- Email: [email protected]
Microwave induced reactions of sulfur dioxide and nitrogen oxides in char and anthracite bed
PERGAMON
Carbon 39 (2001) 1159–1166
Microwave induced reactions of sulfur dioxide and nitrogen oxides in char and anthracite bed Chang Yul Cha a , *, Dong Sik Kim b a
Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, WY 82071, USA b Korea Institute of Energy Research, 71 -2 Jang-dong, Yusung-ku, Taejon, South Korea Received 6 August 1999; accepted 2 August 2000
Abstract Microwaves applied to a pyrolytic carbon matrix enhance the chemical reactions of nitric oxide (NO) and sulfur dioxide (SO 2 ) with carbon to produce nitrogen, sulfur, and carbon dioxide. These microwave-induced reactions were investigated to find the feasibility of applying microwaves to directly destroy NO and SO 2 in the combustion product gases or to minimize the formation of these pollutants during combustion process. The char produced from Wyoming subbituminous coal by the FMC Coke Plant in Kemmerer, Wyoming and Korean anthracite coal are an excellent microwave absorber and were used in this investigation as pyrolytic carbon sources. A complete destruction of NO and SO 2 was achieved by microwaves in both char and anthracite bed. An addition of oxygen to the inlet nitrogen stream up to 10% did not affect NO and SO 2 reactions with carbon. 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Char, Pyrolytic carbon; B. Pyrolysis; C. Chromatography; D. Reactivity
1. Introduction Microwaves applied to a pyrolytic carbon such as activated carbon and char enhance the reaction of sulfur dioxide (SO 2 ) and nitrogen oxides (NO x ) with carbon [1–7]. Simplified reactions of carbon with NO and SO 2 are described below, which will depend on the carbon temperature: microwaves
→
C 1 2NO
microwaves
→
C 1 NO
CO 2 1 N 2
(1)
CO 1 1 / 2N 2
(2)
microwaves
→
C 1 SO 2
CO 2 1 S
microwaves
2C 1 SO 2
→
2CO 1 S
(3) (4)
Reactions (1)–(3) are exothermic and hence favorable at low temperatures. Reaction (4) is endothermic and occurs at high temperatures. Reactions (1) and (2) have much greater heat of reaction than reactions (3) and (4). There*Corresponding author. Present address: CHA Corporation, 372 W. Lyon, P.O. Box 1084, Laramie, WY 82070, USA. Tel.: 11-307-742-2829; fax: 11-307-742-4315.
fore, NO may be decomposed selectively by controlling the carbon-bed temperature. The CHA Corporation, in collaboration with the University of Wyoming has developed a new process utilizing microwaves for removing and destroying SO 2 and NOx from combustion flue gases [1,2,8–10]. Laboratory research work was completed under sponsorship of the United States Department of Energy under phase I and phase II SBIR grants [11]. An integrated prototype system capable of treating exhaust gases produced from a 58-hp diesel engine was designed, constructed and perfected through experimentation. The unit was then transported to McClellan Air Force Base in Sacramento, CA for a weeklong demonstration [12,13]. This field demonstration of the prototype was very successful and the process is ready for a commercial demonstration. During the week of field demonstration, the outlet gas contained less than 1 ppm NOx . However, the inlet gas needs to be cooled below 708C and the exhaust gas treatment system is larger in size for the diesel engine. Because of these two disadvantages, the adsorption / microwave-regeneration system is not suitable to small sources such as diesel engines. If cheap carbon sources such as anthracite coal are available for microwave-induced reactions (reactions (1)–(4)), the exhaust gas does not need to be cooled and equipment
0008-6223 / 01 / $ – see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 00 )00240-2
1160
C.Y. Cha, D.S. Kim / Carbon 39 (2001) 1159 – 1166
sizes will be reduced. Therefore, the main objective of this research was to find technical feasibility of destroying SO 2 and NO x in the flue gases by passing the gas through either the char bed or the anthracite bed that is in the microwave field. Nitric oxide may react with carbon in a combustion process if microwaves are applied to carbon particles in the combustor. Therefore, a proper application of microwaves to coal combustion processes such as fluidized bed coal combustion could minimize NO in the flue gases. Because of high oxygen concentration in flue gases, destruction efficiencies of SO 2 and NO were measured as a function of oxygen concentration. After experimental data are obtained, an economic evaluation should be performed to determine the maximum tolerable oxygen concentration in the combustion product gas that makes the application of the microwave process for treatment of combustion product gases economically feasible.
2. Experimental apparatus and procedure The microwave reactor system is shown in Fig. 1. The microwave energy was generated by Cober model S6F industrial microwave generator, which had a variable operating power setting that ranged from 0 to 6000 W at a fixed frequency of 2450 MHz. The load impedance due to the coal bed in the quartz tube was matched to that of the generator with a three-bolt tuner to minimize the reflected power. Two HP power meters were attached to a 60-dB directional coupler to monitor forward and reflected power. As coal bed is heated, its dielectric properties change, thus causing the impedance to change with time. The tuner was adjusted to compensate for this change.
Table 1 Physical properties of char and anthracite
Fixed carbon (%) Ash (%) Volatile matter (%) Surface area (m 2 g 21 ) Apparent density
FMC char
Korean anthracite
89.0 7.9 3.1 57.31 0.64
74.5 20.9 4.6 0.5 1.08
A quartz tube of 2.54 cm in diameter was vertically centered in the 3.437.2-cm wave-guide. This configuration allows microwaves to radiate a cross-section of the coal bed through the quartz tube wall. All gases were passed through regulators, valves and flow meters from the standard gas cylinders except oxygen. The oxygen was supplied by compressed air. Concentrations of NO and SO 2 of inlet and outlet gases were measured by NO and SO 2 analyzer, respectively. Gas chromatograph was used to measure concentrations of CO and CO 2 in the outlet gas. The char produced from Wyoming subbituminous coal by FMC Coke plant in Kemmerer, WY and Korean anthracite were used as a carbon source and also microwave absorber. Korean anthracite contains a larger amount of ash than any other coals, but it has a unique characteristic of absorbing microwaves. Subbituminous coal does not absorb microwaves but char is an excellent microwave absorber. For preparing the carbon beds, they were sieved and calcined. The sizes of char and coal are 10–20 mesh, and other physical properties are shown in the Table 1. The quartz tube was loaded with 15-cm height carbon materials, which were 30 g of FMC char, or 50 g of
Fig. 1. Microwave experimental apparatus.
C.Y. Cha, D.S. Kim / Carbon 39 (2001) 1159 – 1166
anthracite. Their particle sizes were 10–20 mesh. Before use, the beds were devolatilized for 30 min using 300 W microwave power and N 2 gas. The average inlet gas flow-rate was 2.83 l min 21 except experiments determining the effect of gas velocity. With this gas flow-rate the gas superficial velocity in the tube was 10 cm s 21 and residence time in the bed was 1.5 s. The gases were fed into the fixed bed of carbon materials from top to bottom. This prevented any fluidization of the bed, which might take place according to the gas velocity. First, we tested for destruction of NO or SO 2 in nitrogen stream separately and then for the mixed gases. Second, a series of experiments were performed to determine the effect of oxygen on the destruction efficiency. All experiments were repeated and average values of at least two data points are presented in the following section.
3. Results and discussion
3.1. NO decomposition 3.1.1. Effect of gas velocity The effects of superficial gas velocity on the NO decomposition were determined using input microwave power of 300 W. The inlet NO concentration of 920 ppm was obtained by mixing 10% NO standard gas with nitrogen. The minimum fluidization velocity for 10–20mesh coal bed was 41 cm s 21 . Because of too highpressure drop, gas velocities used in tests were lower than the minimum fluidization velocity. Fig. 2 presents NO destruction efficiency as a function of superficial gas velocity.
1161
Nitric oxide in the nitrogen stream was completely destroyed in both anthracite and char bed with 300 W microwave input power when the superficial gas velocity was lower than 15.5 cm s 21 or the gas residence time was greater than 1 s. As shown in Fig. 2, NO destruction efficiency was decreased with increasing superficial gas velocity. However, 90% of NO was destroyed even when the gas velocity was increased to 30 cm s 21 or the gas residence time was reduced to 0.5 s. Destruction efficiency of NO was slightly greater in the char bed than anthracite bed, mainly due to larger surface area of particles.
3.1.2. Effect of microwave input power Effects of microwave input power on the destruction efficiency of nitric oxide were measured using 10 cm s 21 gas velocity, 920 ppm NO concentration in the nitrogen stream and 15-cm carbon bed. The gas residence time in the carbon bed was 1.5 s with this gas velocity. Due to instability of power, the lowest power used for tests was 100 W. Experimental results are presented in Fig. 3. Nitric oxide in nitrogen stream was completely destroyed for the input power greater than 300 W for both anthracite and char bed. Almost 70% NO was destroyed at 100 W input power. The anthracite and char bed produced about the same NO destruction efficiencies at the given input power. 3.1.3. Effect of inlet oxygen concentration Adding compressed air to nitrogen stream varied the oxygen concentration in the inlet gas from 2 to 10%. All tests were run with 920 ppm NO concentration in 2.83 l min 21 inlet gas and input microwave power of 300 W. Nitric oxide was completely destroyed for all oxygen
Fig. 2. Effect of superficial gas velocity on NO destruction efficiency.
1162
C.Y. Cha, D.S. Kim / Carbon 39 (2001) 1159 – 1166
Fig. 3. Effect of microwave power on NO destruction efficiency.
concentrations tested. This indicates that NO can be destroyed during the combustion of fuel if microwave energy can be applied. As expected, carbon was oxidized by oxygen in the inlet gas.
3.2. SO2 decomposition 3.2.1. Effect of gas velocity Input microwave power of 400 W and 1200 ppm SO 2 in nitrogen were used to determine the effect of superficial gas velocity on the decomposition of SO 2 in both char and Korean anthracite. The volume of carbon in the microwave reactor was 77 cm 3 . Fig. 4 presents SO 2 destruction efficiency as a function of superficial gas velocity for char and anthracite beds. A complete destruction of SO 2 was achieved in the char bed when the superficial gas velocity was lower than 15 cm s 21 or the gas residence time is longer than 1 s. The maximum superficial gas velocities that yield 100% destruction of SO 2 and NO in the char bed are about the same. However, the maximum velocity for the complete destruction of SO 2 in the anthracite bed was only 6 cm s 21 (15 cm s 21 for 100% NO destruction). For higher gas velocities, the nitric oxide reacts with carbon easier than sulfur dioxide in the microwave energy field. For an example, at 30 cm s 21 gas velocity, 90% of NO were destroyed but only 80% of SO 2 were destroyed in the char and anthracite beds. 3.2.2. Effect of microwave input power Effects of microwave input power on the destruction efficiency of sulfur dioxide were investigated using 10 cm s 21 gas velocity, 1200 ppm SO 2 concentration in the
nitrogen stream and 15-cm carbon bed. The gas residence time in the carbon bed was 1.5 s with this superficial gas velocity. Because of power instability, the lowest power used for tests was 100 W. Fig. 5 shows the SO 2 destruction efficiencies at various microwave-input powers. Sulfur dioxide in nitrogen stream was completely destroyed for the input power greater than 400 W for both anthracite and char bed. This minimum microwave power required for the complete destruction of SO 2 is 100 W greater than that for 100% NO destruction. About 50% SO 2 was destroyed at 100 W input power. The char bed produced higher SO 2 destruction efficiencies at the given input power when the power was in the range of 160–400 W.
3.2.3. Effect of inlet oxygen concentration Adding compressed air to nitrogen stream varied the oxygen concentration in the inlet gas from 2 to 10%. All tests were run with 1200 ppm SO 2 concentration in 2.83 l min 21 inlet gas and input microwave power of 400 W. A complete destruction of SO 2 was achieved at all oxygen concentrations tested. As expected, carbon was oxidized by oxygen in the inlet gas. 3.3. Simultaneous destruction of NO and SO2 To find a possible interaction between microwave-induced NO–C reaction and SO 2 –C reaction, a mixture of NO and SO 2 in the nitrogen stream was flowed into a 15-cm carbon bed in the quartz tube reactor. Concentrations of NO and SO 2 in the 2.83 l min 21 inlet nitrogen were equally 800 ppm. Destruction efficiencies of NO and SO 2 were measured at various microwave-input powers for
C.Y. Cha, D.S. Kim / Carbon 39 (2001) 1159 – 1166
1163
Fig. 4. Effect of superficial gas velocity of SO 2 destruction efficiency.
both char and anthracite bed. Experimental results are presented in Fig. 6. A comparison of Fig. 6 with Figs. 3 and 5 shows that when NO and SO 2 react with carbon simultaneously, the minimum microwave powers for the complete destruction remain the same as the minimum powers when these
species react with carbon separately. When the powers lower than 200 W, a simultaneous reaction of NO and SO 2 with carbon produced higher destruction efficiencies of NO but lower destruction efficiencies of SO 2 than separate reactions. Concentrations of CO and CO 2 in the outlet gas were
Fig. 5. Effect of microwave power on SO 2 destruction efficiency.
1164
C.Y. Cha, D.S. Kim / Carbon 39 (2001) 1159 – 1166
Fig. 6. Simultaneous destruction of NO and SO 2 .
measured by gas chromatograph. With these concentration data, the percent of total carbon consumed by reacting with NO and SO 2 to produce CO 2 was calculated and presented in Fig. 7 as a function of microwave power. When the power was lower than 200 W, reactions of NO and SO 2 with carbon produced mainly CO 2 according to reactions (1) and (3). However, as the power increased greater than 250 W, a sharp transition occurred and the main product was CO. It is likely that CO 2 produced from NO and SO 2 reactions with carbon further reacts with carbon to produce CO (C1CO 2 →2CO) at power greater than 250 W.
3.4. Carbon balances for reactions of NO and SO2 To check carbon balances, microwave input power of 400 W and inlet gas containing 800 ppm NO, 1600 ppm SO 2 , and 4% of oxygen were used. A 2.5-cm I.D. microwave reactor was packed with 15 cm carbon. This carbon bed contained 30 g of char or 50 g of anthracite. The gas chromatograph was used to measure concentrations of carbon monoxide and carbon dioxide in the outlet gas. Carbon content of remaining material in the reactor was measured after the experiment was complete.
Fig. 7. Percent conversion of NO and SO 2 into CO 2 at various microwave powers.
C.Y. Cha, D.S. Kim / Carbon 39 (2001) 1159 – 1166
1165
Table 2 Carbon balance for the reaction of SO 2 and NO with char Superficial velocity (cm s 21 )
Input carbon (g)
6 9 10 15 18
29.1 29.1 29.1 29.1 29.1
Output Carbon in remaining solid in reactor (g)
CO (%)
CO 2 (%)
Carbon in gas (g)
23.9 22.3 21.1 20.0 17.6
7.65 6.93 6.82 6.92 6.22
1.08 0.98 0.91 0.93 0.82
5.3 7.2 9.4 11.9 12.8
Total out carbon (g)
% Closure
29.1 29.5 30.5 31.9 30.4
100.0 101.4 104.8 109.6 104.5
Table 3 Carbon balance for the reaction of SO 2 and NO with anthracite Superficial velocity (cm s 21 )
Input carbon (g)
6 9 10 15 18
36 36 36 36 36
Output Carbon in remaining solid in reactor (g)
CO (%)
CO 2 (%)
Carbon in gas (g)
30.2 28.4 26.9 25.7 23.9
7.71 7.35 6.96 6.94 6.49
0.98 1.05 0.92 1.24 0.99
5.3 7.7 9.6 12.4 13.6
Amounts of carbon contained in CO and CO 2 were calculated from outlet gas concentration data. Using carbon contents of raw and spent materials, carbon balances were calculated and presented in Tables 2 and 3 at various superficial gas velocities. Considering highly inhomogeneous nature of char and anthracite coal, carbon balance closures are reasonable.
Total out carbon (g)
% Closure
35.5 36.1 36.5 38.1 37.5
98.6 100.3 101.4 105.8 104.1
• The FMC char reacts with NO and SO 2 better than Korean anthracite at the given microwave power. The presence of oxygen does not affect the destruction of NO and SO 2 by microwave energy in the carbon bed. • At lower microwave powers (,200 W in this investigation), NO–C and SO 2 –C reactions produce mainly carbon dioxide but the increase in the power quickly increases the C–CO 2 reaction to produce carbon monoxide.
4. Conclusions and recommendations Microwave-induced reactions of NO and SO 2 with carbon were investigated in the presence or absence of oxygen to provide a technique that can be used to remove and destroy these air pollutants from combustion product gases. The char produced from Wyoming subbituminous coal by the FMC Coke Plant in Kemmerer, Wyoming and Korean anthracite are an excellent microwave absorbent and were used in this investigation to supply carbon. Following conclusions were drawn from experimental results: • A complete destruction of NO and SO 2 can easily be achieved in both char and anthracite bed by microwave energy in the absence or presence of oxygen. • The minimum power for the complete destruction is greater for SO 2 than NO. • At the given microwave power and gas residence time, the destruction efficiency of NO is greater than that of SO 2 .
Because of simplicity of microwave method for the simultaneous removal and destruction of NO and SO 2 , it is highly recommended that the investigation be continued with using a larger microwave reactor system and real combustion product gases. It is also recommended that a possibility of applying microwave energy directly to combustion process to minimize the formation of NO and maximize the combustion efficiency be investigated. An economic evaluation should be performed to determine the maximum tolerable oxygen concentration in the combustion product gas that makes the application of the microwave process for removal and destruction of air pollutants from combustion product gases economically feasible.
References [1] Cha CY. Res Chem Intermed 1994;20(1):13–28. [2] Cha CY. In: Proceedings microwave-induced reactions workshop, 1993, pp. A21–A229, EPRI TR-102252.
1166
C.Y. Cha, D.S. Kim / Carbon 39 (2001) 1159 – 1166
[3] Cha CY. In: Proc 28th microwave symposium proceedings, Montreal, Canada, 1993, p. 74. [4] Cha CY, Kong Y. Carbon 1995;33(8):1141–6. [5] Kong Y, Cha CY. Carbon 1996;34(8):1027–33. [6] Kong Y, Cha CY. Carbon 1996;34(8):1035–40. [7] Kong Y, Cha CY. Energy Fuels 1995;9(6):971–5. [8] Cha CY. Process for selected gas oxides removal by radio frequency catalysts. US patent 5,246,554, 1993. [9] Cha CY. Process for oxide reactions by radio frequency char catalysts. US patent 5,256,265, 1993. [10] Cha CY. Process for selected gas oxides removal by radio frequency catalysts. US patent 5,269,892, 1993. [11] Cha CY, Carlisle CT, Greaves MJ, editors, Development of
an advanced process for simultaneous removal of SO 2 and NO x from flue gas by electromagnetic method, SBIR phase II final report, Department of Energy, 1993, DE-FG0390ER80898. [12] Cha CY, Carlisle CT. Indirect microwave treatment of diesel exhaust gases for NO x control. In: Proceedings of the 1997 diesel engine emissions reduction workshop (July), 1997, pp. 319–22. [13] Rennie DC, Mook PH, Cha CY. Prototype demonstration of CHA NO x removal system for treatment of stationary diesel exhaust. In: Proc 1999 AIChE spring national meeting, Houston, Texas, March 15–18, 1999.
Carbon 39 (2001) 1159–1166
Microwave induced reactions of sulfur dioxide and nitrogen oxides in char and anthracite bed Chang Yul Cha a , *, Dong Sik Kim b a
Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, WY 82071, USA b Korea Institute of Energy Research, 71 -2 Jang-dong, Yusung-ku, Taejon, South Korea Received 6 August 1999; accepted 2 August 2000
Abstract Microwaves applied to a pyrolytic carbon matrix enhance the chemical reactions of nitric oxide (NO) and sulfur dioxide (SO 2 ) with carbon to produce nitrogen, sulfur, and carbon dioxide. These microwave-induced reactions were investigated to find the feasibility of applying microwaves to directly destroy NO and SO 2 in the combustion product gases or to minimize the formation of these pollutants during combustion process. The char produced from Wyoming subbituminous coal by the FMC Coke Plant in Kemmerer, Wyoming and Korean anthracite coal are an excellent microwave absorber and were used in this investigation as pyrolytic carbon sources. A complete destruction of NO and SO 2 was achieved by microwaves in both char and anthracite bed. An addition of oxygen to the inlet nitrogen stream up to 10% did not affect NO and SO 2 reactions with carbon. 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Char, Pyrolytic carbon; B. Pyrolysis; C. Chromatography; D. Reactivity
1. Introduction Microwaves applied to a pyrolytic carbon such as activated carbon and char enhance the reaction of sulfur dioxide (SO 2 ) and nitrogen oxides (NO x ) with carbon [1–7]. Simplified reactions of carbon with NO and SO 2 are described below, which will depend on the carbon temperature: microwaves
→
C 1 2NO
microwaves
→
C 1 NO
CO 2 1 N 2
(1)
CO 1 1 / 2N 2
(2)
microwaves
→
C 1 SO 2
CO 2 1 S
microwaves
2C 1 SO 2
→
2CO 1 S
(3) (4)
Reactions (1)–(3) are exothermic and hence favorable at low temperatures. Reaction (4) is endothermic and occurs at high temperatures. Reactions (1) and (2) have much greater heat of reaction than reactions (3) and (4). There*Corresponding author. Present address: CHA Corporation, 372 W. Lyon, P.O. Box 1084, Laramie, WY 82070, USA. Tel.: 11-307-742-2829; fax: 11-307-742-4315.
fore, NO may be decomposed selectively by controlling the carbon-bed temperature. The CHA Corporation, in collaboration with the University of Wyoming has developed a new process utilizing microwaves for removing and destroying SO 2 and NOx from combustion flue gases [1,2,8–10]. Laboratory research work was completed under sponsorship of the United States Department of Energy under phase I and phase II SBIR grants [11]. An integrated prototype system capable of treating exhaust gases produced from a 58-hp diesel engine was designed, constructed and perfected through experimentation. The unit was then transported to McClellan Air Force Base in Sacramento, CA for a weeklong demonstration [12,13]. This field demonstration of the prototype was very successful and the process is ready for a commercial demonstration. During the week of field demonstration, the outlet gas contained less than 1 ppm NOx . However, the inlet gas needs to be cooled below 708C and the exhaust gas treatment system is larger in size for the diesel engine. Because of these two disadvantages, the adsorption / microwave-regeneration system is not suitable to small sources such as diesel engines. If cheap carbon sources such as anthracite coal are available for microwave-induced reactions (reactions (1)–(4)), the exhaust gas does not need to be cooled and equipment
0008-6223 / 01 / $ – see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 00 )00240-2
1160
C.Y. Cha, D.S. Kim / Carbon 39 (2001) 1159 – 1166
sizes will be reduced. Therefore, the main objective of this research was to find technical feasibility of destroying SO 2 and NO x in the flue gases by passing the gas through either the char bed or the anthracite bed that is in the microwave field. Nitric oxide may react with carbon in a combustion process if microwaves are applied to carbon particles in the combustor. Therefore, a proper application of microwaves to coal combustion processes such as fluidized bed coal combustion could minimize NO in the flue gases. Because of high oxygen concentration in flue gases, destruction efficiencies of SO 2 and NO were measured as a function of oxygen concentration. After experimental data are obtained, an economic evaluation should be performed to determine the maximum tolerable oxygen concentration in the combustion product gas that makes the application of the microwave process for treatment of combustion product gases economically feasible.
2. Experimental apparatus and procedure The microwave reactor system is shown in Fig. 1. The microwave energy was generated by Cober model S6F industrial microwave generator, which had a variable operating power setting that ranged from 0 to 6000 W at a fixed frequency of 2450 MHz. The load impedance due to the coal bed in the quartz tube was matched to that of the generator with a three-bolt tuner to minimize the reflected power. Two HP power meters were attached to a 60-dB directional coupler to monitor forward and reflected power. As coal bed is heated, its dielectric properties change, thus causing the impedance to change with time. The tuner was adjusted to compensate for this change.
Table 1 Physical properties of char and anthracite
Fixed carbon (%) Ash (%) Volatile matter (%) Surface area (m 2 g 21 ) Apparent density
FMC char
Korean anthracite
89.0 7.9 3.1 57.31 0.64
74.5 20.9 4.6 0.5 1.08
A quartz tube of 2.54 cm in diameter was vertically centered in the 3.437.2-cm wave-guide. This configuration allows microwaves to radiate a cross-section of the coal bed through the quartz tube wall. All gases were passed through regulators, valves and flow meters from the standard gas cylinders except oxygen. The oxygen was supplied by compressed air. Concentrations of NO and SO 2 of inlet and outlet gases were measured by NO and SO 2 analyzer, respectively. Gas chromatograph was used to measure concentrations of CO and CO 2 in the outlet gas. The char produced from Wyoming subbituminous coal by FMC Coke plant in Kemmerer, WY and Korean anthracite were used as a carbon source and also microwave absorber. Korean anthracite contains a larger amount of ash than any other coals, but it has a unique characteristic of absorbing microwaves. Subbituminous coal does not absorb microwaves but char is an excellent microwave absorber. For preparing the carbon beds, they were sieved and calcined. The sizes of char and coal are 10–20 mesh, and other physical properties are shown in the Table 1. The quartz tube was loaded with 15-cm height carbon materials, which were 30 g of FMC char, or 50 g of
Fig. 1. Microwave experimental apparatus.
C.Y. Cha, D.S. Kim / Carbon 39 (2001) 1159 – 1166
anthracite. Their particle sizes were 10–20 mesh. Before use, the beds were devolatilized for 30 min using 300 W microwave power and N 2 gas. The average inlet gas flow-rate was 2.83 l min 21 except experiments determining the effect of gas velocity. With this gas flow-rate the gas superficial velocity in the tube was 10 cm s 21 and residence time in the bed was 1.5 s. The gases were fed into the fixed bed of carbon materials from top to bottom. This prevented any fluidization of the bed, which might take place according to the gas velocity. First, we tested for destruction of NO or SO 2 in nitrogen stream separately and then for the mixed gases. Second, a series of experiments were performed to determine the effect of oxygen on the destruction efficiency. All experiments were repeated and average values of at least two data points are presented in the following section.
3. Results and discussion
3.1. NO decomposition 3.1.1. Effect of gas velocity The effects of superficial gas velocity on the NO decomposition were determined using input microwave power of 300 W. The inlet NO concentration of 920 ppm was obtained by mixing 10% NO standard gas with nitrogen. The minimum fluidization velocity for 10–20mesh coal bed was 41 cm s 21 . Because of too highpressure drop, gas velocities used in tests were lower than the minimum fluidization velocity. Fig. 2 presents NO destruction efficiency as a function of superficial gas velocity.
1161
Nitric oxide in the nitrogen stream was completely destroyed in both anthracite and char bed with 300 W microwave input power when the superficial gas velocity was lower than 15.5 cm s 21 or the gas residence time was greater than 1 s. As shown in Fig. 2, NO destruction efficiency was decreased with increasing superficial gas velocity. However, 90% of NO was destroyed even when the gas velocity was increased to 30 cm s 21 or the gas residence time was reduced to 0.5 s. Destruction efficiency of NO was slightly greater in the char bed than anthracite bed, mainly due to larger surface area of particles.
3.1.2. Effect of microwave input power Effects of microwave input power on the destruction efficiency of nitric oxide were measured using 10 cm s 21 gas velocity, 920 ppm NO concentration in the nitrogen stream and 15-cm carbon bed. The gas residence time in the carbon bed was 1.5 s with this gas velocity. Due to instability of power, the lowest power used for tests was 100 W. Experimental results are presented in Fig. 3. Nitric oxide in nitrogen stream was completely destroyed for the input power greater than 300 W for both anthracite and char bed. Almost 70% NO was destroyed at 100 W input power. The anthracite and char bed produced about the same NO destruction efficiencies at the given input power. 3.1.3. Effect of inlet oxygen concentration Adding compressed air to nitrogen stream varied the oxygen concentration in the inlet gas from 2 to 10%. All tests were run with 920 ppm NO concentration in 2.83 l min 21 inlet gas and input microwave power of 300 W. Nitric oxide was completely destroyed for all oxygen
Fig. 2. Effect of superficial gas velocity on NO destruction efficiency.
1162
C.Y. Cha, D.S. Kim / Carbon 39 (2001) 1159 – 1166
Fig. 3. Effect of microwave power on NO destruction efficiency.
concentrations tested. This indicates that NO can be destroyed during the combustion of fuel if microwave energy can be applied. As expected, carbon was oxidized by oxygen in the inlet gas.
3.2. SO2 decomposition 3.2.1. Effect of gas velocity Input microwave power of 400 W and 1200 ppm SO 2 in nitrogen were used to determine the effect of superficial gas velocity on the decomposition of SO 2 in both char and Korean anthracite. The volume of carbon in the microwave reactor was 77 cm 3 . Fig. 4 presents SO 2 destruction efficiency as a function of superficial gas velocity for char and anthracite beds. A complete destruction of SO 2 was achieved in the char bed when the superficial gas velocity was lower than 15 cm s 21 or the gas residence time is longer than 1 s. The maximum superficial gas velocities that yield 100% destruction of SO 2 and NO in the char bed are about the same. However, the maximum velocity for the complete destruction of SO 2 in the anthracite bed was only 6 cm s 21 (15 cm s 21 for 100% NO destruction). For higher gas velocities, the nitric oxide reacts with carbon easier than sulfur dioxide in the microwave energy field. For an example, at 30 cm s 21 gas velocity, 90% of NO were destroyed but only 80% of SO 2 were destroyed in the char and anthracite beds. 3.2.2. Effect of microwave input power Effects of microwave input power on the destruction efficiency of sulfur dioxide were investigated using 10 cm s 21 gas velocity, 1200 ppm SO 2 concentration in the
nitrogen stream and 15-cm carbon bed. The gas residence time in the carbon bed was 1.5 s with this superficial gas velocity. Because of power instability, the lowest power used for tests was 100 W. Fig. 5 shows the SO 2 destruction efficiencies at various microwave-input powers. Sulfur dioxide in nitrogen stream was completely destroyed for the input power greater than 400 W for both anthracite and char bed. This minimum microwave power required for the complete destruction of SO 2 is 100 W greater than that for 100% NO destruction. About 50% SO 2 was destroyed at 100 W input power. The char bed produced higher SO 2 destruction efficiencies at the given input power when the power was in the range of 160–400 W.
3.2.3. Effect of inlet oxygen concentration Adding compressed air to nitrogen stream varied the oxygen concentration in the inlet gas from 2 to 10%. All tests were run with 1200 ppm SO 2 concentration in 2.83 l min 21 inlet gas and input microwave power of 400 W. A complete destruction of SO 2 was achieved at all oxygen concentrations tested. As expected, carbon was oxidized by oxygen in the inlet gas. 3.3. Simultaneous destruction of NO and SO2 To find a possible interaction between microwave-induced NO–C reaction and SO 2 –C reaction, a mixture of NO and SO 2 in the nitrogen stream was flowed into a 15-cm carbon bed in the quartz tube reactor. Concentrations of NO and SO 2 in the 2.83 l min 21 inlet nitrogen were equally 800 ppm. Destruction efficiencies of NO and SO 2 were measured at various microwave-input powers for
C.Y. Cha, D.S. Kim / Carbon 39 (2001) 1159 – 1166
1163
Fig. 4. Effect of superficial gas velocity of SO 2 destruction efficiency.
both char and anthracite bed. Experimental results are presented in Fig. 6. A comparison of Fig. 6 with Figs. 3 and 5 shows that when NO and SO 2 react with carbon simultaneously, the minimum microwave powers for the complete destruction remain the same as the minimum powers when these
species react with carbon separately. When the powers lower than 200 W, a simultaneous reaction of NO and SO 2 with carbon produced higher destruction efficiencies of NO but lower destruction efficiencies of SO 2 than separate reactions. Concentrations of CO and CO 2 in the outlet gas were
Fig. 5. Effect of microwave power on SO 2 destruction efficiency.
1164
C.Y. Cha, D.S. Kim / Carbon 39 (2001) 1159 – 1166
Fig. 6. Simultaneous destruction of NO and SO 2 .
measured by gas chromatograph. With these concentration data, the percent of total carbon consumed by reacting with NO and SO 2 to produce CO 2 was calculated and presented in Fig. 7 as a function of microwave power. When the power was lower than 200 W, reactions of NO and SO 2 with carbon produced mainly CO 2 according to reactions (1) and (3). However, as the power increased greater than 250 W, a sharp transition occurred and the main product was CO. It is likely that CO 2 produced from NO and SO 2 reactions with carbon further reacts with carbon to produce CO (C1CO 2 →2CO) at power greater than 250 W.
3.4. Carbon balances for reactions of NO and SO2 To check carbon balances, microwave input power of 400 W and inlet gas containing 800 ppm NO, 1600 ppm SO 2 , and 4% of oxygen were used. A 2.5-cm I.D. microwave reactor was packed with 15 cm carbon. This carbon bed contained 30 g of char or 50 g of anthracite. The gas chromatograph was used to measure concentrations of carbon monoxide and carbon dioxide in the outlet gas. Carbon content of remaining material in the reactor was measured after the experiment was complete.
Fig. 7. Percent conversion of NO and SO 2 into CO 2 at various microwave powers.
C.Y. Cha, D.S. Kim / Carbon 39 (2001) 1159 – 1166
1165
Table 2 Carbon balance for the reaction of SO 2 and NO with char Superficial velocity (cm s 21 )
Input carbon (g)
6 9 10 15 18
29.1 29.1 29.1 29.1 29.1
Output Carbon in remaining solid in reactor (g)
CO (%)
CO 2 (%)
Carbon in gas (g)
23.9 22.3 21.1 20.0 17.6
7.65 6.93 6.82 6.92 6.22
1.08 0.98 0.91 0.93 0.82
5.3 7.2 9.4 11.9 12.8
Total out carbon (g)
% Closure
29.1 29.5 30.5 31.9 30.4
100.0 101.4 104.8 109.6 104.5
Table 3 Carbon balance for the reaction of SO 2 and NO with anthracite Superficial velocity (cm s 21 )
Input carbon (g)
6 9 10 15 18
36 36 36 36 36
Output Carbon in remaining solid in reactor (g)
CO (%)
CO 2 (%)
Carbon in gas (g)
30.2 28.4 26.9 25.7 23.9
7.71 7.35 6.96 6.94 6.49
0.98 1.05 0.92 1.24 0.99
5.3 7.7 9.6 12.4 13.6
Amounts of carbon contained in CO and CO 2 were calculated from outlet gas concentration data. Using carbon contents of raw and spent materials, carbon balances were calculated and presented in Tables 2 and 3 at various superficial gas velocities. Considering highly inhomogeneous nature of char and anthracite coal, carbon balance closures are reasonable.
Total out carbon (g)
% Closure
35.5 36.1 36.5 38.1 37.5
98.6 100.3 101.4 105.8 104.1
• The FMC char reacts with NO and SO 2 better than Korean anthracite at the given microwave power. The presence of oxygen does not affect the destruction of NO and SO 2 by microwave energy in the carbon bed. • At lower microwave powers (,200 W in this investigation), NO–C and SO 2 –C reactions produce mainly carbon dioxide but the increase in the power quickly increases the C–CO 2 reaction to produce carbon monoxide.
4. Conclusions and recommendations Microwave-induced reactions of NO and SO 2 with carbon were investigated in the presence or absence of oxygen to provide a technique that can be used to remove and destroy these air pollutants from combustion product gases. The char produced from Wyoming subbituminous coal by the FMC Coke Plant in Kemmerer, Wyoming and Korean anthracite are an excellent microwave absorbent and were used in this investigation to supply carbon. Following conclusions were drawn from experimental results: • A complete destruction of NO and SO 2 can easily be achieved in both char and anthracite bed by microwave energy in the absence or presence of oxygen. • The minimum power for the complete destruction is greater for SO 2 than NO. • At the given microwave power and gas residence time, the destruction efficiency of NO is greater than that of SO 2 .
Because of simplicity of microwave method for the simultaneous removal and destruction of NO and SO 2 , it is highly recommended that the investigation be continued with using a larger microwave reactor system and real combustion product gases. It is also recommended that a possibility of applying microwave energy directly to combustion process to minimize the formation of NO and maximize the combustion efficiency be investigated. An economic evaluation should be performed to determine the maximum tolerable oxygen concentration in the combustion product gas that makes the application of the microwave process for removal and destruction of air pollutants from combustion product gases economically feasible.
References [1] Cha CY. Res Chem Intermed 1994;20(1):13–28. [2] Cha CY. In: Proceedings microwave-induced reactions workshop, 1993, pp. A21–A229, EPRI TR-102252.
1166
C.Y. Cha, D.S. Kim / Carbon 39 (2001) 1159 – 1166
[3] Cha CY. In: Proc 28th microwave symposium proceedings, Montreal, Canada, 1993, p. 74. [4] Cha CY, Kong Y. Carbon 1995;33(8):1141–6. [5] Kong Y, Cha CY. Carbon 1996;34(8):1027–33. [6] Kong Y, Cha CY. Carbon 1996;34(8):1035–40. [7] Kong Y, Cha CY. Energy Fuels 1995;9(6):971–5. [8] Cha CY. Process for selected gas oxides removal by radio frequency catalysts. US patent 5,246,554, 1993. [9] Cha CY. Process for oxide reactions by radio frequency char catalysts. US patent 5,256,265, 1993. [10] Cha CY. Process for selected gas oxides removal by radio frequency catalysts. US patent 5,269,892, 1993. [11] Cha CY, Carlisle CT, Greaves MJ, editors, Development of
an advanced process for simultaneous removal of SO 2 and NO x from flue gas by electromagnetic method, SBIR phase II final report, Department of Energy, 1993, DE-FG0390ER80898. [12] Cha CY, Carlisle CT. Indirect microwave treatment of diesel exhaust gases for NO x control. In: Proceedings of the 1997 diesel engine emissions reduction workshop (July), 1997, pp. 319–22. [13] Rennie DC, Mook PH, Cha CY. Prototype demonstration of CHA NO x removal system for treatment of stationary diesel exhaust. In: Proc 1999 AIChE spring national meeting, Houston, Texas, March 15–18, 1999.