FUELS AND COMBUSTION

FACULTY- ER. GOSTA PANDA

SEMESTAR- 3^RD MECHANICAL ENGG.

SUBLECT- TE-I

Sub Topics

• Fuels –Introduction, definition, classification, calorific value, Gross and Net, theoretical calculation.
• Solid fuel- Coal- classification ,Analysis- Proximate and ultimate
• Carbonisation process-Metallurgical coke-Manufacture of coke by Otto Hoffmann process.
• Liquid fuel-Petroleum processing and fractions-manufacture of synthetic petrol by Bergius process. Knocking in SI and CI engines. Octane and cetane number
• Gaseous fuels - CNG, LPG, Water gas and a producer gas.
• Power alcohol and Biodiesel
• Combustion- Introduction and theoretical calculation of air.- Problems
• Ignition Temperature and explosive range. Flue gas analysis by Orsat method.

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INTRODUCTION�

• Fuels –Introduction, definition, classification, calorific value, Gross and Net, theoretical calculation.
• A fuel is a combustible substance containing carbon as the main constituent which on proper burning gives large amount of heat that can be used economically for domestic and industrial purposes. During the process of combustion of a fuel, the atoms of carbon, hydrogen, etc combine with oxygen with simultaneous liberation of heat. The calorific value of a fuel depends mainly on the two elements.
• C + O2 CO2 + 94 kcals.
• 2H2 + O2 2H2 O + 68.5 kcals.
• So, carbon compounds have been used for many centuries as the source of heat and energy.
• The main source of fuel is coal and petroleum. These are stored fuels available in earth's crust and are generally called fossil fuels because they were formed from the fossilised remains of plants and animals.

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Classification of Fuels

• 1. Primary fuels which occur in nature as such, e.g. coal, petroleum and natural gas.
• 2. Secondary fuels which are derived from the primary fuels, e.g. coke, gasoline, coal gas, etc.
• Both primary and secondary fuels may be further classified based upon their physical state as
• (i) solid fuels (ii) liquid fuels and (iii) gaseous fuels.

CALORIFIC VALUE

• Calorific value of a fuel is "the total quantity of heat liberated, when a unit mass (or volume) of the fuel is burnt completely."

Units of heat :

(1) 'Calorie' is the amount of heat required to raise the temperature of one gram of water through one degree Centigrade (15-16°C).

(2) "Kilocalorie" is equal to 1,000 calories. It may be defined as 'the quantity of heat required to raise the temperature of one kilogram of water through one degree Centigrade. Thus: 1 kcal = 1,000 cal

(3) "British Thermal unit" (B.T.U.) is defined as "the quantity of heat required to raise the temperature of one pound of water through one degree Fahrenheit (60-61°F). This is the English system unit.

1 B.T.U. = 252 cal = 0.252 kcal 1 kcal = 3.968 B.T.U.

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HCV and LCV

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Higher or gross calorific value:

• Usually, all fuels contain some hydrogen and when the calorific value of hydrogen-containing fuel is determined experimen­tally, the hydrogen is converted into steam. If the products of combustion are condensed to the room temperature (15°C or 60°F), the latent heat of condensation of steam also gets included in the measured heat, which is then called "higher or gross calorific value". So, gross or higher calorific value (HCV) is "the total amount of heat produced, when unit mass/volume of the fuel has been burnt completely and the products of combustion have been cooled to room temperature"(i.e., 15°C or 60°F ).

Lower or net calorific value (LCV)

• In actual use of any fuel, the water vapour and moisture, etc., are not condensed and escape as such along-with hot combustion gases. Hence, a lesser amount of heat is available. So, net or lower calorific value (LCV) is "the net heat produced, when unit mass /volume of the fuel is burnt completely and the products are permitted to escape".
• Net calorific value= Gross calorific value - Latent heat of condensation of water vapour produced
• = GCV - Mass of hydrogen per unit weight of the fuel burnt x 9 x Latent heat of condensation of water vapour

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• Dulong's formula for calorific value from the chemical composition of fuel is :

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• HCV = 1/100 [8,080 C + 34,500 (H – O/8)+ 2,240 S] kcal/kg

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• where C, H, O, and S are the percentages of carbon, hydrogen, oxygen and sulphur in the fuel respectively. In this formula, oxygen is assumed to be present in combination with hydrogen as water, and

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• LCV = [ HCV - 9H/100 x 587] kcal/kg = [HCV - 0.09 H x 587] kcal/kg

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• This is based on the fact that 1 part of H by mass gives 9 parts of H2O, and latent heat of steam is 587 kcal/kg.

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Solid fuel �

COAL

• Coal is regarded as a fossil fuel produced from large accumulations of vegetable debris due to partial decay and alteration by the action of heat and pressure over millions of years. Coal is a highly carbonaceous matter that has been formed as a result of alteration of vegetable matter (e.g., plants) under certain favourable conditions. It is chiefly composed of C, H, N, and 0, besides non-combustible inorganic matter.

Classification of Coal

• Coals are classified on the basis of their rank. Rank is defined as the degree or extent of maturation and is therefore a qualitative measure of carbon contents. Peat, lignite and sub-bituminous coals are referred as low rank coals while bitu­minous coals and anthracites are classed as high rank. In European terminology, the lignite and sub-bituminous coals are called soft coals while bituminous coals and anthracite coals are termed as hard coals. In North American terminology, the coal series is written as
• Wood peat lignite bituminous

anthracite.

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Liquid Fuels

The important liquid fuels are petroleum, petroleum products, tar, alcohols. These are naturally found under the sea surface. Liquid fuels are also obtained synthetically from' hydrogenation of coat. Liquid fuels find extensive use in domestic and industrial fields.

Petroleum

• Petroleum or crude oil is a dark greenish brown or black coloured viscous oil found deep in earth's crust. The oil is usually floating over a brine solution and above the oil, natural gas is present. Crude oil containing mixture of paraffinic, olefinic and aromatic hydrocarbons with minor amounts of organic compounds like N, 0 and S. The average composition of crude oil is C =80 - 87 %, H =11-15%, S = 0.1 -3.5%, (N +O) =0.1- 0.5%.

a) Classification of petroleum

• Petroleum is classified into three types based on variation of chemical nature of crude oil found in the earth.
• i) Paraffinic-base type crude oil: It contains saturated hydrocarbons from CH4 to C35H72 and little amount of naphthalenes and aromatics.
• ii) Asphaltic-base type crude oil: It contains mainly cycloparaffins or naphthalenes with smaller amount of paraffins and aromatic hydrocarbons.
• iii) Mixed-base type crude oil : It contains both paraffinic and asphaltic hydrocarbons and are generally in the form of semi-solid waxes.

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Mining and Refining of Petroleum

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• The crude oil obtained from the earth crust contains water, sulphur and some unwanted impurities. After removal of water, sulphur and these impurities, the crude oil is separated into various useful fractions by fractional distillation and finally conveI1ed into desired specific products having different boiling points. The process is called "Refining of Petroleum" and the refining plants are called "Oil refineries". The process of refining involves the following steps.

• Step -I: Separation of water (Cottrell's process)
• The crude oil from the oil well is an extremely stable emulsion 0 f oil and salt water. The crude oil is allowed to flow between two highly charged electrodes, where colloidal water droplets coalesce to form large drops, which is then separated out from the oil.

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• Step - II: Removal of harmful impurities
• a) The presence of NaCI and MgCI in the crude oil can corrode the refining equipment, hence these salts are removed by electrical desalting and dehydration methods.
• b) The sulphur compounds present in the crude oil is removed by treating oil with copper oxide, which results in the formation of copper sulphide (solid), which is then removed by filtration.

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Fractional distillation

• Step - III: Fractional distillation

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• The crude oil is then heated to about 400°C in an iron retort, whereby all volatile substances (except asphalt or coke) are evaporated. The hot vapors are then passed up a fractionating column, which is a tall cylindrical tower containing a number of horizontal stainless steel trays at short distances. Each tray is provided with small chimney covered with a loose cap. (Figure )

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• When the vapours of the oil go up in the fractionating column, they become gradually cooler and get condensed at different heights of column. The fractions having higher boiling points condense at lower trays whereas the fractions having lower boiling points condense at higher trays. The gasoline obtained by the fractional distillation is called straight --run gasoline. Various fractions obtained at different trays are given in table.

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Fractional distillation of Crude oil

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OCTANE NUMBER (or) OCTANE RATING�

• Octane number is introduced to express the knocking characteristics of petrol. It has been found that n-heptane knocks very badly and hence, its anti-knock value has been given zero. On the other hand, iso-octane gives very little knocking and so, its anti-knock value has been given 100.

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• Thus octane number is defined as 'the percentage of iso-octane present in a mixture of iso-octane and n-heptane.‘

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• Iso-octane (Octane number = 100)
• CH3 – CH2 – CH2 – CH2 – CH2 – CH2 - CH3
• n - heptane (Octane number = 0)

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CETANE NUMBER (or) CETANE RATING

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• Cetane number is introduced to express the knocking characteristics of diesel. Cetane has a very short ignition lag and hence its cetane number is taken as 100. On the other hand 2-methyl naphthalene has a long ignition lag and hence its cetane number is taken as zero.
• CH3 – (CH2)14 – CH3
• n-cetane (hexa decane)
• cetane number = 100

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• 2-methyl naphthalene (cetane number = 0)
• Thus the cetane number is defined as "the percentage of hexa decane present in a mixture of hexa decane and 2-methyl napthalene, which has the same ignition lag as the fuel under test".
• The cetane number decreases in the following order.
• n-alkanes > Cycloalkanes > alkenes >branched alkanes >aromatics
• The cetane number of a diesel oil can be increased by adding additives called dopes.
• Important dopes: Ethyl nitrate, Iso-amyl nitrate.
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COMPRESSED NATURAL GAS

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• Natural Gas is obtained from wells dug in the oil bearing regions. When natural gas occurs along with petroleum in oil wells, it is called as ‘wet gas’ and contains gaseous hydro carbons from C1 to C4. The wet gas is then suitably treated to remove propane, propene, butane and butane, which is used as LPG.When the natural gas is compressed, it is called Compressed Natural Gas (CNG). The primary component present in CNG is methane. It is mainly derived from natural gas.
• The natural gas can either be stored in a tank of a vehicle as compressed natural gas (CNG) at 3,000 or 3,600 psi or as liquified natural gas (LNG) at typically 20-150 psi. A suitably designed natural gas engine may have a higher output compared with a petrol engine because the octane number of natural gas is higher than that of petrol.
• Compressed natural gas vehicles require a greater amount of space for fuel storage than convention gasoline power vehicles. Since it is a compressed gas, rather than a liquid like gasoline, CNG takes up more space for each GGE (Gallon of Gas Equivalent). This makes it difficult to design smaller vehicles that look and operate like the vehicles that people are accustomed to.

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• CNG is often confused with liquefied natural gas (LNG). While both are stored forms of natural gas, the key difference is that CNG is in compressed form, while LNG is in liquefied form. CNG has a lower cost of production and storage compared to LNG as it does not require an expensive cooling process and cryogenic tanks. CNG requires a much larger volume to store the same mass of natural gas and the use of very high pressures (3000 to 4000 lbf/in², or 205 to 275 bar).
• CNG has been made mandatory for all public transport in the Indian capital city of New Delhi.

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• Properties
• l. CNG is; the cheapest, cleanest and least environmentally impacting alternative fuel.
• 2. Vehicles powered by CNG produce less carbon monoxide and hydrocarbon (HC) emission.
• 3. It is less expensive than petrol and diesel.
• 4. The ignition temperature of CNG is about 550°C. CNG requires more air for ignition.

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Producer Gas�

• Producer gas is a mixture of combustible gases, CO(30%) and H2(15%) with large amount of non combustible gases N2(56%) and CO2(3%). Its calorific value is 1300kcal/m3

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• It is prepared by passing air mixed with a little steam over a red hot coke maintained at about l100°C in a special reactor called gas producer. It consists of a steel vessel of (3 m in diameter, 4f m in height) inside lined with refractory bricks. It is provided with cup and cone feeder at the top and a side opening for produced gas exit. At the bottom, it has inlets for passing air and steam.

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WATER GAS�

• It is a mixture of combustible gases, CO(41%) and H2 (51%) with small amount of noncombustible gases, CO2(4%) and N2(4%). Its calorific value is about 2800

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• Manufacture
• The water gas producer consists of a tall steel vessel, lined inside with refractory bricks. It is provided with cup and cone feeder at the top and a side opening for water gas exit. At the bottom it is provided with two inlet pipes for passing air and steam
• When steam and little air is passed alternatively over a red hot coke maintained at about 900 - l000°C in a reactor, water gas is produced.

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