Drying Whets Lignite's Prospects

Novel process enables more efficient combustion and lower emissions.

By Seán Ottewell, Editor at Large

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For Germany, lignite — or brown coal — remains an essential source of power. In a country that has few indigenous gas or oil reserves, lignite accounts for more than 25% of primary energy production.

The process reduces lignite's water content to 5–20% from 50%.

According to Brussels-based Euracoal, the European Association for Coal and Lignite, Germany has reserves of nearly 41 billion metric tons of lignite, so it's likely to heavily rely on the fuel for many years to come.

Supporters of lignite point out that it is always available, regardless of international politics, with the added advantage that extraction and processing usually take place in the immediate vicinity.

Indeed, a new report from The German Lignite Industry Association (DEBRIV), Cologne, says that German brown coal not only underpins a quarter of the country's electricity supply but also directly and indirectly provides more than 86,000 jobs.

The organization explains that by applying a complex input/output analysis it found that for every direct job involved with brown coal, 2.5 others are created. The total economic effect of this exceeds €8.1 billion (≈ $11.7 billion) per year.

"The study findings also highlight its importance from a macroeconomic perspective: the only domestic energy source that can be obtained without subsidies, in large quantities, and be used for competitive power generation," says DEBRIV.

However, lignite, while abundant and readily available, has its critics. The Wuppertal Institute for Climate, Environment and Energy, Wuppertal, Germany, which analyzed state aid for lignite on behalf of Germany's Federal Environmental Agency, found that indirect subsidies — such as exemption from water abstraction charges — are costing the German taxpayer almost €1 billion (≈$1.4 billion) per year. This figure rises to €4.5 billion (≈ $6.5 billion) per year when expenditures for damage to the environment and health are taken into account.

Lignite also releases the greatest amount of carbon dioxide per unit when burnt, prompting German Greenpeace to point out that even the most modern lignite-powered power stations emit twice as much carbon dioxide as gas-fired ones.

The problems with lignite largely stem from its water content, which typically accounts for more than 50% of the fuel's weight.

Now, however, a solution might be near, judging by recent pilot-plant results revealed by Vattenfall Europe, Berlin, which has been piloting a lignite-drying process in Schwarze Pumpe, Germany, since 2008. Vattenfall's pressurized fluidized-bed drying (PFBD) process reduces the moisture content of lignite — thereby increasing the efficiency of combustion.

The PFBD process diverts steam from a power plant's turbines to a dryer, where it is used to heat the lignite to between 100°C and 160°C to drive off water. The temperature chosen depends on the moisture content desired, which is determined by the fuel's intended use. Water content can be cut to 5–20%.

The six-stage process begins with feeding raw lignite into the pressure vessel containing the fluidized bed, where steam condenses in a heat exchanger. The transferred heat then vaporizes the water in the lignite. The water vapor exits at the top of the dryer and is dedusted. Its heat can be recovered for use within the process via a vapor compressor or elsewhere. A circulating flow of vapor serves as fluidizing gas. Rotary feeders move dry lignite from the dryer to a dedusting filter before discharge.

"So far, more than 5,000 operational hours have been spent conducting tests on more than 30,000 metric tons of lignite," says Benjamin Jentzsch of Vattenfall R&D, Cottbus, Germany, who is responsible for the program. "The process results in a net efficiency increase of up to 5% and a 10% reduction in carbon dioxide emissions."

"By using PFBD technology, a new, refined product can be produced, with reduced water content and a grain size that has not been seen on the market so far," adds Gerd Sieling, head of refining operation/lignite mining and generation. "This would give us the opportunity to reach new industry customers."

Use of dry lignite also improves the flexibility of a lignite power plant. For example, it can operate at a lower minimum load without having to be shut down. This flexibility, says Vattenfall, will make lignite plants an even-more-suitable method of balancing the increasing share of power production coming from intermittent renewable energy sources such as wind and sunlight.

"PFBD technology is the basis for future low-emission carbon capture and storage power plants and a further step towards increased efficiency and load flexibility," says Joachim Kahlert, head of technical plant management/lignite mining and generation. "Our indigenous lignite therefore significantly contributes as a complementary partner of the renewable energy sources."


Seán Ottewell is Chemical Processing's Editor at Large. You can e-mail him at sottewell@putman.net.

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