PANACEA – Polygeneration fueled with biomass from waste In urban wastewater treatment plants
Responsabile Scientifico: …
Programma: …
Ente finanziatore: Ministero dello Sviluppo Economico – Ricerca di Sistema
Costo progetto: …
Settore/Ambito: …
Periodo: 2016-2018
Codice progetto: …
CUP: B62I16000290005
Sito web progetto: …
Responsabile Scientifico IM-CNR: Ing. Michela Costa
Partecipanti IM-CNR: …
Finanziamento IM-CNR: € 158.683,86
Project details
The purification of urban waste water has become a growing problem both in relation to the need to build new plants and adapt existing ones to respond to the increasingly stringent initiatives of the European Community, and to the energy and environmental costs associated with these systems. Another problem is that of high energy consumption, both electrical and thermal. The analysis of the management costs of different purification plants shows that around 25-30% of the total is connected to the electricity requirement, which is why biogas is used in plants that have an anaerobic digestion section. generated for the production of electricity. However, this production is not sufficient to cover the plant’s energy needs both for the modest amount of biogas that can be produced and for its relatively low calorific value. Further costs, in the measure of 20-25%, are related to the treatment and disposal of the sewage sludge in landfills, to reduce which thermal drying systems are spreading.
At present, the use of vegetable oils, pure or exhausted, is recognized as strategic for limiting the emissions of CO2 into the atmosphere, due to the link between the production cycle and the growth of plants with the relative absorption of carbon dioxide from the ‘atmosphere. OVE are today reused mainly in the production of biodiesel, whose use is limited to the esterified form, mixed with fuel oils of extractive origin, for the calorific value generally lower than that of diesel and for the higher cost, consequence of the more difficult storage and the transesterification process necessary for its production. This process is also extremely expensive in energy and environmental terms for the costs associated with the transport of a biomass that could otherwise be used on a local scale. For this reason, especially in small and medium size electricity generation systems, the use of filtered pure vegetable oils is used, which however present problems related to their chemical-physical constitution (high viscosity). The immediate consequences are the reduction in yield and the increase in specific consumption. At the same time, however, the maximum pressure of the combustion cycle decreases, and, likewise, the peak temperature of the combustion cycle, with a positive effect on the production of NOx which decreases as the thermodynamic efficiency decreases.
With regard to the sludge deriving from the purification of urban waste water, downstream of drying, it is not yet possible to provide a clear indication of the optimal treatment option for their recovery, which can be energy or material. Downstream of the drying process, the sludge has a moderate calorific value so it can be used in co-combustion, incineration, pyrolysis and gasification processes, with the generation of new energy flows. As for the recovery of material, the sludge simply dried or in the form of biochar, downstream of the pyrolysis and / or gasification treatments, can be reused in the agricultural sector in compliance with the limits imposed by the European Directive 86/278 / EEC and related national transpositions.
The fundamental technological objective of this project proposed by the partnership composed by the Motors Institute of the National Research Council, University of Naples “Parthenope”, Society Grastim J.V. S.r.l., GIOTTO – GIOTTO Water S.r.l. and PROTEG – PROTEG S.p.A. it consists in the development of a cogeneration system powered by two biomasses that at present are not used together, neither at the prototype level nor at the commercial level. The consequences of the project are extremely interesting, as they entail an immediate reduction in costs and environmental impact.
The first problem to be addressed will consist in optimizing the energy yield of the cogeneration system. The chemical-physical characteristics of the OVE depend on the type of oil and the conditions of use. Degradation reactions lead to an increase in viscosity, density, free fatty acid content, total polar material, polymerized triglycerides and a decrease in the smoke point and in the number of double bonds. There are two technological solutions proposed in the literature to improve the combustion process of pure vegetable oils, fundamentally acting on viscosity: mixing with diesel oil or preheating. As part of the project, it will be assessed which technique guarantees the best efficiency in the case of combustion of OVE. From a preliminary technical-economic analysis, the preheating solution seems preferable, in relation to the presence of the thermal flows leaving both the cogeneration process and the heat treatment downstream of the drying. In any case, the engine must be started with diesel so a manual or automatic system to change the supply from diesel to OVE will be studied. We will also consider the possibility of equipping the engine with a double tank, one for diesel fuel to be used for thermal regulation and the shutdown procedure, the other for oil to be preheated, together with a valve system to switch between one feed to another (also in emergency procedures).
Particular attention will be paid to the analysis of OVE combustion emissions, as there are still few studies on the subject in the literature. In addition, the tested engines are all small in size, therefore not representative of the case under consideration. The physical-chemical characterization of the exhaust gases of the engine fueled by OVE is also of fundamental importance because the exhaust gases appropriately mixed with the engine cooling air will be used as hot current for drying the sludge. At the same time as studying the emissions of the engine powered by OVE, it will also be necessary to characterize dried sludge from a chemical-physical point of view, to assess the impact of the direct drying process on the substrate in terms of variation in the nutrient content and possible absorption of substances contained in the fumes, so as to identify the best solution for reuse and / or further post-treatment of the dried sludge. At present, in fact, thermal drying in the presence of high temperature thermal waste is generally carried out indirectly, or by means of heat exchangers. Exchangers are used to heat the drying stream for the transfer of heat from the exhausted gases to the air or additional intermediate exchanger, with diathermic oil.
As far as the operation of the dual-fuel engine is concerned, the main problems are generally related to the load regime: at low loads the flame must propagate through a mixture that is too poor, while at high loads it is possible to run into the phenomenon of detonation, i.e. spontaneous ignition of the fraction of the so-called end-gas mixture, located near the cylinder walls and not yet reached by the flame front. Once the load conditions of the engine have been defined, therefore, it will be necessary to evaluate the most appropriate fueling strategy and consider the formation of polluting species such as nitrogen oxides, unburnt hydrocarbons and carbonaceous particulate.