The Phenomena of PC Particle Combustion P M V Subbarao Professor Mechanical Engineering Department A Basis for for Sizing of Coal Particle……

Parameters of Interactive Particle Combustion Number of particles per unit volume Fuel Volume fraction Mass loading Air Fuel Ratio Equivalence Ratio

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 Moisture Qconv Qrad

Transient Conduction Through Coal Particles Lumped Capacitance Model: Where, h is an equivalent heat transfer coefficient due to combined radiation and convection. For low value of Biot number the drying model can be simplified. For large values of Biot number, a semi-infinite solid model or multi-dimensional model are used depending on the time of drying.                          

Heating of Particle The rate of change of internal energy of the particle Removal Moisture starts only when fuel moisture reaches saturation state. Depends on partial pressure of moisture in furnace.

Drying of Coal Particle: LCM Energy loss due to evaporation =

Devolatization (Pyrolysis) Temperature of the particle rises fast after the completion of particle drying. Start of Pyrolysis: Terpens : 225 0C Hemi cellulose : 225 – 325 0C Cellulose : 325 – 375 0C Lignin : 300 – 500 0C

The rate of devolatization of solid fuel Where As the dried particle heats up, volatile gases containing hydrocarbons, CO, CH4 and other gaseous components are released. In a combustion process, these gases contribute about 70% of the heating value of the biomass. Finally, char oxidizes and ash remains. Completion of volatization generates char.

Char Combustion Char is a highly porous solid carbon. 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.

C +1/2 O2 → CO CO+ 1/2O2 → CO2 C+CO2 → 2CO C+H2O → H2O + CO Oxygen reacts with char to produce CO in the lower portion of the bed. 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

The char moves down because the char is shrinking as it burns. Consider a one-dimensional slice dz thick through the bed of corss-sectional area A. The rate at which char particles subside toward the grate equals the rate at which the char is consumed: The char moves down because the char is shrinking as it burns. If bed void fraction is e,

Where

Energy Balance The rate of change of enthalpy of gas is equal to rate of generation of thermal energy due to combustion.

Diameter of Reacting Particle Vs Time

Simplified Equation for Rate of Combustion Burn Rate of single isolated particle: Y show mass fraction of gases and n is the stoichiometric coefficient