Enhancing Fuel Properties of Napier Grass via Carbonization: A Comparison of Vapothermal and Hydrothermal Carbonization Treatments

Abstract

Napier grass is a herbaceous biomass that can be used as biofuel; however, its high ash, potassium, sulfur and chlorine content may cause problems when combusted. Napier grass was submitted to vapothermal carbonization (VTC) and hydrothermal carbonization (HTC) processes at 190 and 220 °C to compare their ability to enhance its fuel properties. The different water distribution between phases in the two processes was verified: up to 14.5% of the water vaporized to steam in the VTC ran at 220 °C, while over 99% of the water remained in the liquid state and in contact with the solids during all HTC runs. Both processes improved the calorific value of the Napier grass (up to 20.6% for VTC220 and up to 29.8% for HTC220) due to the higher C content in the chars. Both processes reduced the sulfur content, removing up to 15.3% of it with VTC190 and 28.5% of it with HTC190 compared to that of Napier grass. In contrast, the two processes had different effects on the ash and chlorine content. While HTC removed both ash and Cl from the Napier grass, VTC concentrated it in the chars (ash: 5.6%wt. Napier grass, 3.3%wt. HTC chars, 7.1%wt. VTC; chlorine: 1.08%wt. Napier grass, 0.19%wt. HTC chars, 1.24%wt. VTC). Only the HTC process leached high percentages of Cl (up to 80%), S (up to 70%), sodium (Na, up to 80%) and potassium (K, up to 90%) into the process water. This may prevent fouling and slagging problems when burning HTC char. The biofuel qualities of the raw Napier grass, VTC, and HTC chars were evaluated using two standards: the international standard for solid biofuels, EN ISO 17225, and the Korean regulation for biomass solid recovered fuels (Bio-SRF). Napier grass and VTC chars presented problems regarding Cl content thresholds for both EN ISO 17225 and Bio-SRF. Both VTC and HTC chars along with the Napier grass fulfilled the requirements for heavy metals (Pb, Ni, Cr, and Cd) except for copper. The choice of process in practical applications will depend on the goal; HTC improves fuel quality and VTC has higher high solid, carbon and energy yields.