Environmental Engineering Reference
In-Depth Information
Gas condenser
Non condensable
gases
Biomass
Biomass
hopper
Hot
gases
Char
collection
Bio-oil
collection
Figure 3.11
Pyrolysis process with a fluidized bed pyrolysis reactor for bio-oil production.
Pyrolysis can generally be divided into two main categories - fast or slow
pyrolysis - depending on the heating regime which ranges from seconds to minutes.
By varying the heating rate, the proportion of the ultimate products also varies. In
fast pyrolysis, bio-oil is the main product. The heating rate of the process can be
as high as 1000-10,000°C s -1 with a peak temperature of about 500°C. At the end
of the process, approximately 50-75% of biomass (on a wet basis) is converted
to bio-oil [61]. To maximize the bio-oil production, a high heating rate, strictly
controlled reaction temperature range, a short residence time (<3 s) of vapour in
the reactor, and rapid quenching of the product gas should be ensured [64]. In
contrast, the heating rate and final temperature of slow pyrolysis is lower, while
residence times of solid and vapour are longer. Bio-char is produced as the primary
product through carbonization in slow pyrolysis. The heating rate of the process is
0.1-2°C s -1 with a peak temperature of around 400°C.
Figure 3.11 illustrates a fluidized bed pyrolysis reactor for bio-oil production
from biomass. Biomass is fed into a pyrolysis chamber (a hot sand bed) through
the hopper and the biomass is heated to the required pyrolysis temperature. The
biomass starts to decompose in this chamber and both the condensable and non-
condensable vapours released from the biomass leave the chamber. The bio-char
formed remains partly in the chamber and partly in the gas. When the condensable
vapour passes through the condenser, it condenses as bio-oil which is collected in
the chamber. While part of the non-condensable vapour leaves the chamber as gas
products, another part of it is returned to the system to provide heat.
3.3.5
Torrefaction
The primary goal of torrefaction is to refine raw biomass to an upgraded solid fuel
with better handling qualities and enhanced combustible properties similar to
those of fossil coal, leading to decreased costs (see Table 3.3). The torrefaction
process reduces oxygen content and thus increases the energy content (high heat-
ing value) from typically 10-17 MJ kg -1 to as much as 19-22 MJ kg -1 (typically
 
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