Page 160 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 4 Torrefaction 137

uniform cross-section shaft of the unit (Figure 4.11). Hot gas, W g , enters the
bottom of the torrefaction section at temperature, T gi , and this along with
other gaseous products leaves the top of the drying section at a temperature
T g0 . Fresh biomass, W f , enters the top of the drier at T 0 , and hot torrefied
biomass, W t , leaves the bottom of the torrefier at T t .
If the reactor volume is V r , the total particle surface area, S r , available for
heat transfer, and the reaction can be calculated for known values of the sur-
face to volume ratio S p of biomass particles and ε p , the voidage in this
section.

S r 5 ð1 ε p ÞV r S p (4.29)
The average value of surface to volume ratio of solid particles is the ratio
of external surface area and volume of the particles, S p 5 (A p /V p ), which is a
function of the particle shape.

Predrying Section We assume that hot gas enters its bottom at temperature,
T g 3 and leaves from the top at T g0 . The biomass enters from the top at T 0

and leaves the bottom at 100 C. Gas to biomass particle heat transfer takes
place across the log mean temperature difference, LMTD, which is given as:

2 T 0 ފ
LMTD 5 (4.30)
2 100Þ 2 ðT g 0
½ðT g 3
2 T 0 Þފ
ln½ððT g 3 2 100Þ=ðT g 0
If the particle gas overall heat transfer coefficient is U t , the heat trans-
ferred to biomass, Q ph , in the predrying section is
Q ph 5 U t S r;ph LMTD ph (4.31)
where the subscript “ph” refers to the values in the preheating section.
The preheat section’s volume, V r,ph , should be such that it accommodates
at least an amount of biomass that would give the required gas particle sur-
face area of S r,ph . Using the relationship (Eq. (4.29)) between particle surface
area and volume occupied by it in this section, we can find the minimum
volume of this section of the reactor, V r,ph , as:
Q ph 1
V r;ph 5 3 (4.32)
U t LMTD ph S p ð1 2 A p Þ
The minimum height of this section, L ph , with cross-sectional area, A r ,is
given as:
V r;ph
L ph 5 (4.33)
A r
Besides the volume, there are two other stipulations for such a moving
bed system.