• Energy Research
• 11. Sustainability close

Laboratory experiments were conducted to investigate the volatilization behavior of heavy metals during pyrolysis and combustion of municipal solid waste (MSW) components at different heating rates and temperatures. The waste fractions comprised waste paper (Paper), disposable chopstick (DC), garbage bag (GB), PVC plastic (PVC), and waste tire (Tire). Generally, the release trend of heavy metals from all MSW fractions in rapid-heating combustion was superior to that in low-heating combustion. Due to the different characteristics of MSW fractions, the behavior of heavy metals varied. Cd exhibited higher volatility than the rest of heavy metals. For Paper, DC, and PVC, the vaporization of Cd can reach as high as 75% at 500 °C in the rapid-heating combustion due to violent combustion, whereas a gradual increase was observed for Tire and GB. Zn and Pb showed a moderate volatilization in rapid-heating combustion, but their volatilities were depressed in slow-heating combustion. During thermal treatment, the additives such as kaolin and calcium can react or adsorb Pb and Zn forming stable metal compounds, thus decreasing their volatilities. The formation of stable compounds can be strengthened in slow-heating combustion. The volatility of Cu was comparatively low in both high and slow-heating combustion partially due to the existence of Al, Si, or Fe in residuals. Generally, in the reducing atmosphere, the volatility of Cd, Pb, and Zn was accelerated for Paper, DC, GB, and Tire due to the formation of elemental metal vapor. TG analysis also showed the reduction of metal oxides by chars forming elemental metal vapor. Cu2S was the dominant Cu species in reducing atmosphere below 900 °C, which was responsible for the low volatility of Cu. The addition of PVC in wastes may enhance the release of heavy metals, while GB and Tire may play an opposite effect. In controlling heavy metal emission, aluminosilicate- and calcium-based sorbents can be co-treated with fuels. Moreover, pyrolysis can be a better choice for treatment of solid waster in terms of controlling heavy metals. PVC and Tire should be separated and treated individually due to high possibility of heavy metal emission. This information may then serve as a guideline for the design of the subsequent gas cleaning plant, necessary to reduce the final emissions to the atmosphere to an acceptable level.

Abstract Sewage sludge, the inevitable by-product of municipal wastewater treatment plant operation, is a key issue in many countries due to its increasing volume and the impacts associated with its disposal. Thermochemical processing offers a new way of managing sewage sludge, not only by providing effective volume reduction, but also enabling transformation of carbon-rich organic fraction into valuable energy and fuel. Owing to some unique properties, sewage sludge differs from other solid fuels such as lignocellulosic biomass and coal, making its thermochemical conversion application somewhat complicated and challenging. This paper reviews the options of converting sewage sludge to energy and fuel via three main thermochemical conversion processes namely pyrolysis, gasification and combustion. The fundamental aspects of sewage sludge and its behaviour in each of thermochemical process are summarised. The challenges in adopting thermochemical conversion technology in sewage sludge management are addressed, and various alternative approaches deserving further consideration, such as the incorporation of pre-processing and co-utilisation, are discussed.

Abstract Substituted polycyclic aromatic hydrocarbons released from sewage sludge pyrolysis bring the concern of the clean utilization for the waste. Herein, the roles of calcium oxide on the release of these compounds, especially the heavy ones containing nitrogen and oxygen atoms were investigated. The results showed that calcium oxide significantly decreased light substituted polycyclic aromatic hydrocarbons in the volatiles from 450 to 850 °C and showed obvious inhibitory effects on the formation of heavy substituted polycyclic aromatic hydrocarbons at 850 °C. To understand the impact mechanisms, pyrolysis experiments of sludge model compounds lipid, protein and polysaccharides were conducted. It was found that calcium oxide could catalyze or react with all three components to decrease the content of light substituted polycyclic aromatic hydrocarbons. Heavy oxygen-containing aromatics in sludge derived volatiles mainly resulted from polysaccharides and protein, and they considerably decreased after mixing with calcium oxide. Oxygen-containing aromatics content in the lipid derived volatiles, however, would increase due to polymerization reactions. The decreasing release of nitrogen substituted polycyclic aromatic hydrocarbons from sludge could be attributed to the synergy effects of CaO and other minerals or the effects of CaO on the interaction between protein and other organics.