Thermal Treatment Of Solid Waste Research Articles

Formation kinetics and reaction behavior of pentavalent chromium formed in the cement kiln co-processing of solid waste

Cement kiln co-processing is becoming the main strategy to dispose of hazardous waste containing Cr. A newly-discovered pentavalent Cr compound, which was proved to be formed during cement kiln co-processing of solid waste, is partly responsible for the water-soluble Cr released from the cement. However, the formation characteristics and the solubility of Cr(V) are still unclear to date. In this study, the reaction kinetics and further redox reactions of Cr(V) at high temperature were examined, and its crystal structure and solubility were also explored. At the temperature range of 1000–1200 °C, the formation rate of Ca5(CrO4)3O0.5 reached over 90 % within 10 min, and then slowly increased to near 100 % from 10 min to 10 h. shows that Ca5(CrO4)3O0.5 is formed by interface reaction at an early period, and by diffusion at a later period. The kinetic analysis indicates that Ca5(CrO4)3O0.5 is initially formed through an interface reaction and subsequently through diffusion. Ca5(CrO4)3O0.5 was identified and assigned as hexagonal crystal group (P63/m). Approximately 0.55 g and 0.15 g of Ca5(CrO4)3O0.5 dissolve in neutral water at 100 °C and 50 °C, and the concentrations of Cr(V) in water reach 550 and 150 mg/L, respectively. Additionally, this study finds that at the temperature range of 400–700 °C Ca5(CrO4)3O0.5 can be oxidized into CaCrO4, and at the temperature higher than 1400 °C, it can be further converted into Ca3(CrO4)2 and reduced into CaCr2O4. This study gives a deep insight into Cr oxidation-reduction reaction during thermal treatment of solid waste. These insights provide a comprehensive understanding of Cr oxidation-reduction reactions during the thermal treatment of solid waste, offering valuable guidance for waste management strategies.

Experimental study of heliothermal processing of municipal solid waste based on solar energy

The article examines the sources of solid waste generation, their impact on the environment and human health. The morphological composition of municipal solid waste (MSW) and the factors influencing the dynamics of their change were analyzed. Based on the results of the study, the physical and thermal parameters of solid waste- ρ, φ were determined and the elemental composition and heat of combustion of solid waste were analyzed. It was found that the physical properties of MSW and the morphological composition of MSW depend on the seasons and other conditions. An energy balance diagram for heliothermal processing of solid household waste with different morphological compositions in a solar device for thermal treatment of solid waste was calculated and it was found that the share of solar energy for thermal processing of solid household waste was 78%. As a result of the thermal processing of organic compounds, which formed the morphological composition of the MSW, it was found that it is possible to obtain alternative fuels and produce thermal energy and electricity in the traditional way.

Oxidation and reduction reactions of (Al/FexCr1-x)2O3 caused by CaO during thermal treatment of solid waste containing Cr

Solid solutions of (AlxCr1-x)2O3 and (FexCr1-x)2O3 are predominant compounds containing Cr in solid waste and are frequently formed during thermal treatment of solid waste. (AlxCr1-x)2O3 and (FexCr1-x)2O3 have superior thermomechanical properties and excellent corrosion resistance. However, oxidation and reduction reactions of the Cr in these solid solutions seriously affect their chemical stabilities and the environmental risks posed by the final products. In this study, first the reaction behaviors of (AlxCr1-x)2O3 and (FexCr1-x)2O3 at high temperatures were analyzed and whether the incorporation of Cr(III) in solid solutions can prevent Cr(III) from being oxidized was determined. Both (AlxCr1-x)2O3 and (FexCr1-x)2O3 without the presence of CaO exhibit good thermal stability at high temperatures. However, the participation of CaO induces Cr(III) oxidation in (AlxCr1-x)2O3 and (FexCr1-x)2O3 at 500–1000 °C. Cr(III) oxidation in these solid solutions is accompanied by the formation of CaCrO4 and Fe2O3 or Al2O3. Al2O3 combines with CaCrO4 and further forms a more stable Cr(VI) compound (e.g., Ca4Al6O12CrO4). While Fe2O3 combines with CaCrO4 at 1000–1200 °C. This is accompanied by the formation of CaCr2O4 and CaFe2O4, which effectively promotes the reduction of Cr(VI). Moreover, part of the CaCr2O4 transforms into a more stable phase (i.e., FeCr2O4) at 1200–1300 °C. Although the incorporation of Cr(III) in these solid solutions cannot prevent Cr(III) oxidation completely at high temperatures, the Cr(III) oxidation in these solid solutions is still suppressed compared with Cr2O3. The results of this study provide further insights into the oxidation and reduction reactions of Cr-hosting compounds during thermal treatment of solid waste.

Thermal Decomposition Kinetic Study of Non-Recyclable Paper and Plastic Waste by Thermogravimetric Analysis

The global net emissions of the Kyoto Protocol greenhouse gases (GHG), such as carbon dioxide (CO2), fluorinated gases, methane (CH4), and nitrous oxide (N2O), remain substantially high, despite concerted efforts to reduce them. Thermal treatment of solid waste contributes at least 2.8–4% of the GHG in part due to increased generation of municipal solid waste (MSW) and inefficient treatment processes, such as incineration and landfill. Thermal treatment processes, such as gasification and pyrolysis, are valuable ways to convert solid materials, such as wastes into syngas, liquids, and chars, for power generation, fuels, or for the bioremediation of soils. Subcoal™ is a commercial product based on paper and plastics from the source segregated waste that is not readily recyclable and that would otherwise potentially find its way in to landfills. This paper looks at the kinetic parameters associated with this product in pyrolysis, gasification, and combustion conditions for consideration as a fuel for power generation or as a reductant in the blast furnace ironmaking process. Thermogravimetric Analysis (TGA) in Nitrogen (N2), CO2, and in air, was used to measure and compare the reaction kinetics. The activation energy (Ea) and pre-exponential factor A were measured at different heating rates using non-isothermal Ozawa Flynn Wall and (OFW) and Kissinger-Akahira-Sonuse (KAS) model-free techniques. The TGA curves showed that the thermal degradation of Subcoal™ comprises three main processes: dehydration, devolatilization, and char and ash formation. In addition, the heating rate drifts the devolatilization temperature to a higher value. Likewise, the derivative thermogravimetry (DTG) results stated that Tm degradation increased as the heating rate increased. Substantial variance in Ea was noted between the four stages of thermal decomposition of Subcoal™ on both methods. The Ea for gasification reached 200.2 ± 33.6 kJ/mol by OFW and 179.0 ± 31.9 kJ/mol by KAS. Pyrolysis registered Ea values of 161.7 ± 24.7 kJ/mol by OFW and 142.6 ± 23.5 kJ/mol by KAS. Combustion returned the lowest Ea values for both OFW (76.74 ± 15.4 kJ/mol) and KAS (71.0 ± 4.4 kJ/mol). The low Ea values in combustion indicate shorter reaction time for Subcoal™ degradation compared to gasification and pyrolysis. Generally, TGA kinetics analysis using KAS and OFW methods show good consistency in evaluating Arrhenius constants.

Temperature dependent reduction of Cr(VI) to Cr(V) aroused by CaO during thermal treatment of solid waste containing Cr(VI)

Cr(VI) compounds at high temperature usually tend to decompose and reduce into Cr(III) due to thermodynamically instability for Cr(VI). This study found Cr(VI) could be reduced into Cr(V) instead of Cr(III) in the presence of CaO during heating solid waste containing Cr(VI). CaCrO4 is prepared and mixed with CaO as simulated solid waste containing Cr(VI). It was found that CaCrO4 reacted with CaO and formed a new product Ca5(CrO4)3O0.5 at temperature range of 800 and 1000 °C. The valence state of Cr in Ca5(CrO4)3O0.5 is determined to be +5 b y XPS analysis, and the color for new formed Cr(V) is observed in green, similar to Cr(III) compounds. The temperature and CaO are two keys to arouse the reduction reaction of Cr(VI) into Cr(V). In particular, the reduction of Cr(VI) into Cr(V) is strongly depended on temperature (800–1000 °C), this reaction can be balanced within 10 min, while prolonging sintering time has little help for promoting the reduction of Cr(VI) to Cr(V). Additionally, it was found Cr(V) can keep stable and not be re-oxidized into Cr(VI) at 800–1000 °C. Above results offers some new understanding and knowledge about the formation of Cr(V) in presence of much CaO or CaCO3 during heating solid waste containing Cr(VI).

Evaluation on isothermal pyrolysis characteristics of typical technical solid wastes

Isothermal pyrolysis characteristics of typical technical solid wastes (polyethylene, paper towel and textiles, and their mixture with a blending ratio of 4:1:1) were carried out in a fixed bed between 475 °C and 550 °C. The influence of the temperature on the pyrolysis index, kinetic parameters and products of the samples was addressed. The interaction between the three individuals during their co-pyrolysis was identified. As the temperature was from 475 °C to 550 °C, the pyrolysis index of the polyethylene increased by about 78 times, while that for paper towel and textiles only increased by about 1 and 6 times, respectively. Solid residues from the mixture pyrolysis were about 11.6 % wt. More alkanes and alkenes were in liquid products of the mixture. The shrinking core model would represent well the mixture pyrolysis. The pre-exponential factor of polyethylene was much higher than other two individuals. There was a significant synergistic effect detected for mixture during the co-pyrolysis. The pyrolysis of mixture was dominated by a third order reaction mechanism. This study can provide basic data for the thermal treatment of solid waste in nuclear power plants.

The Study of Various Models of the Process of Thermal Treatment of Solid Waste

Methods for calculating and modeling individual processes that occur during the thermal treatment of solid waste in reactors with constant and variable temperatures of the medium have been considered. Studies of the influence of various mathematical models of the heating of dispersed material on the results of kinetic analysis of the pyrolysis of wood waste have been carried out. The influence of the type of the mathematical model and its parameters on process characteristics have been shown. For the model of material heating that takes into account the change in the local temperature both with time and with coordinate, the dependences of the average nondimensional temperature of material of spherical and cylindrical form on nondimensional time have been proposed for different values of the Biot number.

Detoxification of solid waste containing Cr(VI) with phosphate by thermal treatment

The remediation of solid waste containing Cr(VI) has always been challenging due to the toxic and carcinogenic properties of Cr(VI). This study proposed a novel method to detoxify solid waste containing Cr(VI) with phosphate by thermal treatment. Thermodynamic calculation proved that thermally reduction of Cr(VI) with phosphate was thermodynamic feasible. When the heating temperature increased to 1000°C, Cr(VI) was effectively reduced to Cr(III) and more than 97% of chromates could be reduced with the addition of (NH4)2HPO4 at n(P)/n(Cr) ratio of 3. Moreover, CaCrO4 was relatively easier to be reduced comparing with Na2CrO4 and K2CrO4. Additionally, phosphate exhibited greater potential to trigger Cr(VI) reduction in comparison to SiO2, because the potential of oxides to trigger the reduction of Cr(VI) was dominated by their acidity. Chromite ore processing residue was mixed with (NH4)2HPO4 at n(P)/n(Cr) rate of 3 and heated at 1000°C. Toxicity characteristic leaching procedure (TCLP) showed that the concentration of Cr(VI) in leachate was below 5mg/L and met the regulatory limit, it indicated that chromite ore processing residue (COPR) could be successfully remedied with phosphate by thermal treatment. These results suggested the thermal treatment of solid waste containing Cr(VI) with phosphate was a promising and reliable strategy for the detoxification of Cr(VI).

EXPERIMENTAL STUDY OF EFFECT OF PRESSURE ON PYROLYSIS OF BIOMASS

Pyrolysis is thermal treatment of solid waste in the absence of an oxidizing agent (in inert atmosphere) which generally yields three products namely pyrolysis-oil (liquid), char (solid residue) and gas. The products obtained in pyrolysis can be further used as energy source. Rice husk and saw dust are considered as solid wastes in this study. The effect of temperature and pressure on pyrolysis was studied in a fixed bed reactor. The experimental runs were performed in nitrogen atmosphere. The temperature was varied from 300°C to 500°C, while the operational pressure was fixed at 4 bar, 7 bar and 10 bar. The products such as solid char, liquid pyrolysis oil and syngas were obtained. The solid char was characterized by CHNS analysis, while the pyrolysis oil was characterized by FTIR analysis,

Effects of Al2O3, Fe2O3, and SiO2 on Cr(VI) formation during heating of solid waste containing Cr(III)

In this study, the influences of Al2O3, Fe2O3, and SiO2 on the Cr(VI) formation during thermal treatment of solid waste containing Cr(III) were studied. Al2O3, Fe2O3, and SiO2 suppressed Cr(III) oxidation by incorporating alkali and alkaline earth metals in the products (i.e. silicates, aluminates, and ferrites). However, the products still have the potential to trigger Cr(III) oxidation, but was less than the original ones. The decrease could be attributed to the interactions of the original alkali and alkaline earth metal salts with Al2O3, Fe2O3 and SiO2, which reducing the basicity of resulting products. Because the potential of alkali and alkaline earth metal salts to trigger Cr(III) oxidation is dominated by their basicity. Additionally, Al2O3, Fe2O3and SiO2 could also trigger Cr(VI) to be reduced, and this ability is related to their acidity. SiO2 is more acidic than Al2O3 and Fe2O3, and can cause Na2CrO4 and K2CrO4 to be reduced at temperatures above 1000°C, but Al2O3 and Fe2O3 cannot. These results suggested that acidic oxides (e.g. SiO2 and P2O5) played an important role in inhibiting Cr(III) oxidation, and that increasing the fraction of acidic oxide was beneficial to suppress the Cr(III) oxidation during thermal treatment of solid waste containing Cr(III).

Economic aspects of thermal treatment of solid waste in a sustainable WM system

This paper offers a systematic review of the literature of the last 15years, which applies economic analysis and theories to the issue of combustion of solid waste. Waste incineration has attracted the interest of economists in the first place concerning the comparative assessment of waste management options, with particular reference to external costs and benefits. A second important field of applied economic research concerns the market failures associated with the provision of thermal treatment of waste, that justify some deviation from the standard competitive market model. Our analysis discusses the most robust achievements and the more controversial areas. All in all, the economic perspective seems to confirm the desirability of assigning a prominent role to thermal treatments in an integrated waste management strategy. Probably the most interesting original contribution it has to offer concerns the refusal of categorical assumptions and too rigid priority ladders, emphasizing instead the need to consider site-specific circumstances that may favor one or another solution.

Effect of HCl/SO2/NH3/O2 and 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. SO2 also 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.

Thermochemical reaction mechanism of lead oxide with poly(vinyl chloride) in waste thermal treatment

Poly(vinyl chloride) (PVC) as a widely used plastic that can promote the volatilization of heavy metals during the thermal treatment of solid waste, thus leading to environmental problems of heavy metal contamination. In this study, thermogravimetric analysis (TGA) coupled with differential scanning calorimeter, TGA coupled with Fourier transform infrared spectroscopy and lab-scale tube furnace experiments were carried out with standard PVC and PbO to explicate the thermochemical reaction mechanism of PVC with semi-volatile lead. The results showed that PVC lost weight from 225 to 230°C under both air and nitrogen with an endothermic peak, and HCl and benzene release were also detected. When PbO was present, HCl that decomposed from PVC instantly reacted with PbO via an exothermal gas–solid reaction. The product was solid-state PbCl2 at <501°C, which was the most volatile lead-containing compound with a low melting point and high vapor pressure. At >501°C, PbCl2 melted, volatilized and transferred into flue gas or condensed into fly ash. Almost all PbCl2 volatilized above 900°C, while PbO just started to volatilize slowly at this temperature. Therefore, the chlorination effect of PVC on lead was apt to lower-temperature and rapid. Without oxygen, Pb2O was generated due to the deoxidizing by carbon, with oxygen, the amount of residual Pb in the bottom ash was significantly decreased.

Thermal treatment of solid waste in view of recycling: Chromate and molybdate formation and leaching behaviour.

Elevated Cr and Mo concentrations are often found in leachates of thermally treated solid waste, but there is no general explanation for this so far. Therefore, we studied the leaching behaviour after thermal treatment as a function of heating temperature and residence time for two types of solid waste: contaminated sludge and bottom ash from municipal solid waste incineration. The leaching behaviour of both waste streams was compared with experiments on synthetic samples, allowing deduction of a general mechanism for Cr and Mo leaching. Cr and Mo showed a similar leaching behaviour: after an initial increase, the leaching decreased again at higher temperatures. Oxidation of these elements from their lower oxidation states to chromate and molybdate at temperatures up to 600 °C was responsible for the increased leaching. At higher temperatures, both Mo and Cr leaching decreased again owing to the formation of an amorphous phase, incorporating the newly formed chromate and molybdate salts, which prevents them from leaching.

Catalytic Pyrolysis/Gasification of Refuse Derived Fuel for Hydrogen Production and Tar Reduction: Influence of Nickel to Citric Acid Ratio Using Ni/SiO2 Catalysts

Gasification technology is an attractive alternative for the thermal treatment of solid wastes, producing a high energy value hydrogen rich syngas. The presence of tar in the produced gas diminishes its quality and potential use in further processes; for this reason the reduction of tar in waste gasification is a major challenge. In this work the pyrolysis/gasification of refuse derived fuel (RDF) from municipal solid wastes, was investigated using a two-stage reaction system with Ni/SiO2 catalysts prepared by a sol–gel method varying the citric acid concentration (CA). The fresh and reacted catalysts were characterised for surface area and pore size distribution, temperature programmed oxidation, and high resolution scanning electron microscopy. The effect of the nickel to citric acid ratio (Ni:CA) was evaluated in terms of the characteristics and performance of the Ni/SiO2 catalysts. The results showed that the prepared Ni/SiO2 catalysts exhibited a relatively high surface area and an increase in pore size distribution as the Ni:CA ratio was increased. The efficiency of the prepared catalysts on tar reduction and hydrogen production was examined during the pyrolysis/gasification of RDF; the results were compared with a blank experiment using a bed of sand. The tar fraction was quantified using gas chromatography/mass spectrometry. A low tar concentration of ~0.2 mgtar/gRDF was attained using the catalysts with Ni:CA ratios of 1:1 and 1:3; additionally a high hydrogen concentration (58 vol%), and low CH4 (2.2 vol%) and C2–C4 concentrations (0.8 vol%), were attained using the catalyst with a Ni:CA ratio of 1:3. A higher tar concentration of ~1.7 mgtar/gRDF was attained using the bed of sand, while the hydrogen production was remarkably decreased. The major tar compounds identified in the tar samples using the Ni/SiO2 catalysts were phenol, cresols, naphthalene, fluorene, and phenanthrene.

Energy from gasification of solid wastes

Gasification technology is by no means new: in the 1850s, most of the city of London was illuminated by “town gas” produced from the gasification of coal. Nowadays, gasification is the main technology for biomass conversion to energy and an attractive alternative for the thermal treatment of solid waste. The number of different uses of gas shows the flexibility of gasification and therefore allows it to be integrated with several industrial processes, as well as power generation systems. The use of a waste–biomass energy production system in a rural community is very interesting too. This paper describes the current state of gasification technology, energy recovery systems, pre-treatments and prospective in syngas use with particular attention to the different process cycles and environmental impacts of solid wastes gasification.