(434c) Detailed Tar Analysis and Char Imaging From White Oak Pyrolysis in a Laminar Entrained Flow Reactor

Authors: 
Nimlos, M. R., National Renewable Energy Laboratory
Daily, J. W., University of Colorado at Boulder


Thermochemical conversion of biomass yields a complex slate of light gases, condensable vapors and refractory aromatic tars. However, the detailed reactions and the kinetic parameters that govern their formation are not well known. To address this problem we have designed a novel particle feeder to deliver milled white oak (<180 µm) to our laminar entrained flow reactor (LEFR) for pyrolysis with detection of vapors by molecular beam mass spectrometry (MBMS) and char analysis by transmission electron microscopy (TEM). Two experiments were conducted. A temperature experiment (450-950 °C, ΔT=25 °C) at fixed residence time (1.0 s), and a kinetic experiment varying residence times (0.2-0.8 s, Δt=0.2 s) at five temperature points (500-900 °C, ΔT=100 °C). The spectra from the temperature experiment were analyzed using multivariate curve resolution with optimization by alternating least squares (MCR-ALS). Spectral data from the flow reactor obtained at small increments of temperature over a wide range allows for finer resolution of the changing product slate, and hence the details of the underlying reaction mechanism. We were able to resolve six pure components from the multivariate analysis. The temperature dependence of the pure component scores (relative concentration) are given showing the progression of biomass pyrolysis from primary products (lignin monomers and polysaccharide fragments), to several stages of cracking leading to reduced molecular weight, and finally molecular weight growth to yield benzene, toluene, and polynuclear aromatic hydrocarbons (PAHs). This final pure component is dominant above 850 °C, which correlates with a maximum in the mean particle diameter and circularity as determined from particle imaging. Char particles were embedded and sectioned to provide a three dimensional picture of the morphological changes occurring during the different stages of pyrolysis and gasification. The results show evidence for cell wall, rather than cell lumen, micropore growth into large pores, which could allow entrapment of pyrolysis products and higher concentrations of PAH precursors (C2-C5 species and radicals). These cenospheres begin to rupture after 800 °C, releasing their contents into the gas phase. Since PAHs can only be formed from bimolecular reactions, the entrapment may result in enhanced PAH production. The kinetic experiment provides a thermo-temporal map of tar formation during biomass gasification. Total tars reach a maximum at 600 °C and 0.4 s, while benzene and PAHs increase steadily after 800 °C. MBMS signals for six prominent tar species are used to calibrate MBMS signal to tar yield (mg tar per g white oak). This information will be used to improve a semi-detailed reaction mechanism of biomass gasification for use in CFD modeling.