Mineral transformation and emission behaviors of Cd, Cr, Ni, Pb and Zn during the co-combustion of dried waste activated sludge and lignite

- Co-combustion of lignite and waste activated sludge (WAS) is conducted.
- Mineral transformation and emission behaviors of heavy metals are determined.
- Anorthite, quartz, pyroxene, and albite are the main components of lignite-WAS ash.
- Effects of O2 concentration and temperature on the volatilization of heavy metals.

Co-combustion of dried waste activated sludge (WAS) and lignite was conducted in a horizontal tube furnace system. The mineral transformation and emission behaviors of Cd, Cr, Ni, Pb, and Zn during combustion were examined. The above heavy metals (HMs) were selected because they are more abundant in WAS than in lignite. In the combustion condition of 1000 °C-21% O2-30 min, the minerals in lignite ash were mainly anorthite (CaAl2Si2O8), quartz (SiO2), pyroxene [Ca(Mg,Fe)Si2O6], and albite (NaAlSi3O8). By contrast, the minerals in WAS ash were mainly quartz (SiO2), anorthite (CaAl2Si2O8), and hematite (Fe2O3). When 90% lignite and 10% WAS were co-combusted, hematite was hardly detected in the combustion product because hematite reacted with quartz and calcium oxide, which are abundant in lignite ash, and generated a large amount of pyroxene. Anorthite and pyroxene were generated continuously during the combustion process, but albite was mainly generated during the first 5 min of combustion. High temperature promoted the generation of complex components, such as sodian anorthite [(Ca, Na)(Si, Al)2Si2O8 or (Ca, Na)(Si, Al)4O8]. High O2 concentration promoted the reaction among the three main mineral elements, i.e., silicon, aluminum, and calcium, and generated a large amount of anorthite. During the co-combustion of WAS and lignite, the volatilization percentages of Cd, Pb, and Zn exceeded 30% after a combustion time of 30 min, and the order was Cd > Pb > Zn. By contrast, the volatilization percentages of Cr and Ni were lower than 15%. High temperature caused an increase in the volatility of HMs, especially Zn and Ni, although the volatilization percentage of Ni remained low. High O2 concentration also caused an increase in the volatility of HMs, except for Cr and Ni, which showed a slight increase or decrease with increasing O2 concentration in the inlet atmosphere.