Transformation Of Heavy Metals During Sewage Sludge Incineration

It is estimated that the average production of dehydrated sewage sludge is around 0.03–0.03 kg every ton of waste water. The total production of sludge in 2003 is 4150 million ton in China. Sewage sludge treatment has become a major issue because sewage sludge contains a lot of organic matters, heavy metals as well as pathogenic or disease-causing micro-organisms. The conventional options for sewage sludge disposal include agricultural utilization, landfill and thermal disposal. However, due to their disadvantages, agricultural utilization and landfill are being progressively restricted. The thermal process offers many advantages such as volume reduction, destruction of organic micropollutants, energy recovery. As a consequence, it has become one of the major technologies for the treatment of sewage sludge. However, sewage sludge incineration would produce many second pollutions such as dioxin, NOx, SO2 and heavy metals, most of the pollutants can be removed by air pollution control device (APCD), whereas, part of the heavy metals may enrich in fine particles and APCD is not very effective in collecting these heavy metals. These heavy metals would badly endanger human health and the environment. Hence, how to control the emission of heavy metals becomes the most serious problem during sewage sludge incineration process. The dissertation studies the combustion characteristics of the sewage sludge, vaporization and condensation of heavy metals during sewage sludge incineration. Both simulation and experimental are conducted to help us understand the behavior of heavy metals during sewage sludge incineration. The results, which listed below, may provide information of the control or recover of heavy metals during sewage sludge incineration. 1 . The combustion characteristics of sewage sludge was studied by a thermogravimetry analyzer combined with thermogravimetry (TG) and differential scanning calorimetry (DSC). Thermogravimetric analysis demonstrated that the incineration process of sewage sludge could be divided into five stages: drying, low temperature decomposition, intermediate temperature decomposition, high temperature decomposition and tar combustion. The reaction mechanism and the parameters of reaction kinetics were studied for each stage in the incineration process. The effects of heating rate on sewage sludge incineration were studied and the results showed that the decomposition temperature increases with the heating rate. Correspondingly, the emission characteristics of heavy metals in the process of sewage sludge incineration were also investigated. At the same time, it was found that the vaporization of heavy metals has no directly relation with the heating rate based on the experimental results. 2.In order to further understand the combustion mechanism of sewage sludge, experiments of sewage sludge incineration were carried out on a bench scale combustor. The fly ash and slug were analyzed by SEM, EDX and XRD. The results disclosure the relation of the size of the slug and time is following: Y =-0.0002X2 -0.1313X + 49.64. At the same time, the transformation of heavy metals during sewage sludge incineration was studied. The distribution of heavy metals in fly ash and the flue gas were investigated at different oxygen concentrations. It was observed that oxygen has a negative effect on heavy metals vaporization. 3.The feasibility study of recovery heavy metals during sewage sludge incineration were conducted on a pilot plant combustor. It was found that the heavy metals could be divided into four groups based on the recovery temperature: group A, which would vaporize during sewage sludge and convert into solid phase above 600℃, includes Zn,Cd and P; The conversion temperature of group B, including Pb and Cu, is above 400℃; Na and Mg are among group C, the capture temperature is lower than 300℃; As for group D, we only found As, with removal temperature between 300℃and 400℃. Therefore, we can recover different heavy metals at different temperatures. In this study, A recovery of 51% Cr, 74% Pb, 62% Cu at 600℃and 70% Zn at 400℃was reached. is well known that chloride has much lower melting and boiling point. We injected calcium chloride with different ratios into the combustor and found that the recovery efficiency of heavy metals increased with the increase of the calcium chloride ratio. However, the conclusion is not fit for all of the heavy metals. 4.A new chemical thermodynamical model, in which the reactions between heavymetals, heavy metals and mineral matters, heavy metals and the composition of flue gas were considered, was established. The model was successfully used to predict the vaporization of heavy metals at different incineration conditions, the condensation of heavy metals in the flue gas, the reactions between heavy metals and other compositions, and the distribution of heavy metals.