Syllabus FUEL

Syllabus FUEL L-1 Fuels Fuel is a combustible substance which during combustion gives large amount of heat There are chemical fuels, nuclear fuels and fossil fuels Classification of Fuels These can be classified on the basis of their occurrence and physical state On the basis of occurrence they are of two types: Primary Fuels: Fuels which occur in nature as such are called primary fuels E.g., wood, peat, coal, petroleum, and natural gas Secondary Fuels: The fuels which are derived from the primary fuels by further chemical processing are called secondary fuels E,g., coke, charcoal, kerosene, coal gas, producer gas etc (ii) On the basis of physical state these may be classified as: Solid Fuels Liquid Fuels Gaseous Fuels Calorific value: It is defined as the total quantity of heat liberated when a unit mass of a fuel is burnt completely Units of Calorific value: System Solid/Liquid Gaseous Fuels Fuels CGS MKS B.T.U Calories/gm k cal/kg BTU/lb Calories/cm3 k cal/m3 BTU/ft3 The quantity of heat can be measured in the following units: (i) Calorie: It is defined as the amount of heat required to raise the temperature of 1gm of water by 1oC calorie = 4.184 Joules (ii) Kilo Calorie: k cal = 1000 cal (iii) British thermal unit: (B T U.) It is defined as the amount of heat required to raise the temperature of pound of water through 1oF B.T.U = 252 Cal = 0.252 k cal (IV) Centigrade heat unit (C.H.U): It is defined as the amount of heat required to raise the temperature of pound of water through 1oC 1k cal = 3.968 B.T.U = 2.2 C.H.U Gross and net calorific Value Gross Calorific Value: It is the total amount of heat generated when a unit quantity of fuel is completely burnt in oxygen and the products of combustion are cooled down to the room temperature As the products of combustion are cooled down to room temperature, the steam gets condensed into water and latent heat is evolved Thus in the determination of gross calorific value, the latent heat also gets included in the measured heat Therefore, gross calorific value is also called the higher calorific value The calorific value which is determined by Bomb calorimeter gives the higher calorific value (HCV) Net Calorific Value: It is defined as the net heat produced when a unit quantity of fuel is completely burnt and the products of combustion are allowed to escape The water vapour not condense and escape with hot combustion gases Hence, lesser amount than gross calorific value is available It is also known as lower calorific value (LCV) LCV=HCV-Latent heat of water vapours formed Since part by weight of hydrogen gives nine parts by weight of water i.e H + 1O → H O 2 2 Therefore, LCV=HCV-weight of hydrogen x x latent heat of steam = HCV-weight of hydrogen x x 587 Determination of Calorific value Determination of calorific value of solid and non volatile liquid fuels: It is determined by bomb calorimeter Principle: A known amount of the fuel is burnt in excess of oxygen and heat liberated is transferred to a known amount of water The calorific value of the fuel is then determined by applying the principle of calorimetery i.e Heat gained = Heat lost Bomb Calorimeter Calculations Let weight of the fuel sample taken = x g Weight of water in the calorimeter = W g Water equivalent of the Calorimeter, stirrer, bomb, thermometer = w g Initial temperature of water = t1oC Final temperature of water = t2oC Higher or gross calorific value = C cal/g Heat gained by water = W x ∆t x specific heat of water = W (t2-t1) x cal The high percentage of ash is undesirable It reduces the calorific value of coal In furnace grate, the ash may restrict the passage of air and lower the rate of combustion High ash leads to large heat losses and leads to formation of ash lumps The composition of ash and fusion range also influences the efficiency of coal When coal is used in boiler, the fusion temperature of ash is very significant Ash having fusion temperature below 1200oC is called fusible ash and above 1430oC is called refractory ash Apart from loss of efficiency of coal, clinker formation also leads to loss of fuel because some coal particles also get embedded in the clinkers Fixed Carbon: Fixed carbon content increases from lignite to anthracite Higher the percentage of fixed carbon greater is its calorific value and better is the quality of coal The percentage of fixed carbon is given by: Percentage of fixed carbon = 100-[% of moisture+volatile matter+ash] Significance: Higher the percentage of fixed carbon, greater its calorific value • The percentage of fixed carbon helps in designing the furnace and shape of the fire-box because it is the fixed carbon that burns in the solid state Ultimate analysis: It is carried out to ascertain the composition of coal Ultimate analysis includes the estimation of carbon, hydrogen, sulphur, nitrogen and oxygen Carbon and Hydrogen: A known amount of coal is taken in a combustion tube and is burnt in excess of pure oxygen C + O → CO 2 H + 1O → H O 2 2 Fig Estimation of carbon and hydrogen 2KOH + CO → K CO + H O 2 CaCl + H O → CaCl H O 2 2 44 g of CO2 contain = 12 g of carbon Y g of CO2 contain = 12 × y 44 y ×100 Percentage of carbon = 12 × 44 weight of coal taken 18 g of water contain = g of hydrogen Z g of water contain = × zg of hydrogen 18 z ×100 Percentage of hydrogen = × 18 weight of coal taken Significance: Calorific value of a fuel is directly related to its carbon content A higher percentage of carbon reduces the size of the combustion chamber High percentage of hydrogen also increases the calorific value of coal The content of hydrogen in coals varies between 4.5 to 6.5 percent from peat to bituminous stage Nitrogen: Nitrogen present in coal sample can be estimated by Kjeldahl’s method Nitrogen + H SO Heat  →( NH ) SO 42 The contents are then transferred to a round bottomed flask and solution is heated with excess of NaOH The ammonia gas thus liberated is absorbed in a known volume of a standard solution of acid used Fig Estimation of nitrogen by Kjeldahl’s method The unused acid is then determined by titrating with NaOH From the volume of acid used by NH3 liberated, the percentage of nitrogen can be calculated ( NH ) SO 2NaOH   → Na SO + NH + H O 42 4 NH + H SO → ( NH ) SO 42 Carbonization of Coal (Manufacture of Coke) It is the process of heating the coal in absence of air to a sufficiently high temperature, so that the coal undergoes decomposition and yields a residue which is richer in carbon content than the original fuel Caking and coking of coals: some coals have a tendency to soften and swell at higher temperatures, to form a solid coherent mass with porous structure Such coals are called caking coals The residue formed is called coke If the coke is hard, porous and strong, than the coal, from which it is formed, it is called coking coal This property is found only in bituminous type of coal Coals with a high percentage of volatile matter are not fit for coking and are used for gas making The coals having 20-30 % volatile matter are good coking coals Process of carbonization: First moisture and occluded gases are driven off At about 260-270oC carbon, water, H2S, some low molecular alkenes and alkanes are evolved At about 350oC the decomposition of coal is accompanied by evolution of gases and elimination of tarry vapours takes place At about 400oC, caking coal becomes soft and plastic At about 700oC, hydrogen is evolved Above 800oC, main gaseous products are evolved Gases evolved from the plastic mass, expand it to give foam like appearance At further high temperatures this foam like mass solidifies to form a solid mass with porous structure called coke Types of carbonization (i) Low temperature carbonization (ii) High temperature carbonization (i) Low temperature carbonization: When the destructive distillation of coal is carried out at temperatures between 500-700oC It is practiced for the production of semi coke Which is also called soft coke The yield of coke is about 75-80 % The coke thus produced contains to 15 % volatile matter The various products of low temperature carbonization are semi coke, low temperature tar, crude low temperature spirit and gas LTC plants normally use low rank coals These low rank coals produce excessive smoke on burning Semi coke from LTC is highly reactive and can be easily ignited into a smokeless flame The gas which is obtained as a byproduct has higher calorific value of about 6500-9500 kcal/m3 (ii) High temperature carbonization: It is carried out at 900-1200oC HTC is used for the production of pure, hard, strong and porous metallurgical coke containing 1-3 % volatile matter The yield of the coke is 65-75% The byproducts-gas and tar have greater amounts of aromatic hydrocarbons The gas which is obtained has lower calorific value of about 5000-6000 kcal/m3 than that produced in LTC; but the yield of the gas is higher The coke obtained is very much harder than the coke obtained from LTC process and hence is called hard coke Metallurgical coke: The properties of coke depend on porosity, reactivity and the amount of volatile matter retained by coke during carbonization Coke is mainly used as a heat source and reducing agent in metallurgy A good coke in metallurgical process should possess the following characteristics: (i) Purity: The metallurgical coke should contain lower percentage of moisture, ash, sulphur and phosphorous (ii) Porosity: The coke should be porous so as to provide contact between carbon and oxygen (iii)Strength: The coke used in metallurgical process should have high strength so as to withstand the weight of the ore, flux etc in the furnace (iv) size: Metallurgical coke should be of medium size (v) Combustibility: Coke should burn easily The combustibility of coke depends on the nature of the coal, carbonization temperature and reaction temperature (vi) Calorific value: It should be high (vii) Reactivity: Reactivity of coke is its ability to react with CO2, steam, air and oxygen The reactivity should not be too high The reactivity toward CO2 represent the reduction of CO2 CO ( g ) + C (s) ⇔ 2CO( g ) [...]...Heat gained by Calorimeter = w (t2-t1) cal Heat liberated by the fuel = x C cal Heat liberated by the fuel = Heat gained by water and calorimeter x C = (W+w) (t2-t1) cal C=(W+W)(t2-t1) cal/g x Net Calorific value: Let percentage of hydrogen in the fuel = H Weight of water produced from 1 gm of the fuel = 9H/100 gm Heat liberated during condensation of steam = 0.09H × 587 cal Net (Lower... (vi) Velocity of combustion: It should be moderate (vii) Nature of the products (viii) Cost of fuel, (ix) Smoke, (x) Control of the process FUEL L-2 Solid Fuels: Primary as well as secondary are widely used in domestic and industrial purposes e.g., wood, coal, charcoal and coke Wood: Wood has been used as a fuel from ancient times Due to large scale deforestation, wood is no longer used except in forest... oxygen in the fuel Therefore, mass of hydrogen available for combustion = Total mass of hydrogen-1/8 mass of oxygen in fuel =H-O/8 Dulong’s formula for calculating the calorific value is given as: Gross calorific Value (HCV) 1 O = [8080C + 34,500( H − ) + 2,240 S ]kcal / kg 100 8 Net Calorific value (LCV) 9H = [ HCV − × 587]kcal / kg 100 = [ HCV − 0.09 H × 587]kcal / kg Characteristics of Good Fuel: (i)... should be added to the observed rise in temperature Therefore, Gross calorific value C=(W+w)(t2-t1+Cooling correction)-[Acid+ fuse corrections] / Mass of the fuel Theoretical calculation of Calorific value of a Fuel: The calorific value of a fuel can be calculated if the percentages of the constituent elements are known Substrate Calorific value Carbon 8080 Hydrogen 34500 Sulphur 2240 If oxygen is... − × 587]kcal / kg 100 = [ HCV − 0.09 H × 587]kcal / kg Characteristics of Good Fuel: (i) Suitability: The fuel selected should be most suitable for the process E.g., coke made out of bituminous coal is most suitable for blast furnace (ii) High Calorific value (iii) Ignition Temperature: A good fuel should have moderate ignition temperature (iv) Moisture content: Should be low (v) Non combustible matter... kcal/kg When wood burns, the ash content is low but the oxygen content is very high This makes even dry wood a fuel of low calorific value Wood charcoal is obtained by destructive distillation of wood The major use of wood charcoal is for producing activated carbon Coal: coal is regarded as a fossil fuel produced from the vegetable debris under conditions of high temperature and pressure over million of... volatile matter 100 × Weight of coal sample taken Significance  A high percent of volatile matter indicates that a large proportion of fuel is burnt as a gas  The high volatile content gives long flames, high smoke and relatively low heating values  For efficient use of fuel, the outgoing combustible gases has to be burnt by supplying secondary air  High volatile matter content is desirable in coal... of Mg wire is also included in the gross calorific value Hence this amount of heat has to be subtracted from the total value (b)Acid Correction: During combustion, sulphur and nitrogen present in the fuel are oxidized to their corresponding acids under high pressure and temperature S + O → SO 2 2 = -144,000 Cal 2SO + O + 2H O → 2∆H H SO 2 2 2 2 4 ∆H = -57,160 Cal 2 N + 5O + 2H O → 4HNO 2 2 2 3 The... very significant Ash having fusion temperature below 1200oC is called fusible ash and above 1430oC is called refractory ash Apart from loss of efficiency of coal, clinker formation also leads to loss of fuel because some coal particles also get embedded in the clinkers Fixed Carbon: Fixed carbon content increases from lignite to anthracite Higher the percentage of fixed carbon greater is its calorific... weight of coal taken 18 g of water contain = 2 g of hydrogen Z g of water contain = 2 × zg of hydrogen 18 z ×100 Percentage of hydrogen = 2 × 18 weight of coal taken Significance: Calorific value of a fuel is directly related to its carbon content A higher percentage of carbon reduces the size of the combustion chamber ...Fuels Fuel is a combustible substance which during combustion gives large amount of heat There are chemical fuels, nuclear fuels and fossil fuels Classification of Fuels These can be classified... calorific value C=(W+w)(t2-t1+Cooling correction)-[Acid+ fuse corrections] / Mass of the fuel Theoretical calculation of Calorific value of a Fuel: The calorific value of a fuel can be calculated... below 12 00oC is called fusible ash and above 14 30oC is called refractory ash Apart from loss of efficiency of coal, clinker formation also leads to loss of fuel because some coal particles also