Dry torrefaction and hydrothermal carbonization of biomass to fuel pellets

Considering the growing economies in the world and the increasing demand for energy, a transition toward sustainable energy resources is pivotal to achieve CO2 mitigation and net zero carbon emissions by 2050. Clean electricity as the future energy can be provided by various renewable resources, especially lignocellulosic biomass. Direct combustion of biomass suffers from its hygroscopic nature, low energy efficiency, corrosion, and challenges related to utilization, transportation, and storage. Thermochemical methods are known as promising pathways converting biomass into different energy carriers. Among energy carriers, sustainable solid fuels are of great significance, due to their potential for substituting conventional coal. Treatment of biomass via dry torrefaction and hydrothermal carbonization provides char materials, which can be further densified into fuel pellets. These treatment methods provide a low-temperature waste valorization platform for solid fuel production, with enhanced energy density and hydrophobicity. This study provides a comprehensive review of process parameters, enhancement of fuel characteristics, and process development concerning dry torrefaction, hydrothermal carbonization, and pelletization. In addition, a comparative literature survey of char-based pellets is conducted to underline the challenges related to char pelletization and physicochemical properties of pellets as highlighted by scientometric analysis. Moisture uptake as a crucial aspect of solid fuels is discussed for raw biomass, treated biomass, and char-based pellets. Recent trends emphasize the need for practical applications of the waste-to-energy platform concerning large scale production, innovative technologies, economic feasibility, and environmental sustainability rather than laboratory experiments and characterization.