Solid Fuels

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Solid Fuels

• Combustion of Coal

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Combustion of Coal

• When a solid fuel particle is exposed to a hot
  gas flowing stream it undergoes three stages of
  mass loss
• Drying
• Devolatilization
• Char combustion
• The relative significance of these three is
  indicated by proximate analysis of coal

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Combustion of Coal

• Drying
• The combustible material generally constitutes
  water e.g. lignites up to 40
• Upon entry into the gas stream, heat is convected
  and radiated to the particle surface and
  conducted into the particle
• The drying time of a small pulverized particle is
  the time required to heat up the particle to the
  vaporization point and drive off the water
• DEVOLATILIZATION
• When the drying of a solid fuel particle is
  complete, the temperature rises and the solid
  fuel begins to decompose
• Devolatilization or pyrolysis is the process
  where a wide range of gaseous products are
  released through the decomposition of fuel.
• The volatile matter (VM) comprises a number of
  hydrocarbons, which are released in steps
• Since the volatiles flow out of the solid through
  the pores, external oxygen cannot penetrate into
  the particle, hence the devolatilization is
  referred to as the pyrolysis stage

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Combustion of Coal

• DEVOLATILIZATION
• The rate of devolatilization and the pyrolysis
  products depend on the temperature and and the
  type of the fuel
• The pyrolysis products ignite and form an
  attached flame around the particle as oxygen
  diffuses into the products
• While water vapour is flowing out of the pores,
  the flame temperature will be low
• For lignite coals, pyrolysis begins at 300-400 C
  releasing CO and CO2
• Ignition of the volatiles occurs at 400-600 C
• CO, CO2, chemically formed water, hydrocarbon
  vapours, tars and hydrogen are produced as the
  temperature reaches 700-900 C
• Above 900 C pyrolysis is essentially complete and
  the char (fixed carbon) and ash remain

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Combustion of Coal

• DEVOLATILIZATION
• For other types of coal, devolatilization
  proceeds differently
• Although the proximate analysis provides an
  estimate of the VM, the actual yield of VM and
  its composition may be affected by a number of
  factors like
• Rate of heating
• Initial and final temperature
• Exposure time at the final temperatures
• Particle size
• Type of fuel
• Pressure

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Combustion of Coal

• CHAR COMBUSTION
• The devolatilized fuel, known as char, burns
  rather slowly.
• For example, it would take 50150 sec for a char
  of size less than 0.2 mm to burn out
• Since it takes this long to burn completely,
  some of the particles may not burn out in the bed
  before leaving.
• The elutriation of these unburnt, fine char
  particles results in combustion losses.

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Combustion of Coal

• CHAR COMBUSTION
• The combustion of a char particle generally
  starts after the evolution of volatiles from the
  parent fuel particle, but sometimes the two
  processes overlap.
• The char, being a highly porous substance, has a
  large number of internal pores of varying size
• Surface areas of the pore walls are several
  orders of magnitude greater than the external
  surface area of the char.
• Oxygen diffuses into the pores and oxidizes the
  carbon on the inner walls of the pores.

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Combustion of Coal

• During the combustion of a char particle, oxygen
  from the bulk stream is transported to the
  surface of the particle.
• The oxygen then undergoes an oxidation reaction
  with the carbon on the char surface to produce
  CO.
• The CO then reacts outside the particle to form
  CO2.
• The mechanism of combustion of char is fairly
  complex.
• Some factors which effect the burning rate are
• Oxygen concentration
• Gas temperature
• Reynolds number
• Char size and porosity

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Combustion Systems for solid Fuels

• Fixed-bed combustion
• Suspension Firing/Pulverized Coal combustion
• Fluidized-bed combustion
• Fixed-bed Combustion
• Fixed-bed systems require least fuel size
  reduction compared to other two systems mentioned
  above
• Crushed coal up to 4 cm in size is used
• Solid fuel handling and feeding are the focus of
  much effort compared with gas or liquid fuels
• A stoker type of boiler is an example of shallow
  fixed- bed combustion system

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Combustion Systems for solid Fuels

• Fixed-bed Combustion
• A continuous fuel feed system is referred to as
  a stoker
• Air flows up through the grate and through the
  bed of ash, char and fuel
• Since the bed is thin, the pressure drop is less
  and the blower costs are reduced
• There are three types of stokers based on the way
  the coal is fed onto the grate
• Overfeed stokers
• Underfeed Stokers
• Cross feed stokers

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Spreader stoker with travelling grate
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Combustion Systems for solid Fuels

• Overfeed stokers
• The flow of fuel and air is counter current. Fuel
  is fed onto the top of the bed and moves downward
  as it is consumed
• Air flows up through the layers of ash, char and
  fresh fuel
• Volatile gases burn above the bed and some fine
  fuel particles burn above the bed
• Overfire air is supplied to complete the
  combustion
• Ash is removed by dumping, shaking, vibrating or
  continuously moving the grate
• The bed is usually 10-20 cm deep
• Fresh fuel is heated by the upward moving gases
  and by radiation from the flame above the bed
• The speed of the grate is adjusted so that the
  coal burns out before it reaches the edge of the
  grate and the ash dumps nto the ash pit below

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Combustion Systems for solid Fuels

• Underfeed stokers
• The flow of fuel and air is upward
• The evolved moisture, volatile matter and air
  pass through the burning fuel layer
• The bed is up to 1 m deep near the centre.
• Fresh fuel is forced from below by a screw
  conveyor
• The grate is usually inclined so that the ash
  automatically moves outwards as the fresh fuel is
  forced from below

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Combustion Systems for solid Fuels

• Crossfeed stokers
• An older type of stoker and grate arrangement
• This type of system is often used for
  hard-to-feed fuels such as unprocessed refuse,
  bagasse, lignite, wood pulp etc
• The fresh fuel is moved to a horizontal platform
  where it ignites
• When the next charge of the fuel enters, the
  ignited fuel moves across a sloping vibrating
  grate
• The air flows upward through the grate
• Such stokers operate with high excess air and
  considerable fuel loss in the ash pit

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Fluidized-bed combustion

• A fluidized bed is a bed of solid particles
  which are set into motion by blowing a gas stream
  upward through the bed at a sufficient velocity
  to suspend the particles.
• The bed appears like a boiling liquid.
• The fluidization occurs when the drag force on
  the particles in the bed due to the upward
  flowing gas just equals the weight of the bed.
• There are two principal types of fluidized bed
  boilers
• 1. Bubbling fluidized bed (BFB)
• 2. Circulating fluidized bed (CFB)

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Quality of fluidization
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Fluidized-bed combustion

• Bubbling fluidized bed (BFB)
• A bubbling fluidized bed boiler comprises a
  fluidizing grate through which primary combustion
  air passes and a containing vessel, which is
  either made of (lined with) refractory or
  heat-absorbing tubes.
• The vessel would generally hold bed materials.
  The open space above this bed, known as
  freeboard, is enclosed by heat-absorbing tubes.
• The secondary combustion air is injected into
  this section
• The boiler can be divided into three sections
• 1. Bed
• 2. Freeboard
• 3. Back-pass or convective section.

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Fluidized-bed combustion

• Bubbling fluidized bed (BFB)
• As the velocity is increased above minimum
  fluidization, bubbles are formed
• The bubbles are referred to as the dilute phase
• The size of the bubbles depend upon the type of
  the distributor plate
• A plate with a few large orifices inlets will
  have larger bubbles while a plate with many small
  inlets will have many bubbles near the plate
• In fluidized bed combustion the bed temperature
  is maintained well below the melting point of the
  ash
• To capture SO2 in the bed, limestone CaCO3 which
  is calcined in the bed to form CaO
• The optimum temperature for CaO reaction with SO2
  to form CaSO4 at atmospheric pressure is 815-900 C

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Fluidized-bed combustion

• Circulating Fluidized Bed Boiler
• In a CFB boiler furnace the gas velocity is
  sufficiently high to blow all the solids out of
  the furnace.
• The majority of the solids leaving the furnace is
  captured by a gassolid separator, and is
  recirculated back to the base of the furnace.
• A CFB boiler is shown schematically in Figure
• The primary combustion air (usually
  substoichiometric in amount) is injected through
  the floor or grate of the furnace
• The secondary air is injected from the sides at a
  certain height above the furnace floor.
• Fuel is fed into the lower section of the
  furnace, where it burns to generate heat.
• A fraction of the combustion heat is absorbed
• by water- or steam-cooled surfaces located in the
  furnace, and the rest is absorbed in the
  convective
• section located further downstream, known as the
  back-pass.

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Bubbling Fluidized Bed (BFB)

• FACTORS AFFECTING COMBUSTION EFFICIENCY
• The combustion efficiency of a bubbling fluidized
  bed (BFB) boiler is typically up to 90 without
  fly-ash recirculation and could increase to
  9899 with recirculation .
• The efficiency of a circulating fluidized bed
  (CFB) boiler is generally higher due to its tall
  furnace and large internal solid recirculation.
  The efficiency depends to a great extent on the
  physical and chemical characteristics of the fuel
  as well as on the operating condition of the
  furnace.
• Factors affecting the combustion efficiency can
  be classified into three categories
• 1. Fuel characteristics
• 2. Operational parameters
• 3. Design parameters

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EFFECT OF Fuel characteristics ON COMBUSTION
EFFICIENCY

• The fuel ratio of a fuel is the ratio of fixed
  carbon (FC) and VM contents of the fuel.
• This ratio has an important effect on the
  combustion efficiency of coal in a CFB boiler
• Higher ratios possibly leading to lower
  combustion efficiencies
• A high rank fuel like anthracite has a higher
  fuel ratio than a low rank fuel like lignite.
• For this reason low-rank fuels (or low fuel
  ratio) like lignite and
• bituminous have higher efficiencies than
  anthracite.
• The fuel ratio is easily computed from the
  proximate analysis of a fuel

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Effect of Operational parameters ON COMBUSTION
EFFICIENCY

• Fluidizing Velocity
• The combustion efficiency generally decreases
  with increasing fluidizing velocity due to higher
• entrainment of the unburnt fines and oxygen
  by-passing.
• Excess Air
• The mixing between fuel and air is never perfect.
  Some areas will be oxygen-deficient and some
• areas even oxygen-starved.
• Ultimately, all fuel particles must have the
  necessary oxygen to complete their burning thus,
  extra oxygen is always provided in FB boilers in
  the form of excess air.
• The combustion efficiency improves with excess
  air, but this improvement is less significant
  above an excess air of 20.
• Bubbling bed boilers may need a slightly higher
  amount of excess airthan CFB boilers.
• Combustion Temperature
• The combustion efficiency generally increases
  with bed temperature because the carbon fines
• burn faster at high temperatures.
• The effect of temperature is especially important
  for less
• reactive particles, which burn under
  kinetic-controlled regimes.

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EFFECT OF DESIGN PARAMETERS ON COMBUSTION
EFFICIENCY

• The combustion efficiency of bubbling FBs is
  affected by several design parameters
• Bed height
• Freeboard height
• Recirculation of unburnt solids
• Fuel feeding
• Secondary air injection