Air Pollution Control Equipment Services, Emissions

Retrofitting comes in small packages

By Amethyst Cavallaro
Online Editor, Power Engineering

CAIR is coming, and it’s influencing the fate of over 400 smaller, aging coal-fired plants. The Clean Air Interstate Rule and the Clean Air Mercury Rule will force coal power plant owners and managers to either install costly emissions reduction equipment or remove the plant from service. But with an energy hungry American population growing, some plant managers have found ways to extend the lives of their plants and not throw out the capacity with the regulations.

AES Corp. is installing multi-pollutant control technology that may reduce SO2 by 95 percent, mercury by more than 90 percent and NOx by 65 to 70 percent at its 161 MW Greenidge station in Dresden, New York.

But making a 1930s-vintage plant come into compliance isn’t the only goal: AES wants to demonstrate this technology can be economically viable for the more than 400 smaller, coal-fired plants around the United States.

The demonstration project, funded in part by a $14.5 million Clean Coal Technology Cooperative Agreement from the U.S. Department of Energy’s National Energy Technology Laboratory, will help reduce emissions at the plant’s 107 MW Unit 4. The total cost of the project is $39 million and is expected to be completed by the end of 2006.

The project participants hope to increase interest in using hybrid emissions systems to get the reduction smaller plants need with a lower overall installed and operating cost.

Cascading Emissions Away
The emissions technology is a blend of methods designed to meet the needs of smaller coal-fired plants. AES asserts that this technology is substantially less capital intensive than conventional equipment and is expected to significantly decrease the costs associated with using conventional emissions reduction systems, making the implementation of clean-coal technology economically feasible for smaller facilities. The plant should come online Nov. 18 and the current estimated cost is $330-$350/kw. Douglas Roll, plant manager at AES Greenidge, estimated that installing a large clean coal type of facility would cost around $1,000-$1,500/kw.

Greenidge is installing a circulating dry scrubber for SO2 control with a baghouse that will complement enhanced SO2 and mercury control. For the mercury, AES is using a power activated carbon injection system where mercury adheres to the carbon particles and goes out of the system with the fly ash. In combination with the circulating dry scrubber, the mercury will have more residence time with the activated carbon than it would with other techniques. Activated carbon has been used in a number of places blown into ductwork, but the ductwork does not have a lot of residence time, said Roll. “The circulating dry scrubber will give the mercury more contact time with the carbon, so we’re looking for a very high reduction rate,” he said.

Instead of installing one piece of equipment for NOx control, AES chose to use a three-phase or “cascade” system. Emission reductions begin in the boiler with the installation of low-NOx burners. The exit gases from the combustion zone then pass through a selective non-catalytic reduction (SNCR) system where urea is injected into the gases, converts into ammonia, which then reacts with the NOx for further reductions. The extra ammonia, or ammonia slip, continues to cascade down to a selective catalytic reduction (SCR) single-layer catalyst system located in the exit gas pass. The SCR uses the ammonia slip as a catalyst to reduce the remaining NOx gases.

The process brings NOx emissions from 0.55 lb/MMBtu to less than .10 lb/MMBtu. “It’s fairly unique in combining three technologies for an overall lower NOx output,” said Roll. “We’re kind of wrapping it all together in a package. For us, it made it a fairly low cost capital improvement.”

“Greenidge is a good project from what we see coming up in the future as far as meeting people’s NOx reduction needs,” said Kevin Dougherty of Fuel Tech.

Babcock Power Environmental was the engineering, procurement and construction (EPC) firm used for the project and hired Fuel Tech as a subcontractor for the SNCR system. Fuel Tech holds the patent for the cascade technology. The technology is based on using urea SNCR in the furnace and the unreacted ammonia slip to feed the downstream SCR without the need for an ammonia injection grid (AIG). The AIG is used just upstream of the catalyst in a conventional SCR system to inject ammonia so it can react with the NOx across the catalyst.

CONSOL Energy, Inc. is the prime contractor under the DOE Cooperative Agreement and will be responsible for project administration, performance testing, and reporting. CONSOL Energy will work with the U.S. Department of Energy for monitoring the project implementation at AES Greenidge.

“The project certainly will demonstrate that you can retrofit an older plant as long as it has been well maintained and has significant capital invested to maintain its reliability and has a heat rate of around 10,000 Btu/KWh to make it economically feasible,” Roll said. He identified about 400 power plants that might be able to use this technology with 80 percent of them located in the eastern United States.

Planning for the Future
CAIR isn’t the only legislation the Greenidge plant is planning for. The plant has been co-firing up to 5 to 10 percent biomass along with its bituminous coal supply since 1995 and may consider including it as a compliance strategy in meeting future possible greenhouse gas emission controls. The plant is engineered to turn the biomass option on and off, giving plant managers the ability to choose what type of fuel is most effective.

Roll said the cost of biomass is currently prohibitive for long-term use. It takes three times the amount of biomass to get the same heat value of coal and biomass usually has a level of moisture content that complicates its preparedness for burning. But he said that in the future, there might be other incentives for burning biomass, which would then make the price AES is willing to pay for biomass higher.

“It provides us another way that we can be flexible in our operations,” he said.

For more information on hybrid SNCR/SCR systems, see the Coal-Gen e-newsletter feature by Thomas Wright and James Cox of Worley Parsons Group.