Wuming Xie

An experimental and thermodynamic equilibrium investigation of the Pb, Zn, Cr, Cu, Mn and Ni partitioning during sewage sludge incineration

The effects of different chlorides and operational conditions on the distribution and speciation of six heavy metals (Pb, Zn, Cr, Cu, Mn and Ni) during sludge incineration were investigated using a simulated laboratory tubular-furnace reactor. A thermodynamic equilibrium investigation using the FactSage software was performed to compare the experimental results. The results indicate that the volatility of the target metals was enhanced as the chlorine concentration increased. Inorganic-Cl influenced the volatilization of heavy metals in the order of Pb>Zn>Cr>Cu>Mn>Ni. However, the effects of organic-Cl on the volatility of Mn, Pb and Cu were greater than the effects on Zn, Cr and Ni. With increasing combustion temperature, the presence of organic-Cl (PVC) and inorganic-Cl (NaCl) improved the transfer of Pb and Zn from bottom ash to fly ash or fuse gas. However, the presence of chloride had no obvious influence on Mn, Cu and Ni. Increased retention time could increase the volatilization rate of heavy metals; however, this effect was insignificant. During the incineration process, Pb readily formed PbSiO4 and remained in the bottom ash. Different Pb compounds, primarily the volatile PbCl2, were found in the gas phase after the addition of NaCl; the dominant Pb compounds in the gas phase after the addition of PVC were PbCl2, Pb(ClO4)2 and PbCl2O4.

Effects of sulfur on lead partitioning during sludge incineration based on experiments and thermodynamic calculations.

Experiments in a tubular furnace reactor and thermodynamic equilibrium calculations were conducted to investigate the impact of sulfur compounds on the migration of lead (Pb) during sludge incineration. Representative samples of typical sludge with and without the addition of sulfur compounds were combusted at 850 °C, and the partitioning of Pb in the solid phase (bottom ash) and gas phase (fly ash and flue gas) was quantified. The results indicate that three types of sulfur compounds (S, Na2S and Na2SO4) added to the sludge could facilitate the volatilization of Pb in the gas phase (fly ash and flue gas) into metal sulfates displacing its sulfides and some of its oxides. The effect of promoting Pb volatilization by adding Na2SO4 and Na2S was superior to that of the addition of S. In bottom ash, different metallic sulfides were found in the forms of lead sulfide, aluminosilicate minerals, and polymetallic-sulfides, which were minimally volatilized. The chemical equilibrium calculations indicated that sulfur stabilizes Pb in the form of PbSO4(s) at low temperatures (<1000 K). The equilibrium calculation prediction also suggested that SiO2, CaO, TiO2, and Al2O3 containing materials function as condensed phase solids in the temperature range of 800-1100 K as sorbents to stabilize Pb. However, in the presence of sulfur or chlorine or the co-existence of sulfur and chlorine, these sorbents were inactive. The effect of sulfur on Pb partitioning in the sludge incineration process mainly depended on the gas phase reaction, the surface reaction, the volatilization of products, and the concentration of Si, Ca and Al-containing compounds in the sludge. These findings provide useful information for understanding the partitioning behavior of Pb, facilitating the development of strategies to control the volatilization of Pb during sludge incineration.

Influence of catalysts on co-combustion of sewage sludge and water hyacinth blends as determined by TG-MS analysis

Effects of the three metal carbonates (K2CO3, Na2CO3, and MgCO3) were quantified on catalytic co-combustion of the sewage sludge and water hyacinth (SW) blend using a thermogravimetric-mass spectrometric (TG-MS) analysis and kinetics modeling. The main dominating steps of the catalysts were the organic volatile matter release and combustion stage. Weighted mean values of activation energy (Em) were estimated at 181.18KJ·mol-1, 199.76KJ·mol-1, 138.76KJ·mol-1, and 177.88KJ·mol-1 for SW, SW+5% K2CO3, SW+5% Na2CO3, and SW+5% MgCO3, respectively. The lowest Em occurred with SW+5% Na2CO3. Overall, catalyst effect on co-combustion appeared to be negligible as indicated by Gibbs free energy (ΔG). The normalized intensities of SW+MgCO3 were strongest. The addition of Na2CO3 and MgCO3 to SW increased flue gases emissions (CO2, NO2, SO2, HCN, and NH3) of SW, whereas the addition of K2CO3 to SW reduced flue gases emissions from the entire combustion process.

Co-combustion of sewage sludge and coffee grounds under increased O2/CO2 atmospheres: Thermodynamic characteristics, kinetics and artificial neural network modeling

(Co-)combustion characteristics of sewage sludge (SS), coffee grounds (CG) and their blends were quantified under increased O2/CO2 atmosphere (21, 30, 40 and 60%) using a thermogravimetric analysis. Observed percentages of CG mass loss and its maximum were higher than those of SS. Under the same atmospheric O2 concentration, both higher ignition and lower burnout temperatures occurred with the increased CG content. Results showed that ignition temperature and comprehensive combustion index for the blend of 60%SS-40%CG increased, whereas burnout temperature and co-combustion time decreased with the increased O2 concentration. Artificial neural network was applied to predict mass loss percent as a function of gas mixing ratio, heating rate, and temperature, with a good agreement between the experimental and ANN-predicted values. Activation energy in response to the increased O2 concentration was found to increase from 218.91 to 347.32 kJ·mol-1 and from 218.34 to 340.08 kJ·mol-1 according to the Kissinger-Akahira-Sunose and Flynn-Wall-Ozawa methods, respectively.

Thermodynamic Equilibrium Calculations on Cd Transformation during Sewage Sludge Incineration.

Thermodynamic equilibrium calculations were performed to reveal the distribution of cadmium during the sewage sludge incineration process. During sludge incineration in the presence of major minerals, such as SiO2, Al2O3 and CaO, the strongest effect was exerted by SiO2 on the Cd transformation compared with the effect of others. The stable solid product of CdSiO3 was formed easily with the reaction between Cd and SiO2, which can restrain the emissions of gaseous Cd pollutants. CdCl2 was formed more easily in the presence of chloride during incineration, thus, the volatilization of Cd was advanced by increasing chlorine content. At low temperatures, the volatilization of Cd was restrained due to the formation of the refractory solid metal sulfate. At high temperatures, the speciation of Cd was not affected by the presence of sulfur, but sulfur could affect the formation temperature of gaseous metals.

Comparative thermogravimetric analyses of co-combustion of textile dyeing sludge and sugarcane bagasse in carbon dioxide/oxygen and nitrogen/oxygen atmospheres: Thermal conversion characteristics, kinetics, and thermodynamics

Thermodynamic and kinetic parameters of co-combustion of textile dyeing sludge (TDS) and sugarcane bagasse (SB) were studied using thermogravimetric analysis in CO2/O2 and N2/O2 atmospheres. Our results showed that the comprehensive combustion characteristic index (CCI) of the blends was improved by 1.71-4.32 times. With the increased O2 concentration, co-combustion peak temperature decreased from 329.7 to 318.2 °C, with an increase in its maximum weight loss rate from 10.04 to 14.99%/min and its CCI by 1.31 times (β = 20 °C·min-1). To evaluate the co-combustion characteristics, thermodynamic and kinetic parameters (entropy, Gibbs free energy and enthalpy changes, and apparent activation energy) were obtained in the five atmospheres. The lowest apparent activation energy of the TB64 blend was obtained in oxy-fuel atmosphere (CO2/O2 = 7/3).

Combustion behaviors of spent mushroom substrate using TG-MS and TG-FTIR: Thermal conversion, kinetic, thermodynamic and emission analyses

The present study systematically investigated the combustion characteristics of spent mushroom substrate (SMS) using TG-MS (thermogravimetric/mass spectrometry) and TG-FTIR (thermogravimetric/Fourier transform infrared spectrometry) under five heating rates. The physicochemical characteristics and combustion index pointed to SMS as a promising biofuel for power generation. The high correlation coefficient of the fitting plots and similar activation energy calculated by various methods indicated that four suitable iso-conversional methods were used. The activation energy varied from 130.06 to 192.95 kJ/mol with a mean value of 171.49 kJ/mol using Flynn-Wall-Ozawa and decreased with the increased conversion degree. The most common emissions peaked at the range of 200-400 °C corresponding to volatile combustion stage, except for CO2, NO2 and NO. The peak CO2 emission occurred at 439.11 °C mainly due to the combustion of fixed carbon.

Thermal degradations and processes of waste tea and tea leaves via TG-FTIR: Combustion performances, kinetics, thermodynamics, products and optimization

The present study characterized the kinetic, thermodynamic and performance parameters, products, factorial interactions, and optimal conditions of combustions of waste tea (WT) and tea leaves (TL) in N2/O2 and CO2/O2 atmospheres through a thermogravimetric/Fourier transform infrared spectrometry (TG-FTIR). The main combustion occurred in the range of 200-600 °C. The increased heating rate increased all the combustion parameters regardless of the fuel and atmosphere type. Activation energy was shown different change tendency with the increasing conversion (α). CO2, H2O, CH4, CO, CO, NH3, and HCN were the main gas products of WT and TL combustions. A three-way interaction among fuel type, atmosphere type and heating rate was found to be significant. The joint optimization of mass loss, derivative TG, and differential scanning calorimetry was achieved using 1049.3 °C, TL, 40 °C/min, and CO2/O2 atmosphere for the operational settings of temperature, fuel type, heating rate, and atmosphere type, respectively.

(Co-)combustion of additives, water hyacinth and sewage sludge: Thermogravimetric, kinetic, gas and thermodynamic modeling analyses

Additives and biomass were co-combusted with sewage sludge (SS) to promote SS incineration treatment and energy generation. (Co-)combustion characteristics of sewage sludge (SS), water hyacinth (WH), and 5% five additives (K2CO3, Na2CO3, Mg2CO3, MgO and Al2O3) were quantified and compared using thermogravimetric-mass spectrometric (TG-MS) and numerical analyses. The combustion performance of SS declined slightly with the additives which was demonstrated by the 0.03-to-0.25-fold decreases in comprehensive combustibility index (CCI). The co-combustion performed well given the 0.31-fold increase in CCI. Kinetic parameters were estimated using the Ozawa-Flynn-Wall (OFW) and Kissinger-Akahira-Sunose (KAS) methods. Apparent activation energy estimates by OFW and KAS were consistent. The addition of K2CO3 and MgCO3 decreased the weighted average activation energy of SS. Adding K2CO3 to the blend reduced CO2, NO2, SO2, HCN and NH3 emissions. CO2, NO2 and SO2 emissions were higher from WH than SS. Adding WH or K2CO3 to SS increased CO2, NO2 and SO2 but HCN and NH3 emissions. Based on both catalytic effects and evolved gases, K2CO3 was potentially an optimal option for the catalytic combustion among the tested additives.

Accelerated crystallization of magnetic 4A-zeolite synthesized from red mud for application in removal of mixed heavy metal ions

To cope with the increasing environmental issues of red mud, an integrated technological route for its comprehensive utilization was developed through the extraction of valuable components and the synthesis of magnetic 4A-zeolite. To accelerate the crystallization process of the synthesized 4A-zeolite, sodium chloride (NaCl) was innovatively employed under hydrothermal treatment. The effects of various parameters, including mass ratio of red mud/NaOH, alkali fusion temperature, alkali fusion time and molar ratio of NaCl/Al2O3, were systematically investigated. The results showed that approximately 81.0% Al, 76.1% Si and 95.8% Fe were utilized from red mud using alkali fusion and acid leaching methods. The optimal conditions of the alkali fusion process were determined as: mass ratio of red mud/NaOH = 1/2, alkali fusion temperature of 800 °C, and time of 90 min. Furthermore, when the molar ratio of NaCl/Al2O3 was kept at 1.5, the crystallization time reduced from 240 min to 150 min, and particle size distributions narrowed from 20-100 μm to 1-10 μm. The practical applications in removal of mixed heavy metal ions (Zn2+, Cu2+, Cd2+, Ni2+, and Pb2+) from wastewater indicated that the as-synthesized magnetic 4A-zeolite is a promising candidate for heavy metals adsorption.