There are immediate benefits to using biofuels, such as reduced SOx, NOx and GHG, but there are challenges too, say Nikolaj Kristensen, R&D Manager, and Bjørn Christian Dueholm, Industrial Ph.D. student, both at Hans Jensen Lubricators.
The two main types of biofuels that are being considered for use in marine two-stroke diesel engines at this time are Hydrotreated Vegetable Oil (HVO) and Fatty Acid Methyl Ester (FAME). These are both drop-in replacements and may be blended at various ratios.
Many of the thermodynamic properties of FAME and HVO are very similar to conventional diesel.
A lot of the thermodynamic properties of these biofuels are very similar to conventional diesel. However, FAME is prone to microbial growth, and the water content has to be strictly controlled when stored in the tank to prevent this. FAME also has quite a high oxygen content which increases the maximum pressure of the combustion, because it speeds up the air fuel mixing rate, resulting in a more violent combustion, which can speed up lubrication oil degradation.
It is important to monitor the lubricity of the fuel blend when using biofuels. FAME has a lubricity below 460μm without additives, which means it is more naturally lubricating. It is possible to use FAME as a lubricity additive in low-sulphur fuels. HVO requires additives to reach under 460μm. If the lubricity of the fuel becomes too high, cylinder liner wear rate can increase.
Typically, biofuels naturally contain acids. A study was done on the corrosion rate of biodiesel compared to conventional diesel, both pure and in different blends. This research has shown that there is a direct correlation between the total acid number (TAN) of the biofuels and the corrosion rate of the metals. This may affect fuel line components such as fuel pumps, pipes, and seals.
The corrosion rate of biodiesel is a lot grater for carbonated steel than for conventional diesel. Cylinder liners are made of grey cast iron which is similar to carbonated steel but with a higher carbon content (<2%). Grey cast iron is considered to be more corrosion resistant than carbonated steel, but this is due to grey cast iron developing a layer of oxidised material preventing deep corrosion. For engine conditions, maintaining such a layer might be difficult.
FAME has a higher TAN than HVO, but the corrosiveness of both is quite low compared to sulphuric acid. Similar to LSFO, where the small amount of sulphur cannot be ignored, the acid introduced by biofuels also has to be kept in check.
Good housekeeping
Generally, more attention should be given to proper housekeeping during handling and storage. This means that a fuel management plan should be made, tanks must be dewatered at all times and storage temperature must be controlled. Do not mix fuels, as incompatibility is a high risk. Do not store biofuel for long periods of time as they will degenerate over time. Do not store too cold, as wax may be formed. Do not store too warm as this will accelerate oxidation, polymerisation and microbial growth. If not handled properly this can adversely affect fuel line components, as well as cylinder condition.
For biofuels, issues arising with cylinder lubrication are similar to that of low sulphur fuel oil, requiring cylinder oil to have low BN and high detergency. The detergency can come from BN, but since the acid in the engine from biofuels is weaker, the cylinder oil should preferably be on the lower side in terms of BN. The oil needs to keep the cylinder clean of small particles and combustion products, while still neutralising the acid introduced by the fuel in the very top of the cylinder.
Thus, HJ SIP, non-intermittent lubrication, automatic cleaning sequence and /delta-timing are advantageous. If the biofuel is used as a blend, the issues relating to lubrication are dependent on the percentage of the blend and what it is blended with. If it is used as a pure fuel, it is assumed that the issues will be similar to LSFO, but there is little operational experience to confirm this assumption.
Therefore, it is imperative to have a flexible cylinder lubrication system. Hans Jensen Lubricators’ offer such a solution, the HJSmartlube 4.0,which offers stepless adjustment of the timing, volume and frequency of injection for each individual injection quill, with capability to utilise both HJ SIP timing and ring pack lubrication within the same engine revolution. New lubrication algorithms are developed continually, with the exact purpose of adapting to new lubrication requirements arising e.g., from the use of biofuels.
