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

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

Qconv Qrad

Moisture

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

)()( 44pgppgpin TThATTAQ

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

dt

umumd dfdfww )(

dt

TcmTcmd pdfdfpww )(

• 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.

• Energy loss due to evaporation =fgw hm

)()()( 44

pgppgpfgwpdfdfppw TThATTAhm

dt

TcmTcmd

)()( 44pgppgpfgw

pdfdf

pwwpww TThATTAhm

dt

dTcm

dt

dTcmTcm

)()()()( 44pgppgp

pdfdfwwfgpww TThATTA

dt

dTcmcmhTcm

Drying of Coal Particle: LCM

)()()()( 44fgwpgppgp

pdfdfww humTThATTA

dt

dTcmcm

dthumTThATTAdTcmcm fgwpgppgppdfdfww )}()()({)( 44

dtdThumTThATTA

cmcmp

fgwpgppgp

dfdfww

)}()()({

)(

44

K

T

p

fgwpgppgp

dfdfwwt

t pin

dry

dThumTThATTA

cmcmdt

373

440 )}()()({

)(

Devolatization (Pyrolysis)

• Temperature of the particle rises fast after the completion of particle drying.

)()( 44pgppgp

pdfdf TThATTAdt

dTcm

dvdv t

K

ppgppgp

dfdft

t

dTTThATTA

cmdt

37344

373 )}()({

• 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 fuelpyrv

v kmdt

dm

p

pyr

RT

E

pyrpyr ekk

,0• 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.

volumeChar

spacevacantofVolumePorosity

,

• Wood char , = 0.9

• Coal char, = 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.

• 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

• 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:

dt

dmdzAV

dt

d csss

The char moves down because the char is shrinking as it burns.

If bed void fraction is ,

1AAs

2121 rrdzVdt

dss

Where

vOe Akr211 75.0

vCOe Akr222 44

12

mv dA

)1(6

Energy Balance

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

332211 HrHrHrhVdz

dggg

dt

COdr 3

Diameter of Reacting Particle Vs Time

Simplified Equation for Rate of Combustion

III

CO

I

O

p

YY

Dd

m

.. 221ln

2

Burn Rate of single isolated particle:

Y show mass fraction of gases and is the stoichiometric coefficient