Ovisnost o uvozu sirovina te utjecaj na okoliš predstavljaju glavne probleme koji definiraju put razvoja cjelokupnog europskog gospodarstva. Europska unija nastoji doskočiti ovim problemima kroz stimuliranje niskougljičnog, održivog, konkurentnog i sirovinski učinkovitog gospodarstva. Razvoj gospodarstva u ovome smjeru moguće je postići kroz povećanje održivosti proizvodnje promoviranjem simbioza industrija te uspostave šireg kružnog gospodarstva što Europska unija promovira kroz strateške planove za razvoj gospodarstva i legislativni okvir, čime je definiran i pristup "zatvaranja petlje". Integrirani sustav gospodarenja otpadom i njegove oporabe, ne samo da može pomoći pri „zatvaranju petlje“ na materijalnoj strani, kroz uporabu sekundarnih sirovina u proizvodnji sekundarnih materijala čime se skraćuje proizvodni lanac, nego i na energetskoj strani. Na ovaj način energija iz otpada se koristi za (djelomično) zadovoljavanje energetskih potreba čitavog sustava. Ova misao je iskorištena kao polazna točka ovoga istraživanja, gdje je korišten energetski pristup procjeni životnog ciklusa razmatranih sustava. Energetska analiza, putem pokazatelja ukupne potražnje za energijom, prihvaćeni je pristup za procjenu održivosti različitih proizvoda i sustava. U ovome je radu, praćenjem svih materijalnih i energetskih tokova te njihovim svođenjem na primarnu energiju, putem pokazatelja ukupne potražnje za energijom, definiran indeks povrata primarne energije (PERI indeks). Dobivene vrijednosti indeksa PERI pokazuju da sustav gospodarenja komunalnim otpadom kroz kombiniranu materijalnu i energetsku oporabu može vratiti u gospodarstvo do 50% resursa koji su ušli u sustav gospodarenja otpadom u obliku otpadnih materijala. Energetski vektori proizvedeni putem energetske oporabe otpada mogu se iskoristiti za zadovoljavanje energetskih potreba samog analiziranog sustava uspostavom povratne petlje kroz energetske sustave. Na ovaj način, smanjuje se utjelovljena energija sekundarnih materijalnih proizvoda što ima direktan utjecaj na njihovu održivost, kao i održivost cjelokupnog sustava. Rezultati pokazuju da se korištenjem materijalnih sirovina, separiranih iz komunalnog otpada, u proizvodnji smanjuje utjelovljena energija proizvedenih (sekundarnih) materijala za prosječno 63% do 74%, u usporedbi s proizvodnjom primarnih materijala, dok korištenje energije iz otpada, u istim scenarijima, dovodi do dodatnog smanjenja utjelovljene energije i to za 19% do 67%. Navedeni rezultati pokazuju da energija iz otpada može dodatno povećati održivost integriranog sustava gospodarenja komunalnim otpadom kao i proizvedenih materijala unatoč tome što se, kroz novi legislativni okvir Europske unije, naglasak stavlja na materijalnu oporabu. Također, može se zaključiti da čak i relativno mala količina energije iz otpada, proizvedene u sustavu koji kombinira materijalnu i energetsku oporabu, može pomoći u „zatvaranju petlje“, unutar gospodarstva Europske unije, povećanjem održivosti recikliranja otpadnih materijala. Sa stajališta lokalnih kreatora politika, ekonomska strana problema gospodarenja otpadom još uvijek predstavlja najznačajniji aspekt. Ovaj aspekt je naglašen činjenicom da troškove ovakvih sustava plaćaju njegovi korisnici (građani) čime ovo pitanje veže za sebe i pitanje socijalne prihvatljivosti. Stoga, posljednji analizirani pokazatelj u ovome istraživanju je pokazatelj ukupnog troška analiziranog sustava, izražen po masi odloženog miješanog komunalnog otpada, kojega plaćaju korisnici ovoga sustava. Rezultati ovoga pokazatelja govore da se ukupno najbolji socioekonomski rezultati ostvaruju kombiniranjem materijalne i energetske oporabe. Korištenjem upravo ovih triju parametara, koji obuhvaćaju više važnih područja koje je potrebno sagledati prilikom analize ovakvih sustava, smanjuje se broj kriterija koje je potrebno razmotriti prilikom multikriterijskog rangiranja rezultata. Sveukupni rezultati pokazuju da energetska oporaba otpada na jednoj razini omogućuje povećanje povrata primarne energije te time smanjuje utjecaj na okoliš, kao i ovisnosti o uvozu sirovina. Na drugoj razini pomaže u „zatvaranju petlje“ u kružnom gospodarstvu te promovira simbiozu među industrijskim granama, čime se direktno povećava održivost materijalne oporabe i proizašlih proizvoda. Na trećoj razini omogućava ekonomsku i socijalnu održivost implementacije promjena u sustavima gospodarenja otpadom i njegove oporabe, čime doprinosi širem europskom stremljenju prema transformaciji postojećeg gospodarstva u okviru koncepta kružnog gospodarstva i „zatvaranja petlje“.
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The European economy is dependent on import of raw materials. Between 20 and 30% of all raw material needs of the European Union are covered by import. Import plays an important role in the supply of energy carriers and industrially important raw materials. Thus, around 42% of European Unions needs for natural gas, 56% for coal and 88% of oil, 50% for copper, 85% for bauxite, 89% for iron ore and 100% of for a wide range of hi-tech metals are covered by import. The European Union identified these problems and its impact on the sustainability of economic development. The ways in which these problems are being tackled are divided into two main approaches which address the problem of energy import dependence and the problem of material import dependence independently of each other. Waste management is identified as the only common point of these two approaches. Waste management is one of the leading problems the European Union is faced with. EU28 member states generate over 1800 kg of waste per capita (excluding mineral wastes) of which 27% is a municipal solid waste. This amount of waste, through its material and energy recovery, enables the (partial) fulfillment of European material and energy needs. The previously identified division of approaches to identified problems, on the material and energy approach, has also been identified in the area of waste management and waste recovery. Therefore, although waste management combines these two approaches, this division is also present in the very concept of a circular economy, which, while recognizing energy recovery, puts an emphasis on the material recovery of waste and gives it the advantage. From this complexity of solving the problem of European raw material dependence, the motivation for this research arose and the research question of this doctoral research is defined: What is the role of energy recovery of waste in the circular economy and the "closing the loop" concept? Hypothesis Through the literature review and a review of corresponding legislation, a wide range of areas are identified in which assessment of the integrated municipal waste management and recovery systems (material and energy) should be performed, in order to obtain a complete response to the research question. Due to the large number of targets which need to be met, a multi-criteria analysis, which takes into account all the relevant parameters that describe the fulfilment of the given goals, is required.
Based on this concept of research, a hypothesis has been made that multi-criteria analysis can justify the inclusion of waste, as a locally available energy source, into a local energy system during the legislative changes in waste management. Methodology and results Due to the complexity of decision-making which needs to take into account a large number of parameters, analyses resulting in single-score indexes, which cover a larger number of issues, have been conducted. These indexes enabled ranking by multiple goals and reduced the number of individual criteria which need to be considered. The first such index is defined by the return of primary energy. Production of secondary (recovered/recycled) products (materials and energy vectors) can be evaluated through the avoided production of primary (virgin) products while the production of different products (material products and energy vectors, produced through technologies for material and energy recovery of waste) can be compared through the primary energy equivalents of identified avoided production,. All input streams, of the considered technologies in the system, can also be reduced to the value of primary energy equivalent. Using the Cumulative Energy Demand (CED) indicator, each input and output stream of a particular technology is reduced to the value of the total energy consumption of the considered flow, taking into account overall energy consumption from “the cradle”, i.e. from the extraction of primary raw materials from the environment, through their processing, transport, transformation and production of the considered product. By using the primary energy equivalents of inputs and outputs per particular technology, it is possible to calculate the primary energy return (PER) factor which represents one of the ways to assess the life cycle of a product or a system. In previous publications, due to its simplicity and better readability of the results, a priority is given to this kind of approach to the life cycle assessment of the municipal waste management system, ahead of the implementation of the full-scale Life Cycle Assessment (LCA). In addition, the primary energy return factor can be used to evaluate a whole set of single score LCA parameters (with which it correlates), thereby, simultaneously showing the environmental burden through the exploitation of natural resources, anthropogenic impact on climate, the influence on the emissions of a broad spectrum emission in the air, water (underground and surface) and soil, as well as the impact on human health and the quality of the ecosystem.
Based on this robust approach, the Primary Energy Return Index (PERI) is designed to analyse the return of primary energy through the context of energy which has entered the analysed system in the form of waste materials. The PERI index represents the share of primary energy, previously contained within the collected waste (waste entering the considered system), that is returned to the economy through material and energy recovery, and represents a more comprehensible indicator then CED primary energy return factor. The primary energy return factor shows an increase in primary energy recovery with the increase in the primary separation of waste and ranks the analysed scenarios (for the analysed periods), while the newly defined PERI index also shows some other trends. While the value of PERI index is increasing over the years (for all scenarios), a faster increase can be observed in the first period, which is even more pronounced in the scenario without integrated secondary separation of waste. This increase is the result of primary waste separation and is not a result of the changes in the waste management system itself. In later years, the convergence of results and smaller changes in the value of PERI index, with the increase of primary separation, can be noticed, especially for scenarios that have shown greater potential for the return of primary energy. This behaviour of the PERI index defines a maximum value, after which further investment in the primary separation of waste is no longer profitable and more concrete changes to the system are needed to increase system sustainability - such as the integration of new technologies. The overall results of the PERI analysis show that the municipal waste management system, through the combined material and energy recovery of waste, can return to the economy up to 50% of the resources that have entered the waste management system in the form of waste materials (expressed as primary energy), which represents a large proportion of previously discarded raw materials whose economic value this way, in some form, remains within the economy. The above-described analysis is a very good and quick tool for assessing a wide spectrum of environmental impacts and for reduction of import dependence, but it does not say anything about "closing the loop", effects on the EU's circular economy, the re-use of the secondary materials and energy vectors, as well as the degree of industrial symbiosis, which is also one of the accentuated goals of European legislation in this area. An integrated waste management system can at the same time, not only strive to "close the loop" by using secondary raw materials, but also through the use of energy recovered from waste to fuel the analysed system. To carry out this analysis, it is necessary to separately calculate the production and consumption of all technologies in the analysed system, by energy vectors. On the basis of the obtained
results, the energy demand coverage factors, of the municipal waste management system and its recovery chain, through the use of energy from waste, by individual energy vectors, were obtained. Satisfying (part of) the energy demand of the overall waste management system and its recovery chain has an impact on the sustainability of manufactured materials and represents a step further in increasing the sustainability of manufactured materials, where the first step is made through the use of secondary raw materials (derived from waste) in production, thus shortening the production cycle. The use of energy produced from waste, via feedback loops established through the energy systems, for the operation of the waste management system and its recovery chain, has a direct impact on the sustainability of the production system and thus on the sustainability of the produced secondary materials, which is reflected in reduction of embodied energy of the produced materials. Based on this approach, an indicator for the reduction of the embodied energy of the produced secondary materials is obtained. Overall results, by years and scenarios, show that the use of separated waste materials in the secondary material production reduces the embodied energy of secondary (recycled) materials by an average of 63% to 74%, compared with the production of primary (virgin) materials. These reductions confirm a thesis that material recovery of waste materials contributes to the increase in the sustainability of the produced secondary materials. However, the integrated energy recovery from waste, in the same scenarios, leads to an additional reduction of embodied energy by 19% to 67%, depending on the scenario and considered timeframe. The results of the reduction in the embodied energy due to material recovery and total possible reduction that can be achieved by combining the material and energy recovery of waste showed that recovered energy from waste could further increase the sustainability of the integrated municipal waste management system as well as produced materials. This conclusion is valid despite the fact that the EU's new legislative framework puts an emphasis on material recovery (recycling). Using this approach, a complete picture of an integrated municipal waste management system is analysed, and it can be concluded that even a relatively small amount of energy produced through the energy recovery of waste, in a system that combines material and energy recovery, can help to “close the loop” within the EU economy by increasing the sustainability of waste recycling. Although the EU encourage its member states to move towards more sustainable waste management systems, the economic side of the waste management problem is still the most
significant aspect on the local level, especially as the question of social acceptability is tied to it, which is the most important question for decision-makers (policy makers), which are elected by the citizens (users and financiers of the considered system). Policy makers must evaluate all of these factors when deciding in which way to continue the development of the municipal waste management and recovery system. Therefore, the last analysed indicator in this study is an indicator of the total cost of the analysed system, expressed by the amount which needs to be paid by the system users per tonne of mixed municipal waste. On the basis of the obtained economic results, it can be concluded that energy recovery yields higher revenue through the energy sale than it can be achieved through the sale of secondary raw materials, therefore, the deciding factors are the investment and operation costs. It is apparent that the thermal treatment of waste results in large annual costs that need to be covered by the increase of the municipal waste disposal fee, whereas anaerobic digestion has lower costs resulting in a total positive balance and in a reduction of the costs for the citizens and, therefore, a positive social effect. Best results among alternative scenarios are achieved by combining material and energy recovery. Although the number of indicators needed for decision making has been reduced by defining and use of single-score indicators and, based on the results of previous analyses it can be concluded which scenarios are more appropriate from, not only the EU standpoint but also from the local point of view, the analysed scenarios are further ranked through a multi-criteria decision-making approach to facilitate unanimous decision-making. In this research, for the final decision-making, the PROMETHEE II method was used, as well as the GAIA approach to presenting the obtained results. The results of the previous analyses were used as inputs for the multi-criteria decision-making – by means of the return of primary energy, the ecological impact as well as the reduction of the total material and energy intensity (impacts on dependence on imports of raw materials) of scenarios, were assessed; by means of the reduction in the embodied energy, impact on the European (circular) economy and promotion of industrial symbiosis was assessed; and by socioeconomic analysis, the impact on economic viability and social acceptability of the analysed scenarios was assessed.
The results show that avoiding energy recovery leads to overall worse results - the scenario that avoids energy recovery shows the worst results. By analysing results by individual criteria, it is apparent that from the aspect of ecological impact and reduction of raw material dependence, as well from the aspect of the impact on the European (circular) economy and the promotion of industrial symbiosis, avoidance of energy recovery also leads to the worst results among all ranked alternative scenarios. Only the socio-economic analysis shows better results of the ranking of such a scenario compared to scenarios that emphasize energy recovery, i.e. requires less funding from system users. Conclusions and scientific contribution From the legislative framework that describes the circular economy, as well as the previously published papers, it is evident that the emphasis is put on reducing the exploitation of material resources, i.e. on material recovery, while energy flows are mostly neglected or observed only through the transition to renewable energy sources or through the reduction of energy consumption via energy efficiency measures. As far as energy recovery for itself is concerned, it is considered as one of the analysed options separately from material recovery. However, this is not the case when the entire integrated waste management and recovery system is looked upon, where waste can be used in parallel as material and energy resource and where these two approaches can complement each other. The results show that such a holistic view on the problem of waste, and its recovery, show the best results. Through this research, a new link between energy and material recovery of waste has been identified through a feedback loop that enables the increase of sustainability of recycled materials by reducing the use of primary (virgin) energy sources. This enables a new point of view at the role on energy from waste in waste management systems as well as at the link between energy and material recovery of waste through energy systems and broadens previous knowledge of the link between energy systems and waste management systems. The conducted research, not only helps in better understanding of the role of energy from waste through quantifying the impact of “closing the loop” on the energy side through mid-term energy analysis of alternative scenarios, but also extends the views of the impact analysis on overall sustainability (ecological, wider economic and socio-economic) of the decision made on the basis of this approach. By that, holistic approach for consideration of the whole spectrum of problems which has to be considered when planning integrated waste management systems
is established. Obtained results emphasize the importance of integrated waste and energy systems analysis. This clearly shows that energy from waste has its role in the circular economy. When applying a multi-criteria approach to system analysis, attention should be paid to the number of criteria which will be taken into consideration. The number of these criteria can be reduced by using criteria with a high degree of interdependence (collinearity) with a number of other significant factors, thus simplifying the entire decision-making process. By this approach to the implementation of the multi-criteria analysis, one more step forward is made compared to the previous analyses. Also, by analysing the obtained results and the presented discussion, it can be concluded that this research also gives answers to others identified deficiencies in the previous research, which further confirms the scientific contribution. Based on the results of multi-criteria ranking, it can be concluded that the implemented approach can justify the use of energy recovery of waste and the use of energy from waste as a local energy source in the time of legislative changes, not only in the field of waste management but also in the field of further development of the European economy. This conclusion confirms the hypothesis of this research.