Environmental Engineering Reference
In-Depth Information
TABLE 12.1 Properties of
solid fuels
Wood
chips
Wood
pellets
Torrefied
wood pellets
Charcoal
Coal
Moisture content
(wt% wet basis)
30-55
7−10
1-5
1−5
10−15
LHV (MJ.kg
-1
) -12
15−17
18-24
30−32
23−28
Volatile matter
(wt% dry basis)
75−85
75−85
55-80
10−12
15−30
Fixed carbon
(wt% dry basis)
16-25
16−25
20-40
85−87
50−55
Bulk density
(kg.L
-1
)
0.2−0.3
0.55−0.65
0.65−0.8
0.18−0.24
0.8−0.85
Volumetric
energy density
(GJ.m
-3
)
1.4−3.6
8−11
12-19
5.4−7.7
18−24
Hygroscopic
properties
Hydrophilic Hydrophilic
(Moderately)
Hydrophobic
Hydrophobic Hydrophobic
Biological
degradation
Fast
Moderate
Slow
None
None
Milling
requirements
Special
Special/
standard
Standard
Standard
Standard
Product
consistency
Limited
High
High
High
High
Transportation
costs
High
Medium
Low
Medium
Low
Torrefaction was primarily conceived as a technology for application in coffee pro-
cessing. The philosophy of designing biomass torrefaction reactors, however, is con-
ceptually dissimilar, since unlike coffee, biomass fuel is not a high value product.
Also, the technology is targeted at processing biomass in quantities that are several
orders of magnitude larger than the quantity of coffee beans roasted. However, just
as in coffee beans roasting, good process control in terms of temperature, residence
time, and mixing is imperative for achieving consistency in product quality. For a cer-
tain reactor configuration, variations in temperature generally have a larger impact on
product quality than variations in residence time. Energetic efficiency is the key to
economic viability and to the overall sustainability of torrefied biomass-based value
chains. This necessitates a proper integration between the several heat sources and
sinks available in the process. Although the torrefaction process itself is not a largely
endo- or exothermic process, the predrying unit requires a substantial energy supply.
Hence, proper design of the predrying unit is also of particular importance in achiev-
ing good overall torrefaction process efficiencies (especially in case the feedstock has
a high moisture content).
Figure 12.3 gives an example of heat integration, where predrying of the biomass
and torrefaction are carried out by utilizing three heat sources, namely, the torrefaction
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