Modern Technologies to Reduce Emissions of Dioxins and Furans from Waste Incineration

Polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) are fonned unintentionally as bypro ducts of waste combustion and many other thelmal industrial processes. Today PCDDIPCDF is known as one of the most toxic groups of organic substances. Furthelmore, they are perceived to be carcinogenic. Although many questions about the toxicological effects of PCDDIPCDF are still unanswered, all possible reduction measures should be investigated to minimize their release into the environment. Consequently, reduction technologies for dioxins and furans are not only needed for densely populated urban areas, but also in rural regions where the population's food is produced. The uptake of PCDDIPCDF via food must considered to be the major pathway of exposure for humans as well as for animals. After a brief presentation of the regulatory and technical background, this paper gives an overview of the formation routes of PCDDIPCDF in combustion and other industrial thelmal processes. The most important sources for PCDDIPCDDF emissions are also described. Secondly, the most commonly applied PCDDIPCDF control technologies will be presented using various types of waste incinerators to demonstrate today's state-of-the-art flue gas cleaning technology. Various modem municipal solid waste (MSW) combustors and a hazardous waste incinerator, as well as an iron ore sintering facility provide actual examples of full-scale systems in commercial operation. The fmal part of the presentation will give an outlook towards new developments in PCDDIPCDF abatement technologies for more economical PCDDIPCDF reduction. 93 1. LEGISLATIVE ACTIVITIES Due to a highly developed environmental sensitivity of the public in Gelmany and other Central European countries, more and more regulations regarding air and water quality are being implemented. Air emission regulations for power generation, waste incineration, crematories and numerous other thelmal industrial processes have created a new market for air pollution control equipment particularly for the effective control of PCDDIPCDF. Around 1990 the governments of Austria, GeImany, the Netherlands, Sweden, Switzerland and other European countries drastically tightened the emission guidelines for many pollutants through implementation of respective legislation. The Gelman 17. BImSch V (17th Implementation Directive to the Federal Gelman Emission Protection Act) [1], which formed the basis for the current legislation within the European Union (EU), is used as an example to outline the implications of such legislation for all kinds of waste incineration plants. Among other things, the minimUl1l acceptable combustion operating conditions were fixed in tenllS of incineration temperature and flue gas residence time as well as maximUl1l peImissible emission concentrations for many air pollutants. One of the most significant achievements of this new directive was the first time introduction of an emission limit for PCDDIPCDF of 0.1 ng I-TEFINm3 (also sometimes referred to as ng 1-TEQINm3). Since the group of PCDDIPCDF consists of 210 individual compounds (75 PCDD congeners and 135 PCDF congeners) with different levels of toxicity, the group is commonly referred to in concentration numbers of toxic equivalents. Several different methods for detelmining toxic equivalents were defmed. Table 1 compares the three most commonly used factors for the calculation of toxic equivalents, namely the International Toxicity Equivalency Factors I-TEF, the World Health Organization's Toxicity Equivalent Factors WHO­ TEF, and the German Umweltbundesamtl Bundesgesundsheitsamt (Federal Environment AgencylFederal Health Agency) Toxicity Equivalency Factors UBAlBGA-TEF. Today, the most commonly used nomenclature is based on I-TEF and expressed in units of ng/m3. However, there are numerous different standards for the term STP standard temperature and pressure, and other parameters such as the 02-content, and the moisture content for the defmition of a m3• Often seen are the terms Nm3 (Normal m3 most commonly used in Europe), Sm3 (Standard m3 most commonly used in the United States and some Asian countries), and Rm3 (Reference m3 commonly used in Canada). All standards are based on dry conditions, thus, the moisture content of the flue gas and the resulting dilution is eliminated, which leads to an increase in the reported over the measured 1-TEF concentration. Further, each standard corrects the measured 1-TEF concentration to a particular oxygen value. As a result, a Nm3 is based on 1 0 1 3 mbar and 0 °C at STP and in Europe is typically further corrected to 1 1 vol. % O2; a Sm3 is based on 1 atm. and 77 F (25 °C) at STP and in the U.S. is typically further corrected to 7 vol.% O2; and a Rm3 is based on 1 atm. and 20 °C at STP and in Canada is typically further corrected to 1 1 vol.% O2, Consequently, the results reported can only be compared in any meaningful way as long as the defmition used for m3 is also reported and all the numbers to be compared are converted to this same standard [2] . The same numerical values of 0. 1 00 ng I-TEFINm3 and 0. 1 00 ng I-TEF/Sm3 are in reality two greatly different concentrations. In order to allow a comparison of both values, both need to be brought to the same basis, i .e. ng I-TEFINni? Thus, the following steps are necessary to convert the numbers to the same basis: 1 . Oxygen Correction: [(2 1 VOl.%02 7 vol.% O2)/(2 1 vol% O2 1 1 vol.% O2)] = 1 .4 2. Temperature Correction: (298 K/273 K) = 1 . 1 3 . Pressure Correction: not necessary, since 1 atm. = 1 0 1 3 mbar 4. Overall Correction: 0. 1 00 ng ITEF/Sm3 * (1 .4 * 1 . 1 Sm3INm3) = 0. 1 54 ng ITEFINm3 94 The example shows that the real difference of the two numerically identical values amounts to more than 50%! Thus, it is of crucial importance for data quality management to ensure the same basis when comparing any data. It is equally important to fully understand which basis was chosen as a standard for a respective piece of legislation. Without this information, no valid knowledge of the technological challenge to achieve compliance of a facility can be developed. Also rounding and truncation of emission limits allows for differences. In some countries, the value of 0. 1 ng 1TEQINm3 is rounded to the extent that even a measured value of 0. 149 ng 1-TEQINm3 is still acceptable for compliance. In other countries the number is truncated after the first digit after the decimal point leading to the fact that even a measured value of 0.199 ng I-TEQINm3 meets compliance. Consequently, no correct evaluation of the applicable process technology solution is possible. The 17th BImSch V also requires certain combustion conditions to be maintained, namely a minimum temperature of 850°C after the last combustion air injection combined with a flue gas residence time at or above said temperature for at least 2 seconds. An oxygen concentration of at least 6 vol.% must be maintained at all times. These requirements for insuring good combustion practice are mandatory for the incineration of MSW, sewage sludge and other such waste, which does not contain significant quantities of halogenated hydrocarbons. In case of other waste containing chlorinated hydrocarbons, the minimum combustion temperature must be raised to at least 1 200 °C with the other 2 requirements remaining unchanged. The reason for these requirements is the significant thermal stability of many halogenated hydrocarbons such as PCDDIPCDF. In order to effectively destroy these compounds, the flue gas must be exposed to sufficiently high temperatures over a long enough period of time while the availability of excess oxygen for the thermal oxidation is ensured at all times. However, the 17th BImSch V also allows for combustion conditions different from the ones required by the directive as long as individual measurements at the individual facility claiming an exception provide proof that the emission concentrations, especially of PCDDIPCDF, polycyclic aromatic hydrocarbons (PAHs), and polychlorinated biphenyls (PCBs), are not higher than at the operating conditions specified in the directive. This provision allows the operator to optimize the actual operation of the individual facility based on economical and other considerations without compromising on environmental compliance. Most hazardous waste incineration facilities in GeImany, which bum large quantities of halogenated hydrocarbons and other Persistent Organic Pollutants (POPs) as defmed by the United Nations, take full advantage of this flexibility given by the 17th BImSch V in order to lower their operating costs. 2. DIOXIN AND FURAN FORMATION ROUTES Since the early PCDDIPCDF measurements at MSW combustors revealed rather high PCDDIPCDF emissions at the stack, it was believed that these toxic substances are fOImed in the furnace. Today, much more about the formation mechanisms of PCDDIPCDF is known. Indeed, a well designed and operated combustion furnace has been fully recognized as the only means of almost complete destruction of the incoming dioxins/furans. Figure 1 details the PCDDIPCDF destruction efficiency of an efficient combustion process, which ensures thorough mixing, adequate temperature and sufficient residence time. Nevertheless, the remaining PCDDIPCDF emissions from waste incinerators were still not considered , acceptable to protect public health and the environment. Since it was unclear where and how the PCDDIPCDF fOImation occurred, a lot of measurements and intensive research was perfoImed and sophisticated emission inventories were developed. After nearly two decades of intense research and testing the answer is clear, although several details still have to be investigated, The reformation of relevant dioxinlfuran concentrations in waste incineration plants takes place downstream of the furnace/combustion chamber in 'the boiler and during dust removal. Figure 2 reveals the two basic mechanisms of reformation of PCDDIPCDF occuring after the combustion process and during cooling of the flue gas. This phenomenon is virtually independent of the actual destruction efficiency of the combustion process and is responsible for significant dioxin/furan concentrations in flue gases downstream of the furnace. The first mechanism occurs between 300-800° C and is a homogenous gas phase reaction. PCDDIPCDF are fOImed through so called "precursors" or "pre-dioxins" (Figure 3). Such precursors are, for example, polychlorinated benzenes, phenols and biphenyls. The second mechanism of reformation is the so called De-Novo-Synthesis of dioxin and furans. It is 95 reasonable to assume that De-Novo-Synthesis contributes the predominant portion to the total PCDDIPCDF emissions from modem MSW incineration plants. For older MSW incineration plants or special incinerators (i.e. hospital waste incinerators or crematories) the individual share of the two fOImation mechanisms can widely differ. Due to very poor combustion operating conditions, extremely high emission values of PCDDIPCDF have been observed. Two main theories are commonly accepted concerning the De-Novo-Synthesis reaction process. Both theories assume dioxinlfuran reformation as a heterogeneous gas-solid phase reaction on the surface of fly ash particles. Inorganic chlorides such as NaCI or HCI in conjunction with catalytic active metallic chlorides like CuCl2 or FeCl3 will form elemental chlorine (CI2) in the presence of oxygen according to the well known Deacon reaction, shown in Figure 4. Subsequently, Cl2 reacts with aromatic components in the flue gas or fractions from the carbon in the fly ash to form chlorinated organic compounds and fragments, which combine to become PCDDIPCDF in the next reaction step. The first theory postulated by Hagenmaier [3] assumes a dualistic principle of catalytic PCDDIPCDF destruction depending on temperature and oxygen concentration (dechlorination/hydrogenation) and catalytic PCDDIPCF reformation by means of chlorine. The destruction of PCDDIPCDF by dechlorination increases exponentially with temperature; whereas the formation is limited with increasing temperature with the reaction velocity of chlorine formation becoming the rate determining step. Due to the mentioned influencing factors such as chlorine concentration and carbon catalytic surface activities, a temperature range results, where the PCDDIPCDF destruction velocity is substantially higher than the fonnation velocity (Figure 5). Thus, a well designed, operated and maintained waste incinerator acts as an overall sink for PCDDIPCDF due to an overall destruction efficiency of over 99.99%. The second theory fOImulated by Griffm [4] assumes a limiting control-mechanism for the chlorination reaction of organic compounds. The in situ formation of chlorine (CI2) gas, according to the copper catalyzed Deacon reaction, increases with decreasing temperature, increasing oxygen concentration and decreasing water vapor concentration. The kinetics of both reactions, i.e. the formation of Cl2 and the chlorination of aromatics, are enhanced with an increase in temperature. These reactions indicate that aromatic ring structures and Cl2 present in the flue gas are the potential ingredients for the reformation and subsequent emissions of PCDDI PCDF. However, chlorination of -aromatics is limited when sulfur is present in the flue gas. If S02 exists in excess relative to C12, the competitive oxidation reaction of S02 to S0 3 predominates. Chlorine is intercepted by S02 and consequently it would not be present in sufficient quantities for the formation of chlorinated aromatics as detailed in Figure 6. Following this theory, a chlorine to sulfur ratio of less than approximately 0 .1 would be sufficient to prevent the reformation of PCDDI PCDF, because the chlorine interception reaction should predominate (Figure 7). Both theories of the De-Novo-Synthesis have been supported by measurements and examples. However, it cannot be defmitely decided which one is right. The De-Novo-Synthesis is most active in a temperature range of 200 500 °C with a maximum at approximately 350 0c. From the theoretical knowledge about PCDDI PCDF destruction during waste combustion and subsequent reformation in the heat recovery boiler, several equipment design and combustion operation principles have been derived as primary measures to minimize PCDDI PCDF emissions from the incinerator before entering the flue gas cleaning plant. Today, such primary measures are consequently applied for the design and construction of new plants. It has also been suggested that these measures should be combined with the addition of inhibitor substances into the boiler to suppress the Deacon reaction. Nevertheless, an emission limit of 0. 1 ng 1-TEQI Nm3 cannot be ensured without additional gas cleaning equipment for the removal of PCDDI PCDF from the flue gas. 3. SOURCES OF DIOXINS AND FURANS According to the above mentioned facts it can be concluded that not only waste combustion plants, but virtually all combustion and thermal industrial process categories in which chlorine occurs in combination with a carbon source at a temperature above 180°C, are potential sources of dioxin and furan formation and emISSIOns. Therefore, in some European countries such as Germany, the Netherlands, Sweden, and Great Britain as well as in North America (Canada and the U.S.) all processes with such conditions precedent were examined. National PCDDI PCDF inventories were established [5] based on very extensive testing campaigns of all potential industrial sources to determine their individual contribution to the total national emissions of PCDDI PCDF. Among these process categories were: • waste incinerators of all kinds; • coal, oil, and wood combustors; • vehicle traffic; • most metallurgical industries, especially sintering processes; • high-temperature processes; • accidental fIres; • chemical production processes; • and numerous others. Some results of intensive measurements as well as information from the literature are presented in Tables 2 and 3. Some 10 years ago, municipal solid waste and its subsequent combustion were among the most important sources of dioxin and furan emissions. The introduction of new, strict emission limits in the early 1990' s led to a dramatic change in this situation. Today, waste incineration has become a rather insignifIcant source of PCDDI PCDF emissions. Up to then, measured dioxinlfuran concentrations in the flue gas stack of MSWI plants varied between 1 and 92 ng 1-TEQINm3 (@ 11 % O2) [6] . Starting with the introduction of the new legislation, most of the existing facilities were retrofItted with extensive flue gas ch:: aning systems. The remaining facilities were either closed down or rebuilt completely. Also additional new waste incineration plants were built. Interestingly, separate studies of the PCDD/PCDF content in the raw waste conducted in the U.S., Canada, and in Germany have indicated that MSWI plants reduce this PCDD/PCDF component by at about _ 90%. That is for every nanogram of PCDDI PCDF in the waste 0.1 nanogram is emitted from the stack making incinerators net reducers of PCDDI PCDF to the environment when compared to landfIll which does not reduce the PCDDI PCDF component of the MSW. Of course, all plants now fully comply with the new stringent standards. Actually; most of the MSWI plants have PCDDI PCDF emissions signifIcantly below the European Union emission limit of 0.1 ng ITEFI Nm3. It is worthwhile to note that the German 17th BImSch V does not differentiate between the various types of waste regarding their emissions from incineration. Thus, the emission limits for municipal waste, industrial waste, hospital waste, sewage sludge, hazardous waste and other types of waste are identical. Consequently, the dioxinlfuran emissions from all