Secondary reactions of flash pyrolysis tars measured in a fluidized bed pyrolysis reactor with some novel design features

Abstract A fluidized bed pyrolysis reactor with variable freeboard height was constructed to investigate the effect of time-temperature history on freshly formed pyrolysis tars. The apparatus allowed direct measurement of tar loss through gas phase reactions in the freeboard, and calculation of the quantities of tar cracked or coked within the fluidized bed itself. At 600 °C about 30 wt% more tar (daf cellulose) appears to crack in the freeboard for 3.5 s residence time compared with 0.25 s. At this temperature, 19 wt% (daf cellulose) tar is destroyed within the fluidized bed before reaching the freeboard. The amount of tar destroyed within the bed was found to increase sharply with temperature, reaching nearly 50 wt% (daf cellulose) at 700 °C. Four other substrates were studied: silver birch, pulverized municipal solid waste, a Turkish lignite (Can), and a low rank British bituminous coal (Linby). Results indicate that the temperatures corresponding to maximum tar yields and thermal stabilities of the tars broadly correlate with substrate rank. At higher reactor temperatures, biomass tars lose their predominantly carbohydrate character and heteroatom content, and become more aromatic in character, with increasing contents of condensed aromatic ring systems.