Electrical conductivity of wood biochar monoliths and its dependence on pyrolysis temperature
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ORIGINAL ARTICLE
Electrical conductivity of wood biochar monoliths and its dependence on pyrolysis temperature Randeep Gabhi1 · Luca Basile2 · Donald W. Kirk1 · Mauro Giorcelli2 · Alberto Tagliaferro2 · Charles Q. Jia1 Received: 6 March 2020 / Accepted: 29 May 2020 © Shenyang Agricultural University 2020
Abstract Biochar is traditionally used as solid fuel and for soil amendment where its electrical conductivity is largely irrelevant and unexplored. However, electrical conductivity is critical to biochar’s performance in new applications such as supercapacitor energy storage and capacitive deionization of water. In this study, sugar maple and white pine were carbonized via a slow pyrolysis process at 600, 800 and 1000 °C and conductivities of monolithic biochar samples along the radial direction were measured using the 4-probe method. Biochars were characterized using an elemental analyzer, scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The solid carbon in biochar samples was found to consist primarily of disordered carbon atoms with small graphitic nanocrystallites that grow with increasing temperature. The bulk conductivity of biochar was found to increase with pyrolysis temperature—1 to ~ 1000 S/m for maple and 1 to ~ 350 S/m for pine, which was accompanied by an increase in carbon content—91 to 97 wt% and 90 to 96 wt% for maple and pine, respectively. The skeletal conductivity of biochar samples carbonized at 1000 °C is about 3300 S/m and 2300 S/m for maple and pine, respectively (assuming solid carbon is amorphous); both values are above that of amorphous carbon (1250–2000 S/m). This work demonstrated the importance of carbonization and graphitization to electrical conductivity and suggested electron hopping as a likely mechanism for electric conduction in biochar—an amorphous carbon matrix embedded with graphitic nanocrystallites. Keywords Biochar · Electrical conductivity · Pyrolysis temperature · Graphitization
1 Introduction Plant-based biomass captures and stores atmospheric carbon dioxide via photosynthesis. It is considered as a source of carbon-neutral fuel (Tilman et al. 2006). Dry plant-based biomass, such as wood and agricultural wastes, typically consists of 50 wt% of carbon. The carbon in biomass can be retained via carbonization and a carbon-rich product biochar is manufactured (Abdul Karim Ghani et al. 2013; Van Zwieten et al. 2010). Large-scale utilization of biochar is considered as an alternative approach to store the atmospheric carbon captured by plants. Pyrolysis of biomass * Charles Q. Jia [email protected] 1
Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, ON M5S 3E5, Canada
Department of Applied Science and Technologies, Politecnico di Torino, C.so Duca Degli Abruzzi, Turin 10129, Italy
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(wood, organic materials, etc.) can be slow or fast depending on the feed and the desired products. The heating rate during pyrolysis affects the quantity and quality of biochar produced (Strezov et al.
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