A bibliographic study of biochar and hydrochar: Differences and similarities

Biomass conversion by thermochemical processes has gained increasing interest due to its carbon-neutral and/or carbon-negative effect. The most common thermochemical approach is pyrolysis, which results in the production of biochar, a well-known carbon-rich material with properties comparable to those of materials derived from fossil sources. In the last decade, hydrothermal carbonization of biomass has attracted attention due to its simple requirements, low cost and lower temperatures, producing a carbon-rich material called hydrochar. This bibliographical study, based on the Scopus database and focused on the last 5 years, without forgetting earlier articles, aims to define the main differences between the two processes and the physico-chemical properties of each char, to summarise the main techniques for their characterisation and to highlight the areas of application according to their properties. This review suggests that it would be strategic to distinguish between the terms hydrochar and biochar to ensure efficient characterisation and appropriate application.