Post Drying Process in PC Coal Firing P M V Subbarao Professor Mechanical Engineering Department Timing of Processes for A True Open System…..

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 reactants "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