Gas 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.
The following types of gaseous fuels exist: Fuels naturally found in nature: - Natural gas - Methane from coal mines Fuel 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 gasification Gases from some fermentation process
Since 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.
LPG is a predominant mixture of propane and butane with a small percentage of unsaturates and some lighter C2 as well as heavier C5 fractions. LPG may be defined as those hydrocarbons, which are gaseous at normal atmospheric pressure but may be condensed to the liquid state at normal temperature by moderate pressures. Although they are normally used as gases, they are stored and transported as liquids under pressure for convenience and ease of handling. LPG vapour is denser than air. Butane is about twice as heavy as air and propane about one and a half times as heavy as air. Consequently, the vapour may flow along the ground and into drains sinking to the lowest level of the surroundings and be ignited at a considerable distance from the source of leakage.
Methane 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 doesn’t 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.
LNG, or liquefied natural gas, is the same natural gas that we use in our homes for heating and cooling - except that, prior to being sent into the nation's pipelines, it is transported and stored in liquid form, rather than as a gas To condense natural gas into a liquid, it must be cooled to approximately 260 degrees Fahrenheit below zero (or minus 162 degrees Centigrade) at atmospheric pressure. When natural gas condenses, it takes up about 1/600th of the volume it did when it was in its gaseous state. LNG is a clear liquid that is odorless, colorless, non-corrosive and non-toxic. The liquefying process removes impurities found in typical pipeline gas resulting in a LNG composition of mostly methane with small amounts of other hydrocarbons and nitrogen.
A typical comparison of carbon contents in oil, coal and gas is given in this table. (Reflection time before continuing)
Fuel and combustion
FUEL AND COMBUSTION Prof R K Patel Dept. of ChemistryFuels and combustion, Fuels-Classification of fuels, calorific value -LCV, HCV; measurement of calorific value using bomb calorimeter(numerical problems). Combustion: Calculation of air qualities(problems). Solid fuel, proximate and ultimate analysis ( problems).Carbonization of coal. Liquid Fuels: Knocking and anti-knocking forpetrol and diesel (octane number and cetane number) - diesel index.Refining of liquid fuels, cracking of petroleum. Gaseous fuels: LPG,natural gas, CNG: Composition and applications. Biofuels: Biodiesel andBiogas -composition and applications. Next generation fuels.
FuelsFuel is a combustible substance which duringcombustion gives large amount of heat.There are chemical fuels, nuclear fuels and fossilfuels.Classification of FuelsThese can be classified on the basis of theiroccurrence and physical stateOn the basis of occurrence they are of two types:Primary Fuels: Fuels which occur in nature as suchare called primary fuels. E.g., wood, peat, coal,petroleum, and natural gas.
Secondary Fuels: The fuels which are derived fromthe primary fuels by further chemical processing arecalled secondary fuels. E,g., coke, charcoal,kerosene, coal gas, producer gas etc.(ii) On the basis of physical state these may beclassified as:Solid FuelsLiquid FuelsGaseous FuelsCalorific value: It is defined as the total quantity ofheat liberated when a unit mass of a fuel is burntcompletely.
Units of Calorific value: System Solid/Liquid Gaseous Fuels Fuels CGS Calories/gm Calories/cm3 MKS k cal/kg k cal/m3 B.T.U BTU/lb BTU/ft3The quantity of heat can be measured in thefollowing units:(i) Calorie: It is defined as the amount of heatrequired to raise the temperature of 1gm of water by1oC 1 calorie = 4.184 Joules
(ii) Kilo Calorie: 1 k cal = 1000 cal(iii) British thermal unit: (B. T. U.) It is defined asthe amount of heat required to raise the temperatureof 1 pound of water through 1oF.1 B.T.U. = 252 Cal = 0.252 k cal(IV) Centigrade heat unit (C.H.U): It is defined asthe amount of heat required to raise the temperatureof 1 pound of water through 1oC.1k cal = 3.968 B.T.U. = 2.2 C.H.U. restore
Characteristics of Good Fuel:(i) Suitability: The fuel selected should be most suitable for the process. E.g., coke made out of bituminous coal is most suitable for blast furnace.(ii) High Calorific value(iii) Ignition Temperature: A good fuel should have moderate ignition temperature.(iv) Moisture content: Should be low(v) Non combustible matter content(vi) Velocity of combustion: It should be moderate(vii) Nature of the products(viii) Cost of fuel, (ix) Smoke, (x) Control of the process
Gross and net calorific ValueGross Calorific Value: It is the total amount of heatgenerated when a unit quantity of fuel is completelyburnt in oxygen and the products of combustion arecooled down to the room temperature.As the products of combustion are cooled down toroom temperature, the steam gets condensed intowater and latent heat is evolved. Thus in thedetermination of gross calorific value, the latent heatalso gets included in the measured heat. Therefore,gross calorific value is also called the higher calorificvalue.The calorific value which is determined by Bombcalorimeter gives the higher calorific value (HCV)
Net Calorific Value: It is defined as the net heatproduced when a unit quantity of fuel is completelyburnt and the products of combustion are allowed toescape.The water vapour do not condense and escape withhot combustion gases. Hence, lesser amount thangross calorific value is available. It is also known aslower calorific value (LCV).LCV=HCV-Latent heat of water vapours formedSince 1 part by weight of hydrogen gives nine partsby weight of water i.e. H + 1O → H O 2 2 2 2
Therefore,LCV=HCV-weight of hydrogen x 9 x latent heat of steam= HCV-weight of hydrogen x 9 x 587Determination of Calorific value1. Determination of calorific value of solid and non volatile liquid fuels: It is determined by bomb calorimeter.Principle: A known amount of the fuel is burnt in excess of oxygen and heat liberated is transferred to a known amount of water. The calorific value of the fuel is then determined by applying the principle of calorimetery i.e. Heat gained = Heat lost
CalculationsLet weight of the fuel sample taken = x gWeight of water in the calorimeter = W gWater equivalent of the Calorimeter, stirrer, bomb,thermometer = w gInitial temperature of water = t1oCFinal temperature of water = t2oCHigher or gross calorific value = C cal/gHeat gained by water = W x ∆t x specific heat of water = W (t2-t1) x 1 cal
Heat gained by Calorimeter = w (t2-t1) calHeat liberated by the fuel = x C calHeat liberated by the fuel = Heat gained by water andcalorimeterx C = (W+w) (t2-t1) calC=(W+W)(t2-t1) cal/g x
Net Calorific value:Let percentage of hydrogen in the fuel = HWeight of water produced from 1 gm of the fuel =9H/100 gmHeat liberated during condensation of steam= 0.09H × 587 calNet (Lower calorific value) = GCV-Latent heat ofwater formed= C-0.09H × 587 cal/gmCorrections: For accurate results the followingcorrections are also incorporated:
(a)Fuse wire correction: As Mg wire is used for ignition, the heat generated by burning of Mg wire is also included in the gross calorific value. Hence this amount of heat has to be subtracted from the total value.(b)Acid Correction: During combustion, sulphur and nitrogen present in the fuel are oxidized to their corresponding acids under high pressure and temperature. S + O → SO 2 2 2SO + O + 2H O → 2H SO ∆H = -144,000 Cal 2 2 2 2 4 2 N + 5O + 2H O → 4HNO ∆H = -57,160 Cal 2 2 2 3
The corrections must be made for the heat liberatedin the bomb by the formation of H2SO4 and HNO3.The amount of H2SO4 and HNO3 is analyzed bywashings of the calorimeter.For each ml of 0.1 N H2SO4 formed, 3.6 caloriesshould be subtracted.For each ml of 0.01 HNO3 formed, 1.43 calories mustbe subtracted.(C) Cooling correction: As the temperature risesabove the room temperature, the loss of heat does
occur due to radiation, and the highest temperaturerecorded will be slightly less than that obtained. Atemperature correction is therefore necessary to getthe correct rise in temperature.If the time taken for the water in the calorimeter tocool down from the maximum temperature attained, tothe room temperature is x minutes and the rate ofcooling is dt/min, then the cooling correction = x × dt.This should be added to the observed rise intemperature.Therefore, Gross calorific valueC=(W+w)(t2-t1+Cooling correction)-[Acid+ fusecorrections] / Mass of the fuel.
JUNKERS GAS CALORIMETERAIM :To determine calorific value of gaseous fuel by Junkers gas calorimeterAPPARATUS: The apparatus mainly consists of a cylindrical shell with copper coil arranged in two passage configuration with water inlet and outlet to circulate through the copper coil, a pressure regulator, a wet type gas flow meter & a gas Bunsen burner.DESCRIPTION: Determination of calorific value (heat value) of combustible gases is essential to assess the amount of heat given away by the gas while burning a known amount of gas to heat a known amount of fluid (water) in a closed chamber.
PROCEDURE:Install the equipment on a flat rigid platform near an uninterrupted continuous water source of ½” size and a drain pipe. Connect the gas source to the pressure regulator, gas flow meter and the burner respectively in seriesInsert the thermometer / temperature sensors, into their respective places to measure water inlet and outlet temperatures and a thermometer to measure the flue gas temperature at the flue gas outletStart the water flow through the calorimeter at a study constant flow rate and allow it to drain through over flow. Start the gas flow slowly and light the burner out side the calorimeter
Regulate the flow of gas at a steady rate to any designed flow (Volume) Insert the burner into the calorimeter and allow the out let water temperature to attain a steady state Swing the out let to a 1000 ml jar and start. The stop watch simultaneously, record the initial gas flow meter reading at the same timeNote down the time taken to fill 1000ml and at the same time the final gas flow reading recorded by the gas flow meterTabulate all the reading and calculate the calorific valve of the gas under testRepeat the experiment by varying the water flow rate or gas flow for different conditions.After the experiment is over stop the gas flow, water flow, and drain the
Theoretical calculation of Calorific value of a Fuel:The calorific value of a fuel can be calculated if thepercentages of the constituent elements are known. Substrate Calorific value Carbon 8080 Hydrogen 34500 Sulphur 2240
If oxygen is also present, it combines with hydrogen toform H2O. Thus the hydrogen in the combined form isnot available for combustion and is called fixedhydrogen.Amount of hydrogen available for combustion = Totalmass of hydrogen-hydrogen combined with oxygen. 1 H 2 + O2 → H 2O 2 1g 8g 9gFixed Hydrogen = Mass of oxygen in the fuelTherefore, mass of hydrogen available for combustion= Total mass of hydrogen-1/8 mass of oxygen in fuel=H-O/8
Dulong’s formula for calculating the calorific value isgiven as:Gross calorific Value (HCV) 1 O = [8080C + 34,500( H − ) + 2,240 S ]kcal / kg 100 8Net Calorific value (LCV) 9H = [ HCV − × 587]kcal / kg 100 = [ HCV − 0.09 H × 587]kcal / kg
Solid Fuels: Primary as well as secondary are widelyused in domestic and industrial purposes.e.g., wood, coal, charcoal and coke.Wood: Wood has been used as a fuel from ancienttimes. Due to large scale deforestation, wood is nolonger used except in forest areas where wood isavailable at a low cost.
Wood when freshly cut contains 25-50% moisture.Normally it is used in air dried condition with 10-15percent moisture content.The calorific value of air dried wood is about 3500-4500 kcal/kg.When wood burns, the ash content is low but theoxygen content is very high. This makes even drywood a fuel of low calorific value.Wood charcoal is obtained by destructive distillation ofwood.The major use of wood charcoal is for producingactivated carbon.
Coal: coal is regarded as a fossil fuel produced from the vegetable debris under conditions of high temperature and pressure over million of years.The transformation of the vegetable debris to coal takes place in two stages:(a)Biochemical or peat stage: During this stage, the plant materials were attacked by various micro organisms.(b)Chemical stage or metamorphism: In this stage, the peat deposit buried under sedimentary deposits lose moisture and volatile components under the effect of high temperature and pressure.The peat gets enriched in carbon whereas its oxygen content decreases.
The spongy peat transforms into hard brittle coalgradually. The time required for the formation ofyoung brown coal is of the order of 107 years.Classification of Coal: Coals are mainly classified onthe basis of their degree of coalification from theparent material, wood. When wood is converted intocoal, there is gradual increase in the concentration ofcarbon and decrease in the percentage of oxygen andnitrogen.Coal is given a ranking depending upon the carboncontent of the coal from wood to anthracite.
Type of Percentage (dry, mineral matter % calorificcoal free basis) moist value ure C H O N VMWood 45-50 5-6 20-40 0-0.5 - 70-90 4000- 4500Peat 45-60 3.5-6.5 20-45 0.75-3 45-75 70-90 4125- 5280 6600-Brown Coal 60-75 4.5-5.5 17-35 0.75-2 45-60 30-50 7100 6600-Bituminous 75-90 4.0-5.5 20-30 0.75-2 11-50 10-20 8800coal 8470-Anthracite 90-95 3-4 2-3 0.5-2 3.8-10 1.5-3.5 8800
Analysis of CoalCoal is analysed in two ways:1. Proximate analysis2. Ultimate analysisThe results of analysis are generally reported in the following ways:As received basisAir dried basisMoisture free basis (oven dried)Moisture and ash free basis
Proximate AnalysisThe data varies with the procedure adopted and hence it is called proximate analysis.It gives information about the practical utility of coal.Proximate analysis of coal determines the moisture, ash, volatile matter and fixed carbon of coal.1. Moisture Content: Air dried moisture is determined by heating a known amount of coal to 105-110 oC in an electric hot air oven for about one hour. After one hour, it is taken out from the oven and cooled in a dessicator and weighed.Percentage of moisture= Loss in weight 100 × Weight of coal taken
•Excess of moisture is undesirable in coal.•Moisture lowers the heating value of coal and takesaway appreciable amount of the liberated heat in theform of latent heat of vapourisation.•Excessive surface moisture may cause difficulty inhandling the coal.•Presence of excessive moisture quenches fire in thefurnace.2. Volatile Matter: consists of a complex mixture ofgaseous and liquid products resulting from the thermaldecomposition of the coal.
It is determined by heating a known weight ofmoisture free coal sample in a covered platinumcrucible at 950 ± 20oC for 7 minutes.Percentage of volatile matter =Loss of weight due to volatile matter × 100 Weight of coal sample takenSignificanceA high percent of volatile matter indicates that a largeproportion of fuel is burnt as a gas.The high volatile content gives long flames, highsmoke and relatively low heating values.
For efficient use of fuel, the outgoing combustiblegases has to be burnt by supplying secondary air.High volatile matter content is desirable in coal gasmanufacture because volatile matter in a coaldenotes the proportion of the coal which will beconverted into gas and tar products by heat.Ash: Coal contains inorganic mineral substanceswhich are converted into ash by chemical reactionsduring the combustion of coal.Ash usually consists of silica, alumina, iron oxide andsmall quantities of lime, magnesia etc.Ash content is determined by heating the residue leftafter the removal of volatile matter at 700 ± 50oC for½ an hour without covering
Weight of the residue left 100 ×Percentage of ash = Weight of the coalAsh can be classified as intrinsic ash and extrinsicash.The mineral matter originally present in vegetablematter from which the coal was formed is calledintrinsic ash. It consists of oxides of Na, K, Mg, Ca andSi.The mineral matter like clay, gypsum, dirt which getsmixed up during mining and handling of coal constitutethe extrinsic ash which remains as a residue after thecombustion. E.g., CaSO4, CaCO3, Fe2O3 etc.
The high percentage of ash is undesirable. Itreduces the calorific value of coal.In furnace grate, the ash may restrict the passageof air and lower the rate of combustion.High ash leads to large heat losses and leads toformation of ash lumps.The composition of ash and fusion range alsoinfluences the efficiency of coal.When coal is used in boiler, the fusion temperatureof ash is very significant. Ash having fusiontemperature below 1200oC is called fusible ash andabove 1430oC is called refractory ash.
Apart from loss of efficiency of coal, clinker formationalso leads to loss of fuel because some coal particlesalso get embedded in the clinkers.Fixed Carbon: Fixed carbon content increases fromlignite to anthracite. Higher the percentage of fixedcarbon greater is its calorific value and better is thequality of coal.The percentage of fixed carbon is given by:Percentage of fixed carbon = 100-[% ofmoisture+volatile matter+ash]Significance: Higher the percentage of fixed carbon,greater its calorific value
•The percentage of fixed carbon helps in designingthe furnace and shape of the fire-box because it is thefixed carbon that burns in the solid state.Ultimate analysis:It is carried out to ascertain the composition of coal.Ultimate analysis includes the estimation of carbon,hydrogen, sulphur, nitrogen and oxygen.1. Carbon and Hydrogen: A known amount of coal istaken in a combustion tube and is burnt in excess ofpure oxygen. C + O → CO 2 2 H + 1O → H O 2 2 2 2
Fig 3. Estimation of carbon and hydrogen 2KOH + CO → K CO + H O 2 2 3 2 CaCl + 7 H O → CaCl .7 H O 2 2 2 244 g of CO2 contain = 12 g of carbonY g of CO2 contain = 12 × y 44
Percentage of carbon = 12 × y ×100 44 weight of coal taken18 g of water contain = 2 g of hydrogenZ g of water contain = 2 × zg of hydrogen 18 Percentage of hydrogen = 2 × z ×100 18 weight of coal takenSignificance:Calorific value of a fuel is directly related to itscarbon content.A higher percentage of carbon reduces the size ofthe combustion chamber
High percentage of hydrogen also increases thecalorific value of coal. The content of hydrogen incoals varies between 4.5 to 6.5 percent from peat tobituminous stage.2. Nitrogen: Nitrogen present in coal sample can beestimated by Kjeldahl’s method. Nitrogen + H SO Heat →( NH ) SO 2 4 42 4The contents are then transferred to a round bottomedflask and solution is heated with excess of NaOH.The ammonia gas thus liberated is absorbed in aknown volume of a standard solution of acid used.
Fig 4. Estimation of nitrogen by Kjeldahl’s methodThe unused acid is then determined by titrating withNaOH. From the volume of acid used by NH3liberated, the percentage of nitrogen can becalculated.
( NH ) SO 2 NaOH → Na SO + 2 NH + 2H O 42 4 2 4 3 2NH + H SO → ( NH ) SO 3 2 4 42 4
Carbonization of Coal (Manufacture of Coke)It is the process of heating the coal in absence of air toa sufficiently high temperature, so that the coalundergoes decomposition and yields a residue whichis richer in carbon content than the original fuel.Caking and coking of coals: some coals have atendency to soften and swell at higher temperatures,to form a solid coherent mass with porous structure.Such coals are called caking coals. The residueformed is called coke. If the coke is hard, porous andstrong, than the coal, from which it is formed, it iscalled coking coal. All coking coals are caking coalsbut all caking coals are not coking coals.This property is found only in bituminous type of coal.
Coals with a high percentage of volatile matter are notfit for coking and are used for gas making. The coalshaving 20-30 % volatile matter are good coking coals.Process of carbonization:First moisture and occluded gases are driven off.At about 260-270oC carbon, water, H2S, some lowmolecular alkenes and alkanes are evolved.At about 350oC the decomposition of coal isaccompanied by evolution of gases and elimination ofvapours takes place.At about 400oC, caking coal becomes soft and plastic.At about 700oC, hydrogen is evolved
Above 800oC, main gaseous products are evolvedGases evolved from the plastic mass, expand it to give foam like appearance.At further high temperatures this foam like mass solidifies to form a solid mass with porous structure called coke.Types of carbonization(i) Low temperature carbonization(ii) High temperature carbonization(i) Low temperature carbonization: When the destructive distillation of coal is carried out at temperatures between 500-700oC.
It is practiced for the production of semi coke. Whichis also called soft coke.The yield of coke is about 75-80 %.The coke thus produced contains 5 to 15 % volatilematter.The various products of low temperaturecarbonization are semi coke, low temperature tar,crude low temperature spirit and gas.LTC plants normally use low rank coals. These lowrank coals produce excessive smoke on burning.Semi coke from LTC is highly reactive and can beeasily ignited into a smokeless flame
The gas which is obtained as a byproduct has highercalorific value of about 6500-9500 kcal/m 3.(ii) High temperature carbonization: It is carried outat 900-1200oC. HTC is used for the production of pure,hard, strong and porous metallurgical coke containing1-3 % volatile matter. The yield of the coke is 65-75%.The byproducts-gas and tar have greater amounts ofaromatic hydrocarbons. The gas which is obtained haslower calorific value of about 5000-6000 kcal/m 3 thanthat produced in LTC; but the yield of the gas is higher.The coke obtained is very much harder than the cokeobtained from LTC process and hence is called hardcoke.
Metallurgical coke: The properties of coke depend on porosity, reactivity and the amount of volatile matter retained by coke during carbonization. Coke is mainly used as a heat source and reducing agent in metallurgy. A good coke in metallurgical process should possess the following characteristics:(i) Purity: The metallurgical coke should contain lower percentage of moisture, ash, sulphur and phosphorous.(ii) Porosity: The coke should be porous so as to provide contact between carbon and oxygen.(iii)Strength: The coke used in metallurgical process should have high strength so as to withstand the
weight of the ore, flux etc. in the furnace.(iv) size: Metallurgical coke should be of mediumsize.(v) Combustibility: Coke should burn easily. Thecombustibility of coke depends on the nature of thecoal, carbonization temperature and reactiontemperature.(vi) Calorific value: It should be high.(vii) Reactivity: Reactivity of coke is its ability to reactwith CO2, steam, air and oxygen. The reactivity shouldnot be too high. The reactivity toward CO 2 representthe reduction of CO2 CO ( g ) + C (s) ⇔ 2CO( g ) 2
Cost: Coke should be cheap and easily available.Manufacture of Metallurgical Coke:(i)
Demerits of Beehive ovens: The demerits are•No recovery of byproducts, which are usefulchemicals and are allowed to escape.•Lower coke yield due to partial combustion•Lack of flexibility of operation(ii) Otto-Hoffmann’s oven or By-product Oven: Thebeehive ovens have been replaced by chamber ovenswhich works on regenerative principle of heateconomy. All the valuable products are recoveredfrom the outgoing flue gases.Construction: It consists of no. of narrow rectangularchambers made of silica bricks.
Fig. 6: A single chamber of Otto Hoffmann’s oven
Working: Coal is charged into the chamber.The coke ovens are heated to 1200oC by burninggaseous fuels.The process of carbonization takes place layer bylayer in the coal charge.As the coal adjacent to the oven walls gets heated, aplastic zone is formed which moves away from thewalls towards the central zone.As the coal is converted into coke, there is decreasein volume. This is because of the removal of volatilematter in the form of tar and gas at about 500 oC. Atfurther high temperature, the plastic mass solidifiesinto hard and porous mass called coke.
Regenerative principle is employed to achieve aseconomical heating as possible.Regenerators are built underneath the ovens.The fluegases pass their heat to the checker brick work ofregenerators until the temperature rises to 1000 oC.Regenerators work on the principle of alternateheating and cooling cycles. This is achieved byperiodically changing the direction of flow of gasesthrough the vertical flues every 30 min or so.Carbonization of a charge of coal takes about 11-18hours. After the process is complete, red hot coke ispushed outside by means of a ram which is electricallydriven. The coke falls into a quenching car. The yieldis 75 % of coal.
Recovery of byproducts: The gases and vapoursevolved on carbonization in coke ovens are notallowed to mix with the combustion and are collectedseparately.The coke oven gas is treated separatelyfor the recovery of the valuable byproducts. Fig. 8: Coke-Oven gas treatment plant
(i) Recovery 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.(ii) 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.(iii) 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 Benzole: The gases are thenintroduced into a light oil or benzol scrubber, wherebenzene along with its homologue is removed and iscollected at the bottom.(v) Recovery of H2S and other S compounds: areremoved from the coke oven gas after the light oil hasbeen separated out. Fe O + 3H S → Fe S + 3H O 2 3 2 2 3 2 2Fe S +4O → 2FeO + 3SO 2 3 2 2 4FeO + O → 2Fe O 2 2 3The SO2 obtained can be used for the manufacturingof sulphuric acid, which can be used to absorb NH 3
Liquid Fuels: The importance of liquid fuels is thefact that almost all combustion engines run on them.The largest source of liquid fuels is petroleum. Thecalorific value of petroleum is about 40000 kJ/kg.There are other supplements of liquid fuels such ascoal tar, crude benzol, syntheic liquid fuel made fromcoal etc.Petroleum: The term petroleum means rock oil. It isalso called mineral oil.Petroleum is a complex mixture of paraffinic, olefinicand aromatic hydrocarbons with small quantities oforganic compounds containing oxygen, nitrogen andsulphur.
The ash of the crude oil is 0.1%.Metals e.g., Silicon,iron, aluminium, calcium, magnesium, nickel andsodium.Crude oil is a mixture of straight chain paraffins andaromatic hydrocarbons e.g., benzene, toluene,naphthalenes etc.Sulphur is present in the form of derivatives ofhydrocarbons such as alkylsulphides, aromaticsulphides etc. Nitrogen is present in the form ofpyridine, quinoline derivatives, pyrrole etc. Cominedoxygen is present as carboxylic acids, ketones andphenols.The objectionable odour of crude petroleum is due tothe presence of sulphur compounds in it.
Classification of Crude PetroleumResidue obtained Name Contentsafter distillationParaffin wax Paraffin Straight chain base hydrocarbons and small amounts naphthenes and aromatic hydrocarbons Aromatic andAsphalt Asphaltic naphthenic base hydrocarbonsParaffin wax and Mixed Paraffins, naphthenes base and aromaticasphalt hydrocarbons
Processing of Crude Petroleum:Petroleum is found deep below the earth crust. The oilis found floating over salt water or brine. Generally,accumulation of natural gas occurs above the oil. Fig. 9: Pumping of oil
Refining of PetroleumCrude oil reaching the surface, generally consists of amixture of solid, liquid and gaseous hydrocarbonscontaining sand and water.After the removal of dirt, water and much of theassociated natural gas, the crude oil is separated intoa no of useful fractions by fractional distillation.The resultant fractions are then subjected topurification known as refining of petroleum.Steps involved in refining of petroleum:(i) Demulsification: The crude oil coming out fromthe well, is in the form of stable emulsion of oil and
The demulsification is achieved by Cottrell’s process,in which the water is removed from the oil by electricalprocess. The crude oil is subjected to an electricalfield, when droplets of colloidal water coalesce to formlarge drops which separate out from the oil.(ii) Removal of harmful impurities: Excessive saltcontent such as NaCl and MgCl2 can corrode therefining equipment. These are removed by washingwith water.The objectionable sulphur compound are removed bytreating the oil with copper oxide. The copper sulphideso formed is separated by filtration.
(iii) Fractional Distillation: It is done in tall fractionatingtower or column made up of steel.In continuous process, the crude oil is preheated to 350-380 oC in specially designed tubular furnace known aspipe still. Fig. 10: Fractional distillation of crude petroleum
The hot vapours from the crude are passed through atall fractionating column, called bubble tower.Bubble tower consists of horizontal trays provided witha no of small chimneys, through which vapours rise.These chimneys are covered with loose caps, knownas bubble caps. These bubble caps help to provide anintimate contact between the escaping vapours anddown coming liquid.The temperature in the fractionating tower decreasesgradually on moving upwards.As the vapours of the crude oil go up, they becomegradually cooler and fractional condensation takesplace at different heights of column.
The residue from the bottom of the fractionating toweris vacuum distilled to recover various fractions Fig. 11: Vacuum distillation of residual oil
There is yet another type of fractional distillation calledTop-flashing. Fig. 11: Top FlashingIn top flashing, there is better control of productcomposition, but requires more pumps andinstruments and hence is an expensive process.
Cracking: Gasoline is the most imp fraction of crudepetroleum. The yield of this fraction is only 20% of thecrude oil. The yield of heavier petroleum fraction isquite high. Therefore, heavier fractions are convertedinto more useful fraction, gasoline.This is achieved by a technique called cracking.Cracking is the process by which heavier fractions areconverted into lighter fractions by the application ofheat, with or without catalyst. Cracking involves therupture of C-C and C-H bonds in the chains of highmolecular weight hydrocarbons.e.g:
C H Cracking→ C H + C H 10 22 5 12 5 10 Decane n - pentane pentene B.Pt =174ο C B.Pt = 36ο C C H Cracking→ C H + C H 8 18 5 12 3 6Nearly 50% of today’s gasoline is obtained bycracking. The gasoline obtained by cracking is farmore superior than straight run gasoline.The process of cracking involves the full chemicalchanges:•Higher hydrocarbons are converted to lower
hydrocarbons by C-C cleavage. The product obtainedon cracking have low boiling points than initialreactant.•Formation of branched chain hydrocarbons takesplace from straight chain alkanes.•Unsaturated hydrocarbons are obtained fromsaturated hydrocarbons.•Cyclization may takes place.Cracking can also be used for the production of olefinsfrom naphthas, oil gas from kerosene. Cracking can becarried out by two methods
Thermal Cracking: When it takes place simply by theapplication of heat and pressure, the process is calledthermal cracking. The heavy oils are subjected to hightemperature and pressure, when the biggerhydrocarbons break down to give smaller molecules ofparaffins, olefins etc. The thermal stability among theconstitutents of petroleum fractions increases asParaffins < naphthenes < aromatics(a) Liquid Phase thermal cracking: The charge iskept in the liquid form by applying high pressures ofthe range 30-100 kg/cm2 at a suitable temperature of476-530 oC. The cracked products are separated in afractionating 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 temperaturescan be cracked. The petroleum fractions of low boilingrange like kerosene oil, are heated at a temp of 670-720 oC under low pressure.Mechanism of thermal cracking: It follows free radicalmechanism.InitiationCH (CH ) CH Heat→ CH (CH ) CH + CH (CH ) CH 3 27 3 3 23 2 2 22 3
PropagationThe free radical formed are thermally unstable andundergo fission at the b-position to yield a new radicaland an olefin. CH 3 − CH 2CH 2 − CH 2 − CH 2 → CH 3 − CH 2 = CH 2Catalytic cracking: Cracking is brought about in thepresence of a catalyst at much lower temperaturesand pressures. The catalyst used is mainly a mixtureof silica and alumina. Most recent catalyst used iszeolite. The quality and yield of gasoline is greatlyimproved by this method.
Advantages of catalytic cracking over thermalcracking:•High temp and pressure are not required in thepresence of a catalyst.•The use of catalyst not only accelerates the crackingreactions but also introduces new reactions whichconsiderably modify the yield and the nature of theproducts.•The yield of the gasoline is higher.•No external fuel is required for cracking.
•The process can be better controlled so desiredproducts can be obtained.•The product contains a very little amount ofundesirable sulphur because a major portion of itescapes out as H2S gas, during cracking.•It yields less coke, less gas and more liquid products.•The evolution of by-product gas can be furtherminimized, thereby increasing t he yield of desiredproduct.•Catalysts are selective in action and hence crackingof only high boiling fractions takes place.•Coke forming materials are absorbed by the catalystsas soon as they are formed.
Knocking and Anti-knockingIn a spark-ignition petrol engine, a phenomenon that occurs whenunburned fuel-air mixture explodes in the combustion chamberbefore being ignited by the spark. The resulting shock wavesproduce a metallic knocking sound. Loss of power occurs, whichcan be prevented by reducing the compression ratio, re-designingthe geometry of the combustion chamber, or increasing the octanenumber of the petrol.(formerly by the use of tetraethyl lead anti-knock additives, but now increasingly by MTBE – methyl tertiarybutyl ether in unleaded petrol). An antiknock agent is a gasolineadditive used to reduce engine knocking and increase the fuelsoctane rating.The typical antiknock agents in use are:Tetra-ethyl lead (phased out)Methyl cyclo pentadienyl manganese tricarbonyl (MMT)Ferrocene, Iron pentacarbonyl, Toluene, Isooctane
Octane rating of a spark ignition engine fuel is ameasure of the resistance to detonation or knockingcompared to a mixture of iso -octane (2,2,4-tri methylpentane, an isomer of octane) and n- heptane. It is anumerical representation of the antiknock properties ofmotor fuel, compared with a standard reference fuel,such as isooctane, which has an octane number of 100.Octane rating does not relate to the energy content ofthe fuel .It is only a measure of the fuels tendency toburn in a controlled manner, rather than exploding in anuncontrolled manner.
Octane number: is defined as the percentage of isooctane present in a mixture of iso-octane and n-heptane, which has the same knocking characteristicsas that of fuel under examination, under same set ofconditions.Thus a gasoline with an octane no of 80, would givethe 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 antiknockproperty of the fuel.Cetane Rating: Fuels required for diesel engine are incontrast to petrol engine fuels, hence a separate scaleis used to grade the diesel oils as they cannot begraded on octane number scale.
The cetane number of a diesel oil is defined as thepercentage of cetane in a mixture of cetane and a-methyl naphthalene which will have the same ignitioncharacteristics as the fuel under test, under same setof conditions.Cetane is n-hexadecaneThe cetane rating of a fuel depend upon the natureand composition of hydrocarbon.The straight chainhydrocarbons ignite quite readily while aromatics donot ignite easily. Ignition quality order among theconstituents of diesel engine fuels in order ofdecreasing cetane no, is as follows:n-alkanes> naphthenes > alkenes > branched alkanes> aromatics
Aniline 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 Index The 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 equationDiesel Index = ( ) ( aniline po int o F x Degrees API gravity 60o F ) 100 In API (American Petroleum Institute) scale, water at 600F has a 0API Of 10. Diesel Index and cetane number are usually about 50. Lower values will result in smoky exhaust
Gaseous Fuels Advantages of gaseous fuels • Least amount of handling • Simplest burners systems • Burner systems require least maintenance • Environmental benefits: lowest GHG and other emissions
Gaseous FuelsClassification of gaseous fuels (A) Fuels naturally found in nature -Natural gas -Methane from coal mines (B) Fuel gases made from solid fuel -Gases derived from coal -Gases derived from waste and biomass -From other industrial processes (C) Gases made from petroleum -Liquefied Petroleum gas (LPG) -Refinery gases -Gases from oil gasification (D) Gases from some fermentation
Gaseous Fuels Calorific value • Fuel should be compared based on the net calorific value (NCV), especially natural gasTypical physical and chemical properties of various gaseous fuelsFuel Relative Higher Heating Air/Fuel Flame FlameGas Density Value kCal/Nm3 ratio m3/m3 Temp oC speed m/sNatural 0.6 9350 10 1954 0.290GasPropane 1.52 22200 25 1967 0.460Butane 1.96 28500 32 1973 0.870
Type of FuelsGaseous Fuels Liquefied Petroleum Gas (LPG) • Propane, butane and unsaturates, lighter C2 and heavier C5 fractions • Hydrocarbons are gaseous at atmospheric pressure but can be condensed to liquid state • LPG vapour is denser than air: leaking gases can flow long distances from the source
Type of FuelsGaseous Fuels Natural gas • Methane: 95% • Remaing 5%: ethane, propane, butane, pentane, nitrogen, carbon dioxide, other gases • High calorific value fuel • Does not require storage facilities • No sulphur • Mixes readily with air without producing smoke or soot
Type of Gaseous FuelsCNGCompressed 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 doesnt increase the concentration of carbon in the atmosphere.CNG is made by compressing natural gas (which is mainly composed of methane [CH4]), to less than 1% of the volume it occupies at standard atmospheric pressure. It is stored and distributed in hard containers at a pressure of 200–248 bar (2900–3600 psi), usually in cylindrical or spherical shapes.Applications Cars Locomotives
Liquefied Natural Gas LNG is natural gas that has been super cooled to minus 260 degrees F becoming liquid for easier storage and shipping LNG is a clear, odorless, colorless, non-corrosive and non-toxic liquid LNG takes up 1/600th of the space –simplifying storage and transportation
COMBUSTIONCombustion reactions are exothermic reactionsaccompanied by evolution of heat and light and thetemperature rises considerably. The amount of oxygenor air required for combustion of a given sample of fuelcan be calculated.Calculation of Air QuantitiesTo determine the amount of oxygen and hence the amount of airrequired for combustion for a unit quantity of fuel, the followingchemical principles are applied.(1) Substances always combine in definite proportions given bymolecular mass. C + O2 → Co2 12 32 4412 g of carbon requires 32 g of oxygen and 44 g of CO2 is formed.
(2) 22-4 L of a gas at 0°C and 760 mm pressure has a mass equal to 1 mol. That is, 22-4 L of oxygen has a mass of 32 g.(3) Air contains 21% oxygen by volume and 23% oxygen by mass. From the amount of oxygen required by the fuel, the amount of air can be calculated. 1 kg oxygen is supplied by 1 x 100/23 = 4.35 kg of air 1 m3 of oxygen is supplied by 1x100/21= 4.76 m3 of air(4) The molar mass of air is 28.94 g mol(5) Minimum oxygen required for combustion is equal to the theoretical oxygen required minus the oxygen present in the fuel.
certain temperature and pressure by assuming that the gas behaves ideally. (PV = nRT) The total amount of oxygen consumed is given by the sum of the amount of oxygen required by individual combustible constituents present in the fuel.
Procedure for combustion calculations:Reaction Weight of oxygen Volume of oxygen required (g) required (m3)C + O2 → CO2 A × 32/12 A×1A gm or m3H2 + 1/2 O2 → H2O B × 16/2 B × 1/2B gm or m3CO + 1/2 O2 → CO2 C × 16/28 C × 1/2C gm or m3S + O2 → SO2 D × 1 × 32/32 D×1D gm or m3CH4 + 2O2 → CO2 + 2H2O E × 2 × 32/16 E×2E gm or m3C2H6 + 3.5O2 → 2CO2 + F × 3.5 × 32/30 F × 3.53H2OF gm or m3C2H4+3O2 → 2CO2+3H2O G × 3 × 32/28 G×3G gm or m3C4H10+6.5O2 → 4CO2+5H2O H × 6.5 × 32/58 H × 6.5H gm or m3Total X YLess O2 in fuel = - w gm = - w m3
Let oxygen required = X – w (g) or Y –w (m3)Since air has 23% oxygen by weight and 21% oxygenby volume Weight of air required = Net oxygen × 100/23 g Volume of air required = Net oxygen × 100/21 gConversion of volume to weight 1 m3 = 1000 LFor air 1 L × (mol/22.4 L) × (28.94/mol) 1 L = 28.94/22.4 gm
Composition of Combustion Volume of 02Fuel gas/m3 Reaction requiredH2 = 0.5 m3 H2+ 1/2 O2 = H2O 0.50 x 0.5 = 0.25 m3C2H6 = 0.06 m3 C2H6 + 3.502 = 0.06 x 3.5 = 0.21 2C02 + 3H20 m3CH4 = 0.30 m3 CH4 + 2O2 = C02 + 0.30 x 2 = 0.6 m3 2H20CO = 0.08 m3 CO + 1/2 O2 = CO2 008 x 0.5 = 0.04 m3Total 1.1 m3 Solution: Volume of air supplied = 1.1 × 100/21 × 120/100 = 6.6 m3 = 6600 L Weight of air supplied = 28.94 × 6600/22.4 = 8.5Kg