Fuels

Cost: Coke should be cheap and easily available.Manufacture of Metallurgical Coke: Beehive OvenOtto Hoffmann oven(i) Beehive oven: A beehive oven is a fire-brick chamber having a dome-shaped structure. The dimensions of a typical oven are 4m and 2.5m high. The roof is provided with a hole for charging the coal from the top. Another hole, the discharging hole is provided in the circumference of the lower part of the wall. A number of ovens are built in a row with common walls between neighbouring ovens.

Fig. 5: Beehive coke ovenDemerits of Beehive ovens: The demerits areNo recovery of byproducts, which are useful chemicals and are allowed to escape.Lower coke yield due to partial combustionLack of flexibility of operation(ii) Otto-Hoffmanns oven or By-product Oven: The beehive ovens have been replaced by chamber ovens which works on regenerative principle of heat economy. All the valuable products are recovered from the outgoing flue gases.Construction: It consists of no. of narrow rectangular chambers made of silica bricks.

Fig. 6: A single chamber of Otto Hoffmanns ovenWorking: Coal is charged into the chamber.The coke ovens are heated to 1200oC by burning gaseous fuels.The process of carbonization takes place layer by layer in the coal charge.As the coal adjacent to the oven walls gets heated, a plastic zone is formed which moves away from the walls towards the central zone.As the coal is converted into coke, there is decrease in volume. This is because of the removal of volatile matter in the form of tar and gas at about 500oC. At further high temperature, the plastic mass solidifies into hard and porous mass called coke.Regenerative principle is employed to achieve as economical heating as possible.Regenerators are built underneath the ovens.The flue gases pass their heat to the checker brick work of regenerators until the temperature rises to 1000oC.Regenerators work on the principle of alternate heating and cooling cycles. This is achieved by periodically changing the direction of flow of gases through the vertical flues every 30 min or so.Carbonization of a charge of coal takes about 11-18 hours. After the process is complete, red hot coke is pushed outside by means of a ram which is electrically driven. The coke falls into a quenching car. The yield is 75 % of coal.

Fig. 7: Otto Hoffmanns Byproduct coke ovenRecovery of byproducts: The gases and vapours evolved on carbonization in coke ovens are not allowed to mix with the combustion and are collected separately.The coke oven gas is treated separately for the recovery of the valuable byproducts.

Fig. 8: Coke-Oven gas treatment plantRecovery of Tar: The gas from the coke ovens is passed through a tower in which liquor ammonia is sprayed.Tar and dust get collected in a tank. The tank is provided with a heating coils to recover back ammonia.Recovery of ammonia: The gases are then passed through a tower where water is sprayed to recover ammonia. The ammonia can also be recovered by dissolving it in H2SO4 to form (NH4)2SO4, which is then used as a fertilizer. Recovery of Naphthalene: The gases are passed through a cooling tower, where water at a low temperature is sprayed.The gas is scrubbed with water until its temp. reduces.(iv) Recovery of Benzol: The gases are then introduced into a light oil or benzol scrubber, where benzene along with its homologue is removed and is collected at the bottom.(v) Recovery of H2S and other S compounds: are removed from the coke oven gas after the light oil has been separated out.

The SO2 obtained can be used for the manufacturing of sulphuric acid, which can be used to absorb NH3 from the coal gas.

Liquid Fuels: The importance of liquid fuels is the fact that almost all combustion engines run on them.The largest source of liquid fuels is petroleum. The calorific value of petroleum is about 40000 kJ/kg.There are other supplements of liquid fuels such as coal tar, crude benzol, syntheic liquid fuel made from coal etc.Petroleum: The term petroleum means rock oil. It is also called mineral oil. Petroleum is a complex mixture of paraffinic, olefinic and aromatic hydrocarbons with small quantities of organic compounds containing oxygen, nitrogen and sulphur. Composition:

ElementCarbonHydrogenSulphurOxygenNitrogenPercentage80-8711.1-150.1-3.50.1-0.90.4-0.9The ash of the crude oil is 0.1%.Metals e.g., Silicon, iron, aluminium, calcium, magnesium, nickel and sodium.Crude oil is a mixture of straight chain paraffins and aromatic hydrocarbons e.g., benzene, toluene, naphthalenes etc.Sulphur is present in the form of derivatives of hydrocarbons such as alkylsulphides, aromatic sulphides etc. Nitrogen is present in the form of pyridine, quinoline derivatives, pyrrole etc. Comined oxygen is present as carboxylic acids, ketones and phenols.The objectionable odour of crude petroleum is due to the presence of sulphur compounds in it.Classification of Crude PetroleumResidue obtained after distillationNameContents

Paraffin wax

Asphalt

Paraffin wax and asphaltParaffin base

Asphaltic baseMixed baseStraight chain hydrocarbons and small amounts naphthenes and aromatic hydrocarbonsAromatic and naphthenic hydrocarbonsParaffins, naphthenes and aromatic hydrocarbonsProcessing of Crude Petroleum: Petroleum is found deep below the earth crust. The oil is found floating over salt water or brine. Generally, accumulation of natural gas occurs above the oil.

Fig. 9: Pumping of oilRefining of PetroleumCrude oil reaching the surface, generally consists of a mixture of solid, liquid and gaseous hydrocarbons containing sand and water.After the removal of dirt, water and much of the associated natural gas, the crude oil is separated into a no of useful fractions by fractional distillation.The resultant fractions are then subjected to purification known as refining of petroleum.Steps involved in refining of petroleum:(i) Demulsification: The crude oil coming out from the well, is in the form of stable emulsion of oil and salt water, which is yellow to dark brown in colour.The demulsification is achieved by Cottrells process, in which the water is removed from the oil by electrical process. The crude oil is subjected to an electrical field, when droplets of colloidal water coalesce to form large drops which separate out from the oil.(ii) Removal of harmful impurities: Excessive salt content such as NaCl and MgCl2 can corrode the refining equipment. These are removed by washing with water.The objectionable sulphur compound are removed by treating the oil with copper oxide. The copper sulphide so formed is separated by filtration. (iii) Fractional Distillation: 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.

Fig. 10: Fractional distillation of crude petroleumThe hot vapours from the crude are passed through a tall fractionating column, called bubble tower.Bubble tower consists of horizontal trays provided with a no of small chimneys, through which vapours rise.These chimneys are covered with loose caps, known as bubble caps. These bubble caps help to provide an intimate contact between the escaping vapours and down coming liquid.The temperature in the fractionating tower decreases gradually on moving upwards.As the vapours of the crude oil go up, they become gradually cooler and fractional condensation takes place at different heights of column.The residue from the bottom of the fractionating tower is vacuum distilled to recover various fractions

Fig. 11: Vacuum distillation of residual oilThere is yet another type of fractional distillation called Top-flashing.

In top flashing, there is better control of product composition, but requires more pumps and instruments and hence is an expensive process.

Fig. 11: Top FlashingCracking: Gasoline is the most imp fraction of crude petroleum. The yield of this fraction is only 20% of the crude oil. The yield of heavier petroleum fraction is quite high. Therefore, heavier fractions are converted into more useful fraction, gasoline.This is achieved by a technique called 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.e.g:

Nearly 50% of todays gasoline is obtained by cracking. The gasoline obtained by cracking is far more superior than straight run gasoline.The process of cracking involves the following chemical changes:Higher hydrocarbons are converted to lower hydrocarbons by C-C cleavage. The product obtained on cracking have low boiling points than initial reactant.Formation of branched chain hydrocarbons takes place from straight chain alkanes.Unsaturated hydrocarbons are obtained from saturated hydrocarbons.Cyclization may takes place.Cracking can also be used for the production of olefins from naphthas, oil gas from kerosene. Cracking can be carried out by two methodsThermal Cracking: When it takes place simply by the application of heat and pressure, the process is called thermal cracking. The heavy oils are subjected to high temperature and pressure, when the bigger hydrocarbons break down to give smaller molecules of paraffins, olefins etc. The thermal stability among the constitutents of petroleum fractions increases asParaffins < naphthenes < aromatics(a) Liquid Phase thermal cracking: The charge is kept in the liquid form by applying high pressures of the range 30-100 kg/cm2 at a suitable temperature of 476-530 oC. The cracked products are separated in a fractionating column.The important fractions are: Cracked gasoline (30-35%), Cracking gases (10-45%); Cracked fuel oil (50-55%).(b) Vapour phase thermal cracking: By this method, only those oils which vapourize at low temperatures can be cracked. The petroleum fractions of low boiling range like kerosene oil, are heated at a temp of 670-720 oC under low pressure.Mechanism of thermal cracking: It follows free radical mechanism.Initiation

PropagationThe free radical formed are thermally unstable and undergo fission at the b-position to yield a new radical and an olefin.

Catalytic cracking: Cracking is brought about in the presence of a catalyst at much lower temperatures and pressures. The catalyst used is mainly a mixture of silica and alumina. Most recent catalyst used is zeolite. The quality and yield of gasoline is greatly improved by this method.

Advantages of catalytic cracking over thermal cracking:High temp and pressure are not required in the presence of a catalyst.The use of catalyst not only accelerates the cracking reactions but also introduces new reactions which considerably modify the yield and the nature of the products.The yield of the gasoline is higher.No external fuel is required for cracking.The process can be better controlled so desired products can be obtained.The product contains a very little amount of undesirable sulphur because a major portion of it escapes out as H2S gas, during cracking.It yields less coke, less gas and more liquid products.The evolution of by-product gas can be further minimized, thereby increasing t he yield of desired product.Catalysts are selective in action and hence cracking of only high boiling fractions takes place.Coke forming materials are absorbed by the catalysts as soon as they are formed.Knocking and antiknockingIn an internal combustion engine a mixture of gasoline vapour and air is used as a fuel. After the reaction is initiated by a spark, a flame should spread rapidly and smoothly through the gas mixture and the expanding gas drives the piston down the cylinder. In certain circumstances the rate of oxidation is so high that the last portion of the mixture detonates, producing an explosive sound called knockingThe tendency of fuel constituent to knock is in the following orderStraight chain>branched>olefins>cycloparaffins>aromaticsAn antiknocking agent is used to increase the octane number of a fuel.TEL ( Tetra ethyl lead)Methyl cyclopentadienyl Manganese tricarbonylFerroceneIron pentacarbonylTolueneIso octane

Anti Knocking agentsOctane number: is defined as the percentage of iso octane present in a mixture of iso-octane and n-heptane, which has the same knocking characteristics as that of fuel under examination, under same set of conditions.Thus a gasoline with an octane no of 80, would give the same knocking as a mixture of iso octane and n-heptane containing 80% of iso octane by volume. Greater the octane number, greater is the antiknock property of the fuel.Cetane Rating: Fuels required for diesel engine are in contrast to petrol engine fuels, hence a separate scale is used to grade the diesel oils as they cannot be graded on octane number scale.The cetane number of a diesel oil is defined as the percentage of cetane in a mixture of cetane and a-methyl naphthalene which will have the same ignition characteristics as the fuel under test, under same set of conditions.Cetane is n-hexadecaneThe cetane rating of a fuel depend upon the nature and composition of hydrocarbon. The straight chain hydrocarbons ignite quite readily while aromatics do not ignite easily. Ignition quality order among the constituents of diesel engine fuels in order of decreasing cetane no, is as follows:n-alkanes> naphthenes > alkenes > aromaticsAniline PointThis is an approximate measure of the aromatic content of a hydrocarbon fuel. It is defined as the lowest temperature at which a fuel oil is completely miscible with an equal volume of aniline. Aniline is an aromatic compound and aromatics are more miscible in aniline than are paraffins.Hence, the lower the aniline point, the higher the aromatics content in the fuel oil. The higher the aromatics content, the lower the cetane number of the fuel.The aniline point can thus be used to indicate the probable ignition behavior of a diesel fuel.Diesel IndexThe Diesel Index indicates the ignition quality of the fuel. It is found to correlate, approximately, to the cetane number of commercial fuels. It is obtained by the following equation

Diesel Index and cetane number are usually about 50. Lower values will result in smoky exhaust

Gaseous FuelsAdvantages of gaseous fuelsLeast amount of handlingSimplest burners systemsBurner systems require least maintenanceEnvironmental benefits: lowest GHG and other emissions36Gas 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.Gaseous FuelsClassification of gaseous fuels (A) Fuels naturally found in natureNatural gasMethane from coal mines(B) Fuel gases made from solid fuelGases derived from coalGases derived from waste and biomassFrom other industrial processes (C) Gases made from petroleumLiquefied Petroleum gas (LPG)Refinery gasesGases from oil gasification(D) Gases from some fermentation37The following types of gaseous fuels exist:Fuels naturally found in nature:- Natural gas- Methane from coal minesFuel gases made from solid fuel- Gases derived from coal- Gases derived from waste and biomass- From other industrial processes (blast furnace gas) Gases made from petroleum- Liquefied Petroleum gas (LPG)- Refinery gases- Gases from oil gasificationGases from some fermentation processGaseous FuelsCalorific value Fuel should be compared based on the net calorific value (NCV), especially natural gasTypical physical and chemical properties of various gaseous fuelsFuel Gas Relative Density Higher Heating Value kCal/Nm3Air/Fuel ratio m3/m3 Flame Temp oCFlame 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 38Since most gas combustion appliances cannot utlilize the heat content of the water vapour, gross calorific value is of little interest. Fuel should rather be compared based on the net calorific value. This is especially true for natural gas, since increased hydrogen content results in high water formation during combustion. Typical physical and chemical properties of various gaseous fuels are given in this table.Type of FuelsGaseous FuelsNatural gasMethane: 95%Remaing 5%: ethane, propane, butane, pentane, nitrogen, hydrogen, CO, carbon dioxide, other gasesHigh calorific value fuelDoes not require storage facilitiesNo sulphurMixes readily with air without producing smoke or soot 39Methane is the main constituent of natural gas and accounts for about 95% of the total volume. Other components are: ethane, propane, butane, pentane, nitrogen, carbon dioxide, and traces of other gases. As methane is the largest component of natural gas, generally properties of methane are used when comparing the properties of natural gas to other fuels. Natural gas is a high calorific value fuel that doesnt require any storage facilities. It mixes with air readily and does not produce smoke or soot. It has no sulphur content. It is lighter than air and disperses into air easily in case of leak. Application of Natural GasNatural gas is finding increasing use as a domestic, transportation and industrial fuel.It is used as a raw material for the synthesis of methanol, formaldehyde and other chemical compounds.It is used as a source of H2Ammonia used for the process for extracting nickel from its ore is made by reacting N2 with natural gas.Varieties of LPG bought and sold include mixes that are primarily propane (C3H8), primarily butane (C4H10) and, most commonly, mixes including both propane and butane and isobutane depending on the season in winter more propane, in summer more butane[. Propylene and butylenes are usually also present in small concentration. A powerful odorant, ethanethiol, is added so that leaks can be detected easilyLPGLiquefied petroleum gas (also called LPG, GPL, LP Gas, or liquid propane gas) is a flammable mixture of hydrocarbon gases used as a fuel in cooking, heating appliances and vehicles.It is used as a feedstock for the manufacture of various chemicals and olefins by pyrolysis.Other industrial application includes its use in portable blow lamps, soldering, welding, annealing hardening, brazing, steel cutting etc. It is increasingly used as an aerosol propellant and a refrigerant, replacing chlorofluorocarbons in an effort to reduce damage to the ozone layer. When specifically used as a vehicle fuel it is often referred to as autogas.Application of LPGCNG 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. It is stored and distributed in hard containers at a pressure of 200248 bar (29003600 psi), usually in cylindrical or spherical shapes.Compressed Natural Gas(CNG)Compressed natural gas (CNG) 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.Cars and locomotives.

Type of FuelsComparing FuelsFuel OilCoalNatural GasCarbon 8441.1174Hydrogen 122.7625Sulphur 30.41-Oxygen 19.89TraceNitrogen Trace1.220.75Ash Trace38.63-Water Trace5.98-45A typical comparison of carbon contents in oil, coal and gas is given in this table. (Reflection time before continuing) Numericals on Combustion of fuel1. Calculate the weight and volume of air required for combustion of 3kg of carbon.2. Calculate the minimum weight of air required for complete combustion of 1kg of fuel containing C=90%, H=3.5%, O= 3.0%, S= 0.5%, water=1%, N=0.5% and rest is ash.3. A gaseous fuel has the following composition by volume: H=20%, CH4= 5%, CO= 20%, CO2= 5% and N= 45%If 50% excess of air is used find the weight of air acutally supplied per m3 of this gas Power Alcohol: Ethyl alcohol is an important liquid fuel and when it is used in an internal combustion engines, it is called power alcohol.Generally gasoline and alcohol are present in the ratio of 4:1. Its octane no is 90.Manufacture of Power Alcohol: Ethyl alcohol can be manufactured from hydrocarbons, carbohydrates, starches and molasses, waste sulphite liquor from paper and pulp mills.Ethyl alcohol is mainly manufactured from molasses. The formation of alcohol involves the following steps:(i) Dilution: High concentration of sugars in molasses (ii) Addition of dilute sulphuric acid: For fermentation the pH of the solution should be kept between 4 to 5.(iii) Addition of ammonium salts: When molasses contain insufficient yeast food, some neutrient substanceslke ammonium sulphate and ammonium phosphate are added to overcome the deficiency of phosphorous and nitrogen.(iv) Addition of yeast: This is the main step in the formation of alcohol from mollases. To the solution a selected strain of yeast is added. The mixture s kept at a temperature of 30 oC for 2-3 days.

(v) Distillation: It is carried out in a specially designed plant called coffey still.

Distillation of wash/wort(vi) Azeotropic distillation: The alcohol obtained after distillation is mixed with benzene or carbontetrachloride and subjected to distillation. The benzene or carbontetrachloride distills over taking away a portion of alcohol and water leaving behind absolute alcohol.Advantages of Alcohol blended gasoline:The octane no of alcohol is quite high. Hence on blending with gasoline, it increases the octane no of gasoline.There is no decrease in power output or increase in specific fuel consumption on blending alcohol with gasoline.3. The starting difficulties because of high boiling point of alcohol is overcome, when it is used in blended form with gasoline.4. Alcohol-petrol blend has a tendency to absorb any traces of moisture present.Disadvantages of Alcohol blended Gasoline:The calorific value of alcohol is low and hence the alcohol lowers the calorific value of petrol.Since the air required for complete combustion of blend is less than the air required for petrol.The ratio of flow of blend is to be increased to 1.56 times to that of pure petrol by altering the size of carburetor jet.