Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
Figure 2. Abrasive wear caused by silicon dioxide deposits ground between rings and liner.
2.3. Presence of other contaminants in the fuel gas There can be other contaminants in the fuel gas, varying from ammonia (e.g. typically present in biogas from livestock manure) to arsenic. Ammonia can potentially attack the non-ferrous metals of the bearings.
2.4. Gas cleaning Technologies have been introduced to remove contaminants from the biogas. Because these technologies are still relatively new, they are quite expensive to install and operate. Therefore the vast majority of the installations runs without such systems.
3. Factors limiting oil life in biogas engines A finished lubricating oil consists of a base oil and selection of additives. Base oils exist in a large variety of qualities, ranging from refinery streams that have received basic treatment, to severely hydro treated streams to fully synthetic fluids. With the help of additives, the lubricating oil manufacturer tries to enhance, suppress or complement certain properties of the base oil. A finished lubricant is therefore a unique combination of base oil and carefully selected additives, designed to provide specific performance attributes that should help the end user to achieve best value for money.
When the oil is in service, its properties will change as a result of degradation processes. These are:
• Oxidation: this is the chemical reaction of the hydrocarbon molecules from the base oil with oxygen. As a result of this reaction weak acids are formed as well as polymerization products.
• Nitration: this is the chemical reaction of the hydrocarbon molecules from the base oil with nitrogen oxides (NOx). As a result of this reaction weak acids are formed as well as polymerization products.
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• Reduction of alkalinity reserve, as indicated by a reduction of the base number (BN) of the oil. BN depletion is normally caused by the neutralization of the acidic products of the oxidation and nitration reactions. However if the fuel gas contains acidic species, which is often the case with biogas, then this will accelerate the depletion of BN.
• Increase of the concentration of acids in the oil. Although most acids formed by oxidation and nitration reactions are neutralized by the alkaline additives in the oil, some acids are so weak that they do not react with these additives. Because of their weakness, and as long as their concentration is not too high, these acids are not harmful to engine (bearing) metals either.
The total acid number (TAN) can be used as an indicator of acids in the oil, where one should realize that even a fresh oil will yield a TAN value when tested, even if there are no acids present at all. The concentration of acids is therefore best represented by the difference between the TAN of the used oil and the TAN of the fresh oil.
An additional measure is the ipH number that describes the strength of the acids accumulated in the lubricating oil.
• Increase of the viscosity of the oil. This is mainly caused by the polymerization products formed during the oxidation and nitration reactions.
• Increased level of contaminants in the oil, such as Si, water, soot, other insolubles, etc.
It is good practice to take regular oil samples. Analysis of these samples will then indicate oil condition, the rate of oil deterioration and will help to determine safe oil drain intervals. This is even more important in biogas applications as fuel quality can vary significantly over time.
Also oil analysis can detect premature wear processes in the engine, notably bearing wear, and cooling water leakage, and can therefore provide additional safety and peace of mind for the operator.
The criteria for oil rejection that Shell applies for this development project are:
• Oxidation by FTIR in abs/cm: maximum 20 • Nitration by FTIR in abs/cm: maximum 20 • BN (ASTM D2896) in mg KOH/g: minimum 50% of fresh oil BN • TAN (ASTM D664) in mg KOH/g: maximum 3 increase over fresh oil TAN
• iPh (GE-Jenbacher method): minimum 4 • Viscosity at 100 °C in mm2/s: SAE40: min 12.0 or max 17.5
A discussion about allowable levels of contaminants is beyond the scope of this paper. But there is one exception: silicon (Si). Accumulation of Si in the lube oil is always seen at plants where siloxanes are present in the fuel gas. Sometimes customers use Si as a criterion for oil drain. Several customers have seen a correlation between Si in oil and engine wear rate. This is correct: the correlation is there, but it is not a causal correlation. Both are a consequence of high siloxanes in the fuel.
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