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

Primary Air

Secondary AirFlame

Green Coal

Incandescent coke

Grate

ASH

p

Fixed Bed

Pressure drop

3

2

32

2 175.1

1150

p

g

p d

U

d

Up

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.

1

5.03

22

1C

gdCC

dU pgpg

gpmf

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