By: Douglas J. Smith IEng, Senior Editor
With almost 30 percent of the world’s reserves of bituminous and anthracite coal, the U.S. has more high quality coal than any other country. It is estimated that at the current rate of consumption the U.S. has enough coal reserves to last for at least the next 250 years. These reserves make the U.S. the Saudi Arabia of coal. Today, coal accounts for 52 percent of the electricity generated in the U.S.
Environmental activists who say coal is dirty and is the major cause of global warming often attack the use of coal for electric power generation. However, with developments in clean coal technology, particularly coal gasification, it is now possible to burn coal with very low emissions of NOx, SOx, CO2, metals and mercury.
The gasification of coal is not a new technology. It has been around since the early 18th century and until the 1940s the gas produced from the gasification of coal and coke in the U.S. was the only gas available for residential, commercial and manufacturing use. In 1792 William Murdock, a Scottish engineer, heated coal in a retort in the absence of air to partially convert coal to a gas with a residue of coke, thus becoming an early pioneer in the commercial gasification of coal.
Integrated Gasification Combined-cycle
In integrated gasification combined-cycle (IGCC) technology, coal is converted into a high value synthesis gas (syngas). After cleanup the gas is used as the primary fuel for the combined-cycle’s gas turbine. In addition to coal, the gasification process can be used to produce syngas from petroleum coke, orimulsion, biomass and municipal waste.
To improve the economics of IGCC, some of the hydrogen and pure sulfur can be extracted from the syngas prior to its combustion in the gas turbine. The hydrogen and sulfur extracted can be utilized to make saleable products including methanol, ammonia, fertilizers and other chemicals. Although this does lower the heating value of the syngas slightly, the sale of the byproducts makes up for this loss, says Dr. Norman Shilling, Leader, Process Power, GE Power Systems.
IGCC generally consist of four separate processes:
- Cryogenic air separation
- Gasification (oxygen blown or air blown)
- Gas clean-up
- Combustion of syngas in the gas turbine
In the process of producing the syngas, coal is fed into a pressurized vessel/gasifier (oxygen blown or air blown), which contains controlled amounts of oxygen and steam or water. The heat and the pressure in the gasifier break apart the chemical bonds in the coal to form syngas, a mixture of hydrogen and carbon monoxide.
After leaving the gasifier the residual fly ash, sulfur compounds, ammonia, mercury and other metals, alkalytes and particulates are removed from the synfuel in the gas cleanup unit. As a result, pollutants are removed from the syngas before it is combusted and not in the post combustion flue gas. According to GE, the IGCC clean-up process is more efficient and has a lower cost than post-combustion clean-up methods.
Polk Power Station IGCC
Under the U.S. Department of Energy’s (DOE) Clean Coal Technology program Tampa Electric Company’s Polk power station was selected as the site for the demonstration of IGCC technology in a greenfield utility application. Polk power station is a nominal 250 MW net IGCC power plant that utilizes a Texaco oxygen blown, entrained flow coal gasifier, integrated with a gas clean-up system, to produce syngas. The syngas is fired in the combined-cycle’s gas turbine, Figure 1.
Detailed design for the project began in April 1993 and construction began in August 1994. The IGCC entered commercial operation on September 30, 1996. According to DOE, since going into commercialization the plant has met its objectives of generating low-cost electricity in a safe, reliable and environmentally acceptable manner. In September 2001 the station completed five years of commercial operation and ended the demonstration phase of the project. The final report was issued in April 2002.
Throughout the commercial operation phase of the project, the IGCC met its production goal of 250 MW. During that time, the gasifier operated more than 29,000 hours and converted 2,300 tons/day of coal into syngas. The gas turbine operated on syngas for more than 28,000 hours, producing 8.6 million MWh of electricity. Carbon burnout exceeded 95 percent and the emissions of SO2, NOx and particulates were well below the regulatory limits set for the Polk plant.
After the raw syngas is cleaned it is sent to the GE model MS 70001FA gas turbine. Nitrogen from the air separation unit, at 98 percent purity, is mixed with the syngas at the combustor inlet. Mixing nitrogen with the syngas increases the mass flow through the gas turbine and increases the power from the gas turbine. This also enhances the overall efficiency of the combined-cycle plant. In addition, the nitrogen reduces NOx emissions and the need for water or steam injection.
Polk Operational Performance
In the fourth and fifth year of operation the gasifier’s capacity factors were 75 percent and 66 percent respectively. Similarly, the gasifier’s availability was 88.7 percent in the fourth year and 84.2 percent in fifth year of commercial operation. For the same two years the air separation unit had an availability of 93.9 percent and 90.5 percent and the power block availability was 86.6 percent and 93.9 percent.
According to DOE the lower availability of the gasifier in the fifth year was due to a longer planned outage to replace the refractory liner. There was also a 28-day forced outage to weld and repair the main compressor in the air separation unit.
In order to achieve high availability, several modifications were made to the original design and procedures. Shortly after startup of the IGCC ash plugging caused failure of some exchangers in the high temperature heat recovery system. After removing the heat exchangers and making compensating adjustments to the rest of the heat recovery system, the problem was resolved. To protect the gas turbine additional particulate removal was also added.
In late 1997, a hot restart procedure was implemented. This eliminated the need to change burners and reheat the gasifier every time the gasifier was shut down. It also reduced the time for restarting the gasifier by 18 hours.
Refractory life, originally estimated at two years, presented another operational concern. Because replacement liners would be expensive and require considerable down time to replace, plant management decided to lower the gasifier’s operating temperature. Lowering the temperature resulted in a significant decrease in carbon conversion efficiency and load restrictions due to capacity limitations in the fines handling system. A slag crusher and a duplicate fines handling system, installed in 1998, solved this problem.
Since completing the demonstration phase of the project, Tampa Electric continues to operate the plant baseloaded and, according to Mark Hornick, general manager Polk Station, they are pleased with its performance as a key part of Tampa Electric’s generation fleet. Some European power producers have contacted TECO Energy to discuss possible technical assistance in using the gasifier technology.
The total capital cost of the Polk IGCC project was $424 million dollars—$1,650/kW. This excludes site acquisition and development, construction management, startup, operator training, project management, permitting and preliminary engineering. It also does not include the off-site cost for constructing a coal truck loading facility. If these costs were included the cost of the Polk IGCC plant would be $2,430/kW.
According to DOE, there are several ways to reduce the cost of today’s first generation IGCC plants including economies of scale, especially in the common or balance of plant systems, and replication of proven configurations to eliminate costly redesign. The DOE estimates that future IGCC power plants, based on mature and improved technology, such as that demonstrated at Polk, will cost in the range of $900-$1,250/kW.
CITGO’s Lake Charles, LA, 670 MW IGCC energy project is scheduled to begin commercial operation in the first quarter of 2005. The plant will use a Texaco gasifier to produce syngas from a petroleum coke feedstock. Three combined-cycle units consisting of a GE 7FA gas turbine, a heat recovery steam generator and steam turbine will use the syngas and refinery fuel gas as the primary fuel. Natural gas will be used as a backup fuel.
The project, located adjacent to the CITGO Lake Charles manufacturing plant, will be interconnected to the Entergy-Louisiana Rose Bluff 500 kV/230 kV substation. This will allow power to be sold from the plant into the Entergy power market. TECO Power Systems and Texaco both have 50 percent ownership in the plant.