Stepwise Fast Pyrolysis of Pine Wood

This paper reports the yields and product composition obtained from the stepwise pyrolysis of pine wood in a fluidized bed reactor. The first step temperature was varied between 260 °C and 360 °C. After the first step, the solid residue was cooled to ambient temperature and pyrolyzed again at 530 °C. If the first step temperature was below 290 °C, the cumulated yields (the sum of steps 1 and 2) were identical to yields of the single-step experiment at 530 °C. This indicates that the chemical and transfer processes taking place below 290 °C do not lead to chemical and structural changes that affect the outcome of the processes occurring in the temperature range from 290 °C to 530 °C. When the first step temperature was higher than 310 °C the cumulated yields of char, water, light organic compounds, and furans were higher, whereas the cumulated yields of volatiles (gases plus organic liquids) were lower than those obtained if the pyrolysis was conducted in a single step at 530 °C. To explain these observations, a mechanism is proposed in which the main emphasis lies on the competition between routes that lead to char formation and routes that release compounds from the biomass particle. Single compounds and lumped groups could not be concentrated with the stepwise approach. A separation into lighter and heavier oils turned out to be possible.