Hydrothermal carbonization of waste biomass: An experimental comparison between process layouts

This paper contributes to the knowledge on waste biomass conversion processes occurring in the presence of hot compressed water. The experimental procedure detailed herein assesses different process schemes based on the low-temperature reaction known as hydrothermal carbonization. The performances of two lab-scale reactor configurations, with and without a downstream flash expansion step, were evaluated and compared. Each setup was tested with six different types of waste biomass. Fir, beech, and olive prunings are representative of lignocellulosic raw materials, while potato, pea, and carrot are representative of non-lignocellulosic wastes from processing in the local agro-food industry. The batch reactions (200 °C, water/solid = 7/1) were carried out for up to 120 min. The hydrochars were characterized by elemental composition, humidity, heating value, and mass and energy yields. The extent of difference between the results obtained for the two procedures varied significantly with the material treated. At a residence time of 30 min, the solid yields increased due to expansion, ranging from 10 to 36% for lignocellulosic material and 50 to 220% for agro-food industry scraps. The downstream flash expansion step causes an increase of the solid yields, especially for hydrochars from lignocellulosic materials, leading to higher energy recovered compared to the configuration without expansion. Lignocellulosic and agro-food wastes behaved dissimilarly, likely because of different hydrothermal reaction pathways. The additional expansion step can considerably increase the efficiency of energy recovery in full-scale plants, the extent of which depends on the biomass waste substrate used.