Thermal History of Wood Particles in Molten Salt Pyrolysis

Abstract

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 °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 °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 β = (k eff/ρ)103(24 + 390e–0.49d ), and the total devolatilization time has been found to follow the empirical correlation t dev = ρ(0.146 e–k eff – 1.09)d 1.05.

On the Experimental Evidence of Exothermicity in Wood and Biomass Pyrolysis

“…In 2003, Di Blasi and Branca observed that the thermal dynamics of relatively small cylindrical beech wood particles (diameter 2–10 mm) pyrolyzed in a fluidized (bubbling) bed at 712–1107 K were qualitatively similar to those observed for thick particles. Successively, the use of a molten salt bath confirmed the same trends. The temperature and weight loss of packed beds (diameter 6.5 cm and height 11.5 cm) of brewer spent grain, fiberboard, and coffee waste particles were measured by Becidan et al .…”

“…However, the attainment of temperature overshoots is related unequivocally to heat generation from chemical reactions. The display of exothermic effects at the center of thick cylindrical or spherical samples heated along the lateral surface has been reported over the years, though only very recently have systematic investigations been undertaken with the main aim to identify the mechanisms responsible for such behavior …”

Combined molten salt–Ni/Al2O3 as synergistic medium for high-quality syngas production

“…Residual solids, such as char and ash, are retained in molten salt for subsequent handy separation and disposal [2]. The potential benefits of molten salt for solar gasification were first demonstrated by Epstein [5], and subsequently investigated by several research groups [6][7][8][9][10][11][12][13]. Recently, the solar gasification of coal and activated carbon with CO 2 in molten salt has also been investigated [9,11,[14][15][16].…”

“…2a reveals typical morphology of pristine c-Al 2 O 3 support and Fig. 2b reveals morphology of the synthesized Ni/Al 2 O 3 catalyst after calcination and reduction at temperature of 1123 K. The SEM image of pristine Al 2 O 3 has relatively round shape with sugar apple-like surface and a nominal diameter of about 80 lm whereas the morphology of Al 2 O 3 was unchanged after calcination and reduction at such high temperature for 4 h. Percentage of Ni on Al 2 O 3 surface, as analyzed with EDS, was 16.1 wt%, which is consistent with the calculated amount of 15 wt% prior to impregnation.Al-Ali et al[28] varied Ni loading(10,15, and 20 wt%) on Al 2 O 3 support for CO 2 reforming of CH 4 in molten carbonate salt and found that a 15 wt% loading provided the highest H 2 yield and the lowest carbon deposition.…”

Solar pyrolysis of cotton stalk in molten salt for bio-fuel production

“…The heating rates of sapwood pyrolysis at 600e900 C in molten salt were 4e10 times higher than those without molten salt, the heating rate can be up to 4100e16700 C/min in a mixture of sodium chloride (NaCl) and potassium chloride (KCl) at 900 C [14]. Maximum heating rates for cylindrical beech wood pellet (30 mm length and 3.5 mm diameter) pyrolysis in LiNaKF salt were 46e56 C/s [15]. The total energy input required for wood pyrolysis at 900 C equal about 20% of its heating value [16].…”