Design Analysis of Furnace Of A Steam Generator

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Design Analysis of Furnace Of A Steam Generator

• P M V Subbarao
• Professor
• Mechanical Engineering Department

Perfection of Primary Cause for All that
Continues..
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Basic Anatomy of A SG
Convection Pass
Transfer only
Furnace
Creation, Generation Transfer of Thermal Energy
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Details of Basic Processes

• Flow of Air and Prepared Fuel
• Fluid Dynamics.
• Generation of Thermal Energy Using Natural
  Resources.
• Combustion Sciences.
• Transfer of Thermal Energy to working fluid.
• Science of Heat Transfer.

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Aerodynamics Dynamics of Stationary Flame

• Along local Normal to Flame Surface
• Burning Velocity gt Flow Velocity Flash Back
  Limit
• Burning Velocity lt flow Velocity Blow Off Limit
• Burning Velocity Flow Velocity Stable Flame.

Burning Velocity
Flow velocity
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Stability Flammability Limits
Rich Mixture
Fuel Flow rate
Flash Back
Stable Flame
Blow off
Lean Mixture
Air Flow rate
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Burning Velocity Residence Time

• Quality of Fuel Fuel Chemistry.
• Air-fuel ratio
• Turbulence level
• Time to be spent by fuel particle in the furnace
  before it burns completely.
• Residence time is inversely proportional to
  burning velocity.
• Fuel particle is continuously moving.
• The distance traveled by the fuel particle should
  be much larger than furnace height.
• Swirl motion will ensure the required residence
  time.
• Internally generated swirl Swirl Burners.
• Externally generated swirl Direct Burners.

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Basics of Coal Combustion Burning Time
Coal is a complex organic polymer consisting of
large polycyclic aromatic clusters of several
fused rings strung together by assorted
hydrocarbon chains of varying lengths and other
hetroatom (O, N, S) linkages.
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Phases of Coal Particle Combustion
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Phases of Combustion

• The first phase is associated with moisture
  evolution and occurs at very low temperatures at
  about 373 K.
• The second phase at a heating rate of 1273 K/sec
  begins at about 723 K.
• This is associated with a large initial evolution
  of carbon dioxide and a small amount of tar.
• The third phase involves evolution of water
  chemically formed in the range 773-973K and
  carbon dioxide as the other significant product.
• The fourth phase involves a final rapid evolution
  of carbon-containing species such as carbon
  oxides, tar, hydrogen, and hydrocarbon gases in
  the temperature range 973-1173 K.
• The fifth phase is the high temperature formation
  of carbon oxides.

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Second Phase of Coal Combustion
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Final Phase of Coal COmbustion
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Modelling of Coal Cloud Combustion
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Typical Flame Speed of PC.
Flame speed m/s
A/F ratio
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Typical Flame Speed of PC.
Flame speed m/s
A/F ratio
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Heat available for Radiation

• Incomplete combustion loss
• Unburned Carbon loss
• Loss due to slag
• Energy brought in by preheated
  air fuel.

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Combustion Limits on Furnace Design

• The lower limit of the furnace volume is
• dominated by the space required for burning the
  fuel completely, or
• to an extent less than the allowable unburned
  fuel loss.
• To complete the fuel combustion within the
  furnace space, the fuel injected into the furnace
  has to reside there for a certain time longer
  than some critical time tr.
• The fuel residence time can be estimated by the
  residence time of the combustion gas produced in
  the furnace.
• An average residence time tr can be proposed.

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Intensity of Heat Release
Combustion Factor Macro Particles Micro Particles
A volumetric combustion intensity, Iv (kW/m3) 250 750 1500 2500
Area Combustion Intensity, IA (kW/m2) 300 1800 Up to 7500
Coal Firing Density Jf,V ( kg/m3.hr) 30 100 1500 3000
Area Firing Density, Jf,A (kg/m2.hr) 40 250 Up to 1000
Air Velocity (m/sec) Up to 0.5 Up to 20
Exhaust Gas Velocity (m/sec) Up to 3 Up to 20
Combustion time (sec) Up to 5000 1
Particle Heating Rate (0C/sec) lt1 10000
Heat Transfer Coefficients (W/m2 K) 10 50 500
Heat Fluxes to Heat exchange (kW/m2) 10 50 500
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• Fuel combustion time is mainly dominated by the
  combustion reaction velocity and the rate at
  which oxygen is supplied into the reaction zone.
• The combustion reaction velocity depends on
  chemical characteristics of the fuel.
• Main technical factors that affect the combustion
  time are
• Combustion characteristics of the fuel.
• Mixing characteristics.
• Fluid flow characteristics of the furnace.
• The combustion time of an oil fuel droplet is
  generally less than 0.1 msec.
• In the case of coal combustion time is much
  longer.

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Design ConstrainsHeat Release Rate

• Heat Release Rate per Unit Volume, qv, kW/m3
• Heat Release Rate per Unit Cross Sectional
  Area,qa, kW/m2
• Heat Release Rate per Unit Wall Area of the
  Burner Region, qb, kW/m2
• The maximum allowable heat flux of the water wall
  is restricted by its water-side burnout (dryout)
  heat flux.

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Heat available to the furnace

• Incomplete combustion loss
• Unburned Carbon loss
• Loss due to slag
• Energy brought in by preheated
  air fuel.
• A part of this total heat should be absorbed in
  furnace.
• The designer should provide an environment for
  the same.