Transcript Combined Cycles Using Coal & Other Solid Fuels

Combined Cycles Using Coal & Other Solid Fuels
P M V Subbarao
Professor
Mechanical Engineering Department
Development of Combustion Technology leading to
development of new Cycles!!!
Fuel Bed Combustion
Flame
Secondary Air
Green Coal
Dp
Incandescent coke
ASH
Grate
Primary Air
Fixed Bed
Pressure drop
U 1   
Dp  150
d p 2  3
2
 gU 2 1   
 1.75
d p 3
Where U is the superficial velocity,  is the void fraction, dp is the diameter
of the particles,  is the sphericity of the bed solids.
  is the viscosity and  is the density of the gas.
• If the gas flow rate through the fixed bed is increased, the pressure drop
continues to rise until a critical velocity.
• This critical velocity is called Minimum Fluidization Velocity (Umf).
• It is the velocity at which the fluid drag is equal to the gravitational pull
on the particle less the buoyancy force.
• At this stage particles feel weightless, and become more mobile.
• The fixed bed transforms into fluidized bed.
Basics of Fluidized Beds
• Fluidization may be defined as the operation through which fine
solids are transformed into a fluid-like state through contact with
either gas or a liquid.
• In fluidized state the gravitational pull on granular solid particles
is offset by the fluid drag.
• The particles remain in a semi-suspended condition.
• A fluidized bed displays characteristics similar to those of a
liquid.
• The static pressure at any height is approximately equal to the
weight of the fluidized particles per unit cross section above that
level.
• The bed surface remains horizontal irrespective of how the bed
is tilted.
• The bed assumes the shape of the vessel.
• The solids from the bed may be drained like a liquid
through a orifice at bottom or other side.
• An object denser than the bed will sink, while one
lighter than it will float.
• Particles are well mixed, and the bed remains a nearly
uniform temperature throughout its body when
heated.
Pressure Drop Vs Velocity of Flow
Bubbling Fluidized Beds
• An increase in velocity of the flow from Umf , don’t result in
any increase in the pressure drop
• The minimum fluidization velocity can be calculated using
force balance on bed.
• Drag force = Weight of the bed – Buoyancy force.
U mf
3 0.5


 g  p   g gd p 
  2
  C1 

C 1  C2

d p  g 




C1 = 27.2 and C2 = 0.0408.
At minimum fluidization, the bed behaves as a pseudo
liquid.
Characteristics of Fluidized Beds
• A further increase in gas flow can cause extra gas to flow throuh
the bed in the form of bubbles.
• The bubble size increases with particle diameter; excess gas
velocity and its position above the grate.
• When the fluidization velocity is increased in a bubbling bed, the
bubble expands.
• Over expansion leads to collapse of bubbles into a continuous
coalescence or deformed bubble.
• This state is called turbulent bed.
• Terminal Velocity: If the gas velocity through the bed is increased
continually another critical velocity Ut, is reached.
• At this velocity the drag on individual particle is equal to weight
less the buoyancy force.
The basic Air Cycle
The Supercharged Boiler Cycle
A PFBC Based Supercharged Cycle