Deptt. Of Applied Sciences Govt. Polytechnic College For Girls Patiala Presented By- Dr.Raman Rani Mittal M.Sc.,M.Phil.,Ph.D. (Chemistry) 1.

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Deptt. Of Applied SciencesGovt. Polytechnic College For Girls

Patiala

Presented By- Dr.Raman Rani

Mittal M.Sc.,M.Phil.,Ph.D.

(Chemistry)

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GASEOUS FUELS

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• Natural Gas• Producer Gas• Water Gas• Coal Gas• Oil Gas• Carburetted Water Gas• Bio Gas • Advantages of gaseous fuels

Contents

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FUELS

Fuel 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

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Classification of FuelsFuels 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

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• 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.

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On the basis of their physical state, fuels are of three types:• Solid fuels• Liquid fuels• Gaseous fuels

1. SOLID FUELS• Natural solid fuels- wood, peat, lignite,

bituminous coal & anthracite coal.• Artificial solid fuels- wood charcoal,

coke.

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2. LIQUID FUELS• Natural liquid fuels- Crude petroleum• Artificial liquid fuels- Various fractions of

petroleum such as petrol, diesel, kerosene oil, coal tar & alcohol etc.

3.GASEOUS FUELS• Natural gaseous fuels- Natural gas or

marsh gas.• Artificial gaseous fuels- Producer gas,

water gas, coal gas & blast furnace gas etc.

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1. Natural Gas

• The naturally occurring gaseous fuel is known as natural gas.

• It is formed by decomposition of organic matter

• It is obtained from wells dug in the oil-bearing regions

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• 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.

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Composition

Methane (CH4) = 70-90%

Ethane (C2H6) = 5-10%

Hydrogen = 3%CO & CO2 = rest

Calorific value12000-14000 kcal/m3

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Uses of Natural Gas

• It is an excellent domestic fuel and can be conveyed over large distances in pipelines

• It has been used in the manufacture of a large number of chemicals by synthetic processes

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• 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

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2. PRODUCER GAS

• Producer 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.

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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 bricks

• It is about 3 m in diameter and 4 m in height

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• 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 steam

• Moreover, at the base it has an exit for the removal of ash formed.

17Manufacture of Producer Gas

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Reactions in Gas Producer

The gas production reactions can be divided into four zones :

1. Ash zone2. Combustion zone3. Reduction zone4. Distillation zone

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(1).Ash zone

• It 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 combustion

• The temperature of supplied air and steam is increased as they pass through this zone

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(2) Combustion zone

• This zone is next to the ash zone• It is also known as oxidation zone• Here carbon burns and forms CO & CO2

C + O2 CO2 + 97 kcal

C + ½ O2 CO + 29.5 kcal

• Temperature of this zone is about 1100oC

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(3) Reduction Zone

• In this zone carbon dioxide and steam combines with red-hot coke and liberates free H2 & CO.

CO2 + C 2 CO – 36 kcal

C + H2O CO + H2 – 29 kcal

C + 2H2O CO2 + 2H2 – 19 kcalAll these reactions are endothermic, so temperature falls to 1000oC

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(4) Distillation Zone

• Temperature of this zone is about 400-800oC

• The incoming coal is heated by the outgoing hot gases

• In the way the volatile matter present in the coal vaporises and mixes with the outgoing gases

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• 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.

24Manufacture of Producer Gas

Ash Zone

Distillation Zone

Secondary Reduction Zone

Primary Reduction Zone

Oxidation Zone

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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%

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Properties of Producer gas

• Burns with a blue flame• Poisonous in nature due to presence of CO• Calorific value is about 1300 kcal / m3

• Calorific value is lowest of all gaseous fuels• In spite of its lowest calorific value, this gas

is the cheapest gaseous fuel per unit heat.

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Uses of Producer gas

• It is used for heating open hearth furnaces in the manufacture of steel and glass

• It is used for heating muffle furnaces and retorts in the manufacture of coke and coal gas

• It is used as a reducing agent in certain metallurgical operations

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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.

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Manufacture of Water Gas

• A water gas generator is a cylindrical vessel made of steel

• It is about 4m in height and 3m wide• Lined inside with refractory bricks• At the top, it has cup & cone feeder for

adding coke

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• Water gas outlet is provided near the top

• At the base, it is provided with inlet pipes for passing air & steam

• At the bottom, it has an arrangement for taking out ash formed

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Manufacture of Water Gas

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ReactionsStep I.• First of all, steam is passed• Steam reacts with red hot coke (coal) at 1000oC

C + H2O CO + H2 - 29kcal (endothermic)

water gas

• CO and H2 (water gas) is formed• The above reaction is endothermic• Hence, the temperature of the bed falls below

1000oC

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Step II.• To raise the temperature of coke bed to

1000oC , steam supply is cut off and air is blown in

• Coke burns in airC + O2 CO2 + 97 kcal

2C + O2 2CO + 59 kcal• As both the reactions are exothermic, so

temperature of the bed rises to 1000oC• Now air entry is stopped & steam is passed to

get water gas

(exothermic)

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• 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 min

• The durations of these periods is adjusted in such a way to get maximum yield of water gas

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Composition of Water Gas

The average composition of water gas is:

Carbon monoxide = 44% Hydrogen = 48%

Carbon dioxide = 4% Nitrogen = 4%

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Properties of Water Gas

• Water gas burns with non-luminous blue flame & hence is called blue water gas

• The temperature of the flame reaches up to 1200o

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• Due to presence of CO, it is poisonous gas

• Calorific value is about 2800 kcal/m3

• When mixed with hydrocarbons , it is called carburetted water gas

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Uses of Water Gas

• Water gas is a good heating fuel because of high calorific value

• A good source of hydrogen on commercial scale

• Used in manufacture of methyl alcohol

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• Used for welding purposes

• When mixed with hydrocarbons , it is called carburetted water gas. It is used for heating & lighting purposes.

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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

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• The gaseous product obtained is a mixture of several combustible gases & is known as coal gas

• Coke is the residue left behind

Coal Coal gas + Coke

Destructive Distillation

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Manufacture of coal gas

• The plant consists of vertical silica retort having airtight hopper at the top

• The retort is heated to about 1350oC by the combustion of preheated producer gas and air mixture

• Coal rich in volatile matter is fed to the retort through hopper which decomposes on heating

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• The gases so produced as a result of decomposition of coal are allowed to pass through a hydraulic main to a big water cooled condensers

• Tar and ammonia get condensed & collected in two separate layers in a tar well below the condensers

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• 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 & ammonia

• The hot gases are chilled by passing them through empty tower sprayed with jets of high pressure water where naphthalene is removed

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• Coal gases are then passed through another scrubber containing creosote oil

• Benzene, toluene & other aromatic compounds are dissolved in oil

• Gases are then passed through purifier containing trays of hydrated iron oxide & lime

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• 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

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Manufacture of Coal Gas

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Composition of Coal gasThe 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%

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Properties• Colourless gas with a characteristic odour• Lighter than air• Slightly soluble in water• Burns with long smoky flame• Poisonous in nature• Calorific value is about 5000 kcal/m3

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Uses•Mainly used as heating fuel• As an illuminant• Used to provide reducing atmosphere in

metallurgical operations• Also used as “town gas” for domestic

purposes

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5. OIL GAS• Oil gas is a mixture of hydrocarbons• Prepared by thermal cracking of kerosene oil • Kerosene oil consists of hydrocarbons ranging

from C11H22to C16H34

C11H22

Thermal Cracking

CH4 + C2H6 + C2H2+ C3H6 + CLower Hydrocarbons Tar

Kerosene Oil

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Manufacture of Oil Gas

• Kerosene oil is allowed to fall drop wise on the bottom of the red hot cast iron retort heated in a furnace

• The oil on coming in contact with red hot bottom, immediately gets cracked into a number of lower gaseous hydrocarbons

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• 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 water

• Tar gets condensed and is removed

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• The extent of cracking is tested time to time by withdrawing some gas samples through testing tap and examining its colour

• A good oil gas appears golden in colour

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• 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 required

• The yield of the gas is about 40 to 50%

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Manufacture of Oil Gas

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Composition of Oil gas

• Composition 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%

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Calorific value Calorific value of the gas is about 4500 – 5400 kcal/m3

Uses of Oil gas• Used as heating fuel in the laboratories of

schools & colleges

• Used to improve the calorific value of water gas and mixture of water gas & oil gas is called carburetted water gas

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6. Carburetted Water GasWater gas enriched with oil gas is called carburetted water gas

Manufacture :• Oil gas is prepared in an auxiliary plant called

carburettor• A checker-work of fire-bricks is heated by

burning a part of water gas• Oil is sprayed over it

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• Oil undergoes thermal cracking & oil gas is produced

• Water gas is passed through it

• The resulting carburetted water gas is passed through hydraulic main

• It is then passed into purifier with the help of exhaust fan followed by scrubbers to remove impurities

62Manufacture of Carburetted Water Gas

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Composition of Carburetted Water gas

Approximate composition of carburetted water gas is :

CO = 30 - 48% H2 = 34 - 38%

CO2 = 2 - 2.5%

N2 = 2.5 - 5%

Hydrocarbons = 30 - 48%

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Properties of Carburetted water gas:• Burns with blue flame• Poisonous in nature due to presence of

carbon monoxide

Uses :• Used for industrial hydrogenation• For manufacture of alcohol• as a fuel in industrial preparation

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7. Bio GasBio gas is produced by the degradation

of biological matter by bacterial action (by anaerobic bacteria) in the absence of

free oxygen

• The cheapest & easily obtainable biogas is gobar gas

• Gobar gas is produced by anaerobic fermentation of cattle dung

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Manufacture of Gobar gas

• The plant for gobar gas is a cylindrical tank made of bricks

• About 6m deep & 2m in diameter• An iron gas holder is suspended upside

down in the tank• Diameter of gas holder is slightly less than

tank

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• 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 pipe

• Used animal waste is taken out in another side tank through an outlet pipe

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To produce gobar gas, gobar ( or human excreta, poultry sweep, plant residue, leaves or any other biomass) is collected

• It is made into slurry with water & is put in the inlet tank• The slurry slides down to the digester

tank through the inlet pipe• The level of the slurry rises in the tank• It undergoes biological degradation by

anaerobic bacteria in the absence of air

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• Gas so produced is collected in gas holder

• Digested slurry (used ) is pushed to the outlet tank

• Digested slurry is dried partially which is an excellent manure for plants

• Gobar gas is easily prepared in summer because optimum temperature for fermentation is 34 - 46oC

70Bio-gas Plant

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Composition of Bio gas

The 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

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Properties of Bio gas

• It is colourless & odourless gas insoluble in water

• It burns with non-luminous blue flame and gives a temperature of 540oC

• It is non-poisnous in nature as it does not contain carbon monoxide

• Calorific value is about 12000 kcal/m3

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Uses of Bio gas• An excellent domestic fuel especially in

villages where dung is available in plenty

• Gas can also be used for lighting and power purposes like running water pumps, chaff cutting machines

• Spent slurry is a richer manure than original dung

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Advantages of gaseous fuels

• Gaseous fuels can be transported through a pipe line to the place of use

• Can be lighted at a moment’s notice• Calorific value is high• Burn with high efficiency and a high

temperature flame can be obtained in no time

• Do not produce any smoke or ash• Burning can be easily controlled

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• Can be preheated by the heat of waste gases & hence their use is economical as far as heat is concerned

• Flame can be made oxidising, reducing by regulating the air supply. This is required in metallurgical operations

• Burn in slight excess of air

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Disadvantages

• Large storage tanks are required for storing

• Highly inflammable, hence risk of fire hazards are quite high

• Are usually suffocating• Cost of production is quite high• Great care is needed for storing &

transportation

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Thank You