The characteristics of torrefied microalgae

► The property of biomass feedstock after torrefaction is approaching to coal. ► The HGI of torrefied microalgal residue can exceed that of sub-bituminous coal. ► Torrefied biomass can be blended with coal directly for firing in PC boilers. In order to utilize microalgae and their residues as biomass solid fuels for co-firing in the existing pulverized coal-fired power plants, this study is to investigate the properties of fuels from those feedstock prepared by the torrefaction process. The microalgae were cultivated by using flue gas emitted from coal-fired power plants of Taipower Co., and the residue was from extraction of microalgae by hot water. The operating conditions for the torrefaction temperature and residence time were set as 200, 250, 300 and 350°C, and 30, 60, 120min, respectively. Also, 30°C/min and 50°C/min as the heating rates were chosen to examine the mass yield of solid torrefied biomass from microalgae. After torrefaction, the ultimate analysis, proximate analysis, and higher heating value analysis of the torrefied biomass were carried out. In addition, the Hardgrove Grindability Index (HGI) of microalgal residue and its torrefied solid were measured. The results show that carbon content, ash content, fixed carbon content, higher heating value, and HGI of torrefied biomass increased with increasing the operating temperature and residence time. At the condition of 300°C and 30min, the higher heating value of torrefied microalgae and microalgal residue were 25.92MJ/kg and 26.76MJ/kg, respectively. Compared to raw materials, the increases were 26.70% and 17.16%, respectively. At the same circumstances, hydrogen content, oxygen content, moisture content and mass yield of solid torrefied biomass decreased with increasing temperature and residence time. After torrefaction at 300°C, the moisture content were all down to 1%, and the mass yield of solid torrefied biomass under 350°C by torrefaction were also kept up to 50%. The HGI of torrefied microalgal residue was 48.53, which exceed the sub-bituminous coal when the operation condition temperature was up to 250°C.