Hydrodechlorination/detoxification of PCDDs,PCDFs, and co-PCBs in fly ash by using calcium polysulfide

Dioxins like polychlorinated dibenzo-p-dioxins (PCSDDs), polychlorinated dibenzofurans (PCDFs) and polychlorinated biphenyls (PCBs) are mainly emitted from waste incinerators (WIs) and have become an international research focus because of its serious concerns over the adverse health effects. The detoxification of PCCDs/Fs and PCBs is very difficult because of their stable chemical structure. A significant hydrodechlorination/detoxification of polychlorinated 1-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and polychlorinated biphenyls (PCBs) were achieved in fly ash by using an aqueous mixture of calcium hydroxide and sulfur. Two different fly ashes were studied: originating from municipal waste incinerator (FA1) and industrial waste incinerator (FA2). They were heated with the aqueous mixture at 150 °C for 30 or 60 min with agitation. Higher decomposition (87%) and detoxification (87.7%) of PCDD/Fs and PCBs were achieved at 150 °C with two runs; every run was for 30 min, compared to one run for 60 min. FA2 gave higher decomposition and detoxification as compared to FA1, which might be due to higher metal content that played a catalytic role to decompose and detoxify the PCDDs, PCDFs and PCBs. The decomposition and detoxification of PCDFs in fly ash was higher than PCDDs and was augmented with increasing number of chlorides on aromatic compounds. As the highly significant decomposition and detoxification of higher concentration of PCDD/Fs and PCBs were achieved in 1 hour without additive catalyst and at low temperature of 150 °C, therefore, the developed method is cost effective and most suitable to apply on commercial/industrial level. The detail results of hydrodechlorination/detoxification of PCDD, PCDFs at different conditions are described and its mechanism is discussed.     

An investigation on pollutant emissions from co-firing of RDF and coal

Pollutant emissions from co-firing of refuse derived fuel (RDF) and coal were investigated in a vortexing fluidized bed combustor (VFBC). RDF-5 was made of common municipal solid waste (MSW). CaCO₃ was injected in the combustor to absorb HCl at 850 °C. The results show that NO_x and HCl emissions increase with RDF-5 co-firing ratio. The NO_x concentration in flue gas at the bottom of the combustor is higher than that at the top. However, the trend of HCl released is reverse compared with NO_x emissions. It was found that the HCl concentration decreases with increasing the molar ratio of Ca/Cl. However, the effect of CaCO₃ addition on HCl retention is not significant when the molar ratio of Ca/Cl is higher than 5. The chlorine content in fly ash increases obviously with the molar ratio of Ca/Cl. PCDD/Fs emissions decrease slightly with an addition of CaCO₃. In this study incomplete combustion is regarded as the main cause for polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) formation.     

Mechanisms contributing to the thermal analysis of waste incineration bottom ash and quantification of different carbon species

The focus of this study was to identify the main compounds affecting the weight changes of bottom ash (BA) in conventional loss on ignition (LOI) tests and to obtain a better understanding of the individual processes in heterogeneous (waste) materials such as BA. Evaluations were performed on BA samples from a refuse derived fuel incineration (RDF-I) plant and a hospital waste incineration (HW-I) plant using thermogravimetric analysis and subsequent mass spectrometry (TG–MS) analysis of the gaseous thermal decomposition products. Results of TG–MS analysis on RDF-I BA indicated that the LOI measured at 550 °C was due to moisture evaporation and dehydration of Ca(OH)₂ and hydrocalumite. Results for the HW-I BA showed that LOI at 550 °C was predominantly related to the elemental carbon (EC) content of the sample. Decomposition of CaCO₃ around 700 °C was identified in both materials. In addition, we have identified reaction mechanisms that underestimate the EC and overestimate the CaCO₃ contents of the HW-I BA during TG–MS analyses. These types of artefacts are expected to occur also when conventional LOI methods are adopted, in particular for materials that contain CaO/Ca(OH)₂ in combination with EC and/or organic carbon, such as e.g. municipal solid waste incineration (MSWI) bottom and fly ashes. We suggest that the same mechanisms that we have found (i.e. in situ carbonation) can also occur during combustion of the waste in the incinerator (between 450 and 650 °C) demonstrating that the presence of carbonate in bottom ash is not necessarily indicative for weathering. These results may also give direction to further optimization of waste incineration technologies with regard to stimulating in situ carbonation during incineration and subsequent potential improvement of the leaching behavior of bottom ash.     

Removal of carbon constituents from hospital solid waste incinerator fly ash by column flotation

Hospital solid waste incinerator (HSWI) fly ash contains a large number of carbon constituents including powder activated carbon and unburned carbon, which are the major source of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in fly ash. Therefore, the removal of carbon constituents could reduce PCDD/Fs in fly ash greatly. In this study, the effects of the main flotation parameters on the removal of carbon constituents were investigated, and the characteristics of the final product were evaluated. The results showed that loss on ignition (LOI) of fly ash increased from 11.1% to 31.6% during conditioning process. By optimizing the flotation parameters at slurry concentration 0.05 kg/l, kerosene dosage 12 kg/t, frother dosage 3 kg/t and air flow rate 0.06 m³/h, 92.7% of the carbon constituents were removed from the raw fly ash. Under these conditions, the froth product has LOI of 56.35% and calorific values of 12.5 MJ/kg, LOI in the tailings was below 5%, and the total toxic equivalent (TEQ) of PCDD/Fs decreased from 5.61 ng-TEQ/g in the raw fly ash to 1.47 ng-TEQ/g in the tailings. The results show that column flotation is a potential technology for simultaneous separation of carbon constituents and PCDD/Fs from HSWI fly ash.     

Generation and distribution of PAHs in the process of medical waste incineration

After the deadly earthquake on May 12, 2008 in Wenchuan county of China, several different incineration approaches were used for medical waste disposal. This paper investigates the generation properties of polycyclic aromatic hydrocarbons (PAHs) during the incineration. Samples were collected from the bottom ash in an open burning slash site, surface soil at the open burning site, bottom ash from a simple incinerator, bottom ash generated from the municipal solid waste (MSW) incinerator used for medical waste disposal, and bottom ash and fly ash from an incinerator exclusively used for medical waste. The species of PAHs were analyzed, and the toxicity equivalency quantities (TEQs) of samples calculated. Analysis results indicate that the content of total PAHs in fly ash was 1.8 × 10³ times higher than that in bottom ash, and that the strongly carcinogenic PAHs with four or more rings accumulated sensitively in fly ash. The test results of samples gathered from open burning site demonstrate that Acenaphthylene (ACY), Acenaphthene (ACE), Fluorene (FLU), Phenanthrene (PHE), Anthracene (ANT) and other PAHs were inclined to migrate into surrounding environment along air and surface watershed corridors, while 4- to 6-ring PAHs accumulated more likely in soil. Being consistent with other studies, it has also been confirmed that increases in both free oxygen molecules and combustion temperatures could promote the decomposition of polycyclic PAHs. In addition, without the influence of combustion conditions, there is a positive correlation between total PCDD/Fs and total PAHs, although no such relationship has been found for TEQ.     

Behavior of TiO2 nanoparticles during incineration of solid paint waste: A lab-scale test

In order to assess the potential impacts posed by products containing engineered nanoparticles, it is essential to generate more data about the release of these particles from products’ life cycle. Although first studies were performed to investigate the release of nanoparticles from use phase, very few data are available on the potential release from recycling or disposal of nano-enhanced products.In this work, we investigated the behavior of TiO₂ nanoparticles from incineration of solid paint waste containing these particles. Solid paint debris with and without TiO₂ nanoparticles were treated in a lab scale incineration plant at 950 °C (combustion temperature) and in oxidizing atmosphere. The obtained ashes were also vitrified with additives and the release of Ti was finally evaluated by leaching test. From our incineration lab-scale experiment, we did not observe a release of TiO₂ nanoparticles into the atmosphere, and Ti was attached to the surface of obtained solid residues (i.e. ashes). The characterization of ashes showed that TiO₂ nanoparticles reacted during the incineration to give calcium titanate. Finally, a very low release of Ti was measured, less 1 mg/kg, during the leaching test of ashes vitrified with glass cullet and feldspathic inert. Our work suggests that TiO₂ nanoparticles added in paints may undergo to physicochemical transformation during the incineration, and that Ti found in ashes may be strongly immobilized in glass matrix. Since this conclusion is based on lab-scale experiment, further research is required to identify which nanoparticles will be emitted to the environment from a real-word-incineration system of household hazardous waste.     

An integrated appraisal of energy recovery options in the United Kingdom using solid recovered fuel derived from municipal solid waste

This paper reports an integrated appraisal of options for utilising solid recovered fuels (SRF) (derived from municipal solid waste, MSW) in energy intensive industries within the United Kingdom (UK). Four potential co-combustion scenarios have been identified following discussions with industry stakeholders. These scenarios have been evaluated using (a) an existing energy and mass flow framework model, (b) a semi-quantitative risk analysis, (c) an environmental assessment and (d) a financial assessment. A summary of results from these evaluations for the four different scenarios is presented. For the given ranges of assumptions; SRF co-combustion with coal in cement kilns was found to be the optimal scenario followed by co-combustion of SRF in coal-fired power plants. The biogenic fraction in SRF (ca. 70%) reduces greenhouse gas (GHG) emissions significantly (～2500 g CO₂ eqvt./kg DS SRF in co-fired cement kilns and ～1500 g CO₂ eqvt./kg DS SRF in co-fired power plants). Potential reductions in electricity or heat production occurred through using a lower calorific value (CV) fuel. This could be compensated for by savings in fuel costs (from SRF having a gate fee) and grants aimed at reducing GHG emission to encourage the use of fuels with high biomass fractions. Total revenues generated from coal-fired power plants appear to be the highest (£95/t SRF) from the four scenarios. However overall, cement kilns appear to be the best option due to the low technological risks, environmental emissions and fuel cost. Additionally, cement kiln operators have good experience of handling waste derived fuels. The scenarios involving co-combustion of SRF with MSW and biomass were less favourable due to higher environmental risks and technical issues.     

Winery waste recycling through anaerobic co-digestion with waste activated sludge

In this study biogas and high quality digestate were recovered from winery waste (wine lees) through anaerobic co-digestion with waste activated sludge both in mesophilic and thermophilic conditions. The two conditions studied showed similar yields (0.40 m³/kgCOD_fed) but different biological process stability: in fact the mesophilic process was clearly more stable than the thermophilic one in terms of bioprocess parameters.The resulting digestates showed good characteristics for both the tested conditions: heavy metals, dioxins (PCDD/F), and dioxin like bi-phenyls (PCBs) were concentred in the effluent if compared with the influent because of the important reduction of the solid dry matter, but remained at levels acceptable for agricultural reuse. Pathogens in digestate decreased. Best reductions were observed in thermophilic condition, while at 37 °C the concentration of Escherichia coli was at concentrations level as high as 1000 UFC/g. Dewatering properties of digestates were evaluated by means of the capillary suction time (CST) and specific resistance to filtration (SRF) tests and it was found that a good dewatering level was achievable only when high doses of polymer (more than 25 g per kg dry solids) were added to sludge.     

Persistent organic pollutant emissions from medical waste incinerators in China

The huge amount of medical waste (MW) has caused a tough challenge to environmental protection in China because of its serious infectious potential. At present, incineration is the most common technology for MW disposal. Unfortunately, the medical waste incinerator (MWI) is considered one of the major sources of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). This study was conducted to investigate the generation and the components of MW; the fingerprint of PCDD/Fs in MWI; and PCDD/F, polychlorinated biphenyl (PCB) and hexachlorobenzene concentrations in residue ash. The estimated annual production of MW was estimated to be 0.97 million tons in China in 2008; in addition, plastic and rubber accounted for 24.5% of MW contents. PCDD/F emissions from MWI could be divided into two main groups according their fingerprints, and the ratio of PCDFs/PCDDs was mostly over 1.5, with a mean value of 3.43. The toxic equivalent of PCDD/Fs was over 30 times that of the value of PCBs in the residue ash, and PCDD/F contents in fly ash accounted for approximately 67% of the total output of PCDD/Fs, which was in line with the UNEP default emission factors for MWI (class 3, 63.7%).     

Energy recovery from waste incineration: Assessing the importance of district heating networks

Municipal solid waste incineration contributes with 20% of the heat supplied to the more than 400 district heating networks in Denmark. In evaluation of the environmental consequences of this heat production, the typical approach has been to assume that other (fossil) fuels could be saved on a 1:1 basis (e.g. 1 GJ of waste heat delivered substitutes for 1 GJ of coal-based heat). This paper investigates consequences of waste-based heat substitution in two specific Danish district heating networks and the energy-associated interactions between the plants connected to these networks. Despite almost equal electricity and heat efficiencies at the waste incinerators connected to the two district heating networks, the energy and CO₂ accounts showed significantly different results: waste incineration in one network caused a CO₂ saving of 48 kg CO₂/GJ energy input while in the other network a load of 43 kg CO₂/GJ. This was caused mainly by differences in operation mode and fuel types of the other heat producing plants attached to the networks. The paper clearly indicates that simple evaluations of waste-to-energy efficiencies at the incinerator are insufficient for assessing the consequences of heat substitution in district heating network systems. The paper also shows that using national averages for heat substitution will not provide a correct answer: local conditions need to be addressed thoroughly otherwise we may fail to assess correctly the heat recovery from waste incineration.     

Thermodynamic analysis on heavy metals partitioning impacted by moisture during the MSW incineration

A thermodynamic calculation was carried out to predict the behavior and speciation of heavy metals (HMs), Pb, Zn, Cu, and Cd, during municipal solid waste (MSW) incineration with the different moisture levels. The calculation was based on the minimization of the total Gibbs free energy of the multi-components and multi-phases closed system reaching chemical equilibrium. The calculation also indicated the reaction directions and tendencies of HMs components. The impacts of chlorine additives (No PVC, 1%PVC, and 5%PVC) and moisture on the behavior of HMs were investigated at different temperature levels in the system (750 °C, 950 °C, and 1150 °C). Furthermore, because the incineration temperature falls down with the increase in moisture in waste, the co-influence of moisture and temperature in combusting MSW on the HMs was also studied with the given chlorine (as 1%PVC + 0.5%NaCl). The results showed that in the non-chlorine system, the impact of the moisture on Pb, Zn, and Cu was not significant, and the ratio of compound transformation was less than 10%, except the Cd compounds at 950 °C and 1150 °C. In the system with low chlorine (as 1%PVC) at constant temperature, the chlorides of HMs (Cd, Pb, Zn, and Cu) transferred to oxides, and when the content of chlorine rose up (as 5%PVC), the ratio of the chlorides of HMs (Cd, Pb, Zn, and Cu) transferring to oxides fell down noticeably. When the moisture varied together with the temperature, the Zn and Cu compounds transferred from chlorides to oxides with increase in moisture as well as decrease in temperature. At the temperature of 700–1000 °C, the impact of temperature on Pb and Cd was little and the moisture was the main factor; while at the temperature of 1000–1200 °C, the impact of increase in moisture and decrease in temperature on Pb and Cd was almost equal and reversed.     

Sample preparation and biomass determination of SRF model mixture using cryogenic milling and the adapted balance method

The biogenic fraction of a simple solid recovered fuel (SRF) mixture (80 wt% printer paper/20 wt% high density polyethylene) is analyzed with the in-house developed adapted balance method (aBM). This fairly new approach is a combination of combustion elemental analysis (CHNS) and a data reconciliation algorithm based on successive linearisation for evaluation of the analysis results. This method shows a great potential as an alternative way to determine the biomass content in SRF. However, the employed analytical technique (CHNS elemental analysis) restricts the probed sample mass to low amounts in the range of a few hundred milligrams. This requires sample comminution to small grain sizes (<200 μm) to generate representative SRF specimen. This is not easily accomplished for certain material mixtures (e.g. SRF with rubber content) by conventional means of sample size reduction.This paper presents a proof of principle investigation of the sample preparation and analysis of an SRF model mixture with the use of cryogenic impact milling (final sample comminution) and the adapted balance method (determination of biomass content). The so derived sample preparation methodology (cutting mills and cryogenic impact milling) shows a better performance in accuracy and precision for the determination of the biomass content than one solely based on cutting mills. The results for the determination of the biogenic fraction are within 1–5% of the data obtained by the reference methods, selective dissolution method (SDM) and ¹⁴C-method (¹⁴C-M).     

Sludge as dioxins suppressant in hospital waste incineration

Nitrogen containing compounds such as ammonia, urea and amines can effectively inhibit the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). Sewage sludge accumulates both sulfur and nitrogen during wastewater treatment so it could be used to reduce PCDD/Fs formation. Indeed, it is observed in this study that the gas evolving from the sludge drying process can significantly suppress chlorobenzene (CBz) and PCDD/Fs formation from fly ash collected from a hospital waste incinerator. For instance, the reduction of hexachlorobenzene (HxCBz) and PCDD/Fs amount was 92.1% and 78.7%, respectively, when the drying gas evolving from 2g sludge flew through 2g fly ash. These tests were conducted in the frame of projects devoted to hospital waste incineration. The disposal technology for hospital waste (HW), developed in this institute, features rotary kiln pyrolysis combined with post-combustion followed by flue gas cleaning. Hence, some preliminary tests were devoted to investigate dioxins suppression by co-pyrolysis and co-combustion of polyvinyl chloride (PVC) and sludge in lab scale. More experimental research will be conducted to appropriately assess these effects of sludge on PCDD/Fs emissions during co-pyrolysis/combustion of HW and sludge.     

Assessing the environmental impact of ashes used in a landfill cover construction

Large amounts of construction materials will be needed in Europe in anticipation for capping landfills that will be closed due to the tightening up of landfill legislation. This study was conducted to assess the potential environmental impacts of using refuse derived fuel (RDF) and municipal solid waste incineration (MSWI) ashes as substitutes for natural materials in landfill cover designs. The leaching of substances from a full-scale landfill cover test area built with different fly and bottom ashes was evaluated based on laboratory tests and field monitoring. The water that drained off above the liner (drainage) and the water that percolated through the liner into the landfill (leachate) were contaminated with Cl^?, nitrogen and several trace elements (e.g., As, Cu, Mo, Ni and Se). The drainage from layers containing ash will probably require pre-treatment before discharge. The leachate quality from the ash cover is expected to have a minor influence on overall landfill leachate quality because the amounts generated from the ash covers were low, <3–30 l (m² yr)^?1. Geochemical modelling indicated that precipitation of clay minerals and other secondary compounds in the ash liner was possible within 3 years after construction, which could contribute to the retention of trace elements in the liner in the long term. Hence, from an environmental view point, the placement of ashes in layers above the liner is more critical than within the liner.     

Characterisation of agroindustrial solid residues as biofuels and potential application in thermochemical processes

In the present work, selected agroindustrial solid residues from Brazil – biosolids from meat processing wastewater treatment and mixture of sawdust with these biosolids; residues from apple and orange juice industries; sugarcane bagasse; açaí kernels (Euterpe oleracea) and rice husk – were characterised as solid fuels and an evaluation of their properties, including proximate and ultimate composition, energy content, thermal behaviour, composition and fusibility of the ashes was performed. The lower heating value of the biomasses ranged from 14.31 MJ kg^?1 to 29.14 MJ kg^?1, on a dry and ash free basis (daf), all presenting high volatile matter content, varying between 70.57 wt.% and 85.36 wt.% (daf) what improves the thermochemical conversion of the solids. The fouling and slagging tendency of the ashes was predicted based on the fuel ash composition and on the ash fusibility correlations proposed in the literature, which is important to the project and operation of biomass conversion systems. The potential for application of the Brazilian agroindustrial solid residues studied as alternative energy sources in thermochemical processes has been identified, especially concerning direct combustion for steam generation.     

Thermal-behavior study of chlorine released from composite refuse derived fuel

In order to reduce secondary pollution during the incineration of composite refuse derived fuel (CRDF), the combustion features and the emission behavior of chlorine in CRDF containing coal were analyzed. The former was analyzed using thermo-gravimetric and the latter by gas chromatography–mass spectrometry. The release rate of inorganic chlorine during combustion reached 90 mass% at temperature between 773.15 and 873.15 K. On the other hand, approximately 84 mass% release rates was resulting from pyrolysis at 723.15 K. When temperature reached above 1073.15 K, it was noticed that higher concentration of organic chlorine in different organic compounds were produced in the processing of pyrolysis compared with those released from the combustion processing. From the thermo-gravimetric analysis using a self-designed system, three distinct phases were detected in the thermal process of CRDF. The first phase occurred at temperature between 473 and 573 K and its mass loss was about 38.50%. The second phase between temperature regions of 673–773 K with a mass loss of 20.35%. The third phase was observed at the temperature between 873 and 1073 K with 22.25% mass loss.     

Environmental impacts of residual Municipal Solid Waste incineration: A comparison of 110 French incinerators using a life cycle approach

Incineration is the main option for residual Municipal Solid Waste treatment in France. This study compares the environmental performances of 110 French incinerators (i.e. 85% of the total number of plants currently in activity in France) in a Life Cycle Assessment perspective, considering 5 non-toxic impact categories: climate change, photochemical oxidant formation, particulate matter formation, terrestrial acidification and marine eutrophication. Mean, median and lower/upper impact potentials are determined considering the incineration of 1 tonne of French residual Municipal Solid Waste. The results highlight the relatively large variability of the impact potentials as a function of the plant technical performances. In particular, the climate change impact potential of the incineration of 1 tonne of waste ranges from a benefit of ?58 kg CO₂-eq to a relatively large burden of 408 kg CO₂-eq, with 294 kg CO₂-eq as the average impact. Two main plant-specific parameters drive the impact potentials regarding the 5 non-toxic impact categories under study: the energy recovery and delivery rate and the NO_x process-specific emissions. The variability of the impact potentials as a function of incinerator characteristics therefore calls for the use of site-specific data when required by the LCA goal and scope definition phase, in particular when the study focuses on a specific incinerator or on a local waste management plan, and when these data are available.     

Experimental investigation of wood combustion in a fixed bed with hot air

Waste combustion on a grate with energy recovery is an important pillar of municipal solid waste (MSW) management in the Netherlands. In MSW incinerators fresh waste stacked on a grate enters the combustion chamber, heats up by radiation from the flame above the layer and ignition occurs. Typically, the reaction zone starts at the top of the waste layer and propagates downwards, producing heat for drying and devolatilization of the fresh waste below it until the ignition front reaches the grate. The control of this process is mainly based on empiricism.MSW is a highly inhomogeneous fuel with continuous fluctuating moisture content, heating value and chemical composition. The resulting process fluctuations may cause process control difficulties, fouling and corrosion issues, extra maintenance, and unplanned stops. In the new concept the fuel layer is ignited by means of preheated air (T > 220 °C) from below without any external ignition source. As a result a combustion front will be formed close to the grate and will propagate upwards. That is why this approach is denoted by upward combustion.Experimental research has been carried out in a batch reactor with height of 4.55 m, an inner diameter of 200 mm and a fuel layer height up to 1 m. Due to a high quality two-layer insulation adiabatic conditions can be assumed. The primary air can be preheated up to 350 °C, and the secondary air is distributed via nozzles above the waste layer. During the experiments, temperatures along the height of the reactor, gas composition and total weight decrease are continuously monitored. The influence of the primary air speed, fuel moisture and inert content on the combustion characteristics (ignition rate, combustion rate, ignition front speed and temperature of the reaction zone) is evaluated.The upward combustion concept decouples the drying, devolatilization and burnout phase. In this way the moisture and inert content of the waste have almost no influence on the combustion process. In this paper an experimental comparison between conventional and reversed combustion is presented.     

Precious metals and rare earth elements in municipal solid waste – Sources and fate in a Swiss incineration plant

In Switzerland many kinds of waste, e.g. paper, metals, electrical and electronic equipment are separately collected and recycled to a large extent. The residual amount of municipal solid waste (MSW) has to be thermally treated before final disposal. Efforts to recover valuable metals from incineration residues have recently increased. However, the resource potential of critical elements in the waste input (sources) and their partitioning into recyclable fractions and residues (fate) is unknown. Therefore, a substance flow analysis (SFA) for 31 elements including precious metals (Au, Ag), platinum metal group elements (Pt, Rh) and rare earth elements (La, Ce, etc.) has been conducted in a solid waste incinerator (SWI) with a state-of-the-art bottom ash treatment according to the Thermo-Re® concept. The SFA allowed the determination of the element partitioning in the SWI, as well as the elemental composition of the MSW by indirect analysis. The results show that the waste-input contains substantial quantities of precious metals, such as 0.4 ± 0.2 mg/kg Au and 5.3 ± 0.7 mg/kg Ag. Many of the valuable substances, such as Au and Ag are enriched in specific outputs (e.g. non-ferrous metal fractions) and are therefore recoverable. As the precious metal content in MSW is expected to rise due to its increasing application in complex consumer products, the results of this study are essential for the improvement of resource recovery in the Thermo-Re® process.     

An innovative example of herb residues recycling by gasification in a fluidized bed

A utilization way of herb residues is designed to convert herb residues to gas fuel in industrial-scale by a circulating fluidized bed gasifier in this paper. The product gas is used in the production of Chinese medicine, and the heat of the flue gas from the boiler can be used in herb residues drying to realize the energy recycling and no herb residues discharge. The gasification characteristics of herb residues in the circulating fluidized bed of 300 kg/h were investigated for about 200 h. The results indicated that the gas composition and tar yield were affected by biomass flow rate, equivalence ratio (ER), moisture content and char circulating. The lower heating value of product gas was 4–5 MJ/m³ using herb residues as feedstock. When mean biomass flow rate was at 5.5 kg m^?2 s^?1 and ER at 0.35, the product gas reached a good condition with lower heating value of 4.89 MJ/m³ and cold gas efficiency of 62.36%. When the moisture content changed from 12.5% to 18.7%, the concentrations of H₂, CO and CO₂ changed from 4.66% to 6.92%, 11.23% to 10.15%, and 16.55% to 17.82% respectively, and the tar content in gas decreased from 15.1 g/m³ to 14.4 g/m³ when the moisture content increased from 12.5% to 15.4%. There are metal oxides in the ash of herb residues, especially CaO, MgO, K₂O, Al₂O₃, and Fe₂O₃ which have obvious function on tar catalytic decomposition. The ash that attaches to the char particles can decrease the tar yield and improve the quality of gas after returning to the gasifier.