America’s coal-fired electricity generating plants have been the workhorses of our electric utility system for more than 50 years.
The overall design is simple. Bring in coal at one end to heat water to make the steam that turns the turbines, and electricity comes out the other end.
While actually making something like that work is frighteningly complicated, the basic idea is relatively easy to build and operate, and provides reliable amounts of power. There are hundreds of these plants operating today. Some have been producing electricity for half a century.
There have been various refinements over the decades. Scrubbers to control certain emissions have been added to older plants, and built-in at newer plants. And engineers have come up with a few extra tricks to get more energy from every ton of coal going in while reducing the amount of waste left over after combustion.
But to make real progress toward controlling emissions, especially greenhouse gases that have been associated with global warming, experts say America’s coal plants need an extreme makeover.
About a year ago, the Electric Power Research Institute (the utility-funded research organization known as EPRI) took a close look at how technology could help reduce carbon dioxide (CO2) emissions within the electric utility sector. This new approach is quite different from what has already been accomplished during the last 35 years to reduce emissions of such things as sulfur dioxide.
Two of the seven areas EPRI investigated, to see how likely it is that we can come up with effective ways to reduce greenhouse emissions, involve coal-fired electricity generating plants. In the next two decades, EPRI says we need to:
• Improve the efficiency of new coal-based generating plants
• Put CO2 capture-and-storage technologies in place at most new coal plants
When a coal plant operates in the most efficient way possible, that can help hold greenhouse gas emissions steady instead of increasing. This month we’ll take a look at how technology might be able to help accomplish this first goal. We’ll save the complex issue of capturing greenhouse gases for next month’s column.
Sharing coal ideas
Not all coal is alike.
America’s coal reserves include three main kinds of coal used for power plants, each with a different energy potential and slightly different mix of components. Bituminous, sub-bituminous, and lignite coals are available in different areas of America. Typically, it’s more economical to use the kind of coal closest to a generating plant. However, these different kinds of coal produce different amounts of energy. Burning each kind of coal also releases a different combination of waste products. So although a power plant in Louisiana that uses lignite coal may look the same on the outside as a power plant in Kentucky that uses bituminous coal, how much electricity and the mixture of emissions each plant produces is quite different.
Those differences mean that no one solution toward improving efficiency will work at every coal plant.
Since the Clean Air Act of 1970, lots of add-ons at standard coal plants have lowered many emissions substantially. But the next leap forward will require a completely new design, not just tweaks to old production methods.
Three years ago, EPRI put together a new program called CoalFleet for Tomorrow to help everyone in the electric utility industry share information about new technologies for coal plants. This initiative will play a vital role in the success of the coal plants of the future. As utilities try these new ideas in real-life situations, they will share engineering details with each other, so that all can benefit from each other’s practical experiences.
Three advanced clean coal technology designs are ready to move into the mainstream. Each one offers advantages and disadvantages.
At an ultra-supercritical pulverized coal (USC PC) generating plant, the basic principle is the same as an old-fashioned coal plant, but with two big changes. The steam is extremely hot—more than 1,100º. And it’s held at extremely high pressure—3,500 pounds per square inch. This new procedure increases efficiency dramatically—and works with all kinds of coal.
At an integrated gasification combined-cycle (IGCC) electricity generating plant, coal is first turned into a gas. This extra step in the process is important. Impurities can be removed from the gas, then the gas is burned to make steam for the turbines. An IGCC power plant emits fewer particulates, less sulfur dioxide, and less mercury. Using coal gas is also more efficient than using plain coal. But this technology only works with sub-bituminous coal.
At a supercritical circulating fluidized bed combustion (shortened to simply CFB by engineers) power plant, the difference involves both fuels and procedures. Several fuel choices are possible. Regular coal can be mixed with limestone. Forms of coal that don’t work in conventional power plants, such as culm (a leftover from anthracite coal, which isn’t usually used at power plants) or gob (a leftover item from bituminous coal), can be used as fuel. Even biomass materials such as waste wood can be used. These different fuels are crushed together and the combustion process begins.
East Kentucky Power Cooperative in Winchester, which generates electricity at several different locations for 16 co-ops in 89 counties, is leading the way with fluidized bed combustion technology. East Kentucky’s first fluidized bed generating plant, Gilbert Unit #3 at Spurlock Station near Maysville, came into service in March 2005. It generates 268 megawatts of electricity. A second unit there is under construction now, and should be producing an additional 268 megawatts in the spring of 2009.
Engineer Craig Johnson, East Kentucky Power Co-op’s vice president for production, says, “Most conventional coal power plants run at about 34 percent efficiency. The Gilbert plant with its new fluidized bed process has been rated at about 36 percent when running at full load. So it’s notable not so much because of the improvement in efficiency, but because of its lower regulated emissions. One of the advantages to this new kind of plant is that it has lower emissions while maintaining fuel flexibility. With this plant we can take advantage of lower quality fuels that conventional units cannot burn, yet when we burn it here it is very clean. The Gilbert Unit has one of the strictest air permits in the United States.”
At existing older-technology coal units at Spurlock, flue gas scrubbers are being added to reduce sulfur emissions, along with a new technology known as “wet ESP,” which will further improve emissions control at these older units. The wet ESP helps remove even more tiny particles, things so small they’re measured in microns. A micron is 39 millionths of an inch—a grain of salt is about 60 microns, so you wouldn’t be able to see a particle of only a single micron. But scientists are concerned about the effect even such tiny bits have on the environment, so preventing them from entering the atmosphere is a good step forward.
Johnson says, “I think we are the only utility in Kentucky so far to install this new wet ESP technology.”
Johnson thinks figuring out what to do with greenhouse gas emissions is going to be a lot more difficult. On paper there are quite a few ideas about how to deal with carbon dioxide, but adding capture equipment will bring a new set of complications.
“It will be a challenge to add this equipment for new coal-fired plants, and it’s not going to be easy to retrofit older coal plants with devices to capture carbon dioxide,” Johnson says, “simply because the technology is still in its infancy.”
Next month: Capturing carbon