Converting Sulfur from Flue Gas into Fertilizer

As environmental regulations tighten—both in the U.S. and around the world—coal-fired power plants continue to look for ways to operate economically. Though reuse and sale of coal combustion by-products has a long history, one new approach could benefit a somewhat unlikely partner industry. 

The history of power plant emissions regulations and control technologies is largely one of preventing elements that are bad for the environment or human health—including sulfur dioxide, particulate matter, and nitrogen oxides—from being dispersed to the environment. But sometimes it’s possible to take advantage of the by-products of the control technologies and put them to good use in the environment. That’s the case with a new process that converts sulfur to fertilizer.

Charah Inc. has developed a technology that allows sulfur captured from power plant exhaust gases to be pelletized into a calcium sulfate fertilizer product that returns vital nutrients to farm fields. To understand why Charah and coal-fired power plants would find this worth doing, you need to understand the role of sulfur in the environment and the economics of the process.

Sulfur’s Ups and Downs

When coal is burned in a boiler to generate electricity, the naturally occurring sulfur in the coal is released into boiler exhaust gases. Before it was regulated, coal sulfur was discharged into the atmosphere through plant stacks. The U.S. Environmental Protection Agency (EPA) first started regulating power plant air emissions in 1971. According to the EPA, these air quality controls covered SO2 because exposure to the gas can cause adverse respiratory effects, it can combine with other gases to produce harmful particulates, and it is a primary cause of acid rain.

Declines in SO2 emissions began soon after enactment of the 1990 Clean Air Act Amendments, which established a national cap-and-trade program for the gas. Because coal-fired units accounted for a large share of SO2 emissions, the program (which also covered NOx) provided an economic incentive for coal-fired power plants to reduce emissions by installing pollution control systems, burning lower-sulfur coal, or generating less electricity.

All plants built after 1978 are required to clean the sulfur from coal combustion gases before they go up the stack. They do so with flue gas desulfurization (FGD) units, commonly called “scrubbers.” The EPA reports that by the end of 2011, 60% of the U.S. coal fleet had FGD scrubbers installed.

As scrubbers began to remove sulfur from exhaust emissions, and some plants switched to low-sulfur coal, the amount of sulfur in the air decreased. EPA data shows that between 1980 and 2012 concentrations of atmospheric SO2 in the U.S. decreased approximately 78% (Figure 1).

PWR_030114_CoalFertilizer_fig1
1. Sulfur reduction. This graph shows SO2 air quality as a national trend from 1980 to 2012 (annual 99th percentile of daily maximum 1-hour average) based on 57 sites. There was a 78% decrease in the national average over that period. Source: EPA

But sulfur need not always be a net negative for coal-fired plants. Since the 1990s, captured sulfur from flue gas has resulted in the production of high-quality gypsum, hydrated calcium sulfate: CaSO4-2H2O. That synthetic gypsum can then be beneficially used in a number of common applications, from plaster and wallboard to cement and fertilizer. Though gypsum occurs naturally (and even lends its name to a town in Colorado with a history of gypsum mining and processing), synthetic gypsum has advantages in that it doesn’t have to be mined, and it recycles what would otherwise be a waste product that power plants would have to pay to dispose of in landfills. Use of synthetic gypsum has also reduced costs for drywall manufacturers.

Coal Country Conversion

Charah Inc.—a Louisville, Ky.–based company that specializes in total ash management, including recycling by-products from coal-fired power plants—has developed a technology that allows sulfur captured from power plant exhaust gases to be pelletized into a calcium sulfate fertilizer product, providing an improvement, it says, over previous forms of fertilizer created from power plant emissions.

Charah’s new facility housing this process is located at the 1,472-MW Louisville Gas and Electric Co. (LG&E) Mill Creek Generating Station, in Jefferson County, Ky. Coal provides the majority of power for Kentucky, and this plant went into commercial operation in 1972 and was LG&E’s first to utilize cooling towers to protect the Ohio River’s aquatic life.

Plant owners are committed to keeping this plant online. Starting in spring 2012, LG&E planned to spend approximately $1.3 billion to modernize the FGD systems and install fabric filter baghouses for increased particulate and mercury control on all units at the plant. This construction project is under way and will continue through 2015. And in November 2012, LG&E officials announced that, as part of the $1.3 billion, they would be spending approximately $940 million on clean coal technology at the station. Mike Kirkland, general manager of Mill Creek Station, told POWER that would include replacing existing scrubbers with new ones, installing new baghouses, and replacing exhaust stacks.

Mill Creek burns approximately 4 million tons of high-sulfur coal annually, primarily sourced from the Illinois Basin. Kenny Tapp, senior by-products coordinator for LG&E and KU Services Co., noted that over 60% of the plant’s fly ash is used in the manufacturing of cement and concrete; the economic value of the fly ash utilization in concrete is estimated to be in excess of $5,000,000 to the regional manufacturers of concrete- and cement-based products. In addition, the plant realizes significant savings on landfill capacity and associated costs, though neither the plant nor Charah would release detailed data on these savings.

The plant has had wet scrubbers and a FGD slurry processing plant on its property since 1978, and its processing plant can dewater up to 1,800 tons of gypsum per day for use in the manufacturing of cement, drywall, or other uses. Now that gypsum has expanded utilization opportunities as fertilizer. This additional use can consume 200,000 plus tons per year of the total gypsum annual production.

From Flue Gas to Gypsum

The sulfur-scrubbing process at a coal-fired power plant typically involves grinding high-calcium limestone to powder and then mixing it with water to form a lime slurry. The lime slurry is then sprayed into a contact chamber, where it combines with boiler exhaust gases and the sulfur reacts with the lime to become chemically bound.

Scrubbers come in two types: wet and dry. In wet scrubbers, the ratio of lime slurry is greater and a slurry by-product is produced. In dry scrubbers, the ratio of slurry to hot exhaust gases is controlled, to dry the lime slurry and result in a dry product. Charah has developed a process to beneficially use the wet scrubber slurry dewatered gypsum to manufacture a sulfur and calcium fertilizer.

Wet scrubbers capture sulfur from all four units at Mill Creek. The lime and sulfur slurry is aerated to create calcium sulfate, dewatered to produce high-quality gypsum, and then processed to make fertilizer at the adjacent Charah facility (Figure 2).

PWR_030114_CoalFertilizer_fig2
2. Conversion site. The Charah product manufacturing facility sits on the Louisville Gas and Electric Co.’s Mill Creek Generating Station property in southwest Jefferson County, Ky. Courtesy: Charah Inc.

Mill Creek produces approximately 600,000 to 800,000 tons per year of calcium sulfate gypsum. The gypsum products are stockpiled onsite, and Charah manages the gypsum on behalf of Mill Creek.

From Gypsum to Fertilizer

The Mill Creek gypsum typically has higher purity than natural gypsum because it has less inert impurities. Mill Creek gypsum is 90+% pure calcium sulfate. Charah utilizes this calcium sulfate gypsum to manufacture a patent-pending fertilizer named “SUL4R-PLUS product” that can be used to replenish the sulfur and calcium in farm soils, turf, and specialty crops (see sidebar). As Danny Gray, executive vice president of Charah, explained, this process essentially closes the cycle loop for the sulfur that once was returned to farm fields with rainfall, but now is removed by the power plant emissions control equipment before discharging the cleaned exhaust gases into the atmosphere.

Sulfur’s Role in AgricultureA key component of agriculture production in the U.S. has been the proper deployment of various types of fertilizers. Historically, the primary fertilizers have been nitrogen (N), phosphorus (P), and potassium (K). High-efficiency farming requires that particular attention be focused on secondary nutrients, which include calcium (Ca), magnesium (Mg), and sulfur (S). Sulfur has become more important to high production and is often referred to as the “fourth major nutrient.” Each of the secondary nutrients is essential for high-intensity farming activities. Though required in smaller quantities than NPK, they are essential for plant growth.As a nutrient, sulfur is needed in significant quantities by many crops that utilize approximately the same amount of sulfur as they do phosphorus. A typical crop, such as corn or soybeans, can extract and remove from the soil 12 to 20 pounds of sulfur per acre (Table 1). The sulfate ion (SO4) is the form of sulfur absorbed by most plants. Replenishment of sulfur is crucial to maintain high production on each acre. Typical sources of sulfur include organic matter, ammonium sulfate, gypsum, zinc sulfate, and elemental sulfur.

PWR_030114_CoalFertilizer_Table1
Table 1. Typical nutrient uptake. Source: Charah Inc.

The Charah plant accepts the gypsum when it discharges from the existing Mill Creek dewatering facility onto a new conveyor that moves it directly into the Charah plant. That gypsum serves as the feed stock for the processing steps that include pelletizing to create the granular SUL4R-PLUS product. Although synthetic gypsum has previously been used as a soil amendment, Charah says it is the first to pelletize the by-product, which makes application easier for the farmer.

That granular product is stored inside the Charah warehouse until it is transported to customers. Custom truck loading is done inside the warehouse facility. Charah also has barge-loading capability, as well as onsite railcar-loading capacity to meet customers’ logistics needs. Because the Kentucky plant is located near the Ohio River, Charah can reach distant markets by barge at economical rates.

The sulfur level of SUL4R-PLUS product is greater than 16%, its calcium level is greater than 20%, and the product looks like and handles like any other granular fertilizer (Figure 3). Farmers can replenish the sulfur depleted by crops from farm soils by applying SUL4R-PLUS product along with their other fertilizers. The product has a unit weight of approximately 50 pounds per cubic foot and spreads in common distribution equipment in a single pass across the field.

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3. From power plant to pelletized fertilizer. Courtesy: Charah Inc.

Win-Win Economics

In nations where power plant emissions are tightly regulated, adding beneficial reuse of by-products is likely to become an increasingly valued option for the future business case. At full capacity, more than 50% of Mill Creek’s gypsum will be beneficially used. By avoiding disposal of the recycled by-products, LG&E realizes lower operating costs, which help lower electricity costs for the utility’s customers.

Additionally, Gray says Charah’s granular fertilizer provides good economic value to the American farmer, as typical prices of SUL4R-PLUS product are 20% to 30% lower than alternative sources of sulfur equivalents.

Charah’s investment of $12 million to $14 million in 2013 has provided a first-of-its-kind manufacturing plant to convert high-grade calcium sulfate into a new agriculture product. The plant is designed to reclaim up to 300,000 tons per year of gypsum and produce up to 250,000 tons of SUL4R-PLUS product fertilizer. It also created up to 25 new jobs in the recycling industry.

At power plants that generate a high-quality gypsum product, Charah says a manufacturing plant can be custom designed and installed within 12 months. Charah provides the capital for SUL4R-PLUS plants and maintains owner and operator status. Agreements between Charah and the host power plant typically extend over five to 15 years. Charah plans to develop and install SUL4R-PLUS manufacturing plants throughout the U.S. at strategic locations to meet the growing demand for agricultural sulfur products. ■

Gail Reitenbach, PhD is POWER’s editor (@GailReit, @POWERmagazine).

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