|Using Limestone for Cleaner Air|
Teaching an Old Stone New Tricks:
by Deborah Galyan
What's bad for Indiana's coal industry may be good for its limestone business. Changes in United States environmental policy have stringently restricted sulfur dioxide emissions caused by coal combustion, resulting in an increased use of low sulfur Western coals. But limestone is the key component in what has become the standard technology for cleaning sulfur dioxide from smokestacks. Wet limestone "scrubbers" use limestone slurries--mixtures of water and very finely crushed limestone--to prevent sulfur dioxide from passing through smokestacks, a process called "flue gas desulfurization." The scrubber mechanism doesn't exactly scrub; a rapid chemical reaction between sulfur dioxide gas and crushed limestone combines the gas, with calcium and oxygen, into a removable solid waste. Wet scrubbers installed in smokestacks are highly efficient and easily adaptable to a variety of coal-burning plants. Utilities and industries are likely to install many more scrubbers over the next few years as they work toward total compliance with the Clean Air Act mandate of a 50 percent reduction in sulfur dioxide emissions by the year 2000.
Indiana, of course, has no shortage of limestone. The state has more than enough to meet the projected demand of 1.5 million tons per year for use in the state's twelve operational scrubbers. But, it turns out, some limestones "scrub" better than others.
Nelson R. Shaffer, a research scientist with the Indiana Geological Survey at Indiana University Bloomington, has spent the last several years studying chemical, mineralogic, and physical properties of Indiana limestones to determine what makes some stones more effective in scrubbers than others. "Over the years, we've found that as regulations and technical specifications for industry change, we need to know more about the properties of the state's resources, such as coal and limestone," Shaffer explains. "In this case, we have an entirely new use for limestone, and that calls for a new way of analyzing which properties affect its performance in scrubbing mechanisms." Research conducted in the late sixties and seventies on the sorptive capacities of limestone in scrubbers showed that stones with similar chemistries didn't always produce similar results. According to Shaffer, similar stones can differ by more than a thousand percent in the amount of sulfur dioxide they can absorb. A further complication is that a particular limestone may scrub effectively in one type of scrubber, but not as well in others.
Geologists describe the effectiveness of limestone in scrubbers in terms of its "reactivity," a measure of how rapidly and completely a particular stone absorbs sulfur dioxide. "The 'wrong' kind of limestone can plug up filters and shut down scrubbing mechanisms," Shaffer explains. "Better limestones react quickly and efficiently, meaning that utilities need less stone to do the job. This makes scrubbing more economical and reduces the amount of solid waste at the end of the process." Cost is a key issue for utilities, given that a single scrubber can consume hundreds of thousands of tons of stone each year--more if it isn't scrubbing efficiently.
To study the composition of limestone in detail, Shaffer employs an empirical "relative-reactivity test," which places careful controls on the chemical processes that take place during wet-scrubbing. "In this test," Shaffer explains, "limestone is the only variable, allowing us to see how certain properties increase or decrease its reactivity." Shaffer's data show that a complex of chemical and physical factors can affect a limestone's scrubbing capacity. Physical properties, including porosity and hardness or "grindability," significantly affects reactivity. Limestone must be crushed to a specified texture for use in a particular scrubber; stones that are too hard scrub less efficiently and cost more to grind. Chemical properties such as the amount of magnesium contained in a limestone also affect its reactivity. Limestones that contain up to 5 percent magnesium work well, but if magnesium is present as dolomite, reactivity can be hurt. The presence of insoluble residues in stones also decreases their scrubbing efficiency.
Researchers can enter data, combined with information about a given power plant's specific combustion equipment and scrubber mechanisms, into a complex computer simulation model that enables them to profile the limestone best suited to a particular scrubbing mechanism. Local utilities such as Indianapolis Power & Light (IPL) have benefited from Shaffer's evaluative model, which is now available nationwide through the Electric Power Research Institute. Support for Shaffer's research came from a partnership between the Geological Survey, the Indiana Department of Commerce, the Indiana Mineral Aggregates Association, and IPL.
Wet limestone scrubber technology may help rehabilitate the "dirty" reputation of Indiana's high-sulfur coal. Shaffer reports that the local limestone industry is already benefiting from increased sales of scrubber stone: two new limestone quarries have opened in anticipation of growing demand. Greater efficiency of scrubbers may encourage Indiana utilities to purchase coal mined in their "own back yards" with the assurance that it is cost-effective and beneficial to the state's economy--Deborah Galyan