Jeffrey Rode, Project Manager, Alstom
Michael Proffit, Project Engineering Manager, Reliant Energy
When Reliant Energy broke ground in 2001 for the $800 million 584 MW Seward power plant in Pennsylvania, it not only began construction of the first major coal-fired power plant to be built in the state in 20 years, but also one which will help to solve a long-running environmental issue – the removal of large piles of reject coal.
The baseload plant will be situated within Pennsylvania’s western coal mining region and will supply power to the PJM Interconnect
In 2001, Texas, USA-based Reliant Energy Inc began construction on the Seward coal fired power plant. The base load plant is one of the first solid fuel based merchant power plants to be built in the USA, and will be situated within and fueled by Pennsylvania’s western coal-mining region.
Seward will supply its power to the mid-Atlantic region’s PJM Interconnect. It will be powered by two Alstom supplied 292 MW (gross) circulating fluid bed (CFB) boilers that will burn local waste coal – a significant environmental issue in its own right – while meeting stringent state and federal clean air emissions standards.
When the new plant begins commercial operation in 2004, it will replace an existing, 80-year-old, 200 MW power facility that will be shut down in late 2003. Due to the combination of CFB technology and other emissions control equipment, the nitrogen oxide (NOx) and sulphur dioxide (SO2) emissions will decrease even though the new station will be capable of producing twice the electricity of the old plant.
Western Pennsylvania has been producing bituminous coal for more than 100 years. Before environmental laws regulating waste coal became more stringent, mine operators dumped the reject coal onto large piles. Water running off the waste piles fouls streams and contaminates water supplies. The Seward plant, which is expected to remove and consume up to 100 million t of waste coal from the Pennsylvania landscape over the project’s life, will eliminate a significant source of acid discharge from the state’s Kiskiminetas-Conemaugh watersheds.
The Seward installation will include Alstom’s new FDA system, which integrates several flue gas desulphurization functions into one unit to achieve 95 per cent SO2 removal
In addition, the alkaline ash produced at the plant will be returned to many of the waste coal sites to neutralize any acids remaining in the soil.
A key driver in obtaining the necessary environmental permits for the plant involved its use of CFB technology. The technology has continued to evolve from the initial clean coal and PURPA project in the US to essentially worldwide market penetration burning a variety of fuels. That fuel flexibility allows Reliant Energy to lower the Seward plant’s fuel costs and subsequently compete in a deregulated marketplace by burning less expensive fuel like Pennsylvania waste coal while still meeting stringent plant emissions requirements.
The environmentally attractive features of the CFB result from its inherent combustion characteristics. Sulphur dioxide is captured in the combustion process by adding limestone to the fluidized bed. Because the combustion temperatures remain in the 815°C-870°C (1500°F-1600°F) range – as compared to 1650°C (3000°F) for conventional pulverized coal technologies – NOx formation is also reduced during combustion.
Reliant Energy owns or partly owns 26 generating plants in Pennsylvania, New Jersey and Maryland. It is also in the process of building an 800 MW gas-fired facility 8 km north of Gettysburg in Adams County
The Seward CFBs are similar in design to other Alstom units either operating or under construction in Puerto Rico (2 x 250 MW) and Kentucky (1 x 268 MW). Each of the Seward units will produce approximately 961 t/hr main steam and 877 t/hr of reheat steam.
To meet Pennsylvania’s air emissions standards, the plant will use an assortment of back-end emissions control technologies to reduce emissions further. Specifically, the CFB technology will be combined with selective non-catalytic reduction (SNCR) equipment for additional NOx reduction, as well as one of the first US applications of Alstom’s patented flash dryer absorber (FDA) system, which in combination with the boiler will reduce SO2 emissions while reducing the amount of limestone required.
An aqueous ammonia-based SNCR will be used on each Seward unit to reduce NOx emissions levels to 45g/MMBTU (0.1 lb/MMBTU) with a resulting ammonia slip not to exceed 10 ppm. In the selective non-catalytic reduction process, aqueous ammonia gas is injected into the flue gas where it thermally reduces the NOx in the flue gas stream to form nitrogen (N2) and water vapour. At the Seward plant, the aqueous ammonia will be injected in the CFB’s gas ducting, providing good mixing and dispersion of reagent.
In addition, the Seward installation will include Alstom’s FDA system which basically integrates several flue gas desulphurization functions into one unit to achieve 95 per cent SO2 removal or better, irrespective of the sulphur content in the fuel. A unique feature of the FDA system is its ability to use ash (which contains unreacted lime) supplied by the CFB (via the flue gas stream) for further sulphur dioxide absorption down stream. The system comprises the patented FDA reactor followed by a fabric filter.
Reliant Energy’s Seward power plant will remove and consume up to 100 million t of waste coal from the Pennsylvania landscape over the entire lifetime of the project
At Seward, the flue gas will be fed into four FDA reactors (per-boiler), where it is mixed with recycled fly ash from the fabric filter. From the FDA reactor, the gas and dust mixture is carried to the fabric filter. Fly ash is recycled from the fabric filter to four mixers, where water is added. The water addition activates the lime content in fly ash originating in the boiler.
The acid gas components in the flue gases react with the lime during intense contact in the reactor. The dust, with its reacted components and captured sulphur, is collected in the fabric filter. The dust collected in the fabric filter falls into two hoppers, where it is recycled to the mixers. The end product is discharged from the filter hopper and transported to a silo via a pneumatic conveying system.
The key parameter to be controlled in any dry FGD process is the humidity of the flue gas in the reaction zone. At a relative humidity of 50 per cent, the hydrated lime is activated and readily absorbs SO2. In a conventional dry FGD process, water and lime are supplied to the flue gas as a slurry (with or without recycle) with a solids content of 35 to 50 per cent. In the FDA, the same amount of water is injected into the flue gas, but it is distributed onto the surface of dust particles with a water content of only a few per cent. As a result, the amount of absorbent (which is recycled) is much greater than in a conventional dry FGD process.
This means that the surface area available for evaporation is much larger than the surface area of a conventional dry FGD. Thus, the time required to dry the dust added to the flue gas is short, which in turn makes it possible to use small reactor vessels. In fact, the volume is an order of magnitude less than the corresponding size for a conventional dry flue gas cleaning system based on spray dryer technology.
The resulting increase in relative flue gas humidity is sufficient to activate the lime for SO2 absorption at operating temperatures typical to a dry FGD: 10 to 20°C above saturation and, in practice, within the temperature range of 65 to 75°C. The activation is considered to proceed fast enough for the SO2 absorption reaction when a layer of one to two water molecules has been formed on the surface of the lime. Water is added to the absorbent in a mixer prior to its introduction into the flue gas. A key feature to the FDA is that all recycled absorbent is subject to wetting in the mixer, which maximizes the use of the recycled absorbent.
After the activation/drying step, the dried recycle dust is separated from the flue gas in a highly efficient dust collector. The separated dust is then again fed to the mixer, concurrent with water being fed to the mixer to hydrate the recycled fly ash before being injected into the reactor.
The control system uses a feed forward signal with back trim, based on the inlet and outlet flue gas temperatures supplemented by a signal indicating the gas flow. The outlet SO2 concentration is controlled in a similar way; the in and outlet SO2 concentrations, plus the flue gas flow, determines the rate of fly ash recycle. The FDA system was evaluated favorably when compared with both conventional dry FGD and wet FGD alternatives.
Compared to conventional dry FGD systems equipped with rotary atomizers or dual fluid nozzles, the FDA process minimizes the need for sophisticated and/or special equipment. There is no rotary atomizer with its high-speed machinery, nor are there any dual fluid nozzles requiring compressed air. Power requirements for the FDA recycle/reagent mixers are much lower than for the corresponding items in a conventional dry FGD system.
The FDA system was also evaluated favourably with respect to its ease of maintenance. All equipment requiring operator attention is placed near ground level, in an enclosure common with the fabric filter. A comparison with a wet FGD system showed lower costs for the FDA since a wet system would require a more expensive wet stack and a separate dust pre-collector for fly ash removal. Finally, pilot tests have shown limestone costs can be reduced by 10 to 30 per cent (by reducing the amount of limestone added to the CFB) while at the same time reducing the amount of disposable ash.
As a result, the combined effect of the CFB technology and plant’s back-end air emissions control systems is expected to be as follows:
•Total NOx emissions in the flue gas will be limited to 45 g/MMBTU (0.1 lb/
MMBTU). The SNCR system using aqueous ammonia will help reduce the NOx emissions to compliance levels.
•Total SO2 emissions in the flue gas will be limited (by the CFB boiler and the FDA) to 272 g/MMBTU (0.6 lb/MMBTU). About 70 per cent of the SO2 formed during the combustion of sulphur in the fuel will be captured internally by the calcium oxide generated from the calcination of the limestone in the CFB combustor.
•A bag house with the FDA will control particulates leaving the flue gas stream from both the CFB and the FDA and keep them at a level of 4.5 g/MMBTU (0.01 lb/MMBTU).
•Total carbon monoxide (CO) emissions will be limited to 6.8 g/MMBTU (0.15 lb/MMBTU) at 70 to 100 per cent load, and 9 g/MMBTU (0.2 lb/MMBTU) at 40 to 70 per cent load. Performance of existing Alstom CFB boiler installations indicates these CO emission levels are achievable without supplemental or post-combustion CO reduction equipment.
•Volatile organic compound (VOC) emissions in the flue gas are limited to 2.3 g/MMBTU (0.005 lb/MMBTU). Performance at existing Alstom CFB installations indicates this VOC emissions level is also achievable without supplemental or post-combustion VOC reduction equipment.
•Emissions of ammonia will be limited to 10 ppm. Limiting the ammonia injection for SNCR NOx control will limit the ammonia emissions.
Reliant Energy owns or partly owns 26 generating plants in Pennsylvania, New Jersey and Maryland. In addition, it is also in the process of building an 800 MW gas-fired facility 8 km north of Gettysburg in Adams County. Together, these new facilities will expand and solidify Reliant Energy’s presence in the mid-Atlantic region – which represents some 23 million people in five states and Washington DC that represent a growing marketplace.
At Seward, the use of CFB technology in combination with state-of-the-art emissions control technologies will result in a power plant that is certain to be an industry standard-bearer for years to come. By eliminating waste coal piles, the plant will:
•Mitigate what has been a difficult, long-term environmental issue in Pennsylvania
•Combust a low-grade fuel inexpensive enough to allow the plant to compete effectively in a deregulated power market and
•Adhere to some of the strictest air emission standards in the country.
In the process, the plant will ensure fuel diversity in the USA’s mid-Atlantic region, thereby providing stability for consumers in an increasingly volatile energy marketplace. Other economic advantages include the direct and indirect preservation of about 130 jobs and the creation of about 400 new ones, according to a study by Pennsylvania State University. In addition, about 1600 construction workers will be on-hand when the peak construction period commences in late 2002 and early 2003.