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CHAPTER 2, , , , FUELS AND COMBUSTION, , , , “Probably foronlyalimited ° : c offs fa ee ‘ed mumberof years tocome, the largest portion of the world’s power will come from the combustion, , , , , , 1 INTRODUCTION, , Fuel isa 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. Wood, charcoal, coal,, kerosene, petrol, diesel, producer 8s, oil gas, etc. are some of the fuels., , During the process of combustion of a fuel (like coal), the, with the sinnultaneous liberation of heat at a rapid rate. ., elections” in these atoms, resulting in the, , compounds have less energy (or heat conte, , atoms of carbon, hydrogen, etc. combine with oxygen, This energy is liberated due to the “rearrangement of valency formation of new compounds (like CO), H20, ete.). These new, nt) in them and, therefore, the energy (or heat) released during the, , eo si process is the difference in the energy of the reactants (C, H, and O, etc. of fuel) and that of the products, formed. :, , FUEL + O, —— PRODUCTS + HEAT, , More heat energy content Lesser heat energy content, , The primary or main source of fuels are coals and petroleum oils, the amounts of which are divindling day-by-day., These are stored fuels available in earth’s crust and are, generally, called ‘fossil fuels’. . :, , 2 CLASSIFICATION OF FUELS, , The fossil fuels have been classified according to their : (1) occurrence (and preparation), and (2) te state of, aggregation. According to the first classification, we have : (a) natural or primary fuels, which are found in nature, as such, ¢.g., wood, peat, coal, petroleum, natural gas, etc. ; ()) artificial or secondary fuels are those which are, prepared form the primary fuels. For example, charcoal, coke, kerosene oil, diesel oil, petrol, coal gas, oil gas,, producer gas, blast furnace gas, etc., , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , CHIMICAL FUELS, PRIMARY OR SECONDARY OR, NATURAL ~ DERIVED, State of “| State of, aggregation : aggregation, ee ed a ey, Solid Liquid Gaseous Solid Liquid Gaseou:, Wood ~ Crudeoil * Natural gas Coke Tar Coal gas, Peat Gi Charcoal Kerosene Water gas, Lignite . Petroleum Diesel Oil eee, Coal coke Petrol Biogas ., Dung : Coal Fuel oil Blast furnace, = briquette Synthetic gas, “ gasolien Coke oven gas, S LP.G., (55), , Scanned with CamScanner

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56, ENGINEERING CHEMistpy, , : (a) solid fuels ; (b) liquid fuels, , , , aniitey seer ar cation is based upon their state of aggregation like, , Ss ue s. The classification of fuels is summarized as on Page +, , of 3:2 Cmwomunn fuel is a suspension finely powdered coal in fuel oil (or any ollwr tin fuel generally it the ratio, , theseparatin ye ually, some stabilizing agent is added during the preparation of col told al fuel in order to ayig, , ; paration of coal from the liquid fuel. In general. Such fuels possess higher calorific values and it is easy is, , ioe them: compared with powdered coal and these such fuels find applications in industrial furnaces for cones, nutacture, brick firing, ceramic firing, metal processing, ele., , 3 CALORIFIC VALUE, , Calorific value of a fuel is “the total quantity of heat lib, the fuel is burnt completely.” ,, , Units of heat : (1) ‘Calorie’ is the amount of heat required to raise Hie, one degree Centigrade (15-16°C),, ae (2) “Kilocalorie” (or “Kilogram Centigrade units”) is equal to 1,0, systent and may be defined as ‘the quantity of hat required to raise the tenipera, one degree Centigrade. Thus :, , , , , , , , , , , , , , , , erated, when a wit mass (or volunie) of, , temperature of one grant of water throw), R, , 100 calories. This is the unit of metric, ture of one kilogram of water throyef,, , = 1 keal = 1,000 cal, (3) “British Thermal unit” (B.Th.U.) is defined as, of one pound of water through one degree Fahrenheit (60-61°F). This is the, 1 B.Th.U. = 252 cal = 0.252 keal, , 1 kcal = 3.968 B.Th.U., (C. H. U.) is “the quantity of heat required to raise the temperature of 1 pound of, , “the quantity of heat required to raise the temperature, English system unit., , or, (4) “Centigrade heat unit, water through one degree Centigrade.” Thus :, , 1 keal = 3.968 B.Th.U. = 2.2 C.H.U., all fuels contain some Hydrogen and when the calorific value, , Higher or gross calorific value : Usually,, of hydrogen-containing fuel is determined experimentally, the hydrogen is converted into steam. If the, 5°C or 60°F), the latent heat of condensation, , products of combustion are condensed to the room temperature (15, of steam also gets included in the measured heat, which is then called “Higher or gross calorific value”. So, gross, , or higher calorific value (HCV) is “the total ammount of heat produced, when unit mass / volume of the fuel has been, , burnt completely and the products of combustion have been cooled to room femperature’” {i.e.,15°C or 60°F )., In actual use 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. So,, net or lower calorific value (LCV) is “He net heat produced, when unit mass volume of the fuel is burnt completely, and the products are permitted to escape”. Alternatively, net or lover calorific value (LCV), , = HCV - Latent heat of water vapour formed, , = HCV — Mass of hydrogen x 9 x Latent heat of steam, because 1 part by mass of hydrogen produces 9 parts by mass of water. The latent heat of steam is 587 keal/kg or, 1,060 B.Th.U./Ib of water vapour formed at room temperature (/.c., 15°C)., Units of calorific value : The calorific value is, generally, expressed in caloric, (kcal/kg) or British thermal wnitflb (B.Th.U/tb) in case of solid or liquid fuel. In case of gaseous, kilocalorielcubic metre (keal/ an?) or BoTh.U cubic feet ( B.Th.UE/, /ft*),, , Lower or net calorific value :, , , , /gram (callg) or kilocalarielkg fuels,, the units used are, , 1 kcal/kg = 1.8 x B.Th.U./Ib, , 1 keal/m* = 0.1077 x B.Th.U./f° wl), 1. B.Th.U./ft* = 9.3 kcal/m*, 4 CHARACTERISTICS OF A GOOD FUEL, ted, , lorific value : A fuel should possess high calorific value, since Hie amount of Ireat libera, , (1) High ca’, by depends upon the calorific value of fuel., , and temperature attained there, , Scanned with CamScanner

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FUELS AND COMBUSTION 7, (2) Moderate ignition t, , he pre-lteated so that it starts hie “Ignition temperature is he lowest temperature to which Teapot, , dec lewiter de ing smoothly, Low ignition temperature is dangerous for storage ie, , (or igniting) the fuel, but the fat ss oe the other hand, high ignition temperature causes difficully in kindling, , 3 o tls safe during stor; rdli nee “, , “moderate” ignition temperature. safe during storage, handling and transport. Hence, an ideal fuel should have, (3) Low moisture content : The, , moisture "1 » fuel reduces the heating : involves i, gloss of money, because it is aid tie ture content of the fuel redices the heating value and involves in, , at the same rate as the fuel. I lence, fucl should have low moisture coufent., , (4) Low non-combusti, , ibl 7 i i, senerally, in the form of ash ee content : After combustion, the non-combustible matter remains,, s 'Yy sit or clinker. The non-combustible matter also reducvs the heating, value, besides, , dditional cost of storage, ha ane ‘, wccble amatterin‘tiel oe handling and disposal of the waste products produced. Each per cent of non-comialtéh ans a heat loss of about 1.5%. Heice, a fuel should have low content of non-combustible, matter., , 5) Moderat i : %, (5) velocity of combustion : If the rate of combustion is low, then the required high, , emperature may 7 2 possible, becaus i, emper eo ee be possible, because a part of the heat liberated may get radiated, instead of raising the, temperature, On the other hand, too high combustion rates are also not requi, , p amid of combustion should not be harmful : Fuel, on burning, should nof give out objectionable Ea harmful gases. In other words, the gaseous products of a combustion should not pollute the, atmosphere. CO ,SO2,H2S, PH, etc., are some of the harmful gases., , , , (7) Low cost : A good fuel should be readily available in bulk at a cheap rate., , (8) Easy to transport : Fuel must be easy to handle, store and transport at a low cost. Solid and liquid, fuels can easily be transported from one place to another. On the other hand, transportation of gaseous fuels, is costly and can even cause fire hazards., , (9) Combustion should be easily controllable, i.c., combustion of the fuel should be easy to start or stop,, when required., , (10) Should no undergo spontaneous combustion : Spontaneous ignition can cause fire hazards., , (11) Storage cost in bulk should be low., , (12) Should burn in air with efficiency, without much smoke., , (13) In case of solid fuel, the size should be uniform so that combustion is regular., , 5 COMPARISON BETWEEN SOLID, LIQUID AND GASEOUS FUELS, , (A) Solid Fuels : Advantages : (1) They are easy to transport.(2) They are convenient to store, without, any risk of spontaneous explosion. (3) Their cost of production is low. (4) They possess moderate ignition temperatures., , Disadvantages : (1) Their ash content is high. (2) Their large proportion of heat is wasted during, combustion. In other words, their ermal efficiency is low. (3) They burn with clinker formation. (4) They, combustion operations caniot be controlled easily. (5) Their cost of handling is high. (6) Their calorific value, is lower as compared to that of liquid fuels. (7) They require large excess of air for complete combustion. (8) They, cannot be used as internal combustion engine fuels., , (B) Liquid Fuels : Advantages : (1) They possess higher calorific value per unit mass than solid fuels., (2) They burn without forming dust, ash, clinkers, etc. (3) Their firing is easier and also fire can be extinguished, easily by stopping the liquid fuel supply. (4) They are easy to transport through pipes. (5) They can be stored, indefinitely, without any loss. (6) The flame produced by burning liquid fuels can easily be controlled by, adjusting the liquid fuel supply. (7) Liquid fuels are, generally, handled by pipes and one man can easily, regulate a large number of furnaces simultaneously. (8) They are clean in use and economic in labour. (9) Loss, of heat to chimney is very low, due to greater cleanliness. (10) They require less excess of air for complete, » combustion. (11) They require less furnace space for combustion. (12) There is no wear and tear of grate bars, and cleaning of fires, etc., unlike solid fuels. (13) They can be used as internal combustion fuels., , x, , Scanned with CamScanner

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58, ENGINEERING CHEMISTRY, , — Disadvanta:, special storage tanks, case of a highly infl, fue, , ges : (1) The cost of liquid fuel is relatively much higher as compared, a oN fuels) Soslly, are required for storing liquid fuels. (3) There is a greater ee offre ne Bently ., sean ammable and volatile liquid fuel. (4) They give bad odour. @) ee fspra om (dur * te, fuel ¢ pecially constructed burners and spraying apparatus are required. (6) acai has, ore is a drawback of oil firing. h pipelines to the actual pl, aseous : é syed easily through pipelines 2 actual place, ei had, tharghsy Siecle dane tise ei cepcaalon @) Shey can be lighted at moment's notice,, (3) They have high heat content and hence, help us if having higher temperatures. (4) They can be pre-heated by, the heat of hot waste gases, thereby affordin, a Sona in heat. (5) Their combustion can readily be controlled, for changes in demand like oxidizing or reducing atmosphere, length ofa toh hanaling ra "0) They, without any soot (or smoke) and are ashless, so there is no Jabour involed in ash handling, “ Aaene va, clean in use. (8) They do not require any special burners. (9) They can be produce! ay teats artis, quantity of coal, etc. (10) They burn without heat loss, due to convection es do not affect thie mG, excess of air supply. (12) They are free from solid and liquid impurities. Hence, ‘toot allution is i Se, of metal produced, when used as a metallurgical fuel. (13) Complete COT Th year ane be used - iit a, due to uniform mixing of air and fuel. (14) They have high calorific values. anil a, combustion engine fuels., , , , for them. (2) They are highly inflammable, so, , Disadvantages : (1) Very large storage tanks are needed pared to solid and liquid fuels., , chances of fire hazards are high in their use. (3) They are more costly a5 Com, 6 BOMB CALORIMETER, This apparatus is used to find the calorific val, Construction: A simple sketch of bomb ca!, , cylindrical stainless steel bomb in which the combustion of, has a lid, which can be screwed to the body of bomb so as wie 2 Petro one of the electrodes,, is provided with two stainless steel electrodes and an oxygen Wt ile can bE supported. The bomb is, , ~ small ring is attached. In this ring, a nickel or stainless steal Lew Set eoater jacket 10 rreetillvat, placed in a copper calorimeter, which is surrounded by an on slectrically operated stirrer fad, losses due to radiation. The calorimeter is provided with an, , Beck ‘s the ete. ‘oO 1 of a degree., pt 1/100th fa deg:, ecknanit hernionieter, which can read accural, , Jue of solid and liquid fuels. as, , i is shown in Fig. 1. It consists of a strong, ret F pul is made to take place. The bomb, ke a perfect gas-tight seal. The lid, , tely temperature difference U), 7, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , 6V, Beckmann's battery,, thermometer Oxygen Electrically, | + ae operated stirrer, H, Electrodes _, to which a ring Copper, is attached | -—~“ calorimeter, Stainless steel Fig. 1. Bomb calorimeter., ‘ —— bomb, , Mg fuse wire, weighed pallet Stainless steel, of given fuel crucible, sample 3, , , , , , , , , , , , , , , , Scanned with CamScanner