Energy Fuels 2010, 24, 4849–4853 Published on Web 08/12/2010
: DOI:10.1021/ef100575v
Fourier Transform Infrared (FTIR) Online Monitoring of NO, N2O, and CO2 during Oxygen-Enriched Combustion of Carbonaceous Materials Astrid S anchez,† Eric Eddings,‡ and Fanor Mondrag on*,† †
Chemistry Institute, University of Antioquia, AA 1226 Medellı´n, Colombia, and ‡Department of Chemical Engineering, University of Utah, Salt Lake City, Utah 84112 Received May 7, 2010. Revised Manuscript Received July 28, 2010
Fourier transform infrared (FTIR) spectroscopy has been used for online monitoring of the gaseous products, resulting from combustion of a small amount of carbonaceous material with a high nitrogen content. The combustion experiments were carried out using a fluidized-bed reactor connected online to a FTIR gas cell. Spectra were collected every 2 s during the transient combustion reaction, and with this information, very detailed and quantitative evolution profiles for NO, N2O, and CO2 were determined. This approach is an option for tracking combustion products in very fast reactions, such as those that take place in combustion reactions with enriched oxygen. The analytical methodology described herein includes sample preparation, selection of experimental parameters, and data processing. The procedure used in this research can be applied to the characterization of gaseous effluents in any continuous process.
that have prompted a large number of studies to provide a better understanding of the behavior of heteroatoms in combustion reactions and, at the same time, to determine methodologies for reducing pollutant emissions. One of the advanced processes for clean coal use is oxy-combustion, wherein combustion is carried out using a mixture of oxygen and carbon dioxide.8 Pure oxygen and coal are fed to the combustor, and the two main products of combustion are carbon dioxide and water. The water is easily condensed to yield a relatively pure carbon dioxide stream that is suitable for further processing or sequestration. A portion of this carbon dioxide is recycled and mixed with the oxygen stream to help control the temperature of the combustion reaction. Many analytical tools can be used for identification of pollutant concentrations during combustion processes, such as oxy-combustion. The selection of an appropriate tool basically depends upon the characteristics of the oxidation reactions. The most common methods for analyzing combustion and pyrolysis products are techniques such as MS, FTIR spectroscopy,9 gas chromatography (GC), chemiluminiscence analysis, and thermogravimetric analysis (TGA).10-12 This work was precipitated by a need to track concentrations of nitrogen oxides and carbon dioxide during oxy-combustion experiments. In these experiments, a few char particles with high nitrogen content were oxidized under various conditions with an increasing oxygen concentration, giving rise to very fast changes in the progress of the reaction. Because differences in particle density, organic composition, pore structure, and mineral matter content can lead to large variations in the kinetic description of the combustion reaction,13 a synthetic char was used. We kept these parameters fixed to facilitate evaluation of combustion
Introduction Online monitoring in any industrial application is of great importance for plant safety and good product quality.1 Current efforts have focused on the use of several analytical techniques to develop online methods and apply them to different processes. Some of the advantages of online monitoring over the conventional techniques are the possibility to obtain near real-time data, to eliminate potential contamination and other errors that can be introduced during sample collection, transportation, and storage.2 Additionally, it can be employed to track the evolution of products and intermediates during reactions and, thus, provide key information toward understanding the reaction mechanism and kinetic rates.3 Some examples of online monitoring of reactions include polymerization reactions, in which Raman spectroscopy was used4,5 to clearly differentiate several stages during the reaction and to determine the time when the reaction had been completed,4 online trace compound determination in medical and environmental applications by mass spectrometry (MS),6 and polycyclic aromatic hydrocarbon measurement in diesel incinerators by means of laser-based resonance-enhanced multi-photon ionization time-of-flight MS in real time.7 Combustion of solid carbonaceous materials is of great importance as a primary energy supply. However, in the case of coal combustion, the reaction has environmental restrictions *To whom correspondence should be addressed. E-mail: fmondra@ udea.edu.co. (1) Westerhuis, J. A.; Gurden, S. P.; Smilde, A. K. Anal. Chem. 2000, 72, 5322–5330. (2) Brukh, R.; Salem, T.; Slanvetpan, T.; Barat, R.; Mitra, S. Adv. Environ. Res. 2002, 6, 359–367. (3) Xu, F.; Armstrong, J. D., III; Zhou, G. X.; Simmons, B.; Hughes, D.; Ge, Z.; Grabowski, E. J. J. J. Am. Chem. Soc. 2004, 126, 13002–13009. € (4) Ozpozan, T.; Schrader, B.; Keller, S. Spectrochim. Acta, Part A 1997, 53, 1–7. (5) Santos, J. C.; Reis, M. M.; Machado, R. A. F.; Bolzan, A.; Sayer, C.; Giudici, R.; Ara ujo, P. H. H. Ind. Eng. Chem. Res. 2004, 43, 7282–7289. (6) Lindinger, W.; Hansel, A.; Jordan, A. Int. J. Mass Spectrom. Ion Processes 1998, 173, 191–241. (7) Oudejans, L.; Touati, A.; Gullett, B. K. Anal. Chem. 2004, 76, 2517–2524. r 2010 American Chemical Society
(8) Wall, T.; Liu, Y.; Spero, C.; Elliott, L.; Khare, S.; Rathnam, R.; Zeenathal, F.; Moghtaderi, B.; Buhre, B.; Sheng, C.; Gupta, R.; Yamada, T.; Makino, K.; Yu, J. Chem. Eng. Res. Des. 2009, 87, 1003–1016. (9) Court, R. W.; Sephton, M. A. Anal. Chim. Acta 2009, 639, 62–66. (10) Gupta, R. Energy Fuels 2007, 21, 451–460. (11) Hosoda, H.; Hirama, T. Energy Fuels 1998, 12, 102–108. (12) Hu, Y. Q.; Kobayashi, N.; Hasatani, M. Fuel 2001, 80, 1851–1855. (13) Benfell, K. E.; Liu, G.-S.; Roberts, D. G.; Harris, D. J.; Lucas, J. A.; Bailey, J. G.; Wall, T. F. Proc. Combust. Inst. 2000, 28, 2233–2241.
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Energy Fuels 2010, 24, 4849–4853
: DOI:10.1021/ef100575v
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Table 1. Proximate and Elemental Analyses for PAN-8 proximate analysis
elemental analysis (daf)
moisture (%)
volatile matter (%)
fixed carbon (%)
ash (%)
C (%)
N (%)
H (%)
1.5
2.4
95.2
0.9
73.9
14.2
1.0
profiles to determine burning rate variations as a function of the oxygen concentration. Our interest was to take advantage of the characteristics offered by online monitoring procedures for the determination and quantification of species evolution profiles for (a) NO, which is a gas that can cause acid rain and, at the same time, contribute to the formation of photochemical smog, (b) N2O, which is a greenhouse gas,14 and (c) CO2, which is a greenhouse gas and major product of combustion. Figure 1. Schematic of the setup used for combustion experiments.
Experimental Section 0.1-0.21 mm from the Merck Company was used as the fluidizing medium. A type-K thermocouple was placed directly into the sand bed, and the sand temperature was kept at 800 °C. The total gas flow employed was 500 standard mL min-1, controlled by means of mass flow controllers. The reactor has a lid with two nuts, which facilitates rapid opening and closing of the reactor. Preliminary tests showed that it was necessary to purge the preheated reactor with argon before each experiment to remove any residual oxygen from the previous experiment. Then, the lid of the reactor is opened, and char particles are place inside the reactor. After this is completed, the lid is closed and the argon gas flow is changed to the reactant mixture at the desired concentration. At this point, collection of the spectra starts. The background for all spectra, unless otherwise stated, was obtained with the reaction gases in their specified concentrations. Water and any particulate matter produced in the combustion reaction were removed using a water trap and a particulate filter. Then, the combustion gases were analyzed by FTIR using a gas cell. Gas Analysis. The spectrometer employed was a Magna-IR 560 from Nicolet with a mercury cadmium telluride (MCT) detector, and the spectral range was 650-4000 cm-1, with spectral resolution of 4 cm-1. After comparison of the results with different numbers of scans and considering the rate and duration of the reaction, a procedure taking 2 scans per spectrum was selected. The software to collect the spectra was Omnic, version 7.0. Because the reactivity of the char samples depends upon the oxygen concentration, the duration of the experiments to reach complete carbon combustion was variable; however, in general, the experiments lasted between 100 and 300 s. The number of spectra taken in each experiment was between 60 and 150. The FTIR signals were processed using a Fortran program written for this purpose.
Materials. To have high nitrogen content in the carbon material, which can facilitate quantification of nitrogen species, polyacrylonitrile (PAN), C3H3N, was employed to prepare the char.15 The PAN sample was placed in a sample holder and was subjected to pyrolysis treatment in a horizontal quartz furnace. Argon at 100 mL min-1 was employed as an inert purge gas. The furnace was initially purged with argon for 30 min to remove any oxygen from the furnace interior, and then the temperature was increased at 20 °C min-1 from room temperature up to 800 °C and held at this temperature for 30 min. A stainless-steel plate with uniform holes of 4.7 mm in diameter was used to obtain PAN char particles with uniform size and shape. Char obtained by pyrolysis of PAN at 800 °C (PAN-8) was approximately 40% of the initial weight of PAN. Proximate and elemental analyses of PAN-8 are shown in Table 1; as expected, PAN-8 has a high nitrogen content that is present mainly in the form of pyridinic and pyrrolic complexes.15 Summation of the compositions found in the elemental analysis for PAN-8 is 89.1%. The balance can be attributed to oxygen from the synthesis procedure of PAN,16 as well as some impurities in the raw material, because elemental analysis of the original, unpyrolyzed PAN indicates that the total content of C, H, and N is 93.1%. Particles have a cylindrical shape with an average weight for each particle of 0.0134 g, with a standard deviation of 0.0010; the average diameter is 3.5 mm, and the average length is 3.7 mm. The surface area of the char particles was determined using the Brunauer-Emmett-Teller (BET) model with N2, obtaining a surface area of 3-4 m2/g. This relatively low value is probably due to the nature of the raw material and the pyrolysis conditions. In each combustion experiment, five char particles were used. The total mass of char in each run was 0.067 ( 0.001 g. Combustion Experiments. Figure 1 shows the experimental configuration used for the combustion reactions. The combustion was carried out in a fluidization regime to obtain data in a reacting system similar to those used in large-scale fluidized-bed combustion.17,18 A stainless-steel tube with a 2.6 cm internal diameter and 76 cm length that was electrically heated was employed as the combustion reactor. A total of 100 g of sand with a particle size of
Results and Discussion Calibration curves for gases of interest were evaluated at room temperature, following the same conditions as those used in the combustion experiments. A total of 500 standard mL min-1 of standard gases passed through the reactor and the gas cell. Three spectra were taken for each concentration. Figure 2 shows the calibration curve for N2O, where each point is the average of the area measurement for three different spectra. Calibration curves for other gases present a linear coefficient higher than 0.95 for the concentration range evaluated. These data were employed to analyze the spectra obtained during the reaction. A typical result obtained for one combustion test is shown in Figure 3. As mentioned previously, spectra were taken every 2 s, to facilitate the visualization of the results; spectra with the most relevant changes at some selected reaction times
(14) Ren, Q.; Zhao, C.; Wu, X.; Liang, C.; Chen, X.; Shen, J.; Tang, G.; Wang, Z. J. Anal. Appl. Pyrolysis 2008, 85, 447–453. (15) Pels, J. R.; Kapteijn, F.; Moulijn, J. A.; Zhu, Q.; Thomas, K. M. Carbon 1995, 33, 1641–1653. (16) Grzyb, B.; Machnikowski, J.; Weber, J. V.; Koch, A.; Heintz, O. J. Anal. Appl. Pyrolysis 2003, 67, 77–93. (17) Komatina, M.; Manovic, V.; Dakic, D. Energy Fuels 2006, 20, 114–119. (18) Leckner, B. Proceedings of the 26th International Symposium on Combustion; Naples, Italy, 1996; pp 3231-3241.
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: DOI:10.1021/ef100575v
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are shown in the figure. At the beginning of the reaction (17 s), it is possible to see CO2, NO, and N2O in low concentrations. As the reaction proceeds, these signals begin to increase; for example, at 24 s of reaction time, the NO2 absorption band becomes more noticeable. Because we are using a small amount of carbon sample, after a period of time (which depends upon the reaction conditions), all of the signals start to decrease again, until they completely disappear, indicating that all of the char material has been totally consumed. All of the combustion spectra were compared to the spectra of standard gases employed for calibration curves. It is possible to see relatively noise-free spectra; the water absorption signal (around the NO2 absorption band) is very low because of the effectiveness of the water trap, which allows, in this way, identification of NO2. In Figure 3, it is possible to see a very large signal around 2100-2400 cm-1 because of the overlapping of main absorption signals for CO2, N2O, and CO. Figure 4 shows individual spectra of these gases. To avoid this overlapping problem, the quantification of N2O and CO2 is carried out using other absorption bands that can be obtained from the calibration curves and, in this way, to monitor their evolution during the combustion reactions. The circles in Figure 4 illustrate the
locations of the alternative signals employed for the analysis of these species. In the case of CO, it was possible to detect its signal at the last stages of the reaction but its concentration was very low, being almost negligible because of the high oxygen concentration. This observation is consistent with blank experiments in gas phase carried out to determine secondary reactions between CO and O2. As expected, CO is readily oxidized by O2 in the gas phase gas at high temperatures. Because of the large number of spectra taken during a reaction, a significant amount of information needs to be processed, because it was necessary to integrate the signal for each species in each spectrum during the reaction to have a complete description of the evolved gases. For example, in the case of the data shown in Figure 3 it is necessary to perform approximately 200 integrations. All of the calculations were carried out with a program written in Fortran to automatically integrate all of the FTIR signals. It was necessary to define the wavenumber region for each species, and at the same time, it was necessary to define the region for the baseline. Because the program takes the average of the points in the baseline and produces a line, it is possible to determine the area under the signal for each species with respect to that horizontal line. Then, this value is converted to concentration by means of the calibration curves. With these data, it is possible to obtain a gas evolution profile for each one of the combustion products, as shown in Figure 5. Determination of the Optimum Number of Scans for Data Acquisition. The results of using 2, 4, 8, and 16 scans per data point were compared to determine the optimal number of scans necessary for the study of these combustion reactions. The time required for 16 scans was too long to provide sufficient temporal detail of the combustion reactions described here. Figure 5 shows a comparison between two and eight scans for every spectrum. As the number of scans increases, the collection time is subsequently higher, and it is apparent in Figure 5 that, for eight scans, some details in the evolution of the species can be lost. Similar observation was made for the data taken with 4 scans. As a consequence, it was decided to use 2 scans per spectrum for the data acquisition as a compromise between accuracy and resolution of the evolution profiles during the reaction. It is also important to note that
Figure 2. Calibration curve for N2O, gas balance Ar.
Figure 3. FTIR spectra for PAN-8 combustion. All spectra are on a common scale. The time indicated in the figure is taken after switching from the argon purge gas to the reacting gas mixture.
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: DOI:10.1021/ef100575v
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Figure 4. FTIR spectra of CO, CO2, and N2O. The circles highlight the signals used for the quantification of CO2 and N2O to avoid the overlapping region between 2100 and 2400 cm-1.
Figure 6. Comparison between CO2 measured by the FTIR method and CO2 emissions estimated from elemental analysis of the char.
Figure 5. Comparison of evolution profiles for CO2, NO, and N2O at 40% O2/60% Ar: (b, 2, and 9), 8 scans per spectrum and (O, 4, and 0) 2 scans per spectrum.
local conditions of temperature and pressure, and the total volume employed, the total amount of CO2 that should be produced for different oxygen concentrations was calculated. This value was compared to the experimental value obtained from integrating the evolution profiles, and the comparison is shown in Figure 6. As shown, there is a consistent offset between the experimental averages and the expected values from the elemental analysis of PAN-8. In all cases, values determined experimentally are higher than the theoretical values. For purposes of comparison, the CO2 concentration in the 50% O2/50% Ar experiment was also determined using a microGC. Because of the fast changes in concentration during the reaction, it was not possible to use the micro-GC online because the running time for the evolution of CO2 is around 2 min and the combustion experiments last between 1.5 and 5 min. Therefore, it would be possible just to take one point in some of the experiments, and it would be very difficult to know the correspondence of that measurement with the real time of the evolved gases during the reaction (see the Supporting Information). To corroborate that the methodology described herein provides reliable concentrations for the gases produced in the combustion experiments, we collected all of the gaseous products during an entire run for a 50% O2/50% Ar experiment using a Tedlar gas sampling bag (18 24, 25.0 L with poly 2-in-1 valve). Then, the bag was connected to the micro-GC, using helium as the carrier gas. CO2 was found to be 8.1% of the total
both profiles in Figure 5 have the same shape and that the total area under the curve is almost the same in both cases, which indicates that the procedure is highly reproducible because the profiles with different scan number represent separate experiments. From Figure 5, it is apparent that N2O and NO have different evolution profiles, with the highest evolution of N2O appearing at the beginning of the reaction, whereas NO is produced in a higher concentration at the middle of the reaction. This kind of information is very important for the development and evaluation of mechanistic insight on the combustion process. For example, the observed behavior may be related to specific steps for each species within the reaction mechanism, or alternatively, it could be a consequence of diffusional differences in these species because of the large size of the particles. Using profiles like those shown in Figure 5, it is possible to obtain an average concentration for every species during the reaction. Each curve can be integrated and divided by the reaction time to obtain an average concentration for the evolved species. Using the data from elemental analysis for PAN-8 and the total weight of char employed for each test, we can obtain the amount of CO2 that would be produced assuming that the CO concentration is very low, as was discussed above. Taking into account the reaction time, the residence time, the 4852
Energy Fuels 2010, 24, 4849–4853
: DOI:10.1021/ef100575v
Sanchez et al.
conventional and oxy-char combustions employing O2/Ar and O2/CO2 as reactant gases will be discussed in detail elsewhere. Conclusions The online FTIR methodology presented here has the capability to successfully monitor in a quantitative way the evolution profiles for NO, N2O, and CO2 from char combustion under different oxygen concentrations using argon as balance gas. The total average concentration for each one of the produced gases can be estimated from gas evolution profiles. It is important to mention that it is not possible to use GC to monitor gases produced in the combustion experiments described here because the running time of the GC is at a comparable time scale to that of the combustion experiments. We are able to demonstrate, however, that quantification of the evolved gases by FTIR is in good agreement with data obtained by GC. The data obtained by the methodology described in this work allows for extraction of important information regarding the formation of nitrogen oxides. NO and CO2 have similar evolution profiles, while N2O has a different behavior, being mainly produced near the beginning of the reaction. The information obtained with the methodology described here can be useful for the study of reaction mechanisms. At the same time, the data treatment can be applied in several continuous processes, such as gasification, conventional combustion to follow the evolution of pollutants, or as a process control technique.
Figure 7. Evolution profile for CO2 at 21% O2, 40% O2, and 100% O2.
gas collected. As shown in Figure 6, this value fits very well with the estimated values, which indicates that the assumptions in the calculations and experimental procedure are in good agreement. Figure 7 shows the CO2 evolution profiles for PAN-8 in different oxygen concentrations in argon. These profiles offer information regarding the reactivity of the char. For example, when the oxygen partial pressure is high (100% O2), the CO2 evolution shows two sharp and large peaks, indicating that consumption of char is very fast (total reaction lasts about 100 s) and that the combustion takes place in two main events, suggesting the presence of two kinds of active sites with different reactivity. On the other hand, when the oxygen partial pressure is low (21% O2), CO2 evolution takes longer (total reaction time of about 280 s); in this case, the CO2 evolution profile shows different small peaks. The methodology described here can be useful in the study of reaction mechanisms, because it is possible to obtain information on stable species produced during combustion reactions. Carbon monoxide evolution was not investigated in this research because of its low concentration. However, it can be quantified by employing a statistical treatment of the individual and mixture gas spectra. The chemistry involved in the NO2 evolution and comparisons of emissions during
Acknowledgment. The authors gratefully acknowledge support from the “Programa Sostenibilidad 2009-2010” of the “Universidad de Antioquia”. We greatly appreciate the technical assistance of Dr. Mauricio Giraldo. A.S. thanks Colciencias and the “Universidad de Antioquia” for her Ph.D. scholarship and the U.S. Department of Energy (DOE) through the Utah Clean and Secure Energy (CASE) Program (FC26-08NT0005015) for her internship. Supporting Information Available: Reproducibility for the same number of scans shown for a gas mixture of 50% O2/50% Ar, taking 2 scans for every spectrum. This material is available free of charge via the Internet at http://pubs.acs.org.
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