A number of countries are planning to transition from coal to gas as the fuel of choice for their power sector, since coal will have some limitations regarding environmental enhancement, quality and supply constraints, noted Marie-Claire Soobramanien, regional stationary natural gas product manager at Lubrizol. Consequently, this should result in higher demand for gas piston engines and gas engine oil additives.

These additives, however, must meet stringent operating conditions and adapt to changing equipment designs. Landfill gas engine oils have an added challenge: curbing the damaging effects of contaminants in landfill gases. Oil formulators, additive makers and an original equipment manufacturer shared insights with LubesnGreases about the unique challenges that landfill gas engine oils face.

Waste Gas Basics

Landfill gas, also called biogas, is generated due to anaerobic decomposition, a process in which bacteria break down biodegradable material in the absence of oxygen to produce methane-the primary component of natural gas-carbon dioxide and other organic compounds.

Municipal solid waste landfills are the third-largest source of human-related methane emissions in the United States, making up about 15.4 percent of these emissions in 2015, according to the U.S. Environmental Protection Agency. The agency estimated that about 575 municipal solid waste sites in the country provide landfill gas to 469 operating power generation projects.

Biogas has the advantage of being a renewable source compared to its fossil fuel counterpart. Landfill gas production starts one to two years after the waste is deposited in the landfill and lasts 15 to 25 years. The continuously decreasing landfill gas volume can be compensated by the disposal of additional waste during this period, United Kingdom-based Clarke-Energy noted on its website. Clarke is a distributor and service provider for General Electrics Jenbacher reciprocating gas engines.

Landfill power generation projects make use of reciprocating engines to convert the gas into electricity. This type of engine contains a cylindrical combustion chamber where a close-fitting piston travels the length of the cylinder when the gas is ignited. The piston connects to a crankshaft, and the linear motion of the piston is transformed into the rotating motion of the crankshaft, according to a fact sheet from the U.S. Department of Energy.

The reciprocating engine is the most commonly used conversion technology for landfill gas electricity applications because of its relatively low cost, high efficiency and size ranges that complement the gas output of many landfills, the EPA expanded.

Gas engine lubricants need to provide protection to valves, gears, bearings, cylinders, liners and piston rings through acid neutralization and control of oxidation and nitration over longer drain intervals. A correctly balanced formulation will not only reduce the risk of ring sticking, bore polish, filter-plugging and bearing damage, but will also help operators keep their engines running efficiently for longer, regardless of fuel or base stock chosen, additive maker Infineum noted in its 2018 Infineum Insight presentation.

Taming Deposits

Biogas from landfills or digester power plants have a number of contaminants: sulfur compounds such as hydrogen sulfide, halides that include fluorides and ammonia, and organosilicon chemicals called siloxanes. All of these contaminants create deposits that generate wear and corrosive acids that can lead to oil degradation and affect the equipments durability, said Hind Abi-Akar, fluids technical expert for Caterpillars fluids engineering unit.

Corrosive acids can accumulate in the gas engine oil and attack copper-containing components inside the engine, including oil coolers and bearings, leading to corrosion, explained Gary Mueller, engineering manager for gas engines at Caterpillar.

The second problem is deposits in the combustion chamber and exhaust system caused by siloxanes in the landfill gas, he continued. These deposits build up on piston crowns, exhaust valves and turbochargers, which reduces the overhaul interval and can cause premature failures.

Often found in decomposed plastic and gel components of biomedical and cosmetic products, siloxanes generate silica deposits that can reach the combustion chamber of a gas engine, coating hot surfaces of engine parts with a thick, hard and abrasive layer, said Virginia Carrick, global stationary natural gas technology manager at Lubrizol.

She explained that high levels of silica deposits in the engine can increase oil consumption, trigger pre-ignition, generate abrasive wear and make top-end overhauls and piston ring and liner replacements more frequent.

To remove silica deposits, landfill engine operators have to mechanically clean them from the engine. This is time-consuming, resulting in increased engine downtime and added maintenance costs for the engine operator, Carrick elaborated.

Its safe to say that the problem will only get worse. The quantity of siloxanes entering landfills-based on trends in patent applications for silicone based products and siloxanes in consumer products-is estimated to increase by approximately 29 percent in 10 years and by 78 percent within 25 years after, found a study by authors B. Tansel and S.C. Surita published in the International Journal of Environmental Science and Technologys April 2017 issue.

Equipment at Risk

To reduce emissions without sacrificing performance, original equipment manufacturers have made changes to their reciprocating gas engine designs, including tighter tolerances and reduced clearances for components, higher compression ratios, increased turbocharger pressures and improved combustion and valve timing, noted Infineum.

Mueller pointed out that Caterpillars landfill gas engine designs are similar to those for natural gas, but that they optimize the lubricating oil and engine coolant temperature, and have additional crankcase ventilation to reduce corrosion generated by landfill gas combustion.

Piston rings, which maintain the compression in the engine to prevent blow-by of contaminants from getting into the oil in the crankcase, are a hard component to lubricate, said Andy Donlon, Americas oil and gas lubrication engineer at ExxonMobil. That top compression ring is the one you have to watch closely and get the right viscosity for, because it has the highest load, the highest temperature and the most friction.

Valves are another component requiring a formulation that can prevent both corrosion and valve recession from occurring. Donlon explained that valves are cushioned by the sulfated ash that forms once the engine oil additives are burned at high temperatures in the combustion process.

This white sulfated ash coats out on the exhaust valve face and cushions it from recessing into the valve seat in the head, he noted. In landfill gas engines, the valves, and specifically the exhaust valve, is the main component attacked by the siloxane.

Oils Adapt

As a result of rising operating temperatures and pressures, as well as the presence of contaminants, both additive and oil suppliers have to continuously develop gas engine lubricants that address the harmful effects of landfill gases on equipment.

The biggest change over the past several years is a preference from North American OEMs such as Caterpillar and Waukesha for oils that contain less than 0.5 percent sulfated ash, said Shawn Ewing, technical coordinator, commercial products at Phillips 66. Historically, higher ash fluids were used in this application to counter the effects of hydrogen sulfide, but the additional sulfated ash led to deposits on the engine valves, resulting in failures, he explained. Phillips 66 released its El Mar LF-D gas engine lubricant in January, which has low ash content and is zinc free to minimize deposits in modern landfill applications.

Similarly, Lubrizol launched its SG9L60 additive package in August, which breaks down silica particles into smaller molecules that are then suspended in the oil. It also enables high-temperature oxidation control, resistance to nitration and deposits, as well as increased phosphorus retention, said Carrick.

Caterpillar recommends equipment operators use gas engine oils with an additive package that has an adequate total base number to address these issues and that can prevent deposits on pistons, liners and cylinder heads, said Abi-Akar. The oils must be full synthetic or semi-synthetic, SAE 40 viscosity with minimal ash content to reduce deposit buildup on valves and turbine wheels of the turbocharger. At the same time, they must demonstrate adequate protection in a controlled, 7,000-hour field evaluation, since there is no industry specification for these types of engines, she added.