Energy & Fuels 2002, 16, 1425-1428
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Analysis of the Reaction Conditions in the NOx Reduction Process by Carbon with a View to Achieve High NOx Conversions. Residence Time Considerations Agustı´n Bueno-Lo´pez,† Jose´ Antonio Caballero,‡ and Avelina Garcı´a-Garcı´a*,† Departamento de Quı´mica Inorga´ nica, Universidad de Alicante, Spain, and Departamento de Ingenierı´a Quı´mica, Universidad de Alicante, Spain Received February 19, 2002
In this work the activity of potassium-containing coal-pellets against NOx at 450˚C was studied. The sample chosen presents high potassium content, 16.8% in wt, and was prepared from a humic acid+KOH mixture and a Spanish high volatile A bituminous coal. The effect of the NOx partial pressure, combined with that of the influence of the residence time as variables for the NOx reduction process were investigated in order to look for reaction conditions where high NOx conversions be reached and maintained during a long time. 100% of NOx conversion was reached and kept during a long time, employing a sample mass of 2 grams (residence time 1.8 s), regardless of the NOx partial pressure tested. A first reaction order, with respect to NOx partial pressure, was found from the fit of all the data, in agreement with the data reported in the literature. The rate constant estimated at the temperture of study was of 1.15 s-1.
The reaction of NO with carbon is of considerable interest both from a fundamental point of view as well as a pollution abatement strategy. This double interest has resulted in a wide variation of previous investigations concerning the effects of carbon surface area,1,2 the feed gas compositions,3,4 and the catalytic effect of metals.5,6 Besides, some works involving the kinetics and mechanisms have been reported.7,8 Regarding the most common kinetic parameters obtained, (more specifically the reaction order), there is a general consensus that this reaction can be considered first order with respect to the partial pressure of NO. However, reaction orders between 0.42 and 0.73 have also been reported, specially at high temperatures (1023-1073 K).8 The works dealing with the estimation of kinetic parameters from the catalyzed NOx-carbon reaction are much less numerous.9 In earlier publications, moderate activity and high selectivity toward NOx against oxygen combustion was * Corresponding author. Address: 99-E-03080-Alicante, Spain. Phone number: +34 965909419. Fax number: +34 965903454. Email:
[email protected]. † Departamento de Quı´mica Inorga ´ nica. ‡ Departamento de Ingenierı´a Quı´mica. (1) Illa´n-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C.; Calo, J. M. Energy Fuels 1993, 7, 146. (2) Calo, J. M.; Suuberg, E. M.; Aarna, I.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C.; Illa´n-Go´mez, M. J. Energy Fuels 1999, 13, 761. (3) Chan, L. K.; Sarofim, A. F.; Beer, J. M. Combust. Flame 1983, 52, 37. (4) Furusawa, T.; Tsunoda, M.; Tsujimura, M.; Adschiri, T. Fuel 1985, 64, 1306. (5) Yamashita, H.; Yamada, H.; Tomita, A. Appl. Catal. 1989, 78, L1. (6) 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. (7) Teng, H.; Suuberg, E. M.; Calo, J. M. Energy Fuels 1992, 6, 398. (8) Aarna, I.; Suuberg, E. M. Fuel 1997, 76, 475. (9) Guo, F.; Hecker, W. C. Prepr. Pap.-Am. Chem. Soc. Div. Fuel Chem. 1996, 41, 179.
reported by potassium-containing coal briquettes, mainly at high loadings of alkali.10,11 In addition to these findings, a very interesting CO/CO2 ratio of the emitted products close to zero was monitored.10,11 These positive aspects lead to: (i) the investigation of reaction variables (not previously analyzed in our laboratories) in order to look for reaction conditions where high NOx conversions be reached and maintained during long times and (ii) the determination of elemental kinetic parameters under the mentioned conditions with a view to scale up this promising process. In line with these issues, the main goal of this research has been to investigate the effect of the NOx partial pressure, combined with that of the influence of the residence time as a variable of the NOx reduction process by potassium-containing coal pellets. The reaction order and the intrinsic rate constant were determined from this set of experiments. The reaction conditions mentioned above were systematically explored to achieve high degrees of NOx conversion. In this research, the sample exhibiting the highest capacity of NOx removal per gram of sample reported in a previous work,11 2.0 gNOx/gsample, at 350 °C, was chosen. This sample is a coal briquette possessing a potassium content of 16.8% in wt, and was prepared from a humic acid + KOH mixture and a Spanish high volatile A bituminous coal. The preparation procedure and the characterization of this sample have been described elsewhere.11,12 The experimental device used to carry out the NOxcarbon reaction consists of a conventional U-shaped (10) Garcı´a-Garcı´a, A.; Illa´n-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martı´nez de Lecea, C. Energy Fuels 2002, 16, 569. (11) Bueno-Lo´pez A.; Garcı´a-Garcı´a A.; Linares-Solano A. Fuel Proc. Technol. 2002, 77, 301. (12) Garcı´a-Garcı´a, A.; Illa´n-Go´mez, M. J.; Linares, A.; SalinasMartı´nez de Lecea, C. Spain Patent P9400104, 1994.
10.1021/ef020041b CCC: $22.00 © 2002 American Chemical Society Published on Web 09/14/2002
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Figure 1. (a) Schematic diagram of the experimental setup and (b) picture of the microreactor and corresponding dimensions.
fixed-bed quartz microreactor (13 mm of inner diameter) coupled to a gas chromatograph Hewlett-Packard 5890 Series 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). The gas line after the microreactor was not heated and the analysis was performed using a gas chromatograph oven temperature of 50 °C. A chemiluminiscence NOx analyzer (Thermo Environmental, Inc., model 42H) was also used for NO and NO2 determination. Figure 1a shows the schematic diagram of the experimental setup and Figure 1b presents a picture of the microreactor. The isothermal NOx-carbon reactions were carried out at 450 °C for 2 h, using a reaction mixture of NOx/ 5%O2/He (100 mL/min). Different NOx partial pressures were tested: 180, 780, and 2000 ppms. Also, the influence of the residence time on the activity at these
Table 1. Reaction Conditions Selected for the NOx Reduction Tests Studied sample mass (g)
bed volume (cm3)
residence time (τ) (s)
0.3 1.0 1.3 2.0
0.46 1.52 1.97 3.04
0.27 0.91 1.18 1.82
different pressures was investigated. The summary of the different sample amounts used together with the estimated residence times are collected in Table 1 (the maximum sample mass tested, 2 g, corresponds to a bed height of 3.5 cm). Due to the inner diameter of the fixed bed reactor used for the experiments and as briquettes present a cylinder-shape geometry (15 mm in diameter × 10 mm in height), samples were ground to a particle size between 0.2 and 1.2 mm to insert them into the reactor. The samples were previously heated in helium
NOx Reduction Process by Carbon
Energy & Fuels, Vol. 16, No. 6, 2002 1427
Figure 2. NOx reduction percentages after 2 h of reaction as a function of the residence time for different NOx initial partial pressures.
until the reaction temperature was reached; 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, (NO/He + O2/He), mixed as close to the microreactor inlet as possible. A number of 12 runs were performed combining the different gas partial pressures together with the residence times. Figure 2 illustrates the percentage of NOx reduction achieved after 2 h of experiment as a function of the residence time (expressed in seconds). Each point corresponds to each kinetic run performed. The sample mass tested was progressively increased until reaching a NOx reduction percentage of nearly 100% at steadystate conditions. As clearly observed, this value was reached by using a sample mass of 2 g (related to a residence time of 1.8 s), regardless of the partial pressure tested. The curve traced by joining the points for every series show increasing profiles until reaching a “plateau”. Interestingly, the NOx conversion values are independent of the NOx partial pressure tested, under our experimental conditions. Results collected in Figure 2 are very valuable from a practical point of view taking into account that flue gases of postcombustion processes present very low NOx concentrations, even as low as 180 ppms, being much lower than those of O2 (5-10%). The theoretical support for the present kinetic analysis is based on a conventional mass balance inside the reactor assuming that the flow within the bed is plug flow:
Q[(NOx)0 + d(NOx)] - Q(NOx)0 ) -k′(NOx)n(O2)n′dV d(NOx) ) -k′(NOx)n(O2)n′dτ d(NOx) ) -k′(NOx)n(O2)n′ dτ and expressed as a function of partial pressures
d(PNOx) dτ
) -k(PNOx)n
(1)
Figure 3. Effects of residence times and NOx initial partial pressures on the NOx-carbon reaction. (Experimental data, solid symbols; estimated values, curves)
where the final k includes PO2, considered constant in all the experiments. By integrating this equation, the following expression is obtained:
PNOx[-τk(1 - n) + P(NOx)0(1 - n)]1/1-n
(2)
As the reaction proceeds, the carbon will be progressively consumed as a consequence of the global gasification reaction. The difference between the amounts of carbon present before and after the 2 h experiment was negligibly small, so that the carbon concentration could be assumed as constant. In fact, the burnoffs estimated after the 12 runs performed ranged between 0.2 and 3.1% (on a total sample basis). Besides, O2 concentration is assumed to be constant throughout the 2 h experiments due to the high selectivity toward NOx exhibited by the sample under study. Values of k rate constant (s-1)-, and n -reaction order with respect to PNOx-, were calculated by minimizing the sum of squares error between the experimental PNOx at each residence time and those calculated by eq 2. Figure 3 shows a combination of two representations providing valuable information about the reaction kinetics. On one hand, the amount of NOx reduced during every experiment (expressed in ppms) is plotted as a function of the residence time for the three different NOx partial pressures investigated. Besides, Figure 3 also gives the fit of the data (solid symbols) to the curves obtained by using expression 2 with the optimized parameters estimated (k of 1.15 s-1 and n of 1.05), resulting to be quite good. The reaction order estimated here (first order) is in agreement with that reported by many authors for the uncatalyzed7,8 and the CaOcatalyzed NO-carbon reaction.9 On the other hand, Figure 3 also shows the relationship between [ln(PNOx)o/
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Figure 4. Variation of the selectivity factor (estimated as µmol NOx reduced/µmol CO2 emitted) in terms of the residence time.
(PNOx)2h] and the residence time. A straight line can be drawn according to the first order reported. The variation of selectivity toward NOx reduction against oxygen combustion is of paramount importance owing to the practical objective of the present research. In agreement with this idea, Figure 4 shows the selectivity factor, obtained from the different experiments performed, versus the residence time. The values estimated are of great relevance, all of them lying inside a very satisfactory interval between 0.18 and 0.31. Only the experiments carried out with the lowest partial pressure (180 ppms) and the highest amounts of sample (1.3 and 2.0 g) fall out of the trend. This finding is in agreement with the inhibitor role of NOx toward oxygen combustion, which has been previously reported.13 If NOx is no longer present in the reaction stream, (because of its complete removal), carbon would be consumed, mainly, by O2 combustion; therefore, the selectivity will decrease. Finally, assessments were carried out about bed temperature uniformity throughout the experiments performed. No uncontrolled increase of temperature due to the exothermicity of the gasification reaction was observed. If the reaction temperature is increased up to 475 °C, an enhancement in temperature is registered. Therefore, 450 °C has been revealed as the maximum value in order to perform an isothermal study. In addition to these assessments, one of the experiments was run for a long time (6 h) in order to check the efficiency of the sample to maintain very high NOx conversion. The results obtained are presented in Figure 5a, where the NOx conversion as well as the temperature record are collected along the experimental time. It is noteworthy to mention that a value close to 100% of NOx conversion is kept during the reaction (when the steady-state conditions are reached) without uncontrolled increase in temperature. Results corresponding to reaction product evolution are shown in Figure 5b. CO evolution was not observed at all, while other undesired reaction products, such as N2O, were emitted as low as 0.7% ratio of nitrogenated products. The (13) Bueno-Lo´pez A.; Garcı´a-Garcı´a A.; Salinas-Martinez de Lecea C.; McRae C.; Snape C. E. Energy Fuels 2002, 16, 997.
Figure 5. Reaction profiles obtained from an isothermal reaction for 6 h: (a) NOx conversion and temperature profiles and (b) N2, CO2, and N2O evolution profiles.
burnoff of the sample was 12.4% (on total sample basis). These data provide an average hourly rate of carbon consumption of 2% and a corresponding average hourly rate of NOx removal of 9 mg/gsample. In this communication, a systematic study of the NOx-carbon reaction was performed, (varying the residence time and the NOx partial pressure) with a sample exhibiting a high selectivity toward NOx against O2 at 450 °C. A first reaction order, with respect to the NOx partial pressure and a rate constant of 1.15 s-1 were reported. The most important finding that can be concluded from the present work is that a NOx conversion level close to 100% can be maintained for long times, without uncontrolled increase of the sample temperature. Besides, undesired product emission, such as CO and N2O was not observed. Nomenclature Q: gas flow (cm3/min) (PNOx)o: initial NOx partial pressure (ppms) (PNOx)2h: NOx partial pressure after 2 h of experiment (ppms) V: bed volume (cm3) τ: residence time (s) k: rate constant (s-1) n: reaction order, dimensionless
Acknowledgment. Support from CICYT under project PB98-0983 is gratefully acknowledged. EF020041B
