Good lubrication is essential for long and reliable operations

The quality of biogas underlies considerable variations. Contaminations, as for example sulphur compounds, put high loads on the gas engine. Therefore it is important to carry out oil changes regularly and in time.

During the fermentation of organic substances methane is developed which can be transformed into electric energy in so-called cogeneration units. However, biogas also contains large amounts of carbon dioxide (25 to 45 percent) and further contaminants such as water, nitrogen, oxygen and hydrogen.

High loads on the engine
The cogeneration unit must cope with these conditions and the varying gas composition. High process temperatures and full-load operation day and night further increase the stress on the gas engine. Besides, contaminants can become even more "aggressive" because of the combustion process: sulphur compounds are transformed into sulphurous acid, for example. If the gas engine oil does not absorb and neutralise these acid components and hydrogen sulphide, this can lead to serious engine damages. In the worst case the cogeneration unit breaks down which causes high costs.

Relying on the right engine oil
The good lubrication of the engine is of utmost importance. Leading manufacturers offer special oils for these applications which display high thermal stability, good ageing characteristics, high operational safety due to engine cleanliness and protection against mechanical and corrosive wear. If such oils are applied, engine lifetimes and oil drain intervals can be prolonged.

But also the best engine oils underlie ageing processes. The possible oil drain interval depends on oil quality, oil volume, gas quality, operating conditions and engine and it varies.

"Oil drain intervals can be between 350 and 1500 operating hours, but also values beyond this range are possible" explains Jürgen Deckert, Head of Research & Development at the engine oil expert Addinol.

Regular checks
He recommends regular analyses of the gas engine oil to monitor its condition and to determine oil change intervals – not only for reasons of guarantee but also to extend engine lifetimes. Samples should be taken according to the recommendations/guidelines of the gas engine manufacturers.

Complex oil analyses (see article "Engine oil analyses" below) are offered by special companies. The costs for one analysis at the Oelcheck GmbH lie between 82.50 and 102.50 Euro (VAT excluded), for example. Oil manufacturers can make exact recommendations for their lubricant based on these data and information.
 
According to Deckert ADDINOL takes into consideration their experiences with gas engine oils and the limits defined by the OEM when determining the oil drain interval. By the help of a sequence of numbers and functions (matrix) the experts make their decision on an extension or shortage of the oil drain interval and the total oil service life. The matrix is based on all values which have been collected in comprehensive field tests with the single manufacturers.
 
Engine oil analyses
By the help of engine oil analyses a number of parameters can be defined and collected which say something about the further applicability of the lubricant.

• The oil condition is defined by the help of viscosity and viscosity increase and further values (e.g. oxidation). Usually a viscosity increase is caused by ageing processes (oxidation). Oxidation can also lead to the formation of organic acids which damage the cylinder-piston-complex.
• Oil acidification: The acid combustion products must be absorbed and neutralised by the engine oil.  The TAN (Total Acid Number or neutralisation number) indicates the acidification of the oil. The TBN (Total Base Number) shows the content of alkaline components in the oil and gives information on how many acid components can be neutralised. The TBN must always be higher than the TAN. For gas engines operated with gases of varying quality, the i-pH value provides additional information on a possible acidification of the lubricant.
• Wear elements: they help to determine the wear condition of the engine.
• Contaminants as for example by silicon in the form of dust often bring about deposits on thermally heavy loaded engine parts leading to power loss, wear and engine damages.

German article published in Österreichische Bauernzeitung 6/2009 (view German article here)

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