Paola Lettieri

Recycling and recovery routes of plastic solid waste (PSW): a review.

Plastic solid waste (PSW) presents challenges and opportunities to societies regardless of their sustainability awareness and technological advances. In this paper, recent progress in the recycling and recovery of PSW is reviewed. A special emphasis is paid on waste generated from polyolefinic sources, which makes up a great percentage of our daily single-life cycle plastic products. The four routes of PSW treatment are detailed and discussed covering primary (re-extrusion), secondary (mechanical), tertiary (chemical) and quaternary (energy recovery) schemes and technologies. Primary recycling, which involves the re-introduction of clean scrap of single polymer to the extrusion cycle in order to produce products of the similar material, is commonly applied in the processing line itself but rarely applied among recyclers, as recycling materials rarely possess the required quality. The various waste products, consisting of either end-of-life or production (scrap) waste, are the feedstock of secondary techniques, thereby generally reduced in size to a more desirable shape and form, such as pellets, flakes or powders, depending on the source, shape and usability. Tertiary treatment schemes have contributed greatly to the recycling status of PSW in recent years. Advanced thermo-chemical treatment methods cover a wide range of technologies and produce either fuels or petrochemical feedstock. Nowadays, non-catalytic thermal cracking (thermolysis) is receiving renewed attention, due to the fact of added value on a crude oil barrel and its very valuable yielded products. But a fact remains that advanced thermo-chemical recycling of PSW (namely polyolefins) still lacks the proper design and kinetic background to target certain desired products and/or chemicals. Energy recovery was found to be an attainable solution to PSW in general and municipal solid waste (MSW) in particular. The amount of energy produced in kilns and reactors applied in this route is sufficiently investigated up to the point of operation, but not in terms of integration with either petrochemical or converting plants. Although primary and secondary recycling schemes are well established and widely applied, it is concluded that many of the PSW tertiary and quaternary treatment schemes appear to be robust and worthy of additional investigation.

Life cycle assessment of energy from waste via anaerobic digestion: A UK case study

Particularly in the UK, there is potential for use of large-scale anaerobic digestion (AD) plants to treat food waste, possibly along with other organic wastes, to produce biogas. This paper presents the results of a life cycle assessment to compare the environmental impacts of AD with energy and organic fertiliser production against two alternative approaches: incineration with energy production by CHP and landfill with electricity production. In particular the paper investigates the dependency of the results on some specific assumptions and key process parameters. The input Life Cycle Inventory data are specific to the Greater London area, UK. Anaerobic digestion emerges as the best treatment option in terms of total CO2 and total SO2 saved, when energy and organic fertiliser substitute non-renewable electricity, heat and inorganic fertiliser. For photochemical ozone and nutrient enrichment potentials, AD is the second option while incineration is shown to be the most environmentally friendly solution. The robustness of the model is investigated with a sensitivity analysis. The most critical assumption concerns the quantity and quality of the energy substituted by the biogas production. Two key issues affect the development and deployment of future anaerobic digestion plants: maximising the electricity produced by the CHP unit fuelled by biogas and to defining the future energy scenario in which the plant will be embedded.

2D and 3D CFD Simulations of Bubbling Fluidized Beds Using Eulerian-Eulerian Models

This paper reports on CFD simulations of freely bubbling gas fluidized beds using CFX-4, a commercial code developed by CFX Ltd. (formerly AEA Technology). Two Eulerian-Eulerian modelling approaches, the granular kinetic model and the particle-bed model (Gibilaro, 2001), have been investigated. The particle bed model has been recently implemented in CFX-4 for 2D simulations and a numerical procedure was developed to allow for a tight control of the fluid-bed voidage at maximum packing during the simulations, see Lettieri et al. (2003). The work has now been extended to 3D simulations and qualitative and quantitative results are presented in this paper for both the 2D and 3D simulations of the bubbling fluidization of a Geldart Group B material. Results on bed expansion, bubble size and bubble hold-up are reported. In particular, simulated bubble size is compared with predictions given by the Darton et al. (1977) equation at different bed heights. The paper shows that the bubble size predicted by both the granular kinetic model and the particle-bed model is in good agreement with the Dartons equation.

Self-segregation of high-volatile fuel particles during devolatilization in a fluidized bed reactor

Abstract The interaction between fuel particles and incipiently bubbling gas fluidized beds during devolatilization has been investigated by X-ray imaging. The fuel consisted of a ligneous biomass ( Robinia pseudoacacia ) reduced into millimeter-sized particles and doped with lead nitrate in order to make particles visible upon X-ray irradiation. A purposely designed single-particle-injector was used to impulsively introduce fuel particles one at a time at a given depth into the fluidized bed. Experiments highlighted three main features of the phenomenology, namely: (a) the formation of ( endogenous ) volatile matter bubbles around devolatilizing fuel particles; (b) the uprise of endogenous bubbles; and (c) the uprise of fuel particles closely associated to endogenous bubble motion. Bubble and particle trajectories and bubble cross sections as functions of time were worked out in order to assess fuel particle segregation times and endogenou s bubble growth rate. The choice of operating under incipient bubbling conditions enabled thorough assessment of interactive processes establishing between gas-emitting particles and the fluidized suspension. The formation, growth and motion of endogenous volatile bubbles and the associated motion of the fuel particle could be characterized without the perturbation caused by exogenous gas bubbles (i.e. bubbles formed under freely bubbling conditions). This represents a first step towards the characterization of the interaction between gas-emitting particles and freely bubbling beds.

Life cycle assessment of integrated waste management systems for alternative legacy scenarios of the London Olympic Park

This paper presents the results of the life cycle assessment (LCA) of 10 integrated waste management systems (IWMSs) for 3 potential post-event site design scenarios of the London Olympic Park. The aim of the LCA study is to evaluate direct and indirect emissions resulting from various treatment options of municipal solid waste (MSW) annually generated on site together with avoided emissions resulting from energy, materials and nutrients recovery. IWMSs are modelled using GaBi v6.0 Product Sustainability software and results are presented based on the CML (v.Nov-10) characterisation method. The results show that IWMSs with advanced thermal treatment (ATT) and incineration with energy recovery have the lowest Global Warming Potential (GWP) than IWMSs where landfill is the primary waste treatment process. This is due to higher direct emissions and lower avoided emissions from the landfill process compared to the emissions from the thermal treatment processes. LCA results demonstrate that significant environmental savings are achieved through substitution of virgin materials with recycled ones. The results of the sensitivity analysis carried out for IWMS 1 shows that increasing recycling rate by 5%, 10% and 15% compared to the baseline scenario can reduce GWP by 8%, 17% and 25% respectively. Sensitivity analysis also shows how changes in waste composition affect the overall result of the system. The outcomes of such assessments provide decision-makers with fundamental information regarding the environmental impacts of different waste treatment options necessary for sustainable waste management planning.

High temperature effects on the dense phase properties of gas fluidized beds

Abstract This paper reports some of the results obtained from an extensive experimental campaign aimed to study the influence of temperature on the fluidization behaviour of solid materials. The fluidization behaviour of a wide range of materials was investigated from ambient conditions up to 650°C. The aim of this work was to highlight the conditions under which the role of the hydrodynamic forces (HDFs) or interparticle forces (IPFs) were dominant, in order to make predictable the fluidization behaviour at elevated temperatures. To this end, the fluidization behaviour of three fresh FCC catalysts was studied. An E-cat FCC catalyst, which contained process residuals, was examined without performing pre-treatments prior to fluidization tests. Furthermore, a highly porous silica catalyst was doped with increasing amount of potassium acetate (KOAc), 1.7, 7 and 10 wt.%, and a sample of glass ballotini was doped with 0.1 wt.% of KOAc. This was done in the attempt of modifying their surface characteristics, thus triggering changes in their fluidization behaviour with increasing temperature. The measured pressure drop across the bed and deaeration tests was used to highlight changes in the fluidization behaviour as a function of temperature. The standardized collapse time (SCT) was obtained from the collapse profiles and was used to distinguish between systems of powders dominated by HDFs and IPFs. Results obtained from analytical techniques such as thermomechanical analysis (TMA), Gas Chromatography Mass Spectrometry analysis (GCMS) and scanning electron microscope (SEM) are also discussed, these techniques were used to investigate physical changes in the particles with increasing temperature.

Two stage fluid bed-plasma gasification process for solid waste valorisation: technical review and preliminary thermodynamic modelling of sulphur emissions.

Gasification of solid waste for energy has significant potential given an abundant feed supply and strong policy drivers. Nonetheless, significant ambiguities in the knowledge base are apparent. Consequently this study investigates sulphur mechanisms within a novel two stage fluid bed-plasma gasification process. This paper includes a detailed review of gasification and plasma fundamentals in relation to the specific process, along with insight on MSW based feedstock properties and sulphur pollutant therein. As a first step to understanding sulphur partitioning and speciation within the process, thermodynamic modelling of the fluid bed stage has been performed. Preliminary findings, supported by plant experience, indicate the prominence of solid phase sulphur species (as opposed to H(2)S) - Na and K based species in particular. Work is underway to further investigate and validate this.

Kinetics and product distribution of end of life tyres (ELTs) pyrolysis: a novel approach in polyisoprene and SBR thermal cracking.

Thermo-chemical treatments (mainly pyrolysis) directed towards energy and products recovery provide a very promising alternative to open space disposal or landfilling, reducing in the process hazardous waste and potential contamination to soil and water resources. In this communication, we present results of end of life tyres (ELTs) pyrolysis via isothermal and dynamic thermogravimetry of two ELT grades. The aim of this study is to demonstrate the possibility of utilizing a pre-set temperature (T(c)=500 degrees C) pyrolysis process (conversion time, t(c), of 120 s), to the benefit of intensifying the global product yields recovered. A novel engineering kinetics approach was undertaken to propose a thermal cracking scheme of four primary and two secondary side reactions. Thermal degradation of ELTs was taken from a depolymerization approach of the present polyisoprene polymer in the tyres, resulting in a high regression of 0.959. The products of ELTs pyrolysis were lumped into four categories, namely aromatics, liquids, char and gases. The thermal cracking model evaluation of kinetic rate constants and lumped products showed a regression ranging between 0.90 and 0.94. Dynamic runs were performed to extend the model derived, taking into account heating rate (beta) influence and products prediction and interaction. The results obtained can be used in designing industrial ELTs pyrolysis units.

Predicting the Transition from Bubbling to Slugging Fluidization Using Computational Fluid Dynamics

Computational fluid dynamics (CFD) modelling has been used to simulate the transition from bubbling to slugging fluidization of a Geldart Group B powder. Simulations have been performed using a commercial code, CFX4.4. The kinetic model for granular flow, which is already available in CFX, has been used during this study. Two-dimensional time-dependent simulations have been carried out to predict the transition velocity from bubbling to slugging fluidization for the material tested. To this end, simulations were performed at four different fluidizing velocities, 0.25, 0.27, 0.30 and 0.33 m s−1. The Baeyens–Geldart criterion was used to compare the simulated transition velocity with prediction. Furthermore, simulated aspects of fluidization such as voidage profiles and bubble size have been investigated. Simulated values obtained for the maximum slugging bed height have been compared with prediction obtained from the Matsen et al. model. The agreement between the theoretical and computational results was found to be within 12%.

A generalization of the Foscolo and Gibilaro particle-bed model to predict the fluid bed stability of some fresh FCC catalysts at elevated temperatures

Abstract The fluid-bed stability of three fresh FCC catalysts has been investigated both experimentally and theoretically as a function of increasing temperature. Values of the voidage at minimum bubbling conditions were obtained from 20°C up to 650°C, and compared with predictions given by the Foscolo and Gibilaro particle bed model. The formulation of the drag force proposed by Foscolo and Gibilaro has been generalized in order to correctly describe the homogeneous expansion of the three FCC catalysts. Consequently, the procedure followed to obtain the Foscolo and Gibilaro stability criterion has been re-examined, and a generalized expression of their criterion is proposed. Predictions obtained from the generalized particle-bed model were found in agreement with experimental data. This paper discusses also the effect of the drag and buoyancy force in predicting correct trends for e mb . To this end, results obtained using the Jean and Fan model are reported. The physical origin of the stability of these catalysts is also discussed, and the role of the hydrodynamic forces and interparticle forces is examined.