Analysis of trace metal elements and chemical forms from waste combustion and its emissions control

点击翻译按钮获取中文摘要

Sewage sludge, pulverized coal and the mixture of them were subjected to combustion in a laboratory-scale drop tube furnace to investigate the emission of suspended particulate matter smaller than 10μm (PM_<10>) and to study the correlation of PM_<10> emission with inorganic species of them. Combustion conditions of 1200 to 1450℃ and 0 to 50% atmospheric oxygen concentration were used and all the carbon was consumed under given conditions. The exiting gas, containing the solid products, was initially quenched with N_2 and collected by a water-cooling probe. Subsequently, coarse ash particles were collected by a cyclone. The suspension of ultrafine particles was further diluted with air, and immediately directed to a Low-Pressure-Impactor (LPI) for size-segregated collection. The LPI used here is composed of 13 stages having aerodynamic cut-off diameters ranging from 0.03 to 12.1 μm. Each stage is composed of a quartz or Teflon filter above a substrate and a substrate holder. All the fi … More lters were analyzed by XRF, XRD, Computer controlled scanning electron microscopy (CCSEM), ICP-AES, XPS and TEM to quantify the elemental composition and chemical forms. The properties of PM_<10> were studied including its concentration, particle size distribution, elemental composition and chemical forms.The results indicated that, both the ash content in the fuels and the oxygen content in gas atmosphere affected PM_<10> concentration significantly. With their increasing, PM_<10> concentration was enhanced exponentially. PM_<10> was formed having a lognormal non-gaussian size distribution. Vaporization of the inherent metals plays the paramount role on PM_<10> formation. The inherent alkali, heavy metals and a portion of Si, P, and Fe initially vaporize the resultant metallic vapors undergo oxidization to form their nuclei around 0.03μm, which subsequently undergo coagulation according to the polydisperse coagulation model. Simultaneously, the heterogeneous coagulation between the different species allowed the formation of new chemical species such as alkali/zinc phosphate and sulfates. In addition, a small amount of particles larger than 1.0μm were also found, which was dominated by the refractory metals resulted from the direct transferring of inherent minerals in the fuels. PM_<1+> was formed by the direct transformation of refractory elements in the fuels. Quartz and aluminosilicates within this portion were formed by transformation without phase change. Meanwhile, calcium/iron aluminosilicate in PM_<1+> was formed by the reaction of calcium or iron with aluminosilicate, which led to a sticky surface of the latter compound. Alkali aluminosilicate was formed as melt droplets in combustion and it condensed into large particulates in PM_<1+>. PM_1 is rich in both sulfates and phosphates of vaporized elements. The degree of vaporization of elements is determined by their content (organically bound and included minerals smaller than 10.0 μm) in the raw fuels. The elemental type also affects their vaporization greatly, which decreases in the order of Na > K > Fe > Mg > Ca > Si > Al. This is fairly consistent with the predictions of thermodynamic equilibrium calculations. The vaporized metals reacted with gaseous S0_2, P_2O_5 and SiO to form their compounds. Less