Energy System Analysis of Waste-to-Energy technologies

Alternative uses of waste for energy production becomes increasingly interesting both from a waste management perspective - to deal with increasing waste amounts while reducing the amount of waste deposited at landfills – and from an energy system perspective – to improve the flexibility of the energy system in order to increase the share of renewable energy and reduce greenhouse gas emissions. The focus of this PhD thesis is the analysis of the optimal use of waste for energy production in Denmark, now and in the future. The object of analysis is waste which is not reused or recycled, but can be used for energy production. Different Waste-to-Energy technologies are analysed through energy system analysis of the current Danish energy system with 13-14% renewable energy, as well as possible future Danish energy systems with 43% (2025) and 100% renewable energy (2050), respectively. The technologies include combustion, thermal gasification, anaerobic digestion, fermentation, and transesterification technologies producing electricity, heat, or transport fuel. The influences on and from the surrounding countries Norway, Sweden, Finland and Germany are included in some of the analyses. The analyses are performed in two Danish energy models: the EnergyPLAN model developed at Aalborg University and the Balmorel model developed at the former TSO, ElkraftSystem. A set of important aspects related to the modelling of waste and Waste-to-Energy technologies have been identified, and both models have been developed and improved in this respect in the course of the PhD project.
Given the assumptions applied, an optimal use of waste in the current and future Danish energy systems is mainly for combined heat and power (CHP) production. It is assessed as feasible to sort out 4% of the mixed combustible waste as a wet organic waste fraction and 19% as refuse derived fuel (RDF) consisting of paper, plastic, and waste wood. The following combination of Waste-to-Energy technologies is found to be optimal:
1) Incineration for CHP of the main amount of waste (77% of total) with the highest possible electricity and heat efficiencies.
2) Biogas production from the full potential of organic household waste and manure, assuming that untreated manure is available equal to 5% of the current untreated potential and that a treatment
price of 3 EUR/GJ can be obtained for organic waste. The biogas should be used for CHP or transport fuel, depending on the CO2 quota costs and declared goal (reduced costs or reduced CO2
emissions).
3) Thermal gasification of RDF for CHP combined with co-combustion of the remaining RDF with coal in new coal-fired power plants, if reduced CO2 emissions are not the main goal. This is under the
assumptions that the new coal-fired plants would, to a large extent, be built anyway; that the efficiencies of the waste incineration plants do not decrease due to a decreased heating value of the mixed waste used for incineration, and RDF is available for free. Affected or “marginal” energy production has been identified as input to life cycle assessments. The main conclusion in this respect is that the
affected energy production always consists of a combination of energy technologies, which can be identified by the use of energy system analysis. Which technologies are affected depends on the time perspective (shortterm or long-term), the energy system analysed, the area analysed (Denmark or Nordic and German electricity markets), as well as on assumptions regarding capacities, efficiencies, costs, and prices. When modelling Denmark along with its surrounding countries and including investments as part of the optimisation, technologies located outside Denmark are affected by the changed uses of waste in Denmark. Furthermore, not only flexible technologies, such as coal-fired power plants, which are capable of reacting to short-term changes in demand, are affected, but also inflexible technologies, such as nuclear power.