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CFB breakthrough

The 460 MW Lagisza project in Poland will feature the world’s first supercritical CFB boiler and represents an important step in clean coal technology development. Several project partners are now aiming to further scale up the technology to 800 MW.

Ilkka Venàƒ¤làƒ¤inen, Foster Wheeler Power Group Europe, Finland and Jouni Hàƒ¤màƒ¤làƒ¤inen, VTT, Energy and Pulp & Paper, Finland

World-wide, coal is the major power generation fuel and is used over twice as extensively as its closest competitor (hydropower). Hard and brown coal play an important and stabilizing role in the European power generation industry and account for about 30 per cent of electricity generation in Europe, and as much as 50 per cent in Germany. Furthermore, under the policies of nations like the USA, China and India, this dependence on coal is set to continue and increase. Consequently, the need for technology that can generate electricity in a clean and sustainable way from fossil fuels must be developed. In support of this, Foster Wheeler is aiming to commercialize supercritical Circulating Fluidized Bed (CFB) technology.

There exists a great need for new power generation capacity worldwide and in Europe alone it is estimated that 550 GWh of new and repowering capacity will be needed during the next 25 years. The fact is that replacement or modernization of existing capacity with modern technology applying high steam values and high efficiencies would result in both fuel savings and a reduction in greenhouse gas (GHG) emissions. Foster Wheeler’s programme to develop supercritical CFB technology aims to provide a competitive option for operators of conventional coal fired boilers to improve both cost effectiveness and environmental performance.

Supercritical CFB

CFB technology has come a long way over the past 20 years. Starting from hard-to-burn fuels, particularly from the forest products industry, it has become a mainstream combustion technology. The benefits of CFB technology – such as its multifuel capability and low-emission performance without the need for secondary systems – have made it an increasingly attractive alternative for many other users, too. The reliance on subcritical steam parameters, however, has limited its potential in true utility-scale applications, with the largest operating CFBs in the order of 300 MW and efficiencies of between 38 and 40 per cent.

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Over the past few years, CFB technology with a once-through supercritical (OTSC) design has been developed through a cooperation project involving Foster Wheeler Energia Oy, Siemens Power Generation, VTT Technical Research Centre of Finland and EnergoProjekt Katowice (EPK), and is now ready for deployment in commercial projects on a medium scale (<500 MW). In fact the first medium-scale OTSC CFB plant, with a capacity of 460 MW, is currently under construction by Foster Wheeler in Lagisza, Poland. Besides being the world’s first supercritical CFB boiler it is also the world’s largest CFB unit. The boiler will start commercial operation in early 2009.

By applying supercritical steam parameters, the fuel use per GWh of electricity generated can be cut by some five per cent compared with conventional drum-based boilers. This, in addition to the fuel flexibility offered by CFB technology, provides operators with further benefits, especially those in competitive markets. The new CFB design can achieve efficiencies in excess of 43 per cent, translating into a reduction in emissions of five per cent compared to conventional designs.

Scaling up

Scaling the CFB technology further up to real utility scale (600-800 MW) is, however, needed to fulfil the future requirements of utility operators. The characteristics of CFB technology – fuel and operational flexibility – will definitely provide an economic advantage in utility-scale applications, improving operators’ competitiveness.

Figure 1. Steam circuitry
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As CFB boilers become larger and larger, not only do the mechanical designs have to be considered but also the processes and the process conditions affecting heat transfer, flow dynamics, carbon burnout, gaseous emission suppression, hydraulic flows and so on must be understood.

Increasing the boiler size also means increasing the furnace dimensions, and there is therefore a need to develop new designs and layouts to account for this, in terms of both furnace performance and construction. In particular, the need for evaporator surface will increase more than the perimeter available in the furnace, which creates a new challenge. Also, other furnace-related components such as hot cyclone, return loop, and solids sealing devices need to be designed and optimized for the increased size. Due to the increased dimensions, the furnace construction needs to be evaluated and optimized for a reliable, safe and cost-effective design. The boiler needs to fulfil safety requirements in all operational situations. For that reason, operation during normal and special situations needs to be analysed to screen out the potential operational risks. In particular, the criteria for allowable metal temperatures need to be analysed and the conditions for heat crises in the water-steam cycle must be eliminated.

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In September 2005 project partners VTT, Foster Wheeler Energia Oy, Endesa SA, Siemens PG, CIRCE and CERTH/ISFTA implemented a programme to continue the development of CFB technology towards utility-scale applications with outputs of 800 MW. The project is a part of the European Union’s Research Fund for Coal and Steel (RFCS) Programme. The three-year project aims to develop a CFB boiler concept up to 800 MW to meet demand for efficient and environmentally-friendly power from coal.

CFB 800

Developing CFB technology to even larger sizes and higher plant efficiencies is an ongoing challenge. After initiating the new CFB800 study, a basic conceptual boiler design has been developed to help understand the feasibility of a large CFB boiler of this size. Imported bituminous coal has been used as the base fuel in the study. The plant will utilize ultra-supercritical steam parameters, with a steam pressure of 300 bar, a superheated steam temperature of 600à‚°C, and a reheat steam temperature of 620à‚°C to maximize the plant efficiency, which is 45 per cent (LHV). In the CFB800 design, the boiler efficiency is further improved by a flue gas heat recovery system, which cools the flue gases down to 90à‚°C.

The study has selected a plant size of 800 MW gross output, which corresponds to 750 MW of net output, utilizing a steam turbine-driven feed water pump. The steam parameters shown in Table 1 have been specified, reflecting current state-of-the-art figures for available materials for the boiler and turbine.

The European Union directive for Large Combustion Plants has been used as the basis for the emissions performance for the CFB800 plant (see Table 2). Emissions of sulphur dioxide are controlled by feeding limestone into the furnace. Nitrogen oxide emissions are controlled by the furnace’s low combustion temperature and staged combustion. An electrostatic precipitator (ESP) is used to control particulate emissions.

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The selected design fuels for the boiler are imported hard coal (main fuel) and petroleum coke (additional fuel). Typical fuel analyses are given in Table 3. An analysis of South African coal has been used for imported coal.

Boiler description

Water and steam circuitry: The feed water in the CFB800 boiler enters the boiler at a temperature of 290à‚°C for preheating in a bare tube economizer. Water is then fed to the enclosure walls of the Intrex fluidized- bed heat exchangers and further to the distribution headers of the evaporator (furnace) walls. The water is heated in the evaporator wall tubes, and dry steam exits at the evaporator outlet. As the boiler operates in sliding pressure mode, dry-out will occur in the subcritical region, as in all once-through designs, at a certain elevation of the evaporator, causing a reduced internal heat transfer coefficient and locally increased tube and fin temperatures. In CFB boilers, the furnace heat flux is considerably lower than in pulverized coal (PC) boilers, and the highest heat flux occurs in the lower furnace, where water is always sub-cooled.

The final superheating is carried out in superheater IV, located in four Intrex superheaters on the other side of the furnace. The main steam temperature is controlled with a two-stage feed water spray, and by adjusting fuel feeding rate.

Figure 2. Furnace heat flux profile
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Steam after the high-pressure turbine is returned to the boiler for reheating. The first-stage reheater is located in the convection pass. Reheater I (RH I) is equipped with a gas side parallel pass, which is used to control the reheat steam temperature with dampers. The final reheater stage consists of four Intrex heat exchangers (RH II).

Furnace design

Flue gas side: The flue gas side of the furnace design for the CFB800 boiler is based on analyses of the imported hard coal and limestone proposed for the unit. Data from these has been fed in the design models to predict the particle size distribution of the circulating material, solids densities, and the heat transfer and gas temperatures. The resulting design has a furnace cross-section of 40 x 12 m, and height of 50 m. The furnace dimensions are slightly larger than those found in existing units.

The furnace has one fluidizing grid, below which there are four separate air plenums for introducing primary air to the furnace. The primary air flow for these four air plenums is measured and controlled separately to ensure equal air flow to all sections of the grid and uniform fluidization. Using a single continuous fluidizing grid makes for simple control, as well as stable and uniform operation of the furnace.

Water/steam side: The water and steam side of the design is based on low mass flux Benson once-through technology licensed from Siemens AG of Germany. This technology is ideal for CFB conditions as it utilizes vertical furnace tubes rather than the spiral-wound tubing used in many other once-through designs. Vertical tubing is the normal arrangement in natural circulation CFB designs.

Figure 4. CFB800 boiler arrangement
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The heat transfer rate in CFB boilers is very low and uniform compared with PC units, and the required water mass fluxes are rather low. Low heat fluxes allow normal smooth tubes to be used in the furnace walls, with a mass flux of 500-700 kg/m2s at full load.

Solids separator

The solids separator design for the CFB800 boiler is based on Foster Wheeler’s second-generation CFB design, with steam-cooled panel wall construction. The solids separator design is optimized for high collection efficiency and low flue gas pressure loss.

The design is based on eight solids separators arranged in parallel, four on either side of the furnace. The separator size is only slightly larger than in the Lagisza boiler, and even larger separators are in commercial operation. As a result, no scale-up is required in this area.

The separators are designed with panel wall sections and have a thin refractory lining anchored with dense studding to minimize the amount of refractory required. The separators are manufactured using panel welding machines, eliminating the need for extensive manual welding. Furthermore, the separator tubes are steam-cooled, forming a second superheater stage. The two flue gas ducts after the solids separators are steam-cooled and connected to the separator tubing.

Intrex heat exchanger

The Intrex exchanger is a fluidized-bed heat exchanger, which extracts heat from the hot circulating bed material that is collected in the solid separators. Additional bed material is taken to the chambers directly from the lower part of the furnace. This provides sufficient bed material across a wide range of loads.

Figure 5. Lagisza will be the site of the world’s first supercritical CFB boiler and also the world’s largest CFB unit
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The CFB800 boiler design includes a total of 12 Intrex heat exchangers. Eight of the heat surfaces serve as superheaters (the third and final stage), and four as final reheaters. The refractory linings in this part of the boiler are also minimized, due to the use of water-cooled casings. This enables the casings to be integrated with the furnace, eliminating the need for expansion joints and minimizing the distances required for transferring hot solids. The flow of hot solids is controlled very simply through the fluidization process, and no valves or other mechanical devices are required.

Future steps

Zero emission power generation using CFB designs with oxyfuel technology is another important Research and Development area for the future. In oxyfuel technology, fossil fuels are combusted with pure oxygen separated from the air. Combustion gas consists mostly of carbon dioxide and water vapour. Carbon dioxide can be liquefied after cooling and stored in geological formations.

VTT is currently studying, in co-operation with Foster Wheeler, the phenomena relating to oxygen combustion using a pilot-scale combustor. The preliminary pilot-scale has shown CFB to be very well suited for oxyfuel combustion. A larger co-operation project, with the objective of demonstrating pure technology in the 30-50 MW size range is being planned for 2010.