Page 190 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 5 Pyrolysis 167

particle by conduction and pore convection (Figure 5.4). The following
modes of heat transfer are involved in this process (Babu and Chaurasia,
2004b).
Conduction inside the particle
Convection inside the particle pores
Convection and radiation from the particle surface.
In a commercial pyrolyzer or gasifier, the system heats up a heat-transfer
medium first; that, in turn, transfers the heat to the biomass. The heat-
transfer medium can be one or a combination of the following:

Reactor wall (for vacuum reactor)
Gas (for entrained-bed or entrained-flow reactor)
Heat-carrier solids (for fluidized bed).
Bubbling fluidized beds use mostly solid solid heat transfer. Circulating
fluidized beds (CFB) and transport reactors make use of gas solid heat
transfer in addition to solid solid heat transfer.
Since heat transfer to the interior of the biomass particle is mostly by
thermal conduction, the low thermal conductivity of biomass (B0.1 W/m K)
is a major deterrent to the rapid heating of its interior. For this reason, even

when the heating rate of the particle’s exterior is as fast as 10,000 C/s, the
interior can be heated at a considerably slower rate for a coarse particle.
Because of the associated slow heating of the interior, the secondary reac-
tions within the particles become increasingly important as the particle size
increases, and as a result the liquid yield reduces (Scott and Piskorz, 1984).
For example, Shen et al. (2009) noted that oil yield decreased with particle
size within the range of 0.3 1.5 mm, but no effect was noted when the
size was increased to 3.5 mm. Experimental results (Seebauer et al., 1997),
however, do not show much effect of particle size on the biomass.

5.5.1 Mass Transfer Effect

Mass transfer can influence the pyrolysis product. For example, a sweep of
gas over the fuel quickly removes the products from the pyrolysis environ-
ment. Thus, secondary reactions such as thermal cracking, repolymerization,
and recondensation are minimized (Sensoz and Angin, 2008).

5.5.2 Is Pyrolysis Autothermal?
An important question for designers is whether a pyrolyzer can meet its own
energy needs or is dependent on external energy. The short and tentative
answer is that a pyrolyzer as a whole is not energy self-sufficient. The reac-
tion heat is inadequate to meet all energy demands, which include heat
required to raise the feed and any inert heat-transfer media to the reaction