Hydrothermally carbonized plant materials: patterns of volatile organic compounds detected by gas chromatography

The nature and concentrations of volatile organic compounds (VOCs) in chars generated by hydrothermal carbonization (HTC) is of concern considering their application as soil amendment. Therefore, the presence of VOCs in solid HTC products obtained from wheat straw, biogas digestate and four woody materials was investigated using headspace gas chromatography. A variety of potentially harmful benzenic, phenolic and furanic volatiles along with various aldehydes and ketones were identified in feedstock- and temperature-specific patterns. The total amount of VOCs observed after equilibration between headspace and char samples produced at 270°C ranged between 2000 and 16,000μg/g (0.2-1.6wt.%). Depending on feedstock 50-9000μg/g of benzenes and 300-1800μg/g of phenols were observed. Substances potentially harmful to soil ecology such as benzofurans (200-800μg/g) and p-cymene (up to 6000μg/g in pine wood char) exhibited concentrations that suggest restrained application of fresh hydrochar as soil amendment or for water purification.