Zhiliang Chen

Dioxins degradation and reformation during mechanochemical treatment

Mechanochemical dechlorination and destruction of polychlorinated dibenzo-p-dioxins and -furans (PCDD/F) on fly ash from Municipal Solid Waste Incineration was tested with and without additives (CaO and CaO/aluminium powder). The first results disappointed because of obvious PCDD/F-reformation and a second test series was conducted after removing soluble salts (NaCl, KCl …) by thorough two-stage water washing. This second test series was successful and demonstrated good destruction results, especially with combined CaO/aluminium powder as additive. In a third test series salt was again added to the water-washed fly ash, and the first, poor results were largely reconstituted. For all tests a fairly complete (94 out of 136 congeners) isomer-specific analysis was conducted and analysed, allowing to differentiate between, e.g., 2,3,7,8-substituted PCDD/F and congeners formed following the chlorophenol route. The first became more important in the samples series Fly Ash, Milled Fly Ash, milling with added CaO, and milling with CaO/aluminium-addition. The second follow the opposite trend. This isomer-specific analysis will form the basis for further study using Principal Component Analysis.

Municipal solid waste incineration in China and the issue of acidification: A review:

In China, incineration is essential for reducing the volume of municipal solid waste arising in its numerous megacities. The evolution of incinerator capacity has been huge, yet it creates strong opposition from a small, but vocal part of the population. The characteristics of Chinese municipal solid waste are analysed and data presented on its calorific value and composition. These are not so favourable for incineration, since the sustained use of auxiliary fuel is necessary for ensuring adequate combustion temperatures. Also, the emission standard for acid gases is more lenient in China than in the European Union, so special attention should be paid to the issue of acidification arising from flue gas. Next, the techniques used in flue gas cleaning in China are reviewed and the acidification potential by cleaned flue gas is estimated. Still, acidification induced by municipal solid waste incinerators remains marginal compared with the effects of coal-fired power plants.

Energy transfer and kinetics in mechanochemistry

Mechanochemistry (MC) exerts extraordinary degradation and decomposition effects on many chlorinated, brominated, and even fluorinated persistent organic pollutants (POPs). However, its application is still limited by inadequate study of its reaction kinetic aspects. In the present work, the ball motion and energy transfer in planetary ball mill are investigated in some detail. Almost all milling parameters are summarised in a single factor—total effective impact energy. Furthermore, the MC kinetic between calcium oxide/Al and hexachlorobenzene is well established and modelled. The results indicate that total effective impact energy and reagent ratio are the two factors sufficient for describing the MC degradation degree of POPs. The reaction rate constant only depends on the chemical properties of reactants, so it could be used as an important index to appraise the quality of MC additives. This model successfully predicts the reaction rate for different operating conditions, indicating that it could be suitably applied for conducting MC reactions in other reactors.

Mechanochemical treatment of hexachlorobenzene: effect of additives

Mechanochemical (MC) methods have shown an excellent performance while decomposing a number of halogenated persistent organic pollutants (POPs). In this study, a number of MC degradation tests were performed on hexachlorobenzene (HCBz), using an all-dimensional planetary ball mill and applying selected additives, including CaO, CaO-SiO2, CaO-Al2O3, CaO-Al and Fe-SiO2. After 8 h of milling, up to 99.7% of the original HCBz was removed by processing with a combined CaO-Al additive. Different ratios of Al to CaO were compared and the highest (99.70%) destruction efficiency of HCBz was achieved after 3 h by the additive CaO-Al (11.1% added), considered to be the most practical pair. Sequential dechlorination of HCBz and polymerisation of the resulting residual benzene should be the main degradation pathway; this conclusion is reached by observing lower chlorinated intermediates and graphite and amorphous carbon as final products. Strong radical signals and micro-spheres as well as tribo-microplasma phenomena were induced by severe mechanical ball milling.

Influence on gaseous pollutants emissions and fly ash characteristics from co-combustion of municipal solid waste and coal by a drop tube furnace

Co-combustion experiments of municipal solid waste and coal were carried out in a drop tube furnace at high temperature 1300 °C. The effect of different simulated municipal solid waste (SMSW) added proportion (0, 7.5, 15, 20 and 25 wt%) in the blend fuels on the characteristics of gaseous pollutants emissions, e.g. CO, HCl, SO2, NOx, and heavy metals, and fly ash were studied. The results indicated that CO and CH4 emission concentrations were at a low level under all conditions. With the increasing proportion of SMSW, the combustion efficiency decreased slightly, the HCl emission increased obviously at 25% conditions while at lower proportion conditions the change was not significant; the NOx emission concentration showed a tendency to rise first and then decrease, while the SO2 showed an exactly opposite trend; besides, Fe, Cl and S content in the fly ash increased obviously. Under all experimental conditions, only a small amount of heavy metals were emitted in the flue gas while most of the heavy metals were retained in the fly ash. The leaching results showed that Ni leaching concentration was beyond the national standard which means the fly ash needs further treatment before they can be disposed of by landfill, while the HCl, NOx and SO2 emissions can easily reach the national emission standard under the desulfurization and denitrification system operating conditions in real power plants. These findings are helpful for the further development of co-combustion with renewable energy in coal-fired incinerators, yet more investigation on heavy metal emission in fly ash is still required to be further conducted in the future.

Formation pathways of PCDD/Fs during the Co-combustion of municipal solid waste and coal

The co-combustion of simulated municipal solid waste (SMSW) and the coal in a drop-tube furnace is studied in five test cases. The concentration and signature evolution of polychlorinated dibenzo-p-dioxins (PCDD) and -furans (PCDF) in both flue gases and fly ashes are monitored at the level of individual congeners, using statistical methods. Special attention is paid to chlorophenol (CP)-route congeners, 2,3,7,8-substitution, and 1,9-substitution, to reveal the formation pathways of PCDD/Fs and the interaction between SMSW and coal. It is identified that the increase of SMSW proportion alters the major formation pathways from CP-route to chlorophenols/chlorobenzenes condensation and de novo synthesis. The coal-induced carbon enhances the adsorption capacity of fly ash particles for PCDD/Fs, yet facilitates the generation of carbon matrixes and polycyclic aromatic hydrocarbons, both of which will significantly boost the de novo synthesis with the increase of SMSW-induced chlorine and catalytic metals. Further investigations about restricting the participation of chlorine in PCDD/Fs synthesis are essential to increase the treatment capacity of MSW and to reduce the PCDD/Fs emission.