BISMARCK, N.D. (AP) -- Officials say production at North Dakota's four large lignite mines dropped for the second year in a row due to scheduled outages at power plants and an increase in hydroelectric production at Garrison Dam on the Missouri River.

The Lignite Energy Council says the Freedom, Falkirk, Center and Beulah mines produced 28.2 million tons of coal in 2011, down about 600,000 tons from the previous year.

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The mines produced 28.8 million tons of lignite in 2010, down about 1 million tons from 2009.

The council says North Dakota's lignite production has been about 30 million tons per year since 1988.

Most of the lignite mined last year was used to create electricity. Smaller amounts were used to produce synthetic natural gas and fertilizer products.