Many types of organic material can be used to produce combustible gas, including organic waste, agricultural biomass, livestock manure, sewage cleaning residue and landfill. In an oxygen-free environment, bacteria digest the organic material to produce what is known as biogas, consisting of methane, carbon dioxide and various trace constituents.
Biogas is converted into electricity by gas engines that typically operate at efficiencies of greater than 40 percent and produce relatively low levels of harmful exhaust emissions. Gas engines can run reliably on different qualities of biogas, although engines running on certain gases require more maintenance than those running on natural gas.
Engines have been running successfully on biogas in a large number of installations for many years. For example in 2007, a leading supplier of biogas engines, General Electric installed more than 1,450 systems in agricultural settings and 1,300 in landfill applications, totaling more than 2,200 megawatts of electrical capacity. In addition, several industry sources predict large growth in the coming decade.
The lubricant in engines burning biogas is often more stressed than the same oil in an engine running on natural gas. The additional stress is caused by trace contaminants in the gas. Depending on the source, the biogas may contain acidic compounds that can corrode engine components if not neutralized by alkaline additives. As these additives are consumed, the oil needs to be changed more frequently.
Other contaminants such as siloxanes have a direct impact on deposit formation in the combustion chamber. To control deposits, the lubricants contribution to deposit formation should be minimized. Therefore, the lubricant should prevent the formation of ash deposits as much as possible.
Biogas Challenges
Biogas from anaerobic bacterial digestion consists of methane, carbon dioxide and a number of trace compounds. Three challenges related to the combustion of biogas are acidic compounds, siloxanes and trace compounds.
Acid producing compounds include hydrogen sulfide, hydrogen fluoride and hydrogen chloride. Hydrogen sulfide is found in all types of biogas, but especially in that produced from agricultural material, manure and sewage. Hydrogen fluoride and hydrogen chloride are typically found in landfill gas.
After combustion and in combination with water, these compounds can form sulfuric acid, hydrofluoric acid or hydrochloric acid that are highly corrosive to engine components such as liners, piston rings, piston ring grooves and bearings. They must be neutralized by the oil to prevent damage.
For this reason, the oil contains alkaline additives that react with acids before they can reach metal surfaces. Neutralization consumes the additives, and the oil needs to be changed when they have been depleted.
The alkalinity reserve of oil is represented by its base number. Every engine burns a small amount of oil, and many types of alkaline additives produce ash when burned that can contribute to deposits forming in the combustion chamber. For this reason, engine manufacturers limit the amount of ash producing additives in the oil.
Most OEMs limit ash to 0.6 percent, and such oils are called low-ash oils. Other OEMs allow oils with up to 1.0 percent ash, and these oils are called medium-ash oils. Because the oil contains a limited amount of alkaline additives, oil life is highly dependent on the amount of acidic compounds in the fuel gas.
Siloxane is a gaseous hydrocarbon molecule containing a silicon atom, and siloxanes are typically found in sewage and landfill gases. When combusted, silicon atoms join with oxygen to form silicon dioxide, the chemical formula of sand and glass, which can deposit on the piston crown, cylinder head flame bottom and valve discs.
These deposits cause a number of problems. Piston crown and valve disc deposits reduce the clearance between these components, potentially causing interference. Deposits also can increase compression ratio, which can promote detonation (also called knocking).
Deposits are hard and abrasive, and they have a different coefficient of thermal expansion than metal. Temperature changes can cause parts of the deposit layer to break off from the piston and cylinder head, and they can get trapped between the ring and liner where they are ground up and contribute to high wear.
Other contaminants include ammonia and arsenic. Ammonia can potentially attack the nonferrous bearing metals.
While technologies have been introduced to remove contaminants from biogas, they are still relatively new and are quite expensive to install and operate. Therefore, the vast majority of installations run without such systems.
Limiting Oil Life
Oil properties change as a result of oxidation, nitration, reduced alkalinity, acid concentration, increased viscosity and increased contaminants. Oxidation and nitration are the reaction of hydrocarbon molecules in the base oil with either oxygen or nitrogen oxides to form weak acids and polymerization products. Reduced alkalinity reserve is normally caused by the neutralization of the acidic products of oxidation and nitration.
Although most acids formed by oxidation and nitration are neutralized by the oils alkaline additives, some acids are so weak that they do not react with the additives. As long as their concentration is low, these acids are too weak to harm engine bearing metals.
Total acid number can be used as an indicator of the amount of acid in the oil. Because even fresh oil will yield a TAN value when tested, the concentration of acids is best represented by the difference between the TAN of the used oil and that of the fresh oil. An additional measure is pH number, which describes the strength of the acids accumulated in the oil. Finally, polymerization products formed during oxidation and nitration can increase oil viscosity.
It is good practice to take regular oil samples to determine oil condition and rate of deterioration and, thereby, to determine safe oil drain intervals. This is even more important in biogas applications because fuel quality can vary significantly over time.
Oil analysis can also detect premature wear in the engine, notably bearing wear and cooling water leakage. Therefore, it can provide additional safety and peace of mind for the operator. General criteria for oil rejection are:
Maximum oxidation of 20 cm-1
Maximum nitration of 20 cm-1
Base number – minimum 50 percent of fresh oil base number
Total acid number – maximum 3 milligrams potassium hydroxide/ gram increase over fresh oil
Minimum pH of 4
Viscosity at 100 degrees C of 12.0 to 17.5 square millimeters per second
A discussion about allowable levels of contaminants is beyond the scope of this article with the exception of silicon. Silicon accumulation in the oil is normal at plants where siloxanes are present in the fuel gas. Sometimes, customers use silicon content as a criterion for oil drain because they see a correlation between silicon level and engine wear rate. However, while the correlation exists, it is not a cause and effect correlation. Both conditions are a consequence of high siloxanes in the fuel.
If the oil filtering system removes particles effectively, any silicon in the oil is harmless. Some OEMs limit silicon in used oil to 300 parts per million; others state that silicon content has no limit.
Lubricant Formulation
Lubricants formulated for gas engines burning biogas must first be suitable for use with gas from any source, including biomass, manure, sewage and landfill. And they must be able to handle the contaminants that these different types of biogas may contain. Also, ash content must comply with the latest OEM requirements, and the oil must provide significantly longer life than conventional oils. Finally, it must provide a high level of engine protection and be suitable for use in engines equipped with exhaust gas catalyst.
The following approach has been used to develop candidate formulations. First, formulate the oil to retard oxidation and nitration. Slower oxidation contributes directly to longer oil life. Second, make base number as high as possible to maximize the oils alkalinity reserve and extend oil life for engines operating on gases containing acidic compounds. Third, minimize the oil contribution to deposit formation by limiting ash producing additives to 0.6 percent and by formulating the base oil and additive combination to be highly resistant to oxidation and carbonization.
To meet all these criteria, modern lubricants are typically formulated with severely processed API Group II base oils. This approach has been endorsed by a leading OEM that explicitly prefers Group II based lubricants for its engines running on biogas.
The table compares the characteristics of three candidate formulations, based on a different additive technology, with a reference oil. Following laboratory bench tests and in-house engine tests, the candidate oils were tested in GEs Jenbacher J312 GSC21 installed on a landfill site. The installation was not equipped with fuel gas cleaning, but fuel acidity was relatively mild. An exhaust gas cleaning catalyst was installed.
The rate of oil oxidation was similar for the three candidates. Even after 2,000 hours, oxidation was below 10, which is considered excellent. However, the oils exhibited very different base number depletion rates. In fact, rapid base number depletion of Oils C and D produced shorter oil life than Shell Mysella S5 S, despite their higher initial base numbers.
With Oil C, oil drain interval would have to be set at 1,200 running hours, and for Oil D drain interval would have to be set at 1,500 hours. Mysella S5 S, however, can safely reach 2,680 hours, and extrapolation of the oil condition trends predicts that it could safely reach 3,000 running hours.
Comparisons of oil life with the benchmark oil at this site show that only Mysella S5 S, despite its low initial base number, provides significantly longer oil life than the benchmark. This is due to its high oxidation resistance, combined with the relatively mild acidity of the fuel gas. If the fuel gas had a high content of aggressive acidic compounds, a slow rate of oxidation would not contribute to the oil life extension. Instead, the oils alkalinity reserve would neutralize the acid compounds to extend oil life.
Many OEMs, however, do not support the use of medium-ash oils and recommend the use of low-ash oil even for aggressive landfill gases. In such cases, the new oil would be expected to provide longer oil life than conventional low-ash oils because its slow rate of oxidation leaves more base number available for acid neutralization.
Protecting the Engine
For engines running on landfill gas, component condition is a function not only of running hours, but it is also strongly dependent on fuel characteristics. Besides neutralizing acid compounds, another important effect is the formation of hard deposits because of siloxanes in the fuel. The oil has limited influence on deposits because they are formed during fuel combustion with minimal interaction of the oil. One area where the oil can contribute is in minimizing the formation of oil-related deposits such as ash and oil coke.
To check the deposit performance, the combustion chamber was inspected by boroscope after 3,670 running hours. The inspection showed that liners were in excellent condition with hardly any wear of the honing pattern. A few liners were scratched by hard silicon dioxide particles. The good condition was also confirmed by the engines low and stable oil consumption. Deposits in the combustion chamber were well controlled, ash layers were relatively thin and no large pieces of solid material broke out of the deposit layer.
Based on this testing, Shell has commercialized Mysella S5 S and expects it to provide good engine protection because it has low ash content, reducing the contribution of lube oil to combustion chamber deposits, and it is formulated with Group II base oil, which helps to minimize ash and carbon deposits.
Richard Holdsworth is Global Product Marketing Manager – Power and Industry, Shell International Petroleum Co. Ltd, U.K., he can be contacted at R.Holdsworth@shell.com.
Joerg Reher is Project Leader Marine & Power Engine Lubricants Development, Shell Global Solutions (Deutschland) GmbH, Germany.
Thijs Schasfoort is Product Application Specialist – Lubricants for Stationary Engines, Shell Global Solutions International B.V., Netherlands.