Humble dryer boosts coal power

Engineers at Great River Energy’s Coal Creek Station in North Dakota have begun the first continuous operation of a clean coal technology drying system. The system uses the plant’s excess heat to wring moisture from the incoming coal.

Initial results show that the system could be the first to demonstrate that pre-drying high-moisture coals before they are fed to a power plant’s boiler offers a practical and economical way to generate more power from a lower quantity of coal. The system could also reduce air emissions.

"Enhancing the fuel value of high moisture coals is critically important," said Jeffrey Jarrett, Assistant Secretary for Fossil Energy. "If we are to reduce our dependence on foreign sources of energy, we must make the most of our abundant coal resources here at home."

The results could benefit power plants that today generate more than 100,000 megawatts of electricity in the United States by burning high-moisture coals. The 279 units that burn these types of coal currently account for nearly a third of the electric power generated in the United States.

In the first phase of the project, a dryer system was designed and constructed to dry about one quarter of the coal fed to a 546-megawatt (MW) unit. Successful operation of the first dryer would facilitate GRE proceeding to commercial demonstration in 2007, which involves final design and construction of a full set of dryers for the complete 546 MW coal-fired unit.

The technology is likely to prove especially beneficial to power plants that burn lignite and Powder River Basin coals, which create operational challenges because of their high moisture content. The lignite entering the Coal Creek Station, for example, can be as much as 40 per cent water measured by its weight.

Until now, the cost of thermally drying these coals has often outweighed any potential gains in the plant’s operational performance. The Coal Creek Station’s approach of capturing and reusing the excess heat, rather than burning additional fuel to generate heat, is proving to be the key to making thermal coal drying commercially practical.

Drying the coal increases its heating value, and that means that less coal is needed to generate the same amount of energy. Less flue gas is also emitted which reduces the workload on other equipment in the plant, such as fans. The result is an estimated increase in efficiency at the Coal Creek Station of about five per cent.

The first dryer has been supplying processed lignite to one of the seven pulverisers for a 546 MW unit at the station for several weeks. Early estimates show that with just one pulveriser using dried coal, the stack flow rate from the unit decreased one per cent, boiler efficiency increased 0.3 per cent, pulveriser power consumption decreased 4.5 per cent, sulphur oxide emissions fell 2.0 per cent, nitrogen oxide emissions decreased 8.5 per cent, and carbon dioxide emissions decreased 0.34 per cent.

The full set of dryers, scheduled to be built in 2007, will operate through 2008 to generate data that could be applied to other high-moisture, coal-burning power plants that operate primarily in western coal regions of the United States and in several other countries. After that, the unit is expected to continue operating as part of the power station’s commercial operations.