The carbonization process is actually the dry distillation process of the material under low temperature conditions. In this process, the material is gradually heated and heated under a certain low temperature range and the condition of air isolation. The low-molecular substances in the material are first volatilized, and then the coal and coal tar pitch are decomposed and solidified. A series of materials will occur during the entire carbonization process Complex physical changes and chemical changes, of which physical changes are mainly dehydration, degassing and drying processes; chemical changes are mainly two types of reactions, thermal decomposition and thermal polycondensation.

During the thermal decomposition and thermal polycondensation reaction of the material, gas and coal tar are precipitated. The oxygen bonding group of the organic compounds in the material is destroyed, and the oxygen element is precipitated with gases such as Hz0, CO, CO: etc. At the same time, aromatic compounds and cross-linked High-strength carbon molecular structure solid; during the carbonization process, due to the discharge of non-carbon materials such as oxygen and hydrogen during the decomposition at high temperature, the carbon atoms after the loss of oxygen and hydrogen are recombined to form an order with a basic graphite microcrystalline structure The crystals consist of hexagonal carbon atom planes, and their arrangement is irregular, thus forming voids between crystallites. These voids are the initial pores of the carbonized material.

Therefore, the purpose of carbonization is to make the material form a secondary pore structure that is easily activated and to give the mechanical strength required to withstand activation. The requirement for the carbonization of materials is that the appearance of the carbonized material obtained through carbonization must meet certain specifications and shape requirements, the internal structure must have a certain initial pore structure, and at the same time have a high mechanical strength.

The carbonization process can generally be divided into the following stages.

(1) The temperature in the drying stage is below 120 ° C, and the external moisture and internal moisture are released from the raw coal. At this time, the appearance of the raw coal is unchanged.

(2) In the pyrolysis stage, the raw coal starts to decompose and release pyrolysis water to form gaseous products (such as CQ, C02, H2S, etc.). Different coal types have different pyrolysis temperatures, and coal with low metamorphism starts to heat. The solution temperature is also low. Northeast peat is about 100-1600, lignite is about 200-3000C, bituminous coal is about 300-4000C, and anthracite is about 300-450C. Because the molecular structure and generation conditions of coal are quite different, the above pyrolysis temperature is just a relative reference value between different coal types.

(3) The temperature in the carbonization stage is 300-600 degrees Celsius, mainly polycondensation and decomposition reactions, the raw coal largely precipitates volatile matter, and almost all the tar and gas products precipitated in the carbonization process are produced in this stage. Cohesive bituminous coal gradually softens and melts at this stage to form a colloidal body with three phases of gas, liquid, and solid, and then turns into semi-coke through the processes of flow, polycondensation, and solidification; non-adhesive forms needle-like semi-coke or lump Shaped half-focus.

The final temperature and rate of carbonization are the main operating conditions controlled by the carbonization process. For different coal types, the tar formation process ends at around 550 ° C. A lot of laboratory research and industrial production experience have shown that 600 ℃ is the best final carbonization temperature. If the temperature is too low, the carbonization product cannot form sufficient mechanical strength. If the temperature is too high, the graphite crystallite structure in the carbonization product will be promoted. Ordering, reducing the gap between crystallites, affecting the activation pore formation process. The carbonization heating rate has a great influence on the yield of carbonized products.

The high heating rate can make the material precipitate more tar and coal gas and reduce the yield of carbonized material. When the heating rate is reduced, the material is heated for a long time in the low temperature region, and the pyrolysis reaction has a strong selectivity. The initial pyrolysis breaks the weaker bonds in the material molecule, and parallel and sequential thermal polycondensation reactions occur, forming a The structure with high thermal stability, thereby reducing the yield of volatiles of the thermal decomposition products at high temperature stage, and obtaining a higher yield of solid carbonized products (ie carbonized materials).

The quality of carbonized materials in the carbonization process is mainly evaluated by volatile matter, coke index, water capacity and strength. The volatility of qualified carbonized materials is 7% -18%, the characteristic index of coke yesterday is 1-3, the water capacity is 15% -25%, and the strength of the ball disc is 90%.

Because the measurement of the above indicators requires a certain amount of time, and during the commissioning of the production site, it is often necessary to quickly adjust the process parameters according to the quality of the carbonized material, so the quality of the carbonized material can also be roughly evaluated through the senses. Qualified carbonized materials should have a smooth, crack-free surface, high strength, and consistent color of the material.

Carbonization process of internal heating rotary furnace

1) Carbonaceous material flow: the shaped particles (raw material) are directly lifted into the charging chamber of the rotary kiln by the conveyor, and fall into the drum by gravity, and are brought to the board along the spiral movement in the drum. Move in the direction of the burner. The material first goes through the preheating and drying stage with a temperature of 200 ° C, and then enters the carbonization stage of 350-550 ° C. During this process, the carbon particles come into contact with the hot air stream for carbonization to discharge moisture and volatile matter, and finally  exported through the export port .

(2) Gas flow: after the tail gas of the furnace is burned in the combustion chamber, a part of the tail gas returns to the furnace head, enters the drum and directly contacts with the countercurrent carbon particles for carbonization; the other part enters the waste heat boiler for heat exchange. The flue gas is discharged from the chimney. Part of the steam generated by the waste heat boiler is sent to the activation process and heat exchange station, and part of it is returned to the furnace head and mixed with the tail gas to enter the carbonization furnace.

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Activated carbon is a processed, porous version of carbon that has many different uses, especially adsorption and chemical reaction needs for water and gas purification. Because activated carbon particles are so porous, they have very expansive surface areas tucked into the holes and tunnels all over their surface.

These areas can be filled with other materials for other purposes as well. For instance, in water purification, silver is mixed into the carbon pores in order to filter contaminants like mercury and organic arsenic from water for domestic drinking purposes.

Because carbon is produced from charcoal through a relatively inexpensive and simple series of activation processes, it can be had in great quantities for many applications.

The Carbon Manufacturing Process – How to Make Activated Carbon

The production process of activated, or active, carbon exists in two forms. A carbonaceous source, which can exist as coal, peat, or any organic carbonaceous material is carbonized, which means the pure carbon is extracted by a heating method known as pyrolysis. Once the material is carbonized, it needs to be oxidized, or treated with oxygen, either by exposure to CO2 or steam, or by an acid-base chemical treatment.

Carbonization

Carbonization is the process of taking a carbon-rich piece of material and converting it to pure carbon through heating. This heating process, called pyrolysis, comes from an ancient technique for making charcoal. Very dense carbonaceous material is used in the beginning, because the end result needs to be extra-porous for activated carbon purposes.

Carbon-rich material is placed in a small (relative to the amount of material) furnace and cooked at extreme temperatures topping 2000 degrees Celsius. What remains is usually 20-30 percent of the beginning weight, and consists of mostly carbon and a small percentage of inorganic ash. This is very similar to “coking,” a method of producing coke from charcoal, a type of carbon-based fuel.

Once the porous form of carbon is produced, it needs to undergo oxidization so it can be adsorbent. This can occur in one of two ways: gas or chemical treatment.

all carbonaceous materials can be converted into activated carbon,materials can be converted into activated carbon, although the properties of the final product will be different, depending on the nature of the raw material used, the nature of the activating agent, and the conditions of the carbonization and activation processes.

During the carbonization process, most of the noncarbon elements such as oxygen, hydrogen, and nitrogen are eliminated as volatile gaseous species by the pyrolytic decomposition of the starting material. The residual elementary carbonstarting material. The residual elementary carbon atoms group themselves into stacks of flat, aromatic sheets cross-linked in a random manner. These aromatic sheets are irregularly arranged, which leaves free interstices. These interstices give rise to pores, which make activated carbons excellent adsorbents.

During carbonization these pores are filled with the tarry matter or the products of decomposition or at least blocked partially by disorganized carbon. This pore structure in carbonized char is further developed andcarbon. This pore structure in carbonized char is further developed and enhanced during the activation process, which converts the carbonized raw material into a form that contains the greatest possible number of randomly distributed pores of various sizes and shapes, giving rise to an extended and extremely high surface area of the product.

The activation of the char is usually carried out in an atmosphere of air, CO2, or steam in the temperature range of 800°C to 900°C. This results in the oxidation of some of the regions within the char in preference to others, so that as combustion proceeds, a preferential etching takes place. This results in the development of a large internal surface, which in some cases may be as high as 2500 m2/g.

Gas Treatment

The activizing of carbon can be done directly through heating in a chamber while gas is pumped in. This exposes it to oxygen for oxidization purposes. When oxidized, the active carbon is susceptible to adsorption, the process of surface bonding for chemicals—the very thing that makes activated carbon so good for filtering waste and toxic chemicals out of liquids and gases. For physical gas treatment, the carbonization pyrolysis process must take place in an inert environment at 600-900 degrees Celsius. Then, an oxygenated gas is pumped into the environment and heated between 900 and 1200 degrees Celsius, causing the oxygen to bond to the carbon’s surface.

Chemical Treatment

In chemical treatment, the process is slightly different from the gas activization of carbon. For one, carbonization and chemical activation occur simultaneously. A bath of acid, base or other chemicals is prepared and the material submerged. The bath is then heated to temperatures of 450-900 degrees Celsius, much less than the heat needed for gas activation. The carbonaceous material is carbonized and then activated, all at a much quicker pace than gas activization. However, some heating processes cause trace elements from the bath to adsorb to the carbon, which can result in impure or ineffective active carbon.

Post Treatment Activated Carbon

Following oxidization, activated carbon can be processed for many different kinds of uses, with several classifiably different properties. For instance, granular activated carbon (GAC) is a sand-like product with bigger grains than powdered activated carbon (PAC), and each are used for different applications. Other varieties include impregnated carbon, which includes different elements such as silver and iodine, and polymer-coated carbons.

How Does Activated Carbon Work?

Physical adsorption is the primary means by which activated carbon works to remove contaminants from liquid or vapor streams. Carbon’s large surface area per unit weight allows for contaminants to adhere to the activated carbon media.

The large internal surface area of carbon has several attractive forces that work to attract other molecules. These forces manifest in a similar manner as gravitational force; therefore, contaminants in water are adsorbed (or adhered) to the surface of carbon from a solution as a result of differences in adsorbate concentration in the solution and in the carbon pores.

Physical adsorption occurs because all molecules exert attractive forces, especially molecules at the surface of a solid (pore walls of carbon), and these surface molecules seek to adhere to other molecules.

The dissolved adsorbate migrates from the solution through the pore channels to reach the area where the strongest attractive forces are located. Contaminants adsorb because the attraction of the carbon surface for them is stronger than the attractive forces that keep them dissolved in solution. Those compounds that exhibit this preference to adsorb are able to do so when there is enough energy on the surface of the carbon to overcome the energy needed to adsorb the contaminant.

Contaminants that are organic, have high molecular weights, and are neutral, or non-polar, in their chemical nature are readily adsorbed on activated carbon. For water adsorbates to become physically adsorbed onto activated carbon, they must both be dissolved in water so that they are smaller than the size of the carbon pore openings and can pass through the carbon pores and accumulate.

Besides physical adsorption, chemical reactions can occur on a carbon surface. One such reaction is chlorine removal from water involving the chemical reaction of chlorine with carbon to form chloride ions.

The most important application of activated carbon

The most important application of activated carbon adsorption where large amounts of activated carbons are being consumed and where the consumption is ever increasing is the purification of air and water. There are two types of adsorption systems for the purification of air.

One is the purification of air for immediate use in inhabited types of adsorption systems for the purification of air. One is the purification of air for immediate use in inhabited spaces, where free and clean air is a requirement. The other system prevents air pollution of the atmosphere from industrial exhaust streams.

The former operates at pollutant concentrations below 10 ppm, generally about 2 to 3 ppm. As the concentration of the pollutant is low, the adsorption filters can work for a long time and the spent carbon can be discarded, because regeneration may be expensive.

Air pollution control requires a different adsorption setup to deal with larger concentrations of the pollutants.

The saturated carbon needs to be regenerated by steam, air, or nontoxic gaseousregenerated by steam, air, or nontoxic gaseous treatments. These two applications require activated carbons with different porous structures. The carbons required for the purification of air in inhabited spaces should be highly microporous to affect greater adsorption at lower concentrations. In the case of activated carbons for air pollution control, the pores should have higher adsorption capacity in the concentration range 10 to 500 ppm.

Marketing of activated carbon

Market The global activated carbon industry is estimated to be around 1.1 million metric ton. Demand for virgin activated carbon is expected to rise by around 10% annually through 2014, worldwide. The U.S is the largest market, which will also pace global growth based on anticipated new federal regulations mandating mercury removal at coal- fired power plants. Nearly 80% of the total active carbon is consumed forfired power plants.

Nearly 80% of the total active carbon is consumed for liquid-phase applications, and the gas-phase applications consume about 20% of the total production. Because the active carbon application for the treatment of waste water is picking up, the production of active carbons is always increasing.

The consumption of activecarbon is the highest in the U.S. and Japan, which together consume two to four times more active carbons than European and other Asian countries. The per capita consumption of active carbons per year is 0.5 kg in Japan, 0.4 kg in the U.S., 0.2 kg in Europe, and 0.03 kg in the rest of the world. This is due to the fact that Asian countries by and large have not started using active carbons for water and air pollution control purposes in large quantities.

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First, the raw material in the form of lumps, pre-sized material, briquettes or extrudates is carbonized by heating in an low oxygen atmosphere so that dehydration and devolatilization of the raw material occurs. Carbonization reduces the volatile content of the source material to under 20%. A coke or charcoal (depending on the raw material) is produced which has pores that are either small or too restricted to be used as an adsorbent.

A variety of methods have been developed but all of these share the same basic principle of initial carbonization at 500-600 degrees C，followed by activation with steam at 800-1,100 degrees C.

Since the overall reaction (converting carbon to carbon dioxide) is exothermic it is possible to utilize this energy and have a self-sustaining process：

• C + H2O (steam) —> CO + H2 (-31 Kcal)
• CO + ½ O2 —> CO2 (+67 Kcal)
• H2 + ½ O2 —> H2O (steam) (+58 Kcal)
• C + O2 —> CO2 (+94 Kcal)

A number of different types of kilns and furnaces can be used for carbonization/activation and include rotary (fired directly or indirectly), vertical multi-hearth furnaces, fluidized bed reactors and vertical single throat retorts.  Each manufacturer has their own preference.

As an example, production of activated carbon using a vertical retort is described below.

Raw material is introduced through a hopper on top of the retort and falls under gravity through a central duct towards the activation zone. As the raw material moves slowly down the retort the temperature increases to 800-1000 degrees C and full carbonization takes place.

The second stage, which can take place later in the same kiln, is activation which enlarges the pore structure, increases the internal surface area and makes it more accessible. The carbonized product is activated with steam at very high temperatures. The chemical reaction between the carbon and steam takes place at the internal surface of the carbon, removing carbon from the pore walls and thereby enlarging the pores.

The activation zone, at the bottom of the retort, covers only a small part of the total area available and it is here that steam activation takes place. Air is bled into the furnace to convert the product gases, CO and H2 into CO2 and steam which, because of the exothermic nature of this reaction, reheats the firebricks on the downside of the retort, enabling the process to be self-supporting.

Every 15 minutes or so, the steam injection point is alternated to utilize the “in situ” heating provided by the product gas combustion. The degree of activation (or quality) of the product is determined by the residence time in the activation zone.

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