Page 147 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

124 Biomass Gasification, Pyrolysis and Torrefaction

when biomass is loaded into multiple containers, which are then slowly drawn
through a long hot but nonoxidizing tunnel furnace.
If particles move with respect to reactor wall, it is called moving bed.
The wall of the reactor can be horizontal, vertical, or inclined. The particles
may be moved, by gravity, by force of a mechanical device like rotating disk
(Figure 4.9C) or vibrating motion of belt (Figure 4.9D). Particles that flow
through the reactor are unidirectional without backmixing. The heat transfer
occurs primarily through gas solid convection that depends on the relative
velocity between the biomass and the hot fluid.
Entrained flow type reactors carry finely ground biomass in hot inert or
low oxygen gas. Because of high heat transfer coefficient between fine (saw-
dust like) biomass and high-velocity gas carrying them, the particles are
heated to torrefaction temperature quickly. As such, it requires much shorter
residence time in the reactor. Rapid heating reduces the solid yield increas-
ing the liquid yield.
Some directly heated convective-type torrefiers use a rotating drum
where the biomass is heated directly by hot gas passing through the tumbling
drum (Figure 4.9E). In this case, the drum simply serves as a mixing device
while heat transfer takes place through gas particle convection.

Fluidized Bed In this type of torrefier, hot inert gas is blown through a bed
of granular heat-carrier solids or appropriately sized biomass particles (Li
et al., 2012) in a way that the solids behave like a fluid. These heat-carrier
particles being in vigorously mixed and agitated state can easily heat up any
fresh biomass particle dropped into it (Basu, 2006). The biomass particles
thus undergo torrefaction in a well-mixed state with uniform temperature dis-
tribution. The system, therefore, ensures a product quality more uniform than
that is available from moving or fixed bed reactors. Separation of heat-
carrier solids from torrefied biomass and entrainment of fine biomass parti-
cles are some of the limitations of this technology.
The dominant mode of heat transfer in a fluidized bed is particle-to-
particle heat transfer. The “torbed” technology works on this principle where
biomass particles are fluidized above a grate of inclined slots and is sub-
jected to cross flow of gas (Figure 4.9H). This type can provide very uniform
quality of the torrefaction product.

Hydrothermal Reactor Here, the biomass is subjected to pressurized heat-
ing in water and thus obviates the need for drying (Yan et al., 2009). It is
especially suitable for high-moisture or wet biomass. The process could
bring about a slight improvement in the hydrophobicity of the biomass
(Medic et al., 2012). The dominant mode of heat transfer in a hydrother-
mal reactor is that between hot water (steam) and biomass. While this
process has several potential advantages, the energy required for pressuri-
zation and movement of a large volume of biomass across a pressure