98,3% Desulphurisation Efficiency - High SO2 Removal Performance using Limestone FGD at retrofit projects Tušimice and Prunerov

Transcription

1 98,3% Desulphurisation Efficiency - High SO2 Removal Performance using Limestone FGD at retrofit projects Tušimice and Prunerov Klaus Bärnthaler 1, Harald Reissner 1 Jan Štancl 2 1 AE&E - Air Pollution Control 2 SKODA PRAHA Invest s.r.o., 1 / 16

2 CEZ RENEWED PLANT PORTFOLIO Improving the horrifying status of environment in Czech Republic, the severe heritage of the past regime, became a high priority of CEZ in the early nineties. Massive investments to ecological programs (mostly FGDs, CFBs and sophisticated control systems) brought immediate improvement [1]. Now, CEZ stands right in the middle of a further key phase of permanent strategic growth, working on project CEZ Group Plant Portfolio Renewal in Czech Republic, managed by the general supplier SKODA PRAHA Invest. One of the most important reasons to start this project was the lifetime expectancy of the existing coal fired power plants. Major part of the installed capacity will reach its lifetime limits within the period 2010 to 2020, whereas additionally installed FGD systems in the years 2015 to Furthermore, more stringent emission limits of NOx (200mg/Nm 3 ) and limits of SO 2 (200mg/Nm 3 ) will come in force starting Last but not least it is essential to cover expected future growth of power demand. On one hand the program brings erection of both new and completely renewed coal fired plants in the northern part of Bohemia (Tušimice II 4 x 200 MWe, Prunerov II 3 x 250 MWe, Ledvice 660 MWe) but on the other hand several ineffective old plants will be shut down. The final balance equals to minus MWe [ 2 ]. This ambitious program appears to be the most extensive investment project in Czech Republic over the decades which should strengthen the CEZ Group position in the European market. Bringing substantial emission reduction, this project is unique even in the European context. Lifetime expectancy of new installations varies case by case between 25 to 40 years (renewed plant, new plant, coal reserves, etc.). Tušimice II plant with an overall efficiency of 38% is scheduled to be fully finished by the end of 2011 (first stage of boiler retrofit including new FGD already in operation (see Picture 1)), Prunerov II which replicates Tušimice project to certain extent should be completed by the end of 2013, however, in this case some uncertainties concerning BAT principle still remain to be resolved. New 660 MWe supercritical plant in Ledvice (efficiency 42%) is well in timeframe, and the scheduled PAC by the end of 2012 seems to be more than achievable (refer to Picture 2 - site ready for starting steel works construction). Overall effect (compared to 2007) of 65% NOx reduction, 67% SO 2 reduction, 48% of fly ash reduction and some 30% CO 2 reduction is expected in the affected region. 2 / 16

3 The other part of CEZ strategy is to the change fuel mix. This means that CEZ has been planning to build several new combined cycle power plants across Europe. A new 880 MWe combined cycle power plant in Pocerady is considered to be the last segment of the project in the Czech Republic for the time being. Other new CCGTs are to be built in Slovakia, Hungary, Turkey and Poland. Last part of the CEZ strategic growth focuses on nuclear power plants. This is a long term goal but preparatory work on three to five new units has already been started. THE TUŠIMICE PROJECT - NEW FGD In addition to the modernization of the 4 x 200 MW boilers of the Tušimice II Power Plant, a new and modern flue gas desulphurization (FGD) plant was installed as well. An FGD plant has been operating at the Tušimice site since the mid-1990s (Jet Bubbling Reactor by Chiyoda). This plant, however, did not meet the operating company s requirements for profitability, easy maintenance and availability. Due to the extension of the range of coals, the coals used in the future will have a higher sulphur content resulting in higher concentrations of SO 2. Furthermore, the emission standards will be adjusted to meet the currently valid EU guidelines. Based on these conditions a new desulphurization plant was put out for tender by Skoda Invest, and AE&E - Air Pollution Control (APC) was assigned as the best bidder with the installation of a new flue-gas desulphurization plant. The process used in the Tušimice II Power Plant is the wet limestone process based on the open spray tower. Here, the flue gas enters radially into the cylindrical absorber. The limestone suspension is finely atomized with many nozzles over several spray banks and absorbs the SO 2 from the flue gas with a counter-current process. The absorbed SO 2 is oxidized in the absorber sump via injection of air and finally converted to gypsum [ 3 ]. Depending on its moisture content and degree of purity the dewatered gypsum is then disposed of or supplied to the building industry. The Tušimice II Power Plant has a total of 4 blocks fired with brown coal, each with an output of 200 MW el. The flue gas from two blocks (a total of approximately 1,690,000 Nm³/h) is combined and flows to one single scrubber (refer to Table 1). The content of sulphur in the brown coal in the region around the Tušimice II Power Plant is rather high when compared with other power plants that are fired with brown coal. The FGD system must therefore be 3 / 16

4 designed for a sulphur dioxide content of 11,600 mg/nm³. The SO 2 content in the purified flue gas was to be, in accordance with specifications, brought down to a value below 200 mg SO 2 /Nm³, which corresponds to a desulphurisation efficiency of over 98%. The entire reconstruction was planned in two stages, during which a new scrubber unit was installed together with the retrofit of two boilers, turbines and electrostatic precipitators (ESP). AE&E s scope of supply essentially included the flue gas ducts (interface after induceddraught fan), the scrubber units, recirculation pumps, oxidation fans with pump station, cleangas duct, residue drainage and process water tanks, the pump house and the electrical facility, as well as the disassembly of the old FGD system. The renewal of the gypsum dewatering system and the electrical installation were carried out by another subcontractor of SKODA PRAHA Invest with the specifications being supplied by AE&E. In the absorber, which is designed as an open spray tower, the limestone suspension is atomized via nozzles positioned at 5 spray banks to create a droplet surface area as large as possible for optimum mass transfer conditions. In addition to the chemical processes, the fluid dynamics also have a considerable effect on the efficiency of a scrubber. This means that not proper designed flue gas inlet, arrangement of spray banks that are not optimal, as well as incorrect arrangement of spray nozzles can lead to bypass effects which considerably reduce the separation efficiency. Based on this knowledge, AE&E - APC has been using the CFD simulation method for calculating the distribution of flow in the scrubbers for many years and of course for the Tušimice scrubbers [ 4, 5]. LONG EXPERIENCE WITH HIGH SULPHUR FGD APPLICATIONS AE&E - APC has long years of experience with the design of scrubber systems for applications with a high sulphur content. Since the 1990 s, around 20 plants in Germany, the Czech Republic, Slovakia and Turkey have successfully been equipped with AE&E s FGD technology. While the first plants were equipped with 7 and 8 spray banks, the design has undergone radical changes, especially due to the use of modern engineering tools e.g. CFD modelling. Analysing L/G ratios of plants in the late 1990 s, not only the overall L/G ratios but especially the number of spray banks, as well as the L/G ratios per spray bank have been reduced [ 6 ]. 4 / 16

5 A list of several plants from AE&E is shown in Table 2 and compares the recirculating suspension flow and the number of spray banks used in the different installations. Analysing the Tušimice plant, only 5 spray banks are used at an L/G ratio of An additional optimization was carried out at the plant in Prunerov where the number of levels was reduced to only 4, the L/G ratio was raised to an L/G ratio of > 27, resulting in a further optimization of investment and operating costs due to the reduced pump height. The Prunerov plant is still in the execution stage - detail engineering - the second FGD project for CEZ from AE&E - APC. CFD-SIMULATION The CFD code fluent forms the basis for the CFD simulation [ 7, 8 ]. This commercially available CFD program was supplemented with user routines, which are the result of the many years of cooperation with the Institute of Process Technology for the Industrial Protection of the Environment at the Montan University in Leoben. Here the approach of AE&E - APC relies on the calculation of the distinctive two-phase flow in the scrubber via a Euler-Lagrange approach. In the simulation the geometrical conditions of the absorber and the installations, the position and essential characteristics of the nozzles, the interaction of the suspension mist with the flue gas and the scrubber wall, as well as the composition of the scrubbing suspension are taken into consideration. In a separate routine the mass transfer of SO 2 from the gas to the liquid phase and the heat transport are calculated, which makes it possible to calculate SO 2 profiles, as shown in Picture 3. Special attention here is focussed on the representation of the spray characteristics of the spray nozzles since the scrubbing fluid constitutes the major part of the pressure loss and has a considerable effect on the gas flow in the absorber. On basis of this knowledge from the CFD simulation, specific areas of the spray levels can be fitted with specific nozzles. The results of the CFD simulation have been verified by means of complex SO 2 profile measurements and the comparison between calculated and measured local concentrations confirms excellent correlation. This has provided AE&E - APC with a fully developed tool that is continually being improved and enables the scrubber to be optimally designed with regard to performance in separation efficiency and profitability. This CFD simulation was used for the design of the absorber for the desulphurization plant in Tušimice. The SO 2 content of over 11,000 mg/nm³ is extremely high and, according to 5 / 16

6 standard design criteria, 6 spray banks would have been necessary here. However, due to the results of the CFD simulation the sixth spray bank could be dispensed with, resulting in a substantial reduction in investment and operating costs. SCRUBBER DESIGN AND COMPONENTS The cylindrical scrubber in Tušimice was constructed from C-steel. For corrosion protection in the entire interior of the scrubber, a bromine-butyl rubber lining was used. The inlet area of the absorber uses high quality Alloy59 as corrosion protection because of the extremely high flue gas temperatures. As already mentioned, each of the two scrubbers in Tušimice is equipped with 5 spray banks, each with more than 10,000 m³/h of suspension volume flow. It is atomized via 100 nozzles per spray bank and absorbs more than 8,000 kg/hr of SO 2 from the flue gas. The arrangement of the different types of nozzles plays an important role in the construction of the spray bank (see Picture 4). Basically there are hollow and full cone spray nozzles. A hollow cone nozzle atomizes the suspension like the casing of a cone. On the other hand, a full cone nozzle sprays the suspension evenly over the entire cross-section of the spray cone and creates higher gas resistances locally than a hollow cone nozzle. These different spray characteristics are used to influence the flue gas flow. The material selected for the spray levels in the Tušimice plant was polypropylene (PP). While glass-fibre reinforced plastic (GRP) may have better strength characteristics, especially at higher temperatures, PP, however, is much more resistant to abrasion, making it more suitable for the use in limestone scrubber systems. A two-stage mist eliminator, also made of PP, is arranged as coarse and fine eliminator to prevent the scrubbing suspension from being discharged into the flue gas flow. Here, the flue gas is fed across curved fins and heavy, sluggish mist is separated by the fins. This reduces the discharge of the mist to a value below 100 mg/nm³. Regular flushing of the individual fields of the mist eliminator prevents the fins from becoming fouled. While the flue gas is fed into the scrubber with an average temperature of over 150 C, the use of heat-sensitive materials does not represent a problem as long as at least one of the recirculation pumps is operating to cool the flue gas. Critical situations can arise when the recirculation pumps fail due to a fault and the hot flue gas contacts the heat-sensitive rubber 6 / 16

7 lining or the PP spray levels and mist eliminator. To protect the installations from overheating during faults, an emergency cooling system was installed. Already in the raw gas duct, at a set minimum distance before the absorber inlet, there is a distribution system over which water can be sprayed in for emergency cooling. To ensure the utmost in safety, two pneumatically operated dampers open automatically when there is a power failure. The water for this emergency cooling comes from containers installed at a height of 40 m, from which the water is pressed gravity-fed into the emergency cooling system. The approximately 100 m³ of water can supply the emergency cooling system until the induced-draught fans and flue gas dampers are returned to an operating condition that is safe for the absorber. As previously mentioned, the gypsum produced in this plant is thickened via a hydrocyclone station and dewatered on vacuum belt filters. While the newly installed components of the gypsum dewatering system were not part of the scope of supply of AE&E - APC, they represent an essential component for the entire plant. The water discharged from the filter is returned to the absorber circuit, which means that the requirement for a plant without the discharge of waste water is being met. Part of the hydrocyclone overflow will only be discharged as waste water if the concentration of chloride in the scrubbing suspension gets too high and gives rise to technical problems or corrosion problems on individual plant components. FLUE GAS DUCTS Other components of the flue gas desulphurization plant worth mentioning are the clean gas ducts. The clean gas flows into the cooling tower and is taken away from there with the existing suction and will then be distributed into the atmosphere. An important advantage of this procedure is that the flue gas does not have to be reheated after the absorber to obtain a suitable distribution of the hazardous substances emitted. Expensive and high-maintenance facilities for heat exchange between the hot raw gas and the cold clean gas can be dispensed with. The clean gas duct is made from glass-fibre reinforced plastic (GRP) and therefore resistant to corrosion from the condensate. The uniqueness of this solution is highlighted by the fact that a duct of this kind made of GRP has never been produced in the Czech Republic. The length of the duct of the first line has a length in excess of 350 m at a diameter of 7.7 m. Since the transport cost for prefabricated 7 / 16

8 duct sections would have been too high, a manufacturing site was set up directly on site of the power plant on which each section of the entire duct was produced piece by piece and assembled (see Picture 5). The prefabrication was carried out on a horizontal winding machine with a diameter corresponding to the duct. Single components with a length of 10 m are wound on a rotating core bar on which alternating layers of glass fibre and resin are filled until the specified wall thickness has been obtained. The individual sections were fitted with reinforcing ribs and flanges and assembled into the appropriate lengths. The prefabricated sections of duct were then installed at the predetermined height of approximately 20 m with spectacular lifting and mounting performances. The types of cranes that were used are not normally used on a construction site of a flue gas desulphurization plant. For example, one section of duct with a weight of more than 45 tons had to be inserted into the shell of the cooling tower from a height of over 100 m. Or the installation of the second section of duct, where the heaviest section of the entire duct with a weight of over 60 tons had to be, due to local circumstances, lifted across a wide space (see Picture 6). RESULTS - COMPARISON DESIGN AND MEASUREMENT The first flue gas was taken over in March/April The following months were used to optimize the operation of the power plant including boiler- and turbine performance but also in the operation of the electrostatic precipitator and the FGD plant. Following information and discussion concentrates only on the performance of AE&E s FGD system. In Picture 7 the main operation parameters of the FGD absorber in Tušimice are shown for a one day operation. Typical inlet conditions for this operation was a flue gas flow of roughly 380 Nm³/s which correspond to Nm³/h. The SO 2 inlet concentration was given at around 7700 mg/nm³ dry at 6% O 2. The Picture shows ph-values between 5.5 and 5.9 which is typical for the operation in the Tušimice plant. The change from 4-spray-bank-operation to 5-spray-bank-operation influences the SO 2 outlet concentration respectively the separation efficiency and the pressure loss of the absorber system. Whereas the SO 2 outlet concentration for the 4-pump-operation is around 150 mg/m³ dry 6% O 2, which corresponds with a separation efficiency of 98.1%, the 5-pump-operation reduces the SO 2 outlet concentration down to 40 mg/m³ dry 6% O 2 corresponding to a separation efficiency beyond 99.5%. The 8 / 16

9 increase of pressure loss in the scrubber due the change from 4- to 5-pump-operation is around 5 mbar. In Table 3 further FGD operation parameters for constant inlet conditions (flue gas flow and SO 2 inlet concentrations) are shown. The analysed operation windows show that the open spray tower based on limestone is able to reach separation efficiencies beyond 99.5% and SO 2 outlet concentrations below 50 mg/m³ dry 6% O2. In Picture 8 the comparison between absorber design and measured operation data can be found. In this diagram the red dotted line defines the calculated design information (separation efficiency versus SO 2 -Cleangas Concentration) for the absorbers in Tušimice (yellow triangles define calculated operation points for 4-pump-operation (1,2,3,4) and 5- pump-operation (1,2,3,4,5) at flue gas conditions summarized in the Flue Gas Data text box). All other coloured points (filled triangles, squares and diamonds) show measured points from the absorbers. In the diagram the not filled coloured points show the measured corresponding power consumption of the recirculation pumps. The difference in the power consumption in 4-pump-operation depending on the selected pumps gives values between 3300 and 3500 KW. The analysis of these first measured operation parameters confirms AE&E s advanced absorber design and shows also very impressive that SO 2 clean gas concentrations below 50 mg/m³ dry 6% O 2 can be reached easily based on SO 2 inlet concentrations of above 7000 mg/m³ dry 6%O 2. CONCLUSIONS CEZ most ambitious program to cover the expected future growth of power demand in the Czech Republic will strengthen the CEZ Group position in the European market. An important part of these extensive investments will be also used for the installation of advanced air pollution control equipment e.g. flue gas desulphurization systems for high sulphur coals, bringing substantial emission reductions. The very complex management of many different suppliers of power plant equipment in a difficult economic environment due to booming markets in the last years was done by the general supplier (EPC) SKODA PRAHA Invest in a professional manner. The execution and finalisation of Stage 1 of the Tušimice FGD project confirms that expected parameters for the desulphurisation efficiency could be exceeded. 9 / 16

10 Advanced absorber design based on modern design tools guarantees high performance, reliability and low maintenance for the operation of a FGD plant. AE&E - APC showed first class performance based on the partnership with CEZ a..s. and SKODA PRAHA invest. Due to this AE&E- APC was also awarded to install new FGD plants in Prunerov and Ledvice in Czech Republic. LITERATURE [ 1 ] Bärnthaler K., Malezek M., Maier H., Design and Start up of FGD Melnik in Czech Republic, May 1999, PowerGen Europe, Frankfurt, Germany [ 2 ] Stancl J., Tušimice - Desulphurisation at High Stage, AE&E Expert Conference on Flue Gas Cleaning, September 2009, Graz, Austria [ 3 ] Reissner H., Development and limits for limestone scrubber systems, European Clean Coal Optimizing the Generation Mix, September 2007, Berlin, Germany [ 4 ] Weiß C.; The liquid deposition of sprays impinging vertical walls and flowing films; International Journal of Multiphase Flow Vol. 31 (2005), pp [ 5 ] Demuth M., Weiß C.; Scientific reports: Workpackage 1: absorber s modelling. In: Gerbec M. et al. (eds.); INCO: International Scientific Cooperation Projects ( ) Low cost flue gas desulphurisation (FGD) technology ; Final Report , European Commission contract number ICA [ 6 ] Reissner H.; 98% Limestone Scrubber Efficiency - Achieving High SO 2 Removal Efficiency using Lime or Limestone; McIlvaine Hot Topic Hours, July [ 7 ] Fluent Inc.; Fluent User Manual. Lebanon, USA, January 11 th [ 8 ] Hochenauer C., Demuth M., Timm W.; Scrubbers for Flue Gas Cleanup. Fluent News, Vol. XV Issue 1, Spring 2006, pp. S10-S / 16

11 PICTURES Picture 1: FGD Tušimice - Absorber Area Picture 2: FGD Ledvice - Absorber Area 11 / 16

12 Picture 3: CFD- Tušimice - Simulation Grid and SO 2 mass fraction distribution Picture 4: Full Cone and Hollow Cone TWIN Nozzles Picture 5: Winding Machine for Clean Gas Ducts 12 / 16

13 Picture 6: Installation of Clean Gas Ducts in Tušimice ,0 650 ph value Flue Gas after FGD [Nm3/s, f.] SO 2 outlet [mg/nm 3 dry, 6%O 2 ] SO 2 - outlet 15:01-15:30 14:01-14:30 13:01-13:30 12:01-12:30 11:01-11:30 10:01-10:30 9:01-9:30 8:01-8:30 Pressure 19:01-19:30 18:01-18:30 17:01-17:30 16:01-16:30 RP 4 RP 5 07:01-07:30 06:01-06:30 05:01-05:30 04:01-04:30 03:01-03:30 02:01-02:30 01:01-01:30 00:01-00:30 23:01-23:30 22:01-22:30 21:01-21:30 20:01-20:30 6,0 5,0 4,0 3,0 2,0 1,0 0,0 Presurre before FGD [kpa] Recycle pumps in operation ph Time [8:00-7:59] GAS FLOW SO2- OUT DP RECP ph Picture 7: FGD absorber operation Tušimice 13 / 16

14 99, ,6 99,4 5 RP in operation spray bank Removal Efficiency [%] 99,2 99,0 98,8 98,6 98,4 98,2 98,0 97,8 97,6 97,4 97,2 Flue Gas Data 4 RP in operation spray bank Volume Flow: 1.69 Mio. m³/h (wet stc) SO 2 -Conc. Inlet: mg/nm³ dry act. O 2 Fuel: Lignite Boiler Size: 2 x 200 MW Design point 5 RP in operation 4 RP in operation spray bank SO2-Removal Efficiency [%] Energy Consumption [KW] 4 RP in operation spray bank ,0 0 30,0 50,0 70,0 90,0 110,0 130,0 150,0 170,0 190,0 SO 2 -Cleangas Concentration [mg/nm³] dry, 6% O2 Design point 4 RP in operation Picture 6: FGD absorber Tušimice - Energy consumption and Separation efficiency Power Consumption Pumps [KW] 14 / 16

15 TABLES Table 1: Flue Gas Data Description Dry gas Nm 3 /h SO 2 in dry gas mg/ Nm 3 Wet gas Nm 3 /h SO 2 in wet gas mg/ Nm 3 Temperature C Average flue gas quantity (per absorber) Flue-gas composition inlet outlet (max.) SO 2 mg/ Nm Cl - mg/ Nm 3 max HF mg/ Nm 3 max concentration of TZL (Solid Particles) mg/ Nm 3 max Further data on flue-gas SO2 removal efficiency % > 98,3 Electr. Consumption kwh/h pressure loss kpa 2,87 / 2,94 availability % > 99 Table 2: High Sulphur FGD plants Plant/Name Country Design Vol. Flow Design SO2-Inlet Design SO2-Outlet Separation Efficiency Start up RCP L/G m³/h stc. mg/m³ dry act. O 2 mg/m³ dry act. O 2 % - - l/am³ FGD Cayirhan 3&4 Turkey < , ,9 FGD Oroszlany Hungary < , ,2 FGD Yagatan Turkey < , ,0 FGD Tusimice Czech Rep < , ,9 FGD Prunerov Czech Rep < , ,5 FGD Karlsruhe *) Germany < 50 98,4-6 20,0 FGD TBK BoA *) Germany < 50 99,1-6 26,0 *) CCS-Study Low SO2-Emissions 15 / 16

16 Table 3: FGD operation - Daily Average Values main parameter Date Flue Gas Inlet SO2 - inlet SO2 - outlet Removal Efficiency RECP in operation Differential Pressure ph Nm³/h wet mg/nm 3 dry 6%O 2 mg/nm 3 dry 6%O 2 % pcs. kpa ,6 98,1 4 2,16 5, ,7 99,2 4 2,34 5, ,5 98,2 4 2,18 5, ,5 99,5 5 2,62 5, ,2 98,3 4 2,31 5, ,6 99,4 5 2,64 5, ,7 98,2 4 2,13 5, ,0 99,3 5 2,64 5, ,1 98,5 4 2,24 5,71 16 / 16