Search

Pyrolysis has been examined as an attractive alternative to incineration for municipal solid waste (MSW) disposal that allows energy and resource recovery; however, it has seldom been applied independently with the output of pyrolysis products as end products. This review addresses the state-of-the-art of MSW pyrolysis in regards to its technologies and reactors, products and environmental impacts. In this review, first, the influence of important operating parameters such as final temperature, heating rate (HR) and residence time in the reaction zone on the pyrolysis behaviours and products is reviewed; then the pyrolysis technologies and reactors adopted in literatures and scale-up plants are evaluated. Third, the yields and main properties of the pyrolytic products from individual MSW components, refuse-derived fuel (RDF) made from MSW, and MSW are summarised. In the fourth section, in addition to emissions from pyrolysis processes, such as HCl, SO2 and NH3, contaminants in the products, including PCDD/F and heavy metals, are also reviewed, and available measures for improving the environmental impacts of pyrolysis are surveyed. It can be concluded that the single pyrolysis process is an effective waste-to-energy convertor but is not a guaranteed clean solution for MSW disposal. Based on this information, the prospects of applying pyrolysis technologies to dealing with MSW are evaluated and suggested., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

With the promotion of classified collection of municipal solid waste (MSW), the heating value of waste sent to incinerators has significantly increased, which affects slagging and ash deposition. Thoroughly studying the slag characteristics in incinerators is important for determining its formation conditions and processes. In this study, slag samples from the furnace and superheaters of a 500 t/d incinerator were comprehensively studied. The slag characteristics were measured using X-ray diffraction (XRD), X-ray fluorescence (XRF), and field-emission high-resolution scanning electron microscopy (SEM); the chemical thermodynamic equilibrium calculations were carried out using FactSage; and the melting kinetics were analysed using differential scanning calorimetry (DSC). It was found that under the high-temperature environment of the furnace, CaO and Fe 2 O 3 /MgO/Al 2 O 3 react with SiO 2 to form low-melting-point eutectic crystals, which are in a molten or semi-molten state and easily adhere to the inner wall of the furnace to form slag that is difficult to remove. CaSO 4 does not melt in the furnace but moves with the flue gas and is mainly deposited on the surfaces of the third- and second-stage superheaters. Na 2 SO 4 and K 3 Na(SO 4) 2 are in a gaseous state under high-temperature conditions in the furnace, and then transform into a liquid or solid state and condense on the surface of the superheaters. The ash melting process in the furnace can be described using zero-order kinetics. These research results provide a theoretical foundation for the detailed prediction of slagging and ash deposition in waste incinerators. • Experiments and equilibrium calculations combined extrapolate the slagging mechanism. • The contents of the main components of the slag changed after MSW classification. • The ash melting process in the furnace can be described using zero-order kinetics. [ABSTRACT FROM AUTHOR]

• A deacidification model was build considering the evaporation process of droplets. • The removal of SO 2 is inhibited by HCl during wet deacidification. • The content and distribution of deacidifier in the droplet were predicted. Deacidification is an indispensable process when cleaning the sludge incineration flue gas. To ensure the flue gas meets the emission standard, guarantee the long-term stable operation of the purification system, and provide theoretical basis for the combined operation of different deacidification methods in the future, a numerical simulation model based on the Euler-Lagrangian method is established to study the wet flue gas deacidification process by using the computational partical fluid dynamic software—Barracuda. The mass transfer process between gas phase and droplets is considered by using the two-film theory, and the effects of evaporation from droplets and vapor condensation in the flue gas during wet deacidification are considered. The model is verified by comparison with experiments, and the calculation results considering the evaporation and condensation process are closer to the experimental results. Besides, the competition mechanism of HCl and SO 2 during the deacidification process and the influence of different operation parameters on the deacidification efficiency are analyzed; the detailed distribution of different substances in the deacidification tower are obtained. The removal efficiency increases when stoichiometric ratio (SR), droplet velocity, and spray angle increased; the removal efficiency decreases when droplet size increased. The results have important guiding significance for the optimization and design of flue gas deacidification tower. [ABSTRACT FROM AUTHOR]

The effective disposal of municipal solid wastes (MSW) and its incineration-derived fly ash (IFA), which contains large amounts of heavy metals (HMs) and chlorine (Cl), is an urgent task. In this study, IFA was used to reform MSW pyrolysis volatiles within 500–800 °C. The changes of reformed pyrolysis products, the migration characteristics of HMs and Cl between IFA and pyrolysis products were investigated. The results indicated that the O- and Cl-containing compounds in pyrolysis oil tended to decrease, light hydrocarbons and its calorific value increased accordingly after reforming; more CH 4 and H 2 gases were produced concurrently. The increase in reforming temperature enhanced these trends. The IFA absorbed Cl from volatiles during reforming, which reduced HCl in the gas product. The toxicity equivalent (TEQ) of PCDD/Fs in IFA decreased dramatically from 0.47 μg/kg to 0.0055 μg/kg after reforming at 500 °C, and it decreased with increasing reforming temperature. Some of the HMs' concentrations in the used IFAs increased, but their leaching capacity all decreased significantly at 800 °C except for Cr. The used IFA at 800 °C (IFA-800) corresponded to the lowest HMs leaching concentrations and could meet the landfill requirements; while the used IFA at 500 °C (IFA-500) corresponded to the maximum carbon deposition of 14.63 wt%, providing the energy source for its melting. Therefore 800 °C was recommended for harmless disposal of IFA, and 500 °C was better for a further melting of IFA., The contamination of pyrolysis liquid caused by inorganic Cl-containing compounds at 500 and 800 °C with much lower levels than the original. This study showed the hazardous properties of IFA can be dampened after interacting with MSW pyrolysis volatiles within the tested temperature range, and provided a good chance for the simultaneous disposal of IFA and recovery of high-quality MSW pyrolysis products. [Display omitted] • IFA used to reform MSW pyrolysis volatiles and being stabilized simultaneously. • The reforming caused 5.20–14.65 wt% of carbon deposition on the used IFAs. • Cl transferred from volatiles to IFA after reforming, but PCDD/Fs in IFA decreased. • The HMs leaching capability of IFA-800 reduced dramatically except for Cr. • 800 °C recommended for IFA stabilization and 500 °C recommended for IFA melting. [ABSTRACT FROM AUTHOR]

[Display omitted] • Promotion effect of the char combustion on its ash melting investigated. • The carbon contained in the MSWC resulted in an earlier melting of the ash in it. • Kinetic parameters of the char combustion and the ash melting were obtained. • Simulation models for combustion and melting established and validated. • O 2 content in the combustion air is more important than carbon content in MSWC. The municipal solid wastes pyrolysis char (MSWC) is a mixture of carbon and ash. Its direct melting contributes to the harmless and most weight-reducing disposal of the municipal solid wastes (MSW). To investigate the impact of the MSWC's combustion on its ash melting, in this research direct MSWC melting was investigated, the apparent activation energies and pre-exponential factors of the char combustion and ash melting reactions were obtained experimentally; reaction models of the MSWC's combustion and its ash melting were established for MSWC in a crucible and for a single particle to investigate the important factors affecting the ash melting. Thermodynamic analysis results showed that the presence of carbon can reduce the ash melting temperature by approximately 40 °C. The apparent activation energies of combustion and ash melting were 148.06 kJ/mol and 878.79 kJ/mol, respectively; but the MSWC combustion rate was much faster than that of ash melting. The combustion heat promoted its ash melting in a crucible inside furnace. The simulation results for a single MSWC particle showed that both the combustion and ash melting processes can be promoted by increasing the surrounding temperature and O 2 concentration in combustion air; while the higher carbon content in the MSWC only slightly improved ash melting when supplying air for combustion. Moreover, a reduction in the MSWC's particle size can make the carbon combustion faster but does not contribute to higher ash melting ratio due to the increase in specific heat dissipation; and particle radius larger than 1.5 mm would result in incomplete combustion of MSWC. These findings can provide guidance for the application of the MSW pyrolysis and char melting technology. [ABSTRACT FROM AUTHOR]

Chars from wastes/biomass pyrolysis are widely used as catalysts. But their catalytic mechanism is not clear enough. To explore the role of inorganics in char-based catalysts, in this research, chars from municipal solid wastes (MSW) and wheat straw (WS) were compared for their catalytic performances and structure changes in reforming volatiles from MSW and WS pyrolysis. Results showed that MSW char (ash: 43 wt%) was more active than WS char (ash: 16 wt%) in reforming both volatiles within 500–700 °C, which facilitated higher syngas yield. 74% of the metals existed as non-evaporable acid-soluble inorganics in MSW char, which favored the transformation of microcrystalline carbon into amorphous carbon by carbon gasification yet increased aromaticity of its carbon matrix; while 27% of metals in WS char were organically bound alkali and alkaline-earth metals (AAEMs), and aromaticity of its carbon lattice increased from 0.39 to 0.67 with increase in reforming temperature. Additionally, WS char was characterized with larger aromatic ring size after reforming temperatures. Rich acid-soluble inorganics in MSW char were found to be favorable to its high activity; while migration of AAEMs caused the lower activity of WS char. These findings will help to improve catalytic activity of char-based catalysts. [Display omitted] • Role of inorganics in different speciation in char-based catalysts explored. • MSW char with more inorganics showed better catalytic performance than porous WS char. • Acid-soluble inorganics hinder microcrystallization and enrich functional groups on the surface. • Migration of organically bound and water-soluble AAEMs led to larger aromatic ring size. [ABSTRACT FROM AUTHOR]