Combustion Processes

1
Fossil Fuels
2
Combustion Processes

• Burning of hydrocarbons in the presence of oxygen
  (high temperature oxidation of hydrocarbon fuels)
  yields a large amount of energy as heat and
  products which are stable oxides
• CH4 2O2 ? CO2 2H2O
• (bond energies ? energy release 810 kJ)

3
Fossil Fuels

• Recall from the carbon cycle that the processes
  of photosynthesis and respiration are closely
  balanced
• cycling of carbon from atmospheric CO2 to reduced
  organic carbon in biological organisms is
  essentially a closed loop

4
Fossil Fuels

• A small fraction of plant matter (1 part in
  10,000) gets buried. Over millions of years,
  this adds up to large amounts of reduced carbon
  compounds
• High temperatures and pressures in the Earths
  crust transform them to coal, oil and gas
• petroleum and gas are of marine origin
• coal is of terrestrial origin

5
Fossil Fuels

• Reduced carbon gets buried - anaerobic bacteria
  digest biological matter releasing N2 and O2
• Temperature and pressure conditions lead to
  organic disproportionation reactions
• produces methane and small hydrocarbons as gases
  and oil from remaining heavier organics
• woody plants decompose and are buried as peat
• temperature and pressure lead to coal formation
  and expulsion of H2O and CO2

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7
Fuel Energy

• Energy release can be calculated using bond
  energies (B.E.s)
• Energy B.E. of products B.E. of reactants
• Petroleum and coal are not pure subtances
• e.g. Petroleum is composed of saturated
  hydrocarbons and some aromatics

8
Fuel Energy

• Combustion of a representative CH2 group in the
  hydrocarbon chain
• 2(- CH2 -) 3O2 ? 2CO2 2H2O
• H/C ratio of 21
• But higher for saturated hydrocarbons (CH3
  terminal groups)
• For aromatics, H/C ratio closer to 11
• Energy content per gram of fuel is determined by
  the H/C ratio

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10
Oil and gas can get trapped in pockets
underground such as where the rocks are folded
into an umbrella shape. Oil and gas can move
through the porous rocks (rocks with gaps between
the grains). The oil and gas move upwards from
the source rock where they were formed. When they
met a layer of cap rock (a rock with no spaces
between the grains) the oil and gas are trapped.
http//www.world-petroleum.org/education/ip1/ip2.h
tml
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A well is drilled so that the crude oil and other
liquids travel up the bore hole. When it comes to
the surface the crude oil has to be moved closer
to where it is needed. Finding oil and gas
trapped deep underground and drilling a well are
very complicated and expensive. It costs millions
to drill a well and only a few are successful.
The liquids found underground can be a
complicated mixture of water, crude oil and gas.
The crude oil and gas need to be separated before
they can be transported safely.
http//www.world-petroleum.org/education/ip1/ip2.h
tml
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Petroleum

• Oil is a complex mixture of hydrocarbons (also
  contains small amounts of sulfur (up to 10),
  oxygen (5), nitrogen (1) and trace amounts of
  metals (V, Ni, Fe, Al, Na, Ca, Cu and U)
• Most hydrocarbons are saturated but up to 10 are
  aromatic
• Molecules range widely in size and are separated
  by boiling point (distillation tower)
• In addition to separation, oil refining also uses
  chemical reactions to enhance yields of certain
  components of crude oil

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Refining is the complex series of processes that
manufactures finished petroleum products out of
crude oil. While refining begins as simple
distillation (by heating and separating),
refiners must use more sophisticated additional
processes and equipment in order to produce the
mix of products that the market demands.
Generally, this latter effort minimizes the
production of heavier, lower value products (for
example, residual fuel oil, used to power large
ocean-going ships) in favor of middle distillates
(jet fuel, kerosene, home heating oil and diesel
fuel) and lighter, higher value products (liquid
petroleum gases (LPG), naphtha, and gasoline).
The first refinery, opened in 1861, produced
kerosene by simple atmospheric distillation. Its
by-products included tar and naphtha. It was soon
discovered that high-quality lubricating oils
could be produced by distilling petroleum under
vacuum. However, for the next 30 years kerosene
was the product consumers wanted. Two significant
events changed this situation (1) invention of
the electric light decreased the demand for
kerosene, and (2) invention of the internal
combustion engine created a demand for diesel
fuel and gasoline (naphtha).
http//www.ocean.udel.edu/oilspill/crudeoil.html
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The composition of crude oil varies depending on
its source. North Sea crude is a light oil. This
makes it valuable because it needs less
processing to make it into the refined products
that are most in demand. These are gasoline
(petrol), diesel, kerosene (including jet fuel)
and fuel oil. It is also low in sulfur which
makes it easier to meet new environmental
standards which demand a very low sulphur
content.
http//www.energyinst.org.uk/education/coryton/pag
e7.htm
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The basic refining processes take place in the
crude distillation unit (CDU) of the Fuels Zone.
Here, crude oil is taken into the atmospheric
distillation tower where it is separated into its
different fractions. The hydrocarbons in crude
oil have different boiling points according to
the number of carbon atoms in each molecule and
how they are arranged. The oil is heated and the
resultant vapours rise up the tower. The vapours
cool as they rise and condense onto trays.
http//www.doitpoms.ac.uk/tlplib/recycling-polymer
s/origin.php
http//www.energyinst.org.uk/education/coryton/pag
e7.htm
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The lightest compounds condense at the top of the
tower and are taken off as LPG (liquefied
petroleum gas). The oil then undergoes further
processing prior to distribution. The octane
rating is increased to improve engine ignition.
Sulfur is removed because when products are used
the sulfur compounds emitted would smell of
rotten eggs and dissolve in rain to form sulfuric
acid. Other strong smelling compounds are also
removed. Heavy residue is taken off at the base
of the tower and reprocessed. In the fluid
catalytic cracker (FCC) the heavy oil is
distilled again, using a chemical catalyst this
time, to produce gasoline and diesel. The
heaviest sticky residue is redistilled in the
vacuum distillation unit then taken to the
Lubricants Zone where it is processed to make
bitumen, lubricating oils and wax.
http//www.energyinst.org.uk/education/coryton/pag
e7.htm
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Products from the lubricants zone
Crude Distillation Unit
LPG is used for bottled gas. Gasoline is used by
cars and lorries. Kerosene is used as aviation
fuel and for lighting and heating. Diesel is used
for road transport and trains. Lubricating oil is
used for cars and machinery. Wax is used for
polish, wax crayons and food packaging. Fuel oil
is used by power stations, factories and ships
engines. Bitumen is used for tarring roads and
coating felt roofs.
http//www.energyinst.org.uk/education/coryton/pag
e7.htm
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Cracking

• Cracking is the name given to the process by
  which large hydrocarbon molecules are broken into
  shorter chains or monomers. This is done under
  high pressures and temperatures, but a catalyst
  allows slightly lower pressures and temperatures
  to be used. The industrially used catalysts are
  zeolites (finely-porous aluminosilicates), which
  are mixed with the naphtha (gasoline range) as a
  fine powder. This is blown through a chamber at
  500ºC.
• C(mn)H2(mn)2 ? CmH2m CnH2n2
• alkane alkene alkane
• (kerosene (gasoline size)
• gas/oil size)

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Cracking

• Once separated the hydrocarbons are filtered to
  purify them. They are passed through a column of
  sulfuric acid, which removes unsaturated
  hydrocarbons (those with carbon-carbon double
  bonds), nitrogen compounds, oxygen compounds and
  residual solids (tars and asphalt). An absorption
  column removes water, then hydrogen-sulfide
  scrubbers remove sulfide and sulfur compounds. An
  example of possible fractions naphtha might split
  into is shown below.

http//www.doitpoms.ac.uk/tlplib/recycling-polymer
s/origin.php
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With the advent of mass production and World War
I, the number of gasoline-powered vehicles
increased dramatically and the demand for
gasoline grew accordingly. However, distillation
processes produced only a certain amount of
gasoline from crude oil. In 1913, the thermal
cracking process was developed, which subjected
heavy fuels to both pressure and intense heat,
physically breaking the large molecules into
smaller ones to produce additional gasoline and
distillate fuels. There are several types of
cracking Thermal - you heat large hydrocarbons
at high temperatures (sometimes high pressures as
well) until they break apart. steam - high
temperature steam (1500 degrees Fahrenheit / 816
degrees Celsius) is used to break ethane, butane
and naptha into ethylene and benzene, which are
used to manufacture chemicals.
http//science.howstuffworks.com/oil-refining5.htm
21
visbreaking - Visbreaking was developed in the
late 1930's to produce more desirable and
valuable products. Residual from the distillation
tower is heated (900 degrees Fahrenheit / 482
degrees Celsius), cooled with gas oil and rapidly
burned (flashed) in a distillation tower. This
process reduces the viscosity of heavy weight
oils and produces tar.
coking - residual from the distillation tower is
heated to temperatures above 900 degrees
Fahrenheit / 482 degrees Celsius until it cracks
into heavy oil, gasoline and naphtha. When the
process is done, a heavy, almost pure carbon
residue is left (coke) the coke is cleaned from
the cokers and sold.
http//science.howstuffworks.com/oil-refining5.htm
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Catalytic Processes. Higher-compression gasoline
engines required higher-octane gasoline with
better antiknock characteristics. The
introduction of catalytic cracking and
polymerization processes in the mid- to late
1930's met the demand by providing improved
gasoline yields and higher octane numbers.
Catalytic - uses a catalyst to speed up the
cracking reaction. Catalysts include zeolite,
aluminum hydrosilicate, bauxite and
silica-alumina. fluid catalytic cracking - a
hot, fluid catalyst (1000 degrees Fahrenheit /
538 degrees Celsius) cracks heavy gas oil into
diesel oils and gasoline. hydrocracking -
similar to fluid catalytic cracking, but uses a
different catalyst, lower temperatures, higher
pressure, and hydrogen gas. It takes heavy oil
and cracks it into gasoline and kerosene (jet
fuel). After various hydrocarbons are cracked
into smaller hydrocarbons, the products go
through another fractional distillation column to
separate them.
http//science.howstuffworks.com/oil-refining5.htm
23
http//www.world-petroleum.org/education/ip1/ip2.h
tml
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The main object of catalytic, or cat, cracking is
to extract gasoline from heavy cuts of
hydrocarbons. The feed for cat cracking is
usually straight-run heavy gas oil and flasher
tops. The feed and fresh catalyst are pumped into
a reaction chamber, where the cracking takes
place.
http//student.britannica.com/comptons/art-53786/T
he-main-object-of-catalytic-or-cat-cracking-is-to
During the cracking process, coke (carbon) ends
up as a deposit on the catalyst and the catalyst
becomes spent, or inactive. To remove the coke,
the spent catalyst is pumped to a regenerator,
where it is regenerated and sent back to the
reaction chamber. Meanwhile the cracked
hydrocarbon is sent to a fractionator, where the
cracked products are separated. The fractionator
bottoms, called cycle oil, are usually mixed with
fresh feed and run through the reaction process
again.
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alkylation

• In alkylation, low molecular weight compounds,
  such as propylene and butylene, are mixed in the
  presence of a catalyst such as hydrofluoric acid
  or sulfuric acid (a by-product from removing
  impurities from many oil products). The products
  of alkylation are high octane hydrocarbons, which
  are used in gasoline blends to reduce knocking
• (catalyzed by strong acids)
• Cracking and alkylation can increase the gasoline
  yield of crude oil by 20 to 40-45.

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The octane rating of gasoline tells you how much
the fuel can be compressed before it
spontaneously ignites. When gas ignites by
compression rather than because of the spark from
the spark plug, it causes knocking in the engine.
Knocking can damage an engine, so it is not
something you want to have happening.
Lower-octane gas (like "regular" 87-octane
gasoline) can handle the least amount of
compression before igniting.
It turns out that heptane handles compression
very poorly. Compress it just a little and it
ignites spontaneously. Octane handles compression
very well -- you can compress it a lot and
nothing happens. Eighty-seven-octane gasoline is
gasoline that contains 87-percent octane and
13-percent heptane (or some other combination of
fuels that has the same performance of the 87/13
combination of octane/heptane). It spontaneously
ignites at a given compression level, and can
only be used in engines that do not exceed that
compression ratio.
http//science.howstuffworks.com/oil-refining5.htm
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Petroleum

• Advantages
• petroleum is a liquid fuel (main advantage)
• (easy to store and transport)
• extremely efficient infrastructure built up
  around the oil industry
• Disadvantages
• oil spills (major pollution source)
• emissions
• CO2 is a greenhouse gas
• photochemical smog (NOx hydrocarbons)

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http//www.channel6.dk/native/uk/page104.html
http//www.greenpeace.org.uk/tags/un?page1
Over half the ocean's waste oil comes from
land-based sources and from unregulated
recreational boating. The heavy development in
this busy California port illustrates one
potential source of petroleum contamination in
coastal waters. (Note dark plume in left
foreground.)
http//www.waterencyclopedia.com/Oc-Po/Oil-Spills-
Impact-on-the-Ocean.html
30
http//www.britannica.com/eb/art/print?id88973ar
ticleTypeId0
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RECOVERING FROM THE EXXON VALDEZ OIL SPILL A
large quantity of crude oil was deposited on
beaches in Prince William Sound and along the
shoreline of the Gulf of Alaska after the Exxon
Valdez tanker wrecked in 1989. The oil waste has
been closely monitored to determine its status
and its effects in the ocean and along the
coast. Initial efforts to remove the oil from
intertidal areas included flushing them with hot
water applied with high pressure, which proved
fatal for much of the marine life involved.
Natural rates of biodegradation and recovery have
been slower than anticipated, and visible residue
may persist for up to 30 years.
http//encarta.msn.com/media_461538361/exxon_valde
z_oil_spill_cleanup.html
http//www.waterencyclopedia.com/Oc-Po/Oil-Spills-
Impact-on-the-Ocean.html
32
Natural Gas

• Can represent as large a resource as petroleum
  but has traditionally been a by-product of oil
  exploration and fuel production
• reserves better understood (gas-bearing
  formations)
• improved recovery techniques

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Natural Gas

• Advantages
• Predominantly CH4
• clean fuel very little processing
• transport via pipelines
• CO2 emission rate is lower than fossil fuels
• contributes less to photochemical smog
• (CH4 less reactive w.r.t. free radicals)

34
Natural Gas

• Disadvantages
• Storage and transport not as easy as liquid fuels
• (high pressures or low temperatures)
• CH4 produces less CO2 than other fuels but it is
  a potent greenhouse gas itself.
• (a methane molecule contributes 20x more to the
  greenhouse effect than CO2)

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Coal

• Coal deposits vary in extent to which original
  woody plant tissue has been transformed to
  non-volatile (fixed) carbon.
• soft coals lignite has higher water and
  small hydrocarbon content.
• 20 moisture 40 fixed carbon
• hard coal anthracite - 80 fixed carbon

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The kinds of coal, in increasing order of
alteration, are lignite (brown coal--immature),
sub-bituminous, bituminous, and anthracite
(mature). Coal starts off as peat. After a
considerable amount of time, heat, and burial
pressure, it is metamorphosed from peat to
lignite. Lignite is considered to be "immature"
coal at this stage of development because it is
still somewhat light in color and it remains
soft. As time passes, lignite increases in
maturity by becoming darker and harder and is
then classified as sub-bituminous coal. As this
process of burial and alteration continues, more
chemical and physical changes occur and a the
coal is classified as bituminous. At this point
the coal is dark and hard. Anthracite is the last
of the classifications, and this terminology is
used when the coal has reached ultimate
maturation. Anthracite coal is very hard and
shiny.
http//www.uky.edu/KGS/coal/coalkinds.htm
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Anthracite
Bituminous
Lignite
The degree of alteration (or metamorphism) that
occurs as a coal matures from peat to anthracite
is referred to as the "rank" of the coal.
Low-rank coals include lignite and sub-bituminous
coals. These coals have a lower energy content
because they have a low carbon content. They are
lighter (earthier) and have higher moisture
levels. As time, heat, and burial pressure all
increase, the rank does as well. High-rank coals,
including bituminous and anthracite coals,
contain more carbon than lower-rank coals which
results in a much higher energy content. They
have a more vitreous (shiny) appearance and lower
moisture content then lower-rank coals.
http//www.scsc.k12.ar.us/2000backeast/ENatHist/Me
mbers/Reynolds/Default.htm
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Coal

• The carbon content of coal supplies most of its
  heat energy per unit weight. The amount of energy
  in coal is expressed in British thermal units per
  pound. A Btu is the amount of heat needed to
  raise the temperature of one pound of water one
  degree Fahrenheit.
• Lignite, the youngest type of coal, has a carbon
  content of 25-35 percent and a heat value between
  4,000 and 8,300 Btus-per-pound. It is mainly used
  for electric power generation.
• Although having a lower carbon content than
  bituminous coal, subbituminous coal is a
  desirable heat source because of its low sulfur
  content. Subbituminous coal has a carbon content
  of 35-45 percent and a heat value of 8,300 to
  13,000 Btus-per-pound. This type of coal is found
  in the Western states and Alaska.
• Bituminous coal is the most abundant coal in the
  United States, with a large deposit found in the
  Allegheny Basin of the East. Bituminous coal has
  a carbon content of 45-86 percent with a heat
  value of 10,500 to 15,000 Btus-per-pound. It is
  used primarily for generating electricity and
  making coke for the steel industry.
• Anthracite has the highest carbon content,
  between 86-98 percent and a heat value of nearly
  1,5000 Btus-per-pound. Anthracite, found in a
  very small supply within the United States, is
  used mainly for home heating (ACF, coal).

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Coal

• Advantages
• Large resource base
• Relatively cheap to mine and transport by rail
• Disadvantages
• Not as convenient as liquid fuels
• Emissioins
• particulates (PAHs)
• SO2 (ultimately acid rain)

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http//en.wikipedia.org/wiki/Coal
Example chemical structure of bituminous coal
Another coal structure
http//www.chemistryexplained.com/Ce-Co/Coal.html
http//www.agen.ufl.edu/chyn/age4660/lect/lect_18
/orgsstruc.gif
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Emission Control

• Limiting atmospheric pollution depends upon two
  strategies
• 1) remove pollutants before they are dispersed
• 2) change conditions to reduce the amount of
  pollutants in the first place
• Both strategies have been tried on most
  pollutants with mixed success.

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Sulfur Dioxide

• Reduction of sulfuric acid aerosols in urban
  areas made by building tall smoke stacks to
  disperse plumes over wider areas
• alleviates local problems
• creates problems downwind
• Abatement requires reducing SO2 emissions and/or
  limiting sulfur content of fuels
• (i) coal-fired power plants use SO2 scrubbers
  in stacks
• gas passes through a slurry of limestone to give
  calcium sulphite
• CaCO3 SO2 ? CaSO3 CO2
• (must be disposed)

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The Inco Superstack in Sudbury, Ontario, with a
height of 380 m (1,247 ft), is the tallest
chimney in Canada and the Western hemisphere, and
the second tallest freestanding chimney in the
world after the GRES-2 Power Station in
Kazakhstan. It is also the second tallest
freestanding structure of any type in Canada,
ranking behind the CN Tower but ahead of First
Canadian Place.
It was constructed by Inco Limited in 1972 at an
estimated cost of 25 million dollars from the
date of its completion until the GRES-2 chimney
was constructed in 1987, it was the world's
tallest smokestack. Between the years 1972-75 it
was the tallest freestanding structure in
Canada. The Superstack sits atop the largest
nickel smelting operation in the world at Inco's
Copper Cliff processing facility in the city of
Greater Sudbury. The structure was built to
disperse sulfur gases and other byproducts of the
smelting process away from the city itself. As a
result, these gases can be detected in the
atmosphere around Greater Sudbury in a 150 mile
radius of the Inco plant.
http//www.answers.com/topic/inco-superstack
http//sudbury.foundlocally.com/Images/default.asp
?Page25Search
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Prior to the construction of the Superstack, the
waste gases caused severe ecological damage in
the area around Sudbury. This included an almost
total loss of native vegetation in some areas,
giving the city a not-entirely deserved
reputation as a barren, rocky wasteland.
Construction of the Superstack allowed the city
to launch an environmental reclamation plan which
has included rehabilitation of water bodies such
as Lake Ramsey, and an ambitious regreening plan
which has seen over three million new trees
planted in the city. In 1992, the city was given
an award by the United Nations in honour of its
environmental rehabilitation programs. Despite
these efforts, much of the environmental damage
to the Sudbury area is permanent, particularly to
exposed rocky outcrops,
which have been dyed jet black by acid rain in a
layer which penetrates up to 3 inches into the
once pink-gray granite. While the Superstack
lowered the ground-level pollution in the Sudbury
area, it also dispersed the gases over a much
larger area. This led to a slow rise in acidity
of lakes in the area, to the point where by the
late 1980s up to 7,000 lakes were severely
damaged due to acid rain. Starting in the early
1990s, a major construction effort started to
dramatically clean the waste gases before pumping
them up the Superstack, removing around 90 of
the sulfur dioxide. The upgrades were completed
in 1994, and emissions from then on are much
reduced. Further reductions in emissions are
planned.
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