3 Contents Natural Gas Producer Gas Water Gas Coal Gas Oil Gas Carburetted Water GasBio GasAdvantages of gaseous fuels
4 FUELSFuel is a combustible substance chiefly containing carbon, which on burning produces large amount of heat that can be used economically for domestic and industrial purposes.Fuel + O2 Products + Heat
5 Classification of Fuels Fuels have been classified according to their (i) Occurrence or preparation (ii) Physical state On the basis of their occurrence, fuels may be of two types (a) Natural fuels or primary fuels (b) Artificial or secondary fuels
6 Natural FuelsFuels which are found in nature as such are called natural fuels.Examples- wood, peat, coal, petroleum, natural gas etc.Artificial FuelsFuels which are prepared artificially fromprimary fuels are called artificial fuels.These fuels are derived from naturally occurring fuels by further chemical processing so these are called secondary fuels. Coke, petrol, kerosene oil, coal gas.
7 On the basis of their physical state, fuels are of three types: Solid fuelsLiquid fuelsGaseous fuelsSOLID FUELSNatural solid fuels- wood, peat, lignite, bituminous coal & anthracite coal.Artificial solid fuels- wood charcoal, coke.
8 2. LIQUID FUELSNatural liquid fuels- Crude petroleumArtificial liquid fuels- Various fractions of petroleum such as petrol, diesel, kerosene oil, coal tar & alcohol etc.3.GASEOUS FUELSNatural gaseous fuels- Natural gas or marsh gas.Artificial gaseous fuels- Producer gas, water gas, coal gas & blast furnace gas etc.
9 1. Natural GasThe naturally occurring gaseous fuel is known as natural gas.It is formed by decomposition of organic matterIt is obtained from wells dug in the oil-bearing regions
10 Before use, the natural gas is purified to remove objectionable impurities such as water, dust, grit, H2S, CO2, N2 and higher hydrocarbons which can be easily liquified.It is a very cheap & convenient fuel.
11 Composition Methane (CH4) = 70-90% Ethane (C2H6) = 5-10% Hydrogen = 3% CO & CO2 = rest Calorific value kcal/m3
12 Uses of Natural GasIt is an excellent domestic fuel and can be conveyed over large distances in pipelinesIt has been used in the manufacture of a large number of chemicals by synthetic processes
13 Natural gas is also used as raw material for the manufacture of carbon black ( a filler for rubber) and hydrogen (used in ammonia synthesis)Synthetic proteins (used as animal feed) is obtained by microbiological fermentation of methane
14 2. PRODUCER GASProducer gas is a mixture of carbon monoxide and nitrogen (CO + N2).It is produced by passing air mixed with little steam over a red hot coke bed maintained at a temperature of about 1100oC.Its calorific value is very low.Among all gaseous fuels, producer gas is the cheapest.
15 Manufacture of Producer Gas The furnace used at for the manufacture of producer gas is known as gas producer.It consists of a steel vessel which is lined inside with refractory bricksIt is about 3 m in diameter and 4 m in height
16 It is provided with a cup and cone feeder at the top Producer gas comes out from an exit near the top.At the base, it has inlet for passing air and steamMoreover, at the base it has an exit for the removal of ash formed.
18 Reactions in Gas Producer The gas production reactions can be divided into four zones :Ash zoneCombustion zoneReduction zoneDistillation zone
19 (1).Ash zoneIt is the lowest zone & contains mainly of ash and hence, is known as ash zone.The ash protects the grate of the producer from the intense heat of combustionThe temperature of supplied air and steam is increased as they pass through this zone
20 (2) Combustion zone This zone is next to the ash zone It is also known as oxidation zoneHere carbon burns and forms CO & CO2C + O2 CO kcalC + ½ O2 CO kcalTemperature of this zone is about 1100oC
21 (3) Reduction ZoneIn this zone carbon dioxide and steam combines with red-hot coke and liberates free H2 & CO.CO2 + C 2 CO – 36 kcalC + H2O CO + H – 29 kcalC + 2H2O CO2 + 2H2 – 19 kcalAll these reactions are endothermic, so temperature falls to 1000oC
22 (4) Distillation Zone Temperature of this zone is about 400-800oC The incoming coal is heated by the outgoing hot gasesIn the way the volatile matter present in the coal vaporises and mixes with the outgoing gases
23 Nitrogen remains unaffected throughout the process. Thus a mixture of CO & N2 with traces of CO2 comes out through the exit at the upper end of the of the producer and is named as Producer Gas.
24 Manufacture of Producer Gas Distillation ZoneSecondary Reduction ZonePrimary Reduction ZoneOxidation ZoneAsh ZoneManufacture of Producer Gas
25 Composition of Producer Gas The average composition of producer gas is : Carbon monoxide (CO) = 30% Nitrogen (N2) = 55% Carbon dioxide (CO2) = 3% Hydrogen (H2) = 12%
26 Properties of Producer gas Burns with a blue flamePoisonous in nature due to presence of COCalorific value is about 1300 kcal / m3Calorific value is lowest of all gaseous fuelsIn spite of its lowest calorific value, this gas is the cheapest gaseous fuel per unit heat.
27 Uses of Producer gasIt is used for heating open hearth furnaces in the manufacture of steel and glassIt is used for heating muffle furnaces and retorts in the manufacture of coke and coal gasIt is used as a reducing agent in certain metallurgical operations
28 3. Water gas Water gas is a mixture of combustible gases (CO & H2) It contains little amount of non-combustible gases (CO2 & N2)It can be prepared by passing alternatively steam and air through a bed of red hot coke or coal maintained at about 900 to 1000oC.
29 Manufacture of Water Gas A water gas generator is a cylindrical vessel made of steelIt is about 4m in height and 3m wideLined inside with refractory bricksAt the top, it has cup & cone feeder for adding coke
30 Water gas outlet is provided near the top At the base, it is provided with inlet pipes for passing air & steamAt the bottom, it has an arrangement for taking out ash formed
32 Reactions Step I. First of all, steam is passed Steam reacts with red hot coke (coal) at 1000oCC + H2O CO + H2 - 29kcal (endothermic)water gasCO and H2 (water gas) is formedThe above reaction is endothermicHence, the temperature of the bed falls below 1000oC
33 Now air entry is stopped & steam is passed to get water gas Step II.To raise the temperature of coke bed to 1000oC , steam supply is cut off and air is blown inCoke burns in airC + O2 CO kcal2C + O2 2CO + 59 kcalAs both the reactions are exothermic, so temperature of the bed rises to 1000oCNow air entry is stopped & steam is passed to get water gas(exothermic)
34 In modern gas plants steam & air are blown alternatively. The period of steam blow (cold blow) is about 4min while period of air blow (hot blow) is of 1 minThe durations of these periods is adjusted in such a way to get maximum yield of water gas
35 Composition of Water Gas The average composition of water gas is: Carbon monoxide = 44% Hydrogen = 48% Carbon dioxide = 4% Nitrogen = 4%
36 Properties of Water Gas Water gas burns with non-luminous blue flame & hence is called blue water gasThe temperature of the flame reaches up to 1200o
37 Due to presence of CO, it is poisonous gas Calorific value is about 2800 kcal/m3When mixed with hydrocarbons , it is called carburetted water gas
38 Uses of Water GasWater gas is a good heating fuel because of high calorific valueA good source of hydrogen on commercial scaleUsed in manufacture of methyl alcohol
39 Used for welding purposes When mixed with hydrocarbons , it is called carburetted water gas. It is used for heating & lighting purposes.
40 4. Coal Gas It is obtained by destructive distillation of coal Coal used should be rich in volatile matter (30 to 40%)The process is carried out in horizontal or vertical closed iron or silica retorts at 1350o C
41 Coke is the residue left behind Coal Coal gas + Coke The gaseous product obtained is a mixture of several combustible gases & is known as coal gasCoke is the residue left behindCoal Coal gas + CokeDestructive Distillation
43 Manufacture of coal gas The plant consists of vertical silica retort having airtight hopper at the topThe retort is heated to about 1350oC by the combustion of preheated producer gas and air mixtureCoal rich in volatile matter is fed to the retort through hopper which decomposes on heating
44 The gases so produced as a result of decomposition of coal are allowed to pass through a hydraulic main to a big water cooled condensersTar and ammonia get condensed & collected in two separate layers in a tar well below the condensers
45 The gases are then led to a scrubber under pressure with the help of exhaust pump where they are scrubbed with water to remove last portions of tar & ammoniaThe hot gases are chilled by passing them through empty tower sprayed with jets of high pressure water where naphthalene is removed
46 Coal gases are then passed through another scrubber containing creosote oil Benzene, toluene & other aromatic compounds are dissolved in oilGases are then passed through purifier containing trays of hydrated iron oxide & lime
47 Impurities like H2S, CS2, CO2 and HCN etc get removed Purified coal gas is then collected in the gas holder over water from where it is supplied
49 Composition of Coal gas The approximate composition of coal gas is: Methane = 32% Hydrogen = 47% Carbon monoxide = 7% Acetylene (C2H2) = 2% Ethylene (C2H4) = 3% Nitrogen (N2) = 4% Carbon dioxide (CO2) = 1% Other hydrocarbons = 4%
50 Properties Colourless gas with a characteristic odour Lighter than air Slightly soluble in waterBurns with long smoky flamePoisonous in natureCalorific value is about 5000 kcal/m3
51 UsesMainly used as heating fuelAs an illuminantUsed to provide reducing atmosphere in metallurgical operationsAlso used as “town gas” for domestic purposes
52 5. OIL GAS Oil gas is a mixture of hydrocarbons Prepared by thermal cracking of kerosene oilKerosene oil consists of hydrocarbons ranging from C11H22to C16H34C11H22Thermal CrackingCH4 + C2H6 + C2H2+ C3H6 + CKerosene OilTarLower Hydrocarbons
53 Manufacture of Oil GasKerosene oil is allowed to fall drop wise on the bottom of the red hot cast iron retort heated in a furnaceThe oil on coming in contact with red hot bottom, immediately gets cracked into a number of lower gaseous hydrocarbons
54 The gaseous mixture emerge out through a bonnet which is fitted at the mouth of the retort with lead seal at one end & water seal at another end.Gaseous mixture goes to a hydraulic main, a tank containing waterTar gets condensed and is removed
55 The extent of cracking is tested time to time by withdrawing some gas samples through testing tap and examining its colourA good oil gas appears golden in colour
56 By proper adjustment of air supply, the gas of required colour can be obtained The gas is finally stored over water in gas holders & is supplied wherever requiredThe yield of the gas is about 40 to 50%
58 Composition of Oil gasComposition of oil gas depends upon the nature of oil used.Approximate composition of the gas is given below :CH4 = 25 – 30%H2 = 50 – 60%CO = 10 – 15%CO2 = 3%
59 Calorific valueCalorific value of the gas is about 4500 – 5400 kcal/m3Uses of Oil gasUsed as heating fuel in the laboratories of schools & collegesUsed to improve the calorific value of water gas and mixture of water gas & oil gas is called carburetted water gas
60 6. Carburetted Water GasWater gas enriched with oil gas is called carburetted water gasManufacture :Oil gas is prepared in an auxiliary plant called carburettorA checker-work of fire-bricks is heated by burning a part of water gasOil is sprayed over it
61 Oil undergoes thermal cracking & oil gas is produced Water gas is passed through itThe resulting carburetted water gas is passed through hydraulic mainIt is then passed into purifier with the help of exhaust fan followed by scrubbers to remove impurities
63 Composition of Carburetted Water gas Approximate composition of carburetted water gas is : CO = % H2 = % CO2 = % N2 = % Hydrocarbons = %
64 Properties of Carburetted water gas: Burns with blue flamePoisonous in nature due to presence of carbon monoxideUses :Used for industrial hydrogenationFor manufacture of alcoholas a fuel in industrial preparation
65 7. Bio GasBio gas is produced by the degradation of biological matter by bacterial action (by anaerobic bacteria) in the absence of free oxygenThe cheapest & easily obtainable biogas is gobar gasGobar gas is produced by anaerobic fermentation of cattle dung
66 Manufacture of Gobar gas The plant for gobar gas is a cylindrical tank made of bricksAbout 6m deep & 2m in diameterAn iron gas holder is suspended upside down in the tankDiameter of gas holder is slightly less than tank
67 Gas holder is always kept in vertical position by a device Animal waste is held in a side tank and is delivered to the tank through an inlet pipeUsed animal waste is taken out in another side tank through an outlet pipe
68 To produce gobar gas, gobar ( or human excreta, poultry sweep, plant residue, leaves or any other biomass) is collectedIt is made into slurry with water & is put in the inlet tankThe slurry slides down to the digester tank through the inlet pipeThe level of the slurry rises in the tankIt undergoes biological degradation by anaerobic bacteria in the absence of air
69 Gas so produced is collected in gas holder Digested slurry (used ) is pushed to the outlet tankDigested slurry is dried partially which is an excellent manure for plantsGobar gas is easily prepared in summer because optimum temperature for fermentation is oC
71 Composition of Bio gasThe average composition of bio gas is : Methane = 55% Carbon dioxide = 35% Hydrogen = 8% Nitrogen = 2% Before using, gobar gas is passed through KOH solution to absorb maximum of CO2
72 Properties of Bio gasIt is colourless & odourless gas insoluble in waterIt burns with non-luminous blue flame and gives a temperature of 540oCIt is non-poisnous in nature as it does not contain carbon monoxideCalorific value is about kcal/m3
73 Uses of Bio gasAn excellent domestic fuel especially in villages where dung is available in plentyGas can also be used for lighting and power purposes like running water pumps, chaff cutting machinesSpent slurry is a richer manure than original dung
74 Advantages of gaseous fuels Gaseous fuels can be transported through a pipe line to the place of useCan be lighted at a moment’s noticeCalorific value is highBurn with high efficiency and a high temperature flame can be obtained in no timeDo not produce any smoke or ashBurning can be easily controlled
75 Can be preheated by the heat of waste gases & hence their use is economical as far as heat is concernedFlame can be made oxidising, reducing by regulating the air supply. This is required in metallurgical operationsBurn in slight excess of air
76 Disadvantages Large storage tanks are required for storing Highly inflammable, hence risk of fire hazards are quite highAre usually suffocatingCost of production is quite highGreat care is needed for storing & transportation