Development of an inverse method to identify the kinetics of heavy metal release during waste incineration in fluidized bed

Emission characteristics and impact factors of air pollutants from municipal solid waste incineration in Shanghai, China

The emission of air pollutants from the municipal solid waste (MSW) incineration is one of the major concerns in air pollution. The up-to-date emission situation for Chinese MSW incineration is largely unknown. The emission factors (EFs) are the key parameters to estimate the emissions from MSW incineration. The localized EFs from MSW incineration in Shanghai, China were established using continuous emission monitoring system data from 2017 to 2019. Our results showed that the EFs were 9.80 g t^-1 of PM, 46.62 g t^-1 of SO₂, 812.68 g t^-1 of NO_x, 25.84 g t^-1 of CO, and 17.49 g t^-1 of HCl for the period 2017-2019, nearly 1.7-24.2 times lower than those in 2010, implying that the current EFs should be updated to avoid overestimation of MSW emissions in China. Compared with 2010, the emissions of PM, SO₂, CO, and HCl in 2019 were significantly reduced by 84%, 69%, 47%, and 72%, respectively, except for NO_x with a 106% increase, although the corresponding MSW incineration amount increased by 356%. The current levels of air pollutants from MSW incineration have already met the current national emission standard as well as the stricter standard of the European Union (98.87%-99.91%). Our findings suggest that China should update the current standards of MSW incineration, which can be a benefit for the prevention and control of MSW incineration in the future. It is still challenging to control NO_x emissions from MSW incineration for Shanghai and even greater China.

The mechanism of hydrating and solidifying green mine fill materials using circulating fluidized bed fly ash-slag-based agent

This study focused on classifying and disposing Circulating fluidized bed (CFB) fly ashes from the level of its origin, and proposed an optimal formulation system for clinker-free cemented backfill materials. CFB fly ash-blast furnace slag (BFS)-based cemented backfill materials with unequal strength grades are used in different locations of the goaf that require more than 1 Mpa and 4 Mpa, respectively, and the leaching levels of all toxic components are lower than the underground III water quality standard limit when the additional amount of CFB fly ash does not exceed 60 wt.%. The stable S/S of Cl^- is due to the combined effect of chemical fixation of HCC and physical adsorption of the C-S-H/C-A-S-H phase. B2(20 wt.% CFB fly ash) exhibits more functional hydration products and higher degree of polymerization with the hydration age extension. Ettringite is the major effective product of CFB fly ash-BFS-based cemented system due to low level of chlorine environment and HCC transformation. CFB fly ash with appropriate active Al₂O₃ can dissolve and promote [AlO₄]^5- to substitute [SiO₄]^4- to form the C-A-S-H phase with longer chains and higher degree of polymerization with increase in Al/Si ratio of C-A-S-H/C-S-H phase.

Bridging the Energy Benefit and POPs Emission Risk from Waste Incineration

Incineration has been the globally controversial and concerned method of solid waste disposal. Energy recovery and volume reduction are the benefits from waste incineration, but risk due to release of persistent organic pollutants is the major public concern in the world. In this study, the emission of organic pollutants including dioxins and polychlorinated naphthalenes from solid waste incineration in China was comprehensively evaluated, and a relationship between energy benefit and pollutant emission was firstly established. The results show that production of medical and industrial waste was smaller than that of municipal waste but yielded comparable or even higher emission of dioxins. The energy benefit-to-emission index for organic pollutants (EBEI_OP) for evaluation of solid waste management on a local or regional scale was proposed. Significant correlations between net energy benefit and pollutant emission for provinces with higher EBEI_OP values were found. Furthermore, higher EBEI_OP values were associated with economic factors while lower values were influenced by emission from incineration of medical and industrial waste. We suggest that an EBEI_OP value of ≥60 can serve as a reference for “profitable” solid waste management, assisting decision making during energy benefit and environmental risk assessment.

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Emission of trace organic pollutants from solid waste incineration in China was comprehensively evaluated.

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The energy benefit-to-emission index for organic pollutants (EBEI_OP) for evaluation of solid waste management on a local or regional scale was proposed.

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Production of medical and industrial waste was smaller than that of municipal waste but yielded comparable or even higher emission of dioxins.

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Higher EBEI_OP values were associated with economic factors, while lower values were influenced by emissions from incineration of medical and industrial waste.

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An EBEI_OP value of ≥60 can serve as a reference for “profitable” solid waste management, assisting decision making during energy benefit and environmental risk assessment.

Study on the effects of catalysts on the immobilization efficiency and mechanism of heavy metals during the microwave pyrolysis of sludge

In order to enhance the immobilization of heavy metals in the bio-char during microwave pyrolysis, the immobilization efficiency and mechanism of heavy metals in the microwave pyrolysis of sludge with different alkaline catalysts were explored. Results showed that the leaching concentrations of heavy metals reduced greatly after pyrolysis, which were lower when catalyzed by CaO than those of Fe₂O₃. CaO was more favorable for the immobilization of Cr, Cu, Zn, Pb and Ni while Fe₂O₃ was more favorable for Cd. Different species distributions of heavy metals in the bio-char affected the leaching concentrations. Adding catalyst could significantly reduce the ecological risks of heavy metals in the bio-char, and CaO (RI = 15.17-20.43) had a better performance than Fe₂O₃ (RI = 16.88-21.79). When catalyzed by CaO, the formation of pores and co-crystal compounds in the bio-char determined the immobilization efficiencies of heavy metals.

Reductive solidification/stabilization of chromate in municipal solid waste incineration fly ash by ascorbic acid and blast furnace slag

Fly ash is a hazardous byproduct of municipal solid waste incineration (MSWI). Cementitious material that is based on ground-granulated blast furnace slag (GGBFS) has been tested and proposed as a binder to stabilize Pb, Cd, and Zn in MSWI fly ash (FA). Cr, however, still easily leaches from MSWI FA. Different reagents, such as ascorbic acid (VC), NaAlO₂, and trisodium salt nonahydrate, were investigated as potential Cr stabilizers. The results of the toxicity characteristic leaching procedure (TCLP) showed that VC significantly improved the stabilization of Cr via the reduction of Cr(VI) to Cr(III). VC, however, could interfere with the hydration process. Most available Cr was transformed into stable Cr forms at the optimum VC content of 2 wt%. Cr leaching was strongly pH dependent and could be represented by a quintic polynomial model. The results of X-ray diffraction and scanning electron microscopy-energy dispersive analysis revealed that hollow spheres in raw FA were partially filled with hydration products, resulting in the dense and homogeneous microstructure of the solidified samples. The crystal structures of C-S-H and ettringite retained Zn and Cr ions. In summary, GGBFS-based cementitious material with the low addition of 2 wt% VC effectively immobilizes Cr-bearing MSWI FA.

Coupling scales for modelling heavy metal vaporization from municipal solid waste incineration in a fluid bed by CFD

Municipal Solid Waste Incineration (MSWI) in fluidized bed is a very interesting technology mainly due to high combustion efficiency, great flexibility for treating several types of waste fuels and reduction in pollutants emitted with the flue gas. However, there is a great concern with respect to the fate of heavy metals (HM) contained in MSW and their environmental impact. In this study, a coupled two-scale CFD model was developed for MSWI in a bubbling fluidized bed. It presents an original scheme that combines a single particle model and a global fluidized bed model in order to represent the HM vaporization during MSW combustion. Two of the most representative HM (Cd and Pb) with bed temperatures ranging between 923 and 1073K have been considered. This new approach uses ANSYS FLUENT 14.0 as the modelling platform for the simulations along with a complete set of self-developed user-defined functions (UDFs). The simulation results are compared to the experimental data obtained previously by the research group in a lab-scale fluid bed incinerator. The comparison indicates that the proposed CFD model predicts well the evolution of the HM release for the bed temperatures analyzed. It shows that both bed temperature and bed dynamics have influence on the HM vaporization rate. It can be concluded that CFD is a rigorous tool that provides valuable information about HM vaporization and that the original two-scale simulation scheme adopted allows to better represent the actual particle behavior in a fluid bed incinerator.

Atmospheric emission characteristics and control policies of five precedent-controlled toxic heavy metals from anthropogenic sources in China

A bottom-up inventory of atmospheric emissions of five precedent-controlled toxic heavy metals (HMs), including mercury (Hg), arsenic (As), lead (Pb), cadmium (Cd), and chromium (Cr), from primary anthropogenic sources in China is established for the period 2000–2010. Total emissions of HMs demonstrate a gradually ascending trend along with the increase of coal consumption and industrial production, which are estimated at approximately 842.22 t for Hg, 4196.31 t for As, 29272.14 t for Pb, 795.29 t for Cd, and 13715.33 t for Cr for 2010. Coal combustion is found to be the primary source of HMs emissions. Owing to the dramatic differences of coal use by industrial and power sectors among provinces, spatial allocation performs remarkably uneven characteristics, and spatial distribution features are demonstrated by allocating the emissions into 0.5° × 0.5° grid cells with GDP and population as surrogate indexes. Further, HMs emissions from specified anthropogenic sources under three different control scenarios for the target year 2015 are projected, and collaborative and specialized control strategies are proposed to meet the demand of emission reduction goals of different regions. In the future, a whole processes control management system will be the most effective way for control of HMs.

Effect of HCl/SO₂₃/NH₃/O₂₃and mineral sorbents on the partitioning behaviour of heavy metals during the thermal treatment of solid wastes

The high concentration of heavy metals in solid wastes may cause serious pollution during thermal treatment. We have investigated, theoretically and experimentally, the effects of several important flue gas species and mineral sorbents on the partitioning behaviour of four major heavy metals (cadmium, lead, zinc and copper) which are often present in municipal solid waste (MSW). Their concentrations in bottom ash, fly ash and flue gas were quantified when model MSW samples were treated thermally under different conditions. The evaporation ratio of the four metals, excluding Cu, increased with decreasing oxygen concentration. The presence of HCl promotes heavy metal evaporation by preventing the formation of stable metallic species, especially for Zn (evaporation of more than 20%). An increase in oxygen concentration has a negative influence on the effect of HCl. In the presence of SO₂, Cd and Pb exhibited a higher evaporation ratio, while Zn and Cu were insensitive to the change. SO₂also inhibits Cd vaporization in an oxidative atmosphere. The effect of NH3 on reducing the metal volatilization rate was established indirectly. Calcium oxide addition enhances metal evaporation except for that of Zn (which shows a decrease of 38%). Although desulphurization by calcium injection decreases the volume of acid gas, calcium affects heavy metal pollution control adversely. The presence or addition of SiO₂- or Al₂O₃-containing minerals can lead to the formation of stable metallic salts. This may favour the control of Cd, Pb, Zn and Cu volatilization up to 13%, 50%, 17.5% and 19%, respectively.

Local CFD kinetic model of cadmium vaporization during fluid bed incineration of municipal solid waste

The emissions of heavy metals during incineration of Municipal Solid Waste (MSW) are a major issue to health and the environment. It is then necessary to well quantify these emissions in order to accomplish an adequate control and prevent the heavy metals from leaving the stacks. In this study the kinetic behavior of Cadmium during Fluidized Bed Incineration (FBI) of artificial MSW pellets, for bed temperatures ranging from 923 to 1073 K, was modeled. FLUENT 12.1.4 was used as the modeling framework for the simulations and implemented together with a complete set of user-defined functions (UDFs). The CFD model combines the combustion of a single solid waste particle with heavy metal (HM) vaporization from the burning particle, and it takes also into account both pyrolysis and volatiles' combustion. A kinetic rate law for the Cd release, derived from the CFD thermal analysis of the combusting particle, is proposed. The simulation results are compared with experimental data obtained in a lab-scale fluidized bed incinerator reported in literature, and with the predicted values from a particulate non-isothermal model, formerly developed by the authors. The comparison shows that the proposed CFD model represents very well the evolution of the HM release for the considered range of bed temperature.

Temporal trends and spatial variation characteristics of hazardous air pollutant emission inventory from municipal solid waste incineration in China

A multiple-year emission inventory of hazardous air pollutants (HAPs), including particulate matter (PM), SO(2), NO(x), CO, HCl, As, Cd, Cr, Hg, Ni, Pb, Sb, and polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs), discharged from municipal solid waste (MSW) incineration in China has been established for the period 2003-2010 by using the best available emission factors and annual activity data. Our results show that the total emissions have rapidly amounted to 28,471.1 t of NO(x), 12,062.1 t of SO(2), 6500.5 t of CO, 4654.6 t of PM, 3609.1 t of HCl, 69.5 t of Sb, 36.7 t of Hg, 9.4 t of Pb, 4.4 t of Cr, 2.8 t of Ni, 926.7 kg of Cd, 231.7 kg of As, and 23.6 g of PCDD/Fs as TEQ (toxic equivalent quantity) by the year 2010. The majority of HAP emissions are concentrated in the eastern central and southeastern areas of China where most MSW incineration plants are built and put into operation. Between 2003 and 2010, provinces always ranking in the top three with largest HAPs emissions are Zhejiang, Guangdong, and Jiangsu. To better understand the emissions of these HAPs and to adopt effective measures to prevent poisoning risks, more specific field-test data collection is necessary.

Back-calculation of the strength and location of hazardous materials releases using the pattern search method

Predicting quickly and accurately the strength and location of hazardous materials releases becomes a critical problem in emergency rescue. A technique that coupled the concentrations observed in the downwind direction of the source with a dispersion model was presented to back-calculate the strength and location of the release source by using the pattern search method. The technique was described as an optimization problem with an objective function constructed from a sum of squared errors between the observed concentrations and the calculated concentrations. The utility of the pattern search method was illustrated by testing the simulation data with practical data. The advantages of the method were demonstrated by a comparison with a gradient-based algorithm and an intelligent optimization algorithm. The computations indicate that this method can achieve optimal solutions in a relatively shorter time, hence more efficiently meeting the needs of emergency rescue.

Kinetic rate laws of Cd, Pb, and Zn vaporization during municipal solid waste incineration

The kinetic rate laws of heavy metal (HM) vaporization from municipal solid waste during its incineration were studied. Realistic artificial waste (RAW) samples spiked with Pb, Zn, and Cd were injected into a fluidized bed reactor. Metal vaporization wastracked by continuous measure ofthe above metals in exhaust gases. An inverse model of the reactor was used to calculate the metal vaporization rates from the concentration vs time profiles in the outlet gas. For each metal, experiments were carried out at several temperatures in order to determine the kinetic parameters and to obtain specific rate laws as functions of temperature. Temperature has a strong influence on the HM vaporization dynamics, especially on the vaporization kinetics profile. This phenomenon was attributed to internal diffusion control of the HM release. Two types of kinetic rate laws were established based on temperature: a fourth- or fifth-order polynomial rate law (r(x) = k0e(-E(A)/RT)p(x)) for temperatures lower than 740 degrees C and a first-order polynomial (r(x) = k0e(-E(A)/ RT(q-q(f) for temperatures higher than 740 degrees C.

Fate of heavy metals during municipal solid waste incineration in Shanghai

The transfer behavior of heavy metals during municipal solid waste (MSW) incineration was investigated based on 2-year field measurements in two large-scale incinerators in Shanghai. Great temporal and spatial diversification was observed. Most of Hg and Cd were evaporated and then removed by air pollution control (APC) system through condensation and adsorption processes, thus being enriched in the fine APC residues particles. Cr, Cu, and Ni were transferred into the APC residues mainly by entrainment, and distributed uniformly in the two residues flows, as well as in the ash particles with different sizes. Pb and Zn in the APC residues were from both entrainment and evaporation, resulting in the higher concentrations (two to four times) compared with the bottom ash. Arsenic was transported into the flue gas mainly by evaporation, however, its transfer coefficient was lower. Though the heavy metals contents in the APC residues were higher than that in bottom ash, more than 80% of As, Cr, Cu, and Ni, 74-94% of Zn, as well as 46-79% of Pb remained in the bottom ash, due to its high mass ratio (85-93%) in the residues. While 47-73% of Cd and 60-100% of Hg were transferred into the APC residues, respectively.

Heavy metal vaporization and abatement during thermal treatment of modified wastes

This study examines the vaporization percentage and partitioning of heavy metals Cd, Pb and Zn during thermal treatment of wastes with added PVC, heavy metals or phosphate, and the efficiency of sorbents for removal of these metallic compounds in flue gas of an industrial solid waste incinerator. Firstly, vaporization experiments were carried out to determine the behavior of heavy metals during combustion under various conditions (type of waste, temperature, presence of chloride or phosphate ...). The experimental results show relatively high vaporization percentage of metallic compounds within fly ash and limestone matrix while heavy metals within sediments treated with phosphoric acid are less volatile. Vaporization of metals increases with increasing temperature and with chloride addition. The thermal behavior of the selected heavy metals and their removal by sorbents (sodium bicarbonate, activated carbon) was also studied in an industrial solid waste incinerator. These pilot scale experiments confirm that heavy metals are concentrated in fly ashes and cyclone residues, thus effectively controlling their release to the atmosphere.