Robert Giglio, Foster Wheeler Global Power Group, USA
|South Korea’s Samcheok Green Power facility will feature eight 550 MWe CFB boilers Source: KOSPO|
Over the past 35 years, circulating fluidised bed (CFB) technology has evolved from robust small-scale industrial boilers for burning difficult fuels to the successful installation and commercial operation in 2009 of the world’s largest CFB boiler, rated at 460 MWe. Thus, for the first time in its history CFB technology can challenge pulverised coal (PC) technology in large-scale electricity generation applications. In addition to the 460 MWe supercritical unit, more than 80 CFB units rated above 200 MWe operate worldwide today.
CFB technology has several established benefits such as improved efficiencies, reduced emissions, high fuel flexibility and lower costs that combine to make this boiler technology a highly competitive option for large-scale utility applications.
Utility-scale PC fired steam generators are designed for a narrow range of fuels, typically coals with a heating value in the region of 6000 kcal/kg. CFB steam generators, in contrast, afford the maximum flexibility in fuel selection and can handle all coal types including low-rank coals, petroleum coke, coal slurries and anthracite culm, as well as biomass and peat. This fuel procurement flexibility for CFB steam generators provides long-term fuel security and full access to arbitrage in the global fuel market.
Another significant feature of CFB technology is its ability to tightly control nitrogen oxides (NOx) and sulphur dioxide (SO2) emissions in the boiler, which can avoid the EPC capital costs associated with the installation of selective catalytic reduction (SCR) and flue gas desulpurisation (FGD) equipment. For a 600 MWe plant, for example, CAPEX savings can exceed $100 million. In addition, the operating costs for ammonia and catalyst management for a SCR and for limestone or lime for a wet or dry FGD system are also avoided.
CFB technology has, in recent years, passed several scale-up milestones in regard to plant capacities for selected fuels and increased steam data as boilers have evolved from subcritical to supercritical.
CFB boilers have undoubtedly now moved into the utility-scale range, with many subcritical units in operation with a capacity of 250–300 MWe. However, the most significant milestone to date has been the successful commercial operation of Poland’s Lagisza 460 MWe power plant, which employs supercritical once-through (OT) technology. This 460 MWe CFB unit, which is owned by TAURON Wytwarzanie SA (formerly Poludniowy Koncern Energetyczny SA) and located in the Katowice area of southern Poland, represents the largest and first-ever supercritical CFB boiler.
The selected steam pressure and temperature (282 bar and 563 °C) for Lagisza were proven in PC supercritical units. The main fuel for the boiler is bituminous coal sourced from ten local coal mines but with a wide range of parameters, including an as-received heating value ranging from 4300 to 5500 kcal/kg, attesting further to the fuel flexibility of the CFB technology. In addition, the design allows provision for the introduction of coal slurry through lances, comprising up to 30 per cent of the total fuel heat input.
The boiler’s OT vertical tube technology produces steam from a single pass of water through the furnace tubes, eliminating the need for a steam drum and the associated steam circulation system. The supercritical steam properties allow high steam heat absorption, achieving net plant efficiency in excess of 43 per cent (LHV) – a marked increase from the 35 per cent efficiency for Lagisza’s original subcritical PC fired plant.
The initial operating experience at Lagisza has met all expectations, with a 28 per cent reduction in carbon dioxide (CO2) emissions, as well as nitrogen dioxide (NO2) and SO2 reductions to levels less than those set by the European Union’s Large Combustion Plants Directive (LCPD).
A second supercritical CFB plant, to be located in Novocherkassk, Russia, is currently under construction. The 330 MWe CFB plant, owned by OAO WGC-6, is slated to enter commercial operation this year. The CFB boiler will be designed to burn anthracite and bituminous coal, and will have the capability to burn lower-quality fuels. CFB combustion temperatures and other parameters do not change significantly with different coals. The steam parameters at 100 per cent load were specified by the owners at 244 bar and 565 °C.
switching focus to biomass
Another significant milestone for CFB technology involves showcasing 100 per cent biomass fuel firing as part of an effort to substantially cut carbon emissions.
The 125 MWe CFB boiler at the Kaukaan Voima Oy plant in Kaukas, Finland, which entered commercial operation in 2010, is one of the world’s largest CFB installations firing 100 per cent biomass. This unit routinely burns peat as well. A second large-scale 100 per cent biomass plant features an 85 MWe CFB boiler at the Soderenergi AB Igelsta plant in Sweden, which came online in 2009. Both of these Scandinavian plants underscore and support Europe’s goal of deriving 20 per cent of its energy from renewable fuels by 2020.
Poland, for example, has now taken an aggressive position in providing incentives for the growth of biomass fired power projects, with an emphasis on maximising the use of difficult-to-burn agricultural wastes. This Eastern European nation has significant sources of biomass usable for direct power generation, including 35 million tonnes per year of wood waste and 12–14 million tonnes per year of agro biomass.
In March 2010, GDF Suez Energia Polska SA awarded a contract to build what is described as the world’s largest 100 per cent biomass fired CFB boiler. The 190 MWe CFB unit will be installed adjacent to the existing units at the Polaniec power station.
The CFB boiler is being designed to co-fire high-alkaline agro biomass with wood-based biomass. Similar to the 100 per cent biomass fired CFB units in Finland and Sweden, the steam properties for the Polaniec plant will be subcritical. The fast-track project is targeting start-up this year to take advantage of Green Certificates for increased use of agro biomass.
USC CFB reaches 550 MW
Last year, the decision by the Korean Southern Power Company (KOSPO), a major South Korean utility, to proceed with its Samcheok Green Power project marked a significant development in clean coal power. The plant will be built in Samcheok City in Gang Won Do province in the northeast of the country. The plant site will occupy 2.5 million m2 of reclaimed coastal land and employ a tiered landscape to minimise total space requirements. The site will also contain a world-leading CO2 research centre employing technical experts from around the globe.
|Poland’s 460 MWe Lagisza unit is currently the largest commercially operating CFB boiler in the world|
When fully complete, the Samcheok project will feature eight 550 MWe ultra-supercritical (USC) CFB steam generators – contributing a total of 4400 MWe – firing imported coals and co-firing up to 5 per cent biomass. In addition to the CFB steam generators, the Samcheok site will generate 600 MW from renewable sources to bring the total site capacity to 5000 MW.
The renewables component will comprise: wind turbines mounted on the plant’s seawall; solar panels/PV arrays covering the coal yard, rooftops and slopes; wave power generation at the seawall; small hydropower at the plant drainage canal; and fuel cells from nearby Korea Gas Corporation. Furthermore, ash from the plant’s electrostatic precipitators will be recycled and used as lightweight aggregate for construction and land reclamation.
KOSPO chose CFB technology over PC technology for the Samcheok project for its greater fuel flexibility, lower combustion temperature and lack of requirement for back-end FGD equipment.
The plant will fire coals with heating values in the range of 4000–6000 kcal/kg. Very low sulphur dioxides (SOx) and NOx emissions (50 ppmv) will be achieved without the need for back-end FGD equipment. The steam data fall into the USC range (270 bar and 602 °C), providing increased efficiency and reduced fuel consumption for a given output. The net plant efficiency is estimated at 42.4 per cent (LHV). CFB technology also mitigates some cost, reliability and operational flexibility issues associated with USC suspension-fired PC boilers.
ARRIVING at 800 MW
An ageing power fleet in Europe and the US – along with significant new plant construction in countries such as China, India and Chile – will increase demand for boilers ranging from 600 MWe up to 800 MWe. Both supercritical and USC CFB technology are at the forefront of addressing this need.
The Lagisza plant’s validation of the supercritical CFB design provided a solid base for scaling up the technology to the 550 MWe Samcheok units. In addition, CFB boiler designs up to 800 MW for bituminous coal have been developed with steam parameters of 300 bar and 600/620 °C, enabling efficiencies up to 45 per cent (LHV) and 43 per cent (HHV).
The scale-up of the dimensions and size of the plant components for the 853 MWe (gross)/800 MWe (net) design is moderate because of a modular approach. The flue gas side of the furnace design for the new CFB800 boiler is based on analyses of the imported hard coal and limestone proposed for the unit. Data from these have been fed into the design models to predict the particle size distribution of the circulating material, solids densities and the heat transfer and gas temperatures.
The design produced a furnace with a cross-section of 40 x 12 metres and a height of 50 metres. These furnace dimensions are slightly larger than those cited for the existing Lagisza 460 MWe supercritical plant and the Samcheok 550 MWe USC units (see Table 1).
The boiler size for CFB units is less affected than that for similarly sized PC steam generators by the fuel choice, which varies between the Lagisza, Samcheok and CFB800 designs. The selected design fuels for the 800 MWe boiler are imported hard coal (main fuel) and petroleum coke (additional fuel). Columbian coal was used to represent the imported coal. Typical fuel analyses for the CFB800 design are given in Table 2.
THe utility contender
When building new plants or repowering old plants, efficiency and environmental impact are key. To achieve these, CFB designs and supercritical steam parameters have established references in place. With the Lagisza supercritical steam unit as the major building block, scaled-up supercritical and USC CFB plants up to 800 MWe are now available, with unit efficiency projected at 45 per cent (LHV), cutting fuel consumption and minimising emissions. With recent technological advances, CFB is without doubt a serious contender for utility-scale power generation.
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