NOx Reduction by Potassium-Containing Coal Briquettes. Effect of

Energy & Fuels 1999, 13, 499-505

499

NOx Reduction by Potassium-Containing Coal Briquettes. Effect of NO2 Concentration Avelina Garcıá-Garcıá, Marıá Jose´ Illań-Go´mez, Angel Linares-Solano,* and Concepcioń Salinas-Martıńez de Lecea Departamento de Quı´mica Inorga´ nica, Universidad de Alicante, Apartado 99-E-03080, Alicante, Spain Received August 26, 1998

The reactions of a potassium-containing bituminous coal briquette and the corresponding catalyst-free char with different NO/O2 mixtures have been investigated at temperatures between 300 and 325 °C. The isothermal reactions have been followed by two techniques: thermogravimetry and gas chromatography + chemiluminescence analysis. A NO/NO2 mixture rich in NO2 results in a high carbon loss compared to that corresponding to the mixture poor in NO2. Despite that, NOx conversion is similar for both mixtures. The extra weight loss in the presence of a high concentration of NO2 is a consequence of NO2 reduction by carbon, producing NO and CO2 evolution.

Introduction The removal of nitrogen oxides is a prevailing priority in our modern society. Recently, a special collection of publications concerning this problem has been assembled in two different journals (Energy Fuels and Fuel). Both special issues contain publications devoted to the basic study of NO reduction over potassiumcontaining carbon,1,2 which shows the peculiarity that is conformed in briquettes. The molded carbons present considerable advantages over conventional powdered or granular carbons.3 The study of the NO-carbon reaction catalyzed by potassium described in these published papers1,2 shows an intrinsic difficulty due to the numerous processes that take part (dissociative NO chemisorption, oxygen retention on the catalyst, transfer of oxygen to carbon surface, and decomposition of oxygen surface complexes). If oxygen coexists in the reaction mixture, this difficulty will be remarkably increased. Flue gases from postcombustion processes contain 500-1500 ppm of NO, depending on the type of combustor, compared to tens of thousands of parts per million of O2.4 In these conditions, the gasification rate of carbon by O2 is 2 orders of magnitude superior to that of NO.5-8 Therefore, the use of carbon as a reductor in NO reaction is limited since it can be consumed more by oxygen (combustion) than by NO (oxidation). In addition, the presence of a catalyst, used to decrease the temperature of the NOx reduction by carbon, would increase both the C-NO and the C-O2 reaction. On this * To whom correspondence should be addressed. (1) Garcıá-Garcıá, A.; Illań-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martıńez de Lecea, C. Fuel 1997, 6, 499. (2) Garcıá-Garcıá, A.; Chinchoń-Yepes, S.; Linares-Solano, A.; Salinas-Martıńez de Lecea, C. Energy Fuels 1997, 11, 292. (3) Durie, R. A. The Science of Victorian Brown Coal: Structure, Properties and Consequences for Utilization; Butterworth-Heinemann: Oxford, 1991.

line, the works of Yamashita et al.9,10 have to be mentioned. These studies report the activity of catalysts based on Ca, Ni, or Cu for NOx reduction at 300 °C. They observed that most of the carbon consumption was due to the O2 reaction. Therefore, the search for a catalyst that shows high selectivity toward the NO-carbon reaction, in the mixture NO + O2, has become a challenge in the past few years. On the other hand, the coexistence of NO and O2 in the reaction gas not only assumes a possible competition by the active sites of carbon, as previously reported,5-7 but also introduces additional complexity to the system because the NO + O2 equilibrium produces NO2 as the reaction product. The physicochemical characteristics of this molecule can determine the global activity because of its very high reactivity. A better understanding of the NOx-carbon reaction needs to consider the coexistence of NO2 and to analyze the parameters that control the NO + O2 equilibrium. Considerations about the Gases of a NO/O2 Mixture. Undoubtedly, the most important reaction of nitric oxide is its oxidation to nitrogen dioxide, therefore, the NO/O2 mixture is formed, at least, by three gases (NO, O2, and NO2):

NO + 1/2O2 f NO2

(1)

Reaction 1 follows a simple third-order rate law over a (4) Stocker, H. S.; Seager, S. L. Quı´mica ambiental: Contaminacio´ n del aire y del agua; Blume: Barcelona, 1981; p 54. (5) Song, Y. H. Ph.D. Thesis, Massachusetts Institute of Technology, 1978. (6) Furusawa, T.; Kunii, D.; Oguma, A.; Yamada, N. Int. Chem. Eng. 1980, 20, 239. (7) Teng, H.; Suuberg, E. M.; Calo, J. M.; Hall, P. J. Extended Abstracts of the Biennial Conference on Carbon 1989, 574. (8) Teng, H.; Suuberg, E. M.; Calo, J. M. Energy Fuels 1992, 6, 398. (9) Yamashita, H.; Yamada, H.; Tomita, A. Appl. Catal. 1991, 78, L1. (10) Yamashita, H.; Tomita, A.; Yamada, H.; Kyotani, T.; Radovic, L. R. Energy Fuels 1993, 7, 85.

10.1021/ef980165h CCC: $18.00 © 1999 American Chemical Society Published on Web 02/02/1999

500 Energy & Fuels, Vol. 13, No. 2, 1999

Garcıá-Garcıá et al.

Table 1. Physicochemical Constants for NO, O2, and NO2

gas

ionization energy (eV)

bond order

bond length (Å)

bond energy (kJ/mol)

NO O2 NO2

9.3 12.1 11.0

2.5 2 1.75

1.150 1.207 1.197

624.6 490.4 466.7

dipolar moment (D) 0.15 0.39

wide range of experimental conditions.11

-d(NO)/dt ) K[NO]2[O2]

(2)

An unusual feature of this reaction is that the rate constant decreases with increasing temperature due to the initial formation of an unstable dimer that reacts with O2 and whose concentration decreases rapidly with temperature:12 O2

2NO f N2O2 98 2NO2

(3)

Three characteristics of the molecules present in the gas mixture (NO, O2, NO2) should be recalled; they are small, reactive, and paramagnetic molecules: NO is the simplest, thermally stable, odd-electron molecule. O2 is a paramagnetic molecule despite having an even number of electrons. NO2 shows a greater trend toward dimerization than NO because of its localized unpaired electron.11,12 A compilation of literature allows one to establish the following reactivity order of these three molecules toward carbonaceous materials:5-8,13-16

RNO < RO2 < RNO2 The NO-carbon reaction rate is 2 orders lower than the O2-carbon reaction,5-8 and the NO2-carbon reaction rate is about 100 times higher than that of the O2carbon reaction.16 Table 1 collects some of the physicochemical constants of these molecules.11,12 Both the bond orders and the bond energies decrease from NO to O2 to NO2, in agreement with the above reactivity order of these molecules with carbon. This is expected since the first step in the reaction is the dissociative chemisorption on the active sites of the carbon surface8 and, hence, is bond breaking. Concerning the properties of NO2, the instability of this molecule with temperature is well-known; at 150 °C it begins to decompose, giving NO and O2; the decomposition is substantially completed at 600 °C.4,11 Both the NO oxidation rate and the instability of NO2 with temperature determine the NO/NO2 ratios of flue gases from postcombustion processes. Thus, gases can get cold, more or less quickly, when leaving the combustion zone and mixing with air, depending on the combustor design. Most of the NOx emissions in electric (11) Bailar, J. C.; Emeleús, H. J.; Nyholm, R.; Trotman-Dickenson, A. F. Comprehensive Inorganic Chemistry; Pergamon Press: Oxford, 1973; Vol. II. (12) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Pergamon Press: Oxford, 1984. (13) DeGroot, W. F.; Richards, G. N. Carbon 1991, 29, 179. (14) Bartek, J. P.; Vastola, F. J.; Walker, P. L., Jr. Extended Abstracts of the Biennial Conference on Carbon, 1973, p 92. (15) Nelson, C. R.; Cox, J. L. J. Phys. Chem. 1985, 89, 892. (16) Arthur, J. R.; Ferguson, H. F.; Lauber, K. Nature 1956, 178, 4526.

power plants are found as NO, with NO2 concentration as low as 10%.4 In the case of mobile sources, the NO2 concentration is ca. 2-3%. Therefore, as NO and NO2 molecules have very different reactivities, the study of the NOx reduction by carbon will be affected by the NO/ NO2 ratio of the gas mixture used. The present paper studies the NOx-carbon reaction using NO/O2 atmospheres with the same NOx percentage value but with different NO/NO2 ratios, with the purpose of linking the study of NO reduction by potassium-containing coal briquettes, performed in the absence of oxygen,1-2 with that performed in the presence of oxygen. Therefore, the aim of this paper is analysis of the NOx-carbon reaction modifying the composition of the reaction mixture (NO + O2) and, as a consequence, both the NO/NO2 ratio and the NO2 concentration. Thereby, a better understanding of the role of the different species involved in the reaction mixture is intended as well as finding the suitable conditions of the gas mixing to perform activity experiments in the presence of oxygen. Experimental Section Samples. The briquetting process to obtain the coal briquettes has been described extensively elsewhere.1,2 Basically, the method consists of mixing a bituminous coal (A3) with potassium-containing humic acid, followed by pressing (1-2 kg/cm2) and pyrolyzing in N2 for 2 h at 700 °C. The coal briquette is designated by the parent coal used, followed by their potassium content in weight percent K (i.e., sample A34.7). For comparative purpose, coal was pyrolyzed at the same conditions and the char obtained was named A3-0. Isothermal Reactions in Different Reaction Atmospheres. With the purpose of analyzing sample reactivities in different reaction gases (NO-O2-NO2), isothermal experiments have been undertaken at temperatures between 300 and 325 °C. The following gas mixtures have been used: (1) 0.5% NO/He, (2) 5% O2/He, (3) 0.5%NO/He + 5%O2/He (mixing streams from two different gas cylinders), (4) 0.5%NO/5%O2/ He (contained in a gas cylinder). In case 3, the concentration of NO2 is low because the gas mixture is the sum of two gas streams mixed as near as possible to the microreactor inlet. On the contrary, in case 4, the mixture of NO and O2 from the same gas cylinder produces a high concentration of NO2. The experiments performed with these gases have been followed by thermogravimetry and gas chromatography + chemiluminiscence analysis. Isothermal Reactions in a Thermobalance. The experimental procedure used is as follows: heating the sample (∼20 mg) in helium (60 mL/min) at 20 °C/min up to the reaction temperature. Then, helium is replaced by the reactive mixture for 3 h. The reactivity of the samples in the different reaction atmospheres has been determined from the slope of the straight line of the plot of the converted carbon fraction versus time. This can be expressed by the equation

R ) 1/mo(dm/dt) where R is the reactivity in h-1 and mo is the free-ash sample mass for reaction time ) 0. This parameter will be used to compare the reactivities of the carbon samples in the different reaction atmospheres. Isothermal Reactions in the Microreactor Followed by Gas-Chromatography + Chemiluminiscence Analysis. The experimental procedure used is as follows: heating

NOx Reduction by K-Containing Briquettes

Energy & Fuels, Vol. 13, No. 2, 1999 501

Figure 1. Weight-change curves in different reaction mixtures for briquette A3-4.7.

Figure 2. Weight-change curves in different reaction mixtures for char A3-0.

the sample (∼300 mg) in helium (60 mL/min) up to the reaction temperature. Then helium is replaced by the reactive mixture for 3 h. The experimental device used consists of a fixed-bed reactor coupled to a gas chromatograph, Hewlett-Packard 5890 Series II, equipped with a switched dual-column system. This system has been designed for the separation and quantification of the numerous reaction gases involved in the present study. The GC system has two serial columns (Porapak Q 80/100, for separation of CO2 and N2O, and Molecular Sieve 13×, for O2, N2, CO, and NO) joined by a six-way valve with a restriction that avoids a pressure drop when the second column is bypassed. The complete analysis is performed in three steps. As NO2 is retained in the Porapak column and its quantification by chromatography is not possible, a chemiluminiscence NOx analyzer (Thermo Environmental Inc., model 42H) is also needed for NO2 determination. With both equipment, all the reaction products can be quantified. Temperature-programmed desorptions, after reaction in different atmospheres, have been also performed to obtain knowledge about the surface chemistry of the samples. These desorptions were accomplished in helium with a heating rate of 5 °C/min, from the reaction temperature up to 900 °C.

concentration of NO2 is achieved. It is well-known that the time required to convert 90% of NO to NO2 in the presence of 20% of O2 is only 36 min.4 Therefore, under our experimental conditions, even though the oxygen content is near 5%, the NO2 species will predominate in the gas mixture coming from an unique gas cylinder. The high reactivity that this molecule exhibits would explain the remarkable enhancement in reactivity, much higher than that of O2. The larger reactivity of carbon in the presence of NO2, found in our study, confirms the results of the literature. The sample reactivity in NO2 is clearly greater than that in O2 and the latter greater than that in NO. Figure 2 presents TG curves for sample A3-0 in NO/ He + O2/He and NO/O2/He mixtures. By comparison of the TG curves in Figures 1 and 2, it can be clearly appreciated that the weight loss is remarkably lower in the potassium-free char. Thus, potassium acts as a catalyst of the carbon gasification in the presence of NOx. Despite considering temperatures as low as 300 °C, two regions can be observed in the TG profiles (Figures 1 and 2): an initial period with a lower slope and a second one after a certain reaction time, with a higher and constant slope for a wide range of weight loss. The reactivity has been estimated in the constant slope region. The first region of low reactivity, typical of carbon oxidation reactions with O2, can be due to the formation of oxygen surface complexes17 created by dissociation of O2, NO2, and NO. These complexes are not desorbed until the oxygen covering of the carbon is high enough to lower the corresponding desorption energy.18,19 Data concerning mass losses during the experiments in distinct mixtures (after 3 h of reaction) and the corresponding reactivities have been collected in Table 2. The enhancement in reactivity attributed to the important contribution of NO2 in the last reaction

Results and Discussion Isothermal Reactions in a Thermobalance. Figure 1 shows the TG curves for briquette A3-4.7 corresponding to isothermal experiments in the following mixtures: NO/He, O2/He, NO/He + O2/He, and NO/O2/ He. It is interesting to note that (i) in the mixture NO/ He, there is no appreciable weight loss even after 12 h of reaction, (ii) in both mixtures O2/He and NO/He + O2/He, the weight loss is small, and (iii) in the mixture NO/O2/He, the weight loss is much more important. It can be concluded that the combined presence of NO and O2 in a gas cylinder raises the reactivity of the gas stream. By mixing the two gas streams (from two cylinders), very close to the microreactor inlet, a low NO2 concentration is obtained because, as it was pointed out, the system NO + 1/2O2 f NO2 cannot achieve the equilibrium. On the contrary, when the mixture NO + O2 is prepared in a gas cylinder several hours before the reaction, the equilibrium is reached and a high

(17) Mahajan, O. P.; Walker, P. L., Jr. Anal. Methods Coal Coal Prod. 1975, 2, 465. (18) Cerfontain, M. Ph.D. Thesis, University of Amsterdam, 1986. (19) Walker, P. L., Jr.; Taylor, R. L.; Ranish, J. M. Carbon 1991, 29, 411.



Table 2. Weight Losses (after 3 h of Reaction) and Reactivities in the Stationary State for Samples Studied in Different Reaction Atmospheres reaction mixture NO/He O2/He NO + O2/He NO/O2/He a

weight loss (%) A3-0 A3-4.7

reactivity (h-1) A3-0 A3-4.7

a a 1.09 5.12

a a 0.004 0.024

a 1.63 3.75 13.61

a 0.009 0.014 0.076

Practically negligible.

Table 3. Data Deduced from the Activity Experiments in a NO/He + O2/He Mixture for Briquette A3-4.7 temp (°C)

weight loss (%)

reactivity (h-1)

300 315 325

3.75 15.39 18.31

0.014 0.079 0.113

Figure 3. Isothermal NOx reduction activity as a function of time for briquette A3-4.7 in the different gas mixtures used at 300 °C.

mixture seems to produce an identical effect for both samples. In fact, normalizing the reactivities (RNO/O2/He/ RNO/He+O2/He), the same value, close to 6, is obtained for both samples. In the presence of the mixture NO/He + O2/He, with a NO/NO2 ratio close to real conditions of postcombustion processes, the reaction for briquette A3-4.7 has been also studied at 315 and 325 °C. The results of these experiments are presented in Table 3. The expected enhancement of carbon mass loss due to the increase in reaction temperature is evident. This fact has a considerable importance from an application point of view. An enhancement of 15 °C (from 300 to 315 °C) produces a remarkable increase in reactivity that could limit briquette application, if carbon is mainly consumed by O2 and not by NOx. Isothermal Reactions in a Microreactor Followed by Gas Chromatography. Similar experiments to those presented in the above section have been carried out in a fixed-bed reactor coupled to a GC and a chemiluminiscence analyzer. The quantification of the reaction products would complete the experiments performed in the thermobalance. Figure 3 contains the NOx reduction percentage versus time for briquette A34.7, at 300 °C, in the three reaction atmospheres (NO/ He, NO/He + O2/He, and NO/O2/He). It can be observed that the presence of O2 in the reaction mixture causes a considerable enhancement in NO conversion, which is practically insignificant in the absence of oxygen after a few minutes of reaction. On the contrary, the activity is maintained throughout 3 h

Figure 4. CO2 evolution as a function of time for briquette A3-4.7 in the different gas mixtures used at 300 °C

of experiment, in the presence of O2, even undertaking a slight enhancement. This increase in reactivity, after a certain reaction time, was also found in the reactivity profile obtained from the thermobalance. The increase in reactivity in the presence of oxygen is in agreement with previous results.10,20-23 It has been reported that the presence of other simultaneous gases can have important effects on the observed kinetics and on the course of the NO-carbon reaction.24 Specifically, in the presence of oxygen, a significant effect on the rate of NO reduction by carbons has been demonstrated.10,23,24 The role of potassium on the reactivity seems to be very important. In fact, comparison of the amount of NOx reduced by briquette A3-4.7 (Figure 3) with that of sample A3-0, after 3 h of experiment, shows significant differences (568 µmol NOx per gram of sample for briquette versus 68 for the char). This catalytic role of potassium will be discussed in more detail in subsequent publications. An important observation that completes the results obtained from the TG data can be extracted from Figure 3. There is no increase in NOx conversion for the mixture NO/O2/He, with a high NO2 concentration. On the contrary an important mass loss in the TG with this gas mixture was observed. A recent work of Suzuki et al.23 concludes that the reactivity of NO2 is so high that it is completely reduced by carbon and that the major product was not N2 but NO. The reduction of NO2 to NO is rather fast, but the further reduction of NO to N2 is much slower. It is concluded from these results and those published by Suzuki et al. that the formation of NO2 by the action of O2 is not beneficial because there is not an enhancement of the amount and the reduction rate of NO. The CO2 evolution, the main oxygenated product during reaction, is presented in Figure 4 for sample A34.7. The other oxygenated reaction products, CO and N2O, appear in very little amounts under these conditions. It can be pointed out that CO2 emission, corresponding to the mixture NO/He, is very low, in agreement with the low reduction capacity in the absence of (20) Urano, K.; Tanikawa, N.; Masuda, T.; Kobayashi, Y. Nippon Kagaku Kaishi 1978, 303. (21) Mochida, I.; Sun, Y.-N.; Fujitsu, H.; Kisamori S.; Kawano, S. Nippon Kagaku Kaishi 1991, 885. (22) Rubel, A. M.; Stencel, J. M.; Ahmed, S. N. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38, 726. (23) Suzuki, T.; Kyotani, T.; Tomita, A. Ind. Eng. Chem. Res. 1994, 33, 2840. (24) Aarna, I.; Suuberg, E. M. Fuel 1997, 76, 475.



Figure 5. NO/NO2 ratios of the reaction mixtures used: (a) before reaction and (b) after reaction.

oxygen; for the mixture NO/He + O2/He, CO2 evolution slightly increases throughout the 3 h interval; finally, for the mixture NO/O2/He, richer in NO2, CO2 evolution is much more important. According to Suzuki et al.,23 NOx conversion capacities are analogous in both mixtures but CO2 evolution for the latter is much higher. The presence of high NO2 concentration in the last reaction mixture results in a high carbon burnoff, as seen in Figure 1, but not a high NOx reduction. As commented on the Introduction, the physicochemical characteristics of this molecule (mainly a high dipolar moment and low bond order) promote high oxidation of the briquette and char. The carbon oxidation by NO2, forming CO2 as the main product, is much faster than that produced by O2, confirming the larger reactivity of NO2 (with regard to O2) against carbon. In fact, CO2 emissions both in O2/He (not shown) and in NO/He + O2/He are analogous. The NO/NO2 ratio for both reaction mixtures (NO/ He + O2/He and NO/O2/He) can be monitored with the chemiluminiscence analyzer. The plot of these ratios before and after the reaction at 300 °C have been represented in Figures 5a and 5b, respectively, as a function of time. Two important aspects can be deduced from these representations: the NO/NO2 ratio obtained from the mixture NO/He + O2/He, before reaction (Figure 5a), is much higher than that from the mixture NO/O2/He. In the case of the ratios measured after reaction (Figure 5b), it is observed that these differences are smaller. In other words, for the mixture NO/O2/He, the NO/NO2 ratio varies remarkably during the reaction, proving that part of the total NO2 is being converted to NO by carbon. This results in an important enhancement in the NO/NO2 ratio and, consequently, an enhancement of the emission of CO2 as a product of carbon oxidation by NO2. The above findings, regarding the enhancement of the NO/NO2 ratios after the reaction with the coal briquette, are not in contradiction with the well-known capacity of a carbon surface to catalyze NO oxidation to NO2.25 From recent studies of NO oxidation to NO2 over ACF,26 it is believed that NO oxidation to NO2 and NO2 reduction to NO by carbon are in competition with each other. At low temperatures, NO oxidation is the dominant reaction and results in high NO conversions. At high temperatures, NO2 reduction with carbon is faster (25) Mochida, I.; Kisamori, S.; Hironaka, M.; Kawano, S.; Matsumura, Y.; Yoshikawa, M. Energy Fuels 1996, 10, 169. (26) Jang, B. W.-L.; Spivey, J. J.; Kung, M. C.; Kung, H. H. Energy Fuels 1997, 11, 299.

than NO oxidation; that is, no apparent NO oxidation was observed at 120 °C. In summary, these authors report that the reduction of NO2 to NO by carbon is too fast at 120 °C to result in any apparent NO oxidation conversion. These speculations could have been supported by the formation of CO2 or/and CO, but they were not measured in the mentioned study.26 The present work, with a remarkable evolution of CO2 in a gas mixture with high NO2 concentration, corroborates the ideas stated in that study.26 Considering the above results and the peculiarities of the NO + O2 equilibrium and the NO/NO2 ratio from electric power plants, our study of NOx reduction by coal briquettes should be studied, achieving a NO/NO2 ratio as high as possible. Temperature-Programmed Desorption after Isothermal Reactions in Different Atmospheres. After the experiments at 300 °C in different atmospheres, TPD measurements were carried out up to 900 °C to evaluate the quantity of stable carbon-oxygen complexes generated during these experiments. The surface groups created after the isothermal reaction can be “labile” or reactive groups, which lead to carbon gasification, and “stable” groups, which do not desorb at gasification temperatures.27-29 TPD technique can help to analyze the stable complexes because they present a desorption energy distribution.28 Figure 6 collects CO2 and CO evolution profiles from TPD experiments in He up to 900 °C for sample A34.7. These spectra reflect the degree of oxidation of the samples after isothermal reactions for 3 h in the different atmospheres (NO/He, O2/He, NO/He + O2/He, NO/O2/He). First, it has to be mentioned that no nitrogen species evolution is observed during these desorption experiments. These results agree with others previously reported,10,30 suggesting that N2 is mainly formed assuming the following stoichiometric reaction:

2NO + 2Cf T 2C(O) + N2

(5)

Although, a fraction of nitrogen can also be trapped on the surface during NOx-carbon reaction, due to C(N) complex formation, this type of complex cannot be released during TPD up to 900 °C. The larger stability of the nitrogen complexes, with regard to oxygen (27) Ranish, J. M.; Walker, P. L., Jr. Carbon 1993, 31, 135. (28) Zhuang, Q.; Kyotani, T.; Tomita, A. Energy Fuels 1996, 10, 169. (29) Lizzio, A. A.; Jiang, H.; Radovic, L. R. Carbon 1990, 28, 7. (30) Illań Go´mez, M. J.; Salinas Martıńez de Lecea, C.; Linares Solano, A.; Phillips, J.; Radovic, L. R. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1996, 41, 293.



Figure 6. TPD spectra for briquette A3-4.7 after reaction at 300 °C in different reaction atmospheres: (a) CO2 and (b) CO.

Figure 7. TPD spectra for char A3-0 after reaction at 300 °C in different reaction atmospheres: (a) CO2 and (b) CO.

complexes, has been attributed to the fact that nitrogen penetrates more deeply into the carbon matrix.23 Second, the amount of oxygenated complexes evolved as a function of temperature is very dependent on the reaction mixture used, and these amounts are observed to grow following the order: NO < O2 < NO + O2 < NO/O2. Therefore, the population of C-O complexes increases as the oxidation character of the gas mixture increases, confirming the above TG and GC results. Yamashita et al.9 also observed that the amount of stable complexes (C-O) grew considerably from NO/He to NO/He + O2/He. In the range 550-700 °C, the shape of the CO2 desorption peak seems to indicate that not only the release of oxygenated groups of carbon is occurring, but also K2CO3 decomposition (created during the reaction and confirmed by XRD)2 is occurring. More CO2 evolution, as a consequence of a more oxidizing character of the reaction mixture, results in more carbonation of the fraction of potassium found as K2O.1 The results in Figure 6 seem to confirm that carbonation of the potassium species really occurs (according to the results of Illań et al.30) and that this process seems to increase with the oxidizing character of the reaction atmosphere. CO evolution also follows the same sequence as that showed by CO2, both (CO2 and CO) reflect the oxidation degree achieved by the sample. The more oxidized samples exhibit a maximum around 750 °C, suggesting that decomposition of C(O) groups as well as partial reduction of K2O are occurring.31 Complementary information is deduced from similar experiments (Figure 7) performed in the potassium-free char (A3-0). In that case, the oxidizing character of the (31) Illań Go´mez, M. J.; Linares Solano, A.; Radovic, L. R.; Salinas Martıńez de Lecea, C. Energy Fuels 1995, 9, 97.

reaction atmosphere also produces an increase in surface complexes. It is noteworthy that the pronounced CO2 peak characteristic of the briquette (and attributed to K2CO3 formation) does not appear in sample A3-0. This observation supports the idea that K2CO3 is created during the NOx-carbon reaction and that its decomposition occurs at around 550-700 °C. The surface complexes able to evolve as CO are considerably greater than those corresponding to briquette A3-4.7. This high concentration of stable C-O species in the catalyst-free sample completely agrees with the data published by Yamashita et al.,10 who reported that irrespective of the nature of the reactant gas, the concentration of stable C-O species on a catalyst-free char was larger than that of a Cu/char sample, even though the extent of carbon conversion was greater for the catalyzed reaction. Previous results concerning TPD spectra of chars oxidized by NO at different temperatures (from 150 to 750 °C)7 revealed that the evolution of CO2 was much lower compared to CO, in the same way as in the present study. Conclusions From the present study the following conclusions can be deduced: (1) The sequence of reactivities of carbon in the different oxidizing atmospheres follows the order NO < O2 < NO + O2 < NO/O2 for potassium-free and potassium-containing samples. The mentioned order agrees with the physicochemical characteristics that determine the reactivity of the molecules involved (NO, O2, NO2). (2) For a given NOx value, the NO2 proportion remarkably affects the reactivity. A high NO2 concentration in the gas mixture produces a high carbon burnoff, although NOx conversion is not enhanced with regard to a low NO2 concentration in the gas mixture.


(3) The high reactivity of NO2 versus carbon is a consequence of its reduction to NO. (4) The population of surface complexes (C-O) created during the reactions increases with the oxidizing character of the mixture used, and it is considerably lower for the briquette than for the original char. To study the NOx-carbon reaction, the activity experiments must be performed mixing the NO and O2 streams very close to the microreactor inlet with the


purpose of achieving a high NO/NO2 ratio, similar to the actual conditions in flue gases from postcombustion processes. Acknowledgment. This study was made possible by the financial support from CICYT (Project No. AMB96-0799) and OCICARBON (C-23-435). EF980165H

NOx Reduction by Potassium-Containing Coal Briquettes. Effect of

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