Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
The lubricating oil in a gas engine burning biogas is often more stressed than the same oil in the same engine running on natural gas. The additional stress is caused by the trace contaminants present in the biogas. Dependent on the source of the biogas, it may contain acidic compounds. These compounds can corrode engine components if not neutralized by alkaline additives in the lubricating oil. Because of the consumption of these additives, the oil in a biogas engine needs to be more frequently changed than the oil in an engine burning natural gas. Other contaminants such as siloxanes have a direct influence on deposit formation in the engine’s combustion chamber. In order to control the total level of deposits, the lubricant’s own contribution to deposit formation should be minimized. It is therefore beneficial if the lubricant is able to prevent formation of ash deposits as much as possible.
In order to serve the increasing market of biogas engine operators better, Shell Lubricants is introducing a new lubricating oil for biogas engines Shell Mysella S5 S.
This paper describes the challenges that different qualities of biogas impose on the engine and the lubricating oil. It describes what properties a lubricating oil for biogas operation should ideally have, and different formulation routes to achieve these. The paper will compare the performance of different candidate formulations, and explain how the new oil will help the operator to reduce cost of operations.
2. Challenges when operating an engine on biogas Biogas from anaerobic digestion consists of methane (CH4) and carbon dioxide (CO2) and a number of trace compounds. There are three particular challenges related to the combustion of biogas in gas engines:
2.1. Presence of acidic compounds in the fuel gas Biogas may contain acid producing species such as hydrogen sulphide (H2S), hydrogen fluoride (HF) and hydrogen chloride (HCl). The H2S is found in all types of biogas, but especially in biogas produced from agricultural material, manure and sewage, whereas HF and HCl are typically found in landfill gas.
After combustion and in combination with water these species can form sulphuric acid (H2SO4), hydrofluoric acid (HF) and hydrochloric acid (HCl). These are highly corrosive to engine components such as liners, piston rings, piston ring grooves and bearings, and must be neutralized by the lubricating oil before doing any harm. For this reason, the lubricating oil contains alkaline additives that will react with the acids when they get into contact with the oil film before they can reach metal surfaces. Because of this neutralization reaction, the alkaline additives in the oil are being consumed whilst the oil is in service, and the oil needs to be changed when the alkaline additives have been depleted.
The alkalinity reserve of a lubricating oil is represented by its base number (BN). Since every engine burns a small amount of lubricating oil, and since many types of alkaline additives are ash producing when burnt, they contribute to the formation of ash
24 LUBE MAGAZINE NO.124 DECEMBER 2014 Figure 1. Severe deposits originating from siloxanes on the piston crown. Potential consequences are:
• The deposits on piston crown and on the valve disc reduce the clearance between these components, and there is the risk that valve and piston crown touch each other.
• As a result of the deposits, the compression ratio increases, which can promote detonation (also called knocking).
• Because of the chemical composition, the deposits are very hard and abrasive. They also have a different coefficient of thermal expansion from metal. As a result of temperature changes, parts of the deposit layer will break off from the surface of piston and cylinder head. These parts may get trapped between ring and liner where they are ground and contribute to high wear rate of these components. This is demonstrated in figure 2.
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deposits in the combustion chamber. For this reason, the engine manufacturers (collectively called OEMs in this paper) limit the amount of ash producing additives in the lubricating oil. Most OEMs limit the ash to 0.6%, such oils are called low ash oils. Some OEMs allow oils with up to 1.0% ash, such oils are called medium ash oils.
Because of the limited amount of alkaline additives in the fresh lubricating oil, the achievable life time of a given oil is highly dependent on the amount of acidic species in the fuel gas.
2.2. Presence of siloxanes in the fuel gas A siloxane is a (gaseous) hydrocarbon molecule with a silicon (Si) atom in it. Siloxanes are typically found in sewage gas and in landfill gas.
When combusted, the silicon atom is joined with oxygen atoms to form silicon dioxide (SiO2), the chemical formula of sand and glass. The silicon dioxide is being formed in the combustion chamber, and deposited on the surfaces in the combustion chamber, such as the piston crown, the cylinder head flame bottom and the valve discs. An example of severedeposits originating from siloxanes is given in figure 1.
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