Coal combustion process and its products

Coal combustion process and its products

Coal combustion process and its products

Fuel is any combination of organic and inorganic material. Combustion is a rapid chemical reaction in which fuel reacts with oxygen to produce heat.

  • Combustion is a rapid chemical reaction in which fuel reacts with oxygen to produce heat
  • Fuel is any combination of organic and inorganic material which during chemical reaction or transformation gives out a large amount of heat
  • Substances which burn in air to produce heat are called combustible substances 
  • Air is necessary for burning
  • The minimum temperature at which fuel catches is called an ignition temperature.
  • Different fuels have different ignition temperatures e.g. ignition temperature of the oil is less than coal
  • Substances which have very low ignition temperature are called inflammable substances e.g. Petrol, LPG, CNG etc.
There are three types of fuels
  • Solid fuels e.g. Wood, coal 
  • Liquid fuels e.g. kerosene, petrol, diesel, oil
  • Gaseous fuels e.g. LPG, CNG, Biogas, Hydrogen
Following three conditions are must for combustion
  • Fuel
  • Air (to supply oxygen)
  • Heat (to raise the temperature more than or equal to ignition temperature)
There are three types of combustion as below:
  1. Rapid combustion: This is a combustion in which a fuel burns rapidly and produces heat with the help of external heat.
  2. Spontaneous combustion: This is a combustion in which fuel burns spontaneously without any external heat.
  3. Explosion: This is a combustion in which fuel burns suddenly and produces heat with the help of external heat or pressure.
Flame:
  • Flame is a zone of combustion
  • Flame has three zones as below:
  1. Inner zone: There are unburnt vapors of fuel
  2. Middle zone: Here partial combustion of vapors of fuel takes place
  3. Outer zone: here complete combustion of vapors of fuel takes place
Characteristics of good fuel 
  • Characteristics of good fuel are as below:
  • Readily available
  • Cheap
  • Easy to store and transport
  • Burns at a moderate rate so that burning can be easily controlled
  • Produces a large amount of heat
  • Does not leave behind any undesirable substance after burning
  • Does not cause pollution
Characteristics of coal  

Coal has got many characteristics of good fuel hence in India about 58% electricity is generated by burning coal.
Coal is different in many ways than other fuels as shown below:
  • Normally fuels other than coal have defined hydrocarbons which means C & H in fuel does not very much.
  •  Coal is a heterogeneous fuel and has only an assumed structure of C & H and varies highly from mine to mine and even in the same mine from seam to seam.
  • C & H in coal can be known only after carrying out ultimate analysis moreover the way in which these hydrocarbons will behave will depend upon their reactivity and how that coal is formed.
  •  Completion of coal combustion will depend upon how the carbon rings are formed and bonded.
  • Some coals burn slowly and require more residence time in the furnace
  • For combustion process temperature, turbulence and time are most important
  • Temperature: Fuel must reach ignition temperature and for stable flame, this temperature must be maintained. For coal, ignition temperature is in the range of 400 to 425-degree centigrade
  • Turbulence: Turbulence helps each particle to quickly contact the necessary oxygen molecule for rapid combustion with minimum excess air
  • Time: Depending on the fuel particle size some time is needed for complete combustion. This time for complete combustion is reduced by turbulence and rise in temperature
  • Combustion is a rapid chemical reaction between fuel and oxygen. During combustion combustible elements of coal like carbon, sulfur, hydrogen combines with oxygen and produces heat along with respective oxides. Source of oxygen is air. By volume 21% oxygen is present in the air and by weight, it is 23.2%
Following chemical reactions takes place in the combustion of coal with the release of heat: 
  • C + O2 = CO2 + 8084 Kcal/ Kg of carbon (33940 KJ/Kg)
  • S + O2 = SO2 + 2224 Kcal/Kg of sulfur (9141 KJ/Kg)
  • 2 H2 + O2 = 2 H2O + 28922 Kcal/Kg of hydrogen (142670 KJ/Kg)
  • 2C + O2 = 2CO + 2430 Kcal/Kg of carbon (10120 KJ/Kg)
  • Combustion of coal produces following products:
  • Ash (bottom ash and fly ash)
  • CO2
  • CO
  • SO2
  • H2O
  • NOX
  • Suspended particulate matter
Harmful effects of coal combustion process

Combustion of coal causes following harmful effect:
  • Incomplete combustion of coal produces carbon monoxide (CO) which is a poisonous gas
  • Complete combustion of coal produces carbon dioxide (CO2) which causes a rise in atmospheric temperature. This is called as a global warming. It causes melting of polar ice, the rise in sea level and flooding of coastal areas.
  • SOX are sulfur dioxide (SO2), Sulfur trioxide (SO3)
  • NOX is nitrogen oxide (N2O), Nitric oxide (NO), Nitrogen dioxide (N2O). The formation of NOX during coal combustion is a complex process. Nitric oxide is formed by atmospheric nitrogen known as thermal NO and chemically bound nitrogen in fuel known as chemical NO
  • Oxides of sulfur (SOX) and oxides of nitrogen (NOX) are produced which gets dissolved in rainwater and forms acid rains harmful to crops, soil and damages buildings.
  • Considering all above harmful effect of coal combustion, coal usage optimization i.e. coal required per unit generation needs to be minimized to all possible extent by adopting all available technological developments. This will also result in a reduction in per unit cost of generation.
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