Heavy Metal Adsorption onto Flyash in Waste Incineration Flue Gases

0957±5820/00/$10.00+0.00 q Institution of Chemical Engineers Trans IChemE, Vol 78, Part B, January 2000

HEAVY METAL ADSORPTION ONTO FLYASH IN WASTE INCINERATION FLUE GASES J. EVANS and P. T. WILLIAMS Department of Fuel and Energy, University of Leeds, Leeds, UK

T

he presence of heavy metals in incinerator ¯ue gases is of environmental concern due to their associated toxic properties. The heavy metals are associated with ¯yash. Several ¯yash samples from a range of municipal solid waste incinerators have been analysed by ICP and coupled SEM-EDX to determine their heavy metal concentration and also heavy metal composition in relation to physical morphology. Flyash is shown to contain a wide variety of heavy metals and consists of complex potassium, sodium and calcium aluminosilicates which form spherical particles with an associated amorphous and crystalline phase with an enriched metal content. An experimental reactor is described which investigates the interaction of heavy metal species with ¯yash under simulated furnace/¯ue gas conditions. The literature is reviewed in relation to the vaporization and condensation reactions of heavy metals under waste incinerator combustion and ¯ue gas conditions. Keywords: incineration; ¯yash; heavy metals; analysis.

INTRODUCTION

In this paper, the heavy metal species found in ¯yash particulates from waste incineration, the in¯uence of process conditions on their adsorption onto ¯yash and proposed adsorption mechanisms are reviewed. Flyash from a range of municipal waste incinerators is analysed and compared. A research programme is outlined which seeks to investigate the adsorption of heavy metals onto ¯yash, in relation to waste incinerator ¯ue gas conditions.

There is much concern over the emission of heavy metals to the environment due to their associated health hazard. Heavy metals exert a range of toxic health effects including carcinogenic, neurological, hepatic, renal and haematological1. There is potentially a wide variety of heavy metal species present in the ¯ue gases from the incineration of municipal solid waste, including the metal and inorganic and organic compounds2. The presence of organo-metallic compounds is particularly signi®cant since this group is considered to be a very high health risk. The source of the heavy metals arises from the range and highly variable concentration of heavy metals in the input wasteÐfor example, Table 1 shows a typical range of metal contents in municipal solid waste2. The metals and metal compounds in the waste, when subject to the high temperatures of the incinerator furnace, may evaporate. The extent of evaporation depends on complex and interrelated factors such as operating temperature, oxidative or reductive conditions and the presence of scavengers, mainly halogens, such as chlorine3. As the gases cool through the post-furnace system the metal species condense, either through the formation of an aerosol of discrete heavy metal particles or through adsorption onto ash particles through a range of processes4. Therefore, heavy metals in the ¯yash tend to be associated with the ®ner size fraction and be adsorbed to the surface of particles5±7. Whilst there has been some experimental and modelling research into heavy metal adsorption onto ¯yash particulate under conditions pertaining to waste incinerators, it is still an area of uncertainty. Indeed, Seeker8 has reviewed the emissions of a range of pollutants from waste incineration, including heavy metals, and showed that there was little detailed data on the fate of metals in waste incineration processes, concluding that further research was required.

MATERIALS AND METHODS Flyash Samples The ¯yash samples were taken from four municipal solid waste incinerators: a modern mid-1990s construction incinerator (Incinerator A); an older pre-1980s design incinerator upgraded to meet 1996 EC emissions legislation (Incinerator B); and an older pre-1980s design incinerator which was subsequently decommissioned since it could not meet the 1996 EC emissions legislation (Incinerator C). The fourth ¯yash sample was a certi®ed reference ¯yash sampleÐCommission of the European Communities, Community Bureau of Reference BCR 176Ðwhich was used for validation of the analytical methodologies. The reference sample was obtained from the electrostatic ®lters of a municipal solid waste incinerator which was supplied with mainly domestic waste. The ¯yash material was further processed to provide a < 40 mm size fraction. Flyash Analysis The four samples of ¯yash were analysed by X-Ray Fluoresence (XRF) and Inductively Coupled Plasma (ICP). XRF was used for major element analysis, the samples of ¯yash were prepared by pressing approximately 5 g of ®ne grained shale into a pellet with an organic adhesive under pressure and placed in an aluminium holder adopting the 40

HEAVY METAL ADSORPTION ONTO FLYASH Table 1. Typical range of trace components in municipal solid waste (g/tonne). Trace component Iron (Fe) Chromium (Cr) Nickel (Ni) Copper (Cu) Zinc (Zn) Lead (Pb) Cadmium (Cd) Mercury (Hg)

Range 25,000±75,000 100±450 50 ±200 450±2500 900±3500 750±2500 10 ±40 2 ±7

method of Brown and Brindley9. The system used was an ARL 9400 sequential spectrometer ®tted with a 3 kW end window Rh target X-ray tube. The analysis was on a semiquantitative basis. Heavy metals were determined using ICP, the samples (approximately 0.5 g accurately weighed) of ¯yash were digested in a 5:1 ratio of hydro¯uoric acid (HF) and nitric acid (HNO3) and evaporated to near dryness. Further nitric acid was added and digested for 8 h. The solution was then ®ltered and quantitaively diluted to 50 ml prior to introduction to the ICP system. The ICP was a Fisons Maxim simultaneous atomic emission spectrometer. The ¯yash samples were also analysed using Scanning Electron Microscopy (SEM) coupled with Energy Dispersive X-ray (EDX) analysis. The system used was a Cambridge Scanning Co. Camscan Series III SEM coupled with EDX and had full computer-based data handling and imaging. EDX graphs provided semi-quantitaive analysis of metals present in the sample. The resolution was approximately 5 nm and magni®cation up to 10,000. Electrons penetrate the sample surface to at least 1 mm and elements must have a concentration of >1% to be displayed as peaks on the EDX trace. Experimental Reactor Figure 1 shows a schematic diagram of the simulated furnace and ¯ue gas reactor to investigate the adsorption of heavy metals onto ¯yash. The reactor consists of two stainless steel tubes independently heated by electrical furnaces, enabling a temperature regime of between 100 and

10008 C to be investigated. The inlet to the reactor enables a range of heavy metals, heavy metal compounds and reactant gases to be introduced. The heavy metals and/or heavy metal compounds are loaded onto a sample boat and when the desired conditions in the reactor are established, the sample boat is introduced. Vaporization of the sample occurs and the heavy metal species pass through the reactor to the ¯ue gas furnace section where deposition onto ¯yash occurs. Mercury is introduced as a vapour from a controlled heated vessel, via a heated transfer line. The inlet gases to be investigated are oxygen (air), hydrogen chloride, nitrogen oxides and sulphur oxides in a make-up gas of nitrogen. The ¯yash was kaolin (aluminium silicate hydroxide, Al2Si2O5(OH)4), powder size <40 mm, to represent a simulated ¯yash matrix, and was held in place with quartz wool. A blank experiment with quartz wool only showed that there was insigni®cant adsorption of the metals onto the quartz wool compared to the ¯yash adsorption. The ef¯uent from the reactor was passed through a sampling train consisting of a series of sulphuric acid and water bubblers to trap the unadsorbed heavy metal species for analysis using atomic absorption spectrometry. After the experiment, the ¯yash sample was removed from the reactor and analysed using SEM-EDX and the acid digestion method outlined earlier. In addition, the reactor tube and sample lines were separately washed with acid and together with the acid and water traps from the condenser system were analysed using atomic absorption spectrometry. RESULTS AND DISCUSSION Table 2 shows the analysis of heavy metals in the ¯yash from incinerators A, B and C and the BCR 176 reference ¯yash sample. In addition, ¯yash analyses for heavy metals from the literature 10±12 and a typical soil sample13 are included for comparison. The concentrations of individual heavy metals are very variable from one incinerator to another, as evidenced by comparison with the data in this paper with those reported in the literature. This is not at all surprizing since the waste input may vary considerably (Table 1) and also the plant con®guration in relation to emissions control systems and the operating parameters of the incinerators would be different. The certi®ed analysis of the reference ¯yash is also given in Table 2 which shows that, in some cases, the analytical method gives quite

Figure 1. Schematic diagram of the heavy metal±¯yash interaction reactor.

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EVANS and WILLIAMS Table 2. Heavy metal concentrations in municipal solid waste incinerator ¯yash (ppm).

Incinerator A1 Incinerator B2 Incinerator C3 Reference incinerator Analysis Certi®ed Literature data Incinerator [Ref. 10] Incinerator [Ref. 11] Incinerator [Ref. 12] Incinerator [Ref. 14] Typical soil [Ref. 13]

Metal Cd

Fe

Cu

± ± ±

67 209 243

1560 13,303 9463

318 702 1153

<2 <2 <2

7900 10,870

100 123

361 470

18,460 21,300

1052 1302

16 31

7200 15,000 4900 27,000 2±300

1800 20 145 70 2±750

64 2000 300 1660 0.01±2

24,000 14,000 ± <1600 ±

510 910 1050 2220 2±250

0.9 4.4 8 9.8 0.01±0.05

Zn

Pb

5418 19,000 32,103

780 8143 8689

21,057 25,770 10,000 110,000 16,600 104,400 1±900

Ni

Hg

1

Incinerator A: Modern incinerator; post-1990 construction Incinerator B: Older incinerator; pre-1980 construction with upgraded ¯ue gas control 3 Incinerator C: Older incinerator; pre-1980 construction now decommissioned 2

different results to the certi®ed analysis. Incinerator ¯yash represents a dif®cult sample for analysis due to the complex matrix in which the heavy metals, mainly in an insoluble form, are contained within a silicate and aluminosilicate matrix. In addition, the presence of the matrix elements in the extracted heavy metal fraction may also cause interference at the analytical stage. Eighmy et al.14 have also shown that, depending on the analytical technique and methodology employed, the analysis of incinerator ¯yash can give signi®cantly different results for heavy metal analysis. It is interesting to compare the municipal solid waste incinerator ash analyses with those of a typical soil13. Whilst generally the heavy metal contents of the ¯yash are much higher than is found in a typical soil, such as Zn, Pb, Cd and Hg, other heavy metals such as Ni and Cu are not too dissimilar. Table 3 shows the major element analyses as metal oxides of the ¯yash samples from the municipal waste incinerators determined by XRF. The major metals identi®ed were Al, Ca, K, Na and Si which represent the silicate and aluminosilicate materials. Calcium is also a dominant metal in the ¯yash from incinerator A since that incinerator incorporates a high degree of lime in the gas clean-up phase of operation. A number of workers have investigated the ¯yash from municipal solid waste incinerators using a variety of analytical techniques14±19. The major constituents in the ash are Cl, K, Zn, Na, Ca, Si, Pb and Al, plus oxygen as the most prominent element in the form of oxides, silicates and aluminosilicates. There are also a large range of trace

elements, including Ag, As, Br, Ce, Mg, Mn, Rb, Ti and Zr. Spherical particles are also common in ¯yash, associated with aggregates of polycrystalline, amorphous and glassy material. Attempts to identify possible mineral phases in the ¯yash show that apart from the amorphous and glassy material, various chemical compounds and mineral species can also be identi®ed which are shown in Table 414±16. The spherical particles common in ¯yash are composed of complex calcium, sodium and potassium aluminosilicates whilst the associated amorphous and crystalline material is enriched in the more volatile elements14. Figure 2 shows a scanning electron microgram of the ¯yash from incinerator A. Also shown is the EDX analysis of the surface scan. The ¯yash samples exhibit the spherical particles and amorphous material characteristic of ¯yash. The EDX analysis also shows that they are very complex materials having a range of metal species present. There is a high calcium content in the sample due to the use of lime in the gas clean-up system for incinerator A. Figure 3 is a close-up of the spherical particles of Figure 2 showing that they are made up of ®ner grained spherical and amorphous material. Analyses of some crystalline material in the ¯yash from incinerator B by SEM-EDX suggested that condensation of the furnace gases as they were cooled to ¯ue gas temperatures resulted in discrete metal fume formation in the form of a particle with a high concentration of lead (Figure 4). Analysis of the reactor ¯yash by acid digestion together with the washings and acid and water traps showed that the majority of the cadmium had been adsorbed onto the kaolin

Table 3. Major element oxide concentrations in municipal solid waste incinerator ¯yash (wt%).

Incinerator A1 Incinerator B2 Incinerator C3 Reference incinerator

Al2O3

CaO

Fe2O3

K2O

2.37 4.55 3.55 4.24

29.19 8.14 6.35 2.40

0.15 1.30 1.51 0.99

4.27 8.70 3.29 7.09

Metal oxide MgO MnO 0.71 0.80 0.65 0.67

0.03 0.07 0.05 0.05

Na2O

P2O5

SiO2

TiO2

10.48 19.31 30.63 33.45

0.50 0.90 0.77 0.52

11.11 14.60 17.38 13.65

0.24 0.79 0.61 0.40

1

Incinerator A: Modern incinerator; post-1990 construction Incinerator B: Older incinerator; pre-1980 construction with upgraded ¯ue gas control 3 Incinerator C: Older incinerator ; pre-1980 construction now decommissioned 2

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Table 4. Compounds identi®ed in ¯yash. Compounds

Reference

Potassium- and sodium-containing compounds NaCl, K2ZnCl4, KClO4, K2PbO4, K2H2P2O5 , KCl Kal(SO4)2, Na2SO4, K2SO4 Calcium-containing compounds CaAl4O7, CaMgV2O7, CaAl2SiO6, Ca3Al6Si2O16, CaSO4, CaCO3, CaO Lead-containing compounds Pb3SiO5, Pb3O2SO4, Pb3Sb2O7, PbSiO4 Cadmium-containing compounds Cd5(AsO4)3Cl, CdSO4 Zinc-containing compounds K2ZnCl4, ZnCl2, ZnSO4 Iron-containing compounds Fe3O4, Fe2O3 Titanium-containing compounds CaTiO3, TiO2 Silica, silicate and aluminosilicate compounds SiO2, CaSiO3,Al2SiO5, Ca3Si 3O9, CaAl2SiO6, Ca3Al6Si2O16, NaAlSi3O8, KAlSi3O8

simulated ¯yash at the experimental conditions of furnace temperature 9538 C and ¯yash temperature 4208 C. Figure 5 shows the SEM-EDX results of the heavy metal adsorption experiments for cadmium in a nitrogen atmosphere, again at the furnace temperature of 9538 C and ¯yash temperature of 4208 C. The surface scan analysis of the sample showed that the cadmium had a relatively even distribution over the ¯yash. In addition, individual discrete crystals of cadmium were seen as shown by the EDX point analysis (Figure 5). At the relatively low ¯yash temperature of 4208 C, the reaction between cadmium and the ¯yash is largely physical adsorption. At higher temperatures of metal vapourЯyash interaction of 8008 C, Uberoi and Shadman22 have shown that in addition to physical adsorption reactions, chemical reaction between cadmium and the aluminosilicate substrate occurs to produce a complex cadmium aluminium silicate.

14, 16, 17, 18, 19, 20 14, 16, 17, 18, 19, 21 14 14, 15 18, 19 17, 18, 19 14, 17 14, 15, 18, 19, 21

However, the chemisorption of cadmium will be a surface phenomenon and once all the potential active chemical sites are occupied and the surface layer is saturated, further condensation of cadmium will be by physical adsorption. Masseron et al.23 have also suggested that after initial chemisorption of cadmium onto aluminosilicate, further adsorption is physical adsorption, indicated by the degree of leachability of the adsorbed cadmium. The chemisorption process has been detailed by Wu et al.24 for high temperature (greater than 8008 C) adsorption of lead onto aluminosilicate. They suggest that the sorption process consists of adsorption of the metal followed by diffusion in the product layer which consists of a molten metalaluminosilicate and ®nally reaction with the aluminosilicate matrix. The resistance to the transport of the metals to the reaction site increases as the sorption reaction progresses.

Figure 2. Scanning electron microgram of the ¯yash from incinerator A with EDX analysis.

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Figure 3. Scanning electron microgram of a spherical particle from incinerator A.

This is due to the formation of the molten lead aluminosilicate layer which causes pore blocking of the void between the aluminosilicate grain particles, resulting in diffusional resistance. Lead in the vapour phase diffuses through the molten layer to reach the unreacted aluminosilicate matrix. Volatilization of Heavy Metals The release of heavy metals from the waste and their incorporation into the ¯ue gases is a function of many

factors, including volatility, combustion conditions and ash entrainment. The distribution of the metals in the various outputs from municipal waste incinerators have been investigated by a number of workers. It has been suggested25 that the partitioning is a function of the physico-chemical properties of the heavy metal elements and their derived compounds, such that volatile mercury and cadmium compounds with high vapour pressures and low boiling points are most likely to be found in the ¯ue gas. Metals with a medium vapour pressure and boiling points, such as lead and zinc, are

Figure 4. SEM-EDX point analysis of a particle from incinerator B showing high lead content.

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Figure 5. Scanning electron microgram of the ¯yash from cadmium±kaolin interaction with EDX analysis from the experimental reactor.

retained better in the slag and are less concentrated in the ¯yash. Other metals with low vapour pressure and high boiling points, such as iron and copper, are almost completely trapped in the bottom ash. The chemical speciation of these metals in the incinerator off-gas is strongly in¯uenced by the presence of compounds of chlorine, sulphur, carbon, nitrogen, ¯uorine and others during combustion and gas cooling. The vaporization process is complicated by simultaneous chemical processes such as decomposition, chlorination, oxidation and reduction26,27. The off-gases containing metals and chlorine species, particularly hydrogen chloride, lead to the formation of metal chlorides. For example, cadmium is easily volatilized during incineration and is oxidized in the presence of hydrogen chloride to form mainly cadmium chloride15. Similarly, the more refractory metals such as nickel will not vaporize under the conditions of incinerator furnaces, but will do so in the presence of chlorine29. Seeker8 has also shown that chlorine in¯uences the volatility of heavy metals via the formation of chlorides. Mercury has also been shown to be present largely in the halogenated form, predominantly mercury (II) chloride and to a lesser extent mercury (I) chloride. Whilst initially mercury is vaporized as the metal in the furnace, it quickly becomes oxidized to the halogenated form and only a small percentage is present as metal vapour28. Alternatively, refractory metal compounds may be released under reducing conditions within the incinerator combustion zone which results in reduction of metal oxides to form compounds which more readily volatilize and diffuse away from the hot zone4. As the gases cool and more oxygenated conditions prevail, secondary reactions result in conversion back to their original more refractory form and condensation4. Heavy metals may also be incorporated into the ¯ue gas stream via entrainment of ®ne ash particles containing heavy metals. The quantity of material entrained is a function of the size, shape and density of the ash particles as well as the incinerator operating conditions4. Trans IChemE, Vol 78, Part B, January 2000

Condensation Reactions of Heavy Metals The deposition process of the heavy metals on cooling of the ¯ue gases has been proposed as homogeneous nucleation via the formation of ®ne fume particles and heterogeneous deposition on ¯yash8. Homogeneous nucleation may occur when the partial pressure of an inorganic vapour species exceeds a certain critical value30. The incineration gases may become supersaturated as a result of rapid cooling of the gas or rapid formation of a new and nonvolatile species. Heterogeneous deposition occurs when surfaces are available for condensation and the supersaturation is low. Particles already in the incineration chamber or chamber walls act as sites for the heterogeneous condensation. In addition to dew points of the condensing species, the relative rates of homogeneous nucleation and heterogeneous deposition will also depend on the time/ temperature gradient experienced by the metal containing ¯ue gas29. It has also been shown that where cooling rates are high, of the order of >600 K/s, homogeneous nucleation would occur even in the presence of existing particles31. Following homogeneous nucleation and heterogeneous deposition, particles will subsequently grow by coagulation. Barton et al.4 have proposed a model based on equilibrium calculations involving condensation and coagulation to describe the proposed condensation of metals in waste incineration. Davis et al.29 investigated the processes involved in the cooling of metal vapours derived from combustion when several metals of differing volatilities are involved. They showed that semi-volatile metals such as lead and cadmium condensed through nucleation followed by coagulation, since they both have similar dew points and number concentrations. However, when cadmium and more refractory nickel were investigated, interaction was not by coagulation because of differences in dew points, resulting in differing particle sizes and number concentrations. At cooler furnace conditions, compositions of particles were

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EVANS and WILLIAMS

determined by condensation of cadmium onto existing nickel particles. Uberoi and Shadman22 investigated the adsorption of cadmium onto different types of adsorbent including, kaolin, silica, alumina and bauxite at 8008 C in a simulated ¯ue gas atmosphere of CO2, O2, N2 and H2O. They used CdCl2 as the source of cadmium and found that the degree of adsorption was dependent on the type of substrate used with alumina and bauxite having the highest adsorption capacity. They showed that the interaction of cadmium with kaolinite resulted in the formation of a cadmium aluminium silicate, CdAl2Si2O8 and where bauxite was used chemical reaction produced a cadmium aluminate and a cadmium aluminium silicate. They therefore suggested that the overall sorption process was not just physical adsorption but a complex combination of adsorption and chemical reaction. Uberoi and Shadman32 have also shown that a similar reaction mechanism occurs for the adsorption of lead with kaolinite at 7008 C with the formation of a complex lead aluminium silicate. The reaction is complicated by the presence of other gases in the combustion ¯ue gasesÐfor example, Scotto et al.33 examined the adsorption of lead by kaolinite and showed that the presence of chlorine as either Cl2 or HCl decreased the adsorption of lead. The research into adsorption of heavy metals onto ¯yash under conditions relating to waste incinerator ¯ue gases is to be further investigated by the authors funded by a UK Engineering and Physical Sciences Research Council (EPSRC) research grant. Initial work outlined in this paper concerns a ®xed bed reactor. However, interaction of heavy metals with ¯yash under real waste incinerator conditions gives reaction times of the order of seconds, whereas the ®xed bed reactor is of the order of minutes. Other experimental studies have also involved a ®xed adsorbate and a ¯ow through of heavy metals which leads to very extended reaction times not typical of those found in incinerator ¯ue gases22,30,32. Therefore, a second phase of the work is to build a simulated ¯ue gas reactor with cocurrent ¯ows of heavy metal vapours and ¯yash to give more realistic reaction conditions. The adsorption of heavy metal species in relation to ¯ue gas conditions will be determined. Scotto et al.33 have used a similar vertical drop tube furnace to investigate the uptake of lead on a commercial kaolinite adsorbate. Similarly, Masseron et al.23 used a vertical furnace to investigate the uptake of cadmium chloride on various mineral substrates. REFERENCES 1. Denison, R. A. and Silbergeld, E. K., 1988, Risk Analysis, 8: 343±355. 2. Williams, P. T., 1998, Waste Treatment and Disposal (John Wiley & Sons Ltd, Chichester). 3. Buekens, A. and Patrick, P. K., 1985, Incineration, in Solid Waste Management; Selected Topics, Suess, M. J. (World Health Organization, Copenhagen). 4. Barton, R. G., Clark, W. D. and Seeker, W. R., 1990, Combust Sci Tech, 74: 327±342. 5. Bouscaren, B., 1988, in Brown, A., Evemy, P. and Ferrero, G. L. (eds), Energy Recovery through Waste Combustion (Elsevier Applied Science, Essex). 6. Greenberg, R. R., Zoller, W. H. and Gordon, G. E., 1978, Environ Sci Technol, 12: 566±573. 7. Buekens, A. and Schoeters, J., 1984, Thermal Methods in Waste

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ACKNOWLEDGEMENTS This research was funded by the UK Engineering and Physical Sciences Research Council (EPSRC) under grant number GR/M 18621; the authors gratefully acknowledge this support. We also thank Leeds University personnel, Ed Woodhouse and Peter Thompson.

ADDRESS Correspondence concerning this paper should be addressed to Dr P.T. Williams, Department of Fuel and Energy, University of Leeds, Leeds LS2 9JT, UK. This paper was presented at the 2nd International Symposium on Incineration and Flue Gas Treatment Technologies, organized by IChemE and held at the University of Shef®eld, UK, 4±6 July 1999.

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