PYROLYSIS KINETICS OF HULLESS BARLEY STRAW USING THE DISTRIBUTED ACTIVATION ENERGY MODEL (DAEM) BY THE TG/DTA TECHNIQUE AND SEM/XRD CHARACTERIZATIONS FOR HULLESS BARLEY STRAW DERIVED BIOCHAR

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Abstract

Abstract The pyrolysis kinetics of hulless barley straw at different heating rates of 5，10, 15, 20 and 30 ºC/min were investigated via thermogravimetry, and the activation energy distribution E and pre-exponential factor k0 were calculated using the Distributed Activation Energy Model (DAEM) from thermogravimetric analysis (TGA) curves, and characterizations of pyrolysis product of biochar were analyzed by techniques of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The pyrolysis process consisted mainly of the dehydration stage (50-150 ºC), the active pyrolysis stage (200-400 ºC) and the passive pyrolysis stage (400-800 ºC). The E ranged from 73.45 to 214.11 kJ/mol within the conversion rate range of 0.10-0.55, and changed from 214.11 to 141.55 kJ/mol within the conversion rate range of 0.55-0.90, and the average value of E was 172.23 kJ/mol. The values of k0 changed greatly with E values at different mass conversion. The wide E and k0 distributions obtained from the kinetic analysis are attributed to the complex chemical reactions of pyrolysis. The structure of biochar was degraded or ruptured due to the increase in temperature. The XRD analysis confirmed that the biochar was amorphous, dominated by disordered graphitic crystallites.

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Dynamic molecular structure of plant biomass-derived black carbon (biochar).

Marco Keiluweit, Peter Nico, Mark Johnson … (2010)

Char black carbon (BC), the solid residue of incomplete combustion, is continuously being added to soils and sediments due to natural vegetation fires, anthropogenic pollution, and new strategies for carbon sequestration ("biochar"). Here we present a molecular-level assessment of the physical organization and chemical complexity of biomass-derived chars and, specifically, that of aromatic carbon in char structures. Brunauer-Emmett-Teller (BET)-N(2) surface area (SA), X-ray diffraction (XRD), synchrotron-based near-edge X-ray absorption fine structure (NEXAFS), and Fourier transform infrared (FT-IR) spectroscopy are used to show how two plant materials (wood and grass) undergo analogous but quantitatively different physical-chemical transitions as charring temperature increases from 100 to 700 degrees C. These changes suggest the existence of four distinct categories of char consisting of a unique mixture of chemical phases and physical states: (i) in transition chars, the crystalline character of the precursor materials is preserved; (ii) in amorphous chars, the heat-altered molecules and incipient aromatic polycondensates are randomly mixed; (iii) composite chars consist of poorly ordered graphene stacks embedded in amorphous phases; and (iv) turbostratic chars are dominated by disordered graphitic crystallites. Molecular variations among the different char categories likely translate into differences in their ability to persist in the environment and function as environmental sorbents.