Transformation Characteristics Of Heavy Metal In Co-combustion Of Municipal Sewage Sludge And Biomass

Municipal sewage sludge (hereinafter referred to as sludge) incineration has the advantages of volume reduction, mass reduction and energy recovery. Fly ash from sludge incineration is considered to be an important source of secondary phosphates due to its high content of phosphorus. However, high contents of heavy metals in the fly ash are the core issue reducing the bioavailability of phosphorus. Therefore, it is of great significance to study reducing the toxicity of heavy metals contained in fly ash for increasing the bioavailability of phosphorus and realising clean utilization of sludge.In this thesis, a new method of co-firing biomass and sludge is proposed to deal with the heavy metal pollution problem. The experiments were carried out in a 5 kW bubbling fluidized bed combustor to investigate the heavy metal characteristics including content distribution, speciation distribution and leachability during co-combustion of biomass and sludge. The chemical thermodynamic analysis software FactSage 6.1 was applied to analyze the speciation transformation and distribution behavior of heavy metals during sludge incineration and co-combustion. The transformation characteristics of heavy metal during co-combustion of sludge and biomass was studied with the experimental and theoretical analysis. The main conclusions are listed as follows:The experimental results in a 5 kW bubbling fluidized bed combustor show that the content of Cr (mixing ratio of 30% and 40%), Cu, Zn, Cd and Pb in the fly ash decrease and the content of As increases during co-combustion of sludge and wheat straw.The speciation stability of Cr (mixing ratio of 50%), As (mixing ratio of 50%), Cu, Zn and Cd (mixing ratio of 30% and 40%) are enhanced and the leaching contents decrease during co-combustion of sludge and wheat straw. Adding wheat straw to sludge weakens the speciation stability of Pb and has little infuence on the leaching content of Pb.The infuence of cotton addition on the heavy metal characteristics such as content distribution, speciation distribution and leachability are basically the same with wheat straw addition(except that the leaching content of As increases when mixing raio of cotton is 50%). However, compared with wheat straw, the contents of heavy metals (except As) in the fly ash are lower, the speciation stability of heavy metals (with the exception of Cd and Pb in the mixing ratio of 50%) is stronger and the leaching amounts are lower (except As) during co-combustion of sludge and cotton.The leaching toxicity of heavy metals in fly ash depends on the leaching content of heavy metals in fly ash, therefore cotton addition is more advantageous to reduce the leaching toxicity of heavy metals in sludge incineration fly ash.The experimental results in a 5 kW bubbling fluidized bed combustor also show that with temperature increasing from 850 ℃ to 950℃, the content of Cr in the fly ash decreases and the speciation stability is enhanced, lowering the leaching toxicity of Cr in the fly ash. The content of As in the fly ash increases and the speciation stability of Cu, Zn, Cd and Pb is weakened with temperature increasing from 850℃ to 950 ℃, resulting in the increasing leaching toxicity of As, Cu, Zn, Cd and Pb in the fly ash.The FactSage 6.1 results show that Cr and Zn are mainly in solid forms with weak volatility during sludge incineration, while all of Cd, Pb and As are transfromed to volatile forms when the temperature is higher than 850℃. Cu is mainly in solid forms at low temperature and exists as CuCl (g) at high temperature. Co-combustion of sludge and wheat straw promotes the volatilizations of Cu, Zn and Pb due to the formation of gaseous matal chlorises and has little effect on the speciation distribution of Cr, As and Cd when incineration temperature is 900℃. The infuence of cotton addition on the heavy metals speciation distribution of Cr, Cu, Cd and Pb is basically the same with wheat straw addition, except that cotton addition has little effect on the speciation distribution of Zn and promotes the formation of solid form of Ca3(AsO4)2.