1
Definition of fuels: Fuel is a combustible substance which on proper burning gives large amount of heat which can be used economically for domestic and Industrial purposes. Examples are wood, coal, petrol 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 (i) On the basis of occurrence they are of two types:
Primary Fuels: Fuels which occur in nature as such are called primary fuels. eg., 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. eg., 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
Classification Chart
2 Characteristics of Good Fuel:
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.
High Calorific value
Ignition Temperature: A good fuel should have moderate ignition temperature.
Moisture content: Should be low Non combustible matter content
Velocity of combustion: It should be moderate Nature of the products
Cost of fuel, Smoke, Control of the process Units of heat:
Calorie: The amount of energy required to warm one gram of air-free water from 15 to 16 °C at standard atmospheric pressure. Experimental values of this calorie ranges from 4.1852 J to 4.1858 J.
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:
British Thermal Unit: The amount of heat required to raise the temperature of one pound (0.45359237 kilograms) of water through one degree Fahrenheit (60-61 °F). Experimental values of 1 B.Th.U.=252cal.
Centigrade heat unit (C.H.U.): The amount of heat required to raise the temperature of one pound of water through one degree centigrade. 1Kcal=3.968B.Th.U=2.2C.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).
CH4 + 2O2 CO2 + 2H2O
steam
Cooled to 15 ⁰C and more energy is released in the process
3 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 do 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).
Since 1 part by weight of hydrogen gives nine parts by weight of water i.e.
Latent heat of steam at room temperature is 587kcal/Kg
Determination of Calorific value: Determination of calorific value of solid and nonvolatile 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.
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Corrections: For accurate results the following corrections are also incorporated:
(a) Fuse wire correction: As Mg wire is used for ignition, the heat generated by burning 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.
(C) Cooling correction: As the temperature rises above the room temperature, the loss of heat does occur due to radiation, and the highest temperature recorded will be slightly less than that obtained. A temperature correction is therefore necessary to get the correct rise in temperature.
Mass of fuel = x g
HCVof the fuel = L cal/g
Heat liberated by burning of x gm of fuel = xL cal (1) Mass of water in calorimeter = W g
Water equivalent of the calorimeter in gram = w Initial temperature of water in calorimeter = t
1Final temperature of water in calorimeter = t
2Heat absorbed by water and apparatus = (W+w)(t
2-t
1) (2) xL = (W+w)(t
2-t
1) from (1) and (2)
Calculation
L= (W+w)(t2-t1)/x cal/g
LCV = L – 0.09H*587 cal/gm
Corrections
a) Fuse wire correction b) Acid correction, tA c) Cooling Correction, tC d) Cotton thread correction, tT
(W+w)(t2-t1)
HCV or L= x cal/g
(W+w)(t2-t1+tC)-[tA+tF+tT]
L= x cal/g
DH = -144,000 Cal DH = -57,160 Cal 4 3
2 2 5 2
2 2 2 2 4
2 2 2
2 22 2
HNO O
H O N
SO H O H O SO
SO O S
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Water Equivalent of Calorimeter: In a reaction the quantity of heat that raises the temperature of some substance/body by some amount, the same quantity of heat can simultaneously raise the same temperature of a certain mass of water. The mass of water is then termed as water equivalent.
The amount of water which has the same heat capacity as that of the body/substance.
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.
If oxygen is also present, it combines with hydrogen to form H2O. Thus the hydrogen in the combined form is not available for combustion and is called fixed hydrogen.
Amount of hydrogen available for combustion = Total mass of hydrogen-hydrogen combined with oxygen.
Fixed Hydrogen = Mass of 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
body
q t
t
Substrate Calorific value
Carbon 8080
Hydrogen 34500
Sulphur 2240
1g 8g 9g O H O H2 2 2
2
1
Dulong’s formula for calculating the calorific value is given as:
Gross calorific Value (HCV)
Net Calorific value (LCV)
kg kcal O S
H
C ) 2 , 240 ] /
( 8 500 , 34 8080 100 [
1
kg kcal H
HCV
kg H kcal
HCV
/ ] 587 09
. 0 [
/ ] 100 587 [ 9
where C, H, O, S refer to % of carbon, hydrogen, oxygen and sulphur respectively.
6 Coal and its Analysis
Coal: Coal is a highly carbonaceous matter that has been formed as a result of alteration of vegetable matter (e.g. plants) under certain favorable conditions. It is chiefly composed of C, H, N and O, besides non-combustible inorganic matter.
The transformation of the vegetable debris to coal takes place in two stages:
(a) Biochemical or peat stage: During this stage, the plant materials were attacked by various microorganisms.
(b) Chemical stage or metamorphism: In this stage, the peat deposit buried under sedimentary deposits loses moisture and volatile components under the effect of high temperature and pressure.
Numerical 1
Calculate the gross and net calorific values of coal having the following Compositions Carbon=85%, Hydrogen=8%, Nitrogen=2%, ash=4%, Sulphur= 1%, Latent heat of steam=587cal/g
1
100 8080*C + 34500(H - ) + 2240*S O
GCV or HCV = 8 Kcal/kg
1
100 8080*85 + 34500(8 - ) + 2240*1 0
HCV = 8 Kcal/kg
HCV = 9650.4 kcal/kg
NCV = HCV- 0.09*H*587 kcal/kg = 9650.4 - 0.09*8*587 = 9227.8 kcal/kg
Numerical 2
0.72g of a fuel containing 80% carbon, when burnt in bomb calorimeter, increased the temperature of water from 27.3 to 29.1 ⁰C. if the calorimeter Contains 250g of water and its water equivalent is 150g, calculate the HCV of the fuel. Give your answer in kJ/Kg.
X=0.72g, W=250g, w=150g, t2=29.1 ⁰ C, t1=27.3 ⁰C
(W+w)(t2-t1)
L= x cal/g
(250+150)(29.1-27.3) 0.72
HCV= cal/g
HCV= 1000*4.2 J/g or kJ/kg =4200 kJ/kg
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Classification of Coal: Various types of coal commonly recognized on the basis of rank or degree of alteration or coalification from the parent material, wood are:
This progressive transformation of wood to anthracite results in:
1. Decrease in moisture content
2. Decrease in H, N, O, and S content with a corresponding increase in C content 3. Decrease in volatile content
4. Increase in calorific value 5. Increase in hardness
1. Peat: is a fibrous jelly like mass. It is regarded as the first stage in the coalification of wood. It is uneconomical fuel, it may contain as much as 80-90% of water but on air-drying, and it burns freely.
Its calorific value is about 5400kcal/kg (on air-dry basis).
2. Lignite (brown coal): are soft brown colored variety of lowest rank coal which consists of vegetable matter decomposed more that in peat. Lignite is compact in texture, containing 20-60% moisture and on air-drying, it breaks up into small pieces. The calorific value is about 6500-7100 kcal/kg.
3. Bituminous coal: are pitch-black to dark-grey coals, which usually soil hands. They show a laminated structure of alternate very bright and dull layers. Its calorific value on ash-free basis is about 8000-8500Kcal/kg.
4. Anthracite: is the highest rank coal, containing highest percentage of carbon (92-98%) and has the lowest volatile matter and moisture contents. They are hardest of all kinds of coals, quite dense and
lustrous in appearance. The calorific value is about is about 8600-8700 Kcal/kg.
Coal analysis: Coal is analyzed in two ways:
1. Proximate analysis 2. Ultimate analysis
Proximate Analysis: Parameters include moisture, volatile matter, ash, and fixed carbon. It provides valuable information in assessing the quality of coal.
Moisture Content: It is an important property of coal, as all coals are mined wet. Groundwater and other extraneous moisture is known as adventitious moisture and is readily evaporated. Moisture held within the coal itself is known as inherent moisture and is analyzed quantitatively.
1g of finely powdered air-dried coal sample is taken in a crucible and heated for 1hour inside a hot air oven maintained at 105- 110⁰C. The sample is then cooled in a desiccator and weighed. Loss in weight is reported as moisture
Wood Peat Lignite Bituminous coal Anthracite
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Volatile Matter: It refers to the components of coal, except for moisture, which are liberated at high temperature in the absence of air. This is usually a mixture of short and long chain hydrocarbons, aromatic hydrocarbons and some sulfur.
The dried sample of coal in (1) is then covered with a lid and placed in an electric furnace (muffle furnace), maintained at 925 ± 20 ⁰C. The crucible is then taken out of the oven after 7 minutes of heating.
The crucible is first cooled in air then in desiccator and loss of weight is measured.
Ash content of coal is the non-combustible residue left after coal is burnt. It represents the bulk mineral matter after carbon, oxygen, sulfur and water (including from clays) has been driven off during combustion.
The residual coal in the crucible (2) is then heated without a lid in a muffle furnace at 700±50 ⁰C for 30 minutes. The crucible is taken out cooled in air first, then in desiccator and weighed. Heating, cooling and weighing is repeated till a constant weight is obtained.
Fixed carbon: The fixed carbon content of the coal is the carbon found in the material which is left after volatile materials are driven off. This differs from the ultimate carbon content of the coal because some carbon is lost in hydrocarbons with the volatiles. Fixed carbon is used as an estimate of the amount of coke that will be yielded from a sample of coal. Fixed carbon is determined by removing the mass of volatiles determined by the volatility test, above, from the original mass of the coal sample.
Percentage of moisture = Loss in weight
Weight of coal taken * 100 (1)
% of volatile matter = Loss in weight due to removal of volatile matter *100
Weight of coal taken (2)
Percentage of ash = weight of ash left
Weight of coal taken * 100
Fixed carbon = 100 – % of (moisture + volatile matter + ash)
9 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: about 1-2g of accurately weighed coal sample is burnt in a current of oxygen in
a combustion apparatus. C and H of the coal are converted CO2 and H2O respectively. The gaseous products of combustion are absorbed respectively in KOH and CaCl2 tubes of known weights. The
increase in weights is measured and percentage of C and H is determined.
Importance of proximate analysis Moisture:
lowers the effective calorific value of the coal
Quenches the fire in furnace
Increase the transportation charges Volatile matter:
Large proportion of volatile matter escapes without burning
Decreases the calorific value
Increases the flame size Ash:
Useless, non combustible matter which reduces the calorific value of coal
Causes hindrance to flow of air and heat, therby lowering the temperature
It forms clinker, which block the interspaces of the grate.
Increases the transportation charges
Wear of furnace walls and burning apparatus
Fixed carbon: Higher the percentage of fixed carbon, greater is its calorific value and better the quality of coal.
Percentage of C = increase in weight of KOH tube * 12 * 100 Weight of coal sample taken * 44 C + O2 CO2
12 44
2KOH + CO2 K2CO3 + H2O
Percentage of H = increase in weight of CaCl2tube * 2 * 100 weight of coal sample taken * 18
H2 + ½ O2 H2O
CaCl2 + 7H2O CaCl2.7H2O
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Nitrogen: About 1g of accurately weighed powdered coal is heated with concentrated H2SO4 along with K2SO4 (catalyst) in a long necked flask (called Kjeldahl’s flask). After the solution becomes clear, it is treated with excess NaOH and the liberated ammonia is distilled over and absorbed in a known volume of standard acid solution. The unused acid is then determined by back titration with standard acid solution.
The consumed acid is related to N in coal as follows.
Sulphur: is determined from the washings obtained from a known mass of coal, used in a bomb calorimeter for determination of a calorific value. During this determination, S is converted into sulphate.
The washings are treated with barium chloride solution and barium sulphate is precipitated. This precipitate is filtered, washed and heated to a constant weight.
Oxygen: it is obtained by difference percentage of O = 100 - % of (C + H + S + N + ash)
Importance of ultimate analysis
Carbon and Hydrogen: greater is the percentage of C and H, better is the coal in quality and calorific value. Higher % of C reduces the size of the combustion chamber required.
Nitrogen:has no calorific value and hence its presence is undesirable.
Sulphur: contributes to calorific value nevertheless it is undesirable as its combustion products (acids from SO2 and SO3) have harmful effects of corroding the equipment's and also cause atmospheric pollution.
Oxygen: it decreases the calorific value and also results in low coking power.
Percentage of N = Volume of the acid used(ml) * Normality * 1.4 weight of coal sample taken
The sample is first digested in strong sulphuric acid in the presence of catalyst
1. Nitrogenous organic compound + conc. H2SO4 (NH4)2SO4
2. (NH4)2SO4+ 2NaOH 2NH3 + Na2SO4 + 2H2O 3. NH3 + HCl NH4Cl + HCl (left back)
Percentage of S = weight of BaSO4 obtained * 32 * 100 weight of coal sample taken * 233 Oxygen: it is obtained by difference
Percentage of O = 100 - % of ( C + H + S + N + ash)
11 Problems
1. Calculate the weight and volume of air required for the combustion of 3kg of Carbon.
2. Calculate the mass of the air needed for the complete combustion of 5kg of coal in which C=80%, H=15% and O=5%.
3. A coal sample was found to contain: C=66.2%, H=4.2%, O=6.1%, N= 1.4%, S=2.9%, moisture=9.7% and ash =9.5% by weight. Calculate the quantity of dry products of combustion, if 1kg of coal coal is burnt with 25% excess air.
Petroleum: The term petroleum means rock oil. It is also called mineral oil.
Petroleum or crude oil is dark greenish-brown, viscous oil found in deep earth’s crust. It is mainly composed of various hydrocarbons (like paraffin, cycloparaffins, naphthalenes, olefins and aromatics), together with small amount of organic compounds containing oxygen, nitrogen and sulphur. The oil is usually found floating upon a layer of brine and has layer of gas on top of it.
Composition:
12 Fraction of petroleum and their uses:
Name of fraction Boiling Range C-contents Applications
Uncondensed Gases Below 30 0C C1-C4 Domestic and industrial uses as L.P.G.
Petroleum Ether 30-70 0C C5-C7 As a solvent
Gasoline/Petrol 40-120 0C C5-C9 Solvent, motor fuel, dry-cleaning Naphtha/solvent spirit 120-180 0C C9-C10 Solvent and dry-cleaning
Kerosene oil 180-250 0C C10-C16 Illuminating oil, jet engine fuel Diesel oil/gas oil/fuel oil 250-320 0C C10-C18 Diesel engine fuel
Heavy oil
1. Lubricating oil 2. Petroleum jelly 3. Grease
4. Paraffin waxes
320-400 0C C17-C30 On cracking yield gasoline
Lubricant
Vaseline, lubricating, cosmetic, medicine
Lubricant
Candle, boot polish, wax paper Residue
1. Asphalt
2. Petroleum coke
Above 400 0C C30- above
Roof proofing, road construction
Fuel, moulding arc light rods
Fractional distillation of crude oil and Uses: It is done in tall fractionating tower or column made up of steel. In continuous process, the crude oil is preheated to 350-380 oC in specially designed tubular furnace known as pipe still.
13 Important fractions of petroleum:
1. Gasoline or petrol:
It is obtained between 40-120 ⁰C
Mixture of hydrocarbons such as C5H12 (pentane) to C8H18 (octane).
Composition C= ~ 84%, H= ~ 15%, N+S+O= ~ 1%.
Its calorific value is about 11,250kcal/kg.
It is volatile, inflammable and used as a fuel for internal combustion engines.
2. Diesel oil
Its fraction obtained between 150-350 ⁰C
It is a mixture of C15H32 to C18H38 hydrocarbons.
Its density is 0.86 to 0.95.
Its calorific value is about 11,000kcal/kg and is used as diesel engine fuel.
The components of diesel fuels are straight run fractions containing paraffinic and naphthenic hydrocarbons, naphtha and cracked gas oils.
3. Lubrication oil:
Length of the hydrocarbon chain varies between 12 to 50 carbon atoms.
The shorter chain oil has lower viscosity than the longer chain hydrocarbons.
These are most widely used as lubricants because they are cheap and quite stable under service condition.
They possess poor oiliness as compared to that of animal and vegetable oils. The oiliness of petroleum oils can be increased by the addition of high molecular weights compounds like oleic acid, stearic acid etc.
Cracking: Cracking is the process by which heavier fractions are converted into lighter fractions by the application of heat, with or without catalyst. Cracking involves the rupture of C-C and C-H bonds in the chains of high molecular weight hydrocarbons.
1. Thermal cracking: the heavy oil are subjected to high temperature (~500 ⁰C) and pressure (~100kg/cm2), when the bigger hydrocarbons molecules break down to give smaller molecules of the paraffin, olefins plus some hydrogen.
C10H22 cracking C5H12 + C5H10
pentane pentene
B.p=174 ⁰C
B.p=36 ⁰C
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(a) Liquid Phase thermal cracking: the heavy oil is cracked at a suitable temperature of 475-530 ⁰C and under a pressure of 100 kg/cm2. The cracked products are then separated in a fractionating column.
The yield is 50-60 %.
(b) Vapour phase thermal cracking: Cracking oil is first vaporized and then cracked at about 600-650
⁰C and under a low pressure of 10-20kg/cm2. This process is suitable for those oils which may be readily vaporized. It requires less time than liquid-phase method.
2. Catalytic cracking: A suitable catalyst like aluminum silicate, Al2(SiO3)2 or alumina (Al2O3) is used to break the high molecular weight hydrocarbons. The use of catalyst reduces the required temperature (420-450 ⁰C) and pressure (1-5kg/cm2) of the cracking. The yield of the petrol is higher and quality of the petrol produced is better.
Advantages of Catalytic cracking:
• The yield of petrol is higher
• Quality of petrol is better
• A much lower pressure is needed
• The product contains less amount of sulphur
• The product contains higher fraction of aromatic thus better antiknock characteristics
Fixed bed catalytic cracking: The oil vapours are heated in pre-heater to cracking temperature (420-450
OC) and then forced through a catalytic chamber (containing artificial clay mixed with zirconium oxide) maintained at 425-450 OC and 1.5 Kg/cm2 pressure. During their passage through the tower, about 40% of the charge is converted into gasoline and about 2-4% carbon is formed. The later get adsorbed on catalytic bed. The vapours produced are then passed through fractionating column, where heavy oil fractions condensed. The vapours are then led through a cooler, where some of the gases are condensed along-with gasoline and uncondensed gases move on. The gasoline containing some gases is then sent to a stabilizer where the dissolved gasses are removed and pure gasoline is obtained.
15 Synthetic Petrol:
Synthetic fuel or synfuel is a liquid fuel obtained from coal, natural gases, oil shales or biomass.
It may also refer to fuels derived from other solids such as plastics or rubber waste.
Common use of the term "synthetic fuel" is to describe fuels manufactured via Fischer Tropsch conversion, methanol to gasoline conversion, or direct coal liquefaction.
Methods for producing synthetic petrol Fischer-Tropsch method
Bergius process
Fischer-Tropsch method
The Fischer–Tropsch process involves a series of chemical reactions that produce a variety of hydrocarbons, ideally having the formula (CnH(2n+2)). The more useful reactions produce alkanes as follows:
(2n + 1) H2 + n CO → CnH(2n+2) + n H2O
where n is typically 10-20. The formation of methane (n = 1) is unwanted. Most of the alkanes produced tend to be straight-chain, suitable as diesel fuel. In addition to alkane formation, competing reactions give small amounts of alkenes, as well as alcohols and other oxygenated hydrocarbons.
2n H2 + n CO → CnH2n + n H2O
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Working Process: Water gas (CO+H2) is mixed with H2 and purified by passing through Fe2O3 (to remove H2S) and then into mixture of Fe2O3.Na2CO3 (to remove organo sulphur compounds). The purified gas is then compressed (~5-25 atm) and then led through a convertor (containing catalyst) maintained at about 200-300 ⁰C. A mixture of saturated and unsaturated hydrocarbons are formed.
(2n + 1) H2 + n CO → CnH(2n+2) + n H2O 2n H2 + n CO → CnH2n + n H2O
The reaction is exothermic, so out coming gaseous mixture is led to a cooler, where the liquid resembling to crude oil is obtained which is then fractionated to give gasoline and high boiling heavy oil.
Bergius process:
One of the main methods of direct conversion of coal to liquids by hydrogenation process is the Bergius process. The reaction can be summarized as follows:
Working Mechanism: The low ash coal is finely powdered and made into a paste with heavy oil and then a catalyst (composed of tin or nickel oleate) is incorporated. The whole is heated with H2 at 450 ⁰C and under a pressure of 200-250 atm for about 1.5 hours, during which H2 combines with coal to form saturated hydrocarbons, which decompose at prevailing temperature and pressure to yield low boiling liquid hydrocarbons. The issuing gases are led to condenser , where a liquid resembling crude is obtained, which is then fractionated to give (1) gasoline, (2) middle oil, (3) heavy oil. The heavy oil is again used for making paste with fresh coal dust. The middle oil is hydrogenated in vapor phase in the presence of catalyst to yield more gasoline. The yield of gasoline is about 60% of the coal dust used.
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Numerical: A petrol sample contains 84% Carbon and 16% hydrogen by weight. Its flue gas compositional data by volume is as under: CO2=12.1%, CO=1.1%, oxygen=1.3%, nitrogen=85.5%.
Calculate weight of minimum air needed for complete combustion of 1Kg of petrol.
GASEOUS FUELS: LPG and CNG:
Gaseous fuels are the most convenient because they require the least amount of handling and are used in the simplest and most maintenance-free burner systems. Gas is delivered "on tap" via a distribution network and so is suited for areas with a high population or industrial density. However, large individual consumers do have gasholders and some produce their own gas
Advantages of Gaseous Fuel Operation 1. Minimum carbon formation.
2. Less sludge in oil.
3. Less valve burning.
4. No wash down bf cylinder wall lubrication during engine starting.
5. No tetra-ethyl lead to foul spark plugs and other engine parts.
6. Environmental benefits: lowest GHG and other emissions 7. Small amount of contaminating residues.
8. A nearly homogeneous mixture in cylinder Classification of gaseous fuels
(A) Fuels naturally found in nature Natural gas
Methane from coal mines (B) Fuel gases made from solid fuel
Gases derived from coal
Gases derived from waste and biomass and from other industrial processes (C) Gases made from petroleum
Liquefied Petroleum gas (LPG) Refinery gases
Gases from oil gasification (D) Gases from some fermentation
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Typical physical and chemical properties of various gaseous fuels are given in this table.
Liquefied Petroleum Gas (LPG):
LPG is a predominant mixture of propane and butane with a small percentage of unsaturated and some lighter C2 as well as heavier C5 fractions.
LPG may be defined as those hydrocarbons, which are gaseous at normal atmospheric pressure but may be condensed to the liquid state at normal temperature by moderate pressures. Although they are normally used as gases, they are stored and transported as liquids under pressure for convenience and ease of handling.
LPG vapour is denser than air. Butane is about twice as heavy as air and propane about one and a half time as heavy as air. Consequently, the vapour may flow along the ground and into drains sinking to the lowest level of the surroundings and be ignited at a considerable distance from the source of leakage.
LPG Applications
Heating: LPG is used an alternative to heating oil and electricity in places where there is no natural gas pipe line.
Cooking: LPG is the most common cooking fuel in India and urban Brazil.
Refrigeration: Gas absorption refrigerators and air conditioning systems use LPG. But its use in motor vehicles for air conditioning has been discouraged due to the risk of fire.
Automobiles: A large number of vehicles, light, medium and heavy duty, around the world are fueled by LPG.
Compressed natural gas (CNG):
It is a fossil fuel substitute for gasoline (petrol), diesel, or propane/LPG. Although its combustion does produce greenhouse gases, it is a more environmentally clean alternative to those fuels, and it is much safer than other fuels in the event of a spill (natural gas is lighter than air, and disperses quickly when released). CNG may also be mixed with biogas, produced from landfills or wastewater, which doesn't increase the concentration of carbon in the atmosphere.
Fuel Gas Relative Density
Higher Heating Value kCal/Nm3
Air/Fuel ratio m3/m3
Flame Temp oC
Flame speed m/s
Natural Gas
0.6 9350 10 1954 0.290
Propane 1.52 22200 25 1967 0.460
Butane 1.96 28500 32 1973 0.870
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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.
CNG Applications:
Automobiles: Any vehicle running on gasoline can be converted to a bi-fuel vehicle (gasoline/CNG). It involves installing a CNG cylinder in the trunk, installing the plumbing system, installing CNG injunction system and the electronics.
Locomotives: Iran, Pakistan, Argentina, Brazil and India have the highest number of CNG vehicles run.
Advantages of using CNG and LPG in vehicles
Lead or benzene is not used. Hence lead fouling of spark plugs is eliminated.
Natural gas vehicles have lower maintenance costs when compared with other fossil fuel-powered vehicles.
The fuel systems are sealed. This prevents any spill or evaporation losses.
CNG does not contaminate or dilute the crank case oil thus increasing the life of lubricating oil.
The emission of greenhouse gases is reduced 80% when compared to gasoline vehicles.
Natural gas disperses easily in air and is not flammable. Hence CNG vehicles are safer than gasoline vehicles.
CNG is cheaper than gasoline or diesel.
LPG
LPG is cheaper than diesel or gasoline.
LPG vehicles have lower maintenance costs.
It has lower emission than gasoline or diesel.
It is non-toxic and non-corrosive.
It requires lesser space than CNG for storage.
20 Lubricants and lubrication:
Lubricants: Any substance introduced between two moving/sliding surfaces with a view to reduce the frictional resistance between them, is known as lubricant. The main purpose of lubricant is to keep the sliding/moving surfaces apart.
Lubrication: the process of reducing frictional resistance between moving/sliding surfaces by the introduction of lubricants in-between them.
Function of lubricants:
1. It reduces surface deformation, wear and tear, because the direct contact between the rubbing surfaces is avoided
2. It reduces the loss of energy in the form of heat 3. It reduces expansion of metal by local frictional heat.
4. It avoids seizure of moving surfaces
5. It reduces running and maintenance cost of the machine 6. It also sometimes acts as a seal
7. It avoids unsmooth relative motion
Mechanisms of lubrication: Basically follows three mechanisms;
1. Fluid-film or thick-film or hydrodynamic lubrication:
The lubricant Film fills the irregularities of the sliding/moving surfaces and forms a thick layer (~1000Å) in-between them, so that there is no direct contact between the material surfaces. This consequently reduces the wear. The resistance to movement is only due the internal resistance between the particles of the lubricant moving over each other. The lubricant chosen should have the minimum viscosity under working conditions and at the same time it should remain in place and separate the surfaces.
Hydrocarbon oils (12-50 carbon atoms) are considered to be satisfactory lubricants for thick- film lubrication. In order to maintain the viscosity of the oil in all seasons of year, ordinary hydrocarbon lubricants are blended with selected long chain polymers
2. Thin film lubrication: This type of lubrication is preferred where a continuous film of lubricant cannot persist. In such cases, the clearance space between the moving/sliding surfaces is lubricated by such a material which can get adsorbed on both the metallic surfaces by either physical or chemical
Thick layer of lubricants
21
forces. This adsorbed film helps to keep the metal surfaces away from each other at least up to a height of the peaks present on the surface.
Vegetable and animal oils and their soaps can be used in this type of lubrication.
Although these oils have good oiliness, they suffer from the disadvantage that they will break down at high temperatures. On the other hand, mineral oils are thermally stable and the addition of vegetable/animal oils to mineral oils, their oiliness can also be brought up. Graphite and molybdenum disulphide are also suitable for thin film lubrication
3. Extreme pressure lubrication: When the moving/sliding surfaces are under very high pressure and speed, a high local temperature is attained under such conditions, liquid lubricants fails to stick and may decompose and even vaporize. To meet these extreme conditions, special additives are added to mineral oils. These are called extreme pressure additives. These additives form more durable films on metal surfaces. Important additives are organic compounds having active radicals or groups such as chlorine (as in chlorinated esters), sulphur (as in sulphurized oils) or phosphorous (as in tricrecylphosphate). These compounds react with metallic surfaces, at existing high temperatures, to form metallic chlorides, sulphides and phosphides.
Classification of lubricants
Lubricants can be broadly classified, on the basis of their physical state as follows:
1. Liquid lubricants and lubricating oils 2. Semi-solid lubricants or greases, 3. Solid lubricants
1. Liquid lubricants and lubricating oils: liquid lubricants are further classified into three categories;
a) animal and vegetable oils, b) Mineral or petroleum oils c) Blended oils
a) animal and vegetable oils:
animal oils are extracted from the crude fat
vegetable oils such as cotton seed oil and caster oils Advantages
These oils possess good oiliness and hence they can stick on metal surfaces effectively even under elevated temperatures and heavy loads.
22 Disadvantages
costly,
undergo easy oxidation to give gummy products
hydrolyze easily on contact with moist air or water Hence they are only rarely used these days for lubrication.
b) Mineral or petroleum oils:
Lower molecular weight hydrocarbons (C-12 to C-50) Advantages
cheap
available in abundance
stable under service conditions hence they are widely used.
Disadvantages
But the oiliness of mineral oils is less, so the addition of higher molecular weight compounds like oleic acid and stearic acid increases the oiliness of mineral oil.
(c) Blended oils: no single oil possesses all the properties required for a good lubricant and hence the addition of proper additives is essential to make perform well. Such additives added lubricating oils are called blended oils.
Example: the addition of higher molecular weight compounds like oleic acid, stearic acid, palmitic acid, etc. or vegetables oil like coconut oil, castor oil, etc increases the oiliness of mineral oil.
Additives in Blended oils
(1) Oiliness carriers: the addition of higher molecular weight compounds like oleic acid, stearic acid, palmitic acid, etc. or vegetables oil like coconut oil, castor oil, etc increases the oiliness of mineral oil.
(2) Extreme Pressure Additives: Under extreme pressure, a thick film of oil is difficult to maintain.
High pressure additives contain materials which are either adsorbed on the metal surface or react chemically with the metal, producing a surface layer of low shear strength on the metal surface.
Examples: fatty ester, acids, organic materials which contain sulphur, organic chlorine compounds, organic phosophorous compounds etc.
(3) Thickeners: such as polystyrene, polyesters etc., are materials of high molecular weight between 300 to 3000. They are added in lubricating oil to increase the viscosity.
polystyrene
23
(4) Antioxidants or inhibitors: When added to the oil retard oxidation of the oil by getting themselves preferentially oxidized. They are particularly added in lubricants used for internal combustion engines. The antioxidants are aromatic phenols, or amino compounds.
(5) Foam Inhibitors: Foaming of lubricants is a very undesirable effect. It can cause enhanced oxidation by the intensive mixture with air.
( 2.) Semi-solid lubricants or grease: Obtained by combining oil with thickening agents.
Lubricating oil: Petroleum oil low to high viscosity Thickeners: soaps of Li, Na, Ca, Ba, Al, etc.
Non-soap thickeners include carbon black, silica gel, polyureas and other synthetic polymers, clays etc.
Grease can support much heavier load at lower speed.
Internal resistance of grease is much higher than that of lubricating oil
It cannot effectively dissipate heat from the bearings, so work at relatively lower temperature.
Classification of grease:
Greases are classified after the soap used in their manufacture.
1. Calcium based greases or cup greases: petroleum oils with calcium soaps. They are prepared by adding calcium hydroxide to hot oil.
They are insoluble in water and are the cheapest 2. Soda based grease: Petroleum oils with sodium soaps.
They are not water resistant, because the sodium soap content is water soluble. They are suitable in ball bearings.
3. Lithium based greases: Petroleum oils thickened by mixing lithium soaps.
They are water resistant and suitable for use at low temperatures.
4. Axle grease: Prepared by adding lime to resin and fatty oils. The mixture is thoroughly mixed and allowed to stand, when grease floats as stiff mass. Fillers like talc and mica also added to them. They are water resistant.
Dimethylsilicones (dimethylsiloxanes)
24 (3) Solid lubricants: They are preferred where;
(1) the operating conditions are such that lubricating film cannot be secured by the use of lubricating oils or grease.
(2) contamination of lubricating oil or grease is unacceptable.
(3) the operating temperature or load is too high.
(4) combustible must be avoided.
They are available as dispersions in non-volatile carriers like soaps, fats, waxes, etc and as soft metal films.
Examples: graphite, molybdenum disulphide, tungten disulphide and zinc oxide.
They can withstand temperatures up to 650 ⁰C. They are also used as additives to mineral oils and greases in order to increase the load carrying capacity of the lubricant.
Graphite: Most common, used either in the powdered form or in suspension. 1. Soapy to touch; 2. Non- flammable and stable upto a temperature of 375 ⁰C.
Graphite has a plate like structure and the layers of graphite sheets are arranged one above the other and held together by van der Waal’s forces. These parallel layers which can easily slide one over other make graphite an effective lubricant. Also the layer of graphite has a tendency to absorb oil.
Molybdenum disulphite: It has a sandwich like structure with a layer of molybdenum atoms in between two layers of sulphur atoms. Poor interlaminar attraction helps these layers to slide over one another easily. It is stable up to a temperature of 400 ⁰C.
25 Selection of lubricants
Selection of lubricants: In selecting a lubricant for a particular job, the service condition requirements are to be related to the properties of the lubricant. The properties of a properly selected lubricant should not change under service conditions. Selection of a lubricant for a few typical jobs is illustrated as follows:
Lubricants for cutting tools: Cutting fluids are lubricants used in cutting, turning, and grinding of metals. The main functions of cutting fluids are: a) to cool the tool, b) to cool the metal work-piece so as to prevent dimensional inaccuracies, c) to reduce power consumption by lubrication action and d) to improve surface finish.
Two situations can be there in cutting tool
For heavy cutting: the most effective lubricants are cutting oils. The cutting oils are essentially mineral oils of low viscosities containing additives like fatty oils, sulphurized fatty oils which by virtue of their polar groups attach themselves to the surface of continuously exposed fresh metals.
In light cutting: the most effective lubricants are oil-emulsions. Oil-emulsions have somewhat smaller lubricating effect than cutting oils, but they are more efficient as cooling media, due to high heat capacity of water, which is present in them as an external phase.
Lubricants for internal combustion engines:
In internal combustion engines, the lubricant is to be exposed to high temperatures. Therefore the lubricant should possess high viscosity index (i.e. low variation of viscosity with temperature) and high thermal stability. Petroleum oils containing additives, which impart high viscosity index and oxidation stability to them, are used as lubricants for internal combustion engines.
Lubricants for gears:
They are subjected to extreme pressures. i) they should possess good oiliness, ii) not to be removed by centrifugal force, iii) possess high resistance to oxidation, iv) have high load carrying capacity. Thick mineral oils containing extreme pressure additives are employed.
Lubricants for transformers:
The functions of the lubricating oil in the transformer are to insulate the windings and to carry away the heat generated. The oil must possess good dielectric properties, chemically inert and low viscosity.
Highly refined mineral oils of high insulating quality, optimum oxidation resistance and chemical stability are employed.
