Pollutant emissions from modern incinerators

Int. .I: Environment and Pollution, Vol. 18, No.4, 2002 336 Pollutant emissions from modern incinerators Rui Lima Instituto politecnico de Braganya (IPB), ESTiG, Gab. 23, Campus de SantaApol6nia, Apartado 134,5301-857 Braganya, Portugal (e-mail: [email protected]) Robert T. Bachmann Department of Chemical and Process Engineering, Sheffield University, Sheffield SI 3m, UK (e-mail: [email protected]) Abstract: Emissions of dioxins are mainly from incinerators, domestic and industrial coal combustion, and traffic. However, the major public concern and research effort are associated with the emissions of organic micropollutants from waste incinerators. This paper gives a brief overview of the more recent research and development for the removal of dioxins and heavy metals from flue-gas streams. Special attention is devoted to the origin and control of emissions from incinerators. It appears that flue-gas cleaning systems in modem incinerators are very reliable in removing almost all polluting emissions from flue-gas streams. However, the fly ash residues generated pose a significant disposal problem, as they are enriched with heavy metals and organic micropollutants. Keywords: dioxins, micropoIlutants. . fly ash residues, heavy metals, incineration, organic Reference to this paper should be made as follows: Lima, R. and Bachmann, R.T. (2002) 'Pollutant emissions from modem incinerators', Int. J. Environment and Pollution, Vol. 18, No.4, pp. 336-345. 1 Incineration The incineration process consists of a number of stages: drying (mainly at 50-200 °C), degasification (mainly at 250-400 °C), gasification (mainly at 400-600 °C) and combustion (mainly at > 600 °C) [I]. In the past, about 30 municipal solid waste incinerators were operated in the UK, some of which had no flue-gas cleaning system. However, over the last 15 years, advances in incinerator research and development as well as more stringent emission limits have resulted in equipping the majority of municipal waste incinerators with fluegas cleaning systems and energy recovery boilers. Energy recovery, via district heating and electricity generation as illustrated in ~Figure I, is an exemplary way of ensuring economic as well as ecological viability of the incineration process. 331 Pollutant emissionsfrom modern incinerators Over the years, the incineration of municipal solid waste (MSW) in 'waste-to-energy' facilities has become a widespread and growing practice in some western European countries, such as Germany, the UK, Denmark, Sweden and Switzerland, and also in Japan [I]. The Sheffield incinerator in the UK is a very good example of how energy recovery has successfully been employed. David Lawrence, Chief Executive of Sheffield Heat and Power (SHP -responsible for the operation of the Sheffield incinerator) pointed out that 'the beauty of Sheffield Heat and Power lies in the simplicity of the product it supplies- energy from waste' [2]. Clinical Waste Incinerator Flue Gls - -Stear1ow Auxiliaryi;s A-Waste Ram Feedel u I Economizer Heat Fabric -I Filter -- Boiler ID '0" ~ n -~vVvV , ~ --,-. I Fly Ash ~ Q~~ Bottom ~trict Heating Ash Reagen Injectio Figure 2 1 Schematic of typical modern municipal waste incinerator. Pollutant emissions to the atmosphere There is a great variety of emissions originating from incinerators, such as heavy metals, carbon monoxide, dioxins, polycyclic aromatic hydrocarbons (PARs), etc. [3]. Table 1 summarizes the most important pollutants associated with the incineration of waste. Waste incineration is reported to be one of the important sources of dioxin and hydrochloric acid emissions into the air [4]. In addition, Nriagu and Pacyna [4] also indicated that waste incineration contributes strongly to the total emissions of heavy metals. However, emission of dioxins, furans and heavy metals can be reduced by efficient abatementtechnologies [5] some of which are listed in Table 2. The concentration of some pollutants is directly related to the composition of the wastethat is feed into the combustion chamber. For example, the oxides of nitrogen (NO,) R. Lima and R.7: Bachmann can be fonned either by the oxygenation of nitrogen in the waste or by fixation of atmospheric nitrogen in a high-temperature flame [3]. Table 1 Pollutantsassociated with wasteincineration[3]. Acidic gases Hydrogen chloride, hydrogen fluoride, nitrogen oxides, sulfur dioxide. Carbon monoxide Heavy metals (Regulatory limits are set for the following metals, either individually or in combination: Cadmium (Cd), mercury (Hg), arsenic (As), chromium (Cr), cobalt (Co), copper (Cu), lead (Pb), manganese(Mn), nickel (Ni), thallium (TI), tin (Sn).) Heavy metals can appear in different chemical forms, including soluble salts such as chlorides and sulfates and less soluble oxides and silicates. Most of the mercury present is released as a vapour, as is some cadmium. Organic material Organic material occurs in volatile fonns, as well as bound to particulates. A large number of organic compounds can be present if combustion has not been complete or if compounds are allowed to fonn after incineration has taken place. An important class of such compounds is dioxins. Particulates Fine particles of inorganic materials, such as silica, have metals and organic material adhering to them. The size distribution, and thus surface area, of such particles can vary widely, as can their chemical composition and their ability to bind other materials. Carbon dioxide If adequately dispersed, is not conventionally regarded as a pollutant relevant to health or the local environment, and is thus not subject to direct regulatory control. It is however a contributor to the greenhouseeffect. The composition of MSW in turn is a function of the industrialization level of the city or the country , geographic location, season,and the recycling capability of a country [ 6] .For example, more than half of the fraction of MSW in Portugal is food and garden waste, while in Finland more than half of the MSW is paper waste. Table 3 summarizes the typical composition of MSW expressedin weight per cent for selected countries. Table 2 Air pollution control methodsandefficiencies[5]. Pollutant Control methods Typical reduction (%) NOx Selective catalytic reduction, selection non-catalytic reduction, flue gas recirculation, combustion control 10 to 60 Acidic gases (SOx and HCI) Wet scrubber, dry scrubber, fabric filter, electrostatic precipitator 50 to 85 SOx 75 to 90 HCI CO Combustion control 50 to 90 Heavy metals Dry scrubber, fabric filter, electrostatic 70 to 95 Particulates precipitator Electrostatic precipitator, fabric filter 95 to 99.99 Toxic organics ~DD, PCD~ Combustion control, combination of dry sc~ and fabric filter 50 to 99.99 340 R. Lima and R. 7: Bachmann 2.2 Dioxins and furans At present, organic micro-pollutants, such as dioxins and furans, are one of the most urgent public concerns, related to pollutant emissions to the atmosphere. Polychlorinated dibenzo-p-dioxins (PCDD), known as 'dioxin', and polychlorinated dibenzofurans (PCDF), known as 'furan', form a family of chlorinated organic compounds [8] (see Figure 2). There are many different compounds in the PCDDIF family, each determined by the number and arrangement of chlorine atoms occupying the outer ring positions of the molecule (see Table 4). Overall, there are about 210 dioxins and furans, 17 of which are very toxic. For example, the 2,3,7,8-tetrachlorinated dibenzop-dioxins are the most toxic dioxins [9, 10]. II 2 PCDD CI2H(8-x)O2Clx PCDF CI2H(8-x)OClx Figure 2 Table 4 The dioxin and furan molecules The homologues Homologue (abbreviation) and the 2,3,7 ,8-tetra isomers. and congeners ofPCDD/Fs [10]. Number of congeners Number of chlorine atoms PCDDs PCDFs Monochloro (M) I 2 4 Dichloro (D) 2 10 16 Trichloro (Tr) 3 14 28 Tetrachloro (T) 22 38 Pentachloro (Pe) 14 28 Hexachloro (Hx) 6 10 16 Heptachloro (Hp) 7 , 4 75 135 Octachloro (0) 8 TOTAL 341 Pollutant emissionsfrom modern incinerators The toxicity of the 2,3,7,8-substituted congeners is generally considered to decrease with increased chlorination of the compound. For instance, 1,2,3,4,7,8-HxCDD (six chlorine atoms) is considered less toxic than 1,2,3,7,8-PeCDD (five chlorine atoms). Therefore, the toxicities of the congeners of PCDD/Fs are defmed as International Toxic Equivalent Factors (I-TEF) expressed as a fraction, with 2,3,7,8-TCDD taken as unity (Table 5). On this scale, other dangerous individual compounds are 1,2,3,7,8-PeCDD (I-TEF = 0.5) and 2,3,4,7,8-PeCDF (1-TEF = 0.5) (10]. Concern about dioxins is primarily due to their carcinogenicity and toxicity. They can persist in the environment for several years, and have a tendency to bioaccumulate in animal tissue; consequently, they may pass into the human food chain (10]. Table 5 2,3,7,8-substituted congenerPCDDIFswith InternationalToxic EquivalentFactors[10]. PCDDs I-TEF PCDFs I-TEF 2,3,7,8-TCDF 1,2,3,7 ,8-PeCDF 2,3,4,7 1,2,3,4,7,8-HxCDD 0.1 1,2,3,6,7,8-HxCDD 0.1 ,8-PeCDF 1,2,3,4,7,8-HxCDF 1,2,3,6,7,8-HxCDF 2,3,4,6,7,8-HxCDF 1,2,3,,7,8,9-HxCDD 0.1 1,2,3,7,8,9-HxCDF 1,2,3,4,6,7,8-HpCDD 0.01 1,2,3,4,6,7,8-HpCDF 1,2,3,4,7,8,9-HpCDF OCDD 0.001 OCDF 2.2.1 Origin of dioxins It is commonly reported that the dioxins are formed as products of incomplete combustion. On the other hand, there are a wide number of theories proposing a combination of conditions responsible for the formation of dioxins during combustion [8]. The following reactions lead to the formation of dioxins: During combustion: formation of dioxins, which, owing to their presence in the waste and because of their thermal stabilities, survive the combustion process particularly at lower temperatures [ II] . During incineration: dioxins are produced via reactions between precursors such as polychlorinated biphenyls (PCB), chlorinated benzenes, etc. These precursors can occur in the waste or be formed via combustion of plastics. However, a number of researchers havedemonstratedthat there is no relationship between PVC plastic or hydrogen chloride (HC1)and dioxin emissions from the incinerator [8]. Post-combustion ('de novo synthesis): it has been claimed, by numerous researchers, that the dominant mechanism of dioxin formation involves reactions between chlorinated and/or the products of 'de novo synthesis'. These reactions take place on the fly ash particles at relatively low temperatures (200-400 °C), which can in the post-combustion environment of the boiler or at arrestment equipment. -mainly copper and iron, are thought to Moreover, precursors, such as p AH and other organics, adsorb onto the R. Lima and R. T. Bachmann surface of ash particles. At active places where metal compounds are present PCDD/F formation occurs by catalysis [10, 11]. Techniquesfor controlling dioxins and emissions to the atmosphere A general understanding of the mechanism of formation of dioxin emissions is fundamental for the design of control systems. Owing to the controversy about the origin of dioxins, as discussed in the previous section, there are generally two main ways of controlling these emissions. Combustion control An efficient combustion control is correlated with a destruction of dioxins of greater than 99.99% [ 11].Combustion control targets parameters such as temperature, combustion air and turbulence. The current legislation in the UK is focused on combustion efficiency with minimum furnace temperatures of 850 °C, for at least 2 seconds in the presence of at least 6% of oxygen [7]. It is generally agreed that thermal destruction increases exponentially with temperature, and that combustion temperatures of 1000 °C, with a residence time of 1 second, are adequate for total destruction [8, 11]. Generally, modern incinerators are equipped with a primary combustion chamber followed by a secondary combustion chamber, in order to ensure a more complete destruction of the waste components. As a result, less unburned products of incomplete combustion (PICs) are formed, and therefore, the potential for heterogeneous reactions involving precursors ofPCDDs, PCDFs, and PARs is reduced [II]. Post-combustion control A large number of researchers, including Hagemnair and Haag [7], have claimed that the ,de novo synthesis' is the dominant formation mechanism for dioxins. Therefore, several techniques have been developed for removing dioxins from flue-gas streams: .Fabric filters (increasingly used due to their greater particle-removal efficiency) [11] .Dry/wet scrubbers with lime as the active scrubbing agent (seems to be another efficient way of removing dioxins; however wet systems are not that effective since some dioxins have a very low solubility in water) [8] .Combination of dry scrubber and fabric filter (typical dioxin emission reduction of 50-99.9%) [5] .Combination of activated carbon injection and fabric filters (guaranteesemissions of less than 0.1 ng/m3 for dioxins) [12] .Combination of electrostatic precipitator, wet scrubber and fabric filter (filsorption stage- filtration, sorption and chemical reaction in fabric filter). Th~ results of this combination are extremely good, as shown in Table 6 [13]. Although almost all dioxins and heavy metals are removed from flue-gas streams, the fly ash residues generatedpose a significant disposal problem. Pollutant emissionsfrom modern incinerators Table 6 Results of emission measurements Concentrations Parameter I- TEQ-dioxin 3 34~ at the incinerator plant in Zimdorf, Germany. in ng/m3 dry gas, 11% O2 [13]. (DecI991) Measurements (MayI992) <0.01 <0.01 Measurements Gennan Regulation 0.1 Contaminated ash State-of-the-art technology in concert with a good design offers the key to meet very high standards of pollution control. However, secondary waste products from the municipal waste combustion process, such as solid residues or wastewater from quenching and gas scrubbing processes,must be managed in an environmentally acceptable way. The solid residues from the incineration process are one of the major concerns due to the presence of heavy metals and organic micropollutants in these ashes. The ash residues are normally classified (see Figure 3) as follows: .Bottom ash -ash from the furnace grate, which represents 85-90% of the total ash. The concentrations of heavy metals and dioxins are generally quite low, therefore this ash is usually disposed directly to landfill. .Fly ash -particulate matter carried over from the combustion chamber, incorporating metals and organic material, including dioxins, volatiles condensed during flue-gas cooling and other products of incomplete combustion. This ash represents 10-15% of the total ash and sometimes is referred to as the air pollution control (APC) residues. Fly ash is usually recovered using both electrostatic precipitators and bag-house filters. .Combined ash -is a mixture of all residue streams [3, 14]. The disposal of both bottom ash and fly ash produced by the combustion process has to be managedin an economical and environmentally friendly way. However, special attention must be devoted to the fly ash as it is enriched in heavy metals and organic micropollutants (dioxins, PARs, etc.). Fly ash is considered as a hazardous material by legislation in many countries [ 15] . Another important aspect is that various pollutants present in the fly ash become increasingly hazardous when placed in landfill, where leaching of the pollutants can be a sourceof groundwater contamination [7].