Particulate and gaseous emissions during fluidized bed combustion of semi-dried sewage sludge: Effect of bed ash accumulation on NOx formation

Combustion of two semi-dried sewage sludges in a 110 mm has been characterized in terms of particulate and gaseous emissions. Sludges differed in that they had been conditioned – at the flocculation stage of wastewater treatment – either with Ca-based inorganics or with polyelectrolytes. Combustion was efficient for both sewage sludges under all the operating conditions tested. Significant differences have instead been observed between the two types of sewage sludges as regards particulate and macro-pollutant gaseous emissions (SO₂, NO_x). NO_x formation is significantly influenced by ash accumulation inside the bed only when sewage sludge conditioned with Ca-based inorganics is fired. The time-resolved profiles of NO_x concentration and the mass flow rate of the elutriated fines have been worked out to evaluate the fuel nitrogen yield to NO_x as a function of ash accumulated inside the bed divided by the air mass feed rate. Experimental results have been compared with data present in literature.     

Characteristics of gas and residues produced from electric arc pyrolysis of waste lubricating oil

An attempted has been made to recover high-calorific fuel gas and useful carbonaceous residue by the electric arc pyrolysis of waste lubricating oil. The characteristics of gas and residues produced from electric arc pyrolysis of waste lubricating oil were investigated in this study. The produced gas was mainly composed of hydrogen (35–40%), acetylene (13–20%), ethylene (3–4%) and other hydrocarbons, whereas the concentration of CO was very low. Calorific values of gas ranged from 11,000 to 13,000 kcal kg^?1 and the concentrations of toxic gases, such as NO_x, HCl and HF, were below the regulatory emissions limit. Gas chromatography–mass spectrometry (GC/MS) analysis of liquid-phase residues showed that high molecular-weight hydrocarbons in waste lubricating oil were pyrolyzed into low molecular-weight hydrocarbons and hydrogen. Dehydrogenation was found to be the main pyrolysis mechanism due to the high reaction temperature induced by electric arc. The average particle size of soot as carbonaceous residue was about 10 μm. The carbon content and heavy metals in soot were above 60% and below 0.01 ppm, respectively. The utilization of soot as industrial material resources such as carbon black seems to be feasible after refining and grinding.     

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.     

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.     

Energy from Waste – Clean,efficient, renewable: Transitions in combustion efficiency and NOx control

Traditionally EfW (Energy from Waste) plants apply a reciprocating grate to combust waste fuel. An integrated steam generator recovers the heat of combustion and converts it to steam for use in a steam turbine/generator set. This is followed by an array of flue gas cleaning technologies to meet regulatory limitations.Modern combustion applies a two-step method using primary air to fuel the combustion process on the grate. This generates a complex mixture of pyrolysis gases, combustion gases and unused combustion air. The post-combustion step in the first pass of the boiler above the grate is intended to “clean up” this mixture by oxidizing unburned gases with secondary air.This paper describes modifications to the combustion process to minimize exhaust gas volumes and the generation of noxious gases and thus improving the overall thermal efficiency of the EfW plant. The resulting process can be coupled with an innovative SNCR (Selective Non-Catalytic Reduction) technology to form a clean and efficient solid waste combustion system.Measurements immediately above the grate show that gas compositions along the grate vary from 10% CO, 5% H₂ and 0% O₂ to essentially unused “pure” air, in good agreement with results from a mathematical model. Introducing these diverse gas compositions to the post combustion process will overwhelm its ability to process all these gas fractions in an optimal manner. Inserting an intermediate step aimed at homogenizing the mixture above the grate has shown to significantly improve the quality of combustion, allowing for optimized process parameters. These measures also resulted in reduced formation of NO_x (nitrogenous oxides) due to a lower oxygen level at which the combustion process was run (2.6 vol% O_2, wet instead of 6.0 vol% O_2, wet).This reduction establishes optimal conditions for the DyNOR? (Dynamic NO_x Reduction) NO_x reduction process. This innovative SNCR technology is adapted to situations typically encountered in solid fuel combustion. DyNOR? measures temperature in small furnace segments and delivers the reducing reagent to the exact location where it is most effective. The DyNOR? distributor reacts precisely and dynamically to rapid changes in combustion conditions, resulting in very low NO_x emissions from the stack.     

Conversion of calcium sulphide to calcium carbonate during the process of recovery of elemental sulphur from gypsum waste

The production of elemental sulphur and calcium carbonate (CaCO₃) from gypsum waste can be achieved by thermally reducing the waste into calcium sulphide (CaS), which is then subjected to a direct aqueous carbonation step for the generation of hydrogen sulphide (H₂S) and CaCO₃. H₂S can subsequently be converted to elemental sulphur via the commercially available chemical catalytic Claus process. This study investigated the carbonation of CaS by examining both the solution chemistry of the process and the properties of the formed carbonated product. CaS was successfully converted into CaCO₃; however, the reaction yielded low-grade carbonate products (i.e. <90 mass% as CaCO₃) which comprised a mixture of two CaCO₃ polymorphs (calcite and vaterite), as well as trace minerals originating from the starting material. These products could replace the Sappi Enstra CaCO₃ (69 mass% CaCO₃), a by-product from the paper industry which is used in many full-scale AMD neutralisation plants but is becoming insufficient. The insight gained is now also being used to develop and optimize an indirect aqueous CaS carbonation process for the production of high-grade CaCO₃ (i.e. >99 mass% as CaCO₃) or precipitated calcium carbonate (PCC).     

Decreased PCDD/F formation when co-firing a waste fuel and biomass in a CFB boiler by addition of sulphates or municipal sewage sludge

Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are formed during waste incineration and in waste-to-energy boilers. Incomplete combustion, too short residence times at low combustion temperatures (<700 °C), incineration of electronic waste and plastic waste containing chlorine are all factors influencing the formation of PCDD/Fs in boilers. The impact of chlorine and catalysing metals (such as copper and iron) in the fuel on PCDD/F formation was studied in a 12 MW_th circulating fluidised bed (CFB) boiler. The PCDD/F concentrations in the raw gas after the convection pass of the boiler and in the fly ashes were compared. The fuel types were a so-called clean biomass with low content of chlorine, biomass with enhanced content of chlorine from supply of PVC, and solid recovered fuel (SRF) which is a waste fuel containing higher concentrations of both chlorine, and catalysing metals. The PCDD/F formation increased for the biomass with enhanced chlorine content and it was significantly reduced in the raw gas as well as in the fly ashes by injection of ammonium sulphate. A link, the alkali chloride track, is demonstrated between the level of alkali chlorides in the gas phase, the chlorine content in the deposits in the convection pass and finally the PCDD/F formation. The formation of PCDD/Fs was also significantly reduced during co-combustion of SRF with municipal sewage sludge (MSS) compared to when SRF was fired without MSS as additional fuel.     

CFD simulation of MSW combustion and SNCR in a commercial incinerator

A CFD scheme was presented for modeling municipal solid waste (MSW) combustion in a moving-grate incinerator, including the in-bed burning of solid wastes, the out-of-bed burnout of gaseous volatiles, and the selective non-catalytic reduction (SNCR) process between urea (CO(NH₂)₂) and NO_x. The in-bed calculations provided 2-D profiles of the gas–solid temperatures and the gas species concentrations along the bed length, which were then used as inlet conditions for the out-of-bed computations. The over-bed simulations provided the profiles of incident radiation heat flux on the top of bed. A 3-dimensional benchmark simulation was conducted with a 750 t/day commercial incinerator using the present coupling scheme incorporating with a reduced SNCR reduction mechanism. Numerical tests were performed to investigate the effects of operating parameters such as injection position, injection speed and the normalized stoichiometric ratio (NSR) on the SNCR performance. The simulation results showed that the distributions of gas velocity, temperature and NO_x concentration were highly non-uniform, which made the injection position one of the most sensitive operating parameters influencing the SNCR performance of moving grate incinerators. The simulation results also showed that multi-layer injections were needed to meet the EU2000 standard, and a NSR 1.5 was suggested as a compromise of a satisfactory NO_x reduction and reasonable NH₃ slip rates. This work provided useful guides to the design and operation of SNCR process in moving-grate incinerators.     

Comparison of different liquid anaerobic digestion effluents as inocula and nitrogen sources for solid-state batch anaerobic digestion of corn stover

Effluents from three liquid anaerobic digesters, fed with municipal sewage sludge, food waste, or dairy waste, were evaluated as inocula and nitrogen sources for solid-state batch anaerobic digestion of corn stover in mesophilic reactors. Three feedstock-to-effluent (F/E) ratios (i.e., 2, 4, and 6) were tested for each effluent. At an F/E ratio of 2, the reactor inoculated by dairy waste effluent achieved the highest methane yield of 238.5 L/kgVS_feed, while at an F/E ratio of 4, the reactor inoculated by food waste effluent achieved the highest methane yield of 199.6 L/kgVS_feed. The microbial population and chemical composition of the three effluents were substantially different. Food waste effluent had the largest population of acetoclastic methanogens, while dairy waste effluent had the largest populations of cellulolytic and xylanolytic bacteria. Dairy waste also had the highest C/N ratio of 8.5 and the highest alkalinity of 19.3 g CaCO₃/kg. The performance of solid-state batch anaerobic digestion reactors was closely related to the microbial status in the liquid anaerobic digestion effluents.     

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.     

High efficiency chlorine removal from polyvinyl chloride (PVC) pyrolysis with a gas–liquid fluidized bed reactor

In this research a gas–liquid fluidized bed reactor was developed for removing chlorine (Cl) from polyvinyl chloride (PVC) to favor its pyrolysis treatment. In order to efficiently remove Cl within a limited time before extensive generation of hydrocarbon products, the gas–liquid fluidized bed reactor was running at 280–320 °C, where hot N₂ was used as fluidizing gas to fluidize the molten polymer, letting the molten polymer contact well with N₂ to release Cl in form of HCl. Experimental results showed that dechlorination efficiency is mainly temperature dependent and 300 °C is a proper reaction temperature for efficient dechlorination within a limited time duration and for prevention of extensive pyrolysis; under this temperature 99.5% of Cl removal efficiency can be obtained within reaction time around 1 min after melting is completed as the flow rate of N₂ gas was set around 0.47–0.85 Nm³ kg^?1 for the molten PVC. Larger N₂ flow rate and additives in PVC would enhance HCl release but did not change the final dechlorination efficiency; and excessive N₂ flow rate should be avoided for prevention of polymer entrainment. HCl is emitted from PVC granules or scraps at the mean time they started to melt and the melting stage should be taken into consideration when design the gas–liquid fluidized bed reactor for dechlorination.     

Circulating fluidized bed incineration of industrial solid wastes

A pilot scale test facility of a circulating fluidized bed incinerator was established to generate design and operation data and help assess the technical feasibility for industrial applications. The use of high turbulence in the combustion zone and feeding an acid-capturer directly into the incinerator to absorb acid gases eliminates the costly afterburner and scrubber. This paper presents some systematic incineration tests of uniform industrial wastes such as paper mill sludge cake, rubber waste and petroleum coke. Sludge cakes with high moisture and low heating values can be treated to a low emission level by co-firing with coal. The high superficial gas velocity has improved combustion efficiency but increases NO_x emissions. However, sulphur content has almost no influence on sulphur retention. The problems of CO/NO_x emissions and circulation stability are also discussed.     

Characteristics of NO2 Pollution in the City of Gothenburg, South-West Sweden—Relation to NO x and O3 Levels, Photochemistry and Monitoring Location

Concentrations of NO₂, NO, and O₃ from a rooftop monitoring station in Gothenburg, Sweden (2002–2006) were analysed to characterise NO₂ pollution. [NO₂] was shown to correlate strongly and non-linearly with [NO _x ] (NO _x ?=?NO?+?NO₂), in line with observations in other cities. The [NO₂] to [NO _x ] fraction fell initially with increasing [NO _x ]. At [NO _x ] levels >200 ppb, the decline in [NO₂]/[NO _x ] with increasing [NO _x ] levelled out and [NO₂]/[NO _x ] converged towards approximately 0.15–0.16, independent of [NO _x ]. Data from a traffic route site showed the same pattern. This value of [NO₂]/[NO _x ] at high [NO _x ] can be interpreted as the NO₂ fraction of the NO _x emissions from vehicle exhaust. Situations with high NO _x pollution and minimum [NO₂]/[NO _x ] were always associated with [O₃] close to zero. Plotting [Ox] (Ox?=?NO₂?+?O₃) vs. [NO _x ] provided a strong linear correlation for situations dominated by local pollution ([NO]/[NO₂]>1). The slope of the regression, a measure of the primary NO₂ fraction in NO _x emissions, was 0.13 during the day and 0.14 during the night. With stronger winds, the rooftop monitoring station became more similar, in terms of NO₂ pollution, to a city street site and a traffic route site, although [NO₂] was almost always higher at the street/traffic route locations. The EU standard for the annual average of [NO₂] (40 μg m^?3) was exceeded, while the hourly standard (200 μg m^?3, not to be exceeded more than 18 times per year by 2010) was not exceeded at any of the sites.     

Pilot-scale studies on the effect of bromine addition on the emissions of chlorinated organic combustion by-products

The addition of brominated organic compounds to the feed of a pilot-scale incinerator burning chlorinated waste has been found previously, under some circumstances, to enhance emissions of volatile and semivolatile organic chlorinated products of incomplete combustion (PICs) including polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDDs/Fs). This phenomenon appears to be sensitive to temperature and combustion conditions. This paper reports on a study to evaluate the emissions of organic combustion by-products while varying amounts of bromine (Br) and chlorine (Cl) are being fed into a pilot-scale incinerator burning surrogate waste materials. The surrogate waste was fed at a constant molar halogen input rate, with varying Br/Cl molar ratios. In these tests, an approximately 30% decrease in the total PCDD/F concentrations due to the addition of Br was observed. This decrease appears to be a decrease only in the chlorinated dioxin and furan species; other halogenated dioxins and furans were formed instead. PCDD/F homologue distribution shifted towards the higher chlorinated species. Perhalogenated or nearly perhalogenated mixed bromo-chloro furans were also observed in quantities that could potentially account for the observed decrease in PCDDs/Fs. This research illustrates the need for careful trial burn planning if Br will be present in the facility's feed-stock during normal operation.     

Mathematical modeling of MSW combustion and SNCR in a full-scale municipal incinerator and effects of grate speed and oxygen-enriched atmospheres on operating conditions

The rising popularity of incineration of municipal solid waste (MSW) calls for detailed mathematical modeling and accurate prediction of pollutant emissions. In this paper, mathematical modeling methods for both solid and gaseous phases were employed to simulate the operation of a 450 t/d MSW-burning incinerator to obtain detailed information on the flow and combustion characteristics in the furnace and to predict the amount of pollutant emissions. The predicted data were compared to on-site measurements of gas temperature, gas composition and SNCR de-NO_X system. The major operating conditions considered in this paper were grate speed and oxygen concentration. A suitable grate speed ensures complete waste combustion. The predictions are as follows: volatile release increases with increasing grate speed, and the maximal value is within the range of 700–800 kg/m² h; slow grate speeds result in incomplete combustion of fixed carbon; the gas temperature at slow grate speeds is higher due to adequate oxygenation for fixed carbon combustion, and the deviation reaches 200 K; NO_X emission decreases, but CO emission and O₂ concentrations increase, and the deviation is 63%, 34% and 35%, respectively. Oxygen-enriched atmospheres promote the destruction of most pollutants due to the high oxygen partial pressure and temperature. The furnace temperature, NO production and CO emission increase as the oxygen concentration increases, and the deviation of furnace exit temperature, NO and CO concentration is 38.26%, 58.43% and 86.67%, respectively. Finally, oxygen concentration is limited to below 35% to prevent excessive CO and NO_X emission without compromising plant performance. The current work greatly helps to understand the operating characteristics of large-scale MSW-burning plants.     

Chemical characterization of emissions from a municipal solid waste treatment plant

Gaseous emissions are an important problem in municipal solid waste (MSW) treatment plants. The sources points of emissions considered in the present work are: fresh compost, mature compost, landfill leaks and leachate ponds. Hydrogen sulphide, ammonia and volatile organic compounds (VOCs) were analysed in the emissions from these sources. Hydrogen sulphide and ammonia were important contributors to the total emission volume. Landfill leaks are significant source points of emissions of H₂S; the average concentration of H₂S in biogas from the landfill leaks is around 1700 ppmv. The fresh composting site was also an important contributor of H₂S to the total emission volume; its concentration varied between 3.2 and 1.7 ppmv and a decrease with time was observed. The mature composting site showed a reduction of H₂S concentration (<0.1 ppmv). Leachate pond showed a low concentration of H₂S (in order of ppbv). Regarding NH₃, composting sites and landfill leaks are notable source points of emissions (composting sites varied around 30–600 ppmv; biogas from landfill leaks varied from 160 to 640 ppmv).Regarding VOCs, the main compounds were: limonene, p-cymene, pinene, cyclohexane, reaching concentrations around 0.2–4.3 ppmv.H₂S/NH₃, limonene/p-cymene, limonene/cyclohexane ratios can be useful for analysing and identifying the emission sources.     

SETTING DIOXIN EMISSION LIMITS FOR MSW INCINERATORS: A MULTIMEDIA EXPOSURE ASSESSMENT FRAMEWORK

Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are emitted in trace amounts from municipal solid waste (MSW) incinerators. The exposure to PCDD/Fs experienced by an individual is dominated by the food chain pathway, which accounts for over 98% of the total uptake. Defining a target daily intake (the World Health Organization TDI of 10 pg I-TEQ kg.bw⁻¹day⁻¹) exposure assessment algorithms were then applied to arrive at the corresponding PCDD/F levels in air, soil, plants, food products, etc., which would allow the target intake to be met while retaining the balance of intake between the various exposure pathways. These concentrations were converted to an ambient air concentration of PCDD/Fs and ultimately, by defining criteria for acceptability, to a guide value for PCDD/F concentration in emissions from the MSW incinerator. This strategy was applied to PCDD/F emissions from MSW incinerators of various sizes against two illustrative criteria for acceptability: an “insignificant” release and a threshold above which the release may require further assessment for environmental effects and for control. Using the criteria developed in this paper, the current PCDD/F emission limit of 0.1 ng I-TEQ m⁻³results in an emission that is classed as “insignificant” for all plant sizes. However, higher emission concentrations can also be accommodated below the threshold for further assessment and control.     

Behavior of PCDD/Fs during open burning of municipal solid waste in open dumping sites

Open dumping sites in Surabaya and Palembang, Indonesia, have been studied to investigate emissions of polychlorinated dibenzo-para-dioxins and dibenzofurans (PCDD/Fs) and dioxin-like polychlorinated biphenyls (DL-PCBs), as well as the resulting soil contamination that might be caused by open burning of municipal solid waste. The emission factors of the waste residue, accounting for the ratio of waste burned, have also been characterized. The concentrations of PCDD/Fs and DL-PCBs in soil from Palembang were 61–310 pg-TEQ/g (dry weight) and 6.3–32 pg-TEQ/g, respectively. In Surabaya, very low levels of PCDD/Fs and DL-PCBs, ranging from 0.075 to 0.098 and 0.00032 to 0.095 pg-TEQ/g, respectively, were observed in soil for an open dumping site that included a top cover layer of soil above the compacted waste. The large difference in concentrations can be explained by the fact that open burning of waste is the source of PCDD/Fs and DL-PCBs. The emission factors for the residue for PCDD/Fs ranged from 27 to 140 pg-TEQ/g, and a sensitivity analysis found that the maximum emission factor to the residue could be 5600 pg-TEQ/g. Our results indicate that emissions of PCDD/Fs can be controlled by modifying the open dumping process to one that involves depositing soil layers on top of the compacted waste.     

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.     

Emission of volatile sulfur compounds during composting of municipal solid waste (MSW)

Volatile sulfur compounds (VSCs) are the main source for malodor from composting plants. In this study, the VSCs generated from composting of 15–80 mm municipal solid waste (T0), kitchen waste (T1) and kitchen waste mixed dry cornstalks (T2) were measured in 60 L reactors with forced aeration for a period of 30 days. The VSCs detected in all treatments were hydrogen sulfide (H₂S), methyl mercaptan (MM), dimethyl sulfide (DMS), carbon bisulfide (CS₂) and dimethyl disulfide (DMDS). Over 90% of the VSCs emissions occurred during the first 15 days, and reached their peak values at days 4–7. The emission profiles of five VSCs species were significantly correlated with internal materials temperature and outlet O₂ concentration (p < 0.05). Total emissions of the VSCs were 216.1, 379.3 and 126.0 mg kg^?1 (dry matter) for T0, T1 and T2, respectively. Among the five VSCs, H₂S was the most abundant compound with 39.0–43.0% of total VSCs released. Composting of kitchen waste from separate collection posed a negative influence on the VSC and leachate production because of its high moisture content. An addition of dry cornstalks at a mixing ratio of 4:1 (wet weight) could significantly reduce the VSCs emissions and avoid leachate. Compared to pure kitchen waste, VSCs were reduced 66.8%.