The pelletizing process resolves some typical problems of biomass fuels: trans­port and storing costs are minimized, handling is improved, and the volumetric calorific value is increased. Pelletization may not increase the energy density on a mass basis, but it can increase the energy content of the fuel on a volume basis. For example, while the energy density on a mass basis for raw wood, torrefied wood, wood pellet, and torrefied pellet was 10.5, 19.9, 16.2, and

21.6 kJ/kg (LHV as received basis), respectively, it was 5.0, 4.6, 10.5, and 18.4 GJ/m3, respectively, on a volume basis (Bergman, 2005c). Thus, pelletiza­tion of torrefied wood greatly increases the transportation and handling cost of biomass. Pelletization of torrefied biomass is better than torrefaction of pelletized wood from the standpoint of process energy consumption and product stability.

Symbols and Nomenclature

A = pre-exponential factor (s-1)

E = activation energy (J/mol) k = reaction rate (s-1) mb = mass of biomass at time t (kg) mc = mass of char residue (kg) mo = initial mass of biomass (kg)

R = universal gas constant (J/mol. K)

T = temperature (K)

Tpyr = pyrolysis temperature (K) heating = heating time (s) tr = reaction time (s)

X = fractional change in mass of biomass dtor = torrefaction temperature (°C)