Regimes of hydrochar yield from hydrothermal degradation of various lignocellulosic biomass: A review

Hydrothermal carbonization (HTC) technique is used to convert lignocellulosic biomass into carbon-rich solid fuel. The product, hydrochar, finds a multitude of applications as a fuel, catalyst, adsorbent, etc. This paper reviews the different feedstock suitable for HTC that are experimentally investigated and classifies them into five categories like woody biomass, plant biomass, agricultural biomass, algae, and municipal solid waste (MSW). The operating regimes and hydrochar yields for each category are systematically reviewed and presented. With 75% hydrochar yield and high calorific value, the food waste of MSW makes an excellent candidate for HTC feedstock. The conversion mechanisms of each constituent of biomass are also discussed. Different parameters that significantly affect the process yield are temperature, residence time, pH, and, water to biomass ratio. Improvement in the quality of hydrochar can be attained by using the assistance of microwaves. Different characterization techniques are discussed briefly and energy content obtained in each type of feedstock is comprehensively presented. The best operating regime for the process having a temperature range of 180–240 °C with a slightly acidic pH in feed condition. Further rigorous and extensive research on surface characteristics and deactivation mechanism for catalyst/adsorbent applications is required to appropriately identify the suitable feedstock. Further research is also required to ascertain the behavior of other organic compounds, their degradation mechanism in biomass, and analyze the liquid and gaseous by-products obtained.