Models for Representation of Fuels

P M V SubbaraoProfessor

Mechanical Engineering Department

Characterization of Natural Resources for Better Design….

Coal Classification• There are two main ways for classifying

coal - by rank and by type.• Coal Rank• Coal Types• Coal Rank : The degree of 'metamorphisrn'

or coalification undergone by a coal, as it matures from peat to anthracite

• This has an important bearing on its physical and chemical properties, and is referred to as the 'rank' of the coal.

• Low rank coals, such as lignite and sub-bituminous coals, are typically softer, friable materials with a dull, earthy appearance; they are characterised by high moisture levels and a low carbon content, and hence a low energy content.

• Higher rank coals are typically harder and stronger and often have a black vitreous lustre.

Composition of Coals

• The natural constituents of coal can be divided into two groups:

• (i) the organic fraction, which can be further subdivided into microscopically identifiable macerals; and

• (ii) the inorganic fraction, which is commonly identified as ash subsequent to combustion, but which may be isolated in the form of mineral matter by low-temperature ashing (LTA).

• The organic fraction can be further subdivided on the basis of its rank or maturity.

Characteristics of Coal

• Sulfur Content : Coal with sulfur > 5% is not recommended for combustion.

• Weatherability : Weathering or Slacking Index .• An indication of size stability. • Denotes the tendency to break on exposure to alternate wet

and dry periods.• Weathering index is the percentage of coal passing through

a sieve having 170 mm2 openings.• Grindability Index : A measure of relative ease of grinding

coals or the power required for grinding coals in a pulverizer.

• G = 6.93 W + 13 -- W is the weight of sample passing through 200 mesh size.

• Burning Characteristics of Coal : • Free burning coals and Caking Coals.• Caking index -- Pulverulent, sintered, weakly caked,

caked and strongly caked.• Ash Fusion temperature -- The temperature where the ash

becomes very plastic. • Design of ash handling system. -- Stoker furnace cannot

use low ash fusion temperature coals.• Dirtiness of furnace walls.

Formation of Oils & Gas

Theory of Oil Formation• The most popular theory is known as the Organic Theory. • This theory states that oil and gas have zoological origins. • Small sea creatures from the days when the earth was mostly covered

in water died and settled to the bottom of the ocean floor. • Layer upon layer of silt, sand and clay built up on top of them over

time. • Through the process of decay, as well as ever increasing heat and

pressure, the former sea creatures were converted to oil and gas. • Over millions of years, continuous pressure actually compressed those

layers of silt and clay into layers of rock. • This is known as "reservoir rock". • The temperature under the earth's surface increases the deeper you go

underground. • At about 600C, oil begins to form. • Oil formation ceases at about 1500 C. • Oil formed at lower temperatures (i.e. closer to the surface) is called

immature and is heavy.

Hydro carbon Chemistry & Classification of Crude Oils

• Paraffin based crudes (a waxy residue)• Asphalt based crudes (an asphalt type

residue)• Mixed type-based crudes ( a combination

residue)• Components of Crude Oils.• Paraffins (CnH(2n+2))• Olefins• Aromatics• Ultimate Analysis• C : 84 -- 87% ; H : 11 -- 16% ; O : 0.3 --

1.8% ; N : 0.1 -- 1.5% ; S : 0.1 -- 3%

Product contents of Crude oils

petroleum refining : Basic refinery processes

• Functions of Refinery Units:• (1) separating the many types of Hydrocarbon present in crude oils into

fractions of more closely related properties,• (2) chemically converting the separated hydrocarbons into more

desirable reaction products, and • (3) purifying the products of unwanted elements and compounds.• Types of Distillation:• Fractional Distillation• Vacuum Distillation• Super fractionation• Thermal Cracking• Catalytic Cracking

Boiling range, and molecule size for typical refinery

• BOILING RANGE # CARBON ATOMS• Refinery Gas <25 oC 3• Gasoline 40-150 oC 4-10• Naptha 150-200 oC 10-12• Kerosene 200-300 oC 12-16• Diesel Fuel 300-400 oC 16-25• Residual Oil >400 oC >25

Properties of Petroleum Derivatives

• Specific Gravity• Calorific Value• Viscosity• Flash Point• Fire Point• Pour Point• Volatility• Ash content• Carbon Residue • Octane Number / Cetane Number / Performance Number

Specific Gravity

• Specific Gravity = (Weight of fuel/unit volume)/(weight of water/unit volume at 15oC)

• API Gravity = 141.5/(SG15.6/15.6oC) - 131.5

• Significance of SG: Origin of the fuel Combustion characteristics

• A high API G : Paraffin fuel with good ignition quality; low c/H ratio.• A high API G : aromatic asphaltic fuel with poor combustion

characteristics• APT G < 10 : Difficult or impossible to separate-out water and solid.• Good quality paraffin straight run fuels : 35 -- 40 (API G)• Aromatic fules : 27 -- 30

• Calorific Value:• HCV (MJ/kg) = {51.916 - 8.792 * (SG)2} * {1 - (M+A+S)} + 9.42 * S • LCV = {46.392 - 8.792 * (SG)2 + 3.187 *(SG)} * {1 - (M+A+S)} +

9.42 *S - 2.449 * M

• Flash Point: The temperature at which the oil must be heated under prescribed conditions for sufficient vapour to be given off to form an flammable mixture with air.

• Determines the type of blend• indicates safe sotrage temperatures.• Gasoline : 40oC; Kerosene: 40oC; Diesel Oile: 90 -- 95 oC

• Fire Point: The temperature at which continuous flame is seen .• Indication of fire risk.

•Ash content: Amount of totally non combustible products. Contaminants such as dirt, sand, rust and scales.

•Solid ash forming compounds can cause

–Severe abrasive wear in IC enginescylinder liners.

–High temperature slagging in fire tubes and super heater tubes.

–Blade deposition on gas turbine blades.

• Viscosity: Kinematic viscosity (Centi Stokes) and Dynamic viscosity (Centi Dynes).

• Design of burners/ IC engine injectors.• Decreases with increasing temperature but becomes constant at 120oC• Heating of fuel helps in atomization.• Maximum viscosity for easy atomization in commercial burners : 25

cStokes.• For easy pumping 1200 cStokes.• Diesel fules : Low viscosity causes exessive leakage. • High viscosity produces coarse drops. -- results in formation of engine

deposits -- incomplete combustion.

• VISCOSITY IS NOT AN PROPORTIONATE PROPERTY.

Gaseous Fuels• Can be easily piped into furnace -- no physical handling is required.• Natural Gas -- True Fossil fuel• Odorless and colorless • Mainly CH4 + heavier HCs• HHV = 55,000 kJ/kg.

• Manufactured Gases• LPG -- light distillates of petroleum. -- Heavier than air!!!• Stored and transported under pressure (0.4 -- 2 Mpa).• SNG : Produced from coal by Hydrogenation -- cheap and clean..• Pressurized Hydrogen at 9000C is combined with coal to produce a

number of light HCs.

• Producer gas, Bio-gas, Water gas, Coke-oven gas etc….

Analysis of Fuel

• Proximate Analysis & Ultimate Analysis.• Proximate analysis - to determine the moisture, ash, volatiles

matter and fixed carbon• Ultimate or elementary analysis - to determine the elemental

composition of the fuel• The Energy content -- CFRI Formulae --• Low Moisture Coal(M < 2% ) -- CV (Kcal/kg) = 71.7 FC + 75.6 (VM-0.1 A) - 60

M• High Moisture Coal(M > 2%) -- CV(kcal.kg) = 85.6 {100 - (1.1A+M)} - 60 M

• Where, M, A, FC and VM denote moister, ash , fixed carbon and Volatile mater (all in percent), respectively.

Fuel Model

Ash Model