“A revolution in humanity‚s use of fossil fuel-based energy would be necessary sooner or later to sustain and to extend modern standards of living.”
‘Fuel is the source of heat energy, it can be stored as potential chemical energy and can be released through combustion.‚
‘Combustible matter having carbon as a major ingredient, produce large amount of heat energy on burning and can be used for heat generation in industry and domestic applications is known as a fuel.‚
‘Any compound or a substance which can produce energy and can be used in the production of power is termed as a fuel.‚
In the combustion process, a fuel reacts with oxygen and releases the energy.
Fuel + O2 → CO2 + H2O + heat
Fuels are broadly classified according to their occurrence and state of aggregation. According to the occurrence they are classified as primary (natural) or secondary (derived) fuels and based on the state of aggregation solid, liquid and gaseous fuels.
1 kcal = 1,000 cal
1 BTU = 252 cal = 0.252 kcal
1kcal = 3.968 BTU
1 kcal = 3.968 BTU = 2.2 CHU
The total quantity of heat liberated by the complete combustion of one unit mass/volume of fuel in oxygen is known as calorific value. This is mainly divided into higher calorific value and lower calorific value.
LCV = HCV – latent heat of water vapour
LCV = HCV – mass of hydrogen × 9 × latent heat of steam
This is because one part by mass of hydrogen gives nine parts by mass of water.
A calorimeter is used for determining calorific value. For determining calorific value of solid and liquid fuels a bomb calorimeter is used and for gaseous fuel Junker‚s calorimeter is used.
Description
Bomb calorimeter consists of strong cylindrical stainless steel bomb with lid. The bomb carries the fuel, and the lid can be screwed to the body of the bomb and make a perfect gas tight seal. The lid has two stainless steel electrodes and an oxygen inlet valve, and among these a small ring is attached to one of the electrodes. A nickel or stainless steel crucible is supported by that right. The bomb is placed in a copper colorimeter, for preventing heat loss by radiation, it is surrounded by air and water jacket. Stirrer which can operated electrically and Beckmann‚s thermometer, having sensitivity to read up to 0.01°C are provided. The set-up is shown in Figure 5.1.
Figure 5.1 Bomb calorimeter
Working
In the clean crucible, a weighted amount (0.5 to 1.0 g) of the fuel is taken and the crucible is supported by a ring, a fine magnesium wire touching the fuel sample is stretched across the electrodes. The bomb lid is tightly screwed, filled with oxygen to 25 atm pressure and then lowered into copper calorimeter, containing known mass of water, and the initial temperature (t1) is noted. Now, the circuit is completed by connecting the electrodes with a 6 V battery. The sample burns, liberates heat and absorbed by water. The water is stirred continuously for maintaining uniform temperature, and hence the final temperature (t2) is noted.
Observations and Calculation
Weight of fuel = x g
Weight of water in calorimeter = wωg
Water equivalent of calorimeter = wt. of apparatus × specific heat = w g
Initial temperature of water = t °C
Final temperature of water = t2 °C
High calorific value of the fuel
Lower calorific value of the fuel
LCV = HCV – 0.09 H × 587 cal/g
Latent of heat of steam = 587 cal/g
Weight of water produced from 1 g of fuel = 9H/100 g = 0.09H g
H = percentage of hydrogen in fuel.
Junker‚s gas calorimeter (Figure 5.2) consists of a vertical cylindrical combustion chamber, and the pressure governor regulates the supply of gaseous fuel. Geometer measures the volume of gas flowing in a particular time and combustion of fuel can be carried out by a Bumen‚s burner. The combustion chamber is surrounded by an annular water space, inside heat exchange coils and outer flues are fitted. Chromium plated outer jacket which prevent the radiative and convective heat loss from calorimeters because it contains air and acts as a very good insulator. Openings of annular space can circulate the water at the appropriate places at constant rate around the combustion chamber. Two thermometers placed at appropriate place can measure the temperatures of the inlet and outlet water.
Figure 5.2 Junker‚s gas calorimeter
In the combustion chamber a known volume of gas is burned at a constant rate in excess of air, produced heat is absorbed by water. From the temperature difference, heat evolved from the gas can be calculated.
Observations and Calculation
Volume of the gas burnt in ‘t‚ at STP = V
Temperature of incoming water = T1
Temperature of outgoing water = T2
Weight of water collected in time t = w
High calorific value
Mass of steam condensed in time ‘t‚ in graduated cylinder from V m3 of gas = m
Latent heat of steam = 587 cal/kg
Lower calorific value
Solution Gross Calorific Value (GCV)
Net Calorific Value (NCV) = (GCV – 0.09H × 587) k cal/kg
= (9650.4 – 0.09 × 8 × 587) k cal/kg = 9227.8 k cal.kg
Solution HCV = (HCV + 0.09H × 587) k cal/kg
= (8490.5 + 0.09H × 587) k cal/kg
= (8490.5 + 52.8H) k cal/kg(i)
Also (ii)
From (i) and (ii), we get
7754.8 + 345H = 8490.5 + 52.8 H
or 292.2H = 8490.5 – 7154.8 = 1335.7
or percentage of H = 1335.7/292.2 = 4.575% (iii)
HCV = (8490.5 + 52.8 × 4.575) k cal/kg [From (i) and (iii)]
= (8490.5 + 241.3) k cal/kg = 8731.8 k cal/kg
Solution Here x = 0.72 gm, W = 250 gm, w = 150 gm, t1 = 273°C, t2 = 29.1°C
Solution Here, wt. of fuel (x) = 0.83 g; wt of water (W) = 3500 g; water equivalent of calorimeter (w) = 385 g; (t2 – t1) = 2.7°C; percentage of hydrogen (H) = 0.7%; latent heat of steam = 587 cal/g.
∴ Gross calorific value
Net calorific value = [GCV – 0.09H × 587] = (12683 – 0.09 × 0.7 × 587) cal/g
= (12683 – 37) cal/gm = 12601 cal/gm
Mass of the coal = 0.6 g
Water equivalent of calorimeter = 2200 gm
Specific heat of water = 4.187 kJ kg–1 °C–1
Rise in temperature = 6.52 °C
Solution Heat liberated by burning 0.6 g coal
= 3.3 kg × 4.187 kJ kg–1 °C–1 × 6.52 °C = 60.06 kJ
∴ Calorific value of coal = 60.06 kJ/0.6 g = 100.1 kJ g–1.
Calculate grass and net calorific values of the coal, assuming the latent heat of condensation of steam as 580 cal/g.
Solution Wt. of coal sample (x) = 0.92 g: wt. of water (W) = 550 g: water equivalent of calorimeter (w) = 2200 g: temperature rise (t2 – t1) = 2.42 °C; acid correction = 50.0 cal, fuse wire correction = 10.0 cal; latent heat of steam = 580 cal/g percentage of H = 6%
Important characteristics of a good fuel are listed hereunder.
Coal and coke are main solid fuels.
Coal is the primary and largest solid fuel used to produce electricity and heat through combustion. Black or brownish black sedimentary rock usually occur as coal beds, composed primarily of carbon along with other elements like hydrogen, oxygen, nitrogen and sulphur, also known as pulverised carbon.
Due to biogeological processes, from the dead plant matter and vegetation fossil fuel coal is formed, and is slowly converted into peat, lignite, bituminous coal and finally to anthracite.
According to carbon and hydrogen ratio, ranking of coal in increasing order is as follows
Proximate and ultimate analysis is carried out to assess and determine the quality of coal.
Practical utility of coal is determined by the proximate analysis. Here, information is obtained regarding moisture, volatile matter, ash and fixed carbon content.
Good quality of coal has more fixed carbon.
Elemental analysis of coal is done by ultimate analysis, and with this analysis carbon, hydrogen, nitrogen, oxygen, sulphur and ash content are determined based on the following procedure.
(12 parts by mass of carbon gives 44 parts by mass of carbon dioxide)
(2 parts by mass of hydrogen gives 18 parts by mass of water)
Solution Mass of moisture in coal sample = 2.500 – 2.415
= 0.085 g
Mass of volatile matter = 2.451 – 1.528
= 0.887 g
Mass of ash = 0.245 gm
Percentage of moisture = = 3.400%
Percent of volatile matter = = 35.48%
Percent of ash = = 9.8%
Percent of fixed carbon = (100 – (3.4 + 35.48 – 9.80))
= 51.32%
C – 80%, H = 15%, O = rest.
Solution 5 kg of coal contains: C = 4 kg; H = 0.75 kg; O = (5 – 4 – 0.75) kg = 0.25 kg
∴ Amount of air required for complete combustion of 5 kg coal
= [5 × (32/12) + 0.75 × (16/2) – 0.25] kg × (100/23)
= [13.333 + 6.000 – 0.25] kg × (100/23) = 82.97 kg
∴ wt. of air reqd. = 2536 g (100/23) = 11026 gm = 11.026 kg.
Solution Carbon undergoes combustion according to the equation.
C + O2 → CO2
12 32
Thus wt. of O2 required for combustion of 12 gm of C = 32 gm.
Hence weight of oxygen required by 1 kg of carbon =
∴ wt. of air (containing 23% oxygen) required =
Now since 32 gm of oxygen occupies 22.4 litres at NTP
∴ 2.667 × 1000 gm of O2 will occupy =
So, volume of air (containing 21% oxygen) required
Solution In one m3 of the gas
Volume of air required for 1 m3 of gas using 50% excess air
Hence, weight of air actually supplied per m3 of the gas,
Solution Volume of components in 1 m3 of gaseous fuel and O2 needed for combustion can be calculated as:
Volume of air required for 1 m3 of gas using 50% excess air
Hence, weight of air actually supplied per m3 of gas
Solution Combustion reaction volume of O2 needed
CO + 0.5O2 → CO2 5 L × 0.5 = 2.5 L
Hence, volume of air required
=
Solution volume of component in 1m3 of gaseous fuel and O2 needed for combustion can be calculated as:
∴ Volume of air required for 1 m3 of gas
=
Solution One kg of coal sample contains:
C = 662 gm; H2 = 42 gm; S = 29 gm; O = 61 gm; H2O = 97 gm
Hence, minimum weight of air required for complete combustion of 1 kg of coal = (1)
(Because the air has 23% (by oxygen weight))
And weight of air supplied for combustion using 25% excess air
(2)
Since, total weight of products of combustion
= Weight of [excess O2 + N2 + H2O + SO2 + CO2] (3)
∴ We should first calculate individual weights of products.
Now, weight of excess O2 = 25% of Net O2 used {equation (1)}
Weight of N2 = 77% of weight of air + weight of N2 present in fuel
Weight of H2O = 378 + 97 = 475 gm
Weight of SO2 = 58 gm
Weight of CO2 = 2427.3 gm
∴ Total weight of products of combustion
= Weight of (excess O2 + N2 + H2O + SO2 + CO2)
= 517.3 + 8659.6 + 475 + 58 + 2427.3 gm = 12137.2 gm
= 12.137 gm
Solution Total weight O2 needed
Less O2 in coal = 126 gm
∴ Net O2 needed = 2321.7 gm
So, minimum weight of air necessary for complete combustion
Dry products of combustion
N2 = 77% of weight of air + in fuel
=
= 7804.5 gm
Total weight of dry products of combustion = weight of (CO2 + SO2 + N2)
= 2764.7 + 26 + 7804.5 = 10595.2 gm
∴ Percentage of .
Percentage of
Percentage of
C = 54%; H = 6.5%; O = 3%; N = 1.8%; moisture = 17.3 and remaining is ash. This coal on combustion with excess of air, gave 21.5 kg of dry flue gases per kg of coal burnt. Calculate percentage of excess air used for combustion.
Solution 1 kg of coal contains
C = 0.54 kg: H = 0.065 kg; O = 0.03 kg; N = 0.018 kg
Minimum weight of air required for combustion
Weight of dry products of combustion
∴ Total weight of dry products combustion = 1.98 + 6.478 = 8.458 kg
Given, the actual weight of dry flue gases is 21.5 kg. so balance must have come from excess air
= 21.5 – 8.458 = 13.42 kg
Hence percentage of excess air =
Solution 1 kg of coal contains
C = 760 gm: H = 52 gm; S = 12 gm; O = 128 gm; N = 27 gm
∴ Net O2 needed for combustion = (O2 needed for combustion)–(O2 is fuel)
Now, weight of air necessary for complete combustion of 1 kg of coal
And volume of air necessary for complete combustion of 1 kg of coal
Weight and percentage of dry products of combustion are calculated below:
O2 = Minimum weight of
Total weight of dry products of combustion = weight of (CO2 + SO2 + N2 + O2)
= 2786.7 + 24 + 11710.9 + 1163.4 = 15684.97 gm
∴ Percentage of
Percentage of
Percentage of
Percentage of
Coke used for metallurgy is called metallurgical coke, and it should have the following good characteristics.
The coke, for metallurgical purposes, is mainly manufactured by two methods. They are (1) Beehive oven and (2) Otto Hoffman‚s by-product oven method.
Figure 5.3 Beehive coke oven
Through the top opening, coal is charged about 0.6 m deep layer, air is supplied from the side opening and the coal ignited. For slow carbonization, combustion is allowed to proceed gradual diminish supply of air, and it will take to complete 3 to 4 days from the top to bottom layer and the volatile matter escapes inside the partially closed door. After completing and carbonization, the hot coke quenched with water and raked out through the side door, leaving the oven hot to start the next charge batch carbonization. The yield is 80 per cent of the charged coal. Many such ovens are arranged in series, and with this waste heat is utilized for heating. Hence, it saves energy, reduces the pollution and is economically beneficial.
Figure 5.4 Otto Hoffman‚s by-product coke oven with regenerators
The oven consists of a number of narrow silica chambers about 10 to 12 m long, 3 to 4 m high and 0.40 to 0.45 m width. These chambers are erected side by side vertically; further, flues in between them form a sort of battery. Each chamber is provided at the top with a charging hole, at the end of chamber a gas off-take and refractory lined cast iron door for discharging coke.
A finely crushed coal is introduced through the charging holes, closed tightly on both the ends to prevent air access. The oven is heated to 1200°C by employing a regenerative principle, with burning of producer gas. During combustion, produced flue gases pass towards sensitive checker brick work until the temperature raises about 1000°C before escaping to chimney. The flow of heating gases is reversed, to serve in the preheat of inlet gases and the cycle goes on. The heating process is continued up to 11 to 18 h, till the carbonization and evolution of volatile matter ceases completely. After complete carbonization, a massive ram pushes the red hot coke into a truck and subsequently quenched.
Liquid fuels are those which are combustible, energy-generating substances and play vital role in transportation and economy. Most widely used liquid fuels are derived from fossil fuel/petroleum/crude oil. Some important liquid fuels are petrol, kerosene, diesel, etc.
Petroleum is a complex mixture of organic liquids (hydrocarbons) also known as crude oil or fossill fuel. It is formed from the fossilized dead plants and animals by exposure to heat and pressure in the Earth‚s crust, and was formed millions of years ago. It is a viscous dark coloured, foul-smelling liquid along with water and soil particles. Hence, it is necessary to separate these hydrocarbons into useful products, and this process is known as fractional distillation. In this process, products are separated depending on boiling points, known as refining of petroleum, and the plant set-up used here are oil refineries as shown in Figure 5.4.
Refining of petroleum involves the following 3 steps.
Step I: Separation of water by Cottrell‚s method: Petrol or crude oil is the emulsion of oil and salt water, and these colloidal water droplets coalesce to form large drops which can separate out from oil when the crude oil is sent through two highly charged electrodes.
Step II: Removal of sulphur compounds: crude oil is treated with copper oxide, sulphur reacts with copper to form copper sulphide precipitate, which is removed by filtration
Doctors sweetening process: The process was described by G. Wendt and S. Diggs. Here, crude oil is treated with sodium plumbate, i.e., doctors solution, converts mercaptans in sour gasoline into disulphide.
Step III: Fractional distillation: In an iron retort, the crude oil is heated to about 400–430°C. Here, all volatile matter are evaporated, components which are not volatile like tar and asphalt are settled at the bottom of the column. The hot vapours are then passed through a distillation column, shown in Figure 5.5.
Figure 5.5 Fractional distillation of crude petroleum
The distillation chamber is a steel cylindrical tube about 31 m height and 3 m in diameter, and inside, the chamber trays are fitted at short distances. Every tray is having many holes and an up going short tube with a bubble cap. At different heights of chamber, the vapours go up, begin to cool and condense in fractions. Fractions which are having higher boiling point condenses first and lower boiling fractions one after other. Various products obtained in distillation are given in Table 5.1.
Petrol can be synthesized by the following methods.
The process of breakdown of high molecular weight hydrocarbons of high boiling points into simple, lower molecular weight hydrocarbons of low boiling points is known as cracking.
Example:
With these we can prepare different fuels having high quality.
Cracking is mainly two types: thermal and catalytic cracking.
In this cracking heavy oils are subjected to high temperature and pressure in the absence of catalyst. In this cracking, the bigger hydrocarbon molecules break down to give smaller paraffins and olefins.
Mechanism of Cracking Process
Cracking processes invoke free radical and carbonium ion intermediates. Thermal cracking mainly goes through the free radical mechanism. In this mainly of three steps they are as follows: Example: Thermal cracking of nonane.
In catalytic cracking, higher molecular weight hydrocarbons breakdown in the presence of catalyst like alumina (or) aluminium sulphate via carbonium ion intermediate. Here, quality and quantity of gasoline can be increased, and it is mainly of two types. They are as follows:
This reaction proceeds via carbonium–ion intermediates.
Figure 5.6 Fixed-bed catalytic cracking
In a catalytic chamber, 40 per cent of oil is converted into petrol and 2–4 per cent of carbon formed is absorbed on the catalyst bed. Catalyst stops function after 8–10 h, and due to carbon deposition it deactivates. This is re-activated by burning off the deposited carbon, During re-activation, the vapours are directed through another catalyst chamber.
Cracked vapours enter into the fractionating column from the catalyst chamber, and different gases are cooled and collected.
Figure 5.7 Moving-bed type catalytic cracking
Oven-heated coke is mixed with hydrogen and passed steam through it, and water gas (CO + H2) is formed. It is purified by passing through first Fe2O3, here H2S is removed, next a mixture of Fe2O3 + Na2CO3, removes organic sulphur compounds. The purified gas is then compressed to 5–25 atm and is sent through a converter containing catalyst. Catalyst is the mixture of 100 parts of cobalt, 8 parts of magnesia, 5 parts of thoria and 200 parts of keirelgular at 200–300°C temperature. A mixture of saturated and unsaturated hydrocarbons is formed.
This reaction is highly exothermic. Hence, formed hot gaseous mixture is sent to a cooler. Here, liquid like crude oil is formed, and passed through a fractionating column. From the column, petrol and heavy oil are formed. Cracking of heavy oil gives and petrol. Schematic diagram of Fisher-Tropsch method is shown in Figure 5.8.
Figure 5.8 Fischer–Trapsch method
A paste of finely powdered low ash coal, heavy oil and tin or nickel oleate (catalyst) is heated with hydrogen at 450°C temperature, and 200–250 atm pressure for about 1.5 h. Here, hydrogen reacts with coal to give saturated hydrocarbons, there are send to condense. Liquid like crude is formed and sent to fractionating column. From the column petrol, middle oil and heavy oil are formed. Heavy oil is used further for making paste with fresh coal. The middle oil is hydrogenated in presence of a solid catalyst in vapour phase to give petrol. The schematic diagram of Bergius process is shown in Figure 5.9.
Figure 5.9 Bergius method
Power alcohol is one of the most important non-petroleum fuels. The first four aliphatic alcohols, methanol, ethanol, propanol and butanol, can be synthesized chemically or biologically and used as a fuel for internal combustion engines. These are not used as a prime fuel, but used in blends as additives.
Chemical formula of power alcohol is CnH2n+1 OH.
Methanol can be prepared from biomass. Ethanol is commonly prepared from various biological organic substances through fermentation process. However, widely it is manufactured from molasses. It is a viscous semisolid material, left after crystallization of sugar from sugar cane juice. It is a mixture of sucrose, glucose and fructose.
The molasses are diluted with water, to reduce sugar concentration from about 50–60 per cent to 10–12 per cent. Nutrients like ammonium sulphate, ammonium phosphate, and some amount of sulphuric acid is added to maintain pH value around 4–5. Right proportions of yeast are added and maintain the temperature of about 30°C. The invertase enzyme of yeast converts entire sucrose into glucose and fructose.
The zymase enzyme of yeast converts entire glucose and fructose into ethyl alcohol and releases carbon dioxide. During this process CO2 produces lot of froth, hence this process is known as fermentation process.
The fermentation process may be completed in about 36–38 h. Depending on the concentration of alcohol, it is named as wash or rectified spirit or absolute alcohol.
Wash: The fermented liquid containing 18–20 per cent of alcohol is known as wash.
Rectified spirit: Fractional distillated wash contains 90–95 per cent alcohol, and it is known as rectified spirit.
Absolute alcohol: The rectified spirit is digested with lime for about 2 days and then distilled to get 100 per cent alcohol which is known as absolute alcohol.
Knocking is the metallic sound produced by a spark ignition petrol engine under certain conditions. The following terms can explain the knocking in better way.
As
The CR obviously indicates the extent of compression of fuel–air mixture by the piston.
The fuel–air mixture gets heated to a temperature greater than its ignition temperature as a result of compression. This leads to spontaneous combustion even before sparking.
It is also possible that the last portion of the fuel–air mixture undergoes self-ignition after sparking. It is due to the heating and compression of the unburned fuel, by the spreading flame-font sweeping across the cylinder.
The resulting shock wave dissipates its energy by hitting the cylinder walls and the piston. In view of the characteristic rattling sound emitted, this is called knocking. The CR at which fuel tends to knock is called critical CR.
To summarise: With the increase in CR, the efficiency of IC engine also increases but after critical CR, tendency to knock also increases.
Consequences of knocking:
Probable mechanisms of chemical reactions that lead to knocking are the following:
Factors on which knocking depend are the following
Aromatic hydrocarbons have higher anti-knocking properties than paraffins and olefins.
In the diesel engine, air is first drawn into the cylinder and compressed to a pressure of about 500 psi (3.52 × 105 kg/m2). This compression is accompanied by a rise in temperature to about 500 °C.
Towards the end of the compression, stroke is injected in the form of finely divided spray into air in the cylinder heated to about 500 °C by compression. The oil absorbs the heat from the air and it ignites spontaneously as it attains ignition temperature. This raises the temperature and pressure. The piston is pushed by expanding gases in the power stroke.
In a diesel engine, combustion of fuel is not instantaneous, as the ignition delay is caused. Ignition delay is the interval between the start of fuel injection and its ignition. This is due to the time taken for the vaporization fuel droplets and attaining of the vapour to its ignition temperature. It depends on the (a) engine design; (b) efficiency of mixing of the spray and air; (c) the injector design; and (d) mostly on the chemical nature of the fuel. Example: Ignition delay is shorter for paraffinic fuel than that of olefinic, naphthalenic and aromatic fuels.
If the ignition delay is long, it will lead to fuel accumulation in the engine even before the ignition. When ignited, an explosion results as the combined effect of increased temperature and pressure. This is responsible for diesel knock. The diesel fuel should have a SIT less than the temperature produced by compression.
As the temperature to which air can be heated by compression is limited by various constraints, it is desirable to have fuels with short ignition delay but the ignition delay must be long enough for the compression stroke to be completed. In order to grade the diesel fuels, cetane rating is employed.
Cetane number: It is a measure of the ease with which a fuel will ignite under compression.
Cetane number of fuel primarily depends on the nature and composition of its hydrocarbons.
For instance, consider the following series: n–alkanes > naphthenes (i.e. cycloalkanes) > alkenes > branched alkanes > aromatics (i.e. cycloalkanes):
As straight chain alkanes such as n–cetane have low ignition delay (high ignition quality) and ignite readily on compression, while aromatics do not ignite readily on compression, so that high cetane number fuels eliminate diesel knock. The cetane number of diesel fuel may be raised by the addition of pre-ignition dopes such as alkyl nitrites such as ethyl nitrite, amylnitrite, etc., 2,2,4,4,6,6,8, 8–hepta methyl nonane (HMN).
With a cetane rating of 15 is now considered as the low-quality diesel in the view of its easy availability and purity. On the revised scale (HMN reference), the cetane number (CN) represents the % cetane, in the blend with HMN plus 15/100 of the % HMN. Thus, a blend of 50% cetane and 50% HMN has a following cetane rating:
Octane number: The resistance offered by gasoline to knocking cannot be defined in absolute terms. It is generally expressed on an arbitrary scale, known as octane rating.
The % of iso-octane in the n–heptane iso-octane blend which has the same knocking characteristics as the gasoline sample under the same set of conditions is known as octane number.
Additives for improving anti-knock properties: Tetra ethyl lead (TEL) and diethyl telluside (C2H5)2Te are the most commonly used additives. TEL gives rise to Pb of PbO during combustion. These particles act as free-radical chain inhibitors as they arrest the propagation of the explosive chain reactions responsible for knocking.
The efficiency of TEL decreases in the presence of sulphur hence desulphurised gasoline is preferred. Pb and PbO2 decrease engine life hence they must be removed along with exhaust gases by adding ethylene dibromide.
Pb, PbO2 + C2H2Br2 → PbBr2
Because PbBr2 formed is volatile its escape into atmosphere. But pollution problem still exists. Another cause of pollution is incomplete combustion leading to the formation of CO, NO, NO2, SO2, SO3, etc. Hence, catalytic converters based on Pt are employed which will catalyse combustion reaction to completion. Example: CO–CO2.
But Pt is poisoned by Pb, so unleaded petrol should be used. Benzene is added for decreasing knocking. Since benzene is carcinogenic, very low concentration of benzene should be used.
Important gaseous fuels are natural gas, producer gas, water gas, coal gas, bio gas, etc.
Natural gas obtained along with petroleum in oil wells is called wet gas. It is purified and removed. Propane, propene, butane, butene, etc. are used for preparing LPG. If the gas is associated with crude oil, it is called dry gas. It is having some of the objectionable ingredients like water, H2S, N2, CO2, etc. and hydrocarbons like propane, butane, propene, butene, etc. are removed.
Composition
Natural gas consists of 70–90 per cent of methane, 5–10 per cent of ethane, 3 per cent of hydrogen and rest of carbon monoxide and carbon dioxide, approximately. Calorific value is about 12,000–14,000 kcal/m3.
Uses
Composition
Producer gas is the mixture of about 20–22 per cent carbon monoxide (CO), 11–13 per cent carbon dioxide (CO2), 20–22 per cent hydrogen (H2), 2.5–3.5 per cent methane (CH4) and 40–42 per cent nitrogen (N2). Hence main composition is CO + H2.
Manufacture
Air is passed through a red hot coal or coke in a gas producer, and maintained temperature is about 1100°C. Producer gas is formed with oxidation and reduction reactions. Initially, oxidation of carbon gives carbon monoxide and carbon dioxide.
Reduction reaction gives producer gas:
Formed gas is distilled and purified. The calorific value of producer gas is about 1300 kcal/m3.
Uses
Composition
Water gas is the mixture of carbon monoxide (40–42 per cent), hydrogen (50–52 per cent), nitrogen (3–4 per cent and carbon dioxide (3–4 per cent).
Manufacture
Steam and little air are passed alternatively through a red hot coal or coke in a reactor maintained at about 1000°C temperature and water gas is formed in the following reactions.
The calorific value of water gas is about 2800 kcal/m3.
Uses
Carbonated water gas: It is a mixture of producer gas and hydrocarbons. Calorific value is about 4500 kcal/m3, and used for illuminating and heating purpose.
Semi-water gas: It is a mixture of water gas and producer gas. Calorific value is about 1700 kcal/m3. Used as a fuel and a source of N2 and H2 in the manufacture of ammonia.
Coal gas is mainly used as an illuminant in cities and towns; hence, it is known as town gas or illumination gas.
Composition
It is a mixture of carbon monoxide (27–29 per cent), carbon dioxide (2.4 per cent), hydrogen (16–18 per cent), nitrogen (49–51 per cent) and methane (0.5–1 per cent).
Manufacture
It is manufactured by destructive distillation of coal in the absence of air, at about 1300°C temperature.
The calorific value of coal gas is about 4900 kcal/m3.
Uses
Composition
Biogas is the mixture of methane (50–60 per cent), carbon dioxide (30–40 per cent), hydrogen (5–10 per cent), nitrogen (2–6 per cent) and trace amount of hydrogen sulphide.
Manufacture
It is produced by the degradation of biological matter like animal dung, poultry waste, vegetable waste, waste paper, plant waste, human excreta, birds‚ excreta, etc. by the anaerobic bacterial action in the absence of free oxygen.
Uses
Flue gas is the mixture of CO2, CO and O2 gases, exhausted from the combustion chamber. Analysis of flue gas gives an idea about efficiency of combustion. Suppose the flue gas contains considerable amount of CO, it indicates incomplete combination and short supply of oxygen, and this will lead to wastage of fuel. If the flue gas contains considerable amount of oxygen, this indicates excess supply of oxygen and results in loss of heat.
With the help of Orsat‚s apparatus flue gas analysis is carried out, as is shown in Figure 5.10. The set-up consists of a horizontal tube, with a three-way stopcock at one end and another end is connected with a graduated burette. For maintaining constant temperature during the experiment, the burette is surrounded by a water jacket. The burette is connected as a set of three absorption bulbs in a series, through a separate stopcock. The lower end of the burette is further connected to a water reservoir through a rubber tube.
Figure 5.10 The orsats apparatus
The water level in the burette can be changed by raising or lowering the reservoir water. One end of the tube, which is connected to a three-way stopcock, is further connected to a U-tube. For drying flue gas and avoiding the incoming smoke particles, the U-tube is packed with fused CaCl2 and glass wool.
Among the three absorption bulbs, first bulb has potassium hydroxide solution and absorbs only CO2. The second bulb contains alkaline pyrogallic acid absorbs only O2 and CO2. The third bulb has ammonical cuprous chloride and can absorb CO2, O2 and CO. For proper analysis of flue gas, first it is passed through potassium hydroxide containing bulb, and here CO2 is absorbed. Then, it is passed through alkaline pyrogallic acid containing bulb, and here O2 is absorbed and it can also absorb CO2, but already it is removed by KOH. Finally, flue gas is passed through the third bulb containing ammonical cuprous chloride, and here CO is absorbed; however, it can absorb CO2 and O2 also but these are already removed.
The entire apparatus is thoroughly cleaned, the steppers are greased, tested for air tightness, the absorption bulbs are filled with their respective solution and the stopcocks are closed. The water reservoir and water jacket are filled with water, and air is excluded from the burette by the raising of reservoir water level till the burette is completely filled with water. For the exclusion of air, the three-way stopcock is opened, next the lowering of water level is done and the fuel gas supply is connected to the three-way stopcock.
Further, 100 ml of the flue gas is carefully sent to the burette with closing of the three-way stopcock. The fuel gas is forced through the first bulb by opening its stopcock and raising the water level in the reservoir. Here, potassium hydroxide absorbs the CO2 flue gas is sent repeatedly 2 or 3 times to the first bulb for complete absorption of CO2. The remaining gas is taken back in the burette and the stopcock of the first bulb is closed. The levels of water in the reservoir and burette are equalized and decreasing volume of gas is noted. This decrease in volume gives the volume of CO2 in 100 ml of flue gas. Similarly, the volumes of O2 and CO are determined by passing the flue gas through the second and third bulbs, respectively. The remaining gas in the burette after absorption of CO2, O2 and CO is nitrogen.
The decrease in volume of flue gas by first bulb = volume of CO2
The decrease in volume of flue gas by second bulb = volume of O2
The decrease in volume of flue gas by third bulb = volume of CO.
[Ans.: Calorie]
[Ans.: 2.2]
[Ans.: Bomb calorie meter]
[Ans.: 588 cal/gm]
[Ans.: Pyrometric effect]
[Ans.: Nitrogen]
[Ans.: Higher]
[Ans.: KOH reduction]
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Ans.: Fuel is a combustible substance containing carbon as the major constituent which on proper burning gives large amount of heat that can be used economically for domestic and industrial purposes.
Examples are coal, petrol, diesel, etc.
Ans.: According to the occurrence, the fuels are classified into natural (primary) and secondary (derived) fuels.
Ans.: Units of heat are calorie, kilo calorie, British thermal unit and centigrade heat unit.
1 kcal = 1000 cal = 3.968 BTU = 2.2 CHU
Ans.: Calorific value is the total quantity of heat liberated by the complete combustion of one unit mass/volume of a fuel in oxygen.
LCV = HCV – latent heat of water vapour formed
Ans.: Bomb calorimeter is used for determining the calorific value of solid and liquid fuels. Junker‚s calorimeter is used for determining the calorific value of gaseous fuels.
Ans.: High calorific value, moderate ignition temperature, low moisture content, low non-combustible matter content, etc.
Ans.: Liquified petroleum gas, gasoline, petrol, kerosene, diesel oil, heavy oil, etc.
Ans.: Beehive oven and Otto Hoffman‚s by-product oven.
Ans.: The process of breakdown of high molecular weight hydrocarbons of high boiling points into simple, lower molecular weight hydrocarbons of low boiling points is known as cracking.
Ex:
Ans.: The mixture of gases like CO2, CO and O2 exhaust of the combustion chamber is called flue gas.
Ans.: The analysis of flue gas either from a furnace or from an engine‚s exhaust would give an idea about the efficiency of the combustion process. If the flue gas contains considerable amount of CO, it indicates that incomplete combustion is occurring and it also indicates the short supply of O2 for combustion, and this will lead to wastage of fuel.
Ans.: It indicates that the O2 supply is very much in excess, and it results in loss of heat.
Ans.: Orsat‚s apparatus.
Ans.: Potassium hydroxide solution – only CO2
Alkaline pyrogallic acid – CO2 and O2
Ammonical cuprous chloride – CO, O2 and CO2
b. A sample of coal contains 60% carbon, 33% oxygen, 6.0% hydrogen, 0.5% sulphur, 0.2% nitrogen and 0.3% ash. Calculate GCV and NCV of coal.
a. Gross and net calorific values
b. Octane number and centane number
b. Give the advantages and disadvantages of coal over gaseous fuels.
b. Distinguish between low-temperature carbonisation and high-temperature carbonisation.
b. Define octane number of gasoline. Why is ethylene dibromide added, when tetra ethyl lead is used as an anti-knock?
a. Explain how fuels are classified with suitable examples.
b. Give the comparison between solid, liquid and gaseous fuels.
c. What are the characteristics of a good fuel?
a. Moisture
b. Volatile matter
c. Ash
d. Fixed carbon
b. Explain the significance of the following constituents present in coal.
b. Calculate gross and net calorific value of a gaseous fuel from the following data. Vol. of gaseous fuel burnt at STP –0.09 m3, weight of water used for cooling 25.0 kg, temperature of inlet water 25 °C, temperature of the outlet water 30.0 °C, weight of water produced by steam condensation 0.02 kg latent heat of steam 587 kcal/kg.
b. Give an account of production of petrol from crude oil.
b. What is meant by calorific value of a fuel? Define calorie and kilocalorie.
a. Total carbon
b. Hydrogen
c. Nitrogen
b. What are different types of fuels? What are the characteristics of a good fuel?
c. Mention the criteria for selecting good fuel.
d. Distinguish between solid, liquid and gaseous fuels.
b. Describe how the calorific value of a solid fuel is determined using a bomb calorimeter.
c. What are the fuels used for determination of water equivalent of bomb calorimeter and why?
b. Discuss the importance of ultimate analysis of coal.
b. How is nitrogen determined in a solid fuel?
c. What is the significance of a volatile matter in coal?
d. How is ranking of coal make based on ultimate analysis?
b. Differentiate between coal and coke.
c. Explain carbonisation of coal.
d. Why is coke preferred to coal in metallurgical purposes?
e. Why are gaseous fuels more advantageous than solid fuels?
b. Why is peat not considered as an economical fuel?
a. What are the structural features of hydrocarbons in unlead petrol and diesel? What are the structural factors that promote its high value?
b. What is the significance of octane number and cetane number and for which these are used? How these can be improved?
c. Why is C2H4Br2 added, when TEL is used as an anti-knock?
d. What types of compounds nowadays are being added to petrol to improve octane rating?
b. What are the advantages of catalytic cracking process? Describe, with a neat diagram, the fixed-bed catalytic cracking process.
c. Differentiate between thermal and catalytic cracking.
d. What are the advantages of catalytic cracking over thermal cracking?
e. What is meant by knocking? How is it related to chemical constitution? Describe the functions of TEL. Explain octane number and cetane number.
b. Write the approximate compositions and calorific values of water gas and producer gas.
b. How distinction can be made between complete and incomplete combustion of fuel?
c. What is leaded petrol?
a. Catalytic converter
b. Flue analysis and its significance
b. How gross calorific value of a solid fuel determined by Bomb Calorimeter? Write Dulong‚s formula for calculating calorific value of fuel from its ultimate combustion data.
c. Discuss Beehives oven method for the manufacture of coke.
b. What do you understand with the knocking of fuel? Report the ways to improve the anti-knocking characteristic of a fuel.
a. Calculate the amount of air required for the complete combustion of 1 kg of the coal.
b. Calculate the gross and net calorific values of the coal sample. Given that the calorific values of C, H and S are 8,060 kcal/kg; 3,400 kcal/kg and 2,200 kcal/kg, respectively.
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