Case study of an MBT plant producing SRF for cement kiln co-combustion, coupled with a bioreactor landfill for process residues

Energy and environmental assessment of solid recovered fuels valorisation: Waste-to-Chemicals options vs co-combustion in cement plants

The increasing interest in Waste-to-Chemical (WtC) technologies operating with Solid Recovered Fuels (SRF) from non-recyclable plastic streams requires a quantitative analysis on the actual convenience of this alternative valorization pathway. This study assesses SRF in selected WtC technologies for hydrogen and methanol production and compares it with the well-established practice of co-combustion in the cement industry. Two case studies are considered: the first one represents the current scenario where SRF is used in co-combustion for cement production meanwhile the chemical is produced by steam reforming; in the second scenario, the cement plant is fed with pet-coke only, leaving SRF as a feedstock for WtC. WtC performance assessment has been carried out in Aspen Plus®, whereas cement production and steam reforming have been characterized based on literature information. The two scenarios have been assessed for two SRF qualities (different LHV and biogenic content) calculating primary energy and fossil CO2 emissions. The results show that SRF from plastic waste as a feedstock in WtC is less effective than its utilization in cement plant: when WtC technology for hydrogen production is adopted, additional 9.1% (SRF-1) and 8.6% (SRF-2) of energy consumption is estimated and 25.8% (SRF-1) and 24.1% (SRF-2) additional fossil CO2 is emitted with respect to the corresponding conventional cases (i.e., chemical from steam reforming and SRF burnt in the cement kiln). When considering methanol production, WtC technology requires 6.2% (SRF-1) and 5.6% (SRF-2) increase of primary energy and 30.2% (SRF-1) and 28.4% (SRF-2) additional fossil CO2 against the conventional cases.

When solid recovered fuel (SRF) production and consumption maximize environmental benefits? A life cycle assessment

Solid recovered fuel (SRF) from non-recyclable waste obtained from source separation and mechanical treatments can replace carbon coke in cement plants, contributing to the carbon neutrality. A life cycle assessment (LCA) of the SRF production from non-recyclable and selected waste was conducted in an Italian mechanical treatment plant to estimate the potential environmental impacts per ton of SRF produced. The analysis would contribute to evaluate the benefits that can be obtained due to coke substitution in best- and worst-case scenarios. The avoided impacts achieved were assessed, together with an evaluation of the variables that can affect the environmental benefits: SRF biogenic carbon content (in percentage of paper and cardboard); transportation distances travelled from the treatment plant to the cement kiln; the renewable energy used in the mechanical facility. On average, about 35.6 kgCO2-eq are generated by the SRF transportation and production phase. These impacts are greatly compensated by coke substitution, obtaining a net value of about -1.1 tCO2-eq avoided per ton of SRF. On balance, the global warming potential due to SRF production and consumption ranges from about -542 kgCO2-eq to about -1729 kgCO2-eq. The research recommended the use of SRF to substitute coke in cement kilns also in low densely-populated areas to mitigate environmental impacts and achieve carbon neutrality at a global level.

Efficiency of biological and mechanical-biological treatment plants for MSW: The case of Spain

Biological and mechanical biological treatment plants combine mechanical and biological treatments to recover the greatest possible amount of materials from municipal solid waste (MSW) and biostabilize the organic fraction to be landfilled or applied in land. These plants handle a high percentage of the MSW generated in Europe. This work presents an exhaustive analysis of the existing plants in Spain which evaluates their typology as well as their performance. In Spain, 137 plants, which receive 13 Mt/year of waste, provide the country with total coverage. Twenty-two types of plants have been identified and grouped into six categories. There are four categories that receive mixed MSW: 1) sorting plants; 2) recovery and composting plants; 3) biodrying and recovery plants; and 4) recovery, biomethanation and composting plants and two that receive separately collected biowaste: 5) composting plants, and 6) biomethanation and composting plants. In plants that receive mixed waste, around 5% of the total input is recovered as recyclable materials (662,182 t/year), of which 29% corresponds to plastics, 27% to metals, and 27% to paper and cardboard. In addition, biostabilized material and/or biogas, and rejects (45-77% of the input) are obtained. In the biowaste plants, high-quality compost (more than 105,000 t/year), a higher biogas yield (43.60 Nm3/t·year) and a lower proportion of rejects (around 29%) are obtained.

Effects of cellulose-based bio-plastics on the aerobic biological stabilization treatment of mixed municipal solid waste: A lab-scale assessment

The aim of this work is to assess how the presence of cellulose-based bio-plastics influence the biological stabilization of mixed Municipal Solid Waste (MSW). For the scope, two cellulose acetate bio-plastics have been mixed with a synthetic mixed waste to create samples with and without bio-plastics. A self-induced biostabilization has been carried out for 7 and 14 days where temperature and off-gas have been monitored continuously. Results about temperature evolution, O₂ consumption, CO₂ production and respiratory quotient did not show a substantial difference regarding both the duration of the process and the presence of cellulose-based bio-plastics on the mixture. On the average, the temperature peak and the maximum daily O₂ consumption and CO₂ production were 52.2 °C, 35.81 g O₂/kg DM *d and 48.95 g CO₂/kg DM *d respectively. Disintegration of bio-plastics samples after 7 and 14 days were comparable (on the average 23.13%). The self-induced biostabilization gave its main contribution after 4 days and resulted almost finished at the end of the day 7 of the process. Results showed that cellulose-based bio-plastics did not give a negative effect on mixed MSW biological stabilization and suggest a possible management, aiming at energy recovery of the outputs.

Life cycle environmental and economic comparison of thermal utilization of refuse derived fuel manufactured from landfilled waste or fresh waste

This paper analyses environmental and economic performance of thermal utilization technologies of two different refuse derived fuel (RDF) manufactured from landfilled waste or fresh municipal waste, including incineration of landfilled RDF (I-LRDF), gasification of landfilled RDF (G-LRDF), replacement of partial coal by landfilled RDF for the cement industry (C-LRDF), incineration of municipal RDF (I-MRDF), and replacement of partial coal by municipal RDF for the cement industry (C-MRDF). The preference among the RDF utilization options is identified from the standpoints of various stakeholders by integrating the life cycle assessment (LCA) and techno-economic analysis (TEA) with the analytic hierarchy process (AHP) and technique for order preference by similarity to ideal solution (TOPSIS) approaches. RDF thermal utilization technologies bring an economic profit of $17.29∼$35.77 per ton of waste. Especially, I-LRDF has the worst effect on ecosystem quality and human health and can yield the greatest economic profit of $35.77 per ton of landfilled waste, while I-MRDF has the least impact on environment. In terms of the five RDF thermal utilization technologies, I-MRDF has the best comprehensive performance from the perspectives of different stakeholders. The improvement of the RDF thermal utilization efficiency is the most critical factor affecting the economic benefits for all cases.

Total organic carbon as a proxy for metal release from biostabilized wastes

In this study, we introduce a simple screening method to predict the metal release from biostabilized wastes as a function of the total organic carbon (TOC) content of the sample. The method relies on a model that simulates the release of dissolved organic carbon (DOC) as a function of the applied liquid to solid (L/S) ratio. The metal release is then estimated using generic empirical DOC to metal correlation coefficients (K_DOC,Me) extrapolated from the literature. Specifically, the results of leaching tests carried out on different types of biowastes and biostabilized wastes were used to calculate the upper and lower K_DOC,Me values that can be expected for common metals of concern (Al, Ba, Cr, Cu, Mo, Ni, Pb, V, and Zn). The statistical analysis of the estimated empirical coefficients highlighted that for most of the investigated metals, the adoption of generic DOC to metal correlation coefficients introduces relatively low uncertainties. The quartiles ratio (QR calculated as the ratio of third and first quartile) of the K_DOC,Me coefficients extrapolated from the literature was indeed below 3 for Cu, Ni, and Zn and below 5 for Al, Ba, Cr, Pb, and V. Only for Mo, the QR was around 10 indicating that for this element, DOC can be a poor indicator of the release expected as a function of the applied L/S. Furthermore, by performing a sensitivity analysis, we found that the experimental conditions only slightly influence the metals release predicted by the model. Based on this evidence, simple nomograms that estimate the cumulative metal release in percolation column tests as a function of the applied L/S are provided. Furthermore, a simple equation that predicts the cumulative metal release expected at L/S of 10 L/kg is presented. The application of the latter to the results of percolation column tests carried out on 8 mechanical biological treatment (MBT) waste samples highlighted that the predicted metals release was close to the measured values with deviations within a factor of 5 for all the investigated metals.

Pelletization of Refuse-Derived Fuel with Varying Compositions of Plastic, Paper, Organic and Wood

The combustible fraction of municipal solid waste (MSW) is called refuse-derived fuel (RDF). RDF is a blend of heterogeneous materials and thus its handling is challenging. Pelletization is an efficient treatment to minimize the heterogeneity. In this research, typical RDF compositions were prepared by mixing several mass fractions of paper, plastic, household organic and wood. The collected compositions were ground, wetted to 20% moisture content (wet basis) and pelletized. Increasing the plastic content from 20% to 40% reduced the pelletization energy but increased the pellet’s calorific value. Pellets with higher plastic content generated more dust when exposed to shaking. Making durable pellets with 40% plastic content needed an increase in die temperature from 80 °C to 100 °C. Increasing the paper content from 30% to 50% increased the durability but consumed higher energy to form pellets. Paper particles increased the friction between pellet’s surface and die wall as was evident from expulsion energy. Force versus displacement curve for material compression revealed that the RDF compositions have rigid material characteristics.

Composting versus mechanical-biological treatment: Does it really make a difference in the final product parameters and maturity

One of crucial waste management problems is the management of organic waste. This activity employs the composting. In case of green waste, its application seems reasonable, whereas the use of selected mixed waste raises problems related to the compost quality. Across countries, the non-sterile organic fraction of municipal solid waste is being separated through the mechanical-biological treatment. The technology is a solution of waste treatment and meets objectives set out in the Landfill Directive. There are many problems associated with the use of output products. The use of compost as a fertilizer requires determination of its impact on the environment. Compost quality can be assessed using analytical methods and phytotoxicity tests. Therefore, the aim of this study was to describe changes in physico-chemical, enzymatic, phytotoxicity and vegetation parameters occurring in composts from two systems - a prismatic installation for green waste, and a mechanical-biological treatment installation. The compost from green waste exhibited greater stability. Values of dehydrogenase activity were lower if compared with the mechanically and biologically treated compost, which indicates lower compost maturity. The biomass production of Brassica napus L. and Fetuca rubra L. was higher in the variant with the application of green compost. The influence on Hordeum vulgare L., Cannabis sativa L., and Sinapis alba L. depended on the plant type and the compost used. Nevertheless, the compost from green waste was less toxic. The evidence from this study suggests that the mechanical-biological treatment had problems associated with the maturation and quality of the final product.

Life cycle costing of energy recovery from solid recovered fuel produced in MBT plants in Italy

The Environmental Life Cycle Costing (LCC) methodology was applied to assess the costs of the processing in Italy of municipal Residual Waste (RW) in Mechanical-Biological Treatment (MBT) plants aimed at the production of Solid Recovered Fuels (SRFs), delivered to dedicated waste-to-energy (WTE) plants or to co-combustion. Two different functional units were defined to consider two different perspectives: 1 tonne of residual waste in input of the MBT plant, and 1 MWh of exergy produced by the energy valorisation of the streams delivered by the MBT plant. Four strategies were analysed that differ for the type of MBT (single stream or separated streams) and the destiny of the SRF (dedicated WTE plant or cement kiln). The results appear to strongly recommend the treatment of the RW in a single stream MBT plant and the production of a SRF with characteristics suitable to meet the technological, economic and environmental needs for co-combustion, substituting fossil fuels.

Experimental simulation and fuzzy modelling of landfill biogas production from low-biodegradable MBT waste under leachate recirculation

In the perspective of a sustainable waste management, biodegradable waste destined to landfilling should be reduced. This work aims to study a combination of waste pretreatments and leachate recirculation. A lab-scale experiment and fuzzy-modelling were chosen to predict cumulative methane production from low-biodegradable waste (LBW) under leachate recirculation. Thanks to moisture increase, the degradation of LBW was reactivated and the cumulative methane production reached 28 NL CH₄ kg^-1 after 442 days. The organic fraction was stabilized with a final chemical oxygen demand (COD) of 81 mg L^-1. Fuzzy model was proposed as an alternative to the common deterministic models, affected by high uncertainties. Eleven inputs (pH, Redox potential, COD, volatile fatty acids, ammonium content, age, temperature, moisture content, organic fraction concentration, particle size and recirculation flow rate) were identified as antecedent, and two outputs, or consequents, were chosen: methane production rate and methane fraction in biogas. Antecedents and consequents were linked by 84 IF-THEN rules in a linguistic form. The model was also tested on six literature studies chosen to test different operational conditions and waste qualities. The model outputs fitted the experimental data reasonably well, confirming the potential use of fuzzy macro-approach to model sustainable landfilling.

Energy, environmental and operation aspects of a SRF-fired fluidized bed waste-to-energy plant

A methodology based on the ISO 14031:2013 guideline has been developed and applied to a full-scale fluidized bed waste to energy plant (WtE) burning solid recovered fuel (SRF). With reference to 3years of operation, the data on energy and environmental performance, on raw materials consumptions such as sand and diesel fuel, accidental reasons of plant shutdown, have been acquired and analyzed. The obtained results have allowed to quantify the energy and environmental performance of the WtE plant under investigation by varying the amount and mixings of the inlet waste, available in form of thickened and fluff (similar to coriander) SRF. In terms of the energy performance, the fluidized bed technology applied to the SRF was able to guarantee an adequate production of electricity (satisfying the market demands), showing a relative flexibility with respect to the inlet waste. In terms of net energy production efficiency, the plant showed values in the range of 13.8-14.9% in line with similar installations. In terms of the environmental performance, the adoption of a cleaning system based on SNCR (Selective Non Catalitic Reduction)+semi-dry scrubbing+Fabric filter generated emissions usually well below the limits set by the EU Directive 2000/76/EC as well as the Italian Law 46/2014 (more restrictive) with reference to all the key parameters. In terms of the plant shutdown, the majority of problems focused on the combustion chamber and boiler due to the erosion of the refractory material of the furnace as well as to the breaking of the superheaters of the boiler. In contrast, the mechanical and electrical causes, along with those related to the control and instrumentation system, were of secondary importance. The sand bed de-fluidization was also among the leading causes of a frequent plant shutdown. In particular, results showed how although the SRF presents standard characteristics, the use of different mixtures may affect the number of plant shutdowns. The full-scale data highlighted how the lower the rate of fluff in the mixture was, the greater the number of plant shutdown due to sand bed de-fluidization was. Finally, the aspects in terms of the energy, environmental protection and raw material consumption have been discussed with reference to similar WtE plants such as Robbins (Chicago, USA), Lidköping (Sweden), Toshima (Tokyo, Japan), Madrid (Spain), Dundee (Scotland, UK) and Valene (Mantes la Jolie, France).

Simulation of the behavior of a refuse landfill on a laboratory scale

The characteristics and properties of waste in a landfill, and its evolution over time, are difficult to estimate because of the heterogeneity of materials, biomass degradation, density, cover material, and infiltration of water. In this work, a lysimeter was used to simulate how refuse from mechanical-biological treatment (MBT) plants evolved in a landfill over a 45-day period. Water was added as a way to imitate the effects produced during rainy seasons. Field capacity and changes in the physical and chemical properties (volatile solids, biomass, and heating value) were analyzed. The results of this research show that the percentage of biomass lowers, and the heating value increases, after bringing about infiltration and percolation of water in the waste mass. Therefore in order to stabilize waste in a landfill, employing irrigation or leachate recirculation could be advisable. As the heating value increases after percolation, it could also be a good idea to recover the fuel material after stabilization.

Environmental performances of different configurations of a material recovery facility in a life cycle perspective

The study evaluated the environmental performances of an integrated material recovery facility (MRF) able to treat 32kt/y of unsorted mixed waste, made of residuals from household source separation and separate collection. The facility includes a mechanical sorting platform for the production of a solid recovered fuel (SRF) utilized in an external waste-to-energy plant, bio-cells for tunnel composting of organic fraction, and a sanitary landfill for the safe disposal of ultimate waste. All the MRF sub-units have been analysed in depth in order to acquire reliable data for a life cycle assessment study, focused on the environmental performances of different configurations of the facility. The study investigated a "past" configuration, including just mechanical sorting, landfilling and biogas combustion in a gas engine, and the "present" one, which includes also a composting unit. Two possible "future" configurations, having a gasifier inside the MRF battery limits, have been also analysed, assessing the performances of two fluidized bed reactors of different size, able to gasify only the residues generated by the sorting platform or the whole amount of produced SRF, respectively. The analysis evaluated the contributions of each unit in the different configurations and allowed a reliable assessment of the technological evolution of the facility. The results quantified the positive effect of the inclusion of an aerobic treatment of the waste organic fraction. The SRF gasification in situ appears to improve the MRF environmental performances in all the impact categories, with the exclusion of that of global warming.