Dominion Energy has just completed a $46-million Carbon Burn Out (CBO) Fly Ash facility at its Brayton Point power plant in Somerset, Mass.
Now in full production, the new CBO system produces a consistent, low-carbon Class F fly ash suitable for use as a substitute for Portland cement in producing
concrete and in other construction applications. Fly ash, used on roads and interstate highways for some 50 years, is a byproduct of the combustion of pulverized coal. More than 68 million tons were produced in 2001 by coal-fired electric and steam generating plants.
Dominion’s Brayton Point is one of those producers. According to Michael Kaczmarek, ash marketing representative for Dominion, the 1,600-megawatt plant is the largest coal- and oil-powered generating station in New England, with 1,160 of the megawatts coming from the combustion of coal.
Kaczmarek said that Brayton Point burns about 3 million tons of coal per year in its coal-fired boilers, adding that all of the coal is delivered by ship.
“The power plant burns 10,000 tons of coal per day when all coal-fired boilers are online. If trucks hauled Brayton Point’s coal it would take 333 18-wheel dump trucks per day to keep it going or a truck every 4-1/2 minutes,” he said.
Coal is pulverized into a powder as fine as talcum powder. It is then pneumatically blown into the boilers where it combusts as well as natural gas. Boiler tubes extract heat from the boilers, cooling flue gasses and causing molten mineral to harden and form ash. The mineral portion of the coal, which does not burn, as well as 8-percent to 12-percent of residual unburned carbon remains. Coarse ash particles — the bottom ash or slag — fall to the bottom of the combustion chamber, while the lighter fine ash particles — the fly ash — remain suspended in the flue gas. Brayton Point produces approximately 250,000 tons of coal ash per year.
Coal fly ash can be used a substitute for Portland cement in making concrete ready mix, but only if the carbon levels are 2-1/2 percent or less, Kaczmarek pointed out.
To produce fly ash of this quality, Dominion invested in the construction of its new CBO facility.
Capable of processing more than 300,000 tons of ash per year, the CBO facility employs the patented technology of Progress Materials Inc. Fly ash, containing residual carbon from the power plant, is fed into a fluidized bed combustor, where it is initially heated by a natural gas startup burner. Once the ash reaches a temperature of approximately 800 degrees Fahrenheit, the carbon within the ash auto-ignites. At that point, the burner is turned off, and the combustion process becomes self-sufficient with the carbon in the ash feed alone. After at least 45 minutes in the CBO unit, the processed ash is removed having the carbon reduced to specification.
Dominion built a 40,000-ton storage dome together with a truck load-out system. The ash storage silo is one of the largest in the United States, and the largest in New England. It stores winter CBO ash production for use in the summer construction season.
The load-out facility has a 200-ton load-out silo with two-truck load-out bays, each with its own truck scale. Automatic equipment transfers fly ash from the 40,000-ton storage dome into the truck load-out silo keeping it full for truck loading.
The load-out facility has the ability to be self-loading or loaded by an attendant. In the self-loading mode the driver pulls his truck into the enclosed truck bay, exits the truck and climbs a safety platform to the top of his pneumatic tanker where he lowers the dustless loading chute into his hatch, and then exits the truck bay. He enters the control room and swipes an electronic ID card that identifies him to the ash marketers’ dispatch system.
This dispatch system processes the information required to load his truck and prints a bill of lading. When all the information is exchanged the driver pushes the load button and the system loads 85 percent of the load into the truck in a fast fill mode, pauses for two minutes to let the ash settle, then loads the last 15 percent in a slow fill mode to the weight sent by the dispatch system.
Fly ash has many applications in highway and other construction, and its use has been promoted since the early 1970s by the Federal Highway Administration. The Environmental Protection Agency has also been supporting the use of the material because of its environmental benefits, since its use increases the life of concrete roads and structures by improving concrete durability. When fly ash is utilized to replace or displace manufactured cement, it results in a net reduction in energy use and greenhouse gas and other adverse emissions. In addition, using fly ash lowers the amount of coal combustion products that otherwise would be disposed in landfills.
Among its many applications, fly ash is used in concrete admixtures to enhance concrete performance, combined with lime and aggregate to produce stabilized road base course, mixed with water and Portland cement to produce flowable fill, and as a borrow material to construct fills and embankments.
Other fly ash uses include adding the material to hot mix asphalt to increase the stiffness of pavements, and to water and other materials as a grout to fill voids under concrete pavements without raising slabs.
Brayton Point’s fly ash is being marketed by Headwater Resources, a subsidiary of Headwaters Incorporated, a nationwide company that supplies materials from coal combustion products. Stephen Berlo of Scituate, Mass., is the company’s New England representative.
Dominion Energy, owner of Brayton Point, is one of the nation’s largest producers of energy, with assets including over 28,000 megawatts of power generation, 6,000 miles of electric transmission, about 6.3 trillion cubic feet equivalent of proved natural gas reserves, 7,800 miles of natural gas pipeline, and about 950 billion cubic feet of storage capacity.
# By Paul Fournier