Wet and dry? Influence of hydrothermal carbonization on the pyrolysis of spent grains

Abstract Brewer's spent grains (BSG) represent a highly abundant byproduct generated in the beer industry. Owing to abundant availability and high water content, BSG constitutes a raw material of interest for thermochemical conversion via hydrothermal carbonization (HTC). Nevertheless, the phyto-toxic effect and low-porosity of hydrochars limit the possible application. Therefore, coupling HTC and pyrolysis may overcome these obstacles. HTC experiments were carried out at 180, 220, and 260 °C and pyrolysis was performed at 400, 600, 800, and 1000 °C to investigate the influence of HTC on the pyrolysis of BSG. Pyrolyzed hydrochars showed lower ash content and higher carbon content compared to char obtained from BSG. The highest surface area (139.5 m2 g−1) was obtained for hydrochar produced at 180 °C and then pyrolyzed at 600 °C. This value was 70% higher compared to BSG pyrolyzed at the same temperature (82 m2 g−1). The coupling of HTC and pyrolysis showed synergies in the char yield, reduction of ash, increasing of carbon content, and porosity. The promising results proved that at selected process conditions, the novel route is beneficial for carbon materials production due to the improvement of the physicochemical properties of pyrochar at similar mass and carbon yields.