저작자표시-비영리-변경금지 2.0 대한민국 ... - KMOU Repository

본 논문에서는 고유황 벙커C 추진엔진의 LNG 이중연료 추진엔진 운용의 안전성과 효율성을 확보하기 위해 디젤 및 가스 모드에서 수집된 데이터와 계산을 바탕으로 성능을 비교 해석하고자 시도하였다. 위의 두 가지 운전 모드에서 수집된 데이터와 계산을 바탕으로 조리개, 출력, 평균 유효 압력, 열효율, 연료 소비율, 배기가스 온도 및 압축 압력을 비교했습니다. 비교 결과, 가스 모드는 디젤 모드와 거의 동등한 효율을 보였다.

Introduction

Background

The revisions to Annex VI to MARPOL include drastic reductions in SOx, NOx and particulate matter (PM) limits from ship emissions. In addition, the 2011 amendments to MARPOL Annex VI introduced mandatory measures to reduce greenhouse gas (GHG) emissions.

Emission Regulations

SOx emission restrictions are expected to be met by limiting the sulfur content of the fuel used. As can be seen, LNG has the lowest ratio between fuel and CO2 per kilograms of fuel.

Fig. 1.1 MARPOL Annex VI NOx Emission Limits [2]

Purpose of Thesis

Relative literature and studies will be presented and discussed to inform the reader of technical information related to the purpose of this article. The author's research data and methodology for evaluating the safety and effectiveness of LNG combustion in large two-stroke marine propulsion engines will be presented to validate the aim of the thesis.

Relevant Literature and Research

Mode Operation

The ME-GI engine can operate in three different fuel modes: 1) gas operation mode or gas mode (with a minimum amount of pilot oil), 2) a specified dual fuel mode (SDF) with an injection of a fixed gas amount, and 3) fuel oil only mode or diesel mode . In gas operating mode, the engine can run with a minimum amount of pilot fuel oil of approx. 3%.

Fig. 2.1 Fuel Index in Gas Operation Mode [4]

Safety of LNG and Fuel Systems

This means that if the amount of CH4 present in the vapor mixture exceeds 15%, the mixture is too rich to burn and if it is lower than 5%, the mixture is too lean. In the event of an LNG spill and a vapor cloud that cannot be dispersed, there is a possibility of fire if an ignition source is present. On the gas supply system of ME-GI engines, the gas pipes are designed with double walls.

The outer tube prevents any gas from escaping into the engine room in the event of a leak or rupture of the inner gas tube. This system enables purging of the fuel gas supply system and the gas system of the engine in the event that a leak is detected.

Table 2.2 Properties of Hydrocarbons [6]

Economics and Availability of LNG

Two fuel systems are required to maintain the ME-GI dual-fuel engine: the fuel oil system and the fuel gas supply system. Furthermore, the gas pipes are connected to an inert gas purification system consisting of 95% nitrogen. Currently, HFO is still one of the cheapest marine fuels that can be burned outside the ECAs.

All these factors play a big role in the LNG market, ultimately deciding whether or not it is a viable resource or not. Fortunately, all of the aforementioned factors are either increasing in number or currently meeting global demand, which means that LNG as an alternative fuel is undoubtedly economically viable [11].

Fig. 2.2 Average Regional Natural Gas Prices [8]

Emissions of Dual-Fuel Engines

Because supply additions outstripped demand growth in 2016, many projects did not go ahead as planned, however, because supply is high, LNG spot prices fell by about $2, making it the cheapest fuel marine available [10] [12]. However, the ME-GI motor design boasts a 0.2% slip at low loads and a negligible slip at loads higher than 15% [14]. The Energy Efficiency Design Index (EEDI), mandated by MEPC, adjusts CO2 grams per transport operation as a function of installed power, specific fuel consumption, DWT and speed.

However, this margin for the MV Ilshin Green Iris is around 23% due to the EEDI and minimum propulsion requirements set by the MEPC and the Maritime Safety Committee.

Fuels and their Properties

High engine performance for the ME-GI engine is still possible even if the lowest calorific value (LCV) of the pilot fuel is about 38 MJ/kg. ME-GI engines are capable of running on a wide range of gas compositions, however, for better engine performance and specific gas consumption (SGC), the ME-GI engine runs by design on gas with an LCV of 50 MJ / kg. This means that the ME-GI engine can operate with less than average LNG composition (C2H6 = 15%) making it a flexible engine.

If the engine is supplied with a very diverse composition of gaseous fuel, this directly affects the amount of pilot fuel injected. This process is called aging and is due to the inevitable flow of heat from the surroundings of the tank.

Table 2.4 ME-GI Engine Guiding Specifications for HFO

Combustion

Before BDC, from 130 to 150 ATDC, the piston exposes the scavenge air ports at the bottom of the cylinder. The remaining exhaust gases are then pushed out of the cylinder by high-pressure cleaning air. Then, from 110 to 150 BTDC, the exhaust valve closes and compression of the charged scavenging air begins.

If all the fuel in the cylinder is burned completely by the end of the steady burn period, the pressure will be even throughout the expansion stroke. The substitution rate describes the amount of energy supplied by LNG expressed as a percentage of the total energy that.

Fig. 2.4 Example of a 2-Stroke Engine Timing Diagram [17]

Mode Transition

Variability of fuel and energy content can contribute significant errors to the control equipment used during mode transition, which is based on fuel energy content and AFR. One method of mode switching control is by using some type of burn index based on operating conditions. The combustion index then provides the desired engine operation at multiple gas and liquid fuel ratios.

Data Analysis and Results

Test Ship and Research Methodology

However, these factors were not taken into account when examining engine data and analyzing engine performance. Two power curves for when the engine was running in diesel and gas mode were plotted. At low load, especially below 10%, the engine will typically run in diesel mode.

It should be noted that this data was recorded sequentially as the engine went from diesel mode to gas mode and then back to diesel mode. The data cutoff represents approximately five hours of continuous engine operation in gas mode and is not shown.

Fig. 3.2 Voyage Route of the MV Ilshin Green Iris

Mean Effective Pressure

This difference, expressed as a difference subtracted from MCR, would mean a loss of approximately 61.68 kW of power when in gas mode. Given that when used in gas mode engine emissions are significantly reduced, this is still not a significant loss, but a small sacrifice to make to meet IMO emission regulatory requirements. However, the Sp and sweep volume are estimated values ​​because the actual surface of the piston has a highly complicated geometry and is guarded information not released to the public by the engine manufacturer.

The BMEP is calculated from the dynamometer power (torque) and is the actual output of the engine at the crankshaft and does take into account the engine efficiency. Because the swept volume and the surface area of ​​the piston are estimated values, and assuming the indicated engine power equation, the true value of IMEP cannot be calculated, so the resulting values ​​are estimates.

Thermal Efficiency

3.6, the gas mode has an overall higher thermal efficiency than the diesel mode, with an average percentage difference of 1.91% in favor of the gas mode. Throttle mode has an overall higher thermal efficiency because the SFOC is lower at any engine load. Gas mode uses less g/kWh of fuel because natural gas has a higher calorific value than diesel.

The main difference between diesel and gas mode is the use of a different fuel for each mode. Because LNG has a higher LCV and a simpler molecular formula than HFO, it would be intuitive to assume that using a diesel engine with LNG would have a higher overall efficiency.

Fig. 3.6 Effect of Load on Thermal Efficiency by Mode

Specific Fuel Oil Consumption

This is also largely due to the fact that LNG has a higher LCV in terms of mass than HFO.

Exhaust Gas

Emissions in kg/s were also calculated against engine load for each mode using the CEAS Engine Calculations platform (assuming ISO parameters as stated earlier). From these calculations, it can be seen that running in gas mode produces less exhaust gas with an average difference of 0.032 kg/s. Over the course of a year, this means that operating in gas mode could result in up to approximately 996 metric tons of emissions reduction.

It is common to see an increase in exhaust gas temperature of 50-60℃ from shop test to sea test due to HFO operation and changed climate conditions. To effectively compare exhaust gas temperatures, we need to consider why there may be deviations during actual engine operation.

PMI Results

3.9, there is little variation between diesel and gas modes in NCR, other than an approximately 10 bar difference in compression pressure between modes. For all six cylinders, the average maximum cylinder pressure for diesel and gas modes was 184.8 bar and 184.5 bar respectively, with a difference of 0.16%. The purge air pressure for diesel and gas mode at NCR was 2.11 bar and 2.03 bar respectively, with a difference of 3.86%.

As with the NCR results, there is also little variation between diesel and gas modes. In NCR, there is no relatively significant distinguishable difference between diesel mode and gas mode when analyzing the P-V% chart.

Fig. 3.9 Cylinder Pressure versus °CA for Diesel and Gas Mode at NCR

Conclusion

International: IMO Marine Motor Regulations.” Emission Standards: International: IMO Marine Engine Regulations, Diesel Net, 2018, www.dieselnet.com/standards/inter/imo.php. LNG safety.” Bring energy where you need it., Prometheus Energy, www.prometheusenergy.com/services-solutions. Fuels - Higher and Lower Calorific Values, Engineering Toolbox, 2003, www.engineeringtoolbox.com/fuels-higher-calorific-values-d_169.html.

ScienceDirect.com | Science, health and medical journals, full-text articles and books., Science Direct, June 2015, www.sciencedirect.com/. Feedback control during mode transition for a marine dual-fuel engine.”ScienceDirect.com | Science, health and medical journals, full-text articles and books., Science Direct, 2015, www.sciencedirect.com/.