Severe gas operation

Right across the globe the use of natural gas for power generation is increasing. And, as well as clean natural gas, waste gases from landfill sites and biogas digesters are being used as a profitable way to recycle man-made and natural resources.

At landfill sites, bacteria decompose the buried solid municipal waste - a process that releases methane gas. Biogas is methane that is generated from the biodegradation of organic material like animal and vegetable waste, sewage, waste paper and wood chips.

Low-value waste gas streams can be captured and burned in reciprocating gas engines to generate electricity, which can offer good returns to gas engine operators.

These engines are run continuously and at full load, which means maintenance downtime time must be minimised in order to avoid revenue loss.

Sour gas challenges

Compared to clean natural gas, which comprises nearly pure methane with other clean-burning alkanes, these so-called ‘sour gasses’ are roughly half methane and half carbon dioxide, and contain a number of impurities. Both types of fuel contain significant quantities of sulphur and landfill gas often contains halogens, both of which react to form strong mineral acids during the combustion process. These acids must be neutralised by the oil to prevent engine damage.

Sour gases contain a number of impurities

In addition, both landfill gas and biogas from sewage treatment works contain siloxanes, which are widely used in the cosmetics industry and in some plastic and food products. As its name suggests, siloxane contains silicon, oxygen and alkane components. On combustion, these produce silica, which can form hard deposits of sand-like particles in the engine. These deposits build up on the cylinder heads and valves and may interfere with proper intake and exhaust valve operation, which can result in valve torching. In addition they can cause abrasive wear and provide an insulating layer, which prevents the lubricant from transferring heat away from the engine.

When the build up of deposits becomes too great, they must be manually removed, which can shorten engine maintenance intervals, increase overall maintenance costs and increase engine downtime – significantly reducing profitability for the operator.

Engine oils tailored for sour gas service must control strong acids and deposits to a much greater extent than those designed for engines burning clean natural gas.

Gas engine oil considerations

One of the main considerations when developing a gas engine oil for use in these conditions is base and acid number control. And a wide variety of components are available to help the gas engine lubricant formulator.

There are for example detergents with different levels of base number (BN) and soap content, different metals and different chemistries (e.g., sulphonates, phenates, salicylates). There are also dispersants with different molecular weights and head groups. And lastly there are a number of antioxidants and corrosion inhibitors to choose from.

Developing an oil suitable for use in severe sour gas applications requires a careful balance to ensure that changing a component to improve one performance attribute does not have a detrimental effect on another.

Acid neutralisation, for example, can be controlled by the proper selection of the type and amount of basic detergent. This key component not only neutralises acids to protect the engine from corrosion but also provides a solid ash residue, which is used to lubricate the intake and exhaust valves. However, it is essential to strike the right balance here. Too little detergent can result in corrosion, shorter oil drain intervals and valve recession, too much detergent can result in excessive ash deposits that may cause valve or cylinder damage or give rise to engine pre-ignition.

Depending on their chemistry, detergents have different base durability, which means some detergents can provide longer protection from corrosive acid attack before their base reserve is depleted, thus enabling longer oil drain intervals.

A well-balanced oil also needs sufficient dispersancy so that silica particles are kept in the oil rather than forming hard deposits on cylinder heads and valves.

Superior chemistry

Infineum uses its unique salicylate technology to formulate gas engine products because it combines cleanliness and antioxidancy benefits together with superior acid neutralisation and deposit protection.

The clear advantages of salicylate technology in the formulation of gas engine oils have been proven in around three million hours of field reliability with all major engine manufacturers. Using this extensive field experience, Infineum has developed carefully tailored products using salicylate chemistry to deliver the right level of protection to gas engines running on sour gas. One of the key parameters in these applications is acid number control and here the new technology performed well in recent field trials, indicating the potential to extend oil drain intervals.

Control of acid number reduces corrosion and offers the potential to increase oil change interval

A number of additional benefits have also been demonstrated in field and laboratory testing including:

• Better viscosity control
• Superior base number retention
• Excellent oxidation control
• Excellent dispersion of silica to prevent deposits
• Superior control of engine wear

The improved control of cylinder head deposits that has been demonstrated in over 8,000 hours of field operation is of particular value since it reduces valve torching and will enable operators to increases the time required between overhauls.

In field testing in a landfill gas-to-energy facility the use of Infineum technology combined with sensible oil management practices has delivered substantial savings in oil costs and reduced maintenance costs, including cylinder head cleaning, top-end overhauls, in-frame overhauls, and major overhauls.

Conclusion

Gas engine oils specifically tailored for sour gas operation can deliver a number of benefits to the end operator. With the key drivers of delivering advanced protection to increase engine up time, Infineum has developed products that can also be tailored to deal with the severity of each gas type in a variety of engines.

The advanced acid number control and deposit prevention mean that on-stream rates can be increased significantly to maximise revenue. At the same time, because the time between cylinder head cleaning, top-end and major overhauls and oil change intervals can all be extended, maintenance and oil costs can be reduced to improve profitability.

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