A life cycle optimization framework for the sustainable design of circular municipal solid waste management systems

RESUMEN: Los objetivos de esta tesis son desarrollar un marco metodológico para determinar la configuración óptima de los sistemas integrados de gestión de residuos bajo una perspectiva del ciclo de vida, e investigar si la economía circular contribuye a la reducción del consumo de recursos y los impactos ambientales y al crecimiento económico. El marco metodológico propuesto se aplicó a la gestión de residuos municipales orgánicos en la Comunidad Autónoma de Cantabria. El modelo del sistema se construyó combinando análisis de flujo de materiales y análisis de ciclo de vida (ambiental y económico). Se formuló un problema de optimización multi-objetivo para maximizar la circularidad de los nutrientes y minimizar el uso de recursos, los impactos ambientales y los costes de gestión de residuos. Los resultados sugieren que mejorar la circularidad de los recursos no implica necesariamente una reducción de costes, del consumo de recursos o de la emisión de cargas ambientales.ABSTRACT: The objectives of this dissertation are twofold: to develop a methodological framework to select the optimal configuration of integrated waste management systems under a life cycle perspective, and to investigate whether adopting a circular economy is an effective measure to attain increased economic benefits and a reduction in resource consumption and environmental impacts. The proposed framework – based the expansion of the boundaries of linear waste management systems – was applied to the management of municipal organic waste in the Spanish region of Cantabria. The system model was constructed combining material flow analysis, life cycle assessment and life cycle costing tools. A multi-objective optimization problem was formulated to maximize nutrient circularity and minimize resource use, environmental impacts and waste management costs. The results suggest that improving resource circularity can lead to increased costs and does not necessarily entail a decrease in the consumption of natural resources or the emission of environmental burdens.The author has been the recipient of two predoctoral fellowships granted by the University of Cantabria and the Spanish Ministry of Education (code FPU 15/01771). Her visit to NCSU was funded by a predoctoral mobility scholarship awarded by the University of Cantabria and by the research project CTQ2016-76231-C2-1-R, whereas her research at Cornell University was sponsored by the FPU program (code EST18/00007). She gratefully acknowledges this financial support

Trade-offs between nutrient circularity and environmental impacts in the management of organic waste

Measuring the circularity of resources is essential to assessing the performance of a circular economy. This work aims at proposing an indicator that quantifies how effective a system is at extending the lifetime of its waste components after they have been discarded. The developed indicator was applied to study the circularity of nutrients within a system that handles the organic waste (OW) generated in the Spanish region of Cantabria. A superstructure was developed to determine the optimal configuration of the system. It is composed of alternative unit processes for (1) the management of OW and (2) the application of the recovered products as soil amendment to grow corn. A multiobjective mixed integer linear programming problem was formulated under two policy scenarios with different source separation rates. The problem was optimized according to six objective functions: the circularity indicators of carbon, nitrogen, and phosphorus, which are maximized, and their associated environmental impacts to be minimized (global warming, marine eutrophication, and freshwater eutrophication). The model was fed with the life cycle assessment results obtained with the Environmental Assessment System for Environmental TECHnologies (EASETECH) version 2.3.6 and the nutrient flows in the agriculture subsystem, which were calculated with Denitrification–Decomposition (DNDC) version 9.5. It was concluded that improving nutrient circularity paradoxically leads to eutrophication impacts and that increasing the SSR of OW has a positive effect on the carbon footprint of the system.The authors acknowledge the financial support from the Spanish MECD (grant no. FPU15/01771) and MINECO (grant no. CTQ2016-76231-C2-1R

From linear to circular integrated waste management systems: A review of methodological approaches

The continuous depletion of natural resources related to our lifestyle cannot be sustained indefinitely. Two major lines of action can be taken to overcome this challenge: the application of waste prevention policies and the shift from the classical linear Integrated Waste Management Systems (IWMSs) that focus solely on the treatment of Municipal Solid Waste (MSW) to circular IWMSs (CIWMSs) that combine waste and materials management, incentivizing the circularity of resources. The system analysis tools applied to design and assess the performance of linear IWMSs were reviewed in order to identify the weak spots of these methodologies, the difficulties of applying them to CIWMSs, and the topics that could benefit from further research and standardization. The findings of the literature review provided the basis to develop a methodological framework for the analysis of CIWMSs that relies on the expansion of the typical IWMS boundaries to include the upstream subsystems that reflect the transformation of resources and its interconnections with the waste management subsystems

Noncooperative game theory to ensure the marketability of organic fertilizers within a sustainable circular economy

To optimize the environmental performance and the conflicting economic interests of the main stakeholders that interact within circular integrated waste management systems (CIWMSs), life cycle analysis and a game-theoretical model-based on the Stackelberg equilibrium-were integrated into a multiobjective optimization framework. The framework was used to determine the operational decisions and the configuration of a CIWMS that simultaneously minimize the total global warming impacts (GWIs) and maximize the profits of (i) the waste managers that valorize the municipal organic waste generated in the Spanish region of Cantabria and (ii) the regional farmers that purchase the resulting organic fertilizers. A bilevel optimization problem was formulated and solved by replacing the lower-level problem with its equivalent Karush-Kuhn-Tucker conditions. The balance between the stakeholders' objectives is reflected in the low prices set for the organic fertilizers (0-2 €·metric ton-1 of compost and 0-1 €·metric ton-1 of digestate). Although the minimal GWIs are constrained by the waste managers' profits, it is possible to improve the values of the objective functions by increasing the waste management tax. The proposed framework proved to be useful to plan for a sustainable circular economy, warranting the profitability of organic fertilizers for both ends of the supply chain.The authors acknowledge the financial support from the Spanish Ministry of Education 567 (EST18/00007 and FPU15/01771

Life cycle assessment of alternative processes to treat fly ash from waste incineration

Unsustainable consumption and production patterns, together with industrialization and population growth, have increased the generation of municipal solid waste (MSW), causing several environmental problems. The European Waste Framework Directive (WFD) sets waste prevention, preparation for reuse and recycling as priority strategies. Nevertheless, still a great amount of MSW ends up in landfills and waste-to-energy (WtE) plants. WtE plants reduces waste volume and allows efficient recovery of energy, however, incineration results in various types of solid wastes, bottom, boiler and fly ashes (FA). Due to the concentration of dangerous substances, FA are treated by means of stabilisation/solidification (S/S), thermal treatments or combined treatments, to reduce their toxicity and to avoid negative impacts on the environment and human health. Among S/S alternatives, stabilisation with cement and carbonation are one of the most popular. To determine the environmental performance of these processes this paper conducted a life cycle assessment (LCA). The study evaluated FA stabilisation with cement and water and FA carbonation for 55 % and 100 % excess of CO2 in the flue gas at the outlet of the reactor, and pressures of 1, 5, 10, 15 and 20 bar. The results showed that the range of pressure between 3 and 4 bar, and 55 % excess of CO2 in the flue gas have an efficient performance. The comparison of FA carbonation and stabilization displayed that the latter has higher impacts than the alternative carbonation due mainly to the cement production and the reduction of lixiviation and CO2 capture in the ash

Carbon footprint of biogenic carbon capture, storage and utilization via electrochemical reduction to methanol

El objetivo de este trabajo es evaluar la huella de carbono de una central termoeléctrica alimentada con biomasa forestal que exporta a la red eléctrica un máximo de 270 MW y captura todo el CO2 generado. Parte del CO2 es almacenado en un acuífero salino. La fracción restante de CO2 capturado es reducido en un reactor electroquímico con electricidad procedente de paneles fotovoltaicos a metanol, que posteriormente será utilizado como combustible en un vehículo. Se evaluó la incorporación al proceso de dos casos de estudio (A y B respectivamente) basados en dos tecnologías de electrorreducción con diferentes configuraciones y materiales electródicos (Albo y col., 2015b; Shironita y col., 2013). Para determinar la huella de carbono del proceso descrito se empleó la metodología del análisis del ciclo de vida. En primer lugar se revisó el estado del arte para la cuantificación de las emisiones de CO2 biogénicas y los procesos de captura y utilización de CO2. A continuación se definieron los límites del sistema estudiados y se desarrolló un modelo matemático basado en los balances de materia y energía a los equipos que integran el proceso, con el propósito de su descripción. Este modelo permitió obtener el inventario de CO2 equivalente del proceso, asignando todas las cargas ambientales del proceso a la electricidad producida. Los resultados obtenidos demuestran que en ambos casos A y B, sin tener en cuenta ningún facto de mejora, la opción con menos emisiones de CO2-eq (0.137 kg CO2-eq·kWh-1), es almacenar todo el CO2 capturado y no derivar ninguna parte a la reducción electroquímica. El caso estudiado A (Albo y col., 2015b) está limitado fundamentalmente por la excesiva demanda energética del proceso de destilación convencional para la separación metanol/agua. Se ha estimado que la concentración de metanol obtenida en el proceso de electrorreducción se debería incrementar unas 10,000 veces para que esta tecnología pueda ser aplicada con menores emisiones que en el caso de referencia. En cuanto al caso B (Shironita y col., 2013), es posible electrorreducir una mayor cantidad de CO2 sin comprometer la eficiencia energética de la planta, ya que por la configuración del cátodo, no requiere incorporar una etapa de purificación al proceso puesto que la corriente de CO2 incorpora la cantidad de agua necesaria para la reacción. No obstante, las emisiones indirectas de CO2-eq son superiores a las del caso A incluso para todas las concentraciones estudiadas, ya que los materiales que componen el electrodo en el caso B tienen una huella de carbono mucho más elevada. Sería necesario mejorar la vida útil del electrodo unas 10 veces en el caso B para que al compararlo con el caso A, en el que la concentración aumenta 4 órdenes de magnitud, las emisiones de CO2-eq sean similares. 2 Si bien la huella de carbono de los procesos de producción de electricidad a partir de biomasa acoplados a un proceso de captura y almacenamiento de CO2 (sin derivación al proceso de electrorreducción) es inferior a, por ejemplo el mix eléctrico de la Unión Europea (0.392 kg CO2-eq·kWh-1), su implementación conjunta con un proceso de valorización electroquímica de CO2 a metanol no es viable en la actualidad para las dos referencias analizadas, bajo las hipótesis asumidas en el presente trabajo. Los principales motivos son las elevadas necesidades de energía derivada de los procesos de destilación para la separación metanol/agua (como consecuencia de la baja concentración obtenida) o la limitada vida útil de los materiales electródicos que emplean metales como platino y rutenio. La investigación futura en relación a los procesos de electrorreducción de CO2 se debería centrar en: i) el aumento de la concentración de metanol (para disminuir el consumo energético en la separación); y ii) el desarrollo de materiales catódicos que impliquen una baja huella de carbono a través de una mayor vida útil.Máster en Ingeniería Químic

Design and simulation of a biomass drying process in a termal power plant

Grado en Ingeniería Químic

Assessing the Environmental Potential of Hydrogen from Waste Polyethylene

In 2019, nearly 370 million tonnes of waste plastic were generated, an amount that has been steadily increasing over the years. Here we assess hydrogen production from waste polyethylene in the context of a circular economy of plastics. Based on the gasification of polyethylene waste (wPG), we performed a Life Cycle Assessment (LCA) study following the ReCiPe method. Our results show that the wPG process coupled with carbon capture and storage (CCS) performs very well environmentally relative to other H2 production routes, outperforming steam methane reforming (SMR) with and without CCS and biomass gasification (BG) in the three endpoint impact categories.ISSN:1570-794

Environmental Sustainability Assessment of Hydrogen from Waste Polymers

The rising demand for single-use polymers calls for alternative waste treatment pathways to ensure a circular economy. Here, we explore hydrogen production from waste polymer gasification (wPG) to reduce the environmental impacts of plastic incineration and landfilling while generating a valuable product. We assess the carbon footprint of 13 H2 production routes and their environmental sustainability relative to the planetary boundaries (PBs) defined for seven Earth-system processes, covering H2 from waste polymers (wP; polyethylene, polypropylene, and polystyrene), and a set of benchmark technologies including H2 from natural gas, biomass, and water splitting. Our results show that wPG coupled with carbon capture and storage (CCS) could reduce the climate change impact of fossil-based and most electrolytic routes. Moreover, due to the high price of wP, wPG would be more expensive than its fossil- and biomass-based analogs but cheaper than the electrolytic routes. The absolute environmental sustainability assessment (AESA) revealed that all pathways would transgress at least one downscaled PB, yet a portfolio was identified where the current global H2 demand could be met without transgressing any of the studied PBs, which indicates that H2 from plastics could play a role until chemical recycling technologies reach a sufficient maturity level.ISSN:2168-048