Integration of the

BENSON Vertical OTU

Technology and the

Compact CFB Boiler

Steve Goidich

Foster Wheeler Energy Corporation

Perryville Corporate Park

Clinton, NJ 08809, USA

Presented at:

PowerGen International

Orlando, Florida

November 2000

INTEGRATION OF THE BENSON VERTICAL OTU TECHNOLOGY AND THE COMPACT CFB BOILER

Stephen J. Goidich, Foster Wheeler Energy International

ABSTRACT

Circulating fluidized bed (CFB) boilers offer increased operational flexibility because of the ability to fire a range of fuels in the same unit. Fuels fired can include waste and low grade fuels which cannot be fired in conventional units. In-furnace capture of sulfur dioxide by limestone addition, and low NOx emissions resulting from low operating furnace temperature (850-900°C) and staged combustion make CFB units an environmentally friendly means to generate steam.

Once-through utility (OTU) boilers can be operated at supercritical pressures [>220 bar (3200 psia)] since they do not rely on the density difference between steam and water to provide circulation through the furnace enclosure tubes; fluid circulation is maintained by the boiler feedpump. Operation at supercritical pressures provides a significant increase in steam cycle efficiency which results in reduced fuel consumption. The coupling of supercritical OTU and CFB technologies offers a very attractive way to produce power in the 150 to 600 MWe size range.

Described in this paper is how the Siemens BENSON Vertical OTU technology is integrated into the Compact CFB boiler configuration offered by Foster Wheeler. Critical to the design of a once-through boiler is the ability to accommodate heat absorption variations without overheating the furnace enclosure tubes. Historically this has been done by designing for high fluid mass flow rates which results in high pressure losses and therefore increased auxiliary power consumption. The BENSON Vertical technology provides a means to design for low mass flow rates with a simple, single vertical pass configuration which can safely accommodate full variable steam pressure operation. The Compact CFB boiler, with its minimal use of refractory which results from having cooled separator, sealing device, and INTREX heat exchanger, allows for rapid load changes to meet the requirements for full variable pressure operation. The unique features of the BENSON Vertical OTU and Compact CFB boiler are discussed as well as start-up system requirements. A design for a 350 MWe Compact OTU CFB boiler is described.

INTRODUCTION

Large scale conventional utility boilers which fire high grade fuels such as pulverized coal (PC), oil, and/or natural gas can be and have been configured as either “drum” or “once-through” types. However, circulating fluidized bed units, with a few exceptions, have primarily been configured as “drum” type units.

In drum type units (see Figure 1) the steam flow rate is controlled by the fuel firing rate. Superheat steam temperature is determined by the proper sizing of the superheater heat transfer surface and controlled by spray water attemperation. In a once-through type boiler, the steam flow rate is established by the feedwater pump, and the superheat steam temperature is controlled by the fuel firing rate. Since the once through boiler does not rely on the density difference between steam and water to provide proper circulation and cooling of the furnace enclosure

tubes, it can be operated at supercritical [>220 bar (3200 psia)] pressures. Operation above the critical pressure significantly increases plant efficiency which not only results in reduced fuel costs, but also has the environmental benefit of less carbon dioxide production (green house effect) and less emission of SOx and NOx (acid rain) that are characteristic of the particular fuels fired.

As CFB boilers move into the large-scale (300 to 600 MWe) utility market, there is a strong incentive to take advantage of the cycle efficiency improvement that supercritical steam conditions offer. To utilize once-through supercritical operating conditions, the steam/water pressure parts of the CFB boiler must be configured to accommodate the selected circulation method. The selection of the circulation method dictates not only the boiler configuration, but also the selection of auxiliary systems as well as the modes and methods of operation and control.

Historically, once through boilers have been designed for high steam/water mass flow rates to minimize peak tube metal temperatures and limit the differential temperature between adjacent enclosure wall tubes. To provide high mass flow rates, the evaporative furnace walls have been designed in a multiple pass arrangement which also limits the heat absorption per pass and therefore potential temperature unbalances within a pass. However, this type of an arrangement requires operation at supercritical pressure over the load range to avoid two-phase flow related problems that can occur when trying to distribute steam/water mixtures between passes. As a result, there is a throttling pressure loss during low load operation.

Another method for achieving high mass flow rates is to incline the furnace enclosure tubes in a single pass, spiral arrangement. This allows fewer tubes to form the required furnace enclosure. Also, since all tubes wrap around through all enclosure walls, heat absorption and therefore tube metal temperature unbalances are minimized. Since a single pass is used, the unit can operate at subcritical pressure during part load or cycling operation which improves part load cycle efficiency and makes it easier to match steam and turbine blade metal temperature for improved steam turbine life. However, the spiral tube arrangement is not acceptable for CFB boiler application because the inclined enclosure tubes would be subject to erosion. In CFB boilers, fuel and sorbent ash are entrained in the flue gas which passes up through the furnace. A significant amount of the entrained solids reflux (fall down) along the furnace walls. Any protrusion which changes the direction of the falling solids can cause an erosive condition.

Figure 1. Utility Boiler Circulation Methods

Fb317

PRINCIPLE NATURAL CIRCULATION (DRUM) ONCE-THRU

SUPERHEATER

EVAPORATOR

ECONOMIZER

OPERATING PRESSURE 10...180 BAR 20...400 BARWATER WALL TUBING VERTICAL SPIRAL OR VERTICAL

The current state-of-the-art technology for once-through boiler design is the BENSON Vertical technology developed by Siemens through extensive research and development, and field-testing. This technology offers significant functional and economic advantages for OTU power generation. In order to incorporate these advantageous features into Foster Wheeler conventional and CFB boilers, FW became a licensee of the BENSON Vertical technology. Features of the BENSON Vertical technology and how it has been integrated into the FW Compact CFB boiler system are described below.

BENSON VERTICAL TECHNOLOGY

“Natural Circulation” Characteristic. The most important requirement for the configuration of the evaporative circuit in a furnace is to minimize peak tube metal temperatures and limit the differential temperature between adjacent enclosure wall tubes. As noted above, this has traditionally been done by ensuring sufficiently high steam/water mass flow rates through the tubes over the once through operating load range.

By maintaining a minimum mass flow rate at the minimum once-through operating load point, a relatively high pressure loss results at full load which increases auxiliary power consumption. This mode of operation has what is termed a “once-through” characteristic because an excessively heated tube will have a reduction in flow because the friction pressure loss is a significant fraction of the total pressure loss. This phenomenon is illustrated in Figure 2. A strongly heated tube will have hotter fluid and therefore a lower density than occurs in the average tube. The pressure loss resulting from hydrostatic head will go down. However, because the fluid density is lower, fluid velocity will increase, increasing the friction pressure loss. Although there is a reduction in hydrostatic head, the increase in friction loss dominates and the circuit total pressure loss increases. To maintain the average pressure loss in the circuit, flow in this excessively heat tube will therefore be reduced. This combination of high heat input with reduced flow can cause an increase in steam temperature and therefore tube metal temperature that can result in tube failure.

In the BENSON Vertical design, the furnace vertical enclosure tubes are selected so that a relatively low mass flow rate (about 1000 kg/m2-s) results at full load. This mode of operation is termed to have a “natural circulation” characteristic because an excessively heated tube will

Figure 2. “Once-Through” Characteristic (Ref. 1)

have an increase in flow because the hydrostatic pressure loss is much greater than the friction loss (see Figure 3). The reduction in hydrostatic head is greater than the increase in friction loss so that the excessively heated tube receives more flow. The steam temperature rise in this circuit is limited because of the corresponding increase in fluid flow through the tube.

With smooth tubes, a low mass flow rate results in a lower steam side film heat transfer coefficient and dryout will occur at relatively low steam qualities. This means that cooling of the furnace tubes will not be as effective as with high mass flow rates. Also, since dryout occurs at a lower steam quality, it occurs lower in the furnace where the heat flux is the greatest. If the deterioration in internal film heat transfer coefficient occurs here, tube failure can result. However, unique to the BENSON Vertical technology is

the use of optimized rifled tubes to eliminate this problem.

As illustrated in Figure 4, dryout in a smooth tube can result at relatively low steam qualities. In the example illustrated, it occurs at about 55% quality at which point there is a sudden increase in tube wall temperature. With an optimized rifled tube, the tube wall can be keep wet to a steam quality over 90% even with low mass flow rates.

Through extensive laboratory testing, Siemens has determined the optimum rifled tube design which uses the best combination of rib lead angle, height, and shape to provide the best combination of heat transfer and pressure loss. This data has been correlated into advanced computerized software for thermal hydraulic analysis. Figure 5 shows how an optimized rifled tube compares to a standard rifled tube and to a smooth tube for the same mass flow rate. As can be seen, the optimized rifled tube results in the lowest tube

Figure 3. “Natural Circulation” Characteristic (Ref. 1)

Figure 4. Rifled Tube Heat Transfer Improvement (Ref. 1)

temperature. The lower plot in Figure 5 illustrates how the mass flow rate of an optimized rifled tube (770 kg/m2-s) can be significantly lower than that for a standard rifled tube (1000 kg/m2-s) and smooth tube (1500 kg/m2-s) to achieve the same level of tube cooling. Because of this, the optimized rifled tube can operate with low mass flow rates to permit operation with a “natural circulation” characteristic.

Considerations for CFB Applications. The BENSON Vertical technology was initially developed for conventional PC, oil, or gas fired utility boilers. A CFB furnace operates under considerably different and less severe heat flux conditions than a conventional furnace. In the CFB furnace a significant portion of the air required for combustion is introduced as primary air through an air distribution grid located on the furnace floor. This air lifts and puts into suspension the solids inventory of fuel and sorbent products resulting from the combustion process. The remainder of combustion air (secondary air) is introduced about 2m above the air distributor to

complete the combustion process and entrain the finer fraction of solid products. The entrained solids pass up through the furnace, are collected in a separator which returns the solids back to the lower furnace and directs the flue gas to the heat recovery area (HRA). This flywheel of circulating solids maintains a relatively uniform vertical and radial temperature distribution throughout the furnace. For optimum capture of SOx by the limestone, the furnace is maintained at a temperature of about 850-900°C. The staged combustion and low operating temperatures also minimize the formation of NOx.

As a result of the low and uniform operating temperature with the CFB furnace, the heat flux to the enclosure walls of the furnace are considerably lower than in a pulverized coal furnace as shown in Figure 6. The lower 4-8 m of the CFB furnace is protected by refractory to prevent corrosion due to the substoichiometric atmosphere, and to prevent erosion due the dense bed of

Figure 5. Optimized vs. Standard Rifled Tubes (Ref. 1)

solids. As a result the heat absorption in this area is minimal. The highest heat fluxes occur just above the refractory protected area. In this transition region, there is a significant amount of refluxing (falling back) of the particles which are too coarse to be completely entrained by the rising flue gas. As a result, the solids concentration and therefore the heat transfer to the furnace walls is highest in this region. However, the peak to average heat flux is considerably lower than in a pulverized coal furnace.

Because of the low and uniform heat fluxes in a CFB furnace, mass flow rates lower than that required for a conventional furnace can be used without concern for departure from nucleate boiling (DNB) or dryout (see Figure 7). A full load mass flow rate in the 500 to 700 kg/m2-s range can be used to achieve the “natural circulation” characteristic. Figure 8 shows that for part load operation with subcritical pressure and with smooth tubes and low mass flow rates (55% of that used for PC design), there is not a significant rise in tube temperature at the dryout point because of the low heat fluxes that are characteristic of a CFB furnace.

Another phenomenon that must be consid-ered in the design is DNB that can occur near the critical steam pressure. As the critical pressure is approached, the Leidenfrost temperature (tube wall temperature above which stable film boiling occurs) approaches the saturation temperature. With the high heat fluxes associated with PC furnaces, DNB can occur near zero percent steam quality (see Figure 9) when operating in the critical pressure range (210-215 bar). Optimized rifled tubing can enhance the heat transfer rate and reduce the tube temperature as shown in Figure 9. For typical CFB operation with smooth tubes, the heat flux is not high enough to increase the tube wall temperature

Figure 6. PC vs CFB Heat Flux

Figure 7. DNB and Dryout

to the level required for film boiling to occur with low steam quality (see Figure 9). Rifled tubing is therefore not typically required. However, project specific requirements that require a wide range of fuel firing capability which includes a high percentage of standalone firing with liquid (oil, desaphalting tar, bitumen, etc.) or gaseous fuels (natural or synthetic gas), may utilize tube rifling.

Additional BENSON Vertical Features. In addition to the “natural circulation” characteris-tic that minimizes differential temperatures that can cause

fatigue cracking, the BENSON Vertical design offers the following advantages:

••••

Low Pressure Loss. Since the evaporative circuit can be de-signed with low mass flow rates, the total pressure loss and therefore power consump-tion is lower. The permanent frictional pressure loss in the furnace tubes is considerably lower with the “natural circu-lation” characteristic when compared to designs with a “forced circulation” charac-teristic as noted in Figure 10.

••••

Simple Support System. The vertical, self-supporting fur-nace enclosure tubes are linked to a standard top support sys-tem that does not require attachment of separate support straps. There is therefore no associated limit on the change rate of waterwall fluid temperature due to fatigue limits of the support straps. Also, the load carrying ability of the furnace is greater in the event that higher than expected furnace solids inventory is required. If repair is required, standard, simple tube replacement procedures can be used.

Figure 8. Dryout at Subcritical Pressure

Figure 9. DNB Near Critical Pressure

••••

Variable Furnace and Superhea-ter Pressure. Full variable pres-sure (see Figure 11) can be used over the once through operating load range to match steam turbine metal temperature for cycling service. The low system operating pressure at low loads reduces pump power consumption and therefore, fuel consumption.

••••

Completely Drainable Water-walls. The vertical tube orientation permits openings, such as those for fuel feed, startup burners, and over-fire air ports, to be formed while allowing all enclosure wall tubes to be completely drainable. This is an advantage if the oxygenated feed-water treatment method is used since there are benefits to completely draining the unit for long term storage.

COMPACT CFB BOILER

Compact Separator. A main distinguishing feature of a CFB boiler is the separating device at the furnace outlet that collects unburned fuel and bed material entrained in the flue gas and returns it back to the lower furnace. The separator can take many forms, but the most common in the industry has been the cyclone.

First generation cyclones consisted of a steel shell lined with one-foot-thick refractory. The capital cost is relatively low, but operating costs are higher due to refractory maintenance and heat loss.

The next major advancement was a cyclone formed with steam-cooled tubing and lined with approximately one-inch-thick refractory held in place with metal studs. Less refractory means less refractory maintenance. In addition, the thin

Figure 10. Furnace Tube Pressure Losses

Figure 11. Steam Pressure vs. Load

layer of cooled refractory operates at a lower temperature which improves its wear resistance characteristics. Also, cooling the cyclone to the same temperature as the furnace mini-mizes differential expansion be-tween the furnace and cyclone, minimizing the number and rela-tive motions of expansion joints, and thereby reducing expansion joint maintenance. The main drawback of the steam-cooled cy-clone is that it requires more complex fabrication which in-creases capital cost.

To eliminate the fabrication complexity issue, Foster Wheeler developed the Compact separator (Ref. 2) that is formed from flat rather than curved tubing panels. The arrangement shown in Figure 12 allows the separator to be positioned adjacent to the furnace and provide a “compact” configuration.

INTREX Heat Exchanger. Another innovation that enhances the Compact CFB boiler design is the Integrated Recycle Heat Exchanger (INTREX) which provides the additional solids cooling needed for larger boilers where the furnace walls are no longer sufficient (Ref. 3). The INTREX is a bubbling bed heat exchanger consisting of one or more tube bundles that further cools the solids collect by the separator before they are returned to the furnace (see Figure 13). In addition to cooling the externally circulated solids, openings in the furnace rear wall provides access for additional solids to internally circulate through the INTREX tube bundles ensuring sufficient hot solids to the INTREX at all loads. Excess solids spill back into the furnace through openings in the furnace rear wall.

The solids flow rate through the tube bundles is controlled by controlling the amount of aeration air added to the lift legs which return the solids to the lower furnace. By controlling the solids flow rate through the INTREX, the heat absorption can be varied

Figure 12. Compact Separator

Figure 13. INTREX Heat Exchanger

giving operational flexibility to control furnace and/or superheat steam temperature. Rapid heat absorption control can also be provided by controlling the fluidization velocity in the INTREX which can vary the heat transfer characteristic to the submerged tube bundles.

The enclosure is formed by water-cooled tubing that is integrated with the furnace circuitry. This integrated configuration allows the INTREX to grow downward with the furnace enclosure so that large maintenance prone expansion joints are not required.

350 MWe COMPACT CFB BENSON VERTICAL BOILER

Figure 14 illustrates a 350 MWe Compact CFB BENSON Vertical boiler. This particular design utilizes two (2) double vortex COMPACT solids separators which are positioned on one side of the furnace. Solids entrained in the flue gas are collected by the separators and are cooled by four (4) INTREX heat exchangers which contain superheat heat transfer surface. The duty of the INTREX is selected to maintain the desired furnace operating temperature for optimum emission control and fuel burnup as well as to efficiently provide final superheat duty.

Feedwater is heated first in the HRA economizer and HRA hanger tubes which support the tube coil elements. From the hanger tubes the feedwater is then passed through the INTREX enclosure walls before it is directed to the furnace enclosure walls which do all the evaporative duty. The furnace enclosure is formed from vertical tubes, all of which are in parallel flow.

Fluid leaving the furnace is passed through the in-line tangential steam separators. From the steam separators, the steam is superheated in the furnace roof, the COMPACT separators, HRA enclosure, the HRA convec-tion super-heaters, and finally in two (2) passes in the four (4) INTREX heat exchangers. The entire reheater is positioned above the economizer in the series pass HRA. Final reheat steam temperature is controlled by the FW patented reheat steam bypass system in which a portion of the reheat steam is bypassed around the lower reheat tube bundles. This design example is configured with a

Figure 14. 350 MWe CFB BENSON Vertical Boiler

reheat steam bypass system. Project specific details will dictate which of the reheat steam temperature control methods will be used to control final reheat steam temperature (reheat steam bypass, parallel pass HRA with gas flow proportioning, or INTREX reheat with solids and fluidization control).

START-UP SYSTEMS

To start-up a once-through boiler, the steam/water pressure parts and the steam turbine must be warmed and brought on-line in a safe and controlled manner that will not cause damage to any component. To do this, a load is defined below which the unit is controlled in a manner similar to a drum type unit (firing for pressure/steam flow). In-line separators are provided to collect steam for warming the superheater pressure parts and the steam turbine. Water collected is returned back to the furnace to maintain a minimum mass flow rate for proper tube cooling. Above this defined minimum load, the unit is operated and controlled as a once-through boiler (firing for steam temperature).

For the Compact CFB BENSON Vertical boiler, the minimum BENSON load is usually established between 35 to 40% load. This requires establishing a minimum mass flow rate of 35 to 40% of the full load flow rate through the furnace walls. To do this, a recirculation pump is used to superimpose a recirculating flow onto the flow provided by the boiler feedpump. Figure 15 illustrates the recirculation pump system.

With the economizer and evaporator cir-cuitry filled with water, and a water level es-tablished in the water collecting vessel, a minimum boiler feed-pump flow rate is es-tablished and is sup-plemented by the recir-culation pump to maintain the minimum load flow rate through the furnace enclosure walls. The water flow leaving the furnace is passed through several tangential steam sepa-rators which are con-figured in parallel (a typical 350 MWe unit would have two sepa-rators). The water collected in the separators is drained to a single water collecting vessel which feeds a single boiler recirculation pump which pumps the water to the economizer feed line. The water level in

Figure 15. Recirculation Pump Start-Up System

the water collecting vessel is controlled by a valve which allows excess flow to be dumped to a flash tank.

Steam collected in the separators flows through the superheater circuitry and is dumped to the condenser via the high pressure bypass station, the reheater, and the low pressure bypass station during the initial start-up phase, and later, via the high pressure turbine, the reheater, and the intermediate/low pressure turbine.

The first formation of steam in the furnace tubes causes a small amount of water to be discharged from the water collection vessel to the atmospheric flash tank when the maximum level in the tank is reached. The water from the atmospheric flash tank flows to the condensate tank and from there is either pumped back to the feed tank via the deaerator or discharged to atmosphere depending on the water quality.

A minimum recirculation pump flow line is provided to protect the pump. The valve in this line is closed when the minimum flow rate is achieved.

A subcooling line is provided to allow low temperature feedwater to be admitted to the saturated water in the water collecting vessel during recirculation. This subcooling of the water prevents the formation of steam bubbles in the recirculating pump suction line even when the rate of pressure reduction is high.

A typical tangential separator is illustrated in Figure 16. Steam enters through either four (4) or six (6) inlet nozzles (depending on unit size) which are positioned tangential around the vessel circumference. The orientation and size of the nozzles in combination with the vessel diameter and position of the vortex finder (upper steam discharge pipe) has been optimized by extensive testing by Siemens to provide a balance between pressure loss and steam separation efficiency. A vortex eliminator is provided near the water drain at the bottom of the vessel. Vessel diameter is limited to about 584 mm to limit vessel thickness so that it does not restrict allowable temperature change rates. Vessel length is typically about 4m.

A typical water collecting vessel is illustrated in Figure 17. The vessel has the same diameter limits as the separators and is about 12m tall. It is equipped with a pressure equalizing line which vents any steam which may be

Figure 16. Tangential Steam Separator

carried along with the separated water. A vent line is connected to the steam discharge line from each separator.

DYNAMIC BEHAVIOR

The Compact CFB BENSON Vertical boiler design permits the use of variable pressure operation with its advantages noted above. As system pressure is lowered, so is the saturation temperature for evaporation which requires changes to the water inventory temperature as well as to the evaporative pressure part temperature. Sliding pressure operation therefore has more inertia or slowness to load change. In addition, CFB boilers have an inventory of solids both in the furnace and INTREX, as well as some refractory that affects the thermal inertia characteristics of the boiler. Experience of load change rates for drum type CFB boilers which operate at constant pressure is therefore not directly applicable to a supercritical OTU CFB boiler.

Extensive modeling work has been conducted by FW to understand the dynamic response characteristics of the Compact CFB BENSON Vertical boiler. The major objective of the work was to develop a control system to coordinate the heat flow to the boiler water/steam according to the energy output requirements placed on the unit.

A major advantage of the CFB boiler is the capability to load and unload the physical and chemical heat of the bed material in an intelligent way. The primary objective is to maintain the appropriate ratio of water/steam system heat input to feedwater flow ratio during fast load change conditions. As schematically shown in Figure 18, the CFB boiler has several parameters for control that increase the operational flexibility of the unit to meet the required dynamic response behavior. Figure 18. Boiler Load Control Parameters

Figure 17. Water Collecting Vessel

For example, although firing rate control may be sluggish in achieving a rapid change, heat absorption rate can be rapidly changed by shifting the solids distribution profile and heat release patterns within the furnace by varying how much and where primary and secondary air are introduced into the furnace. Additional flexibility is available when INTREX heat exchangers are used; heat absorption can be varied by several means (solids flow rate control by lift leg aeration, fluidizing velocity which changes the heat transfer rate to the submerged tube bundles, and spray water attemperation for rapid steam temperature adjustment). As shown in Figure 19, a combination of controlled parameters can provide the required ratio between energy input and feedwater flow.

The dynamic simulation models have shown that the unique control features of a CFB boiler combined with standard control means such as throttling reserve in the variable pressure ramp and throttling of steam extraction to feedwater heaters can be used to make the CFB OTU equal or comparable to conventional PC boiler for step and ramp load changes. Unique to the CFB OTU is that it can accommodate disturbances from the process (fuel rate and quality) because of the stabilizing effect of the inventory and flywheel of circulating solids. Superheat versus evaporative duty distribution can be controlled by varying furnace enclosure, internal furnace heat transfer surface, and INTREX heat absorption characteristics.

CONCLUSIONS

The BENSON Vertical once-through boiler technology, with its “natural circulation” evaporation circuit characteristic, is ideally suited for CFB boiler application. The range of fuels firing capability, low pollutant emissions, and high efficiency provide for cost effective power production. Features such as the Compact separator and INTREX heat exchanger further add to operational flexibility, low maintenance costs, and increased reliability.

REFERENCES 1. Siemens BENSON Boiler Brochure, “Research & Development at the BENSON Test

Rig” by Siemens AG , Power Generation KWU.

2. S. E. Hatch, “Compact CFB’s”, Foster Wheeler Review, Spring 2000.

3. S.J. Goidich, T. Hyppanen, K. Kauppinen, “CFB Boiler Design and Operation Using the INTREX Heat Exchanger, 6th International Conference on Circulating Fluidized Beds, Würzburg, Germany, August 22-27, 1999.

Figure 19. Matching Energy Input and Flow