CFB: scaling up to 600 MWe20 August 1998
Two years' experience with atmospheric circulating fluidized bed (CFB) combustion technology, as implemented at the 250 MWe CFB power plant at Gardanne in southern France, demonstrated that the technology is ready for full-scale industrial application. EDF (Electricité de France) plans to build a 600 MWe CFB plant.
Most of France's generating capacity is nuclear, supplemented by hydropower and fossil fuels for peak demand. Although natural gas is currently very competitive, economic conditions are expected to change over the years ahead. In addition, EDF wants to have available a choice of new power-generating technologies that are both environmentally friendly and economically competitive. Of the possible clean coal technologies, atmospheric circulating fluidized bed combustion offers an excellent cost/pollution control balance, and can be considered to be thoroughly tried and tested.
China, India and various countries in southeast Asia and eastern Europe need to increase generating capacity as their economies expand. However, they face two conflicting requirements. On the one hand, they have extensive local resources of fossil fuels, often of poor to medium quality; on the other hand, there is growing concern about the environment. CFB technology is particularly suitable for these circumstances.
EDF has backed CFB since 1985, with the goals of upgrading and improving its conventional power stations, and at the same time improving its international standing.
Scale-up by stages
Scaling up in stages has enabled EDF to maintain control over its CFB technology development. The first CFB plant, of 125 MWe, was the Carling Emile Huchet Plant, France, built in 1990 with supplier Stein Industrie and with European Community support. This was the subject of a vast test programme which confirmed the soundness of the basic design, enabling the development teams to refine their numerical models.
In 1992, EDF decided to develop the 250 MWe CFB plant at Gardanne, working through subsidiary Soprolif and in cooperation with Stein Industrie. This plant, commissioned in 1996, is the largest CFB power station in the world. The operational experience gained at Gardanne over the last two years has exceeded expectations, and the scale-up from 125 to 250 MWe is regarded by EDF as a complete success.
This repowering project involved the replacement of a pulverized coal boiler by a CFB boiler. Accommodating the new unit was not a problem since, for a given output and level of pollution control, CFB units have a smaller footprint than pulverized coal units. During the first year, shut downs at Gardanne were mainly due to the auxiliaries, not the CFB-specific equipment.
Adopting the same methodology applied at Carling, EDF engineers gathered a large amount of data from the Gardanne plant, in order to improve their understanding of the CFB process under transient and steady-state conditions. The results confirmed the validity of the basic concepts, including the design of improved cyclones, while allowing the development teams to refine their numerical models by comparing predicted results with actual performance.
In cooperation with the manufacturer, new concepts were tested with a view to future scaling up, particularly for the heat exchangers. In addition, the so-called "pant-leg" boiler used at Gardanne is already sized for use as a basic element for larger plants.
The performance data for the Gardanne plant demonstrate that, subject to comparable environmental constraints, CFB is competitive with pulverized coal.
Industrial experience confirms CFB benefits
It is a number of years since the basic principles of CFB were first expounded. Observers have been waiting for proof that the anticipated benefits – particularly low emissions and power scalability – could be achieved under operating conditions.
Gardanne confirms the technology's key advantages. CFB can accommodate a wide range of fuels from low-grade coals and lignite to petroleum residues, biomass, among others. This is illustrated by the fact that the 250 MWe Gardanne plant can burn:coals with high sulphur and ash content and a low heating value (LHV), less than 4000 kcal/kg (ie close to that of a lignite), high-viscosity petroleum residue with a high sulphur content (4.5 per cent) and an LHV of around 9500 kcal/kg, steaming coals with sulphur contents from 1 to 3 per cent and an LHV exceeding 6000 kcal/kg.
The main factors are: the amount of smoke generated, corrosion by combustion products (ie the fuel's chlorine content), and the preparation the fuel requires. CFB can accommodate fuels of varying quality, including sudden drops in combustibility or heating value, with combustion maintained by high thermal inertia. This is a significant advantage over pulverized coal plants, which only maintain combustion under such circumstances by injecting fuel oil.
CFB is intrinsically clean, with emission levels controlled inside the boiler, not outside. Particle recirculation ensures virtually complete carbon burnout (99.6 per cent at Gardanne). In addition to minimizing formation of NOx, the relatively low combustion temperature of 850oC is optimal for desulphurization using injected limestone. There is thus no need for post-combustion emission control as in pulverized coal plants.
Start-up times are primarily determined by the thermal inertia of the refractories. After a short shutdown, restarting takes about two hours, provided the refractories are still hot. After a prolonged shut-down, start-up takes about 12 hours, since the refractories must be heated progressively.
CFB plants offer excellent operational flexibility thanks to good part-load behavior and load-following capabilities without compromising environmental performance. Burning coal alone, the Gardanne plant runs perfectly on part loads down to 35 per cent of its nominal output and can handle load change rates exceeding 4 per cent per minute of its maximum continuous output.
EDF considers the CFB technology as industrially proven and highly reliable. The Gardanne plant has operated commercially since April 1996, with boiler performance and availability exceeding contractual requirements. For the second full year of commercial operation, Gardanne achieved a boiler availability of 87.5 per cent. Following a major overhaul in November 1997, boiler availability from December 1997 to April 1998 was 95.9 per cent.
Scaling up to 600 MWe
With the experience acquired through the Carling and Gardanne projects, EDF began work on the basic design of a supercritical CFB plant generating 600 MWe.
There were two main reasons why 600 MWe was selected. Firstly, scaling up from the 250 MWe Gardanne plant to 600 MWe is comparable to the previous scale-up from 125 MWe. Secondly, EDF considers that 600 MWe represents the lower limit for base load power generation in France.
This raises a number of complex issues requiring advanced knowledge of fluid mechanics. When these problems have been overcome, construction is simpler than for a conventional pulverized coal plant, partly because there are no mills, burners, or post-combustion emission control equipment.
The success achieved in scaling up from Carling to Gardanne has given EDF confidence in its ability to scale-up to 600 MWe. In particular, the pant-leg concept successfully developed for the scale-up from 125 to 250 MWe will be applied directly for the scale-up to 600 MWe. Although the new pant-leg boiler will be deeper, it will have the same length and width as its forerunner. It will therefore operate at the same fast fluidization regime which EDF considers to be tried and tested. In heat transfer mechanics, combustion, and emission control, EDF believes it has the required knowledge.
In addition to the solution adopted for Gardanne, EDF's policy is to have several suppliers working on the competitive design of different industrial implementations of the CFB technology. EDF will subsequently be in position to select the solution that best meets its needs as a power plant operator.
For the plant to operate in supercritical mode, steam will be superheated, and then reheated to 600oC. Superheated steam will enter the turbine at a pressure of 270 bar. The final feedwater temperature will be 290oC. Compared with the Gardanne plant, these operating conditions will contribute to higher overall efficiency.
EDF believes it will be possible to demonstrate that CFB can achieve generating costs per kWh rivaling those of pulverized coal before flue gas cleaning. This will be achieved by combining economies of scale, supercritical operation, reduced thermal inertia, and longer intervals between shutdowns for scheduled maintenance.
Room for improvement
CFB is still a relatively young technology with room for improvement in several areas.Ash recycling. There is little likelihood of a universal solution to ash recycling. This is because fuel variability gives large variations in ash quality, and the feasibility of any particular option depends on the industrial fabric in the plant's vicinity and local environmental regulations. A range of ash recycling options are, however, under investigation. First results are promising, and new applications have already been identified. Though already low, NOx, SO2 and CO2 emissions can be reduced further. Each option investigated must, however, satisfy cost/performance criteria. Improved refractory materials and construction techniques should lead to lower capital and maintenance costs, shorter cold start-up times and longer intervals between major overhauls.
There is still room for improvement in understanding the physical concept behind the CFB technology, reactive diphasic flow.
EDF believes atmospheric circulating fluidized bed combustion will be, over the next decade, increasingly competitive with pulverized coal technologies. CFB is already especially suitable for operators facing evolving environmental constraints and in countries with poor to medium quality fuel resources. Following the success achieved by the last two years with the commercial operation of Gardanne, suppliers including Alstom Stein Industrie, Foster Wheeler and ABB-CE are working on other projects in the 250-300 MWe range.
In undertaking the development of the next generation of CFB plants and scaling up to 600 MWe, EDF aims to improve the industrial implementation of this promising new technology and anticipate future needs.
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| this-will-be-a-440mwe-plant-consisting-of-2-x-220mwe-cfb-boilers-the-units-will-be-provided-by-alstom-in-a-deal-valued-at-80-million-and-the-boilers-will-be-supplied-by-alstom-subsidiary-stein-industrie-the-plant-owner-choctaw-generation-limited-partnership-is-currently-getting-final-emission-permits-construction-is-due-to-start-1-october-1998-and-the-plant-is-expected-to-come-on-line-in-the-4th-quarter-of-2000-with-a-construction-period-of-27-months-engineering-procurement-and-construction-wiTablesTable 1. Performance data for Gardanne 250 MWe CFB plant
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