Energy h Fuels 1992,6,398-406
398
coal samples used for our viscosity determinations were provided by John P. Hurley and Sharon Falcone Miller. Data on the Medicine Bow coals was kindly made available by Robert C. Streeter from work performed at BCR National Laboratory. The sample of Austrian brown coal ash slag and supporting data were provided by Dr. Gernot
Staudinger of the Technische Universitiit Graz. We are grateful for the helpful advice of Karl E. Spear on use and applications of the SOLGASMIXprogram. Registry No. Si02, 7631-86-9;A1203,1344-28-1;TiOz, 13463-67-7; Fe203,1309-37-1; CaO, 1306-78-8;MgO, 1309-48-4; KzO, 12136-45-7. Na20, 1313-59-3;
Studies on the Reduction of Nitric Oxide by Carbon: The NO-Carbon Gasification Reaction Hsisheng Teng,? Eric M. Suuberg,* and Joseph M. Calo Division of Engineering, Brown University, Providence, Rhode Island 02912 Received December 19, 1991. Revised Manuscript Received March 20, 1992
The heterogeneous reduction of NO by carbon was studied in a thermogravimetric analysis system, employing both pseudosteady and transient reaction methods. The reaction was studied at temperatures from near ambient up to 1073 K, and at NO partial pressures in the range 1.01-10.1 kPa. A relatively pure carbon derived from phenolic resin was studied. Gaseous products of rection were measured. The gasification of carbon by NO involves two parallel processes: (1)somewhat slow desorption of relatively stable surface complexes; (2) processes involving NO attack on active unoccupied sites that results in essentially immediate desorption of gaseous products. The first process controls the overall gasification rate at lower temperatures and is governed by a distribution of desorption activation energies, involving mainly surface oxides that yield CO upon desorption. The second process dominates at high temperatures (somewhat arbitrarily defined by T > 650 "C or 923 K)and is suggested to be controlled by the dissociative chemisorptionof NO on the carbon surface. 1. Introduction The mechanisms of the reactions of carbons with 02, COP,and H 2 0 are not yet well understood in many respects. The reactions of carbons with NO are even less well characterized. However, the fact that NO formed during combustion can be heterogeneously reduced by carbonaceous residues produced in situ is well-known.' There have been a modest number of studies of various aspecta of the NO-carbon gasification reaction in the past 30 years2-16(see Table I). A separate literature exists on the process of chemisorption of NO by carbons, and this will be reviewed elsewhere.17 Heterogeneous reactions of NO with carbon can reduce NO to N2 and form CO and C02gaseous products. In general, the overall gasification reaction of carbon with NO has been reported to include the following stoichiometric reactions (see studies cited in Table I): C + 2N0 +C02 + N2
C + NO CO + NO
-
CO + C02 + Y2N2
The last reaction in the sequence is actually carbon-surface-catalyzed oxidation of CO by NO, based upon the observation that NO reduction by carbons is enhanced in the presence of C0.l2J8 At low temperatures, the process may not involve the release of gaseous carbon oxides, as outlined above; instead, stable surface oxides may be
* Corresponding author.
'Present address: Advanced Fuel Research, 87 Church Street, East Hartford, CT 06108.
formed on the carbon via chemisorption. Chemisorption of NO on carbon has recently been studied by our group.17J920 (1)Pereira, F. J.; Beer, J. M.; Gibbs, B.; Hedley, A. B. 15th Symposium (International)on Combustion; The Combustion Institute: Pitts-
burgh, 1975;p 1149. (2)Watts, H.Trans. Faraday SOC.1958,54,93. (3)Smith, R. N.;Swinehart, J.; Lesnini, D. J. Phys. Chem. 1969,63, 544. Also: Smith, R. N.; Lesnini, D.; Mooi, J. J. Phys. Chem. 60,1063 (1956). (4)Edwards, H. W. AIChE Symp. Ser. No. 126 1972,68,91. (5)Lai, C.-K. S.;Peters, W. A.; Longwell, J. P. Energy Fuels 1988,2, 586. (6)Degroot, W. F.; Richards, G. N. Carbon 1991,29,179. (7)Gibbs, B. M.; Pereira, F. J.; Beer, J. M. 16th Symposium (Internatronal) on Combustion; The Combustion Institute: Pittsburgh, 1977; p 461. (8)Song, Y. H.; Beer, J. M.; Sarofim, A. F. Combust. Sci. Technol. 1981,25,237. (9)Levy, J.; Chan, L. K.; Sarofii, A. F.; Be&, J. M. 18th Symposium (International)on Combustion; The Combustion Institute: Pittsburgh, 1981;p 111. (10)Schuler, J.; Baumann, H.; Klein, J. ErdoZ Kohle-Erdgas 1988,41, 296. (11)Matos, M. A. A,; Pereira, F. J. M. A.; Ventura, J. M. P. Fuel 1991, 70, 38. (12)Chan, L. K.; Sarofim,A. F.; Beer, J. M. Combust. Flame 1983,52, 37 _ .I145.
(13)Furusawa, T.; Kunii, D.; Osuma, A.; Yamada, N. Kogaku Kogaku 1978,6,562; Znt. Chem. Eng. 1980,20,239. (14)Bedjai, G.;Orbach, H. K.; Reisenfeld, F. C. I d . Eng. Chem. 1958, 50, 1165. (15)Kapteijn, F.; Mierop, A. J. C.; Abbel, G.; Moulijn, J. A. Proc. Carbone 84,Int. Carbon Conf. Bordeaux, Fr. 1984,84. (16)Suuberg, E. M.; Teng, H.; Calo, J. M. 23rd Symposium (Internotional) on Combustion; The Combustion Institute: Pittsburgh, 1991; p 1199. (17)Teng, H.;Suuberg, E. M., to be submitted to Carbon. (18)Shelef, M.; Otto, K. J. ColZoid. Interface Sci. 1969,31,73. (19)Teng, H.;Suuberg, E. M.; Calo, J. M. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1990,35(3),592.
0887-0624/92/2506-0398$03.~0/0 0 1992 American Chemical Society
Reduction of Nitric Oxide by Carbon carbon sugar charcoal
SA" NA
sugar charcoal Graphon cokee cellulose char coal char coal char coal char coal char coal char
1060 87 NA NA NA 175 20.3 NA NA NA 402 416 3.7 17-20 800-1200 0-1400 NA 2-440
coal char coal char (deashed) graphite coal char activated carbon activated carbon activated carbon0 resin char
Energy & Fuels, Vol. 6, No. 4,1992 399
Table I. Activation Energiee for the NO-Carbon Reaction reactor temp range, K PNO, kPa fixed bed 578-723 0.19-2.9 42.3b packed bed 723-873 K1.1 packed bed 1027-1233 0.27-0.75 fixed bed 623-973 0.18 TGA 773-923 10.1 fluidized bed 900-1200 -0.04-0.08 flow reactor 1250-1750 0.05-0.65 flow reactor, packed bed 800-1750 -0.05-1.0 packed bed 673-1023 0.002-0.04 CSTR 848-1223 0.51 PFR 848-1223 0.51 packed bed 723-1173 0.05-1.0 packed bed 823-1123 0.20 873-1173 0.044.904 packed bed 773-1118 0.034.20 packed bed 773-1118 0.03-0.20 packed bed 648-978 0.16-0.68 packed bed 470-900 packed bed 0.15 770-1073 1.0-10.1 TGA
E, kJ/mol 34.3-54.8
ref Watts2
62.8,' 7~5.7~ 86.6 105 64 67 137 145 120 47j 1928 43) 155' 183r 22s 23Y 68.2: 245' 65.1,m 177" approx 80 70 l 8 O P 63-88'
Smith et Edwards4 Lai et DeGroot et al.B Gibbs et al.' Song et al.B Levy et Schuler et al.l0 Matos et al." Chan et al.12 Furuswa et al.13 Bedjai et al." Kapteijn et al.lS Suuberg et al.16
a mz/g, various techniques (e.g. BET, COz Dubinin). * Copper-catalyzed. Oxidized surface. dHz-treated surface. e Deposited over CaO. fBelow 1003 K. #Above 1123 K. hBelow 1003 K. 'Above 1123 K. jValue for temperatures above 873 K, E is lower for lower temperatures. &Below953 K. 'Above 953 K. "'Mean value. two samdes. - . below 953 K. "Mean value, two samples, above 953 K. OKzCO3 impregnated. P Above 923 K. q Feed contained CO. Below 923 K.
2
At high temperatures, where gasification of carbon by NO is significant,the surface complexes can be desorbed as gaseous products, and several different complexes may form on the surface. It has been noted that the reaction of NO with char parallels in some respects the reaction of O2with chars, in that surface oxide intermediates play a key role in the mechanism. It has been reported3J0" that, at temperatures between 123 and 473 K,carbon-oxygen complexes form on the char surface, with the release of N2 as the main gaseous product. Therefore, an overall reaction in nonstoichiometricterms that summarizes this system is NO + C + C(0) CO + COZ + N2 + C(0)'
N
8
1-
c
0-
B-
a
E
I
0.9
I
I
1.0
1.1
E 1.2
I 1.3
1000/T(K) 2
b
4
where both C and C(0)are involved in the reaction, CO and C02are the gasification products, NO is reduced to N2,and C(0)' denotes a carbon oxide surface product that may be different than the original C ( 0 ) carbon surface oxide. 0.9 1.0 1.1 12 1.3 The NO-carbon reaction studies reported by previous 1000/T(K) workers focus mainly on the global kinetics of the pseuFigure 1. CO/COz product ratio for carbon gasification in 10.1 do-steady-state gasification. The reaction models generally (a) and 4.04 kPa NO (b) data for (b) from ref 16. imply an important role for oxygen surface complexes. The reaction of NO with carbons is generally reported to be et al. first oder with respect to NO partial p r e s s ~ r e , 2 ~ ~inJ ~ ? ' ~ * ~ ~ 2o Song Montana Ligule Char contrast to reactions of carbons with 02,which typically exhibit a fractional order.21 The ratio of CO to C02 in Japanese Coal Furusawa et al.Char and the gasification products of carbon by both 0222-25 N03*4J2-14J6 show similar qualitative trends-the ratio increases with increasing temperature (Figure 1). Chan et. al. In the case of NO gasification, there is a significant change in mechanism at a temperature in the neighborhood of 873-953 K,12J3J6as revealed by a change in apSuvber et a1 parent activation energy for the process. The temperature Resin 8-a
1
-I\ \'
(20) Tena, H.;Suuberg,E. M.R e p.r . Pap.-Am. Chem.SOC.,Diu. Fuel . Chem. 19SK 36(3), 922. (21) Suuberg, E. M.;Wojtowicz, M.;Calo, J. M. 22nd Sympsoium (Internuttonal)on Combustion: The Combustion Institute: Pittaburah,
1988; p 79. (22) Laurendeeu, N. M. R o g . Energy Combust. Sci. 1978, 4, 221. (23) Walker, P. L., Jr.; Rusinko, F., Jr.; Austin, L. G. Adu. Catal. 1969, 11, 133. (24) Mulcahy, M.F. R.; Smith, I. W. Rev. Pure Appl. Chem. 1969,19, 81. (25) Smith, I. W. Fuel 1978,57,409.
.I
.
0.5
0.7
0.9
1.1
1.3
I .5
1000/T(K)
Figure 2. Comparison of NO reaction rates with various types of char. Reactions were performed under different NO pressure ranges in the different studies: 0.05 kPa I P N o I0.65kPa (Song 0.03 kPa I e t al?); 0.05 kPa I P N O 5 1.01 kPa (Chan et PNo 5 0.1 kPa (Furusawa e t a l . 9 ; 1.01 kPa I PNO I 10.1 kPa (Suuberg e t al.16); 1.01 kPa 5 P N O 5 10.1 kPa (Tengm).
Teng et al.
400 Energy & Fuels, Vol. 6, No. 4, 1992
dependence of the rate of the NO-char reaction from our previous studies16*26 and the literaturesJ2J3 is shown in Figure 2. The trend of activation energy with temperature is opposite to that which would be expected if this were simply a case of transition from chemical reaction rate control to mass-transfer control, because higher activation energies are observed at higher temperatures. The activation energies for the reactions of NO with carbons reported by previous workers are shown in Table I. From the global kinetic results, one group12proposed the following reaction model: NO + C C(0) + 1/2N2 (R1) co + C(0) c02 + c (R2) C(0) co (R3) An alternative mechanism was proposed by Smith et 2N0 + 2C 2C(O) + N2 (R4) (R5) NO + C + C(0) C(O***ON)C NO + C(O***ON)C C02 + N2 + C(0) (R6) NO + C(O**.ON)C C02 + N2 + CO (R7) (R8) 2N0 + 2CO -m 2C02 + N2 The original presentation of this model included two reversible steps of the form of (R5), representing the formation of the two different types of C(O-ON)C complexes that react according to (R6) and (R7). There is general agreement that the first step is chemisorption of NO at almost any temperature of relevance. It probably involves addition of NO in an "N-down" configuration,2'followed by release of N2 and formation of carbon oxide surface complexes, as suggested in (R4). We have reason to doubt that the step is as simple as (R4) suggests, however. As we reported sarlier,l6we found that N2 is a significant high temperature (>lo00 K) product during desorption from NO-oxidized chars. This means that stable N-containing surface species must have been present. Although (R5) in the above mechanism represents the existence of long-lived N-containing complexes on the surface, the C(O--ON)C complex with the weak physical bonding suggested by the authors3 most likely cannot represent the long-lived N-containing complexes we have observed.16 It might then be argued that the C(O--ON)C complex is actually a chemisorbed complex and that the desorption of N2 we observed involves the reverse of reaction R5 followed by (R6), (R7), or (R8). We, however, we no evidence of desorption of NO as such, except at very low temperature^.'^^^^ Thus we feel it most likely that reaction R4 does not proceed as indicated in a single step, and that there is some way of forming N-contahing surfam species that are able to desorb as N2in the absence of NO. Furthermore, the Smith et al. model is deficient in not including a desorption route, such aa (R3), for formation of CO, which is usually an important step for practically all oxidizing carbon gasification processes. In our previous study,16the global kinetic parameters of the NO-char reaction, such as activation energy, reaction order with respect to NO, and the CO/CO2 product ratio, were determined. Again, the NO-char reaction can be divided into two different mechanistic regimes aa suggested by Figure 2. The low-temperature regime (arbitrarily 923 K) has a constant activation energy of 180 kJ/mol,ls and the process is also hypothesized to involve a prompt product release step, following the rate-controlling formation of surface complexes formed by reaction with NO. The contribution from desorption of surface complexes (by a route such as (R3))is not dominant in terms of gasification rate, but cannot be neglected, even in this regime. In our previous study,16 oxygen surface complexes formed during gasification were characterized by a model involving a distribution of desorption activation energies. The situation is crudely represented in Figure 3. There are certain low desorption activation energy surface species for which the rate of desorption is quite fast compared to the rate of formation by dissociative adsorption of NO. The sites that form these species are the most active in terms of the rate of gasification. Owing to their high desorption rate, these sites are "empty" under gasification conditions. For the purpoae of this study, the term "rapid turnover site" denotes an unoccupied site with such a high desorption rate during pseudo-steady-state gasification. It has been shown that the gasification product release p r m involving a sequence of NO attack followed by fast product release (so that no stable oxides are formed) contributes significantly to the overall rate of carbon gasification by NO at all temperatures. The type of sites involved in this process remains an interesting, but unresolved, question. By analogy, in the OZ-charreaction, Karsner and Perlmutter2*and Su and Perlmutter* have suggested that besides the oxygen complex formationdesorption reaction, the direct burn-off of unoccupied surface, i.e., (28) Karsner, G. G.; Perlmutter, D. D. Fuel 1982, 61, 29. (29) Su, J.-L.; Perlmutter, D. D. AIChE J. 1985,31(10), 1726.
Energy & Fuels, Vol. 6, No. 4, 1992 401
Reduction of Nitric Oxide by Carbon
-
c + o2 co, coz
039)
makes a significant contribution to the overall rate of char gasification by O2 In summary,according to our previous study, carbon gasification by NO involves the desorption of surface complexes by a process such as (R3), i.e., [surface complexes] CO, COP,or N2 (R10) but the product release process also involves the direct participation of NO in an apparently single-step process: C + NO CO,COz, or N z (RW Note that, in Figure 3, the distinction between CO and C02 yielding sites is for the moment left deliberately vague. This point will be taken up later. Although the kinetics of the desorption process (R10) that occurs during gasification (and which can proceed in an inert environment) has been modeled by using a distributed desorption activation energy model,lethe prompt product release route associated with NO participation (R11) has not yet been much explored. The sites involved in this fast NO attack are unknown. There are other significant unresolved issues as well. For example, why is the apparent order of reaction unity, even though two NO molecules are required to release a COP molecule during gasification in the low-temperatureregime? How does the low-temperature route for forming C02compare to the high activation energy route, forming mainly CO at high temperatures? This paper presents additional evidence which resolves some aspects of these questions.
-
-
2. Experimental Section The carbon samples used in present study were derived from phenol-formaldehyde resins. Thew resins have structural features similar to those in coals, but contain fewer catalytic impurities. These can be controlled to very low levels in the synthesis process.21 The resin char was prepared by pyrolysis of the phenol-formaldehyde resin in a helium environment a t 1323 K for 2 h, then ground and sieved to the desired particle size (106-300 wm). The measurements of reactivity of the chars were performed in a Cahn 113 TGA (thermogravimetric analyzer) system. A sample of char was suspended in a quartz bucket in the heated zone of the TGA, and the temperature in the vicinity of the sample was measured by a small thermocouple probe (type K), placed within a few millimeters of the sample. Prior to each run, the char sample was subjected to degassing and surface cleaning, which involved heating the char to 1223 K and maintaining this temperature for 2 h in ultra-high-purity helium, which was further purified by a cryogenic trap to eliminate traces of oxygen. After surface cleaning, the sample temperature was decreased and adjusted to the desired value. Subsequently, the appropriate NO mixture was admitted into the system. The NO partial pressure used for gasification was between 1.01 and 10.1 kPa,and helium was used for dilution. Oxygen was carefully excluded during the experiment by operation a t positive pressure relative to ambient. Gasification was conducted in a static gas environment of sufficiently large volume that reactant gas consumption was negligible. Reactivity was determined from the weight loss of the char with time. The temperature range of this gasification study was between 773 and 1073 K. External mass-transfer limitations have been determined to be insignificant in the range of reaction rates of interest here;21this is confirmed by the good comparability of kinetic constants obtained by this technique and those obtained by flow techniques under comparable temperature conditions (Table I). In order to obtain the product yield ratio during steady gasification, the products of gasification were subjected to gas chromatographic (GC) analysis, following the reactivity measurements. Surface complex desorption experiments were also performed in the TGA at the desired temperatures, under helium. The process involved pseudo-steady-state oxidation of the char
.
P
:+*,
2 +++;
++SO
+
* *
E
I
5 0.4
0.0 500
+++
+ +
700
900
1100
1300
T(K)
Figure 4. T P D of char gasified in NO a t 873 K, and cooled to 473 K in He (e).TPD of char gasified in NO at 873 K, and cooled to 473 K in NO (+). in NO to build up complexes on the surface, followed by an immediate isothermal desorption in helium. The isothermal desorption products were purged from the system and analyzed by GC. Temperatureprogrammed desorptions (TPD) of surface speciea were performed in this study to determine the composition of surface complexes. Unlike the TPD studies of m a t other workers (e.g., Hall and Calo30),gas chromatography (GC) was employed in this study, instead of mass spectrometry. This procedure involved performing a TPD experiment in the TGA followed by a post-TPD product analysis for each s d temperature interval, since GC provides an integral rather than continuous analysis. The reason for this cumbersome procedure was a desire to accurately determine N2and CO products, both of which give mam 28 signal in mass spectrometry, and both of which were present in inconveniently low concentrations, given the nature of the experiments. Thew TPD experiments were performed in the TGA system, with a linear heating rate of 22.5 K/min under helium flow. Products from TPD were collected in a cryogenic adsorbent trap (Porapak Q a t 77 K), and then subjected to the GC analysis by raising the temperature of the trap. The analysis of NO by this technique is not quantitatively reliable, although its presence or absence is clearly visible. From the data on sample mass, and the GC analysis of other products, the quantity of NO desorbed during T P D can be accurately (
