Fuels and Energy Conversion

1. Fuels and Energy Conversion Pradip Majumdar Professor Department of Mechanical Engineering Northern Illinois University

2. 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

3. Hydrocarbon Fuels • One of the most commonly available forms of fuel is hydrocarbon fuels, which has carbonand 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 gasolineor octane, diesel, methyl alcoholor methanol, ethyl alcoholor ethanol etc.

4. Liquid or Gaseous Hydrocarbons Normally a mixture of many different hydrocarbons. - Gasoline consists of 40 different hydrocarbons

5. 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% Coal Composition

6. Family of Hydrocarbons Isomers Two hydrocarbons with the same number of carbon and hydrogenatoms 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)

7. Liquid Hydrocarbons • Most liquid hydrocarbons are derived from crude oilby 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.

8. 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

9. 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.

10. 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

11. Conversion of Potential Energy • Conversion of potential energy to mechanical energy and then into electrical energy -Hydroelectric power generation using water – impact turbine

12. Internal Energy Forms Includes translation, rotation and vibrational motion of atoms and energy associated with the atoms, molecules and subatomic particles.

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

14. Conversion Internal Energy to Thermal Heat Energy by Chemical Reaction • In a chemical reactionthe 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.

15. 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

16. 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

17. 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,.

18. 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.

19. 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:

20. Conventional Power Generation Conventional power generations are based on heat engine principals developed based on Kelvin-Planck’s 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

21. 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

22. The maximum thermal efficiency of reversible heat engine is given as For a reversible heat engine operating on a Carnot cycle: • 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.

23. Example 1.

24. 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

25. Standard Vapor Power System Turbine Exhaust Vapor or Steam Heat Rejection Boiler Condenser Cooling Tower Furnace Feed Water Heaters Heat Addition 16 Cooling Water Pumps

26. Air- Gas Power System • This gas power systems includes gas turbine, jet propulsion and internal combustion enginesof the spark ignitionand 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.

27. 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.

28. 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.

30. 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.

31. 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