FUEL AND COMBUSTION Prof R K Patel Dept. of Chemistry Fuels 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 for petrol and diesel (octane number and cetane number) - diesel index. Refining of liquid fuels, cracking of petroleum. Gaseous fuels: LPG,
Transcript
FUEL AND COMBUSTIONProf R K Patel
Dept. of Chemistry
Fuels 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 for petrol 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 and Biogas -composition and applications. Next generation fuels.
FuelsFuel is a combustible substance which during combustion gives large amount of heat.
There are chemical fuels, nuclear fuels and fossil fuels.
Classification of Fuels
These can be classified on the basis of their occurrence and physical state
On the basis of occurrence they are of two types:
Primary Fuels: Fuels which occur in nature as such are called primary fuels. E.g., wood, peat, coal, petroleum, and natural gas.
Secondary Fuels: The fuels which are derived from the primary fuels by further chemical processing are called secondary fuels. E,g., coke, charcoal, kerosene, coal gas, producer gas etc.
(ii) On the basis of physical state these may be classified as:
Solid Fuels
Liquid Fuels
Gaseous Fuels
Calorific value: It is defined as the total quantity of heat liberated when a unit mass of a fuel is burnt completely.
Units of Calorific value:
The quantity of heat can be measured in the following units:
(i) Calorie: It is defined as the amount of heat required to raise the temperature of 1gm of water by 1oC 1 calorie = 4.184 Joules
System Solid/Liquid Fuels
Gaseous Fuels
CGS
MKS
B.T.U
Calories/gm
k cal/kg
BTU/lb
Calories/cm3
k cal/m3
BTU/ft3
(ii) Kilo Calorie: 1 k cal = 1000 cal
(iii) British thermal unit: (B. T. U.) It is defined as the amount of heat required to raise the temperature of 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 as the amount of heat required to raise the temperature of 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 Value
Gross Calorific Value: It is the total amount of heat generated when a unit quantity of fuel is completely burnt in oxygen and the products of combustion are cooled down to the room temperature.
As the products of combustion are cooled down to room temperature, the steam gets condensed into water and latent heat is evolved. Thus in the determination of gross calorific value, the latent heat also gets included in the measured heat. Therefore, gross calorific value is also called the higher calorific value.
The calorific value which is determined by Bomb calorimeter gives the higher calorific value (HCV)
Net Calorific Value: It is defined as the net heat produced when a unit quantity of fuel is completely burnt and the products of combustion are allowed to escape.
The water vapour do not condense and escape with hot combustion gases. Hence, lesser amount than gross calorific value is available. It is also known as lower calorific value (LCV).
LCV=HCV-Latent heat of water vapours formed
Since 1 part by weight of hydrogen gives nine parts by weight of water i.e.
OHOH222
12
Therefore,
LCV=HCV-weight of hydrogen x 9 x latent heat of steam
= HCV-weight of hydrogen x 9 x 587
Determination of Calorific value
1. 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
Bomb Calorimeter
Calculations
Let weight of the fuel sample taken = x g
Weight of water in the calorimeter = W g
Water equivalent of the Calorimeter, stirrer, bomb, thermometer = w g
Initial temperature of water = t1oC
Final temperature of water = t2oC
Higher or gross calorific value = C cal/g
Heat gained by water = W x t x specific heat of water
= W (t2-t1) x 1 cal
Heat gained by Calorimeter = w (t2-t1) cal
Heat liberated by the fuel = x C cal
Heat liberated by the fuel = Heat gained by water and calorimeter
x C = (W+w) (t2-t1) cal
C=(W+W)(t2-t1) cal/gx
Net Calorific value:
Let percentage of hydrogen in the fuel = H
Weight of water produced from 1 gm of the fuel = 9H/100 gm
Heat liberated during condensation of steam
= 0.09H 587 cal
Net (Lower calorific value) = GCV-Latent heat of water formed
= C-0.09H 587 cal/gm
Corrections: For accurate results the following corrections 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.
H = -144,000 Cal
H = -57,160 Cal
34
22
25
22
422
22
222
22
HNOOHON
SOHOHOSO
SOOS
The corrections must be made for the heat liberated in the bomb by the formation of H2SO4 and HNO3. The amount of H2SO4 and HNO3 is analyzed by washings of the calorimeter.
For each ml of 0.1 N H2SO4 formed, 3.6 calories should be subtracted.
For each ml of 0.01 HNO3 formed, 1.43 calories must be subtracted.
(C) Cooling correction: As the temperature rises above the room temperature, the loss of heat does
occur due to radiation, and the highest temperature recorded will be slightly less than that obtained. A temperature correction is therefore necessary to get the correct rise in temperature.
If the time taken for the water in the calorimeter to cool down from the maximum temperature attained, to the room temperature is x minutes and the rate of cooling is dt/min, then the cooling correction = x dt. This should be added to the observed rise in temperature.
Therefore, Gross calorific value
C=(W+w)(t2-t1+Cooling correction)-[Acid+ fuse corrections] / 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 series
Insert 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 outlet
Start 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 time
Note down the time taken to fill 1000ml and at the same time the final gas flow reading recorded by the gas flow meter
Tabulate all the reading and calculate the calorific valve of the gas under test
Repeat 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 water from the calorimeter, keep the equipment clean & dry.
Theoretical calculation of Calorific value of a Fuel: The calorific value of a fuel can be calculated if the percentages of the constituent elements are known.
Substrate Calorific value
Carbon 8080
Hydrogen 34500
Sulphur 2240
If oxygen is also present, it combines with hydrogen to form H2O. Thus the hydrogen in the combined form is not available for combustion and is called fixed hydrogen.
Amount of hydrogen available for combustion = Total mass of hydrogen-hydrogen combined with oxygen.
1g 8g 9g
Fixed Hydrogen = Mass of oxygen in the fuel
Therefore, mass of hydrogen available for combustion = Total mass of hydrogen-1/8 mass of oxygen in fuel
=H-O/8
OHOH 222 21
Dulong’s formula for calculating the calorific value is given as:
Gross calorific Value (HCV)
Net Calorific value (LCV)
kgkcalSO
HC /]240,2)8
(500,348080[100
1
kgkcalHHCV
kgkcalH
HCV
/]58709.0[
/]5871009
[
FUEL part -2
Solid Fuels: Primary as well as secondary are widely used in domestic and industrial purposes.
e.g., wood, coal, charcoal and coke.
Wood: Wood has been used as a fuel from ancient times. Due to large scale deforestation, wood is no longer used except in forest areas where wood is available at a low cost.
Wood when freshly cut contains 25-50% moisture.
Normally it is used in air dried condition with 10-15 percent moisture content.
The calorific value of air dried wood is about 3500-4500 kcal/kg.
When wood burns, the ash content is low but the oxygen content is very high. This makes even dry wood a fuel of low calorific value.
Wood charcoal is obtained by destructive distillation of wood.
The major use of wood charcoal is for producing activated 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 coal gradually. The time required for the formation of young brown coal is of the order of 107 years.
Classification of Coal: Coals are mainly classified on the basis of their degree of coalification from the parent material, wood. When wood is converted into coal, there is gradual increase in the concentration of carbon and decrease in the percentage of oxygen and nitrogen.
Coal is given a ranking depending upon the carbon content of the coal from wood to anthracite.
Type of coal
Percentage (dry, mineral matter free basis)
% moisture
calorific value
C H O N VM
Wood
Peat
Brown Coal
Bituminous coal
Anthracite
45-50
45-60
60-75
75-90
90-95
5-6
3.5-6.5
4.5-5.5
4.0-5.5
3-4
20-40
20-45
17-35
20-30
2-3
0-0.5
0.75-3
0.75-2
0.75-2
0.5-2
-
45-75
45-60
11-50
3.8-10
70-90
70-90
30-50
10-20
1.5-3.5
4000-4500
4125-5280
6600-7100
6600-8800
8470-8800
Analysis of Coal
Coal is analysed in two ways:
1. Proximate analysis
2. Ultimate analysis
The results of analysis are generally reported in the following ways:
As received basis
Air dried basis
Moisture free basis (oven dried)
Moisture and ash free basis
Proximate Analysis
The 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 takes away appreciable amount of the liberated heat in the form of latent heat of vapourisation.
•Excessive surface moisture may cause difficulty in handling the coal.
•Presence of excessive moisture quenches fire in the furnace.
2. Volatile Matter: consists of a complex mixture of gaseous and liquid products resulting from the thermal decomposition of the coal.
It is determined by heating a known weight of moisture free coal sample in a covered platinum crucible at 950 20oC for 7 minutes.
Percentage of volatile matter =
Loss of weight due to volatile matter 100
Significance
A high percent of volatile matter indicates that a large proportion of fuel is burnt as a gas.
The high volatile content gives long flames, high smoke and relatively low heating values.
Weight of coal sample taken
For efficient use of fuel, the outgoing combustible gases has to be burnt by supplying secondary air.
High volatile matter content is desirable in coal gas manufacture because volatile matter in a coal denotes the proportion of the coal which will be converted into gas and tar products by heat.
Ash: Coal contains inorganic mineral substances which are converted into ash by chemical reactions during the combustion of coal.
Ash usually consists of silica, alumina, iron oxide and small quantities of lime, magnesia etc.
Ash content is determined by heating the residue left after the removal of volatile matter at 700 50oC for ½ an hour without covering
Weight of the residue left 100 Percentage of ash =
Weight of the coal
Ash can be classified as intrinsic ash and extrinsic ash.
The mineral matter originally present in vegetable matter from which the coal was formed is called intrinsic ash. It consists of oxides of Na, K, Mg, Ca and Si.
The mineral matter like clay, gypsum, dirt which gets mixed up during mining and handling of coal constitute the extrinsic ash which remains as a residue after the combustion. E.g., CaSO4, CaCO3, Fe2O3 etc.
The high percentage of ash is undesirable. It reduces the calorific value of coal.
In furnace grate, the ash may restrict the passage of air and lower the rate of combustion.
High ash leads to large heat losses and leads to formation of ash lumps.
The composition of ash and fusion range also influences the efficiency of coal.
When coal is used in boiler, the fusion temperature of ash is very significant. Ash having fusion temperature below 1200oC is called fusible ash and above 1430oC is called refractory ash.
Apart from loss of efficiency of coal, clinker formation also leads to loss of fuel because some coal particles also get embedded in the clinkers.
Fixed Carbon: Fixed carbon content increases from lignite to anthracite. Higher the percentage of fixed carbon greater is its calorific value and better is the quality of coal.
The percentage of fixed carbon is given by:
Percentage of fixed carbon = 100-[% of moisture+volatile matter+ash]
Significance: Higher the percentage of fixed carbon, greater its calorific value
•The percentage of fixed carbon helps in designing the furnace and shape of the fire-box because it is the fixed 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 is taken in a combustion tube and is burnt in excess of pure oxygen.
OHOH
COOC
2221
2
22
Fig 3. Estimation of carbon and hydrogen
OHCaClOHCaCl
OHCOKCOKOH
27.
227
2
23222
44 g of CO2 contain = 12 g of carbon
Y g of CO2 contain = 12 y44
takencoal ofweight 100
4412carbon of Percentage y
18 g of water contain = 2 g of hydrogen
Z g of water contain hydrogen of zg182
takencoal ofweight 100
182hydrogen of Percentage z
Significance:
Calorific value of a fuel is directly related to its carbon content.
A higher percentage of carbon reduces the size of the combustion chamber
High percentage of hydrogen also increases the calorific value of coal. The content of hydrogen in coals varies between 4.5 to 6.5 percent from peat to bituminous stage.
2. Nitrogen: Nitrogen present in coal sample can be estimated by Kjeldahl’s method.
The contents are then transferred to a round bottomed flask and solution is heated with excess of NaOH.
The ammonia gas thus liberated is absorbed in a known volume of a standard solution of acid used.
42)
4(Heat
42Nitrogen SONHSOH
Fig 4. Estimation of nitrogen by Kjeldahl’s method
The unused acid is then determined by titrating with NaOH. From the volume of acid used by NH3 liberated, the percentage of nitrogen can be calculated.
42)
4(
423
22
32
422
42)
4(
SONHSOHNH
OHNHSONaNaOHSONH
Carbonization of Coal (Manufacture of Coke)
It is the process of heating the coal in absence of air to a sufficiently high temperature, so that the coal undergoes decomposition and yields a residue which is richer in carbon content than the original fuel.
Caking and coking of coals: some coals have a tendency to soften and swell at higher temperatures, to form a solid coherent mass with porous structure. Such coals are called caking coals. The residue formed is called coke. If the coke is hard, porous and strong, than the coal, from which it is formed, it is called coking coal. All coking coals are caking coals but 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 not fit for coking and are used for gas making. The coals having 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 low molecular alkenes and alkanes are evolved.
At about 350oC the decomposition of coal is accompanied by evolution of gases and elimination of vapours takes place.
At about 400oC, caking coal becomes soft and plastic.
At about 700oC, hydrogen is evolved
Above 800oC, main gaseous products are evolved
Gases 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. Which is also called soft coke.
The yield of coke is about 75-80 %.
The coke thus produced contains 5 to 15 % volatile matter.
The various products of low temperature carbonization are semi coke, low temperature tar, crude low temperature spirit and gas.
LTC plants normally use low rank coals. These low rank coals produce excessive smoke on burning.
Semi coke from LTC is highly reactive and can be easily ignited into a smokeless flame
The gas which is obtained as a byproduct has higher calorific value of about 6500-9500 kcal/m3.
(ii) High temperature carbonization: It is carried out at 900-1200oC. HTC is used for the production of pure, hard, strong and porous metallurgical coke containing 1-3 % volatile matter. The yield of the coke is 65-75%.
The byproducts-gas and tar have greater amounts of aromatic hydrocarbons. The gas which is obtained has lower calorific value of about 5000-6000 kcal/m3 than that produced in LTC; but the yield of the gas is higher.
The coke obtained is very much harder than the coke obtained from LTC process and hence is called hard coke.
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 medium size.
(v) Combustibility: Coke should burn easily. The combustibility of coke depends on the nature of the coal, carbonization temperature and reaction temperature.
(vi) Calorific value: It should be high.
(vii) Reactivity: Reactivity of coke is its ability to react with CO2, steam, air and oxygen. The reactivity should not be too high. The reactivity toward CO2 represent the reduction of CO2 )(2)()(
2gCOsCgCO
Cost: Coke should be cheap and easily available.
Manufacture of Metallurgical Coke:
(i) Beehive Oven
(ii) Otto 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 oven
Demerits of Beehive ovens: The demerits are
•No recovery of byproducts, which are useful chemicals 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: 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 Hoffmann’s oven
Working: 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.
Recovery 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 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 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.
OHSFeSHOFe2
3322
332
322
24
232
24
322
OFeOFeO
SOFeOOSFe
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:
Element Carbon Hydrogen Sulphur Oxygen Nitrogen
Percentage 80-87 11.1-15 0.1-3.5 0.1-0.9 0.4-0.9
The 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 distillation
Name Contents
Paraffin wax
Asphalt
Paraffin wax and asphalt
Paraffin base
Asphaltic base
Mixed base
Straight chain hydrocarbons and small amounts naphthenes and aromatic hydrocarbons
Aromatic and naphthenic hydrocarbons
Paraffins, naphthenes and aromatic hydrocarbons
Processing 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 oil
Refining of Petroleum
Crude 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 Cottrell’s 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 petroleum
The 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 oil
There 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 Flashing
Cracking: 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:
6H
3C
12H
5CCracking
18H
8C
Cο36B.Pt Cο174B.Ptpentene pentane-n Decane
10H
5C
12H
5CCracking
22H
10C
Nearly 50% of today’s gasoline is obtained by cracking. The gasoline obtained by cracking is far more superior than straight run gasoline.
The process of cracking involves the full 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 methods
Thermal 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 as
Paraffins < 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
3CH
2)
2(CH
2HC
2HC
3)
2(CH
3CHHeat
3CH
7)
2(CH
3CH
Propagation
The 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.
22322223 CHCHCHHCCHCHCHCH
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.
In a spark-ignition petrol engine, a phenomenon that occurs when unburned fuel-air mixture explodes in the combustion chamber before being ignited by the spark. The resulting shock waves produce a metallic knocking sound. Loss of power occurs, which can be prevented by reducing the compression ratio, re-designing the geometry of the combustion chamber, or increasing the octane number of the petrol.(formerly by the use of tetraethyl lead anti-knock additives, but now increasingly by MTBE – methyl tertiary butyl ether in unleaded petrol). An antiknock agent is a gasoline additive used to reduce engine knocking and increase the fuel's octane rating.The typical antiknock agents in use are:Tetra-ethyl lead (phased out)Methyl cyclo pentadienyl manganese tricarbonyl (MMT)Ferrocene, Iron pentacarbonyl, Toluene, Isooctane
Knocking and Anti-knocking
Octane rating of a spark ignition engine fuel is a measure of the resistance to detonation or knocking compared to a mixture of iso -octane (2,2,4-tri methyl pentane, an isomer of octane) and n- heptane. It is a numerical representation of the antiknock properties of motor 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 of the fuel .It is only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner.
Octane 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-hexadecane
The 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:
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 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
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
100
60int FgravityAPIDegreesxFpoanilineIndexDiesel
oo
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 Fuels
Classification 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 gas
Typical physical and chemical properties of various gaseous fuelsFuel Gas
Relative Density
Higher Heating Value kCal/Nm3
Air/Fuel ratio m3/m3
Flame Temp oC
Flame 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
Type of FuelsType of Fuels
Gaseous 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
• Mixes readily with air without producing smoke or soot
Power generation
Domestic use
Transportation
Fertilizers
Aviation
Hydrogen
Applications
Type of Gaseous Fuels
CNGCompressed 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.
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.
Cars
Locomotives
Applications
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
Liquefied Natural Gas
Comparing Fuels
Fuel Oil Coal Natural Gas
Carbon 84 41.11 74
Hydrogen 12 2.76 25
Sulphur 3 0.41 -
Oxygen 1 9.89 Trace
Nitrogen Trace 1.22 0.75
Ash Trace 38.63 -
Water Trace 5.98 -
COMBUSTION Combustion reactions are exothermic reactions accompanied by evolution of heat and light and the temperature rises considerably. The amount of oxygen or air required for combustion of a given sample of fuel can be calculated.
Calculation of Air Quantities To determine the amount of oxygen and hence the amount of air required for combustion for a unit quantity of fuel, the following chemical principles are applied.(1) Substances always combine in definite proportions given by molecular 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.
(6) Mass of any gas can be converted to volume at 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.
Reaction Weight of oxygen required (g)
Volume of oxygen required (m3)
C + O2 → CO2
A gm or m3
A × 32/12 A × 1
H2 + 1/2 O2 → H2OB gm or m3
B × 16/2 B × 1/2
CO + 1/2 O2 → CO2
C gm or m3
C × 16/28 C × 1/2
S + O2 → SO2
D gm or m3
D × 1 × 32/32 D × 1
CH4 + 2O2 → CO2 + 2H2OE gm or m3
E × 2 × 32/16 E × 2
C2H6 + 3.5O2 → 2CO2 + 3H2OF gm or m3
F × 3.5 × 32/30 F × 3.5
C2H4+3O2 → 2CO2+3H2OG gm or m3
G × 3 × 32/28 G × 3
C4H10+6.5O2 → 4CO2+5H2OH gm or m3
H × 6.5 × 32/58 H × 6.5
Total X YLess O2 in fuel = - w gm = - w m3
Procedure for combustion calculations:
Let oxygen required = X – w (g) or Y –w (m3)Since air has 23% oxygen by weight and 21% oxygen by 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 Fuel gas/m3
Combustion Reaction
Volume of 02 required
H2 = 0.5 m3 H2+ 1/2 O2 = H2O 0.50 x 0.5 = 0.25 m3
C2H6 = 0.06 m3 C2H6 + 3.502 = 2C02 + 3H20
0.06 x 3.5 = 0.21 m3
CH4 = 0.30 m3 CH4 + 2O2 = C02 + 2H20
0.30 x 2 = 0.6 m3
CO = 0.08 m3 CO + 1/2 O2 = CO2 008 x 0.5 = 0.04 m3
Total 1.1 m3
Solution:Volume of air supplied = 1.1 × 100/21 × 120/100 = 6.6 m3 = 6600 LWeight of air supplied = 28.94 × 6600/22.4 = 8.5Kg
