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Note for Applied Chemistry - CHEM by krishna khadka

  • Applied Chemistry - CHEM
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Krishna Khadka
Krishna Khadka
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UNIT III FUELS AND COMBUSTION Calorific value – classification – coal – proximate and ultimate analysis – metallurgical coke – manufacture by Otto-Hoffmann method – petroleum processing and fractions – cracking – catalytic crang and methods – knocking – octane number and cetane number – synthetic petrol – Fischer Tropsch and Bergium processes – gaseous fuels – water gas, producer gas, CNG and LPG – flue gas analysis – Orsat apparatus – theoretical air for combustion. INTRODUCTION: Fuel is a combustible substance, containing carbon as main constituent, which on proper burning gives large amount of heat, which can be used economically for domestic and industrial purposes. Eg., Wood, Charcoal, Coal, Kerosene, Petrol, Producer gas, Oil gas, LPG etc., During the process of combustion of a fuel (like coal), the atoms of carbon, hydrogen, etc. combine with oxygen with the simultaneous liberation of heat at a rapid rate. Fuel + Oxygen ---> Products + Heat CLASSIFICATION OF FUELS: Chemical Fuels: It is of two types viz., Primary or Natural Fuel and Secondary or Derived Fuel. Primary Fuel or Natural Fuel: Solid Liquid Wood Peat Lignite Coal Dung Gaseous Crude oil Natural gas Secondary or Derived Fuel: Solid Liquid Gaseous Coke Charcoal Tar Kerosene Coal Gas Water gas 1

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Petroleum Coke Coal Briquette Diesel Petrol Fuel oil Synthetic Gasoline gas Oil gas Bio gas Blast furnace gas Coke over gas CALORIFIC VALUE Calorific value of a fuel is “the total quantity of heat liberated, when a unit mass (or volume) of the fuel is burnt completely” Units of Heat: (1) Calorie- is the amount of heat required to raise the temperature of one gram of water through one degree centigrade (15-16° C). (2) Kilocalorie – is equal to 1,000 calories. This is the unit of metric system and may be defined as “the quantity of heat required to raise the temperature of one kilogram of water through one degree centigrade. Thus, 1 kcal = 1,000 calories. (3) British Thermal Unit (BTU)- is defined as “the quantity of heat required to raise the temperature of one pound of water through one degree Fahrenheit (6061° F). This is the English system unit. 1 BTU = 252 cal = 0.252 kcal and 1 kcal = 3.968 BTU (4) Centigrade heat unit (CHU)-is “the quantity of heat required to raise the temperature of 1 pound of water through one degree centigrade”. Thus, 1 kcal = 3.968 BTU = 2.2 CHU HIGHER OR GROSS CALORIFIC VALUE: It is the total amount of heat produced, when unit mass/volume of the fuel has been burnt completely and the products of combustion have been cooled to room temperature (15° C or 60° F). It is explained that all fuels contain some hydrogen and when the calorific value of hydrogen containing fuel is determined experimentally, the hydrogen is converted into steam. If the products of combustion are condensed to the room temperature, the latent heat of condensation of steam also gets included in the measured heat which is then called GCV. LOWER OR NET CALORIFIC VALUE: It is the net heat produced, when unit mass/volume of the fuel is burnt completely and the products are permitted to escape. In actual practice of any fuel, the water vapour and moisture, etc., are not condensed and escape as such along with hot combustion gases. Hence, a lesser amount of heat is available. 2

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DETERMINATION CALORIMETER OF CALORIFIC VALUE USING BOMB The calorific value of solid or liquid fuels can be determined with the help of bomb calorimeter. Description: Bomb Calorimeter consists of a strong stainless steel bomb where the fuel sample is burnt. The bomb has oxygen inlet valve and two stainless steel electrodes. A small ring is attached to one of the electrodes. In this ring, a nickel or stainless steel crucible is placed. The bomb is placed in a copper calorimeter containing a known weight of water sample. The copper calorimeter is provided with a Beckmann’s thermometer and stirrer for stirring water. The copper calorimeter is covered by an air jacket and water jacket. Functioning: A known weight of the fuel sample is taken into the crucible. The fine magnesium wire is touching the fuel sample and then stretched across the electrodes. The bomb lid is tightly closed with the help of screw. The bomb is filled with oxygen at 25 atmospheric pressure. The bomb is now placed in a copper calorimeter which containing known weight of water. Initial temperature of the water in the calorimeter is noted (t1°C) after stirring. The electrodes are connected to a battery (6 v). The current is now supplied to the fuel sample which undergoes burning with the evolution of heat. The liberated heat increases the temperature of water in the calorimeter. The maximum temperature of the water during experiment is finally noted (t2°C). From the temperature difference, calorific value of the fuel can be calculated as follows: Calculation: Weight of the fuel sample taken in the crucible Weight of water taken in the calorimeter Weight of calorimeter and stirrer in terms of water Equivalent Initial temperature of water in the calorimeter Final temperature of water in the calorimeter Heat absorbed by the water Heat absorbed the calorimeter Total heat absorbed by the water 3 = = x g W g = = = = = = = A g t1°C t2°C W (t2-t1) cal ----(1) A (t2-t1) cal ----(2) W(t2-t1) + A(t2-t1) cal (W+A) (t2-t1) cal ----(3)

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The relationship between heat liberated by the fuel and HCV is as follows: Heat liberated by the fuel = x x (HCV) ----(4) Therefore, heat liberated by the fuel = Heat absorbed by the water and calorimeter X Weight of fuel Compare equation (3) and (4), we get x x (HCV) = (W+A) (t2-t1) HCV = (W+A) (t2-t1) cal/g X Calculation of Lower Calorific Value (LCV): The percentage of hydrogen in the fuel Weight of water produced 1 g of the fuel = = H 9 H g = 0.09 g 100 Therefore, heat liberated during the Condensation of steam = 0.09 H x 587 cal/g Lower calorific value of the fuel = HCV – Latent heat of water liberated by the fuel LCV = HCV – (0.09 H x 587) cal/g. CHARACTERISTICS OF A GOOD FUEL: High calorific value Moderate ignition temperature Low moisture content Low non-combustible matter content Moderate velocity of combustion Products of combustion should not be harmful Low cost Easy to transport Combustion should be easily controllable Should not undergo spontaneous combustion Storage cost in bulk should be low Should burn in air with efficiency without much smoke In case of solid fuel, the size should be uniform so that combustion is regular. 4

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