explain the process of carbonisation of coal in 850 words
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Carbonization of coal is also known as coking of coal. The process consists of thermal decomposition of coals either in the absence of air or in controlled atmosphere to produce a carbonaceous residue known as coke.
Carbonization of coal can be carried out at the following three temperature ranges.
Low temperature carbonization is normally carried out in the temperature range of 500 deg C to 700 deg C. In this type of carbonization, the yields of liquid products are higher and there is lower gaseous product yield. The coke produced is having higher volatile matter and is free burning.
Medium temperature carbonization is done at temperature range of around 800 deg C. This carbonization produces smokeless soft coke. By products produced are similar in characteristics to high temperature carbonization. Medium temperature carbonization is rarely practiced these days.
High temperature carbonization is carried out at a temperature which is above 900 deg C. This carbonization gives higher yield of gaseous products and lower yield of liquid products. This carbonization produces hard coke and is normally employed for the production of metallurgical coke from coking coals.
Process of carbonization of coal
The coal to coke transformation takes place as the coal is heated. When the state of fusing is reached, the layer of heated coal softens and fuses. From about 375 deg C to 475 deg C, the coal decomposes to form plastic layer. Destructive distillation reactions proceed rapidly in the plastic layer with evolution of volatile products. At about 475 deg C to 600 deg C, there is a marked evolution of tar, and aromatic hydrocarbon compounds. The gas and condensable vapour are entrapped in the plastic mass and, as they expand tend to swell it. As the reactions proceed and as the temperature of the fused zone increases, the plasticity of the coal decreases. With continued heating and evolution of the gas the fused layer gradually resolidifies into semi coke having typical, cellular coke structure. The coke at this stage still contains substantial volatile matter. As the temperature increases further beyond 600 deg C, the destructive distillation reaction continues with the evolution of gas and a little tar. The coke stabilization takes place as the temperature increases from 600 deg C to 1100 deg C. This is characterized by contraction of coke mass, structural development of coke and final hydrogen evolution. At this stage the final reactions take place. These reactions split off hydrogen from extremely complex, high molecular weight hydro- carbons. With increasing temperature, the coke mass shrinks with the development of shrinkage cracks.
The caking mechanism
When the coking coals are carbonized then first the plastic mass of optical isotropic is formed, and thereafter gradually lamellar nematic liquid crystals are formed. This polymeric phase is called mesophase. This is the intermediate phase between the isotropic fluid coal and the solid anisotropic semi coke ultimately formed from the mesophase, and has properties which are intermediate between solids and liquids. If the fluidity of the intermediate phase is quite high then mesophase coalesce immediately into a single larger unit. Over a range of increasing temperature, mesophase is formed continuously, grows in size and ultimately touches each other. Thus the mesophase can solidify and convert from coking coal into optical anisotropic texture of coke.
History of coking coals
Coke was produced in ancient China as per historical sources dating to the fourth century. The Chinese people first used coke for heating and cooking no later than the ninth century. In 1709 a coke-fired blast furnace to produce cast iron was established in Great Britain. , During early 18th century the coke was manufactured by burning coal in heaps on the ground in such a way that only the outer layer burned, leaving the interior of the pile in a carbonized state.
The ‘Hearth’ process of coke making, using lump coal, was continued to be used in many areas during the first half of the 19th century. This process was similar to that of charcoal burning but using a heap of coals covered with coke dust instead of a heap of prepared wood, covered with twigs, leaves and earth.
These led subsequently to the development of beehive ovens of different shapes and sizes to meet the increasing demands of hard coke for iron smelting.
Beehive ovens
A beehive oven is a simple firebrick chamber built with an arched roof so that the shape inside is that of an old-fashioned beehive. Its dimensions are typically 4 m in width and 2.5 m in height. Beehive ovens are usually built in rows, one oven beside another with common walls between neighboring ovens. Such a row of ovens is termed a battery. A battery usually consists of many ovens, sometimes hundreds, in a row
Carbonization of coal can be carried out at the following three temperature ranges.
Low temperature carbonization is normally carried out in the temperature range of 500 deg C to 700 deg C. In this type of carbonization, the yields of liquid products are higher and there is lower gaseous product yield. The coke produced is having higher volatile matter and is free burning.
Medium temperature carbonization is done at temperature range of around 800 deg C. This carbonization produces smokeless soft coke. By products produced are similar in characteristics to high temperature carbonization. Medium temperature carbonization is rarely practiced these days.
High temperature carbonization is carried out at a temperature which is above 900 deg C. This carbonization gives higher yield of gaseous products and lower yield of liquid products. This carbonization produces hard coke and is normally employed for the production of metallurgical coke from coking coals.
Process of carbonization of coal
The coal to coke transformation takes place as the coal is heated. When the state of fusing is reached, the layer of heated coal softens and fuses. From about 375 deg C to 475 deg C, the coal decomposes to form plastic layer. Destructive distillation reactions proceed rapidly in the plastic layer with evolution of volatile products. At about 475 deg C to 600 deg C, there is a marked evolution of tar, and aromatic hydrocarbon compounds. The gas and condensable vapour are entrapped in the plastic mass and, as they expand tend to swell it. As the reactions proceed and as the temperature of the fused zone increases, the plasticity of the coal decreases. With continued heating and evolution of the gas the fused layer gradually resolidifies into semi coke having typical, cellular coke structure. The coke at this stage still contains substantial volatile matter. As the temperature increases further beyond 600 deg C, the destructive distillation reaction continues with the evolution of gas and a little tar. The coke stabilization takes place as the temperature increases from 600 deg C to 1100 deg C. This is characterized by contraction of coke mass, structural development of coke and final hydrogen evolution. At this stage the final reactions take place. These reactions split off hydrogen from extremely complex, high molecular weight hydro- carbons. With increasing temperature, the coke mass shrinks with the development of shrinkage cracks.
The caking mechanism
When the coking coals are carbonized then first the plastic mass of optical isotropic is formed, and thereafter gradually lamellar nematic liquid crystals are formed. This polymeric phase is called mesophase. This is the intermediate phase between the isotropic fluid coal and the solid anisotropic semi coke ultimately formed from the mesophase, and has properties which are intermediate between solids and liquids. If the fluidity of the intermediate phase is quite high then mesophase coalesce immediately into a single larger unit. Over a range of increasing temperature, mesophase is formed continuously, grows in size and ultimately touches each other. Thus the mesophase can solidify and convert from coking coal into optical anisotropic texture of coke.
History of coking coals
Coke was produced in ancient China as per historical sources dating to the fourth century. The Chinese people first used coke for heating and cooking no later than the ninth century. In 1709 a coke-fired blast furnace to produce cast iron was established in Great Britain. , During early 18th century the coke was manufactured by burning coal in heaps on the ground in such a way that only the outer layer burned, leaving the interior of the pile in a carbonized state.
The ‘Hearth’ process of coke making, using lump coal, was continued to be used in many areas during the first half of the 19th century. This process was similar to that of charcoal burning but using a heap of coals covered with coke dust instead of a heap of prepared wood, covered with twigs, leaves and earth.
These led subsequently to the development of beehive ovens of different shapes and sizes to meet the increasing demands of hard coke for iron smelting.
Beehive ovens
A beehive oven is a simple firebrick chamber built with an arched roof so that the shape inside is that of an old-fashioned beehive. Its dimensions are typically 4 m in width and 2.5 m in height. Beehive ovens are usually built in rows, one oven beside another with common walls between neighboring ovens. Such a row of ovens is termed a battery. A battery usually consists of many ovens, sometimes hundreds, in a row
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Answer:the temperature range of the solid carbinizing in digree calci
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