Engineering Chemistry Dr. Payal Joshi

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Fuels & Combustion

Introduction: Fuel is a substance, which on combustion produces large amount of heat that can be used for various domestic and industrial purposes. Fuels commonly used contain carbon as the main constituent and some common fuels are wood, charcoal, kerosene, diesel, producer gas, etc. Fossil fuels are non-renewable energy resources which were stored up millions of years ago by photosynthesis. Fossil fuels are coal, crude oil and natural gas. The process of combustion involves oxidation of carbon, hydrogen, etc of the fuels to CO2, H2O and the difference in energy of reactants and products are liberated as large amount of heat energy which is utilized.

[I] Calorific Value: Total quantity of heat liberated by burning a unit mass or volume of fuel completely. 1. Gross calorific value (Higher calorific value): HCV/GCV: Total amount of heat produced

when a unit mass/volume of fuel has been burnt completely & products of combustion have been cooled to room temperature (150C or 60oF). Fuel (with hydrogen) – burnt – Hydrogen undergoes combustion– steam – combustion products cooled—condensation– water– latent heat evolved– latent heat of condensation of steam liberated is included in HCV.

2. Low Calorific value/ Net Calorific value- LCV/NCV: Net heat produced when unitmass/volume of fuel is burnt completely and products are permitted to escape is LCV. NCV = HCV – Latent heat of water vapour formed = GCV – Mass of hydrogen x a x Latent heat of steam

Definition, Classification, characteristics. Calorific value-Theoretical & Experimental (Bomb calorimeter). Solid Fuels: Coal, proximate and ultimate analysis, Numericals based on analysis of coal. (Dulong formula) and bomb calorimetry. Liquid fuels: Mining of Petroleum, Cracking, Reforming, Knocking in IC engines, anti-knocking agents (TEL and MTBE), octane number of petrol, cetane number of diesel. Gaseous fuels: (LPG, CNG) Composition, properties and applications. Numerical problems based on Combustion of fuels (Calculation of air/oxygen requirement (solid/gaseous fuels and flue gases)

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Since, 1 part by mass of hydrogen produces ‘a’ part by mass of water. Because 1 part by weight of hydrogen produces 9 parts (1 + 8) by mass of water. Thus, NCV = GCV – Mass of hydrogen x 9/100 x Latent heat of steam.

Units of calorific value: calorie/gm (cal/gm) or kilocalorie/kg (kcal/kg) or British thermal unit/lb (B.T.U/lb) in case of solid and liquid fuel. In case of gaseous fuels, units are kcal/m3 or B.T.U./ft3. Calorific value of fuels is determined theoretically by Dulong Formula which is expressed as follows,

( )

where, C,H,O,S are the percentages of carbon, hydrogen, oxygen and sulfur in the fuel. NCV = GCV – Mass of hydrogen (%) x 0.09 x Latent heat of water vapor formed (587 cal/g) Bomb Calorimeter:

Fig1. Bomb Calorimeter Apparatus

1. Bomb Calorimeter is used to find calorific value of solid & liquid fuels. 2. It is a strong cylindrical stainless steel bomb in which combustion of fuels is made to take

place. Bomb has a lid which can be screwed to the body of the bomb so as to make it gas-tight seal. Lid is provided with two stainless steel electrodes and an oxygen inlet valve.

3. To one electrode, a ring is attached. In this ring, Ni or stainless steel crucible is supported. 4. Bomb is placed in a copper calorimeter and surrounded by air-jacket & water-jacket to

prevent heat loss due to radiation. 5. Calorimeter is provided with electrically operated stirrer & Beckmann’s thermometer that

accurately reads temperature difference up to 1/100th of a degree.

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6. Working: A known mass (0.5-1.0g) of a given fuel is taken in a clean crucible. Crucible is supported over a ring. A clean Magnesium wire touching fuel sample is stretched across electrodes. Bomb lid is tightly screwed & bomb filled with oxygen to 25 Atm pressure.

7. Bomb is then lowered into the copper calorimeter containing known mass of water. Stirrer is worked and initial temperature of water is noted.

8. Electrodes are connected to 6V battery & circuit is completed. Sample burns & heat is liberated.

9. Uniform stirring of water is continued and maximum temperature attained is recorded.

Let x = mass in g of fuel taken in crucible W = mass of water in calorimeter w = water equivalent in g of calorimeter, stirrer, thermometer, bomb etc. t1 & t2 are initial & final temperatures of water in calorimeter L = higher calorific value of fuel in cal/g Then heat liberated by burning of fuel = xL Heat absorbed by water & apparatus = (W+w)(t2-t1) But heat liberated = heat absorbed so, xL = (W+w)(t2-t1)

L = (W+w)(t2-t1)cal/g or kcal/kg x If H = % of hydrogen in fuel 9H/100 g = mass of water from 1 g of fuel= 0.09H g So heat taken by water in forming steam = 0.09 H × 587 cal LCV = HCV - 0.09 H × 587 cal/g By considering fuse wire correction, acid correction & cooling corrections,

L = [{(W+w)(t2-t1+ cooling correction)}- {acid +fuse correction}] cal/g or kcal/kg x [II] Characteristics of a Good Fuel: 1. High calorific value: Amount of heat liberated & temperature attained thereby depends on

the calorific value. 2. Moderate ignition temperature: Ignition temperature: Lowest temperature at which the

fuel must be pre-heated so that it starts burning smoothly Low Ignition temperature– dangerous for storage, transport– fire hazard High Ignition temperature—difficulty in igniting fuel– safe during storage Ideal fuel with moderate Ignition temperature

3. Low moisture content: Presence of moisture content of fuel reduces heating value – loss of money, since it is paid for the same rate as the fuel.

4. Low non-combustible matter content: After combustion, non-combustible matter like ash remains – cause reduction in heating value. Each % of non-combustible matter – heat loss by about 1.5%.

5. Products of combustion should not be harmful; objectionable gases– CO, SO2, H2S, PH3 found in coal samples result in toxic emissions on combustion.

6. Easy control on combustion: Spontaneous ignition lead to fire hazard. 7. Low cost, easy to transport and fuel should burn in air without smoke formation.

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Solid Fuels: Coal is the most important solid fuel derived from prehistoric plants. Coal is highly carbonaceous and composed of C, H, N and O and non-combustible inorganic matter. [III] Analysis of Coal A. Proximate analysis of coal: It consists of determination of percentages of (i) Moisture content (ii) Volatile matter, (iii) Ash and (iv) Fixed carbon. 1. Determination of moisture content: 1 g of finely powdered air dried coal sample is taken in a silica crucible and heated in hot air oven at 105-1100C for one hour. The coal sample in the crucible is cooled in a desiccator and weighed. One can determine the loss in weight as moisture on percentage basis as,

High percentage of moisture is undesirable as it reduces the calorific value of coal. It quenches fire in the furnace. Lesser the moisture content; better is the quality of coal as a fuel. 2. Determination of volatile matter: Moisture free coal from above step is taken in Si crucible 2 and covered with a lid. It is then heated in a muffle furnace at 925+/-200C for 7 minutes. Crucible is taken out and cooled in a desiccator and weighed again. Loss in weight as volatile matter on percentage basis is determined as,

Volatile matter is present in form of combustible gases like hydrogen, methane and lower hydrocarbons. Presence of VM gives long flames, high smoke, relatively low heating. Higher the VM, greater the combustion space required– dictates the furnace design. 3. Determination of Ash: Residual coal in Si crucible 2 is heated in a furnace at 700-7500C for 30 minutes. It is cooled in air, desiccated and the process is repeated until a constant weight is obtained.

% of Ash = (Weight of residue left/Weight of coal taken) x 100 Presence of ash reduces calorific value of coal. It causes interference in the flow of air, heat– decreases efficiency and increase transport costs. Hence, lower the ash content, better the quality of coal. 4. Determination of Fixed Carbon: After the determination of moisture content, volatile matter and ash content. Remaining material is known as Fixed Carbon. It is determined by deducting the sum total of moisture, ash and VM matter from 100.

% of Fixed Carbon = 100 - % of (Moisture + Volatile Matter + Ash) Higher the % of Fixed carbon, greater the calorific value of coal, better the quality of coal. It is this fixed carbon that burns in solid state and helps in determining the furnace design.

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B. Ultimate Analysis of Coal: It consists of determining percentages of C, H, O, N, and S. 1. Carbon and Hydrogen:

Fig. 2: Combustion Apparatus

Carbon and hydrogen: Accurately weigh 1-2 gm of the coal sample and burn in a current of oxygen in a combustion apparatus whereby CO2 and H2O are formed. CO2 and H2O are absorbed by previously weighed tubes containing KOH and anhydrous CaCl2. Increase in weight gives the C and H content as: Increase in weight of KOH represents weight of CO2; Increase in weight of CaCl2 represents weight of H2O. C + O2 Æ CO2 and 2KOH + CO2 Æ K2CO3 + H2O 12 44 112g 138 g H2 + ½ O2 Æ H2O and CaCl2 + 7H2O Æ CaCl2.7H2O 2 18 111g 237g

2. Determination of Nitrogen: Nitrogen estimation is done by Kjeldahl’s method. A known amount (1g) of powdered coal is heated with H2SO4 & K2SO4 (catalyst) in a long-necked Kjeldahl’s flask, thereby converting nitrogen of coal to ammonium sulfate. Clear solution is then treated with excess of alkali 50% NaOH and liberated NH3 is distilled over standard 0.1N HCl. Unused acid is back titrated against standard 0.1N NaOH. From volume of acid used by liberated NH3, % N can be estimated.

Fig. 3: Kjeldahl’s Method

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Mass of Coal = X gm Volume of acid in which NH3 is passed = V1 ml Volume of acid unused = V2 ml Volume of acid consumed by NH3 = (V1-V2) ml 1000 ml of 1N HCl ≡ 1 mole of NH3 ≡ 14 g of N2 Thus, (V1 – V2) ml of 0.1 N HCl = 14 × (V1-V2) × 0.1 g of N2 1000 x 1 N X g of coal sample contains = 14 × (V1-V2) × 0.1 g of N2 1000 x 1 N

( ) ( )

x 100 3. Determination of Sulfur: Known amount of coal burnt in current of oxygen. Sulfur gets oxidized to sulfates. Acid extract treated with BaCl2 and BaSO4 precipitate is formed. BaSO4 precipitate is filtered, washed, dried and weighed until constant weight is obtained.

4. Ash determination is performed as per proximate analysis. 5. Oxygen Determination: Sum of percentages of C, H, N, S and ash subtracted from 100 as follows, % Oxygen = 100 – percentage of (C+H+N+S+ash) Significance: Higher percentage of C and H increases the calorific value of coal and hence better is the coal. Higher the percentage of O, lower is the calorific value and lower is the coking power. Also O when combined with H in coal, H available for combustion becomes unavailable. S, though contributes to calorific value, it is undesirable due to its polluting properties as it forms SO2 on combustion. Liquid Fuels: Petroleum: Petroleum/ Crude Oil (Latin: petra = rock, oleum = oil). It is a dark, greenish-brown viscous oil found in deep Earth crust. It is mainly composed of various hydrocarbons along with small amounts of organic compounds with O,N & S. Classification of petroleum: According to the chemical nature, there are three main types of petroleum, viz, i) Paraffin-base crude composed of saturated hydrocarbons upto C35H72 which are semisolids, called waxes. ii) Asphalt-base crude contains mainly naphthenic compounds and cycloparaffins with smaller amounts of paraffins and aromatics. iii) Mixed base crude contains both the above type of compounds but rich in waxes. [IV] Origin of petroleum: Petroleum resulted from partial decomposition of marine animals, vegetable organisms from prehistoric forests. Changes in Earth (volcanic eruptions) had buried these materials underground—subjected to intense pressure & heat during ages of time. Conversion of these materials into various hydrocarbons has been on either under the influence of radioactive substances (Uranium) or by bacterial decomposition in anaerobic conditions.

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1. Mining of petroleum: Crude oil floats over salt water (brine) and natural gas surrounds the porous oil-bearing rocks. Mining is done by drilling holes in Earth’s crust and sinking pipes upto oil-bearing porous rocks. Oil gushes out due to hydraulic pressure of natural gas. Alternatively, compressed air is forced thro’ the outer pipe, whereby oil comes out thro’ inner pipes. Later, the crude oil is conveyed to refinery by system of pipelines.

Fig.4: Mining of Petroleum 2. Refining of Crude Oil/ Petroleum: Problem with Crude Oil: It contains hundreds of different types of hydrocarbons; all mixed together. Easy way to separate out is by the process called Oil Refining. Different hydrocarbon chain lengths have progressively higher boiling points, so they can be separated by distillation. This is what happens in an Oil refinery – Fractional Distillation. Step 1: Separation of Water (Cottrell’s Process): Crude oil is stable emulsion of oil & salt water. Process of freeing oil from water consists in allowing crude oil to flow between two highly charged electrodes is done. Colloidal water droplets coalesce to form larger drops which separate from oil. Step 2: Removal of harmful sulfur compounds: Treat oil with copper oxide, Copper sulfide is formed which is removed by filtration. Oil + CuOÆ CuS (removed by filtration) Step 3: Fractional Distillation: Crude oil is heated to about 4000C in iron retort– all volatile components, except residual coke are evaporated. Hot vapors are allowed to pass into fractionating column. Presence of bubble caps in trays in the column allows maximum contact time between vapor and the liquid. There are temperature differences across the column – hot at the bottom and cool at the top. Substance with the lowest boiling point will condense at the highest point in the column; substances with higher boiling points will condense lower in the column.

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Fig.5: Fractional Distillation unit.

Following are the fractions of distillation of crude oil,

[V] Chemical Processing: Very few components come out of fractional distillation column ready for market. Many of them must be chemically processed to make other fractions. Rather than continually distilling large quantities of crude oil, oil companies chemically process some other fractions from the distillation column to make gasoline. This processing increases yield of gasoline from each barrel of crude oil. You can change one fraction into another by one of three methods:

x Breaking large hydrocarbons into smaller pieces (cracking) x Combining smaller pieces to make larger ones (unification) x Rearranging various pieces to make desired hydrocarbons (alteration)

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1. Cracking: Process of thermal decomposition is known as cracking. Accumulated stocks of higher boiling fractions are converted into valuable lower boiling fractions suitable for spark-ignition engines of automobiles. x Objective: To obtain greater yields of improved gasoline. Cracked gasoline gives better

engine performance. x Higher saturated hydrocarbons are converted to simpler molecules like paraffinic & olefinic

hydrocarbons, eg,

a) Thermal Cracking: Heavy oil is subjected to high temperature & pressure until they break apart to lower hydrocarbons of paraffinic & olefinic series. Yield is generally from 7-30%. i) Liquid Phase Cracking : Heavy oil/gas oil is cracked at 475-530C at pressure of 100 kg/cm2. Cracked products are separated in fractionating column. Octane rating of petrol produced is 65-70. ii) Vapor phase cracking: Oil is first vaporized & then cracked at 600-6500C at pressure of 10-20 kg/cm2. Time required is much lesser. Stability of products is poorer and octane rating of the product higher as compared to liquid phase method. 2. Catalytic Cracking: It uses catalyst to speed up the cracking reaction at much lower temperatures & pressures (300C-4500C; 1-5kg/cm2) i) Fixed Bed catalytic cracking: Heavy oil charge passed through a heater – oil vaporizes by heating to cracking temperature (420-4500C). It enters catalytic chamber (packed with clay & zirconium oxide). Hot vapor passed over fixed bed of catalyst maintained at 425-4500C at 1.5kg/cm2. Cracking takes place in reactor. Cracked vapors enter fractionating column-gasoline vapors & gaseous products are recovered from top; heavy oil fraction condensed at bottom.

Fig.6: Fixed bed catalytic cracking

Vapors get condensed and uncondensed gases move further. Condensate sent to stabilizer– pure gasoline is obtained. After 8-10h, catalyst ceases– removal of carbon deposit by blowing of stream of hot air.

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ii) Fluid Bed/ Moving bed catalytic cracking:

Fig.7: Fluid bed catalytic cracking

Finely powdered catalyst is kept agitated by a gas stream of the vaporized heavy oil feed stock. Thus, the catalyst can be handled like a fluid system and can be pumped like a liquid. This allows close contact between the catalyst and the reactant resulting in efficient cracking. Heavy oil enters reactor and the fluid catalyst is introduced from the regenerator. Cracking takes place at catalytic bed; it circulates with oil vapors at 5300C at 3-5 kg/cm2. Low boiling fraction move to top of reactor & enter fractionating column. Cracked gas & gasoline is removed from top of fractionating column which is later cooled and the condensate is sent into stabilizer to remove dissolved gases. Pure gasoline is recovered. Advantages of Catalytic cracking: Yield and quality of petrol obtained from catalytic cracking is higher. Cracking can be controlled and carried out a much lower pressures. Products obtained from catalytic cracking contain higher amounts of aromatics and hence possess better anti-knock characteristics. Catalysts are selective in their action and result in cracking of only higher boiling hydrocarbons into valuable lower boiling fractions. [VI] Reforming of Petrol/ Gasoline: Reforming is the process of bringing about structural modifications in the components of straight run gasoline with the primary objective of improving its anti-knock characteristics. Reforming or aromatization involves the conversion of open chain (aliphatic) hydrocarbons and/or cycloalkanes in the presence of a catalyst, into aromatic hydrocarbons (arenes) containing the same number of carbon atoms. It is carried thermally (temp 500-6000C, pressure 85 atm) or catalytically [Pt (0.75%) supported over Alumina (temp 460-5300C, pressure 30-35 atm)].

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Dehydrocyclization: Knocking in IC Engines: 1. It is a term referred to internal combustion engine (IC engine) working on petrol. Petrol is a

mixture of different hydrocarbons and diesel is the fifth fraction (250-3200C) after naphtha & kerosene. Diesel is harder to vaporize than petrol and hence is not interchangeable in car engines.

2. In IC engine, mixture of gasoline vapors & air is compressed and ignited by electric spark. 3. Four stroke engines operate on a four-stroke cycle known as the Otto cycle. 4. The four strokes are: intake, compression, combustion and exhaust. To put it in the simplest

of terms, each of the vehicle's piston moves up and down within a cylinder. 5. As the piston moves to the bottom of the cylinder, a mixture of fuel and air flows in. The

piston then moves upward, toward the top of the cylinder, compressing the air and fuel mixture as it does so.

6. Just as the piston reaches the top of the cylinder, the cylinder's spark plug ignites. 7. The spark creates a small, controlled explosion that forces the piston to the bottom of the

cylinder. 8. In the final stroke of the cycle, the piston moves upward to push the exhaust gas out of the

cylinder. 9. Once the exhaust gas has been pushed out, the entire cycle begins again. As long as this

process works as described above, the engine runs smoothly. 10. But occasionally the pressure of the piston itself will cause the air and gas mixture to ignite

prematurely during the compression cycle, creating a smaller, less powerful explosion. This is called pre-ignition and it's the cause of engine knock, the erratic rattling or pinging sound one can hear underneath the car's hood.

11. It means that engine is not running efficiently and if left unchecked, it could eventually cause damage.

12. Knocking is a sharp metallic sound similar to rattling of a hammer which is produced in the internal combustion engines due to immature ignition of the air-gasoline mixture.

13. Knocking causes: a) loss of large amount of energy b) damage to the piston and cylinder. 14. Knocking depends on various factors such as structure of the hydrocarbons present in petrol,

sudden burning of hydrocarbons, engine design like, shape of head, location of plug parts, running conditions.

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15. Thus, tendency of gasoline to knock is in the following order, Straight chain paraffins > Branch chain paraffins > Olefins > Cycloparaffins > Aromatics.

Octane number of Petrol: 1. Engine knock reduces car's performance. Knocking characteristics of fuels can be measured

and expressed as ‘Octane Number.’ 2. Octane number is the measure of its ability to resist knocking. Octane ratings give an

indication of the tendency of fuel to auto-ignite. Lower the octane number, higher the chances of auto-ignition. Thus, high-octane fuels are desirable.

3. Iso-octane (C8H18) has very good combustion characteristics and exhibit little tendency to detonate when mixed with air and ignited at high temperatures. Hence, its octane is taken as 100.

4. n-heptane (C7H16) has a higher tendency to auto-ignite and hence its octane number is taken as zero.

5. Octane number is defined as % of iso-octane in the mixture of iso-octane and n-heptane,

which just matches with knocking characteristics of petrol sample under consideration. 6. Thus, if a sample of petrol gives as much knocking as a mixture of 75 parts of iso-octane and

25 parts of n-heptane, then its octane value is taken as 75. 7. In practice, octane rating of petrol obtained in this manner does not always correspond with

the behavior of fuel when used in a car under normal conditions. Thus, fuel with an octane number of 80 may not be always superior to one always with octane number 75.

Anti-knocking agents: 1. These are compounds which help to increase the octane number of fuel and decrease the

knocking. 2. Anti-knock properties of gasoline is increased by adding lead tetramethyl ethyl lead (TEL), 3. Pb(C2H5)4 and this process of addition and mixing of such anti-knock agents is known as

doping. TEL is a colorless liquid with sweet odour having a specific gravity of 1.62.

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4. It is highly poisonous and care needs to be taken so that it does not enter the body thro’ bruise. Gasoline containing TEL is colored to indicate its poisonous nature.

5. The lead in TEL has the tendency to form PbO2 or free Pb in the cylinder which deposits on the walls of the cylinder and damages the inner walls.

6. To avoid this, a small amount of ethylene dibromide (scavenger) is added which reacts with Pb or PbO2 at 200-300C to form PbBr2.

7. PbBr2 is volatile and gets removed from IC engines along with exhaust gases of engine, but pollutes the air with lead bromide.

Unleaded Petrol: Petrol whose octane number is increased without addition of lead compounds is called unleaded petrol. Alternative method of increasing octane number of gasoline is to add high octane compounds like isopentane, isooctane, isopropyl benzene, methyl tertiary butyl ether (MTBE). MTBE is most preferred since it contains oxygen in form of ether group and supplies oxygen for the combustion of gasoline in the IC engines, thereby reducing the extent of peroxy compound formation. Further, it increases contents of molecules having branched and aromatic ring structures. Advantages of unleaded petrol: Leaded petrol cannot be used in automobile equipped with catalytic converters, since lead present in exhaust gas poisons the catalyst (Rhodium catalyst), thereby destroying active sites of the catalyst. Unleaded petrol is free from lead pollution. [VII] Diesel: It is the fraction obtained between boiling range of 250-3200C. Diesel fuel is heavier and oilier. Diesel fuel evaporates much more slowly than gasoline -- its boiling point is actually higher than the boiling point of water. Diesel fuel has a higher energy density than gasoline. Its calorific value is 11000 kcal/kg. Pros: Diesel emits very small amounts of carbon monoxide, hydrocarbons and carbon dioxide, emissions that lead to global warming. Cons: High amounts of nitrogen compounds and particulate matter (soot) are released from burning diesel fuel, which lead to acid rain, smog and poor health conditions. Diesel engine knocking: 1. Diesel engines are compression ignition engines that do not have spark plug. Fuel is

generally straight chain hydrocarbons with bp=1000-3600C. 2. In the suction stroke only air is drawn into the cylinder. In compression stroke, air is highly

compressed till its temperature is about 5000C. Now fuel is injected as spray into the engines. 3. Diesel vaporizes and attains self-ignition temperature and burns. If vaporization and

combustion are instantaneous, fuel burns smoothly. 4. However, there is always a lag between vaporization and combustion called diesel lag-

ignition delay causing accumulation of diesel in vapor state and whole mixture of diesel vapor and air gets injected with explosion termed as diesel knock.

Cetane Number of diesel: 1. Knocking characteristics of diesel oil is expressed in terms of cetane number. 2. Cetane (C16H34) is a saturated hydrocarbon which has a very short ignition lag as compared

to any other diesel fuel. Hence, its cetane number is taken as 100. Combustion of diesel takes place under compression and heat conditions.

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3. Cetane number is the indicator of the readiness with which a given diesel engine undergoes compression ignition.

4. Straight-chain compounds undergo easy compression ignition and n-cetane is chosen as upper limit of cetane number = 100.

5. 1-methyl naphthalene does not easily undergo compression ignition, hence its cetane number is taken as zero.

6. Cetane number (performance of diesel engines) is defined as percentage by volume of cetane

in a mixture of cetane and 1-methyl naphthalene which exactly matches in its ignition delay characteristics with the diesel under test.

7. Cetane number can be improved by adding small amounts of substances called dopes (generally 2%) like ethyl nitrite, ethyl nitrate, isoamyl nitrate and acetone peroxide.

Gaseous Fuels: LPG: Liquified Petroleum Gas 1. Mixture of n-butane, isobutane, butylene and propane (trace of organic sulfides –

mercaptans- to identify leak) 2. It is bottled gas supplied under pressure in cylinders. 3. Calorific value = 27,800 kcal/m3 4. Used as domestic & industrial fuel 5. Odorless, colorless gas. LPG has only a very faint smell, and consequently, it is necessary to

add some odorant, so that any escaping gas can easily be detected. Ethyl Mercaptan is normally used as stenching agent for this purpose. The amount to be added should be sufficient to allow detection in atmosphere 1/5 of lower limit of flammability or odour level 2 as per IS: 4576.

6. Explosive range of 1.8% to 9.5% volume of gas in air. This is considerably narrower than other common gaseous fuels- making it hazardous gas

7. Auto-ignition temperature of LPG is around 410-5800C and hence it will not ignite on its own at normal temperature.

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CNG: Compressed Natural Gas

1. Principal constituents of natural gas: CH4=70-90%, C2H6 = 5-10%, H2=3%, CO+CO2= rest. Natural gas compressed to high pressure of 1000 Atm.

2. Calorific value of CNG is about 13000 kcal/m3 3. CNG is made by compressing natural gas (which is mainly composed of methane [CH4]), to

less than 1% of the volume it occupies at standard atmospheric pressure. 4. It is stored and distributed in hard containers at a pressure of 200–248 bar (2900–3600 psi),

usually in cylindrical or spherical shapes. Steel cylinder containing 15kg of CNG contains about 2x104 l of natural gas at 1 Atm pressure

5. No unregulated pollutants like smoke, SO2, SO3, benzene, HCHO are formed on combustion and hence CNG is considered a better fuel for automotives. Hence, CNG can be used as substitute for petrol and diesel.