Challenges and alternatives for the adequacy of hydrothermal carbonization of lignocellulosic biomass in cleaner production systems: A review

Hydrothermal carbonization (HTC) is considered an attractive and constantly expanding eco-friendly methodology for thermochemical processing of different types of biomass in energy, carbonaceous materials, structured hybrids, and other chemical products. However, some challenges for the technique to be adequate in clean production need to be overcome. In this way, a comprehensive review is presented regarding the process variables, fundamentals, properties, possible uses of the products obtained and the challenges for cleaner production. This paper stands out in relation to those available in the literature when gathering exclusive information on hydrothermal carbonization with a focus on lignocellulosic biomass, the possibility of reusing the wastewater from HTC in recirculation processes and the challenges of the technique aiming at the sustainability of the process. Initially, we discuss the general concepts and reactional mechanisms involved in the methodology and then it explored the physical-chemical properties and possible applications of hydrochar, the modification methodologies and activation processes to obtain activated carbon in an eco-friendly approach. Current knowledge gaps and the overcoming of challenges such as the recirculation of the aqueous phase of hydrothermal carbonization are also discussed together with new perspectives and alternatives for the technique based on a biorefinery system, in order to enable a wider exploration of hydrothermal carbonization of its great potential in a cleaner production concept.