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Technology | March 2014

Boilers and Turbines |The rise of supercritical boilers

Supercritical boilers using Benson Vertical Tube design offer several advantages over conventional spiral wall design. K Viswanathan shares his technological perspective.

ISGEC, with technical collaboration from Foster Wheeler, offers supercritical boilers using Benson Vertical Tube (BVT) design. This has several advantages over conventional spiral wall design. These advantages include ease of fabrication, construction and maintenance apart from enabling the use of conventional buckstay and self-support design for the furnace walls. In terms of operational performance, the vertical tube design with lower mass flux has inherently lower pressure drop and hence lesser auxiliary power for boiler feed water pump, and natural circulation characteristics, which helps in minimising temperature differential between different tubes in the furnace. The vertical tubes help in minimising the tendency of furnace deposit accumulating over the furnace walls, which is a big plus when the boiler has to fire different types of coal. The paper brings out briefly these benefits of supercritical boilers offered by ISGEC, and also covers an overview of the world´s first and largest pulverised coal-fired supercritical boiler with opposed wall firing, using the BVT feature designed, supplied and successfully commissioned in 2011 by Foster Wheeler in US.

Introduction
Since supercritical once-through boiler technology was introduced in the power industry in the early 1960s, there have been many innovative boiler design configurations and features incorporated to reduce capital and operating costs, simplify operation and maintenance, and increase reliability. A notable example is the introduction of in-line steam/water separators, which eliminated complicated valve manipulations that made every start-up an adventure. Another milestone was use of a spiral furnace tube configuration which, by having a single up-flow configuration, permitted both furnace and superheater to operate on variable pressure mode. This allowed for cycling operation with the benefits of reduced low load auxiliary power and optimum matching of steam and turbine metal temperatures to maximise turbine life. In the 1980s, the spiral configuration became the state-of-the-art for new supercritical power projects. However, the inclined tube configuration requires high (power consuming) mass flow rates to maintain adequate tube cooling, and a special (complex) support system consisting of vertical buckstays and ´finger´ straps to transfer the load of the lower furnace to upper furnace.

In the 1990s, vertical tube configurations with standard rifled tubes were introduced to simplify fabrication, construction, and maintenance while permitting full variable pressure cycling operation with reduced pressure loss. However, with standard rifled tubes there is a minimum fluid mass flow that must be maintained when passing near the critical pressure. This minimum mass flow for standard rifled tubes results in a negative flow characteristic which means that tubes that receive more heat get less flow. To prevent tube overheating, the tubes must be properly orificed to push more flow to the tubes receiving the most heat.

In the mid-1990s, after extensive laboratory testing of many rifled tube rib geometries, Siemens developed and began licensing the BENSON vertical evaporator design for once-through boilers. Foster Wheeler, USA who is the licensor for ISGEC for PC boiler technology in India, has the license from Siemens for BENSON Vertical evaporator once-through design. The BENSON Vertical evaporator includes optimised rifled tubes that provide enhanced tube cooling with very low mass flow rates. With low mass flow rates a positive flow characteristic, similar to a drum type boiler, is achieved. Tubes that receive more heat receive more flow. This self-compensating, low mass flux feature eliminates the need for orificing, and minimises pressure loss and auxiliary power consumption.

In 2002, the BENSON vertical tube technology was first commercially demonstrated in a 300 MWe subcritical PC boiler. Commissioning of the first supercritical CFB boiler using the low mass flux FW-BENSON Vertical tube technology began in early 2009. When the 769 MWe (gross) Longview Power Project (located in Maidsville, West Virginia, USA) was commissioned in 2011, it set another milestone by being the largest and first supercritical PC boiler with opposed wall firing in the world using FW-BENSON low mass flux vertical tube design.

Foster Wheeler North America´s scope of work for the Longview project is for the design and supply of a supercritical once-through pulverised coal (PC) boiler. As part of an extended boiler scope, FW has supplied an SCR system for NOx control, ash handling equipment, and an auxiliary boiler. The plant was built by a consortium of Siemens (whose scope included the steam turbine generator, emissions reduction technology and the power plant control systems) and Kvaerner North America Construction. Foster Wheeler North America supplied the boiler.

Unique OTU boiler design requirements
To reap the high efficiency benefits of the OTU boiler there are special design requirements that must be factored into the configuration of the evaporator circuitry of an OTU boiler. For comparison, in a drum type unit, which operates at sub-critical pressures, large diameter tubes are used to minimise flow resistance so that a sufficient amount of steam and water can flow through the tubing by natural circulation. By designing for a sufficiently high circulation rate, the water passing through the tubing never completely evaporates to steam and a liquid film is maintained on the tube wall so that departure from nucleate boiling (DNB) and/or dry out do not occur. With the high heat transfer coefficient resulting from nucleate boiling, all the evaporator tubes remain at essentially the saturation temperature for the operating pressure of the boiler.

In an OTU boiler, which operates at supercritical pressure, there is no distinction between liquid and vapour phases and there is a continual increase in fluid temperature. With unbalances in heat absorption due to geometric tube position (corner versus centre of a wall), burner heat release pattern, furnace cleanliness, and variations in flow rate due to difference in hydraulic resistance from tube-to-tube, variations in tube temperatures occur. If the unbalance in temperature is not limited, high thermal stresses will result which can lead to tube failure.

FW/BENSON vertical boiler features
The FW/BENSON vertical boiler addresses these requirements in the following unique and effective ways:

Heat Absorption Variations. Historically, heat absorption variations in OTU boilers has been addressed in two different ways:

  • In units with multiple passes in the furnace evaporator, the differential temperature is limited by the fact that each pass picks up a fraction of the total evaporator duty which limits the magnitude of the unbalance and intermediate mixing occurs before the fluid is distributed to the next downstream pass. However, with multiple passes, the furnace must operate at supercritical pressure to avoid the difficulties of uniformly distributing a steam-water mixture to the downstream passes.
  • In units with a spiral tube configuration, the unbalance issue is addressed by having each inclined tube pass through various absorption zones so that each tube absorbs approximately the same amount of heat. With a single up-flow pass, the spiral design can operate with variable pressure steam which minimises part load requirements and allows matching of steam and turbine metal temperature for extended turbine life. However, the spiral tube evaporator configuration requires a special support system for the inclined tubes, which are not self supporting. Inclined tubes are also more prone to slag formation.

In the FW/BENSON Vertical design (Figure 2), the furnace enclosure is formed from a single, upflow pass of vertical tubes (rifled with ´optimised´ rifle configuration in the lower furnace, and smooth-bore in the upper furnace). The tube size and spacing are selected to provide a low fluid mass flow rate of approximately 1000 kg/m2-s. With this low mass flow rate, the frictional pressure loss is low compared to the gravitational head, and as a result, a tube that is heated strongly, i.e., absorbing more heat, draws more flow. With an increase in flow to the strongly heated tube, the temperature rise at the outlet of the tube is limited which limits the differential temperature between adjacent tubes.

As shown in the example in Figure 3, standard rifled tubing will provide an improvement in heat transfer. However, full load mass flow rate of approximately 1500 kg/m2-s would be required at full load to have a sufficiently high heat transfer coefficient at reduced loads when passing through the critical pressure. This mass flow rate would be too high to achieve a ´natural circulation´ flow characteristic as described above. What permits the use of a lower full mass flow rate is an ´optimised´ rifled tube rib configuration that will improve tube cooling as illustrated in Figure 3. Extensive laboratory and field testing has been conducted to define the optimum rib geometry (lead angle, rib height, corner/ edge rounding, etc.) that will provide the best enhancement to heat transfer.

The benefits of the low mass flow rate FW- BENSON Vertical evaporator design can be summarised as follows:

  • Self-compensating to accommodate heat absorption variations
  • Excellent tube cooling with optimised rifled tubes
  • Vertical tube wall construction, which simplifies erection, maintenance and repair, and reduced tendency for slag accumulation
  • Low pressure loss for improved plant efficiency and lower design pressure for pressure parts
  • Full variable furnace/superheater pressure for cycling operation
  • Low minimum once-through load (BENSON load); not limited by minimum mass flux

REFERENCE
Stephen J. Goidich, Richard J.Doccherty, Kenneth P. Melzer, Foster Wheeler North America Corp. Hampton, NJ 08827 ´The World´s First Supercritical FW-BENSON Vertical PC Boiler-The 750 MWe Longview Power Project´ Presented at Power Gen India & Central Asia, New Delhi, India, May 5, 2011.

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