Ind. Eng. Chem. Res. 1994,33,2259-2264
2259
Selective Catalytic Reduction of NOX in the Presence of Oxygen Alvin B. Stiles,' Michael T. Klein, Phillipe Gauthier, Stephen Schwarz, and Jianguo Wang Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delware 19716
A process and catalysts therefor are detailed which will selectively remove 100% of the NOX in a waste effluent (flue gas) also containing oxygen. The process functions on the basis of an adsorbent which efficiently adsorbs the NOX at high space velocities and for long periods (over 9 h). Temperature of adsorption is a very practical 180 "C or above. The catalyst adsorbent can be regenerated for reuse repeatedly by exposing to a gas containing 0.5-1076 H2 at 250 "C or above. A cycle of useregeneration is contemplated for commercial exploitation. Any NOX not reduced as it is desorbed from the adsorbent is reduced to N2 H2O in a downstream catalyst bed thus eliminating 100% of the inlet NOX.
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A process has been developed over the past few years at the Center for Catalytic Science and Technology a t the University of Delaware, which will remove NOX from effluents also containing oxygen. These effluents are from gas-powered turbines, boilers, and electric power generating stations. These gases contain NOX derived either from the fuel or from the extremely high temperature to which the N2 and 0 2 are simultaneously heated. The NOX content may be in the range of 50-1000 ppm and the 02, 0-21 %. The process is unique in that it utilizes an adsorbent comprising primarily transition metal oxides, an alkali, alumina support, and a colloidal oxide as a binder and hardening agent. This intimately mixed complex is made into granules or extrudates, or can be supported on a honeycomb, rings, cylinders, saddles, or any typical wellknown support fabricated from ceramic compositions or metallic sheets or wires. These catalysts are placed in a reactor through which is passed the gases to be freed of NOX. The NOX will be removed over a long time period by a rapid and complete adsorption process. The adsorbed nitrogen oxides, after a period of adsorption, are removed from the adsorbent by regeneration for reuse of the adsorbent. The adsorbent will remove the nitrogen oxides to the extent of 100% at a space velocity exceeding 15 000 and a temperature in the range of 150-300 "C or above. The nitrogen oxides can be quickly reduced in situ or be evolved from the adsorbent as a concentrated stream by passing a gas containing N2 plus 0.5-10 % hydrogen at a temperature of 300-350 "C over the saturated adsorbent. The nitrogen oxides in the concentrated stream are reduced to nitrogen and water at this temperature in a downstream catalyst bed. This reduction of NOX is also 100% complete over a catalyst comprising, for example, chromium, copper, cobalt, or nickel oxide supported on y-alumina or even the same composition as the adsorbent. The adsorbent can be utilized repeatedly in the adsorption-desorption cycle without loss of effectiveness. Both the catalyst adsorbent and reducing catalyst are resistant to small quantities of SOX which may result from the odorant in the natural gas. The process is unique because it can be utilized for adsorption over a period of hours in a gas stream containing oxygen and can readily be regenerated for reuse. A more detailed account of the tests and data will be presented hereinafter.
* To whom correspondence should be addressed.
The Role of Chance and Sleuthing One would prefer to say that the discoveries in this research were the result of careful planning, reasoning, and application of unusual intelligence. However, humbling as it is, one must admit that in this case one of the very critical discoveries resulted from the use of a retained sample of mislabeled material. The facts were discovered when in further testing of the composition using other samples of the material as identified, very poor and puzzling results were observed. As we all are sometimes forced to do, the mystery ingredient was analyzed and adsorbent was made from various types of the genuine article. Superb results were again experienced and then related oxides were examined, and some were found which were even better than the mislabeled ingredient. Confession is purportedly good for the soul, and it is to be confessed that had it not been for the mistake, the outstanding ingredient and its "family" would not have been examined because the "family" is not reported to serve in the function it so admirably now serves.
Background and Other Processes In the nylon synthesis process the adipic acid synthesis involves the use of nitric acid as an oxidizer for cyclohexanol and cyclohexanone. NO2 is a byproduct of the synthesis, and prior to 1950this NO2 (and some N20) was discharged to the atmosphere because the volumes were low. Demand for nylon grew rapidly, so volumes of NO2 gas increased and a decision was made to abate it. There was no 0 2 in the effluent so the NO2 abatement was simple indeed. The effluent was mixed with some purge hydrogen and carbon monoxide from alcohol synthesis units and was passed at 200 "C or above over a used (spent) copper chromite or nickel on alumina catalyst, and the NO2 and N2O were completely removed. In the absence of 0 2 the complete abatement of NOX can be accomplished easily by this simple process. It should be noted that the purge gas contained CO, which serves as well as H2 to reduce NOX. Unfortunately most effluents containing NOX also contain 0 2 in varying amounts. If this 0 2 level is 1%or below, it may be least costly to simply add sufficient reducing gas (H2, CH4, synthesis gas) to remove the 0 2 first and then the NOX. Unfortunately the 02 level in most effluents is too high to remove economically with a reducing gas, so 32 years ago a process was developed by Engelhard using NH3 and a precious metal to selectively remove the NOX with NHs
0888-588519412633-2259$04.50/0 0 1994 American Chemical Society
2260 Ind. Eng. Chem. Res., Vol. 33, No. 10,1994
in the presence of 02 without reacting with the 02 (Anderson et al., 1961). The reaction is as follows: 4NH,
+ 6NOX + 0,
-
5N, + 6H,O
+ 0,
This process has been developed to a commercial scale (Hydrocarbon Process., 1988). The process has the acronym SCR (selective catalytic removal) and in the references the following is given by the Licenser Nippon Kohon KK: Catalyst: Ti02 V20sand iron ore. NOX content of treated initially 100 ppm; posttreatment almost zero dependingontemperatureofoperation200-45OoC. Space velocity is controlled by allowable NH3 and NOX leakage. Catalyst cost for TiO2, V206 catalyst is major capital cost. A noncatalytic SCR process has also been devised using NH3 but at temperatures 750-1000 OC. These data are also from Hydrocarbon Process. references given above, but are described more fully in Power (Makansi, 1993). This latter article also gives a description of NOX abatement by burner control, flue gas recirculation, and combinations of SCR, SNCR (selective noncatalytic removal), flue gas recirculation, and burner control. The major problems are obtaining reliable estimations of cost of a new or retrofit installation, ammonia leakage (or urea or other source of NH& increase of carbon in the ash, changesinperformanceowingtoanewfue1,orunsuspected dust in the flue gas. NOX is abated to the extent of up to 90% in a catalytic process operating at 500 OC in which excess CH. is added to the flue gas at a concentration of only 0.2425% (Hydrocarbon Process., 1988). Also in Hydrocarbon Process. (1988) on page 19, a process for NOX abatement is described which is a version of the SCR process. The unique feature is that it is reportedly capable of simultaneously reducing any dioxin in the effluent. It is also incumbent upon us to report the NOXSO process which is described in literature provided by those offering the technology. The pamphlet is titled, 'NOXSO A No-Waste Emission Control Technology," but is not otherwise identified. The process is somewhat similar to the adsorption process herein reported, but the NOXSO process also removes SO,. A more complete description can be obtained from NOXSO Corporation, P.O. Box 469, Library, PA 15129. The removal is 90+% NOX and 95+% SOX. One notable point is that the regenerationoftheadsorbent is atquite hightemperatures. Air Products of Allentown, PA, recently issued a new release describing their new process for NOX including N20 abatement which uses a metal exchanged zeolite as catalyst andmethane reductant. The proceas is arelatively high temperature operation functioning most efficiently at 500 50 "2. More details can be obtained from Dr. John Armor of Air Products or from his review article on the subject (Armor, 1992). This article has an especially complete and useful list of references.
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More Detailed Description of New Process and Technical Data The removalof NOXfromgaseaadditionaUycontahing large percentages of oxygen, that is oxygen which cannot be eliminated economically with a reducing agent, until now has remained only a partially solved problem. The process herein described however provides a procedure whereby NOX can be removed from a gas stream containing oxygen to the extent of essentially 100%. The process consists of first adsorbing the nitrogen oxide on a highly efficient adsorbent at approximately 200 OC and then
Pigore 1. Individual reamm for admrption4emrption and d u e tion. Reactam 1 and 2 can be combined into r ~ c t 1a by ~ placing reduction catalyst downstresm from adsorbent in reactar I.
desorbing the nitrogen oxide at a slightly higher temperatureusingagasstream whichcontains hydrogen, water vapor, and nitrogen, but no oxygen. The nitrogen oxide can be simultaneously desorbed and reduced to nitrogen and water vapor either by the adsorbent itself acting as a reducing catalyst, or by a separate reactor and catalyst type downstream from the adsorbent which reduces the nitrogen oxide to elemental nitrogen and water vapor. Certain of thecatalyst-adsorbent materials are resistant to sulfur dioxie, but the catalyst is most efficient in the absence ofsulfur dioxide in the gas from which the nitrogen oxides are to be removed. The most effective agent for the adsorption is a transition metal oxide and aluminum oxide coprecipitated to produce a 50/50 mixture of finely divided mixed powder. This powder is milled in a ball mill to produce a paste comprising water, the intimately mixed oxides powder, and some colloidal cerium oxide to actasstrengtheningagent for thedried milledpaste.After the paste has been dried, the granules are derived by crushing and screening the dried paste. The granules are further treated by adding a solution of an alkali-metal carbonate which, on drying, leaves the carbonate completely covering the interior and exterior of the granules. These granules are placed in the adsorption reactor shown in Figure 1 which is heated by an external furnace. Thegas in our testacontainedoxygen,400ppmofnitrogen oxides, water vapor, and the remainder nitrogen, and was pagsed throughthecatalyst in the fumaceat approximately 200 "C at a space velocity of 8 000-15 000. The exit gas was free of detectable nitrogen oxides and remained so for a period of more than 9 h of testing. The adsorbent now containing more than 0.2% NOX by weight was regenerated for reuse by passing a gas containing from 0.5 to 10% hydrogen in nitrogen: both carbon dioxide and water vapor can also be present. The catalyst and reactor were heated to 300 "C, and the aforementioned gas was passed through, simultaneously eitherreducing thenitrogenoxideinsituon theadsorbent and/orpassingit downstream toadifferentcatalyst in the process stream where the desorbed NOX is completely reduced. The reduction catalyst can either be in the downstream portion of the same reactor or in a separate downstream reactor. Economics favor the single reactor.
Ind. Eng. Chem. Res., Vol. 33, No. 10,1994 2261 N,
e.
Figure 2. Reactor system when using two reactom. Sseond reactor cnn be eliminated by placing reduction catalyst downstream from the adaorption catalyst in the asme reactor (no. 1).
After regeneration, the catalyst can he used for adsorption and experience indicates that the time for the amount of nitrogen oxide removed in the second use of the catalyst can exceed the 9 h previously reported for the first use. Inasmuch as the regeneration scheme requires that the adsorbentcatalysthe isolatedfor regeneration, it is obvious that a second reactor in parallel would be required while the first was being regenerated. The scheme is shown in ita entirety in Figure 2. As previously stated, the mixture is a transition metal oxide coprecipitated with aluminum oxide. Some transition metal oxides function better than others with manganese oxide to date being preferred together with aluminum oxide. Although manganese oxide as the transition element oxide appears to he the best, other transition oxides alone or intermixed with the manganese oxide do very well. Aluminais the typicalrefractoryoxidesupportandalso does well. Other refractory oxidessuchas silica,magnesia, calcia,strontia, baria, titania, and zirconia,alsoserve well. Although it is preferredtocoprecipitatetheingredients, battery grade manganese oxide in a slurry of aluminum nitrate from which thealuminum hydroxideis precipitated and occludes the particle of battery grade produces a good product. Of the alkali-metal carbonates, potassium carbonate is preferred, but other alkali-metal carbonates are adequate for shorter times of complete NOX adsorption. Many ofthesubsequent downstreamreductioncatalysts can be used in the final NOX elimination. Examples are oxides of nickel, cobalt, iron, and tin combined with chromiumoxide, gadolinium oxide supported on alumina, silica, titania, ceria, zirconia, and others. Many other hydrogenationcatalysts are effective including the precious metals and the moderated precious metals. Although the temperature of adsorption is described above as approximately 200 'C, the temperature can be
varied from approximately 100 to 500 O C . The reduction eanhec0nductedat200~Ctoashghas500~C. Problems may be encountered when the adsorption is at too low or toohghtemperature,and&o the reductionofthenitrogen oxide may be adversely influenced (may form a small amount of "8) if the reduction is conducted a t temperatures in excess of 350 "C. Instead of, or in addition to, the use of a second (reduction) reactor one can recycle the effluent from the reducer or the adsorher itself, and recycle small quantities of NOX to the high temperature combustion zone or the incoming flue gas to the adsorber for elimination by any of these three means. The present invention differs importantly from the SCR process in that no ammonia is used in the reduction of the NOX. Ammonia is objectionable because it may in itself produce nitrogen oxides or it may be incompletely reacted in the course of the nitrogen oxide abatement and, as a consequence, produce adverse atmcapheric affects. Further points of difference are that the adsorbent catalyst has a uniquely high capacity, in that it will function for long periods of time experimentally determined to he over 9 h. The regeneration of this catalyst can be accomplished in as short a time as 20 min, by choosing the proper gas type and temperature conditions. This makes it possible for the process to be operated on a cycling basis, with high efficiency of NOX adsorption, and high efficiency of reduction of the nitrogen oxide so the gas streams involved ean,afteradsorptionandalsoafterreduction,beexhausted to the atmosphere as harmless gases. This means it would be unnecessary to heat or reheat large volumes of gas because the low temperature of adsorption is essentially identical to that of the flue gas exhaust. As for the reduction gas, as pointed out previously, this is of such low volume that the cost of heating it to the 300-400 OC desired is economically of little concern. It is probably appropriate at this time to mention that the process being described is in a small way similar to the NOXSO process in that an adsorbent is used in both technologies. The uniqueadsorbent being reported herein is more effective both in completeness of NOX removal and in length of time and capacity for complete NOX removal. Thisnew process, however,does not remove SOz, but a previously developed process at the Center for Catalytic Science and Technology does remove both SOX and NOX to the extent of 95+% from flue gas derived from high-sulfur coal. This other process is described in US.Patents5,023,063 (Stiles, 1991)and5,176,888 (Stiles, 1993).
Use-Regeneration-Reuse Procedure A more detailed description of the use-regenerationreuse cycle is aa follows. (Please refer to Figures 1and 2.) The gas to he treated for removal of NOX is passed into anadsorption catalyst bed in reactor 1. The inlet gas may contain NOX, COZ,CO, H 2 0 . 0 ~ .N2, and SOX, hut it is preferred that the SOX not he present. This is possible when treating effluent from gas-fned boilera, of which there are hundreds of thousands as relatively small boilers, and from gas-fired turbines. Adsorption a t a space velocity as high as 18OOO (800012 OOO h-' preferred) and at 180 'C (up to 500 "C can be used) is continued for as long as 9 h for 100% removal. A lower time probably would be desirable to accommodate a use-regeneration-reuse cycle which probably could he advantageously condensed into a 1.5-2-h cycle. As stated in Figures 1and 2, the twereactor arrangement can he condensed into one reactor and a single reduction
2262 Ind. Eng. Chem. Res., Vol. 33, No. 10, 1994
L
NO.2 Adsotter IDlenninantly on Adsorption Stage and Regeneration
Adsorption Stage ard Regeneration Stage
I I
I High Conccnmions of NO,
I
Amwphere M e r Adsorption Step
High Concenmions of NO.
adsohers
Discharge 10 Aunwphere if N 4 is Zem or Recycled 10 Inlet 10 Adsotter If NQ hxnl. Flow Discharge 1%or Lesr of Volume Enlenng Adsarber. I
I
x Convol Valves Directing and Controlling Flow Figure 3. Block flow diagram depicting complete scheme of NOX abatement of NOX from a large volume of gas containing low concentration of NOX. Table 1. Adsorbed Species unused Catalyst adsorbent exposed to NO thermally desorbed gases
species adsorbed nothing detectable all NOX species adsorbed; NO, Nz03, and NO2 NO, Nz03,and NOz
Table 2. Optimization of MnO, to AI208 Ratios. total removal temperature MnOJAl203 of NO (h) ratio ("0 200 f 15 1.0 1090 2.5 200f 15 2575 200 f 15 5.0 40:60 200 & 15 6.5 5050 200 f 15 6040 6.0 200 f 15 75:25 3.5 200 f 15 2.5 9010
space vel (h-l)
m 8000 8000 8000 8000 8000 8000
a Inlet feed NO content is 400 ppm NO. N compound analysis by Antec Titel N compound analyzer reliable to
