Fuels

Fuels and Combustion : Fuels and Combustion By V.S.Saravanamani Assistant Professor in Chemistry Annapoorana Engineering College, Salem

Slide 2 : Introduction Most of the industries depend largely upon the power generated by the combustion of fuels. Any source of heat energy is called as fuel. Fuel is a substance that combines with oxygen and produce large amount of heat.

Slide 3 : Definition Fuel may be defined as any substance which undergoes combustion with oxygen to supply heat energy without producing any objectionable gases.

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Slide 5 : Characteristics of a good fuel A good fuel should have the following characteristics It should have high calorific value. It should have moderate ignition temperature. It should contain low moisture content. It should not give any harmful gases during combustion. It should have low% of non-combustible matter. It should be cheap and readily available. It should not undergo spontaneous combustion. Transportation should be easy.

Slide 6 : Calorific value It may be defined as the amount of heat liberated when unit mass of fuel undergoes complete combustion.

Slide 7 : Solid Fuels Coal Coal is formed by the decay of vegetable and woody materials under high pressure and heat. The process of conversion of woody material to coal is known as coalification or metamorphism.

Slide 8 : Classification of coal by rank Various types of coal can be recognized on the basis of rank from the parent material. Various types of coals are;

Slide 9 : The progressive transformation of coal by rank results in Decrease in the moisture content Decrease in H2,O2 ,N2 and S content Decrease in volatile matter content Increase in 'C' content Increase in calorific value Increase in hardness

Slide 10 : Analysis of coal In order to confirm the quality or rank of the coal the following two methods are carried out. They are, 1) Proximate analysis. 2) Ultimate analysis.

Slide 11 : Proximate analysis This method is used for the determination of i) Moisture content ii) Volatile matter iii) Ash content iv) Fixed carbon

Slide 12 : Determination of moisture A known weight of coal sample is taken in silica crucible and it is heated in an electric hot air oven at 100-110oC for 1 hour. After 1 hour, the crucible is cooled and weighed. The process of heating, cooling and weighing is repeated until the weight of the crucible becomes constant. Loss in weight of coal due to moisture content is calculated as follows.   % of moisture = Loss in weight x 100 Weight of coal taken

Slide 13 : Determination of volatile matter The dried sample of coal from the first experiment is covered with a lid and heated is a muffle furnace at 950oc for 7 minutes. Then, the crucible is cooled and weighed. The process of heating, cooling and weighing is repeated until the weight of the crucible becomes constant. Loss in weight of coal due to volatile matter is calculated as follows. % of volatile matter = Loss in weight x 100 Weight of dried coal

Slide 14 : Determination of ash After the removal of volatile matter, the coal left in the crucible is heated in the muffle furnace without lid at 700-750oC for 30 minutes. Then, the crucible is cooled and weighed. The process of heating, cooling and weighing is repeated until the weight of the crucible becomes constant. The residual weight of coal due to formation of ash can be calculated as follows   % of ash = Weight of ash formed x 100 Weight of dried coal after removed of volatile matter

Slide 15 : Fixed carbon It is determined by deducing the moisture, volatile and ash contents from 100. % of fixed carbon = [100-( % of moisture + % of volatile matter + % of ash)]

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Slide 17 : 2) Ultimate analysis This method is used for the determination of i) Carbon and hydrogen ii) Nitrogen iii)Sulphur iv) Oxygen

Slide 18 : i) Determination of carbon and hydrogen A known weight of coal sample is heated in the presence of cupric oxide in combustion apparatus. The carbon and hydrogen in the coal sample are oxidised to CO2 and H2O respectively. C + O2 CO2 (12) (44) H2 + ½ O2 H2O (2) (18)

Slide 19 : The liberated CO2 and H2O vapour are passed through the tubes containing known weight of KOH solution and anhydrous CaCl2 respectively. When the reaction is over, the tubes are disconnected and weighed. The increase in weight of KOH tube represent the weight of CO2 while increase in weight of CaCl2 tube represents the weight of H2O respectively.

Slide 20 : Knowing the weights of CO2 and H2O, the % of C and H can be calculated as follows:  % of C = 12 x Weight of CO2 formed x 100 44 Weight of coal sample % of H2 = 2 x Weight of H2O formed x 100 18 Weight of coal sample

Slide 21 : Determination of nitrogen Nitrogen can be determined by Kjeldal's method. In this method, a known weight of coal sample is heated with con. H2SO4 is the presence of K2SO4and CuSO4 .Nitrogen present in the coal sample is converted into ammonium sulphate and a clear solution is obtained. The % of N2 present is the coal sample can be determined by volumetric analysis. 2N +3H2 + H2SO4 (NH4)2 SO4

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Slide 24 : The solution is then treated with excess NaOH, the ammonia gas is liberated. (NH4)2 SO4 +2NaOH 2NH3 + Na2SO4 +2H2O The liberated ammonia is distilled over and absorbed by known volume of 0.1N H2SO4.

Slide 25 : The volume of unused 0.1N H2SO4 is determined by titrating it against 0.1N NaOH . Thus, the amount of acid neutralized by liberated ammonia is determined. % of N = (V1-V2 ) ml x Normality x 14 x 100   Weight of coal sample x 1000   % of N = Volume of acid (ml) x Normality x 1.4 Weight of coal sample Where, V1 = Initial volume of 0.1 N H2SO4 V2 = Final Volume of 0.1 N H2SO4 (V1-V2) = Acid neutralized by ammonia.

Slide 26 : Determination of sulphur A known amount coal sample is burnt completely in bomb calorimeter. During burning, sulphur present in the coal is converted as sulphates (SO42-). It is extracted by dilute hydrochloric acid. Then it is treated with barium chloride solution. BaSO4 precipitate is obtained. It is filtered, dried and weighed. From the weight of BaSO4 , the % of sulphur can be calculated.

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Slide 28 : S + 202 SO42- + BaSO4 32 233 233g of BaSO4 contains 32g of sulphur X g of BaSO4 contains =(32 /233) x X g of sulphur W g of coal sample contains = (32 /233) x (X/W) g of sulphur % of sulphur = (32/233) x (X/W) x 100

Slide 29 : (or) % of sulphur = 32 x weight of BaSO4 formed x 100 233 x weight of coal Where, X = weight of BaSO4 formed W = Weight of coal sample taken

Slide 30 : Determination of oxygen It is determined by deducting the C & H, N and S from 100   % of oxygen = [ 100- (% of C & H + % of N + % of S) ]

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Slide 32 : Carbonisation The process of conversion of coal in to coke by strong heating in the absence of air is known as carbonization. This is also called as destructive distillation.

Slide 33 : Caking and coking coals The product obtained from the carbonisation is soft, plastic and coherent mass is called caking coal. On the other hand if the product is hard and strong then the coke is called coking coal.

Slide 34 : Metallurgical coke A coke which is used in metallurgical purposes is called as metallurgical coke. They are hard, strong, porous and coherent.

Slide 35 : Requisites or Characteristics of metallurgical coke It should have minimum percentage of moisture, ash, sulphur and phosphorous. It should have pores in nature, so that the oxygen can easily contact the carbon which ensures the complete combustion It should have high mechanical strength to with stand high pressure.

Slide 36 : It should have low rate of combustion It must be uniform and medium in size. It should have high calorific value It must be cheap and readily available It should burn easily It should have very low reactivity

Slide 37 : Manufacture of metallurgical coke - Otto Hoffmann by product oven Otto Hoffmann oven consists of a number of series of narrow silica chambers. Each chamber is provided with a charging hole at the top. The oven is provided with iron door at each end for discharging the coke.

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Slide 40 : Coal is charged into chambers and the doors are closed. The chambers are heated by a mixture of preheated air and producer gas at 1200oC. The coal undergoes combustion and liberate waste gases. The heat of the waste gases are used for heat the regeneration of oven at about 1000oC before leaving the chamber.

Slide 41 : Therefore, heating of the oven is continued till the liberation of volatile gases is completely. After 24 hours, the coke is removed from the oven and quenched with water. The yield of coke is about 75%. The valuable by products like coal gas, tar, ammonia,H2S benzol etc., are recovered.

Slide 42 : Advantages 1.The carbonization time is less 2.Heating can be done externally by producer gas. 3.Variable by products can be recovered.

Liquid Fuels : Liquid Fuels Types of Petrol Depending upon the type and nature of hydrocarbons (paraffin) present in the crude oil, petrol can be classified into the following three types. 1. Straight run petrol 2. Cracked Petrol 3. Polymer petrol

Slide 44 : Straight run petrol The crude oil on fractional distillation yields only about 15 - 20% gasoline. This is known as straight run gasoline. The quality of straight run gasoline is not so good. It contains mainly straight chain paraffin, which ignite readily and more rapidly than any other hydrocarbons and hence it produces knocking (unwanted sound) in IC engines.

Slide 45 : Cracked petrol. Cracking is defined as “the decomposition of high boiling hydrocarbons of high molecular weight into simpler, low boiling hydrocarbons of low molecular weight.”   C10H22 C5H12 + C5H10 Decane n-Pentane Pentene B.Pt : 174oC B.Pt : 36oC   Thus the petrol obtained by cracking is known as cracked petrol.

Slide 46 : Types of Cracking There are two kinds of cracking 1. Thermal cracking 2. Catalytic cracking

Slide 47 : 1. Thermal Cracking If cracking is carried out at higher temperature and pressure without any catalyst, it is called Thermal Cracking. The petrol so obtained is called cracked petrol. There are two types of thermal cracking. (i) Liquid Phase Thermal Cracking (ii) Vapour phase thermal cracking

Slide 48 : Liquid Phase Thermal Cracking In this method, the heavy oil is cracked at a temperature of 475 - 530oC under high pressure of 100 kg/cm2 to keep the reaction product in liquid state. The cracked products are then separated into various fractions in a fractionating column. The yield of gasoline is about 50-60% and the octane number is 65-70.

Slide 49 : Vapour phase thermal cracking In this method, the heavy oil is first vapourised and then cracked at a temperature of 600 - 650oC under a lower pressure of 10 - 20 kg/cm2. The yield of gasoline is about, 70%. This process is suitable only for those oils which are readily vapourised.

Slide 50 : Catatytic Cracking When cracking is carried out at lower temperature and pressure in the presence of suitable catalyst, it is called Catalytic Cracking. The catalyst used is aluminium silicate or alumina.

Slide 51 : Polymer petrol The gaseous by-products, obtained during cracking, contain olefins (like ethylene, propene and butene) and alkanes (like methane, ethane, propane). These gases undergo polymerization at high temperature and pressure with or without catalyst to give petrol rich in branched alkanes. Thus the gasoline obtained by polymerisation is called polymer petrol.

Slide 52 : Types of polymerisation There are two types of polymerisation.   (a) Thermal polymerisation It is carried out at 500 - 600oC and 70 - 350 kg / cm2 pressure. The products are gasoline and gas oil mixture, from which gasoline is separated by fractional distillation. (b) Catalytic polymerisation. It is carried out at lower temperature of 150 - 200oC in presence of catalyst like H3PO4 .

Slide 53 : Synthetic petrol The synthetic petrol can be obtained by hydrogenation of coal. It is nothing but heating of coal with hydrogen at high temperature and pressure, gasoline or petrol is obtained. The following two methods are available for the hydrogenation of coal. 1.Bergius process (or) direct hydrogenation. 2.Fischer- Tropsch process (or) indirect hydrogenation.

Slide 54 : Bergius process (or) Direct hydrogenation of coal In this process, the finely powdered coal is made in to a paste with heavy oil and nickel oleate catalyst. The coal paste and hydrogen gas mixture is pumped into the converter. The mixture is heated to 400-5000C under 200-250 atmosphere. During this process, the unsaturated coal becomes saturated hydrocarbons.   Coal dust + H2 Catalyst Saturated hydrocarbon 200 - 250 atm 400-4500C

Slide 55 : The saturated hydrocarbon further undergoes decomposition to yield a mixture of lower hydrocarbons. Saturated hydrocarbon Decomposition Crude oil (or) lower hydrocarbons

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Slide 58 : The mixture is passed through a condenser, where the crude oil is obtained. The crude oil is then fractioned to get Gasoline Middle oil and Heavy oil.   The middle oil is further hydrogenated in the presence of catalyst to give more gasoline. The heavy oil is again used for making a paste with fresh coal dust.

Slide 59 : Fisher Tropsch Process or Indirect hydrogenation of coal In this process, coal is first converted to coke. Then, coke is heated and steam is passed over it. Water gas is produced.   C + H2O 1200 C CO + H2 The water gas is mixed with hydrogen and the mixture is passed through chamber containing Fe2O3 to remove H2S. Then it is passed through a chamber containing mixture of Fe2O3 + Na2O3 to remove organic sulphur compounds.

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Slide 62 : The purified gas is compressed to 5 to 25 atmospheres over a cobalt, thorium and MgO catalyst on kieselghur at 200oC. A mixture of straight chain paraffins and olefins are produced. nCO + 2nH2 CnH2n + nH2O olefin nCO + (2n+1)H2 CnH2n+2 + nH2O parafin

Slide 63 : The mixture of olefins and straight chains are passed through a condenser, where the crude oil is obtained. The crude oil is then sent through fractionating column to yields, i) Gasoline and ii) Heavy oil. The heavy oil is used for cracking to get more gasoline.

Slide 64 : Knocking Definition Knocking is defined as a sharp metallic sound produced in the internal combustion engine by immature ignition of air and gasoline mixture.

Slide 65 : Causes of knocking In an internal combustion engine (Petrol engine), a mixture of gasoline vapour and air is used as fuel. The mixture is ignited by an electric spark. But in some cases, the rate of combustion will not be uniform due to unwanted impurities present in gasoline. Therefore, the rate of ignition increases gradually and the final portion of the air fuel mixture gets ignited instantaneously, producing an explosive sound called as knocking. Knocking reduces the efficiency of the engine.

Slide 66 : Chemical structure and knocking The tendency of knocking of a fuel depends upon the molecular structure, design of engine, fuel-air ratio etc., The knocking tendency decreases as follows.   Straight chain alkanes > Monosubstituted alkanes > Cyclo alkanes > alkanes > aromatics.

Slide 67 : Octane number or octane rating The knocking tendency of gasoline is usually expressed in terms of octane number. Isooctane has better resistance to knocking and its antiknock value has been given as 100. On the other hand n-heptane has very poor resistance to knocking and its antiknock value has been given as zero.

Slide 68 : Definition Octane number is defined as the percentage by volume of isooctane present in a mixture of isooctane and n-heptane. Isooctane (octane No = 100) CH3-CH2–CH2-CH2-CH2-CH2-CH3 n- heptanes (Octane Number = 0)

Slide 69 : Anti-knocking agents/leaded petrol Knocking can be minimized or prevented by addition of suitable additives such as tetraethyl lead (CH2)4Pb to petrol. This petrol is called leaded petrol and the process is called as sweetening of petrol.

Slide 70 : Mechanism of prevention of knocking Proper combustion causes knocking due to formation of free radical mechanism. When addition of TEL to petrol, it under goes thermal decomposition to form ethyl free radical. It combines with the free radicals in knocking process and thus the chain growth is stopped.

Slide 71 : Disadvantages of TEL During thermal decomposition of TEL, certain amount of lead oxide and metallic lead may deposit on the plug and cylinder wall. In order to avoid this, ethylene dibromide is added to dilute the lead pollution. CH – Br2 Pb + CH2 = CH2 + PbBr2 CH – Br2

Slide 72 : Diesel oil It is obtained from fractional distillation of petroleum between 250 - 320oC.It is a mixture of C15H32 to C18H38 hydrocarbons. Its calorific value is about 11000 kcal/kg. It is used as a very good diesel engine fuel.

Slide 73 : Ignition lag or Ignition delay The combustion of a fuel in a diesel engine is not instantaneous and the time between injection of the fuel and its ignition is called as ignition lag or ignition delay.

Slide 74 : Diesel index The quality of a diesel oil is indicated by diesel index number using the following formula. Diesel index number Specific gravity (API) x Aniline point in Fo 100 Aniline point and specific gravity are noted from API. (American Petroleum Institute) scale.

Slide 75 : Cetane number (or) Cetane rating The knocking tendency of diesel is usually expressed in terms of cetane number. Cetane has better resistance to knocking and its antiknock value has been given as 100. On the other hand 2-methyl naphthalene has poor resistance to knocking and its antiknock value has been given as zero. CH3- (CH2)14- CH3 Cetane (or) hexa decans (Cetene number = 100)   2 Methyl naphthalene (Cetane number = 0)

Slide 76 : The cetane number is defined as “the percentage by volume of hexa decane present in a mixture of hexa decane and a - methyl naphthalene, which has the same ignition lag as the fuel under test “ The cetane number decreases in the following order Straight chain alkanes > naphthalene > alkenes > branched alkanes > aromatic. The cetane number of diesel can be improved by addition of additives such as ethyl nitrate or isoamyl nitrate to diesel. This process is called as doping.

Slide 77 : Gaseous fuels Producer gas The average composition of the producer gas is as follows. CO 30% N 52-55% H 8-12% CO 3% It's calorific value is 1300 Kcal/kg

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Slide 79 : Manufacture The furnace consists of a tall steel vessel lined with silica bricks inside. It is provided with cup and cone arrangement to feed coal into furnace. It is also provided with inlet and outlet for passing air and steam and removes the ash respectively. The low grade coal is fed in to the furnace as shown in the figure. Then a mixture of air and steam is passed over a red hot coke at 11000C; the producer gas is produced. The following reactions take place in different zones.

Slide 80 : i) Ash zone The lowest zone consists mainly of ash. It is used to preheat the incoming air and steam. ii) Oxidation zone In this zone, the coal undergoes oxidation to form CO and CO2. It is an exothermic reaction. The temperature of this zone is 11000C. C + ½ O2 CO + N2 (Air) + N2 Exothermic C + O2 CO2

Slide 81 : iii) Distillation zone This is the upper part of the fuel bed. The incoming coal is heated by outgoing gases to remove volatile matter. The temperature of this zone is 400-5000 C. Uses 1. It is used for heating open hearth furnace, glass furnace, muffle furnace etc., 2. It can be used as reducing agent in metallurgical operation.

Slide 82 : Water gas The average composition of water gas is as follows. CO = 40 - 42 % H = 48 - 51% 2 CO = 3 - 5 % 2 N = 3-6 % 2   It's calorific value is about 2800 Kcal/kg.

Slide 83 : Manufacture The furnace consists of a tall steel vessel lined with silica bricks inside. It is provided with cup and cone arrangement to feed the coke into furnace .At the bottom, it is provided with two inlets for passing air and steam. The coke is fed into the furnace as shown in the figure. When steam and air are passed alternatively over a red hot coke at 900 - 10000C, water gas is produced.

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Slide 85 : Reaction The reaction of water gas production involves the following steps.   i. Air is passed through the coke bed, carbon burns to CO2 and the temperature of the fuel bed increase to over 10000C. C + O2 CO2 (Exothermic)

Slide 86 : Steam is passed through the red hot coke; CO and H2 are produced. The reaction is being endothermic, the temperature of the fuel and falls below 10000c. C + H2O CO + H2 ( Exothermic) In order to maintain the temperature of the fuel bed above 10000c, the air and steam are passed alternatively.

Slide 87 : Uses 1. It is used as a source of hydrogen. 2. A mixture of water gas and producer gas is used for the manufacture of ammonia. 3. It is mixed with hydrogen to produce methanol. 4. It is used for the preparation of carbureted water gas (Water gas + gaseous hydrocarbon) which can be used for lighting and heating purposes.

Slide 88 : Liquefied Petroleum Gas (LPG) It is obtained as a by - Product during fractional distillation of crude petroleum oil or by cracking of heavy oil. It consists of propane and butane. It can be readily liquefied under pressure, so it can be economically stored and transported in cylinders. The average composition of LPG is as follows.

Slide 89 : Constituents n - Butane = 38.5 % Iso butane = 37 % Propane = 24.5 % Its calorific value is about 25,000 kcal /kg   Uses 1. It is used as a domestic and industrial fuel. 2. It is also used as a motor fuel.

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