Method of conditioning flue gas

The collection characteristics of particles entrained in a particle-laden gas for collection by an electrostatic precipitator are improved by injecting finely divided sodium and ammonium phosphate salts into a particle-laden gas stream formed by the burning of coal. Sufficient additive is injected to provide 24-1200 grams per metric ton of coal burned to form the gas. After injection, the stream is directed through a heat exchange means and finally into the precipitator to collect the particles therein.

BACKGROUND OF THE INVENTION 
This invention relates generally to the separation of particulate material 
from a gas stream and specifically to a method of chemically conditioning 
a particle-laden gas stream so that the particles may be efficiently 
removed in an electric field. 
DESCRIPTION OF PRIOR ART 
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 grounded 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, low sulfur coal, i.e., less than one percent sulfur, 
produces particles having high resistivity, e.g., 10.sup.13 
ohm-centimeters resistivity; coal with 3-5 percent sulfur produces 
particles having 10.sup.8 -10.sup.10 ohm-cm resistivity and poor 
combustion of coal produces particles having 10.sup.4 -10.sup.5 ohm-cm 
resistivity. 
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 it 
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 used 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 
references 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. 
U.S. Pat. No. 3,523,407 describes a process for injecting water, ammonia 
and, when it is not present as a combustion product, SO.sub.3, to alter 
the resistivity of entrained dust and make it easier to collect in an 
electrostatic precipitator. The water and ammonia are injected, preferably 
separately, prior to the passage of the flue gas through the preheater in 
an area where the temperature is at least 400.degree. F. (204.degree. C.) 
and preferably at least 450.degree. F. (232.degree. C.). The disadvantages 
of this approach are obvious. First, depending on the gas to be treated 
one needs either two or three complete injection systems, and one must 
handle at least one and sometimes two toxic gases. Second, a relatively 
large amount (i.e., 40-80 gals.) of water must be injected per million 
cubic feet of flue gas, and the amount of water must be varied depending 
on the SO.sub.3 content of the gas being conditioned. Third, the 
conditioning depends on a chemical reaction occurring in the flue; e.g., a 
molecule each of ammonia, water and sulfur trioxide combining to form 
ammonium bisulfate. 
U.S. Pat. No. 3,284,990 describes the use of phosphoric acids to reduce the 
resistivity of fly ash and enhance its collectability in an electrostatic 
precipitator. The phosphoric acids are formed in situ by injection of 
elemental phosphorus into the flue gas stream. The phosphorus burns to 
give phosphorus pentoxide which subsequently reacts with the water present 
in the flue gas and produces various phosphoric acids that act as the 
actual resistivity-modifying agents. The effectiveness of phosphorus is 
attributed to the extremely hygroscopic nature of phosphorus pentoxide 
initially formed. Because of its hygroscopicity, phosphorus pentoxide 
extracts water from the flue gas to form phosphoric acids which coat the 
fly ash particles with a highly conductive layer and thereby reduce the 
resistivity. It is also stated that the phosphoric acid is significantly 
less corrosive to boiler surfaces than sulfuric acid formed by the 
reaction of sulfur trioxide with water when sulfur trioxide is used as a 
fly ash conditioning agent. 
The effects of phosphorus pentoxide on the performance of electrostatic 
precipitators have also been reported in a paper presented at the American 
Power Conference in April, 1977*. In this study precipitator power input 
was found to decrease with increasing phosphorus pentoxide content of the 
fly ash. The conclusion was drawn that "the presence of high levels of 
phosphorus in the fuel ash exerts a strong detrimental effect on 
precipitator electrical operation and plume opacity." This conclusion is 
in direct contrast to the observations of the present invention in which 
the use of phosphate salts as conditioning agents greatly enhances 
precipitator performance. 
FNT *A. B. Walker, Operating Experience with Hot Precipitators on Western Low 
Sulfur Coals, American Power Converence, Chicago, Ill., April, 1977. 
Sodium salts have been used to reduce fly ash resistivity and enhance 
electrostatic precipitator performance, but in a manner different from 
that described in the present invention. This work, reviewed by R. E. 
Bickelhaupt**, involved the incorporation of Na.sub.2 O as an integral 
part of fly ash by addition of sodium compounds to the coal before 
combustion, thereby lowering the bulk resistivity of fly ash produced from 
the coal. This method has the disadvantage of: (1) requiring an 
uneconomically high concentration of conditioner (up to 2.5% added 
Na.sub.2 O based on the ash); (2) possibly increasing the fouling or 
slagging potential of the coal because of the high sodium concentration. 
In contrast, since the method of the present invention alters only the 
surface resistivity of the fly ash, a much lower conditioner concentration 
is required (typically equivalent Na.sub.2 O=0.11% of the ash at a rate of 
300 grams of disodium phosphate per metric ton and an ash content of 12%). 
Also, since the conditioner is added to the flue gas stream well past the 
combustion zone of the boiler, it does not alter the slagging or fouling 
tendency of the fly ash. 
FNT **R. E. Bickelhaupt, Sodium Conditioning to Reduce Fly Ash Resistivity, 
Environmental Protection Agency Technology Serial, EPA-650/2-74-092, 
October, 1974. 
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. 
Another object is to provide such a method where only one injection system 
is needed to inject the conditioning agent. 
A further object is to provide such a method where there is no necessity to 
handle one or more toxic gases. 
It is also an object to provide such a method using a conditioning agent 
which is much less corrosive to boiler surfaces than either sulfuric or 
phosphoric acids. 
It is a further object to provide a method which conditions the 
particle-laden gas stream using a much smaller quantity of conditioning 
agent than hitherto through possible and without the risk of boiler 
slagging or fouling.

SUMMARY OF THE INVENTION 
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 the formation of a mixture of the particle-laden gas and finely 
divided ammonium or sodium phosphate salts such as (NH.sub.4).sub.2 
HPO.sub.4, NH.sub.4 H.sub.2 PO.sub.4, Na.sub.2 HPO.sub.4, NaH.sub.2 
PO.sub.4, Na.sub.3 PO.sub.4 which contains 24-1200 grams of the phosphate 
salt per metric ton of coal burned to form the gas. Preferably the mixture 
contains 60-480 grams of phosphate salt per metric ton of coal burned to 
form the gas, this being a significantly low weight range compared to 
prior art additives. The phosphate salt may be added to the gas in the 
form of either a dry powder or an aqeous solution. The location of the 
area of injection of the phosphate salt into the flue gas stream should be 
chosen to provide adequate dispersal of the powder or volatilization and 
dispersal of the aqueous solution prior to passage of the flue gas stream 
through the electrostatic precipitator. 
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 diammonium 
phosphate as a 20-40% aqueous solution into a stream of particle-laden gas 
formed by the burning of coal. Sufficient diammonium phosphate is injected 
to provide 24-1200 and preferably 60-480 grams of diammonium phosphate per 
metric ton of coal burned to form the gas. After injection, the gas stream 
is directed through the heat exchange means and finally into the 
precipitator to collect the particles therein. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The conditioners useful in the present invention are finely divided 
phosphate salts (e.g., diammonium phosphate, (NH.sub.4).sub.2 HPO.sub.4 ; 
monoammonium phosphate, NH.sub.4 H.sub.2 PO.sub.4 ; disodium phosphate, 
Na.sub.2 HPO.sub.4 ; monosodium phosphate NaH.sub.2 PO.sub.4 ; trisodium 
phosphate, Na.sub.3 PO.sub.4, and mixtures thereof. The conditioner may be 
utilized either in dry form (for example, as a powder of finely divided 
particles) or preferably as an aqueous solution. 
The amount of conditioner to be injected into the gas stream 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 24-1200, and preferably the quite 
low values of 60-480 grams of the conditioner agent (e.g., diammonium 
phosphate) per metric ton of coal burned to form the gas. Since the weight 
of flue gas is dependent on the weight of coal burned, another way of 
expressing this value is about 2.3-115, and preferably 5.8-46, parts by 
weight of conditioner per million parts by weight of flue gas, and in 
particular this would be an appropriate way to designate conditioner 
amount when the gas was not a product of coal combustion. 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 conditioner determined according to the foregoing criteria 
is preferably added in the form of an atomized aqueous solution, 
preferably a 20-40% by weight solution, depending upon the solubility 
limits of the specific salt used. Higher or lower concentration may be 
used; however, as the function of the water is merely to facilitate 
injection of the conditioner 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 mechanism by which the conditioner of the present invention changes the 
resistivity of the particles in the gas stream is not fully understood. 
One possible explanation is analogous to that advanced in U.S. Pat. No. 
3,523,407, i.e., that the entrained dust particles become enveloped in a 
film or coating of the phosphate salt. Since the phosphate salt is a 
better conductor of electricity than the minerals normally present in fly 
ash, electric current can flow over the surface of the ash particles 
rather than through them. The effect of this phenomenon is to lower the 
apparent resistivity of the fly ash and improve its collectability by an 
electrostatic precipitator. 
Regardless of the operative mechanism, it can be readily shown that the 
present method represents a significant improvement over previous methods 
employing phosphoric acids or combinations of reagents requiring in situ 
formation of the conditioner. FIG. 1 shows the results of laboratory 
resistivity determinations on fly ash coated with various conditioning 
agents at a level of 0.5 wt. % under controlled conditions. This level 
corresponds to a treatment rate of 500 g. of conditioner per metric ton of 
a coal containing 10 percent ash. Although phosphoric acid reduces the 
resistivity of the fly ash, several sodium and ammonium phosphate salts 
tested were even more effective. Within the range of 
120.degree.-160.degree. C., the average operating temperature range of an 
electrostatic precipitator, diammonium phosphate, which is the preferred 
conditioner of the present invention, gave a resistivity of about 
10.sup.11 ohm-cm, which is lower than the 10.sup.12 ohm-cm resistivity 
observed for phosphoric acid by a factor of more than ten. The other 
additives of the present invention show an improvement factor of five or 
greater. In addition, the conditioners of the present invention are less 
corrosive to boiler surfaces than either sulfuric or phosphoric acids. 
FIG. 2 shows the results of laboratory resistivity measurements on a 
different fly ash sample before and after treatment with Na.sub.3 PO.sub.4 
in accordance with the present invention. A decrease in resistivity by 
greater than a factor of 100 is indicated in the usual operating range of 
125.degree.-150.degree. C. 
Another important advantage of the present invention arises out of the fact 
that the conditioners are effective irrespective of the chemical content 
of the gas being conditioned; that is, their effectiveness does not depend 
on dust particles or the gas including a particular initial chemical 
substance (such as an oxide of sulfur) which would then combine with the 
condition in situ to condition the particles. Such dependency upon an in 
situ chemical reaction was one shortcoming of several 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 sulfur fuels. 
It will be recognized that an important feature of the present invention is 
the injection of the conditioner into a gas stream having the proper 
temperature range. It is probable that the gas temperature at the point of 
injection must be sufficiently high to insure proper volatilization of 
water carrier when present and dispersal of the conditioner prior to 
contact of the conditioner with the air preheater means or any other heat 
exchange unit which the conditioner might deposit upon and/or clog. When 
the gas stream at the point of injection is at least 200.degree. C., the 
specified quantities of conditioner volatilize and disperse with 
sufficient speed for this purpose, but at least diammonium phosphate works 
well when injected at temperatures as low as 100.degree.-120.degree. C. 
Whether or not, or the extent to which, these temperatures produce 
volatilization of the water carrier is not known for certain, but the 
operability of the process at those temperatures is known. Of course, if 
there are not heat exchange units intermediate the point of injection and 
the collector, even somewhat lower injection temperatures may be tolerated 
provided they are effective to disperse the conditioner 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 dispersed conditioner and any of its decomposition products with the 
particles entrained in the gas stream. 
The maximum temperature of injection should also be regulated since 
excessively high temperatures will result in decomposition of the 
conditioner to less effective reaction products. Loss of activity can also 
result from reaction of the conditioner with the fly ash, particularly 
when the conditioner is introduced into an area of the boiler where the 
fly ash is in a molten state. In general, a maximum of about 900.degree. 
C. is appropriate. 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 the 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 steam entering the 
ball-room is typically slightly under 900.degree. C., and the temperature 
of the gas stream entering the air preheater is typically about 
300.degree. C. In this situation, the preferred location for the injection 
ports for the conditioner would be somewhere between the ball-room entry 
duct and the entrance to the air preheater. 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 include the temperature of the gas stream at such 
points, the accessibility 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, since 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 be alternatively injected on 
an intermittent or periodic basis. 
The following examples will serve to illustrate the application of the 
present invention. Particulate emission levels, expressed in the examples 
as kilograms per hour, are conveniently measured by the procedure given in 
EPA Method #5 as described in the Federal Register, Vol. 36, No. 247, Part 
II, pp. 24, 888-24-890 (Dec. 23, 1971). 
EXAMPLE I 
A 125 Megawatt design capacity forced draft boiler with two Ljungstrom 
heaters had been equipped with an American Standard electrostatic 
precipitator designed for 98% efficiency at 125 Megawatts when burning a 
coal containing 4.6% sulfur and 15% ash. Because of environmental 
restrictions on SO.sub.2 emissions, this boiler was switched to a coal 
containing 0.6% sulfur and 11% ash. While burning the high sulfur coal, 
precipitator efficiency had been quite good, but with the slow sulfur coal 
the particulate emissions reached an unacceptable level of 800-1000 
kilograms per hour. To lower the emission level, a 25% aqueous solution of 
diammonium phosphate was injected into the superheat section of the boiler 
where the flue gas temperature was about 700.degree. C. 
As indicated by the data recorded in Table 1, for a treatment rate of 360 
grams of diammonium phosphate per metric ton of coal burned, the 
particulate emissions were reduced to about 12% of the untreated level at 
equivalent boiler loads. This reduction in emissions was accomplished 
without significant increase in air heater differential pressure 
indicating that no air heater pluggage occurred during treatment. 
In addition to particulate emission levels, in situ fly ash resistivity 
measurements were made. The observed reduction in fly ash resistivity from 
an untreated level of 7.88.times.10.sup.11 ohm centimeters to 
4.92.times.10.sup.10 ohm centimeters during treatment accounts, at least 
in part, for the observed improvement in precipitator efficiency. 
TABLE 1 
______________________________________ 
Fly Ash 
Treatment Rate 
Emissions, Resistivity 
Grams/Metric Ton 
Kilograms/Hour Ohm-Centimeters 
______________________________________ 
None 866 7.88 .times. 10.sup.11 
360 103 4.92 .times. 10.sup.10 
______________________________________ 
EXAMPLE II 
A 390 Megawatt capacity balanced draft boiler was designed to burn coal 
containing 2.5% sulfur and 13% ash. After passing through two horizontal 
Ljungstrom air heaters, flue gas from the boiler was directed first 
through a mechanical fly ash collector and finally through an 
electrostatic precipitator. A change to coal containing only 1.2% sulfur 
resulted in a deterioration in precipitator performance, and, 
consequently, an increase in particulate emissions. 
An improvement in precipitator efficiency was achieved by injection of a 
25% aqueous solution of diammonium phosphate into the boiler in the 
superheat area where flue gas temperatures of 540.degree.-620.degree. were 
observed. The reduction in particulate emissions due to injection of 
diammonium phosphate into the flue gas is shown in Table 2. At equivalent 
boiler conditions particulate emissions were reduced by 24% from an 
untreated level of 306 kilograms per hour to a treated level of 234 
kilograms per hour while using diammonium phosphate at a rate of 120 grams 
per metric ton of coal burned. In situ fly ash resistivity measurements 
showed a reduction from the untreated level of 1.72.times.10.sup.11 ohm 
centimeters to 6.93.times.10.sup.10 ohm centimeters during injection of 
diammonium phosphate. 
TABLE 2 
______________________________________ 
Fly Ash 
Treatment Rate 
Emissions, Resistivity 
Grams/Metric Ton 
Kilograms/Hour 
Ohm-Centimeters 
______________________________________ 
None 306 1.72 .times. 10.sup.11 
120 234 6.93 .times. 10.sup.10 
______________________________________ 
EXAMPLE III 
755 Megawatt balanced draft boiler with two Ljungstrom air heaters and a 
tubular air heater had been equipped with a Research Cottrell 
precipitator. In order to meet particulate emissions requirements the 
precipitator was designed for greater than 97% collection efficiency when 
burning coal containing 0.6% sulfur and 18-20% ash. Because of an increase 
in ash content of the coal to 21-24% and some deterioriation of the 
precipitator, collection efficiency had decreased to about 95%, which was 
insufficient to maintain compliance emission levels. In order to reduce 
the particulate emission level, a 25% aqueous solution of diammonium 
phosphate was injected into the primary superheat area of the boiler where 
the temperature was about 600.degree.-700.degree. C. 
The data in Table 3 show that at a treatment rate of 120 g. of diammonium 
phosphate per metric ton of coal burned particulate emissions were reduced 
by 35% at equivalent boiler loads. This degree of improvement represented 
an increase in efficiency from the untreated level of 95% to about 96.7% 
which was sufficient to achieve compliance emission levels. 
TABLE 3 
______________________________________ 
Treatment Rate, 
Emissioons, Fly Ash Resistivity 
Grams/Metric Ton 
Kilograms/Hour Ohm Centimeters 
______________________________________ 
none 2499 3.6 .times. 10.sup.11 
120 1631 2.3 .times. 10.sup.11 
______________________________________ 
Although a significant improvement in precipitator efficiency was observed 
during the injection of diammonium phosphate, fly ash resistivity 
measurements made in this case did not reveal a substantial change 
compared to untreated fly ash. It is not clear why, in this instance, the 
measured fly ash resistivity figures did not show a change of the same 
magnitude as in Examples I and II despite the fact that the precipitator 
efficiency was signficantly improved. There are, however, several other 
mechanisms which may be at work here--the diammonium phosphate may cause 
agglomeration of the particles, or the diammonium phosphate may affect the 
overall nature of the fluid system by producing a space charge effect 
which will aid the electrostatic precipitator. The precise mechanism here 
operative is not known, but the improvement in precipitator efficiency is 
marked. 
From the above, it will be seen that the use of sodium and potassium 
phosphate salts as conditioning agents to improve the action of the 
electrostatic precipitator on particles entrained in a particle-laden gas, 
and particularly in a particle-laden flue gas has several significant 
advantages. They are useful over a very wide temperature range, they 
provide significant precipitation improvement even when used in quantities 
which are very low compared with prior art conditioners, they do not 
present the corrosion problems that many of the prior art conditioners 
present, they have no undesirable tendency toward boiler slagging or 
fouling, and they do not produce toxic or noxious gases. 
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. Accordingly, the spirit and 
scope of the present invention is to be limited only by the appended 
claims, and not by the foregoing disclosure.