Energy & Fuels 2002, 16, 569-574
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NOx Reduction by Potassium-Containing Coal Briquettes. Effect of Preparation Procedure and Potassium Content Avelina Garcı´a-Garcı´a, Marı´a Jose´ Illa´n-Go´mez, Angel Linares-Solano,* and Concepcio´n Salinas-Martı´nez de Lecea Departamento de Quı´mica Inorga´ nica, Universidad de Alicante, Apartado 99-E-03080, Alicante, Spain Received June 7, 2001. Revised Manuscript Received November 20, 2001
The activity of potassium-containing bituminous coal briquettes for NOx reduction in an oxygenrich environment has been investigated at temperatures of 300 and 350 °C. Two different procedures have been used for sample preparation: adding the alkali as the humic acid, or in a combined method (KOH + humic acid). The reaction has been followed by gas chromatography and chemiluminescence analysis. The main reaction products are N2 and CO2. The potassiumcontaining coal briquettes improve the performance of the original char (without potassium) in the following aspects: (i) increasing the NOx conversion; (ii) decreasing the CO/CO2 ratio in the reaction products, and (iii) enhancing the selectivity toward the NOx reduction against combustion by oxygen. In this sense, a very interesting potassium-containing coal briquette has been prepared by using KOH + humic acid, as precursors. This sample exhibits a medium NOx conversion level, a high selectivity factor, and a low burnoff during an experiment carried out at 350 °C.
Introduction The interest in NOx-carbon reaction has become increasingly important in connection with the reduction of nitrogen oxide pollution. The use of low cost carbon materials could be an interesting solution for the removal of nitrogen oxides, because carbon is a reducing agent which would be gradually consumed by combustion exhaust gases and, consequently, an additional reducing agent would not be required.1-3 In previous publications, an effective removal of dilute NO was reported by using potassium-containing coal briquettes in the absence of oxygen.4-7 The effect of variables involved in the preparation process of these materials on the NO reduction was previously analyzed in detail.4-8 Potassium-containing coal briquettes are promising reducing agents for NOx reduction with many potential advantages, such as: (i) low cost, (ii) easy availability of coal, (iii) high efficiency, (iv) simplicity of process, (v) no secondary pollution (i.e., as ammonia “slip” in SCR * Author to whom correspondence should be addressed. (1) Mochida, I.; Ogaki, M.; Fujitso, H.; Komatsubara, Y.; Ida, S. Fuel 1985, 64, 1054. (2) Teng, H.; Suuberg, E.; Calo, J.; Hall, P. Proceedings of the 19th Carbon Conference, Pennsylvania State University, 1989; p 574. (3) Illa´n-Go´mez, M. J.; Linares, A.; Salinas-Martı´nez de Lecea, C.; Calo, J. M. Energy Fuels 1993, 7, 146. (4) Garcı´a-Garcı´a, A.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1996, 41, 293. (5) Garcı´a-Garcı´a, A.; Illa´n-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C. Fuel 1997, 6, 499. (6) Garcı´a-Garcı´a, A.; Chincho´n-Yepes, S.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C. Energy Fuels 1997, 11, 292. (7) Garcı´a-Garcı´a., A.; Illa´n-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C. Fuel Process. Technol. 1999, 61, 289. (8) Garcı´a-Garcı´a, A.; Illa´n-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C. Spain Patent 1994, P9400104.
technology), (vi) no need to recover the catalytic agent from the gasification residue, (vii) the possibility of obtaining samples with proper geometry and mechanical resistance to withstand abrasion phenomena, and (viii) no significant evolution of undesirable reaction products such as N2O and CO at temperatures around 300 °C.9,10 In the presence of oxygen, an enhancement in the NOx reduction by carbon has been observed.10-15 However, the major disadvantage in the use of carbon is the consumption by combustion with oxygen. In fact, considerable amounts of CO and CO2 were produced during this reaction (even at temperatures as low as 300 °C), yielding carbon losses of 30% throughout 2 h of reaction.13 Most of the carbon consumption is due to the combustion with O2, because the concentration of O2 in the feed is much higher than that of NO in the outlet gas12,13 and because of the presence of NO2, which is more reactive than O2 and which is formed by reaction of NO and O2. The large combustion of carbon, that reduces its usefulness, justifies the search for a metalcarbon system exhibiting a high activity for the NO(9) Garcı´a-Garcı´a, A.; Illa´n-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C. Coal Sci. Technol. 1995, 24, 1787. (10) Garcı´a-Garcı´a, A.; Illa´n-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C. Energy Fuels 1999, 13, 499. (11) Inui, T.; Otowa, T.; Takegami, Y. Ind. Eng. Chem. Prod. Res. Rew. 1982, 21, 56. (12) Yamashita, H.; Yamada, H.; Tomita, A. Appl. Catal. 1989, 78, L1. (13) Yamashita, H.; Tomita, A.; Yamada, H.; Kyotani, T.; Radovic, L. R. Energy Fuels 1993, 7, 85. (14) Ma´rquez-Alvarez, C.; Rodriguez-Ramos, I.; Guerrero-Ruiz, A. Carbon 1996, 34, 1509. (15) Illa´n Go´mez, M. J.; Salinas Martı´nez de Lecea, C.; LinaresSolano, A.; Radovic, L. R. Energy Fuels 1998, 12, 1256.
10.1021/ef0101208 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/24/2002
570 Energy & Fuels, Vol. 16, No. 3, 2002
carbon reaction with a relatively low activity for the O2carbon reaction. It has been demonstrated that NOx can be effectively reduced by potassium supported on carbon between 300 and 500 °C.15-17 The influence of different variables, such as rank of the coal precursor, heat treatment of the samples, and the corresponding surface areas, were properly analyzed.15 However, questions concerning the possibility of increasing the selectivity toward the NOxcarbon reaction against the O2-carbon reaction and the influence of potassium content on the reaction, with regard to the non-catalyzed reaction, remain still unsolved. Thus, the purpose of this paper is to analyze the effect of the potassium content and the catalyst addition procedure in the NOx reduction by coal briquettes in the presence of oxygen. For that, and relying on the results obtained in previous papers in the absence of oxygen,4-7 two different series of potassium-containing coal briquettes have been prepared, adding the alkali as an humic acid, or in a combined method (KOH + humic acid). Experimental Section Sample Preparation and Characterization. A Spanish high volatile A bituminous coal, (A3 with a 7.7 wt % ash content), was used as coal precursor for this study. The commercial humic acid used in this study has a total humic extract of 16% w/w and a potassium content of approximately 0.049 g/cm3. Its characterization has been presented elsewhere.5 The method of coal-briquette preparation has been described in detail in the literature.5,6,8,9 For the present study, two series of briquettes have been prepared: (i) for the first series, (named HA), 10 g of coal was impregnated with a variable binder volume (4-12-24 mL, respectively), and (ii) for the second series of briquettes, (named HO), different amounts of KOH were dissolved in humic acid using a fixed humic acid/ coal ratio (12 mL/10 g of coal). All the potassium-coal slurries were mixed for 30 min with stirring, dried at 110 °C, pressed at 1-2 kg/cm2, and pyrolyzed in N2 for 2 h at 700 °C. The highest KOH-content sample of series HO was used directly or after a water-washing step, consisting of stirring with distilled water for 30 min. Potassium content was determined after the pyrolysis step by ICP-AES. For this purpose, the metal was extracted from the samples by refluxing them in 1 M HCl for 8 h. The briquettes are designated by the corresponding series (HA or HO) and the name of the parent coal, A3, followed by their potassium content (in weight percent). Ash contents of samples were determined by burning them in a muffle furnace at 650 °C over a period of 12 h. The textural characteristics of fresh and selected reacted samples were determined by physical adsorption of CO2 (at 0 °C) and N2 (at -196 °C) in an automatic volumetric system (Autosorb-6, Quantachrome). The identification of the mineral components in coals and the catalyst species in briquettes was carried out by X-ray diffraction (2002 Seifert powder diffractometer, using a CuKR radiation with graphite monochromator and Na (Tl) scintillation detector, 35 mA and 42 kV). The scanning rate was 2°/min for 2θ from 6° to 90°. (16) Kapteijn, F.; Mierop, A. J. C.; Abbel, G.; Moulijn, J. A. J. Chem. Soc. Commun. 1984, 1085. (17) Illa´n Go´mez, M. J.; Raymundo-Pin˜ero, E.; Garcı´a-Garcı´a, A.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C. Appl. Catal. B 1999, 20, 267.
Garcı´a-Garcı´a et al. Table 1. Sample Preparation and Characterization sample
HA/coal KOH/coal yield (mL/g) (g/g) (%)
HA-A3-1.8 HA-A3-4.7 HA-A3-9.7
0.4 1.2 2.4
HO-A3-6.6 HO-A3-9.4 HO-A3-14.7 HO-A3-5.8wash
1.2 1.2 1.2 1.2
a
0.056 0.113 0.169 0.169
ash (%)
SN2 SCO2 (m2/gC) (m2/gC)
73.2 72.0 68.0
14.3 19.6 26.8
(a) (a) (a)
561 352 286
64.6
23.7 28.1 36.2 20.8
(a) (a) (a) 71
161 272 (a) 648
70.7 70.7
Practically negligible.
Finally, the evaluation of the surface potassium/carbon ratio in selected samples was determined by XPS. The corresponding spectra have been obtained with a VG-Microtech Multilabel electron spectrometer, by using the Mg KR (1253.6 eV) radiation of twin anode in the constant analyzer energy mode with pass energy of 50 eV. Pressure of the analysis chamber was maintained at 5 × 10-10 mB. The binding energy and the Auger kinetic energy scale were regulated by setting the C1s transition at 284.6 eV. The accuracy of BE and KE values was (0.2 and (0.3 eV, respectively. The BE and KE values were obtained by using the Peakfit Program implemented in the control software of the spectrometer. NOx-Carbon Reaction Study. The experimental device used to carry out the NOx-carbon reaction consists of a fixed bed reactor coupled to a gas chromatograph Hewlett-Packard 5890 Serie II, equipped with a switched dual columns system (Porapak Q 80/100, for separation of CO2 and N2O, and Molecular Sieve 13X, for O2, N2, CO, and NO) joined by a sixway valve with a restriction that avoids a pressure drop when the second column is by-passed. A chemiluminiscence NOx analyzer (Thermo Environmental Inc., model 42H) is also used for NO2 determination. The isothermal NOx-carbon reactions have been carried out at 300° and 350 °C for 3 h, using a reaction mixture of 0.5%NO/ He + 5%O2/He (60 mL/min) and a sample mass of 300 mg. Due to the inner diameter of the fixed bed reactor used for the experiments and as briquettes present a cylinder-shaped geometry (15 mm in diameter × 10 mm in height), samples were ground to a particle size between 2 and 2.83 mm to introduce them into the reactor. Experiments performed with other particle sizes (1.5 < Φ < 2.0 mm or Φ < 1.5 mm) showed that reaction rate control existed, therefore, the NOx activity and the selectivity were not significantly changed using different particle sizes under our experimental conditions. The samples were previously heated in helium until the temperature of reaction was reached, and then, the experiment was initiated by substituting He by the reaction mixture, whose NO/NO2 ratio is very high because the gas mixture is the sum of two streams mixed as near as possible to the microoreactor inlet. The isothermal reactivity results have been expressed as: the % NOx reduced after 3 h of experiment, and a selectivity factor, F, which has been used to determine the extent of NOx reduction against oxygen combustion.15 This value is calculated according to the expression:
F ) (µmol NOred)/(2 µmol CO2 + µmol CO) F shows values in the range of 0-1, a selectivity factor of 1 means that the carbon consumption is due to the NOx reduction, while a factor of 0 means that the carbon is consumed only by combustion with oxygen.
Results and Discussion Sample Characterization. Table 1 shows the sample nomenclature, including the potassium contents in weight percent, the variables involved in the prepara-
NOx Reduction by K-Containing Coal Briquettes
tion of the samples (the humic acid-to-coal ratio and the KOH-to-coal ratio), the yields of the pyrolysis process (700 °C), the ash content, and the specific surface areas of the briquettes. By modifying the preparation variables, the following trends should be noted: (i) as expected, the potassium content of the briquettes increases with increasing humic acid/coal ratio and KOH/coal ratio; (ii) there is a dramatic decrease in potassium content (and also in the ash content) after performing a washing step with water for 30 min, (from 14.7 to 5.8% in K) in agreement with previous results concerning activated carbons with potassium remaining from the activation process with KOH;15 (iii) the pyrolysis yields slightly decrease with the increase in humic acid/coal ratio, due to the higher content in volatile matter of the binder with respect to the coal A3 (48.1 against 28.1%); (iv) an increase in the humic acid/coal ratio produces an increase in the briquette ash content due to the high ash content of the humic acid (21%); and (v) negligible surface areas, measured by N2 adsorption, are shown by all the samples. On the other hand, the micropore surface area, measured by CO2 adsorption at 273 K, decreases with the binder contentsfrom 561 to 286 m2/g for the series HA, according to the idea that a binder can block the narrow microporosity.18 Regarding the series HO, the micropore surface areas are even lower, reaching a negligible value for the highest potassium content sample. In the washed sample, the micropore and supermicropore surface area increase upon potassium removal by water washing (sample A3-5.81 wash). Concerning the characterization and identification of the potassium species in samples, XRD analysis provided interesting information. Samples with a potassium content high enough to be well-characterized, such as HA-A3-9.7, HO-A3-6.6, HO-A3-9.4, and HO-A314.7, show the presence of K2SO4 (arcanite) and KAlSiO4 (kalsilite) (as a consequence of the interaction between potassium and the mineral matter of the coal and/or the humic acid, in fact, quartz, kaolinite, and gyspum have been identified in the original coal19). Also the presence of KHCO3 has been evidenced. Previous works concerning potassium-containing coal chars reported the presence of potassium carbonate in potassium-rich samples.15 On the other hand, the washed briquette, HO-A35.8wash shows the presence of arcanite and kalsilite, but no carbonated species are observed; therefore, this species was removed during the water-washing step. In fact, the solubility of KHCO3 is higher than those attributed to K2SO4 and KAlSiO4.20 NOx-Carbon Reaction. Figure 1a compares the NOx reduction capabilities of the different HA series briquettes with the potassium-free char (A3-0), as a function of time. It is observed that the potassium incorporated in the humic acid is an active catalyst for the reduction of NOx in the presence of oxygen. A detailed analysis of Figure 1a reveals that the activity increases with the potassium content, up to a certain level. Similar behavior was observed with different series of activated carbons doped with potassium and coal briquettes during the NO reduction by carbon in (18) MacDonald, J. A. F.; Quinn, D. F. Fuel 1999, 77, 61. (19) Garcı´a-Garcı´a, A. Ph.D. Thesis, University of Alicante, 1997. (20) Lide, D. R. Handbook of Chemistry and Physics, 74th ed.; CRC Press: Boca Raton, Fl, 1993-1994.
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Figure 1. NOx reduction at 350 °C (a) HA series, (b) HO series.
the absence of oxygen.6,21,22 Also, Cerfontain et al.,23 for the case of carbon gasification by CO2 catalyzed by potassium, observed that the corresponding gasification rate did not increase continuously with the catalyst content. On the contrary, there was a content of catalyst, for which an additional increase did not result effective. This behavior seemed to be a consequence of the enlargement in the size of potassium “clusters”, and therefore, their diminution in activity. Regarding the activity profile of the two highestpotassium-content samples (HA-A3-4.7 and HA-A39.7), a curious trend is observed: low values of activity are shown at the first step of the reaction, followed by a rising in activity until reaching a maximum value, which is maintained for HA-A3-4.7. This behavior could be justified as follows. At the beginning of the reaction, the access of the reactive gases to the active sites is hindered because the binder can block the porosity and, a fraction of the catalyst can be nonaccessible to the reaction gases; but, becoming accessible when the carbon is being progressively consumed. These types of observations are supported by the work of Meijer et al.24 for the CO2 gasification in alkaline carbonate-carbon black systems, where the increase of burnoff profiles with the reaction time was attributed to a higher availability of the catalyst when increasing the consumption of carbon. Thus, to verify that hypothesis, data concerning porosity of the samples after the NOx reaction were determined. The analysis of the surface areas included in Tables 1 and 2 shows a remarkable enhancement in the adsorption capacity after the reaction (21) Illa´n-Go´mez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartı´nez de Lecea, C. Energy Fuels 1995, 9, 97. (22) Illa´n-Go´mez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartı´nez de Lecea, C. Energy Fuels 1995, 9, 104. (23) Cerfontain, M. B. Ph.D. Thesis, University of Amsterdam, 1986. (24) Meijer, R.; Weeda, M.; Kapteijn, F.; Moulijn, J. A. Carbon 1991, 29, 929.
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Table 2. N2 and CO2 Surface Areas of Briquettes after NOx-Carbon Reaction at 350 °C sample HA-A3-1.8 HA-A3-4.7 HA-A3-9.7 HO-A3-6.6 HO-A3-9.4 HO-A3-14.7 HO-A3-5.8wash
SN2 (m2/gC)
SCO2 (m2/gC)
439 371 71
465 447 502
5 37 (a)
559 320 (a)
a Nondetermined because of the extension of sample gasification.
of these two briquettes. This confirms that the reaction produces an opening of the porosity. Consequently, the reaction gases will access to new active sites and the activity will increase.25 Figure 1b collects the evolution of NOx reduction curves at 350 °C for the HO series, including the sample HA-A3-4.7 as a reference, and those prepared adding different amounts of KOH to a fixed HA/C ratio (HOA3-6.6, HO-A3-9.4, HO-A3-14.7, and HO-A3-5.8wash). The profile of the curve exhibited by the sample HOA3-5.8wash is very different to the rest and will be discussed later on. Nevertheless it is quite similar to those presented by potassium-containing coal chars, reported in a previous paper.15 For the rest of the samples, it can be seen that as the KOH loading increases, the samples display a decrease in activity. Regarding the porosity of these samples after the reaction, they also exhibit an increase in the porosity. For all the samples, the N2 surface area is lower than CO2 surface area, even after the reaction. This fact is a consequence of activated diffusion of N2 at 77 K in the very narrow micropores and has been also reported in a kinetic study of the NO-char reaction at low carbon conversions.26 If the activity of series HA and series HO is compared, using HA-A3-9.7 and HO-A3-9.4, because they exhibit very similar potassium contents, it can be seen that their reduction profiles are very different. The briquette belonging to series HA, presents an increase in activity, reaching a maximum of almost 50% of reduction during 20 min, followed by a progressive decrease in activity. Conversely, the briquette belonging to series HO exhibits a very slight and progressive increase in activity during the experiment. Analysis of the products evolved during the reaction (see Figure 2) provides valuable complementary information about the different behavior of these samples. Figures 2a and 2b present the results for sample HAA3-9.7 and HO-A3-9.4, respectively. The evolution of the reactive compounds (NOx and O2) and the corresponding products (N2, N2O, CO, and CO2) for briquette HA-A3-9.7 at 350 °C is presented in Figure 2a. The reaction rate increases with the reaction time up to a top level, manifested by a decrease in the NOx and O2 values. The level of N2, main nitrogen product, increases with the reaction time as well as CO and CO2 (oxygen products). N2O evolution is not significant, in fact the ratio N2O/N2 is around 0.06, which (25) Bansal, R. Ch.; Donnet, J.; Stoeckli, F. Active Carbon; Marcel Dekker: New York, 1988. (26) Li, Y. H.; Radovic, L. R.; Lu, G. Q.; Rudolph, V. Chem. Eng. Sci. 1999, 54, 4125.
Figure 2. Product evolution during NOx reduction at 350 °C (a) briquette HA-A3-9.7, (b) briquette HO-A3-9.4.
Figure 3. Loss of weight curves during NOx reduction at 350 °C of HA and HO series samples.
results very positive, owing to the current concern originated by the emission of N2O and its relation to the “greenhouse effect”.27,28 After the maximum in activity, product evolution (N2 and CO2) starts decreasing. In agreement with the NOx activity profile exhibited by sample HO-A3-9.4, the evolution of the reaction products increases in a slight and progressive form. Interestingly, the level of CO and CO2 emitted by this briquette is much lower than the briquette HA-A3-9.7; however, the percent of NOx reduced at the end of the experiment presents the same value for the two samples (35%). To observe the extent of carbon gasification during reaction, Figure 3 collects the global weight losses (estimated from the evolved amount of CO and CO2 (27) Takeshita, M.; Sloss, L. L.; Smith, I. M. N2O emissions from coal use, IEA Perspectives IEAPER/06; IEA Coal Research: London, 1993. (28) Houghton, J. T.; Jenkins, G. J.; Ephraums, J. J. Climate change; The IPCC Scientific Assessment, IPPC.; Cambridge University Press: Cambridge, U.K., 1990.
NOx Reduction by K-Containing Coal Briquettes
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during the reaction at 350 °C). A gradual “inhibition” of carbon gasification when increasing the loading of potassium hydroxide of briquettes belonging to series HO is observed. Sample HO-A3-5.8wash (obtained from A3-14.7 after a washing step) has been also included to show the important effect of the washing step on the enhancement of the carbon gasification. These results are in agreement with those reported by Kapteijn et al.,29 they observed that, apparently, a high loading of K2CO3 prevents the attack of the carbon by NO, and therefore the carbon consumption decreases. It is suggested that at such high loadings, the catalyst particles may hinder the access to active sites. The removal of the catalyst in excess promotes a better accessibility of the reaction gases to the carbon active sites.30 In fact, sample HO-A3-5.8wash presents higher values of surface area than its precursor (HO-A3-14.7). The more accessible porosity of the washed sample joined to the absence of KHCO3, (see sample characterization section), possibly preventing the attack of the carbon by the oxidant gases, as previously reported,29 can explain the relatively high activity of this sample compared with the unwashed samples. As previously commented, sample HA-A3-9.7 presents a more accentuated gasification than sample HOA3-9.4. In an attempt to gain more direct information regarding the phenomena which govern the differences between the behavior exhibited by these two samples, and excluding attributions due to a very different microporosity or to a different nature of the potassium species involved in the reaction (see sample characterization section), X-ray photoelectron spectroscopy was used to analyze the K state for these selected samples. Interestingly, both samples, although presenting a very similar potassium and ash contents, exhibit a quite different surface potassium/carbon ratio (0.17 and 0.38, respectively). The sample with lower global gasification (HO-A3-9.4) presents a higher K/C ratio (0.38). This seems to indicate that the carbon surface could be more “protected” against oxygen combustion. Table 3 summarizes the activity at two different reaction temperatures, 300 and 350 °C, for HA and HO series. Three parameters are included: the percentage of NOx reduced at 3 h of experiment (% Red), the
selectivity factor (F) and the CO/CO2 ratio of the products evolved. First, the catalytic activity of potassium has to be outlined, whatever the sample preparation procedure. The NOx reduction capacity of a bituminous coal char (A3-0) is considerably increased by addition of potassium. On the other hand, an increase in the reaction temperature supposes an increase in NOx conversion (except for the sample HA-A3-1.8, which presents a more or less constant activity). The selectivity factor, (F), exhibits a remarkable decrease with temperature. This means that, as the temperatures increases, the oxygen combustion is more favored than the NOx reduction. In the absence of potassium (see data corresponding to sample A3-0) the F factor varies in an opposite way. Considering each series, (HA and HO) the F factor increases with the potassium content. This allows us to assert that potassium exhibits a certain selectivity toward the NOx reduction against oxygen combustion or, in other words, high potassium contents seem to inhibit combustion by O2. Other reaction times studied (concretely 6 and 12 h for the case of sample HA-A3-4.7), did not change the value of selectivity significantly. The possibility of modifying the selectivity results was very interesting and it was not expected, considering that both reactions are related to the gasification of carbon. The beneficial effects of potassium as catalyst for the NOx-carbon reaction can be extended also to another important aspect, affecting the CO/CO2 ratio of the emitted products. The analysis of this parameter is very interesting due to the practical purpose of this research. The search of the optimization of the process requires a low CO/CO2 ratio. The minimization of this ratio is important due to two fundamental reasons: (i) the very well-known toxicity of CO and (ii) the conversion of carbon is minimum if most of the product is CO2. Table 3 also summarizes the corresponding CO/CO2 ratios of the products evolved at 300 and 350 °C, respectively, for all the briquettes and the char. At 300 °C the production of CO is negligible, however at 350 °C there is a certain proportion of that compound. But, the increase of potassium loading in briquettes, by binder addition or by KOH addition, produces a clear and favorable decrease of the CO/CO2 ratio, reaching a value nearly negligible for the sample with the highest potassium content. There is a general agreement concerning carbon-oxygen gasification reactions that both CO and CO2 are primary products during carbon oxidation. One of the most extended hypotheses states that the mechanism governing the ratio of products is CO production at the edge carbons of the graphitic planes and CO2 at inorganic sites.31,32 The decrease in the CO/CO2 ratio shown by the briquettes, with an increase in the catalyst content, are supported by the commented mechanisms. Comparing samples with similar potassium content but belonging to different series, samples HA-A3-9.7 and HO-A3-9.4, it is observed that the former is much less selective than the latter, despite of presenting both a similar reduction capacity (35.3%). One of the possible
(29) Kapteijn, F.; Mierop, A. J. C.; Moulijn, J. A. Proceedings of the 17th Carbon Conference, Kentucky, USA, 1985; p 181c. (30) Kosky, P. G.; Lamby, E. J.; McKee, D. W.; Spiro, C. L. Proceedings of the 16th Carbon Conference, San Diego, CA, USA, 1996; p 180.
(31) Du, Z.; Sarofim, A. F.; Longwell, J. P. Fundamental Issues in Control of Carbon Gasification Reactivity; Lahaye, J., Ehrburger, P., Eds.; NATO ASI Series: Cadarache, France 1991, p 192. (32) Phillips, R.; Vastola, F. J.; Walker, P. L., Jr. Carbon 1970, 8, 205.
Table 3. Kinetic Data Related to the HA and HO Series 300 °C
350 °C
CO/CO2c % reda
% reda
Fb
A3-0 HA-A3-1.8 HA-A3-4.7 HA-A3-9.7
0.5 13.6 7.4 2.4
0.02 0.26 0.65 0.70
(d) (d) (d) (d)
HO-A3-6.6 HO-A3-9.4 HO-A3-14.7 HO-A3-5.8wash
6.0 11.4 6.7 33.6
0.54 0.99 0.97 0.09
(d) (d) (d) (d)
sample
Fb
CO/CO2c
8.8 10.4 57.8 35.3
0.10 0.06 0.07 0.09
0.34 0.18 0.08 0.05
44.9 35.3 20.4 38.1
0.16 0.22 0.60 0.06
0.02 0.01 (d) 0.06
a % NO reduced after 3 h of experiment. b Selectivity factor (F): x calculated as (µmol NOred)/(2µmol CO2 + µmol CO), all of them estimated as integrated values. c Ratio of the integrated values (CO and CO2) emitted over a 3 h reaction. d CO signal practically negligible.
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responsibles of this phenomenon seems to be a different potassium distribution on the carbon surface, evaluated by measuring the surface potassium/carbon ratio by XPS. Therefore, a control of the selectivity can be attained by an appropiate choice of the preparation procedure of the briquettes. By combining these two factors, increasing the potassium content and using the preparation procedure of series HO, a highest selectivity has been achieved. In fact, sample HO-A3-14.7 exhibits a selectivity factor of 0.6 at 350 °C. It appears that the extent of the selectivity toward NOx reduction can be controlled by simply increasing the potassium loading by KOH. Conversely, the washed sample obtained from HO-A314.7, presents a 10 times lower value of selectivity with regard to the unwashed sample, at both temperatures studied (0.09 against 0.97 and 0.06 against 0.6, at 300 and 350 °C, respectively). These observations verify that the washing step is not beneficial in these conditions. In summary, important improvements related to the increase in selectivity have been made by controlling the preparation procedure of briquettes, compared to previous works.15 The “selective” behavior presented by briquettes at high potassium loadings are of paramount importance in order to search a proper system for a practical application. Moreover, the preparation procedure (binder + KOH) seems to be critical in terms of selectivity. Presently, the search of an effective and selective catalyst for the NOx-carbon reaction has become an important challenge. The most studied catalysts have been Co, Fe, Cu, Ca, and Ni.11-14,16 The emission of considerable amounts of CO and CO2, as a consequence of carbon gasification during the catalyzed NOx-carbon reaction, promotes the high values of burnoffs. This makes, in principle, unfeasible the practical application of this reaction.12 The finding of low burnoffs for certain briquettes (for example 1.5% for sample A3-14.7 at 350 °C) joined to the satisfactory values of selectivity reached and, additionally, the important decrease in the CO/CO2 ratio establish very
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hopeful results to lead to expanding the study of these processes. Conclusions In the study of the NOx reduction by potassiumcontaining coal briquettes the following conclusions were reached: (i) The potassium added during the preparation of coal briquettes, without performing additional heat treatments previous to the reaction, catalyzes NOx reduction in the presence of oxygen. The potassium-containing coal briquettes improve the performance of the original char in the following aspects: (1) increasing NOx conversion; (2) decreasing the CO/CO2 ratio in the evolved products; and (3) enhancing the selectivity toward the NOx reduction against oxygen gasification. (ii) A combined method of potassium addition using KOH and humic acid is advantageous with respect to the single humic acid addition. Samples with similar potassium contents, prepared from these two methods, show similar reduction capacities, but the selectivity is clearly higher for briquettes prepared with the humic acid + KOH method, with weight losses as low as 1.5% during an experiment of 3 h. These values show a clear improvement with regard to those previously published in the literature for the catalyzed NOx-carbon reaction. (iii) A surface enrichment of potassium species has been observed for the most selective briquette, consequently, a washing step, in the preparation process of the briquettes, is not beneficial because it decreases the selectivity for NOx reduction. The high activity of the washed sample toward oxygen and NOx promotes an important carbon gasification, with a dramatic weight loss of the sample that produces a remarkable deactivation of the sample during the reaction. Acknowledgment. This study was made possible by financial support from CICYT (PB98-0983). EF0101208
