Products BENSON Boilers Experience in Nearly 1000 Plants and Innovative Design Promise Continuing Success Manfred Klein, Rudolf Kral, Eberhard Wittchow The unique advantages of the are its suitability for use over a pressure range of 100 to more than 350 bar without modifying the evaporator system, and its exceptional suitability for sliding pressure operation. An innovative evaporator design now combines these operational advantages with the structural ones of a vertical-tube combustion chamber. A low-cost BENSON boiler with outstanding operating characteristics can therefore be realized. In Central Europe, oncethrough boilers have been the dominant design for more than 30 years, and this boiler design has also become established in Japan and South Korea. Growth countries, too, such as those in Southeast Asia, are now beginning to switch from subcritical drum boilers to once-through steam generators. The BENSON boiler is the most common implementation of the once-through design, and it is in use in nearly 1000 plants worldwide. Based on a patent registered to Mark Benson, it was developed in the 1920s by Siemens, who initially manufactured this type of boiler. Its introduction brought highpressure design to the field of power plant operation. After Siemens closed down its boiler manufacturing operations, it awarded licenses for the manufacture of s beginning in 1933. Today, the leading manufacturers of once-through boilers worldwide are licensees (Fig. 1). The activities handled by Siemens, which cover both the boiler itself and its incorporation into power plants, have resulted in important innovations in power plant construction such as oncethrough boilers with variable evaporation end point, generously dimensioned furnaces, the sin- Siemens is licensor for s Siemens activities include: Boiler concepts, thermal-hydraulic design, arrangement of heating surfaces, start-up systems, control systems, interaction of boiler and turbine, operating modes, and R&D. Licensees Austria Austrian Energy & Environment (SGP/Waagner-Biró) Denmark Burmeister & Wain Energi England Mitsui Babcock Energy Germany Deutsche Babcock Steinmüller Italy Ansaldo Japan Babcock-Hitachi Kawasaki Heavy Industries USA Babcock & Wilcox Sublicensees Denmark Vølund Energiteknik Finnland Tampella Mexico Babcock & Wilcox de Mexico Netherlands Stork Boilers South Africa Babcock Africa South Korea Halla Hyundai Spain Babcock Espanola Fig. 1 s: Manufactured by licensees and sub-licensees around the world. 26 Siemens Power Journal 1/96
gle-pass design, sliding-pressure operation and spiral-wound tubing. The has reached an extraordinarily high degree of technical refinement and exhibits excellent operating characteristics (Fig. 2). In addition to ongoing optimization, Siemens has developed an innovative boiler design, and is developing measures to increase plant efficiency. Current Status of BENSON Boiler Technology in Germany A variety of different designs and firing systems make BENSON boilers exceptionally well-suited for use in today s different power plant environments: In addition to the trend toward increased efficiency and reduced investment costs, these boilers also provide high availability, long times between overhauls, can be used with a broad range of fuels, and are characterized by low pollutant emissions. Highest efficiency Enthalpy Modes of operation Critical point Pressure (load) 100 to over 350 bar Suitable for subcritical and supercritical pressure without changing the evaporator system 7 Use of all coal grades 5 6 Burners Wide scope in design, e.g., oversized combustion chamber or slag tap furnace (shown) Economical, low-stress operation Temperature (545 580 C) Load Constant main steam temperature regardless of load, fuel and degree of fouling. Economical, low-stress start-up. 1 2 3 4 5 6 7 8 9 10 11 12 13 Flexible operating mode Load 4 6 %/min Burner Combustion chamber Evaporator tubes Ash hopper Convection heating surfaces Support bars Start-up system Recirculation pump deno x system Air heater Flue gas Air intake Coal mills 9 Time Rapid load changes in sliding pressure operation Fig. 2 s: Well-suited for efficient, economical and flexible operation. The current state of the art in Germany is represented by the in the 550-MW Unit 5 of the Staudinger power plant (Fig. 3): 3 The single-pass design of the boiler is structurally straightforward, which among other things eliminates the need to weld enclosure walls to one another, which exhibit different temperatures in operation, causing thermal stresses. All the superheaters can be drained, which allows the system to be started up quickly. The ash concentration in the flue-gas flow is uniformly distributed, which minimizes wear on the tubes of heating surfaces. 8 1 2 4 13 12 10 11 Fig. 3 Unit 5 of the coalfired Staudinger power plant: The 100-meter-high, 550-MW singlepass BENSON boiler is equipped with an opposed firing system. Siemens Power Journal 1/96 27
Fig. 4 The unrestricted design of the combustion chamber in the BENSON boiler with its water/steam section is particularly well matched to the broad range of firing system requirements. The can therefore be used with a number of different firing systems. Tangential 930 MW Lignite Burner Opposed 930 MW Hard coal Corner 400 MW Hard coal Slag tap 770 MW Anthracite Dry vertical 350 MW Anthracite Fig. 5 The Deutsche Babcock whirlstage burner is characterized by exceptionally low NO x emissions with levels down to 130 ppm (measured downstream of the steam generator). Ignition burner Ignition lance Core air IR flame monitor Secondary air Tertiary air Swirl plates Pulverized coal UV flame monitor Stabilizer Swirl insert At full load, the boiler operates at supercritical pressure and steam conditions of 260 bar/545/562 C. At these temperatures, it is still possible to use low-cost martensitic steels for the final stages of the high-pressure and reheater heating surfaces. The optimum feedwater temperature was determined to be 276 C. Net unit efficiency was measured at 43 percent. The airflow and flue-gas passages are designed as single trains. This design, which has also proven effective in several 700-MW units, has been extended at the Staudinger plant to include the scrubber in the flue-gas desulfurization system as well. This design simplifies operation while maintaining the same level of availability. One interesting solution was implemented in the 770-MW unit at the Ibbenbüren plant. The slagtap firing system used in that unit burns anthracite and is the largest of its kind in the world. The unit can be operated under part-load conditions at down to 40 percent of full-load capacity without backup firing, and with only six percent volatile matter (water- and ash-free).the BENSON boiler also operates at full load with supercritical pressure in the evaporator. A Variety of Different Firing Systems Are Possible One of the advantages of the is the unrestrict- 28 Siemens Power Journal 1/96
ed design of the combustion chamber, the walls of which are designed as evaporator heating surfaces, allowing the water/ steam section to be particularly well matched to the requirements of the firing system. Fig. 4 shows the firing system designs which have to date been used in highcapacity s. Design features implemented since the end of the 1970s to reduce NO x emissions on the primary side have brought about fundamental changes in tured thus far, NO x values measured downstream of the steam generator have ranged from 400 mg/m 3 (200 ppm) down to 260 mg/m 3 (130 ppm), depending on the composition of the fuel. The new burner design has a greater influence than previous models on NO x formation directly at the point of origin. It features special inserts in the pulverizedcoal pipe and a swirl flow of combustion air on the secondary and tertiary sides. In terms of combustion, the burner promotes in the combustion chamber walls (Fig. 6). While this design has reached an exceptionally high level of technical refinement, higher manufacturing and installation costs put it at a disadvantage as compared to a verticaltube design. Siemens has further developed the vertical-tube design for the (Fig. 6) not only in order to reduce investment costs, but also to facilitate the introduction of boilers operating at supercritical pressure in countries in Drum boiler Subcritical operation Vertical tubing Subcritical and supercritical operation Spiral tubing Start-up system Recirculation pump to economizer Subcritical and supercritical operation Spiral tubing Start-up system to flash tank (Diagram showing two-pass design) Fig. 6 The innovative concept of the verticaltube combustion chamber with rifled evaporator tubes combines the advantages of the with evaporator tubes in spiral configuration and those of the drum boiler. Hence, a low-cost BENSON boiler with outstanding operating characteristics can be realized. intensive and early coal pyrolysis in a low-oxygen zone. This also leads to very stable ignition, and has the advantage of allowing the use of a wide variety of fuel grades without having to change the burner settings. Evaporator Design with Vertical-Tube Combustion Chamber A typical feature of BENSON boilers has been the spiral configuration of the evaporator tubes the design of pulverization and firing systems, including finer pulverization and graded airflow with supplementary turbulence and over-fire air supply, to name but a few. The whirl-stage burner designed by Deutsche Babcock (Fig. 5) is representative of the high level of technical refinement which has been achieved. This burner design meets the pertinent requirements in terms of flame stability, coal burnout and minimum NO x emissions. In the more than 200 burners manufacwhich the drum boiler (Fig. 6) has been the dominant design. By using rifled tubes with high heat transfer capability, it was possible to reduce the mass velocity in the tubes from a value of 2000 to 2500 kg/m 2 s, which is typical of the spiral evaporator tube configuration, to less than 1200 kg/m 2 s in the case of the vertical-tube design. Siemens has made more than 80,000 measurements during this development initiative. The resulting advantages in plant operation are: Siemens Power Journal 1/96 29
51 % 50 49 48 47 46 45 44 43 42 41 Net efficiency 167 bar 538/ 538 C F 12 Fig. 7 Higher steam conditions and higher efficiencies are made possible through the use of new materials. 250 bar 540/ 560 C 1,5 F 12 The evaporator system exhibits a flow characteristic similar to that of a natural-circulation boiler (drum boiler). In this design, an increase in the heat input to individual tubes increases the flow rate through them; thus the outlet temperature is only marginally affected. This in turn tends to 270 bar 585/ 600 C 1,1 P 91 300 bar 600/ 620 C 0,7 315 bar 620/ 620 C 0,4 NF 616 Austenitic steel 350 bar 700/ 720 C 2,1 Inconel Material make the outlet temperatures of the tubes uniform despite varying heat input. An evaporator heating surface based on this design was successfully tested for a period of more than 10,000 hours by Siemens together with Deutsche Babcock and Steinmüller in the 320-MW Farge unit. The minimum load in oncethrough operation can be reduced from the previous value of 35 to 20 percent without reduced main steam temperatures, thus avoiding frequent start-ups and shutdowns. This is of particular advantage in the case of hightemperature systems with their thick-walled components, which can only be started up and shut down slowly. Early changeover to oncethrough operation mode keeps start-up losses so low that recirculation pumps are not required. The vertical-tube design, therefore, combines the structural advantages of the drum boiler and the operating advantages of the with evaporator tubes in a spiral configuration. This vertical-tube design can be used for capacities of around 300 MW up to more than 1000 MW, and it has proven to be a successful, innovative step toward a lowcost, structurally simple boiler design with superior operating characteristics. Enhanced Efficiency through New Materials The potential increase in efficiency through the use of new materials and hence the scope for attaining higher steam conditions is shown in Fig. 7. Over the last several years, it has been possible to develop non-austenitic materials with increased creep rupture strength. In power plants currently under construction, thick-walled components are now able to withstand main-steam temperatures of 585 C and reheat temperatures of 600 C, and a further increase to 630 C is expected. Given these new developments, it makes little sense to continue to use austenitic steel, with its unfavorable behavior, for headers, piping, and turbine shafts and casings, since the potential increase in temperature to 650 C would result in only a marginal increase in efficiency.the European power plant industry is therefore looking at the possibility of achieving steam temperatures of 700 C, and thus net efficiencies of more than 50 percent, through the use of nickel-base alloys. Manfred Klein received a degree (Dipl.-Ing.) in mechanical engineering. Since 1970 he has been employed by Babcock Lentjes Kraftwerkstechnik GmbH, where he is involved in the design of boiler systems for power plants, and since 1993 has served in the capacity of manager for bid generation with responsibility for the design and sales & marketing of large-capacity boilers. Rudolf Kral joined KWU in 1970 after receiving a degree (Dipl.-Ing.) in mechanical engineering. Since then he has worked in the field of BENSON boiler technology, and is currently responsible for advising licensees on technical issues, and for continued BENSON boiler development. Eberhard Wittchow joined Siemens in 1958 after receiving a degree (Dipl.-Ing.) in energy economics. He has previously worked in the fields of design and commissioning of steam generators and power plants. At present, he is responsible for the areas of licensing and special steam power plant issues. 30 Siemens Power Journal 1/96