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IR-CFB Boilers: Supercritical Once-through Developments for Power Generation

Technical PaperBR-1883

Authors:K.J. McCauleyD.L. KraftM. MaryamchikD.L. WietzkeK.C. Alexander Babcock & Wilcox Power Generation Group, Inc.Barberton, Ohio, U.S.A.

Presented to:Power-Gen Asia

Date:October 3-5, 2012

Location:Bangkok, Thailand

Babcock & Wilcox Power Generation Group 1

IR-CFB Boilers: Supercritical Once-through Developments for Power Generation

K.J. McCauley, D.L. Kraft,M. Maryamchik, D.L. Wietzke

K.C. AlexanderBabcock & Wilcox

Power Generation Group, Inc.Barberton, Ohio, USA

Presented at: Power-Gen AsiaBangkok, ThailandOctober 3-5, 2012

BR-1883

AbstractThe paper provides an update on Babcock & Wilcox

Power Generation Group, Inc. (B&W PGG) Internal Recir-culation-Circulating Fluidized-Bed (IR-CFB) boiler operat-ing experience, new commercial projects, and developments in large-scale supercritical boiler design and process.

B&W PGG IR-CFB boilers feature a proven and unique two-stage solids separator. The primary stage is an impact solids separator located at the furnace exit which collects the bulk of the solids and returns them to the furnace. The primary separation stage is arranged as an array of water-cooled, segmented, U-shaped vertical elements (U-beams). The secondary separation stage, typically a multi-cyclone dust collector (MDC), is located in a lower gas temperature region of the boiler. With three-years of successful commer-cial operation, the latest IR-CFB design is being expanded to higher capacity boilers of 300 MWe.

Operating experience of IR-CFB boilers confirms their efficient performance and high reliability and availability. Scale-up to 300 to 600 MWe, with higher steam tempera-tures, increases these efficiencies and benefits. This allows for a highly competitive utility-scale power plant for central power generation using local coals. Pilot-scale operations are currently underway to finalize the design of an advanced IR-CFB that will utilize in-bed heat exchangers (IBHX). This will allow for use of a greater range of fuels at higher capacities (>300 MWe) and for supercritical once-through designs. Current results of the pilot testing and boiler de-signs will be discussed, along with applicability to various markets throughout Asia.

IntroductionOver the years there has been a continuing need to provide

steam and/or electric power to drive our economies, and the use of available solid fuels has always been a key motiva-tor and enabler. From the 1800s when B&W first provided plants with grate (stoker) technology, to the 1970s with the introduction of bubbling fluid-bed (BFB) technology, and finally to the introduction of circulating fluid beds, techni-cal advances have met the ever changing needs of industry. Modern grate (stoker) technology is used successfully today for waste-to-energy power plants, and BFB technology is used for niche applications such as high volume biomass (and in recent utility power plants for renewable energy generation).

Likewise, the circulating fluid-bed (CFB) combustion process has been a market-wide success. Hundreds of CFB boilers operate today and provide high availability, low maintenance and reliable operation for both industrial and utility applications. The development of the CFB technology has evolved over the past three decades. The latest efforts are focused on developing large capacity, supercritical once-through CFB boilers for utility electric power generation. The drivers for these scale-up efforts include the need for higher plant efficiencies firing waste coal, low volatile an-thracite coal, high moisture lignite coal, highly erosive coal and possible applications for oxy-fuel combustion.

A CFB boiler circulates solid particles within the com-bustion process to transfer heat from the chemical process to the boiler water-cooled tube enclosure and other heating surfaces (see Figure 1). In doing so, the furnace gas tempera-

2 Babcock & Wilcox Power Generation Group

tures are lowered for the sulfation reaction of calcium oxide (principally) and to prevent agglomeration of the solids. Therefore, the primary function of the solids is to control the furnace temperature to 815 to 900C (1500 to 1650F) for the reduction of sulfur dioxide (SO2) by adding limestone to the circulating solids. The solids that exit the furnace are captured and returned to the furnace to maintain adequate solids inventory to control the gas temperature.

The B&W PGG design of the furnace exit solids return system is an array of U-beams, or impact solids separators, that discharge solids directly back to the upper furnace. The majority of the solids attempting to exit the furnace are captured and returned within the furnace enclosure. There is no major external solids return system. The B&W PGG design provides recycling of a minor solids stream from a multi-cyclone dust collector (MDC) located in the convection pass; however, this solid stream is quite small compared to the solids collected by the U-beam separators. The MDC impacts the upper furnace heat transfer rate, and thus overall bed temperature, and provides improvement in carbon and lime utilizations.

Supercritical steam cycle historyB&W has been providing steam generation systems

since the original water-tube boiler designs in the 1800s utilizing the Rankine cycle. Over the years the technology has continually been improved with a significant milestone being the introduction of the supercritical steam cycle by B&W in the 1950s. Since that time B&W has continued to lead the way in supercritical boiler applications, with the largest being the 1300 MWe series of plants built in the United States (U.S.). Today, B&W PGG has more than 135 supercritical units in service providing over 90,000 MWe of electric power.

The Rankine cycle is the most widely used steam/water cycle used for power generation, and is typical for solid fuel-fired power plants. The process is carried out reversibly since

the fluid cycles among liquid, two-phase and vapor states. A closed condensing cycle provides enhanced efficiency, as it also allows for close control of water chemistry required for high pressure, high temperature cycles, and favors a mini-mum of makeup water. An additional improvement is the use of regenerative feedwater heating, which uses extraction steam from various stages in the turbine to heat the feedwater as it is pumped from the condenser to the economizer.

Cycle thermodynamic efficiency is improved by increas-ing the temperature of the heat source for a constant heat sink temperature. This temperature can be increased when the feedwater pressure is increased because the boiler inlet pressure sets the saturation temperature in the Rankine cycle (see Figure 2). If the pressure is increased above the critical point of 220 bar (3200 psi), the addition of heat no longer results in a typical boiling process in which there is defined interface between the steam and water. Rather, the fluid can be treated as a single phase. This is referred to as a super-critical steam cycle, originally referred to as the Benson® Super Pressure Plant when first proposed in the 1920s. The first commercial unit featuring a supercritical cycle and two stages of reheat was placed in service in 1957 using B&W technology. In general, with equivalent plant parameters (fuel type, heat sink temperature, etc.) the supercritical steam cycle generates about 4% more net power than the subcritical pressure regenerative Rankine steam cycle.

These factors, along with component design limitations, must be considered in a cycle analysis where the objective is to optimize the thermodynamic efficiency within the physi-cal and economic constraints of the equipment. In addition, there are constraints imposed by the economics of fuel selec-tion and the environmental impacts of fuel combustion that impact the design of the boiler/turbine regenerative system.

Bubbling fluid-bed technologyThe BFB combustion process has been successfully ap-

plied in smaller industrial boiler applications (see Figure 3). In a smaller furnace, the coal can be fed over the top of the bed, and the bed gas velocities can be lowered without a large cost penalty, lowering the erosion rate. In smaller, lower velocity beds, the surface can be arranged to eliminate the need for complicated tube supports. Because of the lessons

Fig. 1 B&W PGG IR-CFB.

Fig. 2 Steam temperature and pressure versus heat available.


learned from coal-fired applications, improvements provide smaller capacity BFB boilers capable of using a wide range of biomass fuels, and many BFB industrial projects have been successful for many years.

The BFB process was introduced to the electric power industry in the 1970s. The work during the first ten years was limited to studies and pilot-scale testing. Between the late 1970s and mid 1980s, two demonstration facilities were built, and three commercial installations were built.

All three commercial projects were successful and met the owners’ needs. However, there were three significant lessons with the utility size application of the BFB combus-tion process:

1. The coal distribution was difficult for large or multiple beds. The coal was injected into the bottom of the bed with under-bed pneumatic systems. These systems plugged and eroded and the availability of the system was limited.

2. The beds had in-bed surface to control the bed tempera-ture. The surface was generating surface that utilized forced or pumped circulation and final superheater surface. These tube bundle surfaces eroded over time due to the erosive impact of the bed material. Tube sec-tions had to be replaced in 5 to 10 years of operation.

3. The tube bundles required support. The tube supports were uncooled and typically operated at 850C (1560F). The first supports required constant maintenance which affected availability. Later, the tube supports were increased in mass and mechanically tied together (no welds), which proved to be more successful, but quite expensive.

At this same time the CFB combustion process was being developed which addressed these deficiencies. The coal feed was simply fed into the lower furnace bed using a chute, and the higher fluidizing velocities typical of the CFB process effectively mixed the coal, bed material and air. There were no in-bed tube bundles to erode. The CFB process carried the solids to the vertical furnace enclosure walls to transfer the heat. High solids circulation rates allow uniform furnace temperature. Without in-bed tube bundles, supports, and their inherent maintenance, were not required.

IR-CFB technologyB&W PGG IR-CFB boilers feature a two-stage solids

separator. The primary separation stage is an impact solids separator located at the furnace exit collecting the bulk of the solids (95 to 97%) that are then returned to the furnace by gravity. The primary separator is arranged as an array of U-shaped vertical elements (U-beams). The secondary separation stage, typically an MDC, is located in the lower gas temperature region of the boiler convection pass, i.e., 250 to 510C (480 to 950F).

The U-beam separator design has evolved through several generations (see Figure 4), starting with 11 rows installed externally to the furnace with solids recycle through non-mechanical, controllable L-valves, to the current design featuring a total of 4 rows, two of which are located in the furnace. While each U-beam in earlier designs was made as a single piece supported from the top, the current design includes segments, each being supported independently from a water-cooled tube (see Figure 5).

Fig. 3 Bubbling fluidized-bed boiler.

Fig. 4 U-beam separator design generations.


During the same period, the design of the MDC separator has been modified for improved efficiency, reliability and maintainability. The MDC solids recycle system has evolved from dense-phase pneumatic transport to gravity conveying.

Operating experience of IR-CFB boilers has clearly confirmed their efficient performance and high reliability and availability.

Proven design featuresThe design of a solids separator is the core of IR-CFB

combustion technology because it has a major impact on the boiler layout, cost, fuel and sorbent utilization, operational flexibility, and reliability. B&W PGG CFB boilers with two-stage solids separation provide the following design features that positively benefit each of these key parameters:

High solids collection efficiencyThe collection efficiency of the two-stage solids separa-

tor is intrinsically high due to the greater efficiency of the MDC. Higher solids collection efficiency helps to achieve greater inventory of fine circulating particles in the furnace that provides: a) higher furnace heat transfer rate, b) better control of furnace temperature, and c) better carbon and sorbent utilization due to the increased residence time of fine particles.

Controlled furnace temperatureThe furnace temperature is controlled in response to load

changes and variations of fuel and/or sorbent properties by controlling the solids recycle rate from the MDC. The recycle rate at high boiler loads is set to achieve the upper furnace density required to maintain the target furnace temperature. At low loads, the recycle rate directly controls the dense bed temperature.

Low auxiliary powerThe auxiliary power requirement is lower for impact

separator-type boilers since the total pressure drop across the two-stage separator (U-beams and MDC) is typically only 1 kPa (4 in. wc). In addition, high-pressure air blowers for fluidization of returning solids are not needed.

Uniform gas flowThe gases exiting the furnace to the U-beam separators

positioned across the furnace width provide for a uniform two-dimensional gas flow pattern. This allows placement of in-furnace heat transfer surfaces as needed over the entire furnace height and width, including the region adjacent to the rear wall in the upper furnace. Selection of the furnace height can be based on combustion and sulfur capture considerations rather than heating surface requirements. Combined with high collection efficiency of the two-stage solids separator, this allows reduced furnace height.

High solids separator reliabilityThe U-beams and MDC have high reliability and low

maintenance since they do not include any maintenance-intensive components such as refractory, solids return seal, expansion joints, or vortex finders. The U-beam design that has evolved through twenty-five years of operating experi-ence requires essentially no maintenance, due to the falling ash capturing incoming ash with no ash to metal impact.

Minimal refractory useThe amount of refractory used in B&W PGG CFB boilers

is 80 to 90% less than that used for similar capacity CFB boilers with non-cooled hot cyclones and 40 to 50% less than CFB boilers with cooled cyclones. The startup time

Fig. 5 Segmented U-beam particle separators.


for B&W PGG CFB boilers is not limited by the rate of temperature rise of the refractory; instead it is limited by adding heat to the pressure parts, like all other non-fluid-bed boiler technologies.

Low maintenanceA distinct feature of B&W PGG IR-CFB boilers is low

maintenance. Among the factors contributing to this feature are: low overall amount of refractory, lower furnace refrac-tory interface Reduced Diameter Zone (RDZ) design (see Figure 6), low furnace exit velocity, and an absence of hot expansion joints.

Dynamic load changeEnhanced dynamic load change response is possible due

to the reduced refractory and the ability to adjust furnace inventory using a variable ash recycle rate from the MDC.

Wide turndown ratioA wide turndown ratio (5:1) without auxiliary fuel is

possible due to the selection of furnace velocity and control-lable solids recycle.

Operating experienceB&W PGG has a long history of boiler reliability in the

U.S. and worldwide. The long-term availability of CFB units supplied by B&W PGG's licensee in India, Thermax Limited, is shown in Table 1 (Maryamchik 2005). Thermax Limited has been very active in the CFB market and has successfully sold over 50 CFBs in India and 4 CFBs outside

India. Twenty of these units are in commercial operation, while the rest are in various stages of design, fabrication, construction and commissioning.

New commercial projectsA substantial number of new B&W PGG CFB units have

been in operation or will be coming online in the next few years. Two of the units (AGP, U.S. and Alunorte, Brazil) featuring the latest design of the U-beam particle separator (see Fig. 5) are in commercial operation. The design and performance characteristics of these units were described in an earlier paper (Maryamchik, 2008).

The highest capacity IR-CFB units are sold for Meenakshi Power in Krishnapatnam, Andhra Pradesh, India, which were in the early stages of commissioning at the time this paper was being written. The units are designed for firing Indian and Indonesian coals (moisture = 25 to 45%, ash = 5 to 19%, sulphur = 0.6 to 0.7%, HHV = 13,250 to 19,990 kJ/kg (5700 to 8600 Btu/lb)). The main boiler performance characteristics are provided in Table 2 and its arrangement is shown in Fig. 7.

The U-beam particle separator (Fig. 5) system is com-prised of four rows (two in-furnace and two external) of U-beams. Each beam assembly consists of segments supported from a water-cooled tube. The design allows independent thermal expansion of each segment. The U-beam segments are made of stainless steel material, which has proved suitable for U-beam fabrication in previous B&W PGG CFB projects. Solids collected by the U-beams fall along the beams and return to the furnace directly (from the first U-beam row) or by sliding along the U-beam zone floor. Superheater and reheater banks are located downstream of the U-beams followed by the economizer.

The MDC is the second stage of the solids collection system and is located immediately downstream of the economizer followed by the tubular air heater. The air heater is a gas-through-tube type and is side-split for primary and secondary air. After the air heater, gas flows through an electrostatic precipitator (ESP), and an induced draft (ID) fan to a stack.

Solids collected by the MDC are recycled back to the furnace through recycle lines utilizing conveyors and gravity feed. Controlling the MDC solids recycle rate allows precise and effective furnace temperature control.

New developments in B&W PGG CFB design

300 MW IR-CFB boilerWith a three -year proof of successful commercial op-

eration of key design features, the latest IR-CFB design is being expanded to higher capacity boilers (see Figure 8). The 300 MW IR-CFB boiler features a top-supported furnace and horizontal convection pass enclosure made of gas-tight

Fig. 6 Reduced Diameter Zone (RDZ) design for erosion protection at the upper refractory edge.


membrane walls. It contains full height water-cooled panels, or division walls (about 1/3 the depth of the furnace), and steam-cooled wing walls. All other design features are the same as the lower capacity B&W PGG IR-CFB boilers.

Supercritical once-through CFB with IBHX

HistoryIn the beginning applications of the CFB for the power

industry, the boilers were small capacity for low steam pressure and temperature conditions. As such, the furnaces were quite small and the relationship of the furnace surface to volume was quite large. That is, there was a high amount of furnace surface for a relative small amount of gas volume, which lowers or cools the furnace to a lower temperature than lower surface-to-volume ratio furnace designs (larger boilers). Therefore, the furnace enclosure surface was ad-equate to control the gas temperatures in the furnace to a relatively constant 840 to 900C (1550 to1650F) throughout the height of the furnace.

As the CFB design scaled up, the surface-to-volume ratio decreased, which necessitated adding more surface in the furnace to maintain the furnace temperature profile. One method used by several manufacturers to add surface in the furnace was to cool solids collected by the hot cyclone using a BFB heat exchanger between the hot cyclone outlet and the furnace return chute. Since these solids left the furnace to be separated from the flue gas and returned to the lower furnace, these solids are considered external to the furnace.

These external heat exchangers can have solids bypass conduits and a valve to control the amount of solids that flow to the heat exchanger. The bypassed solids and the cooled solids recombine and return to the furnace. By controlling the bypass flow and the resultant equilibrium temperature after mixing, the furnace temperature can be controlled. That is, the lower surface-to-volume ratio is compensated

by the additional heat exchanger surface in the external heat exchanger.

One design integrated the external heat exchanger into the lower furnace. The total amount of solids captured by the hot cyclone still flows into the integrated heat exchanger. The integrated heat exchanger shares a wall between the CFB lower furnace and the heat exchanger. There are openings in this shared wall that allow translation of solids between the furnace to the integrated heat exchanger. At full load, the majority of the solids through the heat exchanger come from the hot cyclone return solids. Essentially, the heat exchanger functions as an external heat exchanger since the solids and heat to the heat exchanger come from the external hot cyclone. At lower loads, the cyclone solids are significantly lower and the furnace solids flow into the heat exchanger to supplement the heat requirements. In this mode the heat ex-changer essentially functions as an internal heat exchanger.

B&W PGG designed an In-Bed Heat Exchanger (IBHX) that is totally internal to the furnace (see Figure 9). The heat exchanger utilizes only solids from the lower furnace because there is no major solids stream from an external source. This makes the heat exchanger a 100% internal heat exchanger over the entire load range of the CFB. Since B&W PGG’s IBHX is totally reliant on solids internal to the furnace, the location of the heat exchanger is not dependent on alignment with furnace exterior walls and the outlet of a hot cyclone.

Table 1 Plant Availability (all data in % of total time available)

Kanoria 1(commissioned in 1996)

Kanoria 2(commissioned in 2005)

Indian Rayon(commissioned in 2006)

Saurashtra Cement(commissioned in 2008)

Years reported 1997-2011 2006-2011 2007 – 2011 2009 – 2011

Plant availability 90.9 94.3 96.3 95.9

Table 2 Meenakshi CFB Performance Characteristics per

Project SpecificationMain steam flow @ MCR, klb/hr (t/hr) 1091 (495)Reheat steam flow @ MCR, klb/hr (t/hr) 886 (402)Main steam pressure, psig (barg) 2020 (139)Reheat steam pressure, psig (barg) 378 (26)Main steam temperature, F (C) 1004 (540)Reheat steam temperature, F (C) 1004 (540)

Fig. 7 General arrangement of 150 MW Meenakshi Power CFB boiler.


The IBHX is used to provide the final superheat and re-heat steam temperatures. The BFB style provides the greatest temperature differential between the solids and tube metal temperature for optimal heat transfer. Because 100% of the solids enter the IBHX from the furnace inventory, the avail-ability of solids is greater than required for the final superheat and reheat absorption duties throughout a wide load range. Therefore, these steam temperatures can be maintained to boiler loads as low as 30%.

In addition to compensating for the surface-to-volume ratio issues for larger furnaces, the IBHX allows for the opti-mization of the final superheat and reheat surface. If the final superheat and reheat steam temperatures were achieved in a combination of upper furnace and convection pass surfaces, the overall heat transfer coefficient would range narrowly from the upper furnace to the convection pass values. The heat transfer coefficient in the BFB is multiple times higher, which reduces the amount of surface directly by the increase in the coefficient. Therefore, the amount of the highest alloy material is significantly reduced by the ratio of the overall heat transfer coefficient. As the steam conditions advance to higher temperatures, the cost saving for this higher coef-ficient becomes more significant and becomes a major cost driver for the implementation of a heat exchanger.

To summarize, the IBHX compensates for the lower surface-to-volume ratio of larger furnaces, the steam tem-perature control turndown capabilities of the IBHX with 100% internal solids is not affected by the diminishing solids availability from the hot cyclone, and the amount of the most expensive metallurgy is significantly reduced by the high heat transfer coefficient in the BFB.

Supercritical once-through CFB design studyB&W PGG completed a study comparing the differ-

ences between a CFB utilizing a hot cyclone and its design

that utilizes a two-stage solids separation system (U-beams and MDC). The supercritical once-through full load steam conditions are 597C/594C/310 bar (1106F/1101F/4500 psi) and the capacity is 400 MWe net. There are two areas of significant differences between the two designs: the furnace height and the overall cost of the solids collection system.

The furnace height from the bubble caps to the roof for the hot cyclone design is significantly greater than the height of the B&W PGG design (up to fifty feet). The furnace height for the hot cyclone design is typically set to obtain 2600 kJ/kg (1100 Btu/lb) enthalpy leaving the furnace circuit, which is a standard criterion for once-through technology. The absorption in both furnaces must, by definition, be identi-cal to obtain the same steam dry-out condition required of once-through technology.

The difference in height is explained by how the hot cyclone furnace is surfaced as compared to the B&W PGG furnace surfacing. With B&W PGG’s U-beam design, the flue gas flows through the same open plan area through the

Fig. 8 General arrangement of 300 MW IR-CFB boiler.

Fig. 9 Internal Bed Heat Exchanger (IBHX).


U-beams as through the upper furnace. That is, there is no acceleration of the flue gas/solids mixture. With the hot cyclone design, the flue gas and solids accelerate from the large furnace plan area to the much smaller entrance area of the cyclone acceleration flue to the cyclone. Due to this acceleration of the flue gas/solids, pendent surface must not be near the cyclone entrances to prevent erosion. The internal vertical surface, division walls and platens can only be installed in the center of the furnace and away from the cyclone entrance walls, which constrains the design. For the U-beam technology, these surfaces can be installed directly in front of the furnace exit without a concern for erosion because the flue gas/solids are not accelerated when enter-ing the U-beams.

Therefore, the B&W PGG design permits installation of the required surface to obtain the required enthalpy in a shorter furnace height by concentrating the wing walls in front of the U-beams. The only way the hot cyclone design can install enough surface to satisfy the enthalpy requirement is to the increase the furnace height.

Another issue related to the taller furnace height is predicting performance accurately. Predicting the furnace absorption for a once-through design is critical. Over absorp-tion means higher tube temperatures in the furnace outlet circuits. Under absorption means the outlet enthalpy would be lower than design dry-out conditions for the primary superheater. B&W PGG uses the MDC ash recycle to trim the furnace overall temperature and wall absorption. Vary-ing the MDC recycle within the capability of the recycle equipment can vary the furnace temperature by +/- 50C (+/- 120F) and consequently vary the wall absorption. This, coupled with the supercritical once-through furnace height being maintained at normal height, demonstrates not only the uniqueness of the B&W PGG design, but the lower risk of furnace absorption changes because the solids density is not as expected at the taller furnace heights.

The second area of significant difference is the cost of the hot solids collection systems. The hot cyclone construc-tion is membraned superheater surface and the total area of the cyclone enclosure and flue is more than the enclosure surface area of the furnace. These cyclones are large and after applying pin studs and thin refractory, the total cost can be four to five times higher than the U-beams, MDC and MDC recycle system.

B&W PGG supercrtical once-through CFB design

Following the study, B&W PGG completed a detailed supercritical once-through CFB design for a waste coal plant in China. The design conditions are 565C/565C/240 bar (1050F/1050F/3500 psig) at 350 MWe net capacity. An actual turbine balance is used for the steam and feedwater conditions. The pressure reduces with load down to 86 bar (1250 psig) at 27% load. At this load, the main superheat is maintained at 565C (1050F) and the reheat is maintained at 565C (1050F) down to 40% load.

Water/steam circuitryThe water/steam flow path begins when the feedwater

enters through the economizer followed by the IBHX en-closure and furnace enclosure. The flow continues upward through the furnace division walls, furnace wing walls, separator and into the convection pass enclosure and U-beam cooled supports. The steam path then continues through the convection pass superheater, several passes for attempera-tion, and the furnace wing walls and finally the IBHX final superheat. The reheat path follows to the convection pass reheat and IBHX final reheat.

Flue gas pathThe flue gas leaves the IBHX section and the primary

zone and combines above the secondary air nozzles. There are dual outlets at the top of the furnace. The primary outlet superheater sections are in the convection pass that aligns with the final superheat IBHXs on the front wall. The reheat convection pass sections align with the final reheat IBHXs on the rear wall. The split flue gas exits the CFB convec-tion passes and flows to separate MDCs. The flue gas then recombines before flowing through the common convection pass containing the economizer and regenerative air heater.

The requirement for the dual outlet is utilized for the U-beam height and entering flue gas velocity. The flue gas temperature continues to decrease from the bottom to the top of the U-beams. Halving the U-beam height minimizes the temperature imbalance entering the superheater sections. Additionally, for a single outlet design and maintaining a constant maximum U-beam height, the furnace depth would have to increase and furnace width decrease to maintain the same flue gas velocity in the furnace and entering the U-beam area. The depth is the most expensive dimension for the boiler; mostly caused by extremely long superheater headers and long furnace buckstay spans for pressure con-tainment. The dual outlet optimizes performance and cost for large capacity CFBs.

Reheat temperature controlThe dual furnace outlet design and reheat configuration

in only one of the flow paths allow for some amount of flue gas biasing for reheat temperature control, within the toler-ance of the solids return systems.

In-bed heat exchanger developmentAs discussed previously, the two primary reasons for the

BFB heat exchangers are compact surfacing within the fur-nace to compensate for the surface-to-volume ratio for large furnaces, and placing the most expensive tube metallurgy in the highest heat transfer area of the boiler to minimize the cost of final superheat and reheat tubes. To this end, B&W PGG has developed the patented IBHX.

A cold model and a hot pilot plant have aided in the characterization and confirmation of performance. The


cold model has been built and performance characterized. A 2.5 MWt hot pilot facility has been designed (see Figure 10), and the construction, commissioning and testing will be completed in 2012.

Results of the cold model have been successful. Some of the features that have been confirmed by the cold model are:

1. The patent-pending control of solids flow method (un-der-flow conduit) through the heat exchanger confirmed the solids flow is variable and controllable.

2. The solids entering through the top of the IBHX are finer than the primary zone solids, which increases the heat transfer and lowers the required fluidization velocity.

3. The amount of solids entering the IBHX is greater than required for the desired absorptions.

4. The solids overflow nozzles are adequate to remove the excess solids from the top of the bed.

5. Fluidization of the bed can be varied from minimum fluidization to bubbling fluidization regimes while flow-ing solids through the under-flow conduits.

The 2.5 MWt pilot plant will confirm the hot performance including the heat transfer coefficient, solids flow, and over-all heat absorption duty.

IBHX designThe IBHXs are located on the lower furnace front and

rear walls. They are not full width, but are segmented to allow room for coal feed chutes and startup burners. There are four heat exchangers on two opposing sides. The heat exchangers on one side are final superheat and the heat exchangers on the other side are final reheat. The width of the tube sections is set by the maximum length of the tubes to be end-supported only; that is, no middle supports and the issues associated with uncooled supports are eliminated.

The tubes forming the wall between the CFB furnace and the IBHX form overfire air nozzles at the top of the IBHX. This leaves the top opening fully open for solids to enter the IBHX. These nozzles also allow placement of the secondary air penetration at its proper location for mixing at the top of the primary zone. There are overflow nozzles above the

bed to drain excess solids. The remainder of the solids flow down through the in-bed tube bundle, transferring heat to the steam and exiting through the underflow conduits to the lower CFB furnace.

The bed fluidization in the IBHX is capable of the range from minimum-to-bubbling fluidization regimes. The size distribution of the solids in the IBHX is quite fine as com-pared to the CFB primary zone, so the bubbling-bed veloci-ties are significantly lower than the bed velocities associated with the first BFBs.

The three major issues associated with previous utility-scale experiences with BFBs were solved in the IBHX BFB design.

1. The fluidizing velocities are low so the erosion potential is low.

2. There are no uncooled major supports requiring main-tenance.

3. There is no direct firing of coal in the BFB, so distribu-tion of coal is not an issue.

Open-bottom bed drain systemB&W PGG utilizes its open-bottom bed drain system un-

der the IBHX. B&W PGG has applied the same technology to the IBHX for the effective removal of oversized material, rocks and agglomerates, from the floor of the IBHX to reduce blockage of the underflow conduits.

The open-bottom construction is a hopper that is filled with bed material and is effectively 100% open to the floor of the bed. Air pipes with bubble caps enter from one side for the primary air to fluidize the BFB compartments. Double disk batching valves located on the bottom of the hoppers cycle to intermittently drain material. The drain action al-lows oversized material to sink below the bubble caps and no longer interfere with the BFB fluidization process. Once the material is in the hopper, the heat in these solids can be dissipated or recuperated in various ways depending on the frequency of the batch drain cycles. The solids leaving the bed drain hopper can be screened and returned to the CFB bed or simply transported by a drag chain conveyor to the ash removal system.

ConclusionThe use of a supercritical once-through design for

advanced CFBs for electric power generation is a natural evolution of a proven product to meet the ever-changing worldwide energy needs. The supercritical once-through design uses technology that has been proven since the 1950s, including supercritical Rankine water/steam cycles, BFB technology and CFB technology. Development of these into a unique product provides for sustainable use of natural resources, a lower carbon impact for electric power generation, and an environmentally conscious application.

Fig. 10 IBHX pilot plant.


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References• M. Maryamchik and D.L. Wietzke, Proceedings of 18th

International Conference on Fluidized Bed Combustion, ASME, Toronto, Ontario, Canada, May 22-25, 2005, FBC 2005-78004.

• M. Maryamchik, Proceedings of the 9th International Conference on Circulating Fluidized Beds, Hamburg, Germany, May 13-16, 2008.

• S.P. Ganehsan, M. Maryamchik and D.L. Wietzke, Proceedings of the 2010 PowerGen – India Conference, New Delhi, India, April 21-23, 2010.

• M. Maryamchik and D.L. Wietzke, Proceedings of the 2010 Coal-Gen Conference, Pittsburgh, Pennsylvania, August 10-12, 2010.

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