Influence of SO2 in the Reduction of NOx

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Energy & Fuels 2005, 19, 94-100

Influence of SO2 in the Reduction of NOx by Potassium-Containing Coal Pellets A. Bueno-Lo´pez and A. Garcıá-Garcıá* Department of Inorganic Chemistry, University of Alicante, Spain, Ap. 99, E-03080 Alicante, Spain Received February 23, 2004. Revised Manuscript Received July 4, 2004

The influence of SO2 in the reduction of NOx by different potassium-containing coal pellets with catalyst loading between 7.9 and 16.8% was studied. For comparative purpose, a catalystfree sample was also checked. At several temperatures between 350 and 550 °C, 2-h isothermal reactions were carried out under NOx/O2/N2 and NOx/SO2/O2/N2 gas mixtures, and the results revealed that SO2 inhibits drastically the potassium-catalyzed NOx-carbon reaction. The amounts of SO2 retained on samples indicate that there is a direct relationship between these quantities and the potassium loading on samples. X-ray fluorescence, Fourier transform infrared-in situ, and X-ray diffraction techniques were used in order to prove the presence of sulfur on samples after the reactions with NOx/SO2/O2/N2 and to elucidate the nature of the sulfur species formed. The cause of the loss in activity seems to be related to the SO2 irreversible chemisorption on the catalyst, originating the formation of potassium-sulfur compounds.

1. Introduction Combustion is the prevailing mode of fossil energy utilization, and coal is the principal fossil fuel used for electric power generation. Due to its low cost and broad availability, it is expected that this fuel will be essential during the twenty-first century. However, emissions originated by coal combustion present negative effects to environment, and actual legislation requires the development and application of advanced coal combustion clean technologies.1 The main pollutants emitted to the atmosphere during the combustion of coal are SO2, a major contributor to acid rain, and NO and NO2 (generally denoted by NOx), which also promotes acid rain, reduction of atmospheric visibility, and production of tropospheric ozone.2 Considerable effort has been made and many techniques have been developed to reduce the emission of NOx from the combustion processes.2-10 Some of these techniques have been widely implemented. However, * To whom correspondence should be addressed. FAX: 965903454. E-mail: [email protected]. (1) Bee´r, J. M. Prog. Energy Combust. Sci. 2000, 26, 301-327. (2) Hu, Y.; Naito, S.; Kobayashi, N.; Hasatani, M. Fuel 2000, 79, 1925-1932. (3) Bleckner, B. 26th Symposium (International) on Combustion; The Combustion Institute, Pittsburgh, PA, 1996; p 3231. (4) Yamagishi, K.; Nozawa, M.; Yoshie, T. 15th Symposium (International) on Combustion; The Combustion Institute, Pittsburgh, PA, 1974; p 1157. (5) Wendt, J. O. L.; Sterling, C. V.; Matovich, M. A. 14th Symposium (International) on Combustion; The Combustion Institute, Pittsburgh, PA, 1973; p 897. (6) Zhou, C. Q. 26th Symposium (International) on Combustion; The Combustion Institute, Pittsburgh, PA, 1996; p 2091. (7) Gilot, P.; Guyon, M.; Stanmore, R. Fuel 1997, 76, 507-515. (8) Soud H. Suppliers of FGD and NOx Control Systems; IEA Coal Research: London, 1995. (9) Kohl, A.; Nielsen, R. Gas Purification; Gulf: Houston, TX, 1997; p 866.

minimization of the pollutant emissions by these technologies is generally not compatible with high combustion efficiency, and advanced methods for further reduction of NOx are needed to meet stricter requirements.2 Carbonaceous materials have been proposed as potential inexpensive reducing agents for NOx reduction under suitable operating conditions.11 This motivates the study of the NOx-carbon reactions, and many reports and several reviews examining kinetic and mechanistic aspects have been published.12-17 Our contribution to the topic of the NOx-carbon reaction is focused on the potential application of potassiumcontaining coal briquettes and pellets to the abatement of NOx in post-combustion gas streams. The influence of the preparation procedure of these materials has been extensively investigated, optimizing several variables such as the raw coal, catalyst loading, presence and nature of binder, and pyrolysis temperature.18-21 The effect of the gas composition in the NOx reduction process has also been studied, concretely, the influence (10) Hjalmarsson, A. NOx Control Technologies for Coal Combustion; IEA Coal Research: London, 1990. (11) Tomita, A. Fuel Process. Technol. 2001, 71, 53-70. (12) Moulijn, J.; Kapteijn, F. Carbon 1995, 33, 1155-1165. (13) Yamashita H.; Yamada H.; Tomita, A. Appl. Catal. 1991, 78, L1-L6. (14) Aarna, I.; Suuberg, M. Fuel 1997, 76, 475-491. (15) Illań-Go´mez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartinez de Lecea, C. Energy Fuels 1996, 10, 158-168. (16) Li, Y. H.; Lu, G. Q.; Rudolph, V. Chem. Eng. Sci. 1998, 53, 1-26. (17) Teng, H.; Suuberg, E. M.; Calo, J. M. Energy Fuels 1992, 6, 398-406. (18) Garcıá-Garcıá, A.; Illań-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martıńez de Lecea, C. Energy Fuels 1997, 11, 292-298. (19) Bueno-Lo´pez, A.; Garcıá- Garcıá, A.; Caballero-Sua´rez, J. A.; Linares-Solano, A. Fuel 2003, 82, 267-274. (20) Bueno Lo´pez, A.; Garcıá-Garcıá, A.; Salinas Martıńez de Lecea, C.; McRae, C.; Snape, C. E. Energy Fuels 2002, 16, 997-1003. (21) Garcıá-Garcıá, A.; Illań-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martıńez de Lecea, C. Fuel Process. Technol. 1999, 61, 289297.

10.1021/ef049950m CCC: $30.25 © 2005 American Chemical Society Published on Web 12/07/2004

Reduction of NOx by K-Containing Coal Pellets

Energy & Fuels, Vol. 19, No. 1, 2005 95

Table 1. Sample Characterization sample

N (%)

C (%)

H (%)

S (%)

asha (% )

Ktotal (%)

Ksurface (%)

pyrolysis yield (%)

SN2 DR (m2/g)

SCO2 DR (m2/g)

A3 A3-0 A3-7.9 A3-10.5 A3-16.8

1.7 1.5 1.6 1.4 1.9

80.1 76.0 70.0 65.3 59.0

3.2 3.0 1.2 1.2 1.6

0.7 0.2 1.7 1.4 1.0

7.7 13.4 21.1 23.4 31.8

b b 7.9 10.5 16.8

b b 15.7 19.2 33.0

66.8 69.6 70.6 68.8

b b b b b

221 370 669 709 b

a

Dry basis. b Negligible.

of NO2, O2, CO2, and H2O.22,23 SO2 is another important compound whose presence must be investigated for practical application of this technology. SO2 is generated during coal combustion due to the oxidation of sulfur,1 the principal organic impurity in coal. The objective of this study is to investigate the influence of SO2 in the reduction of NOx emissions by potassium-containing coal pellets. Concretely, the activity of a set of potassium-containing coal pellets for NOx reduction in the presence of SO2 and O2 and the interaction between SO2 and the potassium species that catalyze the NOx-carbon reaction have been analyzed. 2. Experimental Section 2.1. Sample Preparation. Three potassium-containing coal pellets with different metal loadings and one catalyst-free char were used in this study. The coal-pellet preparation procedure was described in detail elsewhere.19,24 A Spanish high-volatile A bituminous coal (A3) with a particle size of 0.1-0.2 mm and 7.7% (w/w) of ash content was used as coal precursor. A commercial humic acid, which contains a total humic extract of 16 wt % and 5 wt % (w/w) of potassium, was used as binder for pellet conformation (1.2 mL/gcoal). Different amounts of KOH, ranging from 0 to 0.17 gKOH/gcoal, solved in the minimum amount of water (about 1 mL/gcoal), were added to the coal/ binder mixture in order to increase the loading of catalyst introduced by the humic acid solution. A previous publication reported that this preparation method was better than that consisting of the addition of increasing amounts of humic acid as a unique source of potassium, in terms of selectivity toward NOx.25 The slurries were stirred during 30 min, dried at 110 °C, and conformed in pellets with 2 mm diameter and 8 mm average length. Finally, the pellets were pyrolyzed in N2 for 2 h at 700 °C. Pellets are denoted by A3-7.9, A3-10.5, and A3-16.8, 7.9, 10.5, and 16.8 being the potassium percentages (w/w) after the pyrolysis step, respectively. The catalyst-free sample was prepared by pyrolysis of the original coal under conditions similar to those used for pellets. This sample is denoted by A3-0. 2.2. Sample Characterization. For the estimation of the total amount of potassium, the metal was extracted from samples by refluxing them in 1 M HCl for 8 h. The resulting solutions were analyzed by ICP-AES. The ash contents after the pyrolysis step were gravimetrically determined by burning the samples at 650 °C in a muffle furnace for 12 h. The textural characteristics of the 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 apparent N2 and CO2 surface areas were estimated by applying the equation of Dubinin-Radushkevich to the experimental points of the adsorption isotherms. (22) Bueno-Lo´pez, A.; Garcıá-Garcıá, A. Carbon 2004, 42, 15651574. (23) Garcıá-Garcıá, A.; Illań-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martıńez de Lecea, C. Energy Fuels 1999, 13, 499-505. (24) Garcıá-Garcıá, A.; Illań-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martıńez de Lecea, C. Spanish Pat. No. P9400104, 1994. (25) Garcıá-Garcıá, A.; Illań-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martıńez de Lecea, C. Energy Fuels 2002, 16, 569-574.

The surface potassium contents were determined by X-ray photoelectron spectroscopy (XPS). The spectra were obtained with a VG-Microtech Multilabel electron spectrometer, by using the Mg KR (1253.6 eV) radiation of a 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, setting the C 1s transition at 284.6 eV. The accuracies of BE and KE values were (0.2 and (0.3 eV, respectively. The BE and KE values were obtained using the Peak-fit Program implemented in the control software of the spectrometer. The amounts of sulfur on samples before and after selected NOx reduction experiments were determined by X-ray fluorescence in a Philips-PW1480 device, and the nature of the species on samples before and after selected NOx reduction experiments were determined by X-ray diffraction (XRD) in a 2002 Seifert powder diffractometer, using a Cu R radiation with graphite monochromator and Na (Tl) scintillation detector. The scanning rate was 2°/min. 2.3. NOx-Carbon Reaction Study. Most of the NOx reduction tests were carried out at atmospheric pressure in a tubular quartz reactor coupled to a set of three FisherRosemount NDIR-UV specific gas analyzers for NO and NO2 (model BINOS 1004), CO and CO2 (model BINOS 100), and O2 and SO2 (BINOS 1001) monitoring. At several temperatures between 350 and 550 °C, 2-h isothermal reactions were performed. A 1-g amount of sample was used for each experiment. Before reactions, the samples were heated in nitrogen until the reaction temperature was reached and then the reaction mixture replaced the inert gas. Two reactive gas mixtures with a total flow of 620 mL/min were used: 0.2/5/ 94.8 NOx/O2/N2 and 0.2/0.1/5/94.7 NOx/SO2/O2/N2. A Fourier transform infrared (FTIR) spectrophotometer (model Infinit MI60 from Mattson) with a diffuse reflectance accessory (model COLLECTOR from Spectra Tech) was used for in situ monitoring of an isothermal reaction at 350 °C under the NOx/SO2/O2/N2 mixture. The spectra were recorded between 700 and 4000 cm-1. Sample A3-16.8 was used for this test. 2.4. SO2 Chemisorption Experiments. Isothermal experiments of SO2 chemisorption on ashes of selected samples were performed at 350 and 550 °C in a thermobalance StantonRedcroft (Series 780) using the gas mixture 0.2/99.8 SO2/N2 (total flow, 90 mL/min). The experiments were carried out with 10 mg of ashes, and they were conducted until constant weight. The ashes were obtained by burning the samples in a muffle furnace at 550 °C for 12 h.

3. Results and Discussion 3.1. Sample Characterization. Table 1 lists the set of samples along with their elemental compositions (N, C, H, and S), ash percentage, total and surface potassium percentage, and the yields of the pyrolysis process on a fresh sample basis. For comparative purposes, data of the original coal A3 have also been included in Table 1. Results of the characterization indicate that the higher the potassium loading, the higher the ash

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Energy & Fuels, Vol. 19, No. 1, 2005

Figure 1. NOx, O2, and SO2 profiles during isothermal reactions (2 h) at 350 °C with A3-7.9 under (a) NOx/O2/N2 and (b) NOx/O2/SO2/N2 (dotted lines: references).

contents and, in consequence, the lower the C content, as expected. Whatever pellet, the Ksurface percentage is higher than the Ktotal one, which points out the accumulation of catalyst on the surface. The pyrolysis yields remained more or less constants, which indicate that the elimination of carbon during the pyrolysis step was not affected by the potassium content (assuming that the metal losses were negligible under these conditions). The apparent surface areas determined by CO2 adsorption at 0 °C increased with the potassium loading, which is reasonable considering that KOH is an habitual activating agent used for the preparation of high surface area carbon materials.26 However, when activated carbons are prepared with KOH, the potassium species must be removed from carbon by acid washing after the pyrolysis step; otherwise all or part of the porosity remains closed.26 This seems to be the case of the highest potassium-loading sample, of which the surface area, determined by CO2 adsorption, is under the detection limit of this technique (about 2 m2/g). This is in accordance with the negligible values obtained by N2 adsorption at -196 °C for all the samples. These results indicate that the gas diffusion limitations affect N2 in all the samples and CO2 in the highest potassiumloading sample. Differences observed between N2 and CO2 characterization must be mainly attributed to the temperature used to perform the adsorption isotherms (-196 °C vs 0 °C, respectively). 3.2. NOx-Carbon Isothermal Reactions. At several temperatures between 350 and 550 °C, 2 h isothermal reactions were carried out with the set of potassiumcontaining pellets and the catalyst-free char. As an example, parts a and b of Figure 1 show the NOx, CO2, and SO2 (only Figure 1b) profiles obtained during the experiments performed at 350 °C with the sample A3(26) Lozano-Castello´, D.; Lillo-Ro´denas, M. A.; Cazorla-Amoro´s, D.; Linares-Solano, A. Carbon 2001, 39, 741-749.

Bueno-Lo´ pez and Garcıá-Garcıá

Figure 2. Isothermal reactions (2 h) carried out with potassium-containing coal pellets: (a) mg of NOx/gsample and (b) burnoff (%) (open symbols, NOx/O2/N2 mixture; solid symbols, NOx/SO2/O2/N2 mixture).

7.9 and the gas mixtures NOx/O2/N2 and NOx/SO2/O2/ N2, respectively. Full lines represent data corresponding to these experiments, and dotted lines represent the reference data obtained in blank experiments (without sample). In Figure 1a, the NOx profile reached early the stationary level, which was lower than the reference value. The NOx removal after 2 h was 10%. The emission of CO2 during the reaction proved the gasification of carbon (note that CO evolution was negligible, in accordance with our previous results23). On the other hand, the reduction of NOx as well as the CO2 evolution stopped after 15 min approximately in the presence of SO2 (Figure 1b), and little SO2 consumption was observed. These experiments evidenced that SO2 inhibits the potassium-catalyzed NOx-carbon reaction. The amounts of NOx reduced during all the isothermal reactions performed with the set of potassium-containing pellets and their burnoffs estimated from CO2 emissions have been included in Figure 2a,b, respectively. For comparison, data corresponding to experiments performed with the catalyst-free char have also been collected in Figure 3a,b. Whatever the temperature, the inhibition by SO2 affected the potassium-catalyzed NOx-carbon reaction, which is deduced if data in the Figure 2a corresponding to the NOx/O2/N2 and NOx/SO2/O2/N2 gas mixtures are compared. As expected, NOx reduction is improved by temperature in experiments performed with the NOx/ O2/N2 mixture, while similar and little amounts of NOx were reduced in experiments performed with the NOx/ SO2/O2/N2 mixture. According to our previous results reported in ref 22, the inhibition by SO2 is much more important than that promoted by CO2 if present in the reaction mixture. For example, the presence of 4% CO2 in the gas mixture delayed the onset temperature of NOx reduction by A3-16.8 from 325 °C (NOx/O2/N2) to 425 °C (NOx/CO2/O2/N2), but the sample remained active above this temperature.



Figure 4. Amounts of SO2 removed from the reactive gas flow during isothermal reactions (2 h).

Figure 3. Isothermal reactions (2 h) carried out with the catalyst-free char: (a) mg of NOx/gsample and (b) burnoff (%) (open symbols, NOx/O2/N2 mixture; solid symbols, NOx/SO2/ O2/N2 mixture).

It is important to pay special attention to data in gray color included in Figure 2. These data correspond to experiments in which an uncontrolled increase of temperature was observed. This phenomenon affects sample A3-7.9 (lowest potassium loading) above 500 and 450 °C in the NOx/SO2/O2/N2 and NOx/O2/N2 gas mixtures, respectively. Note than only appreciable sample burnoffs (Figure 2b) are reached in these experiments. The error in the estimation of the NOx-reduced values in the experiments with uncontrolled increase of the temperature is about 15%, while in the rest of the experiments it is less than 5%. According to our previous studies,20,23 the uncontrolled increase of temperature is a consequence of the exothermic O2-carbon reaction. In these studies, we observed that the potassium catalyst induces the selective gasification of carbon with NOx in NOx/O2 gas mixtures; that is, this catalyst inhibits the O2carbon reaction and favors the NOx-carbon one, which is decisive for the successful abatement of NOx by coal pellets. The catalyst loading is a key factor in this selective behavior, and the higher the potassium loading the higher the sample selectivity toward NOx. The uncontrolled increase of the temperature must be avoided in practical application, both for safety hazard and for the efficiency of the NOx reduction process. Therefore, pellets with high potassium loading (optimum about 16.8 wt %) must be used, and the proper tools for heating removal must be included in the reactors. The catalyst-free char (Figure 3) showed different behavior than pellets. At 350 and 400 °C the reduction of NOx and the char burnoffs were not affected by SO2, and very low and similar data were obtained with the NOx/SO2/O2/N2 and NOx/O2/N2 gas mixtures (black symbols). During these experiments the temperature remained stable. However, above 450 °C, the uncontrolled increase of temperature was observed. Therefore, to establish a comparison above 450 °C is not possible, and the differences observed in Figure 3 due to SO2 cannot be correctly interpreted.

The amounts of SO2 removed from the gas flow during all the isothermal reactions were quantified, and they have been compiled in Figure 4. The x-axis indicates the sample’s potassium loading. Whatever the temperature, results corresponding to char A3-0 are almost zero, and there is a relationship between potassium loading and SO2 removal, which leads us to think about the catalyst deactivation by poisoning with SO2. It is important to remember that sulfur compounds, mainly H2S and SO2, commonly poison metallic catalysts.27 In Figure 4, the only data out of the linear trends (gray symbols) correspond to experiments in which uncontrolled increase of the temperature was observed. These values are much higher than the expected amounts considering the linear trends. In an attempt to elucidate the causes of this irregular behavior, Figure 5 shows the NOx, O2, and SO2 reduction curves corresponding to experiments carried out with the different potassium-containing coal pellets at 550 °C. Temperature profiles during reactions are also included. NOx and O2 consumption were observed at the beginning of the reactions performed with the high potassium content pellets (A3-16.8 and A3-10.5; Figure 5a,b, respectively), which decreased progressively until negligible values were reached. The sample burnoffs after 2 h were only 1.9 and 1.7% for A3-16.8 and A3-10.5, respectively. Note that the temperature remained stable throughout these experiments. SO2 removal was important at the beginning of these experiments, and it maintained measurable values after 2 h. This behavior supports that NOx reduction ceases as SO2 “blocks” or “poisons” the catalyst active sites of potassium. Profiles obtained with A3-7.9 (Figure 5c) were significantly different. During an early period of a few minutes, the SO2, NOx, and O2 profiles in Figure 5c were qualitatively similar to the counterpart profiles in Figure 5a,b. However, the temperature increased during the experiment performed with A3-7.9, which improved the SO2, NOx, and O2 conversions. Note that NOx and O2 showed parallel profiles (Figure 5c) with maximum values after 40 min of reaction. During this period, significant CO2 emission was detected and the sample burnoff increased from 2% (measured at treacn ) 10 min) up to a 50% (measured at treacn ) 50 min). Note that the SO2 profile in the Figure 5c is different from those of NOx and O2, which seem to indicate that different chemical processes governed the SO2 removal and the NOx and O2 removals. Differences between the SO2 profile and the NOx and O2 ones and the fact that SO2 (27) Bartholomew, C. H. Appl. Catal. A 2001, 212, 17-60.

98



Figure 6. Total sulfur amounts on samples before and after isothermal reactions (2 h) at 350 °C under the NOx/O2/N2 and NOx/SO2/O2/N2 mixtures, determined by XRF.

Figure 7. FTIR-in situ spectra obtained at 350 °C under the NOx/SO2/O2/N2 mixture with sample A3-16.8. Figure 5. NOx, SO2, and O2 reduction percentages during isothermal reactions (2 h) at 550 °C carried out with (a) A316.8, (b) A3-10.5, and (c) A3-7.9.

removal by the catalyst-free char is almost zero (Figure 4) lead us to reject the possibility of carbon gasification by SO2 under our experimental conditions. Finally, note that the SO2 profile in the Figure 5c shows a second stage of SO2 removal after 60 min of reaction. At this moment, the sample burnoff is 70%, and, therefore, the sample’s ashes are easily accessible. It is important to mention that a similar SO2 profile (removal in two steps) was observed during the experiment performed with A3-7.9 at 500 °C, where uncontrolled increase of temperature was also observed. Therefore, the irregular behavior of A3-7.9 at 500 and 550 °C, which data in Figure 4 are out of the linear trends, must be attributed to the SO2 removal improvement during the first stage (due to the uncontrolled increase of the temperature) and the participation of ashes once high sample burnoffs were reached. 3.3. Assessment of Sulfur Retention on Samples by FRX. To confirm that SO2 was retained in the samples during the reactions carried out in the presence of SO2, the sulfur contents on samples before (original samples) and after isothermal reactions at 350 °C were determined by X-ray fluorescence. Figure 6 collects these data. X-ray fluorescence provides information about the total sulfur content, and, therefore, the term Sx includes both inorganic and organic species.

The amounts of sulfur on pellets before and after reaction under the NOx/O2/N2 mixture are very similar, which is reasonable considering the low mass variation during these experiments (less than 1% whatever sample). These amounts of sulfur come from the sulfur in the raw coal and in the humic acid binder. On the contrary, the amounts of sulfur on pellets increased considerably after reaction under the NOx/SO2/O2/N2 mixture, in agreement with the hypothesis of an irreversible SO2 retention in the catalyst. Besides, if the slopes of the lines drawn in Figure 6 are compared, it can be deduced that the higher the potassium loading, the higher the sulfur uptake (if not, both lines would be parallel). This finding confirms the relationship between potassium loading and SO2 removal from the gas mixture observed in Figure 4. 3.4. Identification of the Nature of the S-Species Created as a Consequence of the SO2 Retention Process. Once the SO2 retention in the samples is confirmed, the next step is to identify the sulfur species formed. Two cases will be considered: (i) the nature of the species formed at low sample burnoffs and (ii) the nature of the species formed at high sample burnoffs (after the second stage of SO2 retention, as shown in Figure 5c). 3.4.1. Low Sample Burnoff. An isothermal reaction at 350 °C carried out with sample A3-16.8 and the gas mixture NOx/SO2/O2/N2 was monitored by in situ FTIR. Figure 7 shows a detail of the corresponding spectra between 1000 and 1250 cm-1. The spectrum denoted as “original” was recorded before the reaction (in helium



Table 2. SO2 Chemisorption on Ashes at 350 and 550 °C mg of SO2 retained/gash sample

at 350 °C

at 550 °C

ashes of A3-0 ashes of A3-7.9

0 6

0 136

at 350 °C) and 0′, 10′, 20′, 40′, and 50′ denote spectra after different times of reaction under NOx/SO2/O2/N2. As can be seen, the original sample does not present any peak in this range of wavenumbers. However, when the reactive mixture replaced the inert gas, the formation of a peak around 1100 cm-1 was observed, whose intensity increased with the reaction time. This peak could be attributed to the absorption of the “SdO” functionality. For example, the main adsorption peak characteristic of K2SO4 may appear at 1100 cm-1, which suggests that some potassium-sulfur compound is formed during the reaction. It is important to mention that this is the only peak that can be assigned to potassium-sulfur compounds in the wavenumber range recorded (700-4000 cm-1) and that other bands typical of sulfur-potassium species were not detected under our experimental conditions. This finding suggests that the inhibition of the potassium-catalyzed NOx-carbon reaction when SO2 is present in the gas mixture is related to the chemisorption of SO2 on the catalyst and creation of potassiumsulfur compounds. It is reasonable to assume that this species, due to its stability, does not present catalytic activity. As previously reported in the literature,16,28,29 the catalytic activity of potassium (and also other metals) is based on a redox cycle in which the catalyst is consecutively oxidized by gas species (NOx or O2) and reduced by carbon. The catalytically active species of potassium have postulated to be mainly suboxides with unknown stoichiometry (KxOy). It is expected that this redox cycle does not occur in highly stable potassiumsulfur species such as K2SO4. 3.4.2. High Sample Burnoff. To identify the sulfur species created at high sample burnoff (during the isothermal reactions at 500 and 550 °C carried out with sample A3-7.9), different experiments were performed by thermogravimetry and X-ray diffraction. 3.4.2.1. SO2 Chemisorption Studied by Thermogravimetry. SO2 retention in the second stage (see Figure 5c), which arises at high sample burnoffs, could be caused by SO2 capture on the sample ashes. To clarify this aspect, SO2 chemisorption experiments were performed using ashes both of the potassium-free char (A3-0) and sample A3-7.9. Table 2 is a compilation of the amounts of sulfur dioxide captured during the chemisorption experiments at 350 and 550 °C. Data presented in Table 2 indicate that SO2 was not retained in the ashes of the char. On the contrary, the ashes of pellets are able to capture SO2, supporting that the SO2 retention at high sample burnoffs could occur on the potassium species of ashes. The retention increased with temperature, as expected considering that the temperature usually favors the chemisorption processes. (28) Kapteijn, F.; Moulijn, J. A. Fuel 1983, 62, 221-227. (29) Moulijn, J. A.; Kapteijn, F. In Carbon and Coal Gasification; Figuereido, J. L., Moulijnm, J. A., Eds.; Martinus Nijhoff: Dordrecht, The Netherlands, 1986; p 181.

Figure 8. XRD characterization of A3-7.9 (original sample and after isothermal reactions (2 h) at 350 and 550 °C under NOx/SO2/O2/N2).

3.4.2.2. Elucidation of the Nature of the Sulfur Species Created. Sample A3-7.9 was analyzed by XRD after reaction at 550 °C under the NOx/SO2/O2/N2 mixture. For comparative purpose, diffractograms of the original sample and of the sample after reaction at 350 °C were also recorded and collected in Figure 8. The diffractogram recorded after reaction at 350 °C showed peaks that could be assigned to K2SO4, and peaks potentially originated by other potassium-sulfur species were not observed at this temperature. This diffractogram jointly with the NDIR results suggests that K2SO4 is formed at 350 °C. On the contrary, after reaction at 550 °C different peaks were observed, proving the presence of other species: Fe2O3. This species comes from the iron-rich mineral matter of the raw coal. K2Fe2O3/K2Fe2O4. These species are formed by interaction between the mineral matter of coal and the potassium precursor during the pyrolysis step (700 °C) and/or during the isothermal reaction. K2S2O3/K2S2O7. Three of the peaks shown in the diffractogram recorded after reaction at 550 °C could be assigned to these species. Considering that the reactive gas mixture is highly oxidizing, the formation of K2S2O7 seems most provable. However, the correct identification of the potassium-sulfur species formed would need further assessment. 3.5. Interpretation of the Mechanism of Catalyst Deactivation. There are many paths for catalyst decay, which can be grouped into six intrinsic mechanisms: poisoning, fouling, thermal degradation, vapor formation, vapor-solid/solid-solid reactions, and attrition/ crushing.27 Results presented in this study indicate that the mechanism of the potassium-catalyst deactivation by SO2 is poisoning. This mechanism is a chemical mechanism, and it can be described as a strong chemisorption of species on catalytic sites, thereby blocking these sites for catalytic reaction with a loss over time of catalytic activity. Despite only one concentration of SO2 that has been tested (0.1%) in this study, it is reasonable to assume that the SO2 concentration may affect the poisoning rate and the lifetime of pellets for NOx reduction. In the process studied in this work, the causes of the catalyst inhibition by SO2 poisoning could be two: (i) the chemisorption of SO2 to yield some pottasium-sulfur species decreases the number of active positions and (ii) the higher size of the SO2 molecule

100



Table 3. Percentages of Potassium Sulfated at Different Temperatures until Complete NOx-Carbon Reaction Inhibition Percentage of Potassium Sulfated sample

350 °C

400 °C

450 °C

500 °C

550 °C

A3-7.9 A3-10.5 A3-16.8

0.9 0.8 1.2

2.0 2.0 2.5

3.3 3.1 4.4

a 3.3 4.3

a 5.6 6.1

a

During these tests, the temperature increased out of control.

if compared with NO could block several active sites, otherwise available for catalytic activity. It is interesting to estimate the fraction of potassium atoms that become inactive until the NOx reduction stops completely. Table 3 is a compilation of these amounts, which have been calculated from the amounts of SO2 removed from the gas flow during the 2 h NOx reduction experiments. The total amounts of SO2 removed from the gas mixture have not been considered for these calculations but only the amounts since the beginning of the experiment until the NOx reduction activity ceases. The stoichiometric ratio S:K of 1:2, (corresponding to the formation of K2SO4) was assumed. Data have been expressed as a percentage of potassium poisoned with respect to the total amount of potassium in the sample. The percentages in Table 3 do not increase proportionally with the total amount of potassium on the samples but increase with temperature. This is expected considering that the chemisorption processes are usually promoted at higher temperatures. These data indicate that only a few percentage of the potassium in the samples (between 0.9 and 6.1%) is catalytically active. Causes are diverse and could be related to the low dispersion of the catalyst due to the low carbon surface

areas, the fact that only a few atoms of potassium are expected to be in contact with the carbon, or the fact that only a few atoms of potassium are allowed to follow the previously described redox cycle because of their own chemical nature. However, despite only a few percentage of potassium being active catalytically, the whole amount of potassium in the pellets plays a role in the reaction. Our previous studies19 pointed out that highloading pellets (16.8 wt % of potassium) are more selective toward NOx than low-loading ones (in example, 7.9 wt %). The role of the high percentage of potassium inactive catalytically is not completely understood at this moment and needs further studies. 4. Conclusions The presence of SO2 in the reactive gas mixture inhibits totally the potassium-catalyzed NOx-carbon reaction. A clear relationship is observed between the SO2 uptake during isothermal reactions and the loading of catalysts. The whole results obtained lead us to confirm that a process of catalyst deactivation is occurring, defined as a catalyst poisoning with creation of inactive sulfur-potassium species. The SO2 uptakes until complete NOx-carbon reaction inhibition has been determined. These amounts are directly related to the fraction of potassium catalytically active, which is lower than 6% whatever temperature (350-550 °C) and total potassium loading (7.9-16.8 wt %) tested in this study. Acknowledgment. This study was made possible by financial support from the Fifth Framework Program (TOMERED Project, No. ENK5-2002-00699) and MCYT (Project PPQ 2002-01025). EF049950M

Influence of SO2 in the Reduction of NOx

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