In recent years, liquefied natural gas (LNG)-fueled ships have gained international attention as a potential eco-friendly propulsion system to reduce exhaust gas emissions from ships, which is regulated by the Marine Environment Protection Committee (MEPC) in the International Maritime Organization (IMO). The types of dual-fuel engines classified into a high-pressure Diesel cycle and low-pressure Otto cycle are important in the design of fuel gas supply system (FGSS) for LNG [1].

The Diesel cycle engine (e.g., MAN B&W’s MEGI) has an autoignition method with a high compression ratio and high thermal efficiency but requires a high-pressure gas fuel supply above approximately 300 bar and a nitrogen oxide (NO_x) emission reduction device such as selective catalyst reduction (SCR) [2]. In contrast, the Otto cycle engine (e.g., Winterthur Gas & Diesel’s X-DF) has a low operating pressure of approximately16 bar and satisfies the exhaust gas regulation standards without an after-treatment device, so it is preferred in the market in terms of economic feasibility [3]. However, since the compression ratio for the Otto cycle engine is limited by the occurrence of knocking, special attention is required in fuel quality control.

As a fundamental study, the critical compression ratio implying the occurrence of light knock in a Cooperative Fuel Research (CFR) F-2 engine was investigated as a function of methane number (MN) of alternative gaseous fuels [4]. Although recent research on engine combustion simulation has suggested that the appropriate minimum methane number is 70 [5], most engine manufacturers recommend MN ≥ 80 to achieve fuel economy for high power and low emissions [6]. The commercial Otto cycle engine (XDF) adopts active combustion control for the low MN conditions, resulting in MN ≥ 65 [7].

For LNG fueled ships with the Otto cycle engine, the steady and unsteady process simulations for the fuel gas supply systems were conducted for global oceangoing LNG carriers and propulsion vessels considering different MNs according to relevant sources [8][9]. The process control strategies were also studied to deal with MN fluctuations for different LNG compositions [10]. In this paper, we report the effect of periodic bunkering operations on methane number (MN) variations in LNG propulsion systems.

The LNG fueled vessel was modeled with a storage tank volume of 3,600 m³ with a boil-off rate (BOR) of 0.32%/day. A main engine and three generators of the low-pressure Otto cycle engine consumed LNG along with a gas boiler as summarized in Table 1.

Figure 1 shows a process flow diagram for an LNG FGSS comprising vaporization, boil-off gas (BOG) treatment, vent, and bunkering equipment, which was modeled with ASPEN HYSYS software. The details of the operating conditions for each state are listed in Table 2. Here, the mass flow in the BOG (NG) pipeline occurred several hours after bunkering. Further, GW stands for glycol water. Table 3 shows representative gas compositions used in the simulation cases. The representative operating conditions in Table 2 were simulated with the initial gas composition with respect to the separator temperature of −50 ℃. MN was calculated as a function of the hydrogen/carbon ratio, x, as shown in Equation (1) [11].

Figure 1: 
Process flow diagram of the LNG fuel gas supply system (FGSS)

where MON is the motor octane number fitted by the third-order polynomial.

In addition, the Wobbe index (WI) or Wobbe number was calculated with the volumetric higher heating value, HHV_v, and relative density, RD, from Equation (2) [12].

In this study, dynamic simulations were conducted to understand the effect of periodic bunkering operations on the MN variations in LNG propulsion systems. The time-dependent MN and WI for the engine and boiler devices were derived and compared with the changes in the representative gas compositions and separator temperatures. The main findings of this study are summarized as follows:

• ① Although the rich gas composition for bunkering has MN < 65, the requirement of a commercial engine with MN ≥ 65 could be satisfied by setting the separator temperature to −50 ℃.
• ② In the rich and −50 ℃ case, the occurrence of abnormal operation after the fifth bunkering implied that continuous bunkering with the rich gas composition should be discouraged.
• ③ All the simulation conditions indicated that the main and generator engines were operated with WI < 56. However, because the boiler was operated in a wide range of MN ≒ 37–85 and WI ≒ 53–80, the gas boiler should be designed to cover both natural gas and LPG fuels sufficiently.