Environ. Sci. Technol. 2009, 43, 2512–2518
Influence of Combustion Temperature on Formation of Nitro-PAHs and Decomposition and Removal Behaviors in Pilot-Scale Waste Incinerator MAFUMI WATANABE* AND YUKIO NOMA Research Center for Material Cycles and Waste Management, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan
Received December 12, 2008. Revised manuscript received February 3, 2009. Accepted February 6, 2009.
To gain a better understanding of the formation and decomposition behaviors of nitro-polycyclic aromatic hydrocarbons (nitro-PAHs) in solid waste combustion, incineration experiments were conducted using a pilot-scale incinerator. NitroPAHs were formed during primary combustion, although the amounts formed were several orders of magnitude lower than those of the PAHs, PCDD/Fs, and the dioxin-like PCBs. Increasing the temperature of primary combustion from 690 to 890 °C resulted in a significant decrease in the formation of most of the nitro-PAH compounds studied. More than 99% of nitroPAHs formed in the primary combustion zone were decomposed in the secondary combustion chamber at 900 °C with a 3-s residence time. The results indicate that appropriate secondary combustion conditions are the key to controlling emissions of nitro-PAHs. Under optimized conditions, the amounts of nitroPAHs in the final off gases and in the ashes were significantly lower than those present in the incinerator input. Overall destruction efficiencies of nitro-PAHs reported in this study were 95.81-98.33%, indicating that emission of nitro-PAHs from solid waste combustion can be minimized by appropriate combustion control.
Introduction Nitro-polycyclic aromatic hydrocarbons (nitro-PAHs) have been identified as important environmental contaminants because of their high mutagenic and carcinogenic potentials (1). Eight nitro-PAH compounds have been classified under group 2B (possibly carcinogenic to humans) by the International Agency for Research on Cancer (2). They have been detected in both ambient air and indoor air samples (3, 4). Studies have also demonstrated the presence of nitro-PAHs in other environmental media, such as soil and sediments (5, 6) and in foods and beverages (7, 8). In addition, Tokiwa et al. (9) have reported the detection of nitro-PAHs in human tissue. In most of these studies, the concentrations of nitro-PAHs found were considerably lower than their parent PAH compounds. Nitro-PAHs, however, are considered as being of great concern because of their considerably higher mutagenic and carcinogenic potentials as compared to the * Corresponding author phone: 81-29-850-2203; fax: 81-29-8502759; e-mail:
[email protected]. 2512
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parent PAH (1). In addition, most nitro-PAHs demonstrate relatively high direct mutagenic activities in the absence of metabolic activation. While some nitro-PAHs compounds have been produced industrially as intermediates in the manufacture of explosives, colorants, and dyes, nitro-PAHs detected in the environment originate primarily as direct or indirect products of combustion sources (1). Indirect production of some nitro-PAHs involves their atmospheric formation from PAHs originating largely from combustion processes (10, 11). Various combustion processes have been identified as direct emission sources of nitro-PAHs into the environment. Nitro-PAH emissions in diesel exhaust have been well studied (1). Concentrations and patterns of nitro-PAHs emitted by diesel engines were found to vary according to the type of engine, the fuels, and fuel additives used, driving conditions, and the presence of a catalytic converter (1). Nitro-PAHs have also been detected in gasoline engine and airplane engine emissions, in emissions from the combustion of heating oils, and so forth (1). Furthermore, incineration of municipal and industrial wastes is also considered as significant emission sources of nitro-PAHs (1). Nitro-PAH emissions from waste incinerators have been studied as mutagenic activities using bioassay techniques, such as the Ames test (12-14), because some studies indicated that nitro-PAHs were considered to be an important contributor to the overall mutagenic activity of emissions from municipal waste incineration (12). The major mutagenic fraction was released into the environment via gaseous emissions (13). Yoshino and Urano (14) compared different incinerators and found that exhaust gas from fluidized bed type showed a higher mutagenicity than emissions from stoker-type incinerators, and those from old plants also showed a higher mutagenicity. Most studies, however, did not perform a quantitative analysis of individual nitro-PAHs. A few studies have quantitatively determined nitro-PAH compounds in emissions (stack gases or ash) from waste incinerators, but these have targeted either only single nitroPAHs (usually 1-nitropyrene) or, at best, a limited number of those potentially present (15-17). These studies reported that some nitro-PAHs examined were found at a concentration on the order of milligrams per kilograms in ash and on the order of nanograms per cubic meter in the stack gases of waste incinerators. Because of the increasing global utilization of waste incineration in waste management strategies and because of the significance of nitro-PAHs with respect to the mutagenic and carcinogenic potential of air emissions from such processes, there is an urgent need for more comprehensive studies on emissions of these toxic compounds and on methods for the minimization of their release. To our knowledge, there has been, to date, no study that has addressed the important issues of formation and degradation behaviors of nitro-PAH in waste incinerators. This information, however, is crucial for effective emission control of these compounds in waste incineration. To gain an initial insight into this important topic, we performed combustion tests using a pilot-scale incinerator. To understand the formation and decomposition behaviors of nitro-PAHs in solid waste combustion, we conducted determination for 27 nitro-PAHs at different positions in the incinerator. In addition, we studied the effect of primary combustion temperature on the formation of nitro-PAHs and compared it with the formation of PAHs, PCDD/Fs, and dioxin-like PCBs in the same experiments. This study has, 10.1021/es8035169 CCC: $40.75
2009 American Chemical Society
Published on Web 03/05/2009
for the first time, investigated the formation and destruction behaviors of nitro-PAHs during a waste incineration process. A second aim of the study was to evaluate whether the higher combustion temperatures specified by regulations for emission control of PCDD/Fs and dioxin-like PCBs (DRCs) by the Japanese regulations (18) and by the global Best Available Technology (BAT) process specified by the Stockholm Convention (19) could have a negative influence on the emissions of nitro-PAHs.
Materials and Methods Pilot Incinerator and Combustion Conditions. In this study, the pilot-scale incinerator at the National Institute for Environmental Studies, Japan, was used. Details of this facility were described previously (20). Briefly, this facility consists of a rotary kiln serving as a primary combustion unit and a vertical secondary combustion chamber connected to a gascooling zone (Figure S1 in the Supporting Information). The air pollution control section includes a bag filter, an activated carbon adsorption tower, and a wet scrubber. This system has the principal design of a modern Japanese municipal solid waste (MSW) incinerator. Three experiments using homogenized MSW as fuel were carried out at different temperature settings of the primary combustion chamber as follows: 690, 790, and 890 °C. In all the experiments, secondary combustion was maintained at a temperature of 900 ( 10 °C with a residence time of 3 s. The temperatures in the primary and secondary combustion chambers were maintained by electrical heating. The off gases were rapidly cooled at a rate of approximately 230 °C/s to 150 °C. While the combustion conditions, except for primary combustion temperature, were comparable among the three experiments in this study, the air flow rate in the primary combustion chambers and the MSW feeding rates had to be increased with increasing temperatures in the primary combustion chamber to maintain stable combustion (Table S1 in the Supporting Information). Sampling and Chemical Analysis. After stable combustion conditions were established at each temperature, the flue gases were collected for 4 h at four sampling points as follows: kiln exit (primary combustion exit), bag filter inlet (secondary combustion exit), bag filter exit, and at the very end of the flue gas line (Figure S1 in the Supporting Information). Sampling was performed according to the method specified in the Japanese Industrial Standard (JIS) report K0311 (21). This was slightly modified by placing the filter and the XAD cartridge after the liquid gas samplers. This setting was chosen to avoid the artificial formation of nitro-PAHs in the sampling train (22). At each sampling location, nitrogen oxides (NOx) were continuously monitored by normal pressure chemoluminescence (Shimadzu NOA-7000) during flue gas sampling. Bottom and fly ashes were collected at the end of each experiment and analyzed. In this study, 27 compounds of nitro-PAHs (mononitro and dinitro-PAHs; Table S2 in the Supporting Information), 17 PAHs (biphenyls and 16 PAHs regulated by the U.S. EPA), and DRCs (tetra- to octachlorinated dibenzo-p-dioxins and dibenzofurans, and dioxin-like PCBs) were analyzed. The extraction of nitro-PAHs, PAHs, and DRCs in flue gas samples and in input materials (MSW) and ash samples was performed according to the methods of JIS K0311 (21) and Noma et al. (20), respectively. An aliquot of the extract was used for quantitative analysis of nitro-PAHs and spiked with deuterium-labeled nitro-PAHs as internal standards. An activated silica-gel column was used as a first cleanup step. The column was rinsed with hexane, and the nitro-PAHs were eluted with dichloromethane/hexane (1/1). This fraction was concentrated to about 1 mL using a rotary evaporator. As a second cleanup step, a multilayered silica-gel column (0.5% AgNO3impregnated silica-gel, silica-gel, and 2% H2SO4-impregnated
silica-gel) was used. The nitro-PAHs were again eluted with dichloromethane/hexane (1/1). After concentration, 13C12labeled 2-monochlorodibenzofuran was added as a syringe spike. Identification and quantification of nitro-PAHs was performed using a GC (Agilent 6890 series, Agilent Technology) chromatograph fitted with a high-resolution mass selective (HRMS) detector (AutoSpec Ultima, Micromass) at a resolution of >8000. Separation was accomplished by DB-5 ms fused silica capillary columns (J&W Scientific Inc., 60-m length, 0.25-mm id., 0.25-µm film thickness) and a DB-17ht column (J&W Scientific, 30-m length, 0.25-mm id., 0.15-µm film thickness). Nitro-PAHs were detected using selective ion monitoring mode at ions of [M]+ and [M + 1]+, except for 2,2′-dinitrobiphenyl and 1,8-dinitronaphthalene, which were detected using ions of [M - NO2]+ and [M - NO2 + 1]+. Each nitro-PAH was quantified using the isotope dilution method with the corresponding internal standard. Cleanup procedure and quantification of DRCs were performed according to the methods of JIS K0311 (21). For analysis of the 17 PAHs, an aliquot of the extract was diluted and spiked with the 13C-labeled corresponding PAHs as an internal standard. The 17 PAHs included all parent compounds of the monitored nitro-PAHs. Identification and quantification of PAHs were performed using GC/HRMS at resolution of >10 000 with a DB-5 ms fused silica capillary column (J&W Scientific, 60-m length, 0.32-mm id., 0.25-µm film thickness). The input material (MSW) was analyzed in duplicate. Because the measured concentrations between the duplicates were considered to fall within acceptable margins of sampling and analytical error (30%), the average values from the duplicate analyses were used as concentrations in the input materials (MSW) (Table S3 in the Supporting Information). Concentrations of nitro-PAHs, DRCs, and PAHs were recalculated to give milligrams per ton of input material to compensate for the slightly different feed rates of MSW in each of the three experiments. The real concentrations (nanograms per gram for MSW and ashes or nanograms per cubic meter for flue gases) of those compounds are shown in Table S4 in the Supporting Information.
Results and Discussion Influence of Primary Combustion Temperature on Formation of Nitro-PAHs. In the MSW input material used in the experiments, only nitronaphthalenes were detected at a low concentration of 1.0-2.0 mg/ton (Table 1). However, in kiln exit gases, a range of nitro-PAHs was detected in all experiments at concentrations significantly higher than those detected in the input material. It was concluded, therefore, that the nitro-PAHs were formed in the primary combustion process. Further, the concentrations of PAHs and DRCs at the primary combustion exit were several orders of magnitude higher than those detected in the waste input. It was concluded, therefore, that they were also formed in the primary combustion zone. Figure 1 shows the total concentrations of nitro-PAHs, PAHs, DRCs, and NOx in kiln exit gases at each of the three primary combustion temperatures tested. Concentrations of nitro-PAHs in kiln exit gas were 51-360 mg/ton on an input material equivalent basis. PAHs were formed at concentrations 3 to 4 orders of magnitude higher (93 000-1 300 000 mg/ton) and DRCs at concentrations 1 to 2 orders of magnitude higher (2600-3000 mg/ton) than that of nitro-PAHs. This is, to our knowledge, the first comparison of emissions of key organic pollutants (PAHs, nitro-PAHs, PCDD/Fs, and dioxin-like PCBs) formed from primary combustion process. These values give a basic insight into the concentration ranges of these key pollutants likely to be emitted from incineration processes having only a primary combustion chamber, as is often the case in VOL. 43, NO. 7, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Concentrations (Milligrams per Ton on Input Material Basis) of Nitro-PAHs, PAHs, DRCs, and Major Nitro-PAH Compounds and Their Parent PAHs in Input Material (MSW), Flue Gases, and Ashesa flue gases compounds 1-nitronaphthalene 2-nitronaphthalene 3-nitrobiphenyl 1-nitropyrene total nitro-PAHs naphthalene biphenyl pyrene total PAHs PCDFs PCDDs dioxin-like PCBs total DRCs
a
primary combustion temp (°C) MSW: input material 690 790 890 690 790 890 690 790 890 690 790 890 690 790 890 690 790 890 690 790 890 690 790 890 690 790 890 690 790 890 690 790 890 690 790 890 690 790 890
(2) (2) (2) (1) (1) (1)
