The preceding discussions assume that the heat or mass transport rate is too high to offer any resistance to the overall rate of pyrolysis. This is true in the temperature range of 300 to 400 °C (Thurner and Mann, 1981), but at higher temperatures heat and mass transport influence the overall rate and so cannot be neglected. This section deals with heat transport during pyrolysis.

During pyrolysis, heat is transported to the particle’s outer surface by radia­tion and convection. Thereafter, it is transferred to the interior of the particle by conduction and pore convection (Figure 3.4). 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 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 (~0.1 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 reactions 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 to 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.