Microwave-assisted wet co-torrefaction of food sludge and lignocellulose biowaste for biochar production and nutrient recovery

Microwave-assisted wet co-torrefaction of food sludge (FS) and six widely produced lignocellulose biowaste samples were performed for biochar production. Factors including the torrefaction temperature, reaction time, biowaste type, and blending ratio of sludge and biowaste were evaluated. Biochar produced from FS torrefied at 150 degrees C for 20 min exhibited the greatest enhancement of carbon content and higher heating value (HHV). Blending sludge with macadamia husk at a ratio of 25/75 (db%) under the same wet torrefaction condition further improved biochar quality, enhanced the fixed carbon content, caused the highest HHV (19.6 MJ/kg), and considerably decreased the ash content. Thermogravimetric analysis indicated that sludge and biowaste degradation followed first-order kinetics, and the resultant biochar exhibited improved thermal stability, combustion efficiency, and safety for biofuel applications because of the increase in the activation energy, frequency factor, and ignition and combustion temperatures. Biochar is an excellent coal substitute for power generation because of its enhanced energy efficiency (energy return on investment: 7.4) and environmental friendliness (45.2 % reduction in greenhouse gas emissions compared with using bituminous coal). Moreover, the protein and carbohydrate contents in the supernatant of FS co-torrefied with biowaste at different blending ratios increased by 1.8-9.0 and 0.8-3.4, respectively. These contents can be recycled back to the activated sludge unit as a nutrient source, rendering the process green and sustainable without any waste production. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.