Xiaoqing Lin

Inhibition of the de novo synthesis of PCDD/Fs on model fly ash by sludge drying gases

Sludge drying gases (SDG), evolving from drying and mild thermal decomposition (<300°C) of raw sewage sludge contain NH3 and SO2 as well as other N- and S-compounds. All of these are potential PCDD/Fs suppressants. It is indeed observed that these SDG suppress 2,3,7,8-substitued PCDD/Fs formation on Model Fly Ash (MFA) with an efficiency up to 97.6% in wt. units and 96% in I-TEQ, respectively. This suppression is strong for (the bulk of) PCDD/Fs, adsorbed on the model fly ash; conversely, sludge drying gases enhance PCDD/Fs desorption from MFA. Moreover, TCDD/Fs are suppressed least, possibly following stepwise dechlorination of higher chlorinated PCDD/Fs. Characteristics, such as the type, origins and amount of sludge, its moisture-, nitrogen- and sulfur content and the nature of the thermal treatment applied are all expected to influence upon the suppression capabilities. In this study three types of dry sludge are tested and applied as suppressant in four different amounts or modes. The quality of the sludge drying gases is continuously monitored: the Gasmet results reveal that NH3 and SO2 are the most important components of SDG. The MFA reaction residue is scrutinized by Scanning Electron Microscope (SEM) and Energy Dispersive Spectrometer (EDS) analysis. A large number of particles attaching to the surface of model fly ash are observed by SEM. Moreover, EDS analysis reveals that part of the chlorine in MFA is carried away with the SDG and replaced by sulfur, so that eliminating chlorine may be part of the inhibition mechanism. However, further research is still needed to establish the optimum operating modes and to confirm the role of both inorganic and organic nitrogen and sulfur compounds in the suppression of PCDD/Fs formation on model fly ash.

Development of new transition metal oxide catalysts for the destruction of PCDD/Fs.

Various transition metal oxide and vanadium-containing multi-metallic oxide catalysts were developed for the destruction of PCDD/Fs (polychlorinated dibenzo-p-dioxins and furans). A stable PCDD/Fs generating system was installed to support the catalytic destruction tests in this study. Nano-titania supported vanadium catalyst (VOx/TiO2) showed the highest activity, followed by CeOx, MnOx, WOx and finally MoOx. Multi-metallic oxide catalysts, prepared by doping WOx, MoOx, MnOx and CeOx into VOx/TiO2 catalysts, showed different activities on the decomposition of PCDD/Fs. The highest destruction efficiency of 92.5% was observed from the destruction test over VOxCeOx/TiO2 catalyst. However, the addition of WOx and MoOx even played a negative role in multi-metallic VOx/TiO2 catalysts. Characterizations of transition metal oxides and multi-metallic VOx/TiO2 catalysts were also investigated with XRD and TPR. After the catalysts were used, the conversion from high valent metals to low valence states was observed by XPS.

Simultaneous suppression of PCDD/F and NOx during municipal solid waste incineration

Thiourea was tested as a dioxins inhibitor in a full-scale municipal solid waste incinerator with high capacity (34 t h(-1)). The suppressant, featuring a high S- and N-content, was converted into liquor and then injected (35 kg h(-1)) into the furnace (850 °C) through the inlets already used for Selective Non-Catalytic Reduction (SNCR) of flue gas NOx. The first results show that thiourea reduces the dioxins in flue gas by 55.8 wt.%, those in fly ash by 90.3 wt.% and the total dioxins emission factor by 91.0 wt.%. The concentration of PCDD/Fs was 0.08 ng TEQ Nm(-3), below the national standard of 0.1 ng TEQ Nm(-3). The weight average chlorination degree of dioxins decreases slightly after adding the inhibitor, indicating that it suppresses both the formation and the chlorination of dioxins. Analysis of fly ash by scanning electron microscope (SEM) suggests that the particle size becomes larger after adding the inhibitor. Further analysis using an energy dispersive spectrometer (EDS) reveals that the sulphur content in fly ash rises, but the chlorine content declines when adding thiourea. These results suggest that poisoning the metal catalyst and blocking the chlorination are probably responsible for suppression. NOx reduction attains 42.6 wt.%. These tests are paving the way for further industrial application and assist in controlling the future emissions of dioxins and NOx from MSWI.

Suppression of dioxins after the post-combustion zone of MSWIs

Thiourea was selected as representative of combined S- and N-inhibitors and injected after the post-combustion zone of two full-scale municipal solid waste incinerators (MSWIs) using a dedicated feeder. Firstly, the operating conditions were scrutinised by monitoring the concentrations of SO2, NH3 and HCl in the clean flue gas. The suppression experiment showed that in MSWI A thiourea could reduce the total I-TEQ value in flue gas by 73.4% from 1.41ng I-TEQ/Nm(3) to 0.37I-TEQ/Nm(3), those in fly ash by 87.1% from 14.3ng I-TEQ/g to 1.84I-TEQ/g and the total dioxins emission factor by 87.0wt.%, with a (S+N)/Cl molar ratio of 9.4. The suppression efficiencies of PCDD/Fs in flue gas and fly ash in MSWI B could be up to 69.2% and 83.0% when the (S+N)/Cl molar ratio attained 7.51. Furthermore, the congener distributions of dioxins were also analysed in the flue gas and fly ash, before and after addition of thiourea, to find cues to some suppression mechanism. In addition, the filtered fly ash was explored by the Scanning Electron Microscope (SEM) and Energy Dispersive Spectrometer (EDS) analysis of fly ash. These results suggest that poisoning the metal catalyst and blocking the chlorination are most probably responsible for suppression.

Influence of organic and inorganic flocculants on the formation of PCDD/Fs during sewage sludge incineration

Flocculants are widely used to improve the properties of sludge dewatering in industrial wastewater treatment. However, there have been no studies conducted on the influence of flocculants on the formation of polychlorinated dibenzo-p-dioxin and dibenzofurans (PCDD/Fs) during sewage sludge incineration. This paper selected three typical kinds of flocculants, including polyacrylamide (PAM), poly-ferric chloride (PFC), and polyaluminum chloride (PAC) flocculant, to study their influences on the formation of PCDD/Fs during sewage sludge incineration. The results indicated that PAM flocculant, which is an organic flocculant, inhibited the formation of PCDD/Fs in sewage sludge incineration, while inorganic flocculant, such as PFC and PAC flocculant, promoted the formation. The most probable explanation is that the amino content in the PAM flocculant acted as an inhibitor in the formation of PCDD/Fs, while the chlorine content, especially the metal catalyst in the PFC and PAC flocculants, increased the formation rate. The addition of flocculants nearly did not change the distribution of PCDD/F homologues. The PCDFs contributed the most toxic equivalent (TEQ) value, especially 2, 3, 4, 7, 8-PeCDF. Therefore, the use of inorganic flocculants in industrial wastewater treatment should be further assessed and possibly needs to be strictly regulated if the sludge is incinerated. From this aspect, a priority to the use of organic flocculants should be given.

Removal of PCDD/Fs and PCBs from flue gas using a pilot gas cleaning system

A 100 Nm3/hr capacity pilot scale dual bag filter (DBF) system was tested on the flue gas from an actual hazardous waste incinerator (HWI), the removal efficiency of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) and polychlorinated biphenyls (PCBs) was also studied. The first filter collected most of the fly ash and associated chlorinated organic; then activated carbon (AC) was injected and used to collect phase chlorinated organic from the gas. Concentrations of PCDD/Fs and PCBs after the DBF system were 0.07 and 0.01 ng TEQ/Nm3, respectively, which were both far below the national emission standard. Comparing with the original single bag filter system, the PCDD/Fs concentration dropped a lot from 0.36 to 0.07 ng TEQ/Nm3. Increasing AC feeding rate enhanced their collection efficiency, yet reduced the AC utilization efficiency, and it still needs further study to select an appropriate feeding rate in the system. These results will be useful for industrial application and assist in controlling emissions of PCDD/Fs and other persistent organic pollutions from stationary sources in China.

Suppression of dioxins in waste incinerator emissions by recirculating SO2.

Sulphur is an effective inhibitor of the formation of Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzo-furans (PCDD/Fs), as was proven in laboratory and pilot plant studies. In this study, a pilot-scale system with capacity 300 N m(3) h(-1) was situated at the bypass of an actual hazardous waste incinerator (HWI) and tested to reduce the emission of PCDD/Fs. Activated carbon was used as a medium to adsorb SO2 from flue gas and release it again at the higher temperature of filtered ash detoxification to achieve SO2 circulation in the system. Most PCDD/Fs in the filtered ash are decomposed by thermal treatment. Experimental results indicate that the system is capable of stable operation with SO2 accumulation at a high level of concentration and a high reduction efficiency of PCDD/Fs. A reduction of more than 80% was already achieved without addition of other sulphur compounds. When pyrite (FeS2) was added the reduction of PCDD/Fs could reach 94%, with a residual PCDD/Fs concentration in the flue gas as low as 0.13 ng TEQ N m(-3). This SO2 recirculating and suppression technology potentially provides significant progress for dioxin emission control in waste incineration and could be useful for controlling emissions of PCDD/Fs and other chlorinated organic chemicals in China.

Levels and profiles of dioxins from circulating fluidised bed incineration

In China, minimising the emissions of polychlorinated dibenzo-pdioxins and dibenzofurans (PCDD/F) from municipal solid waste incineration (MSWI) remains full of challenges. This paper presents a comprehensive diagnosis of formation, destruction and removal of dioxins in MSWI, after finding out their main area of formation and the parameters influencing their level and signature. The PCDD/F loads significantly increase between furnace exit and the entrance of the air pollution control system (APCS), i.e., in the temperature range of 450 to 250°C. In addition, co-combustion of low-calorific municipal solid waste (MSW) with sulphur-containing coal provides pathways to reduce PCDD/F creation by improving combustion conditions, reducing precursors and unburned carbon, and suppressing PCDD/F formation by the SO2 liberated. Adequate activated carbon (AC) feeding systems and adsorption ability of AC are essential for removing dioxins.

Emission characteristics and relationships among PCDD/Fs, chlorobenzenes, chlorophenols and PAHs in the stack gas from two municipal solid waste incinerators in China

An extensive investigation was conducted to understand polychlorinated dibenzo-p-dioxin and furan (PCDD/F) formation mechanisms and their relationships with chlorobenzenes (CBzs), chlorophenols (CPhs) and polycyclic aromatic hydrocarbons (PAHs) in the stack gas from two fluidized bed municipal solid waste incinerators in China. The toxic equivalent quantity (TEQ) value and the concentration of target compounds changed with the incinerator operating conditions. CPhs and PAHs were much more sensitive to operation conditions and were affected more easily by change. Only 2-monochlorophenol revealed a negative linear correlation (R2 ≥ 0.7). More than half of the PAHs revealed an adequate correlation model with PCDD/F concentration (R2 > 0.6), while CBzs showed almost perfect correlations with PCDD/Fs (R2 ≥ 0.8, significance level α ≤ 0.05). 123-Trichlorobenzene, 1234-tetrachlorobenzene and pentachlorobenzene revealed the best positive linear correlation (R2 > 0.9). PCDFs were revealed to be the best target compounds for indication due to the similar formation variation trend to that of other precursors. Unary and multiple linear regression equations with high coefficients of determination between several CBz, PAHs and PCDD/Fs, TEQ and PCDFs were established. The detailed relationships among PCDD/F homologues, isomers and other compounds and their formation mechanism were also discussed.

High temperature suppression of dioxins.

Combined Sulphur-Nitrogen inhibitors, such as sewage sludge decomposition gases (SDG), thiourea and amidosulphonic acid have been observed to suppress the de novo synthesis of dioxins effectively. In this study, the inhibition of PCDD/Fs formation from model fly ash was investigated at unusually high temperatures (650 °C and 850 °C), well above the usual range of de novo tests (250-400 °C). At 650 °C it was found that SDG evolving from dried sewage sludge could suppress the formation of 2,3,7,8-substituted PCDD/Fs with high efficiency (90%), both in weight units and in I-TEQ units. Additionally, at 850 °C, three kinds of sulphur-amine or sulphur-ammonium compounds were tested to inhibit dioxins formation during laboratory-scale tests, simulating municipal solid waste incineration. The suppression efficiencies of PCDD/Fs formed through homogeneous gas phase reactions were all above 85% when 3 wt. % of thiourea (98.7%), aminosulphonic acid (96.0%) or ammonium thiosulphate (87.3%) was added. Differences in the ratio of PCDFs/PCDDs, in weight average chlorination level and in the congener distribution of the 17 toxic PCDD/Fs indicated that the three inhibitors tested followed distinct suppression pathways, possibly in relation to their different functional groups of nitrogen. Furthermore, thiourea reduced the (weight) average chlorinated level. In addition, the thermal decomposition of TUA was studied by means of thermogravimetry-fourier transform infrared spectroscopy (TG-FTIR) and the presence of SO2, SO3, NH3 and nitriles (N≡C bonds) was shown in the decomposition gases; these gaseous inhibitors might be the primary dioxins suppressants.