Phases in Combustion of Travelling Coal Particles P M V Subbarao Professor Mechanical Engineering Department Timing Processes for A True Open System…..

Mill Mass Balance : A SSSF Open System Flow of Crushed Coal ~20 mm Tempering Air, Tatm Low Moisture pulverized coal + Air + Moisture Flow of Hot air

Mill Energy Balance Hot air Heat loss Puliverizer frictional Coal Dry pulverized coal + Air + Moisture Puliverizer frictional dissipation Motor Power Input Heat loss Tempering Air, Tatm Energy Balance for Drying of Coal: Heat Transfer to coal particle due to convection = The rate of Change of internal Energy of the particle + Rate of Energy used for evaporation of moisture in coal.

Drying of Coal Particle: Energy Balance The rate of change of internal energy of the coal particle Moisture

Drying of Coal Particle Energy loss due to evaporation = Instantaneous rate of evaporation:

Moisture Content in Coal at Mill Exit Moisture removed in Mill:

Mill Operation Window Erosion Limit PC Transport Limit Tampering Limit Erosion Limit PC Transport Limit Drying Limit Milling Capacity Limit

Suggested Moisture Level in Coal at Mill Exit Min  Mexit

Pulverized Coal Distribution System

Transport of coal particles from Mill to Furnace Pneumatic conveying of coal particles are known as flows with low-solids concentration ("dilute-phase flow"). The ratio of coal to carrying gas is determined by systems and combustion considerations and is usually in the range of 0.5-0.6 kg of coal/kg of air. Assuming a coal density rc = 1.5 x 103 kg/m3, and the density of the carrying gas as rg = 0.9 kg/m3, the volume fraction of the coal can be shown to be very small, 0.036 %.

Coal Particle Transport Velocity An important aerodynamic characteristic of the particles is their terminal velocity. This is defined as the free-fall velocity in stagnant air. For a spherical particle of d = 100 m has an approximate value of Vt = 0.3m/sec. Local velocities of air flow must impart a velocity to coal particles. If Vparticle < Vterminal : Falling of particles If Vparticle = Vterminal : Floating of particles If Vparticle > Vterminal : Transport of particles Due to non-uniformities of flow behind bends, and to avoid settling of solids in horizontal sections of the transport line, a minimum air velocity of V = 16 - 20 m/sec is recommended.

Introduction of Coal Particles into Furnace ????

Introduction of Coal Particles into Furnace ???? Combustion is Chemistry Fuel transport is fluid mechanics Calculations for Chemistry ???? Robert Bunsen Bunsen’s developed a gas burner. By introducing air into the gas in the correct proportion before it burns, a clean, soot-free, almost colorless flame is produced. Using his burner, Bunsen used flame tests to analyze substances much more reliably than ever before. The burners he designed were made by Peter Desaga, his laboratory assistant.

The Burners Bunsen published the design of the burner in 1857, but did not patent his design. He did not wish to make profits from science; he believed the intellectual rewards were more than enough. His burner is now used not only for flame tests. It is used to heat samples and to sterilize equipment in medical laboratories all over the world. Burner Governs: Fuel Ignition Aerodynamics of Fuel air mixture Generation of combustion conditions.

Simple Bunsen Burner Burning Velocity Flow velocity Air Fuel

Positioning of Flame in A Furnace

Velocity of Planar Laminar Flames

Flame Speed & Rate of Combustion Laminar Turbulent

Stable Flame, Flame Speed & Rate of Combustion : Coal Quality VM=30% & A=5% VM=20% & A=5% VM=15% & A=5%

Stable Flame, Flame Speed & Rate of Combustion VM=30% & A=15% VM=30% & A=5% VM=30% & A=30% VM=30% & A=40%

Stability & Flammability Limits Burning Velocity > Flow Velocity : Flash Back Limit Burning Velocity < flow Velocity : Blow Off Limit Burning Velocity = Flow Velocity : Stable Flame. Rich Mixture Fuel Flow rate Flash Back Stable Flame Blow off Lean Mixture Air Flow rate

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.

Frugal Solutions for Turbulent Combustion

Tangentially Fired Burnes External Swirl……. A tiny Tornado … Generation of a Whirling Fireball …

Anatomy of Whirling Fireball Vortex core – Quasi-solid Zone. Vortex annulus – Quasi-equivalence zone.

Anatomy of Fireball Tangential Velocity in the furnace

Stream line in Furnace Cross-section

Three dimensional Fire Ball

Burner Parameters Primary and secondary air velocities should be decided to locate the point of ignition from the exit plane of burner. Low (16-20 m/s) are not suitable for high volatile coals. Low velocities are recommended for low volatile coals. Slagging and thermal distortion of the burners can result in variations in velocities. Large particles throw active combustion region into the wall region. This increases slagging and increase in Unburned carbon losses. For bituminous secondary : primary air velocity ratios are 1.4-1.5. This gives sufficient reserve for load decrease without significantly impairing the aerodynamics of P.F. Jet.

Particle size distribution Particles less than 60 mm burn before entering the cyclone. Particles 60 mm to 100 mm are likely to be carried inside the cyclone. The majority of the particles larger than 150 mm will be flung to the wall and will start to burn there.

Thermo-physics of Traveling Coal Particle in A Furnace Coal Particles are dragged into furnace by air and continuously moving towards furnace exit. The hot gas environment changes the thermo-physics of the coal particle. Last stage Drying Devolatization & Pyrolysis Char Ignition and combustion.

Drying of Coal Particle: Energy Balance The rate of Change of internal Energy of the particle + Rate of Energy loss due to evaporation of moisture = Energy gain due to convection +Radiation Energy gain: Qin = Qconv + Qrad Qconv Qrad Moisture

Drying Time for Coal Particle & Furnace Size Qconv Qrad Moisture