Process

Figure 12.5 depicts a typical underfeed bubbling bed boiler. There is a bottom/grid/distribu­tion plate consisting of sand/ash and fuel. Evenly spaced air and fuel nozzles as well as ash nozzles for ash discharge are attached to the bottom plate. After preheating, air is admit­ted to fluidize the bed, followed by the fuel feeding. The volatiles, the small light-weight particles, and moisture leave the fuel to burn in the freeboard above, whereas the fixed carbon (FC) portion of the fuel burns in the bed. The bed coil, immersed in the expanded bed, extracts heat and keeps the bed at the desired temperature. The hot flue gases generated in the bed, on completion of combustion, leave the freeboard along with dust particles and flow over the convection sections such as superheater (SH), boiler bank (BB), economizer (ECON), and airheater (AH) to progressively yield heat like any other boiler.

A cyclone-type dust collector dedusts the gases and returns the dust particles to the bed to capture and burn carbon. The gases, after being cooled to their optimum temperature, are cleaned in a bag or electrofilter before they are released into the atmosphere.

The distinguishing feature of a BFBC boiler is the bed coil meant to extract the heat and keep the bed temperature low, although for fuels with very high moisture or high ash, bed coils may not be required. Located in the violently churning bed material, the bed coil is subject to high heat transfer from gas convection and radiation but, in this case, more predominantly from conduction due to bed material impingement. The impingement is also the cause for erosion. The permissible bed velocity is limited by what the bed tubes can safely withstand. For high-ash coals, it normally does not exceed 2.5 m/s (—8 ft/s). Bed velocity or superficial bed velocity is the gas velocity derived by dividing the volume of gas passing through the bed at the bed temperature by the plan area of the bed.

Excess air requirement is 20%. Around 10% air is given as secondary air (SA),

• For providing an air curtain to curtail the entrainment of fine particles

• For reducing the fan power

With an in-bed coil, PA should not be less than the stoichiometric level to prevent the cor­rosion of coil.

Freeboard should normally give —2.5 s residence time for free-burning fuels and —3 s for slow-burning and low-volatile fuels. This translates into a freeboard height of 2.5-6 m (8-20 ft) in most cases.

The residence time in bed is a concept that does not apply to other modes of firing. Here it refers to the time taken by a coal particles entering the bed to burn and reduce to a size that permits them to be airborne. It is usually —0.5 s.

Bed density depends on the voidage. A slumped bed typically has bulk density of

14.5 kg/m3 (—90 lb/ft3). When expanded the voidage is usually 65% and the corresponding bulk density is —9.3 kg/m3 (58 lb/ft3).

Normally the design for the bed is of 600 mm (2 ft) depth in slumped condition, made of mostly sand or ash. When fluidized it assumes a height of —1200 mm (4 ft). The ratio of expanded to slumped bed is called the bed expansion factor. Usually it is 2. In the shallow bed operation, which is explained later, the thickness of the slumped bed is 300 mm.

Underbed versus Overbed Feeding

The two BFBC designs based on the type of fuel feeding are underbed and overbed feeding, as shown in Figure 12.6 and compared in Table 12.1.

Bunker

FIGURE 12.6 Underbed versus overbed feeding arrangements.

Vw

In

Feeders Mechanical Spreaders

— Freeboard

-Bed

-Air

Plenum

Overbed feeding (mechanical)

TABLE 12.1

Comparison of Underbed and Overbed Feeding

Fuel

Overbed Firing

Underbed Firing

Top size

Moisture (surface)

Fines

CaCO3 <10 mm

<6%

<30% through 1 mm 5 mm maximum, 0.8-1.4 mm average

< 30 mm <20%

< 20% through 1 mm 5 mm maximum,

0.8-1.4 mm average

Underbed Feeding

1. In the underbed design, the fuel is admitted from the bottom of the bed, allowing more residence time for the same bed thickness.

2. There is less carryover as compared to an overbed feeding, where the fuel is dis­tributed from above the bed, as in a spreader stoker (SS).

3. There is comparatively less of burning of fines and VM above the bed. Most of the FC and a large part of the VM burn in bed, giving a slightly higher temperature that, with more bed residence time, aids carbon burnup. Thus, it has an efficiency advantage of 0.5-1.0%.

4. As fuel is not spread from above, it has a higher tolerance for fines and more fines are burnt in the bed.

5. Sulfur capture is also better because the fuel that burns above the bed does not react with limestone particles in the bed and this proportion is smaller in under­bed designs and the Ca/S ratio is smaller.

6. NOX generation is also slightly lower, as very little fuel burns above the bed, resulting in a lower freeboard temperature.

7. The furnace dimensions are not governed by the spreader throw, and there is a greater flexibility in arranging the fuel feed points, making an economical narrow boiler with a deep furnace.

The limitations of underbed design are as follows:

1. The multiple feeding points and long fuel pipes are highly erosion prone. If the surface moisture of fuel is more than 6%, the fuel pipes and feeders choke up,

Disrupting the combustion. Covered storage of incoming and crushed fuel can solve this problem but it occupies a lot of space, and sheds for fuel are not practi­cal for larger sizes. Hot air for fuel conveying, proper insulation, and heating are other measures.

2. In case of interruption of power, the fuel in the pipes settles at the bottom and restoring the unit requires manually emptying the pipes.

3. Also, the fuel has to be crushed to a lower size of <6 mm, leading to more fine generation, whereas the overbed accepts 20 mm or even larger top size, similar to a stoker.

4. Multiple coal pipes clutter the arrangement of coal bunkering and feeding.

5. Conveying air at high temperature also requires a high pressure of 3500-5000 mm wg to overcome the line losses and push the fuel against a bed pressure of —900 mm wg. Suitable Rootes blowers are required, which are power-consuming and noisy.

6. For bed temperature and automatic combustion control, the regulation of numer­ous feeders and fuel nozzles can be challenging.

Overbed Feeding

The overbed feeding has the advantages of

• Yielding a better layout with fewer bunkers and feeders and no coal pipes.

• No restarting after a power trip.

• Less crushing of fuel, as the fuel size is usually <20 mm and, at times, even <30 mm. Placing overfeed BFBC boiler alongside stoker-fired boilers is not a problem.

• Better tolerance to surface moisture of up to 20%.

• Easy regulation of fuel feeders for automatic combustion control.

The disadvantages are as follows:

• The boiler is wider and heavier.

• It is slightly less efficient.

• It is less tolerant to fines.

The overbed design is adopted

• For highly reactive fuels such as lignite or low-sulfur fuels or high-ash coals where combustion efficiencies are inherently high

• Where the desulfurization requirement is low or absent, where sulfur in fuel is negligible or emission norms are liberal

• Where simplicity and operational convenience are preferred to a small increase in efficiency, particularly if fuel and limestone costs are low

A combination of overbed and underbed is effective, particularly in places that experi­ence high rainfall. To obtain the efficiency advantage and avoid disruptions during rainy periods, underfeed boilers are built with overfeed systems as standby. This adds cost but improves the availability.

Bed Regulation for Part-Load Operation

One of the major drawbacks of BFBC boilers is that at part loads of <70%, as the fuel input is reduced, the bed cools down disproportionately, since the bed coil made of saturated tubes extracts almost the same quantum of heat. This increases the unburnt carbon to unacceptable levels. There are three methods available for boiler part-load operation.

1. Bed height variation. Here, the expanded bed height is lowered along with the reduc­tion of fuel and air with reducing loads. Since the height of expanded bed only marginally more than bed coil height, the tubes get uncovered and bed tempera­ture remains nearly constant, creating an effective variation of evaporator heating surface (HS). The drained hot ash is stored in a silo and fed into the bed when the load has to be increased, requiring the bed to be raised. Thus, the bed temperature is constant but the bed height is varied.

2. Velocity turndown. This is the simplest method effective up to 70% load. Here, the fuel and air are reduced over the entire bed. The bed temperature reduces, and the combustion efficiency is lowered. This method is suitable for small boilers or boilers for base load.

3. Bed slumping. The bed height is much more than the coil height and the bed is compartmentalized. Up to —70% loads, the fuel and air are reduced proportion­ately, with bed temperatures decreasing. Thereafter a bed is taken out of operation by totally diverting air and fuel to the other compartments and defluidizing the chamber. With the reduction in cooling surface, the bed temperature recovers. The process continues as the load is lowered further. Here the bed temperature fluc­tuates as the beds are cut in and out. Thus, the bed height is nearly constant but the bed temperature is varied. The bed slumping method for load control is more prevalent.

Bed Coil

The bed coil is the most intensely heated and stressed surface, immersed in a violently churning bed of solids. In-bed surface is unique to BFBC boilers and not found in any other type of boiler construction, except for external fluid bed exchangers in some CFBC boilers.

Heat transfer. How intense is the heating can be inferred from the fact that the bed coil absorbs nearly a third of the entire heat input into the boiler. Heat transfer rate at —255 W/m2 °C or 220 kcal/m2 h °C or 45 Btu/ft2 h °F is five to eight times the heat transfer rate in other areas. Thus, the bed surface is the cheapest HS and the most effective one too.

Sizing. Bed coil sizing determines one of the most important parameters in BFBC, namely the bed temperature. Lower bed temperature gives the boiler

• Lower turndown

• Reduced combustion efficiency

• Larger SH

• Lower NOX and higher CO

The effects are in reverse order for higher bed temperature.

A temperature of 750°C (1380°F) during low loads acts as a limiting temperature for the bed to decrease unburnt loss. The optimum temperature for lime-sulfur reactions is 850°C (1560°F). A rough guide for bed temperature selection can be as follows:

• 800°C (—1470°F) for fuels with low-melting compounds in ash such as lignite

• 850°C (—1560°F) for fuels needing sulfur removal

• 900°C (—1650°F) for difficult-to-burn low-volatile fuels such as anthracite with more FC and ashy bituminous coals

More bed coil surface is required for cooling the bed to a lower range.

Access. In smaller boilers operating at —900°C, the bed coil can be small enough to permit full access for both the tubes and the air nozzles from the top of the coil. For larger boilers, this may not be possible and it is necessary to provide access cavity of —600 m between the nozzle top and bottom of the bed tube.

Bed tube inclination. The bed tubes are usually inclined at —5° to horizontal to ensure an adequate flow of water at all conditions. But in larger boilers, with more HS, the inclined bed tubes may not be accommodated within the height of the expanded bed. Circulating pumps are then employed for a positive flow and the tubes are made horizontal to contain the height of bed coil.

Bed coil SH. When final steam temperatures >500°C are required, the final SH tubes are placed in the bed in one compartment. Extreme care is needed in the selection of tube materials and supports. High-mass steam flows are required to ensure proper cooling of tubes. Skilled operation is required during startup, load swings, and shut-down. A conser­vative design favors pendant SH to in-bed SH.

Stainless steel (ss) is not the preferred material for the SH bed tubes due to the risk of corrosion from the chlorides in ash, but alloy steels (AS) such as T22 or T91 prove satisfac­tory. For evaporator bed tubes, carbon steel (CS) is adequate because the rate of boiling heat transfer of water is very high. T11 tubes or rifled/ribbed tubes have been used to prevent departure from nucleate boiling (DNB). It is usual to provide 1 mm of sacrificial thickness as a first measure of erosion protection.

Deep versus Shallow Bed

Forced draft fans consume maximum power in a BFBC boiler and the reason is the huge amount of bed material to be fluidized. This auxiliary power can be reduced if the bed height and velocity are lowered without sacrificing the combustion efficiency. A deep bed of 1000-1200 mm is necessary to provide

• Adequate residence time

• Turbulence needed for desulfurization reaction

• Burning of fuels rich in FC

But quick-burning fuels such as sub-bituminous coals or lignites, with low sulfur can use a shallow bed of 600 mm to properly cover the bed tubes.

Erosion in BFBC Boilers

Prior to the commercialization of BFBC boilers it was believed that combustion at 800-900°C, well below the ash-softening temperature would create

1. No slagging in furnace and no fouling in convection banks

2. No erosion of tubes and refractory, as the ash does not melt and recrystallize to develop sharp edges

Although the first expectation materialized and the boilers were supplied without soot blowers (SBs), the second benefit was not realized. Erosion was experienced

• Most heavily by bed tubes

• By tubes placed in the gas turns

• By the first row of tubes in a bank

Bed coil erosion was brought under control by

1. Reducing bed velocities from ~3 to 2.5 m/s (—10 to 8 ft/s) or lower.

2. Protecting bed tubes by erosion shields or studding to deflect ash streams away from tubes. (Note: Studding increases the HS and, unless planned from the beginning, the bed temperature would be lowered, resulting in performance alteration.)

3. Protecting the other tubes by erosion shields, which are described in Section 3.5.1.2.

Because erosion behavior is not predictable, particularly if the fuel is sourced from sev­eral mines, it is cheaper to carry an inventory of bed tubes that can be changed periodically. The furnace wall tubes around the bubbling bed are exposed to severe erosion, almost on par with bed tubes. They suffer less because they are vertical, but they need protection. Good heat transfer is also desired. This area is usually studded and covered with refractory. Silicon carbide is a better choice because of its high thermal conductivity and extreme hard­ness. In the remaining areas of the boiler, erosion has been substantially overcome by

• Lowering the gas velocities to <10 m/s

• Protecting the first tubes of the banks by suitable erosion shields

• Avoiding gas turns over tubes

Bed

The bed consists of two layers:

1. The active, fluidized layer above the air jets

2. The static layer of —100 mm thickness all around air nozzles, which acts as an insulation protecting the bottom plate from the heat of the bed

Bed material is usually sand, ash, or crushed refractory in a size range of 0.5-1.2 mm. Sand should be rounded river or lake sand with no abrasive alpha quartz to avert any chances of erosion. Sea sand should not be used becuase it contains alkalies and chlorides.

Fuels with ash >15% do not require bed ash replenishment. For firing fuels with lower ash, a bed material silo and a feeding system are required. Crushed refractory for bed material is less aggressive but more expensive than sand. Stoker ash, properly sized, suits very well.

High underbed pressure in FBC boilers drives away most of the ash, and only the heavier particles, which are fuel impurities such as stones and shale separate out as bed ash. Bed ash usually contains very little carbon (<1%) in case of coals and forms <10% of total ash. Periodic draining is needed to remove this burden to maintain bed height. Usually, one ash nozzle of 150 NB is considered for 10-20 m2 (—100 to 200 ft2) of bed area suitable to drain an area within 3.5-5 m (—10 to 15 ft) of radius.

A cost-effective disposal is by discharge into a water-impounded hopper or an equiva­lent wet system, provided the local pollution laws permit it and the bed is not dosed with limestone for desulfurization. Lime produces gypsum on contact with water, which solidi­fies into a hard rock and only dry disposal should be considered. With a large bed, a BFBC boiler would have several ash outlets carrying relatively small amounts of ash and need­ing many ash pipes, making the arrangement clumsy.

Bed height control by overflow is another simple method adopted. Overflow pipes are arranged on both the sidewalls, —100 mm above the bed level, to spill over and maintain the height.

Self-closing flaps at the end of the pipes prevent ingress of tramp air into the freeboard.

One fuel nozzle is required to be placed at every 2-3 m2 (—20 to 30 ft2) in underbed feeding. Usually fuel nozzles, 150-200 mm NB, are of T-shape or with annular openings, injecting air-fuel mixture at a velocity of —15 m/s.

Freeboard

Freeboard is the chamber between the top of the expanded bed and the convection sur­faces. For easy-burning fuels, it should give a residence time of 2.5 s, and for slow-burning fuels, the time should be 3 s. Fines and volatiles burn here, and despite good heat absorp­tion by radiation, the exit temperature is —30 to 50°C higher than the bed temperature. For overfeed firing, the difference is at the higher end, as all fines burn in suspension. SA nozzles are provided on opposite walls.

Ash Recirculation

There are two reasons for recirculating ash from the back ends in FBC boilers.

1. Fine particles escaping combustion in freeboard get caught in the hoppers and mechanical dust collectors (MDCs) or in bags and electrofilters. The fines are rich in carbon and can be burnt if returned to high furnace temperature once again. This improves the carbon burn-up efficiency. In less reactive and high calorific value fuels such as bituminous coal or anthracite, the dust particles contain a lot of carbon and it is vital that this is returned for refiring to improve efficiency.

2. Ash recirculation is also necessary for better utilization of limestone and lowering of Ca/S ratio.

However, very high-ash fuels such as washery rejects and highly reactive fuels such as lignite burn out quickly, and the fine dust is mostly ash with practically no carbon. Further, if there is no limestone savings due to absence of desulfurization, there is no practical gain in returning the ash to bed.

Carbon burnup increases with an increase in recycle ratio, as shown in Figure 12.7, and beyond a point, the effect starts tapering. But the dust load also increases, enhancing the chances of erosion. It is normal to adopt a ratio of 2.0 to 2.5 as a compromise.

Recycle ratio (kg/kg) FIGURE 12.7 Effect of ash recycle on underbed and overbed feeding.

Air Nozzles

Air has to be uniformly distributed at the bottom at a pressure sufficient to lift the bed and fluidize it. Typically an air nozzle is made of 20 mm NB pipe of ss, 160-180 mm long, closed at the top end and weld prepared at the bottom to be attached to the bottom plate or membrane strip. Starting —25 mm from the top, fine air jets of 1.5 mm diameter around the circumference are drilled downward at two or three levels. Fine holes help prevent backflow of ash into the windbox, a common occurrence in FBC boilers. If the problem still persists, possibly because of the ash particles breaking down to finer levels, a shroud has to be fixed to the nozzle. Stainless steel ensures the survival of the nozzle during bed slumping when the nozzle attains nearly the same temperature as the bed with no airflow inside. Nozzle spacing is —150 to 125 mm (4-5 in.) or 60-80 nozzles/m2. Typical air nozzles are shown in Figure 12.8.

The air pressure to be provided at the windbox is arrived at in the following manner:

• It is the sum of the air pressure losses in bed and air nozzle.

• The bed pressure loss is 0.70-0.75 times the expanded bed height in mm wg.

• The air nozzle is designed for a loss of —300 mm wg.

Thus, for a 1200 mm expanded bed, the air pressure works out to be 1200 mm wg.

Salient Process Parameters of BFBC Boilers

Table 12.2 lists process parameters of BFBC boilers.

Applications

BFBC boilers have two areas of application, namely,

1. Fossil fuels

2. Biofuels

R-~

1

• I •

*

F

FIGURE 12.8

Salient Process Parameters of Bubbling Fluidized Bed Boiler

Typical air nozzle.

Parameter	Value	Remarks
Bed temperature	800-950°C (1470-1740°F)	Normal temperature with desulfurization is —850°C (1560°F)
Bed velocity	1.8-3.0 m/s	For fuels with erosion tendency, velocity is limited to 2.5 m/s (—8 ft/s)
Bed particle size	0.5-1.5 mm	Average size —1 mm
Bed depth	600/1200 mm (2/4 ft)	Slumped/expanded for deep beds, for shallow it is 300/600 mm (1/2 ft)
Bed pressure drop	500-1000 mm w. g. (20-40 ft)	Percentage drop = 0.8-1.0 of the expanded bed height
Bed density	720 kg/m3 (45 lb/ft3)
Ash recycle ratio	0-5:1	Normal range is 1:2-2.5
Excess air	20-35%	For most coals it is 20%
SA as % of total	<10%	SA can be in any opposite walls
Residence time	2.5-3 s	For bed and freeboard. Bed residence time is —0.5 s
Carbon loss	4-8%
Ca/S ratio	2.5-3.0	For 90% S capture
Gas velocities	<10 m/s (—35 ft/s)	Limits are similar to those of pulverized fuel boilers, as gases are dust laden
SO2	<300 ppm dv	At —85% desulfurization
CO	—250 ppm dv
NO,	<200 ppm dv

TABLE 12.2

Evaporation (~ tph) 100 200 300 400 500 600 1800 Heat release (MW/h) FIGURE 12.9 Capabilities of stoker, bubbling fluidized bed combustion (BFBC), and circulating fluidized bed combustion (CFBC).

The most common application is coal firing in developing markets. In absence of strict pollution norms, particularly with NOx, bubbling beds continue to be sold only in devel­oping markets for solid fuels such as coal and lignite. They have practically replaced the stokers and entered the CFBC boiler market at the lower end. They are not accepted in the developed markets, where stricter pollution norms prevail for fossil fuels. However, their popularity for biofuels is on the rise, particularly for very high moisture (—70%) fuels such as industrial sludges and process wastes.

Stokers, BFBC, and CFBC burn solid fuels. See Figure 12.9.

BFBC Boiler Design Principles

• Bubbling bed boilers are short and stocky because

O The low heat release rates make the plan area large.

O A residence time of 2.0-2.5 s makes the freeboard only —6 m, even for larger boilers.

• With nearly 60-70% of evaporation taking place in the bed coils, the boiler bank load is greatly reduced. It is normal to adopt single-drum boilers from 60 kg/cm2 or even lower level.

• Freeboard temperatures are lower than 1000°C (1830°F) and the heat flux is low and more uniform, making it possible to employ widely spaced membrane panels with 50 mm membrane bars.

• The dust levels in the flue gases at the freeboard exit are —300 g/N m3 (higher than the PF boiler dust loading), and the gas velocity for high-ash coals is —6 to 10 m/s (—20 to 35 ft/s) at bank inlet. Erosion protection by way of tube shields on the first row is normal.

Area	Approximate Heat Transfer Rates	Gas Velocity m/s (maximum)
W/m2 h	Kcal/m2 h	Btu/ft2 h °F
In-bed	44	215	250	3
Surrounding	32	155	180
Lined fluid bed	5.3	26	30
Unlined fluid bed	26	129	150
Superheater	9	43	50	8
Boiler	7	35	40	8
Economizer	10-11	50-55	55-65	10

• The full load bed velocities for high-ash coals are between 1.8 and 2.2 m/s (—6 to 7 ft/s) for large units, whereas the gas speeds are required to be closer to 10 m/s over convective surfaces. Narrowing the furnace outlet is done for this reason.

• A dust collector is usually placed after the ECON to reduce its size. Recirculation of grit back to the furnace is needed to reduce the carbon losses. The grit collector should have only 90% collection efficiency in order not to increase fines in the sys­tem. Dust recirculation is between 2.0 and 2.5 times the coal inputs for coals with medium ash.

• Heat transfer rates in various areas of the boiler are approximately as given in Table 12.3. The highest rate is in the bed tubes, coil, and walls, where the heat is transferred by radiation of gas and convection by gas and dust. The contributions of gas by convection and radiation are approximately 0-5% and 20-30%, respec­tively, with the rest coming from dust.

BFBC Boiler Designs

1. Bottom-supported bidrum boiler design (Figures 10.40 and 10.41) is modified to accommodate underbed firing. The floor is replaced by a nozzle plate with air, ash, and fuel nozzles. The tubes in the BB are reduced and made into a single pass. Superheater, ECON, and AH are added as required. Boilers up to 60 tph on coal can be economically built in this way.

2. Top- or bottom-supported single-pass standard stoker-fired boilers are suitably modified to accommodate overfired BFBC. Underfeed systems can also be adopted.

3. Single-drum field-erected oil-fired boilers are suitable for higher capacities of 100 tph and above, as shown in Figure 12.10.

Solid fuel firing BFBC boilers cover a range of 5-150 tph for process industries and smaller power plants of up to 30 MWe. From 60 kg/cm2 and above, single-drum boilers are popular.

Figure 12.10 depicts a single-drum top-supported radiant boiler with overbed feeding. A limited grit recirculation is incorporated because the coal has high ash and less FC.

Figure 12.11 shows a single-drum underfeed boiler. Due to space restriction the bag fil­ter and the fans are mounted on top of the boiler. There is an extensive ash recirculation because the coal has less ash and high FC.

FIGURE 12.10 Single-drum overfeed bubbling bed boiler. AH, airheater; ECON, economizer; FD, forced draft; SH, superheater.