817 Lecture Notes 3 Fuels2

7/27/2019 817 Lecture Notes 3 Fuels2

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ME 422

Introduction to Combustion

Fall 2013

Instructor: Dr. Elisa Toulson

Lecture 3-Fuels

Homework

Due September 12th:

Chapter 17 Questions 17.1, 17.2, 17.3, 17.8


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Natural Gas

Primarily methane with smaller quantities of otherlight (C2-C8) hydrocarbons,N2, CO2, He, hydrogen

sulfide, water, O2also present.

Natural gas usually forms underground and

rises towards the surface until it gets

trapped by some geological formation such

as layers of porous sedimentary rock with a

denser impermeable layer of rock on top.

Gas in these reservoirs is generally under

pressure so once a hole is made in the

impermeable rock the gas will escape on it

own.

Natural Gas UsesIn the U.S. natural gas is mostly used in

the transportation sector for fleets but

as of 2008 there were 9.6 million natural

gas vehicles worldwide, led by Pakistan

(2.0 million), Argentina (1.7 million),

Brazil (1.6 million), Iran (1.0 million), and

India (650,000)


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Biogas Biogas: a gas produced by the biological breakdown

of organic matter in the absence of oxygen

Biogas is produced by the anaerobic digestion or

fermentation of biodegradable materials such as

biomass, manure, sewage, municipal waste, green

waste, plant material, and crops.

Biogas comprises primarily methane (CH4) and

carbon dioxide (CO2) and may have small amounts

of hydrogen sulphide (H2S).

Compound Chem %

Methane CH4 5075

Carbon dioxide CO2 2550

Nitrogen N2 010Hydrogen H2 01

Hydrogen

sulfideH2S 03

Oxygen O2 00

Typical Composition of Biogas


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Synthetic Gas (Syngas)Gasification process for syngas production

C + H2O CO + H2

H298 = 175.3 kJ/molC + O2 CO2

H298 = -393.5 kJ/mol

CO2 + C 2CO

H298 = 172.5 kJ/mol

More than a million Wood Gas Vehicles were used

when fuel was rationed during World War 2.


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Fisher Tropsch Liquid Fuels

Interest in gas-to-liquid or Fischer-Tropsch diesel fuels hasincreased in recent years because of their potential to displace

imported petroleum.

Fisher-Tropsch process was invented in the 1920s in Germany

and during World War 2 production accounted for an

estimated 9% of German war production of fuels and 25% of

the automobile fuel.

The price of oil determines whether Fisher-Tropsch synthesis

fuels are viable as capitol costs are high.

Commercial Fisher-Tropsch synthesis plants have been in

operation in South Africa for more than 50 years where an

abundance of coal and government policy makes them viable.

Fisher Tropsch

Liquid Fuels

Synthetic Liquid Fuels can be generated from the syngas derived

from various feedstocks using the Fisher-Tropsch synthesis process.

The FischerTropsch process involves a series of chemical reactions

that produce a variety of hydrocarbons molecules .

The more useful reactions produce alkanes as follows:

(2n+1) H2+ n CO CnH(2n+2) + n H2O

Most of the alkanes produced tend to be straight-chain, suitable as

diesel fuel. In addition to alkane formation, competing reactions give

small amounts of alkenes, as well as alcohols and other oxygenated

hydrocarbons.


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Fisher-Tropsch Reactor

Hydrogen Hydrogen gas is highly flammable and will

burn in air at a very wide range ofconcentrations between 4% and 75% byvolume

When hydrogen combusts in air, theproducts are water and nitrogen, but thereis potential to form nitrogen oxides (NOx).

The main advantages of hydrogen are thatit burns easily, it can be used almostdirectly in systems that are well developedand reliable, and it can significantly reducefossil fuel consumption.

Hydrogen can be produced two ways: bythe decomposition of water throughelectrolysis or by the reformation of fossilfuels

The Space Shuttle Main

Engine burnt hydrogen with

oxygen, producing a nearly

invisible flame at full thrust.


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Considerations for the Choice of Fuels

For most combustion processes, air is used as theoxidizer because air is free and available almosteverywhere on earth.

Choice of fuel depends on purpose of thecombustion process and is subject to local safety

and emission regulations: Energy content per volume or per mass

Safety

Combustion and fuel properties

Cost
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Identification of Fuel by Molecular

Structure: IUPAC

The identification of a fuel can be best defined

by its molecular structure.

For organic chemistry, the convention adopted

by International Union of Pure and Applied

Chemistry (IUPAC) is well established and

should be used.

Naming Conventions for HydrocarbonFuels Commonly used for Combustion


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Naming Conventions

Prefixes for Hydrocarbon Fuels

Naming Conventions The word endingsane, -ene, -

yne indicate how the carbon

atoms are bound in the

molecule

Saturated is used to denote

hydrocarbon molecules in

which the maximum number of

hydrogen atoms is associatedwith carbon atoms (i.e. the

molecule has no double or

triple bonds)

Alkanes are saturated, Alkenes

and Alkynes are unsaturated


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Naming Conventions

n-octane C8H18iso-octane C8H18

2,2,4-trimethylpentane

Octane has

18 isomers

Alkanes with 3 or more carbons can have branches, and such alternative fuel

structures are called isomers. Straight chains are denoted as normal (n-)

Isomers are molecules with the same chemical formula and often the same kinds

of chemical bonds between atoms, but in which the atoms are arranged

differently Larger molecules tend to have more isomers.

Many isomers share similar if not identical properties in most chemical contexts.

Combustion characteristics of isomers, particularly their ignition properties, may

be quite different.

ExampleWhat is the structural formula for:

2,2,4-trimethyl pentane (iso-octane)
http://en.wikipedia.org/wiki/File:Isooctane-3D-vdW.pnghttp://en.wikipedia.org/wiki/File:Isooctane-3D-balls.pnghttp://en.wikipedia.org/wiki/File:2,2,4-Trimethylpentane.svg


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Question

What is the structural formula for

2-2 dimethylpropane ?

Answer


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Question

What is the structural formula for

2-4-5-trimethyl-3-ethyloctane?

Answer


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The basic molecular structure of Cyclanes is a

closed ring, with all carbon atoms singlybonded.

More complex cyclanes are formed by the

substitution of parraffinic groups for H atoms

Cyclanes

Methylcyclohexane: most common cycloalkane found in petroleum

Aromatics Aromatic or benzene family is based on a ring of 6 carbons but with

only one hydrogen atom associated with each carbon atom.

The resulting 6 free valence electrons from resonance hybrid bonds

among the six carbon atoms in the ring so that all of the bonds are

equivalent, with the bonding electrons delocalized over several atoms

The term 'aromatic' was assigned before the physical mechanism

determining aromaticity was discovered, and was derived from

the fact that many of the compounds have a sweet scent.

ethylbenzene

or

Toluene or

methylbenzene

Asprin

Napthalene


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Alcohols (ROH) Common alcohols are formed by the substitution of a

hydroxyl group (OH) for an H atom in an alkane molecule. Alcohols are generically designated as ROH, where R is the

hydrocarbon radical

Aldehyde

Aldehydes can be emitted from diesel engines and

those fueled with alcohols.

Aldehydes contribute to photochemical smog

Formaldehyde, Volatile Organic Compound (VOC)

SMOG


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Ketones

Ketones are used as solvents and are formed

as intermediate species of Hydrocarbon

combustion

Acetone is a common Ketone

Ether (R-O-R)

The general structure of an ether is

or Dimethyl ether

Diethyl ether

or


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Esters

Esters are chemical compounds consisting of a carbonyl adjacentto an ether linkage which are derived by reacting an oxoacid with

a hydroxyl compound such as an alcohol or phenol

Ester names are derived from the parent alcohol and the parent

acid

Most naturally occurring fats and oils are the fatty acid esters of

glycerol

Esters- Important constituents ofbiodiesel

Methyl linoleate a

methyl ester produced

from soybean or canola

oil and methanol

Ethyl stearate an ethyl ester

produced from soybean or

canola oil and ethanol

The five primary methyl esters found in biodiesel

fuels produced from soy and rapeseed


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Relative amounts of various chemical classes in dieselfuel and possible compounds to represent thesechemical classes in a diesel surrogate fuel

General Distribution of Hydrocarbonsin Gasoline

Gasoline can consist of more that 500 different types ofhydrocarbons between 5 and 12 carbons.


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Properties of Fuels

Ignition Characteristics

Volatility

Energy Density

Flash Point

Pour Point

Cloud Point

Spark Ignition Engine Ignition Ignition Quality refers to the prevention of Knock

Knock is the spontaneous ignition (auto-ignition or detonation)of the endgas (unburned

mixture) ahead of the flame.

Detonation is characterized by an instantaneous, explosive ignition of at least one pocket

of fuel/air mixture outside of the flame front. A local shockwave is created and the cylinder

pressure may rise sharply beyond its design limits and cause engine damage.

Pre-ignition is when the mixture in the cylinder ignites before the spark plug fires. Pre-

ignition is initiated by an ignition source other than the spark

Engine damage due to knock


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Normal

Combustion

Pre-Ignition

Knock

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817 Lecture Notes 3 Fuels2