Activated carbon has been used since Roman times to remove harmful impurities such as organic pollutants from water. Activated carbon is the generic term used to describe a family of carbonaceous adsorbents with a highly amorphous form and an extensively developed internal pore structure.
Activated carbon is extremely porous and has a very large surface area, which makes it an effective adsorbent material. This large surface area relative to the size of the actual carbon particles makes it easy to remove large amounts of impurities in a relatively small confined space.
The approximate ratio of surface area is one square meter per gram. The intermolecular attraction in the smallest pores leads to adsorption. Contaminant molecules in water are adsorbed onto the activated carbon surface by physical or chemical attraction. Physical attraction does not change the molecular structure of the adsorbent; chemisorption causes a change in the molecular structure of the adsorbed material. Some people prefer to refer to these two phenomena as physical adsorption and chemisorption.
The two mechanisms by which a chemical substance adsorbs onto activated carbon are that it "doesn't like" water or that it is attracted to the activated carbon. Activated carbon adsorption proceeds through three basic steps.
1. The substance adsorbs to the outside of the carbon surface.
2. The substance enters the carbon adsorption pores with the highest adsorption energy.
3. The substance is adsorbed onto the internal graphite flakes of the carbon.
Many natural substances are used as the base material for the manufacture of activated carbon. Among the most common ones used for water purification are lignite, bituminous coal and anthracite as well as peat, wood and coconut shells.
Different substrates and activation processes result in unique pore sizes and pore distributions. The grouping of pores in carbon is usually described by their pore size. Macropores (diameters greater than 50 nanometers [nm]); mesopores (diameters 2 to 50 nm) and micropores (diameters less than 2 nm). Another way to consider and visualize the pore structure is transport and adsorption pores: transport pores carry the adsorbate to the adsorption pores.
Activated carbons based on coconut shells are the least dusty. They are mainly microporous and well suited for organic chemisorption. Compared to other types of activated carbon, coconut shell-based carbon has the highest hardness, which makes it an ideal carbon for water purification.
In terms of substrates, coconut shells and wood are renewable resources. Although billions of coconut shells are used each year for activation, coconut plantations with millions of acres of land continue to provide all the benefits of green trees for our environment.
Activated carbon is produced from coconut shells through a two-step process.
The first step of activation involves carbonizing the shell, driving about two-thirds of the volatiles out of the shell to form a carbonaceous material filled with tiny pores.
In the second stage, this carbonized substrate is activated at high temperatures (1,100°C/2,012°F) in steam. The activation temperature and activation time are important to create the internal pore network and to impart certain surface chemistry (functional groups) inside each particle. In essence, the entire activation process imparts unique adsorption properties to the carbon.
The process of carbonization is the conversion of coconut shells into charcoal or charcoal.
The carbonization process (charcoal manufacturing) is known as pyrolysis, which is the chemical decomposition of the husk by heating in the absence of oxygen.
During the carbonization of coconut shells, volatiles equivalent to 70% of the shell mass are released into the atmosphere, producing 30% of the shell mass as charcoal. The volatiles released during carbonation are methane, CO2, water vapor, and a variety of organic vapors.
Coconut shells are carbonized in an ancient process commonly referred to as the open air method. In this process, the earth is used as an insulator and heats the shells in the absence of oxygen. The pit carbonization cycle consists of three stages.
1. Pyrolysis stage. Over 12 hours, when the gases are released.
2. Pacification phase. When the pit is closed and the char is cooled for 12 hours.
3. Unloading. The final step is to unload the charcoal and load fresh shells for the next production cycle.
The temperature maintained during the process is critical for complete pyrolysis. Experiments have shown that charcoal production is directly proportional to the temperature in the pit.
In addition, the amount of methane released to the atmosphere is directly related to the temperature in the pit.
In total, the pits produce 30% of the char, which means that their temperature is maintained at 500°C (932°F). Several experiments have been conducted to measure the amount of gas released; on average, one million tons (MT) of coconut husk releases about 12 to 15 kg of methane into the atmosphere.
Greenhouse gases keep the Earth safe from the cold of space. As incoming solar radiation is absorbed and re-emitted from the Earth's surface as infrared energy, greenhouse gas (GHG) in the atmosphere prevents some of the heat from escaping into space and instead re-emits the energy back to further heat the surface.
Human activity is amplifying the natural greenhouse effect. Our greenhouse gas emissions are altering the Earth's energy balance between the incident solar radiation and the heat released back into space, leading to climate change.
Climate change alters temperature, precipitation, and sea level, and may adversely affect human and natural systems, including water resources, human health and settlements, and the biodiversity of our ecosystems.
The apparent acceleration of climate change models over the past 50 years and the growing confidence in the results of global climate models add to the compelling evidence that the climate is being affected by anthropogenic greenhouse gas emissions. If this is true, we must all do what we can to limit greenhouse gases.
There are several greenhouse gases, of which water vapor, methane and carbon dioxide are both naturally and industrially produced. CO2 and methane come primarily from fossil fuel combustion. Land use change and deforestation are important sources of CO2 emissions.
India, the Philippines, Sri Lanka and Indonesia are the countries where coconut shells are mainly charred for water and air purification. In recent years, several countries in the Association of Southeast Asian Nations (ASEAN) region have also begun to do so.
A conservative estimate is that about 350 metric tons/year of methane is emitted into the atmosphere through pit charring in the four major countries. This is equivalent to the CO2 emitted by 350,000 medium-sized vehicles traveling 20,000 miles per year. Compared to CO2, methane is four times more effective (and therefore harmful) as a greenhouse gas than CO2.
In the pit method of charring coconut shells, greenhouse gases are emitted into the atmosphere without any control or treatment of the emitted gases. Recently, a company in collaboration with the Indian Institute of Science (Bangalore) has developed a new process for charring coconut shells in a reactor to capture greenhouse gases and use them to generate heat under controlled conditions. Coconut shells contain cellulose, hemicellulose and lignin, with an average composition of C6 H10 O5 varying slightly depending on the nature of the biomass. Theoretically, the air-fuel ratio (defined as the stoichiometry (the exact ratio of substances belonging to or involved in a particular reaction required for combustion) required for complete combustion of the shells is 6:1 to 6.5:1 The end products are CO2 and H2O.
In this new process, combustion takes place under sub stoichiometric conditions, with air-fuel ratios of 1.5:1 and 1.8:1. The gas thus obtained is called generator gas and is combustible. This process is carried out in a device called "charring reactor" with limited air supply. Two reactions occur: oxidation and reduction.
The first part of the sub-chemometric oxidation (air with shell) leads to the loss of volatiles from the shell and is exothermic. It leads to a peak temperature of 800°C (1,472°F) and produces gaseous products such as carbon monoxide and hydrogen (in the same ratio) as well as carbon dioxide and water vapor, which are partially reduced to the charcoal hot bed and hydrogen produced during the carbon monoxide gasification process. The reduction reaction is heat absorbing; as shown below, combustible products such as CO, H2 and CH4 are produced.
C + CO 2 -> 2CO
C + H 2 O -> CO + H 2
C + 2H 2 -> CH 4
Since char is produced during the gasification process, the whole operation is self-sufficient.
The development of the most advanced technology is the key factor, in which the shell is fed from the top through the company's double inlet re-combustion process. The process consists of a fuel and ash handling system, a reactor for the gasification system, and a gas cooling and cleaning system. The process is unique in that it prevents the formation of tar during pyrolysis.
The process produces consistently high quality char compared to char produced by the open pit method. It has a consistently high iodine value and is free of contaminants from the pit, such as soil, silica and pebbles. This char forms a good base material for the activation process and allows for high performance.
