Comparison of direct and indirect contact heat exchange to improve recovery of bio-oil

Abstract This study investigates whether the rate of cooling of pyrolysis vapors affects the composition of the resulting bio-oil. Pure cellulose was pyrolyzed in a laboratory-scale fluidized bed reactor at 500 °C and the bio-oil collected in either an indirect contact heat exchange (conventional water-cooled condenser system) or a direct contact heat exchange (liquid quench) system developed in our laboratory. The liquid quench system was estimated to achieve a seven-fold increase in cooling rate compared to the water-cooled condensers. Direct contact cooling in the quench system also eliminated temperature gradients experienced by films of bio-oil running down the walls of the water-cooled condensers. The combination of these two factors helped reduce secondary decomposition of primary pyrolysis products, especially anhydrosugars such as levoglucosan. The quench system increased the yield of levoglucosan by over 20% while minimally effecting yield of other compounds. The concept of direct contact cooling was applied to a pilot-scale, lignocellulosic biomass pyrolysis plant using water as a more practical quench media than liquid nitrogen. As with the liquid nitrogen quench, the water flashed to gas while the heavy ends of the bio-oil condensed to liquid. The quench vessel was operated above the dew point of the water to assure that it left the vessel as gas along with produced water and light ends of bio-oil, which were recovered in a condenser as an aqueous phase. In pyrolysis experiments with red oak, the quench vessel increased the yield of heavy ends by 15% compared to conventional condensers. These results encourage the design of bio-oil recovery systems that can rapidly quench products to achieve high yields and improve the quality of bio-oil.