Fuels and Combustion Arunava Agarwala.

Fuels and Combustion Arunava Agarwala

Fuels Fuels are combustible substance, containing carbon as main constituents, which on proper burning gives large amount of heat, which can be used economically for domestic and industrial purposes. Alternately, Materials which possess chemical energy is known as fuel. For example Wood, Charcoal, coal, Kerosene, petrol diesel, oil gas etc.

During the process of combustion of a fuel, the atom of carbon,
hydrogen etc. combine with oxygen with simultaneous liberation of Heat at rapid rate. This energy is liberated due to the “rearrangement of valence electrons” in these atoms, resulting in the formation of Compounds like CO2 and H2O. Theses compounds have less energy (heat content) as compared to reactants. FUEL O2 PRODUCTS + HEAT More heat energy content Lesser heat energy content C (s) + O2(g) CO2(g) + DH= -94.1Kcal/mole

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. On the basis of physical state these may be classified as: Solid Fuels: woods, Peat, Lignite etc. Liquid Fuels: Crude oil, Tar, Diesel, Petrol, Kerosene. Gaseous Fuels: Natural gas, coal gas

Calorific value: The heating value (or energy value or calorific value) of a substance, usually a fuel or food (see food energy), is the amount of heat released during the combustion of a specified amount of it. The energy value is a characteristic for each substance. Units of Calorific value: 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

The quantity of heat can be measured in the following units:
Calorie: It is defined as the amount of heat required to raise the temperature of 1gm of water by 1oC calorie = Joules (ii) Kilo Calorie: the quantity of heat required to raise the temperature of one kilogram of water by 1oC 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 = 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 = B.T.U. = 2.2 C.H.U.

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.

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 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 = m 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 Specific heat of water = S Higher or gross calorific value = C cal/g Heat gained by water = W x Dt x S = W (t2-t1) S

Heat gained by Calorimeter = w (t2-t1) S
Heat liberated by the fuel = m θ cal Heat liberated by the fuel = Heat gained by water and calorimeter m θ = (W+w) (t2-t1) S cal θ=(W+w)(t2-t1) S cal/g m Note: the water equivalent of calorimeter is determined by burning a fuel of known calorific value and using above equation. The fuel used for this Purpose is benzoic acid (HCV =6,325cal/g) and napthalene (HCV= 9,622cal/g) Salicylic acid=5269cal/g

Net Calorific value: Let percentage of hydrogen in the fuel = H Weight of water produced from 1 g of the fuel = 9H/100 g Heat liberated during condensation of steam = 0.09H  587 cal Net (Lower calorific value) = GCV-Latent heat of water formed = θ-0.09H  587 cal/gm

Corrections: For accurate results the following corrections are also incorporated:
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. Acid Correction: During combustion, sulphur and nitrogen present in the fuel are oxidized to their corresponding acids under high pressure and temperature. ΔH= -144,000 cal ΔH= -57,160,000 cal

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 0.1 HNO3 formed, 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

Solution: here X=0.72 g, W=250 g; w=150g; t1= 27.3◦C; t2=29.1◦C
Some problems Q: gram of a fuel containing 80% carbon, when burnt in a bomb calorimeter, Increased the temperature of water from 27.3◦C to 29.1◦C. If the calorimeter contains 250 grm of water and its water equivalent is 150 grms, calculate the HCV of the fuel. Give your answer in Kj/Kg. Solution: here X=0.72 g, W=250 g; w=150g; t1= 27.3◦C; t2=29.1◦C HCV = (W+w)(t2-t1)/X Kcal/Kg = ( )( )/0.72 = 1,000 kcal/Kg = 1,000×4.2 KJ/Kg = 4200KJ/Kg

Some problems On burning 0.83 gram of a solid fuel in a bomb calorimeter, the temperature of 3500 g water icrease from 26.5˚C to 29.2 ˚C. water equivalent of calorimeter and latent heat of steam is 385 g and 587 cal/g respectively. If fuel contain 0.7% hydrogen, calculate the gross and net Calorific value

Some problems A sample of coal contain c = 93%, H= 6% and ash = 1%. The following Data were obtained when the above coal was tested in bomb calorimeter Weight of coal burnt = g Weight of water taken = 550 g Water equivalent of bomb calorimeter = 2200 g Rise in temperature = 2.42 ˚C Fuse wire correction = 10 cal Acid correction = 50 cal Calculate gross and net calorific value. Assuming that latent heat of vapor is s 580 cal/g.

Some problems The following data is obtained in a bomb calorimeter Weight of crucible = g Weight of crucible and fuel = g Water equivalent of bomb calorimeter = 570 g Rise in temperature = 2.3 ˚C Fuse wire correction = 3.6 cal Acid correction = 3.8 cal Cooling correction = ˚C Calculate gross and net calorific value if the fuel contain 6.5% hydrogen.

Some problems Determine the water equivalent of the bomb calorimeter apparatus which gave the following data in the experiments Weigh of Benzoic acid = g Weight of calorimeter = g Weight of calorimeter and water = g Initial temperature = ˚C Final Temperature = ˚C Cooling correction = ˚C Heat from fuses = 22 cal Calorific value of benzoic acid = 6324 cal/g

BOYS OR JUNKERS GAS CALORIMETER
AIM :To determine calorific value of gaseous fuel by Junkers gas calorimeter

BOYS OR JUNKERS GAS CALORIMETER
AIM :To determine calorific value of gaseous fuel by Junkers gas calorimeter APPARATUS: It consists of following Bunsen Burner: special type of Burner clamped at the bottom. It can be pulled out of the combustion chamber or pushed up in chamber during the carrying out combustion. Gasometer: It is employed to measure the volume of gas burning per unit time. This attached with manometer fitted with the thermometer so that pressure and temperature of the gas before burning can be read. Pressure governor: It can control the supply of quantity of gas at give pressure. Gas Calorimeter/ Combustion chamber: It is a vertical cylinder, which is surrounded by annular space for heating water and interchange coils. The entire is covered by an outer jacket in order to reduce the heat loss by radiation and convection.

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

When the steady condition are established, then the reading are taken
simultaneously of: The volume of gaseous fuel burnt (V) at given temperature and pressure in certain period of time (t). The quantity of water (W Kg) passing through the annular space during the same interval of time The steady rise in temperature (T2-T1) The mass of water (steam) condensed (in Kg) in the outlet water.

Calculation: Volume of gas burn at STP in certain time (t)= V Mass of the cooling water used in time t = W Temperature of inlet water = T1 Temperature of outlet water = T2 Mass of steam condensed in time t in graduated cylinder = m Higher calorific value of fuel = L Specific heat of water = S Heat absorbed by circulating water = W(T2-T1)×Specific heat of water (s) Heat produced by combustion of fuel = VL Thus VL = W(T2-T1)×S HCV (L) = W(T2-T1)×S/V LCV = 1 cm3 of water = 1 g of water

Problem-1 The following were obtained in the Boy’s gas calorimeter experiments Volume of gas used = 0.1 m3 at NTP Weight of water heated = 25 Kg Temp. of inlet water = 20 ˚C Temp. of outlet water = 33 ˚C Weight of steam condensed = kg Calculate the gross and net calorific value per m3 at NTP. Take the heat liberated in condensing water is 580 Kcal/Kg

Some problems During the determination of calorific value of a gaseous fuel by Boy’s Calorimeter. The following results are obtained Vol. of gaseous fuel burnt at NTP = m3 Weight of water used for cooling the combustion products = 30.5 kg Weight of steam condensed = kg Temp. of inlet water = 26.1˚C Temp. of outlet water = 36.5˚C Determine the gross and net calorific value of the gaseous fuel per cubic meter at NTP provided that the heat liberated in condensation of water Vapor is 587cal/g

problems Calculate the gross calorific value and net calorific value of a gaseous fuel, 0.012m3 of which when burnt raised the temperature of 3.5kg of water by 8.2K. Specific heat of water is 4.2 kJ kg-1K-1. Latent heat of steam is 2.45 kJ kg-1. The volume of water collected is 6.5cm3 . V = volume of the gas burnt = m3 W = mass of water = 3.5 kg t2- t1 = rise in temperature = 15.6 K s = specific heat of water = 4.2kJ kg-1K-1 v = volume of water collected = 6.5 cm3

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
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 g g 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

Dulong’s formula for calculating the calorific value is given as:
Gross calorific Value (HCV) Net Calorific value (LCV)

Calculate the gross and net calorific value of coal having the following
composition carbon = 85%, hydrogen = 8%, sulphur = 1%, nitrogen = 2%, Ash = 4 %, latent heat of steam 587cal/g.

Characteristics of Good Fuel:
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. High Calorific value Ignition Temperature: A good fuel should have moderate ignition temperature. Moisture content: Should be low Non combustible matter content Velocity of combustion: It should be moderate Nature of the products (viii) Cost of fuel, (ix) Smoke, (x) Control of the process

Solid Fuels: Wood: Wood has been used as a fuel from ancient times. Due to large scale deforestation, wood is no longer used. Freshly cut wood contains 25-50% moisture. Air dried wood contains about 10-15% moisture content. The calorific value of air dried wood is about 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 formation At various times in the geologic past, the Earth had dense forests in low-lying wetland areas. Due to natural processes such as flooding, these forests were buried under the soil. As more and more soil deposited over them, they were compressed. The temperature also rose as they sank deeper and deeper. As the process continued the plant matter was protected from biodegradation and oxidation, usually by mud or acidic water. This trapped the carbon in immense peat bogs that were eventually covered and deeply buried by sediments. Under high pressure and high temperature, dead vegetation was slowly converted to coal. As coal contains mainly carbon, the conversion of dead vegetation into coal is called carbonization

The wide, shallow seas of the Carboniferous Period provided ideal conditions for coal formation, although coal is known from most geological periods. The exception is the coal gap in the Permian–Triassic extinction event, where coal is rare. Coal is known from Precambrian strata, which predate land plants — this coal is presumed to have originated from residues of algae

Coal: coal is highly carbonaceous fossil fuel that has been produced as a result of vegetable debris under favorable conditions of high temperature and pressure over million of years. It is chiefly composed of C, H, N and O besides non-combustible inorganic matter. The transformation of the vegetable debris to coal takes place in two stages: Biochemical or peat stage: During this stage, the plant materials were attacked by various micro organisms. 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.

Peat, considered to be a precursor of coal, has industrial importance as a fuel in some regions, for example, Ireland and Finland. In its dehydrated form, peat is a highly effective absorbent for fuel and oil spills on land and water. It is also used as a conditioner for soil to make it more able to retain and slowly release water. Lignite, or brown coal, is the lowest rank of coal and used almost exclusively as fuel for electric power generation. Jet, a compact form of lignite, is sometimes polished and has been used as an ornamental stone since the Upper Palaeolithic.

Sub-bituminous coal, whose properties range from those of lignite to those of bituminous coal, is used primarily as fuel for steam-electric power generation and is an important source of light aromatic hydrocarbons for the chemical synthesis industry. Bituminous coal is a dense sedimentary rock, usually black, but sometimes dark brown, often with well-defined bands of bright and dull material; it is used primarily as fuel in steam-electric power generation, with substantial quantities used for heat and power applications in manufacturing and to make coke. “Steam coal" is a grade between bituminous coal and anthracite, once widely used as a fuel for steam locomotives. In this specialized use, it is sometimes known as "sea-coal" in the US. Small steam coal (dry small steam nuts or DSSN) was used as a fuel for domestic water heating.

Anthracite, the highest rank of coal, is a harder, glossy black coal used primarily for residential and commercial space heating. It may be divided further into metamorphically altered bituminous coal and "petrified oil", as from the deposits in Pennsylvania. The classification of coal is generally based on the content of volatiles. However, the exact classification varies between countries Graphite, technically the highest rank, is difficult to ignite and is not commonly used as fuel — it is mostly used in pencils and, when powdered, as a lubricant. _____

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. Wood Peat Lignite Bituminous coal Anthracite Increase in carbon contents, Calorific value and hardness Decrease in H, O, S, N, contents and volatile matter

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 3-4 20-40 20-45 17-35 20-30 2-3 0-0.5 0.75-3 0.75-2 0.5-2 - 45-75 11-50 3.8-10 70-90 30-50 10-20

Selection of coal Calorific value should be high
Moisture content should be low Ash content should be low Sulphur and phosphorus contents of coal should be low Size of coal should be uniform

Analysis of Coal In order to assess the quality of coal, the following two types of analysis are made: Proximate analysis Ultimate analysis The results of analysis are generally reported in the following ways: As received basis Air dried basis Moisture free basis (oven dried)

Proximate Analysis Proximate analysis of coal determines the moisture, ash, volatile matter and fixed carbon of coal. It gives information about the practical utility of coal. Moisture Content: About 1 g of finely powered air dried coal sample is weighted in crucible. The crucible is placed in electric hot air oven, maintained at oC. The crucible is allowed to remain in electric hot air oven for about one hour and then taken out, cooled in a desiccator and weighed. Loss in weight is reported as moisture Importance of Proximate analysis: Excess of moisture is undesirable in coal. Moisture lowers the calorific value of coal because it takes away appreciable amount of the liberated heat in the form of latent heat of vaporization. Excessive surface moisture may cause difficulty in handling the coal. Presence of excessive moisture quenches fire in the furnace.

2. Volatile Matter: It is determined by heating a known weight of moisture free coal sample in a covered platinum crucible with lid at 950  20oC for 7 minutes in an electrical furnace. The crucible is cooled in a desiccator and weighed. Loss in weight is reported as volatile matter 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. 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.

(3) 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. The crucible is taken out, cooled first in air, then inside a desiccator and weighed. The residue is reported as ash on percentage basis. Significance 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.

(4) Fixed Carbon: The percentage of fixed carbon is given by:
Percentage of fixed carbon = 100- [% of moisture + volatile matter + ash] Significance: 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 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 as certain the composition of coal.
Ultimate analysis includes the estimation of carbon, hydrogen, sulphur, nitrogen and oxygen. Carbon and Hydrogen: A known amount of coal is taken in a combustion tube and is burnt in excess of pure oxygen. C and H of coal are converted into CO2 and H2O respectively. Theses gaseous products are absorbed respectively in KOH and CaCl2 tubes of known weight. Fig. Estimation of carbon and hydrogen

44 g of CO2 contain = 12 g of carbon
Y g of CO2 contain =

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.

(4) Ash is determined as described in proximate analysis
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. (3) Sulfur is determined conveniently from the bomb washing from combustion of a known mass of coal in bomb calorimeter experiment. The washing contain Sulfur in the form of sulfate which it is precipitate as BaSO4 (4) Ash is determined as described in proximate analysis (5) % Oxygen = % of (C+H+N+Ash)

A sample of coal is analyzed as follows: Exactly 2
A sample of coal is analyzed as follows: Exactly 2.5 g was weighted into silica crucible. After heating for an hour at 110˚C, the residue weighed 2.415g. The crucible next was covered with a vented lid and strongly heated For exactly seven minutes at 950 ˚C. The residue weighed g. the Crucible was then heated without the cover, until a constant weight was Obtained. The last residue was found to weight g. calculate the percentage result of above analysis.

Liquid Fuels: The importance of liquid fuels is the fact that almost all combustion engines run on them. Benefits of Liquid fuel: They are easy to handle, store and transport. After burning they do not leave any applicable amount of ash Liquid can easily kindled. The combustion can be started or stopped at once The rate of combustion can easily controlled as desired Less excess air is needed in case of liquid fuels as compare to solid fuel The furnace space is required is lesser than solid fuel Operation is cleaner The largest source of liquid fuels is petroleum. The calorific value of petroleum is about kJ/kg. There are other supplements of liquid fuels such as coal tar, crude benzol, synthetic liquid fuel made from coal etc.

Petroleum: The term petroleum means rock oil
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: Carbon = 79.5% to 87.1% Hydrogen = 11.5% to 14.8% Sulphur = 0.1% to 3.5% Nitrogen and oxygen = 0.1% to 0.5% 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. Combined oxygen is present as carboxylic acids, ketones and phenols. The ash of the crude oil is 0.1%.Metals e.g., Silicon, iron, aluminium, calcium, magnesium, nickel and sodium.

Classification of crude petroleum:
Paraffinic base type crude oil: These category of oil is mainly composed of saturated hydrocarbons from CH4 to C35H72 and little of naphthalene and aromatics. The hydrocarbon from C18H38 to C35H72 are semi solids called waxes. Asphaltic Base type crude oil: these category of oil contains mainly cycloparaffinis, or naphthalene with smaller amount of paraffinis and aromatic hydrocarbons. Mixed-based type crude oil: these category of oil contains both paraffinic and asphaltic Hydrocarbons and are generally rich in semi solid waxes.

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. : 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 number of useful fractions by fractional distillation and finally converted into desired specific products. The process of refining involves the following steps Step 1: Separation of water (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 color. 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. Step 2: 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 (solid) so formed is separated by filtration

(iii) Fractional distillation is the separation of a mixture into its component or fractions on the basis of their boiling point by heating them to a temperature at which one or more fractions of the compound will vaporize. It is done in tall fractionating tower or column made up of steel. Fractional column consists of horizontal trays provided with a no of small chimneys, through which vapors rise. These chimneys are covered with loose caps, known as bubble caps. These bubble caps help to provide an intimate contact between the escaping vapors 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.

˚C ˚C ˚C ˚C 40-120˚C 30-70˚C Below 30˚C

The residue from the bottom of the fractionating tower is vacuum distilled to recover various fractions Fig. 11: Vacuum distillation of residual oil

Cracking Gasoline (petrol, C4-C12)) is the most important 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.” 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 naphtha's, oil gas from kerosene.

Cracking can be carried out by two methods
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. this process can be carried either liquid phase or vapour phase Liquid Phase thermal cracking: The heavy oil is cracked at a suitable temperature oC and under pressures of the range kg/cm2. The cracked products are then separated in a fractionating column. The important fractions are: Cracked gasoline (30-35%), Cracking gases (10-45%); Cracked fuel oil (50-55%). Vapour phase thermal cracking: by this methods, only those oils can be cracked which vapourised at low temperature. The cracking oil is first vapourised and then crackred at about oC and under a low pressures of kg/cm2.

Advantages of catalytic cracking over thermal cracking:
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. 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.

There are two methods for catalytic cracking:
(a) Fixed bed catalytic cracking: Gasoline + Dissolved gas Heavy oil charge Pre Heater ( ˚C) Catalyst chamber Fractionating column oil Cracked vapour Cooler Gasoline Stabilizer The heavy oil are heated in a pre heater to cracking temperature ( ˚C) and then forced through the catalytic chamber (containing artificial clay and mixed with zirconium Oxide) maintained at ˚C and 1.5 Kg/cm2 pressure. During their passage through the tower about 40% heavy oil converted into gasoline and about 2-4% carbon formed. The vapour produced then passes through fractionating column, where heavy oil fraction condensed. The vapour are then led through a cooler, where some of the gas are condensed along with the gasoline and uncondensed gas move on. The gasoline containing gas is then sent to stabilizer, where the dissolved gas are removed and pur gasoline are obtained

(b) Moving-bed catalytic cracking:

Procedure: (1) The solid catalyst is very finely powered, so that it behave almost as a fluid, which can be circulated in a gas stream. (2) The vapours of cracking stocks mixed with fluidized catalyst is forced up into a large reactor in which heavier molecules are cracked into lighter molecules. Near the top of he reactor, there is a centrifugal separator (called cyclone), which allow only the cracked oil vapours to pass on to fractionating column, but retain all the catalyst powder in the reactor itself (3) The catalyst powder gradually becomes heavier, due to coating with carbon and settles to the bottom from where it is forced by an air blast to regenerator (maintained 600˚C) (4) In regenerator, carbon is burnt and the regenerated catalyst then flow through a stand pipe for mixing with fresh batch of incoming cracking oil. (5) At the top of regenerator, there is an separator, which permits only gases (CO2 etc.) to pass out, but holds back catalyst.

Zeolites Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents

Reforming Reforming is a process of bringing about the structural modification in components of straight run gasoline (prepared by the fractional distillation of the crude oil), with primary object of improving the octane number. Reforming can be accomplished by increasing in volatility (reduction of molecular size) or by the conversion on n-paraffins to isoparafins, olefins, Aromatics and naphthens to aromatics. Reforming is carried out either thermally or in the presence of catalyst: Thermal reforming: thermal reforming is carried out in a reactor, heated at ˚C and a pressure of 85atm. The fed stock is straight run gasoline. To avoid the formation of gas (at the expanse of gasoline), the condition are controlled by quenching (rapid cooling) the products by spread of cold oil. (2) Catalytic reforming: To get better grade and yield of gasoline, catalyst reforming [Pt (0.75%) supported on alumina] is carried out by using either fixed bed or fluid bed methods at ˚C and atm. The following are the main reaction during catalyst reforming:

Knocking In an internal combustion engine, a mixture of gasoline vapours and air is used as fuel. After the initiation of the combustion reaction by spark in the cylinder, the flame should be spread rapidly and smoothly through the gaseous mixture, thereby the expanding gas drive the piston down the cylinder. The power output and efficiency of an IC engine depends on the Compression ratio which is the ratio of the volume of the cylinder at the end of the suction stroke to the volume of the cylinder at the end of the compression stroke. Volume of cylinder at end of suction stroke Compression ratio = Volume of cylinder at end of compression stroke The efficiency of IC engine increase with increase in compression ratio, which is dependent upon the nature of constituents present in gasoline.

What is reason behind knocking?
Under ideal conditions, The hydrocarbons in gasoline undergo complete combustion and the flame propagates smoothly. “In certain circumstances, the rate of oxidation becomes so great that the last portion of fuel-air mixture gets ignited instantaneously, producing an explosive violence, known as Knocking.” the knocking results in loss of efficiency. What is reason behind knocking?

Under ideal conditions C2H6 + 7/2 O2  2 CO2 + 3H2O
Due to deposits of carbon on the walls of the cylinder, some hydrocarbons of gasoline converted into peroxy compounds. These accumulated peroxides decompose suddenly and burst into flames producing shock waves. The shock wave hits the walls of the engine and the piston with a rattling sound. The reactions that take place in an IC engine are as given below (taking ethane as an example for the hydrocarbon present in gasoline): Under ideal conditions C2H /2 O  CO H2O Under knocking conditions C2H O  CH3 –O-O- CH3 (Dimethyl peroxide) CH3 –O-O- CH  CH3CHO + H2O CH3CHO + 3/2 O  HCHO + CO2 + H2O HCHO + O  H2O CO2

The tendency of fuel constituents to knock in the following order:
Straight chain paraffins > Branched chain paraffins > olefins > cycloparaffins (i.e. naphthalenes) > aromatics Thus, olefines of the same carbon chain length possess better anti-knocking Properties than the corresponding paraffins and so one Effects of knocking: Decreases life of engine Consumption of fuel is more

Improvement of anti-knocking characteristics of fuel
(Anti-knocking agents) Addition of tetraethyl led (TEL): Tetraethyl lead is converted into lead oxide particles in the cylinder and these particles reacts with the hydrocarbon peroxides molecules formed, thereby slowing down the chain oxidation reactions and thus decrease the chances of knocking. In the process, lead gets deposited on the inner walls of the engines and at spark plugs, which is harmful for the engine life. Hence dichloroethane and dibromoethane are added along with tetraethyl lead. These convert the lead into lead halides, which are volatile and escape with exhaust gases. But, elease of lead compounds pollutes the atmosphere. Catalytic converters (rhodium catalyst) are used in IC engines to convert CO in the exhaust to CO2. Lead tetraethyl used as anti knocking agent poisons the catalyst and hence leaded petrol is not advisable in such IC engines.

Addition of MTBE (Unleaded petrol):
Methyl terta-butyl ether (MTBE) is added to petrol instead of TEL to boost its octane number. The oxygen of MTBE brings about complete combustion of petrol preventing peroxide formation and hence knocking is prevented. Power alcohol: When ethyl alcohol is used as fuel in internal combustion engine, it is called as power alcohol. Generally ethyl alcohol is used as its 5-25% mixture with petrol. Advantages of power alcohol: Ethyl alcohol has good anti-knocking property and its octane number is 90, while the octane number of petrol is about is 65. Therefore, addition of ehtyl alcohol increases the octane number of petrol. Alcohol has property of absorbing any traces of water if present in petrol. (3) Alcohol contains higher percentage of oxygen than MTBE and hence brings about complete oxidation of petrol more effectively.

Octane rating Octane rating of a engine fuel is a measure of the resistance to knocking. It as been found that n-heptane knock very badly and hence its anti-knock value has been given zero, while on the other hand isooctane (2,2,4-trimethyl pentane) gives very little knocking, so its anti knocking value has been given as 100. Octane number is the percentage of isooctane present in a mixture of isooctane and n-heptane has the same knocking characteristics as that of fuel under examination, under same set of conditions. It is a numerical representation of the antiknock properties of motor fuel, compared with a standard reference fuel. 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. Greater the octane number, greater is the antiknock property of the fuel.

Cetane Number: The suitability of diesel fuel is defined by its cetane number. The cetane number of a diesel oil is defined as the percentage of n-hexadecane in a mixture of n-hexadecane and 2-methyl naphthalene which will have the same ignition characteristics as the fuel under test, under same set of conditions. 2-methyl naphthalene (C.N. =0) n-Hexadecane (C. N. = 100) 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: n-alkanes> naphthenes > alkenes > branched alkanes > aromatics

Synthetic Petrol Synthetic petrol can be produced from coal by the following methods (1) Fisher-Tropsch Process: In this process, coke is heated and steam is passed over it, thereby, water gas (CO + H2) is formed Water gas is purified by passing through the Fe2O3 (to remove H2S) and then into Fe2O3 . Na2O3 (to remove organic sulfur compounds). This purified gas is compressed to 5 to 25 atm and then led through convertor (containing a catalyst, consisting of a mixture of 100parts cobalt, 5 parts thoria, 8 parts magnesia) maintained at about ˚C. A mixture of saturated and unsaturated hydrocarbons results

(1) Bergius Process: The low ash coal is finely powered and made into a paste with heavy oil and then a catalyst (composed of tin and nickel oleate) is incorporated. The whole is heated with hydrogen at 450˚C and under pressure atm for about 1.5 h, during which hydrogen combines with coal to form saturated and unsaturated hydrocarbon, which decompose at prevailing at high temperature and pressure to yield low boiling liquid hydrocarbons. The issuing gas are led to condenser, where a liquid resembling crude oil is obtained, which fractionated to get (a) gasoline (b) middle oil (c) heavy Oil. The heavy oil is used again for making paste with fresh coal

Producer gas Producer gas is essentially a mixture of combustible gases, CO, H2 etc. associated with large percentage of non combustible gas like CO2, N2 etc. It is prepared by passing air mixed with little steam (about 0.35 kg/kg coal) over a hot coke maintained at 1100˚C in special a reactor called ‘gas producer’. It consists of steel vessels about 3m dia and 4 m in height. The vessel is lined inside with refractory bricks. Exit for ash Air mixed with little steam Producer gas outlet coke Combustion zone Reduction zone Distillation zone Coal at 1100˚C

The producer gas production reactions can be divided into four zone as follows
Ash zone: It is lowest zone consisting mainly of ash. The ash protects the grate from the intense heat of combustion. Moreover the temperature of supplied air and steam is increased as they pass through this zone. (b) Combustion zone: It is the zone next to the ash zone. It is also known as a oxidation zone. Here carbon burn and forms CO and CO2. the temperature of this zone is about 1100˚C. (c) Reduction zone: In this zone, CO2 and steam combines with red-hot coke and liberated free hydrogen and CO.

Calorific values = 1300 Kcal/m3
(d) Distillation Zone: In this zone ( ˚C), the down coming coal is heated by the out going gases, since the gases give their sensible heat to the coal. The heat give by the gases and heat radiated from reduction zone helps to distill the fuel. Compositions: CO = 22-30% H2 = 8-12% N2 = 52-55% CO2 = 3% Calorific values = 1300 Kcal/m3

Water Gas (Blue Gas) Water gas is mixture of combustible gases, CO and H2 with little amount of non combustible gas like CO2 and N2. It can be made by passing alternatively steam and little air through a bed of red hot coal or coke maintained at about 900 to 1000˚C in a reactor, which consists of steel vessels Exit for ash Steam supply Water gas outlet coke Coal at 1000˚C Air supply

Reactions: Supplied steam reacts with red hot coal at ˚C to form carbon monoxide and hydrogen. The reaction is endothermic , so the temperature of bed falls. In order to rise the coal bed at about 1000˚C, the steam supply is temporarily blown in, when the following exothermic reaction occurs. The temperature again rise bed to about 1000˚C. the cycles of steam run and air blow are thus repeated alternatively to maintain proper temperature.

Average composition of water gas :
H2 = 51% CO = 41% N2 = 4% CO2 = 4%

Natural gas Natural gas is obtained from wells dug in the oil bearing regions. When natural gas occurs along with petroleum in oil wells is called “wet gas’. The wet gas is treated to remove propane, propene, butane and butene, which are used as LPG. When the gas is associated with the crude oil, It is called dry gas. The approximate composition of natural gas CH4= 70-90% C2H6= 5-10% H2= 3% CO+CO2= rest Calorific value= to kcal/m3

H2S gas, if present in natural gas, is removed by scrubbing with
Monoethanol amine (NH2-CH2-CH2-OH) 2 NH2-CH2-CH2-OH + H2S  (NH2-CH2-CH2-OH)2.H2S Use It is an excellent domestic fuel and can be conveyed over very large distances in pipelines. It has recently been used in the manufacture of number of chemicals by synthetic processes. It is also used as raw material for the manufacture of carbon black (a filler for rubber) and hydrogen (used for ammonia synthesis).

Compressed natural gas (CNG) is a fossil fuel substitute for gasoline (petrol), Diesel fuel, and propane/LPG. Although CNG's combustion does produce greenhouse gases, it is widely considered a more environmentally "clean" alternative to conventional fuels; plus, it is much safer than other fuels in the event of a spill (as natural gas is lighter than air, and disperses quickly when released). CNG may also be mixed with biogas (produced from landfills or wastewater).

Coal gas

Oil Gas

C (s) + O2(g)  CO2 (g) (12) 2H2 (g) O2(g)  2H2O (g) (2X2=4) X18=36 S (s) O2(g)  SO2 (g)

Air contains 21% of oxygen by volume and mass percentage is 23

Fuels and Combustion Arunava Agarwala.

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Fuels and Combustion Arunava Agarwala.

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