Flammability and mechanical properties of biochars made in different pyrolysis reactors

Highlights: • Birch wood was carbonised in three different reactors with analogous reaction conditions. • Hydrothermal, vertical and horizontal tube reactors were used to produce the biochars. • Vertical and horizontal tube biochars had most properties identical to each other. • Hydrothermal biochar resisted combustion the most but had low mechanical property. • Different reactors had varied effect on the nanoindentation properties of biochars. The effect of pyrolysis reactors on the properties of biochars (with a focus on flammability and mechanical characteristics) were investigated by keeping factors such as feedstock, carbonisation temperature, heating rate and residence time constant. The reactors employed were hydrothermal, fixed-bed batch vertical and fixed-bed batch horizontal-tube reactors. The vertical and tube reactors, at the same temperature, produced biochars having comparable elemental carbon content, surface functionalities, thermal degradation pattern and peak heat release rates. The hydrothermal reactor, although, a low-temperature process, produced biochar with high fire resistance because the formed tarry volatiles sealed water inside the pores, which hindered combustion. However, the biochar from hydrothermal reactor had the lowest nanoindentation properties whereas the tube reactor-produced biochar at 300 °C had the highest nanoindentation-hardness (290 Megapascal) and modulus (ca. 4 Gigapascal) amongst the other tested samples. Based on the inherent flammability and mechanical properties of biochars, polymeric composites' properties can be predicted that can include them as constituents.