Process for simultaneous removal of SO.sub.2 and NO.sub.x from gas streams

A process for simultaneous removal of SO.sub.2 and NO.sub.x from a gas stream that includes flowing the gas stream to a spray dryer and absorbing a portion of the SO.sub.2 content of the gas stream and a portion of the NO.sub.x content of the gas stream with ZnO by contacting the gas stream with a spray of an aqueous ZnO slurry; controlling the gas outlet temperature of the spray dryer to within the range of about a 0.degree. to 125.degree. F. approach to the adiabatic saturation temperature; flowing the gas, unreacted ZnO and absorbed SO.sub.2 and NO.sub.x from the spray dryer to a fabric filter and collecting any solids therein and absorbing a portion of the SO.sub.2 remaining in the gas stream and a portion of the NO.sub.x remaining in the gas stream with ZnO; and controlling the ZnO content of the aqueous slurry so that sufficient unreacted ZnO is present in the solids collected in the fabric filter to react with SO.sub.2 and NO.sub.x as the gas passes through the fabric filter whereby the overall feed ratio of ZnO to SO.sub.2 plus NO.sub.x is about 1.0 to 4.0 moles of ZnO per of SO.sub.2 and about 0.5 to 2.0 moles of ZnO per mole of NO.sub.x. Particulates may be removed from the gas stream prior to treatment in the spray dryer. The process further allows regeneration of ZnO that has reacted to absorb SO.sub.2 and NO.sub.x from the gas stream and acid recovery.

FIELD OF THE INVENTION 
The invention includes a process for the simultaneous removal of SO.sub.2 
and NO.sub.x (NO and NO.sub.2) from gas streams. ZnO is the material used 
for gas cleaning together with a spray dry scrubbing system. 
BACKGROUND OF THE INVENTION 
The use of ZnO for flue gas cleaning was presented as early as 1940 by 
Johnstone and Singh. Johnstone, H. F., and Singh, A. D., "Recovery of 
Sulfur Dioxide from Waste Gases--Regeneration of the Absorbent by 
Treatment with Zinc Oxide", Ind. Eng. Chem., 32 (8), 1037-1049 (1940). 
They proposed a type of regenerable double alkali process in which ZnO is 
reacted with sodium sulfite and bisulfite from an SO.sub.2 absorber to 
precipitate zinc sulfite. The zinc sulfite is dried and then calcined to 
give pure SO.sub.2 and ZnO, the latter being recycled. 
Lowell, et al. performed a thermodynamic analysis on the oxides of 47 
elements for use as sorbents for FGD in processes based upon thermal 
regeneration of the sorbent. Lowell, P. S., et al., "Selection of Metal 
Oxides for Removing SO.sub.2 from Flue Gas", Ind Eng. Chem., Process Des. 
Develop, 10 (3), 384-380 (1971). Under flue gas conditions, thermodynamic 
analysis predicts that ZnO forms a sulfite in the presence of SO.sub.2 at 
a temperature of 248.degree. F. and below. Furthermore, the decomposition 
temperature for zinc sulfite is 374.degree. F., which is lower than for 
the sulfite of any metal oxide except BeO and Mn.sub.2 O.sub.3. However, 
zinc sulfite both disproportionates and decomposes to the oxide when 
heated in an inert atmosphere. The zinc sulfite disproportionates to 
sulfate and sulfide. Sulfate can also be formed as a result of sulfite 
oxidation by oxygen present in the flue gas. Zinc sulfate thermally 
decomposes to the oxide at 1364.degree. F., but with intermediate 
formation of zinc oxysulfate at 1130.degree. F. In a regenerable flue gas 
desulphurization process (FGD), it would be highly desirable to suppress 
the disproportionation and oxidation reactions in order to simplify the 
regeneration process. 
Bienstock and Field studied the reaction between SO.sub.2 and ZnO by 
passing simulated flue gas (without NO.sub.x or fly ash) through a fixed 
bed of ZnO at both 265.degree. and 625.degree. F. Bienstock, D., and 
Field, F. J., "Bench-Scale Investigation on Removing Sulfur Dioxide from 
Flue Gases", J. Air Pollut. Control Assoc., 10 (2), 121-125 (1960). In 
both cases, they found that the loading was less than 1 g SO.sub.2 /100 g 
ZnO when breakthrough of SO.sub.2 occurred. This result is not surprising 
in view of the thermodynamic analysis which predicts a maximum temperature 
of 248.degree. F. for zinc sulfite formation. However, DeBerry and Sladek 
studied the ZnO-SO.sub.2 reaction in the range of 77.degree. to 
1472.degree. F. by thermogravimetric analysis; the reaction rate was 
immeasurably small over the entire temperature range. DeBerry, D. W., and 
Sladek, K. J., "Rates of Reaction of SO.sub.2 with Metal Oxides", Can. J. 
Chem. Eng., 49 (6), 781-785 (1971). The composition of their feed gas was 
14.3 v/o CO.sub.2, 3.4 v/o O.sub.2, 2.0 v/o H.sub.2 O, 0.10 to 0.35 v/o 
SO.sub.2, and balance N.sub.2. Their H.sub.2 O concentration was 
considerably lower than in actual flue gas. 
Graefe, et al. studied the ZnO-SO.sub.2 reaction at 131.degree. F. with 
simulated flue gas saturated with H.sub.2 O and obtained SO.sub.2 loadings 
of about 30 g/100 g ZnO. Graefe, A. F., et al., "The Development of New 
and/or Improved Aqueous Processes for Removing SO.sub.2 from Flue Gas", 
Vol. II, Envirogenics report to NAPCA, PB 196 781 (October, 1970). Their 
most interesting conclusions are that water vapor is required for 
absorption to occur, high humidities give higher absorptions, and a liquid 
water phase is not required. It was not stated how the reaction products 
were identified, but the main product was reported to be ZnSO.sub.3. 
21/2H.sub.2 O. Some zinc sulfate was formed mainly because of the presence 
of NO.sub.2. Graefe, et al. also found that zinc sulfite can be decomposed 
below 572.degree. F. without disproportionation and without oxidation in 
the absence of air and preferably in the presence of steam. They found 
that zinc sulfate can be decomposed at 1832.degree. F. Graefe, et al. 
proposed a ZnO fluidized bed system for the absorption and regeneration of 
SO.sub.2, with separate thermal regenerators for sulfite and sulfate. 
In Japan, Mitsui Mining and Smelting Company, Ltd., has developed a 
ZnO-based SO.sub.2 scrubbing system that is in commercial use at their 
electrolytic zinc plants. An aqueous slurry of ZnO is fed to an absorption 
tower for reaction with SO.sub.2 in tail gas from an acid plant to form 
zinc sulfite and bisulfite. A bleed stream from the tower is reacted with 
sulfuric acid to produce zinc sulfate and SO.sub.2. The SO.sub.2 gas is 
returned to the acid plant and the zinc sulfate solution is used at the 
ore roasting plant or for the production of zinc sulfate. 
Several papers discuss the use of ZnO as a catalyst to reduce or decompose 
NO. Kortum and Knehr found that activated ZnO is capable of reducing NO at 
room temperature to N.sub.2 O. Kortum, V. G., and Knehr, H., 
Reflexionsspektroskopische Untersuchungen im IR uber die Adsorption von NO 
an Zinkoxid", Berichte der BunsenGesellschaft, 77 (2), 85-90 (1973). 
Yur'eva, et al. found that ZnO catalytically decomposes NO in the range of 
1200.degree. to 1380.degree. F. Yur'eva, J. M., et al., "Catalytic 
Properties of Metal Oxides of Period IV of the Periodic System with 
Respect to Oxidation Reactions - II. Decomposition of Nitric Oxide", 
translated from Kinetika i Kataliz, 6 (6), 1041-1045 (1965). Alkhazov, et 
al. found that ZnO can act as a catalyst for the reduction of NO by CO in 
the range of 210.degree. to 930.degree. F. Alkhazov, F. G., et al., 
"Catalytic Activity of Transition-Metal Oxides for the Reaction of Nitric 
Oxide with Carbon Monoxide", translated from Kinetika i Kataliz, 16(5), 
1230-1233 (1975). 
Although a conceptualized flue gas desulphurization process using ZnO has 
been described in the literature by Graefe et al., it is not the optimum 
process. A fluidized bed reactor would not be the best contacting device 
for flue gas subsaturated with water. In addition, there is a complete 
lack of data on the reaction between ZnO and SO.sub.2 from flue gas in the 
temperature range of most interest (150.degree. to 250.degree. F.). 
Operation in this temperature range corresponds to flue gas subsaturated 
with water and would allow the use of contacting devices such as a spray 
dryer and/or a fabric filter or a partial gas quench followed by a bed 
filter. Also, it would be possible to eliminate the need for a separate 
sulfate regenerator as described later. 
Previous work on the use of ZnO for combined SO.sub.2 /NO.sub.x removal 
from flue gas could not be found in the literature, but the possibility 
was alluded to by Graefe, et al. Data on the reaction between ZnO and 
NO.sub.x from flue gas could not be found for any temperature range. 
Currently, the leading flue gas desulfurization process in terms of 
installed utility capacity is wet limestone scrubbing. The process employs 
a limestone slurry to contact the flue gas and react with the SO.sub.2 to 
produce a calcium sulfite/sulfate sludge for waste disposal. In addition 
to the waste disposal requirements, process reliability is still a problem 
area. Plugging and scaling in the system cause a high amount of 
maintenance work and corrosion can be severe. It is still difficult to 
select an adequate material to withstand the conditions in the outlet duct 
and stack. The maintenance and materials problems can add significantly to 
the process costs. Also, the process does not have any NO.sub.x removal 
capability, and with high-sulfur coal, it is difficult to achieve the 90 
percent SO.sub.2 removal required by the New Source Performance Standards. 
Alternatives for increasing the SO.sub.2 removal include the use of 
additives to enhance the limestone reactivity, or the use of slaked lime 
which is more reactive than limestone. These alternatives add to the 
process cost. 
The need for FGD processes that are more reliable, environmentally 
acceptable, and less costly has led to the development of the spray dry 
scrubbing process. Spray nozzles or centrifugal atomizers in a spray dryer 
create a mist of fine droplets of slaked lime slurry (limestone does not 
possess sufficient reactivity) into which the SO.sub.2 is absorbed as the 
flue gas mixes intimately with the spray. Water in the droplets is 
evaporated by the sensible heat in the flue gas, so that the calcium 
sulfite and sulfate reaction products leave the scrubber as a dry powder 
entrained in the flue gas. The reaction products are collected along with 
the fly ash in a fabric filter or electrostatic precipitator following the 
spray dryer. The quantity of water fed to the spray dryer is regulated so 
that evaporation does not cool the flue gas closer than about 20.degree. 
F. to its adiabatic saturation temperature (typically about 125.degree. 
F.). Because the gas remains unsaturated and the slurry pumping 
requirements are greatly reduced (all of the water is evaporated rather 
than being recycled), many of the operating problems associated with wet 
scrubbers are avoided. 
However, there is still a dry waste product for disposal and lime, rather 
than less expensive limestone, must be used as the sorbent. Also, it is 
difficult to achieve 90 percent SO.sub.2 removal with high-sulfur coal 
unless the lime feed rate is increased to the point where the cost may 
become prohibitive. The NO.sub.x removal is nil; in fact there is 
considerable evidence that a visible plume is produced at certain 
temperatures because NO is oxidized to NO.sub.2 as it passes through the 
solids layer in a fabric filter. The process can be modified to achieve 50 
to 70 percent NO.sub.x removal by using sodium hydroxide as an additive 
and operating at a spray dryer outlet temperature of 200.degree. F. 
(75.degree. F. above saturation). However, sodium hydroxide is expensive 
and the waste product contains soluble salts that can cause surface and 
groundwater contamination. 
Thus far, NO.sub.x emissions from stationary sources have not received as 
much attention as SO.sub.2 emissions. It is possible to meet the current 
EPA regulations for NO.sub.x emissions through combustion modifications. 
However, NO.sub.x control will probably receive increased attention in the 
future. NO.sub.x emissions are believed to have a deleterious effect on 
human health and visibility and are believed to be significant 
contributors to the formation of both nitrate and sulfate acid 
precipitation. Furthermore, the total annual NO.sub.x emission from 
stationary sources has been steadily increasing, while SO.sub.2 and 
particulate emissions have been leveling off or decreasing. 
The most advanced technology for NO.sub.x removal from flue gas is 
selective catalytic reduction (SCR) with ammonia. This process operates at 
700.degree. F. and, therefore, is difficult to retrofit. In Japan, flue 
gas cleaning on a coal-fired boiler includes an SCR system for NO.sub.x 
removal, an electrostatic precipitator for fly ash removal, and a wet 
limestone FGD system for SO.sub.2 removal. Since the combined cost of the 
separate processes for SO.sub.2 and NO.sub.x control is relatively high, 
there is incentive for the development of a process for the simultaneous 
removal of SO.sub.2 and NO.sub.x. If such a process were regenerable, 
i.e., did not produce a waste product for disposal, it would be even more 
attractive. 
Several other processes for the simultaneous removal of SO.sub.2 and 
NO.sub.x from flue gas have been proposed and tested, but none of them 
have achieved commercial significance. These processes include the use of 
activated carbon, copper oxide, or sodium aluminate as dry sorbents, the 
use of electron beam radiation in conjunction with spray dry scrubbing, 
and the use of iron sulfide (pyrites) in a wet scrubber. Each of these 
processes has disadvantages that are absent from the ZnO process. The 
activated carbon process requires ammonia for NO.sub.x removal, involves 
the circulation of a large inventory of solids, and requires thermal 
regeneration at high temperature. The copper oxide process also requires 
ammonia for NO.sub.x removal and a reducing gas such as hydrogen for 
conversion of copper sulfate to copper and SO.sub.2 ; it is not suitable 
for retrofit because the operating temperature is about 700.degree. F. as 
compared with a 300.degree. F. flue gas exit temperature from the boiler 
train. The sodium aluminate process requires a reducing gas for 
regeneration and a gas-solid contacting device that would have a 
relatively high pressure drop. The electron beam process requires a source 
of high energy electrons and uses either ammonia, which is converted to 
fertilizer, or lime, which produces solid waste, including water-soluble 
calcium nitrate. The chemistry of wet scrubbing with pyrites is extremely 
complex and the process requires thermal regeneration at high temperature. 
In contrast, the ZnO process requires no raw materials other than makeup 
ZnO, produces no waste product, uses conventional equipment currently in 
operation on utility FGD systems, and requires moderate temperatures for 
thermal regeneration. For these reasons, the ZnO process should cost 
substantially less than the other simultaneous removal processes. The cost 
of the ZnO process is best compared with the combined cost of SCR and wet 
limestone FGD for application on high-sulfur coal-fired boilers. However, 
it must be remembered that the latter more conventional scheme produces 
solid waste for disposal; hence, power plants in urban locations may not 
be able to utilize a non-regenerable FGD process, or will have to incur 
substantial costs over those estimated for disposal. 
The current invention describes a regenerable process for the simultaneous 
removal of SO.sub.2 and NO.sub.x from waste gas streams. Further, the 
range of conditions for the efficient simultaneous removal of these 
compounds has been discovered that makes the process amenable to 
widespread use. 
BRIEF DESCRIPTION OF THE INVENTION 
The current invention utilizes ZnO for gas cleaning and offers the 
advantages of spray dry scrubbing without the disadvantages mentioned 
previously. The process can remove greater than 90 percent of the SO.sub.2 
and up to 90 percent of the NO.sub.x from waste gas streams. There is no 
waste product because the spent sorbent is thermally regenerated, 
producing a concentrated stream of SO.sub.2 and NO.sub.x for further 
processing into useful byproducts. The regeneration temperature is 
considerably lower than that required in other regenerable FGD processes 
so that the energy consumption is less.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENT 
A flow sheet for the ZnO process of the current invention is shown in FIG. 
1. As with any regenerable process, prior particulate collection is 
required to prevent the former from entering the regeneration loop and 
contaminating the byproducts. The type of particulate collection device 
100 used depends upon the amount of particulates that can be conveniently 
separated by downstream processing. An electrostatic precipitator provides 
a high particulate removal efficiency, but it may be possible to use a 
less expensive device and tolerate a lower removal efficiency as described 
later. Waste gas from the particulate collection device 100, typically at 
a temperature of 250.degree. to 500.degree. F. depending upon the source 
of the gas and the type of pretreatment (quenching or heat exchange may be 
required for a hot gas) enters a spray dryer 110 where it is contacted 
with a slurry of ZnO that has been atomized to fine droplets using dual 
fluid nozzles or centrifugal atomizers. The spray dryer outlet temperature 
is controlled in the range of 125.degree. to 250.degree. F., and 
preferably 150.degree. to 200.degree. F., to optimize the removal of both 
SO.sub.2 and NO.sub.x in the spray dryer and in a downstream fabric filter 
120. The difference between the spray dryer outlet temperature and the 
adiabatic saturation temperature of the gas stream is termed the approach 
to saturation. The adiabatic saturation temperature of a gas stream is 
determined by its initial temperature and moisture content. In the case 
described, the approach to saturation is in the range of about 0.degree. 
to 125.degree. F., and preferably about 25.degree. to 75.degree. F. 
The outlet temperature of the spray dryer 110 is controlled by the amount 
of water in the feed slurry, and the SO.sub.2 and NO.sub.x removal 
efficiencies are controlled by the amount of ZnO in the feed slurry. The 
amount of ZnO fed to the spray dryer 110 is such that sufficient unreacted 
ZnO is present in the solids collected in the fabric filter 120 to react 
with SO.sub.2 and NO.sub.x as the flue gas passes through the filter cake. 
The overall stoichiometric ratio of ZnO to SO.sub.2 plus NO.sub.x, as 
specified by the following chemical reactions is in the range of 1.0 to 
4.0, and preferably 1.2 to 2.0: 
EQU SO.sub.2 +ZnO.fwdarw.ZnSO.sub.3 (1) 
EQU SO.sub.2 +1/2O.sub.2 +ZnO.fwdarw.ZnSO.sub.4 (2) 
EQU 2NO+3/20.sub.2 +ZnO.fwdarw.Zn(NO.sub.3).sub.2 (3) 
EQU 2NO.sub.2 +1/2O.sub.2 +ZnO.fwdarw.Zn(NO.sub.3).sub.2 (4) 
Some of the ZnSO.sub.3 and ZnSO.sub.4 reaction products may be present as 
hydrates. The above reactions are simplified in that there is a 
synergistic effect in removal efficiency of both SO.sub.2 and NO.sub.x 
when both species are present, although individual compounds containing 
both sulfur and nitrogen have not been found. Also, it is possible that 
some of the NO and NO.sub.2 are decomposed to elemental nitrogen and 
oxygen as the gas stream flows through the system. It has been shown that 
ZnO is far more effective for NO.sub.x removal than are the reaction 
products of ZnO and SO.sub.2. Although SO.sub.2 and NO.sub.x removal occur 
in both the spray dryer 110 and the fabric filter 120, SO.sub.2 removal 
predominates in the former and NO.sub.x in the latter. However it is 
important that not all of the SO.sub.2 be removed in the spray dryer 110 
in order to take advantage of the synergistic effect of SO.sub.2 on 
NO.sub.x removal in the fabric filter 120. 
A general description of the process without particulate removal or 
regeneration would be flowing the gas stream to a spray dryer and 
absorbing a portion of the SO.sub.2 content of the gas stream and a 
portion of the NO.sub.x content of the gas stream with ZnO by contacting 
the gas stream with a spray of an aqueous ZnO slurry; controlling the gas 
outlet temperature of the spray dryer to within the range of a 0.degree. 
to 125.degree. F. approach to the adiabatic saturation temperature; 
flowing the gas, unreacted ZnO and ZnO with absorbed SO.sub.2 and NO.sub.x 
from the spray dryer to a fabric filter and collecting any solids therein 
and absorbing a portion of the SO.sub.2 remaining in the gas stream and a 
portion of the NO.sub.x remaining in the gas stream with ZnO; and 
controlling the ZnO content of the aqueous slurry so that sufficient 
unreacted ZnO is present in the solids collected in the fabric filter to 
react with SO.sub.2 and NO.sub.x as the gas passes through the fabric 
filter whereby the feed ratio of ZnO to SO.sub.2 plus NO.sub.x is about 
1.0 to 4.0 moles of ZnO per mole of SO.sub.2 and about 0.5 to 2.0 moles of 
ZnO per mole of NO.sub.x. 
In the alternative the feed ratio using one to four times the 
stoichiometric ratio of reactions (1) through (4) above could also be 
used. 
The gas stream from the fabric filter passes into an induced draft fan 130 
to overcome the pressure losses of the spray dryer 110, the fabric filter 
120, and the associated ductwork. From the fan 130, the gas stream is 
exhausted via the stack 135. 
The solids collected in the fabric filter 120 are stored for regeneration 
in a reaction product bin 140. It may be possible to separate residual 
particulates from ZnO and its reaction products by a dry screening or air 
classifying technique. If so, the upstream particulate removal 
requirements would be less stringent so that less expensive multicyclones 
could be used instead of an electrostatic precipitator for the particulate 
device 100. 
The spent solids are fed to a static, rotary, or fluidized bed calciner 150 
for thermal regeneration. The regeneration temperature is dependent upon 
the amount of sulfate formed in the absorption step. The sulfite and 
nitrate can be decomposed at temperatures in the range of 450.degree. to 
550.degree. F. according to the reverse of Reactions 1, 3, and 4. However, 
the sulfate decomposes at about 1400.degree. F. according to the reverse 
of Reaction 2. Several alternatives to operating the calciner 150.degree. 
at 1400.degree. F. are available depending upon the percent of the sulfite 
that is oxidized to sulfate. If the level of oxidation is low, the sulfate 
can be allowed to reach a steady-state concentration in the recycled 
solids. This will occur because some zinc compounds will be lost from the 
system on each recycle and will have to be made up by adding fresh ZnO. 
Another alternative is to add coke to the calciner to lower the 
decomposition temperature of sulfate according to the following reaction: 
EQU ZnSO.sub.4 +1/2C.fwdarw.ZnO+SO.sub.2 +1/2CO.sub.2. (5) 
A third alternative is to extract the sulfate with water prior to 
calcination; zinc sulfate is highly soluble while the oxide and sulfite 
are relatively insoluble. This alternative may not be economically 
attractive because of the added cost of drying the zinc compounds after 
the extraction step and the need for disposing of the separated zinc 
sulfate. 
The regenerated ZnO is cooled in a cooler 160 and classified and ground as 
needed by classifier 170 and grinder 180. It is stored for reuse in a feed 
bin 190. Regenerated ZnO and makeup ZnO are slurried in water in a feed 
preparation tank 200 and fed to the spray dryer 110 through a feed pump 
210. The ZnO concentration in the slurry is determined by the desired mole 
ratio of ZnO to SO.sub.2 plus NO.sub.x in the flue gas and the spray dryer 
outlet temperature. The ZnO particle size is determined by the surface 
area required for high removal efficiency of SO.sub.2 and NO.sub.x. The 
surface area should be in the range of 1 to 30 m.sup.2 /g, preferably 5 to 
20 m.sup.2 /g. The classifier 170 and grinder 180 are provided at the exit 
of the regenerated ZnO cooler to handle any particle agglomeration that 
may occur during the calcination step. However, if the calcination is 
carried out in the range of 450.degree. to 550.degree. F., agglomeration 
is not expected to occur. 
The off gas from the calciner 150, which contains a high concentration of 
SO.sub.2 and NO.sub.x, is fed to conventional gas cleaning equipment 220 
for removal of particulates and trace impurities. The type of equipment 
used depends upon the degree of cleaning required. The ZnO particulate 
carryover from the calciner 150 may be less for a rotary type than a 
fluidized bed type. The particulates can be removed in an electrostatic 
precipitator or a fabric filter. If necessary this equipment can be 
preceded with a cyclone followed by a waste heat boiler depending upon the 
calcination temperature. The recovered ZnO is sent to the ZnO cooler 160 
for eventual recycle to the spray dryer 110. 
The clean gas, containing a high concentration of SO.sub.2 and NO.sub.x, is 
fed to an acid plant 230 which uses a modified contact or chamber process 
to produce sulfuric and nitric acids. In the contact process, the gas is 
fed to catalytic converters to oxidize the SO.sub.2 to SO.sub.3. The 
SO.sub.3 is absorbed by sulfuric acid to produce more acid. In this case, 
the NO.sub.x will also be absorbed by the sulfuric acid and must be 
removed by thermal stripping. The separated NO.sub.x is then fed to an 
absorption tower for conversion to nitric acid. The tail gas streams from 
the catalytic converter and the absorption tower are fed to the spray 
dryer inlet for removal of residual SO.sub.2 and NO.sub.x, respectively. 
In the chamber process, NO.sub.x is recovered for reuse, but in this case, 
the NO.sub.x that is continuously supplied by absorption from the flue gas 
is converted to nitric acid, while excess NO.sub.x that is required for 
SO.sub.2 oxidation is recycled. The recovered sulfuric and nitric acids 
are sold for industrial use. 
The invention finds utility in coal or oil fired electric power generating 
plants, in coal or oil fired boilers for steam production, in waste 
incineration, in smelters, in the steel industry and in metal finishing. 
Process Variations 
In an aqueous cleaning process, the gas temperature and moisture content 
are not completely independent variables because they are related by the 
type of gas conditioning used prior to or during the process. At a 
coal-fired boiler, for example, flue gas leaves the air preheater at a 
temperature of about 300.degree. F. and a moisture content of about 7 
volume percent. These would be the initial conditions for an add-on gas 
cleaning process with or without prior fly ash collection in a particulate 
collection device 100. The gas stream can be changed from the initial 
conditions by water injection, steam injection, and/or indirect heat 
exchange as discussed below. Heat balance calculations were performed for 
the case of a coal-fired boiler to outline the relationship between 
moisture content and gas temperature for the various types of gas 
conditioning. The results of these calculations are illustrated in FIG. 2 
for the initial conditions of 7 volume percent water vapor and 300.degree. 
F. The principles would be the same for gas with different initial 
conditions, such as from a waste incinerator. 
Water injection at the initial conditions (Line A in FIG. 2) corresponds to 
the use of the spray dryer 110 absorber. A quencher or humidifier (not 
illustrated) could precede the spray dryer 110 or replace the spray dryer 
110 as discussed below. The spray dryer 110 is usually not operated to 
saturation because the objective is to obtain a dry product. If saturated 
steam injection (Line B) is used prior to the spray dryer 110 (Line C), 
the moisture content of the gas stream can be increased at any given 
temperature. If heat is extracted from the gas stream by indirect heat 
exchange (Line D) prior to the spray dryer 110 (Line E), the gas stream 
can be saturated and then reheated to subsaturated conditions with the 
previously extracted heat (Line F) between the spray dryer 110 and the 
fabric filter 120. 
The point is that the techniques described above can be used to optimize 
SO.sub.2 and NO.sub.x removal with ZnO in the spray dryer 110 and the 
fabric filter 120. Steam injection allows the spray dryer 110 and/or 
fabric filter 120 (depending upon the point of injection, point 1 or 2 of 
FIG. 1) to operate at a higher moisture content for a given gas 
temperature. A higher gas temperature (up to about 200.degree. F.) 
enhances NO.sub.x removal, but a high moisture content is required for 
good SO.sub.2 removal. Another alternative is to operate the spray dryer 
110 at or near the adiabatic saturation temperature to maximize SO.sub.2 
removal in the spray dryer 110, and reheat the gas between the spray dryer 
110 and the fabric filter 120 (with the regenerative technique described 
above, with steam coils in line 111, point 2 of FIG. 1, or with hot air 
injection at point 2 of FIG. 1) to maximize NO.sub.x removal in the fabric 
filter 120. It is important that sufficient unreacted ZnO and SO.sub.2 
leaves the spray dryer 110 and enters the fabric filter 120 for reaction 
with NO.sub.x. 
Both SO.sub.2 and NO.sub.x removal can be enhanced by the use of additives 
to the ZnO in the feed preparation tank 200. One class of additives, of 
which sodium hydroxide is an example, would increase the reactivity of ZnO 
by conversion to Zn(OH).sub.2. Also, the presence of sodium would enhance 
the synergistic effect of SO.sub.2 on NO.sub.x removal through the 
formation of sodium sulfite which can react with NO.sub.x to form 
imidodisulfonates or sulfamates. The weight ratio of NaOH to ZnO should be 
about 1:10, but can be as high as 1:1, especially if the NaOH can be 
regenerated along with ZnO (by heating and reaction with water). 
Another class of additives, of which high surface area silica (silica gel) 
is an example, would enchance SO.sub.2 and NO.sub.x removal in the fabric 
filter 120 by absorbing water on a filter cake that forms in the fabric 
filter 120. The absorbed water enhances the reactions among ZnO, SO.sub.2, 
and NO.sub.x as the gas passes through the filter cake. The silica gel 
should have a surface area in the range of 200 to 800 m.sup.2 /g, and the 
weight ratio of silica gel to ZnO should be in the range of 1:10 to 1:1. 
The silica gel would be thermally regenerated (removal of the absorbed 
water) prior to or simultaneously with the ZnO. 
The NO.sub.x removal can be enhanced by oxidizing NO to the more reactive 
NO.sub.2. This can be done by ozone injection into the gas stream or, more 
preferably, by oxidation with oxygen already present in the gas. The 
oxidation with oxygen is favored by low temperature and long residence 
time. An oxidation chamber 115 or 116 can be used before or after the 
spray dryer 110; the downstream location has the advantage of lower 
temperature. A catalyst can be used to greatly reduce the required 
residence time. The catalyst would have to be designed to handle the 
particulate loading in the gas stream (parallel passage design). It is 
advantageous to use a catalyst upstream of the spray dryer 110 because the 
particulate loading would be lower than downstream where ZnO and its 
reaction products are present in the gas stream. However, it may also be 
possible to use a parallel passage type of catalyst downstream from the 
spray dryer 110. In any event, the NO.sub.2 would be removed by reaction 
with ZnO as the gas passes through the filter cake in the fabric filter 
120. If the oxidation occurs upstream of the spray dryer 110, a portion of 
the NO.sub.2 will be removed by reaction with ZnO in the spray dryer 110. 
Another technique for increasing NO.sub.x removal is to minimize SO.sub.2 
removal in the spray dryer 110 so that the SO.sub.2 can exert a 
synergistic effect on NO.sub.x removal in the fabric filter 120. This can 
be achieved by feeding finer droplets (less than 50 microns) to the spray 
dryer 110 so that the water in the ZnO slurry will evaporate faster, 
operating the spray dryer 110 at an outlet temperature above 150.degree. 
F., and/or injecting part of the ZnO as a dry powder into the fabric 
filter 120 to precoat the bags after each cleaning cycle and/or into the 
gas stream line 111 between the spray dryer 110 and the fabric filter 120. 
If all of the ZnO is injected as a dry powder, the spray dryer 110 can be 
replaced by a quencher to condition the gas stream to the appropriate 
temperature and humidity prior to the fabric filter 120. 
Other process variations include the use of a quencher followed by a 
parallel passage, moving bed, fixed bed, or fluidized bed reactor rather 
than a fabric filter 120. A parallel passage reactor utilizes wire screens 
to hold the ZnO reactant in thin envelopes. The gas passes through spaces 
between the envelopes and the SO.sub.2 and NO.sub.x react with the ZnO. 
Regeneration would occur by passing a hot combustion gas through the 
reactor. The advantage of this configuration is a lower pressure drop than 
a conventional packed or fixed bed. A moving bed reactor offers the 
advantage of a continuous process; i.e., gas flow to the reactor does not 
have to be interrupted for the regeneration cycle. A fluidized bed also 
can be operated continuously but the gas entering the bed must be 
significantly above the adiabatic saturation temperature. However, a 
fluidized bed has the disadvantage of requiring a downstream particulate 
collection device, and generally a higher pressure drop than the other 
gas-solid contractors mentioned above. 
EXPERIMENTAL RESULTS 
The reactions between ZnO and SO.sub.2 /NO.sub.x were studied in a 
fixed-bed reactor for application to spray dry scrubbing. 
The experimental conditions used to study the ZnO process are outlined 
below: 
Gas Composition (Dry Basis) 
CO.sub.2 12.00 vol. percent 
O.sub.2 5.25 vol. percent 
SO.sub.2 3,000 ppmv 
NO 700 ppmv 
NO.sub.2 70 ppmv 
N.sub.2 82.37 vol. percent 
Moisture Content of Gas 
Selected based on operating temperature (125.degree. to 200.degree. F.) and 
Line A in FIG. 2. 
Gas Flow Rate 
Volumetric 1,200 cm.sup.3 /min 
Superficial velocity 2 ft/min 
Characteristics of ZnO 
Type AZO-77S 
Surface area 8.00 m.sup.2 /g 
Weight used in bed 5.00 g 
Characteristics of Diluent 
Type American Foundry Sand No. 61; Glass beads 
Particle size 250 microns; 800 microns 
Weight used in bed 100 g or 0 g 
The removal efficiencies obtained for SO.sub.2 and NO.sub.x at 135.degree., 
150.degree., and 200.degree. F. are shown in Table 1 for a diluted bed of 
ZnO. The results indicate that greater than 90 percent SO.sub.2 removal 
should be attainable under conditions corresponding to spray cooling of 
the flue gas to a temperature between 135.degree. and 150.degree. F., or a 
12.degree. to 27.degree. F. approach to the adiabatic saturation 
temperature. This represents a practicable operating condition for the 
spray dryer 110 and the fabric filter 120. However, the NO.sub.x removal 
efficiencies obtained in the fixed bed under these conditions were low. 
The use of a diluted bed of ZnO to prevent channeling probably results in 
the rapid formation of a layer of zinc sulfite reaction product on each 
particle of ZnO. The lack of available ZnO surface area may hinder the 
absorption of NO.sub.x. Accordingly, three experiments were performed with 
undiluted beds of ZnO at a temperature of 190.degree. F. and 18 percent 
relative humidity corresponding to a point on Curve A of FIG. 2. One bed 
consisted of 5.0 g of fresh ZnO, one bed consisted of 5.0 g of ZnO 
previously exposed to simulated flue gas at 150.degree. F. and 55 percent 
relative humidity also corresponding to a point on Curve A of FIG. 2 for 
6.5 hours, and one bed consisted of 2.5 g ZnO previously exposed to 
simulated flue gas under the same conditions plus 2.5 g of fresh ZnO. The 
SO.sub.2 and NO.sub.x removal results are summarized in Table 2. Not 
surprisingly, the SO.sub.2 removal efficiency increases with the amount of 
fresh ZnO in the bed. However, the NO.sub.x removal efficiency also 
increases with the amount of fresh ZnO in the bed. These results are a 
strong indication that ZnO is a more effective reactant for NO.sub.x than 
is zinc sulfite. The results also indicate that the reaction products 
between ZnO and SO.sub.2 do not enhance NO.sub.x removal. 
Two duplex experiments were performed in an attempt to simulate the dry 
portion of a spray dryer 110 plus a fabric filter 120. The experiments 
were performed at 190.degree. F. and 18 percent relative humidity. In the 
first portion of each experiment, the packed bed consisted of 5.0 g of ZnO 
diluted with 0.8 mm diameter glass beads at a weight ratio of 20/1. After 
exposure to simulated flue gas for about 7 hours, the "used" ZnO was 
separated from the glass beads and put back into the reactor. The 
undiluted "used" ZnO was then re-exposed to flue gas with the SO.sub.2 
concentration adjusted to account for prior removal. In one experiment, 
5.0 g of fresh ZnO was added to the "used" ZnO to simulate a 
stoichiometric ratio greater than one in the spray dryer. The SO.sub.2 and 
NO.sub.x removal results are summarized in Table 3. In the experiment with 
the fresh ZnO added after separation from the glass beads, the maximum 
overall removal efficiencies were 93 percent for SO.sub.2 and 58 percent 
for NO.sub.x. It should be possible to achieve higher NO.sub.x removal in 
an actual spray dryer 110 and fabric filter 120 because the ZnO would be 
in a continuous flow system. Further, the process parameters could be 
manipulated to adjust the SO.sub.2 removal in the spray dryer 110 so as to 
optimize the overall NO.sub.x and SO.sub.2 removal efficiencies. 
A sample of a spent bed of undiluted ZnO was subjected to differential 
thermal analysis (DTA) and the results are shown in FIG. 3. There are 
three distinct peaks at 426.degree., 531.degree., and 1393.degree. F., 
respectively. It is postulated that the lower temperature peaks represent 
SO.sub.2 and NO.sub.x from the decomposition of sulfite and nitrite or 
nitrate. The high temperature peak most likely represents SO.sub.2 from 
the decomposition of sulfate, as the decomposition temperature of 
ZnSO.sub.4 is about 1418.degree. F. 
A bed of spent ZnO was subjected to a regeneration test by differential 
thermal heating in the absorption reactor while being purged with air. The 
off gas was continuously monitored for SO.sub.2 and NO.sub.x and the 
results are shown in FIG. 4. Evolution of SO.sub.2 and NO.sub.x began a 
little above 400.degree. F. These results are in substantial agreement 
with the DTA data and confirm the regenerable nature of the process. 
While the forms of the invention herein disclosed constitute presently 
preferred embodiments, many others are possible. It is not intended herein 
to mention all of the possible equivalent forms or ramifications of the 
invention. It is to be understood that the terms used herein are merely 
descriptive rather than limiting, and that various changes may be made 
without departing from the spirit or scope of the invention. 
TABLE 1 
______________________________________ 
SO.sub.2 AND NO.sub.x REMOVAL WITH DILUTED ZnO.sup.(a) 
Relative Maximum Maximum 
Temperature, 
Humidity, SO.sub.2 Removal, 
NO.sub.x Removal, 
F. percent percent percent 
______________________________________ 
135 70 100 20 
150 50 71 7.0 
200 12 7.5 13 
______________________________________ 
.sup.(a) ZnO: 5.00 g; foundry sand: 100 g. 
TABLE 2 
______________________________________ 
SO.sub.2 AND NO.sub.x REMOVAL WITH UNDILUTED ZnO.sup.(a) 
Maximum Maximum 
SO.sub.2 Removal, 
NO.sub.x Removal, 
Type of Packed Bed 
percent percent 
______________________________________ 
Fresh ZnO, 5.0 g 65 45 
"Used" ZnO, 5.0 g.sup.(b) 
20 25 
"Used" ZnO, 2.5 g.sup.(b) plus fresh 
.sup. 50.sup.(c) 
40 
ZnO, 2.5 g 
______________________________________ 
.sup.(a) Temperature: 190 F.; relative humidity: 18 percent. 
.sup.(b) Exposed to simulated flue gas at 150 F. and 55 percent relative 
humidity for 6.5 hours. 
.sup.(c) SO.sub.2 deleted from flue gas prior to achieving maximum 
removal. 
TABLE 3 
__________________________________________________________________________ 
SO.sub.2 AND NO.sub.x REMOVAL IN DUPLEX EXPERIMENTS.sup.(a) 
Maximum SO.sub.2 
Maximum NO.sub.x 
Maximum SO.sub.2 
Maximum NO.sub.x 
Maximum 
Maximum 
Diluted Bed 
Undiluted Bed 
Removal in 
Removal in 
Removal in 
Removal in 
Overall 
Overall NO.sub.x 
Composition,.sup.(b) 
Composition,.sup.(c) 
Bed A, Bed A, Bed B, Bed B, Removal, 
Removal, 
Bed A Bed B percent percent percent percent percent 
percent 
__________________________________________________________________________ 
Glass beads, 100 g 
Bed A, 2.5 g 
55 30 35 15.sup.(d) 
71 41 
ZnO, 5 g 
Glass beads, 100 g 
Bed A, 3.4 g 
55 30 85 40.sup.(e) 
93 58 
ZnO, 5 g ZnO, 5 g 
__________________________________________________________________________ 
.sup.(a) Temperature: 190 F.; relative humidity: 18 percent: superficial 
gas velocity: 2 ft/min. 
.sup.(b) Exposure time of about 7 hours. 
.sup.(c) Separated from 0.8 mm glass beads. 
.sup.(d) SO.sub.2 concentration reduced to 800 ppmv. 
.sup.(e) SO.sub.2 concentration reduced to 1,300 ppmv.