Abstract
From the working data of a dual-fuel marine engine, in this paper, we optimized and compared two waste-heat-recovery single-pressure steam plants—the first characterized by a saturated-steam Rankine cycle, the other by a superheated-steam cycle–using suitably developed simulation models. The objective was to improve the recovered heat from the considered engine, running with both heavy fuel oil and natural gas. The comparison was carried out on the basis of energetic and exergetic considerations, concerning various aspects such as the thermodynamic performance of the heat-recovery steam generator and the efficiency of the Rankine cycle and of the combined dual-fuel-engine–waste-heat-recovery plant. Other important issues were also considered in the comparison, particularly the dimensions and weights of the steam generator as a whole and of its components (economizer, evaporator, superheater) in relation to the exchanged thermal powers. We present the comparison results for different engine working conditions and fuel typology (heavy fuel oil or natural gas).
Highlights
Most engines used for ship propulsion still consist of diesel engines fueled by traditional fuel types, such as Heavy Fuel Oil (HFO) and Marine Diesel Oil (MDO)
Line 3 of Table 8 shows that Rankine cycle efficiency was very similar in the cases of the engine powered by natural gas (NG) or HFO; a greater cycle efficiency of 6.91% was observed in the case of NG fuel for the superheated-steam plant compared to the saturated-steam one, and this difference dropped to 4.41%
The comparison evidenced substantial equivalence between them with regards to the efficiency of the DF engine–WHR combined plants in all considered engine powers for the engine fueled by NG or HFO
Summary
Most engines used for ship propulsion still consist of diesel engines fueled by traditional fuel types, such as Heavy Fuel Oil (HFO) and Marine Diesel Oil (MDO). The growing demand for a reduction in the polluting emissions of ships, promoted by increasingly restrictive international regulations, has favored interest in engines powered by natural gas that could significantly reduce emissions of sulfur oxides, nitrogen oxides, particulate matter, and carbon dioxide [1]. The reduction of CO2 emissions can be achieved by using fuels characterized by low carbon content, such as natural gas [5], and by improving the efficiency of marine propulsion systems, with positive effects on the reduction of fuel costs. An example of a typical reduction of pollutant emissions of an NG-fueled engine in comparison with a diesel engine fueled with HFO was reported in [6]: 25 ÷ 30% carbon dioxide, 25% carbon monoxide, Energies 2020, 13, 985; doi:10.3390/en13040985 www.mdpi.com/journal/energies
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