Pyrolysis of biomass: Temperature effects on oil yields and quality

A pilot scale fluidized bed fast pyrolysis set-up was designed and operated. The fluidized bed temperature (TFB) was varied between 400° and 530°C and the vapor phase temperature between 400° and 540°C to study the effect of these temperatures on product yields and quality. The gas yield increased and the char yield decreased with increasing fluidized bed temperature. The oil yield was constant at TFB = 450°-530°C and a small decrease in oil yield was observed at TFB = 400°C. Water production was constant between TFB = 400°-530°C. With increased vapor phase temperature, the gas yield increased at the expense of fast pyrolysis vapors due to vapor cracking reactions. A lower TFB resulted in oils containing more light organic compounds and a lower maximal molecular weight. For TFB = 400°C, no molecules with a MW higher than 500 gram/mole were detected in the oil while at 530°C, a considerable amount of very large molecules were measured. These large molecules are unfavorable for some further pyrolysis oil upgrading processes like hydrotreating to obtain transport fuel precursors. Thus, fast pyrolysis temperatures above 450°C at which the oil yield is maximal, not always result in better oil quality. This is an abstract of a paper presented at the 8th World Congress of Chemical Engineering (Montréal, Quebec, Canada 8/23-27/2009).