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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
