Permittivity and chemical characterization of woody biomass during pyrolysis and gasification

Online measurements of key information such as the decomposition stages of biomass gasification/pyrolysis in a fixed-bed reactor is currently limited with existing technologies. In this study a proof of concept for using Electrical Capacitance Tomography (ECT) for monitoring the decomposition stages of woody biomass is done. In practice, ECT measures the permittivity of the material under test. Therefore, the relation between the permittivity of the biomass in terms of the extent of thermal treatment is essential. Pyrolysis and gasification experiments were conducted to different extent of time and temperatures at a lab scale gasifier using wood chips. The permittivity of the samples was measured in-vitro using impedance analyzer at different excitation frequencies (20 kHz to 3 MHz). At a critical pyrolysis temperature of 700 °C the permittivity starts to increase significantly. In addition, for air-aided gasification the permittivity was found to reach its maximum in holding time of 10 or 30 min. Further examination of these samples in an ECT sensor revealed the same trend, implying that the permittivity results can be used for converting ECT reading to decomposition state. Chemical characterizations were done via elemental, thermogravimetry analysis, X-ray diffraction and Fourier transform infrared (FTIR) Spectroscopy. It was found that graphitization of the material is a prominent factor that control the increase of the permittivity. Analyzing quantitatively the aromatic and aliphatic carbon from the FTIR results disclose that it has a strong relation to the permittivity change when the aromatic carbon is higher than 95%. Therefore, in this range, it was possible to correlate these two parameters by an inverse relation which lead to a good match under frequencies higher than 200 kHz.