Study of dioxins formation in the basic oxygen furnace during organic waste disposal

Transcription

1 IOP Conference Series: Materials Science and Engineering PAPER OPEN ACCESS Study of dioxins formation in the basic oxygen furnace during organic waste disposal To cite this article: S N Kuznetsov et al 2016 IOP Conf. Ser.: Mater. Sci. Eng View the article online for updates and enhancements. This content was downloaded from IP address on 19/10/2018 at 03:28

2 Study of dioxins formation in the basic oxygen furnace during organic waste disposal S N Kuznetsov, E P Volynkina and E V Protopopov Institute of Metallurgy and Material Science, Siberian State Industrial University, 42 Kirova Street, Novokuznetsk, , Russia kaf.erc@sibsiu.ru Abstract. Incineration is one of the most common methods used for organic waste utilization. However, there is a danger of the secondary generation of such supertoxicants as dioxins and furans. The results of the investigations of experimental and comparative converter meltings with the use of paper and plastic wastes under conditions of JSC EVRAZ ZSMK show the absence of influence of wastes on the concentration and isomeric profile of dioxins and furans in the converter gases. 1. Introduction High temperature incineration is one of the most widely accepted methods used in organic wastes disposal [1-5]. As noted in [1], the incineration is applicable to virtually all types of organic wastes, since it transforms the main elements of most organic compounds (carbon, hydrogen, oxygen) into environmentally safe CO 2 and H 2 O, and in the presence of chlorine into chlorine hydride recycled in the wet cleaning system. However, to ensure the full decomposition of hazardous organic compounds it is necessary to observe a number of conditions: incineration temperature not less than 1200 С; residence time of incineration products in the incineration chamber is not less than 2 s (rule excess of oxygen in the combustion zone; rapid cooling of converter gases to a temperature below 200 C, which makes secondary generation of dioxins impossible. Failure to comply with these requirements may result in the formation of polycyclic aromatic hydrocarbons, many of which are carcinogens, such as benzopyrene and high toxic polychlorinated dibenzodioxins (PCDDs) and dibenzofurans (PCDFs). Over the past decade incineration technology in the world has come a long way from an open incineration and simple incinerators, which heavily polluted the environment, to the modern efficient incinerators equipped with a complex system of flue gases purification and limiting emissions of pollutants into the environment. Nowadays, in Russia several types of incinerators are produced. They are offered for incineration of different kinds of solid and liquid organic wastes, including chemical, medical wastes, biological, oil sludge, as well as organochlorine wastes that belong to the list of POPs, such as PCB oils, pesticides, herbicides. Incinerator is a chamber lined with bricks and equipped with a diesel / natural gas burner. In the incinerators usually a two-step technique of waste disposal is used, that is Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by Ltd 1

3 incineration of wastes at a temperature C, post-incineration of gases at a temperature C followed by mechanical and chemical cleaning. Rotary cement furnaces are widely used abroad for disposal of hazardous organic wastes. In 2003 in the Report of the first session of the expert group on the best available techniques and best environmental practices of the UNEP UN [6] cement furnaces are recommended for decomposition of PCDDs and PCDFs. In cement furnaces it is technologically possible to meet the described above requirements and ensure the complete decomposition of organic compounds in wastes, including organochlorine compounds. A well-known international document containing data on PCDDs and PCDFs emissions in various technological processes is Standardized Toolkit for Identification and Quantification of Dioxin and Furan Releases developed by UNEP in the framework of UN environment program [7]. It states that PCDDs and PCDFs can be supplied for incineration alongside with the disposed wastes, be generated in the process of incineration or after its completion during the flue gas cooling. In this regard, [6] states the following primary methods and measures for optimization of the production process to reduce PCDDs / PCDFs formation: continuous supply of fuel and wastes with the registration and accounting of the content of heavy metals, chlorine, sulfur; maintenance of control over the materials loaded into the furnace; pre-treatment of wastes in order to provide a more uniform load, which contributes to efficient incineration under more stable conditions (drying, fining, mixing, grinding); loading of wastes into the main chamber or the secondary one in the furnaces with preliminary heating (temperature > 900 C); wastes cannot be loaded alongside with a raw material mixture, if they contain organic substances; stabilization of technological parameters (constancy of fuel properties, constant dosage, an excess of oxygen, CO control); wastes loading at the stages of furnace start-up and shut-down is not permitted; rapid cooling of flue gases to temperatures below 200 C. The possibility of achieving PCDDs content in the flue gases at a level less than 0.1 ng/m3, permitted by European regulations, due to primary measures at the existing plants has been proved. If all these methods do not result in the value decrease to the level less than 0.1 ng/m3, in [6] as secondary measures it is recommended to improve the purification methods of flue gases from dust, for example, to use activated carbon filters or selective catalytic reduction. However, the cement production technology in rotary furnaces cannot guarantee the complete decomposition of organic wastes. In cement furnaces there are several temperature zones continuously transforming into each other: heating zone ( C), calcination ( C) zone, sintering area (1450 C), cooling zone ( C). Only one of these zones ensures the prevention of PCDDs and PCDFs formation, but only by maintaining the time conditions. However, it cannot be guaranteed, because the flue gases pass successively through all temperature zones, and the time spent in each zone is unlikely to be sustained precisely because of their continuity. According to the EU standards the geometry of the furnace hot zone should provide residence of gases in the zone at a temperature not lower than 850 C for at least 2 seconds (rule of 2 seconds) with an oxygen concentration not less than 6 %. This is a very stringent requirement which is not easy to meet. The increase in emissions is observed at the beginning of furnace operation, so the EU standards allow the beginning of pyrolysis or waste incineration only after the furnace is heated to 850 C [6]. High-temperature steel units provide a more safe incineration of combustible waste components than incinerators or cement furnaces. It is conditioned by the fact that the incinerators and cement furnaces cannot compete with high-temperature metallurgical aggregates such as blast furnace or basic oxygen furnace (BOF) in conditions of toxic wastes disposal, that have temperature up to C, active oxidizing or reducing atmosphere, intensive heat- and mass-transfer, availability of high active basic slags, providing binding of non-combustible residue left from the waste incineration and 2

4 its vitrification. In such circumstances dioxins formation is practically impossible. Besides, the powerful metallurgical units are equipped with a full set of equipment for the capture and cleaning of the released gases, including the wet gas cleaning and a closed water supply system. The UNEP Standardised Toolkit for Identification and Quantification of Dioxin and Furan Releases [7] gives the following values of PCDDs / PCDFs emissions in various technological processes (Table 1). Table 1.Emissions of PCDDs / PCDFs from a variety of sources, µg TEQ / tonne of product. Type of a source Air Products Wastes Ash Slag Solid waste incineration in the waste incineration plant with a complex system of gas purification 0.5 n/t** Medicine waste incineration in the incinerators with a complex system of gas purification 1 n/t Cement furnace n/a*** Blast furnace 0.01 n/t - n/t BOF 0.06 n/t * TEQ International toxic equivalent ** does not transform *** data is not available These data indicate that high-temperature metallurgical aggregates (blast furnace, basic oxygen furnace) provide minimal emissions of dioxins and furans by one-two orders of magnitude below of any known most modern units, currently used in the world for incineration of hazardous organic waste. At the same time, incineration of organic waste in metallurgical units provides additional heat generation and save traditionally used energy supplies (coke, coal, fuel oil, natural gas, etc.). Technologically in the blast furnace neutralization of liquid and solid organic waste with sizes less than 100 mm is possible, and in the BOF solid organic waste with size from 5-10 mm. 2. Experimental research and results Let us consider the technological possibility of neutralization of hazardous organic wastes in the BOFs of ZSMK. Waste incineration in the converter proceeds in the following temperature conditions: period of scrap heating C; period of hot-metal charging over 1380 C; blowing period from 1440 to over 2000 C. Wastes incineration in the converter occurs in the atmosphere of commercially pure oxygen with a purity not lower than 99.5 %, supplied with high intensity and providing a high rate of heat- and masstransfer. It allows an effective complete combustion of all organic components of the fuel and raw material and destruction of the lattice structure of complex organic compounds, including dioxins. Wastes reside in the converter during the whole period of smelting, complete incineration takes place during scrap heating and subsequent blowing. From the very first moment of waste input into the converter they are in close contact with fluxes, the main of which is lime. During blowing the lime ensures formation of the basic Ca-containing slag. The presence of significant amounts of lime in the converter accelerates oxidation and decomposition reactions of complex organic compounds and destruction of dioxins. Residues from waste incineration transforms into the molten slag, which absorbs all the products of combustion with subsequent vitrification during cooling and is not harmful to the environment. Converters are equipped with gas exhaust ducts serving for extraction, cooling, purification and post-incineration of converter gases. Gases, exiting the converter, go through the boiler-cooler, where the temperature drops to 850 C, and then are directed to the first stage of purification irrigated gas duct. In the irrigated gas duct the further cooling of gases to C takes place, as well as coarse 3

5 cleaning and wetting until their complete saturation. Saturated gases then pass the I and II stages of rectangular Venturi tubes, which is their primary treatment, and, after the drops separator drained enter the post-incineration device. Wet cleaning of converter gases ensures a high degree of dust collection 99.5 %. The process water in the wet gas cleaning system is characterized by high ph value at the level of units, that indicates a high content of lime. High alkalinity of the process water will contribute to the high capture efficiency of chlorine-containing gases (chlorine hydride, gaseous chlorine) from the converter gases. Thus, in the existing gas treatment system of converters there are favorable conditions for collecting chlorine-containing impurities from the flue gases at the moment of their formation, for example, when products from PVC get into the wastes or disinfection of wastes by chlorine-containing disinfectants. Table 2 provides the comparative characteristics of the conditions of dioxins formation in various technological processes used for the incineration of hazardous organic wastes incinerator, rotary cement furnace, as well as in the high temperature metallurgical unit (BOF). Table 2.Comparative characteristics of the conditions of dioxins formation and technological parameters of units used for the incineration of hazardous organic wastes. Technological parameters Method of waste charging Incineration temperature, С Conditions of dioxins formation Charging of large amounts incinerator Charging of large amounts Technological units rotary cement furnace Wastes pretreatment, that provides even charging of small amounts Less than 900 С С Temperature zones from 500 С to 1450 С BOF Charging of small amounts with consumption from 0.1 to 2 kg/tonne of steel, that along with the high rate of oxygen blast ensures good heat- and mass-transfer during their incineration High temperature incineration mode at temperatures С О 2 content, % Lack of oxidizer Excess oxygen, oxygen blast 99.5 % О 2 Purification of converter gases Purification in electrofilter Post-incineration of converter gases Formation of ash/slag Dust collectors, functioning at high temperatures (more than 200 С) Absence Dry purification, in some cases wet one in the scrubber with dust removal efficiency 93 % Post-incineration in the flare with temperature 1500 С Absence Three-stage wet purification, providing high degree of dust removal (99.5 %) and rapid gases cooling to С Post-incineration in the flare with temperature 1500 С Ash Ash High-active liquid slag, containing more than 50% of СаО, creates good conditions for sulfur and halogens absorption. During solidificationvitrification. 4

6 The given data show that the smelting conditions in the steel converter process comply with the conditions that ensure non-dioxins waste disposal: high-temperature incineration mode at temperatures above 1250 C; intense heat- and mass-transfer in the converter; active oxidizing atmosphere with an excess of commercially pure oxygen; high-active liquid slag, absorbing sulfur and halogen containing compounds, during solidification vitrified slag; rapid cooling of the flue gases in a wet gas purification system to С; post-incineration of gases in the flare with a temperature 1500 С. Three-stage wet purification of converter gases, providing a high degree of dust removal (99.5 %) and their rapid cooling to C, as well as post-incineration of converter gases exclude the possibility of dioxins re-synthesis. Thus, high-temperature metallurgical units, in particular oxygen converters, are the safest among currently known units used for incineration of hazardous organic wastes including chlorine-containing ones. Under conditions of EVRAZ ZSMK the experimental-industrial smelts were conducted using specially prepared pilot lots of solid municipal wastes (SMW): compacted waste paper, cardboard and polyethylene; crushed electronic equipment enclosures, consisting of polystyrene, with a particle size less than 20 mm. The experimental meltings were performed in the Oxygen Converter Plant 2 on the 350 tonne converter No. 4. Blast of the experimental and comparative melts was carried out according to the standard operating procedures. SMW consumption ranged from 1.0 to 8.8 kg/tonne of steel. It was established that the basic technological parameters of experimental and comparative meltings slightly differ. Meltings with SMW were carried out practically without use of carbon-containing heat-transfer materials, the steel temperature of the turn-down exceeded the steel temperature of comparative meltings. As a result of the calculations of material and heat balance of the converter melting it was found that by introduction of combustible components of SMW into the converter with SMW consumption rate 7.2 kg/tonne of steel additionally kj of heat energy is introduced into the converter, or 4.21 % of the total heat generation. The technology of experimental meltings presupposes supply of the compressed bales of paper and bags of polyethylene with the crushed polystyrene into the furnace alongside with scrap and fluxes (lime). After charging the burden it is heated by wastes incineration in the jet of technically pure oxygen for 5-6 minutes. Then, the liquid pig iron was poured into the converter, and then the melting was carried out by standard method of melt blowing by oxygen for min. and addition of fluxes during smelting. The results of industrial tests, described in detail in [8-10], showed that the basic technological parameters of experimental and comparative meltings differed very slightly. A higher steel temperature of the turn-down during experimental meltings was noted 1706 and 1655 C, while during comparative meltings it reached accordingly and 1629 C, that indicates a higher level of temperatures in the pool during melting due to the high heat of waste combustion. The average dust level of the converter gases after purification was 117 mg/m3. During the experimental and comparative meltings the dust analysis at the outlet of gas emissions flow for dioxins was made. The analysis of the collected dust was carried out by an accredited laboratory of Bashkir Republican Scientific Research Ecological Center in Ufa using the USEPA methodology 1613, adapted to the Russian conditions. The methodology consisted in the dioxins extraction by solvents from the objects in the environment, dioxins extraction from the matrix, their cleaning from the impurities present on the sorbents. The quantitative analysis was performed by the method of isotope dilution using chromatography-mass spectrometer of high resolution. All 17 isomers of dioxins and furans marked 5

7 by the isotope of carbon C 13 were used as internal standards. We studied 5 isomers of tetra-chlorinated dioxins 1,3,6,8 TCDD, 1,2,3,4 TCDD, 1,2,3,7 TCDD, 2,3,7,8 TCDD, 1,2,3,9 TCDD. The results of quantitative isomer-specific analysis of polychlorinated dibenzo-p-dioxins and dibenzofurans are shown in Table 3. Table 3.Quantitative isomer-specific analysis of polychlorinated dibenzo-p-dioxins and dibenzofurans in the collected dust of the experimental and comparative meltings. SampleNo. 1 (experimental melting) SampleNo. 2 (comparative melting) PCDDs/Fs Concentration, pg/g of dust TEQ-WHO, pg/g of dust Concentration, pg/g of dust TEQ-WHO, pg/g of dust 2378-Tetra-CDD Penta-CDD Hexa-CDD Hexa-CDD Hexa-CDD Hepta-CDD Octa-CDD Tetra-CDF Penta-CDF Penta-CDF Hexa-CDF Hexa-CDF Hexa-CDF Hexa-CDF Hepta-CDF Hepta-CDF Octa-CDF Total amount of PCDDs, pg/g of dust Total amount of PCDF, pg/g of dust Total amount of PCDD/F, pg/g of dust For each sample the concentrations of isomers in the unit of measurements pg ( g) per 1 g of dry sample weight are given, as well as the toxicity coefficients of each isomer in terms of toxicity 2,3,7,8 TCDD in accordance with factors of equivalent toxicity (TEF). As a result of the studies no significant differences in the concentrations or in the isomeric profile for both samples were found. Value of the cumulative rate of sample toxicity TEQ (amount of the reduced toxicity of all isomers) also differed slightly and amounted to pg/g TEQ for the experimental melting and pg/g for the comparative melting. The experimental data on the content of PCDDs / Fs in the ash samples can be used to assess the pollution of the converter gases. If to assume that the analyzed samples are the settled dust emitted from the pipe with the flue gases into the atmosphere, then the amount of dioxins emitted, connected with dust particles, will depend on the concentration of suspended particles in the converter gases. So, if the concentration of suspended particles in the converter gases is 117 mg/m3, according to the study the total content of dioxins and furans in the flue gases of experimental melting was pg/m3, comparative melting pg/m3. It should be noted that the European standard for flue gases from stationary sources is 0.1 ng/m3 (1 ng=1000 pg= g) or 100 pg/m3. 3. Conclusions 6

8 Thus, the analysis of the conditions of high-temperature processes in the metallurgical units and the research results show that high-temperature metallurgical units are by far the safest for disposal of hazardous organic wastes, including those containing such hazardous to human health supertoxicants as dioxins and furans, as well as carcinogenic hydrocarbons that are formed in large amounts even during incineration of some kinds of wastes at incineration plants, a fortiori, during un-organized burning at landfills. 4. References [1] Dawson G and Merser B 1996 Neutralization of Toxic Wastes (M.: Stroyizdat) p 288 [2] Kraynov I P 2002 Ecol. Tech. & Res [3] Orris P, Chary L K, Perry K and Astury J 2000 Persistent Organic Pollutants (POPs) and Human Health (Washington WEPHA) p 38 [4] Birman Yu A and Vurdovoy N G 2002 Environmental Engineering (M.: Association of Construction Universities) p 269 [5] Vlasov O A, Mechev V V and Mechev P V 2015 MSW [6] Report of the Co-chairs of the Expert Group on Best Available Techniques and Best Environmental Practices [7] Standardized Toolkit for Identification and Evaluation of Emissions of Dioxins and Furans. UNEP [8] Volynkina E P, Klimovskaya I A and Protopopov E V et al 2008 Proc. 2nd Int. Conf. on Waste Management Basis for the Restoration of the Ecological Balance in Kuzbass (Novokuznetsk: SibSIU) pp [9] Kuznetsov S N, Volynkina E P and Protopopov E V 2015 Bulliten of Universities. Ferrous Metallurgy [10] Kuznetsov S N, Volynkina E P, Protopopov E V and Zorya V N 2014 Metallurgical Technologies for Disposal of Technogenic Deposits, Industrial and Domestic Wastes (Novosibirsk: SB of the RAS) p 294 7