Viswa Group, in its latest technical update has published the white paper focusing on the ignition combustion properties of VLSFOs determined using the Fuel Ignition/Combustion Analyzer (FIA -100/FCA).
Abstract
New 2020 VLSFOs
The new 2020 Very Low Sulfur Fuels(VLSFOs) have become the most widely used fuel in 2020 since the IMO sulfur regulations.
Complex chemical nature
The chemical nature of VLSFOs has been very complex, owing to the fact that these are hybrid fuels made by blending multiple product streams. However, it is known that these fuels are far from trouble-free.
While there are other problems with the use of VLSFOs, around 14% of the problems reported to us were related to liner wear and scuffing.
Viswa’s analysis
In order to determine the effect of the ignition and combustion properties of VLSFOs, a detailed analytical study of ignition and combustion parameters was carried out.
The Viswa Group has carried out ignition and combustion analyses on 120 VLSFOs from different sources and characteristics, grouped as paraffinic and aromatic.
Findings
Most of the VLSFOs had very good ignition and combustion properties as well as a normal after-burning-period (ABP). The study attempts to understand the ignition and combustion properties using FIA-100 (FCA). Some problems encountered while using these fuels such as liner wear and scuffing are also addressed.
Compliant fuel transition
Ignition-combustion properties
Compliant 2020 fuels Very Low Sulfur Fuels (VLSFOs) can be blended from various non-traditional sources with wide variability in their formulations. Mixtures of components in varying blend ratios will result in unknown ignition-combustion properties of a fuel.
Not a smooth transition
The transition to compliant fuels so far has not been very smooth. The below is the list of the main issues that were reported to Viswa Group in the last 9 months from its customers: purifier and filter choking, fuel solidification and wax deposition, liner wear, scuffing issues & black smoke.
Out of the problem samples reported to us, around 14% of them have been related to liner wear, scuffing and piston ring breakage issues.
Fuel’s impact on cylinder liner
Many groups in the maritime industry have been working to understand the mechanism of liner wear & scuffing with VLSFOs and this topic is being widely discussed. Ignition and combustion properties of the fuel have a direct impact on the cylinder liner and exhaust.
Scuffing issues & smoke emissions cases
The Viswa Group has received 50 liner wear and scuffing issues and a few black smoke emissions cases in 2020. In order to investigate if the problem is related to VLSFO fuel quality, we analyzed the ignition and combustion properties of these fuels. 120 VLSFOs were tested by FIA-100 to evaluate their ignition and combustion characteristics. The purpose of this paper is to evaluate the ignition-combustion characteristics of VLSFOs and compare them to high sulfur fuel oils (HSFOs).
Determination of Ignition & Combustion properties of Fuel Oil
Parameters
Ignition-combustion properties are indicative of the fuel oil quality which is a vital aspect in maintaining optimized engine performance and operation. While there are other important parameters such as engine design, engine condition, settings, applied load, ambient condition, and fuel pretreatment, both ignition and combustion properties are very much dependent on the composition and quality of the fuel.
Calculated Carbon Aromaticity Index (CCAI)
The first parameter introduced to evaluate the fuel ignition combustion properties was the Calculated Carbon Aromaticity Index (CCAI). A correlation between the calculated carbon aromaticity index (CCAI) and ignition delay was developed in the 1980s.
At that time, the marine fuel oils composed of straight run residue from atmospheric distillation processing. However subsequently, due to variations in the chemical composition of high sulfur fuel oil (HSFO), the correlation between CCAI and ignition delay became weak.
As a result, CCAI may not be an appropriate parameter to predict the fuel ignition quality of complex blends of 2020 fuels because of the wide variation in the fuel characteristics.
Estimated Cetane Number (ECN)
The Cetane Number is a measurement of the ignition quality of diesel fuel. Since there is no Cetane Number defined for the high sulfur fuel oil, Estimated Cetane Number (ECN) is used to understand the ignition and combustion properties of HSFOs.
This is determined from the IP 541 test method using the Fuel Ignition/Combustion Analyzer (FIA-100 FCA)2. The estimated cetane number (ECN) is calculated using the ignition parameter – Main combustion delay (MCD) via the FIA/FCA.
Experimental procedure
ECN Testing
In this study, the ignition combustion properties are evaluated based on the IP541/06 method by using FIA-100 Fuel Combustion Analyzer (FCA). The pressure increase is measured in order to determine the parameters related to the main phases; Ignition, main combustion, and after burning (After burning period) (Shown in Figure 1).
Figure 1. DESCRIPTION OF PARAMETERS FROM TEST METHOD IP541/06
Ignition delay (ID) period is the period of inactivity between the time when the first drop of fuel hits the hot air in the combustion chamber and the time it starts the actual burning phase/combustion. Main combustion delay (MCD) shows the time delay that the main combustion process begins and reaches 10% of the maximum generated pressure of the entire process. A long ignition delay and main combustion delay may result in the accumulation of unburned fuels in the combustion chamber. Furthermore, a long combustion period and after burning period may result in exposing the cylinder liner to a high temperature which will cause liner wear, scuffing, and engine damage.
Combustion property of fuel corresponds with the second half to the end of combustion (EC) and extended combustion period while the ignition property is defined based on the start of the combustion. Long After Burning Period (ABP) leads to increased probability of soot and particulate matter emissions, deposits, and clogging of the exhaust gas system.
Figure 2 shows the recommended operational ranges for the ECN parameter by the CIMAC committee Working Group on Fuels for HSFO. The case for VLSFOs can be extrapolated from this. It is not possible to give definite thresholds for the ignition performance of VLSFOs due to various influencing factors such as changing components making up a VLSFO, engine sensitivity, engine ignition performance as well as engine load.
Figure 2. RECOMMENDED ECN OPERATIONAL RANGES
Results and discussion
VLSFOs have wide variability in their characteristics such as viscosity, which can range between 2.5cst to 700cst; density that can range from 830 to 1037 kg/m3, and MCR that can range from 0.01% to 17% (The Viswa Group data). For a better understanding of the ignition & combustion properties, the samples considered for the study were divided into two categories:
- Paraffinic and
- Aromatic VLSFOs.
The criteria for fuel to fall in these two categories is given below.
Paraffinic VLSFOs have low to medium viscosity and medium-density. They have a higher content of long saturated hydrocarbon chains. Conversely, Aromatic VLSFOs have higher density and viscosity. Aromatic VLSFOs contain more benzene rings within their structure, which makes them more stable and less prone to thermal degradation in comparison with the paraffinic VLSFOs. However, higher aromatic content reduces the combustibility of the fuels and increases the unburnt component in the combustion chamber. Therefore, the VLSFOs with higher aromaticity are more likely to have poor ignition combustion properties.
ECN distribution
The distribution of ECN values is divided into the following ranges, <10 (poor ignition combustion properties), 10-15, 15-30, 30-40, and >40 (very good ignition combustion properties) and is shown in Figure 3. For this study, The Viswa Group had tested around 120 VLSFOs from different suppliers and sources (both paraffinic and aromatic). Based on this data, only 2% of all the tested samples had ECN values below 10, which is considered as having poor ignition quality. 66% of all tested VLSFOs had ECN values above 30 which is an indication of very good ignition properties.
Surprisingly, some of the fuels that had caused issues such as liner wear & black smoke also had ECN values considered to be good. Investigations will continue as more and more VLSFO fuels are received.
Figure 3. DISTRIBUTION OF ECN VALUES FOR VERY LOW SULFUR FUELS*
*Based on 120 samples
The distribution of ECN values of the two categories of VLSFOs, the Paraffinic, and the Aromatic is shown in Figure 4. Paraffinic VLSFOs are expected to have better ignition and combustion quality, which is consistent with the results that were obtained on the ignition combustion studies on these VLSFOs. None of the Paraffinic VLSFOs had poor ECN values, while 4% of the Aromatic VLSFOs had ECN values below 10.
FIGURE 4. ECN VALUES DISTRIBUTION FOR PARAFFINIC AND AROMATIC VLSFOs
The Viswa Group has so far tested over 50,000 number of VLSFOs from numerous sources, multiple blending procedures, and several global bunkering ports. 65% of these fuels tested contained a more paraffinic base and were classified as Paraffinic VLSFOs. Consequently, they had better ignition combustion properties in comparison to the Aromatic VLSFOs.
Impact of fuel’s poor combustion property
Poor combustion property of fuel can be as a result of a prolonged combustion period. This causes an increased level of soot deposition in the combustion chamber, on the exhaust valves, in the turbocharger system, the exhaust after-treatment devices, waste heat recovery units, and other exhaust system components.
Additionally, when there is an extended combustion period, the cylinder liner surface is exposed to high temperatures seen during combustion for a longer time.
Unburnt fuel deposit
Accumulation of unburnt fuel and deposits in the piston ring groove will restrict the free movement of the piston ring which leads to disruption of the cylinder lubricant film, abrasive wear, and scuffing.
Engine damage
Fuels with poor ignition combustion properties have been reported to cause engine damage and total breakdown in extreme cases.
Table 1 shows the average values of ignition – combustion parameters calculated based on 120 VLSFOs vs 415 HSFOs.
The test results
The results indicate that all the VLSFO ignition combustion parameter values (ID, MCD, EC, and ABP- these parameters are shown in Figure 1) are less than HSFOs. Consequently, VLSFOs have better ignition (shorter MCD) and combustion properties (shorter ABP) when compared with HSFOs. Examples of fuels with good and poor ignition and combustion properties are shown in Figure 5.
TABLE 1. AVERAGE VALUES FOR IGNITION COMBUSTION PARAMETERS OF HSFOS (415 SAMPLES) AND VLSFOS (120 SAMPLES)
ID, Ignition delay: The ignition delay is the time from the fuel injection to that point in time where the pressure increase, relative to the starting pressure, reaches 1% of the maximum pressure recorded at end of combustion.
MCD, Main combustion delay: The main combustion delay is the time from the fuel injection to that point in time where the pressure increase, relative to the starting pressure, reaches 10% of the maximum pressure recorded at the end of the combustion.
ABP, After burning period: After burning occurs when the final phase of combustion extends over a long period. After burning creates high cylinder and exhaust temperatures and may cause overheating of the engine in severe cases.
FIGURE 5. EXAMPLES OF FUELS WITH GOOD AND POOR IGNITION AND COMBUSTION PROPERTIES3.
Conclusion
Based on FIA testing of 120 VLSFOs from paraffinic and aromatic sources, we can conclude that:
- Most of the tested VLSFOs have very good ignition and combustion properties as well as a normal after burning period.
- Only 2% of the tested VLSFOs have ECN values below 10, which is an indication of poor ignition combustion properties.
- The VLSFOs are a heterogeneous group that can be divided into two groups namely Paraffinic and Aromatic VLSFOs. The Paraffinic VLSFOs have better ignition combustion quality when compared with Aromatic VLSFOs. Among the Paraffinic VLSFOs, there are no fuels with ECN values below 10 and only 4% of Aromatic VLSFOs have ECN values below 10, validating the generally good ignition combustion properties of VLSFOs as a group.
- However, several cases of liner wear damage, some scuffing damage & a few cases of black smoke have been reported to us after the use of VLSFOs in the last 9 months and the cause ascribed has been related to poor ignition combustion properties of the VLSFOs. Our study shows that VLSFOs as a group generally have good ignition combustion properties. We have covered the comprehensive description and analysis of these problem cases in our white paper titled “Excessive liner wear while using the new VLSFO fuels” and the Addendum and clarifications to our white paper titled “Excessive liner wear while using the new VLSFO fuels”. Research carried out by Viswa Group indicated that Cylinder Lubricating Oil (CLO) quantity, TBN number, and feed rate of 0.6 g/KwH of CLO may be too high for VLSFOs which may be the cause of liner wear and scuffing issues. Therefore, it is our belief that it is necessary to choose the correct CLO with TBN and feed rate in order to avoid liner wear. We also believe that the scuffing issues which can result in deposit formation and accumulation on piston ring tips are unlikely to be related to poor ignition and combustion properties of the fuel. The lube oil manufacturers have created a new base cylinder lube oil which is expected to have improved detergency properties. Further studies need to be carried out with these new cylinder lube oils to confirm that the improved detergency properties will help reduce liner wear and scuffing issues. Other problems experienced such as black smoke issues will also need further investigation when using VLSFOs.
Acknowledgment
Viswa Group gratefully acknowledges materials used from CIMAC and MAN articles.
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Source: Viswa Group
