Activated Carbons Prepared from Extracted-Oil Palm Fibers for Nitric

In this paper, the preparation of activated carbons from extracted-oil palm fibers for the reduction of nitric oxide was studied. Preparation methods ...
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Energy & Fuels 1998, 12, 1089-1094

1089

Activated Carbons Prepared from Extracted-Oil Palm Fibers for Nitric Oxide Reduction Aik Chong Lua* and Jia Guo Division of Thermal and Fluids Engineering, School of Mechanical and Production Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798 Received May 5, 1998. Revised Manuscript Received August 17, 1998

In this paper, the preparation of activated carbons from extracted-oil palm fibers for the reduction of nitric oxide was studied. Preparation methods included two-step CO2 activation, one-step CO2 activation and chemical activation with KOH impregnation. The effects of the heating temperature and hold time on the properties of the chars and activated carbons were investigated. For chemical activation, the effects of KOH concentration and soak time were also studied. The activated carbon with the highest BET surface area was obtained by pyrolysis at 750 °C for 2 h and subsequent activation with CO2 at 850 °C for 30 min for extracted-oil palm fibers impregnated with 0.10 M KOH for 24 h. Experimental results from the thermogravimetric study on the reduction of nitric oxide showed that activated carbons prepared from extracted-oil palm fibers could reduce NOx appreciably either by reaction with NO at high temperatures or by simultaneous reaction and adsorption for a mixture of NO and O2. These activated carbons can be potentially used for NOx reduction in flue-gas streams from high-temperature combustors or downstream of electrostatic precipitators and in some chemical plants operating at room temperature.

Introduction A large amount of solid wastes, such as extracted-oil palm fibers, palm peels, palm stones, and empty fruit bunches, are produced during the palm-oil milling process. For example, in Malaysia, which is the largest palm-oil producer in the world, more than 2 million tons of extracted-oil palm fiber, known also as palm-cake fiber, are generated annually and used as fuel or building materials.1-2 To economically utilize this cheap and abundant solid waste, it is proposed to use it as a prospective starting material for the preparation of activated carbon because of its relatively high fixedcarbon content (about 18 wt %), low ash content (less than 1.0 wt %), and the presence of inherent pore structures. A large number of carbonaceous agricultural solid wastes, such as coconut shell, 3 almond shell and grape seed,4 walnut shell,5 sawdust,6 extracted rockrose,7 and various fruit stones,8-10 have been successfully converted into activated carbons by pyrolysis, followed by (1) Yeoh, B. G.; Idrus, A. Z.; Ong, K. S. J. Sci. Technol. Dev. 1988, 5 (1), 1-13. (2) Tay, J. H. Resour. Conserv. Recycl. 1991, 5, 383-392. (3) Laine, J.; Calafat, A.; Labady, M. Carbon 1989, 27 (2), 191195. (4) Gergova, K.; Petrov, N.; Eser, S. Carbon 1994, 32 (4), 693-702. (5) Hu, Z. H.; Vansant, E. F. Carbon 1995, 33 (5), 561-567. (6) Ferraz, M. C. A. Fuel 1988, 67, 1237-1241. (7) Pastor-Villegas, J.; Valenzuela-Calahorro, C.; Bernalte-Garcia, A.; Gomez-Serrano, V. Carbon 1993, 31 (7), 1061-1069. (8) Caturla, F.; Molina-Sabio, M.; Rodriguez-Reinoso, F. Carbon 1991, 29 (7), 999-1007. (9) Rodriguez-Reinoso, F.; Molina-Sabio, M.; Gonzalez, M. T. Carbon 1995, 33 (1), 15-23. (10) Lussier, M. G.; Shull, J. C.; Miller, D. J. Carbon 1994, 32 (8), 1493-1498.

either physical or chemical activation. No studies on the preparation of activated carbons from extracted-oil palm fibers have been reported in the literature. However, Renouprez and Avom had characterized activated carbons from palm-tree cobs by using nitrogen adsorption and small-angle X-ray scattering (SAXS).11 For carbons prepared physically at different temperatures, the surface areas measured by SAXS were significantly higher than those by nitrogen adsorption. The presence of some submicropores (diameter smaller than 0.4 nm) and mineral impurities which blocked up the pore entrance had resulted in reduced nitrogen adsorption capacity. For activated carbons prepared from cobs with acid pretreatment, their BET area was 970 m2/g, which was close to the X-ray results. Shamsuddin and Williams investigated the devolatilization behavior of palm shell and fiber in a thermogravimetric analyzer over a temperature range between room temperature and 950 °C at a purge gas (N2 or CO2) flow rate of 50 cm3/min with heating rates of 25, 40, and 80 °C/min. Two groups of reactions were evident in the devolatilization process for both fiber and shell. It was found that the yield of char was affected by the material particle size, the type of purge gas, and the devolatilization temperature.12 Activated carbons are widely used as adsorbents for the removal or reduction of pollutants from the exhaust gases of industrial sources such as power stations, kilns, smelters, and nitric acid production plants. Among them nitric oxides (NOx), one of the major air pollutants, has (11) Renouprez, A.; Avom, J. Characterization of Porous Solids; Unger, K. K et al., Eds.; Elsevier Science: Amsterdam, 1988; pp 4954. (12) Shamsuddin, A. H.; Williams, P. T. J. Inst. Energy 1992, 65, 31-34.

10.1021/ef9801118 CCC: $15.00 © 1998 American Chemical Society Published on Web 10/02/1998

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drawn much global attention as the emission standards become more and more stringent. Lu and Toh studied the NO-activated carbon reaction at high temperatures and moderate NO concentrations using a thermogravimetric analyzer.13 The experimental results confirmed that the NO-activated carbon reaction was first order in the temperature range 700-900 °C. It was also observed that the reaction rate increased with carbon consumption, reached a maximum, and then decreased. Hoppe and Huschenbett investigated the adsorption of NO2 onto two different types of activated carbons in a fixed-bed configuration at 30 °C.14 The measured breakthrough curve for WL2-type carbon was found to more closely approximate the ideal curve than the R4-type, which exhibited significant diffusion resistance. They attributed this phenomenon to diffusion resistance of relatively large N2O4 molecules through the meso- and macropores of the activated carbons. Kaneko and Imai also obtained the NO2 adsorption isotherms for cellulose- and pitch-based activated carbon fibers.15 They observed that the amount of NO2 adsorbed at a relative pressure of 0.3 was approximately proportional to the micropore volume of the fiber. Hence, the adsorption of NO2 may be regarded as typical micropore filling. They also found that the micropores swelled during NO2 adsorption and that a part of the adsorbed NO2 was chemisorbed. The aim of this work was to study the preparation of activated carbons from these oil palm wastes to be used for the removal of NOx gaseous pollutant. The influences of the heating temperature and hold time on the chars and final products were investigated. For chemical activation, the effects of solution concentration and soak time were studied. The reduction of NO by (i) its reaction at high temperatures in an atmosphere of N2 as an inert gas, (ii) adsorption of a mixture of NO and O2 at various temperatures, and (iii) simultaneous reaction and adsorption of NO2 was studied using a thermogravimetric analyzer. Experimental Section The extracted-oil palm fibers were obtained from an oil palm mill in Selangor, Malaysia. The as-received fibers were first dried at 110 °C for 24 h to reduce the moisture content. They were then cut, ground, and sieved. The size fraction of 0.51.0 mm was used in this study. Both pyrolysis and activation were carried out in a stainless steel vertical reactor (length of 550 mm and 38 mm i.d.), which was placed in a tube furnace (818P, Lenton) with a programmable PID temperature controller (Figure 1). For chemical activation, the extracted-oil palm fiber was first impregnated with a potassium hydroxide (KOH) solution of various concentrations for various soak times and then washed and dried for further preparation. Nitrogen gas (N2) of 99.9995% purity at a flow rate of 150 cm3/ min and carbon dioxide gas (CO2) of 99.998% purity at a flow rate of 100 cm3/min were used during pyrolysis and activation, respectively. In both cases, a heating rate of 10 °C/min was used. The two-step preparation procedures for activated carbon included pyrolysis of the starting material at 450-950 °C for 0.5-4.5 h. After pyrolysis, the furnace was cooled to room temperature with N2 flushing through the sample. The resulting char was characterized. The pyrolyzed char was then (13) Lu, G. Q.; Toh, K. C. Gas Sep. Purif. 1993, 7 (4), 225-229. (14) Hoppe, H.; Huschenbett, R. Luft Kaeltetch. 1977, 13, 267-270. (15) Kaneko, K.; Imai, J. Carbon 1989, 27, 954-955.

Lua and Guo

Figure 1. Pyrolysis and activation system.

Figure 2. Thermogravimetric analysis system. activated at 500-900 °C for 15-60 min. Before activation with CO2, N2 was flushed through the sample to provide inert conditions. The one-step preparation procedure involved direct activation of the starting material at 700-900 °C for 30-120 min in the presence of CO2. Characterizations of the chars and activated carbons were carried out by nitrogen gas adsorption at -196 °C with an accelerated surface area and porosimetry system (ASAP-2000, Micromeritics). The BET surface area was calculated from N2 adsorption isotherms using the Brunauer-Emmett-Teller (BET) equation.16 The cross-sectional areas for nitrogen molecules were assumed to be 0.162 nm2. The Dubinin-Radushkevich (DR) equation was used to calculate the micropore volume from the N2 adsorption data, and the micropore surface areas were determined from the values obtained for the micropore volumes.17 The pore-size distribution was determined using the BJH model,18 in which the apparent pore radius was estimated from the Kelvin equation.19 The average pore diameter was calculated from 4 times the total pore volume over the Langmuir surface area. The reduction of NO and a mixture of NO and O2 (99.9995% purity) by activated carbons prepared from extracted-oil palm fibers with KOH impregnation was carried out in a thermogravimetric analyzer (TA-50, Shimadzu). The thermogravimetric analyzer system is shown in Figure 2. Since NO is a supercritical gas at room temperature and, therefore, does not adsorb well,20 the study on NO by itself will be shifted to its reaction with carbon at higher temperatures rather than adsorption by activated carbon. NO of various concentrations was obtained by diluting 1000 ppm NO (balanced by N2) with pure nitrogen (99.9995%) through a gas-mixing panel. The gasmixing panel consisted of three rotameters and three fine (16) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: New York, 1982; p 85. (17) Dubinin, M. M. Progress in Surface and Membrane; Academic Press: New York, 1975; Vol. 9. (18) Barrett, E. P.; Joyner, L. C.; Halenda, P. H. J. Am. Chem. Soc. 1951, 73, 373. (19) Defay, R.; Prigogine, I.; Bellemans, A.; Everett, D. H. Surface Tension and Adsorption; Longmans: Essex, U.K., 1966; p 218. (20) Gray, P. G. Gas Sep. Purif. 1993, 7 (4), 213-224.

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Energy & Fuels, Vol. 12, No. 6, 1998 1091

Table 1. Proximate Analyses of Some Typical Samples (wt %)a sample

moisture

raw material N2-650°C-3.5h N2-850°C-3.5h N2-650°C-3.5hfCO2-900°C-15min N2-850°C-3.5hfCO2-900°C-15min CO2-800°C-2h KOH(0.10M)-24hfN2-750°C-2hf (CO2-850°C-30min

0.24 0.22 0.15 0.17 0.31 0.20 0.18

volatile fixed matter carbon 80.57 33.18 8.56 9.09 9.21 5.43 6.60

18.23 61.36 82.84 87.59 81.28 89.72 84.45

ash 0.96 5.24 8.45 3.15 9.20 4.65 8.77

a The notation KOH(0.10M)-24hfN -750°C-2hfCO -850°C2 2 30min means that the material impregnated with 0.10 M KOH for 24 h was subjected to pyrolysis at 750 °C for a hold time of 2 h and then activation with CO2 at 850 °C for a hold time of 30 min.

adjustable valves to control various gas-flow rates and one mixing chamber (dead volume) to allow a homogeneous mixing of various gases. The rotameters were calibrated by the soapfilm burette technique.21 At the beginning of each run, about 20 mg of activated carbon was placed on a platinum sample holder, which was suspended in the furnace. The furnace was subjected to a heating rate of 50 °C/min. Once the furnace temperature had reached a desired reaction temperature, the NO and N2 gas mixture was introduced and, hence, reaction started. The subsequent sample weight change was recorded by data acquisition software. In the presence of O2, some amounts of NO were homogeneously oxidized into NO2, which could be adsorbed strongly by activated carbons since it is a condensable gas. Therefore, both adsorption and reaction (only at high temperature) of NO (also balanced by N2) in the presence of different concentrations of O2 were also studied.

Results and Discussion The results of the proximate analyses for some typical samples are given in Table 1. The proximate analyses showed that the raw material had high volatile matter and low ash contents, and the final products from both physical and chemical activation had relatively high fixed-carbon contents. The influences of heating temperature and hold time on the properties of the chars and activated carbons for both the two-step and one-step procedures are given in Table 2. It could be seen from Table 2 that the yields of chars pyrolyzed at various temperatures with different hold times were around 30%. With activation by CO2, the yields dropped significantly to as low as 5.2% for the case of activation at 900 °C for 60 min, due to reaction of carbon with CO2. The final products from the one-step procedure involving direct CO2 activation also had relatively low yields. By increasing the pyrolysis temperature from 550 to 850 °C for a hold time of 3.5 h (Table 2), the BET as well as micropore surface areas of the chars increased. However, when the pyrolysis temperature was 950 °C, both the BET and micropore surface areas decreased due to a sintering effect and subsequent shrinkage of the sample, resulting in the realignment/attachment of the carbon structure, consequently producing less microporosity. For the pyrolysis temperature of 850 °C, a relatively long hold time, such as 3.5 h, was needed to enhance microporosity as well as to clear the blocked (21) Pursley, W. C. Meas. Control (Flow measurement special issue) 1986, 19 (5), 37-45.

entrances of the micropores. At these optimum pyrolysis conditions, the BET and micropore surface areas were found to be at a maximum. However, if the char pyrolyzed at 850 °C for 3.5 h was used for further activation with CO2 at 900 °C with hold times of 15-60 min, the BET and micropore surface areas decreased. Hence, activation of this char at these conditions was detrimental to pore surface area development. The average pore size increased to as large as 12.4 nm for a hold time of 60 min. CO2-activated carbons prepared from chars pyrolyzed at 650 °C for 3.5 h had the highest BET and micropore surface areas. As the pore structures within the low-temperature chars were not fully developed, activation further enlarged the pores with additional development of new pores, resulting in increases of both the BET and micropore surface areas (almost doubled those of chars alone). For chars pyrolyzed at 550 °C for 3.5 h, the BET and micropore surface areas also increased by more than 50% with activation. For the one-step procedure, CO2 activation tests at 700-900 °C for 30-120 min were carried out. The maximum BET surface area of 243.0 m2/g was obtained at an activation temperature of 800 °C for a hold time of 120 min. At an activation temperature of 900 °C, the BET surface area decreased very significantly, resulting in no micropore surface area. Therefore, the one-step CO2 activation method was unsuitable for the preparation of activated carbons from extracted-oil palm fibers because of the incomplete combustion of the volatile matters with CO2, inhibiting the release of volatile residues and forming tar residues within the carbon structures, which were not favorable for the development of micropores. The influences of activation temperature and hold time on the properties of the activated carbons from oil palm fibers impregnated with 0.10 M KOH for 24 h are given in Table 3. The main purpose of KOH impregnation is to remove as much of the oil contents as possible that are retained in the oil palm fibers after extraction and to prevent the formation of tar or any other liquids that are likely to clog the pores in the pyrolysis process. The chars prepared by pyrolysis at 750 °C for 2 h from fibers previously impregnated with 0.10 M KOH for 24 h already had a BET surface area of 357.3 m2/g. If followed by activation at 850 °C for 30 min, the highest BET surface area of 894.7 m2/g was obtained. However, at a higher activation temperature of 900 °C or a longer hold time of 60 min, the BET and micropore surface areas were reduced due to eventual combustion and shrinkage of carbon contents, leading more microporosities to mesoporosities or even macroporosities. For chemical activation, the influences of KOH concentration and soak time on the properties of activated carbons are given in Table 4. After impregnation, all the samples were pyrolyzed at 750 °C for 2 h, followed by activation with CO2 at 850 °C for 30 min. It could be seen from Table 4 that for the same activation conditions; the overall yield of activated carbons from extracted-oil palm fibers increased with increasing KOH concentration or soak time. The highest BET surface area was obtained for 0.10 M KOH impregnation for 24 h. If the concentrations were too low, the oil contents could not be removed properly, while if the concentra-

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Table 2. Characterization Results of Samples Subjected to Physical Activationa sample

overall yield (%)

BET surface area (m2/g)

micropore surface area (m2/g)

total volume (cm3/g)

average pore size (nm)

N2-550°C-3.5h N2-650°C-3.5h N2-850°C-0.5h N2-850°C-3.5h N2-950°C-3.5h N2-850°C-3.5hfCO2-500°C-15min N2-850°C-3.5hfCO2-900°C-15min N2-850°C-3.5hfCO2-900°C-60min N2-550°C-3.5hfCO2-900°C-15min N2-650°C-3.5hfCO2-900°C-15min CO2-700°C-2h CO2-800°C-2h CO2-900°C-0.5h

33.1 31.1 31.5 28.8 27.8 18.6 12.3 5.2 14.4 13.7 20.9 15.2 11.4

182.9 324.4 246.6 520.6 264.9 310.2 402.2 13.1 303.0 618.5 32.1 243.0 8.8

127.4 235.2 186.8 366.4 161.7 209.7 296.3 0.8 195.6 450.6 15.8 182.1

0.08 0.19 0.14 0.35 0.17 0.18 0.21

1.84 1.65 1.91 1.97 1.84 2.03 3.12 12.43 1.88 1.65 1.87 1.58 11.94

0.19 0.38 0.01 0.09

a The notation N -850°C-3.5hfCO -900°C-15min means that the material was subjected to pyrolysis at 850 °C for a hold time of 3.5 2 2 h and then activation with CO2 at 900 °C for a hold time of 15 min.

Table 3. Characterization Results of Samples Subjected to Chemical Activationa sample

overall yield (%)

BET surface area (m2/g)

micropore surface area (m2/g)

total volume (cm3/g)

average pore size (nm)

KOH(0.10M)-24hfN2-750°C-2h KOH(0.10M)-24hfN2-750°C-2hfCO2-850°C-15min KOH(0.10M)-24hfN2-750°C-2hfCO2-850°C-30min KOH(0.10M)-24hfN2-750°C-2hfCO2-900°C-30min KOH(0.10M)-24hfN2-750°C-2hfCO2-850°C-60min

35.3 22.6 18.5 16.8 17.2

357.3 641.2 894.7 815.0 838.4

254.5 393.0 628.2 523.9 583.8

0.16 0.42 0.67 0.59 0.64

1.46 1.68 1.52 1.76 1.65

a The notation KOH(0.10M)-24hfN -750°C-2hfCO -850°C-30min means that the material impregnated with 0.10 M KOH for 24 h 2 2 was subjected to pyrolysis at 750 °C for a hold time of 2 h and then activation with CO2 at 850 °C for a hold time of 30 min.

Table 4. Activated Carbons Prepared by Various KOH Impregnation Treatmenta KOH conc (M)

soak time (h)

overall yield (%)

BET surface area (m2/g)

micropore surface area (m2/g)

total volume (cm3/g)

average pore size (nm)

0.02 0.04 0.10 0.20 0.40 0.80 0.10 0.10 0.10

24 24 24 24 24 24 12 48 72

15.6 16.9 18.5 20.2 21.5 23.0 17.3 18.4 19.1

543.2 650.3 894.7 842.5 853.2 784.3 865.1 832.6 811.0

308.5 386.3 628.2 501.7 524.6 495.2 544.8 504.4 487.3

0.34 0.37 0.67 0.54 0.56 0.48 0.56 0.52 0.47

1.62 1.54 1.52 1.57 1.50 1.61 1.59 1.58 1.52

a

All samples were pyrolyzed at 750 °C for a hold time of 2 h, followed by activation with CO2 at 850 °C for 30 min.

tions were too high, the residues would inhibit pore development. The influence of soak time on the properties of activated carbons by chemical activation was relatively small. The isotherms of nitrogen adsorption at -196 °C for chars pyrolyzed at 850 °C for 3.5 h and activated carbons produced by two-step activation, one-step activation, and chemical activation are shown in Figure 3. The shape of the adsorption isotherm can provide qualitative information on the adsorption process and the extent of the surface area available to the adsorbate. On the basis of an extensive literature survey, Brunauer et al. found that all adsorption isotherms would fit into one of five basic types (Types I-V).22 All isotherms in Figure 3 are of Type I isotherms, which are associated with microporous structures. The pore size distributions of a char and some typical activated carbons are shown in Figure 4. All pore size distribution curves had sharp increases as the pore diameter decreased below 2 nm, indicating the formation of micropores. Therefore, it is confirmed that these activated carbons are suitable for (22) Brunauer, S.; Deming, L. S.; Deming, W. E.; Teller, E. J. Am. Chem. Soc. 1940, 62, 1723.

Figure 3. Adsorption isotherms of some typical chars and activated carbons.

applications in gaseous pollutant adsorption. Activated carbon impregnated with 0.10 M KOH for 24 h, pyrolyzed in N2 at 750 °C for 2 h and activated with CO2 at 850 °C for 30 min was used in subsequent tests for NO reduction.

Activated Carbons for Nitric Oxide Reduction

Energy & Fuels, Vol. 12, No. 6, 1998 1093

Figure 4. Pore size distributions of some typical chars and activated carbons.

Figure 6. Sample weight gain at low temperatures for NO in the presence of O2.

Figure 7. Sample weight loss at high temperatures for NO in the presence of O2. Figure 5. Sample weight loss at different temperatures for various concentrations of NO.

The weight change characteristics for activated carbon versus time for various NO concentrations at different temperatures in NO reduction tests are shown in Figure 5. At a room temperature of 25 °C, no adsorption was observed even for a NO concentration as high as 1000 ppm since the critical temperature of NO is -94 °C. At 25 °C, NO is not a vapor and, therefore, was not adsorbed. If the temperature was raised to 800 or 900 °C, which would be the common operation temperature of a combustor, NO would react with carbon to form N2 and CO as gaseous products.23 This could, therefore, cause significant weight losses in the samples. CO would then be oxidized to CO2 on the catalytic surface of the activated carbon. The overall reaction could be expressed as

C + 2NO f CO2 + N2

(1)

It could be seen from Figure 5 that both the NO concentration and temperature had significant effects on sample weight loss. Both increasing temperature and increasing NO concentration reduced the sample weight because of the increased reaction between NO and carbon. (23) Furusawa, T.; Kunii, D.; Oguma, A.; Yamada, N. Int. Chem. Eng. 1980, 20, 239-243.

The weight-change characteristics for activated carbon versus time for mixtures of 1000 ppm NO and different concentrations of O2 at low temperatures (2560 °C) are shown in Figure 6. Some chemical plants use selective catalytic NOx reduction at such operating temperatures. In the presence of O2, a certain amount of NO is oxidized into NO2, which can then be adsorbed by the activated carbon as NO2 is a condensable gas. The homogeneous gas-phase reaction between NO and O2 follows third-order kinetics and is slow (the rate constant of the NO-[O] reaction is 7.1 × 10-2 s-1 while that for the NO-O2 reaction is 1.4 × 10-3 s-1).24 However, in this case, the conversion of NO to NO2 is accelerated by the catalytic oxidation action of activated carbon. Oxygen in the gas phase is first adsorbed into the activated carbon in the form of some surface oxygen complex and then reacts with NO to form NO2. The reactions are as follows:25

O2(gas) T 2[O](complex)

(2-a)

NO(gas) + [O](complex) f NO2(adsorbed)

(2-b)

It could be seen from Figure 6 that the O2 concentration had a great influence on the adsorption process. At a temperature of 25 °C, as the O2 concentration decreased (24) Fogler, H. S. Elements of Chemical Reaction Engineering; Prentice Hall: New York, 1992; p. 820. (25) Richter, E.; Knoblauch, K.; Juntgen, H. Gas Sep. Purif. 1987, 1, 35-43.

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Figure 8. Sample weight loss for NO2 desorption.

from 21% to 5%, the amount of NO2 adsorbed also reduced progressively, as seen by reducing weight gains. This was because a higher O2 concentration promoted the conversion of NO to NO2, resulting in greater NO2 adsorption onto the activated carbon surface. The temperature also had an influence on NO2 adsorption. The higher the temperature, the lesser would be the amount of NO2 adsorbed since adsorption is an exothermic process. Figure 7 shows the sample weight-loss characteristics versus time for a mixture of 1000 ppm NO and various concentrations of O2 at high temperatures (100-200 °C), which are normal temperatures of flue gases downstream of electrostatic precipitators. At temperatures higher than 100 °C, a different reaction from that of low temperatures (Figure 6) occurred. NO and NO2 react with carbon forming N2 and CO2 in the presence of O2.26 The following overall reactions occur:

2NO + 2C + O2 f N2 + 2CO2

(3-a)

2NO2 + 2C f N2 + 2CO2

(3-b)

It could be seen from Figure 7 that with increasing temperature from 100 to 200 °C, an increasing sample weight loss occurred. This was because at a higher (26) Kong, Y.; Cha, C. Y. Carbon 1996, 34 (8), 1027-1033.

temperature, the reaction between NO/NO2 and carbon would be greater. However, there was no significant influence of the O2 concentration on the reaction of NO/ NO2 and carbon as the O2 concentration increased from 5% to 21%. This was possibly due to the O2 concentration of 5% being sufficient for (i) the conversion of NO into NO2, or (ii) the reaction between NO and carbon, at this temperature, or these two reactions to occur simultaneously. Figure 8 shows the weight-loss characteristics due to NO2 desorption. NO2 desorption was carried out in a flow of purified nitrogen gas at a temperature of 50 °C higher than that for adsorption. For samples from lowtemperature adsorption (Figure 6), all of the NO2 adsorbed could be desorbed completely, indicating a pure adsorption process for the mixture of NO and O2. For samples from NO and O2 reactions at high temperatures (100 and 150 °C in Figure 7), a very small amount of weight loss occurred during the desorption process, due to the release of NO2 adsorbed. This observation indicated that the reduction in NO for the tests in Figure 7 was due to slight adsorption of NO2 but mainly due to the reactions of NO and NO2 with carbon. Conclusions Experimental results showed that it was feasible to produce activated carbons from extracted-oil palm fibers by a two-step procedure involving either physical activation or chemical activation with KOH impregnation. The activated carbon with the highest BET surface area was obtained by pyrolysis at 750 °C for 2 h, followed by activation with CO2 at 850 °C for 30 min from the fibers impregnated with 0.10 M KOH for 24 h. From tests of NO reduction, it could be seen that activated carbon could reduce NO by reaction with NO alone at high temperatures of 800-900 °C. In a mixture of NO and O2, NO reduction was also possible either by adsorption (at low temperatures of 25-60 °C) or by simultaneous reaction and adsorption (at high temperatures of 100200 °C). To verify these different reactions for NO reduction, actual chemical analyses of inlet and outlet gases to and from the activated carbons will be useful. EF9801118