Physical and Combustion Properties of Binder-Assisted Hydrochar Pellets from Hydrothermal Carbonization of Tobacco Stem

Hydrothermal carbonization (HTC) combined with pelletization is an efficient upgrading process to convert biomass into energy-dense pellets. Binders usually serve an important role in the quality of biofuel pellets. Here, hydrochar was produced from tobacco stem through HTC, and was pelletized in the presence of various binders. The effect of four kinds of binders (K₂CO₃, CaCO₃, xanthan gum and carboxymethyl cellulose) on the basic fuel properties, mechanical strength, moisture adsorption and combustion characteristics was investigated. The results showed that K₂CO₃ assisted pellet had decreased compressive strength, increased hygroscopicity and high combustion residues. The compressive strength of pellets increased with increasing amount CaCO₃ binder probably due to the hardening effect of enhanced adhesion attraction forces between adjacent particles. Meanwhile, the CaCO₃ assisted pellet exhibited the lowest moisture uptake behavior. In addition, the introduction of organic binders led to improved mechanical strength, especially for xanthan binder, which could be ascribed to the formation of solid bridges and enhancement of adhesion forces between particles. TGA results suggested that the binder-assisted pellets exhibited desirable combustion characteristics such as decreased ignition temperature and increased comprehensive combustibility index. Kinetic parameters of the combustion process of binder-assisted pellets were also determined by employing the model free isoconversional Kissinger–Akahira–Sunose method. This study demonstrated that HTC of tobacco stem combined with pelletization in the presence of appropriate binders provide an alternative for solid biofuel pellets production. Hydrothermal carbonization (HTC) of tobacco stem combined with pelletization in the presence of appropriate binders provide an alternative for solid biofuel pellets production.