Thermal History of Wood Particles in Molten Salt Pyrolysis

Molten salt pyrolysis is a thermochemical conversion process in which biomass is fed into and heated up by a molten salt bath. Molten salts have very high thermal stability, good heat transfer characteristics, and a catalytic effect in cracking and liquefaction of large molecules found in biomass. In this study, the heat transfer characteristics of molten salts are studied by recording the thermal history of wood particles in molten salt pyrolysis. Experiments have been carried out with cylindrical beech and pine wood particles with constant length (L = 30 mm) and varying diameter (d = 1-8 mm) in a FLiNaK melt with a temperature of 500 degrees C. The thermal history at the particle center has been used to evaluate the reaction temperatures, the heating rates, and the devolatilization times. Results have been compared with a similar study in a fluidized sand bed. It is found that FLiNaK gives significantly higher heating rates for cylinders with d <= 4 mm. For larger cylinders, the process is dominated by heat transfer within the wood particle, and the heat transfer medium is of less importance. For the smallest cylinders (d = 1 mm), heating rates as high as 218 +/- 6 and 186 +/- 15 degrees C/s were observed for beech and pine wood, respectively. The average heating rate for wood cylinders until the main degradation takes place has been found to follow the empirical correlation beta = (k(eff)/rho) 10(3) (24 + 390 e(-0.49d)), and the total devolatilization time has been found to follow the empirical correlation t(dev) = rho(0.146 e(-keff) - 1.09)d(1.05).