Incineration of doped sludges in fluidized bed. Fate and partitioning of six targeted heavy metals. I. Pilot plant used and results

Incineration of sewage sludge doped with several heavy metals was studied at small pilot plant scale in a bubbling fluidized bed of 15 cm i.d. and 5.2 m height. Some ceramic and metallic filters were tested at a relatively high temperature (600–700°C) to check their usefulness for partitioning of heavy metals in the flue gas. The work was focused on the fate of six selected heavy metals (Cr, Cd, Ni, Zn, Cu, Pb). In this process, there were four exit flows or discharges for these metals: bottom ash, coarse fly ash, cake filter or fine fly ash and flue exit gas. The distribution or partitioning of each heavy metal (HM) among these four exit flows was studied. Only cadmium and sometimes lead showed any difference between the different HMs considered. All other HMs seems to have the same fate, distribution or partitioning. Such distribution is governed or ruled by the fluid dynamics in the incinerator, cyclone and ceramic filter. Most of the HMs do not have enough residence time in this incinerator type to diffuse out of the ash particle and so remain in the particle. The amount of each HM in each exit flow in this process is governed by fluid dynamics and kinetics and not at all by thermodynamics.     

Partitioning behavior of trace elements during pilot-scale fluidized bed combustion of high ash content lignite

This study describes the partitioning of 20 trace elements (As, B, Ba, Cd, Co, Cr, Cu, Hg, Li, Mn, Mo, Ni, P, Pb, Sb, Se, Sn, Tl, V, Zn) and eight major and minor elements (Al, Ca, Fe, K, Mg, Na, Si, Ti) during the combustion of high ash content lignite. The experiments were carried out in the 0.3 MW(t) Middle East Technical University (METU) atmospheric bubbling fluidized bed combustor (ABFBC) test rig with and without limestone addition. Inert bed material utilized in the experiments was bed ash obtained previously from the combustion of the same lignite without limestone addition in the same test rig. Concentrations of trace elements in coal, limestone, bottom ash, cyclone ash and filter ash were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). Partitioning of major and minor elements are influenced by the ash split between the bottom ash and fly ash and that the major proportion of most of the trace elements (As, Ba, Cr, Hg, Li, Mo, Ni, Sn, V, Zn) are recovered in fly ash. Limestone addition shifts the partitioning of Ba, Cr, Mo, Ni, Sn, V, Zn from bottom ash to fly ash.     

Development and combustion characteristics of microwave plasma-assisted fluidized bed combustor

A novel waste treatment method that can efficiently decompose waste, suppress by-product generation, and operate at a low cost is urgently required. Herein, a microwave plasma-assisted combustor was developed and its combustion characteristics were investigated for application in solid waste treatment. The experimental conditions for obtaining good states of fluidization, mixing, and plasma formation were examined prior to the combustion experiments. Subsequently, the optimal experimental conditions, such as the filling amount of bed particles, bed particle diameter, microwave irradiation position, and microwave output, required for good combustion were achieved. Combustion experiments based on these conditions revealed that a good fluidization state is required to obtain a good combustion state in this device, although the combustion condition does not necessarily depend on the system pressure at each O₂ flow rate. Comparison of the conditions with similar fluidization states at O₂ flow rates of 1–4 L/min revealed a maximum fuel conversion ratio at 4 L/min owing to the combustion promotion caused by increased O₂ partial pressure. The fuel did not remain in the fluidized bed combustor after the combustion experiments.     

Characterization of low chromium ferritic steels for use in fluidized bed combustion systems

There is evidence from certain materials evaluation studies that suggests that low chromium ferritic steels are susceptible to oxide scaling at a gr:ater rate than would have been expected if the steels had been expectsed m a. conventional coal-fired combustor. Such a result is of economic sigmfwance since it tmplles that the changeover point in a superheater from low alloy ferritic to austenitic tubing would have to be at a lower metal temperature than would otherwise be necessary, with consequent Cast penalties. There were, however, some anomalies in the data because of uncertamty over the precise metal temperatures of the steels included in the studtes. A programme of work was therefore established to resolve this problem.

Ferritic steel samples were exposed at carefully controlled metal tempe.ratures under steady state conditions in an AFBC test rig. The scaling characteristics of these samples were compared with data produced under controlled. condttwns m atr. In contrast to earlier, less closely defined tests, the results of thts present study suggest that the oxidation rates of 2.25 wt% Cr-lwt% Mo steel are similar to those expenenced in air and in conventional combustion systems. On this basis the upper temperature limit for use of this steel for in-bed superheater tubes would be set at 560°C. In contrast 9 wt% Cr-l wt% Mo steel showed enhanced oxidation rates, greater than those found in either air or conventional combustion systems, and would appear to offer no advantage over 2.25 wt% Cr-lwt% Mo steel. The possible impact on these recommendations for systems where high chlorine coals will be burned, or where erosion-corrosion effects are likely, is discussed.     

Solubility of heavy metals added to MSW

This paper aims to investigate the six heavy metal levels (Cd, Cr, Cu, Pb, Ni and Zn) in municipal solid waste (MSW) at different pHs. It intends to provide the baseline information of metals solubility in MSW co-disposed or co-digested with MSW incinerator ashes in landfill or anaerobic bioreactors or heavy metals contaminated in anaerobic digesters. One milliliter (equal to 1mg) of each metal was added to the 100ml MSW and the batch reactor test was carried out. The results showed that higher HNO3 and NaOH were consumed at extreme pH of 1 and 13 compared to those from pH 2 to 11 due to the comparably higher buffer capacity. Pb was found to have the least soluble level, highest metal adsorption (%) and highest partitioning Kd (lg(-1)) between pH 3 and 12. In contrast, Ni showed the highest soluble level, lowest metal adsorption (%) and lowest Kd (lg(-1)) between pH 4 and 12. Except Ni and Cr, other four metals seemed to show the amphibious properties as comparative higher solubility was found in the acidic and basic conditions.     

Experimental and numerical investigation of catalytic PM combustion in a fluidized bed type PM removal device for low-temperature continuous regeneration

A fluidized bed filter can perform highly efficient PM collection and low-temperature continuous regeneration. However, to further reduce continuous regeneration temperature, a rough surface bed particle was selected herein. It is expected that the rough surface increases and stabilizes doped catalyst on bed particle even in fluidized bed. This bed particle can stably support 9.48 g-catalyst/kg-bed particle of doped catalyst versus 1.58 g-catalyst/kg-bed particle in previous research. This increase in catalyst amount increases the probability of good PM-catalyst contact, and collection efficiency can easily maintain its initial value due to catalytic PM combustion. PM combustion also depends on fluidization. Thus, combustion kinetics in a fluidized bed was investigated via a newly developed thermogravimetric analyzer that considered PM-gas relative velocity, and a constructed kinetic model was applied to numerical simulation. PM combustion obeyed an Arrhenius relationship, and the effect of PM-gas relative velocity was included in the kinetic model as a mass transfer term. A continuous regeneration experiment was conducted under optimal conditions, and the continuous regeneration temperature is 330 °C. As water vapor occurs in combustor exhaust, we added 10 vol% water vapor and found that the continuous regeneration is further reduced to 300 °C.     

Modeling electrowinning process in an expanded bed electrode

A theoretical model has been developed to describe the flow behavior of conducting particles in a fluidized bed electrode for electro winning of metal ions present in the dilute solution. Model equations have been developed for potential and current distributions and mass transfer rates. The influence of operating parameters on particle growth has been critically examined. It has been observed from the present investigation that the particle size increased with electrolysis time. The present model simulations have been compared with the experimental data reported in the literature and observed that the model predictions satisfactorily match with the reported experimental findings.     

Factorial experimental design for recovering heavy metals from sludge with ion-exchange resin

Wastewaters containing heavy metals are usually treated by chemical precipitation method in Taiwan. This method can remove heavy metals form wastewaters efficiently, but the resultant heavy metal sludge is classified as hazardous solid waste and becomes another environmental problem. If we can remove heavy metals from sludge, it becomes non-hazardous waste and the treatment cost can be greatly reduced. This study aims at using ion-exchange resin to remove heavy metals such as copper, zinc, cadmium, and chromium from sludge generated by a PCB manufacturing plant. Factorial experimental design methodology was used to study the heavy metal removal efficiency. The total metal concentrations in the sludge, resin, and solution phases were measured respectively after 30 min reaction with varying leaching agents (citric acid and nitric acid); ion-exchange resins (Amberlite IRC-718 and IR-120), and temperatures (50 and 70 degrees C). The experimental results and statistical analysis show that a stronger leaching acid and a higher temperature both favor lower heavy metal residues in the sludge. Two-factors and even three-factor interaction effects on the heavy metal sorption in the resin phase are not negligible. The ion-exchange resin plays an important role in the sludge extraction or metal recovery. Empirical regression models were also obtained and used to predict the heavy metal profiles with satisfactory results.     

Distribution of extractable fractions of heavy metals in sludge during the wastewater treatment process

Sludge samples were collected from different treatment steps of Gaobeidian wastewater treatment plant (WWTP) of Beijing City, PR China, to investigate the distributions of total and chemical fractions of Fe, Mn, Ni, Cu, Zn, Cr, Pb, and Mo in different sludges. The highest total concentrations were found for Fe, Mn, Pb, and Mo in digested sludge (DS), Ni and Cr in thickened sludge (TS), Zn in dewatering sludge (DWS), and Cu in active sludge (AS). The lowest concentrations were observed in AS, except for Cu in TS. Significant differences of total metal concentration were observed between AS and TS (or DS), suggesting that sludge thickening and digesting treatments significantly influenced the total metal concentrations. Fe, Cu, Ni, Cr, Mo, and Pb distributed principally in the residual fraction in all sludges, while Zn and Mn presented in a highly available fraction. For same metal in different sludges, the portion of easily mobile fraction decreased significantly along the wastewater treatment process, and metals in AS presented in the highest available fraction. Organic matter contents, TN, and TP of sludges exhibited a significant positive correlation with the concentrations of exchangeable and reducible fraction of Pb, Mo, Cr, Cu, and Fe, while sludge pH demonstrated significant negative correlations with the concentrations of these metals.     

Kinetic modeling of PM combustion with relative velocity at low-temperature and numerical simulation of continuous regenerating type PM removal device that uses a fluidized bed

The size of particulate matter (PM) generated by combustion has decreased with the improvement of combustion technology. While small PM has a significant negative impact on the human body, it is difficult for a conventional PM removal device to collect small PM. We developed a fluidized bed type PM removal device with a focusing adhesion force. This device collects small PM effectively and can be operated as a continuous regeneration device at low temperature. To further develop this device, it is important to investigate the PM combustion characteristics in this device. The kinetic model constructed in conventional thermogravimetry could not accurately represent the combustion rates of the solid fuel in the fluidized bed. Therefore, a new thermogravimetric apparatus was constructed in this study that generates the direct collision of air with carbon to reproduce the fluidized bed combustion. The influence of the relative velocity between PM and gas on the combustion rate was investigated. The effect of relative velocity was represented as the mass transfer coefficient of kinetic model. It is observed that the combustion rate shows Arrhenius behavior, and kinetic parameters were determined by fitting. The kinetic model was applied to the numerical simulations of the PM removal device. The numerical collection efficiency was in good agreement with the experimental data. PM adhesion and combustion characteristics were investigated in numerical simulations. It is observed that the adhesion rate is high at a low void fraction and that the combustion rate is high at a high relative velocity. The PM combustion amount is high for the high adhesion and combustion rates. The total combustion amount is determined to be 55% of the total amount of PM deposition after 180 min at each set of conditions.     

Emissions of polycyclic aromatic hydrocarbons from fluidized and fixed bed incinerators disposing petrochemical industrial biological sludge

This study investigated the emissions of polycyclic aromatic hydrocarbons (PAHs) from two fluidized bed incinerators (FLBI_A and FLBI_B) and one fixed bed incinerator (FIBI) disposing biological sludge generated from the petrochemical industries in Taiwan. The results of 21 individual PAHs (including low (LM-PAHs), middle (MM-PAHs) and high molecular weight PAHs (HM-PAHs)) were reported. The LM-PAHs mainly dominated the total-PAHs in the stack flue gases, whereas the LM- and HM-PAHs dominated the total-PAHs in the bottom fly, fly ash and WSB effluent. Due to high carcinogenic potencies (= total-BaP_eq concentrations) in the bottom ash (195 ng g⁻¹) and WSB effluent (20,600 ng L⁻¹) of the FIBI, cautious should be taken in treating them to avoid second contamination. Lower combustion efficiency and elevated fuel/feedstock (F/W) ratio for the FIBI led to the highest total emission factor of total-PAHs (38,400 μg kg⁻¹). Lower total-PAH removal efficiencies of wet scrubber (WSB) (0.837–5.89%), cyclone (0.109–0.255%) and electrostatic precipitator (ESP) (0.032%) than those reported elsewhere resulted in high fraction in PAH contributions from the stack flue gases. Lower total-PAH emission factor was found for FLBI_A (2380 μg kg⁻¹ biological sludge) with higher combustion efficiency compared to those for FLBI_B (11,500 μg kg⁻¹) and FIBI (38,400 μg kg⁻¹ biological sludge), implying that combustion efficiency plays a vital role in PAH emissions.     

Stabilization of heavy metals in ceramsite made with sewage sludge

In order to investigate stabilization of heavy metals in ceramsite made with sewage sludge as an additive, the configuration of heavy metals in ceramsite was analysed by XRD and while leaching tests were conducted to find out the effect of sintering temperature (850 degrees C, 900 degrees C, 950 degrees C, 1000 degrees C, 1100 degrees C, and 1200 degrees C), pH (1, 3, 5, 7, 9, and 12), and H2O2 concentration (0.5molL(-1), 1molL(-1), 1.5molL(-1), 3molL(-1), and 5molL(-1)) on stabilization of heavy metals (Cd, Cr, Cu, and Pb) in ceramsite. The results indicate that leaching contents of heavy metals do not change above 1000 degrees C and sintering temperature has a significant effect on stabilization of heavy metals in ceramsite; leaching contents of heavy metals decrease as pH increases and increase as H2O2 concentration increases. XRD analysis reveals that the heavy metals exist in steady forms, mainly Pb2O(CrO(4)), CdSiO3, and CuO at 1100 degrees C. It is therefore concluded that heavy metals are properly stabilized in ceramsite and cannot be easily released into the environment again to cause secondary pollution.     

3D simulation on pressurized oxy-fuel combustion of coal in fluidized bed

Pressurized oxy-fuel combustion technology is considered as a perspective carbon capture technology in industrial process. A computational fluid dynamics (CFD) model based on Multi-Phase Particle-In-Cell (MP–PIC) method was developed to predict pressurized oxy-coal combustion process in fluidized bed. The heterogeneous and homogeneous combustion reactions of coal were considered in this model. The predicted results were validated the accuracy of this model with experimental data from a 15 kW_th pressurized fluidized bed combustor in terms of the gas component and temperature characteristics. The characteristics of gas–solid flow and combustion under different pressure (0.1–2 MPa) and oxygen atmosphere were studied in this work. The predicted results show that the intensity of particle motion and the expansion degree in the fluidized bed was gradually decreased with an increase in pressure. A correlation was proposed based on the simulation results to maintain suitable fluidization conditions in pressurized circulating fluidized bed at different pressures. The temperature of particle phase region gradually increased with combustion pressure and inlet O₂ concentration increased. In addition, the CO₂ concentration in outlet increased while the emission of CO and NO_x decreased as the combustion pressure increased.     

The influence of heavy metals on the formation of organics and HCl during incinerating of PVC-containing waste

The heavy metal, organic pollutants and HCl emissions from incineration pose a grave threat to human health. However, submicro metal particles and organics in gas phase are removed with difficulty by conventional air pollution control apparatus, and could be released into the atmosphere along with the flue gas. This research studied the formation of organics and HCl (hydrogen chloride) during incineration when the feedstock contained different amounts of PVC and heavy metals. The experiments were performed in a bubbling fluidized bed incinerator with two cyclones and scrubbers. Experimental results indicated that the greatest amount of organic compounds produced were aromatic compounds. The amount of organic compounds and HCl increased when the feedstock contained PVC (polyvinyl chloride), and decreased when the feedstock contained heavy metals.     

Influence of clinoptilolite rock on chemical speciation of selected heavy metals in sewage sludge

The chemical speciation of Cd, Cu, Pb, Cr and Ni in Torun municipal sewage sludge is investigated with addition of a natural sorbent (clinoptilolite rock). The total contents of the heavy metals in the sludge are substantially lower than the corresponding limits established by European or Polish legislation excepting nickel only. But the metals concentrations excepting lead exceed significantly the natural background (average contents in soils and in the Earth's crust) in dozens. Application of the sequential chemical extraction indicated that the metals in the sewage sludge are bound mainly (over 50%) in the residual fraction. The metals form the following order by parts of the mobile form: Ni> Cd> Cr> Cu> Pb. Addition of the clinoptilolite to the sludge leads to the metals contents fall in all four fractions of the sequential procedure. Concentrations of mobile forms of cadmium, chromium, copper and nickel decrease by 87, 64, 35 and 24%, respectively, as a result of addition of 9.09% of the clinoptilolite. The total decreases of the metals amount after 9.09% clinoptilolite addition to the sludge are around 11, 15, 25, 41 and 51% for copper, nickel, chromium, cadmium and lead, respectively.     

Structural characterization of fluidized bed nitrided steels

The nitriding behaviour of 34CrAlNi7, 42CrMo4 and 40CrMnMoS86 steels was investigated nitrided in the fluidized bed processes. The nitriding processes were carried out at a temperature of 575 °C for treatment times of 6, 12 and 18 h. The nitrided samples were fully characterized using metallographic, microhardness and XRD techniques. Test results indicated that thickness of the compound layer on the steel surface changed in the range from 10 to 18 μm depending on steel type and treatment time, and γ′-Fe₄N and ε-Fe₂₋₃N formed in the compound layer. The hardness of the diffusion layer was over 1000 HV. Depending on the chemical composition of steels, the case depth ranged from 155 to 525 μm. Kinetics studies showed that the effective diffusion coefficients are 298×10⁻¹⁴, 525×10⁻¹⁴ and 68.8×10⁻¹⁴ m² s⁻¹, for 34CrAlNi7, 42CrMo4 and 40CrMnMoS86 steels, respectively. The fluidized bed process realizes the highest hardness of the case layer, 1095 HV, with fairly high growth rates, 27 μm/h.     

Use of experimental designs to optimize fluidized bed granulation of maltodextrin

The goal of this study was to use the experimental design approach in order to determine which process parameters are the most influent to granules properties. The agglomeration process was performed with a fluidized bed processor equipped with a top-spray tilted nozzle. The granules were obtained by binding fluidized particles of maltodextrin (DE 12) with an aqueous solution of maltodextrin. The variables considered in the experimental design were fluidization air flow rate, drying air temperature, spraying pressure and binder flow rate. The physical properties of the granules were evaluated in terms of granule size, particle size distribution, dissolution time and flowability. Moreover, the amount of binder for each experiment was noted. The statistical analysis performed with Statgraphics® Centurion version XVI software indicated that the best operating conditions were found for low fluidization air flow rate, high drying air temperature, high spraying pressure and high binder flow rate. The study also revealed the difficult task of acquiring all the optimum properties at the same time.     

Agglomerate behavior in a recirculating fluidized bed with sheds: Effect of sheds

A Radioactive Particle Tracking (RPT) technique was used to study the effects of the internal baffles in the stripping section of the Fluid Coker?, called sheds, have on the behavior of wet agglomerates that are formed when residual oil is injected into the Coker. Vapor emitted by reacting wet agglomerates below the sheds rises and causes shed fouling. The release of vapor from agglomerates can be estimated by combining the RPT results with a coking reaction model. The study found that the sheds reduce the time agglomerates spend in the shed zone, which in turn reduces the amount of organic vapor that reaches the sheds, but at the same time increase the wetness of the agglomerates that exit to the recirculation line, which results in the loss of valuable liquid. The research also found that the best type of shed, from the point of view of agglomerate motion, is the mesh-shed. Finally, experimental data indicate that reducing the cross sectional area of the sheds from 50% to 30% increases the time that the agglomerates spend above the shed zone, and thus reduces the flow of vapor emitted below the sheds.     

Particle size distribution control and related properties improvements of tungsten powders by fluidized bed jet milling

Fine grinding process of different particle size tungsten powders was carried out by fluidized bed jet milling. The results showed that the jet milling treatment caused deagglomeration of tungsten powders, which led to particles sufficient dispersion and narrow particle size distribution. Grinding gas pressure of 0.70 Mpa made the particles achieve high speed which promoted the particles collision contributing to particle dispersion and shape modification. For tungsten powder with particle size of 3 μm FSSS, a higher packing density with 5.54 g/cm³ was obtained, compared with the 3.71 g/cm³ of the original powder. For tungsten powder with particle size of 1 μm FSSS, the big agglomerates disappeared and the particle size distribution become narrower, while small aggregates about 2–3 μm still exist after the jet milling process. For tungsten powder with particle size of 5 μm and 10 μm FSSS, different medium diameter particle size and narrow particle size distribution of monodisperse tungsten powders can be produced by the optimized jet milling parameters. More important, the effective dispersion, favorable shape modification and precise classification have been achieved in the simple process.     

Incineration of a small particle of wet sewage sludge: a numerical comparison between two states of the surrounding atmosphere

The main goal of this paper is to elaborate a mathematical model that represents the physics and chemistry involved when a small particle of wet sewage sludge is incinerated. Compared to existing models, our study includes both drying and heterogenous combustion of the pyrloysis residue of the processed sludge. This model relies on the assumption of homogeneous composition and temperature for the particle under study. It includes drying, pyrolysis (controlled by a four successives steps reaction pathway) and combustion of the resulting char. The ability of the model is illustrated using it in two different process conditions (representing thermogravemetric analysis and fluidized bed conditions) in order to investigate the influence of the surrounding atmosphere. It is found, that fluidized bed conditions reduce the burnout time of a small particle by enhancing the rate at which heat is transferred to that particle. It is also shown that high heating rates enhance the tar yield.