be noted that a number of other independent variables affect acid flow in the aforementioned system. If the viscosity of the acid varies (as it does with temperature changes), the flow will vary.

Also, any small particles partially blocking nozzles or valves will vary the flow.

To provide the operator with as much information as possible, the control panel 28 also includes a tank liquid level indicator 118, a tank temperature indicator 119, a system "on" indicator 120, a power "on" indicator 121, pressure gauges 122-125 for inlet fluid, inlet air, outlet fluid and outlet air, respectively. A gauge 126 is a temperature set point gauge for setting the lowest temperature of the flue gases at which the system will operate. This keeps the system from operating below the dew point of the acid. Gauges 127, 128 indicate the manual and automatic control pressure, respectively, of the air acting on the control valve 109 in response to actuation of the pressure regulator 116 and the selector valve 117. Although only two rotameters 112 and two lances 20 are shown in the diagram of Fig. 5 for simplicity, it would be common to have 6-14 of each to achieve uniform acid dispersion in a large duct.

Fig. 6 illustrates a schematic flow diagram of a gas-conditioning system incorporating the invention which might be typically used in a commercial situation.

The principal distinction between a socalled "commercial" unit and the "pilot plant" unit shown in Figs. 1 and 5 are that the pilot plant unit uses a single gear driven pump 105 to feed acid to anywhere from one to fourteen nozzles, depending on the size and capacity of the flue duct 24. The "commercial" system illustrated in Fig. 6 utilizes a plurality of metering pumps 205 which are positive displacement pumps.

The number of pumps used depends on the total flow required, with a different pump being used for each single lance or each pair of lances. The pumps pull in a given quantity of acid from a day tank 212 on the suction stroke and force it through the outlet on the pressure stroke. The quantity of acid delivered to and by each pump is determined by pneumatic output valves 216 which are operated by a load signal 218 in response to a parameter such as the volume of flue gases or the amount of SO3 in the flue gases downstream of the precipitator.

Thus, the downstream piping resistance to flow does not materially effect the flow output of the metering pumps. This is in contrast to the gear type pump 105 used in the "pilot" unit wherein the flow varies depending on the downstream resistance.

The metering pumps 205 distribute the flow uniformly to a plurality of nozzles 222 arranged in a duct 224 without any need for trim valves.

Referring to Fig. 6 in more detail, the acid day tank 212 includes high and low level alarms 226 and a level indicator 228.

The acid is piped from the day tank 212 to the metering pump or pumps 205. As previously mentioned, the number of pumps 205 depends on the total flow required. The acid is piped from the pumps 205 through a pulsation dampener 230 which serves to smooth out the flow due to the piston strokes of the pump. A line 232 between the pulsation dampener 230 and the nozzles 222 includes a pressure gauge 234 for measuring the pressure, a pressure switch 236 which serves as an alarm for abnormal pressure conditions, a backpressure valve 238 which produces the pressure needed for the pump check valves (not show) to close tightly, a flow indicator 240 to indicate that the pump is actually pumping and a shut-off valve 242 to close off the acid line. The acid line 232 would also include a heated section of tubing 244 and lances 246 which would be constructed similarly to the identical elements in the pilot system of Figs. 1 through 5. The air is shown as coming from a compressed air supply 250 rather than a separate air compressor since a plant commonly has a source of compressed air. Air dryers 252 are provided for cleaning and drying the incoming air before it passes to an air receiving tank 254 which holds a supply for limited times when the supply 250 may fail.

After the air leaves the tank 254 it passes through a pressure regulator 256, a pressure gauge 258, a pressure switch 260, and a flow indicator 262 before passing through lines 264 which are connected to the lances 246.

The optimum acid injection rate for the gas-conditioning apparatus is the one that produces the best results in fly ash collection without acid carrying over past the precipitator. This rate is generally between 15-30 ppm acid (by weight) to flue gas. The exact ratio will vary, however, with the flue gas rate, the coal analysis, plant operation, precipitator condition, and other variables.

One method of determining and controlling the acid injection rate is as follows. For a given coal, the plant is operated at full rate and the acid injection through the nozzles 22 (Fig. 1) is increased to the point of maximum precipitator collection efficiency as determined by observing the stack, observing the precipitator electrical performance parameters and/or taking flue gas samples.

After the correct rate for the plant at full load is known, a signal provided to the conditioning unit by the plant which is roughly proportional to the flue gas flow rate should provide automatic injection of the correct amount of acid. This signal is transmitted to the air control valve 109 (Fig. 5) and permits the acid injection rate to drop proportionally to any drop in the flue gas flow rate. Thus, the amount of acid being injected can be kept in constant proportion to the flue gas. If a plant is operated near full load most of the time and uses a single type of coal the aforementioned control system is very dependable. If the plant burns several types of coals with different optimum acid injection rates for the different ones, a more sophisticated control system, such as one dependent on the SO3 content of the flue gas entering the precipitator can be used.

It is important that acid not be permitted to condense on the duct or precipitator provided excellent results, the orifice opening 58 had a diameter of 0.053 inches while the holes 64 through which the acid passes had a diameter of 0.029 inches.

Although there are pressure drops in the system which vary with the length of the lines 16, 18, a typical operating pressure for a nozzle might be 2.5 gph flow and 4 psig pressure for the acid and 2.5 scfm flow and 38 psig pressure for the air. These pressures would be measured at the flange location 44 where the lance penetrates the ductwork.

WHAT WE CLAIM IS: 1. Method of injecting an acidconditioning agent into a flue gas stream containing fly ash to be conditioned to enhance the efficiency with which the fly ash can be electrostatically precipitated comprising the steps of: passing a liquid acid-conditioning agent into the flue gas stream by feeding the agent under pressure through a first line into a lance incorporating a sonic atomizing nozzle producing sonic vibrations capable of breaking up said liquid acid into a mist having a mean droplet size no greater than 10 microns; heating a portion of said first line adjacent said lance so that said acid will be heated and will enter said lance at a temperature higher than ambient but lower than its vaporizing temperature; and passing a gas under pressure through a second line into said nozzle, said gas pressure being at least 10 psig higher than the liquid acid pressure.

2. The method of claim I wherein said nozzle breaks up said liquid acid into a mist having a mean droplet size no greater than 5 microns.

3. The method of claim I wherein said conditioning agent comprises H2SO4 injected at a rate of 15-30 ppm (by weight) acid to flue gas.

4. The method of claim 3 wherein the volume of acid injected is automatically varied with changes in the flow rate of the flue gas stream.

5. The method of claim 1 wherein the acid is heated sufficiently to increase its temperature to at least 930C. before it leaves the nozzle.

6. An apparatus for injecting a mist of liquid acid into a flue gas stream to condition fly ash in the stream and enhance the efficiency with which the fly ash can be electrostatically precipitated downstream of the apparatus comprising: a reservoir for liquid acid; means to supply pressurised air; at least one sonic atomizing nozzle, able to break up the liquid acid by sonic vibrations into a mist having a mean droplet size no greater than 10 microns, a lance mounting the nozzle and adapted, in use of the apparatus to position the nozzle in the flue gas stream; a pump for delivering acid to said lance and nozzle under pressure; said nozzle and lance being connected to said reservoir for acid and said means to supply pressurised air by flow lines, the flow line for the acid being surrounded by heating means for a portion of its length immediately adjacent said lance which heating means is adapted in use of the apparatus to raise the temperature of the acid in the line to a value less than the vaporization temperature.

7. The apparatus of claim 6 wherein said nozzle includes an axially positioned venturi shaped orifice through which air is directed in an axial direction and a plurality of radial openings extending outwardly from said orifice to an acid-containing chamber for delivering acid to said orifice where it is entrained in said air and carried outwardly where it impinges into the sonic energy field and is dissociated.

8. The apparatus of claim 6 or claim 7 wherein the pressurised air supply means can deliver air to the nozzle at a pressure at least 10 psig higher than the pressure of the acid delivered to the nozzle by said pump.

9. The apparatus of any of claims 6 to 8 wherein the acid is continually pumped so that a portion recirculates to the reservoir through a backpressure valve and a portion is delivered to said at least one nozzle, the pressure on the portion delivered to said at least one nozzle being controlled by a flow control valve.

io. The apparatus of claim 9 wherein said flow control valve is operated automatically by a signal which varies in response to the flow rate of flue gases past said nozzle or nozzles.

11. The apparatus of any of claims 6 to 10 wherein the reservoir for acid is a day tank and the means to supply pressurised air is a compressor, said day tank and compressor being mounted on a portable skid member along with said pump.

12. The apparatus of any of claims 6 to 11 wherein a plurality of nozzles are provided and a main rotameter is positioned downstream of said flow control valve to control the total flow to said plurality of nozzles, auxiliary rotameters being provided downstream of said main rotameter for permitting the flow to each individual nozzle to be controlled.

13. The apparatus of any of claims 6 to 12 wherein said pump is a positive displacement metering pump.

14. The apparatus of claim 13 wherein a plurality of nozzles and a plurality of positive displacement metering pumps are provided.

15. The apparatus of any of claims 6 to 14 wherein said nozzle is made of tantalum in

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (18)

**WARNING** start of CLMS field may overlap end of DESC **. provided excellent results, the orifice opening 58 had a diameter of 0.053 inches while the holes 64 through which the acid passes had a diameter of 0.029 inches. Although there are pressure drops in the system which vary with the length of the lines 16, 18, a typical operating pressure for a nozzle might be 2.5 gph flow and 4 psig pressure for the acid and 2.5 scfm flow and 38 psig pressure for the air. These pressures would be measured at the flange location 44 where the lance penetrates the ductwork. WHAT WE CLAIM IS:

1. Method of injecting an acidconditioning agent into a flue gas stream containing fly ash to be conditioned to enhance the efficiency with which the fly ash can be electrostatically precipitated comprising the steps of: passing a liquid acid-conditioning agent into the flue gas stream by feeding the agent under pressure through a first line into a lance incorporating a sonic atomizing nozzle producing sonic vibrations capable of breaking up said liquid acid into a mist having a mean droplet size no greater than 10 microns; heating a portion of said first line adjacent said lance so that said acid will be heated and will enter said lance at a temperature higher than ambient but lower than its vaporizing temperature; and passing a gas under pressure through a second line into said nozzle, said gas pressure being at least 10 psig higher than the liquid acid pressure.

2. The method of claim I wherein said nozzle breaks up said liquid acid into a mist having a mean droplet size no greater than 5 microns.

3. The method of claim I wherein said conditioning agent comprises H2SO4 injected at a rate of 15-30 ppm (by weight) acid to flue gas.

4. The method of claim 3 wherein the volume of acid injected is automatically varied with changes in the flow rate of the flue gas stream.

5. The method of claim 1 wherein the acid is heated sufficiently to increase its temperature to at least 930C. before it leaves the nozzle.

6. An apparatus for injecting a mist of liquid acid into a flue gas stream to condition fly ash in the stream and enhance the efficiency with which the fly ash can be electrostatically precipitated downstream of the apparatus comprising: a reservoir for liquid acid; means to supply pressurised air; at least one sonic atomizing nozzle, able to break up the liquid acid by sonic vibrations into a mist having a mean droplet size no greater than 10 microns, a lance mounting the nozzle and adapted, in use of the apparatus to position the nozzle in the flue gas stream; a pump for delivering acid to said lance and nozzle under pressure; said nozzle and lance being connected to said reservoir for acid and said means to supply pressurised air by flow lines, the flow line for the acid being surrounded by heating means for a portion of its length immediately adjacent said lance which heating means is adapted in use of the apparatus to raise the temperature of the acid in the line to a value less than the vaporization temperature.

7. The apparatus of claim 6 wherein said nozzle includes an axially positioned venturi shaped orifice through which air is directed in an axial direction and a plurality of radial openings extending outwardly from said orifice to an acid-containing chamber for delivering acid to said orifice where it is entrained in said air and carried outwardly where it impinges into the sonic energy field and is dissociated.

8. The apparatus of claim 6 or claim 7 wherein the pressurised air supply means can deliver air to the nozzle at a pressure at least 10 psig higher than the pressure of the acid delivered to the nozzle by said pump.

9. The apparatus of any of claims 6 to 8 wherein the acid is continually pumped so that a portion recirculates to the reservoir through a backpressure valve and a portion is delivered to said at least one nozzle, the pressure on the portion delivered to said at least one nozzle being controlled by a flow control valve.

io. The apparatus of claim 9 wherein said flow control valve is operated automatically by a signal which varies in response to the flow rate of flue gases past said nozzle or nozzles.

11. The apparatus of any of claims 6 to 10 wherein the reservoir for acid is a day tank and the means to supply pressurised air is a compressor, said day tank and compressor being mounted on a portable skid member along with said pump.

12. The apparatus of any of claims 6 to 11 wherein a plurality of nozzles are provided and a main rotameter is positioned downstream of said flow control valve to control the total flow to said plurality of nozzles, auxiliary rotameters being provided downstream of said main rotameter for permitting the flow to each individual nozzle to be controlled.

13. The apparatus of any of claims 6 to 12 wherein said pump is a positive displacement metering pump.

14. The apparatus of claim 13 wherein a plurality of nozzles and a plurality of positive displacement metering pumps are provided.

15. The apparatus of any of claims 6 to 14 wherein said nozzle is made of tantalum in

at least the portions thereof which are contacted by heated acid.

16. The apparatus of any of claims 6 to 15 wherein the lance and the portions of the flow lines which carry heated acid are lined with polytetrafluoroethylene.

17. An apparatus for injecting a finely particled mist of liquid acid in to a flue gas stream containing fly ash substantially as hereinbefore described with reference to Figures I to 4, with or without Figure 5 or Figure 6, of the accompanying drawings.

18. A method for injecting an acidconditioning agent into a flue gas stream containing fly ash carried out substantially as hereinbefore described or exemplified.