Steam–O2 Blown Circulating Fluidized-Bed (CFB) Biomass Gasification: Characterization of Different Residual Chars and Comparison of Their Gasification Behavior to Thermogravimetric (TG)-Derived Pyrolysis Chars

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Studies of the pyrolysis of biomass fuels and their residual char gasification are important to optimize and model gasification processes. In this paper, the characterization of circulating fluidized-bed chars (CFB-chars) was primarily performed. They were sampled from Agrol, willow, and dried distiller’s grains with solubles (DDGS) gasification using an atmospheric pressure 100 kWth steam–O2 blown CFB gasifier. The physical and chemical properties of char samples were studied by powder X-ray diffraction (XRD), X-ray fluorescence (XRF), N2 adsorption/desorption at −196 °C, and scanning electron microscopy (SEM) coupled with energy-dispersive scattering (EDS). Sequentially, the pyrolysis behavior of the Agrol, willow, and DDGS chars was investigated using thermogravimetric analysis (TGA) coupled with a Fourier transform infrared (FTIR) spectrometer. Gasification reactivities of CFB-char and char obtained after pyrolysis (PYR-char) were further investigated. The influences of the pyrolysis temperature (750 and 850 °C), heating rate (10, 30, 50, and 70 °C/min), CO2 concentration (10, 20, and 30 vol %), and gasification temperature (900, 1000, and 1100 °C) on the reaction rate of the char–CO2 reaction were studied. The volumetric reaction model (VRM) and the shrinking core model (SCM) were applied to determine kinetic parameters. SEM images showed that Agrol chars were very porous, with different superficial cavities and thin walls. Willow chars had a more compact agglomerated structure, while DDGS chars had a macroporous structure with rounded pores of different sizes and some particulates on the surface. The results observed from EDS analysis revealed that the composition of the chars was not completely homogeneous. XRD patterns showed that char samples had a disordered graphite-like structure. Agrol and willow showed similar pyrolysis behavior, and their related weight loss rates increased with an increasing heating rate. The XRF-analyzed results showed that the inorganic elements of the Agrol char were formed by Ca, Fe, K, Mg, and Si. The willow char was mainly composed of Ca and K, with minor amounts of Fe, Mg, P, and Si. However, DDGS char was mainly dominated by K and P, with a lesser amount of Ca, Mg, and Na. The char gasification rate increased with an increasing gasification temperature, CO2 concentration, and heating rate, while it decreased with an increasing pyrolysis temperature. Generally, the calculated activation energy (Ea) values using the SCM were slightly lower than those using the VRM. The calculated Ea value for PYR-char using both models was in the range of 90–210 kJ/mol, while the calculated Ea values for CFB-char were in the range of 55–120 kJ/mol. The predicted results using both models showed a reasonably good agreement with experimental results, in particular, with those obtained at a lower gasification temperature and lower CO2 concentration.