Scrubber Residue Research Articles

Variation in chemical composition of MSWI fly ash and dry scrubber residues

Our society generates extensive amounts of municipal solid waste (MSW), which are mainly incinerated for volume reduction and energy recovery. Though, MSW incineration generates hazardous air pollution control (APC) residues that must be treated and deposited in appropriate landfills. An alternative to landfilling is material recovery, leading to regeneration of valuable products and reducing hazardous waste amounts. The chemical composition of APC residues, stemming from MSW, makes the waste attractive for metal and salt recovery, but its variation makes the development of material recovery processes challenging. This study investigates results from 895 X-ray fluorescence analyses of fly ash and dry scrubber residue samples originating from Norway and Sweden between 2006 and 2020 to explore variation in chemical composition within and between different incineration plants. The average relative standard deviation of elemental concentration in APC residue was estimated to 30% within plants. The variation in elemental concentration between grate fired incineration plants is about half of the average variation within the plants. The study also clarifies compositional differences from APC residues originating from fluidized bed incinerators and grate incinerators. Also, reported concentrations of APC residues from other countries than Sweden and Norway showed significant differences in chemical composition. The presented variations clarifies the importance of holistic approaches for waste valorization processes which can substitute stabilization processes for landfilling.

Advanced Oxidation Processes to the Remediation of Liquid Residue from the Thermal Treatment of Oily Sludge

This work reports the remediation of a liquid residue derived from the thermal treatment of petrochemical residues by advanced oxidation processes. Firstly, degradation studies of phenol aqueous solutions by heterogeneous photocatalysis, Fenton, and photo-Fenton were conducted. Fenton processes showed a higher efficiency, mainly in the UVA photo-assisted mode (94% in 3 min), and henceforward it was chosen for the remediation of the residue. However, the high organic load of the raw residue (6,000 mg L-1 of chemical oxygen demand (COD)) limited the efficiency of the Fenton processes. Hence, the raw liquid residue was diluted with the scrubber residue, produced during the thermal treatment of the petrochemical residues, reducing the COD to values close to 2,000 mg L-1. The photo-Fenton treatment allowed the removal of approximately 95% of the COD load, decreasing the residual concentration of phenolic compounds from 213 to 0.5 mg L-1. At the same time, the concentration of BTEXs (benzene, toluene, ethylbenzene and xylene) were reduced to values lower than the limit of quantification (LOQ) of the chromatographic technique. Therefore, the photo-Fenton process is an efficient alternative to the remediation of residues with high complexity and with high COD values, like the ones obtained in the petrochemical industry.

Treatment-oriented characterization of dry scrubber residue from municipal solid waste incineration

Dry scrubber residue from municipal solid waste incineration (MSWI) was characterized to identify critical inorganic pollutants and to suggest a conceptual treatment method. The key methods used were thermal analysis, including thermogravimetry (TG) and differential thermal analysis (DTA), pHstat titration, qualitative X-ray diffraction (XRD), scanning electron microscopy (SEM), chemical equilibrium calculations, and statistics such as error propagation, principal component analysis (PCA), and empirical modeling based on factorial designs. Based on EU directives, the major inorganic pollutants Cd, Cr, Pb, and Zn were found. In addition, the pH was too high. With dry scrubber residue stabilization in mind, the impact of carbonation and hydration was assessed and judged to be encouraging. In particular, chemical equilibrium calculations showed that carbonation has considerable potential to lower the pH and the availability of Pb, Zn, and Cr. The impact of carbonation on the mobility of Cd was found to be small. During carbonation, a metal-trapping calcium aluminosilicate hydrate (C–A–S–H) phase is also formed. Both processes together have the potential to lead to a robust, reliable, and reasonable stabilization method for dry scrubber residue. However, to control these processes, the decisive factors need to be identified and their effects need to be quantified. Ca, Cl, Na, and K might be abundant components which would be mobile even after stabilization.

Analysis of Naturally-occurring Radionuclides in Coal Combustion Fly Ash, Gypsum, and Scrubber Residue Samples

Coal combustion residues from coal-fired power plants can be advantageous for use in building and construction materials. These by-products contain trace quantities of naturally occurring radionuclides from the uranium and thorium series, as well as other naturally occurring radionuclides such as K. Analysis was performed on samples of coal fly ash, flue gas desulfurization, gypsum and scrubber sludges, fixated scrubber sludges, and waste water filter cakes sampled from multiple coal-fired power plants in the United States. The radioactive content of U and Th decay series nuclides was determined using gamma photopeaks from progeny Pb at 352 keV and Tl at 583 keV, respectively; K specific activities were determined using the 1,461 keV photopeak. The samples were hermetically sealed to allow for secular equilibrium between the radium parents and the radon and subsequent progeny. Samples were analyzed in a common geometry using two high purity germanium photon detectors with low energy detection capabilities. The specific activities (Bq kg) were compared to results from literature studies including different building materials and fly ash specific activities. Fly ash from bituminous and subbituminous coals had U specific activities varying from 30-217 Bq kg (mean + 1 s.d. 119 ± 45 Bq kg) and 72-209 Bq kg (115 ± 40 Bq kg), respectively; Th specific activities from 10-120 Bq kg (73 ± 26 Bq kg) and 53-110 Bq kg (81 ± 18 Bq kg), respectively; and K specific activities from 177 to 928 Bq kg (569 ± 184 Bq kg) and 87-303 Bq kg (171 ± 69 Bq kg), respectively. Gypsum samples had U, Th, and K specific activities approximately one order of magnitude less than measured for fly ash samples.

Heavy metals in wet scrubber residue from a medium-sized (32 MW) municipal district heating plant incinerating wood and peat in a fluidised bed boiler

Due to the low total concentrations of heavy metals and extractable compounds, it is recommended that the wet scrubber residue be used as a construction agent instead of being disposed of in landfills. According to five-stage sequential extraction, the highest concentrations of most of the heavy metals occurred in the residual fraction (i.e., HF + HNO3 + HCl), which means that the heavy metals in the wet scrubber residue are tightly bound to the matrix. This is a favourable phenomenon, if the residue is utilised in earth construction.

An emerging pollutant contributing to the cytotoxicity of MSWI ash wastes: Strontium

In this study, we used the multiple toxicity characteristic leaching procedure to test the long-term leaching behavior of bottom ash, scrubber residue, and baghouse ash from a municipal solid waste incinerator (MSWI). We used the short-term viability percentage of African green monkey kidney cells (Vero cells) as a bioindicator to investigate the cytotoxicity of the leachates from the MSWI ash wastes. We found that strontium was a significant contributor to the cytotoxicity of the bottom ash.

Using Demercurated Lighting Phosphor and MSWI Scrubber Residues to Prepare High Performance Plastic Concrete

This study aimed to prepare a high performance plastic concrete made of epoxy resin and Portland type-I cement mixed with at least one inorganic solid waste of demercurated lighting phosphor powder or municipal solid waste incineration scrubber residue. The ratio between liquid epoxy resin and cement was 1:2; the scrubber residue and demercurated phosphor powder were added as modifiers for cement component in order to improve the strength and thermal properties of synthesized plastic concrete. The results indicate that, the addition of scrubber residue causes a decrease in both strength and thermal properties; whereas, the demercurated phosphor powder can replace 100% of the contents of cement without any significantly change in either strength or thermal properties. Atomic force microscopy and Raman spectroscopy were used to characterize the chemical structure of cured concrete and the results indicate that the surface softness increases with an increase in the mixed percentage of epoxy resin.

Weathered Fly Ash Does Not Affect Soil And Biosolid Carbon Mineralization

Fly ash and biosolid wastes can be mixed and applied to soil as a means of disposal. A significant decline in soil respiration following waste application indicates restricted activities of functional microbial populations. Weathering decreases salinity and neutralizes alkalinity in fly ash, but there is little information on the effects of unweathered fly ash and biosolid mixtures on soil carbon (C) mineralization. The objective of this study was to determine the effects of a weathered fly ash–limestone scrubber residue (LSR) mixed with an aerobically digested biosolid on soil respiration in a laboratory incubation study. Biosolids significantly increased carbon dioxide (CO2) production (p < 0.05), but up to 6.75% (w/w) fly ash did not. Mean total C mineralization was 770 mg CO2‐C kg−1 soil in the control and 3,810 mg CO2‐C kg−1 soil in the 6.75% (w/w) biosolid treatment. Fly ash with neutral pH and low salinity appears unlikely to affect soil and biosolid C mineralization.

Cytotoxicity of municipal solid waste incinerator ash wastes toward mammalian kidney cell lines

In this study, three municipal solid waste incinerator (MSWI) ash wastes—bottom ash, scrubber residue, and baghouse ash—were extracted using a toxicity characteristic leaching procedure (TCLP) extractant. These so-called final TCLP extracts were applied to African green monkey kidney cells (Vero), baby hamster kidney cells (BHK-21), and pig kidney cells (PK-15), multi-well absorption reader analysis was performed to test how the cytotoxicity of the incineration ashes would affect the digestive systems of animals. Ion-coupled plasma analyses indicated that the baghouse ash extract possessed the highest pH and heavy metal concentration, its cytotoxicity was also the highest. In contrast, the bottom ash and the scrubber residue exhibited very low cytotoxicities. The cytotoxicities of mixtures of baghouse ash and scrubber residue toward the three tested cell lines increased as the relative ratio of the baghouse ash increased, especially for the Vero cells. The slight cytotoxicity of the scrubber residue arose mainly from the presence of Cr species, whereas the high cytotoxicity of the baghouse ash resulted from its high content of heavy metals and alkali ions. In addition, it appears that the dissolved total organic carbon content of these ash wastes can reduce the cytotoxicity of ash wastes that collect in animal cells.

Optimization of sprayed lime amount in the semi-dry scrubbing system of MSWI

This paper reports the characterization of residues collected from air pollution controlling devices (APCDs) at seven large-scale municipal solid wastes (MSW) incineration plants. The amount of lime being sprayed in plants is a function of free CaO content in scrubber residues; a higher free CaO content in scrubber residue causes more leaching of lead. The proper amount of lime in the semi-dry scrubbing system was found to be 5 kg/ton of MSW waste. Under these conditions, both the free CaO content and lead leaching became extremely low in scrubber residue.

High temperature behavior of electrostatic precipitator ash from municipal solid waste combustors

Municipal solid waste (MSW) flue gas residues require further treatment prior to disposal or reuse, and vitrification is one of the main solidification–stabilization processes. This paper investigates the high temperature behavior of MSW flue gas residues, performed in laboratory experiments up to 1400 °C, and coupled with thermogravimetric analyses, X-ray diffraction, chemical and electron microprobe analyses. Melting temperatures of electrostatic precipitator (ESP) ash are in the range of 1202–1272 °C, whereas semi-dry scrubber residues melt between 1900 and 2300 °C. We show that the mean liquidus temperature of flue gas residues can be simply evaluated from their CaO content, by using the CaO SiO 2 Al 2O 3 ternary diagram. For ESP ash, the liquidus phase is a Zn-rich aluminous spinel, followed by anorthite at 1225 °C, and melilite at 1190 °C. The total mass loss reaches 18 wt.% at 1300 °C. Moreover, 90% of evaporation takes place below 1000 °C, linked to evaporation of C, Cl, S, Na, K, and of the toxic metals Hg, Cd, Pb, Cu. Due to the high partial pressure of chlorine during heating, chloride is the most probable form of evaporation for Cd, Pb, and Cu. However, most of Zn, Cr, Ni, Sb and Sn remain in the vitrified product.

The effect of isosaccharinic acid (ISA) on the mobilization of metals in municipal solid waste incineration (MSWI) dry scrubber residue

Co-landfilling of incineration ash and cellulose might facilitate the alkaline degradation of cellulose. A major degradation product is isosaccharinic acid (ISA), a complexing agent for metals. The impact of ISA on the mobility of Pb, Zn, Cr, Cu and Cd from a municipal solid waste incineration dry scrubber residue was studied at laboratory using a reduced 2 5−1 factorial design. Factors investigated were the amount of calcium isosaccharinate (Ca(ISA) 2), L/ S ratio, temperature, contact time and type of atmosphere (N 2, air, O 2). The effects of pH and Ca(ISA) 2 as well as other factors on the leaching of metals were quantified and modelled using multiple linear regression ( α = 0.05). Cd was excluded from the study since the concentrations were below the detection limit. The presence of Ca(ISA) 2 resulted in a higher leaching of Cu indicating complex formation. Ca(ISA) 2 alone had no effect on the leaching of Pb, Zn and Cr. A secondary effect on the mobilization was predicted to occur since Ca(ISA) 2 had a positive effect on the pH and the leaching of Pb, Zn and Cr increased with increasing pH. The leaching of Pb varied from 24 up to 66 wt.% of the total Pb amount (1.74 ± 0.02 g(kg TS) −1) in the dry scrubber residue. The corresponding interval for Zn (7.29 ± 0.07 g(kg TS) −1) and Cu (0.50 ± 0.02 g(kg TS) −1) were 0.5–14 wt.% of Zn and 0.8–70 wt.% of Cu. Maximum leaching of Cr (0.23 ± 0.03 g(kg TS) −1) was 4.0 wt.%. At conditions similar to a compacted and covered landfill (4 °C, 7 days, 0 vol.% O 2) the presence of ISA can increase the leaching of Cu from 2 to 46 wt.% if the amount of cellulose-based waste increases 20 times, from the ratio 1:100 to 1:5. As well, the leaching of Pb, Zn, and Cr can increase from 32 to 54 wt.% (Pb), 0.8–8.0 wt.% (Zn), and 0.5 to 4.0 wt.% (Cr) depending on the amount of cellulose and L/ S ratio and pH value. Therefore, a risk ( α = 0.05) exists that higher amounts of metals are leached from landfills where cellulose-containing waste and ash are co-disposed. This corresponds to an additional 29 t of Pb and 17 t of Cu leached annually from a compacted and covered landfill in the north of Sweden.

An Opportunity for Recycling Semi-dry Scrubber Residues Directly from Municipal Solid Waste Incineration Plants without Concern of Leaching Toxicity

The leaching behaviors of heavy metals from semi-dry scrubber (SDS) residues of municipal solid waste incineration (MSWI) plants are re-investigated. The most leachable heavy metals in semi-dry scrubber residues from municipal solid waste incinerators are found to be chromium ions (Cr(III) and Cr(VI)), but neither lead nor cadmium. Both of the leachabilities of Cr(III) and Cr(VI) in MSWI scrubber residue are a function of the CaO/Al2O3 ratio of residues. The pH-dependent leaching behaviors are only observed in the case of Cr(III), but not for Cr(VI). Our results show that it is worth recycling SDS residues which possess a lower chemical composition ratio of CaO/Al2O3, in order to reduce the potential risk of toxic leaching of chromium species.

Coleaching behavior of lead of incineration ash waste mixtures

In this paper, the authors reported the coleaching behavior of lead of incineration ash waste mixtures, i.e., fabric filter residues [called fly ash (FA)] and semidry scrubber residues [called reaction product (RP)]. The leaching potential of lead metal of those mixtures was estimated by toxicity characteristic leaching procedure (TCLP) with acidic extractant. And it was found to be under the controls of dissolution rate and interparticulate interactions. The author has developed a linear regression equation to fit the coleaching behavior well, the equation is basing on the chemical compositions of (RP/FA) mixtures. The impact of this study is that the leaching toxicity of mixture could be reduced to be nonhazardous, just only to adjust the mixing ratio of these two ash wastes. And the authors are able to use these ash wastes for wider recycling applications without any intermediate stabilization treatment.

Chemical Speciation of Amphoteric Ions on Semi‐Dry Scrubber Residue and Bag‐House Ash from MSWI

AbstractTwo series of size‐fractionated combusted residues, a bag‐house ash and a scrubber residue, from a municipal solid waste incinerator (MSWI) were prepared and extracted by means of sequential chemical extraction (SCE) procedures proposed by Tessier. Concentrations of water‐soluble phase, exchangeable phase, carbonated phase, Fe/Mn oxide phase, organic matter phase, and residual phase bound amphoteric metals (Pb and Zn) were obtained, and have been compared with those of single batch extraction of toxic characteristic leaching procedure (TCLP). In each size fraction of baghouse ash or scrubber residue, lead or zinc shows a common phenomena that the TCLP value is always lower than the content of the water‐soluble fraction. For lead, baghouse ash has a significant size‐dependent distribution on the water‐soluble phase to perform a size‐dependent test of lead in the TCLP test. The zinc TCLP data having less size dependence might be due to that neither the baghouse ash nor the scrubber residue has a size‐dependent distribution on the water‐soluble zinc phase.

Comparison of immobilisation of air pollution control residues with cement and with silica

Cement as agent for immobilising Pb from air pollution control residues is compared with the use of different silica-containing materials. The DIN 38414-S4 leaching test was used to control Pb leachability and to compare obtained Pb leachate concentrations with the landfill limit of 2 mg/l for Pb. Firstly, one scrubber residues was treated with cement and micro-silica. With cement, the Pb leachability could be reduced with a factor ranging from 3 to 50 depending on the type and amount of cement used and depending on the curing time. The landfill limit of 2 mg/l was, however, never attained. From all tested silica-containing additives, aerosil could reduce the initial Pb leaching (101.3 mg/l) to below the detection limit at a dosage of 0.13 g aerosil/g residue. Second best and an economically preferable silica-containing additive was micro-silica: a reduction from 101.3 to 0.7 mg/l was observed at a dosage of 0.4 g micro-silica/g residue. The formation of Ca-silicates was found to be responsible for the decreased Pb leachability. To generalise the findings, the Pb leachability of five cement-treated and five micro-silica-treated air pollution control residues were compared. For three scrubber residues, 2–20 times lower Pb leachate concentrations were measured for micro-silica-treated samples (cured for 5 weeks) than cement-treated samples. For a fly ash and a boiler ash the difference was, respectively, 48 and 17 times. pH-dependent leaching tests showed that at pH=2.5, Pb leaching is 250 times lower for the micro-silica-treated residue than for the cement-treated residue and almost seven times lower at pH 12.4.

Immobilization of lead and zinc in scrubber residues from MSW combustion using soluble phosphates

The immobilization of MSWI-scrubber residues with soluble PO 4 3− was studied and compared to the immobilization using cement. The DIN 38414-S4 leaching protocol and pH dependent leaching were used to evaluate the leaching of Pb and Zn. Four different scrubber residues from MSW combustion (Pb concentration: 2.8–4.8 mg/g; Zn concentration: 3.0–12.3 mg/g) were mixed with water and cement or Na 2HPO 4 as source of soluble PO 4 3− at dosages of at least 0, 0.1, 0.2, 0.3 and 0.4 g per g residue. With cement as well as with PO 4 3− a reduction in Pb and Zn leaching was observed. With 0.4 g cement per g residue, the Pb leaching was reduced by a factor ranging from 70 to 100, but still exceeded the Pb landfill limit of 2 mg/l. With PO 4 3− the Pb leaching was reduced with a factor of 100–300 to below 2 mg/l. The Zn landfill limit (10 mg/l) was only exceeded by one untreated residue. Adding 0.2 g cement or 0.1 g PO 4 3− per g of that residue was enough to reduce leaching below 10 mg/l. However, when 0.6 g Na 2HPO 4 per g residue was added to a lime based scrubber residue, an increase in Zn leaching up to 12.5 mg/l was observed due to an increase in pH of up to 13.0. When using NaH 2PO 4 and H 3PO 4 no such increase in Zn leaching was observed. pH dependent leaching performed on one of the four residues showed that in the pH range of 2.5–6, Pb leaching was 100–50 times lower with Na 2HPO 4 treatment than with cement. In the pH range from 7–11, almost equal results were obtained for cement treated and Na 2HPO 4 treated residue. Above a pH of 12, Pb leaching was three times lower for the PO 4 3−-treated residue than for the cement treated residue. With soluble PO 4 3−, Pb leaching below 2 mg/l could be attained at a dosage of 0.27 g PO 4 3−/g residue. With cement, Pb leaching was never below 2 mg/l.

Phosphate stabilization of municipal solid waste combustion residues: geochemical principles

Abstract The use of orthophosphate (PO 4 3 −) as a chemical stabilization agent for municipal solid waste (MSW) combustion residues is widespread in Japan and North America. The application of this technology to MSW ashes generally parallels its use with other metal contaminated wastes (e.g., soils, sediments, smelter dusts, slags, wire chopping wastes, mine tailings), especially Pb-contaminated soils. The technology relies on the fact that PO 4 3 − forms very insoluble and stable minerals for a variety of divalent metal cations (e.g., Pb, Cd, Cu, and Zn). Extensive data from phosphate-treated contaminated soil systems suggest that stabilization involves surface immobilization reactions involving sorption, heterogeneous nucleation and surface precipitation, and/or solution phase precipitation involving homogeneous nucleation and precipitation. A geochemical basis for use of PO 3− 4 in ash systems is presented with a focus on the wide theoretical pH distribution, pH-pE predominance and redox stability of Cd, Cu, Pb, and Zn phosphates within complex bottom ash pore water systems. Stabilization mechanisms in bottom ashes, scrubber residues, and vitrification dusts are similar to those observed in soil systems. Some longer term leaching behaviour of phosphate-stabilized ashes are presented. The roles of Ostwald ripening, solid solutions (e.g., (Pb,Ca) 5 (PO4) 3 OH), and kinetically controlled reaction pathways probably are more important than what is presently envisioned in phosphate-stabilized ash systems.

A study on the cementlike properties of municipal waste incineration ashes

This paper describes the origin of various ash wastes arising from municipal solid waste (MSW) incinerators, especially bag filter precipitator (BP) residue [called fly ash (FA)] and scrubber residue [called reaction product (RP)], which has cementlike compositions. This makes them suitable for immobilization for cement. The cementlike compositions are potential reuse as well as for stabilization treatments. The relations of ash wastes' alkalinity and salinity with lead leachability are correlated with composition.

Mechanisms for the aging-induced reduction of lead solubility in scrubber residues from municipal solid waste combustion.

This manuscript elucidates the mechanisms responsible for aging-induced reduction in lead leaching from scrubber residues. Leaching tests (JLT13) were conducted on 48 types of scrubber residues and lead solubility was found to be significantly reduced independent of incinerator type or type of gas treatment method. Reaction kinetics that result in lead solubility reduction were shown to be proportional to carbon dioxide partial pressure and in many cases were limited by mass transfer to the residue. With forced gas convection through the residue and a CO2 partial pressure of 0.3%, the concentration of lead in leachate was reduced from 84 mg L(-1) to < 0.5 mg L(-1) in two days. Ettringite analogs (Ca6Al2 (SO4) 3(OH)2 . 26H2O) were identified by X-Ray Diffraction in the wetted scrubber residues early in the aging process; these appear to have been converted into gypsum (CaSO4 . 2H2O) and vaterite (CaCO3) in the aged material. All of these solid phases are believed to substitute lead into their crystal structure and evidence for this attenuation mechanism is presented.