Hydrochar production from waste lignocellulosic biomass with recirculation of process water

The utilization of biowaste and its transformation into alternative sources of energy is a trend, which could 
help reduction of carbon emissions and support increasing demand for energy, as well as to solve the 
growing  environmental  problems  related  to  solid waste.  One  of  the  alternative  sources  of  energy  is 
hydrochar (HC) which is produced using hydrothermal carbonization (HTC) of various wet biomass wastes. 
During the HTC process, process water in subcritical state is used as a reaction medium to convert biomass 
into HC. Process water from HTC represents a liquid by-product, rich in organic (polyphenols, flavonoids, 
dietary fibers, etc.) and inorganic compounds that can have negative impacts on the environment if the 
water is not properly managed. One of the ways to avoid contamination by releasing process water into 
the environment, as well as the application of expensive treatments for its purification, is to repeatedly 
use it in HTC process (i.e. recirculation). The reuse of process water in HTC has been gaining interest due 
to its potential contribution to the circular economy (closing the water loop), environmental protection, 
and health. In this paper, the characteristics of the HCs obtained with the recirculation of process water 
in four consecutive cycles are examined. The HTC process was carried out in a commercial stainless-steel 
reactor at 300 °C, a pressure of 20 bar (autogenous pressure), while a solid/liquid ratio was 1:15. Waste 
wood biomass was used as feedstock. The yield of HC in the first run was 40%, while the carbon content 
was amounted to 79 wt.%. Furthermore, the obtained HC was characterized with relatively high higher 
heating value (HHV) of 2561 kJ/100 g. In the case of the HTC process with process water recirculation, an 
increase in the HC yield was recorded for all the runs (up to 10%), while the carbon content remained at 
the approximately same value. During the process with recirculation, HHV value increased with increasing 
the number of recirculation cycles (up to 3095 kJ/100 g in the fourth cycle), which indicates that organic 
matter from the liquid phase has been incorporated into the HC structure contributing to its yield, as well 
as its quality as an alternative source of energy.