深度脱水污泥焚烧过程中调理剂的热转化行为及其对重金属挥发的影响

采用化学热力学平衡分析方法,应用污泥实测数据模拟计算了深度脱水后污泥焚烧过程中调理剂的热转化过程及逸出产物,并预测了含Cl调理剂与CaO耦合作用对重金属迁移转化及排放的影响.研究结果表明,污泥添加单一调理剂Al_2(SO_4)_3、Fe_2(SO_4)_3、FeSO_4、AlCl_3及FeCl_3后,Fe和Al在高温下主要以Al_2O_3(s)和Fe_2O_3(s)存在,主要气态产物是HCl和SO_2.调理剂CaO和FeCl_3共存且含量增加时,焚烧过程中HCl(g)逸出过程变慢,并且Fe_2O_3(s)易于和CaO(s)作用导致Fe_2O_3(s)存在温度区间变窄.污泥未添加调理剂焚烧过程中,重金属Cd、Pb、Cr、Tl、Hg、As易于以Cd(OH)_2(g)、PbO(g)、CrO_2(OH)_2(g)、AsO(g)、Hg(g)、Tl(g)形式挥发,而Cu、Zn、Ni以其氧化物形式存在而难挥发.调理剂FeCl_3及CaO添加到污泥后,重金属(Cd、Pb、Cu、Zn、Ni、Hg、Tl)的分布受FeCl3的影响大于CaO,导致其形成重金属氯化物而挥发,而Cr和As与CaO作用生成CaCr_2O_4(s)、Ca_3(AsO_4)_2(s)热稳定化学物,抑制了Cr、As的挥发.当脱水污泥与含硫化物掺烧时,重金属(Pb、Cu、Zn、Ni)挥发受到Cl的影响大于S,在低温区间其主要以硫酸盐为主,在中高温区间以氯化物存在,而Hg和Tl不受硫化物影响.

Thermal conversion of conditioner and its effect on the volatilization of heavy metals during the dewatering sludge incineration

Thermodynamic equilibrium calculations were performed to reveal the partitioning of the conditioners and their emissions during sewage sludge incineration. Actual compositions of sewage sludge and various combustion conditions were applied in simulations. Effects of co-existence of FeCl₃ and CaO on the migration of heavy metals were also considered. Results showed that Fe and Al were in the form of Al₂O₃(s) and Fe₂O₃(s) respectively at high temperature after single conditioners (Al₂(SO₄)₃, Fe₂(SO₄) ₃, FeSO₄, AlCl₃ and FeCl₃) mixed with the sludge. The major gaseous products were HCl(g) and SO₂(g). The emission concentration of HCl(g) decreased when CaO and FeCl₃ were added to sludge simultaneously and the formation temperature range of Fe₂O₃(s) was suppressed owing to the reaction of Fe₂O₃(s) and CaO(s). Without addition of conditioners, heavy metals were easy to volatilize in the forms of Cd(OH)₂(g), PbO(g), CrO₂(OH)₂(g), AsO(g), Hg(g) and Tl(g), and it was notably that Cu, Zn and Ni oxides were difficult to volatilize. When FeCl₃ and CaO mixed with the sludge, the distribution of heavy metals (Cd, Pb, Cu, Zn, Ni, Hg, and Tl) was more significantly influenced by FeCl₃ than CaO. Metals (Cd, Pb, Cu, Zn, Ni, Hg and Tl) formed as chlorides were easy to volatilize, but volatilizations of Cr and As were inhibited because of generation of CaCr₂O₄(s) and Ca₃(AsO₄)₂(s). Additionally, heavy metals (Cu, Pb, Ni and Zn) were affected by Cl than S when the sludge was mixed with sulfide, FeCl₃ and CaO. In that case, metal sulfates mainly existed at low temperature, but metal chlorides dominated in the mid-to-high temperature ranges. Particularly, metals such as Hg and Tl did not react with S.