Energy & Fuels 1995,9, 971-975
971
NO, Abatement with Carbon Adsorbents and Microwave Energy Yougen Kong and Chang Yul Cha*$+ Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, Wyoming 82071 Received March 23, 1995@
Nitrogen oxides (NO,) are formed during the combustion of coal and other fuels. The Clean Air Act Amendments of 1990 require reductions in NO, emission by all industrial processes. In this NO, abatement process, NO, in flue gas are adsorbed by a carbon adsorbent and then reduced by carbon t o nitrogen and carbon oxides with microwave energy. Six carbon adsorbents were tested in this study to investigate the effects of the microwave treatment on the NO, adsorption capacity and rate. The surface areas of FMC calcined char and FMC coke increased from about 100 m2/g to around 800 m2/g. This is due to the carbon consumption in the microwave-induced NO, reduction reactions. Through increasing the internal surface area and nitrogen content, the microwave treatment improved the NO, adsorption capacity and rate for all carbon adsorbents, except the commercial activated carbon. The FMC calcined char showed the best imprwement in NO, adsorption capacity and rate by microwave regenerations and was thus studied further to obtain the mechanisms of NO, adsorption on char and their reduction in the microwave energy field. The experimental results show that in the presence of oxygen and moisture, NO is converted to NO2 and HNO3 on carbon surfaces. The NO, adsorbed on carbon adsorbents can be reduced using microwave energy with the product gas at room temperature. Nearly 90% of NO, adsorbed on FMC calcined char is reduced in this process.
1. Introduction
Combustion of fuels with air leads to the formation of nitric oxides (NO,). The Clean Air Act Amendments of 1990 require reductions in NO, emission by all industrial processes. The reduction of NO by carbon has been extensively studied by Teng1-3 and Catalysts are often used in the selective catalytic NO, reduction (SCR) to accelerate reactions between NO, and carbon or a m m ~ n i a . ~ - ~ In the NO, abatement process developed by the Cha Corp., Laramie, WY, and the University of Wyoming, microwaves and carbon adsorbents are used. In this process, NO, are removed by passing the flue gas through a bed of pyrolytic carbon adsorbent which selectively adsorbs NO, and other pollutants. When the NO,-saturated carbon adsorbent is radiated by microwaves, the NO, adsorbed on the adsorbent reacts with carbon to produce nitrogen gas and carbon oxides. Since carbon is consumed in microwave-inducedNO, reduc?
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* To whom all correspondence should be directed.
Abstract published in Advance ACS Abstracts, October 15, 1995. (1)Teng, H. S.; Suuberg, E. M.; Calo, J. M. Energy Fuels 1992,6, 398-406. (2)Teng, H.S.;Suuberg, E. M. J.Phys. Chem. 1993,97,478-483. (3) Teng, H.S.; Suuberg, E . M. Ind. Eng. Chem. Res. 1993,32,416423. (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)Illan-Gomez, M. J.; Linares-Solano, A,; Salinas-Martinetz de Lecea, C.; Calo, J . M. Energy Fuels 1993,7,146-154. (7)Lai, C. K S.; Peters, W. A.; Longwell, J. P. Energy & Fuels 1988, 2, 586-588. (8)Wood, S.C.Chem. Eng. Prog. 1994,January, 32. (9)Heck, R.M; Chen, J. M.; Speronello, B. K. Enuiron. Prog., 1994, 13, 221-225. @
Table 1. Characterizationof Carbon Adsorbents FMC FMC calcined PRB char char
FMC coke
activated carbon
Mitsui coke
Geneva coke
particle size, mesh
10-20
> 10
10-20
8-30
10-20
ash content,
5.82
13.96 9.85
9.30
cylinder, 5-10 mm 9.80
7.91
101
1069
119
1
90.31 0.92
85.91 1.92
89.85 1.72
wt%
BETsurface 82 area, mZ/g carbon, % 87.02 nitrogen, % 1.34
135
64.58 84.45 1.61 1.22
tion reactions, the carbon adsorbent becomes more porous and its internal surface area increases. Therefore, the microwave-regenerated carbon adsorbent is used again to adsorb more NO,. In this study, six carbon adsorbents were tested in this NO, abatement process. The adsorption and microwave-induced regeneration process was repeated nine times for each adsorbent to find the effects of the microwave treatment on the NO, adsorption capacity and rate. FMC calcined char was found to be the best carbon adsorbent in terms of improvement in the adsorption capacity and rate by microwave regenerations. Therefore, this adsorbent was used to investigate the mechanisms of NO, adsorption and reduction using microwave energy.
2. Esperimental Section Six carbon adsorbents were first tested in this study. Their properties are summarized in Table 1. FMC calcined char was made through the devolatilization of sub-bituminous coal at about 770 "C by the FMC Coke Plant in Kemmerer, WY. The other carbon adsorbents were FMC Powder River Basin char (PRB char), FMC coke (both from the FMC Coke Plant),
0887-0624/95/2509-0971$09.00/00 1995 American Chemical Society
Kong and Cha
972 Energy & Fuels, Vol. 9, No. 6, 1995 Power meters Forward Reflected
Power meters Forward Reflected
, Microwave generator
Figure 1. Microwave reactor system-usedfor comparison of different carbon adsorbents. Geneva metallurgical coke, Mitsui activated coke, and a commercial granular activated carbon (GAC 830, made by Atochem). The adsorption column used for NO, adsorption was made of glass tube with an inner diameter of 2.85 cm. Initially, a carbon adsorbent of 310 g was used to adsorb NO, from a gas mixture containing 1000 ppm NO, 610 ppm NO2, and balance air which contained 0.95% of water vapor. This gas mixture was made by mixing pure NO and air, and a part of NO in the air was oxidized into NO2. The feed gas flow rate was held constant at 4.15 Umin with a corresponding superficial velocity of 0.11 d s . The adsorption was carried out at room temperature (about 20 "C). The residence time varied with different adsorbents and adsorptionhegeneration cycles due to sample density differences and the decrease of adsorbent bed height after microwave regenerations. When the feed gas passed through the adsorbent bed, NO, were captured by the porous carbon adsorbent while other gases left the bed without being considerably adsorbed. The adsorption experiment was carried out until NO, were no longer adsorbed. The time required for saturation of sample with NO, was approximately 1week. The NO,-saturated carbon adsorbent was then regenerated by microwave energy. The microwave regeneration system is shown in Figure 1. The microwave is produced by a Cober Model S6F industrial microwave generator and has a variable operating power setting which ranges from 0 to 6000 W at a fixed frequency of 2450 MHz. The load impedance due t o the adsorbent bed in the reactor is matched to that of the generator by the aid of a three-bolt tuner. The reflected power is minimized thereby maximizing the forward power. Two HP power meters (Model 432A) are attached to a 60 dB directional coupler to monitor forward and reflected power. As the adsorbent bed is heated, its dielectric properties change, thus causing the impedance t o change with time. The tuner is adjusted to compensate for it. A quartz tube is vertically centered in the reactor and the NO,-saturated adsorbent is held in the quartz tube. The microwave enters the quartz tube reactor through the top and radiates a large cross section of the NO, saturated adsorbent through the quartz tube wall. The energy output of microwave generator was set a t 480 W. The regeneration was nearly completed within 5 min which was determined by the time when no significant amount of gas was produced. However, regeneration was continued for 30 min to ensure that all saturated sample was regenerated. This adsorption-regeneration cycle was repeated nine times. No additional carbon adsorbent was added to the saturated or regenerated adsorbent during the 9-cycle experiments. At the end of each cycle, the adsorbent adsorption capacity for NO, and weight loss during microwave regeneration were recorded. The NO, adsorption capacity was calculated from the adsorbent weight gain subtracted by the weight of water collected during microwave regeneration. Surface areas of carbon adsorbents were determined by the standard liquid N2 BET method. A carbon-hydrogen-nitrogen determinator (CHN-600,made by Leco Corp.) was used to measure carbon
rFTl -,--a
NOxChar saturated
Microwave generator
-
GC
Figure 2. Microwave reactor system used to investigate NO, reduction reactions. and nitrogen contents in these carbon adsorbents. Samples for the above analysis were taken every three cycles. FMC calcined char was used to investigate the mechanisms of NO, adsorption on char and reduction by carbon using microwave energy. A devolatilized char of 36.1 g was filled into a quartz tube which had an inner diameter of 13.8 mm. The height of the char bed was 50.8 cm. The quartz tube was set on a digital balance which was connected t o a PC in order t o continuously record the sample weight change. An NO monitor, with a measurement range of 1000 ppm, and an NO2 monitor, with a measurement range of 500 ppm, were used to continuously measure the concentrations of NO and NO2 in the outlet gas. The feed gas was made by mixing pure NO with air which contained 0.95%of water vapor. The superficial velocity was same as in the previous tests, namely O . l l d s , which corresponds to a residence time of 2.3 s. The concentrations of NO and NO2 in the feed gas were 585 ppm and 464 ppm, respectively. After the char was saturated with NO,, the sample was stored in a sealed bottle. A devolatilized FMC calcined char of 91.9 g was put into a quartz tube of 2.82 cm in diameter which was placed inside the waveguide (Figure 2). Then helium was used to purge N2 and 0 2 out of the microwave regeneration system. The NO,-saturated char was placed on the top of the devolatilized char. A helium flow of 100 cm3/min was then used to purge 0 2 out of the char bed for 2 min. This amount of purging with helium did not cause considerable amount of NO, desorption. However, there was some N2 left in the system. Therefore, the measured concentration of N2 in the product gas was not used in the following gas analysis. Then the microwave generator was turned on and the input power was set a t 480 W. After 30 min, the microwave was turned off and 850 cm3/min of helium was applied to purge the product gases for 5 min. Since the heat transfer in the microwave heating is from inside to outside, it is not possible to measure the temperature inside particles where NO, reduction reactions take place. Instead, the temperature of product gas was measured with a thermocouple. The flow rate of the product gas was measured with a rotameter. A GC equipped with a TCD was used to measure the concentration of the product gas.
3. Results and Discussion
As stated earlier, when a carbon adsorbent saturated with NO, is exposed to microwave energy, NO, reacts with carbon to produce nitrogen and carbon oxides. Carbon is consumed in these reactions as shown in Figure 3. According to this figure, more carbon in the FMC calcined char and FMC coke was consumed in the NO, reduction reactions. On the other hand, other carbon adsorbents did not reduce so much NO,. When the carbon is consumed by the NO, reduction reactions, micropores of adsorbent particle are enlarged and also more pores are formed. Therefore, its internal surface area increases with the microwave regeneration
NO, Abatement with Carbon Adsorbents
4 0
p
c-
g
3
280 250 220 190
Energy & Fuels, Vol. 9, No. 6,1995 973
k FMC coke
160
130
9
100
O
I
3
0
70 3
0
6
Figure 3. Change of carbon weight with adsorptiodregeneration cycle.
Figure 6. Change of nitrogen content with adsorption/ regeneration cycle. -
p
30r
3
6
9
Adsorptioflegeneration Cycle
PRB char
1'
I
0
3
6
9
AdsorptionRegenerationCycle
Figure 5. Change of amount of nitrogen with adsorption/ regeneration cycle.
except for the activated carbon, as shown in Figure 4. For example, the BET surface area of FMC calcined char increased from 82 t o nearly 800 m2/g. The internal surface area decrease of the activated carbon was probably caused by the coalescence of micropores due to consumptionof carbon in the NO, reduction reactions. Figure 5 shows that the amount of nitrogen in the carbon adsorbent increased at first and then decreased. This suggests that some nitrogen compounds were produced from the adsorbed NO, during the first few microwave regenerations. This agrees with the observation of Suzuki et al.1° that a considerable amount of nitrogen is trapped in the carbon matrix during the C-NO, reaction. After a few microwave regenerations, some nitrogen was lost. This phenomenon might be explained by the formation of stable nitrogen-containing surface species C(N)from the dissociative chemisorption of NO on carbon adsorbent1 and the desorption of C(N) under microwave radiation: 2C
+ NO, - CCO), + C(N) 2C(N) - N, + 2C*
(1) (2)
where C ( 0 , represents the COXforming surface oxides. According t o Figure 6, FMC carbon adsorbents had higher nitrogen contents than the activated carbon. (10) Suzuki, T.; Kyotani, T.; Tomita, A. Ind.Eng. Chem. Res. 1994, 33, 2840-2845.
FMGcoke
3 5 7 AdsorptionRegenerationCycle
FMC char
* 9
Figure 7. Change of adsorption capacity for NO, with cycle.
Figure 4. Change of adsorbent BET surface area with adsorptiodregeneration cycle.
a
PRB char
Geneva coke
0
1
0
9
AdsorptionRegenerationCycle
9
AdsorptioflegenerationCycle
-
6
Nitrogen-containing compounds have been used as catalysts for NO, reductionll by carbon. This explains why FMC carbon adsorbents after three microwave regenerations had higher adsorption capacities for NO, (Figure 7) than the activated carbon, although the BET surface areas of activated carbon were still higher than those of FMC carbon adsorbents. It is worth noticing that after six microwave regenerations, PRB char reached the highest nitrogen content (Figure 6). This sample had the highest adsorption capacity a t the seventh adsorption cycle (Figure 7). The nitrogen content decreased after the sixth cycle and the adsorption capacity decreased correspondingly. Similar phenomenon was observed in the case of FMC coke. This observation indicates again that higher nitrogen content can improve the adsorption capacity for NO,. Therefore, it may be concluded that both the internal surface area and nitrogen content play an important role in the adsorption capacity. To find the effect of microwave regenerations on the NO, adsorption rate, breakthrough curves for the fresh samples and samples after nine regenerations were measured for all carbon adsorbents except Geneva coke. The adsorption conditions were same as in the previous tests but a much shorter adsorbent bed (16.5 cm) was used. The FMC calcined char sample aRer nine adsorptiodregeneration cycles removed all NO, for about 90 min (Figure 81, and then the NO content in the outlet gas increased very slowly, while the fresh sample removed about 82% NO, a t the beginning and the NO, content went up quickly afterwards. The same tendency held for PRB char, FMC coke, and Mitsui coke (Figures 9-11). The activated carbon, in contrast, behaved quite differently. Figure 12 shows that the fresh activated carbon not only adsorbed more NO, but also faster than the sample after nine adsorptiodregeneration cycles. This is in agreement with the decrease of its BET surface area. Therefore, microwave regeneration decreases its adsorption ability for NO, by reducing its surface area, whereas the microwave treatment greatly (11) Singoredjo, L.; Kapteijn, F.; Moulijn, J. A.; Martin-Martinez, J. M.; Boehm, H. P. Carbon 1993,31, 213-222.
Kong and Cha
974 Energy & Fuels, Vol. 9, No. 6, 1995 m-
.oa Ea
800
8
600 400
v
Ann.
1000
*
-0
200
gi
0
0
2
6
4
20
10
0
8
30
40
SO
60
70
Time (hour)
Time (hour)
Figure 8. Adsorption breakthrough curves of FMC calcined char.
Figure 13. Breakthrough curves of NO, adsorption on FMC calcined char. M
4.5
v
3g 3 0
3 3
0
9
6
12
1.5
0 0
10
Time (hour)
1000
8 3a
800 400
v
gm
200 0 0
2
6
4
8
10
Time ( hour)
Figure 10. Adsorption breakthrough curves of FMC coke. JK
,s .g
g3 2
u -
alter 9th cycle
800 600 400 200 t
2
0
1
2
3
4
S
6
Time (hour)
Figure 11. Adsorption breakthrough curves of Mitsui coke. 800 c
a -
6 ,G
4
2
:5
V-8
-P
600
400 200
0 0
3
6
9
40
50
60
70
Time (hour)
Figure 9. Adsorption breakthrough curves of FMC PRB char. +
30
20
12
15
18
Time (hours)
Figure 12. Adsorption breakthrough curves of activated carbon.
enhances both surface areas and adsorption capacities for the FMC carbon adsorbents. FMC calcined char was then used to investigate the mechanisms of NO, adsorption on char and reduction with microwave energy. The breakthrough curves of NO and NO2 adsorption on this char are shown in Figure 13. The amount of NO, removed from the feed gas was calculated as 0.067 04 mol from these break-
Figure 14. Weight gain of FMC calcined char during adsorption of NO,.
through curves by integration. Figure 13 shows that neither NO nor NO2 was detected during the first 4.7 h. However, NO is a supercritical gas a t the ambient temperature and very little NO adsorbs on porous adsorbents.12 Therefore, NO in the gas phase was converted to some compounds which were easily adsorbed on char. Rubel et al.I3 found that the adsorbed species was NOz. Mochida et aL14reported the catalytic oxidation of NO into NO2 over activated carbon fibers. Thus the further oxidation of NO into NO2 must have taken place on the char surface with at least one species in the adsorbed form. Okuhara and Tanaka15 found that the adsorption of NO2 on the K-doped carbon and the adsorption of NO on the oxygen pre-adsorbed K-carbon gave the same NO desorption. Therefore, it may be assumed that oxygen is first adsorbed on the char and then reacts with NO in gas phase to form NO2. The assumed reaction is
NO(g) + O(a)
-
N02(a)
(3)
where g is the species in the gas phase and a represents the adsorbed species. If all the removed NO, were adsorbed as NO2, the calculated weight gain of char would be less than that recorded by the balance (Figure 14). Since the NO2 adsorbed in the carbon is highly hygroscopic, some adsorbed NO2 reacts with the water vapor in gas phase to produce HNO3:
+
-
3N02(a) H20(a)
2HNO,(a)
+ NO(g)
(4)
This reaction is verified by the observation that more water was adsorbed in the presence of NO,. If 89.17 mol % of the removed NO, was adsorbed as HNO3 and the rest adsorbed as NOz, the calculated weight gain of (12) Kaneko, K.; Imai, J. Carbon 1989,27, 954-955. (13) Rubel, A. M.; Stewart, M. L.; Stencel, J. M. Symposium on NO, Reduction, 207th National Meeting, American Chemical Society, San Diego, CA, 1994, pp 137-140. (14) Mochida, I.; Kisamori, S.; Hironaka, M.; Kawano, S.; Matsumura, Y.; Yoshikawa, M. Energy FueZueEs 1994,8, 1341-1344. (15)Okuhara, T.; Tanaka, K. J. Chem. Soc., Faraday Trans. 1 1986, 82,3657-3666.
NO, Abatement with Carbon Adsorbents
Energy & Fuels, Vol. 9, No. 6, 1995 975
I\
0.007f
I
0.006 0.005 0.004 0.003 0.002 0.001
wave regeneration of the char saturated with HNO3 and NO,, we infer that CO should have been produced from reactions between HN03 and carbon at these hot spots, as shown in the following reaction:
+
4HN0, 3C
-
+ 2N0, + C 0 2 + 2CO + 2 N 0
2H,O
(5)
0
0
10
5
IS
20
25
30
35
Time (minute)
Figure 15. Flowrate of product gases in microwave regeneration of NO,-saturated char.
The water vapor which was produced in the regeneration condensed on the tube wall and was thus not detected by the GC. The reactivity of NO2 is so high that it will be completely reduced by carbon and the major product was not N2 but Nolo 3N0, 4-2 c
- 3 N 0 + co
4- CO,
(6)
At elevated temperatures, some adsorbed NO2 will also desorb from the char surface and gaseous NO2 will subsequently decompose into NO and 0 2 : 2N0,
1 0
5
10
15 20 Time (minute)
25
30
Figure 16. Temperature of product gas of microwave regeneration of NO,-saturated char.
-
2N0
+ 0,
The above three reactions contribute to the fast release of NO after the microwave was applied t o the NO,-saturated char. The gaseous NO will also react with carbon at hot spots inside char particles according to reactions 8 and 9. The gas analysis shows that
+ 2C - N, + 2CO 2 N 0 + C - N, + CO,
2N0 char matches reasonably well with that obtained from the balance (Figure 14). The maximum error of this match is 11.75%of the total weight gain. Although both HNO3 and NO2 are adsorbed on char, we still call the char after saturation with NO,-containing feed gas as NO,-saturated char. This NO,-saturated char was regenerated with microwave energy as described in the Experimental Section. The analysis of the product gas is shown in Figure 15. At about 10 s after the microwave generator was turned on, a high NO peak was detected by the NO analyzer with a measurement range of 1000 ppm. There was only very little NO2 released together with NO. After that time, CO and COZstarted coming out and there was no detectable amount of NO, in the product gas. A small amount of (2% in the product gas was probably due t o devolatilization of char. No other gases were detected by the GC. From Figure 15, the CO/CO2 ratio is calculated as 3.5, which implies that these NO, reduction reactions took place at high temperatures. However, the temperature of product gas was a t room temperature for the first 20 min and then increased slowly to 47 "C (Figure 16). This may be due t o the fact that the adsorption sites of NO2 and HN03 could adsorb much more microwave energy and heat up very fast, and the reduction reactions took place at these hot spots. On the other hand, the rest of the char particle did not adsorb so much microwave energy, and the whole char particle remained "cold". Therefore, the temperature of the product gas was still low. Richter et a1.16found that NO, NOz, COZ,and water vapor were the products of reactions between HNo3 and carbon at temperatures between 350 and 450 K. Since there was more CO than COZ produced in the micro(16) Richter, E.; Kleinschmidt, R.; Pilarczyk, E.; Knoblauch, R ; Juntgen, H. Thermochim. Acta 1986,85, 311-314.
(7)
(8)
(9)
reaction 9 is insignificant and can be neglected. Calculations from Figure 15 show that nearly 90% of adsorbed NO, was reduced into N2 and carbon oxides. 4. Conclusion
The microwave regeneration of NO, saturated carbon adsorbent increases both the NO, adsorption capacity and rate, except for the activated carbon. This is accomplished by increasing the internal surface area and nitrogen content. In the presence of moisture and oxygen, NO is converted to NO2 and HNO3 on adsorbent surfaces, which are then adsorbed on the adsorbent. The NO, adsorbed on carbon adsorbents can be reduced using microwave energy with the product gas at room temperature. Nearly 90% of the NO, adsorbed on FMC calcined char can be reduced into Nz and carbon oxides. This NO, abatement process does not need reducing agents such as NH3 and has a high reduction rate of NO,. Another advantage of this process is that it can produce a valuable byproduct, namely activated carbon. This microwave char process can be used t o effectively clean up flue gas. This process can be applied to small sources such as diesel power generators and industrial heaters where it is not economical to convert the adsorbed NO, and HNO3 t o valuable nitrogen chemical compounds. Acknowledgment. The authors gratefully acknowledge the financial support of the National Science Foundation (Grant Number EHR-910-8774), and the experimental assistance of Mr. Hu Wang and Miss Melanie Gores.
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