300 MWe demonstration CFB takes shape at JEA’s Northside power plant

It is only relatively recently that CFB boilers have been considered for larger utility power plants. At nearly 300 MWe each, the JEA’s two CFB plants will be the world’s largest.

Among the attractions of CFB technology is its ability to burn efficiently a wide range of fuels while achieving low emissions. The low combustion temperature allows SO2 capture via limestone injection, while minimising NOx emissions. The technology is flexible enough to handle a wide range of coals plus petroleum coke as well as blends of coal and coke.

JEA is the eighth largest public power company in the USA and is currently experiencing a load growth rate of over 3 per cent per year. Its Northside Generating Station includes three oil/gas fired steam plants, units 1-3 (in addition there are four diesel-fired combustion turbine units at the site).

The demonstration project involves repowering unit 2. This unit was originally completed in 1972 but has not operated since 1983 due to boiler availability problems. It was originally equipped with a 275 MWe oil/gas fired boiler and will be repowered with a 297.5 MWe CFB boiler. DOE is contributing $73.1 million from its Clean Coal Technology Program, with JEA providing the remainder of the total cost. The DOE cost sharing includes two years of demonstration test runs, during which both coal and coal/petcoke blends will be fired.

JEA also plans to repower unit 1, which is currently operating and whose original boiler came on-line in 1966, with an identical CFB boiler to that being installed at unit 2. The unit 1 repower will be privately financed. Unit 3, a 564 MWe oil-fired unit, with a boiler that came on line in 1977, will continue to operate.

Units 1 and 3 currently fire relatively high cost fuels and so capacity factor has been limited. After the repower, units 1 and 2 will fire relatively low cost solid fuels, and plant power production is expected to increase to over twice current levels. At the same time, total plant emissions of NOx, SO2 and particulate matter will be at least 10 per cent below current levels.

Preconstruction activities for the project began in August 1999, with initial operation of unit 2 envisaged for January 2002, followed by unit 1 in May 2002. The two year demonstration programme is scheduled to start in April 2002, following which the repowered unit 2 will continue in commercial operation. The unit 1 project is about three months behind unit 2.

JEA has contracted with Foster Wheeler to supply the extended boiler island scope of the project. The rest of the project will be implemented by JEA staff, supplemented by Black & Veatch Corporation through a pre-existing alliance with JEA for engineering services. Procurement, construction and related services will be provided through other pre-existing alliances between JEA and Zachry Construction Company, Fluor-Global, W.W. Gay Mechanical Contractor, Inc, and Williams Industrial Services Inc. This work will include an upgrade of the existing turbine island equipment, construction of the receiving and handling facilities for the fuel and reagent required to convert the plant from oil/gas firing to solid fuel firing, upgrade of the electrical switchyard facilities and an ash management system.

CFB boilers are generally capable of removing over 98 per cent of SO2. However, to improve the overall economics and environmental performance, a polishing scrubber will be employed to minimise reagent consumption while firing petcoke containing up to 8.0 per cent sulphur. The relatively low furnace operating temperature of about 1600°F (870°C) would inherently result in appreciably lower nitrogen oxide emissions compared with conventional coal-fired power plants. However, the project will also include a new selective non-catalytic reduction (SNCR) system to further reduce emissions of nitrogen oxides. Over 99.8 per cent of particulate emissions will be removed by a new baghouse.

In addition to the CFB combustor itself and the air pollution control systems, the project will also entail a new stack plus new fuel, limestone, and ash handling systems. The project will also require overhaul and/or modifications to existing systems such as the steam turbines, condensate and feedwater systems, circulating water systems, water treatment systems, plant electrical distribution systems, the switchyard, and the control systems.

New construction associated with the CFB project will occupy about 75 acres (30 hectares) of land at the site. Solid fuel delivery will be accommodated by construction of new receiving, handling, and storage facilities. Limestone and ash storage and handling facilities also will be required.

The 300 MWe class CFB boiler is designed to fire 100 per cent bituminous coal and 100 per cent high sulphur petroleum coke. The tables above show the steam conditions and fuel analysis on which the design is based.

CFB boiler design features

The boiler contains a single, water-cooled furnace. An INTREX (Integrated Recycle Heat Exchanger) receives the ash flow in the return leg from each cyclone, and contains intermediate and finishing superheater surface. Three steam-cooled cyclones are provided. The backpass is of parallel pass design and contains primary superheater, reheater, and economiser surface. A tubular air heater with flue gas inside the tubes follows the economiser in the gas path.

Furnace

The main influences on CFB boiler configuration are the specified steam conditions and the fuel type. Compared with industrial boilers, the superheat and reheat duty of utility boilers is a greater percentage of the total input due to higher steam pressure and temperature. Higher feed water temperature in the utility boiler further increases the furnace heat duty due to a larger air heater duty which is transferred to the furnace.

The evaporative duty is provided by the enclosure, division, and wing walls of the furnace. The furnace is a gas-tight enclosure formed from membrane tube panels cooled by natural circulation. Water-cooled partial division walls divide the furnace into three zones and so help evenly distribute gas and solids to the three cyclone separators. Six wing walls inside the furnace will provide additional evaporative surface. There will be no superheat or reheat surface located in the furnace. This arrangement of furnace and INTREX heat exchanger surface gives uniform heat removal and minimises temperature variations.

The furnace temperature can be effectively controlled by changing the solids loading in the upper furnace, by varying the primary/secondary air ratio and by changing the solids flow over the INTREX heat exchanger superheat surface.

To avoid erosion, a thin refractory lining is applied over metal studs in the lower furnace and around the openings to the cyclone. A patented tube arrangement at the top of the lower furnace refractory lining avoids local erosion. The critical dimensions of furnace height and depth have been controlled within Foster Wheeler’s experience base to minimise scale-up risk.

Steam-cooled cyclone

The separator is one of the most important components in CFB boilers. Its efficiency is vital for the proper operation of the boiler. Proper efficiency will capture sufficient solids to ensure good bed quality which is manifested by proper furnace temperature and low temperature drop in the furnace, low carbon loss and low emissions.

The JEA CFB boiler uses three steam cooled cyclones. Each cyclone is lined with 1 in (25 mm) thick refractory held in place with metal studs to protect against erosion. Operating experience in all Foster Wheeler steam cooled cyclones shows this refractory lining design is virtually maintenance-free.

INTREXTM heat exchanger

In large CFB boilers, about 25 per cent of total superheat (SH) duty is absorbed in the hot solids circulating loop, via in-furnace surfaces such as wing walls or via surface in an external heat exchanger. As a boiler reaches utility sizes, Foster Wheeler uses the innovative and patented INTREX heat exchanger.

Solids returning from the cyclones flow into the inlet channels of the INTREX heat exchangers. During normal operation, the solids are passed into the SH cells by fluidising both the inlet channels and the SH cells. During start-up the SH cells are bypassed by fluidising only the inlet cells. By changing the mode of fluidisation in the inlet channels and SH cells, solids flow to the SH cells can be controlled to change the superheat pickup in the INTREX heat exchanger, hence furnace temperature.

The INTREX heat exchanger enclosure comprises an inlet channel, superheat bundle cells, and a return channel to distribute solids evenly back to the furnace. The INTREX heat exchanger design for the JEA CFB is based on the INTREX heat exchanger used at the NISCO (Nelson Industrial Steam Company) plant. Over eight years of NISCO operating experience has shown that the design works well and provides the following advantages:

l Reduced corrosion and erosion. High temperature SH surface located in the INTREX heat exchanger is not exposed to corrosive elements in the flue gas stream. This makes the INTREX heat exchanger excellent for firing corrosive fuels. Very low fluidisation velocity in the SH cells (< 1.0 ft/s (0.3 m/s)) and very fine particle sizes (~200 micron) eliminate the potential for erosion to the SH tubes.

l Furnace temperature control. As already noted, the change of fluidisation mode can effectively adjust the furnace temperature.

Parallel pass backpass

The backpass contains two parallel gas passes; the front pass houses the reheat surface and the rear pass houses the primary superheater. The hot gas is biased by two gas pass dampers located underneath the backpass. Reheat temperature control is achieved without water spray by controlling the gas flow passing over the reheater, which causes no reduction in cycle efficiency compared with spray RH control. This design is proven on Foster Wheeler utility boilers up to 930 MWe.

Start-up duct burners

Duct burners, firing natural gas (with oil as backup) will be used for start-up to preheat the primary air stream which in turn uniformly preheats the bed material to the temperature needed for solid fuel combustion. This preheating method maximises the efficiency of bed preheating and so minimises the amount of start-up fuel required, saving about 40 per cent start-up fuel compared with start-up burners located on the furnace wall.

Fluidised ash cooler

The bottom ash cooler is required to maintain the desired furnace inventory and cool the ash to the temperature required by the bottom ash handling system. The JEA CFB boiler uses the Foster Wheeler patented stripper/cooler design which is typically used for large CFB boilers or for high ash fuel in smaller boilers. The stripping(classifying)/cooling process consists of draining material from the bed and then, fluidising this material in the stripper zone at a velocity sufficient to strip the required amount of fines from the stream and return these fines to the furnace. The balance of the material, which is primarily coarse, will pass through the next cooling zones to the ash drain in the floor of the last zone. These zones are fluidised and cooled by air from the air heater and from the primary fan. The stripper section is important for returning the fines (typically unburned carbon and unutilised limestone) to the furnace thereby increasing carbon burnup efficiency and reducing limestone consumption. The stripper/cooler also raises the boiler efficiency significantly by recovering the heat from bottom ash, as compared to cooling devices which do not recover this thermal energy.

Fuel feed system

The fuel feed system is designed to accommodate a positive pressure condition with the furnace balance point set at the cyclone inlets. Seal air is provided from the primary air fans to the belt feeders. These fans also provide air to the air swept fuel distributors. The air swept fuel distributor adds horizontal momentum to the fuel to assist in injecting it into the boiler. Seal legs of material are provided in the downspouts above the belt feeders. These legs are of sufficient height to seal against the maximum furnace pressures anticipated.

The proven air swept fuel distributors have been carefully designed to propel the fuel into the furnace in such a manner as to avoid hang-ups and back flow from the furnace and to distribute the fuel throughout the bed. They are the result of an extensive research programme involving numerous flow models and operating experience. Air is admitted into each distributor at two locations in a carefully designed manner to maintain the proper velocity and flow pattern.

Reducing emissions

To optimise overall plant performance, the JEA project incorporates a polishing SO2 scrubber. The CFB boiler provides about 90 per cent SO2 capture via limestone injection, with the remaining capture from a semi-dry polishing scrubber via injection of lime. Overall SO2 capture is over 98 per cent.

Although CFB boilers can achieve 98 per cent SO2 removal, limestone utilisation is reduced as removal efficiencies exceed 90-95 per cent. The polishing scrubber allows reduction in the overall sorbent use, such that the savings in operating cost (sorbent, ash disposal) could offset the capital and operating costs of the polishing scrubber. However, another consideration in the decision to add the scrubber was the enhanced environmental performance regarding trace elements provided by the scrubber. While this benefit cannot be quantified, JEA decided the added capital cost was justified.

Various types of polishing scrubbers were evaluated. The design selected is a spray dryer/baghouse combination. The spray dryer utilises a dual fluid nozzle atomised with air and the baghouse is a pulse-jet design. A key feature of the polishing scrubber is a recycle system which adds flyash to the reagent feed, thus utilising the unreacted lime in the flyash from the CFB and reducing the amount of fresh lime required. Further, provisions will be made for a future recycle system for the CFB bottom ash.

The SO2 emissions limit is 0.15 lb/MMBtu (220 mg/Nm3), which requires over 98 per cent SO2 capture when firing petcoke with the maximum sulphur content of 8 per cent. This emissions limit is achieved via limestone injection in the CFB plus lime (and recycle ash) injection in the polishing scrubber.

NOx emissions are inherently low due to the low CFB furnace temperatures and staged combustion.

Additional NOx control will be provided via an SNCR deNOx system whereby aqueous ammonia is injected into the flue gas stream at the cyclone inlet. The cyclone provides for efficient mixing of the flue gas and ammonia and sufficient gas residence time at the optimum temperature for effective NOx reduction. The combination of low NOx at the furnace outlet with the additional reduction provided by the SNCR system will result in emissions below 0.09 lb/MMBtu (130 mg/Nm3).

Particulate control is accomplished by a pulse jet baghouse. Outlet emissions of particulate matter (total and size fraction less than 10 micron) will be less than 0.011 lb/MMBtu (20 mg/Nm3).

CFB technology: the basic process

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