Herbert M. Kosstrin, Ph.D., R. W. Beck
Coal is no longer a dirty word. Prior to 2002, natural gas-fired combined-cycle power plants represented the power plant of choice for both utilities and independent power producers in the U.S., while coal-fired plants, though always reliable stand-bys, were far from being favorites. The subsequent rise in natural gas prices and their continuing volatility, however, have led to a resurgence in the planning and development of coal-fired generation in the last two years.
This renewed interest in using the county’s most abundant fossil fuel comes hand-in-hand with the need to produce electricity in an environmentally acceptable manner. The use of coal presents environmental challenges that can be addressed either with technologies that are added on to the back end of the coal boiler or with the “new” technology of coal gasification.
Coal gasification, coupled with a combined-cycle power plant, results in an IGCC (integrated gasification combined-cycle) power plant. The IGCC plant allows for the control of most of the coal contaminants (sulfur, chlorine, particulates, mercury) in the concentrated fuel gas stream, and produces a clean fuel gas to be burned in the combined-cycle portion of the power plant. The other major contaminant, nitric oxide (NOx), is controlled in the combustion process without the use of expensive catalysts and ammonia, which most straight combustion coal technologies require. This sequential use of the environmental control functions results in the cleanest coal-fired power plants available today. IGCC plants also virtually eliminate any solid waste generation, and they substantially reduce water usage, compared to other coal combustion plants.
If future environmental regulations require reductions in carbon dioxide emissions, coal gasification is ready. Using the typical IGCC configuration and proven technologies, approximately 20 percent of the CO2 can be removed for sequestration. As combustion turbine manufacturers produce engines that will burn a pure hydrogen stream, over 90 percent carbon dioxide removal can be achieved.
Utilities such as American Electric Power and Cinergy have recognized that IGCC represents the best hope of the future for transforming dirty coal into clean power. Recently, both utilities announced plans to build large-scale IGCC plants. One of their compelling reasons was the ability to control both current and future environmental regulations, while also gaining an efficient means of using the nation’s vast coal resources.
back to the future
Modern coal gasification, which is actually more than 50 years old, started with the Lurgi units in South Africa. It has proven itself over the years in a variety of applications. Domestically, there are now seven operating coal or petroleum coke gasification plants producing chemicals, hydrogen, and methane, as well as power. Current IGCC power plants meld the new generation of coal gasifiers with the advanced combined-cycle combustion turbines to arrive at a highly efficient power plant. Designs using eastern high-sulfur coals and “F” class turbine technology are predicted to reach efficiencies of approximately 40 percent (8500 BTU/kWh). Improvements in these operating efficiencies will come from both advanced combustion turbines (the “H” class machines) and improvements in the overall gasification system.
Although the environmental advantages of IGCCs are clear, and operating efficiencies are currently on a par with, or better than, the most advanced pulverized-coal ultra-supercritical steam cycles, the level of capital costs of IGCCs needs to be confirmed when the first units are deployed. Many of the current IGCC designs are built around the amount of fuel gas required for a single combustion turbine. This leads to the incorporation of multiple gasification trains (each generating about 260 MWe) to achieve the larger IGCC plants that generate 500, 750, and 1,000 MWe. Putting multiple modules at a site would result in economies of scale in both capital cost deployment and in operations.
The current generation of entrained-flow slagging coal gasifiers being proposed for the initial deployment of IGCC power plants has been operating
since the mid-1990s. These demonstration plants-the Wabash River IGCC plant (E-Gas technology), the Tampa Polk IGCC Power Station (Texaco technology) and Nuon (Shell technology)-had government support. They have worked through their early “technical bugs” and are now ready for full commercial implementation.
Since coal gasification is a combination of several technologies (oxygen production, gasification, gas clean-up and, if desired, chemicals production), advances can be achieved in different areas to enhance the technologies. Programs to advance these technologies, which are supported by the U.S. Department of Energy, are continuing to reduce costs and enhance outputs.
Coal gasification goes beyond power generation. The syngas produced (primarily hydrogen, carbon monoxide, carbon dioxide, with some methane), can be used to produce methanol, hydrogen for ammonia production, methane, liquid fuels, and chemicals. Small amounts of these “other” products could be co-produced with power (ie., poly-generation). Or, power could be co-produced in a plant that would produce, say, methane. Re-powering of existing natural gas-fired combined-cycle power plants (NGCCs) as IGCC plants is also being considered.
The flexibility of IGCC technology opens up new vistas in the drive for clean use of domestic coal resources, while also producing valuable by-products. Perhaps most important, it also moves us further down the road toward energy independence.
Kosstrin is a principal and senior director at R. W. Beck.