TS-75
Combustion Characteristics of Bunker Fuel from the Viewpoint of Distribution of Distillation Temperature
Koji TAKASAKI*, Tatsuo TAKAISHI**, Kazuyuki MAEDA****, Michirou MATSUBARA***, Hiroshi TAJIMA*, Masayoshi NAKASHIMA* and Hiroyuki ISHIDA**
ABSTRACT
Low-grade fuels with unusual distribution of distillation temperature and a high rate of aromatic hydrocarbons are currently being supplied at certain ports. Sometimes these fuels cause damage i.e. abnormally heavy wear of cylinder liners to marine diesel engines. In this study, a sample of such a fuel is obtained and its properties and composition are in detail analyzed. Moreover combustion characteristics are examined using various testing devices including visual engine and visual combustion chamber. These are new facilities for study of bunker fuel oil. According to results of such examinations, a cause of the heavy wear of the cylinder liners can be guessed with strong foundation.
Key words: Bunker Fuel, Dumbbell Fuel, Distillation Temperature, Aromatics, After-burning, Flame-length
1. INTRODUCTION
Medium and large-sized diesel engines have two economic advantages: high efficiency and the ability to use cheap fuels, the latter being made possible by the use of bunker fuel oil (BFO). Lately, a kind of low-grade bunker fuel oil is being supplied for marine diesel engines at certain ports. Because the combustion control of this fuel is very difficult, some marine engines have been damaged.
The authors have got the sample of such a fuel that was bunkered in the West Coast and after that damaged five container ships on voyage. According to an analysis of the distillation temperature, it has become clear that the cutter stocks of this fuel are composed of lighter distillate than conventional bunker fuel oil. And according to the chemical analyses, this fuel contains much more aromatic hydrocarbons than the conventional.
In this research, the combustion characteristics of this fuel have been examined by following methods.
・Examination of the ignition characteristics using a FIA (Fuel ignition Analyzer)
・High speed visualization of the combustion process using a visual engine
・High speed visualization of the single spray/flame using a visual combustion chamber
2. EXPERIMENTAL PROCEDURE
2.1 Experimental Apparatus
2.1.1 FIA (Fuel Ignition Analyzer)
The Fuel Ignition Analyzer was recently developed in Norway and is explained in [1]. Fuel is injected ten times into air with a pressure of 4.5 MPa and a temperature of 450 ℃. Afterwards the distribution of ignition delay is displayed.
2.1.2 Visual Engine
The visual engine, explained in detail in [2], is used to visualize the combustion within the whole combustion chamber. It actually is a supercharged, 2-stroke-cycle single-cylinder engine with a bore of 190 mm. Pmi and engine speed at the visualization test are 15 bar and 400 rpm respectively.
2.1.3 Visual Combustion Chamber
The visual combustion chamber shown in Fig. 1 is used to visualize a single spray/flame and examine its spatial flame-length and combustion duration. The single fuel spray is injected into the chamber in which air at a pressure of 2.5 MPa and a temperature of 600〜670 ℃ is charged. As the distance from the injection nozzle to the bottom wall of the chamber is about 300 mm, relatively long free spray/flame can be observed. Photos of spray/flame are taken through the glass window fitted on the side of the chamber. A special fuel injection system for bunker fuel oil, which is actuated by hydraulic pressure, is applied to this chamber. Bunker fuel oil can be injected with a maximum injection pressure of 100 MPa.
2.2 Fuels
Two kinds of bunker fuel oils are used in the experiments. Table 1 shows the characteristics of these fuels. The two fuels (BFO-S. BFO-A) are common bunker fuel oils on the market. Five low-speed marine engines driven with BFO-A have suffered abnormally heavy wear of the cylinder liners in a short time, whereas the same engines have not experienced any problems with BFO-S at all. According to Table 1 there are only a few differences between BFO-S and BFO-A. The sulfur content of BFO-A (1.3%) is much lower than BFO-S (3.5%), and the nitrogen content of BFO-A (0.76%) is much higher than BFO-S (0.28%). Seeing the residual carbon and asphaltene percentages, BFO-A contains rather smaller amount of them than BFO-S. CCAI value sometimes used to estimate the ignitability of bunker fuel oil is about the same for the two fuels.
