Advances on in-situ analysis of char structure evolution during thermochemical conversion of coal/biomass: A review

Structure is the basis of the reaction. Compared with the ex-situ analysis of char structure evolution during thermochemical conversion of carbonaceous substance, in-situ analysis shows great advantages in reducing the inaccuracy of the results originated from sample cooling and pre-treatment as well as an effective way to avoid the multiple batches of experiments for the sample preparation. Thus, in-situ analysis is essential for an accurate and in-depth understanding of the reaction mechanism. In this review paper, in-situ structure characterization techniques, such as in-situ diffuse reflection Fourier transform infrared spectrometer (in-situ DRIFTS), in-situ Raman, in-situ synchrotron small-angle X-ray scattering (in-situ SAXS), heating stage-optical microscope combined system (HSOM), and visualization technology inside the gasifier as well as related analysis methods have been systematically described. Additionally, the recent research progress on in-situ analysis of char structure (chemical functional groups, carbon structure, pore structure, apparent structure, particle swarm evolution, etc.) evolution during the pyrolysis, gasification, or combustion of coal/biomass have been summarized in detail. Moreover, the challenges and prospects for in-situ analysis of char structure evolution have been outlined from technical and research levels based on this technical review.