The severity factor as a useful tool for producing hydrochars and derived carbon materials

Area of Science:

• Materials Science
• Chemical Engineering
• Biomass Conversion

Background:

• Olive stones (OS) are abundant lignocellulosic biomass with potential for valorisation.
• Hydrothermal carbonisation (HTC) converts biomass into hydrochar, a coal-like material.
• Optimizing HTC conditions is crucial for tailoring hydrochar properties.

Purpose of the Study:

• To investigate the impact of time and temperature on olive stone HTC.
• To introduce and utilize the severity factor for describing HTC process conditions.
• To fully valorise olive stones by analysing both solid hydrochar and liquid fractions.

Main Methods:

• Hydrothermal carbonisation (HTC) of olive stones at temperatures from 160-240 °C and times from 3-48 h.
• Thermogravimetric analysis (TGA) and elemental analysis of produced hydrochars.
• Analysis of liquid fractions for high-value products and further carbonisation of hydrochars at 900 °C.

Main Results:

• Hydrochar yield ranged from 70% to 50%.
• Furfural and 5-hydroxymethylfurfural were identified in liquid fractions.
• Hydrochars produced at severe conditions exhibited lignite-like elemental composition.
• Further carbonisation yielded materials with surface areas up to 1200 m²/g and narrow pores.

Conclusions:

• The severity factor effectively describes HTC conditions and their influence on hydrochar yield, composition, and surface area.
• HTC of olive stones can produce valuable products, including precursors for activated carbons.
• The severity factor is a valuable, though uncommon, tool for materials production studies involving HTC.