Fuels, Combustion and Pollution

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Fuels, Combustion and Pollution

• Overview

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Definitions

• Fuels are substances which, when heated,
  undergo chemical reaction with an oxidizer,
  typically oxygen, to liberate heat.
• Commercially important fuels contain carbon and
  hydrogen and their compounds, which provide
  heating value

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Definitions (contd)

• Fuels may be solid, liquid or gaseous
• Fuels may be fossil (non-renewable) or biomass
  (renewable)
• Fossil fuels may be coal, petroleum-crude
  derived or natural gas.
• Biomass fuels may be wood, refuse or
  agricultural residues.

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Definitions (contd)

• World-wide production of fossil fuels in 1994
• Coal 180 x 1015 kJ
• Petroleum crude 114 x 1015 kJ
• Natural gas 98 x 1015 kJ
• Biomass fuels provide about 20 x 1015 kJ to
  world energy production
• Fossil fuels provide about 85 of world energy
  production. Balance provided by hydroelectric,
  nuclear and biomass.

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Some statistics

• Middle East and Eastern Europe have 70 of
  worlds natural gas reserves
• Middle East has 67 of worlds crude oil reserves
• Canada has approx. 1 trillion barrels of oil in
  tar sands
• North America, Eastern Europe and China have the
  largest coal reserves

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Some statistics

• The US with a small fraction of the worlds
  population consumes 25 of the worlds crude oil,
  25 of the worlds natural gas and 21 of the
  worlds coal production. They also have a third
  of the worlds motor vehicles
• Each American uses the same energy as 3 Japanese,
  38 Indians and 531 Ethiopians!

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Forms of Fuels
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Solid Fuel Analysis

• Proximate analysis (ASTM D3172)
• Sample of known mass, to determine
• Moisture dried at 105 to 110oC in an oven
• Volatile combustible matter heated to 900oC in
  a covered crucible
  Fixed carbon heated to 750oC in
  an open crucible
  Ash the final residue

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Solid Fuel Analysis

• Ultimate Analysis (ASTM D3176)
• Provides the major elemental composition of the
  fuel, that is usually reported on dry, ash-free
  basis
• Carbon includes organic carbon carbon from
  mineral carbonates
• Hydrogen includes organic hydrogen hydrogen
  from moisture mineral hydrates
• Other elements include oxygen, nitrogen, sulfur
  and others like chlorine.

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Wood

• A Renewable Fuel

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Typical Proximate Analysis of Wood compared to
Coal
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Typical Ultimate Analysis of Some types of Wood
in
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Typical Ultimate Analysis of Some types of Bark
Species in
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Typical Values of Calorific Values in kJ/kg of
Wood Fuels
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Wood Storage

• Wood fuels undergo losses in net available energy
  as storage time increases due to
• Moisture accumulation with time and reaches
  saturation.
• Loss of volatiles due to evaporation 15 of net
  available energy is lost this way.
• The pH of wood is reduced making it acidic
  leading to corrosive effects
• Last in, first out (LIFO) must be followed.

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Wood Combustion

• Surface undergoes thermal breakdown vapors,
  gases, mists (combustibles) are evolved. Exists
  up to 200oC.
• More gases are evolved. Heat liberation reactions
  occur but no flaming. Occurs from 200 to 280oC.
• Gases continue to evolve and heat is liberated.
  Flaming starts. Occurs up to 500oC.
• Above 500oC all gases and tar are driven off.
  Pure carbon (charcoal) remains. Further heating
  will result in combustion of charcoal.

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Combustion Characteristics of Wood

• It is easily ignited.
• Does not burn in large pieces because layers of
  semi-fused ash forms on the surface.
• Produces a long, non-smoky flame when burned in
  excess air. With limited air, it burns with a lot
  of smoke.
• As saw dust it burns readily. Saw dust can be
  made into binderless briquettes at pressures of
  up to 8 kg/mm2.

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Alternate fuels from Wood

• Charcoal
• A carbonized form of wood. Involves the
  decomposition of the wood in the absence of air.
  Three methods are known
• a. An ancient process in pits.
• b. Low temperature carbonization in metal
  retorts, at about 350oC.
• c. High temperature carbonization in retorts,
  at around 1000-1200oC.
• Charcoal is easily ignited. Used as reducing
  agent for iron ore, domestic cooking and to
  manufacture producer gas.

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Alternate fuels from Wood

• Charcoal (Continued)
• Typical Ultimate analysis on wet basis with ash
• Carbon 85.2
• Hydrogen 2.9
• OxygenNitrogen 3.5
• Ash 2.5
• Moisture 5.9
• Calorific Value 31,400 kJ/kg

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Alternate fuels from Wood

• Substitute Natural Gas (SNG) and Methanol
• Obtained by gasifying wood to carbon monoxide and
  hydrogen after moisture is removed.
• Wood has self generating water supply and low ash
  and sulfur, making its gasification superior to
  coal gasification.
• CO and H2 are synthesized to form SNG over a
  catalyst or methanol. Methanol can be converted
  to gasoline by the MTG process.

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Alternate fuels from Wood

• Producer gas
• In India, producer gas from wood is used as a
  fuel. Yield from about 500 kg wood is about 7400
  m3 and calorific value is about 5600 kJ/ m3.

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Peat

• Beginning of Fossilization

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Peat

• Peat is the first stage in the formation of coal.
• It is regarded as the borderline between
  vegetation (biomass) and a fossil fuel.
• It is a brown, fibrous mass of partially decayed
  plant material accumulated in situ under
  water-logged conditions.
• Composition depends on type, depth of deposit and
  age. The oldest peats are about 1 million years
  old.
• Peat is believed to have formed from wood. When
  wood is subjected to bacterial processes under
  nearly stagnant water, the cellulose, lignin and
  protein are decomposed. Residuals combine to form
  dopplerite.

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Peat (Continued)

• Contains 70-90 dopplerite and 5-30 resins and
  waxes.
• Wet peat contains 95 moisture.
• Reduces to 90 when cut.
• Reduces to less than 25 when air dried.
• Ash is about 3.
• Calorific value varies between 16,700 and 20,900
  kJ/kg.

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Peat (Continued) Ultimate Analysis
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Peat (Continued) Combustion Characteristics

• Its low calorific value and high moisture content
  reduces furnace temperature and efficiency of
  combustion.
• Its low bulk density (320 kg/m3) reduces capacity
  of furnace and increases storage and handling
  capacity due to its high volume.
• Its friable nature (can be easily crumbled)
  causes appreciable loss in handling.
• It may be used as a powder or may be briquetted
  without any binder.

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Peat Carbonization

• Like wood, it may be carbonized at low
  temperature in metal retorts. Yields
• Charcoal 30
• Gases 19-30
• Moisture 30-40
• Tar 6-7
• Gases used to provide heat for carbonization. Tar
  yields was and oil. Moisture yields ammonium
  sulfate, calcium acetate and methanol.

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Ultimate Analysis of Peat on wet basis with ash

• Carbon 84.2
• Hydrogen 1.9
• OxygenNitrogen 7.8
• Ash 3.1
• Moisture 3.0
• Calorific Value 29,300 kJ/kg

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Producer gas from Peat

• Gives producer gas at an efficiency of 80-85. No
  water needed as in case of coal. Gives high yield
  of gas and ammonia.
• Typical composition
• Carbon monoxide 17.
• Hydrogen 10.9
• Methane 2.5
• Nitrogen 55.7
• Carbon dioxide 13.3
• Gas yield 2550 m3/tonne of peat
• Calorific value 4100 kJ/m3
• Ammonium Sulfate 55 kg/tonne of peat

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Lignite
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Lignite

• Forms the first phase of fossilization of
  vegetable matter.
• It is an immature form of coal.
• Believed to be between 10 and 40 million years
  old.
• It is intermediate in composition between peat
  and bituminous coal.
• Most immature lignites are chemically similar to
  most mature peats.

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Composition of typical lignites

• Carbon 64.5-78.5
• Oxygennitrogensulfur 16.5-30
• Water (as mined) 20-75
• Water (dried) 12-20
• Ash 3-30
• Volatile matter 40-50
• Sulfur 1-12
• Calorific value (dry) 20,900-29,300 kJ/kg
• Used raw or dried in furnaces
• Pulverized and used in mills
• May be used in briquetted forms as well

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Coal

• A Fully Fossilized Fuel

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Coal A Heterogeneous Mineral

• Consists principally of carbon, hydrogen, and
  oxygen, with lesser amounts of sulfur and
  nitrogen.
• Other constituents are the ash-forming inorganic
  compounds distributed throughout the coal.
• Coal originated through accumulation of wood and
  other biomass that was later covered, compacted
  and transformed into rock over a period of
  millions of years.

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Coal Classification

• There are a number ways to classify coals.
• One way is to Rank the coal. It indicates the
  degree or extent of maturation.
• It is a qualitative measure of carbon content.
• Thus lignites and sub-bituminous are low rank
  coals
• While bituminous and anthracite are high rank
  coals.
• Rank is not synonymous with grade which implies
  quality.
• Low rank coals may not be suitable for some
  applications as the higher ranked ones
• Although they may be superior to them in other
  applications

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Rank of Coal

• With increasing Rank, the following
  characteristics are noticed
• Age of coal is increased. This increases with
  increase in depth of deposit.
• A progressive loss of oxygen, hydrogen and in
  some cases sulfur, with a corresponding increase
  in carbon.
• A progressive decrease in equilibrium moisture
  content.
• A progressive loss of volatile matter.
• Generally, a progressive increase in calorific
  value.
• 6. In some cases, a progressive increase of ash
  content.

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Proximate Analysis of some typical anthracite
coals
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Proximate Analysis of some typical bituminous
coals
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Proximate Analysis of some typical sub-bituminous
coals
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Proximate Analysis of some typical Lignites
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Typical oxygen, water and ash content in solid
fuels
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Ultimate Analysis of some typical anthracite coals
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Ultimate Analysis of some typical Carbonaceous
and Bituminous coals
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Ultimate Analysis of Some Typical Lignite, Peat
and Wood
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Mineral Elements and Chlorine in Pine and
Bituminous Coals
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More on coal

• Coal may be banded or non-banded.
• A banded coal is not homogeneous but consists of
  alternate layers or bands of bright-black,
  dull-black and gray vegetal matter. Exists in all
  types of coal.
• Attributed to different kinds of wood and plant
  substances in various stages of decay.
• Non-banded coals are uniform and compact in
  structure.

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Co-existence of coal and petroleum

• Where coal and petroleum co-exist, increasing
  temperature affect in opposite ways.
• Coal gradually loses its volatility and goes
  deeper whereas petroleum becomes progressively
  lighter as it cracks and rises.
• Thus the best coals are deeper in the ground
  whereas the best petroleum are nearer the ground
  level.

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Coal Combustion

• When heated to progressively higher temperatures
  in inert atmosphere (very little oxygen present),
  coal decomposes.
• Evolves water, tar and gas, and leaves a solid
  residue whose composition and properties depend
  on heat treatment temperature.
• Temperature range in which volatilization
  proceeds very rapidly is 350-500oC.
• But thermal decomposition begins at a much lower
  temperature.
• Can be divided into 3 stages.

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Stages of Coal Decomposition

• Below 200oC decomposition is slow. Release of
  small quantities of chemically combined water,
  oxides of carbon and hydrogen sulfide.
• Begins between 350 and 400oC and ends around
  550oC. About 75 of all volatile matter is
  released, including all the tar.
• Termed secondary degasification, is
  characterized by gradual elimination of
  hetero-atoms, and ends when the char is
  transformed into a graphitic solid. Principal
  products include water, oxides of carbon,
  hydrogen, methane, and traces of C2 hydrocarbons.
• As carbon content increases, active thermal
  decomposition occurs at progressively higher
  temperature.
• In this stage, there is progressive
  aromatization of the char, i.e., increasingly
  large hexagonal carbon platelets.
• Where residue is a coke, heat treatment up to
  1000oC also leads to marked increase in
  mechanical strength.

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Solid fuels from Coal

• Coal can be used as mined or after treatment.
• Coal can be briquetted or converted to coke.
• Briquetting. Done because
• (i) to convert cheap and waste coal dust to
  lump fuel.
• (ii) to use coal more effectively on the
  grate of furnace, and
• (iii) to produce smokeless fuel from fine coal.

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Briquetting (Continued)

• Briquetting may be done as follows
• Without binder for sub-bituminous coal, lignite
  or peat.
• With binder like pitch for bituminous,
  carbonaceous and anthracite coals.
• Other inorganic binders like sodium silicate,
  magnesium oxychloride and lime silica may be
  used.
• Cereal binders like starch and ground maize may
  also be used.
• Inorganic binders are easy to use but will
  increase the ash content when burned.

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Solid fuels from Coal (Continued)

• Coke. Formed by the carbonization of coal.
• Yields benzole, oils and tar. Gaseous products
  include coal gas.
• Yield and chemical nature of the products depend
  on rank of coal carbonized and duration of
  carbonization.

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Coke (Continued)

• Two commercial processes are available
• Low temperature carbonization at about 600oC and
• High temperature carbonization at temperatures
  above 900oC.
• Coal is heated in retorts. Evolves gases like
  carbon monoxide, methane, unsaturated
  hydrocarbons, and hydrogen.
• Tar forms up to about 500-600oC.
• Coals for converting to coke must have carbon
  content from 83 to 90.
• Coke is used in iron and steel industries
  (metallurgical coke), foundries, and as a
  domestic (smokeless) fuel.

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Coal Liquefaction

• Coal can be converted into a clean liquid fuel by
  reducing its molecular weight with a substantial
  reduction in the C/H ratio. Four methods are
  possible
• Pyrolysis.
• Direct Liquefaction. Examples are the SRC
  (Solvent Refined Coal), the Synthoil and H-coal
  processes.
• Indirect Liquefaction. The Fischer-Tropsch
  synthesis. Example is the SASOL process developed
  in South Africa.
• Chemical Synthesis.
• Liquefaction entails use of large quantities of
  water and there is the problem of ash disposal
  and slag removal plus elimination of sulfur
  dioxide emissions if the coal contains large
  quantities of sulfur.