Environmental benefits of LNG as a marine fuel reduced by 'slippage'

Nov 22, 2011 12:00 AM

The use of liquefied natural gas (LNG) to power vessels has been touted as one of the best means for the maritime industry to meet strict new emissions regulations. The use of LNG as a fuel eliminates 100 percent of sulfur dioxide emissions and particulates and 80 percent of nitrogen oxides. It has been claimed that the use of LNG in engines could reduce greenhouse gas emissions by up to 25 percent, compared with conventional fuels.

However, recent research shows that the environmental benefits of LNG fuel may not be as significant as advertised. The incomplete combustion of methane in engine cylinders, known as methane slippage, can decrease, or in some cases, negate the CO2 reduction benefits of using LNG as a maritime fuel. That is because methane is much more powerful than CO2 as a greenhouse gas. Consequently, release of even small volumes of methane easily spoils potential environmental gains from reduced emissions of CO2.

"I think people automatically say that we can go to LNG and clean up the environment," said Jose Femenia, professor of engineering at the United States Merchant Marine Academy at Kings Point, N.Y. "You have to take that with a grain of salt."

Using LNG as a fuel"has to be done in a manner that is conscious of the fact that the slippage can simply negate the impact of using the LNG fuel," Femenia said.

There are many methods used to analyze the environmental impact of LNG as a marine fuel, compared with conventional fuels. An analysis of greenhouse gas emissions through the entire supply chain (known as "well-to-propeller") still gives LNG the advantage, according to the report "Environmental and Economic aspects of using LNG as a fuel for shipping in The Netherlands," released March 1 by the Dutch research organization TNO. "Well-to-propeller greenhouse gas emissions with the most logical LNG chains are about 10 percent lower than the diesel fuel chains," the authors of the report wrote. More improvement is possible by lowering the methane emissions in marine engines, the authors of the report state.

However, a report titled "Life cycle assessment of marine fuels: A comparative study of four fossil fuels for marine propulsion," released in April and written by Selma Bengtsson, Karin Andersson and Erik Fridell, of the Chalmers University of Technology in Gothenburg, Sweden, concludes that the greenhouse gas benefits of LNG compared with diesel is about the same.

"Marine transportation with LNG fuels can be attributed to comparable or a little lower global warming potential than other fuels, depending on modeling choices," Bengtsson, Andersson and Fridell wrote.

But it is the tank-to-propeller phase of marine transportation that has the highest impact on the total life cycle performance of a fuel, according to Bengtsson. This is where methane slippage comes into play. The problem of methane slippage has a tremendous impact on the greenhouse gas emissions generated by LNG as a fuel for vessels. That is because the heat trapping effect of methane in the atmosphere is 25 times greater than the same amount of CO2.

If more than 2 percent of the LNG used for transportation leaks during the life cycle, then the LNG fuel has a higher global warming potential than crude oil fuel, Bengtsson, Andersson and Fridell wrote in an article titled "A comparative life cycle assessment of marine fuels: Liquefied natural gas and three other fossil fuels," which appeared in the Journal of Engineering for the Maritime Environment. However, the methane slip during combustion with a four-stroke gas engine is uncertain and different emission rates were found in tests by Wärtsilä and in a study done in 2000 by the Environmental Protection Agency, according to citations in this article.

"In order for the LNG alternative to have a lower global warming potential, the engine efficiency should be at least the same as for corresponding diesel engines," the authors wrote in the Chalmers report. Leakage of methane must be kept to a minimum level. "Even small diffuse leakages during the life cycle can make LNG a worse alternative when it comes to climate change potential." A 5 percent lower efficiency of the LNG-fueled engine will increase the global warming potential for LNG to the same levels of crude-oil-based fuels, Bengtsson, Andersson and Fridell wrote.

Their report was written in the context of the promotion of LNG as a marine fuel in Sweden, Bengtsson said. "LNG is good to reduce local pollutants," she said in an interview. "It will probably not lead to a negative effect to change to LNG, but it does not reduce the climate impact as much," she said.

But this problem is being addressed by engine makers, according to Tony Teo, business development director of Det Norske Veritas (DNV) North American Maritime. "They are fully aware, because there is a public noise about this from the environmental community," he said. The engine makers "have done a lot of research to fine tune their engines."

Arto Järvi of Finland examined Wärtsilä's progress in this area in a paper titled "Methane slip reduction in Wärtsilä lean burn gas engine," presented at the 2010 convention of the International Council of Combustion Engines.

According to Järvi, there are three major causes of methane slip: the timing of the gas admission into the intake valve; poor combustion along the walls of the cylinder where the engine temperature might not be as high as in the middle of the chamber; and crevices in the combustion chamber where combustion does not take place, such as the clearance between the piston top and liner, the piston ring and pack crevices and crevices around valve seals.

There are several methods to reduce methane slip, according to Järvi, which were tested on Wärtsilä's W32DF engine (a duel-fuel LNG/diesel engine) and the W34SG engine (a LNG-only engine).

One method involves using a lower boost pressure, close to engine knock, and earlier timing, which reduces methane emissions. Increasing receiver air temperatures also improves combustion of methane, but increases NOx emissions. Skip firing, which uses a control algorithm to feed only part of the cylinders with fuel, improves methane slip emissions remarkably, according to Järvi's article. In addition, piston ring crevice and gasket ring crevice volumes have been reduced in Wärtsilä gas engines.

However, Wärtsilä engineers have found that you cannot just combine different measures to improve combustion. Resulting emission levels are not simply superimposed from the different test results.

"Even higher reduction rates can be achieved by changing engine components or adopting the latest part load enhancing technologies," Järvi wrote.

Wärtsilä has reduced methane slip by as much as 30 percent with these reduction methods. Treatment after combustion offers even more reductions. "Secondary methods offer a great potential to achieve close to zero methane emission from gas engines," Järvi wrote.

These secondary methods include running exhaust gases through a bed of sand. In addition, a pre-turbine oxygen catalyst could be used before combustion.

But some in the industry believe that claims that methane slip reduces or eliminates the environmental benefits of LNG are misleading.

Lars Petter Blikom, LNG segment director at DNV, wrote in his blog about a news article in Norway stating that the ferry industry in that country is emitting more CO2 because it has converted to LNG. The article just does not make sense, he said.

"Consequently, environmental groups think that LNG as marine fuel is a scam, and are attacking the government for backing this effort," Blikom, wrote in an April 1 blog post.

These misperceptions come from a variety of sources, he said. Some critics base their claims on some of the first LNG engines made. "The new engines currently being promoted by the manufacturers and installed in new ships greatly reduce the discussed methane slip," Blikom wrote in his DNV blog on June 17. "For lifecycle emissions from well to engine, there is no basis for shredding the performance of LNG," he wrote.

And even with methane slippage, "the improvement in air quality in local emissions is just massive, compared to diesel engines," Blikom said in an interview. "I think this whole discussion has been a sidetrack; it has nothing to do with the real case at hand." It would be a mistake, he continued, to "stop this development — because maybe the improvement is not as big as politicians had been hoping for."

Teo, of DNV, sums up the case for LNG as marine fuel succinctly. Even accounting for methane slippage, by using LNG "at the end of the day, you still get a lot more benefits in the reduction of CO2," he said.

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