UNIT – IV Fuels and Combustion Dr. P. KRISHNAMOORTHY Assistant Professor DEPARTMENT OF CHEMISTRY KONGU ENGINEERING COLLEGE PERUNDURAI 10-Feb-22 UNIT – IV Fuels and Combustion: Introduction – classification of fuels - characteristics of a good fuel combustion - calorific values – gross and net calorific values - Dulong‘s formula (simple problems) - Flue gas analysis by Orsat‘s method - ignition temperature spontaneous ignition temperature - explosive range - solid fuels - coal and its varieties – proximate and ultimate analysis – significance – metallurgical coke Otto-Hoffman byproduct method - liquid fuel - refining of petroleum – manufacture of synthetic petrol - hydrogenation of coal - Bergius process knocking - octane number – cetane number - gaseous fuel - water gas Fuels Any substance used to produce heat or power by combustion. Combustion Any chemical process accompanied by the evolution of light or heat is called combustion. Fuel + Oxygen → Combustion products + Heat Example of Fuels: wood, Charcoal, Coal, Kerosene, Petrol, Diesel, Producer gas, Oil gas Classification of based on physical state Solid Fuel: ü It is characterised by its mass, composition, specific heat, combustion residue and other thermal properties. Liquid Fuel: ü It is characterised by its composition, density, viscosity, specific heat and other thermal properties. Gaseous fuel: ü It is characterized by its composition, volume and pressure and thermal properties. Classification of based on Occurrence Primary or natural fuels: ü These fuel exist in their natural form. Secondary or Synthetic fuels: ü These fuels are synthesized from naturally occurring primary fuels. Characteristics of Good Fuels: A good fuel possess ü High Calorific Values: It should give more heat on combustion. ü Moderate Ignition Temperature: The temperature at which the fuel starts to burn without further addition of heat. Very low ignition temperature leads to fire hazards. Very high disfavours the starting of combustion. ü Low Moisture Content: The moisture content in the fuel will consume some amount of heat for its own evaporation and also reduces the calorific value. ü Low Ash Content: Non combustible matter reduces the calorific value of the fuel. ü Moderate Velocity of Combustion: Low rate of combustion leads to unnecessary loss. High rate of combustion leads to out of control burning. ü Should not produce harmful products ü Less Cost ü Easy Storage & Transportation Calorific Value: It is defined as “the total amount of heat liberated, by the complete combustion of a unit mass of fuel”. • It is used to identify “the Efficiency of a Fuel. The calorific value is measured by the following units : Calorie Kilocalorie British Thermal Unit (B.T.U) Centigrade Heat Unit (C.H.U) Calorie: It is the amount of heat required to raise the temperature of 1g of water by 1˚C (i.e. 15 to 16 ˚C) TYPES OF CALORIFIC VALUES Lower or Net Calorific Value It is defined as the amount of heat liberated, when one unit of the fuel is burnt completely and the combustion product are allowed to escape. Net or Low C.V. = Gross C.V. – loss due to water formed (Because 1 part by weight of hydrogen produces 9 parts (1 + 8) by weight of water) Calculate the gross and net calorific value of a coal which analyses: C 74%, H 6%, N 1%, O 9%, S 0.8%, moisture 2.2% and ash 8%. 7362.01 FLUE GAS ANALYSIS (ORSAT APPARATUS) ü The mixture of gases like CO2, O2, CO etc., coming-out from the combustion chamber is called flue gases. ü The analysis of a flue gas will give an insight into the complete or incomplete combustion process and also the efficiency of the engine. ü The flue gases are estimated by using Orsat’s method. ORSAT’S APPARATUS Construction and working Ø It consists of a horizontal tube having a 3-way stopcock at one end and water-jacketed measuring burette at the other end. Ø The horizontal tube also connected to 3-different absorption bulbs for the absorption of CO2, O2 and CO respectively. The lower end of the burette is connected to water reservoir (rubber tube) Ø The bulb I: Potassium hydroxide solution. Ø The bulb II: Alkaline pyrogallol solution. Ø The Bulb III: Ammonical cuprous chloride solution Absorption of gases in reagent bottles: v It is quite necessary to follow the order of absorbing the gases, CO2 - first, O2 - second and CO - last. v This is because the absorbent used for O2 (i.e: alkaline pyrogallol) can also absorb some amount of CO2 and the % of CO2 left would be less. (i) Absorption of CO2: CO2 present in the flue gas is adsorbed by KOH. (ii) Absorption of O2 : O2 present in the flue gas is adsorbed by alkaline pyrogallol. (iii) Absorption of CO: CO present in the flue gas is adsorbed by ammonical cuprous chloride. Significance of Flue-gas Analysis Ø This analysis gives an idea about the complete or incomplete combustion process. Ø The presence of a high % of CO in the flue gases shows incomplete combustion of the fuel and also indicates the short supply of oxygen. Ø If the flue gases contain considerable amount of oxygen it indicates an excess supply of oxygen and possibility of complete combustion. COMBUSTION Combustion is defined as the process of burning of a substance in the presence of air (or) oxygen with the liberation of light and heat. Ignition temperature Explosive range Concepts Flame Ø Ignition Temperature It is defined as the lowest temperature at which the fuel must be heated, so that it starts burning smoothly. Spontaneous Ignition Temperature •The minimum temperature at which a substance catches fire spontaneously without external initiator such as a flame or a spark. Explosive range •The minimum and maximum amounts of fuel vapour that needs to be present in air for the fuel vapour to ignite. ü The limiting composition of a gas-air mixture beyond which the mixture will not ignite and continue to burn - limit of ignitability. ü For every combustible gas there are two limits ü Lower limit: representing the smallest portion ü Upper limit: the largest proportion of combustible gas that when mixed with air, will sustain combustion. SOLID FUELS ü Solid fuels refers to various forms of solid material that can be burnt to release energy, providing heat and light through the process of combustion. ü Common examples of solid fuels include wood, charcoal, peat, coal etc. ü Solid fuels have been used throughout human history to create fire and solid fuel is still in widespread use throughout the world in the present day. COAL Coal is a complex organic compound derived form the dead plants which had been buried for seeral million years. • The dead plants due to pressure and temperature, and contact with silt and water were subjected to various chemical and biochemical reactions resulting the formation of coal. • The degraded matter, as the time elapsed, was converted into various types of coal: peat, bituminous and anthracite, of varying maturity. • The process of conversion of degraded matter into various form of (Peat to anthracite) coal is called as coalification. • Peat: Ø Precursor of coals, Lower rank of coal with the lowest carbon content and with lowest calorific value. Found in west bengal (Sunderbans) and Nilgiri Hills Tamilnadu. Lignite: Ø Also known as brown coal, is the lowest grade coal with 25-30 % of carbon and 60% moisture content. Used for generating electricity. Lignite is mined in Neyveli in Tamil Nadu also found in Rajasthan and Kashmir. Sub-bituminous coal: Ø Next low rank coal is sub-bituminous coal (contains 35-45% carbon). It is black in color and has a higher calorific value than lignite, primarily as fuel for steam electric power generation. Bituminous coal: Ø Bituminous coal is a middle rank coal between sub-bituminous and anthracite. Contains 45-86% carbon. Bituminous usually has a high calorific value and is the most common type of coal used in electricity generation. Anthracite: Ø The highest rank of coal. It is a hard, brittle, and black lustrous coal, often referred to as hard coal, containing 86-97% percentage of fixed carbon and a low percentage of volatile matter. Peat is the most immature coal, hence it is lowest in rank whereas anthracite is the most matured coal, and hence it is highest in rank. Analysis of coal Proximate analysis Ultimate analysis To determine the Percentage of moisture, ash, volatile matter and fixed carbon content To determine the chemical composition of coal like percentage of C, H, N and S Proximate analysis Estimation of moisture content It is determined by heating about 1g of finely powdered coal at 100◦ C to 105 ◦C for an hour in a hot-air oven. The loss in weight is reported as moisture. % "# $"%&'()* = ,-.. /0 12/345 -6 7-8, ×100 9*%:ℎ' "# <"=> %?%'%=>>@ '=A*? Significance of analysis Ø Decreases calorific value of fuel Ø Lengthens the time of heating The lesser the moisture content, the better the quality of coal Estimation of volatile matter For determining volatile matter, a known weight of moisture free coal is taken in a crucible with properly fitting lid. It is then heated at 950 ◦C ±20 ◦C for exactly seven minutes in previously heated muffle furnace. The loss in weight is due to volatile matter which is calculated as % "# $"%&'(%) *&')+ ($*) = !"## $% &'$()* +"$#*,-' .-'' /"0! &'$()* ". /"0! $%$*$0!!1 *02'% ×100 Significance of analysis ü Decreases calorific value of fuel ü Forms smoke and pollutes air A good quality coal should have lesser volatile matter Estimation of ash in coal A known weight of coal is taken in a crucible and the coal is burnt completely at 700 ◦C-750 ◦C for 30 minutes in muffle furnace. The crucible is taken out and cooled and weighed. Repeat the heating, cooling and weighing until a constant weight is obtained. The residue left in the crucible is ash content in coal which is calculated as % -6 8.4 /0 7-8, = 12/345 -6 8.4 6-EF2G ×100 12/345 -6 7-8, /0/5/8,,H 58I20 Significance of analysis ü Ash is non combustible matter, which reduces the calorific value. ü Which causes obstruction to air supply: thus causes incomplete combustion of coal. ü It increases the transport, handling, storage and disposal cost. A good quality coal should have lesser ash content Estimation of Fixed Carbon % of Fixed Carbon = 100-[% of moisture content + % of Volatile Matter + % of ash content] Significance of Analysis Higher the fixed carbon content, greater is the quality of coal. ü Higher the fixed carbon content, greater is the calorific value. ü It is useful for designing the furnace and firebox. ü Ultimate Analysis Estimation of Carbon and Hydrogen: ü About 1-2 g of accurately weighed coal sample is burnt in a combustion apparatus by supplying oxygen. ü The carbon is converted into CO2, and Hydrogen is converted into H2O. C + O2 12 g 2H +1/2O2 2g CO2 44 g H2O 18g % "# $%&'"( = *($&+%,+ -( .+-/ℎ1 "# 234 × 12 ×100 .+-/ℎ1 "# $"%8 1%9+(×44 % "# 4<=&"/+( = *($&+%,+ -( .+-/ℎ1 "# >%>8!× 2 ×100 .+-/ℎ1 "# $"%8 1%9+(×18 üThe gaseous products i.e., CO2 and H2O absorbed in KOH and CaCl2 tubes of known weights respectively. üIncrease in the weight of the tubes is used to % of carbon and hydrogen. Estimation of Nitrogen Ø Ø Ø Ø Ø Ø About 1g of weighed coal is heated with Conc H2SO4 along with K2SO4 in a long necked flask called Kjeldahl’s flask. The obtained clear solution is treated with excess of NaOH (NH4) 2 SO4 + 2NaOH Na 2 SO4 + 2NH3 The liberated ammonia is distilled over and absorbed in a known solution of acid solution. The unused acid is then determined by back titration with standard NaOH solution. From the volume of acid consumed by liberated ammonia the percentage of N in coal sample is calculated.. % "# @-1&"/+( = A"8BC+ "# %$-= $"(,BC+= '< @4" D @"&C%8-1< "# %$-= D 14 D 100 E+-/ℎ1 "# $"%8 1%9+( Organic Compound +Conc H2SO4 + K2SO4 Estimation of Sulphur A known quantity of coal is burnt in bomb calorimeter for the determination of a calorific value. Ø Residue ash is then treated with HCl Ø During this determination, S is converted into sulphate. Ø S + O2 SO42- + BaC l2 32 g BaSO4 233 g Ø The washings are treated with barium chloride solution to precipitate as barium sulphate. Ø This precipitate is filtered, washed and heated to constant weight. %F = E+-/ℎ1 "# G%F3#D 32 D 100 E+-/ℎ1 "# >"%8 1%9+( D 233 Significance and importance of ultimate analysis Carbon and Hydrogen Ø Greater the percentage of C and H better the quality of coal Nitrogen Ø Nitrogen is an inert and noncombustible gas, its presence is not desirable. Sulphur Ø Oxidation products of Sulphur (SO2 and SO3) , have corrosive effect on the equipment and also cause atmospheric pollution. Ø Presence of sulphur is not desirable in coal. Oxygen Ø High oxygen content coals are characterized by high moisture content. Ø A good quality coal should contain lesser moisture content. Carbonisation of coal Ø It is the process of heating the coal in the absence of air to a high temperature to produce a residue coke. Metallurgical coke Ø When bituminous coal is heated strongly in the absence of air, the dense strong, porous mass obtained is called metallurgical coke. Ø Characteristics of Metallurgical coke:I. Purity: It should contain less percentage of moisture, ash, phosphorus and sulphur. II. Porosity: It should be porous, so that combustion should be uniform and complete. III. Strength: The mechanical strength of coke should be very high. IV. Size: Coke should be have medium size. V. Calorific value: Coke should possess a very calorific value. VI. Cost: Coke should be cheap and easily available. VII. Combustibility: Coke should burn easily. VIII. Reactivity: The reactivity of coke should not be very high. Metallurgical coke is superior to coal for the following reasons: Coke is stronger and more porous than coal. • Coke contains lesser amount of sulphur than coal. • Coke does not contain much volatile matter. • Manufacture of Metallurgical coke by Otto-Hoffmann’s method: Significance of Otto-Hoffman’s method : ü To increases the thermal efficiency of the carbonization process. ü To recover the valuable by products (like coal gas, ammonia, benzyl oil, etc). Otto-Hoffman byproduct coke oven Ø The oven consists of narrow rectangular oven chambers are about 14 m long, 3 m height and 0.4 to 0.45 m wide. Ø They are tightly closed in order to achieve anaerobic heating. Ø The oven may heated by using producer gas and coal gas produced in the coke oven itself to 1000oC. Ø The coking time ranges from 12 h to 20 h, with a yield about 70 %. Otto-Hoffman by-product coke oven onia m m a r Liquo O NH3+H2 H NH4O ater w d eum l o C petrol By-products recovery Removal of H2S Fe2O3+ 3 H2S --------> Fe2S2+ 3H2O Recovery of variable byproducts Coal tar: ü The gas from the oven is made to pass through a tower where liquid ammonia is sprayed over it. ü This causes the collection of dirt and dust at the bottom of the tank ü The ammonia is regenerated by heating the liquid through steam coils. Ammonia: ü The ammonia from the gas is converted into NH4OH or (NH4)2SO4 by passing through the tower sprayed with water or dilute sulphuric acid. NH3 + H2O 2NH3 + H2SO4 NH4OH (NH4)2SO4 Naphthalene: ü When the gas are passed through the tower sprayed with cold water, the naphthalene gets condensed and can be separated. Light oil or crude oil: The gases are passed through the tower sprayed with petroleum to separate benzene and its homologues. H2S: The gases are allowed to pass into a chamber containing Fe2O3 in moist form: Fe2O3 + 3H2S Advantages High thermal efficiency Lesser carbonization time ü By-products recovery ü Heating is done externally by producer gas Ø ü ü Fe2S2 + 3H2O Liquid fuels – Petroleum ü Petroleum is made from the remains of plants and animals buried millions of years ago. ü It is a non-renewable resource. ü It contains straight or cycloparaffins such as, methane, ethane, propane, etc., ü Olefins such as ethylene, butene, butadiene, etc., ü Aromatics such as benzene, naphthalene, cyclohexane etc., ü Other organic compounds containing N, O and S. Constituents Percentage (%) The average composition of crude oil is as follows C 80 – 87 H 11 – 15 S 0.1 – 3.5 N+O 0.1 – 0.5 10-Feb-22 Production from refining crude oil Ø The petroleum obtained by mining is viscous and dark colored liquid Ø Due to the presence of sulphur it has an unpleasant smell. Ø It also contains sand, brine or sea water as impurities. Ø The petroleum with more sulphur content are called as sweet crude oil. Ø The sulphur content of crude oil is removed by adding copper oxide. Ø The crude oil after the removal of impurities subjected to fractional distillation. 10-Feb-22 Purification of petroleum Separation of water (Cottrell’s Process) ü The crude oil from oil well is an extremely stable emulsion of oil and salt water. ü The crude oil is allowed to flow between two highly charged electrodes, where colloidal water droplets combine to form large drops, which is then separated out from the oil Removal of harmful sulphur compounds & Salts ü Sulphur compounds are removed by treating the crude oil with copper oxide. The copper sulphide formed is separated out by filtration. ü Electrical desalting are used to remove NaCl and MgCl2. Refining of Petroleum ü Crude oil is mixture of hydrocarbons, mainly alkanes and it is difficult to separate the individual compounds. ü When fractional distillation is performed, the crude oil is separated into a number of fractions having different and definite boiling ranges. ü The fractions find immense use in industries and in domestic purposes. Fractional Distillation Ø The fractional distillation of petroleum carried out continuously in a specially designated fractionating tower or column made of steel. Ø The oil is preheated in a furnace to about 400oC and introduced in this tower. Ø The tower is hot towards the lower end and comparatively cooler at the upper end. Ø The fractions having higher boiling points condense at lower trays whereas the fractions having lower boiling points condense at higher trays. Ø The gasoline obtained by this fractional distillation is called straight-run gasoline. Various fractions obtained at different trays are given in table Various fractions, compositions and their uses SYNTHETIC PETROL ü It is a mixture of alkanes with composition resembling that of petrol, obtained artificially from coal. ü Coal contains about 4.5% hydrogen, but petroleum contains about 10-15 % of hydrogen. ü Coal is a hydrogen deficient compound. If coal is heated with hydrogen at high temperature under high pressure, it is converted into gasoline. ü The preparation of liquid fuels from solid coal is called hydrogenation of coal (or) manufacture of synthetic petrol. There are two methods available for the hydrogenation of coal vBergius process (or direct method). vFischer-Tropsch process (or indirect method). Bergius process (direct method) ü In this process, the finely powdered coal is made into a paste with heavy oil and a catalyst powder (tin or nickel oleate) is mixed with it. ü The paste is pumped into the converter, where the paste is heated to 400 − 450°C under 200 − 250 atm pressure in the presence of Hydrogen. ü During this process hydrogen combines with coal to form saturated higher hydrocarbons, which undergo further decomposition at higher temperature to yield mixture of lower hydrocarbons. Coal dus + H2 Catalyst 450oC, 200atm Mixture of Hydrocarbon (1) H2, (2) cracking Crude oil The crude oil is then fractionated to yield (i) Gasoline (ii) Middle oil (iii) Heavy oil. The middle oil is further hydrogenated in vapour phase to yield more gasoline. The heavy oil is recycled for making paste with fresh coal dust. The yield of gasoline is about 60% of the coal used. KNOCKING ü Knocking is a kind of explosion due to rapid pressure rise occurring in an IC engine. ü Causes of knocking in S.I (Spark Ignition) Engine [Petrol engines] ü In a petrol engine, a mixture of gasoline vapour and air at 1:6 or 1:10 ratio is used. ü In a SI engine fuel must be ignited at the beginning of the power stroke. Ø The rapid compression of fuel-air mixture heats the engine, it causes the ignition fuel at compression stroke (2nd stroke) without the spark (before power stroke). Ø This causes violent jerk to the piston giving a metallic rattling sound, known as knocking. The knocking tendency decreases as follows n-alkanes < isoparaffins < olefins < naphthenes < aromatics (aromatics have highest antiknock value whereas n-alkanes have lowest antiknock value) So, the presence of maximum quantity of aromatics and minimum quantity of nalkanes is desirable in petrol. Octane Number Octane number is used for gasoline(petrol). It is the % of iso-octane in a mixture of iso-octane and n-heptane, which matches the same knocking characteristics of gasoline mixture test sample. Octane Number is rated from 0 to 100. Comparisons are made with blends of two pure hydrocarbons, n-heptane and iso-octane (2,2,4-trimethylpentane). Octane Number Ø Higher octane number fuel will have lesser chance of knocking. Hence such fuel can be compressed to higher extent without observing any knocking. Ø Lesser octane number fuel will have higher chance of knocking. Hence such fuel can't be compressed to higher extent. Ø Octane number actually measures the resistance to auto-ignition. Higher the Octane Number, Higher will be the resistance to auto-ignition. For example, o Gasoline with a knocking ability that matches that of a blend of 90% isooctane and 10% n-heptane has an octane number of 90. 1. 2. How octane number of a fuel can be increased ? By using special additives into the fuel which discourage auto ignition. By blending high-octane fuels in with the ordinary petrol. Anti-knocking additives • Anti-knocking additives are substances which reduce the tendency of a fuel to auto-ignite, and so increase the octane number. • Small amounts of lead compounds have been used as economical and effective antiknock additives. tetraethyl lead • But it damage the environment. Other Additives Ø Toluene , Xylene, tertiary-butyl-ether (MTBE) , Methanol or Ethanol Alcohol and Tertiary Butyl Alcohol CI Engine In a CI engine, Ø Air alone gets compressed. Ø Diesel is sprayed which must ignite spontaneously Ø The product of combustion increases pressure and pushes the piston out and expels the exhaust gases from the cylinder. Knocking in CI Engine In Diesel Engine, Sometimes, in some cases, Even after the compression stroke is over and even after the diesel oil is sprayed, burning may not start. Ø So more and more fuel is injected automatically and sudden ignition may occur . This is called delayed-ignition. Ø This delayed ignition causes explosion. Ø This is called knocking. Chemical Structure and Knocking The knocking tendency increases as follows n-alkanes isoparaffins olefins naphthenes aromatics (n-alkanes have highest antiknock value whereas aromatics have lowest antiknock value) So, the presence of maximum quantity of n-alkanes and minimum quantity of aromatics is desirable in diesel. Cetane Number Ø Cetane number is used for diesel. Definition: It is the % of n-hexadecane (n-cetane) in a mixture of n-hexadecane and 1-methyl naphthalene, which matches the same knocking characteristics of diesel mixture test sample. Cetane Number is rated from 0 to 100. Comparisons are made with blends of n-hexadecane and 1-methyl naphthalene. Additives (Improvement of cetane number) Ø Ethyl nitrate and iso amyl nitrate CONCLUSION OCTANE NUMBER AND CETANE NUMBER Ø Octane Number and Cetane Number are the standards to measure the tendency of fuel to ignite spontaneously. Ø Octane number measures the performance of gasoline while Cetane number measure the performance of the diesel. Ø The fuel having high octane number has the low cetane number and high cetane number fuel has low octane number. Ø This is one reason why we can’t use petrol in a diesel engine and diesel in petrol engine. Comparison of gasoline oil and diesel oil Gaseous FuelWATER GAS vIt is a mixture of CO and H2 with small amount of N2. v The average composition of water gas is as follows vIts calorific value is about 2800 kcal/m3 Manufacture ü The water gas producer consists of a tall steel vessel, lined inside with refractory bricks. ü It is provided with cup and cone feeder at the top and a side opening for water gas exit. ü At the bottom it is provided with two inlet pipes for passing air and steam. When steam and little air is passed alternatively over a red hot coke maintained at about 900 − 1000°C in a reactor, water gas is produced. C + H2O CO + H2 Various Reactions The reactions of water gas production involves the following two steps. I - Step In the first stage, steam is passed through the red hot coke, where CO & H2 gases are produced. The reaction is endothermic. Hence, the temperature of the coke bed decreased. II - Step In the second stage, in order to raise the temperature of the coke bed to 1000°C, the steam supply is temporarily cut off and air is blown in. The reaction is exothermic. C + O2 CO2 +97 kcal 2C + O2 2CO +59 kcal Thus the steam-run and air-blow are repeated alternatively to maintain proper temperature. Since both CO and H2 burn with a non-luminous or blue flame, water gas some times known as “blue gas” Properties: Ø High Calorific values Ø Burns with nonluminous flame Ø Flame is short but very hot 10-Feb-22 Uses vIt is used for the production of H2 and in the synthesis of ammonia. vIt is used to synthesis gasoline in Fischer Tropsch process. vIt is used as an illuminating gas and a fuel. vIt is also used in the manufacture of power alcohol and carbureted water gas (water gas + oil gas).
