Effect of Temperature in Fluidized Bed Fast Pyrolysis of Biomass: Oil Quality Assessment in Test Units

Abstract

Pine wood was pyrolyzed in a 1 kg/h fluidized bed fast pyrolysis reactor that allows a residence time of pine wood particles up to 25 min. The reactor temperature was varied between 330 and 580 °C to study the effect on product yields and oil composition. Apart from the physical−chemical analysis, a pyrolysis oil quality assessment has been performed by using two applications. The pyrolysis oils were tested in a laboratory scale atomizer and in a hydrodeoxygenation unit for upgrading/stabilizing of the pyrolysis oil. The pyrolysis oil yield increases from 330 to 450 °C, is nearly constant between 450 and 530 °C, and decreases again at a pyrolysis temperature of 580 °C. At temperatures of 360 and 580 °C, total pyrolysis oil yields of, respectively, 58 and 56 dry wt % can still be obtained. The produced amount of water is already significant at a reactor temperature of 360 °C and becomes constant at a temperature of 400 °C. At a temperature of 580 °C, the water production starts to decrease slightly. Initially the number average molecular weight of the pyrolysis oil increases at increasing temperatures, which is ascribed to the observed increase in concentration of water insoluble compound in the pyrolysis oil. At a temperature of 580 °C, the number average molecular weight, viscosity, and the amount of produced water insoluble compounds decreases. The oil obtained at a pyrolysis temperature of 360 °C produced less char, 2 versus 5 wt %, compared to the oil obtained at a pyrolysis temperature of 530 °C in our atomizer/gasifier. About 100% of the carbon goes to the gas phase compared to 84% for the oil obtained at a pyrolysis temperature of 530 °C. Therefore, the 360 °C oil has a better quality for this unit under the applied conditions (850 °C and droplet sizes of 50± μm) Testing the three pyrolysis oils (pyrolysis temperatures of 330, 530, and 580 °C) in the hydrodeoxygenation unit showed that pyrolysis oil with a lower viscosity resulted in deoxygenated oil of lower viscosity. The oxygen content of the three oils was almost the same, but the yield of the deoxygenated oils obtained at a pyrolysis temperature of 330 °C was significantly lower. Together with chemical and physical analyzes of the pyrolysis oils, feeding the pyrolysis oil into a test units relevant for applications, direct information on the effect of varied pyrolysis process parameters on the quality and applicability of the pyrolysis oil is obtained.