Energy & Fuels 1999, 13, 1145-1153
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The Role of Carbon Monoxide in the NO-Carbon Reaction Indrek Aarna† and Eric M. Suuberg* Division of Engineering, Brown University, Providence, Rhode Island 02912 Received February 16, 1999
The enhancement of the NO/carbon reduction reactions by carbon monoxide has been demonstrated for carbons of widely varying nature (coal char, phenolic resin-derived char and graphite). The evidence supports enhancement of NO reduction via a carbon-surface-catalyzed reaction such as NO + CO f 1/2N2 + CO2. This reaction appears to be characterized by an activation energy of around 116 kJ/mol, and by zero order with respect to CO, in the range of CO pressures examined here (up to order 500 ppm). This suggests that an oxide stripping reaction, e.g., CO + C(O) f CO2 + C*, cannot be invoked in its usual form, to explain the rate enhancement. The reaction appears to be first order with respect to NO, at high temperatures and at NO concentrations of above roughly 100 ppm, but kinetic analysis is complicated by the fact that the NO-carbon reaction itself does not have a unique order at low temperatures. It is also shown that kinetic analysis can be greatly complicated in the presence of other oxidizing gases or surface oxides deposited by these gases.
Introduction The reactions of nitric oxide with carbons or chars are of current interest with regard to their possible role in reducing NO emissions from combustion systems. They also offer new useful insights into the oxidation reactions of carbons, generally. There has developed a large literature concerning these reactions, as evidenced in two recent reviews1,2 and by the recent publication of many papers in the area. These works have suggested considerable complexity in the mechanisms of NO reduction and a large variability in reported kinetics. The main oxidation reactions of NO with carbons may be stoichiometrically represented as
NO + C f 1/2N2 + CO
(R1)
2 NO + C f CO2 + N2
(R2)
One interesting aspect of this system is that the product CO can, in turn, react further with NO in a separate surface-catalyzed reaction which may be represented as
NO + CO f 1/2N2 + CO2
(R3)
The fact that a product of a primary reduction reaction for NO, (R1), may become a reductant for NO via a secondary reaction (R3) has the potential for greatly complicating the kinetic and mechanistic analysis of experiments intended for exploring only the primary * Author to whom correspondence should be addressed. Telephone: 1-401-863-1420. Fax: 1-401-863-1157. E-mail:
[email protected]. † Present Address: Thermal Engineering Department, Tallinn Technical University, Ehitajate Road 5, Tallinn, Estonia. (1) Li, Y. H.; Lu, G. Q.; Rudolph, V. The Kinetics of NO and N2O Reduction over Coal Chars in Fluidized Bed Combustion. Chem. Eng. Sci. 1998, 53, 1. (2) Aarna, I.; Suuberg, E. M. A Review of the Kinetics of the Nitric Oxide - Carbon Reaction. Fuel 1997, 76, 475.
reactions. It has been suggested that this also makes difficult the determination of reaction order with respect to NO in packed bed experiments.2 It should, however, be noted that in practical combustion systems, the small amount of CO produced in (R1) will most likely be inconsequential in comparison with CO from other sources. This paper is mainly concerned with the role of reaction (R3) in systems in which (R1) and (R2) occur at the same time. There have been many reports concerning the catalysis of (R3) by various types of surfaces, including quartz,3,4 impure quartz sands,5 calcined limestone and dolomite,6-9 sulfided limestone,10 transition-metal oxides,11 other mixed catalysts,12,13 carbon-supported tran(3) Wittler, W.; Schu¨tte, K.; Rotzoll, G.; Schu¨gerl, K. Heterogeneous Reduction of Nitric Oxide by Carbon Monoxide on Quartz Surfaces. Fuel 1988, 67, 438. (4) Berger, A.; Rotzoll, G. Kinetics of NO Reduction by CO on Quartz Glass Surfaces. Fuel 1995, 74, 452. (5) Schoderbo¨ck, P.; Lahaye, J. The Influence of Impurities Contained in Quartz Sand on the Catalytic Reduction of Nitric Oxide by Carbon Monoxide. Appl. Surf. Sci. 1996, 93, 109. (6) Tsujimura, M.; Furusawa, T.; Kunii, D. Catalytic Reduction of Nitric Oxide by Carbon Monoxide over Calcined Limestone. J. Chem. Eng. Jpn. 1983, 16, 132. (7) Olanders, B.; Stro¨mberg, D. Reduction of Nitric Oxide over Magnesium Oxide and Dolomite at Fluidized Bed Conditions. Energy Fuels 1995, 9, 680. (8) Shimizu, T.; Tachimaya, Y.; Fujita, D.; Kumazawa, K.; Wakayama, O.; Ishizu, K.; Kobayashi, S.; Shikada, S.; Inagaki, M. Effect of SO2 Removal by Limestone on NOx and N2O Emissions from a Circulating Fluidized Bed Combustor. Energy Fuels 1992, 6, 753. (9) Hansen, P. F. B.; Dam-Johansen, K., Johnsson, J. E.; Hulgaard, T. Catalytic Reduction of NO and N2O on Limestone During Sulfur Capture Under Fluidized-Bed Combustion Conditions. Chem. Eng. Sci. 1992, 47, 2419. (10) Furusawa, T.; Koyama, M.; Tsujimura, M. Nitric Oxide Reduction by Carbon Monoxide over Calcined Limestone Enhanced by Simultaneous Sulfur Retention. Fuel 1985, 64, 413. (11) Shelef, M.; Otto, K.; Gandhi, H. The Oxidation of CO by O2 and by NO on Supported Chromium Oxide and Other Metal Oxide Catalysts. J. Catal. 1968, 12, 361.
10.1021/ef9900278 CCC: $18.00 © 1999 American Chemical Society Published on Web 09/21/1999
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Aarna and Suuberg
sition metals,14,15 and carbon-supported alkali.16 It has also been reported that coal ash has low catalytic activity and CFBC bed ash can have high activity.17 No attempt has been made to be inclusive of all studies on the topic of NO/CO catalysis; there exists an enormous literature on noble-metal-based automotive catalysts not considered here. There exist in the literature apparent inconsistencies concerning catalysis of (R3), such as comments to the effect that quartz has no catalytic effects on this reaction.18 These have been subsequently resolved when the key role of impurities in the quartz5 and CO concentration3 were clarified. A similar claim as to the absence of a catalytic effect has been made concerning alumina,19 but that too is apparently not universally accepted.20 The inhibitory influence of carbon dioxide on the catalytic efficacy of limestone has also been noted to be an important consideration when working with those materials.21 The key conclusion is that a wide variety of materials is known to be catalytic toward (R3), and that sometimes it is difficult to identify the actual catalytic agents in materials which are not pure. Surface catalysis of the NO/CO reaction has been identified by several workers as playing a role in the NO-carbon reaction system.2,15,18,19 Actually, two distinct hypotheses have been proposed for the role of CO in these systems. One simply involves separate parallel reaction channels for (R1)/(R2) and for (R3). The second involves coupling the reactions through a mechanism which invokes the stripping of oxides from the surface by CO.19,20,22 This reaction may be represented as
CO + C(O) f CO2 + C*
(R4)
where the C(O) represents a surface oxide and the C* is a free active site, which can react with NO. The effect of CO on NO reduction has been seen to decrease with increasing temperature.22 This would be consistent with a role for (R4), since the desorption processes involving (12) Mergler, Y. J.; van Aalst, A.; Nieuwenhuys, B. E. NO Reduction by CO and H2 over a Pt/CoOx/SiO2 Catalyst - Effect of CoOx on the Activity and Selectivity. Prepr. Pap.sAm. Chem. Soc., Div. Petr. Chem. 1994, 39 (1), 161. (13) Srinivas, G.; Chuang, S. S. C.; Debnath, S. Interaction of NO and CO on Rh/SiO2 and Ce-Rh/SiO2 Catalysts: A Transient In Situ Infrared Spectroscopic Investigation. In Environmental Catalysis; Armor, J. N., Ed.; ACS Symp. Ser. No. 552, 1994; p 157. (14) Strengega, S.; Mierop, A. J. C.; de Vries, C.; Kapteijn, F.; Moulijn, J. A. Reactivity of Carbon Supported Catalysts for NO Reduction and CO Oxidation. Proc. 19th Biennial Conf. Carbon; The American Carbon Society, 1989; p 74. (15) Shelef, M.; Otto, K. Simultaneous Catalytic Reaction of O2 and NO with CO and Solid Carbon. J. Coll. Interface Sci. 1969, 31, 73. (16) Kapteijn, F.; Mierop, A. J. C.; Abbel, G.; Moulijn, J. A. Catalytic Reactions Over Alkali Metal/Carbon Systems: Reduction of NOx and Methanation of CO. Proc. Carbone ’84, Bordeaux, 1984; p 60. (17) Johnsson, J. E. Formation and Reduction of Nitrogen Oxides in Fluidized Bed Combustors. Fuel 1994, 73, 1398. (18) Furusawa, T.; Tsunoda, M.; Tsujimura, M.; Adschiri, T. Nitric Oxide Reduction by Char and Carbon Monoxide. Fuel 1985, 64, 1306. (19) Chan, L. K.; Sarofim, A. F.; Beer, J. M. Kinetics of the NOCarbon Reaction at Fluidized Bed Combustor Conditions. Comb. Flame 1983, 52, 37. (20) DeSoete, G. G. Reduction of Nitric Oxide by Solid Particles. In Pulverized Coal Combustion: Pollutant Formation and Control 19701980; U.S. EPA Report EPA-600/8-90-049, 1990. (21) Dam-Johansen, K.; Hansen, P. F. B.; Rasmussen, S. Catalytic Reduction of Nitric Oxide by Carbon Monoxide over Calcined LimestonesReversible Deactivation in the Presence of Carbon Dioxide. Appl. Catal. B- Environ. 1995, 5, 283. (22) Levy, J. M.; Chan, L. K.; Sarofim, A. F.; Beer, J. M. NO-Char Reactions at Pulverized Coal Flame Conditions. 18th Symp. (Int.) on Combust., [Proc.]; The Combustion Institute, Pittsburgh; 1981; p 111.
the surface oxides themselves are faster at higher temperatures, and the stripping reaction, (R4), would thus be less important. There remains, however, some uncertainty in the carbon gasification literature as to when (R4) is significant,23 so it is unclear whether it plays a role in the case of NO destruction in the presence of carbon. It is clear that there exist catalytic routes for the NO/CO reaction on other materials, which cannot involve the same type of C(O) complexes as on carbon. This leaves open the possibility that (R4) is not the main pathway for the NO/CO reaction in carbon systems. Various possibilities will be discussed in the context of new data presented below. Experimental Section Reactor System. A fixed bed reactor was the main experimental tool employed in this study. It consisted of a quartz packed bed reactor tube of 4 mm internal diameter and 500 mm overall length, which was used to hold a bed of, typically, 20-500 mg of carbon or char particles, in a predetermined length, again typically, in the range from 1 to 50 mm. The reactor was heated by an electrical tube furnace and a chromel-alumel thermocouple read bed temperature. During the reactivity measurements, the effluent gases were continuously analyzed for NO and NO2 using a chemiluminescence analyzer. Nitric oxide/helium mixtures of the desired concentration levels were prepared using a KIN-TEK precision calibration system. The desired NO concentration was usually in the range 10-300 ppm of NO in a carrier gas which was either pure helium or in helium mixed with another gas (e.g., CO2 or O2). The desired NO concentration range could be obtained by controlling the flow rate of the He carrier gas mixture at around 100 cm3/min, and adjusting the absolute temperature of the permeation membrane and the NO partial pressure in the KIN-TEK permeation cell. In experiments in which the effect of CO concentration was explored, the desired CO concentration (50500 ppm of CO in NO/He mixture) was obtained by preparing a CO/He mixture and controlling its addition rate to the main NO/He carrier mixture. There was concern that the CO-NO reaction, (R3), might possibly be catalyzed by the quartz reactor tube.4 To examine this possibility, reactor blanks were run with NO- and CO-containing mixtures. These tests showed no significant NO reduction at temperatures below 1273 K. This is consistent with the cited literature, in which it was found that relatively pure quartz is not catalytic, and that reasonably high concentrations of CO are required to see a significant amount of reaction on this surface. The reactor containing the carbon bed was outgassed prior to each run by vacuum pumping for 1 h at room temperature. This was followed by thermal surface cleaning of the carbon (at 1273 K for 1-2 h in He) to remove surface oxides. In some cases, transient NO reduction behavior was then examined. In other cases, pseudo-steady-state reactivities were of interest. These NO reduction reactivities were determined following attainment of pseudo-steady state, which typically (23) Brown, T. C.; Haynes, B. S. Energy Fuels 1992, 6, 154.
The Role of Carbon Monoxide in the NO-Carbon Reaction
required 1-2 h from the introduction of NO to the bed. This long equilibration time makes it clear that results of experiments from different laboratories can only be reliably compared if comparable experimental times were employed. Materials Examined. The carbonaceous solids examined in this study were a char prepared from phenolic resin, graphite, and a char prepared from Wyodak coal. The phenolic resin char was synthesized in-house,24 and carbonized in a tube furnace at 1273 K. The polycrystalline graphite samples (particle size 250-325 µm) were prepared from graphite rods from Great Lakes Carbon. Both the resin char and graphite had very low impurity contents. The coal char used in this work was prepared from Wyodak subbituminous coal, which was obtained from the Argonne Premium Coal Sample Bank.25 This coal contains 6.3% ash and has a particle size
