Influence of biomass torrefaction parameters on fast pyrolysis products under flame-equivalent conditions. (December 2018) - Pielsticker, S.
Abstract: Pretreating raw biomass via torrefaction changes fuel specific properties like grindability, volatile content, energy density and biochemical stability and thus enables an enhanced fuel replacement for pulverized fossil fuel fired furnaces. In this study, the influence of torrefaction temperature on devolatilization behavior is investigated in a small-scale fluidized bed reactor approximating flame-equivalent conditions. Therefore the pyrolysis products of two different biofuels with varying degree of torrefaction are determined via ex-situ FTIR gas analysis in an N 2 atmosphere in the temperature range from 873 to 1473 K. Furthermore, the mass fraction of residual char particles is determined by adding O 2 to the fluidizing gas and analyzing the burnout products. Char fraction and volatile composition are used to estimate the energy release distribution between homogeneous volatile combustion and heterogeneous char burnout. The experiments revealed enlarging char yields at the expense of volatile yields with increasing degree of torrfaction at all investigated pyrolysis temperatures. Furthermore, torrefaction favors higher fractions of CO 2 and lower fractions of CO and C 2 H x in the light gas. Further on, no significant impact of torrefaction conditions on the tar composition could be identified. The calculation of higher heating value (HHV) based on char yield and gas composition reveals an overall increase of HHV, while the relative contribution from the volatile fraction decreases with increasing degree of torrefaction. Following this, an increase of torrefaction degree will shift combustion from a high intense volatile combustion in the near burner region towards a less intense but prolonged char conversion in the far burner region. Highlights: High temperature pyrolysis of differently torrefied biomass is investigated. Torrefaction increases pyrolysis char yields at the expense of gas and tar yields. No significant impact of torrefaction on tar composition can be observed. Pyrolysis gas species distribution is impacted by chemical equilibrium reactions. HHV values indicate a shift from homogeneous volatile combustion to char burnout.