Selective collection as a pretreatment for indirect solid recovered fuel generation

Effective selective collection (SC) has been increasing in many countries of the European Union. As a consequence the composition of residual municipal solid waste (RMSW) is changing not only because of economic development, but also due to the collection system. The input of RMSW treatment plants is thus not homogeneous. This paper analyses two case studies involving a total of five SC scenarios and their impact on the generation of solid recovered fuel (SRF) with/without bio-drying, post-treatment and packaging take back programs (TBPs). These case studies are based on two types of SC: kerbside and drop-off. The latest regulation on SRF classification is taken into account: energy content, Cl and Hg concentrations are assessed and discussed. The role of the respirometric index (RI) is also analyzed. Results show that when SC is highly efficient RMSW can be classified directly as SRF, but only if the introduction of RI does not set stringent respirometric targets. The role of packaging TBPs is important as the residual waste remaining after the packaging has been processed, can be valorized increasing the lower heating value (LHV) of the residual RMSW and minimizing the streams that should be landfilled. The source separation of food waste has a significant impact on the suitability of bio-drying: when the SC of food waste is very efficient, its percentage in the RMSW may be too low for effective bio-drying.     

A study of Cr(VI) in ashes from fluidized bed combustion of municipal solid waste: leaching, secondary reactions and the applicability of some speciation methods

The use of the fluidized bed technique for the combustion of municipal solid waste is a rather new concept. This type of combustor produces ash residues with somewhat different properties than the residues generated from the traditional mass burn techniques. Therefore, chemical characterization and the investigation of toxic metals behavior during ash water reactions are necessary for the safe disposal of these residues. In the present work, the total elemental composition, mineralogy and leaching behavior of ashes from the combustion of municipal solid waste in a fluidized bed combustion boiler have been investigated. The cyclone ash and, in particular, the filter ash contained considerable amounts of soluble substances, thus giving leachates with high levels of Cl-, Na+, K+, Ca2 + and Al(IIl). On the other hand, the two ash fractions taken in the boiler, the bottom and hopper ashes, were much more stable with respect to the release of salts and heavy metals. Since Cr(VI) is mobile and toxic its release from combustion residues can pose environmental problem. Even though the total Cr contents were similar in all ashes studied, the bottom ash gave about a thousand times higher levels of Cr(VI) in test leachates than the hopper, cyclone and filter ashes. However, it was found that the leached amount of Cr(VI) from the bottom ash decreased significantly when bottom ash was mixed with the hopper ash. The most probable cause for this decrease is the coupled oxidation of Al(0) to Al(III) and reduction of dissolved Cr(VI) to Cr(III). This finding that the mixing of two ash streams from the same boiler could result in the immobilization of Cr may point at a simple stabilization method. Selective extraction of water soluble, exchangeable and sparingly soluble forms of Cr(VI) was also investigated. Extraction methods were evaluated for their suitability for ash matrixes. It was found that interferences due to the presence of reducing substances in some ash materials may occur.     

Mass,energy and material balances of SRF production process. Part 2: SRF produced from construction and demolition waste

In this work, the fraction of construction and demolition waste (C&D waste) complicated and economically not feasible to sort out for recycling purposes is used to produce solid recovered fuel (SRF) through mechanical treatment (MT). The paper presents the mass, energy and material balances of this SRF production process. All the process streams (input and output) produced in MT waste sorting plant to produce SRF from C&D waste are sampled and treated according to CEN standard methods for SRF. Proximate and ultimate analysis of these streams is performed and their composition is determined. Based on this analysis and composition of process streams their mass, energy and material balances are established for SRF production process. By mass balance means the overall mass flow of input waste material stream in the various output streams and material balances mean the mass flow of components of input waste material stream (such as paper and cardboard, wood, plastic (soft), plastic (hard), textile and rubber) in the various output streams of SRF production process. The results from mass balance of SRF production process showed that of the total input C&D waste material to MT waste sorting plant, 44% was recovered in the form of SRF, 5% as ferrous metal, 1% as non-ferrous metal, and 28% was sorted out as fine fraction, 18% as reject material and 4% as heavy fraction. The energy balance of this SRF production process showed that of the total input energy content of C&D waste material to MT waste sorting plant, 74% was recovered in the form of SRF, 16% belonged to the reject material and rest 10% belonged to the streams of fine fraction and heavy fraction. From the material balances of this process, mass fractions of plastic (soft), paper and cardboard, wood and plastic (hard) recovered in the SRF stream were 84%, 82%, 72% and 68% respectively of their input masses to MT plant. A high mass fraction of plastic (PVC) and rubber material was found in the reject material stream. Streams of heavy fraction and fine fraction mainly contained non-combustible material (such as stone/rock, sand particles and gypsum material).     

Evolution of heavy metals in municipal solid waste during bio-drying and implications of their subsequent transfer during combustion

Bio-drying has been applied to improve the heating value of municipal solid waste (MSW) prior to combustion. In the present study, evolution of heavy metals in MSW during bio-drying and subsequent combustion was studied using one aerobic and two combined hydrolytic-aerobic scenarios. Heavy metals were concentrated during bio-drying and transformed between different metal fractions, namely the exchangeable, carbonate-bound, iron- and manganese-oxides-bound, organic-matter-bound and residual fractions. The amounts of heavy metals per kg of bio-dried MSW transferred into combustion flue gas increased with bio-drying time, primarily due to metals enrichment from organics degradation. Because of their volatility, the partitioning ratios of As and Hg in flue gas remained stable so that bio-drying and heavy metal speciation had little effect on their transfer and partitioning during combustion. In contrast, the partitioning ratios of Pb, Zn and Cu tended to increase after bio-drying, which likely enhanced their release potential during combustion.     

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.     

Nitrogen evolution during the co-combustion of hydrothermally treated municipal solid waste and coal in a bubbling fluidized bed

Nitrogen evolution was studied during the co-combustion of hydrothermally treated municipal solid wastes (HT MSW) and coal in a bubbling fluidized bed (BFB). HT MSW blending ratios as 10%, 20% and 30% (wt.%) were selected and tested at 700, 800, 900 °C. Emissions of NO and N₂O from blends were measured and compared with the results of mono-combustion trials. Moreover, concentrations of precursors like NH₃ and HCN were also quantified. The results are summarized as follows: NO emissions were predominant in all the cases, which rose with increasing temperature. The blending of HT MSW contributed to the NO reduction. N₂O emissions decreased with temperature rising and the blending of HT MSW also presented positive effects. At 30% HT MSW addition, both NO and N₂O emissions showed the lowest values (391.85 ppm and 55.33 ppm, respectively at 900 °C). For the precursors, more HCN was detected than NH₃ and both played important roles on the gas side nitrogen evolution.     

Mass,energy and material balances of SRF production process. Part 1: SRF produced from commercial and industrial waste

This paper presents the mass, energy and material balances of a solid recovered fuel (SRF) production process. The SRF is produced from commercial and industrial waste (C&IW) through mechanical treatment (MT). In this work various streams of material produced in SRF production process are analyzed for their proximate and ultimate analysis. Based on this analysis and composition of process streams their mass, energy and material balances are established for SRF production process. Here mass balance describes the overall mass flow of input waste material in the various output streams, whereas material balance describes the mass flow of components of input waste stream (such as paper and cardboard, wood, plastic (soft), plastic (hard), textile and rubber) in the various output streams of SRF production process. A commercial scale experimental campaign was conducted on an MT waste sorting plant to produce SRF from C&IW. All the process streams (input and output) produced in this MT plant were sampled and treated according to the CEN standard methods for SRF: EN 15442, EN 15443. The results from the mass balance of SRF production process showed that of the total input C&IW material to MT waste sorting plant, 62% was recovered in the form of SRF, 4% as ferrous metal, 1% as non-ferrous metal and 21% was sorted out as reject material, 11.6% as fine fraction, and 0.4% as heavy fraction. The energy flow balance in various process streams of this SRF production process showed that of the total input energy content of C&IW to MT plant, 75% energy was recovered in the form of SRF, 20% belonged to the reject material stream and rest 5% belonged with the streams of fine fraction and heavy fraction. In the material balances, mass fractions of plastic (soft), plastic (hard), paper and cardboard and wood recovered in the SRF stream were 88%, 70%, 72% and 60% respectively of their input masses to MT plant. A high mass fraction of plastic (PVC), rubber material and non-combustibles (such as stone/rock and glass particles), was found in the reject material stream.     

A comparative assessment of waste incinerators in the UK

The uptake in Europe of Energy from Waste (EfW) incinerator plants has increased rapidly in recent years. In the UK, 25 municipal waste incinerators with energy recovery are now in operation; however, their waste supply chains and business practices vary significantly. With over a hundred more plant developments being considered it is important to establish best business practices for ensuring efficient environmental and operational performance. By reviewing the 25 plants we identify four suitable case study plants to compare technologies (moving grate, fluidised bed and rotary kiln), plant economics and operations. Using data collected from annual reports and through interviews and site visits we provide recommendations for improving the supply chain for waste incinerators and highlight the current issues and challenges faced by the industry. We find that plants using moving grate have a high availability of 87–92%. However, compared to the fluidised bed and rotary kiln, quantities of bottom ash and emissions of hydrogen chloride and carbon monoxide are high. The uptake of integrated recycling practices, combined heat and power, and post incineration non-ferrous metal collections needs to be increased among EfW incinerators in the UK. We conclude that one of the major difficulties encountered by waste facilities is the appropriate selection of technology, capacity, site, waste suppliers and heat consumers. This study will be of particular value to EfW plant developers, government authorities and researchers working within the sector of waste management.     

Studies on the pyrolysis behavior of gasification and melting systems for municipal solid waste

 It is important to investigate the pyrolysis processes of municipal solid waste (MSW) in the same way as for any mixture comprised of multiple substances. In this article, a two-reaction model for a variety of MSW mixtures is proposed to predict mass changes due to pyrolysis. In order to formulate the model based on pyrolysis kinetics, we conducted experiments to determine the kinetic model parameters. By thermal analysis of the typical components of MSW, mass changes attributable to the pyrolysis reaction were found at about 350°C for paper, 400°–500°C for plastics, and 200°–400°C for garbage (dry condition). Activation energies were obtained by the Ozawa method based on the mass changes in pyrolysis. Thus, the pyrolysis behavior is formulated as a function of temperature. Then the pyrolysis mass change of the mixture can be predicted by using a weighted sum of the individual components. The model proved useful in experiments with real waste (refuse-derived fuels). Furthermore, the weight yields (pyrolysis gas, tars, solid residues) of the mixture can be calculated by their additive property after measuring the mass balance of each component. Received: May 11, 2001 / Accepted: November 16, 2001     

Comparing urban solid waste recycling from the viewpoint of urban metabolism based on physical input-output model: A case of Suzhou in China

Investigating impacts of urban solid waste recycling on urban metabolism contributes to sustainable urban solid waste management and urban sustainability. Using a physical input-output model and scenario analysis, urban metabolism of Suzhou in 2015 is predicted and impacts of four categories of solid waste recycling on urban metabolism are illustrated: scrap tire recycling, food waste recycling, fly ash recycling and sludge recycling. Sludge recycling has positive effects on reducing all material flows. Thus, sludge recycling for biogas is regarded as an accepted method. Moreover, technical levels of scrap tire recycling and food waste recycling should be improved to produce positive effects on reducing more material flows. Fly ash recycling for cement production has negative effects on reducing all material flows except solid wastes. Thus, other fly ash utilization methods should be exploited. In addition, the utilization and treatment of secondary wastes from food waste recycling and sludge recycling should be concerned.     

An LCA model for waste incineration enhanced with new technologies for metal recovery and application to the case of Switzerland

A process model of municipal solid waste incinerators (MSWIs) and new technologies for metal recovery from combustion residues was developed. The environmental impact is modeled as a function of waste composition as well as waste treatment and material recovery technologies. The model includes combustion with a grate incinerator, several flue gas treatment technologies, electricity and steam production from waste heat recovery, metal recovery from slag and fly ash, and landfilling of residues and can be tailored to specific plants and sites (software tools can be downloaded free of charge). Application of the model to Switzerland shows that the treatment of one tonne of municipal solid waste results on average in 425 kg CO₂-eq. generated in the incineration process, and 54 kg CO₂-eq. accrue in upstream processes such as waste transport and the production of operating materials. Downstream processes, i.e. residue disposal, generates 5 kg CO₂-eq. Savings from energy recovery are in the range of 67 to 752 kg CO₂-eq. depending on the assumptions regarding the substituted energy production, while the recovery of metals from slag and fly ash currently results in a net saving of approximately 35 kg CO₂-eq. A similar impact pattern is observed when assessing the MSWI model for aggregated environmental impacts (ReCiPe) and for non-renewable resource consumption (cumulative exergy demand), except that direct emissions have less and no relevance, respectively, on the total score. The study illustrates that MSWI plants can be an important element of industrial ecology as they provide waste disposal services and can help to close material and energetic cycles.     

Evaluating the biochemical methane potential (BMP) of low-organic waste at Danish landfills

The biochemical methane potential (BMP) is an essential parameter when using first order decay (FOD) landfill gas (LFG) generation models to estimate methane (CH₄) generation from landfills. Different categories of waste (mixed, shredder and sludge waste) with a low-organic content and temporarily stored combustible waste were sampled from four Danish landfills. The waste was characterized in terms of physical characteristics (TS, VS, TC and TOC) and the BMP was analyzed in batch tests. The experiment was set up in triplicate, including blank and control tests. Waste samples were incubated at 55 °C for more than 60 days, with continuous monitoring of the cumulative CH₄ generation. Results showed that samples of mixed waste and shredder waste had similar BMP results, which was in the range of 5.4–9.1 kg CH₄/ton waste (wet weight) on average. As a calculated consequence, their degradable organic carbon content (DOCC) was in the range of 0.44–0.70% of total weight (wet waste). Numeric values of both parameters were much lower than values of traditional municipal solid waste (MSW), as well as default numeric values in current FOD models. The sludge waste and temporarily stored combustible waste showed BMP values of 51.8–69.6 and 106.6–117.3 kg CH₄/ton waste on average, respectively, and DOCC values of 3.84–5.12% and 7.96–8.74% of total weight. The same category of waste from different Danish landfills did not show significant variation. This research studied the BMP of Danish low-organic waste for the first time, which is important and valuable for using current FOD LFG generation models to estimate realistic CH₄ emissions from modern landfills receiving low-organic waste.     

LCA of local strategies for energy recovery from waste in England, applied to a large municipal flow

An intense waste management (WM) planning activity is currently undergoing in England to build the infrastructure necessary to treat residual wastes, increase recycling levels and the recovery of energy from waste. From the analyses of local WM strategic and planning documents we have identified the emerging of three different energy recovery strategies: established combustion of residual waste; pre-treatment of residual waste and energy recovery from Solid Recovered Fuel in a dedicated plant, usually assumed to be a gasifier; pre-treatment of residual waste and reliance on the market to accept the ‘fuel from waste’ so produced. Each energy recovery strategy will result in a different solution in terms of the technology selected; moreover, on the basis of the favoured solution, the total number, scale and location of thermal treatment plants built in England will dramatically change. To support the evaluation and comparison of these three WM strategy in terms of global environmental impacts, energy recovery possibilities and performance with respect to changing ‘fuel from waste’ market conditions, the LCA comparison of eight alternative WM scenarios for a real case study dealing with a large flow of municipal wastes was performed with the modelling tool WRATE. The large flow of waste modelled allowed to formulate and assess realistic alternative WM scenarios and to design infrastructural systems which are likely to correspond to those submitted for approval to the local authorities. The results show that all alternative scenarios contribute to saving abiotic resources and reducing global warming potential. Particularly relevant to the current English debate, the performance of a scenario was shown to depend not from the thermal treatment technology but from a combination of parameters, among which most relevant are the efficiency of energy recovery processes (both electricity and heat) and the calorific value of residual waste and pre-treated material. The contribution and relative importance of recycling and treatment/recovery processes change with the impact category. The lack of reprocessing plants in the area of the case study has shown the relevance of transport distances for recyclate material in reducing the efficiency of a WM system. Highly relevant to the current English WM infrastructural debate, these results for the first time highlight the risk of a significant reduction in the energy that could be recovered by local WM strategies relying only on the market to dispose of the ‘fuel from waste’ in a non dedicated plant in the case that the SRF had to be sent to landfill for lack of treatment capacity.