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