Post on 26-Oct-2014
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2. Decomposition of biomass under inert atmosphere2.1 Effect of particle size of raw material biomass on torrefaction
What is torrefaction?Torrefaction is a low temperature treatment for lignocellulosic biomass (LCB), and is carried out under these conditions:• Absence of oxygen (inert conditions such as N2)• Low temperature between 200 to 300 ˚C • Retention time of 30-90 minutes
Products from torrefaction
Products are: •Torrefied lignocellulosic biomass (LCB)•Tar•Non condensable gases
Torrefied LCB may be: •Utilized as a solid fuel with improved quality•Converted into syngas or bio-oil
Torrefaction
Hemi-celluloseCellulose
LigninWater
Decomposed hemi-celluloseCellulose
Lignin
From the view point of solid reaction:
CO2, H2O etc
Solid reactant
Solid product
Gas & liquid product
Endothermic
Appearance change of raw materials by torrefaction
Fresh Torrefied at220ºC
Torrefied at250ºC
Torrefied at300ºC
Kernel Shell
MesocarpFiber
EFB
Fresh Torrefied at220ºC
Torrefied at250ºC
Torrefied at300ºC
Kernel Shell
MesocarpFiber
EFB
Three possible controlling steps
Controlling Factor
External Heat Transfer
Kinetics
Internal Heat Transfer
Biomass
Gas film
Energy & mass balance for biomass decomposition
))(()2
()(2
2
tq
r
T
rr
TTC
t p
Pyle, D. L. and C. A. Zaror, “Heat transfer and kinetics in the low temperature pyrolysis of solids,” Chem. Eng. Sci., 39, 147-158 (1984).
T = temperature [K]; t = time[s]; a=external heat transfer coefficient [W/(m2 K)]; ∙ k =reaction rate constant [s-1]; r = density [kg/m3]; Cp = heat capacity [J/(kg K)]; ∙ rp = radius [m]; l = thermal conductivity of biomass [W/(m K)]∙
Basic equations
)(
k
t
)( TTr
Tf
))((3)(2
2
tq
r
TTC
t p
t>0, r=rp
00
rr
T All t
All t, r=0
Boundary conditions
Pyle and Zaror’s scheme
PP rCktransferheatexternal
reactionBiPyPy
)(
)('
Pr
transferheatexternal
transferheaternalBi
)(
)(int
For determining the rate controlling step:
2)(int
)(
PP rCktransferheaternal
reactionPy
Pyle, D. L. and C. A. Zaror, “Heat transfer and kinetics in the low temperature pyrolysis of solids,” Chem. Eng. Sci., 39, 147-158 (1984).
a=external heat transfer coefficient [W/(m2 K)]; ∙ k =reaction rate constant [s-1]; r = density [kg/m3]; Cp = heat capacity [J/(kg K)]; ∙ rp = radius [m]; l = thermal conductivity of biomass [W/(m K)]∙
Pyle and Zaror’s scheme
Controlling Factor
Range
Bi (∞rp) Py (∞rp-2) Py’ (∞rp
-1)
External Heat Transfer Small Small
Kinetics Small Large
Internal Heat Transfer Large Small
Pyle, D. L. and C. A. Zaror, “Heat transfer and kinetics in the low temperature pyrolysis of solids,” Chem. Eng. Sci., 39, 147-158 (1984).
Example of torrefaction mass yieldwith EFB of 0.375, 1.5, 3 and 6 mm in size
0
20
40
60
80
100
3 9 15
Mas
s Yie
ld a
t 573
K [%
]
O2 Concentration [%]
0.375mm 1.5mm 3mm 6mm
at 0% O2
1. In this result, there is no significant effect of biomass size on torrefaction yield.2. The mass yield is expected to increase when biomass size reached a certain level.
What is the threshold diameter at which transition from intrinsic reaction to heat transfer controlling
occurs?
1' PP rCk
Py
1
PrBi cmrP 1
cmrP 6.0
Conditions:Nitrogen gas 100mL/min, reactor ID=2.7cm, 300 ˚C, wood
1 cmHeat transfer controlIntrinsic reaction Pr
Summary for torrefaction
For measuring intrinsic reaction rate of torrefaction:-> rp<1 cm
For industrial torrefaction:-> Depending on the purpose