Science solution—pure and simple
EnergyLines July 2016
How environmental science at Merom helps create gypsum for ag, cement industries
One of the byproducts of modern coal plant operations is a fluffy, cakelike substance called gypsum.
Thicker than sand, but porous, synthetic gypsum is a much sought-after product for the wallboard, cement and agricultural industries. Rich in calcium sulfate, gypsum is manufactured from the very substance that the scrubber system removes from coal plant exhaust gas – sulfur dioxide.
Its chemical makeup, combined with its texture and other properties make it highly desirable as a soil amendment. When wet, it absorbs like those paper towel commercials, preventing runoff and soil erosion. Heat it, then add water and it is rock solid, a desirable attribute for the cement industry.
Gypsum occurs naturally in sedimentary rock formations and has been used by farmers for years. Ben Franklin began applying what he called “land plaster” to his crops in the late 1800s after returning from a trip to Europe. Thomas Jefferson also reportedly used gypsum on his farm fields, but historically its use has been limited due to mining and transportation costs.
Fast-forward to today’s agri-economy and demand for gypsum has grown. The reason lies in a coal pile.
Environmental scrubber technology used by coal plants to control sulfur dioxide emissions can also produce an environmentally safe byproduct – synthetic gypsum.
Merom began exploring the potential production of resale quality gypsum in the early ‘90s, but the scrubber technology at the time didn’t make it feasible to produce.
Then two things occurred, making this secondary reuse market an attractive prospect.
Upgrades to Merom’s advanced flue gas desulfurization (FGD) process in 2012, which removes about 98 percent of the sulfur dioxide from the plant’s air emissions, made the production of resale quality gypsum more practical. When Indianapolis Power & Light decided in 2014 to convert its Harding street plant on the southside of Indianapolis from coal to natural gas, an opportunity opened in the gypsum supply chain for southern Indiana.
Soon third parties were approaching Hoosier Energy and asking whether the Merom Station could supply the product.
Plant personnel began perfecting the waste center to process the synthetic gypsum separately from fly ash and a new market began to take off.
“We definitely are taking the crawl, walk, run approach to these beneficial reuse markets,” says Will Kaufman, Manager of Fuels.
But what started as a crawl quickly turned into a gallop. Production of resale quality gypsum began in March and has already met annual targets for the first year.
For Merom, what was once bound for the landfill is now filling up a holding pile for sale to the cement and agricultural industries. As of June 1, more than 12,000 tons of gypsum had been sold.
Demand is so great, the plant is turning away business.
“Right now, we try to balance the amount of trucks with amount of pure product that we produce. It’s really a kind of just-in-time production,” Kaufman says.
During the FGD process, a mixture of water and crushed limestone slurry is injected into the scrubber system where it reacts with exhaust gas from the boiler, effectively removing sulfur dioxide from the gas. Through a process called forced oxidation, the calcium carbonate in the limestone reacts with the sulfur dioxide to form calcium sulfate dihydrate, or gypsum.
The resulting byproduct is not only safe, but rich in calcium sulfate – properties that can turn the claylike southern Indiana soil into more tillable land.
Those environmental benefits aren’t lost on anyone at the plant.
Herb Abbott, Area Coordinator for FGD and a 30-year veteran at Merom, says the plant saw the value in perfecting the waste center system to separate the synthetic gypsum from fly ash. “A lot goes into removing sulfur dioxide from the flue gas stream of a coal-fired power plant that folks just don’t know about,” he says.
Both air and waste products from a coal plant, including the byproducts, are heavily regulated, says Lon Petts, Environmental Team Leader for Hoosier Energy. “The beneficial reuse of synthetic gypsum has been recognized by the U.S. Environmental Protection Agency and the U.S. Department of Agriculture,” says Petts, adding that research by the Electric Power Research Institute points to the agricultural advantages of applying synthetic gypsum to fields. “Less runoff keeps phosphate in the soil and the fields improve.”
While historically Merom has produced an average of 400,000 – 500,000 tons of gypsum a year, it was used to mix with fly ash and bottom ash and bound for the landfill, which follows strict state and federal environmental regulations.
Now, the gypsum produced through the upgraded FGD process is not only using chemistry to “recycle” what would have been an air emission into an environmentally safe product, but in the process, benefitting society while reducing handling and landfill costs.
At $1-2 a ton for landfill space and handling, that can add up. “Selling gypsum adds to Merom’s cost competitiveness by reducing handling and landfill costs as well as providing agricultural benefits to the communities we serve,” Kaufman says.
The more Merom runs, the more opportunity there is for turning waste into environmentally safe products.
“The one thing that amazes me is instead of spending millions to maintain a landfill and cap it, we can now load most of this up on a truck and put it on a field,” Abbott says. “It’s not waste anymore. It’s something good and it came from coal.”