French firms band together to bring large CFB dream into reality
250 MW power plant in France uses a circulating fluidized bed boiler to reduce environmentally harmful emissions
By Pierre Lucat
GEC Alsthom Stein Industrie
A 250 MW power plant using the world`s largest circulating fluidized bed (CFB) boiler is in operation in Gardanne-Meyreuil, in the south of France. Designed and manufactured by the French utility boiler manufacturer GEC Alsthom Stein Industrie, the plant burns a variety of fuels, particularly coal with a high ash and sulfur content, while reducing sulfur and NOx emissions. Its ability to burn fuel in the furnace and scrub fumes at the same time, at a competitive cost, makes this method for generating electricity a highly attractive alternative to conventional pulverized coal (PC) boilers.
GEC Alsthom Stein Industrie has long been interested in the potential of CFB boilers. The French coal board, Charbonnages de France, is also attracted to them because the board is seeking the best method for using low-grade fuels, such as coal-washing residues and coal with a high sulfur content.
Electricit? de France (EDF), the French electric utility, also wants to carefully evaluate clean coal technology as part of its plan to build 600 MW coal-burning power units in France, as well as for the overseas power stations. According to EDF, it undertook the Provence power plant project as a step toward preserving its ability to provide a generation mix of nuclear, fossil-fired and hydroelectric power that best meets the demand for power consumption. EDF has 54 nuclear units with four more under construction, 49 conventional thermal units and a few hydroelectric plants, with a total installed capacity of 23,300 MW. Coal is considered a critical element of the fuel mix for EDF because coal reserves are both plentiful and well distributed around the world, making it both affordable and available as a power generation resource. Due in part to the European Union`s new requirements for environmental protection, the utility accelerated its development of new generating units using clean coal technology. EDF had been working on the design of a 250 MW CFB power plant since 1987, basing its studies on the integration of such a boiler into an existing old oil-fired unit.
EDF, GEC and the French coal board eventually combined their capabilities and common interest in developing clean coal power plants.
CFB vs. PC
CFB boilers consist of a furnace, highly efficient cyclones and siphon seals from which hot particles are recycled to the furnace. The furnace receives coarsely ground coal and the limestone necessary for sulfur removal. Combustion and desulfurization take place in the furnace within the bed, a large mass of highly agitated fine ash particles, at a relatively low temperature. These particles, which are also called solids, are held in suspension, or fluidized, by an upward flow of air blown into the bottom of the furnace.
The bed completely fills the furnace. Cyclones capture solids leaving the furnace which are then recycled through fluidized siphon seals back into the furnace, thus completing a recirculation loop. The flue gases then flow through a conventional heat recovery section, the air heater and dust-collecting system before being discharged at the stack.
Compared to conventional PC furnaces where particles simply follow gas streams, CFB systems are characterized by high gas/solids slip velocity, strong agitation and the mixing of particles both internally and externally. These characteristics greatly improve contact between gases and solids, with solids` residence time in a CFB system many times longer than in a PC furnace.
CFBs have excellent properties regarding heat and mass transfer, which explains why coal particle combustion (ignition as well as complete char burnout) is much easier in a CFB than in a PC boiler. Combustion in a CFB boiler doesn`t depend on fuel analysis, requires much less fuel preparation and can be efficiently achieved at a much lower furnace temperature. Its reduced temperature prevents slagging troubles which can affect PC units. The absence of hot spots also prevents ash buildup. In addition, their reduced furnace temperature ensures much lower NOx emissions than with PC units (Table 1).
The advent of this new technology in the power station market occurred in 1985, with the commissioning of the world`s first reheat CFB boiler in Duisburg, Germany. It was a 96 MWe, single furnace unit. Since that time, more than a dozen commercial units from approximately 100 MW and up to 170 MW have been placed in operation in Europe and North America. This already represents a fairly fast penetration of the market. An even larger share could be expected in the future now that 250 to 350 MW CFB boilers are available on a commercial basis, as such unit sizes match the needs of many power stations around the world.
External heat exchanger
The optimum reaction temperature is around 850 C for fluidized beds. In practice, most CFB boilers require extra heat transfer surface in the recirculation loop to attain this temperature level. To provide this extra surface, the GEC design uses a fluid bed external heat exchanger (EHE) concept which avoids heat exchange panels immersed in the CFB furnace, thus ensuring precise furnace temperature control in all operating conditions. The flow of solids through the EHE, and therefore the heat pick-up of the exchanger, is controlled by a cone valve located on the siphon seal.
One advantage of this concept is that the low fluidization velocity selected for EHE operation avoids all risk of heating surface erosion. Another advantage is that operators have an easy means of controlling the recirculation loop`s heat pick-up without interfering with bed characteristics or fluidization. The bed temperature automatically adjusts the position of the cone valve, which in turn controls the flow of solids to the EHE.
The bed operating temperature results from the heat balance over the recirculation loop. Roughly, this is heat input vs. outgoing flue gas heat content at bed temperature plus heat pick-up by the loop surfaces, such as furnace waterwalls.
Experience has confirmed that a tight bed temperature control is critical for optimizing the environmental performance of CFB boilers. When top performance is needed, for instance with high-sulfur fuels, it is extremely important to be able to control the recirculation loop heat pick-up in all operating conditions.
Heat pick-up is difficult to control because it depends on heat transfer coefficients and a bed`s characteristics. If this heat pick-up is not correct, operators can attempt to alter the natural bed characteristics in several ways, but there are limits to the possible adjustments.
The Provence power station near Gardanne was a 25-year-old 250 MW PC unit burning high-sulfur coal from an adjacent mine (Table 2). At the end of 1992, a consortium led by EDF gave GEC and CdF the task of replacing this unit with a CFB boiler having identical steam characteristics–250 MW, 700 t/h of steam at 163 bar, 1,049 F. The new boiler was designed to burn imported coal as well as the local high-sulfur coal. Gardanne coal is a unique, young sub-bituminous high-ash coal that is close to a lignite, with a high sulfur content that is almost constant when expressed in relation to the fuel heating value. Significant limestone inclusions lead to a high natural Ca/S ratio which requires an extra large furnace to accommodate its high slagging/fouling tendencies when fired in a PC boiler.
Provence`s record-size boiler features a single furnace which is 49 feet deep by 38 feet wide by 121 feet high with a “pant-leg” bottom. This means that the lower furnace is divided into two “legs,” each with its own fluidization grid. The furnace discharges into four cyclones, two on each side, and each cyclone is associated with a large EHE. One EHE, including medium-temperature superheaters, is used for bed/furnace temperature control, and the other one, including the final reheater, is used for reheat steam temperature control. The furnace is completely cooled by water through waterwalls, fluidization grids and windboxes. Taking into account Gardanne coal characteristics, the Provence furnace rating is equivalent to 270 MW with regular steam coal.
The companies completed construction of the boiler in accordance with their initial schedule (Table 3). GEC began manufacturing pressure parts in March 1993 and started erecting the steel structure as soon as the construction permit was obtained in November 1993. The boiler drum was lifted in May 1994 and the hydrotest was completed in November 1994. The first firing of the induct burners came at the end of May 1995, and at the end of August, boiler chemical cleaning, refractories drying, steam blowing and safety valve setting were completed. The boiler generated its first power on Oct. 29, 1995, and commercial operation began at the close of 1995.
The Provence CFB boiler is owned by the Soprolif company, a joint venture between EDF, 55 percent; Endesa, 25 percent; Houlleres du Bassin du Centre et du Midi, 10 percent; and GEC (Table 4). This new company is structured as an independent power producer and sells its energy to EDF. Engineering and construction of the facility were performed by EDF in close collaboration with the engineering group of Charbonnages de France.
GEC plans to continue CFB boiler development, with a goal of increasing unit size to 500 to 600 MW units while improving plant efficiency and minimizing all dangerous emissions. GEC`s studies show that the Provence design can be safely adjusted to 350 MW units.
It is quite conceivable to design very large CFB boilers for ultra-supercritical steam parameters, such as operating at 300 bar, 580 C with double reheat. High-temperature corrosion phenomena in EHE conditions are being investigated by testing alloyed steel tube sections at temperatures controlled between 600 and 680 C. Other approaches include combining CFB steam generators and gas turbines into combined cycles. With this configuration the CFB boiler would use the gas turbine exhaust as secondary air. A further concept integrates such combined cycles with coal gasification. The Air Blown Gasification Cycle, previously known as the British Coal Topping Cycle, is under development by a European consortium.
Currently, the global coal-fired power plant market is still dominated by conventional PC boilers. However, as users must equip these boiler with flue gas treatment systems to meet more stringent environmental regulations, the CFB boiler will be a more attractive alternative. CFB boilers are efficient, simple to operate and maintain and are able to fire a wide range of low-quality fuels.
The Provence/Gardanne plant with its circulating fluidized bed boiler during construction. Photo courtesy of Gerard Halary, EDF.