Torrefaction - Biomass Gasification, Pyrolysis and Torrefaction - page 91

while minimizing the char yield. The objective of carbonization, on the other
hand, is to maximize fixed carbon and minimize hydrocarbon content of the
solid product, while that of torrefaction is to maximize energy and mass
yields with reduction in oxygen to carbon (O/C) and hydrogen to carbon
(H/C) ratios.
Carbonization is similar to torrefaction in many respects, but there are
some important differences between them. For example, carbonization
drives away most of the volatiles, but torrefaction retains most of it, driv-
ing away only the early volatilized low energy dense compounds and
chemically bound moistures. Therefore, during torrefaction the carboniza-
tion reactions that remove the volatiles should be avoided (Tumuluru
et al., 2011). Table 4.3 compares the properties of products of torrefac-
tion and carbonization. Here, we see a comparison of the properties of
carbonized and torrefied wood with those of raw wood with coal as a
reference.

TABLE 4.3 Comparison of Typical Properties of Raw Wood, Torrefied

Wood, Charcoal, and Coal

Typical
Wood a

Torrefied
Wood

Charcoal
(Carbonization)

Coal
(Bituminous) b

Temperature ( C)

200 300 300

.

Moisture (%)(wb) 30 60 1 5 1 5 3 20

Volatile (%db) 70 80 55 65 10 12 28 45

FC (%db) 15 25 28 35 85 87 45 60

Mass yield B 80% B 30%

18 a

Energy density (db)
(MJ/kg)

B 20 24 30 32 24 33

5.8 a 40 c

Volumetric energy
density (GJ/m 3 )

6.0 10.0 18.5 19.8 30

B

680 e 500 d Apparent density

(kg/m 3 )

350 300 600 640 1100 1350

Hydrophobicity Hygroscopic Hydrophobic Hydrophobic Hydrophobic

a Bergman, P.C.A., 2005. Combined torrefaction and pelletisation. ECN Report: ECN-C-05-073.
b Steam 41 edition, p. 9

6, 9 10, 12 7

c Based on 1225 kg/m 3 .
d Mullins and McNight, p. 75.
e Jenkins 1989, p. 866.
wb- wet basis; db- dry basis.
Source: Adapted from Kleinschmidt (2010).