Release properties of semi-volatile heavy metals in sewage sludge/coal co-incineration under O₂/CO₂ atmosphere

The effects of six factors involving incineration temperature (t_c), sludge blending ratio (X_s), O₂ concentration(φ_O2), initial moisture content (φ_H2O), incineration time (τ) and chlorine content (φ_Cl) on the release properties of semi-volatile heavy metals (SVHMs) (such as Zn, Pb, Cd, Cu, Ni and Cr) in co-incineration of sewage sludge and coal under O₂/CO₂ atmosphere were investigated in a fixed-bed incinerator based on orthogonal experiment. The results indicate that the influence of six factors on the release fraction of SVHMs is ordered as t_c >> X_s > φ_H2O > φ_O2 ≈ τ > φ_Cl. The release fraction of Zn is the largest, followed by Pb and Cd, and the release fractions of Cu, Ni and Cr are the least at the same co-incineration condition. Increasing temperature is helpful for SVHMs release, and the promotion at high temperatures of 1000-1100 ℃ on SVHMs release is significantly stronger than that at the low temperature of 700-900 ℃. The release fractions of Zn and Pb increase remarkably from 36.1% and 12.2% to 70.9% and 63.5% with increasing t_c from 700 to 1100 ℃, respectively, and the release fraction of Cd achieves the maximum of 40.0% at t_c=900 ℃, however, the release fractions of Cu, Ni and Cr mostly keep below 20.0%. The influence of incineration temperature on the heavy metal release fraction is ordered as Pb > Zn > Cd > Cu > Cr > Ni. The release fractions of SVHMs decrease with increasing sludge blending ratio, but present a wave-like trend with increasing initial moisture content. The lower release fraction of SVHMs is achieved at φ_H2O=0 or 40%. O₂ concentration has a certain influence on SVHMs release, and the lowest release fraction of SVHMs is achieved at φ_O2=30%. The influence of incineration time and chlorin content on the release of Pb is significantly stronger than that of other five SVHMs. The suggested optimal conditions of co-incineration of sludge and coal in O₂/CO₂ atmosphere are: the incineration temperature is 900-1000 ℃, the sludge blending ratio is about 25%, the O₂ concentration is 30%, the initial moisture content is less than 10%, and the incineration time is reduced as far as possible.