Effects of gaseous NH(3) and SO(2) on the concentration profiles of PCDD/F in flyash under post-combustion zone conditions

The influence of gaseous ammonia and sulphur dioxide on the formation of 2378-substituted PCDD/F on a reference flyash from a municipal waste incinerator has been investigated using a laboratory scale fixed-bed reactor. The reference flyash samples (BCR-490) was reacted under a simulated flue gas stream at temperatures of 225 and 375°C for 96h. The experiments were carried out in two series: first with simulated flue gas alone, and then with injection of NH(3) or SO(2) gas into the flue gas just before the reactor inlet. It was found that the injection of gaseous ammonia into the flue gas could decrease the concentration of both PCDD and PCDF by 34-75% from the solid phase and by 21-40% from the gas phase. Converting the results to I-TEQ values, it could reduce the total I-TEQ values of PCDD and PCDF in the sum of the flyash and exhaust flue gas by 42-75% and 24-57% respectively. The application of SO(2) led to 99% and 93% reductions in the PCDD and PCDF average congener concentrations, respectively in the solid phase. In the gas phase, the total reductions were 89% and 76% for PCDD and PCDF, respectively. Moreover, addition of SO(2) reduced the total I-TEQ value of PCDD and PCDF in the flyash and exhaust flue gas together by 60-86% and 72-82% respectively. Sulphur dioxide was more effective than ammonia in suppressing PCDD/F formation in flyash under the conditions investigated.     

Characterization and mass balance of dioxin from a large-scale municipal solid waste incinerator in China

The input and output samples from existing large-scale municipal solid waste incinerator (MSWI) were collected and analyzed for polychlorinated dibenzo-р-dioxins and dibenzofurans (PCDD/Fs) in this study, aiming to evaluate PCDD/F characteristic and the corresponding mass balance through the whole system. The grate-type MSWI is equipped with semi-dry scrubber, activated carbon injection, and bag filter as air pollutant control devices (APCDs). Results showed that on the output side, the stack gas, bottom ash and fly ash presented their mean dioxin levels of 0.078 ng I-TEQ/Nm(3), 12.94 ng I-TEQ/kg and 858 ng I-TEQ/kg, respectively, and showed large similarities in congener profiles. Instead, on the input side, the municipal solid waste (MSW) presented a mean dioxin level of 15.56 ng I-TEQ/kg and a remarkable difference in congener profiles compared with those of the output. The dioxin mass balance demonstrated that the annual dioxin input value was around 5.38 g I-TEQ/yr, lower than the total output value (7.62 g I-TEQ/yr), signifying a positive dioxin balance of about 2.25 g I-TEQ/yr.     

Catalytic destruction of PCDD/F in a fabric filter: experience at a municipal waste incinerator in Belgium

In order to comply with dioxin and furan (PCDD/F) emission regulations in Belgium, the IVRO municipal waste incinerator adopted catalytic filters for use in the plant's two existing fabric filters. This system replaced the injection of powdered activated carbon (PAC) because of concerns that PAC, used at temperatures above 200 degrees C, would ignite and lead to fires and plant down-time. The performance of the catalytic filter system, since its installation in 1997, is described. PCDD/F emissions are controlled to well below the regulatory limit of 0.1 ng I-TEQ/Nm3. The amount of PCDD/F entering and exiting the fabric filter is quantified. The resulting mass balance shows that greater than 99.5% of the gaseous PCDD/F entering the fabric filter is destroyed by the catalyst within the filter media.     

Characterization of PCDD/Fs and heavy metals from MSW incineration plant in Harbin

The characterization of PCDD/Fs and heavy metals in the flue gas and fly ash of Harbin municipal solid waste (MSW) incineration plant, located in the northeast of China, was investigated in this study. The MSW was treated in a twin internal fluidized (TIF) bed incinerator. The results indicate that the emission of PCDD/Fs into the environment is 0.02 ng I-TEQ/m3 and the level of PCDD/Fs in the fabric filter fly ash is 0.7982 ng I-TEQ/g. The leachability levels of Pb, Cd and Hg in the fly ash are below the limits of environmental protection standard in China. However, the contents of Cu, Zn, and Hg are high in the fly ash. This suggests that the fly ash is a hazardous waste that requires special treatment and disposal. The practice of more than four years of operation shows that the TIF bed incinerator is very suitable and practical for China.     

De-novo formation of dioxins and furans and the memory effect in waste incineration flue gases

The formation of dioxin (polychlorinated-rho-dibenzodioxins - PCDD) and furan (polychlorinated dibenzofurans - PCDF) in waste incineration flyash in relation to temperature has been investigated in a bench scale reactor. The PCDD/PCDF released into the exhaust gases of the reactor and trapped in a condensation and vapour phase adsorption system were also determined. The concentrations of PCDD/PCDF in the flyash were analysed in detail for all congeners and isomers including the mono-, di- and tri-PCDD/PCDF. De-novo synthesis of PCDD/PCDF was clearly seen in the flyash with large changes in concentrations of PCDD/PCDF with only small changes in temperature. Exhaust gas PCDD/PCDF were detected at all temperatures, with the maximum emission occurring at a flyash temperature of 350 degrees C, where the equivalent of over 42 times the total dioxin/furan concentration of the original flyash was lost to the vapour phase per unit mass of initial flyash. Although the desorbed species were mainly the lower mono- to tri-congener group of dioxins and furans, the desorbed dioxin/furans had a considerable toxic equivalent value.     

Decreased PCDD/F formation when co-firing a waste fuel and biomass in a CFB boiler by addition of sulphates or municipal sewage sludge

Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are formed during waste incineration and in waste-to-energy boilers. Incomplete combustion, too short residence times at low combustion temperatures (<700 °C), incineration of electronic waste and plastic waste containing chlorine are all factors influencing the formation of PCDD/Fs in boilers. The impact of chlorine and catalysing metals (such as copper and iron) in the fuel on PCDD/F formation was studied in a 12 MW_th circulating fluidised bed (CFB) boiler. The PCDD/F concentrations in the raw gas after the convection pass of the boiler and in the fly ashes were compared. The fuel types were a so-called clean biomass with low content of chlorine, biomass with enhanced content of chlorine from supply of PVC, and solid recovered fuel (SRF) which is a waste fuel containing higher concentrations of both chlorine, and catalysing metals. The PCDD/F formation increased for the biomass with enhanced chlorine content and it was significantly reduced in the raw gas as well as in the fly ashes by injection of ammonium sulphate. A link, the alkali chloride track, is demonstrated between the level of alkali chlorides in the gas phase, the chlorine content in the deposits in the convection pass and finally the PCDD/F formation. The formation of PCDD/Fs was also significantly reduced during co-combustion of SRF with municipal sewage sludge (MSS) compared to when SRF was fired without MSS as additional fuel.     

Effects of combustion parameters on polychlorinated dibenzodioxin and dibenzofuran homologue profiles from municipal waste and coal co-combustion

Variation in polychlorinated dibenzo-p-dioxin and polychlorinated dibenzofuran (PCDD and PCDF) homologue profiles from a pilot scale (0.6 MW_t, 2×10⁶ Btu/h), co-fired-fuel [densified refuse derived fuel (dRDF) and high-sulfur Illinois coal] combustion system was used to provide insights into effects of combustion parameters on PCDD and PCDF pollutant formation. A 24-run, statistically designed test matrix varied dRDF and/or coal firing rates (at a constant targeted energy release rate) along with a range of process variables including calcium hydroxide injection, hydrogen chloride (HCl) concentration, flue gas temperature, quench, and residence time such that the results would be relatable to a wide variety of combustion conditions. Statistical analysis of the molar homologue profiles enabled interpretation based on non-confounding variables. A multivariate, generalized additive model, based on transformations of the design variables, described 83% of the variation of the profiles characterized by log ratios of the homologue molar concentrations. This method identifies the operating parameters that are most significant in determining the PCDD/F homologue profiles. The model can be exercised to predict homologue profiles through input of these system-specific operating parameters. For example, both higher HCl and sulfur dioxide concentrations favor higher relative formation of the lower chlorinated PCDF homologues.     

Comparison of coal/solid recovered fuel (SRF) with coal/refuse derived fuel (RDF) in a fluidized bed reactor

An experimental study was undertaken to compare the differences between municipal solid waste (MSW) derived solid recovered fuel (SRF) (complying with CEN standards) and refuse derived fuel (RDF). Both fuels were co-combusted with coal in a 50 kW fluidised bed combustor and the metal emissions were compared. Synthetic SRF was prepared in the laboratory by grinding major constituents of MSW such as paper, plastic, textile and wood. RDF was obtained from a local mechanical treatment plant. Heavy metal emissions in flue gas and ash samples from the (coal + 10% SRF) fuel mixture were found to be within the acceptable range and were generally lower than that obtained for coal + 10% RDF fuel mixture. The relative distribution of heavy metals in ash components and the flue gas stream shows the presence of a large fraction (up to 98%) of most of the metals in the ash (except Hg and As). Thermo-gravimetric (TG) analysis of SRF constituents was performed to understand the behaviour of fuel mixtures in the absence and presence of air. The results obtained from the experimental study will enhance the confidence of fuel users towards using MSW-derived SRF as an alternative fuel.     

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 m³/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.     

An investigation on pollutant emissions from co-firing of RDF and coal

Pollutant emissions from co-firing of refuse derived fuel (RDF) and coal were investigated in a vortexing fluidized bed combustor (VFBC). RDF-5 was made of common municipal solid waste (MSW). CaCO₃ was injected in the combustor to absorb HCl at 850 °C. The results show that NO_x and HCl emissions increase with RDF-5 co-firing ratio. The NO_x concentration in flue gas at the bottom of the combustor is higher than that at the top. However, the trend of HCl released is reverse compared with NO_x emissions. It was found that the HCl concentration decreases with increasing the molar ratio of Ca/Cl. However, the effect of CaCO₃ addition on HCl retention is not significant when the molar ratio of Ca/Cl is higher than 5. The chlorine content in fly ash increases obviously with the molar ratio of Ca/Cl. PCDD/Fs emissions decrease slightly with an addition of CaCO₃. In this study incomplete combustion is regarded as the main cause for polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) formation.     

SETTING DIOXIN EMISSION LIMITS FOR MSW INCINERATORS: A MULTIMEDIA EXPOSURE ASSESSMENT FRAMEWORK

Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are emitted in trace amounts from municipal solid waste (MSW) incinerators. The exposure to PCDD/Fs experienced by an individual is dominated by the food chain pathway, which accounts for over 98% of the total uptake. Defining a target daily intake (the World Health Organization TDI of 10 pg I-TEQ kg.bw⁻¹day⁻¹) exposure assessment algorithms were then applied to arrive at the corresponding PCDD/F levels in air, soil, plants, food products, etc., which would allow the target intake to be met while retaining the balance of intake between the various exposure pathways. These concentrations were converted to an ambient air concentration of PCDD/Fs and ultimately, by defining criteria for acceptability, to a guide value for PCDD/F concentration in emissions from the MSW incinerator. This strategy was applied to PCDD/F emissions from MSW incinerators of various sizes against two illustrative criteria for acceptability: an “insignificant” release and a threshold above which the release may require further assessment for environmental effects and for control. Using the criteria developed in this paper, the current PCDD/F emission limit of 0.1 ng I-TEQ m⁻³results in an emission that is classed as “insignificant” for all plant sizes. However, higher emission concentrations can also be accommodated below the threshold for further assessment and control.     

Emission of dioxins/furans and other U-POPs from test burns of non-POP pesticides in a hazardous waste incinerator

This paper presents the results of test burns for obsolete pesticides (OPs) of the permethrin group in a high temperature incinerator (HTI) in Southeast Asia. Three test burn runs were conducted, a baseline run when no OP was fed to the incinerator, and two test runs with different mixtures of OP compounds (formula 1 and 2, refer to Table 1 for detail) containing chlorine in the feeding wastes. The unintentional formed persistent organic pollutants (U-POPs) including 17 dioxins/furans, 12 dioxin-like PCBs and 12 chlorobenzenes (CBs) were monitored in all input materials and all discharges (flue gas, scrubbing liquid and solid residues). The results show relatively high levels of the U-POPs in the flue gas emission with total dioxins/furans of 4.4, 3.4, and 8.4 ng I-TEQ/m³ in the baseline, test run 1 and test run 2, respectively, which are above international acceptable emission levels. The PCB levels in flue gas were, respectively, 0.01, 0.3 and 0.4 ng I-TEQ/m³. The baseline thus had similar U-POPs levels with the OP test runs.In stack emission, approximately 90% of the U-POPs and 100% of CBs were present in gas phase. The emission factors, mass of pollutants per metric ton (tonne) of input waste, of U-POPs associated with fly ash (from bag house) were the highest, followed by flue gas and bottom ash while those associated with scrubbing liquid were relatively low. Among the waste input material only the black toner power contained U-POPs, but at low levels. The profiles of the dioxins/furans and PCBs in the toner waste were significantly different from that in the discharges. Despite the overall good destruction and removal efficiency of permethrin (better than 90%) the high emission of U-POPs and CBs from the test burn is of another more serious concern. To our best knowledge the findings of this study are the first of this kind for the Southeast Asia. The findings emphasize that if not properly conducted a standard destruction technology of a non-POP chemical can lead to a release of a range of more dangerous U-POPs into the environment.     

Pyrolysis of waste tyres: A review

Approximately 1.5 billion tyres are produced each year which will eventually enter the waste stream representing a major potential waste and environmental problem. However, there is growing interest in pyrolysis as a technology to treat tyres to produce valuable oil, char and gas products. The most common reactors used are fixed-bed (batch), screw kiln, rotary kiln, vacuum and fluidised-bed. The key influence on the product yield, and gas and oil composition, is the type of reactor used which in turn determines the temperature and heating rate. Tyre pyrolysis oil is chemically very complex containing aliphatic, aromatic, hetero-atom and polar fractions. The fuel characteristics of the tyre oil shows that it is similar to a gas oil or light fuel oil and has been successfully combusted in test furnaces and engines. The main gases produced from the pyrolysis of waste tyres are H₂, C₁–C₄ hydrocarbons, CO₂, CO and H₂S. Upgrading tyre pyrolysis products to high value products has concentrated on char upgrading to higher quality carbon black and to activated carbon. The use of catalysts to upgrade the oil to a aromatic-rich chemical feedstock or the production of hydrogen from waste tyres has also been reported. Examples of commercial and semi-commercial scale tyre pyrolysis systems show that small scale batch reactors and continuous rotary kiln reactors have been developed to commercial scale.     

Verification results of jet resonance-enhanced multiphoton ionization as a real-time PCDD/F emission monitor

A jet resonance-enhanced multiphoton ionization (REMPI) monitor was tested on a hazardous-waste-fired boiler for its ability to determine concentrations of polychlorinated dibenzodioxins and dibenzofurans (PCDDs/Fs). Jet REMPI is a real-time instrument capable of highly selective and sensitive (from parts per billion to parts per trillion) detection of a broad range of aromatic compounds, including a number of air toxic compounds. The PCDD/F toxic equivalency (TEQ) value was derived from a predetermined correlation (R ² = 0.74) with monochlorobenzene (MClBz). This relationship was applied to nine subsequent jet REMPI on-line measurements of MClBz and parallel, standard extractive sampling for PCDD/F TEQ. For high waste-firing rates, with a range of PCDD/F TEQ values between 3.9 and 6.0 ng TEQ/m³, the TEQ values predicted by jet REMPI had a relative difference of 26% with the standard EPA Method 23 results. At low waste-firing rates (0.9–1.6 ng TEQ/m³), the relative difference increased to 219%. This limited testing shows that jet REMPI has promise as an on-line diagnostic monitor, providing feedback on the effects on PCDD/F emissions of operating parameter changes such as fuel feed interruptions or air pollution control failures.     

Correlation study of hazardous waste characteristics among various chemical processes in Republic of Korea

To harmonize with international standards, the Republic of Korea is in the process of converting its current hazardous waste classification system and setting up regulatory standards for all toxic substances present in hazardous waste. Detailed characterization of each form of hazardous waste belonging to five chemical processes and their correlations were studied. In the present work, the concentrations of 13 heavy metals, F^?, CN^?, 7 PAH compounds, total PCDD/F and 7 PCB isomers present in the hazardous waste generated among chemical processes such as synthetic rubber (SR), man-made fibers (MF), organic dyes and pigments (DP), pharmaceuticals and cosmetics were analyzed along with their leaching characteristics. Comparing all the processes, most of the heavy metal concentrations were high in SR waste. Naphthalene was the dominant PAH in most of the chemical process waste. PCDD/F concentrations of the samples were in the range of 0.001–0.003 ng I-TEQ/g. PCB isomer-101 and isomer-118 were found to be slightly higher than the permissible limit in the SR filter cake sample. SR process wastes doesn’t show any resemblance with the other process waste in either the heavy metals and PAH trend. Each sample from DP and MF were suitable only for hazardous waste landfill.     

Coal fly ash based carbons for SO2 removal from flue gases

Two different coal fly ashes coming from the burning of two coals of different rank have been used as a precursor for the preparation of steam activated carbons. The performance of these activated carbons in the SO₂ removal was evaluated at flue gas conditions (100 °C, 1000 ppmv SO₂, 5% O₂, 6% H₂O). Different techniques were used to determine the physical and chemical characteristics of the samples in order to explain the differences found in their behaviour. A superior SO₂ removal capacity was shown by the activated carbon obtained using the fly ash coming from a subbituminous–lignite blend. Experimental results indicated that the presence of higher amount of certain metallic oxides (Ca, Fe) in the carbon-rich fraction of this fly ash probably has promoted a deeper gasification in the activation with steam. A more suitable surface chemistry and textural properties have been obtained in this case which explains the higher efficiency shown by this sample in the SO₂ removal.     

Dioxin emissions from thermal waste management in Medellín, Colombia: present regulation status and preliminary results

Preliminary results of a study undertaken to characterize dioxin and furan releases from waste incineration plants operating in Medellín-Colombia are presented. Emission and fly ash samples were collected from representative plants burning medical and industrial residues to characterize PCDD/PCDF levels. Analyses were carried out following European standards for stationary gas emissions EN-1948:1996 and US EPA 1613 (fly ashes). Final extracts were analyzed by both high resolution gas chromatography coupled to high resolution mass spectrometry (HRGC-HRMS) and high resolution gas chromatography coupled to ion trap tandem mass spectrometry (HRGC-ITMS/MS). Preliminary results revealed emission levels of 1-30.3 ng I-TEQ/Nm3 depending on whether or not an air pollution control system (APCS) was installed. Fly ashes contained 8.5-68 ng I-TEQ/g. Critical issues that should be addressed in an assessment and in developing waste management plans in Colombia are discussed.     

Treatment of tar pond sludge in a circulating fluidized bed combustor

A series of tests to burn mixtures of tar pond sludge and coal was carried out using a mini‐circulating fluidized bed combustor (mini‐CFBC). During the tests, carbon dioxide, oxygen, carbon monoxide, sulfur dioxide, and nitrogen oxides in the flue gas were monitored continuously. Stack gas sampling was carried out for hydrochloric acid, metals, particulate matter, volatile organic compounds (VOCs), total hydrocarbons, semivolatile organic compounds (SVOCs), dioxins and furans (PCDD/Fs), and polychlorinated biphenyls (PCBs). Results showed that hydrochloric acid, mercury, particulate matter, PCDD/F, and metal concentrations were all below both the current limits and the gas‐release limits to be implemented in 2008 in Canada. The new 2008 emissions limits will reduce the maximum allowable concentrations of most pollutants by half. Thus, the maximum concentration for particulate matter will be 5 mg/m³ (from the current maximum concentration of 10 mg/m³);the maximum concentration for hydrochloric acid will be 5 mg/m³ (from 10 mg/m³); and the‐maximum concentration for dioxins and furans will be 0.032 ng/m toxic equivalent (from 0.08 ng/mcurrently). Sulfur capture efficiency was 89–91 percent. The percentage of fuel nitrogen converted to nitrogen oxides was of the order of 4.7 to 6.1, which is significantly lower than that of conventional pulverized coal‐fired boilers and well within the normal range for fluidized bed combustors (FBCs). PCB and polycyclic aromatic hydrocarbon (PAH) emissions levels were comparable or lower than levels reported in the literature for industrial‐scale FBCs. VOC concentrations were low except for benzene, for which the concentration was higher than that reported for pulverized coal‐fired utility boilers. In addition, carbon monoxide concentration was high at 1,200 to 2,200 parts per million. However, these carbon monoxide concentrations are typical of the mini‐CFBC firing coal. The trials showed that for 10 percent by weight tar pond sludge mixed with 90 percent by weight coal, the combustion was both stable and efficient. The tests demonstrated that CFBC technology is an environmentally sound option for eliminating tar pond waste sludge. © 2005 Wiley Periodicals, Inc.     

Sludge as dioxins suppressant in hospital waste incineration

Nitrogen containing compounds such as ammonia, urea and amines can effectively inhibit the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). Sewage sludge accumulates both sulfur and nitrogen during wastewater treatment so it could be used to reduce PCDD/Fs formation. Indeed, it is observed in this study that the gas evolving from the sludge drying process can significantly suppress chlorobenzene (CBz) and PCDD/Fs formation from fly ash collected from a hospital waste incinerator. For instance, the reduction of hexachlorobenzene (HxCBz) and PCDD/Fs amount was 92.1% and 78.7%, respectively, when the drying gas evolving from 2g sludge flew through 2g fly ash. These tests were conducted in the frame of projects devoted to hospital waste incineration. The disposal technology for hospital waste (HW), developed in this institute, features rotary kiln pyrolysis combined with post-combustion followed by flue gas cleaning. Hence, some preliminary tests were devoted to investigate dioxins suppression by co-pyrolysis and co-combustion of polyvinyl chloride (PVC) and sludge in lab scale. More experimental research will be conducted to appropriately assess these effects of sludge on PCDD/Fs emissions during co-pyrolysis/combustion of HW and sludge.     

Persistent organic pollutant emissions from medical waste incinerators in China

The huge amount of medical waste (MW) has caused a tough challenge to environmental protection in China because of its serious infectious potential. At present, incineration is the most common technology for MW disposal. Unfortunately, the medical waste incinerator (MWI) is considered one of the major sources of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). This study was conducted to investigate the generation and the components of MW; the fingerprint of PCDD/Fs in MWI; and PCDD/F, polychlorinated biphenyl (PCB) and hexachlorobenzene concentrations in residue ash. The estimated annual production of MW was estimated to be 0.97 million tons in China in 2008; in addition, plastic and rubber accounted for 24.5% of MW contents. PCDD/F emissions from MWI could be divided into two main groups according their fingerprints, and the ratio of PCDFs/PCDDs was mostly over 1.5, with a mean value of 3.43. The toxic equivalent of PCDD/Fs was over 30 times that of the value of PCBs in the residue ash, and PCDD/F contents in fly ash accounted for approximately 67% of the total output of PCDD/Fs, which was in line with the UNEP default emission factors for MWI (class 3, 63.7%).