A better burn

Building on the continuous operation of a prototype coal dryer that uses waste heat to remove moisture from coal, the

(DOE) has given the go-ahead to

to conduct the first-ever full-scale demonstration of the utility company's innovative technology.

Great River Energy will soon begin the demonstration at its Coal Creek Station near Underwood, North Dakota, during the second phase of a cost-shared project with DOE. The $31.5m project, which received $13.5m in funding from DOE, was one of eight projects selected in the first phase of DOE's Clean Coal Power Initiative, a 10-year $2bn commitment to advance of clean coal technologies and an integral part of the US administration's National Energy Policy. The projects are managed by DOE's National Energy Technology Laboratory.

‘This unique coal-drying system enables the nation to tap into vast resources of high-moisture coal while simultaneously providing environmental benefits,’ according to Jeffrey Jarrett, Assistant Secretary for Fossil Energy. ‘The successful demonstration of the system will further indicate that the nation can cost-effectively use its most abundant resource and still contribute to the President's Clear Skies Initiative.’

During the first phase of the project, Great River Energy used its prototype dryer to supply about 14 percent of the coal fed to a 546-megawatt unit at the station. Positive results from the operation indicated that the stack flow rate decreased 1 percent, boiler efficiency increased 0.3 percent, pulverizer power consumption decreased 3.5 percent, sulphur oxide emissions fell 1 percent, nitrogen oxide emissions decreased 7.6 percent, and carbon dioxide emissions fell 0.9 percent.

The full-scale demonstration planned for Phase 2 of the project will include the final design and construction of a four-dryer integrated system that will supply all of the coal (high-moisture lignite) to the 546-megwatt unit. The demonstration of the integrated system is expected to occur sometime in 2008 and will operate through that year, generating data that will be useful to operators of other power plants that burn high-moisture coal. After 2008, the unit is expected to operate as an integral part of the utility's power grid.

The likely benefactors of a successful demonstration would be those power plants, primarily in the western United States, that burn the lignite and Powder River Basis coals. These coals have a high moisture content and create challenges for utility operators. As an example, Great River Energy's Coal Creek Station burns lignite that can contain as much as 40 percent water by weight.

Until Great River Energy's project, the cost of thermal coal drying often exceeded any predicted gains in a plant's operational performance. At the Coal Creek Station, however, the approach of capturing and reusing the excess heat, rather than burning additional fuel to generate the heat, holds the promise for commercial application of thermal coal drying.

Today in the United States, power generation units that burn lignite and Power River Basin coal have a combined capacity of 115 gigawatts. Over the next 20 years, the capacity of those types of units is projected to increase by another 100 gigawatts. If the coal-drying system is applied at power stations totalling just 10 gigawatts, the annual reduction in air emissions will be nearly 7,000 tons of nitrogen oxides, more than 18,000 tons of sulphur dioxide, more than seven million tons of carbon dioxide, more than 9,000 tons of particulates, and nearly 300 pounds of mercury.

The advantages inherent in Great River Energy's system is that drying the coal increases its heating value and requires less coal to generate the same amount of energy. The system also emits less flue gas and thereby reduces the workload on other equipment in the plant, such as fans. The overall result is an estimated increase in efficiency of about 5 percent, considered a significant improvement in plant performance and cost savings.