Page 148 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 4 Torrefaction 125

barrier could be an issue. One could, however, avoid this by adopting
batch process as used for fermentation reactors. Multiple digester-type
reactors could be used to produce a torrefied product that is dewatered
using some conventional means.
Another form of hydrothermal reactor could use hot liquid (waste liquid
or superheated steam) as the heat carrier. If the liquid is inexpensive and has
some heating value like waste oil, it could be a viable heating medium that
does not have to be removed from the product. On the other hand, super-
heated steam may condense making the product less attractive.

Indirectly Heated Reactors
In indirectly heated torrefiers, the heat-carrying medium does not contact the
biomass directly. Heat is transported across a wall or through electromag-
netic radiation. So, here it is relatively easy to avoid contact with oxygen
and therefore avoid undesired combustion during torrefaction. Such reactors
have two major advantages: one that the heating fluid and medium can be
anything hotter and the other is that the volatiles released during torrefaction
are not diluted by the heating medium passing through it. So, the gaseous
product of torrefaction can be combusted separately to supplement the ther-
mal load of the reactor.
Since the heat is conducted slowly from the biomass layer in contact with
the hot reactor wall to the core of biomass pack, one could expect a tempera-
ture gradient resulting in nonuniform heating of the biomass inventory in the
reactor. A microwave that heats by electromagnetic irradiation may also
result in nonuniform heating of the biomass particles (Basu et al., 2012).

Rotating Drum Such torrefiers use a indirectly heated rotating drum that
tumbles the biomass in an environment of inert gaseous medium (Figure
4.9E). The biomass is heated by hot drum walls or by hot internals in the
drum. The heat transfer from the wall to the biomass particles is the primary
controlling factor and not the heat transfer from gas to particle. Biomass is
generally dried separately.
There is another version of such indirectly heated torrefier. Here, the bio-
mass is contained in thermally conducting containers, which are carried
through a hot furnace in an inert ambience at a slow speed. Heat conducted
into the biomass through the heated wall slowly torrefies the biomass.
Because of the relatively low heat exchange coefficient, residence time for
this type is large and could be in the order of hours.

Screw or Stationary Shaft Here, the torrefaction reactor (circular or rect-
angular cross section) is stationary, and it could be vertical, horizontal, or
inclined (Figure 4.9F). The reactor is generally heated from outside to
avoid contact with hot gases, though some may have holes for the products