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Lubricating the Biogas Engine

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Biogas is an age-old byproduct drawing new interest from fuel-hungry power generators. Sewage plants, landfills, coal mines, decaying wood chips and other plant waste, even manure piles from livestock – all are rich sources of this flammable gas, which often is flared off as a nuisance. Once cleaned of impurities, the renewable fuel burns easily in natural gas engines to generate heat and electricity. But wider success with this fuel requires overcoming numerous hurdles, including the unique challenges that biogas presents to engine oils.

The principal component in biogas is methane, which is the same as fossil-fuel natural gas but is created by organic decomposition (rotting). Despite this chemical similarity, comparing natural gas engines to biogas engines shows strong differences in their operation and reliability, according to Rudiger Krethe of the condition monitoring firm Oel Check GmbH. Biogas also comes with a host of unwanted impurities, such as siloxane, sulfur and other minerals, and water.

Natural gas is a very clean fuel, with stable qualities, he explained. Biogas, by contrast, is not so stable in content. It contains lots of impurities which can be very aggressive. The quality varies according to the production process. And the cleaning process also has an effect on quality.

Over the past five years, installations of biogas power plants have bloomed, as industries and municipalities realized that their unwanted methane gas could be converted into saleable electricity. In the United States, stricter limits on uncontrolled emissions of methane also provide an incentive to capture the gas and harness it.

According to the U.S. Energy Information Administration, more than 400 landfill gas energy projects are operating in the United States now, led by California with 73 installations in place, Illinois (36) and Michigan (27). Industrial engine manufacturer Dresser Waukesha estimates that at least 600 more landfill sites in the United States are ripe for power installations. Additional projects extract gases from sewage, agricultural waste and lumber mills.

In Europe, the phenomenon has grown even faster. The number of engines burning biogas in Germany, for example, escalated sharply as power generators were encouraged to adopt renewable and carbon-neutral sources of energy. Ten years ago, there were fewer than 1,000 biogas engine units operating in Germany, said Krethe, who is based in Munich. Now there are more like 5,000 units in operation. From just 50 megawatts of biogas power generated in 1998, Germany now gets 1,600 MW – a 32-fold increase.

The gas engines used at cogeneration facilities may range from about 500 kilowatts to more than 5 MW in scale. And their uptime is being closely watched. Biogas engines are expected to operate 24/7 at high performance, particularly those used for power generation and combined heat-and-power cycles, Krethe pointed out in January to the International Colloquium Tribology at Germanys Technische Akademie Esslingen. In his address, Krethe presented research from his Oel Check colleagues Carsten Heine and Steffen Bois, and Jo Ameye of Fluitec in Belgium.

Their investigations suggest that bio-gas engine oil formulations present a big challenge for traditional oil monitoring practices. Longstanding oil analysis tools such as total acid number (TAN), total base number (TBN), elemental analysis and contamination counts are not sufficient any more to set oil change intervals or diagnose oil condition with biogas fueled engines, Krethe warned.

HOTTER, DIRTIER

To start, gas engine oils (whether biogas or natural gas) differ from those used in diesel and gasoline engines. The clean-burning process means theres less need for dispersant/detergent additives, and preventing viscosity increase is more critical. The process burns hotter (ranging from 165 to 235 degrees C), so oxidation and nitration are of high concern. Poor oxidation control can lead quickly to sludge formation, oil thickening, blocked filters, deposits and increased acidity. Other additives are needed to maintain alkalinity (to neutralize acids), combat foam and protect against wear and corrosion, but over-additizing increases the ash content, which may lead to more emissions and deposits.

For biogas operations, Krethe said, many headaches can be averted by using an oil made with good quality base oil, so synthetic and semi-synthetic engine oils are gaining favor. The oils also need to have high neutralization capability, and multiple antioxidants such zinc dithiophosphates, amines, phenates, phenols and salicylates.

The oil formulations components are like an orchestra, so if one cannot play well with others, you notice it very easily, he said. But the oil aging process with biogas is caused not only by oxidation. There are also three sub-processes at work to degrade the oil, including contaminants, additive depletion and base oil degradation, and each of these influences the others.

Oil change frequencies for natural gas engines usually vary from 500 to 1,500 operating hours. The use of contaminated biogases, plus high load factors on the engines, can significantly reduce these drain intervals, Krethe said. But why, and how much?

DRAINING QUESTIONS

To find answers, Oel Check began using Fluitecs hand-held Ruler device, which is based on linear sweep voltammetry, to track biogas engine oil antioxidant levels and compare them to the fresh oils. The researchers view antioxidancy as a clear measure of remaining oil life, and suggested that by mapping the Ruler data (along with TAN, TBN and other results), biogas engine operators can get a better picture of their oils condition and start to set appropriate drain intervals.

Other factors to monitor include wear metals, viscosity and contamination, but Krethe stressed that oxidation stability is one of the main limiting factors for the oil drain interval. Having this information early may alert users to take action before sludge, varnish and oil thickening can worsen.

Field trials in Germany with different biogas types and engine designs are beginning to bear out this proposal. One trial looked at eight Deutz 630-KW engines. With each ones reservoir holding 320 liters (84.5 gallons) of oil, these units burn methane gathered from fermenting vegetable waste. The testers tracked one of the engines for 6,500 hours, sampling the oil every 100 hours, and found strong correlation between the Ruler and TAN/TBN measurements. Additionally, the Ruler data helped pinpoint more rapidly which key antioxidants were losing ground. If you monitor only base number and acid number, you wont see all thats going on, Krethe noted.

Another field trial monitored a Jenbacher 312 biogas engine operating on a modern, zinc-free engine oil formulated with aminic and phenolic antioxidants. A subsidiary of General Electric, Jenbacher is one of the leaders in biogas engines and estimates it has 1,250 units operating on landfill gas worldwide.

The 312 is a natural gas-type engine holding 450 liters of engine oil, and rated for 3,000-hour drain intervals. In the biogas trial, its oils base number and acid number were still acceptable after 2,000 hours – but a closer look showed that the oils viscosity already was rising to unacceptable levels, long before the scheduled drain interval. By contrast, the Rulers measure of antioxidant depletion correlated well with the viscosity increase, and gave a far more accurate picture, earlier. Here we saw that when antioxidancy decreased, viscosity began to rise very strongly, so the operator could react earlier than the base number and acid number indicated, Krethe noted.

Positive results came from another field trial, with Jenbacher J 612 biogas engines, while a fourth example employed Waukesha F18 natural gas engines, which have a 400-liter oil reservoir. In both cases, good correlations could be seen between the antioxidant values and vital oil conditions such as viscosity and pH.

Wrapping up, Krethe reminded the colloquium that oil analysis requires a mix of different methods, especially as gas engines and fuels continue to proliferate. Theyre more in number and type, so were seeing more changes also to the gas engine oils. Acid number and base number alone dont give detailed information about the engine oil aging process any more.

Especially, monitoring the levels of individual antioxidants, along with other parameters, can give better understanding of the processes going on during the lifetime of biogas engine oils, he concluded.

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