Page 130 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 4 Torrefaction 107

controlled by heat conduction into it and that may lead to a higher tempera-
ture in the biomass core.

4.5 DEGREE OF TORREFACTION

The torrefaction process is similar to that of roasting of coffee beans. The
higher the roasting temperature the darker is the color of the bean. The price
and taste of roasted coffee depends to some degree on the roasting tempera-
ture. Table 4.5, for example, presents a range of roasting temperature with the
corresponding name of the coffee brand produced from it. One can thus see
how the degree of roasting varies with the torrefaction temperature. So, for
such a type of process, a gradation is indeed necessary for the quantification
of the process of torrefaction of biomass. A tentative gradation or severity of
the process (not the product) may be offered as below (Chen and Kuo, 2011):

1. Light torrefaction: Occurs at a temperature of 200 240 Cor at 230 Cwhen
only hemicellulose is degraded leaving lignin and cellulose unaffected.

2. Medium torrefaction: Occurs at a temperature of 240 260 C or at about

250 C, when cellulose is mildly affected.
3. Severe torrefaction: Occurs at a temperature of 260 300 Corat 275 Cchar-

acterized by depolymerization of lignin, cellulose, as well as hemicellulose.
TABLE 4.5 Degree of Coffee Roasting Classified by Roasting Temperature
Name of the Coffee Product Roasting Temperature ( C)

Green unroasted coffee 75
Arabic coffee 165
Cinnamon roast 195
England roast 205
American roast 210
City roast 220
Full city 225
Vienna roast 230
French roast 240
Italian roast 245
Spanish roast 250
Immanent fire 497
Source: From wikipedia.org/wiki/Coffee_roasting.