Enhancement of fuel properties of yard waste through dry torrefaction

Abstract

Yard waste or green waste is an abundant yet underutilized source of biomass around the world. Yard waste is rich in complex organic compounds, which is beneficial when used as feedstock for chemicals production, but undesirable components within the biomass are detrimental to the quality of the products. In this work, the prospect of yard waste upgrading as a solid fuel source by dry torrefaction (DT) was studied. Dry torrefaction of yard waste was conducted at 170 °C, 200 °C, 250 °C, and 300 °C under N2, CO2, and flue gas (25:75 vol% mixture of CO2:N2) atmosphere in a tubular reactor. As temperature was increased, the mass yield of the torrefied yard waste decreased while the HHV increased. The mass yields obtained were in the order of nitrogen < carbon dioxide < flue gas. HHV was in the order of flue gas < nitrogen < carbon dioxide. Overall, carbon dioxide was shown to be the best carrier gas for energy intensification, where HHV was enhanced from 15.6 to 22.2 MJ/kg at 300 °C, with an energy yield of 98.1%. Flue gas showed no visible improvement in properties up to temperatures of 250 °C. Thermogravimetric analysis (TGA) highlighted that within the given temperature range, hemicellulose was completely degraded as temperature approach 300 °C, while cellulose was partially degraded. Fourier transform infrared spectroscopy (FTIR) emphasised that flue gas was unable to degrade various functional groups to the same degree as the other gases. Field Emission scanning electron microscopy with Energy Dispersive X-ray analysis (FESEM-EDX) revealed that flue gas was unable to degrade the biomass as much as nitrogen or carbon dioxide.

Keywords

• Carbondioxide torrefaction
• Dry torrefaction
• Energy yield
• Flue gas torrefaction
• Yard waste