Gordana Stefanović

Sustainability assessment and comparison of waste management systems: The Cities of Sofia and Niš case studies:

Sustainability assessment of a waste management system is a very complex problem for numerous reasons. Firstly, it is a problem of environmental assessment, economic viability and social acceptability, and also a choice of the most practical waste treatment technique, taking into account all the specific areas in which a waste management system is implemented. For these reasons, among others, it is very important to benchmark, cooperate and exchange experiences in areas with similar characteristics. In this study, a comparison of waste management scenarios in the Cities of Niš and Sofia was performed. Based on the amount and composition of municipal solid waste, and taking into account local specifics (economic conditions, social acceptance, etc.), different scenarios were developed: landfilling without energy recovery, landfilling with energy recovery, mechanical-biological treatment, anaerobic digestion with biogas utilization and incineration with energy recovery. Scenario ranking was done using multi-criteria analysis and 12 indicators were chosen as the criteria. The obtained results show that the most sustainable scenario in both case studies is the mechanical-biological treatment (recycling, composting and Refuse Derived Fuel production). Having in mind that this scenario is the current waste management system in Sofia, these results can help decision-makers in the City of Niš in choosing a successful and sustainable waste management system.

Closed vessel combustion modelling by using pressure-time evolution function derived from two-zonal approach

In this paper a new method for burned mass fraction – pressure relation, x-p relation, for two-zone model combustion calculation is developed. The main application of the two-zone model is obtaining laminar burning velocity, SL, by using a pressure history from a closed vessel combustion experiment. The linear x-p relation by Lewis and von Elbe is still widely used. For linear x-p relation, the end pressure is necessary as input data for the description of the combustion process. In this paper a new x-p relation is presented on the basis of mass and energy conservation during the combustion. In order to correctly represent pressure evolution, the model proposed in this paper needs several input parameters. They were obtained from different sources, like the PREMIX software (with GRIMECH 3.0 mechanism) and GASEQ software, as well as thermodynamic tables. The error analysis is presented in regard to the input parameters. The proposed model is validated against the experiment by Dahoe and Goey, and compared with linear x-p relation from Lewis and von Elbe. The proposed two zone model shows sufficient accuracy when describing the combustion process in a closed vessel without knowing the end pressure in advance, i. e. both peak pressure and combustion rates can be sufficiently correctly captured.