Activated carbon

Activated carbons (ACs) are carbon with highly microporous structure, high specific surface areas (SSA) and good adsorption properties. ACs allows the gas/liquid access into internal pore surface and high degree of surface reactivity. ACs is an attractive material use in various applications such as wastewater treatment, harmful gases removal in the air and solvent recovery and ground water improvement. Nowadays, the agricultural by-products

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have proved to be promising raw materials for the production of ACs because of their availability at a low-cost, renewable and environmental friendly.

Lignocellulosic biomass is one of abundant agricultural wastes to produce ACs that used for water and air pollution treatment. The advantages of ACs from lignocellulosic biomass over the ACs from fossil sources are less emission of CO2 due to its carbon-neutral cycle in the conversion process, reduce the amount of abundantly agricultural wastes and low cost. Generally, the main components of lignocellulosic biomass are comprises with cellulose, hemicellulose and lignin. Among those components, lignin is identified as the useful component for the adsorption process due to the rich carbon content in lignin. Note that the worldwide production of lignin-based biomass is 40 to 50 million tons per year.

ACs with high adsorption capacity can be produced from numerous sources of lignocellulosic biomass such as coconut shell, durian shell, hazelnut shell, rubber seed shell, palm kernel shell, almond shell, cotton stalks, plum stones, rice husk, pistachio-nut shell, walnut shell, wood, etc. Lignocellulosic ACs can be used for chemical processes, petroleum refining, waste water treatment, air pollution treatment and volatile organic compounds (VOC) adsorption. Moreover, ACs obtained from Lignocellulosic provides an effective way for gas phase applications such as for purification, separation, deodorization, storage and catalysis.

To produce the ACs, the carbonization or pyrolysis process is firstly requires to converse the char from biomass. In this process step, moisture and volatile compounds are removed from the biomass. After the char producing, ACs can be fabricated using three different processes: physical activation, chemical activation and physiochemical activation.

Physical activation is related to the gas-activating agents such as steam and CO2. The chemical activation involves the presence of chemical agents such as metal oxide, alkaline

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metal and acid. After the activation process, ACs with high porosity, large surface area and high pore volume can be obtained.

1.10.1 Activation process

In the present days, the carbonization of lignocellulosic biomass to produce ACs is widely study. Typically, the carbonization was done at the temperature below 800^oC in the absence of oxygen ambient. After that, the activation process is required to increase the surface area and pore volume of ACs. There are two different activation processes called physical activation and chemical activation.

1.10.2 Carbonization

The carbonization process is a thermal decomposition process that can eliminate a non-carbon species and thus enrich the carbon content in carbonaceous material. The initial porosity of char that obtained from carbonization process is still comparatively low.

Therefore, the porosity of char in activation process should be further developed due to the products of this process step are significantly effect on the final product.

Among the carbonization process parameters such as heating rate, nitrogen flow rate and the residence time, the carbonization temperature is the most important parameter.

Normally, high carbonization temperatures in the range between 600 to 700^oC can reduce the yield of char but can increase the liquid and gases release rate. Higher temperature is preferred to obtain high quality char due to an increasing of amount of ash and fixed carbon content with lower amount of volatile matter. Unfortunately, high carbonization temperature can also decrease the yield due to the reduction of primary decomposition of biomass, the decreasing of residence times of primary vapors inside the cracked particle and secondary decomposition of char residue at high temperature. Moreover, high carbonization

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temperatures also increase ash and fixed carbon content due to the decreasing of volatile matter.

Char with a high fixed carbon content is requires for producing ACs. Low volatilization with a high char yield can be obtained by using low carbonization heating rates of 10 to 15^oC/min. The low heating rate increases the dehydration and improves the stabilization of the polymeric components ^[2-3]. However, the microporosity of char is independent to the precursor composition and the carbonization heating rate. Table 1-4 shows the proximate and ultimate analysis of several lignocellulosic biomass materials. It was found that the carbonization is an important process to develop the initial pore structure in the char.

This can be explained by the release of volatile compounds from the carbon’s matrix.

Regarding to the pore development in the char has a great influence on the pore characteristics of subsequently ACs production, the carbonization parameters should be taken into account prior to activation process.

Table 1-4 Ultimate and proximate analysis of lignocellulosic biomass used for air pollution control.

26 1.10.3 Activation

The activation process is use to increase the pore volume by increase the diameter of pores, and thus increasing the porosity of ACs. ACs can be performed by three different methods: physical activation, chemical activation and physiochemical activation (a combination of physical and chemical activation). Physical activation use steam or CO2 while the chemical activation uses various chemicals. The preparation of ACs from lignocellulosic precursors by using various activation conditions is shown in Table 1-5. In the activation process, unorganized carbon is removed during the first stage. Hence, the exposing of lignin to the activating agents can lead to the development of micro-porous structure. In the second stage of the reaction, the existing pores are widened and large size pores are formed. The walls between the pores are simultaneously burnt-off. Completely burnt-off the wall of the pores can increases the transitional pores and macro-porosity but also decreases the volume of micro-pores. Therefore, the extension of burn-off carbon material is an important parameter in activated carbon production.

During activation, the temperature is typically set between 800 to 1000^oC to increase the porosity and surface area of lignocellulosic carbon. In the physical activation, steam is more effective than CO2 due to the smaller molecule of water can diffuse within the porous of char faster than CO2 molecule. Therefore, steam activation is two or three times faster than CO2 at the same conversion process. By using the steam, ACs with a relatively high surface area compared to CO2 can be produced.

In the chemical activation, various chemical agents such as ZnCl2, H3PO4, KOH and NaOH are used to develop the porosity. Generally, the chemical activation is takes place at the temperature of 300 to 500^oC, which is lower temperature than physical activation. The dehydration and degradation mechanism of chemical agents can improves the development of pore in carbon structure by using shorter treatment duration compared to physical activation.

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Table 1-5 Various activation conditions for preparation of lignocellulosic chars.

In addition, the chemical activation process can form the ACs with larger surface area with smaller ranges of micro-porosity compared to physical activation process. Furthermore, the carbon yield of chemical activation is higher than that of physical activation.

1.10.4 Applications of activated carbon from lignocellulosic biomass

The rapid development in industrial activities that follows the growth of the world population severely degraded the air quality due to high amount of pollutant emissions to atmosphere. Therefore, air pollution control is a crucial step to achieve a sustainable energy development. Currently, scrubbing gaseous pollutants using the adsorption method by adsorbents such as ACs is widely used due to it has a suitable pore size in the micropore region (< 2 nm) for gas adsorption and it has a large surface area for rapid reaction. A summary of gaseous pollutants removal by various lignocellulosic ACs such as SO2, NOx, H2S, volatile organic compounds (VOCs) and CO2 is presented in Table 1-6.

1.10.4.1 Removal of SO2

SO2 is the main precursors for acid rain generation, which is the most serious global environment problem. The utilizing of ACs for SO2 adsorption through physical adsorption and chemical adsorption takes a several advantages compared to the earlier methods. The

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utilizing of metal oxides components in ACs that impregnated with the chemicals method to remove SO2 from coal and oil combustion exhaust has been studied.

Table 1-6 Preparation method and adsorption capacity of various activated carbons from lignocellulosic biomass.

Table 1-7 Characteristics of activated carbon used for removal of SO2.

29 1.10.4.2 Removal of NO2

The ACs with high porous structure that obtained from Lignocellulosic biomass is widely used for minimizing the emission of NO2 gas. In addition, the surface chemistry that defines by the type, number and chemical arrangement of heteroatoms on their surface are considering. The dry adsorption process has better adsorption capacity compared to other method due to the reaction mechanism is significantly changed and difficult to control in the water. The micropores of activated carbon produced under optimum condition contributed up to 96% of total pore volume. Preparation conditions and characteristics of activated carbon for removal of NO2 are summarized in Table 1-8.