Advancing hydrochar production and application: A critical examination of microwave irradiation and blended feedstocks synergy

Sustainable and efficient hydrochar production via microwave-assisted hydrothermal carbonization (MW-HTC) and co-hydrothermal carbonization (co-HTC) has garnered increasing attention. This review offers a comprehensive analysis of recent advancements, primarily focusing on the performance and potential applications of hydrochar produced through these two processes. MW-HTC of lignocellulosic biomass yields hydrochar in the range of 30–80 dwt.%, with higher heating values of 19–23 MJ/kg and energy yields of 50–85%. Its adsorption capacity reaches around 50 mg/g for heavy metals and 1–5 mg/g for organic pollutants. Principal component analysis reveals significant differences in MW-HTC treatment of lignocellulosic and non-lignocellulosic materials. In the co-HTC process, sewage sludge and lignocellulosic biomass are the most studied feedstocks. Synergistic and antagonistic effects on hydrochar yield were identified, with most studies reporting a synergistic effect, particularly when protein and carbohydrate co-exist, potentially due to the Maillard reaction. The adsorption capacities of co-HTC hydrochars depend on the raw material mix, process parameters, and target contaminants. This review highlights the promising potential of MW-HTC and co-HTC for enhanced hydrochar production and application, emphasizing the necessity for future advancements, including machine learning for prediction of hydrochar yield, HTC process optimization, and feedstock's single model components-based HTC to elucidate the MW effect and synergism/antagonism.