Physical Activation of Rice Husk Pyrolysis Char for the Production of

Article pubs.acs.org/IECR

Physical Activation of Rice Husk Pyrolysis Char for the Production of High Surface Area Activated Carbons Jon Alvarez, Gartzen Lopez, Maider Amutio, Javier Bilbao, and Martin Olazar* Department of Chemical Engineering, University of the Basque Country UPV/EHU, P.O. Box 644, E48080 Bilbao, Spain ABSTRACT: A study has been performed on the production of high quality activated carbon from rice husk char obtained by flash pyrolysis in a conical spouted bed reactor. In order to enhance both the quality of the adsorbents obtained and the economy of the process, the amorphous silica contained in the char has been recovered with Na2CO3. Subsequently, the resulting carbonaceous material has been subjected to physical activation at 800 °C using steam and carbon dioxide as activating agents. Although the maximum BET surface areas obtained with carbon dioxide and steam are similar, 1514 and 1365 m2 g−1, respectively, significant differences are observed in the performance of the two oxidizers. Thus, steam gasification kinetics is faster than that of carbon dioxide. The carbons produced by activation using both steam and carbon dioxide have a wide pore size distribution, with microporous structure being more developed when the latter is used. m2 g−1). However, the difficulties associated with the recovery of the activating agent as well as the need for a stringency wash of the adsorbents produced in order to avoid the corrosion of the equipment and interferences in the adsorption process would increase the overall cost of the process and hinder its implementation at large scale.29 This paper approaches the physical or thermal activation of the rice husk char obtained by flash pyrolysis4 and subjected to a desilication treatment.5 Thermal activation is known to be environmentally more friendly and less costly than chemical methods. This carbonaceous precursor is subjected to partial gasification using steam and carbon dioxide as activating agents. The reactions of carbon with steam and carbon dioxide are endothermic and easy to control:

1. INTRODUCTION Rice is one of the most common food crops cultivated in numerous countries and a staple food for more than one-third of the global population.1 According to recent estimations the global production of rice is around 685 Mt, with production of rice husk per kg harvested being between 0.2 and 0.33 kg.2 Rice husk is usually handled as a waste and burnt in open fields, but this practice involves serious environmental and human health problems due to the formation of fine crystalline silica particles, which remain suspended in the air.3 Fast pyrolysis is a feasible alternative for the thermo-chemical conversion of rice husk to produce high bio-oil yields and char.4 Rice husk char usually contains more than 60% silica, which may be efficiently recovered as amorphous silica.5 Recovery of this silica contributes to solving the problem of waste disposal and guaranteeing the long-term availability of natural silicon resources.6 Furthermore, it promotes the economy of rice husk valorization by pyrolysis. The silica from rice husk char has been widely used as construction material to produce concrete. In fact, the pozzolanic role of this material improves cement properties.7,8 It has also been used in the production of catalysts,9,10 amorphous silica nanoparticles,11 silica gel12 (rice husk is considered to be the cheapest route) or glass.13 In recent years, there has been a growing interest in the production of activated carbons from agricultural byproducts14−16 and residual wastes.17,18 In this sense, rice husk char has been widely used as a precursor for the production of activated carbons, which are applied in an aqueous medium for the adsorption of different pollutants, such as phenols,19 metals ions20,21 and volatile organic compounds.22 The main challenge for the application of this material for the production of activated carbon lies in its high ash content, and therefore silica removal is a key factor for the quality of the adsorbent obtained.23,24 Chemical activation of rice husk char has been carried out using different activating agents, such as alkali hydroxides (NaOH/KOH),25−27 inorganic acids (H3PO4, HCl and H2SO4)19,28 or ZnCl2.27 In general, the carbons produced are mainly microporous with high specific surface areas (up to 3000 © 2015 American Chemical Society

C + CO2 → CO

ΔH = 159 kJ mol−1

C + H 2O → CO + H 2

ΔH = 117 kJ mol−1

(1) (2)

In addition, the following equilibrium involving steam may occur around 800 °C: CO + H 2O ↔ CO2 + H 2

ΔH = − 41 kJ mol−1

(3)

Both steam20,22,24,30,31 and carbon dioxide32,33 have been used to produce activated carbons from rice husk, even though the carbons produced in some of these runs were of limited quality (with BET surface areas below 400 m2 g−1) due to the high ash content. However, Deiana et al.24 obtained an activated carbon of high surface area (1180 m2 g−1) by applying a leaching treatment with HF. The production of high quality activated carbon for use as adsorbent or catalyst support34 is a complementary objective to amorphous silica separation,5 which contributes to improving Received: Revised: Accepted: Published: 7241

April 28, 2015 June 30, 2015 July 2, 2015 July 2, 2015 DOI: 10.1021/acs.iecr.5b01589 Ind. Eng. Chem. Res. 2015, 54, 7241−7250

Article

Industrial & Engineering Chemistry Research

Figure 1. Experimental procedure followed for the production of amorphous silica and carbonaceous precursor.

Figure 2. Schematic diagram of the activation unit.

with 15 wt % Na2CO3 solution. Once the suspension has been filtered, the precipitate is the carbon material to be used in the production of activated carbon (precursor). Therefore, the filtrate obtained in the carbonation process is a solution of sodium silicate. 2.2. Physical actIvation in a Bench Scale Plant. A scheme of the activation plant used in this study is shown in Figure 2. The same unit has already been used for the physical activation of the pyrolysis char derived from waste tires.35 The activation took place in a fixed bed reactor placed inside a radiant oven that provides heat to operate at temperatures of up

the perspectives for the economy of the full scale valorization of rice husk by flash pyrolysis.

2. EXPERIMENTAL SECTION 2.1. Silica Extraction Procedure. In order to improve the properties of the rice husk char as activated carbon promoter, the amorphous silica was removed and the procedure followed is described in detail a previous study.5 Figure 1 shows a scheme of the procedure followed. First, rice husk char was treated with HCl (1 M) to remove most of the impurities and improve silica purity. The next step was to remove the silica 7242

DOI: 10.1021/acs.iecr.5b01589 Ind. Eng. Chem. Res. 2015, 54, 7241−7250

Article

Industrial & Engineering Chemistry Research to 1000 °C. The reactor supplied by Autoclave Engineers has an internal diameter of 13.1 mm and a total length of 340 mm. The unit is provided with a pressure meter to ensure that pressure in the reactor is not above 1.2 atm during the activation. Both the reactor and the oven are located in a forced convection oven (hot box) at 270 °C in order to avoid the condensation of the volatiles released during the carbonization step. Nitrogen and carbon dioxide are fed into the reactor by means of two mass flow meters. When steam is used as activating agent, a high precision Gilson 307 pump is used to control the water flow rate. Furthermore, at the inlet of the hot box there is a heating coil with an electric cartridge in order to ensure the immediate and homogeneous vaporization of the water stream. Finally, the nonreacted steam was condensed at the outlet of the reactor in a Peltier cooler. 2.3. Experimental Procedure of the Activation Process. In each activation run, 1 g of desilicated rice husk char was used. The sample was heated at a rate of 15 °C min−1 to the activation temperature (800 °C), in an inert atmosphere (nitrogen flow). Once this temperature has been reached, the nitrogen flow was maintained for 1 h in order to ensure complete carbonization of the sample. When the carbonization process was completed, the activating gas mixture was continuously fed into the reactor with a flow rate of 200 cm3 min−1 measured at normal conditions. The activating gas was composed of either carbon dioxide or steam diluted in nitrogen at a ratio of 75:25. The effect of activation time has been studied by varying the residence time of the sample under activation conditions from 15 to 60 min. Once the final activation time has been reached, the reactor was cooled under nitrogen atmosphere and the activated carbon was then removed from the reactor and weighed in order to determine the burnoff undergone in the reaction. The burnoff refers to the difference between the weight of the desilicated char (before carbonization) and the activated carbon divided by the weight of desilicated char, i.e: burn‐off (%) =

W0 − WAC × 100 W0

isotherms obtained in Micromeritics ASAP 2000 by following Brunauer−Emmett−Teller (BET) and Barret−Joyner−Halenda (BJH) methods, respectively. Prior to analysis, each sample was thoroughly degassed at 150 °C for 15 h under a vacuum pressure below 10−3 mmHg. Surface morphology and characteristics of the rice husk char and activated carbons have analyzed in a JEOL JSM-6400 scanning electronic microscope (SEM). The surface organic functional groups have been determined by Fourier transform infrared spectrometer (FTIR), Thermo Nicolet 6700.

3. RESULTS AND DISCUSSION 3.1. Rice Husk Char Production and Characterization. The carbonaceous material used for producing activated carbons was rice husk char obtained by continuous flash pyrolysis carried out in a conical spouted bed reactor. The char sample used for activation was obtained operating at 500 °C, with its yield being 26 wt % at this temperature and those of gas and bio-oil 6 and 68 wt %, respectively.4 The excellent performance of the spouted bed reactor for pyrolysis and gasification of biomass,36−38 plastics,39,40 and waste tires41,42 has been proven in previous studies. This reactor is characterized by a high solid circulation rate, which gives way to high heating rates43 and, therefore, bed isothermicity. Furthermore, the residence time in the reactor is as short as hundreds of milliseconds, which minimizes secondary reactions in the reactor. These pyrolysis conditions lead to low char yields, but of relatively high quality, given that they avoid deposition of carbonaceous residues on the char surface and thus minimize pore blocking.41 Furthermore, char particles are continuously removed from the bed throughout continuous operation, which avoids their accumulation in the bed. The fountain region of the conical spouted bed reactor is characterized by the segregation of different density materials.41 In this region, the solids of lower density (char particles) describe higher and wider trajectories, whereas the heavier particles (sand and unreacted rice husk) record lower heights. Based on this segregation, the removal of char from the reactor is carried out through a lateral pipe placed higher than the bed surface. Table 1 shows the proximate analysis, ultimate analysis, and other rice husk char and precursor characteristics. The most significant feature of the rice husk char is its high ash content (51.3 wt %). This content hinders the application of this material as either fuel or active carbon. Table 2 shows the

(4)

where W0 is the weight of the desilicated char and WAC is the mass of the carbon after the activation. A previous study dealing with the steam activation of rice husk char showed that 800 °C was an adequate temperature for the activation process.5 In fact, higher temperatures caused an excessively fast and uncontrolled gasification of the sample. In order to ensure the reproducibility of the results, certain activation runs have been repeated and burnoff and surface properties of the resulting carbons determined. The differences in the burnoff values were below 2% in all cases. The differences observed in the surface properties of the activated carbons were slightly higher, but never higher than 5%. 2.4. Analysis of Physical, Chemical, and Morphological Properties. The ultimate and proximate analyses of the chars and active carbons have been carried out in a LECO CHNS-932 elemental analyzer and in a TGA Q500IR thermogravimetric analyzer, respectively. The high heating value has been measured in a Parr 1356 isoperibolic bomb calorimeter. The chemical composition of the ash from the char, including silica and major metal compounds was analyzed and quantified by X-ray fluorescence (model AXIOS, PANalytical). Surface area and pore volume and distribution have been determined from nitrogen adsorption−desorption

Table 1. Properties of the Raw Rice Husk, Pyrolysis Char, and the Char after Silica Removal

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ultimate analysis (wt %)

rice husk

char

precursor

carbon hydrogen nitrogen oxygen Proximate Analysis (wt %) volatile matter fixed carbon ash BET surface area (m2 g−1) pore volume (cm3 g−1) higher heating value (MJ kg−1)

42.0 5.4 0.4 39.3

45.2 1.5 0.4 1.7

83.1 2.9 0.7 6.9

70.5 16.6 12.9

12.8 36 51.3 25.6 0.05 14.4

26.7 67 6.3 227 0.17 32.0

16.8


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3.2. Physical Activation with Carbon Dioxide and Steam. The main properties of the carbons produced by physical activation with steam and carbon dioxide for different times are listed in Table 3. Figure 4a compares the evolution of burnoff with activation time when steam and carbon dioxide are used as activating agents at 800 °C. As observed, the gasification rate is higher when steam is used as an activating agent. This trend is especially remarkable for short activation times; in fact the slopes of both curves are similar after 30 min activation. The faster kinetics of steam activation than carbon dioxide has been observed by several authors in the activation of different carbonaceous materials.45−51 This difference is usually related to the reaction energies when different activating agents are used.51 The initial burnoff value shown in Figure 4a is 23.5%, and this mass loss is related to the carbonization step of the sample and subsequent release of volatiles due to the difference in temperature between pyrolysis (500 °C) and carbonization (800 °C). Furthermore, the carbonization step is associated with a significant pore development in the sample. Thus, the carbon residue obtained after silica removal has a BET surface area of 227 m2 g−1, but that of the carbonized sample is as high as 486 m2 g−1 (see Table 3). Other authors have also observed a positive effect of carbonization temperature increase on the surface properties of the rice husk char.52 Apart from the different activation kinetics throughout burnoff evolution, there are also significant differences regarding the development of the porous structure when steam or carbon dioxide are used as gasifying agents (see Figure 4b and c). Figure 4b shows the evolution of BET surface area with activation time. The BET surface area peaks (1365 m2 g−1) for an activation time of only 15 min using steam as activation agent, whereas the optimum activation time in the case of carbon dioxide is 45 min (1514 m2 g−1). Furthermore, a dramatic decrease in the BET surface area was observed in the steam activation for times longer than 15 min. Thus, subsequent to 30 min activation the BET area was as low as 905 m2 g−1. The reduction in the BET area is not so pronounced in the case of carbon dioxide when the activation time is increased from 45 to 60 min (1326 m2 g−1). Based on the evolution of the BET surface area with burnoff (see Figure 4c), the differences between steam and carbon dioxide are not so evident. The maximum BET surface area was obtained in the case of steam for a burnoff level of 62%, but in the case of carbon dioxide the surface area increased up to a burnoff value of 74%. Although the BET surface area is determinant in the price of the activated carbon obtained,44 the yield should also be taken into account. Thus, the optimum activated carbon with steam was obtained for a lower burnoff level than that obtained with carbon dioxide. Furthermore, the product of activated carbon yield by BET surface area (BET surface area obtained by mass unit of the original rice husk char) for the activated carbons of maximum BET surface areas is clearly higher in the case of steam than carbon dioxide, 519 vs 393 m2g−1. Furthermore, the value obtained with steam for low burnoff values cannot be reached in the case of carbon dioxide. The BET surface areas obtained in this study are relatively high compared to other physical activation studies of rice husk char in the literature. Thus, Kumagai et al.33 studied the activation with carbon dioxide of different samples of rice husk char previously pyrolyzed at 400 °C. Under the optimum conditions, 850 °C and 60 min, the maximum BET surface area obtained was 473 m2 g−1, with burnoff being 73%. Hsi et al.22

chemical composition of the rice husk ash, which is almost entirely composed of silica (98%). Table 2. Chemical Composition of the Rice Husk Ash compound

wt %

SiO2 Al2O3 Fe2O3t MnO MgO CaO Na2O K2O TiO2 P2O5 others

98.02 0.52 0.11 0.01 0.11 0.23 0.10 0.38 0.02 0.08 0.42

The BET surface area of the rice husk char is as low as 25.6 m2 g−1. However, once silica has been extracted, the surface area of the carbon material precursor reaches a value of 227 m2 g−1; that is, the removal of silica form the carbonaceous matrix gives way to the formation of a relatively well developed porous structure.44 Furthermore, the silica extraction procedure allows recovering up to 88% silica from the char. As mentioned in the Introduction, this material has several applications, and therefore its removal from rice husk char not only enhances the properties of the resulting adsorbents, but also improves the overall economy of the process for rice husk char valorization. The rice husk char obtained by fast pyrolysis and the precursor have been characterized by FTIR spectrometry and the spectra of both materials are shown in Figure 3. The main

Figure 3. Infrared absorption spectra corresponding to the original rice husk char and the char after silica extraction (precursor).

difference between the spectra of the two materials is the absence of the band at 1100 and 790 cm−1 in the precursor corresponding to the vibration of silicon−oxygen bonds.25,28,44 Furthermore, vibration of the CC bond in aromatic hydrocarbons at 1600 cm−1 and the band around 3440 cm−1 related to the stretching of O−H bonds in hydroxyl groups20,28,44 are observed in the char before and after silica removal. Moreover, small bands near 1450, 1700, and 2150 cm−1 are found in both samples, which are assigned to  CH2, CO (ketones, aldehydes, and carboxyl groups), and CC stretching vibrations, respectively.20,44 7244


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Table 3. Surface Characteristics and Chemical Analysis of the Activated Carbons Produced for Different Times Using Carbon Dioxide and Steam As Activating Agents ultimate analysis (wt %) sample

burn-off (wt %)

ABET (m2g−1)

micropore vol. (cm3 g−1)

pore vol. (cm3 g−1)

ash (wt %)

C

H

N

O

carbonized CO2-15 CO2-30 CO2-45 CO2-60 H2O-15 H2O-30 H2O-45 H2O-60

23.5 47.2 65.8 74.0 81.0 61.9 74.6 82.5 86.4

486.7 882.7 1174.2 1514.0 1325.7 1364.5 905.4 809.0 728.6

0.151 0.248 0.339 0.398 0.369 0.347 0.195 0.172 0.185

0.290 0.581 0.763 1.045 0.861 1.160 0.987 0.881 0.636

8.1 11.7 18.1 23.6 32.6 16.2 24.3 35.3 45.1

80.5 76.3 72.5 66.9 58.9 73.2 64.9 56.3 47.7

2.1 2.3 1.6 1.5 1.2 2.3 3.2 2.0 1.3

0.5 0.6 0.8 0.7 0.7 0.4 0.2 0.1 0.1

8.8 9.1 7.1 7.3 6.6 7.9 7.4 6.4 5.8

H3PO4 and ZnCl2, to produce activated carbons of high surface areas by applying a process to reduce the ash content of the rice husk char.27,28 However, the results obtained by Kennedy et al.19 using H3PO4 as activating agent were poorer. The maximum BET surface area was of 439 m2 g−1, which is explained by the high ash content because no leaching process was performed. The FTIR spectra of activated carbons produced by steam and carbon dioxide activation are shown in Figure 5. As observed, the spectra of both carbons are made up of similar bands. Some of these bands were previously observed in the spectra of the rice husk char (Figure 3). Thus, the band at 3440 cm−1 related to the stretching of the O−H bond in hydroxyl groups, the vibration of C−H bond at 2920 cm−1 and the C C bond in the aromatic structures at around 1600 cm−1 were clearly observed in both the rice husk char and the activated carbons.20,25,28,44,54 The bands at 1050 cm−1, attributed to−OH alcohol groups,28 and at 670 cm−1, assigned to the vending vibration of the C−H bonds,44 are observed in the activated carbons, but not in the rice husk char (Figure 3), due to the presence of silicon that masks these bands. Furthermore, the bands observed in the rice husk char and the precursor around 2150 and 1450 cm−1 (Figure 2), which correspond to the stretching vibrations of CC and CH 2  groups, respectively, were not observed neither in the steam nor in the carbon dioxide activated carbons. 3.3. Morphology of the Activated Carbons. Figure 6a compares the nitrogen adsorption−desorption isotherms for the activated carbons obtained under optimum conditions using carbon dioxide and steam, which correspond to 45 and 15 min of activation, respectively. As observed, the shape of both isotherms is similar, i.e., the active carbon samples follow type IV isotherms, which evidence the presence of a well-structured pore network. Under low relative pressures, the adsorption takes place in the micropores and the adsorption capacity under these conditions is higher for the carbon obtained using carbon dioxide. When relative pressure is higher than 0.5, hysteresis loops are evident in both cases. This may be caused by a capillary condensation in the mesoporous structures of the activated carbon. Furthermore, the presence of mesopores is more evident in the carbon obtained by steam activation. Figure 6b shows the pore size distributions of the same active carbons analyzed in Figure 6a. The values were deduced by the BJH method (based on the desorption branch). As observed, there are clear differences between the pore size distributions obtained using steam and carbon dioxide as activation agents. Thus, the activated carbon produced for 45 min activation with carbon dioxide is characterized by a structure made up of very

studied the steam activation (with a mixture of steam and nitrogen 50/50 vol.) of rice husk char (previously carbonized at 400 °C) at 800 °C. These authors obtained a maximum BET surface area of 362 m2 g−1 for 30 min treatment, with burnoff being around 75%, but they observed a loss of surface properties for longer activation times. Fu et al.30 also carried out activation with a mixture of steam and nitrogen 50/50 vol., with both the carbonization and the activation steps being carried out at 900 °C. These authors observed an increase in the BET surface area for burnoff values of up to 49% (244 m2 g−1), but surface area decreases for higher mass losses. Zhang et al.20 studied the effect of pyrolysis temperature, steam activation temperature and steam concentration and found that under the optimum conditions (700 and 750 °C for pyrolysis and activation, respectively, and a steam concentration of 6%) a maximum BET surface area of 244 m2 g−1 was obtained. Similar results have been obtained by Malik31 in a two-step process of pyrolysis (400 °C) and steam activation at relatively low temperature (600 °C), i.e., the surface area reached was 272 m2 g−1. As observed, the values obtained in the aforementioned studies are much lower than those in the present study. The explanation lies in the lower ash content of the precursor used in the present study once silica has been recovered, being the ash content of 6 wt %, compared to those of the mentioned papers, above 40 wt % in all cases. Although rice husk silica has a considerable external porous structure, around 130 m2 g−1,53 the high ash content in the rice husk char is a serious drawback for the direct production of activated carbons.23,24,28,44 The results obtained in the literature together with those in the present study emphasize the interest of silica removal for improving the quality of rice husk derived activated carbons. In fact, Deiana et al.24 compared steam activation of rice husk with and without leaching with HF and the differences were significant. Thus, the direct activation yielded an activated carbon with a maximum BET surface area of 290 m2 g−1, but this value was improved to 1180 and 820 m2 g−1 when the ashes were removed before and after the steam activation step. The production of activated carbons derived from rice husk char by chemical activation has been widely studied. Thus, Guo et al.26 produced activated carbons with surface areas higher than 3000 m2 g−1 using NaOH and KOH as activating agents in the 650−850 °C range and by removing silica during the activation process. An et al.25 used recovered Na2CO3 to remove the silica present in rice husk char. They then used KOH as activating agent to obtain active carbons of high quality, with the maximum surface area being 1936 m2 g−1. Other authors used other chemical activating agents, such as 7245


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Figure 5. Infrared absorption spectra of the activated carbons produced with steam and carbon dioxide for 15 and 45 min activation, respectively.

Figure 4. Evolution with activation time of burnoff (a) and BET surface area (b) and evolution of BET surface area with burnoff (c) for the two activating agents. Figure 6. Adsorption−desorption isotherms (a) and pore size distribution (b) of the active carbons obtained with carbon dioxide (for 45 min) and steam (for 15 min) under optimum conditions.

small micropores, i.e., the contribution of standard micropores and mesopores is low. However, the carbon produced by steam activation (15 min treatment) has a wider pore size distribution, with the presence of micropores being significant, 7246


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widening and destruction. These results are consistent with those obtained by other authors, who observed that carbon dioxide favors the development of new fine porosity followed by pore enlargement, whereas steam causes widening of micropores from earlier activation stages.58,59 The wide pore size distribution of the activated carbons obtained by using both steam and carbon dioxide is also related to the presence of a well developed porous structure in the precursor. Scanning electron micrographs are shown in Figure 8 for the activated carbons produced using steam and carbon dioxide as activating agents for 15 and 45 min, respectively. As observed in Figure 8a, corresponding to carbon dioxide activation, an external corrugated structure characteristic of rice husk60 is observed even after undergoing activation and subsequent mass loss (above 80 wt %). Thus, this structure is similar to that reported in previous studies for the rice husk char obtained once silica has been extracted.5 Therefore, the external structure of the carbons remains unchanged throughout the activation process and the morphology of the sample is similar to the original rice husk char, which has also been observed by Zhang et al.20 in the steam activation of rice husk. However, other authors have observed a fragmentation of the original structure after the activation process.25,44,54 This difference is probably related to the more aggressive nature of the chemical activation methods. Figure 8b and c show in detail the different surface morphology for the activated carbons obtained undergoing steam activation (Figure 8b) and carbon dioxide activation (Figure 8c). Pore development by oxidation of the precursor’s carbonaceous structure is clearly observed in both cases, in which a high number of macropores are observed at a scale of 10 μm. Figure 8c seems more porous because it corresponds to the cross-section of the material, whereas Figure 8a and b show the longitudinal section and, due to the irregularity of the rice husk char, the pore structure is not so clearly observed.

in addition to a well developed mesoporous structure. This different pore size distribution is clearly reflected in the average pore sizes obtained for the carbons, 43.4 and 60.1 Å for carbon dioxide and steam activation, respectively. These results are consistent with those by other authors.55−57 However, this difference can be minimized by reducing steam partial pressure in the reaction environment, and therefore leading to the development of highly microporous carbons.49,50 The effect carbon dioxide activation time has on the nitrogen adsorption isotherms and pore size distribution are shown in Figure 7. The figures corresponding to the steam activation

4. CONCLUSIONS The activation of previously desilicated rice husk char by partial gasification with either steam or carbon dioxide allows producing activated carbons of high BET surface area and suitable porous structure for their application as adsorbent or catalyst support. These results are encouraging for the economy of the rice husk flash pyrolysis, complementing the interest of amorphous silica recovery from rice husk char. It is noteworthy that the optimum activated carbon with steam was obtained at a shorter time and has a higher BET surface area by mass unit of the original rice husk than that corresponding to the optimum activated carbon with carbon dioxide. Significant differences have been observed in the development of the porous structure when these two activation agents are used. Thus, steam only generated new very small pores in the initial stages of activation, which subsequently became larger and finally porous structure destruction occurred. However, carbon dioxide is able to create new micro and mesopores for longer treatments. These differences are considerable for the use of these carbon materials in the preparation of acid catalysts (activated by impregnation with H3PO4 or as metallic catalyst support), which require significant mesopore content in order to minimize deactivation by micropore blocking.61

Figure 7. Adsorption−desorption isotherms (a) and pore size distribution (b) of active carbons obtained at different burnoff levels with carbon dioxide.

process can be found elsewhere.5 Carbon dioxide activation gives way to a continuous micropore formation, at least up to 45 min reaction. Subsequent to 60 min activation, only widening of pores bigger than 1000 Å is observed. The situation is completely different when steam is used as activating agent; that is, the number of micropores peaks for 15 min activation and subsequently there is a significant pore 7247


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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was carried out with financial support from the Ministry of Science and Education of the Spanish Government (CTQ2013-45105-R), the FEDER funds, the Basque Government (IT748-13), and the University of the Basque Country (UFI 11/39). J.A. also thanks the Basque Government for his research training grant (BFI2010-206).

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(1) Jung, D. S.; Ryou, M. H.; Sung, Y. J.; Park, S. B.; Choi, J. W. Recycling rice husks for high-capacity lithium battery anodes. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 12229. (2) Lim, J. S.; Abdul Manan, Z.; Wan Alwi, S. R.; Hashim, H. A review on utilisation of biomass from rice industry as a source of renewable energy. Renewable Sustainable Energy Rev. 2012, 16, 3084. (3) Vitali, F.; Parmigiani, S.; Vaccari, M.; Collivignarelli, C. Agricultural waste as household fuel: Techno-economic assessment of a new rice-husk cookstove for developing countries. Waste Manage. 2013, 33, 2762. (4) Alvarez, J.; Lopez, G.; Amutio, M.; Bilbao, J.; Olazar, M. Bio-oil production from rice husk fast pyrolysis in a conical spouted bed reactor. Fuel 2014, 128, 162. (5) Alvarez, J.; Lopez, G.; Amutio, M.; Bilbao, J.; Olazar, M. Upgrading the rice husk char obtained by flash pyrolysis for the production of amorphous silica and high quality activated carbon. Bioresour. Technol. 2014, 170, 132. (6) Shen, Y.; Zhao, P.; Shao, Q. Porous silica and carbon derived materials from rice husk pyrolysis char. Microporous Mesoporous Mater. 2014, 188, 46. (7) Ganesan, K.; Rajagopal, K.; Thangavel, K. Rice husk ash blended cement: Assessment of optimal level of replacement for strength and permeability properties of concrete. Constr. Build. Mater. 2008, 22, 1675. (8) Givi, A. N.; Rashid, S. A.; Aziz, F. N. A.; Salleh, M. A. M. Assessment of the effects of rice husk ash particle size on strength, water permeability and workability of binary blended concrete. Constr. Build. Mater. 2010, 24, 2145. (9) Chen, K.; Wang, J.; Dai, Y.; Wang, P.; Liou, C.; Nien, C.; Wu, J.; Chen, C. Rice husk ash as a catalyst precursor for biodiesel production. J. Taiwan Inst. Chem. Eng. 2013, 44, 622. (10) Hello, K. M.; Mihsen, H. H.; Mosa, M. J.; Magtoof, M. S. Hydrolysis of cellulose over silica-salicylaldehyde phenylhydrazone catalyst. J. Taiwan Inst. Chem. Eng. 2015, 46, 74. (11) Wang, W.; Martin, J. C.; Zhang, N.; Ma, C.; Han, A.; Sun, L. Harvesting silica nanoparticles from rice husks. J. Nanopart. Res. 2011, 13, 6981. (12) Prasad, R.; Pandey, M. Rice husk ash as a renewable source for the production of value added silica gel and its application: An overview. Bull. Chem. React. Eng. Catal. 2012, 7, 1. (13) Lee, T.; Othman, R.; Yeoh, F. Y. Development of photoluminescent glass derived from rice husk. Biomass Bioenergy 2013, 59, 380. (14) Blanco Castro, J.; Bonelli, P. R.; Cerrella, E. G.; Cukierman, A. L. Phosphoric Acid Activation of Agricultural Residues and Bagasse from Sugar Cane: Influence of the Experimental Conditions on Adsorption Characteristics of Activated Carbons. Ind. Eng. Chem. Res. 2000, 39, 4166. (15) Mohanty, K.; Jha, M.; Meikap, B. C.; Biswas, M. N. Preparation and Characterization of Activated Carbons from Terminalia Arjuna Nut with Zinc Chloride Activation for the Removal of Phenol from Wastewater. Ind. Eng. Chem. Res. 2005, 44, 4128. (16) Márquez-Montesinos, F.; Cordero, T.; Ródriguez-Mirasol, J.; Rodríguez, J. J. CO2 and steam gasification of a greapefruit skin char. Fuel 2002, 81, 423.

Figure 8. SEM images of activated carbons produced by carbon dioxide (a and c) and steam (b).

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REFERENCES

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Physical Activation of Rice Husk Pyrolysis Char for the Production of

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