Pneumatic Transport and Modelling Coal a Scientific Perspective

1
Pneumatic Transport and Modelling Coal a
Scientific Perspective

• Professor William J EassonSchool of Engineering
  and ElectronicsThe University of Edinburgh

2
Pneumatic Conveyance

• Size Matters,
• but does Shape?

3
Applications

• Food Processing (Powders)
• Agriculture (Straw)
• Energy (Pulverised Coal)

4
Coal-Fired Power Generation

• Coal is ground to c. 100?m
• Conveyed through main pipes to a number of
  burners
• Drop-out causes blockage, occasionally explosions
• Ropes of particles form, leading to uneven
  distribution to burners

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Modelling Parameters

• Air Flow
• mean velocity, turbulence levels, structures
• Particle Loading
• Particle Size Distribution
• Wall Roughness
• Particle-Wall Interaction
• Air-Particle Interaction
• Particle-Air Interaction
• Particle-Particle Interaction

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Particle Shape

• Almost every numerical and experimental
  investigator uses spherical particles
• Drag of non-spherical (but not irregular)
  particles has been studied
• Coal is crystalline and comes in many shapes

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Objective

• Does Shape Matter?
• Do particles rope?
• Is the velocity field affected?
• Is the transport velocity affected?

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Particle Shapes
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Measuring techniques

• Cross-Correlation Particle Image Velocimetry
  (PIV)
• Flow illuminated by pulsing light sheet - pulse
  pairs at rate of around 10Hz
• Two frames, typically separated by 100?s
• Computer assesses mean shift of particle images
  between frames

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Schematic of Test Rig
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Working Section
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Typical Image and Vector Map
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Effect of Load on Particle Velocity
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Regular profiles
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Irregular profiles
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Effect of Size on Mean Velocity
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Effect of Shape on Mean Velocity
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Conclusions(1)

• Particle velocity profiles change according to
  the mass loading
• Irregular particle velocity profiles are more
  sensitive to loading than spherical particle
  velocity profiles
• Using irregular particles results in transport
  velocities which are 5-10 greater than for
  spherical particles

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Particles in Free Fall
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Modelling Parameters

• Air Flow
• mean velocity, turbulence levels, structures
• Particle Loading
• Particle Size Distribution
• Wall Roughness
• Particle-Wall Interaction
• Air-Particle Interaction
• Particle-Air Interaction
• Particle-Particle Interaction

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Free-fall of Irregular Particles

• Objective
• Experimental investigation of streams of
  irregular and spherical glass particles
  free-falling in air

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Particle distributions
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Equipment
1
Fibre
1 hopper 2 vibrating system 3 mesh 4 glass
box 5 light sheet 6 scanning box 7 camera
3
6
5
4
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Hopper
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Spherical and irregular particles in free fall
Irregular particles d150-180 mm
Spherical particles d150-180 mm
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Vector Maps
Spherical Particles
Irregular Particles
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Terminal Velocity Vt versus Vertical Position
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Self-similar region
Particle diameter d150 to 180 mm
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Developing Region
Particle diameter d150 to 180 mm
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Terminal Velocity Vt versus Radial Position
Developed region a) Mass flow rate 0.3 b)
Mass flow rate 0.45 c) Mass flow rate 0.85
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Terminal velocity versus Reynolds number
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Drag coefficient for different particle sizes
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Drag coefficient for different mass flow rate
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Conclusions(2)

• Velocity profiles of the free falling jet showed
  that significant differences in the profiles
  occur due to particle shape
• Terminal velocity of irregular particles is lower
  than for spherical particles. The terminal
  velocity of irregular particles also evolves into
  a more pronounced profile, and this profile is
  achieved more quickly than for spherical
  particles.
• Velocity data have been attained for different
  mass flow rates
• at lower mass flow rates the terminal velocity is
  highest at the flow centreline for both spherical
  and irregular particles
• for higher mass flow rates, the terminal velocity
  profiles show a low velocity at the flow
  centreline and two distinct peaks in the
  immediate vicinity
• The velocity data is shown to be in reasonable
  agreement with the empirical correlation of
  Haider and Levenspiel