Production of Activated Carbon from Biochar Using Chemical and

Chapter 29

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Production of Activated Carbon from Biochar Using Chemical and Physical Activation: Mechanism and Modeling Ajay K . Dalai and Ramin Azargohar Department of Chemical Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada

Biochar, a solid product of fast pyrolysis of biomass, was converted to activated carbon by physical (steam) and chemical (potassium hydroxide) activation. The effects of operating conditions on the B E T surface area and the reaction yield of physically and chemically activated carbons were investigated. Two models for B E T surface area and reaction yield of each activated carbon were developed. Using these models, the optimum operating conditions for production of activated carbons with large surface area and high yield were determined. The B E T surface area and yield of products predicted by models and from experiments at optimum operating conditions showed good agreement. The effects of activating agent on the chemical structure of biochar, during chemical activation, were investigated by thermogravimetric method and infrared spectroscopy.

© 2007 American Chemical Society

In Materials, Chemicals, and Energy from Forest Biomass; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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464


Introduction Although the most familiar forms of carbon are cubic diamond and hexagonal graphite [7] and recently discovered fullerenes [2, 3], most carbon materials have less ordered structures. According to their crystallographic structure, they are categorized into graphitic and non-graphitic carbons [4]. The latter is classified as graphitizable (cokes) and non-graphitizable carbons (chars) [5]. Chars do not pass through a fluid phase during pyrolysis (carbonization) [4] and are the main precursors of activated carbon. These carbons are porous materials with highly developed internal surface area and porosity. They are used as catalyst [6, 7] and catalyst support [4, 7, 8] as well as in many adsorption processes for removal of impurities from liquids and gases. Activated carbons can be produced from most carbon-containing organic materials [9,10], but commercial processes which make activated carbon use precursors, which originate from either degraded and coalified plant matter (e.g. peat, lignite and all ranks of coal) or botanical origin (e.g. wood, coconut shells and nut shells) [11,12]. These materials have a high content of carbon and are inexpensive [5]. Coal is commonly used for producing high yields of activated carbon [13, 14]. Materials from botanical origin or in other words, lignocellulosic materials, have low inorganic and relatively high volatile content. The first characteristic results in producing activated carbon with low ash and the second characteristic helps to control the production process [P]. The physical and chemical activation methods are commonly applied in the production of activated carbon [5, 15]. In physical activation, char is produced during the first step, by carbonization (pyrolysis) of the precursor. This step removes non-carbon species [5] and produces char with a high percentage of carbon. Because of the blockage of the pores by tars [10], the internal surface area of char is too low. The second step of physical activation is high temperature gasification (activation) using oxidizing agents such as steam or carbon dioxide, which produces activated carbon with high porosity [5, 9, 10]. Porosity development is due to the penetration of the oxidizing agent into the internal structure of char and the removal of carbon atoms by reaction which results in the opening and widening of inaccessible pores [9, 10]. The overall reaction between steam and carbon (including heterogeneous water-gas reaction, shift reaction and methanation) [16] and the reaction between carbon dioxide and carbon [9] are endothermic. Oxygen is not used as an oxidizing agent because of the exothermic reaction between carbon and oxygen, which makes it difficult to control the process temperature and prevents the development of high porosity due to external burning of carbon particles [5, 9]. The chemical activation of the precursor with a chemical (dehydrating) agent is another important industrial process for producing activated carbon. The most common activating agents are potassium hydroxide, phosphoric acid



465 and zinc chloride. In comparison with the mechanism in physical activation, the mechanism in chemical activation is not well understood [17] It seems that the chemical agent dehydrates the sample, inhibits the tar formation and volatile compounds evolution, and therefore enhances the yield of the carbonization process [5, 18], After impregnating the precursor with the chemical agent followed by a heat treatment of the mixture, the chemical agent is eliminated by washing with acidfàase and water. The washing step allows the creation of a pore structure [9]. For quality control of the activated carbon produced by the activation process, characterization methods are used to specify the physical and chemical structure of this product. Porosity is the most important physical characteristic of activated carbon which is used to study the textural properties. I U P A C (International Union of pure and Applied Chemistry) classifies porosity into three groups: micropores (width less than 2 nm), mesopores (width between 2 to 50 nm) and macropores (width greater than 50 nm) [19], The main technique for studying the porosity is measurement of adsorption isotherms. A n adsorptiondesorption isotherm indicates the measured quantity of gas that is adsorbed or desorbed on the surface of a solid at different equilibrium pressures and constant temperature [20]. The chemical structure of activated carbon has a great effect on its adsorptive capacities. Due to the presence of many heteroatoms chemically bonded to the surface of activated carbon, many surface groups can be formed [10]. Oxygen surface groups, such as carboxylic, lactone, phenol, carbonyl, have the predominant role in the surface adsorption behavior of activated carbon [7]. Many methods, such as Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and chemical titration methods, can be used to determine the surface chemistry of activated carbons [21]. Biochar was used as the precursor in this study. It was provided by Dynamotive Canada Inc. (Vancouver, B C , Canada). This char is produced by fast pyrolysis of biomass. Agricultural wastes such as bagasse and wheat straw, and forest residues such as sawdust and bark are the main sources of biomass. Large amount of agricultural wastes and forest residues can be recycled and converted to value added products [22]. Biomass is converted to a mixture of liquid organic compounds, gases and biochar by fast pyrolysis [23], which is the rapid thermal decomposition of organic compounds in an inert atmosphere. The liquid product of fast pyrolysis (biooil) is used as a clean burning fuel in boilers and turbines instead of fossil fuels such as diesel oil and natural gas. Gaseous product is recycled to the process to supply some of the heat required for pyrolysis. The biochar yield in this process is 20-30 wt% [9]. This char is used as a high heating-value solid fuel in kilns and boilers. The porosity developed during activation process depends on the type of precursor, type of activation process and operating conditions. Biochar, due to



466 its availability, low price and low amount of ash content (inorganic contents), is a suitable precursor for activated carbon. Ash (mineral) content does not contribute in the formation of porosity and therefore its presence reduces the development of surface area and pore volume per unit mass [14, 24]. Steam and carbon dioxide are the most commonly used gases for physical activation. Carbon dioxide develops and widens micropores, but steam develops micropores as well as mesopores, and produces a wider range of pore size [5, 25]. Zinc chloride, phosphoric acid and potassium hydroxide are commonly used for chemical activation. The use of ZnCl2 is decreasing due to environmental and corrosion problems [26]. Phosphoric acid produces finer pores in comparison to Z n C l [5] and needs lower activation temperature in comparison with two other agents [9]. Potassium hydroxide develops highly microporous activated carbons [9]. The purpose of this study is to convert biochar to a high value-added product such as activated carbon and to further optimize the operating conditions for the production of activated carbon with large surface area and high yield. The Biochar used in this work was produced by fast pyrolysis of biomass in a bubbling fluidized bed reactor by Dynamotive Energy System Corporation. The as-received char was sieved, and particles between 150 - 600 μην (100-30 mesh) were collected for activation. 2

Physical Activation Steam was used as the oxidizing agent for physical activation in order to produce activated carbon from biochar. Steam, because of the smaller dimensions of water molecule in comparison with that of C 0 , provides faster diffusion into a porous network, easier access into the micropores and a faster reaction rate [9]. The effect of three parameters, activation temperature (T), mass ratio of steam to char (S/C) and activation time (t), on the process was studied. The range of activation temperature, mass ratio of steam to char and activation time were 600 - 900 °C, 0.4 - 2 and 0.9 - 4 h, respectively. The experiments were designed with a statistical method called central composite design (CCD). This method enables us to optimize the effective parameters with a minimum number of experiments, as well as to analyze the interaction effect between parameters. This method includes three kinds of run; factorial runs (2 ), axial or star runs (2k) and center runs ( rv=six replicates) [27], where k is the number of parameters. Therefore, the number of runs required for studying the effect of three parameters is as follows: 2

k

N = 2 +2k k

+ n =2 c

3

+ 2 * 3 + 6 = 20

(1)

This method was used for the physical activation of the char. With such details presented elsewhere \2S\.


467 Using this statistical method and design-expert software, two models for B E T surface area (Y ) and reaction yield (Y ) were developed, and the optimum operating conditions were obtained by using these models. These data are required for scaling-up the laboratory results to pilot-plant or full-scale levels [29]. Fig. 1 shows the schematic diagram of the experimental setup. The details of the reactor setup are mentioned elsewhere [28]. For each run, 20 g of biochar was used. The temperature of the reactor was increased to the desired activation temperature, at a rate of 3 °C/min, under nitrogen flow followed by the steam injection. At the end of the run, the reactor was cooled to room temperature by flowing nitrogen.


{

2

Figure 1. The schematic diagram of reactor set-up.

Results and Discussion The ultimate and bulk ash analyses of biochar were done by Loring Laboratories, in Calgary, Canada. The ultimate analysis of biochar (moisture free) was: carbon-83.07, hydrogen-3.76, nitrogen-0.11, sulphur-0.01, oxygen9.60, and ash-3.44 wt %. The bulk ash analysis of biochar was: Si0 -53.48, A l 0 - 7 . 7 3 , TiO -0.10, Fe 0 -2.52, CaO-17.98, MgO-4.20, Na O-2.07, K 0 6.93, P O -0.94, SO3-I.O6 and undetermined -2.99 wt %. The p H of biochar was 7.64 and its B E T surface area was less than 10 m /g. Physically activated carbons produced in this study have an average pore diameter in the range of 13-26 °A, a maximum B E T surface area of more than 950 m /g„ a reaction yield in the range of 17 - 79 wt% and maximum pore volume of more than 0.83 cc/g. 2

2

3

2

2

2

3

2

5

2

2


2

468 The pH of activated carbons was in the range of 9-11. The pH was measured according to A S T M D 3838-80. After performing 20 experiments, two models were obtained for B E T surface area ( Y 0 and reaction Yield ( Y ) , as shown in formulas (2) and (3), respectively. From a statistical point of view, three tests are required to evaluate the model; the test of significance of factors and interactions, the Rsquared test and the lack-of-fit test. These tests showed that these models can navigate the experimental data. Downloaded by UNIV MASSACHUSETTS AMHERST on August 6, 2012 | http://pubs.acs.org Publication Date: April 16, 2007 | doi: 10.1021/bk-2007-0954.ch029

2

25

6

7

5

(Y - 25) = - 3.600 * 10 +5226.098 * Γ-2.040 * 10 * (S IC)+7.871 * 10 * t t

(2)

+35146.501*r*(S/C) 2

Y = 94.85-0.01* Γ+3 3.65 * (S I Q+0.31 * f+1.49 * t -0.06 *T*(S/Q-0.0 2

\*T*t (3)

Figure 2a is the three-dimensional plot of B E T surface area model and shows the effects of temperature and mass ratio on the B E T surface area of activated carbons prepared at a constant activation time of 2.46 hours.

(a) Figure 2. a) 3-D plot of BET surface area for physically activated carbons prepared at t=2.46 hrs b) 3-D plot of BET surface area for chemically activated carbons prepared at nitrogen flow rate of 165 cc/min. As can be inferred from this figure, increasing the temperature increases B E T surface area which is expected from an overall endothermic process such as steam activation. In addition, increasing the mass ratio, i.e., using more oxidizing agent, has a similar effect. The influence of time can be shown by the amounts of B E T surface areas of three activated carbons prepared at the same temperature and mass ratio (T=750 °C and S/C=1.2), but at different activation times (0.6, 2.46, 4.32 hrs). For these samples, B E T surface area is increased by increasing the time and these samples have surface areas equal to 554, 657 and 730 m /g, respectively. This trend is in agreement with the equation shown above in 2


469


formula 2 (2). In industry, it is more conventional to express these porosity characteristics per unit mass of the raw material (biochar) instead of product [9, 30]. Seven activated carbons were prepared at constant steam to feed mass ratio (S/C=1.2) and activation time (t=2.46 h) but at different temperatures, and their B E T surface areas per unit mass of activated carbon and per unit mass of biochar are plotted against burn-off degree (percentage of feed mass loss due to activation) of these samples (see Fig. 3 a).

Figure 3. a) The effect of burn-off degree for activated carbons prepared at S/C=1.2 andt=2.46 hrs b) Typical isotherm plots for chemically activated carbon(l) and physically activated carbon(2). It shows that the B E T surface area (per unit mass of activated carbon) is increased by increasing the burn-off degree but for B E T surface area per unit mass of biochar the curve exhibits a maximum in the range of 40-50 wt % of burn-off. Total pore volumes of these samples show similar trend, which was also observed by other researchers [25, 30, 31, 32]. It is due to widening of the porosity or the external ablation of the carbon particles at high degree of burnoff [5, 9]. The following constraints were used to find the optimum operating conditions for producing activated carbon with large B E T surface area and high yield : (1) B E T surface area > 600 m /(g of activated carbon); (2) 50 < Yield (wt %) < 60; and (3) 0.91 < activation time (hr) < 1.5 . The activation time should be as low as possible to obtain a good overall yield from a batch process. The optimum operating conditions were calculated based on the constraints, by Design Expert software, and are as follows: 2

Τ = 792 °C, Mass ratio = 1.06, t = 1.39 h A n experiment was performed at these operating conditions. The observed and predicted B E T surface area and reaction yield were 664 and 643 m /g and 56.6 and 56.9 wt %, respectively. Therefore, the difference of B E T surface area and 2


470 product yield for experiment and model are less than 4 % and 0.6 %, respectively.


Chemical Activation Potassium hydroxide was used as an activating agent for this process. In this study, the effects of three parameters on the B E T surface area and reaction yield were investigated by C C D : activation temperature (T), mass ratio of K O H to biochar (R), and nitrogen flow-rate (F). The ranges of T, R, and F were 550-800 °C, 0.25-3.00, and 80-250 cc/min., respectively. For each run, a specified amount of potassium hydroxide was impregnated in biochar. The char was mixed with 100 ml of water having a desired concentration of K O H . This mixture was kept for 2 hours at room temperature to ensure the access of K O H to the interior of the biochar, and subsequently dried at 120 °C in an oven overnight. Then, 20 g of the prepared sample was placed in the fixed-bed reactor under a nitrogen flow and heated to 300 °C at 3 °C/min, and held at this temperature for l h to prevent carbon loss through the direct attack of steam [33]. Then, the temperature was increased at 3 °C/min to reach the desired activation temperature. The chemical activation was carried out for 2 h at this temperature before cooling down. Then, the products were thoroughly washed with water, followed by treatment with 0.1 M HC1, and finally by distilled water to remove the soluble salts [9] and the potassium compounds [17]. The chlorine ions were eliminated with distillated water [34], when p H of the washed solution was between 6 and 7. Then the sample was dried at 110 °C for 12 h and characterized for its physical and chemical property measurements.

Results and Discussion Chemically activated carbons produced in this study have an average pore diameter in the range of 13 - 15 °A, a maximum B E T surface area of more than 1500 m /g„ a reaction yield in the range of 50 - 82 wt% and a maximum pore volume of more than 0.75 cc/g. The following models were obtained for B E T surface area (Y ) and reaction yield (Y ): 2

3

4

4

3

(-^)=0.155-1.248 MO" * Γ-0.0586 * R -2.263 * 10" * F 3

2

(4)

5

+7.431 * 10" * R +3.794 *\0~ *T*R 2

Y =13.05-3.5* A-X0.25* B-5.21* B -3M* A* D 4


(5)

471 Where A , Β and D are as follows;


A =-l + (T-550)1125,Β

= -\ + (R-0.25)11375,

D = -l + (F-SO)/&5

(6)

As shown in Fig. 2b, the three-dimensional plot of B E T surface area model showing the effects of temperature and mass ratio on the B E T surface area of activated carbon (prepared at constant nitrogen flow rate of 165 cc/min), it can be inferred that increasing the temperature increases the B E T surface area. The B E T surface area increases with an increase in the mass ratio up to a maximum, e.g., mass ratio of 1.93 at 800 °C, and then decreases. This phenomenon has been reported by some researchers for other precursors such as Spanish anthracite and Australian coal [77, 35]. It can be due to predominately pore widening at higher mass ratios [9] or severe gasification on the external surface of the carbon reducing the number of the pores [36], Experimental data show that activated carbons prepared at a constant temperature of 675 °C and a mass ratio of 1.63, but at different nitrogen flow rates of 72, 165, and 258 cc/min, have B E T surface areas equal to 582, 927, and 1210 m /g, respectively. Therefore, the samples prepared at a higher nitrogen flow rate have a larger B E T surface area. This could be due to faster removal of reaction products from the reactor at higher nitrogen flow rates [77]. The optimum operating conditions were defined according to the following constraints: (1) B E T surface area > 700 m /g; (2) reaction yield > 70 wt % .The solution is as follows: 2

2

Τ = 680°C, KOH/biochar = 1.23, N flow rate = 240 cc/min 2

The experiments were performed at these operating conditions and the B E T surface area of product and reaction yield were measured. The observed and predicted B E T surface areas were 836 and 783 m /g, respectively. The observed reaction yield was 78.0 and the predicted one was 75.3 wt %. Therefore, the difference of B E T surface area and product yield for experiment and model are less than 7 % and 4 %, respectively. Since mass ratio of K O H to biochar equals 1.23, more than 50 wt% of the feed is impregnating agent (KOH). The price of this chemical is several times higher than biochar price. Therefore, the recovery of the chemical agent and its related cost have strong effects on the economy of chemical activation [9] and an economical recovery method by acid washing should be developed for the production of low cost activated carbon by K O H chemical activation. Fig. 3b shows typical isotherm plots for physically (curve # 2) and chemically (curve # 1) activated carbons. The isotherm plots of physically and chemically activated carbons are type IV, which shows the formation of mesopores, [37] and type I, which is due to the development of microporous material. 2



472 The effect of activating agent (KOH) on the chemical structure of biochar was studied by some characterization methods such as T G / D T A (Thermogravimetric/Differential thermal analysis), DRIFTS and S E M . The T G / D T A study was carried out to investigate the effects of K O H on the reaction yield by a pyres-diamond T G / D T A (Perkin-Elmer instruments) under the flow of argon. T G / D T A analysis, after heating to 700 °C, for biochar and a mixture of biochar and K O H , with KOH/biochar ratio of 1.63, showed that weight loss of biochar was 8 wt% more than the mixture. It can be obviously inferred that the addition of the activating agent increases the carbon yield. K O H influences the pyrolitic decomposition, inhibits tar formation and therefore increases carbon yield [21]. One of the possible effects of activating agent in the chemical activation is cross-linking and aromatization of the precursor structure which reduces tar formation and increases carbon yield [26]. Therefore, DRIFTS was used to examine the effect of activating agent on the formation of aromatic structure during activation process. Spectra were obtained by a FTIR spectroscope (Spectrum G X , Perkin-Elmer) at a resolution of 4 cm" . Undiluted activated carbons, in the form of powder, were used for FTIR study in the scanning range of 450-4000 cm" . 1

1

The IR bands which can be associated with aromatic hydrocarbons are as follows: peak between 900 - 675 cm' (out-of-plane bending of the ring C - H bonds), peak between 1300-1000 cm" (in-plane bending bands), peaks between 1600-1585 cm' and 1500-1400 cm" (skeletal vibrations involving carbon to carbon stretching within the ring), peak between 3100-3000 cm" (aromatic C - H stretching bands), peak between 2000-1650 cm" (weak combination and overtone bands). Fig. 4 shows DRIFTS results for virgin biochar, activated carbon (prepared at 675 °C, K O H to biochar mass ratio of 1.63 and nitrogen flow rate of 165 cc/min), and processed biochar (without any activating agent and heated up to 675 °C). The effect of activating agent on the aromatization of the product can be seen from the spectra of biochars and activated carbon. This figure shows that there are low levels of aromatic compounds in biochar, which disappear after heat treatment without any activating agent. However, the aromatic structure in the product had developed due to use of the activating agent at the same heat treatment and gas flow-rate. Fig. 5 shows the effect of activation temperature on the aromatization of activated carbons prepared at mass ratio of 1.63 and gas flow rate of 165 cc/min. It can be inferred from this figure that an activation temperature of up to 675 °C increases the apparent aromatization of the structure of activated carbon, after which the aromaticity decreases. The scanning electron micrographs were taken for biochar and activated carbon, prepared at temperature of 675 °C, K O H to biochar mass ratio of 1.63 and nitrogen flow rate of 165 cc/min (see Fig. 6). According to these micrographs, biochar does not have a porous structure, but activated carbon produced after acid washing step shows a highly porous structure. 1

1

1

1

1

1



473

4000

2800

2000

1600

1200

800

400

Wave number (1/cm) Figure 4. Effect of KOH/biochar mass ratio on the surface chemistry.

4000

2800

2000

1600

1200

800

400

Wave number (1/cm) Figure 5. Effect of activation temperature on the surface chemistry (All activated carbon prepared atF= 165 cc/min).



474

Figure 6. Scanning micrographs of biochar and chemically activated carbon.

Conclusions Biochar as one of the products of the fast pyrolysis of biomass, was used as a precursor for the production of activated carbon. Physically and chemically activated carbons prepared from this material had B E T surface area in the range of 300-950 and 180-1500 m /g, respectively. The B E T surface area of physically activated carbon was increased by an increase in activation temperature, mass ratio of steam to char and activation time, whereas the B E T surface area for chemically activated carbon was increased by an increase in temperature and nitrogen flow rate. A n increase in mass ratio of K O H to biochar to a certain limit, increased surface area as well. The optimum operating conditions for physical and chemical activation methods were calculated using models developed for B E T surface area and reaction yield. The B E T surface area and reaction yield of activated carbons prepared at optimum operating conditions were in good agreement with model predictions. Physically activated carbons had a relatively wide range of average pore diameter (13-26 °A). Chemically activated carbons, although had a narrow range of average pore diameter (13-15 °A), showed a highly microporous structure. Thermogravimetric study of chemical activation showed that activating agent increased activated carbon yield. DRIFTS confirmed the development of aromaticity by chemical activation which resulted in cross linking, reduction of tar formation and higher carbon yield. 2


475

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Production of Activated Carbon from Biochar Using Chemical and

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