A comparative review of biochar and hydrochar in terms of production, physico-chemical properties and applications Harpreet Singh Kambo, Animesh Dutta n School of Engineering, University of Guelph, Guelph, Canada N1G-2W1 article info Article history: Received 11 January 2014 Received in revised form 8 January 2015 Accepted 12 January 2015 Available online 14 February 2015 Keywords: Biochar Hydrochar Hydrothermal Carbonization Pyrolysis abstract Slow-pyrolysis of biomass for the production of biochar, a stable carbon-rich solid by-product, has gained considerable interest due to its proven role and application in the multidisciplinary areas of science and engineering. An alternative to slow-pyrolysis is a relatively new process called hydrothermal carbonization (HTC) of biomass, where the biomass is treated with hot compressed water instead of drying, has shown promising results. The HTC process offers several advantages over conventional dry-thermal pre-treatments like slow-pyrolysis in terms of improvements in the process performances and economic efficiency, especially its ability to process wet feedstock without pre-drying requirement. Char produced from both the processes exhibits significantly different physiochemical properties that affect their potential applications, which includes but is not limited to carbon sequestration, soil amelioration, bioenergy production, and wastewater pollution remediation. This paper provides an updated review on the fundamentals and reaction mechanisms of the slow-pyrolysis and HTC processes, identifies research gaps, and summarizes the physicochemical character- istics of chars for different applications in the industry. The literature reviewed in this study suggests that hydrochar (HTC char) is a valuable resource and is superior to biochar in certain ways. For example, it contains a reduced alkali and alkaline earth and heavy metal content, and an increased higher heating value compared to the biochar produced at the same operating process temperature. However, its effective utilization would require further experimental research and investigations in terms of feeding of biomass against pressure; effects and relationships among feedstocks compositions, hydrochar characteristics and process conditions; advancement in the production technique(s) for improvement in the physicochemical behavior of hydrochar; and development of a diverse range of processing options to produce hydrochar with characteristics required for various industry applications. & 2015 Elsevier Ltd. All rights reserved. Contents 1. Introduction ........................................................................................................ 360 1.1. Lignocellulosic composition and combustion properties of biomass...................................................... 360 1.2. Origin and definition of biochar and hydrochar...................................................................... 361 2. Production of biochar and hydrochar .................................................................................... 361 2.1. Feedstock for the production of biochar and hydrochar ............................................................... 361 2.2. Biochar and hydrochar production technologies ..................................................................... 362 2.2.1. Pyrolysis .............................................................................................. 362 2.2.2. Dry Torrefaction ........................................................................................ 362 2.2.3. Gasification............................................................................................ 363 2.2.4. Hydrothermal carbonization .............................................................................. 363 3. Chemical reaction mechanism(s) behind the production of hydrochar and biochar ............................................... 364 4. Characterization of biochar and hydrochar ............................................................................... 365 4.1. Solid yield and chemical composition characterization of biochar and hydrochar........................................... 365 4.2. Morphological characterization of biochar and hydrochar ............................................................. 366 4.2.1. Tailoring the structure of chars via physical and chemical activation .............................................. 368 5. Potential benefits and applications of biochar and hydrochar ................................................................ 369 Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/rser Renewable and Sustainable Energy Reviews http://dx.doi.org/10.1016/j.rser.2015.01.050 1364-0321/& 2015 Elsevier Ltd. All rights reserved. n Correspondence to: School of Engineering, 50 Stone Road East, University of Guelph, Guelph, Ontario, N1G 2W1 Canada. Tel.: þ1 519 824 4120x52441; fax: þ1 519 836 0227. E-mail address: adutta@uoguelph.ca (A. Dutta). Renewable and Sustainable Energy Reviews 45 (2015) 359–378