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Energy & Fuels 1996, 10, 1245-1249
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Microwave-Induced Regeneration of NOx-Saturated Char Yougen Kong* and Chang Yul Cha† Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, Wyoming 82071 Received April 17, 1996X
The rate of microwave-induced regeneration of nitrogen oxides (NOx)-saturated char was studied. A small amount of NOx-saturated char sample was placed in the microwave field, and its weight was measured as a function of time by a digital balance. The weight loss data were used to calculate the activation energy of C-NO reaction under microwave radiation. The experimental results showed significant surface complex formation from the C-NOx reaction during the regeneration of NOx-saturated char. An increase of power input not only lowered the amount of surface complexes but also enhanced the regeneration rate. When the input power was >300 W, the regeneration was actually completed within 1 min. The activation energy of the microwave-induced C-NO reaction was estimated as 17 kJ/mol, much less than those obtained by others using conventional heating (63-68 kJ/mol).
1. Introduction Selective catalytic nitrogen oxides (NOx) reduction (SCR) using ammonia as the reducing agent has received wide attention.1-3 Carbon has also been found to be effective in reducing NOx.4-10 However, a major disadvantage in the use of carbon is that carbon is consumed by combustion with oxygen present in flue gas. The experimental results of Suzuki et al.11 show that the presence of oxygen (5%) can initially enhance the NOx conversion rate significantly. Then the NOx conversion rate drops rapidly due to the consumption of activated carbon in the carbon-oxygen reactions. An effective solution to this problem is to use a carbon adsorbent to selectively adsorb NOx from flue gas.12 However, the regeneration of NOx-saturated adsorbents by conventional heating results in the desorption of the adsorbed NOx and produces concentrated NOx streams. In practice, NOx need to be destroyed except there are * Author to whom correspondence should be addressed [telephone (307) 766-2837; fax (307) 766-6777]. † Present address: Kenonic Controls Inc., 5119 Office Park Dr., Bakersfield, CA 93309. X Abstract published in Advance ACS Abstracts, October 15, 1996. (1) Heck, R. M.; Chen, J. M.; Speronello, B. K. Environ. Prog. 1994, 13, 221-225. (2) Singoredjo, L.; Kapteijn, F.; Moulijn, J. A.; Martin-Martinez, J. M.; Boehm, H.P. Carbon 1993, 31, 213-222. (3) Ness, S. R.; Dunham, G. E.; Weber, G. F.; Ludlow, D. K. Environ. Prog. 1995, 14, 69-74. (4) DeGroot, W. F.; Richards, G. N. Carbon 1991, 29, 179-183. (5) DeGroot, W. F.; Osterheld, T. H.; Richards, G. N. Carbon 1991, 29, 185-195. (6) Teng, H. S.; Suuberg, E. M.; Calo, J. M. Energy Fuels 1992, 6, 398-406. (7) Lai, C. K. S.; Peters, W. A.; Longwell, J. P. Energy Fuels 1988, 2, 586-588. (8) Illan-Gomez, M. J.; Linares-Solano, A.; Salinas-Martinetz de Lecea, C. Energy Fuels 1993, 7, 146-154. (9) Illan-Gomez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartinetz de Lecea, C. Energy Fuels 1995, 9, 97-103. (10) Mochida, I.; Ogaki, M.; Fujitsu, H.; Komatsubara, Y.; Ida, S. Fuel 1985, 64, 1054-1057. (11) Suzuki, T.; Kyotani, T.; Tomita, A. Ind. Eng. Chem. Res. 1994, 33, 2840-2845. (12) Rubel, A. M.; Stewart, M. L.; Stencel, J. M. Symposium on NOx Reduction. Presented at the 207th National Meeting of the American Chemical Society, San Diego, CA, March 13-18, 1994.
large quantities of NOx to be converted to nitric acid (HNO3). Therefore, it becomes a practical challenge to reduce the NOx adsorbed on carbon adsorbents to N2. Microwave energy has been used to regenerate NOxsaturated carbon adsorbents.13 Since microwaves can penetrate dielectric materials, the maximum temperature of the material being heated by microwave energy is only dependent upon the rate of heat loss and power applied. For solids of low thermal conductivity, the center or an interior section of the mass may be hundreds of degrees hotter than the external surface.14 As a result, most NOx, which are adsorbed in the micropores of carbon adsorbents, will react with carbon when microwave energy is applied. The objectives of this work are to investigate the microwave-induced regeneration of NOx-saturated char and the kinetic parameters of C-NO reaction using the char weight loss during the regeneration. The weight loss during the regeneration was continuously monitored by a digital balance. The average temperature of the char sample under microwave radiation was calculated on the basis of the energy balance. Then the activation energy assuming the first-order reaction was estimated and compared to those obtained by others using conventional heating. 2. Experimental Section The char used in the tests was produced by the FMC Coke Plant in Kemmerer, WY. This calcined char was made through the devolatilization of sub-bituminous coal at about 770 °C. The received char was further devolatilized in a microwave reactor described in a previous paper.13 A batch of 325 g of char with sizes of 10-20 mesh was placed in the microwave reactor for 1 h to further remove volatiles and adsorbed moisture. The microwave generator was set at 480 W during the devolatilization. The sample of 313.5 g was left after the microwave treatment and was then stored in a sealed bottle. Its BET surface area was 163 m2/g as measured by (13) Kong, Y.; Cha, C. Y. Energy Fuels 1995, 9, 971-975. (14) Peterson, E. 28th Microwave Symposium Proceedings; Montreal, Canada, 1993; p 89.
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Kong and Cha by a gas chromatograph (GC). The flow rate of the produced gas was not measured either, due to the experimental setup.
3. Theory The rate of weight loss of NOx-adsorbed char sample during the microwave regeneration is assumed to be represented by
-dw/dt ) k(w - w∞)
(1)
d(w0 - w)/dt ) k[(w0 - w∞) - (w0 - w)]
(2)
or
Figure 1. Experimental setup. the liquid nitrogen adsorption method. Then, 34.88 g of char was filled into a quartz tube having an inner diameter of 13.8 mm. The feed gas was prepared by mixing pure NO with air at a relative humidity of 40%. A part of the NO was homogeneously oxidized into NO2 due to the presence of oxygen. The mixing tube was opaque and long enough so that the gas phase conversion of NO to NO2 inside the tube had reached equilibrium before the gas mixture entered the char bed. The concentrations of NO and NO2 in the feed gas were 815 and 230 ppm, respectively. The feed gas flow rate was 974 cm3/min, which corresponds to a residence time of 2.3 s. The adsorption was performed at room temperature (20 °C). When there was no further weight gain observed, the char was regarded as saturated with NOx. The weight of NOx-saturated char was 37.29 g, which corresponds to a relative weight gain of 6.9%. The NOx-saturated char was regenerated with microwave energy in an experimental setup shown in Figure 1. The microwave energy was produced by an industrial microwave generator and had a variable operating power setting which ranged from 0 to 3000 W at a fixed frequency of 2.45 GHz. The load impedance due to the char sample and the water load was matched to that of the generator with a three-bolt tuner to minimize the reflected power. Three power meters were used to measure the input power, reflected power, and transmitted power which was absorbed by the water load. In this experimental setup, there was almost no reflected power and most of the input power was absorbed by the water load. A quartz tube with an inner diameter of 2.2 cm was placed inside a waveguide (WR 284, TE10). In the middle of the tube there was a sample holder made of porous ceramic (FAO-160 produced by the Ferro Corp.). This ceramic material was tested in a microwave oven together with the same amount of char for 1 min. The ceramic was a little warm and the char was glowing. Therefore, it can be safely assumed that the sample holder is nearly transparent to microwaves. The inner diameter of the sample holder was 1.6 cm. The sample holder was supported with a quartz bar since quartz does not absorb microwave energy. A base holding the quartz bar sat on the plate of a digital balance, which had a sensitivity of 0.0001 g. The weight data were recorded by a personal computer connected to the balance. For each regeneration test, 0.3078 g of the NOx-saturated char was placed in the ceramic sample holder. The height of the char sample was 1.3 cm. A nitrogen flow of 20 cm3/min was used to prevent the gases produced during the regeneration from coming down to the balance. The microwave energy of different power inputs was applied for 20 min. Analysis of the gas produced from the microwave-induced regeneration of NOx-saturated char was performed and reported in our previous paper.13 The concentrations of the produced gases in these experiments were too small to be measured accurately
where w is the weight of char sample at time t, k is a rate constant, w∞ is the weight of char sample after regeneration, and w0 is the weight of NOx-saturated char sample before regeneration. Let ∆w∞ ) w0 - w∞ and ∆w ) w0 - w. Then eq 2 becomes
d∆w/dt ) k(∆w∞ - ∆w)
(3)
Dividing the above equation with ∆w∞ gives
d(∆w/∆w∞)/dt ) k[1 - (∆w/∆w∞)]
(4)
Let w* ) ∆w/∆w∞. Then eq 4 can be written as
dw*/dt ) k0e-Ea/RT(1 - w*)
(5)
where k0 and Ea are the frequency factor and activation energy, respectively. Taking the natural logarithm to eq 5 and rearranging the resulting equation, we obtain
Ea
1 ) ln k [1dw*/dt - w*] R T
ln
0
(6)
Plotting ln[(dw*/dt)/(1 - w*)] against 1/T, the activation energy and frequency factor can be estimated from the slope and intercept of the line. Values of w* can be calculated from w, w0, and w∞, which were measured with a digital balance. Since almost all NOx are adsorbed in the interior of char particles, the NOx reduction reactions by carbon under microwave radiation actually take place in the interior of these char particles. When the microwave energy is applied, the temperature in the interior should be much higher than at the surface and in the bulk gas phase.14 Unfortunately, it is not possible to measure interior temperatures of a particle. To estimate the activation energy with eq 6, the average temperatures of the char sample under microwave radiation were calculated on the basis of the energy balance. In the calculation, the following energy items involved in the regeneration were considered. (1) Microwave Energy Absorbed by the Char Sample. The microwave energy absorbed by a dielectric material is expressed as15
P1 ) (0.556 × 10-10)f′′E2rmsV
(7)
where f is the microwave frequency, V is the volume of material, and the loss factor ′′ is the imaginary part of (15) Metaxas, A. C.; Meredith, R. J. Industrial Microwave Heating; 1983.
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Energy & Fuels, Vol. 10, No. 6, 1996 1247
the complex dielectric constant of a dielectric material:
P4 ) eδAc(T4 - T4w)
* ) ′ - j′′
where e is the emissivity of char (0.85), δ is the StefanBoltzman constant [5.67 × 10-8 W/(m2‚K4)], Ac is the external surface area of the char sample (4.247 × 10-4 m2), T is the average temperature of the char sample (K), and Tw is the temperature of the waveguide (K). During the regeneration, the temperature of the waveguide, Tw, was measured with a thermocouple and was found to be constant at room temperature (293 K). (5) Heat Used To Increase the Temperature of Char Sample. The heat consumed to increase the temperature of char sample is
(8)
where ′ is also referred to as the dielectric constant. In eq 7, Erms is the root mean square value of the electric field strength in the material, calculated by the equation16
Erms )
1 (2Pinµ0ν0/ab) ′x
(9)
where Pin is the input power, µ0 is the permeability of air, v0 ) 3 × 108 m/s is the velocity of light in air, and a ) 3 in. and b ) 1.5 in. are the dimensions of a WR 284 waveguide. (2) Heat Produced by Chemical Reactions during the Regeneration. The heat of reaction was calculated using the composition of product gas measured in a separate experiment in which 38.51 g of the same NOx-saturated char was regenerated with 500 W of microwave energy in a similar setup. From the moles of adsorbed NOx on char and the concentrations of CO, CO2, and N2 in the product gas, the total heat produced in 20 min of reaction was calculated as -18 140 J with the heat of formation of each species. Assuming that the heat of reaction was proportional to the amount of NOx-saturated char, the average rate of heat generated by chemical reaction during the regeneration of 0.3078 g of NOx-saturated char can then be calculated as
P2 ) (-1.814 × 104) ×
1 0.3078 × ) 38.51 20 × 60 -0.121 J/s (10)
The heat of reaction is only 5.2% of the microwave energy absorbed by the char sample for five different input powers used in this study. Therefore, the error introduced in eq 10 due to the assumptions is negligible. (3) Heat Removed by the Product Gas. In the separate experiment mentioned above, the total heat removed by the product gas in 20 min was calculated as 1595 J. Assuming that the heat removed by the product gas was proportional to the weight of NOxsaturated char sample, the heat removed by the product gas during the regeneration of 0.3078 g of NOxsaturated char can then be extrapolated as
1 0.3078 P3 ) 1595 × × ) 0.011 J/s 38.51 20 × 60
(11)
The heat removed by the product gas is only 0.5% of the microwave energy absorbed by the char sample. Therefore, the error introduced in eq 11 due to the assumptions is negligible. (4) Energy Lost through Radiation to the Surroundings. Since the volume of the char sample is so small compared to the waveguide, this system can be regarded as a small convex object in a large cavity. Thus, the heat loss through radiation from the hot char sample can be written as17 (16) Kong, Y. Investigation of Microwave-Char NOx Abatement Process. Ph.D. dissertation, University of Wyoming, 1996. (17) Incropera, F. P.; DeWitt, D. P. Fundamentals of Heat and Mass Transfer; 1990.
P5 ) WcCp,c(T - T0)
(12)
(13)
where Wc is the weight of char sample (0.3078 g), Cp,c is the heat capacity of char [1.51 J/(g‚K)], and T0 is the initial temperature of char (293 K). (6) Heat Lost due to Convection. The heat loss through convection can be calculated as
P6 ) hAc(T - Tq)
(14)
where Tq is the temperature of the quartz tube assumed at room temperature (293 K) and h is the convection heat transfer coefficient. The quartz tube-char sample system is a concentric tube annulus. Consequently, the corresponding Nusselt number is given by
Nu ) hDh/kb
(15)
with the hydraulic diameter Dh ) Dq - Dc ) 0.6 cm, where Dq is the inner diameter of the quartz tube, kb is the thermal conductivity of bulk gas, and Dc is the diameter of the cylindrical char sample. The velocity of the purge gas in the annulus is 0.2 cm/s. The corresponding Reynolds number is 0.72, which indicates the presence of a laminar flow. In this case, the Nusselt number is about 5.3.17 Thus, h ) 22.9 W/(m2‚K). From the energy balance
P1 ) P2 + P3 + P4 + P5 + P6
(16)
the average temperature of the char sample at any time can be calculated. 4. Results and Discussion When microwave energy is applied to the NOxsaturated char, the char sample heats up and the adsorbed NOx react with carbon to produce carbon oxides, N2, and a small amount of NO.13 Therefore, the weight of char sample decreases with time. The regeneration was first investigated with a power input of 100 W. The percentage of weight loss is shown in Figure 2. This curve shows that the sample weight decreased continuously after the microwave energy was applied. The total weight loss after 20 min was only 2.7% of the weight of char, even less than the weight gained after the NOx adsorption (6.9%). If all adsorbed NOx are desorbed or react with carbon and all products are released as gases, the total weight loss should be greater than the weight gain after the NOx saturation. To explain this experimental observation, the average temperature of char sample after 20 min was calculated from eq 16 as 495 K. At this temperature, the adsorbed
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Figure 2. Weight loss of char sample during microwave regeneration under different power inputs.
Figure 3. Temperature of char sample during microwave regeneration at 30 s and 20 min, respectively, after microwave energy was applied.
NOx should either react with carbon to produce N2, CO, and CO2 or be desorbed. At relatively low temperatures, however, gaseous carbon oxides (CO and CO2) produced from char-NO reactions may not be released; instead, stable surface oxides may be formed on the char via chemisorption.6,18 Therefore, it is believed that some carbon oxides produced in the reactions between the adsorbed NOx and carbon are adsorbed on the char surface since the particle temperature was not high enough to release these gases. As a result, the sample weight after the regeneration was even 4.2% greater than the weight of char before saturation. When 300 W of microwave energy was applied to the same amount of NOx-saturated char, the rate of weight loss at the beginning was greater than when 100 W of input power was applied. Nevertheless, the sample weight loss after 20 min was only 4.1%, still less than the weight gain after the saturation with NOx (6.9%). In other words, there was a significant amount of carbon oxides on the char surface when the temperature of the char sample reached 673 K (Figure 3). The rate of weight loss at the beginning and end increased with the input power (Figure 2). When the input power was 900 W and the sample temperature was 920 K at 20 min (Figure 3), the sample weight loss was 6.5%, still less than the weight gain after the saturation with NOx (6.9%). In other words, there were some carbon oxides adsorbed on the char surface even at temperatures as high as 920 K. However, the differences in the final weight are continuously reduced as the input power increases. To study the regeneration of NOx-saturated char further, 0.3078 g of NOx-saturated char was first regenerated with microwave energy of 100 W for 20 min, and then 500 and 900 W of microwave energy was applied in series for 10 min each. The weight loss of the char sample is shown in Figure 4. After 20 min at an input power of 100 W, the weight loss of char sample (18) Furusawa, T.; Kunii, D.; Oguma, A.; Yamada, N. Int. Chem. Eng. 1980, 20, 239-244.
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Figure 4. Weight loss of char sample when microwave energy of different power inputs was applied in series.
Figure 5. Estimation of the activation energy of NO-char reaction under microwave radiation.
reached a constant. However, when the input power was increased to 500 W, the weight loss of char sample increased abruptly and then approached a constant again. The increase of power input from 500 to 900 W did not cause significant desorption of surface complexes. This experimental result clearly indicates that surface complexes are formed during the microwaveinduced regeneration of NOx-saturated char and are desorbed at higher temperatures. Figure 2 shows that the regeneration was actually completed within 1 min when the input power was >300 W. Therefore, the rate of sample weight loss at 30 s in each test was used in eq 6 to estimate the kinetic parameters. The average temperatures of char sample at 30 s were calculated with eq 16 and are shown in Figure 3. Plotting ln[(dw/dt)/(1 - w)] against 1/T gives a reasonably good straight line (Figure 5). From the slope and intercept of this line, the activation energy and frequency factor in the temperature range of 495920 K were calculated as Ea ) 17 kJ/mol and k0 ) 97 s-1, respectively. According to our previous paper,13 NOx are adsorbed on char in the forms of NO2 and HNO3 in the presence of oxygen and moisture. When the NOx-saturated char is regenerated with microwave energy, the adsorbed HNO3 and NO2 are reduced by carbon to NO, which reacts with carbon to produce N2, CO, and CO2. The reduction of NO2 to NO is rather fast, but the further reduction of NO to N2 is much slower.11 Correspondingly, the reduction of HNO3 by carbon to NO is supposed to be rather fast compared to the reduction of NO to N2. Therefore, the reaction between NO and carbon actually controls the C-NOx reaction. These results lead us to believe that the activation energy of the C-NO reaction under microwave radiation is close to 17 kJ/mol in the temperature range of 495-920 K. DeGroot5 investigated the rate of the C-NO reaction also with the weight loss during the gasification of char by NO in a thermogravimetric analyzer (TGA) and
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found that the activation energy and frequency factor of the NO-char reaction in the temperature range of 773-923 K are 64 kJ/mol and 105, respectively. Teng6 studied the NO-char reaction with a TGA and found that this reaction can be divided into two different mechanistic regimes: the low-temperature regime (923 K), which has a constant activation energy of 180 kJ/mol. Both activation energies are much higher than that estimated in this study (17 kJ/mol). The experimental results of a previous study13 show that sites occupied by NOx may absorb more microwave energy than pure char. Consequently, these adsorption sites heat up more rapidly and may turn into hot spots in char. The reduction reactions of adsorbed NOx by carbon take place at these hot spots. However, it is impossible to measure or estimate the temperatures of these hot spots. Instead, the average particle temperature was estimated on the basis of the energy balance and used in the calculation of activation energy. Obviously, the average temperature of char is lower than those of the hot spots in char where C-NO reaction takes place. This may be a part of the reason the activation energy obtained in this study is smaller than those in the literature.
Energy & Fuels, Vol. 10, No. 6, 1996 1249
5. Conclusion The experimental results showed significant surface complex formation from NOx-char reactions during the regeneration of NOx-saturated char. An increase of power input not only lowered the amount of surface complexes but also enhanced the regeneration rate. When the input power was >300 W, the regeneration was actually completed in the first minute. Therefore, the rates of the sample weight loss at 30 s were used to estimate the activation energy and frequency factor. The average temperatures of char sample under microwave radiation were calculated on the basis of the energy balance. The activation energy of the microwaveinduced C-NO reaction was estimated as 17 kJ/mol, much less than those obtained by others using conventional heating (63-68 kJ/mol). This difference in the activation energy may be partly due to the difference between the calculated temperatures and those of the hot spots inside the char where chemical reactions actually took place. Acknowledgment. We thank the National Science Foundation of the United States for financial support (Grant EHR-910-8774) and Dr. Pradeep K. Agarwal for valuable suggestions. EF960060J
