Plastic waste gasification using oxygen-enriched air and steam: Experimental and model results from a large pilot-scale reactor.

Advanced thermochemical technologies for plastic waste valorization represent an interesting alternative to waste-to-energy options. They are particularly appealing for waste-to-hydrogen and waste-to-chemicals applications, with autothermal steam-oxygen gasification in fluidized bed reactors showing the greatest market potential. The study describes a series of experimental tests carried out on a large pilot-scale fluidized bed gasifier, using steam and O2-enriched air, with increasing fractions of oxygen. Different values of the main operating parameters are varied: equivalence ratio (0.22-0.25), steam-to-carbon ratio (0.7-1.13), and steam-to-oxygen ratio (up to 3.2). The fuel consists of real mixed plastic waste coming from separate collection of municipal solid wastes. The data obtained are used to investigate in depth the role of the main operating parameters and to improve and validate a recently developed one-dimensional kinetic model for waste gasification. The validation shows a good agreement between experimental data and model results, suggesting the reliability of the model to predict the reactor behavior under conditions of pure steam-oxygen gasification, relevant to many industrial applications. It has been found that the equivalence ratio is the parameter that most affects the syngas composition. At a constant equivalent ratio, the molar fraction of oxygen in the enriched air shows a limited influence on syngas composition while the steam is crucial in controlling the temperature along the reactor. Provided that the steam-to-carbon molar ratio is larger than 1.5, steam affects mainly the reactor temperature rather than the syngas composition, qualifying the steam-to-oxygen molar ratio as an instrumental parameter for smooth plant operation.

Waste-derived catalysts for tar cracking in hot syngas cleaning.

Catalytic tar cracking is a promising technique for hot syngas cleaning unit in gasification plants because it can preserve tars chemical energy, so increasing the syngas heating value. The cost associated with catalyst preparation is a key issue, together with its deactivation induced by coke deposition. Iron is a cheap and frequently used catalyst, which can also be found in some industrial wastes. The study aims to assess the catalytic efficiency for tar cracking of two waste-derived materials (red mud and sewage sludge) having high content of iron. The catalysts were supported on spheres of γ-Al2O3, and their efficiency was compared to that of a pure iron catalyst. The role of support was investigated by testing pure red mud, with and without the support. A series of long-term tests using naphthalene as tar model compound were carried out under different values of process temperatures (750 °C-800 °C) and steam concentrations (0 %-7.5 %). The waste derived catalysts showed lower hydrogen yields compared to pure iron catalyst, due to their lower content of iron. On the other hand, the conversion efficiencies of all the tested catalysts resulted rather similar, since the Alkali and Alkaline-Earth Metallic species present on the surface of waste-derived catalyst help in preventing coke deposition. The iron oxidation state appears to play an important role, with reduced iron more active than its oxidised form in the tar cracking reactions. This indicates the importance of tuning steam concentration to keep constant the reduced state of iron while limiting coke deposition.

An LCA answer to the mixed plastics waste dilemma: Energy recovery or chemical recycling?

The study focuses on mixed plastics waste (MPW), whose complex and unpredictable composition (due to high polymer heterogeneity, additives, and contaminants) makes its valorisation a true technical, environmental, economic, and regulatory challenge. Chemical recycling by means of advanced thermochemical treatments (ATT) could be a successful strategy, able to support the transition from a carbon intensive to a carbon negative sector, and alternative to the current treatments of energy recovery or mechanical downcycling. Some of these ATTs provide an efficient recovery of valuable resources, such as fuels and chemicals, but their role is mainly limited by time necessary to complete the process optimization and implement the required infrastructures. A reliable identification of the best alternatives is thus crucial. A specific LCA approach quantifies the environmental performances of a selected set of ATT technologies for resource recovery from MPW. It includes plastics-to-energy, by combustion or gasification; plastics-to-methane and plastics-to-hydrogen, by gasification; and plastics-to-oil, by thermal pyrolysis. The results highlight the crucial role of carbon capture and storage (CCS) units, which partially reduces that of the specific thermochemical treatment. The best performances, particularly for Climate Change category, are those of the MPW-to-hydrogen by gasification, followed by those of MPW-to-energy by combustion or gasification, all equipped with CCS. The sensitivity analysis considers the evolution of the European energy mix, characterised by a larger utilisation of renewable energy sources, and highlights the corresponding increased sustainability of chemical recycling by ATTs. This suggests that the MPW dilemma should be definitively solved in a close future.

How to enhance the environmental sustainability of WEEE plastics management: An LCA study.

A new management scheme of plastics from waste of electrical and electronic equipment (WEEE), which includes novel treatments of sorting, dissolution/precipitation, extrusion, catalytic pyrolysis, and plastic upgrading, is proposed. Its environmental performances are quantified by an attributional Life Cycle Assessment and compared with those of European currently adopted schemes, which include conventional mechanical recycling and thermal treatments as well as improper options of dumping and open burning, largely applied to WEEE plastics exported to developing countries. The proposed innovative scheme greatly enhances the environmental sustainability of WEEE plastics management, by increasing the annual amounts of polymers sent to recycling (from 390 kt/y up to 530 kt/y), decreasing residues to be sent to combustion (from 360 kt/y up to 60 kt/y), and reducing the potential impacts of all the midpoint categories under analysis (up to 580% for that of Global Warming). These results are mainly related to the adoption of a dissolution/precipitation process, which allows recovering target polymers such as ABS, HIPS and PC, with improvements in terms of Global Warming, Non-Carcinogens, and Carcinogens equal to 246%, 69% and 35%, even when the stages of polymer upgrading and catalytic pyrolysis are not included in the analysis. The sensitivity analysis shows that advantages of the new approach substantially disappear if the awful contributions of exportation outside Europe are taken into account. This clearly indicates that the first step to enhance the sustainability of WEEE plastics management is a strong limitation of improper treatments applied to exported wastes.

About the environmental sustainability of the European management of WEEE plastics.

A huge increase of waste of electrical and electronic equipment (WEEE) is observing everywhere in the world. Plastic component in this waste is more than 20% of the total and allows important environmental advantages if well treated and recycled. The resource recovery from WEEE plastics is characterised by technical difficulties and environmental concerns, mainly related to the waste composition (several engineering polymers, most of which containing heavy metals, additives and brominated flame retardants) and the common utilisation of sub-standard treatments for exported waste. An attributional Life Cycle Assessment quantifies the environmental performances of available management processes for WEEE plastics, those in compliance with the European Directives and the so-called substandard treatments. The results highlight the awful negative contributions of waste exportation and associated improper treatments, and the poor sustainability of the current management scheme. The ideal scenario of complete compliance with European Directives is the only one with an almost negligible effect on the environment, but it is far away from the reality. The analysed real scenarios have strongly negative effects, which become dramatic when exportation outside Europe is included in the waste management scheme. The largely adopted options of uncontrolled open burning and illegal open dumping produce huge impacts in terms of carcinogens (3.5·10+7 and 3.6·10+4 person⋅year, respectively) and non-carcinogens (1.7·10+8 and 2.0·10+6 person⋅year) potentials, which overwhelm all the other potential impacts. The study quantifies the necessity of strong reductions of WEEE plastics exportation and accurate monitoring of the quality of extra-Europe infrastructures that receive the waste.

Environmental performances of a modern waste-to-energy unit in the light of the 2019 BREF document.

The study compares the environmental performances of a new-generation, large scale, combustion-based waste-to-energy unit, active since 2010, with those of different "virtual" units, defined in the light of the Best Available Techniques REFerence document (BREF) for Waste Incineration published by the European Community on December 2019. The average performances of these units have been evaluated in terms of air emissions, material consumptions and energy recovery, based on data related to 355 "existing" European waste incineration lines and those established for the future "new" plants. An attributional Life Cycle Assessment has been used to compare and quantify the environmental performances of the selected units, all equipped with a moving grate furnace and similar air pollution control systems. A sensitivity analysis quantifies how even more severe requests for emission and energy performances as well as the evolution of the European electricity mix until the year 2030 can affect the comparative assessment. The results indicate that the considered large scale waste-to-energy plant has good environmental performances, even in an electricity mix characterised by 45% of renewable sources. This allows an easy compliance with the Best Available Techniques Associated Emission Levels of the new waste incineration BREF document. Possible further improvements of its performances should be focused mainly on a further increase of the energy efficiency, provided that it is economically viable.

Biowaste-to-Biomethane: An LCA study on biogas and syngas roads.

Biomethane produced from waste-derived biomass (biowaste) is a clean and renewable fuel, which offers substantial reductions of greenhouse gas emissions and resource consumption. Biomethane is currently produced via the "biogas road", which includes the anaerobic digestion of wet biowaste and a successive upgrading of obtained biogas, with good environmental performance. An alternative production strategy is the "syngas road", which includes the gasification of dry or semi-dry biowaste followed by cleaning, conditioning, methanation, and final upgrading of obtained syngas. It is still at a demonstration level but appears of great interest for the highest values of energy efficiency and carbon utilisation. The study acquired technical data from existing plants of both these strategies, and developed a quantitative environmental assessment by means of holistic tools of sustainable engineering: material and substance flow analyses and an attributional life cycle assessment. The technical and environmental performance of the two biomethane roads have been then compared, in terms of energy and process efficiency as well as potential impacts in the main midpoint categories (global warming, respiratory inorganics, and non-renewable energy). The syngas road appears to have higher levels of carbon utilisation and better environmental performance, even though an extended sensitivity analysis shows different results if alternative plant configurations and energy mix are considered. This suggests that R&D studies and policies of economic incentives have to be further implemented for both the analysed strategies, even because they deal with different kind of biowaste, having different availability in different countries.

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.

A life cycle assessment of environmental performances of two combustion- and gasification-based waste-to-energy technologies.

An attributional life cycle analysis (LCA) was developed to compare the environmental performances of two waste-to-energy (WtE) units, which utilize the predominant technologies among those available for combustion and gasification processes: a moving grate combustor and a vertical shaft gasifier coupled with direct melting. The two units were assumed to be fed with the same unsorted residual municipal waste, having a composition estimated as a European average. Data from several plants in operation were processed by means of mass and energy balances, and on the basis of the flows and stocks of materials and elements inside and throughout the two units, as provided by a specific substance flow analysis. The potential life cycle environmental impacts related to the operations of the two WtE units were estimated by means of the Impact 2002+ methodology. They indicate that both the technologies have sustainable environmental performances, but those of the moving grate combustion unit are better for most of the selected impact categories. The analysis of the contributions from all the stages of each specific technology suggests where improvements in technological solutions and management criteria should be focused to obtain further and remarkable environmental improvements.