February 1, 2009
A new use for waste limestone fines not only provides a potential market opportunity, it’s also one way producers can go green.
by Bill Langer
Earth is a cool place to call home, largely because of Earth’s unique combination of moderate temperatures, atmospheric gases, and the presence of liquid water that make it an inhabitable planet for life, including humans.
The bad news is that carbon dioxide generated by we humans may be causing adverse climatic and environmental changes. Indeed, during their 2008 presidential campaigns, the two major presidential candidates both called for reductions in U.S. carbon dioxide emissions.
The good news is that producers of crushed limestone have an important resource – their waste fines – that might be used in the green fight to reduce carbon dioxide emissions.
Part of the reason Earth is a cool place to live is that our atmosphere contains just the right amount of carbon dioxide – not too little, not too much. Without a trace of carbon dioxide in our atmosphere, the oceans would freeze and life as we know it would cease to exist. By having a small amount of carbon dioxide, the greenhouse effect keeps us warm. But too much carbon dioxide and we would be toast.
Fortunately, the natural weathering of limestone helps mitigate atmospheric carbon dioxide. When it rains, carbon dioxide from the atmosphere mixes with rain water and creates carbonic acid. When that acid rain falls on limestone, it dissolves the rocks and forms a bicarbonate solution. Rivers carry the bicarbonate solution to the oceans where corals and other life forms convert the bicarbonate back into limestone.
This is where crushed limestone producers and their waste fines enter the picture. Scientists from Lawrence Livermore National Laboratories have patented a method using waste limestone fines to capture and sequester carbon dioxide from the chimneys of carbon dioxide emitters such as fossil fuel-fired power plants. The method is referred to as Accelerated Weathering of Limestone (AWL).
The AWL process speeds up the natural weathering of limestone by purposely contacting water and limestone fines at the source of the carbon dioxide. Just like in nature, carbon dioxide is hydrated with seawater to produce carbonic acid, which is reacted and neutralized with limestone, thus converting carbon dioxide gas to calcium bicarbonate in solution. AWL waste products can be disposed of in the ocean where they may enhance growth of corals and other calcifying marine organisms that are threatened by increases in ocean acidity.
Approximately one third of man-made carbon dioxide emissions come from fossil fuels used for generating electricity. Although AWL is a low-tech method to sequester carbon dioxide, it requires very large amounts of sea water, thus limiting AWL facilities to within about 10 kilometers of the coastline. About 12 percent of carbon dioxide emissions from U.S. electricity production occur at power plants within 10 kilometers of the coastline.
AWL also requires huge amounts of limestone fines: about 2,190,000 tons of limestone are needed each year to sequester 20 percent of the carbon dioxide from a 500 MW coal-fired plant. Studies show that the majority of those U.S. coastal power generating facilities are within economical transport distance of limestone resources, that sufficient waste limestone fines are being produced, and that transportation systems and material-handling capabilities currently exist to meet the huge demand for limestone fines that could be used by AWL, should it ever be implemented.
The best news is that Accelerated Weathering of Limestone presents opportunities for the crushed stone industry, electrical utilities, and research scientists to work together to help keep the Earth a cool place to live.
The author will be presenting a paper on AWL at the SME Annual Meeting in Denver, Colo., on Feb. 25, 2008 (www.smenet.org).