Page 149 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

126 Biomass Gasification, Pyrolysis and Torrefaction

of torrefaction to escape. Heat is conducted to the biomass by means of
conduction across reactor walls. A rotating screw churns and moves the
biomass through the reactor to enhance heat transfer between the wall and
the bulk of the biomass and at the same time to move the biomass along its
length (Figure 4.9F).

Microwave Microwave irradiation involves electromagnetic wave in the
range of 300 MHz to 300 GHz. Typical microwave ovens or microwave
reactors work at a frequency of 2.45 GHz. The microwave irradiation pro-
duces efficient internal heating by direct coupling of microwave energy with
the molecules of biomass. The electric component of electromagnetic micro-
wave radiation causes heating by two main mechanisms: dipolar polarization
and ionic conduction. The heating depends on the ability of the materials
being heated to absorb microwaves and convert it into heat (Figure 4.9A).
Metals, for example, reflect microwave, while biomass absorbs it.
The microwave reactor (Figure 4.9A) is different from other indirectly
heated reactors, where biomass particles are heated externally, that is, heat
from the reactor wall arrives at the surface of biomass particles, and then it
is conducted into the interior of the biomass. Contrary to this, biomass parti-
cles in a microwave reactor are heated from within. Microwave heating bio-
mass may not be very efficient because biomass is a poor thermal conductor.
In a microwave reactor, the heating is internal; every part of the biomass in
the path of microwave radiation are heated simultaneously. Limited data
available (Basu et al., 2012) show that microwave torrefaction creates an
extremely fast rate of heating of the biomass interior, a matter of seconds.
So, it does not allow heat to be conducted adequately to its exterior, and it
causes a large temperature gradient in wood sizes of 25 mm or larger. Some
investigators (Ren et al., 2012) found more encouraging results like 67 90%
energy yield with 79 88% overall energy recovery.

4.7.1.2 Classification on Mode of Gas Solid Mixing
In chemical processing plants, reactors are often classified by their gas solid
contacting modes. As such, this classification better helps understand the
mixing that is vital for the reactor. In the list of reactors shown in
Table 4.10, one can identify the following four modes of contact.
1. Plug flow (gas percolates through static solids; gas and solid both move
unidirectional)
2. Partial back-mixed (e.g., fluidized bed, where gas is unidirectional but
solids are back-mixed)
3. Tumbling (solids tumbles or moves around in a drum or cylindrical
tunnel)
4. Entrained (solids are pneumatically transported by gas).