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Ind. Eng. C h e m . Res. 1989, 28, 1171-1177
Dual-Catalyst System To Broaden the Window of Operability in the Reduction of NO, with Ammonia F. G . Medros,t J. W. Eldridge,* and J. R. Kittrelll Department of Chemical Engineering, University of Massachusetts, Amherst, Massachusetts 01003
The objective of this research was to determine if a dual-catalyst system for NO reduction with NH3 can achieve a given percent NO reduction over a wider range of temperatures and space velocities than either catalyst used alone in the same total reactor volume. Hydrogen mordenite (20/32 mesh) and copper-ion-exchanged hydrogen mordenite (2.25% Cu) were used in series at temperatures from 200 t o 600 “C and space velocities from 100000 to 450000 h-’ (STP). The superiority of the dual-catalyst system was demonstrated experimentally, and a model was developed which predicted its performance very well from data on the individual catalysts. A technique was then developed for predicting quantitatively the dual-catalyst enhancement of the space velocity versus temperature window for achieving a given percent NO conversion. The higher the conversion required, the greater is the enhancement provided by this system. Research indicates that nitrogen oxides (NO,) are among the most troublesome air pollutants. The present levels of NO, emissions already pose a significant threat to our health and environment in urbanized areas. There has been increasing public and government concern over the environmental impact of acid rain, to which nitrogen oxides contribute a substantial percentage. Industrial plants based on combustion processes are by far the largest producers of NO, pollution. NO, control technologies generally fall into two categories: those based on combustion modifications and those based on the elimination of NO, from flue gas. DeNO, combustion modification technologies all strive to limit the formation of NO during combustion (primarily by lowering the flame temperature and controlling the fuel/air ratio). With expectations that more severe NO, emission standards are to be promulgated in the near future, major NO,-producing industries (e.g., the electric utility industry) must seriously consider more effective methods of control. The selective catalytic reduction process (SCR), using ammonia as the reducing agent, has achieved the most notable success in treating combustion flue gas for the removal of NO,. SCR technology has been used for more than a decade in Japan for meeting stringent NO, emission standards. Generally, anhydrous ammonia is injected directly into the NO,-containing flue gas, and the resulting mixture is passed over a catalyst. The NH3 selectively reduces the NO, in the presence of the catalyst to molecular nitrogen and water. This SCR process has been singled out as the most attractive and promising back-end control approach mainly because of its high NO, removal capabilities (Pruce, 1981). The primary objective of this study was to determine experimentally if a series combination of hydrogen and copper mordenite catalysts provided a “better” (larger) window of operability for SCR with NH3than either of the individual catalysts used alone. The incentive for this work at the University of Massachusetts stemmed from the encouraging results obtained by Phillibert (1985) and Nam (1983) for these catalysts. Both researchers concluded that mordenite catalysts (hydrogen or copper) are extremely active for the SCR of NO, with NH3. However, the region -?Present address-Thermedics, Inc., 470 Wildwood St, Woburn, MA 01888-1799. $Present address: KSE, Inc., P.O. Box 368, Amherst, MA
01004.
of maximum activity for each of these catalysts occurs within a different temperature range. Hence, the objective of this study is to determine if, in a given total reactor volume, a dual-catalyst system is superior to a singlecatalyst system by broadening the temperature range within which a given high NO, removal efficiency can be achieved. For the present dual-catalyst system, the selective catalytic reduction of NO by NH3 primarily involves three chemical reactions: 6N0
+ 4NH3
4
5N2 + 6H20
-
+ 4NH3 + O2 4N2 + 6H20 4NH3 + 502 4N0 + 6H20
4N0
-
(1)
(2)
(3)
Reactions 1 and 2 are the desired NO reduction reactions producing only molecular nitrogen and water. Reaction 3 is an unwanted side reaction which takes place primarily at higher temperatures ( z
2 40+ 0
20
,
,
500
550
-
FEED NO 1500ppm
0
T'510'C
NHJ/NO
A
T=597'C
10
E
('C)
Figure 3. Effect of space velocity on temperature dependence of NO conversion with CuHM catalyst.
Experimental Section Catalysts. The hydrogen mordenite (HM) used in the present study was a commercial catalyst, Zeolon 900H, obtained in granular, 20/50-mesh form from the Norton Company. This catalyst was screened to obtain a 20132mesh particle size for use in the investigation. The copper mordenite (CuHM) used in the present study, also 20/ 32-mesh granules, was formulated at the University of Massachusetts from the same lot of HM and contained 2.2% copper by weight. Reactor System. The kinetic data used in the present study were obtained by using a packed-bed, tubular reactor
system (Figure 2 ) . The reactor, a 9.5-mm-0.d. aluminum tube, was installed in a vertical downflow arrangement within a Lindberg three-zone furnace. The flue gas was generated by a Utica furnace burning LPG. Nitric oxide (NO) was added to the flue gas to increase its concentration to 1500 ppm, typical of diesel exhaust. Ammonia was added to approximately equal the concentration of nitric oxide, and the resulting flue gas was passed through the catalyst bed. Thermocouples were installed inside and outside the reactor to monitor the reaction temperature, which was varied from 200 to 600 "C. Analytical Measurements. Concentrations of NO and NHB entering and leaving the reactor were measured with
Ind. Eng. Chem. Res., Vol. 28, No. 8, 1989 1173 a Thermo-Electron Model lOAR NO gas analyzer, which was calibrated against standard gas compositions from separate NO and NH3 calibration cylinders. The NH, measurements from this chemiluminescence analyzer were also checked independently with Matheson-Kitagawa ammonia indicator tubes.
100
\
f.1
Experimental Results Temperature Effect. As a basis for the analysis of the dual-catalyst system, the performance of the individual catalysts was investigated as a function of the two most important kinetic variables, temperature and flow rate (space velocity). Presented in Figures 3 and 4 are conversion versus temperature profiles for different flow rates for CuHM and HM, respectively. CuHM displays a pronounced bell-shaped conversion versus temperature curve, with the maximum NO conversion occurring at approximately 400 "C. For temperatures greater than 400 OC, the decline in NO conversion is a result of the ammonia being partially consumed through oxidation. On the other hand, with HM, NO conversion increased steadily with increasing temperature. It should be apparent from Figures 3 and 4 that CuHM is a much more active catalyst for NO reduction a t lower temperatures than is HM. The objective of a dual-catalyst configuration is to have a catalyst system that takes advantage of both the lowtemperature activity of CuHM and the high-temperature activity of HM. In the present study, HM was located upstream of CuHM. This means that the flue gas containing NO and NH, contacted the HM first and then the CuHM. If CuHM were located upstream of HM, then at high temperature it would oxidize all the NH3 and give a low conversion of NO with no NH, left for HM (located downstream) to convert the remaining NO. Thus, for high temperatures with CuHM located upstream, nothing is to be gained by adding HM downstreams, as the NO will pass through it unchanged. For the purpose of this study, equal weights of CuHM and HM were used arbitrarily. Space Velocity Effect. For the purpose of this study, the performance of both HM and CuHM was investigated as a function of space velocity. In the case of CuHM, two distinct temperature regions may be defined: a lower temperature region (
