A thermochemical conversion method known as hydrothermal carbonization (HTC) is appealing, because it may convert wet biomass directly into energy and chemicals without the need for pre-drying. The hydrochar solid product’s capacity to prepare precursors of activated carbon has attracted attention. HTC has been utilized to solve practical issues and produce desired carbonaceous products on a variety of generated wastes, including municipal solid waste, algae, and sludge in addition to the typically lignocellulose biomass used as sustainable feedstock. This study aims to assess the in-depth description of hydrothermal carbonization, highlighting the most recent findings with regard to the technological mechanisms and practical advantages. The process parameters, which include temperature, water content, pH, and retention time, determine the characteristics of the final products. The right setting of parameters is crucial, since it significantly affects the characteristics of hydrothermal products and opens up a range of opportunities for their use in multiple sectors. Findings reveal that the type of precursor, retention time, and temperature at which the reaction is processed were discovered to be the main determinants of the HTC process. Lower solid products are produced at higher temperatures; the carbon concentration rises, while the hydrogen and oxygen content declines. Current knowledge gaps, fresh views, and associated recommendations were offered to fully use the HTC technique's enormous potential and to provide hydrochar with additional useful applications in the future.

A recent advancement on hydrothermal carbonization of biomass to produce hydrochar for pollution control
    Abstract
    1 Introduction
    2 Hydrochar
    3 Hydrochar production
        3.1 Hydrothermal carbonization
        3.2 Hydrothermal liquefaction
        3.3 Hydrothermal gasification
        3.4 Microwave-assisted hydrothermal carbonization
    4 Reaction mechanism of hydrochar
    5 Role of water in HTC
    6 Process parameters variables
        6.1 Temperature
        6.2 Residence or retention time
        6.3 Pressure
        6.4 pH
        6.5 Feedstock–water ratio
    7 Hydrochar composition
    8 Hydrochar activation methods
        8.1 Physical activation
        8.2 Chemical activation
    9 Functionalization of hydrochar
    10 Hydrochar vs biochar
    11 Hydrochar applications in various fields
        11.1 Heavy metal adsorption
        11.2 Dye adsorption
        11.3 Emerging contaminants
        11.4 Electrochemical applications
        11.5 Soil amendments
        11.6 Greenhouse gas emission
        11.7 Carbon sequestration
    12 Hydrochar benefits
    13 Future scope
    14 Conclusion
    References

• R. Sivaranjanee(Department of Chemical Engineering, Sri Sivasubramaniya Nadar College of Engineering, Kalavakkam 603110, Tamil Nadu, India, Centre of Excellence in Water Research (CEWAR), Sri Sivasubramaniya Nadar College of Engineering, Kalavakkam 603110, Tamil Nadu, India)
• P. Senthil Kumar(Department of Chemical Engineering, Sri Sivasubramaniya Nadar College of Engineering, Kalavakkam 603110, Tamil Nadu, India, Centre of Excellence in Water Research (CEWAR), Sri Sivasubramaniya Nadar College of Engineering, Kalavakkam 603110, Tamil Nadu, India)
• Gayathri Rangasamy(Department of Sustainable Engineering, Institute of Biotechnology, Saveetha School of Engineering, SIMATS, Chennai 602105, India, School of Engineering, Lebanese American University, Byblos, Lebanon)