Page 243 - Biosystems Engineering

Biomass Pyr olysis and Bio-Oil Refineries 221

Three controlling mechanisms or extreme cases have been used to
describe the devolatilization of biomass particles: (1) external heat
transfer controlled, (2) kinetically controlled, and (3) internal heat
transfer controlled. Each mechanism is briefly explained here.
External Heat Transfer–Controlled Regime This occurs when the rate of
external heat transfer is very slow compared with internal heat trans-
fer. Thus, the temperature inside the biomass particle will be essen-
tially uniform but will gradually climb over time. Accordingly, the
reaction rate will be uniform across the particle (Pyle and Zaror
1984).
Kinetically Controlled Regime The reaction rate is very slow compared
with the heat transfer to and within the biomass particle. The particle
temperature will be very similar to the reactor temperature. The ther-
mal degradation of biomass particles with diameters less than 2 mm
in fast pyrolysis reactors happens in this regime (Pyle and Zaror
1984).
Internal Heat Transfer–Controlled Regime If the internal heat transfer is
very slow compared with the reaction rate, there will be large gradi-
ents of temperature inside the particles. The reaction zone will be
very narrow; consequently, the process can be regarded as if a char-
coal front is advancing into the virgin biomass at “a rate controlled by
the velocity at which the thermal wave propagate” (Pyle and Zaror
1984). Most of the models tested (Pyle and Zaror 1984) suppose that
the reaction happens at temperatures around 430°C. This regime
occurs when large biomass particles are converted in slow pyrolysis
reactors.
Pyle and Zaror (1984) proved that the controlling mechanism
during the thermal degradation of a single biomass particle can be
identified if the following three dimensionless numbers are known:

Biot number: Bi = hR/K (7.1)
2
Pyrolysis number: Py = K/(k ρ c R ) (7.2)
p
Pyrolysis number: Py’ = h/(k ρ c R) (7.3)
p
where h = external heat transfer coefficient (J/s m K)
2
R = particle radius (m)
K = thermal conductivity of biomass (J/s m K)
k = rate of biomass thermal-degradation reactions (1/s)
3
ρ= biomass bulk density (kg/m )
c = biomass specific heat (J/g K)
p
The Biot number compares the rate of heat transfer to the surface by
convection with the internal conduction resistance (Graham et al. 1984).
Large woody materials processed in pyrolysis reactors will have large