Migration and transformation of trace elements during sewage sludge and coal slime Co-combustion.

Co-combustion of sewage sludge (SS) and coal slime (CS) could improve the combustion properties of the two materials, however, high levels of trace elements (TEs) can be released from the two wastes, resulting in secondary pollution. The migration and transformation behavior of As, Cr, Pb, Zn, and Mn during co-combustion is explored in current research. The results showed co-combustion could inhibit the emission of Zn, As, Pb, and Mn, and the effect was more pronounced for Zn, As and Mn. Meanwhile, minerals like kaolinite and gypsum were found to generated in the ash from co-combustion but not solo-combustion. Model experiments demonstrated that kaolinite captured As, Pb and Mn, while gypsum captured Zn, As and Mn but facilitated the emission of Pb and Cr. This well explained the distinct TEs emission characteristics between co-combustion and solo combustion. As the temperature elevated, kaolinite in co-combustion ash decomposed and the generation of gypsum was promoted. In this way, the emission ratios of Zn, As, and Mn initially increased but subsequently decreased between 700 and 1300 °C, whereas Pb and Cr emission ratios increased by twofold within the same temperature range. Leaching characteristics and risk assessment code on co-combustion ashes were also conducted in this study. The results indicated a marginal elevation in the risk associated with trace elements (TEs) following co-combustion, provided that all five TEs remained within the limits of national standards.

Heavy tar evolution characteristics during advanced sludge pyrolysis and biomass gasification integrated process.

Sludge pyrolysis and biomass gasification integrated process (SPBG) is an attractive route for the comprehensive utilization of the two materials but more tar is produced in this process compared to traditional biomass steam gasification. Nitrogen-containing compounds in the tar bring threatens to the environment and heavy components in the tar contributes to undesired coke formation. In current study, the evolution of heavy tar, especially the nitrogen-rich components, during SPBG is revealed for the first time. It was found that heavy components were mainly distributed in the mass range of 150-450 Da, where aromatics consisted of carbon, hydrogen and nitrogen atoms were the most abundant. Deamination (NH3) and the combination of quinoline accompanied with the generation of the heavy components. Organics from sludge could react with biomass to form heavier oxygen-containing molecules. Meanwhile, steam from sludge promoted heavy components to crack by tar reforming reactions and consumed radicals in bio-char to inhibit the catalytic cracking of tar. Under the combination of above reactions, more heavy molecules were generated at low sludge volatile/biomass ratio and the aromatic content in the heavy tar decreased at high sludge volatile/biomass ratio.

Effects of reaction conditions on the emission behaviors of arsenic, cadmium and lead during sewage sludge pyrolysis.

Sewage sludge is an important class of bioresources whose energy content could be exploited using pyrolysis technology. However, some harmful trace elements in sewage sludge can escape easily to the gas phase during pyrolysis, increasing the potential of carcinogenic material emissions to the atmosphere. This study investigates emission characteristics of arsenic, cadmium and lead under different pyrolysis conditions for three different sewage sludge samples. The increased temperature (within 723-1123K) significantly promoted the cadmium and lead emissions, but its influence on arsenic emission was not pronounced. The releasing rate order of the three trace elements is volatile arsenic compounds>cadmium>lead in the beginning of pyrolysis. Fast heating rates promoted the emission of trace elements for the sludge containing the highest amount of ash, but exhibited an opposite effect for other studied samples. Overall, the high ash sludge released the least trace elements almost under all reaction conditions.

Inhibitory effects of CaO/Fe2O3 on arsenic emission during sewage sludge pyrolysis.

This work aimed to investigate effects and reaction mechanisms of CaO/Fe2O3 on emission behaviors of arsenic during sewage sludge pyrolysis. The results showed that 24.8-54.2%, 26.4-60.4% and 27.7-63.1% of arsenic escaped from three samples when pyrolysis process happened at 723, 923 and 1123K respectively. And the sludge which contained higher calcium and iron contents released less arsenic than others. External CaO and Fe2O3 were added into the sewage sludge to study their effects on arsenic emissions during pyrolysis, where both of them inhibited arsenic emission effectively, especially at high temperatures. With the help of thermogravimetry analysis and X-ray fluorescence, inhibitory mechanisms of CaO/Fe2O3 on arsenic emission during sewage sludge pyrolysis were studied. CaO could react with As2O3, As2S3 and NaAsO2 to form nonvolatile substances, such as Ca(AsO2)2; while Fe2O3 could react with NaAsO2 to generate certain substances which was stable below 1123K.