Improved methods are provided for the efficient and economic removal of sulfur dioxide from gases, which comprise contacting the gas with neutralizing values obtained from Type S hydrated lime, i.e. calcined dolomite, slaked with water under elevated temperature and pressure. The Type S hydrated dolomitic lime may be used in conjunction with a wet scrubber to provide base and neutralizing magnesium values, in a spray dryer or dry scrubber, or directly introduced into the boiler. The use of Type S hydrated dolomitic lime greatly enhances the efficiency of sulfur dioxide removal, providing for enhanced utilization of base values and more rapid rate of reaction.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
Sulfur is found admixed with a wide variety of fossil fuels and metal ores. 
When oxidizing the naturally occurring minerals and fuels through heating 
or burning, the sulfur oxidation product is sulfur dioxide. Sulfur dioxide 
is a serious pollutant, since in itself it forms sulfurous acid, which in 
itself and also upon oxidation to sulfuric acid causes serious 
contamination and injurious effects to the environment. 
One of the ways to prevent sulfur dioxide in waste gases from being vented 
to the atmosphere is to treat sulfur dioxide containing gaseous effluents 
with base, either in the liquid or vapor phase. Various bases have been 
used, such as soda ash, lime and the like. 
There are a number of considerations in the choice and manner in which the 
base is employed. The first consideration is economics. There are a number 
of factors involved with the economics. One factor is the cost of the 
alkaline or basic material. A second factor is the efficiency of the 
alkaline material. The less efficient the material, the more costly the 
removal of sulfur dioxide will be, in requiring greater amounts than 
stoichiometric to provide for the desired reduction in sulfur dioxide in 
the effluent and in requiring larger plant, particularly storage, metering 
and recovery equipment. A third consideration is the degree to which the 
sulfur dioxide may be removed with a given amount of the base. 
Besides the base which is employed, another consideration is the equipment 
required for processing and the effect of the materials employed on the 
equipment. There is also the nature of the sludge or residue and the 
manner in which the residue may be disposed of, as well as the condition 
e.g., temperature, of the effluent gas. 
In developing a process for removing sulfur dioxide from effluent gases, 
normally flue gases, all of the above considerations are involved for an 
effective process. In view of the very large volumes of gas which exit 
from power plants, processing plants, and other installations burning or 
oxidizing sulfur containing materials, small improvements in efficiency 
can result in dramatic savings. It is therefore desirable to provide for 
simple and efficient processes utilizing comparatively inexpensive 
materials, which can rapidly reduce sulfur dioxide content in waste gases 
and the like to acceptable levels while producing residues which are 
readily disposable. 
2. Description of the Prior Art 
Type S hydrated dolomitic lime, which is prepared from calcined dolomite, 
is available as a structural material from Flintkote Lime Products. 
Description of the preparation of Type S hydrated lime may be found in 
Boynton, Chemistry and Technology of Lime and Limestone, Interscience 
Publishers, New York, 1965, pages 167, 288-9, 302-307, 317-318, and 
333-338. U.S. Pat. No. 4,046,856 describes a sulfur dioxide removal 
process employing magnesium with recycling of the magnesium as magnesium 
hydroxide. Other patents of interest describing processing of flue gas 
with basic materials include U.S. Pat. Nos. 2,068,882, 3,883,639, 
3,941,378, 3,919,393, 3,991,172, 4,011,299, and 4,018,868. See also, C.A. 
81, 6803u, 82, 63922r, 82 174821b, 84, 155093r, 84, 155095t and Ger. 
Offen. 2,412,372.

SUMMARY OF THE INVENTION 
An improved method is provided for the removal of sulfur dioxide from 
sulfur dioxide containing gases, particularly flue gases from the burning 
of fossil fuels, which comprises contacting the gases with a sufficient 
amount of sulfur dioxide neutralizing values derived from Type S hydrated 
dolomitic lime to substantially reduce the sulfur dioxide content of the 
gas. The contacting can be carried out under various conditions, such as a 
wet scrubber, spray drying, or boiler injection, where the sulfur dioxide 
is rapidly and efficiently neutralized to a product, which may be further 
treated to provide an environmentally acceptable waste product. 
In one embodiment, employing a wet scrubber, Type S hydrated dolomitic lime 
is employed as a source of magnesium sulfite which reacts with sulfur 
dioxide to provide a mixture of magnesium sulfite-bisulfite. A sidestream 
containing the magnesium sulfite-bisulfite is oxidized to sulfate, and the 
magnesium sulfate converted to magnesium hydroxide, which is combined with 
the wet scrubber effluent to provide magnesium sulfite. 
In the "dry" removal of sulfur dioxide from flue gases, a dispersion of 
Type S dolomitic lime is sprayed into the hot flue gases as fine particles 
in an amount and at a rate which provides for the desired level of sulfur 
dioxide reduction while maintaining the temperature of the effluent 
sufficiently above adiabatic saturation to avoid condensation. Use of the 
Type S hydrated dolomitic lime avoids recycling of partially spent 
neutralizing values, while providing efficient sulfur dioxide 
neutralization over a wide range of sulfur dioxide concentrations and 
rapid reaction allowing for smaller equipment and ease of operation. 
DESCRIPTION OF THE SPECIFIC EMBODIMENTS 
An improved and highly efficient method is provided for the removal of 
sulfur dioxide from sulfur dioxide containing gases, particularly flue 
gases, which comprises contacting the sulfur dioxide containing gas with a 
sufficient amount of sulfur dioxide neutralizing values derived from Type 
S hydrated dolomitic lime. The Type S hydrated dolomitic lime may be used 
directly or indirectly, by itself or in combination with other alkaline 
values. Both wet and dry techniques may be employed, such as wet 
scrubbing, spray drying, and boiler injection. 
The sulfur dioxide neutralizing values are at least in part derived from 
Type S hydrated dolomitic lime, which is calcined dolomite, hydrated under 
conditions of elevated temperatures and pressures. As provided by 
Flintkote, Type S hydrated dolomitic lime has a calcium hydroxide content 
of about 55%, magnesium hydroxide content of about 40%, magnesium oxide 
content of about 2% and water of about 0.2%. The settling rate to 1/2 
volume in minutes, (ASTM C-110) is about 225, while the specific gravity 
is 2.24. The sieve analysis shows that 100% of the particles passed 20 
mesh, while 79% of the particles pass 325 mesh. 
As described in Chemistry and Technology of Lime and Limestone, supra, page 
336, Type S dolomitic lime is hydrated in an autoclave at pressures of 
from about 25 to 150 psi, and temperatures from about 250.degree. to 
400.degree. F. 
In the subject method, a sulfur dioxide containing gas is contacted with 
sulfur dioxide neutralizing values derived at least in part from Type S 
hydrated dolomitic lime where essentially all magnesium present is derived 
from Type S hydrated dolomitic lime, the total sulfur dioxide neutralizing 
values being present in sufficient amount to reduce the level of sulfur 
dioxide in the flue gas to substantially less than about 10% of the 
original level, preferably less than about 70 ppm, more usually to a value 
below about 55 ppm. 
Depending upon the mode employed for sulfur dioxide removal, the Type S 
hydrated dolomitic lime will normally be employed as a powder or slurry. 
The amount of solids in the slurry will also vary depending upon the mode 
of sulfur dioxide reduction employed. Usually solids content in slurries 
for wet scrubbing will be at least about 10 weight percent, more usually 
at least about 40 weight, and generally not more than about 80 weight 
percent. 
The wet scrubbing method for sulfur dioxide removal will be considered 
first. In general terms, the sulfur dioxide containing flue gas is 
contacted with a magnesium sulfite solution in a scrubbing zone at a pH in 
the range of about 6-7.5 to produce magnesium bisulfite. The scrubbing 
zone effluent is divided into two streams, a major cycle stream, and a 
minor regeneration stream. The regeneration stream is subjected to 
oxidation to oxidize bisulfite and sulfite to sulfate and at least a major 
portion of the oxidized solution made alkaline with Type S hydrated 
dolomitic lime and other hydrated lime to produce magnesium hydroxide and 
inert insoluble calcium sulfate. The basic stream is then combined with 
the major recycle stream to neutralize the magnesium bisulfite to 
magnesium sulfite and restore lost magnesium values. A second minor side 
stream is taken from the regeneration stream between the scrubber effluent 
and alkalization, usually from the recycle tank or oxidizer, as a solids 
removal stream, which is separated into a sludge fraction rejected and the 
liquid fraction, with the sludge fraction rejected and the liquid fraction 
combind with the major recycle stream. 
The subject wet scrubbing method provides many advantages: calcium is not 
present as an absorbent in the scrubber which avoids scaling; only minor 
amounts of environmentally undesirable soluble magnesium salts are 
discharged; efficient utilization of sulfur dioxide neutralizing values is 
achieved; all of the absorption and neutralization reactions are rapid due 
to the use of Type S hydrated dolomitic lime and regenerated magnesium as 
absorbent, so that the rapidity of the reactions coupled with the 
efficient use of the neutralizing values allows for smaller equipment; and 
sulfur dioxide is efficiently removed from the gas stream to 
environmentally acceptable values. 
While Type S hydrated dolomitic lime has many advantages as described 
above, the subject wet scrubbing process could use any source of make-up 
magnesium which is convertible, either directly or indirectly, to 
magnesium sulfite. 
The wet scrubbing operation will now be considered in greater detail. A wet 
scrubbing unit may be employed of conventional design, such as a 
venturi-type wet scrubbing unit, a spray absorber, a packed tower, a tray 
tower or the like. Continuously fed to the scrubbing tower is the effluent 
from the scrubbing tower mixed with an alkaline stream from the reactor, 
the latter to be described in more detail subsequently. The slurry to the 
scrubber will contain soluble magnesium sulfite, magnesium bisulfite, 
magnesium sulfate, insoluble calcium sulfate, and other calcium and 
magnesium salts, and may also include fly ash or fly ash reactant 
products. The pH of the slurry will be below 8, generaly in the range of 6 
to 7.5. The various insoluble salts will be maintained at a predetermined 
level of suspended solids. Once the system is in equilibrium, the solids 
content of the slurry may be maintained fairly constant without 
significant modification of the various parameters. 
A strongly alkaline scrubbing medium is avoided, preventing the formation 
of basic magnesium carbonate. The magnesium sulfite which is present 
rapidly and efficiently reacts with the sulfurous acid formed from sulfur 
dioxide and water to form a magnesium sulfite-bisulfite mixture, resulting 
in an acidic effluent, with concurrent formation of magnesium sulfate. The 
pH of the exiting effluent will usually be 6.5 or below. Substantially 
complete reaction of the sulfur dioxide takes place, so that substantially 
stoichiometric amounts of the make-up neutralizing materials are provided 
for the amount of sulfur dioxide removed in the scrubber. Neutralization 
base values may not only be derived from the magnesium sulfite, but fly 
ash may provide alkalinity for neutralization of the sulfur dioxide. 
After contacting the scrubbing stream, the effluent gas will pass through a 
mist eliminator. The mist eliminator may be washed with water or 
individual or combinations of streams from other sources, including the 
pond return liquor, thickener overflow, or cooling tower blowdown water. 
The mist eliminator removes entrained solid particles and liquor droplets 
and the substantially sulfur dioxide free gaseous stream is discharged 
from the mist eliminator. 
The effluent slurry exits from the scrubber to a slurry recycle tank. To 
the slurry in the slurry recycle tank is added a stream from the reactor 
tank, which provides the base values for reaction with the sulfur dioxide. 
The base reacts with the magnesium bisulfite to produce magnesium sulfite 
and restore the pH to greater than 6. 
A portion of the slurry in the slurry recycle tank is removed as a minor 
side stream and pumped to the oxidizer. The oxidizer is employed to 
oxidize sulfite to sulfate, primarily magnesium sulfite-bisulfite to 
magnesium sulfate. Conveniently, oxidation is carried out with compressed 
air. 
A variable bleed stream from the recycle tank or oxidizer, preferably the 
latter, is directed to the thickener or settling pond. The rate of flow of 
this sidestream is chosen to provide the desired suspended solids content 
in the system, generally from about 10 to 15 weight percent. The sludge is 
dewatered by conventional means. The overflow containing solubles from the 
sludge separation is transferred to an overflow or holding tank to which 
is also directed the mother liquor from the sludge dewatering unit, which 
may be a pond, if the pond is used for sludge dewatering, or a sludge 
dewatering unit. As indicated previously, the overflow tank contents may 
be used for washing the mist eliminator. 
The reactor tank is employed to provide the desired base or neutralizing 
values. Into the reactor tank is introduced the stream from the oxidizer, 
optionally a small underflow slurry stream from the thickener in 
sufficient amount to maintain the desired concentration of suspended 
solids, and sufficient alkali. The solids serve as nucleating agents for 
the rapid formation of particles, inhibiting scale formation. The base is 
normally Type S hydrated dolomitic lime and other hydrated lime, 
preferably prepared in substantially the same manner as the Type S 
hydrated dolomitic lime to enhance its reactivity. 
The Type S hydrated dolomitic lime is added in sufficient amount to make up 
the lost magnesium, which is lost primarily with the sludge. The other 
hydrated lime, calcium hydroxide, is added in sufficient amount to 
transform a major portion of the non-absorbent magnesium sulfate to 
magnesium hydroxide. The combined Type S hydrated dolomitic lime and the 
calcium hydroxide are added in sufficient amount to provide the necessary 
base values of magnesium hydroxide to react with the magnesium bisulfite 
in the recycle tank to provide adequate sulfur dioxide neutralization 
capacity in the scrubber. 
The slurry formed in the reactor is then transferred to the recycle tank to 
be mixed with the recycle slurry from the scrubber. The slurry may also 
include, besides the various calcium and magnesium compounds, fly ash or 
fly ash products. The Type S hydrated dolomitic lime may be added as a 
powder or slurry or by any other convenient means which provides for the 
desired concentration in the reactor. By employing specially hydrated 
calcium oxide, faster reaction can be achieved, so that the residence time 
in the reactor may be reduced as compared to using normally hydrated lime. 
However, the special hydration is not required, but provides for a more 
efficient process. 
Turning now to a consideration of the other methods for sulfur dioxide 
removal, improved results can also be obtained by spray drying or boiler 
injection with a Type S hydrated dolomitic lime slurry directly injected 
into the flue gas, where the solids content of the slurry is sufficient to 
minimize the temperature reduction to less than about 100.degree. F. 
Normally, exit flue gases will have temperatures in the range of about 
250.degree. to about 300.degree. F. A sufficient amount of the Type S 
hydrated dolomitic lime is introduced to reduce the sulfur dioxide content 
to the desired level. 
As the flue gas passes through a spray dryer, the flue gas is contacted 
with the Type S hydrated dolomitic lime slurry. Contact times can be very 
short, normally involving fractions of a second to a few seconds. The flue 
gas after contact with the Type S hydrated dolomitic lime is then passed 
through a particle separator, which can be a bag filter, precipitator, 
cyclone, scrubber, etc. Under the conditions of the neutralization, much 
of the sulfite which forms will be oxidized to sulfate. 
The spray dryer method of removing sulfur dioxide from a flue gas will be 
described. Commonly, the flue gas which is to be treated will have a 
flow-rate of about 2.5 to 4 cfm/kw, depending upon the heating value of 
the fuel and on the amount of excess air. The temperature will normally be 
in the range of about 240.degree.-350.degree. F., depending upon the 
sulfur content in the fuel. The lower the flue gas exit temperature, the 
higher the boiler efficiency. The composition of the gas will normally 
vary having the following constituents in percent by volume: 
______________________________________ 
FLUE GAS COMPONENT 
RANGE, VOLUME PERCENT 
______________________________________ 
Carbon Dioxide 11-16 
Oxygen 2-6 
Nitrogen 66-80 
Moisture 4-12 
Sulfur Oxides 0.04-0.4 
Nitrogen Oxides 0.04-0.08 
______________________________________ 
In addition, there will normally be from about 0.01 to 1 weight percent of 
fly ash. 
Without flue gas cleaning, the sulfur dioxide emission would vary from 
about 400 ppm for low sulfur fuel containing from about 0.3-0.5% sulfur to 
4,000 ppm for high sulfur fuel containing 3-5% sulfur. The EPA regulations 
limit the sulfur dioxide emission for new large units to 1.2 pounds sulfur 
dioxide/MM BTU and 90% sulfur dioxide removal for high sulfur fuel and to 
0.6 pounds sulfur dioxide/MM BTU and 70% sulfur dioxide removal for low 
sulfur. In some special cases as in a Class I zone, the sulfur dioxide 
emission is limited to 0.1 pounds sulfur dioxide/MM BTU, which is 
equivalent to an emission of 50 ppm sulfur dioxide maximum in flue gas. 
The Type S hydrated dolomitic lime will be employed at a weight percent of 
suspended solids in the range of about 3-15%, depending upon the sulfur 
dioxide concentration in the flue gas. The Type S hydrated dolomitic lime 
has a low settling rate. The low concentration which can be used offers 
many advantages. Among these advantages are that the dispersion may be 
easily atomized to provide fine particles, which permit more efficient 
utilization of the neutralizing values. In addition, there is 
substantially reduced concern with clogging of the feed system. Various 
atomizers can be used, such as rotary atomizers or fluid nozzle atomizers 
to provide droplets in the range of about 5-50, more usually about 10-25 
microns in size. These small diameter particles dry rapidly, so that 
shorter residence times can be employed in the spray dryer. By contrast, 
more conventional lime particles are less reactive and require loading to 
a higher degree in the slurry which in turn introduces more water, and 
makes it difficult or impossible to use them with high sulfur removal 
requirements without decreasing the plant efficiency by starting with a 
higher temperature flue gas. 
The slurry will be introduced into the boiler effluent stream at a greater 
than stoichiometric ratio, generally from about 1.1 to 1.5 mole ratio, 
more usually from about 1.2 to 1.4 mole ratio,. With the effluent from low 
sulfur coal, the stoichiometry will generally be about 1.4, while with 
high sulfur coal, the mole ratio will generally be about 1.1-1.2. The 
effluent from the spray dryer will generally be at a temperature in the 
range of about 145.degree. to 165.degree. F. 
The particles are blown in a fine mist into the effluent and carried 
through the spray dryer to a separation zone. The resulting fine particles 
may be separated by conventional ways, such as bag filters, electrostatic 
precipitators, combinations thereof, or the like. The particles will be 
comprised of calcium sulfite and sulfate, magnesium sulfite and sulfate, 
any unreacted lime, and fly ash, if any. The dry powder can be 
pneumatically or mechanically conveyed and removed to a waste disposal 
area, where it may be wetted or pelleted to prevent wind dispersion. The 
resulting particles which will be a combination of magnesium sulfite and 
sulfate, which are water soluble, and calcium hydroxide, which has 
completely reacted due to stoichiometric excess, will react to modify the 
soluble magnesium salts to insoluble magnesium hydroxide. Thus, the 
particles will be substantially pollutant free, in that they are 
substantially insoluble. 
In order to demonstrate the advantages of using Type S hydrated dolomitic 
lime for removal of sulfur dioxide, Type S hydrated dolomitic lime was 
titrated at a rate of 3 ml/per min with 10% by weight aqueous sulfuric 
acid to a pH of 6. The lime solution had 10 g of the Type S hydrated 
dolomitic lime in 90 ml of water. When pH 6 was reached, approximately 126 
ml of sulfuric acid had been added, with 92% of the dolomitic lime 
neutralized. By comparison, when the same experiment was carried out 
replacing the Type S hydrated dolomitic lime with Type N hydrated lime, at 
pH 6, approximately 56 ml of sulfuric acid was added for an alkali 
utilization of 44%. This experiment demonstrates the much higher 
efficiency of the Type S hydrated dolomitic lime in neutralizing acid in a 
short period of time. 
In the next study, a wet scrubbing pilot plant was employed, where a small 
side stream of flue gas was taken upstream from the electrostatic 
precipitator from a coal burning power plant. The pilot plant employed a 
conventional venturi scrubber. The process was monitored for sulfur 
dioxide, pH, and volume flows. Both Type S hydrated dolomitic lime and 
Type N hydrated lime were tested. The following parameters were 
maintained: flue gas flow rate in the scrubber was about 3,340 acfm at 
250.degree. F. and 13.66 psia; slurry recycle rate was 45 gpm into the 
venturi (L/G=15) (L--liquid rate gallons/min; G--gas rate, thousands of 
cubic feet/min) and 55 gpm into the absorption spray (L/G=18) or a total 
of L/G=33; percent suspended solids in the recycle slurry was about 12% by 
weight; residence time in the recycle tank was about 8 min; pressure drop 
in the venturi for fly ash removal was about 17 inches water; mist 
eliminator washing was about 3 gpm (0.5 gpm/ft.sup.2); wash tray feed was 
11 gpm and wash tray underspray was about 2.4 gpm. 
The process for this study was performed as follows: Flue gas from a 
pulverized coal fired boiler containing about 2 grains of fly ash per dry 
standard cubic foot and about 600-1,000 ppm of sulfur dioxide was passed 
to a venturi scrubber where the gas was contacted with a recycle tank 
slurry flow of 45 gpm. The high gas velocity in the venturi throat (200 
ft/sec) atomized the liquor into fine droplets which contacted the fly ash 
fine particles and removed them from the gas stream. The gas and slurry 
were then directed to the absorption spray vessel. The flue gas passed 
through the slurry absorption spray (55 gpm), where most of the sulfur 
dioxide was removed. The gas temperature dropped to about 125.degree. F. 
and the gas was water saturated. The flue gas with the entrained droplets 
was then passed through the wash tray, where the entrained slurry droplets 
were removed from the gas bubbles. The gas stream was then passed through 
the mist eliminator where most of the liquid mist was removed. 
The gas was then heated to a temperature of about 175.degree. F. by means 
of heat exchange with a contribution of about 12.degree. F. resulting from 
the I.D. fan. The clean reheated flue gas was then exhausted to the 
atmosphere through a stack. 
The slurry from the venturi tank and from the absorption spray was directed 
by gravity into a recycle tank. 
A stream of about 2-3 gpm of slurry was bled from the recycle tank to the 
oxidizer, where compressed air was bubbled through the stream to oxidize 
all of the magnesium sulfite and bisulfite into magnesium sulfate, as well 
as minor amounts of calcium sulfite and bisulfite to calcium sulfate. The 
overflow stream from the oxidizer was divided into a first stream of about 
1 gpm which was directed to a thickener tank or settling tank, while a 
second stream of about 2 gpm was directed to a reactor tank. The first 
stream was employed to maintain a constant solids inventory in the system. 
With the present parameters, about 12% suspended solids was maintained. 
The solids provided for nucleation in the reaction tank for rapid 
precipitation of supersaturated calcium sulfate, thus avoiding scaling in 
the reactor tank. 
The underflow or sludge from the thickener was pumped about every two hours 
into a barrel from which the supernatant was pumped into the recycle tank 
and the sludge containing about 50% moisture was pumped to an outside 
pond. 
Into the reactor tank was directed Type S hydrated dolomitic lime and Type 
N hydrated lime as dry solids. If desired, slurries could be employed in 
place of the dry solids and preferably the hydrated lime would be hydrated 
in the same manner as the Type S hydrated dolomitic lime. The ratio was 
about three parts by weight of hydrated lime to one part by weight of 
hydrated dolomitic lime. For an inlet flue gas sulfur dioxide content of 
about 1,050 ppm, the feed rate of hydrated dolomitic lime was about 35 
g/min, while the feed rate of hydrated lime was about 140 g/min to provide 
a final flue gas sulfur dioxide content of about 50 ppm (0.1 lb sulfur 
dioxide per million BTU heat input). For a sulfur dioxide inlet flue gas 
content of about 750 ppm, with the same sulfur dioxide emission level, the 
feed rate was about 28 g/min hydrated dolomitic lime and 90 g/min hydrated 
lime. 
In the reactor tank, the soluble magnesium sulfate reacted with the calcium 
hydroxide to produce magnesium hydroxide and calcium sulfate, both of low 
solubility. The effluent from the reactor rank was pumped to the recycle 
tank at a rate of about 2.5 gpm. Conveniently, the addition of the 
hydrated dolomitic lime and hydrated lime may be followed by the pH of the 
mixture in the reactor tank, with the pH in the reactor tank varying 
between 9 and 10, which provides a pH in the recycle tank of between about 
6 and 7. 
The ratio of hydrated dolomitic lime to hydrated lime is varied depending 
upon the magnesium sulfate concentration in the thickener tank underflow, 
increasing the ratio when the magnesium sulfate concentration is less than 
40,000 ppm and vice versa. 
To insure the desired solids content in the reactor a portion of the 
underflow from the thickener may be continuously transferred to the 
reactor tank. 
It should be noted that the fly ash had residual basicity as basic calcium 
and reacted with the magnesium sulfate in the liquor to produce magnesium 
hydroxide and calcium sulfate. 
The amount of base provided as hydrated dolomitic lime and hydrated lime 
will vary depending upon the sulfur dioxide concentration and the flue 
gas. At 700-750 ppm of sulfur dioxide, approximately 90% of stoichiometric 
is employed, while at 1050 ppm of sulfur dioxide, approximately 100% of 
stoichiometric is employed. However, if the fly ash were to be removed and 
the sludge dewatered to about 20% moisture, the total base added would be 
about 110% of stoichiometric. 
In accordance with the above described procedure two runs were carried out, 
where the base was provided as a mixture of Type S hydrated dolomitic lime 
and hydrated lime in one run, as compared to another run where base was 
provided as hydrated lime. The following table indicates a number of the 
parameters of the run over one day and the results. 
TABLE I 
______________________________________ 
average SO.sub.2 
lbs SO.sub.2 
ppm MBTU 
inlet outlet outlet 
______________________________________ 
Type S Hydrated 
Dolomitic Lime & 
Hydrated Lime 1039 50.7 0.101 
Hydrated Lime 920 394 0.79 
______________________________________ 
Lower outlet SO.sub.2 levels than 0.79 lbs SO.sub.2 /M BTU would be 
difficult to realize without scaling under the process conditions selected 
when hydrated lime was employed. However, the use of Type S hydrated 
dolomitic lime and hydrated lime together in the reactor tank made it 
possible to operate in the scrubber with soluble magnesium sulfite (a very 
good SO.sub.2 absorbent) and, therefore, to achieve higher SO.sub.2 
removal efficiency without scaling. 
The subject wet scrubber method employing the Type S hydrated dolomitic 
lime has a number of advantages which are extremely important in a system 
where very large amounts of materials are required, large volumes of 
liquid must be pumped and recycled, and the waste material discharged 
should be relatively free of pollutants. In the subject method, very high 
sulfur dioxide removal efficiency is achieved to a level of less than 0.2 
pound SO.sub.2 /per million BTU emission rate. In addition, the sulfur 
dioxide is captured as magnesium bisulfite, which is then oxidized to 
magnesium sulfate, which greatly reduces or eliminates the problem of 
scaling which is typically encountered when calcium hydroxide reacts with 
sulfur dioxide. 
The subject wet scrubber method has relatively low operating costs, because 
it involves relatively low pumping rates, relatively small material 
transfers, and high alkali utilization. Because of the greater efficiency, 
smaller equipment may be employed, so that capital costs are reduced. 
A further advantage is that cooling tower blowdown water may be employed as 
make-up water. Because the cooling tower blowdown water has relatively 
high levels of dissolved calcium, the potential for supersaturation of 
calcium in the medium is enhanced when calcium hydroxide is employed, with 
greater possibilities for scaling. In the subject system, where the 
calcium hydroxide exists in the reactor only to react with magnesium 
sulfate to provide magnesium hydroxide, together with calcium sulfate 
which is substantially insoluble and inert, the problem of supersaturation 
potential of the calcium is greatly reduced, and thus the potential for 
scale formation is reduced. 
In the next study, Type S hydrated dolomitic lime was used in a dry system. 
To demonstrate the efficiency of the subject system, a number of runs were 
carried out in a spray dryer pilot plant to compare the results with Type 
S hydrated dolomitic lime and Type N hydrated lime. The pilot test system 
was designed to treat 400 acfm of flue gas. The system consisted of: (1) 
an air heater including a heater shell and a gas burner to produce the hot 
flue gas; (2) a spray dryer to remove sulfur dioxide; (3) a fabric filter 
to collect dry FGD solids, as well as remove additional sulfur dioxide; 
(4) an induced draft fan to move the flue gas through the system; (5) a 
feed tank with an agitator and a metering pump to supply the alkaline 
slurry to the system; (6) a sulfur dioxide cylinder on the weight scale 
and a delivery system to provide the amounts of sulfur dioxide to the flue 
gas for tests at different inlet sulfur dioxide concentration; and (7) 
duct work for flue gas transport. 
The pilot spray dryer was an insulated Stork-Bowen standard laboratory unit 
conical type, 2'-6" diameter x 2'-9" high cylinder section with 65.degree. 
conical bottom equipped with a variable speed rotary disc atomizer as 
manufactured by Stork-Bowen Engineering, Inc. (Type AT-4 complete with 0.5 
hp air turbine and Type CSC 2" atomizer rated maximum 50,000 rpm at no 
load). The rotary atomizer can be replaced by a two-fluid type nozzle 
atomizer located in the drying chamber with the head (co-current air flow) 
provided with different types of spray nozzles. 
The spray dryer is also provided with a process air-heater. The hot flue 
gas, into which is injected sulfur dioxide, enters the spray dryer chamber 
from an overhead plenum and is then directed to fixed-vanes, which impart 
turbulent spiral gas flow into the chamber. 
The slurry fed to the atomizer disc or to the two fluid nozzle atomizer is 
dispersed directly into the gas flow, providing an optimized gas-liquid 
contact. The turbulent mixing of a flue gas and finely atomized droplets 
of absorbent slurry effects rapid sulfur dioxide absorption and 
evaporation of the droplets. The scrubbed flue gas containing fine dry 
particles leaves the dryer via an outlet elbow and duct work to the fabric 
filter. 
During testing, conditions are held as fixed values until the system 
operation responses, particularly sulfur dioxide removal efficiency and 
temperature drop, are measured. System responses to changes in 
stoichiometry are observed on a sulfur dioxide analyzer, which samples 
both inlet and outlet streams from the spray dryer system. A close 
material and heat balance is observed. Thus, the flue gas flow rate 
through the spray dryer is determined, based on the heat balance around 
the spray dryer. The slurry concentration is measured and the weight of 
the slurry flowing into the spray dryer is recorded every 10 minutes. The 
amount of sulfur dioxide injected into the flue gas is also regularly 
recorded. 
In demonstrating the high efficiency of the Type S hydrated dolomitic lime, 
using the commonly employed Type N hydrated lime, the following variables 
were studied: Absorbent/sulfur dioxide stoichiometry; inlet sulfur dioxide 
concentration; inlet gas temperature; outlet gas temperature; and type of 
atomization. 
The range of pilot test variables is set forth in the following table. 
TABLE II 
______________________________________ 
Pilot Test Range 
Variables Min. Max. 
______________________________________ 
inlet SO.sub.2 conc., ppm 
500 2,700 
stoichiometry in, 
absorbent/SO.sub.2 mole ratio 
0.5 2.4 
inlet flue gas, T .degree.C. 
140 160 
outlet flue gas, T .degree.C. 
65 80 
inlet flue gas flow rate, ACFM 
250 400 
absorbent slurry feed conc., in. % 
5.0 15.0 
recycle ratio, 
recycle/make up absorbent lb/lb 
0 2 
rotary atomizer 
disc diameter, in 2 2 
disc speed, rpm 30,000 40,000 
two-fluid nozzle atomizer 
type of mixing internal external 
______________________________________ 
Based on the individual data obtained, the following overall results were 
observed and conclusions derived. 
Maintaining the inlet sulfur dioxide concentration at 1,000 ppm, while 
employing a stoichiometric ratio of absorbent/SO.sub.2 in the range of 
0.67-0.75, the Type S hydrated dolomitic lime removed 82.5% of the sulfur 
dioxide, while the Type N hydrated lime removed only 67.5%. 
Also, the high sulfur dioxide removal does not appear to depend upon the 
sulfur dioxide concentration of the inlet flue gas when the Type S lime is 
used at a stoichimetric ratio in the range 1.2-1.5. However, the sulfur 
dioxide removal shows a considerable fall-off in performance at higher 
sulfur dioxide concentrations in the inlet flue gas, when Type N hydrated 
lime is used as shown by the following table. 
TABLE III 
______________________________________ 
Inlet SO.sub.2 SO.sub.2 Removal, % 
ppm Type S Type N 
______________________________________ 
.sup. 500 86.5 75.0 
1,000 84.5 67.5 
1,500 83.5 59.0 
2,700 82.0 -- 
______________________________________ 
The Type S hydrated lime shows only a very small drop-off in efficiency 
with increasing amounts of sulfur dioxide, while by contrast the Type N 
hydrated lime suffers substantial inefficiency to the point of questioning 
its utility with effluents from high sulfur fuel. 
The Type S lime was shown to have a high percent utilization in the spray 
dryer at a mole stoichimetric ratio of 1.2-1.5, as contrasted with the 
Type N hydrated lime. This is particularly evident where recycling of the 
effluent product is added to the fresh hydrated lime. Since the Type S 
hydrated dolomitic lime is efficiently used, the residual neutralizing 
values in the effluent product are relatively low and since the fresh Type 
S hydrated dolomitic lime is already at high efficiency at sulfur dioxide 
removal, there is little enhancement in sulfur dioxide removal. 
By contrast, there is significant enhancement when recycling the FGD 
product in conjunction with the Type N hydrated lime, since there is a 
substantial amount of residual neutralizing value in the FGD product. With 
Type N hydrated lime, it becomes important economically to recycle the FGD 
product. However, the need to recycle the FGD product creates many 
concerns. The FGD product includes fly ash and is therefore very abrasive, 
so that it can lead to rapid deterioration of equipment. Furthermore, 
recycling requires additional equipment in that the recycled product has 
to be blended with the Type N hydrated lime. 
Another variable of importance in the spray dryer is the temperature 
approach to adiabatic saturation. At adiabatic saturation, the particles 
remain wet and stick to the equipment and clog the filter. In addition, at 
lower temperatures the effluent loses its buoyancy and settles to the 
earth, rather than rising and being dispersed by prevailing winds. It is 
therefore important to be able to work at a temperature sufficiently 
higher than the adiabatic saturation temperature to avoid these concerns 
and reduce the need for careful monitoring. The following table compares 
Type S and Type N limes at two different adiabatic saturation 
temperatures. 
TABLE IV 
______________________________________ 
Inlet SO.sub.2 
Temperature* SO.sub.2 Removal, % 
ppm .DELTA. .degree.C. 
Type S Type N 
______________________________________ 
550 18 86.2 73.4 
500 13 87.5 77.5 
______________________________________ 
*.degree.C. above adiabatic saturation. 
The above data demonstrate that the efficiency of SO.sub.2 removal when 
using Type N hydrated lime is increased as the saturation adiabatic 
temperature is approached. However, there is little difference in the 
efficiency of the Type S hydrated lime at the two different approaches to 
adiabatic saturation. Thus sulfur dioxide removal can be efficiently 
performed at a substantially higher exit gas temperature than with Type N 
hydrated lime. 
In performing the study, the pilot plant was operated in the following 
manner. The stream of the flue gas amounted to 1,220 lbs/hr and had a 
sulfur dioxide concentration (based on volume) as indicated, with the gas 
free of fly ash. At 2.25 lbs/hr, hydrated lime is added as a 5 weight 
percent suspension, except when studies were being made with higher 
stoichiometric ratios, when 10 weight percent suspensions were employed. 
Where recycling was employed, the suspension had 20 weight percent solids, 
10 weight percent of the fresh lime and 10 weight percent of the FGD 
recycled product. The temperature of the flue gas fed to the spray dryer 
was 155.degree. C. and the temperature leaving the spray dryer was 
70.degree. C., with a residence time of 3.3 sec. The effluent temperature 
was modified for the study of the effect of approach to adiabatic 
saturation, lowering the effluent temperature from 70.degree. C. to 
65.degree. C. An effluent temperature of 70.degree. C. is 18.degree. C. 
above the saturation temperature of the gas. 
Based on the observed results, the use of Type S hydrated dolomitic lime 
provides a highly efficient and economical process for the removal of 
sulfur oxides from the burning of both high and low sulfur containing 
fuel. The efficiency is achieved at moderate stoichiometric ratios without 
requiring recycling of partially spent particles. At a stoichiometric 
ratio of 1.2-1.5, the Type S hydrated dolomitic lime is far superior to 
the Type N hydrated lime, regardless of the sulfur dioxide concentration 
in the inlet flue gas. The high performance of sulfur dioxide removal at 
low stoichiometric ratios can be accomplished at a higher approach to 
adiabatic saturation, avoiding the requirements of flue gas bypassing for 
reheating and limiting the risk of particle build-up and clogging in the 
spray dryer. The very fine porous particles which one can obtain with the 
Type S hydrated dolomitic lime in the feed slurry expose a very large 
surface area for reaction with efficient diffusion of sulfur dioxide into 
the droplets and efficient utilization of the neutralizing values present. 
A finer atomization is achieved with the Type S hydrated dolomitic lime, 
permitting faster reaction and shorter residence times, so that equipment 
size can be reduced as compared to the use of Type N hydrated lime. 
Furthermore, because of the high efficiency in utilization of the 
neutralizing values in a single pass, no recycling of the FGD product is 
required. This avoids the problems of abrasion resulting from the fly ash 
which is present in the recycled product, as well as the additional 
equipment associated with the recycling of the FGD product. 
The use of the Type S hydrated dolomitic lime in a spray dryer becomes 
essential, both for efficiency and economic reasons, as high sulfur fuels 
are burned and EPA regulation requirements continue to reduce the 
permissible amounts of sulfur dioxide in the effluent gas. The economic 
and efficient use of the Type S hydrated dolomitic lime greatly expands 
the fuel types which can be used without pre-treatment to extract the 
sulfur which is present in the fuel. It is evident from the above results, 
that Type S hydrated dolomitic lime provides numerous efficiencies and 
advantages in the removal of sulfur dioxide from effluent gases. 
Although the foregoing invention has been described in some detail by way 
of illustration and example for purposes of clarity of understanding, it 
will be obvious that certain changes and modifications may be practiced 
within the scope of the appended claims.