Micro-pyrolysis of various lignocellulosic biomasses in molten chloride salts

In fast pyrolysis, the use of molten salts (i.e. eutectic mixtures) has shown potential for improving the heating rates of biomass, which is essential when targeting high bio-liquid yields. Techniques that allow faster screening of suitable eutectic salt mixtures for biomass pyrolysis before scaling up, are thus necessary. To validate a technique for rapid molten salt pyrolysis, micro-scale pyrolysis (py-GC-MS) has been carried out at 350 °C. Different chloride salt compositions (melting point of ∼ 200 °C) were tested in micro-pyrolysis of cellulose, pinewood, wheat straw, and softwood/hardwood Kraft lignin, at biomass to salt ratios from 1 to 20 (wt% biomass/wt% salts). Chloride molten salts pyrolysis highly promoted the depolymerization and dehydration of cellulose resulting mainly in the production of furfural and acetic acid, which are of interest when targeting the production of value-added chemicals. However, some of the eutectic mixtures used were not stable because they hydrolyzed (HCl detection) and volatilized during pyrolysis. Hydrolysis of salts was counteracted partially by adding ZnO, and fully by using less ZnCl₂ in the eutectic mixture. On the other hand, the volatilization of salts inside the micro-reactor was suppressed by limiting the time at which the salts were kept at reaction temperatures. It was observed that the salts had a major catalytic effect on the pyrolysis volatiles when compared to those from pure lignocellulosic biomass. Molten salts pyrolysis of Lignoboost Kraft lignin selectively produced guaiacol and syringol, whereas pinewood and wheat straw, like cellulose, resulted mostly in the production of furfural and acetic acid. Overall, the yield of volatiles from all lignocellulosic feedstock was less in the presence of chloride molten salts, which indicated that the salts favored the production of char and/or non-condensable gases, instead of volatiles.