What are dioxins (PCDD/Fs)? Why are they important?

Transcription

1 Dioxin formation and emission control Prof. em. Dr. ir. A. Buekens Now with: Department of Energy Engineering, Zhejiang University. Hangzhou, P.R.China Haldor Topsoe Meeting "Catalysis in new environmental processes 27- Contents What are dioxins? Difficulties of sampling and analysis? Routes of formation? Ways of reduction and destruction? Outlook? Annexe I. About the Author (and dioxins) Annexe II. About MINIDIP and other Projects Dibenzo-p-Dioxin (DD) Structure Mono-to octa-chlorination is possible in positions 1 to to 9 What are dioxins (PCDD/Fs)? Why are they important? DibenzoFuran (DF) structure Mono-to octa-chlorination possible in positions 1 to to 9 In thermal processes 2,3,4,7,8 PeCDF is first in I-TEQ values! 2,3,7,8-TCDF

2 Dioxins Dioxins = sum of (P = 1 to 8) PCDD/Fs 75 Polychlorinated dibenzo-para-dioxins or PCDDs, 135 Polychlorinated dibenzofurans or PCDFs Analysis almost always limited to P = 4 to 8, or even only to the dirty 17, i.e. 7 out of 75 distinct PCDD congeners, and 10 out of 135 distinct PCDF congeners. Dirty 17 Dioxins 7 Polychlorinated dibenzo-p-dioxins PCDDs 10 Polychlorinated dibenzofurans PCDFs with substitution at the positions 2,3,7,8, leading to a planar structure with each a toxicity-equivalency factor (I-TEF). Generally, ca. 40% of dioxin-toxicity derives from 2,3,4,7,8 PeCDF. Dioxin-like toxicity is also exhibited by PCBs. Polychlorinated biphenyls (PCBs) There are 209 PCB-congeners, with P = 1 to non-ortho, mono-ortho PCBs are also capable of assuming a planar structure. In thermal processes PCBs sign for ca. 5% of the total WHO dioxins toxicity. PCBs were widely used, e.g. as thermal oil and dielectric fluid in transformers and capacitors. PCB production was first banned in 1976 (USA). Toxicity Equivalency Scales Toxic Equivalent (TEQ) schemes weigh the toxicity of less toxic compounds as fractions of the toxicity of the most toxic 2,3,7,8-TCDD, which is given a reference value of 1. The I-TEF (NATO-CCMS) system is used most. In thermal sources TEQ mainly derives from PCDF. 0.pdf

3 PCDD/F fingerprints Characteristic fingerprints may link specific emissions to their sources. Examples are: An atmospheric fingerprint, mainly OCDD and HpCDF. A thermal fingerprint, with more PCDF than PCDD-toxicity. A chemical fingerprint, e.g. in PCBs, electrode sludge... Importance of dioxins PeCPh, Agent Orange... was contaminated with dioxins. Industrial accidents, such as at Monsanto (1949, Nitro), BASF (1952), Philips-Duphar (1963), Seveso (1976), often involved herbicide production Municipal Solid Waste Incineration (MSWI) Metallurgy, almost all thermal processes, except those based on sulphide roasting. Cold sources, such as chlorine pulp bleaching, water disinfection, and chlorine chemistry at large. Brominated Fire Retardants (BFR) lead to mixed brominatedchlorinated dioxins PCBrDD/Fs. Some Facts Persistent Organic Pollutants, or POPs, regulated by the Stockholm Conference (2002). PCDD/Fs and PCBs are POPs, almost insoluble in water, semi-volatile, lipophilic, bio-magnifying. In thermal sources PCDD/F (and PCBs) appear all together, unless derived from specific precursors, such as chlorophenols (Agent Orange, Seveso). Production techniques PCDD/Fs were never produced commercially, except for scientific purposes. They occur as by-products in a series of chemical syntheses, of herbicides, dyestuffs, etc. They appear as a by-product of any thermal process. Annual rates of generation reached ca. 1 kg/year in some dioxin-proficient countries. PCBs were used in numerous, safety-related applications, until being phased out. Inventories are important! Media attention PCBs are important dioxin-like contaminants. More than 2 M tonnes was commercialised! The Seveso Disaster (1976) changed the face of industry (E.U.-Directive). Greenpeace accused PVC to be at the origin of dioxins from MSWI and launched a campaign against chlorine industry and MSWI.

4 In summary PCDD/Fs occur in all thermal processes and as a by-product of few chemical syntheses. PCBs were produced industrially. They are less toxic, yet inventories are huge. PCDD/Fs and PCBs are highly toxic substances. Their occurrence in the environment must be reduced. Exposure principally occurs via the food chain. Methods and difficulties of analysis? Sampling & Analysis Dioxins occur at very low levels of concentration: emission limit values are e.g. 0.1 ng I-TEQ/m 3 (s.t.p.), i.e. the order of 0.1 ppt. Contaminated solids are loaded with 1 pg to 10 ng I-TEQ/g. They appear together with hundreds of other organics. Clean-up procedures are lengthy, cumbersome, and costly. There is no continuous analyser for dioxins. These are to be sampled in a reproducible and isokinetic manner. Off-gas is often stratified! Surrogates or Indicator compounds PCDD/F are Products of Incomplete Combustion (PICs), such as CO, TOC, PAHs! PCDD/F often occur together with PAHs as well as chlorinated benzenes (PCBz), naphthalenes (PCN), biphenyls (PCB), phenols (PCPh)... as well as with hetero-compounds, featuring sulphur or nitrogen. Such surrogate compounds may reach higher concentrations and be easier to analyse! Precursors In principle, almost any organic compound may become a dioxin precursor. In fact, any combination of C, H, O, and Cl will yield some PCDD/F! The term precursor is normally reserved for compounds with strong structural similarity with dioxins. Dioxins may result from a condensation of chlorophenols, oxidation of PCBs, oxi-chlorination of PAHs, etc. Scrambling tests suggest distinct pathways for forming PCDD and PCDF.

5 Dioxins analysis Dioxins occur as minute traces in a host of different matrices: e.g. in off-gases (1 pg to 100 ng/nm 3 ) Residues (1 pg to 100 ng/g) Ambient air (fg/nm 3 ) Milk fat (0.4 pg to 6 pg/g fat). Sampling & analysis are very tricky and difficult! Theories of Formation MSW-incineration In 1977, dioxins were found on fly ash from MSWI in Amsterdam (Dr. Olie, Prof. Hutzinger). Early research centred on: Confirming and explaining this discovery. Improving sampling & analysis. Inventing formation pathways. Developing reduction & abatement methods. MSW-incineration MSWI long has been the principal known source. The amount of dioxins entering MSWI was comparable with that leaving the plant. Dioxins entering are effectively destroyed. Formation of new dioxins starts in the boiler and peaks in hot electrostatic precipitators. Reduction and abatement methods were imposed even earlier than they were demonstrated at large-scale! Theories of Formation The appearance of dioxins in MSWI has prompted much concern, as well as research: Trace Chemistries of Fire. Precursor routes, starting from phenol or phenoxy radicals, toluene, PAHs, quinones... High temperature formation of basic structures, followed by low temperature chlorination. de novo route. Trace Chemistries of Fire (1980)... dioxins are present in particles from many types of combustion sources and in dust and soil in the vicinity of combustion sources. The data... suggest the hypothesis that chlorinated dioxins result from trace chemical reactions occurring in fire. Authors: Bumb, R. R.; Crummett, W. B.; Cutie, S. S.; Gledhill, J. R.; Hummel, R. H.; Kagel, R. O.; Lamparski, L. L.; Luoma, E. V.; Miller, D. L.; Nestrick, T. J.; Shadoff, L. A.; Stehl, R. H.; Woods, J. S. Science, Volume 210, Issue 4468, (1980)

6 Precursor theories Condensation of 2 chlorophenol molecules (or phenoxy radicals) yields PCDD, e.g. 2 PeCPh gives OCDD. 2,3,7,8-TCDD was generated during a reactor run-away accident at Seveso, involving TrCPh. PCDFs occur as a trace impurity in PCBs. Some structurally relevant PAHs and quinones can be converted into PCDD/F with appreciable yields. However, the typical incineration fingerprint is not generated from such precursors! de novo formation Major Factors of Influence Time: formation linearly increases with time. Temperature: maximum rate at C. Oxygen: (quasi-)linear rate increase with O 2. Chlorine: only rarely rate-determining, but plays several roles! Carbon: structure relevant, surface not. Precursors: e.g. PAH, PCPh, PCB... The latter are also de novo products! Catalysts: Cu, Heavy Metals, Surfaces, Matrix. Suppressants. Ratio of S/Cl. Memory effects. de novo formation The first de novo experiments demonstrated that MSWI fly ash, when annealed in air at 300 C, generates new dioxins, sometimes 10 times more than those originally present. Formation continues for hours at least. Important factors are: carbon, chlorides, oxygen, the fly ash matrix and intrinsic catalysts. de novo catalysis The de novo theory, as developed by the late Dr. L. Stieglitz, requires two types of catalysis: Capability of chlorinating carbon. Oxidising chlorinated structures (incompletely). Copper chloride (oxychlorination) is most active, but transition metals all show some activity. Iron compounds are also cited. The real catalyst has never been identified!

7

8 Carbon Many types of carbon have been investigated: Activated carbon is the most typical model substance. Polycyclic Aromatic Hydrocarbons (PAHs) are also converted into PCDD/F; yield varies strongly with parent structure. Graphite is least active. (Amorphous) structure is important, not specific surface. Mineral matrix MSWI Fly Ash is heterogeneous and has a highly complex composition. Laboratory surrogates are silica, alumina, Mgsilicates, etc. Real fly ash, when added to carbon, conveys superior de novo activity! Copper occurs at a level of wt.% (mechanical grate plants) or more (fluid bed units) Chlorine Chlorine is an ubiquitous element. NaCl, KCl, Zn, Pb, Cu... considerably enrich in fly ash. HCl is less active in de novo synthesis. Cl 2 is a strong chlorinating agent. The Deacon equilibrium is most favourable at 450 C. de novo synthesis is most active at 350 C. Role of temperature During laboratory experiments a single or a double optimal temperature of formation is identified. Above 350 C decomposition becomes faster. High temperature formation (<500 C) requires residence times remaining short. Formation reactions slow down considerably below 300 C. Yet, slow reaction rates may be compensated for by long residence times, also Haldor causing Topsoe Meeting memory "Catalysis in effects. new environmental processes 27- Rate determining factors Rate determining factors may vary with circumstances. Most important are: Suitable catalysts for chlorination and oxidation. Reactive carbon/matrix interfaces. Adequate oxygen supply. Limited destruction activity. Chlorine is rate determining only when scarcely available, which is not the case in MSWI.

9 Memory Effects The formation of CO, TOC, and PICs by combustion upsets may cause PCDD/Fs, hours or days afterwards. During real-scale studies, the link between operating parameters and output thus may fail. Effects on dioxin emissions always remained obscure. Formation from fly ash deposits continues for long time periods: memory effects. Wet scrubbers also cause almost lasting memory effects: dioxins dissolve into the scrubber materials, seeping out slowly! Testing methods Dioxin formation has been tested by numerous research teams. Mechanisms were verified using C 13, O 18, Cl 37. PCDFs are not synthesized from single aromatic units; PCDDs in part show scrambling! Formation rates are still impossible to predict! My own research focuses on small-scale testing of fly ash + full-plant testing. Testing methods Sample characterization: SEM, ERF, XRF Conventional analysis of PCDD/F, PCB, PCBz, PCPh and PAH DSC and TGA Standard de novo test (Real-time monitoring using Jet-REMPI) CFD simulations PCDD/F-balances, e.g. over iron ore sintering plant Chemometrics Our present testing SEM/ERD, to see the sample DTA (or DSC), showing carbon reactivity TGA/MS, with real-time monitoring of surrogate compounds Wider approach in fingerprinting, with interest in PAHs,... A universal approach is still elusive...!

10 Quality of Dioxins sorption Gas/particulate distribution Sorbed dioxins are removed easily. Sorption depends on The quality, temperature, and (less) duration of the gas/solids contact. The supplemental contact in the dust layer, provided by baghouse sleeves. The quality of filtration (by-pass leaks, sleeve puncturing, wear, dust migration). The initial concentration and dust! Elimination of dust from a gas flow Pre-requisites are: A deep dust removal, e.g. (typical limit value) 10 mg dust/nm 3 x 4 ng I-TEQ/g = 40 pg I-TEQ/Nm 3 or 40% of the MSWI-limit value! A low filtration temperature (< 200 C). Dust residence times on ESP collector plates or on fabrics may be substantial. Hot ESP acted as a dioxins factory! Dioxin deposits in flues generate memory effects. Gas/Particulate Distribution System dependent! Dust-laden gases feature more dioxins-in-dust. Common filter dust is only moderately adsorptive, with a specific surface of few m 2 /g only. Old MSWI featured adsorptive fly ash. Modern plant less so, because of a superior carbon burnout. Both Activated Carbon and Lignite Cokes are used for sorbing dioxins. Lower gas temperature leads to more dioxins-in-dust. Dioxins and Dust At ambient temperature, dioxins sorb to dust. Also in thermal plant, such sorption is important. Is it really dissociated from formation? Complete extraction (solvent or thermal) is difficult; it may be a path towards formation! Prevention

11 Survey of Methods for Dioxin reduction Better Combustion (works) Hot Particle Removal (works not) Rapid Quench (works) Deep heat recovery low exit temperature (works) Low Oxygen (works, yet potential trade-off: formation of CO, PICs, PAHs, soot) Use of suppresants or inhibitors (works) Better Combustion Better quality of combustion is expressed by residual TOC, CO (gas), Carbon-in-ash (fly ash). Less primary air, less fly ash entrainment. Less excess air, longer residence time. Less oxygen, reduces PCDD/F formation. Better Combustion leads to less carbon-in-ash and precursors. Temperatures in the flues Formation proceeds fastest at 300 to 350 C. This temperature window must be avoided! Possibilities are: Rapid quench from 500 to 200 C. Lower ESP temperatures. Less deposits on boilers and in flues. Low exit temperatures shift PCDD/F from vapour phase towards particulates! Suppressants - inhibitors The possible use of suppressants has been demonstrated successfully, both at the lab and the industrial scale. Useful inhibitors are: Urea, ethanolamines, other N-compounds. Sulphur compounds. Basic compounds (NaOH, lime). The use of S-compounds is difficult in metallurgy. N-compounds may lead to fumes. Destruction of Dioxins in the gas Phase Thermal destruction Treatment for 1 or 2 s at temperatures of >850 C ensure a deep destruction of PCDD/F and PCBs. For cement kilns destruction is virtually complete, when dioxins are introduced at the discharge side. However, during cooling new dioxins will form. This is a limitation to treatment processes, such as sintering, vitrification... of dioxin-laden fly ash.

12 Best Available Technology MSWI = (possibly) preliminary dust removal. Neutralising acid gases by means of semi-wet lime slurries or dry NaHCO 3. Injection of activated carbon ( mg/nm 3 ) Filtration using a baghouse filter, retaining all spent chemicals + AC. Dust collection Multicyclones (almost only fluid bed units) Electrostatic precipitator: lost importance, as a prime producer of dioxins! Baghouse filter: in general use, operating temperature generally C in MSWI. Panel bed filter: rarely used. Ceramic or metal candles: H.T.-filtration. Acid gas neutralisation Conventional stoker furnaces: Dry lime (large excess required) or NaHCO 3 (ideally suited for feedforward control). Semi-wet lime slurry. Wet scrubbing. Voest Alpine s Airfine (R) and Lab s scrubber both claim PCDD/F removal capabilities. High temperature limestone neutralisation is unusual in MSWI. Adsorption units First units featured fixed or moving bed. In situ regeneration may be possible (Sumitomo). Bubbling or circulating fluid beds are also possible. Today, injection, co-current flow until baghouse filtration, is BAT. Adsorption on A.C. removes PCDD/Fs, PAHs, but also Hg, SO 2 and HCL! DeNOx units SNCR allows attaining the E.U. threshold emission values. However, local tighter limits may make SCR mandatory. An important fringe benefit is the PCDD/Fs and PAHs destruction capability. The low-dust solution is generally preferred, so that reheating is often required, typically from 160 C to 220 C. Alternatives (I) DeNOx catalysts also destroy dioxins, by sorption, followed by oxidation, on typical Ti/V/W-oxides catalyst, or on Pt on carriers. The contact process for converting SO 2 into SO 3 is also believed to destroy dioxins. H 2 O 2 has been proposed for catalytic oxidation. Atmospheric UV-photolytic destruction via OH. Reburning has also been proposed.

13 Alternatives (II) Sorption in or onto Activated Carbon or Lignite Coke. Oil (CRM patent). Plastics (FZ-K tested). Mineral sorbents, solves the problem of filter fires (pyrophoric metallurgical dust). Sorptive power is less than for carbon based sorbents. Sorption immobilizes dioxins, without destruction! Alternatives (III) In Japan, since 1999, MSWI became obsolete in new units, being replaced by a new generation of thermal processes, based on gasification or pyrolysis and featuring vitrification of fly ash and possibly bottom ash. Tai-wan now follows suit. Catalytic DeDiox Method developed by Prof. Hagenmaier, Prof. Hiraoka (1989) and applied industrially. Best (patented) catalyst uses V/W-oxides on TiO 2. Typical operating conditions: SV = 3000 h -1 (granular) to 20,000 h -1 (honeycomb). Temperatures C. Below 180 C only adsorption. Ammonia concentration should be limited. Alternatives are: Pt on Molecular Sieves, etc. Destruction of Dioxins on Solids Methods proposed Biological: at research level. Chemical: with Na, NaOH... applied in Japan for PCBs. Mechanical: at research level (ball mill). Extraction (Soxhlet, supercritical): standard part of analytical procedures. Thermal: in air sintering vitrification: applied de facto. in nitrogen: Hagenmaier drum. in sub- and supercritical water: at research level. Hagenmaier drum Dioxins and PCBs are thermally destroyed in a rotary kiln, operating Under inert gas conditions, At C, With residence times of 1 to 3 hours. Numerous plants are operating in Japan.

14 Conclusions (I) Dioxins stimulated scientific research in numerous disciplines, as well as much hot debate. Human mortality is not at stake (epidemiological study), yet the effects of exposure (prenatal, stilling mothers) are not known with certainty. Following precautionary principles, emissions must be reduced, dispersion prevented, and effects further investigated. The food chain should be shielded from both PCDD/F and PCBs. This involves lots of controls! Conclusions (II) Origins are sufficiently researched, but all industrial sources are not necessarily known. Worst source at present is uncontrolled combustion. Dioxins may be formed by various processes, such as: Chlorination of DD and DF, formed as PICs. Precursor route (chemical processes, conditions conducive to conversion of PCBz into PCPh). de novo route (thermal & pyrometallurgical processes). Mechanisms are known. Some issues are not clarified. Conclusions (III) Open issues are: The nature of the catalytic system, responsible for chlorination and oxidation. The precise mechanism of suppression by S- and N- compounds. The interactions between carbon, matrix, and catalyst. The effects of similar substances, such as PCNs, PXCDD/Fs, thiophene and pyridine derivatives, etc. Dioxins are no longer problematic in MSWI! Conclusions (IV) Dioxins caused a revolution in MSWI and related flue gas cleaning: Emissions are continuously monitored. Combustion (burnout) greatly improved. Yet, more is required for curtailing PCDD/F emissions! Dust collection moved from ESP towards baghouse filtration. CO, TOC, PAHs, dust emissions decreased dramatically! PCDD/Fs sorption and oxidation compete. Industrial sources Sintering belt

15 About the Author Prior experience Petrochemical processes, including thermal cracking and steam reforming of naphtha and methane. Environmental techniques in Air, Water, and Soil Pollution. Waste Management, in particular thermal techniques (incineration, gasification, pyrolysis, slagging). Off-gas treatment, in particular regarding prevention and abatement of dioxins. Safety of chemical & thermal plant. Dioxins experience Incineration, international experience (B, F, Japan, NL) Metallurgy, worldwide activities. Concentrating on formation, prevention and abatement. Support from 4 E.U.-Projects: 1. Upcycle Project (Treatment of fly ash for sound material utilization). 2. Cycleplast Project (Chemical pyrolytic - and mechanical recycling of Plastics and composites) 3. Minidip Project 4. Haloclean Project MINIDIP is an Acronym for : MINImization of Dioxins in thermal Industrial Processes MINIDIP Minidip MINIDIP: Minimization of dioxins in thermal industrial processes: mechanisms, monitoring and abatement. VUB coordinated, with partners BIfA, Augsburg: Dr. H. Fiedler Deutsche Forschungsanstalt fûr Luft- und Raumfahrt, DLR: Dr. H. Grotheer Forschungs Zentrum Karlsruhe: Dr. L. Stieglitz Leiden University: Prof. R. Louw Rheinbraun AG: Mr. Wirling Umeå University: Prof. St. Marklund Several industrial processes were evaluated with respect to their generating of dioxins. Dust samples from thermal and metallurgical plants were studied, e.g. carbon content and its reactivity, original load and ability to form supplemental dioxins, distribution between particulate and gas phase, potential effects of primary and secondary countermeasures.

16 MINIDIP Major achievements in: Confronting precursor with de novo theory of formation. Studying dioxins in metallurgical processes, formation, reduction and abatement, in particular relating to sintering, the Waelz process... Solving the problems at major sources in Belgium, France, and Germany. Dioxins experience Disputes between GREENPEACE and the chlorine vinyl chloride PVC industry Comprehensive activities and studies in: Ore sintering (int. al. iron ore, manganese ore) Waelz process (converting Zn-bearing dust from EAF, etc.) Scrap metal and mother alloys High temperature electrolysis of magnesium chloride Advisor to the Council for Prosecution in the Belgian Dioxin Crisis (actually a PCB contamination of animal fodders) MSW Incineration Flanders: strong curtailment of emissions from MSW Incineration. Each excess in emissions lead to immediate closure of the plant at fault. A thorough expert file was required to re-open the plant. In Belgium: each plant has steady sampling + analysis (AMESA system) Later, this competence was used in France. Iron Ore sintering Largest industrial source. First identified in Germany (1992) during systematic scrutiny of industrial sources Concentration between 0.5 and 50 ng I-TEQ per Nm 3 of sintering belt off-gas. Volumetric flow enormous: M.Nm 3 /h Reasons: metal catalyst, chlorine traces in ore and more in recycle streams, oxygen, cokes Aminal Study, 2001/2 (1) dress an Inventory of all dioxin-laden residual streams from Thermal and Metallurgical Processes in Flanders (2) register their Pathways of Elimination, Recycling or useful Application, and concomitant Risk Factors, (3) Best Available Technology for Treatment & Disposal of dioxin-laden residual streams, (4) identify issues of further concern, and (5) advise on eventual legal measures to be taken. Dioxin Monitoring in Flanders (II) Permanent Control of Fodder for PCBs Permanent Control of Dairy Products Less attention at present to: Other compounds with a dioxin-like toxicity, e.g. co-planar PCBs, PCN, PXDD/F, or to precursors (PCBz, PCPh) Wastewater, sludge, soil, chemicals or commercial products