Fumitake Takahashi

Mineralogical characterization of municipal solid waste incineration bottom ash with an emphasis on heavy metal-bearing phases.

Municipal solid waste incineration (MSWI) bottom ash contains a considerable amount of heavy metals. The occurrence and uneven distribution of these heavy metals in bottom ash can increase the complexity of such residues in terms of long-term behavior upon landfilling or recycling. Bottom ashes sampled from three stoker-type incinerators in Japan were analyzed in this paper. This study presents detailed information on the mineralogical characterization of bottom ash constituents and the weathering behavior of these constituents by means of optical microscopy and scanning electron microscopy. It was revealed that bottom ash mainly consists of assorted silicate-based glass phases (48-54 wt% of ash) and mineral phases including melilites, pseudowollastonite, spinels, and metallic inclusions (Fe-P, Fe-S, Fe-Cu, Cu-Sn, Cu-Zn, Cu-S, and Cu-Pb dominated phases), as melt products formed during the incineration process. The compounds embedded in the glass matrix, e.g. spinels and metallic inclusions, played the most important role in concentration of heavy metals (Pb, Zn, Cu, Cr, Mn, Ni, etc.). Other phases such as refractory minerals and ceramics, frequently found in ash, were of less significance in terms of their influence on the involvement of heavy metals. Analysis of lab-scale artificially weathered and 10-year landfilled bottom ash samples revealed that secondary mineralization/alteration of the bottom ash constituents principally carbonation and glass evolution substantially decreased the potential risk of the heavy metals to the surrounding environment.

Metal nickel nanoparticles in situ generated in rice husk char for catalytic reformation of tar and syngas from biomass pyrolytic gasification

This paper aims to propose a novel catalytic pyrolytic gasification technology for the in situ conversion of tar and syngas, accompanied by the silica-based nickel nanoparticles generated in situ and the highly dispersed rice husk char (RHC), namely RHC Ni. Partially oxidized nickel oxides (i.e., NiO) in the carbon matrix of biochar can be carbothermally reduced to metallic nickel (Ni0) nanoparticles by reducing gases (e.g., CO) or carbon atoms during biomass pyrolysis. Moreover, due to its strong reducibility, the addition of sodium borohydride (NaBH4) can significantly promote the generation of Ni0 by the reduction of NiO, improving the biochars catalytic activity. An ultra-low tar yield can be achieved by pyrolysis of RH Ni and RH Ni–B at 750 °C, in terms of the high tar conversion efficiencies of 96.9% and 98.6%, respectively, compared with the pyrolysis of raw RH. It is noteworthy that the condensable tar could be catalytically reformed into the small molecules of non-condensable tar or gases, which contributes to improving the syngas fuel characteristics in the favor of power generation systems, corresponding to the lower heating value (LHV) of syngas increasing from 10.25 to 11.32 MJ m−3. In addition, the increase of the polymolecular Ni0 was most possibly caused by the disproportionation reaction and strong reducibility of NaBH4. In addition, the produced RHC Ni showed a good performance for the catalytic conversion of tar (conversion efficiency, 96.5%) through co-pyrolysis with biomass. After deactivation, the waste RHC Ni might be easily regenerated via thermal treatment or directly catalytically gasified into the applicable syngas, accompanied by the production of the silica-based nickel nanoparticles.

Alteration of municipal solid waste incineration bottom ash focusing on the evolution of iron-rich constituents

Municipal solid waste incineration (MSWI) bottom ash contains a considerable amount of Fe-rich constituents. The behaviors of these constituents, such as dissolution and precipitation, are quite important as they regulate the distribution of a series of ions between the liquid (percolated fluid) and solid (ash deposit) phases. This paper studied both fresh and weathered MSWI bottom ash from the mineralogical and geochemical viewpoint by utilizing optical microscopy, scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX), and powder X-ray diffraction. The analysis results revealed that for the fresh bottom ash, iron preferentially existed in the chemical forms of spinel group (mainly Fe(3)O(4), and a series of Al- or Ti- substituted varieties), metallic inclusions (including Fe-P, Fe-S, Fe-Cu-Pb), hematite (Fe(2)O(3)) and unburned iron pieces. In the 1-20 years weathered bottom ash collected from a landfill site, interconversions among these Fe-rich constituents were identified. Consequently, numerous secondary products were developed, including goethite (α-FeOOH), lepidocrocite (γ-FeOOH), hematite, magnetite, wustite (FeO), Fe-Si-rich gel phase. Of all these transformation products, hydrous iron oxides were the most common secondary minerals. Quantitative chemical analysis of these secondary products by SEM/EDX disclosed a strong association between the newly formed hydrous iron oxides and heavy metals (e.g. Pb, Zn, Ni, and Cu). The results of this study suggest that the processes of natural weathering and secondary mineralization contribute to reduction of the potential risks of heavy metals to the surrounding environments.

Statistical estimate of mercury removal efficiencies for air pollution control devices of municipal solid waste incinerators

Although representative removal efficiencies of gaseous mercury for air pollution control devices (APCDs) are important to prepare more reliable atmospheric emission inventories of mercury, they have been still uncertain because they depend sensitively on many factors like the type of APCDs, gas temperature, and mercury speciation. In this study, representative removal efficiencies of gaseous mercury for several types of APCDs of municipal solid waste incineration (MSWI) were offered using a statistical method. 534 data of mercury removal efficiencies for APCDs used in MSWI were collected. APCDs were categorized as fixed-bed absorber (FA), wet scrubber (WS), electrostatic precipitator (ESP), and fabric filter (FF), and their hybrid systems. Data series of all APCD types had Gaussian log-normality. The average removal efficiency with a 95% confidence interval for each APCD was estimated. The FA, WS, and FF with carbon and/or dry sorbent injection systems had 75% to 82% average removal efficiencies. On the other hand, the ESP with/without dry sorbent injection had lower removal efficiencies of up to 22%. The type of dry sorbent injection in the FF system, dry or semi-dry, did not make more than 1% difference to the removal efficiency. The injection of activated carbon and carbon-containing fly ash in the FF system made less than 3% difference. Estimation errors of removal efficiency were especially high for the ESP. The national average of removal efficiency of APCDs in Japanese MSWI plants was estimated on the basis of incineration capacity. Owing to the replacement of old APCDs for dioxin control, the national average removal efficiency increased from 34.5% in 1991 to 92.5% in 2003. This resulted in an additional reduction of about 0.86Mg emission in 2003. Further study using the methodology in this study to other important emission sources like coal-fired power plants will contribute to better emission inventories.

Co-gasification kinetics of coal char and algae char under CO2 atmosphere

ABSTRACT In this study, Newlands coal char and spirulina algae char were prepared separately in a fixed bed reactor with a pyrolysis temperature of 1000 ºC. The isothermal CO2 co-gasification experiment was done using a thermogravimetric analyzer in the temperature range of 800–1000 ºC. The volumetric model (VM), the shrinking core model (SCM), the random pore model (RPM) and the modified random pore model (MRPM) were applied to describe the gasification kinetics of the samples. The results show that synergetic effects were observed for all ratios while the 5:5 blend demonstrates the best performance which may be attributed to the high content of potassium included in the algae char which in turn promotes the catalytic effect. Among the above-mentioned models, RPM was found to be predicting the conversion profile best among all tested models except for the algae sample which failed to fit the data by RPM and hence MRPM was applied for the algae sample only. RPM and MRPM were adopted to calculate the kinetics parameters. The activation energy and the pre-exponential factor were determined using the Arrhenius equation. The activation energy for all chars was found to be in the range of 100–200 kJ/mol.

Characterization of chlorine and heavy metals for the potential recycling of bottom ash from municipal solid waste incinerators as cement additives

AbstractBottom ash is an inevitable by-product from municipal solid waste (MSW) incineration plants. Recycling it as additives for cement production is a promising disposal method. However, the heavy metals and chlorine are the main limiting factors because of the potential environmental risks and corrosion of cement kilns. Therefore, investigating heavy metal and chlorine characteristics of bottom ash is the significant prerequisite of its reuse in cement industries. In this study, a correlative analysis was conducted to evaluate the effect of the MSW components and collection mode on the heavy metal and chlorine characteristics in bottom ash. The chemical speciation of insoluble chlorine was also investigated by synchrotron X-ray diffraction analysis. The results showed that industrial waste was the main source of heavy metals, especially Cr and Pb, in bottom ash. The higher contents of plastics and kitchen waste lead to the higher chlorine level (0.6 wt.%–0.7 wt.%) of the bottom ash. The insoluble chlorine in the MSW incineration bottom ash existed primarily as AlOCl, which was produced under the high temperature (1250°C) in incinerators.

Tar removal capacity of waste cooking oil absorption and waste char adsorption for rice husk gasification

ABSTRACT Tar removal by oil absorber and char adsorber known as the physical method is a promising technology for cleaning synthesis gas from biomass gasification offering high tar removal efficiency, low cost, uncomplicated operation and waste-free facility. However, it is essential to maintain tar removal efficiency to a satisfying level in order to prevent downstream application breakdown. Therefore, periodic changing of absorbent and adsorbent are highly required. In this paper, the tar removal capacity of waste cooking oil (WCO) and waste char (WC) is investigated to maximize waste utilization for gas cleaning pre-treatment systems. For WCO, the gravimetric tar removal reached the breakthrough point in the second hour and the capacity was 14.4 g-tar/L-WCO (80.6% and 94.6% of gravimetric tar and naphthalene removal performance on average). For WC, the breakthrough point was in the second hour for naphthalene removal and the capacity was 0.15 mg-naphthalene/g-waste char (76% of naphthalene removal performance) while it could also adsorb heavy tar with the capacity of 48.8 mg-tar/g-waste char. This increased the gravimetric tar removal efficiency by 3.1% when connecting the WC bed after the WCO scrubber.

Atmospheric mercury emissions from waste combustions measured by continuous monitoring devices

Atmospheric mercury emissions have attracted great attention owing to adverse impact of mercury on human health and the ecosystem. Although waste combustion is one of major anthropogenic sources, estimated emission might have large uncertainty due to great heterogeneity of wastes. This study investigated atmospheric emissions of speciated mercury from the combustions of municipal solid wastes (MSW), sewage treatment sludge (STS), STS with waste plastics, industrial waste mixtures (IWM), waste plastics from construction demolition, and woody wastes using continuous monitoring devices. Reactive gaseous mercury was the major form at the inlet side of air pollution control devices in all combustion cases. Its concentration was 2.0–70.6 times larger than elemental mercury concentration. In particular, MSW, STS, and IWM combustions emitted higher concentration of reactive gaseous mercury. Concentrations of both gaseous mercury species varied greatly for all waste combustions excluding woody waste. Variation coefficients of measured data were nearly equal to or more than 1.0. Emission factors of gaseous elemental mercury, reactive gaseous mercury, and total mercury were calculated using continuous monitoring data. Total mercury emission factors are 0.30 g-Hg/Mg for MSW combustion, 0.21 g-Hg/Mg for STS combustion, 0.077 g-Hg/Mg for STS with waste plastics, 0.724 g-Hg/Mg for industrial waste mixtures, 0.028 g-Hg/Mg for waste plastic combustion, and 0.0026 g-Hg/Mg for woody waste combustion. All emission factors evaluated in this study were comparable or lower than other reported data. Emission inventory using old emission factors likely causes an overestimation. Implications Although waste combustion is one of major anthropogenic sources of atmospheric mercury emission, estimated emission might have large uncertainty due to great heterogeneity of wastes. This study investigated speciated mercury emissions from the combustions of municipal solid wastes, sewage treatment sludge with/without waste plastics, industrial waste mixtures, waste plastics from construction demolition, and woody wastes using continuous monitoring devices. Reactive gaseous mercury was the major form in all combustion cases and its concentration in the gas had large fluctuation. All emission factors evaluated in this study were comparable or lower than other reported data. Emission inventory using old emission factors likely causes an overestimation.

Geochemically structural characteristics of municipal solid waste incineration fly ash particles and mineralogical surface conversions by chelate treatment

Leaching behaviors of heavy metals contained in municipal solid waste incineration (MSWI) fly ash have been studied well. However, micro-characteristics of MSWI fly ash particles are still uncertain and might be non-negligible to describe their leaching behaviors. Therefore, this study investigated micro-characteristics of MSWI fly ash particles, especially their structural properties and impacts of chelate treatment on surface characteristics. According to SEM observations, raw fly ash particles could be categorized into four types based on their shapes. Because chelate treatment changed the surface of fly ash particles dramatically owing to secondary mineral formations like ettringite, two more types could be categorized for chelate-treated fly ash particles. Acid extraction experiments suggest that fly ash particles, tested in this study, consist of Si-base insoluble core structure, Al/Ca/Si-base semi-soluble matrices inside the body, and KCl/NaCl-base soluble aggregates on the surface. Scanning electron microscope (SEM) observations of the same fly ash particles during twice moistening treatments showed that KCl/NaCl moved under wet condition and concentrated at different places on the particle surface. However, element mobility depended on secondary mineral formations. When insoluble mineral like gypsum was generated and covered the particle surface, it inhibited element transfer under wet condition. Surface characteristics including secondary mineral formation of MSWI fly ash particles are likely non-negligible to describe trace element leaching behaviors.

The weathering of municipal solid waste incineration bottom ash evaluated by some weathering indices for natural rock

The weathering of municipal solid waste incineration (MSWI) residues consists of complicated phenomena. This makes it difficult to describe leaching behaviors of major and trace elements in fresh/weathered MSWI bottom ash, which was relevant interactively to pH neutralization and formation of secondary minerals. In this study, mineralogical weathering indices for natural rock profiles were applied to fresh/landfilled MSWI bottom ash to investigate the relation of these weathering indices to landfill time and leaching concentrations of component elements. Tested mineralogical weathering indices were Weathering Potential Index (WPI), Ruxton ratio (R), Weathering Index of Parker (WIP), Vogts Residual Index (V), Chemical Index of Alternation (CIA), Chemical Index of Weathering (CIW), Plagioclase Index of Alternation (PIA), Silica-Titania Index (STI), Weathering Index of Miura (Wm), and Weatherability index of Hodder (Ks). Welchs t-test accepted at 0.2% of significance level that all weathering indices could distinguish fresh and landfilled MSWI bottom ash. However, R and STI showed contrasted results for landfilled bottom ash to theoretical expectation. WPI, WIP, Wm, and Ks had good linearity with reclamation time of landfilled MSWI bottom ash. Therefore, these four indices might be applicable as an indicator to identify fresh/weathered MSWI bottom ash and to estimate weathering time. Although WPI had weak correlation with leachate pH, other weathering indices had no significant correlation. In addition, all weathering indices could not explain leaching concentration of Al, Ca, Cu, and Zn quantitatively. Large difficulty to modify weathering indices correctly suggests that geochemical simulation including surface sorption, complexation with DOM, and other mechanisms seems to be the only way to describe leaching behaviors of major and trace elements in fresh/weathered MSWI bottom ash.