Activated Charcoal 3: Production from Biomass Wastes
Published Date: 25th January 2021
Introduction
Preferably, low-cost materials, rich in carbon, and low in inorganic constituents are used as raw materials for activated charcoal production. Some of the popular, and widely used materials include agro-industrial byproducts, which are characterized by their renewability, high mechanical strength, cheapness, abundance, and low ash content. In practice, however, the economics, and the required product properties are important determining factors in selecting the appropriate raw materials. This is because the physicochemical nature of the precursor material has a significant impact on the final product properties, including pore size distribution, and volume, hardness, and purity. Specifically, the optimal adsorptive capacity of activated charcoal essentially depends on the inherent nature of the raw material, and its production processes. Several studies have reported the utilization of agricultural wastes, and other biomass residues in the production of activated charcoal. These biomass precursors include rice, wheat, corn, bean and sorghum husk; sawdust, wood, bamboo, and palm trunk wastes; palm kernel, coconut, hazelnut, walnut, and mangosteen shells; corn cobs, watermelon, tobacco stems, banana, and pineapple wastes, and peels; palm press fiber, empty palm fruit bunch, abattoir wastes, and animal dung among many others. Some of these raw materials may however need to undergo pre-processing steps, usually to control their sizes, forms, moisture content, and other properties. They may therefore be crushed, milled, briquettes, or mixed with binders, and extruded prior to the carbonization processes.
Several studies have divided activated charcoal production into four main processes, which are the pyrolysis process, physical, and chemical activation process, and carbonization and steam/thermal activation. However, the most frequently adopted approach at the small-scale production units has been simplified into pyrolysis or carbonization of the precursors at high temperature, followed by the activation process. The activation process could be achieved through physical, and chemical methods. The physical activation process involves the treatment of char obtained from the carbonization of the biomass with oxidizing gases using steam or carbon dioxide at high temperatures (400 - 10000C) depending on the type of precursor. In chemical activation, the precursor is mixed with activating reagent, and the mixture heated usually at a lower temperature, and activation time, with the final product having better adsorption properties than the physically activated product. However, traces of the activating material may remain in the activated charcoal, which may require washing to remove the impurities, and may also result in higher production costs due to the use of additional chemicals. The activation process can also be by physiochemical, and microwave-assisted approaches, involving heat, and chemical treatment, and microwave radiation respectively. A simple schematic representation of the carbonization and activation processes is shown in figure 1.
When activated charcoal or biochar is the targeted product, thermos-chemical conversion of the biomass precursor by pyrolysis or gasification is employed. However, the pyrolysis process can be manipulated to influence the charcoal yield or to produce other products such as bio-oil, and biogas in addition to the charcoal as shown in figure 2. For example, through slow pyrolysis, a higher amount of biochar can be produced from the biomass precursor, indicating that the same precursor, can be used to produce different types of activated charcoals by using the activating agents under different operating conditions. Pyrolysis/gasification can also be applied to generate bio-oil, and biogas from biomass precursors of different properties like rice husk, benth fruit shell, Jatropha curcas fruit, and seed residues, while the biochar residue can subsequently be converted into activated charcoal by physical activation. Again, recent research attempts at improving the adsorptive efficiency of activated charcoal in organic, and inorganic compounds from aqueous solutions have been made through the loading of nanoparticles on the surface of the materials.
Production of activated charcoal from biomass wastes
Combustion is a two-step process starting with pyrolysis, and followed by combustion proper, which is an exothermic oxidation reaction. The heating of any cellulosic material or macromolecule containing carbon, hydrogen, and oxygen, undergoes a complex chemical change or cleavage often referred to as thermal degradation. This results in the release of combustion gases or oxidizable volatiles such as alkanes, alkenes, ketones, carbon monoxide, aldehydes, alcohols, furans, and other combustible gases, leading to an increase in the carbon content of the residue. The generation of this mixture of combustible gases marks the end of pyrolysis. The behavior of any biomass or flammable organic substance in responses to heating is dependent on many factors, among which are the moisture content, and the density or pore content. The presence of metals in the biomass will also influence its thermal conductivity, ignition time, flame spread, afterglow time, burnout time, and smoky density, especially those that are in briquette form. Combustion on the other hand is characterized by two types of fire namely, flaming combustion, and smoldering combustion.
The use of biomass wastes as precursors of activated charcoal usually requires different pretreatment steps before the actual carbonization, and activation processes. For example, there may be the need to sieve, and or wash the biomass material in order to remove impurities. There is also the need to standardize the moisture content of the material by drying in order to standardize the carbonization process. It is indeed important that all biomass materials intended for pyrolysis be dried to about 10 percent moisture content, because during pyrolysis, excess water vapors operating under the vapor phase tends to dilute the oxidizable volatiles, and flammable pyrolysates, thereby reducing their concentrations in the flame or the combustion zone. Again, milling, and sieving the biomass precursor to standardize the particle size is essential for the optimal carbonization process. Prior to the carbonization process, the amount of ash, and minerals in the biomass materials could be reduced by leaching with acidic or basic solutions. This is usually done when activated charcoals of low ash content are targeted, especially those used in catalytic applications. Samples are mostly demineralized in a stepwise treatment with concentrated hydrochloric acid, hydro fluoric acid, and again, hydrochloric acid to remove the metal oxides, and silica in the biomass material.
Carbonization/pyrolysis: Carbonization or pyrolysis has been defined as the thermal decomposition of biomass materials in a furnace, under an inert gas atmosphere in order to expel the volatilely, non-carbon elements like nitrogen, oxygen, and hydrogen, and with the aim of enriching its fixed carbon content, and production of biochar. It is a simultaneous, and irreversible process that changes the physicochemical nature of the biomass materials at a temperature range of 400 – 8000C. During pyrolysis, the narrow or micropores in the structure of the biomass precursor start to widen, thereby leading to the deposition of tarry substances, which are formed as a result of the increasing temperature. This widening of the existing pores creates the intermediate or mesopores (2 nm - 50 nm), and macropores (> 50 nm), and reduces the volume of the micropores (< 2 nm) as shown in figure 3.
The important parameters that influence the process, and quality of the final product are the carbonization temperature, the retention time, heating rate, and nitrogen flow rate. Carbonization temperature, however, has the most significant effect on the process, because, increase in temperature results in the release of more volatile species, and an increase in the fixed carbon, and ash contents of the final product. Thus, an increase in the carbonization temperature results in increases in both the gases, and liquid percentage yields, but a reduction in the biochar yield (Figure 2). The process however produces better biochar, which is a carbon skeleton of graphite-like pore structure. The biochar prepared at low temperatures, however, has more yield, minor compactness of the aliphatic compounds, and is amorphous. The biochar obtained at the end of carbonization, has relatively low surface area (300 m2/g), and adsorption capacity due to the fact that the pores are locked by tarry substances, which can be removed by activation. A recent video demonstrating the steps in biochar production can be viewed at https://youtu.be/svNg5w7WY0k
Physical activation: Physical activation is a two-step process that involves carbonization of the biomass material, and subsequent activation of the resulting char at higher temperatures in the presence of CO2, steam, air, or their mixtures serving as oxidizing gases. In this case, the carbonization temperature is usually between 400 and 8000C, while activation is usually at a higher temperature of about 800v-11000C. In some industrial units, single-step carbonization, and activation is adopted, in which the temperature is kept in the range of 600 - 8000C for a specified duration, and the activation process achieved just by switching the inert gas to oxidizing gas. This approach has the advantage of skipping the cooling phase after the carbonization, and reducing the physical effort, electrical consumption, cost, and operating time. The main aim of activation is to transform the carbon structure into a highly porous solid of activated charcoal. Therefore, during the physical activation, new pores develop on the surface of the biochar, while the narrow or micropores are widened into meso, and macropores, thereby enhancing the porosity, and surface area of the carbonaceous material. Steam, and CO2 are the ideal oxidizing agents, and have been shown to give similar optimal surface areas of up to 1000 m2/g. At a near activation temperature, steam is however reported to react four times faster with carbon than CO2, indicating that steam activation is better, both in terms of high surface area production, and shortness of activation time. Carbon dioxide activation, however, has the advantage of favoring the development of new pores, while steam activation favors the widening of micro-pores into meso, and macropores.
In steam activation, the biomass waste materials are either heated at a temperature range of 500 – 700°C under a flow of pure steam or heated at a higher temperature range of 700 – 800°C under a flowing stream. Steam pyrolysis has been successfully applied on different types of agricultural biomass wastes, such as rice husk, sawdust, tropical wood waste, palm shells, durian peel, corn cobs, coconut shells, tobacco stems, hazelnut shell, banana peels, etc. In a process described by Hidayu and Muda (2016), a palm kernel shell sample was loaded into a stainless reactor, and heated by an electrical tube furnace at an initial temperature of 3000C for 30 minutes, and later increased to 8000C to achieve complete pyrolysis of the shells. Water was then injected at the rate of 120 ml per hour to the reactor to activate the sample, with the reaction between the steam, and the biochar generating the pores. Thereafter, the reactor was cooled down, and the sample taken out, washed with distilled water, and subsequently dried as activated charcoal. For small-scale production, a simple six steps procedure demonstrated in https://www.wikihow.com can be adopted. The procedure involves heating the prepared precursor in a metal pot over an open fire for 3 to 5 hours to achieve the carbonization process; followed by water introduction to achieve the physical activation, and then cooling the activated charcoal. The activated charcoal is then ground to the desired size, dried for 24 hours, and stored in durable a plastic bag or container.
Chemical activation: Chemical activation of carbonaceous material requires only one step, since both the carbonization, and activation processes take place in a single step. The precursor is impregnated with a chemical catalyst, and heated at high temperature, with the catalyst acting as an oxidant, and dehydrating agent. The chemical catalysts commonly used are FeCl2, ZnCl2, H3PO4, H2SO4, K2S, HNO3, H2O2, KMnO4, NaOH, KOH, and K2CO3. Thus, the activation can be basic, acidic, or neutral activation. For example, the basic activation of biomass with metal alkaline hydroxides such as KOH, and NaOH produces activated charcoals of very high surface areas that can reach up to 2000 m2/g. This is usually ideal for biochar production, since it is relatively more susceptible to metal alkaline hydroxides activation, and helps produce activated charcoals of better surface properties. Acidic activation using H3PO4 has however been reported to be more economical, and results in a higher activated charcoal percentage yield as well as better surface area, and porosity than those obtained with the neutral activation (Figure 4). Nonetheless, metal alkali hydroxides, and acids have the disadvantages of being corrosive, toxic, hazardous, and explosive, thereby requiring both safeties, and environmental precautions when using them as activating agents.
The final stage in chemical activation is washing of the activated charcoal with acid or alkali depending on the chemical reagent used in the preparation, and followed by washing with water. Chemical activation has the advantages of needing only a single activation step, lower activation temperature (< 8000C), shorter activation times, higher yield, and good porous characteristics. Therefore, chemical activation may be the preferred activation method in terms of the development of the surface area, macropores, and higher carbon yield among its other economic benefits.
Physiochemical activation: Physiochemical activation is essentially a double-step activation. Activated charcoal production by this activation process has a two-method approach, which is by chemical treatment prior to carbonization or pre-carbonization method, and chemical treatment subsequent to carbonization or post-carbonization method. In the pre-carbonization method, the biomass precursors are first carbonized to produce biochar, which is subsequently impregnated with an activating agent, and then subjected to thermal treatment in the presence of oxidizing gas or further carbonization in an inert environment before switching to oxidizing gas for physical activation at a high temperature of range 600 – 8500C. In the post-carbonization method, the biomass precursors have subjected to chemical activation before thermal treatment, and physical activation, such that the sequence of chemical activation significantly affects the quality, and textural characteristics of the activated charcoals developed.
Studies have shown that the biochar produced with the pre-carbonization method has a superior surface area, and pore volume to the one produced with the post-carbonization method, due to the pore-blocking effect of the dehydrating agent. For example, the surface areas of activated charcoals produced through physiochemical activation changed from 1035 to 1653 m2/g, and 554 to 1213 m2/g as a result of activation with H3PO4 and CO2, and sole CO2 activation respectively. Physiochemical activation is therefore recommended in situations where the dehydrating agent remains on the surface of biochar after washing thereby necessitating additional physical activation to improve the porosity of the biochar. Essentially, such double-step activation produces activated charcoals of better textural characteristics and quality.
Microwave-assisted activation: Microwave-assisted activation employs a combination of physical and/or chemical activation to produce activated charcoals of better quality through either a one-step or two-step activation process. Carbonization, and activation are the two major procedures for one-step microwave activation in a reactor, while the two-step microwave activation process involves the carbonization, and activation of biochar under microwave radiation. The quality of the product is influenced by the biomass sources, the temperature of the pyrolysis, power of radiation, running time, and the nature of the additives. Microwave activation is an advanced procedure for producing top-grade activated charcoals of the superior pore, and textural characteristics. It is increasingly being preferred over the conventional activation approaches because of its unique features such as selectively, quick, even, and volumetric heating, indirect interaction between the heat source, and heated material, and also the prompt, and precise regulation of the process.
Structural, and chemical characteristics of activated charcoal
During the carbonization process, the carbon molecules in the biomass material are rearranged into their most stable structure, which is a graphite plate. However, the specific arrangements of the angles, spacing, and gaps within the structure vary with the type of raw material used in the preparation of the activated charcoal. Structurally, activated charcoal or biochar contains micropores (< 2 nm), mesopores (2 - 50 nm), and macropores (> 50 nm) that play significant roles in determining its important property of material absorbance. Physically however, it is characterized according to its density, surface area, and pore volume. It is the astonishingly large surface area, and pore volume that confers on it the unique adsorption capacity. Commercial food-grade activated charcoal has a surface area range of 300 to 2,000 m2/g, while values as high as 5,000 m2/g have been recorded. Activated carbon is also graded into three different types, powdered, granular, and extruded, each with its own physical attributes that make it ideally suited for specific types of applications.
Activated charcoal is made up of fixed carbon, and other minor constituents like ash, moisture, and volatile matter. The presence of other atomic elements such as hydrogen, oxygen, nitrogen, phosphorus, and sulfur on the surface of activated charcoal influences its type and quality and has been used to determine its chemical properties. For example, the concentration of the oxygen atoms on the surface basically influences the adsorption capacity of the activated charcoal. Several measures such as iodine, molasses, methylene blue, and tannin numbers have been used to determine the quality of activated charcoal. The iodine number (500 to 1200 mg/g) measures the adsorption capability for small molecules; the molasses number (95 to 600 mg/g) measures the adsorption capability for large molecules; methylene blue number (11 to 28 g/100g) shows the adsorption capability for medium-sized molecules, while tannin number (200 to 362 ppm) determines the capability to adsorb mixtures of different molecular sizes. Other quality measures include the carbon tetrachloride activity (45 to 70% by weight) that measures the activated charcoal capacity for porosity for air/vapor applications, and dechlorination, which determines the ability of the activated charcoal to remove half the chlorine from a liquid stream. The mineral fraction of activated charcoal comprises macro- and micro-elements which may act as a source of minerals for micro-organisms living in the soil.
Conclusion
Carbonization/pyrolysis is an environment-friendly method of converting biomass wastes into biochar, and activated charcoal. Physical, and chemical activation are the most commonly used methods of biochar activation because of their simplicity, lower cost, and shorter period of preparation, although the chemical method generates more superior products. Specifically, the chemical activation process may be the preferred activation choice in terms of the development of a better surface area, micro-pores, and higher carbon yield among its other economic benefits. The double-step activation employed in the physicochemical activation method also produces activated charcoals of better textural characteristics, and quality.
Bibliographic references
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