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Energies 2020, 13, 4014 2 of 18 Basically, high-temperature waste heat sources are available in refineries (from the process of fluid catalyst cracking, for example, at 700 to 800 ◦C), in cement plants (at temperatures from 300 to 500 ◦C) and in steel and glass manufactory processes. The dust content of the available flue gases is highly variable, depending on the industrial process. For example, in flue gases from cement plants, the dust content may range from 30,000 to 90,000 mg Nm−3. The cement industry is one of the largest contributors to carbon dioxide emissions and is very energy-intensive: the production of one ton of cement requires an energy consumption of about 4–5 GJ, with an average emission value of 0.81 kgCO2 /kgcement [7,8]. Therefore, it is certainly appropriate, necessary and useful to recover energy from flue gases of the cement industry for the production of electricity, given the high available temperatures. Heat recovery from dusty flue gases, which sometimes also contain sulphur compounds—and are therefore potentially corrosive—is a challenging problem, but the correct materials and suitable technologies are currently available [9,10]. A second important aspect of the problem is the use of advanced high-efficiency thermal power thermodynamic engines. Usually, Rankine cycles with organic fluids are proposed, with good practical results [11–14]. The steam cycle is another well-known accepted possibility for energy recovery. In recent years, interest in trans-critical and supercritical thermodynamic cycles using carbon dioxide as a working fluid has grown. In [15], for example, a summary of proposals in the literature of carbon dioxide cycles for heat recovery from waste heat is presented. In [7], the authors report a literature overview of power cycles that were proposed for heat recovery in cement plants. The energy efficiencies of the thermodynamic cycles listed in the paper range between 11% and 30%, according to the temperature of the considered heat source (270 to 400 ◦C), the thermodynamic cycle and working fluid (organic fluids, steam or carbon dioxide). With regard to carbon dioxide cycles, in [16], a good review of the main advantages and drawbacks of the use of carbon dioxide as a working fluid is presented. Briefly, one advantage is that a very high power density is produced due to the high pressure levels; furthermore, in the supercritical configuration, reasonably high cycle efficiencies even at moderate maximum temperatures can be achieved, but with the need of a large recuperator. However, a corrosive behavior at temperatures greater than 500 ◦C is exhibited, requiring the use of nickel alloys. About 10 ten testing facilities are operating today globally, and many studies investigating this process are underway. On the other hand, as regards Rankine cycles operating with organic fluids, there are now at least 1500–2000 installed engines globally with a wide range of applications (geothermal, biomass, solar and heat recovery) and powers (from a few tens of kW to units of a few tens of MW). According to the analysis developed in [17], carbon dioxide cycles represent a potentially more effective solution than ORC for heat source temperatures greater than 350 ◦C; this is particularly true if cold water is available, making condensation possible. In the following, we consider as a case study the heat recovery from flue gases available at 450 ◦C (a typical maximum reference value for a cement plant) with a mass flow of 100 kg s−1. Usually, in a cement plant, two different hot gas streams are available (see, for example [7,11]): from kiln preheaters and from clinker coolers at different exhaust temperatures and with different exhaust mass flows. Here, for simplicity, we assumed the recovery of heat only from the flue gas flow at higher temperature. Normally, an intermediate heat transfer fluid (diathermal oil or pressurised water) is interposed between the flue gas and the working fluid used in the thermodynamic cycle. Here, to simplify the scheme and the calculations, we considered a direct heat exchanger. On the other hand, some examples of direct heat exchangers are present in industrial practice. Assuming a simple recuperative supercritical carbon dioxide cycle (the simplest plant configuration considered in literature) as a reference, we developed a performance comparison of transcritical ORC cycles with cycles adopting binary mixtures of carbon dioxide as a working fluid.PDF Image | CO2 Mixtures as Working Fluid for High-Temperature Heat Recovery
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