WO1993000984A1

WO1993000984A1 - A LOW TEMPERATURE, SUPPORTED METAL OXIDE CATALYTIC NOx AND SOx REMOVAL PROCESS FOR RETROFIT TO WET FLUE GAS DESULFURIZATION SYSTEMS

Info

Publication number: WO1993000984A1
Application number: PCT/US1992/005553
Authority: WO
Inventors: Raul Eduardo Ayala
Original assignee: General Electric Company
Priority date: 1991-07-02
Filing date: 1992-07-01
Publication date: 1993-01-21

Abstract

A wet flue gas desulfurization system is retrofitted with an NOX catalytic reactor which permits substantially simultaneous removal of any SOx and NOx present in the flue gas. Such retrofitting allows the system to be operated at lower temperatures without adversely affecting the reactor and the wet scrubber.

Description

A LOW TEMPERATURE. SUPPORTED METAL OXTDE

CATALYTTC NO_χ AND SO REMOVAL

PROCESS FOR RETROFIT TO WET FLUE GAS

DESULFURTZATΪON SYSTEMS

Cross Reference to a Related Application

This application is related to commonly assigned U.S. Patent Application Serial No. _{07/725 f 857} (RD-20,981), entitled "SUPPORTED METAL OXIDE CATALYSTS FOR THE SIMULTANEOUS REMOVAL OF NO_x AND SO_x FROM GAS STREAMS".

Background of the Invention

Field of the Invention

This invention relates to wet flue gas desulfurization systems, hereinafter referred to as wet FGD systems with catalytic NO_x removal. Such systems of this type generally remove substantially all of the SO_x and NO_x found in the exhaust gases of coal burning systems.

Description of the Related Art

Selective catalytic reduction, hereinafter referred to as SCR, is a recent method employed for the removal of NO_x from gas streams by injecting a reducing agent such as ammonia (NH3) in me presence of a conventional oxide catalyst. The difficulty in trying to couple SCR systems with existing wet scrubbers is that the SCR must operate at around 400'C(752^*F), temperatures too high for wet scrubbers and usually found upstream of the airpreheater and pre-scrubber where the dirty flue gas typically contains fly ash, chlorides, and high levels of SO2. Because of the relatively large amounts of these contaminants, the SCR catalyst is easily poisoned, eroded by particulates, and rendered useless or in need of frequent regeneration.

An often common alternative for the simultaneous NO_x removal in wet scrubbers is to operate with aqueous lime-based solutions containing calcium ions (for SO2 removal) and in addition to introduce additives such as EDTA, yellow phosphorus, or amines for NO_x control. Unfortunately, these techniques result in major operational changes to the existing scrubber and are not easily implemented as a retrofit. Moreover, the chemistry of NOχ in solution is not well understood, and often these processes end up with a mixture of nitrates, nitrites, and other nitrogen byproducts from NO_x removal in the liquid phase that are not easily disposed of or reclaimed. A more advantageous system, then, would be presented if the catalyst were made to be more durable and the process for NO_x removal were more easily retrofitted to existing wet FGD systems.

It is apparent from the above discussion that there exists a need in the art for wet FGD systems which remove the amount of SO_x found in the exhaust gases, and which at least equal me SO removal characteristics of the known systems, but which at the same time also substantially eliminate the NO_x found in the exhaust gases without any major operational changes to the existing system. It is a purpose of this invention to fulfill these and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.

Summary of the Invention

Generally speaking, this invention fulfills needs for simultaneous removal of SO_x and NO_x by providing a wet flue gas desulfurization system, comprising an exhaust gas means providing an exhaust gas, a heater means for adjusting a temperature of said exhaust gas, a pre-scrubber means attached to said heater means, a scrubber means attached to said pre-scrubber means and said heater means, and a catalytic reactor means to said heater means.

In certain preferred embodiments, the system is capable of being operated at less than 200'C, preferably, between 150' and 200^*C without adversely affecting the catalytic reactor. Also, the catalytic reactor means is easily retrofitted to me wet scrubber system.

In another further preferred embodiment, substantially all of me SO and NO_x contained in the exhaust gases is simultaneously eliminated by the wet scrubber system.

The preferred system, according to this invention, offers the following advantages: easy retrofit; good chemical stability; improved durability; good economy; high strength for safety; and improved NO_x removal. In fact, in many of the preferred embodiments, these factors of retrofit, durability and NO removal are optimized to an extent considerably higher than heretofore achieved in prior, known wet FGD systems.

Brief Description of the Drawings The above and other features of the present invention which will become more apparent as the description proceeds are best understood by considering the following detailed description in conjunction with the accompanying drawings wherein like characters represent like parts throughout the several views and in which:

Figure 1 is a schematic drawing of a wet FGD system, according to the prior art; Figure 2 is a schematic drawing of a wet FGD system having catalytic NO_x and

SOχ removal, according to the present invention;

Figure 3 is a schematic representation of the apparatus for constructing the NO_x and SOχ catalytic reactor; and

Figure 4 is a side view of the NOχ and SO_x catalytic reactor. Detailed Description of the Invention

With reference first to Figure 1, there is illustrated a prior art wet FGD system 2.

System 2 includes conventional conduits 4, 8, 16, 24 and 26, heater 6, pre-scrubber 10 having liquid 12, which usually is water (H2O) and wet scrubber 18 having liquid 20 which usually is a calcium-based or sodium-based slurry, preferably, a limestone (CaCO3) slurry. In particular, heater 6, typically, is constructed so mat it will cool the exhaust gases emitted from the coal burner (not shown), from 150'C down to approximately 120'C. The exhaust gases leaving the burner along conduit 4 and entering heater 6 also usually contain approximately 2000 ppmv SO2 and 250 ppmv NO_x. Heater 6 cools down the exhaust gases to 120^*C so that as the exhaust gases enter pre-scrubber 10 from conduit 8, the temperature of d e exhaust gases will not cause the liquid 12, to boil off too rapidly. It is to be understood mat prescrubber 10 may not be utilized if, for example, chlorides are not present in the exhaust gases.

After the exhaust gases enter pre-scrubber 10 and liquid 12, substantially all of the hydrochloric acid (HCl) and particulates found in the exhaust gases are removed. The exhaust gases are then transported along conduit 16 to wet scrubber 18 to remove the SO_x present in the exhaust gases.

Wet scrubber 18 usually is constructed of one or two different designs. In one design, the exhaust gases are introduced into the bottom of the wet scrubber and the gases flow to me top of the scrubber. As the gases rise, a conventional slurry 20 of CaCO3 is sprayed by well known techniques through the exhaust gases. In the other design, scrubber 18 contains a reservoir 20 and the exhaust gases are bubbled up through the reservoir 20 from the bottom of scrubber 18. In these two designs, 90% or more of the SOχ found in the exhaust gases is removed. After the S0_X is removed from the exhaust gases in wet scrubber 18, the exhaust gases, typically, are at a temperature of approximately 52^*C. The exhaust gases which leave scrubber 18 then proceed along conduit 24 to heater 6 where the gases are heated to approximately 85^*C. The exhaust gases leaving heater 6 also contain, typically, 150-200 ppmv SOχ and 250 ppmv NO_x. The exhaust gases leaving heater 6 then are transported along conduit 26 to the stacks (not shown) and emitted into the atmosphere.

While it is clear that the amount of SO_x emitted into the atmosphere is substantially reduced (2000 ppmv to 150-200 ppmv), the amount of NO_x emitted is not reduced in any appreciable amount Therefore, a more advantageous system would be one which not only reduced the amount of SO_x found in the exhaust gases but, could further reduce the amount of NOχ found in the exhaust gases as well.

Figure 2 illustrates a wet FGD with catalytic NO_x removal system 50. In particular, system 50 includes conventional conduits 4, 8, 16, 24, 26 and 54, heater 6, pre-scrubber 10 having fluid 12, which typically is H2O, wet scrubber 18 having fluid 20 which typically is CaCθ3 and catalytic reactor 52. Conduits 4, 8, 16, 24 and 26, heater 6, pre- scrubber 10 having fluid 12 and wet scrubber 18 having fluid 20 are constructed substantially the same as in the prior art system 2. However, reactor 52 has been retrofitted to the end of conduit 26 so as to substantially remove the NOχ and any remaining SO_x found in me exhaust gases before the exhaust gases are transported along conduit 54 to the exhaust stacks (not shown).

It is to be understood that reactor 52 is prepared by the same techniques as set forth in U.S. Patent Application Serial No. 07/725 ,857 (RD-20,981), entitled

"SUPPORTED METAL OXIDE CATALYSTS FOR THE SIMULTANEOUS REMOVAL OF NOχ AND SOχ FROM GAS STREAMS". In particular, with reference to Figure 3, there is illustrated a catalyst coating apparatus 100. Apparatus 100 includes enclosure 103, injection port 106, preferably, used for introducing ammonium metal salts such as ammonium metavanadate (NH4VO3) into enclosure 103, injection port 108, preferably, used for introducing nitrogen and oxygen (N2 and O2) into enclosure 103, and injection port 110, preferably, used for introducing ammonia (NH3) into enclosure 103. Substrate 104 which can be constructed of Tiθ2, activated carbon carbonaceous materials, or LEXAN® polycarbonate is placed in apparatus 100 so substrate 104 can be coated. It is to be understood that the substrate can also be a high surface area powder in which case a support structure in the shape of a fiat plate or honeycomb pattern is needed for added mechanical strength. The role of the substrate is to synergistically interact with the metal so as to result in catalytic activity greater than metal and substrate acting separately.

Substrate 104 is coated with an ammonium metal salt layer 114 (Figure 4) and substrate 104 and layer 114 are dried in air at a temperature of less dian lOO'C. After layer 114 and substrate 104 are dried, they are put back into enclosure 103 and a gas mixture of balanced nitrogen and oxygen are introduced into enclosure 103 through port 108. Next, ammonia (NH3) is placed into enclosure 103 such that me NH3 absorbs itself into layer 114 to create a molecular layer 114 having molecules 116 of NH3.

With respect to Figures 3 and 4, the production of catalytic reactor 52 will be discussed wim reference to the following preferred example.

EXAMPLE 1

Substrate 104, preferably constructed of ΗO2 is placed inside enclosure 103.

Substrate 104 is then wet impregnated by conventional wet impregnation techniques with a solution of an ammonium metal salt, preferably, ammonium metavanadate (NH4VO3). The NH4VO3 is introduced through port 106. It is to be understood mat if ΗO is used as the substrate, the solution should be a water solution but if a polymeric material is used as the substrate, the solution should be an alcohol/water solution.

After substrate 104 is coated with the ammonium metal salt layer 114 (Figure 4), substrate 104 and layer 114 are dried in air in a conventional oven (not shown) at a temperature less than lOO'C, preferably, 70"C for at least 8-12 hours or until layer 114 is dried. This drying of the layer 14 causes layer 114 to begin to activate. Once layer 114 and substrate 104 are dried, they are put back into enclosure 103 and a gas mixture of balanced nitrogen and oxygen having a percentage of total volume of the gas of approximately 1-5%, is placed in enclosure 103 through port 108. Substrate 104 is subjected to mis nitrogen/oxygen gas mixmre at a temperature of less than 200^*C, preferably, between 150^* and 200"C for approximately 1 hour. The nitrogen/oxygen gas mixmre is heated in enclosure 103 by heaters 118 which, preferably, are conventional heater strips. This step also aids in activating layer 114 be converting the ammonium metal salt into a metal oxide. Once substrate 104 and layer 114 are sufficiently dried, the ammonia (NH3) is introduced into enclosure 103 that already contains the nitrogen/oxygen mixmre through port 110. The amount of ammonia added is, preferably, up to 500 ppm. The NH3 absorbs itself to layer 114 to create molecules 116 (Figure 4). The purpose of layer 114 having molecules 116 is that the NO_x contained within the gas stream will react with molecules 116 in sim to substantially decompose the NO present in the exhaust gases into nitrogen and water.

After substrate 104 is coated with layer 114, catalytic reactor 52 is formed. Reactor 52 can men be used to decompose NO from a gas stream. In order to further decompose SOχ from the gas stream by chemical reaction, additional NH3 must be injected by conventional techniques such as those used in the SCR method into the area near reactor 52. The preferred concentration of NH3 injected is a concentration which is stoichiometric to the amount of SO_x present in the gas stream with the amount of SO_x being measured by conventional techniques. The preferred amount of NH3 injected into the area around reactor 52, typically, being approximately 500-2000 ppm. During the operation of system 50, after the exhaust gases have been transported from wet scrubber 18 where a large portion of the SO_x contained in me exhaust gases has been removed, the exhaust gases are then heated in heater 6 to approximately 150'-200"C. The exhaust gases leaving heater 6 and proceeding along conduit 26 contain approximately 150-200 ppmv of SO2 and 250 ppmv NOχ. After the exhaust gases enter reactor 52, the exhaust gases are treated in reactor 52 according to the mediod set forth above, such that the SO2 concentration leaving reactor 52 along conduit 54 is approximately 0-20 ppmv while the concentration of NO_x is approximately 0-20 ppmv. Also, the amount of NH3 present in the exhaust gases in conduit 54 is approximately 15-20 ppmv. The present invention is an improvement over the prior art because reactor 52 can be operated at a lower temperature (150-200'C) than me SCR device (~400^*C) which allows reactor 52 to be placed after scrubbers 10 and 18 rather than before scrubber 8 as is the practice with the SCR device so mat the contaminants and particulates can not poison and erode reactor 52. Also, reactor 52 can be easily retrofitted to system 50 by using conventional connectors and conduits. Finally, system 50 substantially simultaneously removes virtually all of the SO_x and NO_x found in the exhaust gases. In fact, any residual SO2 in the exhaust gases leaving scrubber 18 and heater 6 are converted to powdered ammonium sulfate on the inner surface of reactor 52, which further increases the efficiency of the SOχ removal system or else the minor amount of residual SO2 simply leaves reactor 52 where it is emitted into the atmosphere through the exhaust stack (not shown). Once given the above disclosure, many other features, modifications or improvements will become apparent to the skilled artisan. Such features, modifications or improvements are, therefore, considered to be a part of this invention, the scope of which is to be determined by me following claims.

Claims

WHAT IS CLAIMED IS:

1. A wet flue gas desulfurization system, said system comprising: an exhaust gas means providing an exhaust gas; a heater means for adjusting a temperature of said exhaust gas; a pre-scrubber means attached to said heater means; a scrubber means attached to said pre-scrubber means and said heater means; and a catalytic reactor means attached to said heater means.

2. The system, according to claim 1, wherein said catalytic reactor means is further comprised of: a substrate having first and second sides; a layer of a metal salt having first and second sides such mat said first of said layer is adjacent to said second side of said substrate; and a layer of ammonia having first and second sides such that said first side of said ammonia layer is adjacent to said second side of ammonium metal salt layer.

3. The system, according to claim 2, wherein said substrate is further comprised of: titanium dioxide.

4. The system, according to claim 2, wherein said substrate is further comprised of: a polymeric material.

5. The system, according to claim 4, wherein said polymeric material is further comprised of:

LEXAN® polycarbonate.

6. The system, according to claim 2, wherein said substrate is further comprised of: activated carbon.

7. The system according to claim 2, wherein said substrate is further comprised of: carbonaceous material.

8. The system, according to claim 2, wherein said metal salt is further comprised of: an ammonia layer on a metal oxide.

9. The system, according to claim 8, wherein said ammonium metal salt is further comprised of: vanadium-based metal salt.

10. The system, according to claim 2, wherein said metal salt layer is further comprised of: a chromium-based metal salt.

11. The system, according to claim 2, wherein said substrate is further comprised of: a sheet of material.

12. The system, according to claim 11, wherein said sheet of material is further comprised of: a honeycombed sheet.

13. The system, according to claim 2, wherein said substrate is further comprised of: a powdered materiaL

14. A method for substantially simultaneously removing SO_x and NOχ from exhaust gases including a heater means, a pre-scrubber means ,a scmbber means, and a catalytic reactor means, said method comprising the steps of: transporting said gases to said heater means; heating said gases; transporting said gases to said pre-scrubber means; treating said gases in said pre-scrubber means to substantially remove any particulates or hydrochloric acid present in said gases; transporting said gases to said scrubber means; treating said gases in said scrubber means to remove substantially any SO_x present in said gases; transporting said gases to said heater means; performing a second heating of said gases in said heater means; transporting said heated gases to said catalytic reactor means; treating said gases in said reactor means to remove substantially any NO_x and SO_x present in said gases; and transporting said gases into the atmosphere.

15. The method, according to claim 14, wherein said step of performing a second heating of said gases is further comprised of the steps of: heating said gases to approximately 150^* to 200'C.

16. The method, according to claim 14, wherein said step of treating said gases in said reactor is further comprised of the steps of: introducing said gases into said reactor, introducing ammonia into said reactor, interacting said ammonia and said reaαor with any SO_x and NO_x present in said gases to remove the SO_x and NO_x present in said gases; and exhausting said gases from said reactor.