Fuels and Energy Conversion

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Fuels and Energy Conversion

• Pradip Majumdar
• Professor
• Department of Mechanical Engineering
• Northern Illinois University

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Fossil Fuels
Remains of vegetations deposit of past geological
ages after subjected to biochemical reactions,
high pressure and temperature.

• Categories
• - Coal
• - Liquid hydrocarbon
• - Gaseous hydrocarbon

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Hydrocarbon Fuels

• One of the most commonly available forms of fuel
  is hydrocarbon fuels, which has carbon and
  hydrogen as the primary constituents.
• The hydrocarbon fuels exits in different phases
  such as liquid like gasoline, solid like coal,
  and gas like natural gas.
• Some of the common hydrocarbon fuels are gasoline
  or octane, diesel, methyl alcohol or methanol,
  ethyl alcohol or ethanol etc.

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Liquid or Gaseous Hydrocarbons

• Normally a mixture of many different
• hydrocarbons.
• - Gasoline consists of 40 different
  
• hydrocarbons

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Coal Composition

• Coal is mainly composed of carbon, sulfur, oxygen
  and hydrogen with varying compositions.
• Composition changes from location to location.
• Analysis given on a mass
• basis, relative moisture
• content, volatile matter,
• fixed carbon and ash.

• Composition of coals from
• western USA - mass
• H 3.5
• C 48.6
• S 0.5
• N 0.7
• O 12.0
• Ash 5.8

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Family of Hydrocarbons

• Isomers
• Two hydrocarbons with the same number of
  carbon and hydrogen atoms and different
  structures.
• Family identified by Suffix
• Paraffin Family - ane ( as propane or
  Octane)
• Olefin Family - ylene or -ene (propene
  and
• 
  Octene)
• Diolefin Family - diene (as butadiene)
• Napthene Family Prefix with cyclo
  (cyclopentane)
• Has the same chemical formula as the olefin
  family, but has a ring rather than chain
  structure)

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Liquid Hydrocarbons

• Most liquid hydrocarbons are derived from
  crude oil by distillation or Cracking Processes
  Gasoline, Kerosene, diesel etc.
• Each type is characterized by its distillation
  curve.
• The distillation curve is obtained by slowly
  heating the crude so that it vaporizes and
  condenses.
• The more volatile component is vaporized first.

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Gaseous Hydrocarbons

• Sources
• 1. Natural gas wells
• 2. Chemical manufacturing processes
• Major constituents Natural gas consists of
  methane, carbon dioxide, hydrogen, nitrogen and
  oxygen with varying composition.
• Typical Composition
• Methane 93.9
• Ethane 3.6
• Propane 1.2
• Butanes Plus 1.3
• Present effort is to produce gaseous fuel or
  liquid hydrocarbons fuel from coal, Oil Shales
  and Tar sands

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Energy Forms

• Total energy content of a system is
  classified into three basic categories
• 1. Kinetic energy,
• - Associated with the translation
  velocity of
• the system
• 2. Potential energy,
• - Associated with the elevation the
  system
• from some reference level
• 3. Internal energy
• - Include all energy forms associated
  with the
• atomic and molecular structures and
  orientations.

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Conversion of Kinetic Energy

• Conversion of kinetic energy to mechanical energy
  and then into electrical energy
• - Wind Energy Generation using wind
• turbine
• - Tidal Energy Generation
• - Wave Energy Generation
• - Jet Propulsion Thrust

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Conversion of Potential Energy

• Conversion of potential energy to mechanical
  energy and then into electrical energy
• -Hydroelectric power generation using
• water impact turbine

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Internal Energy Forms

• Includes translation, rotation and vibrational
  motion of atoms and energy associated with the
  atoms, molecules and subatomic particles.

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Internal Energy forms

• Internal energy is also classified in
  different forms
• Latent energy associated with the phase of
  
• the substance
• Chemical energy associated with the atomic
  
• bonds in a molecular structure.
• Nuclear energy associated with the binding
  
• force within the nucleus of the atom.

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Conversion Internal Energy to Thermal Heat Energy
by Chemical Reaction

• In a chemical reaction the bond structure of the
  reactants are modified to form new bond structure
  and in the process electronic configuration
  within the atoms are changed and chemical energy
  is released.
• Amount of chemical energy released is the
  difference between the internal energy content of
  the original molecular structure of the reactants
  and the internal energy content of the molecular
  structures of the products.

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Combustion

• Combustion process is chemical reaction in which
  a fuel is oxidized and a large quantity of
  chemical energy is released.
• In the combustion of hydrocarbon fuel, carbon,
  hydrogen and any other constituents in the fuel
  that are capable of being oxidized reacts with
  oxygen.

In this reaction, one-kmol (32 kg) of Oxygen
reacts with one-kmol (12 kg) of Carbon and forms
one-kmol (44 kg) of Carbon dioxide Mass Balance
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Combustion with Air

• Oxygen often supplied as air rather than in a
  pure form as it is free and available in
  abundance.
• Even though air is composed number of different
  gases such as oxygen, nitrogen, argon , it is
  assumed primarily composed of 79 nitrogen and
  21 oxygen by volume for analysis purposes,
• i.e. for each k-mole of oxygen there are
  79/21 3.76 k-mole of nitrogen. The reaction
  methane with air is then written as

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• In this reaction nitrogen is assumed as inert and
  does not undergo any chemical reaction.
• Nitrogen thus appears on both sides of the
  equation and simply effects the product
  temperature by absorbing part of the released
  chemical energy and raising its own internal
  energy.
• In some high temperature and pressure reactions,
  nitrogen may undergo reaction and form air
  pollutants such as nitrogen oxide, or nitrogen
  dioxide, or nitric oxide,.

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Incomplete Combustion

• In general, air is supplied as 100 theoretical
  air or stoichiometric air that supplies
  sufficient amount of oxygen for complete
  combustion of all elements.
• In a complete combustion, all carbon oxidizes to
  form , all hydrogen oxidizes to from
  and sulfur oxidizes to form .
• In an incomplete combustion reaction the product
  may contain some fuel as un burnt fuel, some
  carbon in the form of CO and even as carbon
  particles.

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• Incomplete combustion is caused by insufficient
  supply of oxygen as well as inadequate mixing of
  fuel and air in the mixture.
• In a real reaction process, air is supplied in
  excess to achieve complete combustion.
• A combustion reaction with 50 excess air, i.e.
  150 theoretical air or stoichiometric air is
  represented as follows

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Conventional Power Generation

• Conventional power generations are based on
  heat engine principals developed based on
  Kelvin-Plancks statement of second law of
  thermodynamics

High Temperature Source
Heat Addition
High Temperature
Machine
Work, W
Heat Addition
Heat Rejection
Work, W
Machine
Low Temperature Sink
Impossible
Possible
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Carnot Engine-Maximum Possible Performance

• Consist of Four Ideal Processes
• - Reversible isothermal heat addition, Qh
• - Reversible adiabatic Expansion (Work), W
• - Reversible Isothermal heat rejection, Qc
• - Reversible adiabatic compression

Qh
Thermal Efficiency,
W
Qc
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• For a reversible heat engine
• operating on a Carnot cycle

The maximum thermal efficiency of reversible heat
engine is given as

• The lower temperature reservoir in heat engine
  
• power cycle is limited by the ambient
  condition.
• The high temperature is limited by the
• temperature of vapor in the boiler in a vapor
  
• power cycle or the temperature of the product
  of
• combustion in the internal combustion engine.

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Example

• 1.

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Vapor Power Systems

• Vapor power cycles uses working fluids that
  alternately vaporized and condensed.
• In a vapor power system the combustion takes
  place out the system in a furnace

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Standard Vapor Power System
Turbine
Vapor or Steam
Exhaust
Heat Rejection
Boiler
Condenser
Cooling Tower
Furnace
Feed Water Heaters
Heat Addition
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Cooling Water Pumps
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Air- Gas Power System

• This gas power systems includes gas turbine, jet
  propulsion and internal combustion engines of
  the spark ignition and compression-ignition
  types.
• All these systems are internal combustion types
  with combustion taking place inside the system in
  contrast to vapor power systems where combustion
  takes place out the system.

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Reciprocating Internal Combustion Systems

• Two principal types of reciprocating internal
  combustion engines are the spark-ignition engine
  and the compression-ignition engine.
• In a spark ignition engine, a mixture of fuel and
  air is ignited by a spark plug.
• In a compression-ignition engine, air is
  compressed to a high enough pressure and
  temperature that combustion occurs spontaneously
  when fuel is injected.

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Ideal Cycle for Spark Ignition Internal
Combustion

• Four Processes
• 1-2 Isentropic compression as the piston
  moves
• from the crank-end dead center to
  head-end
• dead center.
• 2-3 Heat addition at constant volume when
  piston is
• momentarily at rest at head-end
  dead center.
• 3-4 Isentropic expansion as piston moves from
  
• head-end dead center to crank-end
  dead center
• (Work output).
• 4-1 Rejection of heat when piston is at the
  crank-
• end dead center.

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Why Alternative Energy?

• High cost and higher risk of un-interrupted
  supply imported oil.
• Increased demands for energy and fossil fuels due
  to continuing economic growth in countries such
  as China and India.
• Global warming caused by emission of carbon or
  other greenhouses gases from consumption of
  fossil fuels such as coal and oil used in power
  generations and transportations.
• Cleaner forms of energy are essential to reduce
  carbon and greenhouse gas emissions.
• Increased concern over climate change and
  increased effort to use low-carbon energy to
  reduce greenhouse gas emission.

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Alternative Energy Sources and Power generation

• Alternative energy sources that emit little or
• no carbon and greenhouse gasses are
• - Solar Power - Tidal power
• - Wind Power - Hydrogen Power
• - Geothermal - Hydroelectric
  
• - Fuel Cell