Torrefaction of combustible construction and demolition waste: Evolution of fuel performance, heavy metal migration and combustion characteristics

Combustible construction solid wastes (CSW) derived from construction and demolition (C&D) wastes is an important part for waste-to-energy system. Torrefaction experiment of five CSW components involving plastic (PL), wood (WD), textile (TX), paper and board (PB), and rubber (RB) were conducted in a fixed-bed reactor at torrefaction temperature of 200-400 degrees C and holding time of 30 min. Fuel properties and heavy metal migration of CSW components via torrefaction are characterized by proximate analysis, ultimate analysis, caloric value determination, and the determination of heavy metal contents in solid, liquid and gas products. Combustion characteristics of CSW components and their torrefied solid products were evaluated by TG-FTIR method. The obtained results indicated that torrefaction has a significant influence on volatile matter, fixed carbon, calorific value, and H/C and O/C molar ratios of CSW components. The ash content in high ash CSW components increases and energy yield decreases significantly when torrefaction temperature is larger than 250 degrees C. Heavy metals of Zn, Pb, Cu and Mn in CSW components is partly migrated into the liquid products, but Hg and As are almost entirely present in the liquid products at different torrefaction temperatures. The combustible index and burnout index of PL, WD, PB and RB decrease gradually when torrefaction temperature is more than 250 degrees C, and the lower yields of SO2, NO, HCl, CH4 and CO of torrefied CSW are achieved at torrefaction temperature of 250 degrees C. The optimum torrefaction temperature of 250 degrees C is proposed for improving fuel properties, heavy metal migration and combustion characteristics of CSW components.