Abstract
Data from a two-stage axial vapor cryogenic compressor on the dual-fuel diesel–electric (DFDE) liquefied natural gas (LNG) carrier were measured and analyzed to investigate compressor energy and exergy efficiency in real exploitation conditions. The running parameters of the two-stage compressor were collected while changing the main propeller shafts rpm. As the compressor supply of vaporized gas to the main engines increases, so does the load and rpm in propulsion electric motors, and vice versa. The results show that when the main engine load varied from 46 to 56 rpm at main propulsion shafts increased mass flow rate of vaporized LNG at a two-stage compressor has an influence on compressor performance. Compressor average energy efficiency is around 50%, while the exergy efficiency of the compressor is significantly lower in all measured ranges and on average is around 34%. The change in the ambient temperature from 0 to 50 °C also influences the compressor’s exergy efficiency. Higher exergy efficiency is achieved at lower ambient temperatures. As temperature increases, overall compressor exergy efficiency decreases by about 7% on average over the whole analyzed range. The proposed new concept of energy-saving and increasing the compressor efficiency based on pre-cooling of the compressor second stage is also analyzed. The temperature at the second stage was varied in the range from 0 to −50 °C, which results in power savings up to 26 kW for optimal running regimes.
Highlights
The harmful pollutant emissions significantly increase over the last years in industry, marine and other energy sectors [1]
The results show that when the main engine load varied from 46 to 56 rpm at main propulsion shafts increased mass flow rate of vaporized liquefied natural gas (LNG) at a two-stage compressor has an influence on compressor performance
The exergy destruction of the two-stage compressor is higher comparing to energy losses where peak destruction is raised to about 320 kW
Summary
The harmful pollutant emissions significantly increase over the last years in industry, marine and other energy sectors [1]. Most LNG carriers use boil-off gas (BOG) from the cargo tanks for its propulsion [9]. Entropy 2020, 22, 115 possible upgrades [14] Such dual-fuel diesel engines require a compressor unit which delivers LNG vapors from the cargo tanks to the engine. The authors investigate entire processes with LNG vapors, exact compressor types are not mentioned. The authors made several variations of the entire LNG fuel gas supply system, but the BOG compressor (or more of them) type is not presented. Gasified LNG is taken from the cargo tanks to the inlet of the two-stage compressor unit and delivered to the engines for completion and continuation of the combustion process [25,26]. The engine load was gradually increased until the point of normal continuous rating at main propulsion shafts, which equals 56 rpm at each propulsion shaft
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