Conversion of water caltrop husk into torrefied biomass by torrefaction

In this work, water caltrop husk (WCH), a special agricultural residue in tropical and subtropical Asian countries, was used as a potential precursor for preparing torrefied biomass at different temperatures (i.e., 200, 240, 280, 320, and 360 °C) and residence times (i.e., 0, 30, 60, and 120 min). To best of our knowledge, this is currently the first study on the thermochemical characteristics of WCH-torrefied products. The mass yields and energy yields of resulting products indicated a decreasing trend with increasing temperature. By contrast, their calorific values and carbon contents generally increased at higher temperatures and longer residence times. These findings were consistently verified by the energy dispersive X-ray spectroscopy (EDS). Based on the thermochemical characteristics, the optimal WCH-torrefied product, which corresponded to mass yield of 45.2%, carbon content of 65.23%, calorific value of 25.3 MJ/kg and energy yield of 65.7%, was obtained at 320 °C for 60 min. According to the classification of solid fuels by the van Krevelen diagram, the optimal torrefied product showed a lignite-like feature. However, this lignite-like biomass fuel would not be appropriate to be directly used in boilers because of its relatively high minerals. Alternatively, it may be blended with coal in existing coal-fired power plants.