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

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

Simple Bunsen Burner Burning Velocity Flow velocity Air Fuel

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 m burn before entering the cyclone. Particles 60 m to 100  m are likely to be carried inside the cyclone. The majority of the particles larger than 150 m 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 of 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

Post Drying Process in PC Furnace

Timings & Phases of pulverized coal burning The time of pulverized coal-air mixture in a furnace can be divided into three periods: 1. Drying, Devolatilization and ignition of char particles, which require 0.2–0.3 s. 2. Intensive mixing and burning of pulverized coal-air jet during 0.5–1.5s. Leads to formation of with formation of flame kernel with a temperature 1500–1600 °C at a distance of 1–5 m. 3. Burnout of larger coal particles and cooling of flue gas during1–3 s over a distance 2/3 of the furnace height.

Drying Time for Coal Particle & Furnace Size Qconv Qrad Moisture

Onset of Devolatization & Pyrolysis Temperature of the particle rises fast after the completion of particle drying. As the dried particle heats up, volatile gases containing hydrocarbons, CO, CH4 and other gaseous components are released. Start of Pyrolysis: Terpens : 225 0C Hemi cellulose : 225 – 325 0C Cellulose : 325 – 375 0C Lignin : 300 – 500 0C

Time Taken by Devolatization & Pyrolysis The rate of devolatization/pyrolysis of solid fuel Where The released combustible gases get ignite and burn outside the particle. In a combustion process, these gases contribute about 70% of the heating value of the biomass.

Char Combustion Char is a highly porous mixture solid carbon and ash. Wood char , f = 0.9 Coal char, f = 0.7 Internal surface area : 100 sq. m. per gm. – coal char. : 10,000 sq.m per gm – Wood char. Oxygen is first absorbed from the gas volume on the surface of particles. Absorbed oxygen reacts with carbon to from complex carbon-oxygen compounds : CxOy. These complex compounds dissociated into CO2 & CO.

Mechanism of Char Combustion Oxygen reacts with char to produce CO in the lower portion of the furnace. The CO reacts rapidly inn the gas to form CO2. The CO2 in turn is reduced by the char. The latter reaction causes CO buildup when oxygen is depleted. The resulting reactions: C +1/2 O2 → CO CO+ 1/2O2 → CO2 C+CO2 → 2CO C+H2O → H2O + CO

Combustion of Char Cloud Furnace Exit Plug Flow Zone Heat release Zone Recirculation Zone Elevation Z=0

The Process of PC Cloud Combustion The heat release zone comprises of the "firing" slices. Each slice is capable of releasing a certain percentage of the available calorific value. The mass of products "created" by the firing slices directly proportional to the percentage of heat release. The flow of reactants originates in the heat release zone, circulates through the hopper region. The contribution from each "firing" slice to the recirculation or "back-mix" flow is assumed proportional to the mass of combustion products generated in the firing slice.

Generation & Exchange of Microscopic Kinetic Energy Popularly known as the Sensible Heat Inventory of A slice. SHI of ith Slice: Sensible Heat Leaving Slice "i" ith Slice Heat Release in Slice "i" Sensible Heat Entering Slice "i"

Distribution of Heat Release Rate

Energy Balance The rate of change of enthalpy of gas is equal to rate of generation of thermal energy due to combustion of several volatile hydro carbon compounds and solid carbon. Furnace

Distribution of Heat Flux Average Heat flux can be calculated using: As the temperature and emissivity of flame is not uniform in the furnace volume, the local heat flux is not uniform. Actual local wall temperature depends on the value of local heat flux. Special experiments are carried out to find the heat flux distribution on water wall.

Control of Local Heat Flux Distribution

Diabatic Furnace Wall : Water Wall Furnace Exit Hot Exhaust gases Flame Heat Radiation & Convection Burners

The Basics of Flow Boiling Supercritical Steam Generation Subcritical Flow Boiling

Religious to Secular Attitude of Water