Characterization of residual carbon influencing on de novo synthesis of PCDD/Fs in MSWI fly ash

Study of the memory effect of PCDD/F during the combustion of several biomasses in a moving grate boiler

The objective of this work was to study the influence of temperature on the PCDD/Fs behavior stored in biomass ashes issued from an automatic pilot-scale wood boiler. A mixture of these ashes with a dioxin mass ratio dry based of 5.9 μg kg^-1 was used for thermal treatment at temperatures ranging between 200 and 500°C. First tests were carried out in a macro thermogravimetric analysis instrument while a couple of other tests were carried out in a quartz-lined crossed fixed-bed reactor. These treated ashes were afterwards analyzed and their dioxin mass ratio was determined. Results obtained from these both experiments are radically different: ashes treated in macro-TGA contain 18 times more PCDD/Fs than the initial non-treated ashes while those from the reactor have 84% less of PCDD/Fs. Ninety-one percent of stored PCDD/Fs in the initial ashes were eliminated after 1h at 400°C in crossed fixed-bed reactor. Macro-TGA experiments are a representation of what happens during the shutdown and cooling phase of the boiler, some areas reaching temperatures for which the formation of PCDD/Fs is optimal. Without cleaning the boiler, a memory effect on further tests is observed.

Inhibition on de novo synthesis of PCDD/Fs by an N-P-containing compound: Carbon gasification and kinetics

In this study, an N-P-containing compound (ammonium dihydrogen phosphate (ADP)) and an auxiliary material (CaO) were used to inhibit the formation of polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs). ADP significantly inhibited the formation of PCDD/Fs by the inhibition efficiencies of 98.45% for total concentration and 96.55% for toxic concentration. ADP was the best single inhibitor on toxic PCDFs (96.55%), and the inhibition on toxic PCDDs improved after ADP (90.91%) coupled with CaO (95.69%). In the temperature range of 300-350 °C, ADP inhibited the carbon gasification by reducing CO₂ and CO (400%-500% (20 K/min)), which could attributed to the formation of Cu₂P₂O₇ and copper nitrides from the Cu deactivation by P and N, respectively. However, the synergy of ADP and CaO decreased CO and CO₂ by 200%-300% (20 K/min), because CaO could promote carbon gasification. In addition, the apparent activation energy (Ea) increased from 78.50 kJ/mol to 102.04 kJ/mol with the addition of ADP but decreased to 73.92 kJ/mol after adding ADP and CaO. These results revealed that one inhibition route of de novo synthesis was the inhibition of carbon gasification by ADP, while CaO mainly inhibited de novo synthesis via the consumption of HCl and Cl₂. Furthermore, a reaction mechanism function in model fly ash was built as f(α)=2α^-1/2/3, which included carbon gasification and de novo synthesis. The results pave the way for further research on the inhibition kinetics of PCDD/F and development of other inhibitors.

Variations of PCDD/Fs emissions from secondary nonferrous smelting plants and towards to their source emission reduction

Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) are cancerogenic organic pollutants that priority controlled by Stockholm Convention with globally 183 signatories now. Secondary nonferrous smelting plants are confirmed to be important sources in China due to its large industrial activities and high emissions of PCDD/Fs. It is important to prioritize source to achieve source emission reduction by conducting field monitoring on typical case plants. Here, the emission profiles and levels of PCDD/Fs were investigated in 25 stack gas samples collected from three secondary copper production (SeCu), two secondary zinc production (SeZn) and two secondary lead production (SePb). Both average mass concentration and toxic equivalency quantity (TEQ) concentrations of PCDD/Fs all generally decreased in the order: SeCu > SeZn > SePb. It is noteworthy that the mean TEQ concentration in stack gas from SeCu with oxygen-enrich melting furnace technology, at 2.7 ng I-TEQ/Nm³, was much higher than the concentrations of other smelting processes. The average emission factors and annual release amounts of PCDD/Fs from SeCu, SePb and SeZn investigated were 28.4, 1.5, 10.4 μg I-TEQ/t and 1.03, 0.023, 0.17 g I-TEQ/year, respectively. The ratios of 2,3,7,8-TCDF to 1,2,3,7,8-PeCDF and OCDD to 1,2,3,7,8,9-HxCDD varied to large extent for three metal smelting, which could be used as diagnostic ratios of tracing specific PCDD/Fs sources. Addition of copper-containing sludge into the raw materials might lead to higher PCDD/Fs emissions. It is important to emphasize and reduce the PCDD/Fs emissions from oxygen-enrich melting furnace from secondary copper productions.

Thermochemical formation of polychlorinated dibenzo-p-dioxins and dibenzofurans on the fly ash matrix from metal smelting sources

Metal smelting processes are important sources of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). The present work aims to clarify the formation characteristics of PCDD/Fs by heterogeneous mechanisms on fly ash from typical multiple secondary aluminum (SAl), secondary lead (SPb) smelting, and iron ore sintering (SNT) sources in China. The formation characteristics of PCDD/Fs on fly ash were studied in the temperature range 250-450 °C for 10-150 min. Substantial thermochemical formation of PCDD/Fs on SAl and SNT ash was observed. The maximum increase of PCDD/F concentrations under 350 °C for 30 min was 604 times greater than the initial concentration in SAl ash. The concentration of PCDD/Fs was 77 times greater than that of SNT fly ash under 350 °C for 30 min. However, the maximum increase of PCDD/F concentrations was less than 8 times that in raw SPb ash under 350 °C. Contents of total organic carbon (TOC), Cu, Al, Zn and Cl, which are widely recognized as important elements for promoting PCDD/F formation, were obviously higher in SAl and SNT ash than in SPb ash. This may explain the greater observed formation times of PCDD/Fs on SAl and SNT ash than that on SPb ash. It was found that several congeners tended to form at higher temperatures than those for SAl ash. Activation energy calculation according to the Arrhenius equations could explain the dominant formation of those congeners at much higher temperatures on SAl ash.

Source contributions to total concentrations and carcinogenic potencies of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) in ambient air: a case study in Suzhou City, China

The potential source categories and source contributions of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) in ambient air from Suzhou City, China, were performed by principal component analysis-multiple linear regression (PCA-MLR) and positive matrix factorization (PMF). The carcinogenic potencies of PCDD/Fs were quantitatively apportioned based on the positive matrix factorization-toxic equivalent concentration (PMF-TEQ) method. The results of the present study were summarized as follows. (1) The total concentrations and toxic equivalent concentrations of PCDD/Fs (∑PCDD/Fs and TEQ) in ambient air from Suzhou City were 1.34-42.80 pg N m^-3 and 0.081-1.22 pg I-TEQ N m^-3, respectively. (2) PCA-MLR suggested that industrial combustion (IC), electric arc furnaces (EAFs) and secondary aluminum smelters (ALSs), unleaded gas-fueled vehicle sources (UGFVs), ALSs, and hazardous solid waste incinerators (HSWIs) could be the primary PCDD/F contributors, accounting for 13.2, 16.7, 35.5, 19.4, and 15.2% of ∑PCDD/Fs, respectively. (3) PMF and PMF-TEQ indicated that EAFs (carbon steel), UGFVs, IC, ALSs, municipal solid waste incinerators (MSWIs) and hospital waste incinerators (HWIs), and HSWIs contributed 10.9, 10.9, 42.8, 11.3, 10.7, and 13.4% to ∑PCDD/Fs, but contributed 8.3, 12.3, 50.3, 12.7, 6.0, and 10.4% to carcinogenic potencies of PCDD/Fs. This study was the first attempt to quantitatively apportion the source-specific carcinogenic potencies of PCDD/Fs in ambient air.

PCDD/Fs distribution characteristics and health risk assessment in fly ash discharged from MSWIs in China

This study provided distribution and health risk information of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in fly ash from 4 municipal solid waste incinerators (MSWIs) in four seasons from four sites, including Zhengzhou City in Henan Province, Chuzhou City in Anhui Province, Jilin City in Jilin Province and Zibo City in Shandong Province. The toxic equivalent concentration (I-TEQ) values of PCDD/Fs ranged from 0.0707 to 0.7742ng I-TEQ/g, and no identical sequence occurred during four seasons in different sampling sites. The stabilization process might efficiently reduce the content and toxicology of PCDD/Fs in fly ash. The value of PCDD/PCDF in fly ash ranged from 0.145 to 0.787 after solidification. The characteristic index (DCI) of 2,3,4,7,8-P₅CDF was 0.803 with 6.6% under 95% probability for fly ash samples discharged from MSWIs. The 95th percentile carcinogenic risks (CRs) for onsite workers were lower than the threshold value (10^-5), suggesting that the cancer risk levels of PCDD/Fs in fly ash for onsite workers were acceptable. The 95th percentile non-carcinogenic risks (non-CRs) for onsite workers were lower than 1, suggesting no obvious non-carcinogenic effect was developed for onsite workers. This paper provide an overview information on the distribution of PCDD/Fs in fly ash during four seasons, and it could be used as an important fingerprint to distinguish the fly ash sources. Thus, the research could provide basic information for fly ash management in environment.

Composition profiles, monthly changes and health risk of PCDD/F in fly ash discharged from a municipal solid waste incinerator (MSWI) in Northeast China

Monthly changes and health risk of PCDD/Fs in fly ash which was one of hazardous wastes from municipal solid waste incinerator (MSWI) were studied.

In-filter PCDF and PCDD formation at low temperature during MSWI combustion

This case study investigated PCDF and PCDD emissions from a 65 MW waste-to-energy plant to identify why an air pollution control system remodeling to accommodate increased production resulted in increased TEQ concentrations. Pre- and post-filter gases were collected simultaneously in four sample sets with varying filter temperatures and with/without activated carbon injection. Samples were analyzed to determine total PCDF and PCDD concentrations, as well as homologue profiles, and concentrations of individual congeners (some remained co-eluted). The total post filter PCDD concentrations where found to increase while the concentrations of PCDF and 2,3,7,8-substituted congeners declined. An investigation of the individual congener concentrations revealed that the increase of PCDD concentrations were due to a few congeners, suggesting a single formation route. The study also concludes that vital information about the formation could be obtained by not restricting the analysis to just the 2,3,7,8-substituted congeners.

Removal of carbon constituents from hospital solid waste incinerator fly ash by column flotation

Hospital solid waste incinerator (HSWI) fly ash contains a large number of carbon constituents including powder activated carbon and unburned carbon, which are the major source of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in fly ash. Therefore, the removal of carbon constituents could reduce PCDD/Fs in fly ash greatly. In this study, the effects of the main flotation parameters on the removal of carbon constituents were investigated, and the characteristics of the final product were evaluated. The results showed that loss on ignition (LOI) of fly ash increased from 11.1% to 31.6% during conditioning process. By optimizing the flotation parameters at slurry concentration 0.05 kg/l, kerosene dosage 12 kg/t, frother dosage 3 kg/t and air flow rate 0.06 m(3)/h, 92.7% of the carbon constituents were removed from the raw fly ash. Under these conditions, the froth product has LOI of 56.35% and calorific values of 12.5 MJ/kg, LOI in the tailings was below 5%, and the total toxic equivalent (TEQ) of PCDD/Fs decreased from 5.61 ng-TEQ/g in the raw fly ash to 1.47 ng-TEQ/g in the tailings. The results show that column flotation is a potential technology for simultaneous separation of carbon constituents and PCDD/Fs from HSWI fly ash.

Dioxins and their fingerprint in size-classified fly ash fractions from municipal solid waste incinerators in China--mechanical grate and fluidized bed units

The distribution of polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs), in brief dioxins, has seldom been addressed systematically in fly ash from municipal solid waste incinerators (MSWIs). This study shows the amount and fingerprint of PCDD/Fs in fly ash from four different Chinese MSWIs, that is, three mechanical grate units and one circulating fluidized bed unit. In these fly ash samples, dioxins-related parameters (international toxic equivalent quantity, total amount of PCDD/Fs, individual isomer classes, and 17 toxic 2,3,7,8-substituted congeners) all tend to increase with decreasing particle size for mechanical grate incinerators, yet only for the finest fraction for fluidized bed units. Moreover, the fluidized bed incinerator seems superior to grate incineration in controlling dioxins, yet a comparison is hampered by internal differences in the sample, for example, the fluidized bed fly ash has much lower carbon and chlorine contents. In addition, the presence of sulfur from mixing coal as supplemental fuel to the MSW may poison the catalytic steps in dioxins formation and thus suppress the formation of dioxins. With more residual carbon and chlorine in the fly ash, it is easier to form dioxins during cooling. Nevertheless, there is no apparent relation between Fe, Cu, and Zn contents and that of dioxins in fly ash.

The reduction of dioxin emissions from the processes of heat and power generation

The first reports that it is possible to emit dioxins from the heat and power generation sector are from the beginning of the 1980s. Detailed research proved that the emission of dioxins might occur during combustion of hard coal, brown coal, and furnace oil as well as coke-oven gas. The emission of dioxins occurs in wood incineration; wood that is clean and understood as biomass; or, in particular, wood waste (polluted). This paper thoroughly discusses the mechanism of dioxin formation in thermal processes, first and foremost in combustion processes. The parameters influencing the quantity of dioxins formed and the dependence of their quantity on the conditions of combustion are highlighted. Furthermore, the methods of reducing dioxin emissions from combustion processes (primary and secondary) are discussed. The most efficacious methods that may find application in the heat and power generation sector are proposed; this is relevant from the point of view of the implementation of the Stockholm Convention resolutions in Poland with regard to persistent organic pollutants.

Effect of temperature and oxygen on the formation of chlorobenzene as the indicator of PCDD/Fs

Sampling and analysis of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) are both time-consuming and expensive. In principle, real-time monitoring of chlorobenzene (CBz) as an indicator for PCDD/Fs could be useful to control and optimize incinerator operating conditions. To test this strategy, CBz was analyzed together with PCDD/Fs in flue gas samples collected from a hospital waste incinerator. Moreover, lab experiments were conducted to investigate the effect of temperature and oxygen on CBz formation from fly ash from the same incinerator. The experimental results demonstrate that chlorobenzene (in particular PeCBz) correlate well with PCDD/Fs, in line with previous research. The optimum temperature of CBz formation is in a range of 350 to 400 degrees C and CBz yield increases significantly with oxygen, in line with PCDD/Fs formation. This study is useful for confirming the de novo mechanism and defining correlations between CBz and PCDD/Fs.