Exergy Balance of Material Recycling and Energy Generation of Waste Materials: Case Studies
The Saubermacher Dienstleistungs AG is active in the field of waste management and is inter-ested in an energy and exergy balance of the material recycling and energy generation of differ-ent waste materials, to find out, what is the most appropriate waste management option.
The aim is to find out the frame work conditions under which the material recycling or the energy gener-ation makes more sense and if the exergy balance gives different results compared to the energy balance. For the following four case studies - aluminum, waste plastic, waste paper and house-hold waste - the comparison was made based on life cycle assessment (LCA) and the main in-fluencing factors were analyzed. The exergy and energy balances were analyzed based on a life cycle assessment, which is a method to estimate the environmental impacts of a product in its entire life cycle - "from cradle to grave". Starting with the type and amount of energy (e.g. heat, electricity) and auxiliary materials used the amount of primary energy was calculated, which are necessary for the waste treatment options. All the different primary energy carriers including the heating value of the waste are included in the energy and exergy balance of the material recy-cling and energy generation of the considered waste materials in the four case studies.
The following results were obtained for the case studies:
- The comparison of the material recycling and energy generation of waste materials can only be done based on a life cycle perspective.
- The comparison must be based on the same amount of waste material (e.g. 1 t) and the same amount of services provided by the waste management option (e.g. x MWh electricity, y MWh heat, z t product) which means that within the system boundaries of the material re-cycling an energy system and in the system boundaries of the energy generation a product system must be added.
- The cumulated primary energy demand calculated on a life cycle bases of the different waste management option can be used to assess the energy and exergy efficiencies.
- In general the assessment of the exergy efficiency leads to the same results compared to the energy efficiency in the comparison of the different waste management options.
- Aluminum: the recycling of aluminum has significantly higher energy efficiency than the production of virgin aluminum.
- Household waste: The energy and exergy efficiencies of the energy generation from house-hold waste are similar in the cement industry and in waste incineration plants. The efficien-cy in cement industry is significantly higher, if only electricity and no district heat is pro-duced in the waste incineration plant. In contrast a high utilization rate of the heat from the waste incineration plant gives a higher efficiency compared to the use of household waste in the cement industry. The results strongly depend on the electric and thermal efficiency of the waste incineration plant.
- Waste plastics: The energy and exergy efficiencies of the energy generation from waste plastic are only slightly higher compared to the material recycling. If most of the waste plas-tic can be recycled to new products and the energy demand of recycling is low, than the en-ergy and exergy efficiency of material recycling of waste plastic might be higher compared to the energy generation.
- Waste paper: The energy and exergy efficiencies of the energy generation of waste paper is only significantly higher, if the unused wood in case of material recycling is not used for energy and remains in the forest for natural oxidation.
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