Influence of reaction conditions and feedstock on hydrochar properties

Hydrothermal carbonization (HTC) is a biomass conversion process to produce a renewable solid fuel (hydrochar). The reaction conditions, such as temperature, time, and water/biomass ratio have key effects on hydrochar characteristics. However, it has not been fully investigated to establish and compare models of hydrochar properties (solid yield, carbon content and HHV) for different biomass HTC at different reaction conditions. These models and the corresponding analytical methods are favorable to optimize operating parameters and process design of HTC. In this work, HTC experiments from corn stalk, longan Shell and NaOH-pretreated longan Shell were carried out at 210°C, 250°C and 290°C for 30min, 240min and 480min with different water to biomass ratios. New models of the hydrochar properties of corn stalk, longan Shell and NaOH-pretreated longan Shell were established based on severity parameter (combined time and temperature) and dose-response function. Also, data of wood, olive stone and grape marc hydrochars (collected from literatures) were used for comparison. The first and second derivative methods were also employed to analyze and compare the variation of these hydrochar properties. The results showed that the hydrchar yield, carbon content, and HHV curves decrease monotonically and can be divided into three significant stages with increasing reaction severity. The water to biomass ratio has a significant effect on the hydrochar yield. With increasing the water to biomass ratio, both the maximum decrease rates and the variation regions of hydrochar yields for corn stalk and longan shell shift to lower severities. The chemical composition of the feedstock has also a significant effect on the hydrochar properties. However, the maximum decrease rates and the variation regions of hydrochar properties (determined by the first and the second derivative methods) show similar profiles for different feedstock. The maximum variation rate of the hydrochar properties for six biomass samples can be found at severity of 5.8–6.4.