Activated carbon preparation from eucalyptus wood chips using continuous carbonization–steam activation process in a batch intermittent rotary kiln

The production of activated carbon from eucalyptus wood chips by steam activation in a 2000 kg batch intermittent rotary kiln with continuous carbonization–steam activation process conducted at 500 °C to 700 °C was studied. The activated carbon products were characterized by FTIR, SEM–EDS, Raman spectroscopy, and BET analysis. Percent yields, iodine number, and methylene blue number of the produced activated carbon materials were measured as well. It was shown that the percent yields of the activated carbon materials made in the temperature range from 500 to 700 °C are 21.63 ± 1.52%–31.79 ± 0.70% with capacities of 518–737 mg I2/g and 70.11–96.93 mg methylene blue/g. The BET surface area and micropore volume of the activated carbons are 426.8125–870.4732 m2/g and 0.102390–0.215473 cm3/g, respectively. The steam used in the process could create various oxygen containing surface functional groups such as –CO and –COC groups. In addition, it could also increase the amorphous nature of the activated carbon product. These properties of the activated carbon products are increased with increasing steam activation temperature from 500 to 700 °C. As a result, the activated carbon materials produced at activation temperatures of 600 °C and 700 °C exhibit higher adsorption.


Introduction
Nowadays, the environmental problems in the world are increasing, especially the pollution of wastewater that needs to be improved in terms of quality for discharge and consumption. Therefore, there is a need to nd new methods and materials to be used in the water treatment process. One of the most widely used materials for the treatment of water is activated carbon, which should have good absorbent quality.
It is cheap and has a lot of precursor raw materials. Biomass is a suitable material for producing good quality activated carbon, which is due to its good structural porosity and the ease of adding further micropores by activation. It is also a precursor material that is continuously made in nature and industry.
Wastes from the wood industry are low cost and bulk materials. Therefore, it is likely to be a feasible option for the production of activated carbon. Eucalyptus wood waste is a waste material from the wood industry made in abundance and which has been used in many areas, such as briquette fuel 1 , torre ed solid fuel 2 , sugar and gasoline 3 , or biooil 4 . Normally, the activated carbon production has two stages.
These are the carbonization and activation stages. After the carbonization step is completed and the char is obtained, the furnace is turned off and the reactor is cooled down to ambient temperature prior to the activation stage. The activation methods for activated carbon production include chemical and physical activation. Steam activation for activated carbon production is one method of physical activation, which usually operates between 700 °C and 1000°C 5 . The production of activated carbon with steam is a low cost, green, viable, and effective environmental remediation tool 6 . The activated carbon from Miscanthus sacchari orus prepared with steam activation at 800 °C is high surface area material with increased density of micropores. This is because steam removes carbon atoms from within the solid carbon network, resulting in the creation of new pores and opening of clogged pores 7 . The pyrolysis reactors, which are most widely used for activated carbon production, can be divided into two types by method and load speeding. These are xed-bed and movable-bed reactors. The movable-bed reactors are further subdivided into pneumatic (bubbling, spouted, circulating, or transport uidized beds), mechanical (rotary kiln, rake, auger, ablative, stirred) reactors, and reactors in which the charge moves under gravity 8 . For xed-bed reactors, a 1000 kg scale tubular type xed bed reactor has been used for activated carbon production from spent mushrooms, which obtained activated carbon product of high quality 6 . However, the rotary kiln was selected for activated carbon production in this research. This is because it has many advantages over other types of reactors such as easily adjusted residence time of solids in the reactor, good mixing of materials, good heat transfer during slow rotation of the inclined kiln, which gives uniform pyrolytic products, and the possibility of continuous operation, even though solid materials have various shapes, sizes and calori c values 8 .
In this research, eucalyptus wood chips were used as the precursor material for the production of activated carbon by steam activation on a large scale (2000 kg). The 2000 kg pyrolysis reactor is an intermittent rotary furnace. It is adjusted to operate continuously without temperature reduction after the carbon production stage. When the charcoal production process is complete, the steam is introduced immediately to reduce costs and processing time. The effect of steam activation temperature of 500 °C, 600 °C, and 700 °C on the properties of the activated carbon from eucalyptus wood chips was

Results And Discussion
Proximate analysis and percent yield.
Proximate analysis ( Table 1) has indicated that raw eucalyptus wood chips constitute a good biomass for activated carbon production with high xed carbon content and very low ash content. The yield of the activated carbon has decreased from 31.79 ± 0.70% to 21.63 ± 1.52% when the steam activation temperature was increased from 500 °C to 700 °C. Concomitantly the volatile matter content has decreased. On the other hand, the xed carbon content has increased with increasing steam activation temperature. These results are due to the thermal degradation of volatile matter and decomposition of carbon as CO, CO 2 , and CH 4 9 . Thermal behavior (TGA) of dried raw eucalyptus wood chips was investigated from 40 °C to 1000 °C at the rate of 10 °C/min in oxidizing atmosphere and the results are shown in Fig. 1. The gure depicts fourstage weight loss. The weight loss during the rst stage of TGA, below 200 °C, was found to be 7.90%. This is due to the removal of water content from the cellular structure of eucalyptus wood chips. The second heating stage from 200 °C to 342 °C resulted in a weight loss of 20.04%, which can be attributed to the decomposition of carbohydrates, cellulose, and hemicellulose components of eucalyptus wood chips. The hemicelluloses, which consist of an amorphous structure with a low degree of polymerization, were decomposed within a temperature range of 240-295 °C, while cellulose, which is a crystalline material consisting of large chain molecular-weight-polymer, was decomposed from about 300 to 342 °C in the second stage 10 . The third heating stage from 342 °C to 900 °C resulted in the weight loss of ∼69% which is mainly due to the signi cant decomposition of lignin, which comprises a complex aromatic polymer structure, carbonaceous structures, and inorganic impurities. In the last step, beyond 900 °C, there is very minimal weight loss resulting in residue ash weight of 1.80%. The TGA results are in line with the carbon content of the activated carbon materials obtained after carbonization and steam activation under an inert atmosphere as reported in the proximate analysis results.

EDS analysis.
The results of energy dispersive X-ray analysis ( Table 2) show that the raw eucalyptus wood chips contained 51.99% of carbon and 35.35% of oxygen together with other elements such as Na, Si, K, and Ca. After the carbonization-steam activation, the carbon content has increased while the oxygen content has decreased for all activated carbon materials, which were prepared by carbonization-steam activation 500°C-700°C. This is attributed to the removal of volatile matter and oxygen containing functional groups by thermal degradation during the carbonization stage and steam activation stage, respectively. The resulting oxygen content in the steam activated materials was expected to be from oxygen containing functional groups on the surface of steam activated carbon materials. These results indicate that the steam activated carbon materials exhibit high carbon content and some oxygen containing surface functional groups, which are providing an active surface for the attachment of the organic pollutants.
Furthermore, the content of certain other elements has also decreased with increasing steam activation temperature. It is assumed that these were more readily dissolved and eluted by steam at higher activation temperatures.   (Fig. 2b-d) by dehydration effect during the carbonization stage 13 . The steam activated carbon materials prepared at 500-700 °C, spectra of which are shown in Fig. 2b-d, exhibit newly created bands or peaks. It can be seen that the vibration bands, which have maxima located at 1593 cm − 1 and 1720 cm − 1 , are demonstrating the presence of C = C aromatic ring stretching vibrations and C = O stretching vibrations of carbonyl and carboxylate ion groups 13 , respectively. The presence of C = C aromatic ring stretching vibration, which has shifted from 1593.21 cm − 1 in raw eucalyptus wood chips (Fig. 2a) to 1504.88 cm − 1 in the activated carbon materials (Fig. 2b-d), may be attributed to reduction of sidechain aromatic ring-oxygen-containing surface functional groups and high degree of graphitization after carbonization at high temperature 9 . On the other hand, the peak of C = O groups found at 1720 cm − 1 in raw eucalyptus wood chips has transformed to a weak peak at about 1700 cm − 1 after activation. Furthermore, the results indicate that C = C aromatic ring stretching vibration tends to increase when C = O vibration decays more extensively with increasing activation temperature. On the other hand, the weak bands of the -CO group at 1250 cm − 1 and -COC group at 1160 cm − 1 have also appeared in the spectra after steam activation, which is due to partial oxidation. In addition, three very weak bands of surface aromatic -CH groups between 750 cm − 1 and 898 cm − 1 were found on of the activated carbon products after steam activation and tend to increase with increasing activation temperature. This indicates that the aromatic content of the ring structures 14 has increased with increased temperature of steam activation. The content of carbonyl groups and substitutions in aromatic rings has also increased. The FTIR results show that the activated carbon materials from eucalyptus wood chips prepared with steam activation should be able to interact with polar and non-polar adsorbates by surface chemical groups 12 .
Raman spectroscopy analysis.
The Raman spectra of eucalyptus wood chips and activated carbon products prepared with steam activation at temperatures of 500-700 °C are shown in Fig. 3. The D band and G band are located at 1350 cm − 1 and 1587 cm − 1 , respectively. It is known that the D band at 1350 cm − 1 is corresponds to disordered and structurally defected regions consisting of the sp 3 -hybridized carbons. On the other hand, the G band at 1587 cm − 1 corresponds to the stretching of the C = C bond in graphitic carbons with sp 2 hybridized carbon systems 11 . The ID/IG ratio is therefore indicative of amorphous degree of the material.
The results indicate that ID/IG ratios increase with increasing activation temperature from 500 °C to 700°C . The observed values are 0.74 (500 °C), 0.77 (600 °C), and 0.83 (700 °C). This shows that the surface defects or amorphous degree of the activated carbon products increased with increasing activation temperature, which has been proven to be bene cial for adsorbent applications. This result demonstrates that the surface carbons of eucalyptus wood chip chars were more partially oxidized by steam (H 2 O) to CO 2 and CO at higher activation temperatures. The surface porosity of activated carbon has also increased with increasing the activation temperature due to the increasing disorder and defects of the carbon structure. This points out that the steam-activated carbon from eucalyptus wood chips has mixed characteristics of amorphous and graphitic carbons.
XRD analysis. Figure 4 shows X-ray diffraction pattern of eucalyptus wood chip activated carbon materials prepared at temperatures of 500 °C, 600 °C, and 700 °C. It can be seen that the carbon structure of all activated carbon products is mainly amorphous with the presence of some crystalline material. This is demonstrated with XRD broad bands at ∼ 26° and ∼ 44°, respectively, which correspond to an amorphous structure composed of randomly oriented aromatic graphene-like sheets 15 . This is because the steam activation process causes the graphitic structure to be destroyed and the remaining carbon sheets are predominantly disordered with pores on the surface area of steamed activated carbon. This result is in good agreement with the Raman spectroscopy results. Functional groups on the surface of the activated carbon have been shown to have a large effect on the reaction of polar and non-polar substances. On the other hand, the porous structure of the activated carbon governs its adsorption capacity. Therefore, the surface porous properties of activated carbon materials are also an important factor in adsorption. In this study, the surface areas and porosity of the eucalyptus wood chip activated carbon materials resulting from steam activation at different temperatures were investigated and are shown in Table 3. It can be seen that the BET surface area and micropore volume increase with increasing temperature of the steam activation step from 500 °C to 700 °C. This indicates that increasing the steam activation temperature enhances the development of pores and creates new pores. This is due to more extensive thermal degradation of surface carbon on eucalyptus wood chip char concomitant with partial oxidation by steam as a result of higher activation temperature. This conclusion is supported by the Raman and XRD analyses results. In addition, it can be seen that the content of pores with dimensions in the range of mesopores and macropores (17 Å and 3,000 Å) also increases with increasing steam activation temperature. This may be a result of new micropores being created and existing micropores being enlarged as the walls between pores collapsed by the action of steam, which transformed them to mesopores and macropores. The increase in porosity with steam activation temperature (500 °C − 700 °C) is due to the higher rate of water molecules diffusion into the interior of the char structure, which developed a wide ranging pore network by partial steam oxidation. In addition, some surface carbons were decomposed to a higher degree by thermal degradation with increasing activation temperature.
Morphology by Scanning Electron Microscope.
The SEM images of raw eucalyptus wood chips and eucalyptus wood chips activated carbon products prepared at 500 °C − 700 °C are shown in Fig. 5. For the raw eucalyptus wood chips, the SEM image (Fig. 5a) clearly shows the dense texture, irregular structure with some large pores, and large particles on its surface. However, the activated carbon materials also clearly exhibit large wall-mounted tube structure with very low content of sheet particles after carbonization-steam activation (Fig. 5b-d). Furthermore, it can be seen that the number of sheet particles on the surface of activated carbon product decreases with increasing steam activation temperature from 500 °C to 700 °C. In the case of activated carbon prepared with steam activation at 500 °C, it can be seen that the tube structures are blocked with a large number of sheet particles. However, the number of sheet particles present on the surface of carbon material prepared with steam activation at 600 °C is reduced and the tube structure becomes partially unblocked and some walls of the tube structure are cracked. Furthermore, the surfaces of tube structure of activated carbon prepared with steam activation at 700 °C exhibits a very low amount of sheet particles. This is because ash and disorganized carbon, which formed from decomposition of organic matter and blocked the pores and tube structures during carbonization, were washed out by vapor steam 16 or burnt out after steam activation. This results in the activated carbon surface being smooth, wrinkled, and exhibiting open holes. These results con rm the results of the investigation of surface properties. In addition, eucalyptus wood chips steamed activated carbon is composed mainly of tube structures, which should promote the velocity of liquid diffusion for the adsorption process. Iodine and methylene blue number of activated carbon.
The iodine number and methylene blue number are considered as a measure of adsorption capability of activated carbon and represent the amount of micropores and mesopores 17 , respectively. In this work, it was observed that all steam activated carbon materials exhibited low methylene blue number in comparison to the iodine number (Table 4). This demonstrated that all activated carbon materials contain a high amount of micropores with low amount of mesopores, which is suitable for adsorption of small particles and some big organic materials as well. Furthermore, both numbers increase with increasing steam activation temperature. This indicates that the surface area and pore volume of the steam activated carbon materials both increase with increasing steam activation temperature, which is in line the results of investigation of surface properties and SEM. Thus, it can be concluded that the steam activation contributes to the formation of both micropores and mesopores on steam activated carbon materials. In this work, the iodine number values of the steam activated carbon materials, which were prepared under the steam activation temperatures of 600 °C and 700 °C, are higher than the value in the Thai industrial standard, which sets the minimal value as 600 mg/g 18 .

Conclusions
The production of activated carbon from eucalyptus wood chips by steam activation in 2000 kg batch intermittent rotary kiln with continuous carbonization -steam activation process was investigated. The effect of steam activation temperatures of 500°C to 700 °C was investigated and it was shown that it could enhance the properties of the activated carbon product for adsorption process. The products exhibit high xed carbon content and very low ash content. The percent yields of activated carbon materials produced in the activation temperature range of 500°C to 700 °C are 21.63 ± 1.52% − 31.79 ± 0.70% (432.6 kg − 635.8 kg), which are su cient for each batch production. The steam activation has also created some oxygen containing surface functional groups and micropores -mesopores, which provide the active surface for the attachments of the organic pollutants on the surface of the activated carbon product. The activation process also increases the surface area and volume pore of the produced carbon materials. In addition, the steam also dissolves ash and disorganized carbon from the activated carbon during steam activation. As a result, the activated carbon materials formed at activation temperatures of 600 °C and 700 °C exhibit a higher iodine number than of the Thai industrial standard. However, the activated carbon products from eucalyptus wood chips made by continuous carbonizationsteam activation process in this research are still in need of development to have better adsorption properties.

Materials And Methods
The sun dried eucalyptus wood chips (Fig. 6a) without bark (size 25 mm X 37.5 mm x 5 mm) were collected from a wood factory in Chaiyaphum Province, Thailand. A mass of 2000 kg of eucalyptus wood chips (about 40% of the kiln capacity) was loaded into the rotary kiln (diameter of 2.6 m and a length of 6.6 m (Fig. 6c) by a conveyor belt (Fig. 6b) followed by a fan blowing system. Then, the rotary kiln was closed and wood chips were burned in a wood stove under the rotary kiln ( Fig. 6d) with kiln rotation rate of 0.25 rpm. The temperature in the kiln reactor was measured by two thermocouples which were located at the front and end of the kiln (Fig. 6e). The heating rate was 2.5 °C/min until the desired temperature (500 °C, 600 °C, and 700 °C) was reached. After that, continuous steam activation was performed by introducing the steam from a boiler (capacity size 1500 kg) into the kiln at a pressure of not more than 3 bar for about 2 hours using a 1:1 weight ratio of steam: raw wood chips. The volatile pyrolysis products left the reactor via an exhaust pipe with a diameter of 25 mm (Fig. 6f) and were allowed to vent to the atmosphere after passing the condenser and liquid-gas separator. After steam activation at the desired temperature for 2 hours, the wood stove and steam were turned off and the kiln was let to cool down overnight. The activated carbon materials were discharged from the kiln and ground for characterization studied.

Characterization of eucalyptus wood chips and activated carbon
The sun dried eucalyptus wood chip and activated carbon materials were subjected to approximate analysis using standard methods. Both were also characterized by FTIR (Spectrum GX, Perkin Elmer), SEM-EDS (Mode LEO 1455 VP, LEO Electron Microscopy Ltd, England), Raman spectroscopy (Bruker, MultiRAM), XRD (PW 3040/60 X'Pert PRO Console, Philips, Netherlands), TGA (PW 3040/60 X'Pert PRO Console, Philips, Netherlands) and BET (Micromeritics TriStar II). Iodine and methylene number were measured for the activated carbon products. Finally, the percent yields of activated carbon materials obtained from steam activation at temperatures of 500 °C, 600 °C, and 700 °C were calculated.