First published in Cornerstone, Volume 4, Issue 3
By Malgorzata Wiatros-Motyka, Author and Analyst, IEA Clean Coal Centre
The Łagisza power plant in Będzin, Poland, is home to the world’s first 460-MW supercritical circulating fluidized bed boiler (CFB), which remains the largest of its kind outside China. Since beginning commercial operation in June 2009, the plant has attracted considerable interest from all over the world. Experience gained from its design, construction, and operation has been a valuable stepping stone in further developing the technology and implementing it in other countries.
The power plant is currently owned by Tauron Wytwarzanie S.A., the second largest energy company in Poland. The first subcritical units at Łagisza were built in the 1960s. At the turn of this century, when Łagisza consisted of seven 120-MW pulverized coal-fired boilers, the decision was taken to build a new, larger coal-fired unit to replace the smaller, less efficient ones. As described by Szymon Jagodzik,Łagisza’s Deputy Director and Chief Energy Generation Engineer, various options were initially considered, including both pulverized (PC) and CFB combustion designs. All the possibilities were carefully evaluated before the company decided to build a supercritical CFB unit—even though, at the time, no such boilers were operating anywhere in the world. A number of factors influenced the decision. First, it was calculated that the total plant investment cost for the CFB was approximately 15% lower than for a comparable pulverized coal-fired boiler. Second, a CFB would not require the installation of expensive wet flue gas desulfurization (FGD) and selective catalytic reduction (SCR) systems as both sulfur dioxide (SO ) and nitrogen oxides (NO ) could be removed from within the boiler. Third, CFB units have greater fuel flexibility than pulverized coal combustion units.
Foster Wheeler Energy Polska and Foster Wheeler Energia OY (currently Amec Foster Wheeler) designed and built the boiler. To keep costs down, a number of suppliers and contractors were chosen, both locally and from abroad. Alstom Power supplied the turbine set and Elektrobudowa S.A. Katowice provided the electrical system. The ash handling and limestone sorbent systems came from Mostostal Kraków and the Energo–Eko-System Katowice consortium. The CiepŁo–Serwis Będzin and PURE Jaworzno consortium provided the coal-feed system, and the distributed control systems (DCS) came from the consortium of Metso Automation Finlandia and Metso Automation Polska. This strategy was successful as the unit was completed below the budget price; the total cost was about 1.9 mld zl (€0.422B, $0.594B). The money was raised by the company, bonds, and various Polish government environmental funds. It took three and a half years from the start of construction in January 2006 to commissioning of the unit in June 2009. Between 1500 and 2000 people were involved in its design, construction, and commissioning.
THE CFB UNIT
The design of the Łagisza unit was based on Foster Wheeler’s second-generation CFB technology, which features solids separators (cyclones) constructed from water- or steam-cooled panels integrated with the furnace combustion chamber. Prior to Łagisza, Foster Wheeler’s largest second-generation CFB boilers were the 262-MW units at the Turów power plant, also in Poland. The main design parameters of the boiler are listed in Table 1 and a schematic is shown in Figure 1.
Although CFB boilers have considerable fuel flexibility and can fire many low-grade fuels, including low-rank coals, biomass, and different types of waste, the boiler at Łagisza plant was designed specifically for locally mined hard coal and the limestone used for desulfurization. In 2015, the average parameters of the fired coal were as follows: calorific value 20,522 kJ/kg; 19.21% ash; 1.03% sulfur; and 14.49% moisture content.
The most significant design features of the Łagisza CFB unit are the boiler’s compact size, its once-through operation mode, the single fluidizing grid, the integrated steam-cooled solids separator, the INTREXfluidized bed heat exchanger, and the flue gas heat recovery system.
The parameters of the coal and limestone to be used were analyzed extensively, which led to the design of a compact boiler 27.6 m wide, 10 m deep, and 28 m high. In fact, it is only slightly larger than the boilers designed for Foster Wheeler’s subcritical 235-MW CFB units 1–3 at Turów power plant (22 m wide, 10.1 m deep, 42 m high).
The unit uses a single fluidizing grid in the bottom of the boiler, with four separate air plenums for the primary air flows. The primary air flow to each plenum is measured and controlled separately to ensure equal air flow to all sections of the grid and uniform fluidization as well as simple control.
The application of vertical Benson tubing (low mass flux once-through technology) and Siemens supercritical steam flow technology allows steady operation of the boiler at variable load conditions (40–100% load).
The unit has eight integrated steam-cooled solids separators arranged in parallel, four separators on two opposite furnace walls. This arrangement allows a high collection efficiency with low flue gas pressure loss. The inlet is tall and narrow in shape to provide a uniform flow of flue gas and solids, thus avoiding high local velocities. The result is a collection efficiency equal to the best conventional cyclones with substantially lower loss of pressure. To minimize the required amount of refractory material, the separators are designed with panel wall sections and have a thin refractory lining anchored with dense studding. The separator tubes are steam cooled, forming a third superheater stage.
Foster Wheeler’s integrated recycle heat exchanger (INTREX) incorporates the heat exchanger water wall with the furnace water steam system and the return channel. As well as cooling the externally circulated solids, openings in the furnace’s rear wall provide access for additional solids to circulate internally through the heat exchanger tube bundles, ensuring sufficient hot solids to the INTREX™ heat exchanger at all loads. As the system is located in the solids return part of the solids separator, corrosion from high temperature and the acidic flue gas component is avoided.
The flue gas heat recovery system (HRS) cools the flue gas from 130°C to 85°C and improves the total efficiency of the unit by around 0.8 %. The HRS operates in the clean gas after the electrostatic precipitator (ESP) and induced draft fans. The flue gas is cooled in a heat exchanger made of PFA tubing to avoid corrosion problems. After HRS cooling, the flue gas is conducted to the cooling tower via a fiberglass duct. The recovered heat is transferred by a primary water circuit to the combustion air system of primary and secondary air. As the combustion air temperature before the rotary air preheater is increased, the incoming cold combustion air flow is not able to absorb all the heat from the flue gases. Hence part of the flue gas is directed to a separate low-pressure bypass economizer that allows the heat from the flue gas to be used to heat the main condensate.
The temperature of the flue gas after heat recovery is relatively low and would cause problems for a traditional stack. Thus a decision was made to construct a 133.2-m-high “cooling stack”. This was more economical than constructing a cooling tower and a separate stack. More importantly, it allows higher and better dispersal of the flue gas than would be achieved with a stand-alone stack.
The advantages that resulted from these applied solutions include significant fuel flexibility and a low combustion chamber temperature of 800–900°C. This means that screen tube slagging is avoided, as well as high-temperature corrosion.
Łagisza’s CFB unit operates with much greater efficiency and emits significantly less carbon dioxide and other air pollutants than the 120-MW pulverized units it replaced (see Table 2).
OPERATION OF THE BOILER
Currently, Łagisza power plant consists of the 460-MW CFB and two subcritical 120-MW pulverized coal-fired units. The plant employs 326 people, of which approximately 60 are required to operate the CFB unit. In 2015, the unit was in operation for 6000 hours, used 905,000 tonnes of local coal, and generated 2.3 TWh of electricity. It operated at 65–100% load, with an average load of 85% (392 MW). Obviously, variations in the load translate to variation in the boiler’s efficiency. As shown in Figure 2, when operating at full load, net efficiency is in the region of 43% (lower heating value, LHV) and net power output is 439 MW.
Sulfur dioxide emissions are controlled by feeding limestone into the boiler; last year, 62,500 tCaCOwere used for that purpose. NO emissions from a CFB combustion unit are only around one fifth of those produced by pulverized coal combustion as the combustion occurs at lower temperature and thus less NO is formed. Hence, NO emissions are effectively controlled by staged combustion as well as the addition of ammonia as part of a selective non-catalytic reduction process. In 2015, 2885 tonnes of ammonia were used. An ESP system is used to control PM emissions. Consequently, in 2015 the emissions were as follows: 17,000.2 kg PM (<30 mg/Nm ); 454,962.3 kg SO (<200 mg/Nm ); 521,274.3 kg NO (<200 mg/Nm ); and 721,367 tCO These pollution control options enable compliance with the relevant EU legislation.
Fuel is delivered to the boiler by 14 screw-type feeders, as reported by Jagodzik. Coal is crushed to 1–30 mm; 80% of it to between 1 and 10 mm to allow seamless combustion. About 40–50 kg of coal is delivered each second to the boiler, corresponding to around 4000 tonnes of fuel being used per day. When in operation, the total mass of solids (fuel, sand, sorbent, ash) in the boiler is about 200 tonnes. Although the CFB boiler can fire different types of fuel, unsurprisingly it is most efficient when using the fuel for which it has been designed: local hard coal.
It is worth noting that the Łagisza CFB unit also has the Dual-Reflux Vacuum-Pressure Swing Adsorption (DR-VPSA) COcapture pilot installation in place. The installation is operated by Częstochowa University of Technology and Eurol Innovative Technology Solutions Sp. z o.o. with the participation of Tauron staff. When in operation, the installation utilizes a slipstream of the CFB flue gas (100 m /h) to investigate two adsorbents (activated carbon and zeolite types) for their potential to remove CO .
STEPPING STONE FOR FURTHER DEVELOPMENT OF CFB
As noted by Lockwood,despite attaining the status of “cleaner” coal technology because the emissions of NO and SO emissions are more easily controllable, the use of CFB combustion at the utility scale has been limited by smaller boiler sizes than those used in pulverized coal combustion. However, scale-up and optimization over recent years have allowed CFB boilers to benefit from economies of scale. Larger units have been built since the commissionsing of the Łagisza CFB unit, and CFB combustion is beginning to provide a viable alternative to pulverized coal combustion for utility power generation, especially where low-grade fuel will be used. The successful operation of the world’s first supercritical CFB boiler at Łagisza power plant in Poland has been crucial to this progress. Łagisza has validated Foster Wheeler’s supercritical CFB design platform, providing a solid base for its development of units of up to 600–800-MW capacity.
Tauron Wytwarzanie S.A. has generously shared its knowledge and experience gained during the operation of the world’s first supercritical 460-MW CFB unit. The company has hosted many tours and provided training and learning opportunities for plant operators from around the world. This included training for a team from KOSPO, South Korea, which is expected to commission four 550-MW units in Samcheok later this year. And as Szymon Jagodziknoted, as Tauron staff train others during such interactions, they are also open to suggestions because they never stop improving operation of their coal-fired fleet.
After over six years of operation, the decision to build the world’s first supercritical CFB unit in Łagisza appears to have been both economically and environmentally successful. Łagisza’s operating experience has provided a good knowledge base for further development of CFB units all over the world. The Łagisza CFB unit is predicted to be in operation until 2046 and there are plans for it to produce heat as well as electricity. The amount of heat to be produced is not yet known, as it will depend on local demand. Nevertheless, the future of Łagisza CFB unit looks good.