Biomass derived chars for energy applications

Biomass-derived chars present energy density values close to those of fossil fuels and therefore they are good candidates in electricity or heat production plants with only minor drawbacks compared to fossil fuels. Even if co-firing seems the most attractive solution for near-term applications, processes based on combustion and gasification (which are competing in dependence of the need of heat or electricity) are receiving renewed attention. Thanks to their high carbon content, and their high specific surface area and developed porous structure, biomass-derived chars can be treated and converted into activated carbons and applied in many different field (as energy storage materials for gaseous fuels, mainly hydrogen and methane, or as electrodes). They can constitute the raw materials for preparing synthetic graphite, which can be used in some types of batteries and fuel cells, and in carbon electrodes for electrochemical capacitors. The performances in terms of capacitance, electrical conductivity, potential, charge and discharge rates, power density, etc. have been reported to be very close to those of commercial devices. The recent progress in the activation protocols brought to higher fuel gas storage capacities, especially in cryogenic conditions and under high pressure, and opened the possibility to apply these materials in new application fields. In catalysis, advances in the use of biomass-derived chars and active carbons have been made thanks to the improvement of the modification techniques. The optimization of the engineering methodologies allows to lower the cost of the activation processes of biomass-derived chars and to tune the char properties to adapt them to the final application. The present paper aims to give a comprehensive survey of already-well-established or future potential energy applications of biomass-derived chars. A critical comparison of their use in different processes is reported and their modification by various catalytic, physical and chemical routes is detailed. •Competition of combustion and gasification in energy recovery from chars.•Tar, oil and hydrocarbon reforming increase the chars' energy recovery output.•Chars can be applied as adsorbents in fuel-gas storage systems.•Char based electrodes ensure good performances in electrochemical storage systems.•Chars can be used as catalysts in green energy applications.