Environ. Sci. Technol. 2005, 39, 795-799
Effect of Nitrogen-Containing Compounds on Polychlorinated Dibenzo-p-dioxin/Dibenzofuran Formation through de Novo Synthesis SHUNSUKE KUZUHARA,* HIROSHI SATO, NAOTO TSUBOUCHI, YASUO OHTSUKA, AND EIKI KASAI Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1, Katahira, Aobaku, Sendai, 980-8577, Japan
An experimental study was conducted to clarify the suppression effect of nitrogen-containing compounds, that is, ammonia and urea, on the formation of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) through the de novo synthesis reaction. In the experiment, graphite and copper chloride contained in a mixture were used as sources of carbon and chlorine, respectively. The granulated sample mixture was charged as a packed-bed in the glass tube and heated at 300 °C in the flow of Ar-O2 gas mixture. In some cases, urea was added as aqueous solution to the sample, while ammonia was added to the gas flowed through the sample bed. The amount of PCDD/Fs formed decreases significantly by the addition of both ammonia and urea. Particularly, the addition of urea reduces the amount of PCDD/Fs discharged in the outlet gas by approximately 90%. The oxidation rate of carbon in the early stage of the experiment, that is, the heating period, is promoted by the addition of nitrogencontaining compounds. However, soon after the temperature reaches 300 °C, the formation rate becomes lower than that of the case without the addition of nitrogen-containing compounds. On the other hand, organic compounds containing amino (-NH2) or cyanide (-CN) groups and those containing nitrogen within the carbon ring frame were detected in the outlet gas in the case of urea addition. Typically observed aromatic compounds are chlorobenzonitriles, chlorobenzeneamines, and chloropyridines. This suggests a possibility that hydrogen and/or chlorine combined with PCDD/Fs are also substituted by such nitrogencontaining groups, and this decreases the formation rate of their frame of carbon rings. This phenomenon was also consistent with the fact that a significant reduction was observed in the amount of PCDD/Fs released to the outlet gas when urea was added.
Introduction Generally, the following three routes are pointed out for the formation of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in the combustion processes (1): * Corresponding author phone: +81-22-217-5154; fax: +81-22217-5154; e-mail:
[email protected]. 10.1021/es049040j CCC: $30.25 Published on Web 12/18/2004
2005 American Chemical Society
(i) from precursor substances such as chlorobenzenes (CBzs) and chlorophenols (CPs) (2), (ii) by the decomposition/cleavage reaction of polycyclic aromatic hydrocarbons (PAHs) (3-5), and (iii) “directly” from carbonaceous materials such as soot, which has a macrocarbon structure and was produced normally by the condensation of unburned carbon and hydrocarbon components (6). For reaction route i, condensation reactions of precursors contribute to the formation of PCDD/Fs. Previous reports suggest that specific congeners may be formed through both routes i and ii depending on the structure of the starting compounds. For example, 1,2,8,9-T4CDF and 1,4,6,9-T4CDF are mainly formed from chlorinated coronene through route ii. Route iii is referred to as “de novo synthesis.” In the hightemperature combustion process, the formation of these compounds occurs in the cooling stage of waste gas containing fly ash particles and many researches have been trying to clarify the mechanism of PCDD/Fs formation (79). In practical combustion processes, “soot” particles are usually observed as complex structures containing various metal elements and chlorides. However, their composition largely depends on their formation process. De novo synthesis reaction is known to proceed in an oxidative atmosphere at lower temperatures, that is, between 200 and 500 °C, and the maximum reaction rate is observed at about 300 °C (10-16). Although the oxidation reaction proceeds slowly at such low temperatures, coexisting metals/metallic compounds promote its rate considerably. The promotion effect of metallic chlorides is usually larger than that of the oxides (17). Further, metallic chlorides can also act as a source of chlorine in the formation reaction of chlorinated organic compounds such as PCDD/Fs. Copper chlorides, CuCl and CuCl2, are catalytic compounds that have high ability to promote PCDD/Fs formation (18). We found a linear relationship between the rates of carbon oxidation and organic chlorine formation when copper chloride coexists with carbon (19). Currently, the control of PCDD/Fs emissions in practical processes is generally conducted by “end of pipe” technologies such as activated carbon injection, rapid cooling scrubber, and/or catalytic decomposition tower for organic compounds. However, such technologies sometimes lead to a substantial increase in the process cost and difficulties of heat recovery from waste gas. To establish stable emission control and to comply with future emission regulation, it is an important subject to develop an effective reagent/inhibitor that reliably suppresses PCDD/Fs emissions even when used in small quantities. So far, nitrogen- and sulfur-containing compounds (20-22), hydroxides/oxides of alkali metals (23), and so forth, have been examined as inhibitors. Particularly, with regard to nitrogen-containing compounds, noteworthy suppression effects have been confirmed for urea (21, 24, 25), ammonia (20), monoethanolamine (26), triethanolamine (26), and EDTA (ethylenediaminetetraacetic acid) (27). The formations of Cu-N bond and a complex composed of Cu and N have been proposed as the mechanisms involved in the suppression of PCDD/Fs formation by the addition of monoethanolamine and urea, respectively (28, 29). However, fundamental information to understand the details of the mechanism is still insufficient. In the present study, the effect of the addition of urea to the carbonaceous materials and ammonia to the gas flowed in the de novo synthesis reaction was examined, focusing on the species of organic compounds formed and the surface state of carbon. Temperature-programmed desorption (TPD) measurement was conducted to evaluate the substances VOL. 39, NO. 3, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Elemental Composition of the Graphite Reagent and Its Ash component
graphite
component
ash of graphite
ash content (%) volatile (%) C (%) Cu (ppm) K (ppm) Cl (ppm) N (ppm) S (ppm) O (ppm) H (ppm)
0.08 0.39 99.3
