CA2112634A1 - Integrated particulate/soxnox removal matter/processes and systems - Google Patents

Abstract

INTEGRATED PARTICULATE/SOx/NOx REMOVAL MATTER/PROCESSES AND SYSTEMS

ABSTRACT OF THE DISCLOSURE
The removal of particulate material, SOx, and NOx from contaminated gases. SOx is removed by react-ing it with an alkali reagent, and particulate matter may be removed by filtration. A low-temperature selective catalytic reduction (SCR) process is used to remove NOx.

Description

INTEGRATED PARTICULATE/SOX/NOX
REMOVAL MATTER/PROCESSES AND SYSTEMS

RELATION TO OTHER APPLICATIONS
This application is a continuation-in-part of application No. 07/835,215 ~iled 13 February 1992 by Jianping Chen et al. for LOW TEMPERATURE CATALYTIC
REDUCTION OF NITROGEN OXIDES. Application No.
07/835,215 is a division of application No. 07/547,766 which was filed 3 July 1990 by the same inventors, has the same title, and has since matured into patent No.
5,106,102 dated 21 April 1992.
., The present invention relates to novel integrated processes for removing particulate materi-3 al, SOx, and NOX from contaminated gases and to novel systems in which those processes may be carried out.
The process feedstock is hereinafter common-ly referred to as a "gas" for the sake of convenience.
3 In actual practice the feedstock may, and will typi-cally, be mixtures of gases; and the singular term is fully intended to cover such mixtures.

BACKGROUND OF THE INVENTION

i The emission of particulate matter, SOx, and NOX, especially from coal-fired power plants, has for a long time posed serious environmental pollution problems. Strategies for controlling these emissions - have evolved over four decades. Each control technol-. .
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ogy was developed in response to concerns for theenvironmental implications of a particular one of these three emissions and associated regulations restricting emissions. As a result, these control technologies, generally developed independently, do not represent an integrated system capable of control-J ling all three emissions simultaneously. Attempts to develop combined control concepts are ongoing but have largely been unsuccessful.
In fact, operating axperience has shown that the technologles available for controlling particulate material, S0x~ and N0x can interfere with each other, increasing operating costs and reducing emission control efficiency. The most frequently cited inter-action is between electrostatic precipitators (ESP's) ;, for collection of fly ash and so2 scrubber operation.
-1 As the ESP is located upstream of the SO2 system, the fly ash collection efficiency can affect S02 scrubber performance~ Also, ammonia-based selective catalytic reduction (SCR) technologies -- located ahead of both ESP's and S02 scrubbers -- can similarly adversely affect the performance of the latter systems.
Specifically, ESP's can remove up to 99.9+
% of the fly ash emitted from a conventional pulver-ized coal-fired boiler, depending on the operating temperature, the chemical composition of the ash and i~ flue gas, and the ESP design. Where discharge of process waste water is either restricted or limited, the carryover to the S02 scrubber of trace quantities of fly ash can inhibit S02 removal performance and increase already unacceptably high operating costs by ~i significantly elevating power and reagent require-ments.
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The advent of ammonia-based selective catalytic reduction (SCR) processes for N0x control adds a further complication to the operation of ESP's and conventional flue gas desulfurization (FGD) processes. This technology employs injected ammonia in the presence of a catalyst to reduce N0x to nitrogen which exits the process along with the process byprod-ucts residual ammonia (NH3~ and sulfur trioxide (S03).
Conventional SCR technology requires relatively high (700-F) temperature flue gas for significant (e.g.
80%) N0x removal. The requirement for flue gas operating temperatures significantly higher than those required for ESP's and FGD processes (e.g. 300 F) demands that the SCR reactor precede the FGD and ESP
systems. As a consequence, the residual NH3 and byproduct S03 can enter the ESP and FGD processes and Iinhibit operation and performance.
¦A seemingly viable alternative to employing SCR, ESP, and FGD technology sequentially would be the use of similar components in one control strategy. It would appear that the individual processes could be selected so that any interactions enhanced -- rather ithan interfered with -- total system performance. In particular, a process that could operate at relatively low flue gas tem~eratures (~300-F and below) would minimi2e the complication of placing the necessary components within a given site plan. An evaluation of ,conventional combined control technologies has identi-jfied numerous combined N0x and S02 controls but none i!30 that simultaneously offer control of particulate emissions and none that could be carried out at flue gas temperatures amenable to equipment retrofit (300-F).
'Two other processes that are economically I ' .

12~3 1 attractive for operation in the desired temperature range are sodium injection for SO2 and particulate control and low temperature selective catalytic reduction (SCR) for NOX control.
Injection of an alkali material for S02 control has been tried in a number of cases with significant success in terms of achieving high levels of SO2 control. However, this approach has an "Achil-les' heel", the formation of NO2 as an unwanted byprod-uct of the process. NO2 in sufficient amount~ forms an unwanted visible plume. Even the use of ammonia or urea as a NO2 control additive has not heretofore ! proven to be a satisfactory solution because there is a tradeoff between excess ammonia emissions and unacceptably high NO2 levels due to inadequate amounts of NH3.
The low temperature SCR process is disclosed in above-cited patent No. 5,106,602. This process may however be less than optimal in particular cases because of the formation of ammonium-sulfur precipi-tates by the reaction of NH3 with SO3 present in the flue or other gas being treated. This material can ~ result in plugging of the catalyst and a significant I reduction in catalyst life.
In short, there is a demonstrated and j continuing need for a process which is capable of removing particulate material, SOx, and NOX from contaminated gases in an efficient, cost effective manner and for systems in which such processes can be effectuated in the manner just described.

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-- 21i~&3'1 SUMMARY OF THE INVENTION

There have now been invented and disclosedherein certain new and novel pollution control pro-cesses and systems which combine the use of alkaliinjection to reduce SOX (So2 and S03) with filtration to remove particulate material and low-temperature catalytic NOX reduction catalyst to remove NO and the NO2 formed during the SOx removal step.
10Injection of the alkali reagent upstream of the filter facilitates the use of the low-temperature catalyst and the reduction of NOX by decreasing the concentration of SOx in the gase~ being treated.
Likewise, the low-temperature catalyst facilitates the use of alkali injection for SOx removal by alleviating the above-discussed NO2 problem which conventional SCR
NOX reduction processes have. As a consequence, the present invention makes it possible to efficiently remove all three pollutants of concern -- particulate material, SOx, and NOX -- from flue and other gases efficiently and in a cost effective manner.
Locating the SCR after the particulate matter removal system is also very advantageous.
Because, the gas reaching the SCR reactor is in, or approaches, a particle-free condition, a catalyst geometry with a high surface area/volume ratio and consequent compact design can be employed.
The objects, features, and advantages of the invention will be apparent to the reader from the foregoing and the appended claims and as the ensuing detailed description and discussion proceeds in conjunction with the accompanying drawings.

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2 ~ 3 1 BRIEF DESCRIPTION OF THE DRAWINGS
'-, FIG. 1 is a schematic diagram of one system which is constructed, and usable, in accord with the principle~ of the present invention to remove a combination of particulate matter, SOx, and NOx from contaminated gases; and FIG. 2 is a more detailed but still general-ly schematic view of a reactor which may be employed in the system of FIG. 1 in the extraction of NOx from the gas or gases being treated.

3 '1 DETAILED DESCRIPTION OF THE INVENTION

The initial step in removing particulate matter, SOx, and NOx from a contaminated gas by an integrated proces~ emhodying the principles of the present invention is to introduce into and mix with the contaminated gas an alkali reagent capable of reacting with SOx present in the gas to form an insoluble precipitate which can be mechanically removed from the gas. Representative of the materials which can be employed for this purpose are sodium bicarbonate and those identified and characterized in Table 1.

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``` 2 ~ , 3 (1 In general, the materials of Table 1 are white crystalline powders, stable in dry air but slowly decomposing in moist air. They are readily available in a range of grades and particle sizes and 5 can easily be pulverized to extreme fineness if desired. Typically preferred because of its suitabil-ity for use in a unit employing dry desulfurization is a sodium bicarbonate manufactured by solution carbon-ating or dry carbonating sodium carbonate that has 10 been refined from sodium sesquicarbonate (trona).
Nahcolite is a naturally occurring form of ~ sodium bicarbonate found in large underground deposits I in northwestern Colorado. This ore can be mined with I conventional room and pillar techniques and benefici-15 ated at the surface, or it can be solution mined and beneficiated. The terms nahcolite and sodium bicar-bonate are sometimes used interchangeably.
Trona is a naturally occurring sodium sesquicarbonate. Commercial sodium sesquicarbonate is i 20 normally obtained by refining trona ore. It is an j intermediate in the manufacture of sodium carbonate i~ (soda ash). Trona ore is dissolved in a liquor, allowing removal of insoluble impurities. Crystalli-zation then results in a pure sodium sesquicarbonate 25 product. The sesquicarbonate is water soluble and y decomposes with heat to yield sodium carbonate, carbon dioxide, and water. Tremendous deposits of trona, located in the State of Wyoming, are currently being 3~ mined and refined into soda ash for use in a great30 number of commercial products. The terms trona and ~ ~odium sesquicarbonate are sometimes used interchange-i ably.

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The alkali reagent is employed in a concen-tration dependent on the concentration of SOx (includ-ing the acid forms such as H2SO4) in the contaminated gas and the level to which the SOx concentration needs to be reduced to comply with pertinent regulations and/or to meet other objectives.
Temperatures at which the reactions between the alkali reactant and SOx are effected will typically be between 200 to 450'F and perhaps most often in the 10 range of 250 to 350-F. Optimum temperatures depend, at least in part, on the particular alkali reactant being employed. For example, sodium sesquicarbonate is more effective at lower temperatures; and sodium bicarbonate is better at higher temperatures.
Ammonia or an ammonia source is preferably added to the contaminated gas in the course of the SOx ¦ removal step. Via a mechanism that is not fully } understood, the ammonia inhibits the formation of NO2 -, and keeps an unwanted visible plume from developing.
Ammonia can be injected directly into the gas being treated.
Urea, which thermally decomposes into '~ ammonia (one mole of NH3 per one mole of urea), is a suitable alternative to ammonia and may be preferred for ease in handling, for example. This reactant may be mixed in pellet form with the alkali SOx removal reactant, ground to a fine powder, and added to the ` contaminated gas with the alkali. Alternativeiy, the urea may be slurried with water and added separately.
` 30 At least urea addition rates ranging from 0 to 0.2 pounds of urea per pound of alkali reactant may be ; used.

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In any case, the ammonia or ammonia source will be employed in a concentration which is propor-tional to that of the NO in the gases being treated and the inhibition of N0 to N02 conversion required for the removal of SOx to proceed efficiently. This ammonia may also be relied upon to reduce a signifi-cant proportion (typically ca. 10 percent) of the N0x in the gas to nitrogen in the particulate matter removal system, thereby facilitating the removal of N0x from the gas in a subsequent process step.
Another option that can often be employed to advantage is to add water to (humidify) the gas being treated after the alkali reactant has bee~ added.
Humidification increases the efficiency of the S0x removal process and helps to prevent the conversion of N0 in the contaminated gas to unwanted, interfering N02. Humidification is also used to condense gas and vapor phase contaminants so that those contaminants can be removed from the gas with other particulate material. Another important use of humidification is to, as appropriate, reduce the temperature of the contaminated gas to a level at which N0x removal will proceed efficiently.
The gas being treated with its burden of particulates -- including SOx precipitates and perhaps condensed gas and vapor phase contaminants -- proceeds to a station, typically a baghouse, where the particu-late material is removed from the gas. A conventional pulse jet fabric filter may advantageously be employed because of its ability to efficiently remove particu-late material found in gas streams of the character which the present invention is designed to treat as well as particulate material of the character generat-~11 2~

ed by the S0x removal and optional humidification stepsdescribed above. It is, however, not essential that this particular type filter, or indeed a filter of any type be employed for the removal of the particulate material. Electrostatic precipitators, for example, I are acceptable alternatives.
The particulate material collected in the baghou~e can be safely disposed of in a sanitary landfill. Alternatively, because of its sulfate and nitrogen values, the collected material may, in appropriate instances, be employed for its plant nutrient values with only minimal processing. Also, it may in at least some cases be advantageous to recover and regenerate the alkali reactant for recy-cling.
From the baghouse or other particulateremoval station, the now at least partially, and typically primarily, Sox- and particle-free gas proceeds to a low-temperature SCR reactor where N0x is removed from the gas by the selective catalytic reduction of the N0x to nitrogen.
The SCR catalysts are composed of transition metal sulfates which have sufficient Br0nsted activity ~ to effect N0x conversion with an efficiency of at least 3 25 50 percent at a temperature not exceeding 250 C.
Iron, cobalt, and nickel sulfates are preferred for their greater activity.
The catalyst can be employed in either a , supported or unsupported form. The support, if the catalyst is supported, is employed primarily to provide a high surface area. Which support is em-ployed is not critical as long as the support is porous and is physically and chemically stable at the :~
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112~3'1 sreaction temperature. Supports fabricated from commercially available alumina, silica, and titania are all suitable.
The weight percent of the transition metal ;5 sulfate can range from 1-30 based on the total weight of the supported catalyst. The main criteria are that the surface of the support be covered and that the , pores of the support not be blocked.
Supported catalysts are prepared by impreg-10 nating the selected support with an aqueous solution of the selected transition metal sulfate or with an aqueous solution of nitrate or other thermally decom-;~posable salt of the selected transition metal. The concentration of the aqueous solution may range from 0.01-10 normal. Repeated applications of the solution may be required if the solution is dilute.
After the impregnation step has been com-pleted, the impregnated support is dried in those applications in which a transition metal sulfate is 20 employed. Temperatures below those at which the sulfate will decompose for periods of at least 0.5 hour are used.
If the impregnant contains a salt other than a sulfate, the impregnated catalyst is dried as just r,,~25 described and then heated to a temperature (typically i600-C) high enough to decompose the salt into the 3corresponding oxide. Sulfur dioxide and oxygen are 1then contacted with the oxide to convert the latter to ", ~ .
~ithe wanted transition metal sulfate.
The reactor in which the N0x reduction i~
carried out may vary widely in character and appear-ance, depending upon the particular application of the invention. It need be nothing more than a simple tube ,., .
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of temperature resistant material with an inlet and an outlet and a section for the transition metal cata-lyst. It is preferred, however, that the reactor include, or have associated therewith, a temperature 5 controlled heater for maintaining the wanted tempera-ture in the reaction zone.
In power plants, the conversion will typi-cally, and preferably, be carried out at flue gas temperatures of 200 -35n F (33--177-C). This allows 10 the NOX conversion reactor to be conveniently located at the exit of th,e steam generator's air heater or in the base of the flue gas stack, in both cases minimiz-ing capital investment and process costs. The poten-tial for lower process cost is due in these instances 15 to: (1) smaller flue gas volume, and (2) improved equipment access.
Ammonia is maintained in the reaction zone in an amount ranging from 100 to 200 percent of the ~ stoichiometric amount to effect efficient NOX reduc-¦ 20 tion. The optimum amount of ammonia is ca. twice the ! stoichiometric amount. Higher concentrations can perhaps be employed, but it is not expected that this would be beneficial.
The ammonia may be mixed with nitrogen or 25 air in a concentration of (preferably) less than 10 percent before the mixture is introduced into the gas ~, being treated at a location upstream from the SCR
3 reactor. Also, the ammonia/nitrogen mixture is preferably heated. The dilution of the ammonia and ~ 30 the heating of the ammonia/nitrogen mixture preclude } the formation of ammonium sulfide or chloride salts before the mixture is added to the gas being treated.
The formation of those salts is undesirable because ., ~112~3~ ~

they may plug the pores of the catalyst employed for N0x removal.
It was pointed out above that it may be s~advantageous to introduce ammonia into the gas being '~5 treated upstream from the system in which particulate ¦matter is removed from the gas. The ammonia required in the SCR reaction zone for the reduction of N0x may also be supplied in this manner by providing an excess ¦instead of being added to the gas being treated after tlo the gas is discharged from the particulate matter i removal system.
iIt is in appropriate cases also advantageous to cool and humidify the gas being treated before that ,7,gas is introduced into the SCR reactor. This is done 15 in order to lower the temperature of t7he gases reach-ing the SCR reactor to a level resulting in efficient N0x removal in those cases where the incoming gases 3have a higher temperature. The water may be added in a conventional humidifier or with an air atomizing 20 water injection nozzle, for example. The water is added in the amount needed to reach that goal. -The gas discharged from the SCR N0x removal reactor will typically have an at least 90 percent lower concentration of particulate material than the 25 gas reaching the baghouse or other particulate matter separation system. The process will typically remove at least 70 percent of the S02 and at least 80 percent ;~of the sulfuric acid in the incoming gas, at least 60 -~percent of the N0x in that gas and at lest 90 percent i30 of the particulates.
:7Referring now to the drawing, FIG. l depicts ~an exemplary integrated system 20 which is constructed ;~in accord with and embodies the principles of the "

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present invention and is designed to remove particu-late matter, S0x, and N0x from a contaminated gas inputted to the system. The major components of system 20 include: (1) an inlet duct 22 for the contaminated gas; (2) a variable feed gravimetric screw feeder 24 for a particulate alkali reagent capable of reacting with, and precipitating, SOx in the gas being treated; (3) filters 26 (shown in block diagram form) for removing particulate matter, includ-ing precipitated S0x compounds, from the gas; (4) a discharge duct 28 for the at least relatively parti-cle-free gas; and (5) a compact second reactor 30 in which N0x is removed from the gas being treated by a low-temperature SCR process. Additional, major , 15 components of system 20 include: (6) an inlet line 32 I for adding ammonia to the incoming contaminated gas in 7 inlet duct 22; (7) a water/air inlet line and atomiz-ing nozzle system 34 between gravimetric feeder 24 and filters 26 for humidifying the gas by atomizing and supplying the water optionally employed to optimize the temperature window for S0x and N0x removal reac-tions and to promote the condensation of contaminants in the gas before it reaches the filter; (8) a second water/air system 36 between filters 26 and SCR reactor 30 which can be used to add water to the gas in discharge duct 28 and thereby control the temperature in the N0x removal reactor; and (9) a supply or inlet line 38 for adding to the gas in discharge duct 28 the ammonia utilized in reactor 30 in reducing, and 3C thereby removing, Nox from the gas being cleaned.
As discussed above, filters 26 will typical-ly be conventional, pulse jet fabric filters. A set of filters is hung in a baghouse 40 from a tube sheet !

f~ ~. ,i, f~j~ & 3 4 42. Particulate material removed from the contaminat-ed gas and collected on filters 26 is blown from the filters into a hopper 44 by pulses of air directed against the filters from a plenum 46 at a rate regu-lated by pulse control 48. The hopper is heated toavoid condensation as this would cause the collected particles to clump, making it difficult to remove the collected material from the hopper.
As shown in FIG. 2, SCR reactor 30 may be a tube 50 with a fritted glass support 52 for a bed 54 of transition metal sulfate catalyst. The reactor has an inlet duct 56, tapped into baghouse discharge duct 28, for the reactant mixture of ammonia and the gas being treated in system 20 and an outlet duct 58. A
lS resistance heater 60 wound around inlet duct 56 I preheats the reactant mixture supplied to reactor 30 ¦ through that duct.
Reactor 30 may be housed in a tubular heater 62 which closely surrounds the reactor to maintain an ~ 20 optimal N0x reduction temperature in the reactor if the 3' incoming gas is not hot enough to achieve that objec~
tive. The operation of the heater, which can be used to raise the temperature in reactor 30 by 25-50-F, is regulated by a programmable temperature controller 64.
25The particulate system 20 illustrated in FIG. 2 is appropriately instrumented. Thermocouples ! 66 and 68 inserted into the gas streams are used to measure the temperature of the gas being treated at the baghouse 46 inlet and outlet. An orifice meter 70 in the baghouse discharge duct 28 is used to measure the contaminated gas flow rate through s~stem 20, and a valve 72 in gas inlet duct 28 is used to control the flow rate of the contaminated gas through the system.

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21i~3'1 A slipstream of the gas downstream from orifice meter 70 flows through the smaller 5CR inlet duct 56 to SCR reactor 30, then back to the baghouse outlet duct 28 through SCR reactor outlet duct ~8. A
second, smaller-diameter orifice meter 74 is used to measure the flow rate of this slipstream, and a butterfly valve 76 in the baghouse outlet duct 28 between the take-off and return for SCR reactor inlet and outlet ducts 56 and S8 is adjusted to divert the required amount of gas through the SCR reactor. Only approximately 5 to 15 acfm of the gas in duct 28 i~
routed through SCR reactor 30 in the particular system 20 shown in FIG. 1.
Gas samples are continuously extracted from , 15 ports 78, 80, and 82 in contaminated gas inlet duct -, 22, baghouse outlet duct 28, and SCR reactor outlet ,~ duct 58 for SOx analysis and from ports 84, 86, and 88 in the same ducts for NOX analysis.
Sx removal efficiency is measured by single , 20 subtraction with inlet and outlet measurements cor-i, rected for dilution attributable to the leakage of air 3 into the baghouse.
I The samples taken from ports 84, 86, and 88 are routed through a chiller/knockout (not shown) to 25 remove moisture and then to a chemiluminescence NOX
~,~ monitor.
The effectiveness of system 20 in removing ' NOX is determined by: (1) measuring baseline concen-trations for these species before any alkali reagent 30 i5 fed to the contaminated gas upstream of baghouse 40 and before any ammonia is injected into the gas stream upstream of SCR reactor 30, and (2) comparing these ' baseline levels with levels measured during the ,...

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"` 2 t 1"`~) ~ 3 ll addition of the alkali and ammonia reagents. Theeffectiveness of the SCR unit 30 in removing N0x is determined by measuring concentrations of these species in the gas with and without the ammonia reagent being added to gas stream upstream of the SCR
reactor.
For some tests, an N02 monitor based on the principles of ultraviolet spectroscopy is used to measure N02 concentration in the gas discharged from SCR reactor 30. This is a more direct measurement of the concentration of this specie than the alternative, which i6 to subtract the N0 concentration in the gaæ
from the total N0x concentration as measured by the chemiluminescence monitor.
The amounts of ammonia and nitrogen or air diluent added to the gas being treated between bag-house 40 and SCR reactor 30 in system 20 are measured with individual rotameters and controlled with manual valves. The temperature of the ammonia/diluent mixture is measured with a thermocouple and controlled ¦ with feedback controller 64 which regulates the flow ~ of current to the resistance heater 60 utilized to ¦ heat the ammonia/diluent mixture.
The components just described have for the mo~t part not been illustrated in FIG. 1 as they are not part of the present invention and because showing ~, them would therefore not be in the interest of brevity and clarity.
The efficacy of system 20 was confirmed in tests employing flue gas isokinetically extracted from a coal-fired boiler at the New York State Electric and Gas Company Kintigh Station at the boiler air heater exit ductwork. The flue gas contains about 1000 to ~, .. , 1~00 ppmv (wet) of SO2, 300 to 400 ppmv (dry) of N0x, 6 to 8 volume percent moisture, and 1 gr/acf of particulate matter and is at a temperature of approxi-mately 300-F.
The terms "ppmv" and "acf" as employed in the preceding paragraph and hereinafter respectively stand for parts per million on a volumetric basis and actual cubic feet.
The SCR catalyst was 10% nickel on alumina, the SOx removal reagent was sodium bicarbonate with a mean particle diameter of approximately 7.3 ~n, and ammonia was added to the contaminated gas through supply line port 38. The baghouse inlet temperature was 295 F, the SCR reactor inlet temperature was ~ 15 220-F, and the rate-of-flow of the flue gas into I ~ystem 20 was 7600 l/hr.
Data obtained from the tests is presented in Table 2.

Table 2 i Run l Run 2Run 3 lSodium Bicarbonate Feed Rate (lb/h)2.7 2.72.7 j 25 Total NH3/N2 Injection ~i~ Rate (cc/min) 758 14151485 '4 ' (50 NH3, ~30 NH3, (70 NH3, 708 N2) 1415 N2) 1415 N2) .. 30 , ., .

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' ifi 3 ~1 Run 1 Run 2 Run 3 _x_Data ~Across Baghouse 40 SO2In(ppmv) 1000 5O2Out(ppmv) 200 2NSR 1.2 SO2Rem(%) 80 -x Data ~Across Baghouse 40 NOIn(ppmv) 290 NO~iN(ppmv) 5 NOOut(ppmv) 152 f NO2Out(ppmv) 25 NORemoval(%) 40 ' 15 Across SCR Reactor 30 i SpaceVol(1/hr) 7600 7600 7600 2NSR 0.5 1.0 2.4 NOOut(ppmv)125 ~2 52 NO2Out(ppmv)23 18 12 NORemoval(%)18 46 66 NO2Removal(%)8 28 52 NOxRemoval(%)16 44 64 Across Total System NOxRemoval(~)50 66 78 -~-1. Same for all runs 2. NSR = normalized stoichiometric ratio. One mole i~ of ammonia is needed to react with each mole of ~ NO in the gas being treated, and two moles of :' ammonia are needed for each mole of NO2. An NSR of :.
1 means that enough ammonia is added to provide 1 mole of ammonia for each mole of NO and 2 moles of ammonia for reaction with each mole of NO2.

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,~ , ;, . ,i ~i1 2~3 1 The tests showed that system 20 and the integrated process carried out in that system had the capability of reducing S02 (80~ removal) and N0x (50-78% removal) with a normalized stoichiometric ratio of NH3/N0x of 0.5-2.4~ Of the N02 generated in the baghouse, 8-52% was removed in SCR reactor 30. The duration of the test~ was 16 hours, during which no noticeable buildups of deposits were observed on the catalyst.
The invention may be embodied in many forms other than those disclosed above without departing from the spirit or essential characteristics of the invention. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicat-ed by the appended claims rather than by the foregoing description; and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

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Claims (34)

What is claimed is:

1. A method of removing both SOx and NOx and particulate contaminants from a contaminated gas, said method comprising the steps of sequentially:
introducing an alkali reagent into said gas in an amount effective to accomplish a removal of SOx from the gas;
removing particulate contaminants from the gas; and effecting the removal of NOx from the gas.

2. A method as defined in claim 1 in which the alkali reagent introduced into the gas being treated to remove SOx therefrom is a particulate sodium bicarbonate.

3. A method as defined in claim 1 in which the particulate solids removed from the gas being treated include those generated by reactions between SOx in the gas and the alkali reagent and in which the particulate solids are removed from the gas by filtra-tion.

4. A method as defined in claim 3 in which the solids are removed with a pulse jet fabric filter.

5. A method as defined in claim 1 in which gas and/or vapor phase contaminants in the gas are condensed and thereby made removable from the gas with other particulate material.

6. A method as defined in claim 1 in which the gas and/or vapor phase contaminants are condensed by introducing water into the contaminated gas.

7. A method as defined in claim 1 in which NOx is removed from the contaminated gas by reducing it in a reaction zone with a low-temperature SCR
catalyst.

8. A method as defined in claim 7 in which the catalyst comprises a transition metal sulfate which has sufficient Bronsted activity to effect NOx conversion with an efficiency of at least 50 percent at a temperature of not more than 250 C.

9. A method as defined in claim 8 in which the catalyst is an iron, cobalt, or nickel sulfate.

10. A method as defined in claim 7 in which ammonia is maintained in the reaction zone in an amount effective to bring about the reduction to nitrogen of NOx introduced into said zone.

11. A method as defined in claim 10 in which ammonia is maintained in the reaction mixture in an amount ranging from 100 to 200 percent based on the stoichiometric amount of the NOx in said zone.

12. A method as defined in claim 11 in which the ammonia is maintained in the reaction zone by introducing a mixture of ammonia and nitrogen or air diluent into the contaminated gas before it reaches the reaction zone, the concentration of diluent being such as to dilute the ammonia to an extent effective to inhibit the formation of salts which might clog the pores of the SCR catalyst.

13. A method as defined in claim 10 in which the ammonia is maintained in the selected concentration in the reaction zone by introducing ammonia into the contaminated gas.

14. A method as defined in claim 13 in which the ammonia is introduced into the contaminated gas at a location upstream from a location where particulate material is removed from the gas.

15. A method as defined in claim 7 which includes the step of regulating the temperature of the reaction zone by heating the ammonia/contaminated gas mixture.

16. A method as defined in claim 1:
in which the NOx is removed from the contami-nated gas in a SCR reaction zone; and which includes the step of regulating the temperature in the reaction zone by adding water to the contaminated gas before it reaches the reaction zone.

17. A method as defined in claim 1:
in which the NOx is removed from the contami-nated gas in a reaction zone defined by a SCR reactor;
and which includes the step of regulating the temperature in the reaction zone by heating said reactor.

18. A method as defined in claim 1 in which:
NOx is removed from the contaminated gas by catalytic reduction; and ammonia or urea is added to the gas being treated at a location upstream from that where partic-ulate contaminants are removed from the gas being treated in an amount which is effective to remove SO3 from the gas being treated and thereby keep that compound from forming precipitates which might clog the catalyst.

19. A method as defined in claim 1 in which water is introduced into the contaminated gas at a location upstream from a location where particulate contaminants are removed from the gas, the water being added in an amount effective to maintain a temperature at which SOx can be efficiently removed from the gas and to potentiate the condensation of contaminants and the subsequent removal of said contaminants from the gas being treated along with other particulate contam-inants.

20. An integrated system for removing the combination of particulate matter, SOx, and NOx from a contaminated gas, said system comprising:
a first reactor means in which SOx can be removed from the gas by reaction with an alkali reagent;
a particle collection means for thereafter removing particulate matter from the gas;
a second reactor means in which NOx can be removed from the gas by a low-temperature SCR process;
and means for transferring the gas after treat-ment thereat from the particle collection means to the second reactor means.

21. A system as defined in claim 20 in which the particle collection means comprises a filter means.

22. A system as defined in claim 21 in which the filter means is a pulse jet fabric filter.

23. A system as defined in claim 20 which includes means upstream from the first reactor means via which the alkali reagent can be added to the gas.

24. A system as defined in claim 23 in which the means for adding the alkali compound to the gas is an adjustable rate, particulate solids feeder.

25. A system as defined in claim 20 in which the second reactor means comprises a transition metal sulfate catalyst which has sufficient Bronsted activity to effect NOx conversion with an efficiency of at least 50 percent at a temperature of not more than 250 C.

26. A system as defined in claim 25 in which the transition metal sulfate is a nickel, cobalt, or iron compound.

27. A system as defined in claim 20 which comprises means upstream of the first reactor for adding a nitrogen compound to the gas to effect the reduction of NOx in gas and thereby keep said NOx from interfering with the reactions which effect the removal of SOx from the gas.

28. A system as defined in claim 20 which includes means for maintaining ammonia in the second reactor means in an amount effective to bring about the reduction to nitrogen of nitrogen oxides intro-duced into said reactor means.

29. A system as defined in claim 20 which comprises means for adding water to the contaminated gas upstream from the first reactor in an amount effective to maintain a temperature at which SOx can be efficiently removed from the gas and to potentiate the condensation of contaminants and the subsequent removal of said contaminants from the gas being treated along with other particulate contaminants.

30. A system as defined in claim 20 which comprises means for regulating the temperature in the second reactor means.

31. A system as defined in claim 30 in which the temperature regulating means comprises heater means and means for regulating the operation of the heater means.

32. A system as defined in claim 30 in which the temperature controlling means comprises means for introducing water into the gas at a location downstream from the particulate matter removing means.

33. A method as defined in claim 30:
which includes means for introducing ammonia into the contaminated gas at a location upstream from the second reactor means; and in which the means for regulating the temperature in the second reactor means comprises means for heating the mixture of ammonia and contami-nated gas.

34. A method as defined in claim 30:
which includes means for introducing ammonia into the contaminated gas at a location upstream from the second reactor means; and means for diluting the ammonia with nitrogen or air before it is introduced into the contaminated gas.