Co-combustion between sewage sludge and maize stover and content analysis of heavy metals in ash

Abstract: Incineration is an alternative way to treat sludge, but heavy metals and other pollutants will be generated thereby. Besides, landfill of incinerated sludge ash will cause soil pollution, so reducing heavy metals in sludge combustion ash is an urgent problem to be solved. Based on this issue, this paper studies the co-combustion between sludge and maize stover at 350-650 ℃ in a laboratory scale tubular furnace. The main purpose is to explore the influence of different reaction temperatures and mixing ratios of corn stalk on the content of eight heavy metals (Zn, Mn, Pb, Sb, As, Ni, Cr, Cu) in the (co-)combustion ash. The results showed that the content of Zn, Pb and Sb in the ash decreases with the addition of maize stover. Comparing with the combustion ash of corn stalk, the content of Zn, Pb and Sb within sludge combustion ash decrease by 93%, 90% and 82% at 350 ℃. At 350-550 ℃, the content of Mn and Cr in the co-combustion ash is higher than that of sludge or maize stover ash alone. Comparing with the combustion ash of maize stover, the content of Mn and Sb within co-combustion ash decrease by 20% and 37% at 350 ℃. While the changing trend of Cu and Ni are just opposite. Comparing with the combustion ash of maize stover, the content of Cu and Ni within co-combustion ash decrease by 30% and 50% at 450 ℃. Then, it can be concluded from the testing results of XRD that the maize stover combustion ash contains more KCl (45.4% at 350 ℃), which could be converted to HCl under certain conditions, and then reacts with heavy metals to form chloride which promotes the volatilization of Zn, Pb and Sb. However, KCl is not found in the co-combustion ash, which indicates that complete conversion of KCl may occur during co-combustion, thus it could promote the volatilization of Cu and Ni. Meanwhile, it also shows that the decomposition of silicate to produce a large amount of SiO2 (76.4% at 650 ℃) could happen during co-combustion, which is conducive to retention of Mn and Cr in the ash. Finally, the optimal fitting relationship (Ploy2D) between the contents of eight heavy metals in combustion ash and reaction conditions were given, which can be used to predict the contents of heavy metals in the ash, This study can provide technical support for sludge recycling treatment.