6

Fluidized Bed Combustion

6.1 INTRODUCTION TO FLUIDIZED BED COMBUSTION

Due to the availability of low calorific value, high ash content coal in India, it is essential to use technology-driven fluidized bed combustion (FBC) concept over the conventional stoker firing system. The benefits an FBC offers are as follows:

1. Compact boiler design
2. Flexibility in using fuel
3. Higher combustion efficiency
4. Reduced emission from pollutants such as NO_x and SO_x

6.2 REGIMES OF COMBUSTION

FBC works on the principle by passing an evenly distributing air or gas current upward through a finely divided bed of solid particles such as sand supported on a fine mesh. Referring to the Figure 6.1, when the air velocity is gradually increased, the individual particles are suspended in the air stream and the bed is called “fluidized”. Further, increase in air velocity bubbles form with vigorous turbulence and rapid mixing. This results in the formation of dense bed surface containing solid particles that exhibit the properties similar to a boiling liquid. This stage is referred to as “bubbling fluidized bed”. As velocity is increased further, bubbles disappear and particles are blown out of the bed, and hence some amounts of particles are re-circulated to maintain a stable system. This stage is referred to as “circulating fluidized bed”. [1]

Fig. 6.1 Principles of Fluidization

Fluidization mainly depends on the particle size and the air velocity as shown in Figure 6.2. It can be observed from the figure that the mean solids velocity increases at a slower rate than the gas velocity, and this difference between the mean solid velocity and mean gas velocity is known as slip velocity. In order to have good heat transfer and intimate contact, it is essential to maintain maximum slip velocity between the solids and the gas. By heating the sand particles in fluidized state to the ignition temperatures of fuel, which could be rice husk, coal or bagasse, and by injecting the fuel continuously into the bed, a uniform temperature bed temperature is attained. Typically, FBC takes place at about 840°C to 950°C. The problems associated with the melting of ash are avoided in an FBC system because the bed temperature is much below the ash fusion temperature. Since mixing is rapid in a fluidized bed, the rate of heat transfer in bed tubes and walls of the bed is higher. In order to ensure a stable operation of the bed and avoid particle entrainment in the gas stream, it is always necessary that minimum fluidization velocity and particle entrainment velocity be maintained.

In an FBC, turbulence promoted by fluidization results in improved mixing and even distribution. In addition, due to higher particle resident time when compared to conventional stoker firing system, heat generation is rapid and more, even at lower temperature. If a fuel contains more sulphur content, a limestone particle bed is used to control SO_x and NO_x formation.

Fig. 6.2 Variation of Gas Velocity with Solid Velocity

6.3 FLUIDIZED BED BOILERS – CLASSIFICATION

FBC boilers are generally classified into the following three basic types: (1) atmospheric FBC (AFBC) system or bubbling FBC, (2) atmospheric circulating (fast) FBC (CFBC) system and (3) pressurized FBC (PFBC) system. [3]

6.3.1 Atmospheric FBC System or Bubbling FBC

Atmospheric FBC (AFBC) system is a versatile boiler used for burning various fuels including agricultural residues (rice husk or bagasse) and even low-quality coal. This type of boiler is used for combined heat (process industries) and power generation application.

In AFBC boilers, the fuel is sized based on the type of fuel and fuel feeding system before feeding into the combustion chamber. If coal is used as a fuel, it is crushed to a size of 1–10 mm before burning. The hot atmospheric air, which acts as both the fluidization air and combustion air, is delivered under pressure through the nozzles at a velocity of 1.2 to 3.7 m/sec.

Almost all AFBC/bubbling bed boilers use in-bed evaporator tubes in the bed of limestone, sand and fuel for extracting the heat from the bed to maintain the bed temperature. The bed depth is usually 0.9–1.5 m deep and the pressure drop averages about 1 inch of water per inch of bed depth. Since a small amount of solid particles leave the bubbling bed (about 2–4 kg of solids per ton of fuel burned), this is recycled. Figure 6.3 shows a cut section view of an AFBC boiler.

Fig. 6.3 A Cut Section View of AFBC Boiler

In the AFBC, the combustion gases pass over the super heater tube sections initially and then flow past the economizer, the dust collectors and the air pre-heaters before exiting through the chimney. However, the operating conditions of these boilers are restricted by a relatively narrow temperature range within which the bed must be operated. With coal, if the bed temperature exceeds 950°C chances of clinker formation are more and if the temperature falls below 800°C in the bed, the combustion efficiency drops. For efficient sulphur retention, and combustion efficiency, the ideal working temperature range is 800–850°C.

General Arrangements of AFBC Boiler

AFBC boilers comprise of following systems, and many of these systems are common to all types of FBC boilers:

1. Fuel feeding system
2. Air distributor
3. Bed and in-bed heat transfer surface
4. Ash-handling system

(a) Fuel feeding system

This system is used to feed fuel and adsorbents such as limestone or dolomite, using any of the following two methods: under bed pneumatic feeding and over-bed feeding.

• Under bed pneumatic feeding

In under bed feeding, coal is crushed to 1–6 mm size and pneumatically conveyed from feed hopper to the combustor through a feed pipe piercing the distributor. For high capacity boilers, multiple such feeders are used at multi-points to ensure uniform distribution.

• Over-bed feeding

In over-bed feeding, crushed coal, 6–10 mm size is conveyed from coal bunker to a spreader by a screw conveyor. The spreader distributes the coal over the surface of the bed uniformly. Oversized coal particles can be used in this system. Over-bed feeding system is quite economical and ideal for rice husk and other agricultural residues.

(b) Air distributor

Air distributor plates are used to uniform distribute the fluidizing air across the bed cross section. This ensures constant motion of solid particles, preventing the formation of de-fluidization zones within the bed. The distributor plates (metallic) with a number of perforations (known as nozzles) are fixed to the furnace floor in a definite geometric pattern. The nozzles or nozzles with bubble caps prevent solid particles from flowing back into the space below the distributor. The distributor plate is protected from high temperature of the furnace by a refractory lining or by providing a static layer bed material or by means of water cooled tubes.

(c) Bed and in-bed heat transfer surface

As mentioned earlier, the bed material has an average size of about 1 mm and can be sand, ash, crushed refractory or limestone. Depending on the bed height either a shallow bed or a deep bed could be used. Even though the fluidizing velocity remains same, the two ends fluidize differently, with different heat transfer rates from the surfaces. The shallow bed system has distinct advantage of lower bed resistance, and hence a lower pressure drop and lower fan power requirements over a deep bed.

In-bed heat transfer surface is made of tube bundles, or coils with horizontal, vertical or inclined positions. However, the rate of heat transfer in the bed depends on parameters such as bed pressure and temperature, particle size, superficial gas velocity, heat exchanger and distributor plate design.

(d) Ash-handling system

In the FBC boilers, the bottom ash constitutes roughly 30–40 per cent of the total ash, the rest of it being the fly ash.

• Bottom ash removal

Bottom ash removal is essential in order to eliminate over-sized particles and to avoid accumulation and consequent de-fluidization. To maintain constant bed height, bed ash is removed continuously and bottom ash is removed periodically.

• Fly ash removal

Since the fly ash contains 60–70 per cent of total ash, due to elutriation of particles at high velocities, it is removed in a number of stages: initially in convective section, then from the ­bottom of air pre-heater/economizer section and finally in dust-collecting systems such as cyclone separators, bag house filters, electrostatic precipitators (ESPs) or a combination of all of these.

6.3.2 Circulating FBC

Some of the drawbacks associated with conventional bubbling bed combustion system are overcome by circulating FBC (CFBC). A circulating FBC (CFBC) system shown in Figure 6.4 is preferred when the boiler capacity is large or medium, inferior quality coal is handled and it is essential to contain pollution due to NO_x formation.

Fig. 6.4 Circulating Fluidized Bed Combustion

Advantages of a CFBC boiler are the following:

1. Higher combustion efficiency and processing capacity
2. Lower NO_x formation due to constant operating temperature of around 850^oC
3. Low-pressure combustion air, 10–14 kPa as against 20–35 kPa is used in an AFBC system
4. Heat transfer surfaces are parallel to flow, which enables minimum erosion and corrosion of tubes. In an AFBC system, heat transfer surfaces are perpendicular to the flow
5. Better turndown ratio as compared with a bubbling system

In a CFBC system, crushed coal of 6–12 mm size and limestone is injected into the furnace combustor. About 60–70 per cent of total air moving in the upward direction through the nozzles of the distributor plates holds the particles in suspension. The air velocity is limited to 3.7–9 m/s here. The remaining air contributes as secondary air and is admitted from above the furnace bottom. Finer particles of < 450 μ are elutriated out of the furnace along with flue gases moving at 4–6 m/s. These particles are collected in solid cyclone separators (50 kg–100 kg/kg of fuel burnt) and circulated back into the furnace.

These boilers can handle 75–100 tph of steam, but they require big cyclone separators to handle large amount of bed material. The boiler is taller when compared with AFBC boilers. Recirculating the particles improves heat transfer efficiency and better calcium to sulphur utilization 1.5:1.0 as against 3.2:1, compared with AFBC boilers.

In a CFBC system, the bed is designed in such a way that most of the heat transfer takes place outside the combustion zone with no steam generation tubes present in the bed section. External heat exchangers are also used sometimes to achieve this.

6.3.3 Pressurized FBC System

Pressurized FBC (PFBC) system is preferred in handling large-scale coal-burning systems in a power plant at a bed operating pressure of around 16 bar.

Figure 6.5 shows a typical PFBC system [2] used for co-generation plant. The hot flue gases coming from the combustion chamber of the PFBC system is expanded in the gas turbine. Part of the power generated by the gas turbine is used to run the compressor that supplies air to the combustion chamber. The expanded gases are then passed through a waste heat recovery system to heat the incoming condensate water from the steam turbine condenser. The steam generated in the PFBC system is expanded in a steam turbine to generate power. By the combined cycle operation, the overall efficiency of the system increases by around 5–8 per cent.

Fig. 6.5 A PFBC System Used for Co-Generation (Source: Bureau of Energy Efficiency) [4]

6.4 ADVANTAGES OF FBC SYSTEM

FBC boilers offer the following advantages:

(a) High efficiency

FBC boilers have high combustion efficiency of 92–95 per cent and overall efficiency of 82–86 per cent.

(b) Fuel flexibility

FBC boilers can burn variety of fuels from agricultural products to low-grade high ash content coal with high efficiency. Coal fines of less than 6 mm size can be easily burnt, which otherwise is difficult to burn in conventional boilers.

(c) High heat transfer rate

High rate of heat transfer is possible within a small area, reducing the boiler size compared with a conventional system.

(d) Low erosion and corrosion

Erosion and corrosion problems are minimized due to the lower operating temperature. In addition, chances of clinker formation are reduced drastically in these boilers.

(e) Less pollution

Due to the lower operating temperature, formation of NO_x is reduced. Also, addition of limestone or dolomite for coal containing high percent sulphur reduces the SO₂ formation.

(f) Quick startup and fast response to load variations

Boiler startup and shutdown time is less, as turbulence in the combustion zone is rapid. Response to varying load demand is very good.

(g) Easy atomization

Complete atomization of fuel and ash-handling systems is possible. Micro-processor-based automatic ignition systems give better combustion control and fuel economy.

(h) High reliability and reduced maintenance

As no moving parts are present in the combustion zone, system is highly reliable with reduced maintenance cost.

6.5 CONTROL OF OXIDES OF NITROGEN

Production of NO_x is an endothermic reaction and its concentration is temperature dependent. NO_x emissions can be reduced by lowering the combustion temperature and by eliminating hot spots in the furnace. It can also be inhibited by lowering the air–fuel ratio or by employing exhaust gas recirculation. Lowering the air–fuel ratio restricts the amount of oxygen available for combustion while exhaust gas recirculation reduces the combustion chamber temperature.

6.6 DESULPHURIZATION TECHNOLOGY

Flue gas desulphurization (FGD) is a technology that extracts sulphur dioxides from flue gases produced in coal-based thermal power plants, where sulphur content in coal is more than 0.5 per cent. The coal produced from Indian mines contains only 0.4 per cent sulphur contents; therefore in India, this technology was not required much. Now, a lot of companies in India are importing coal from other countries such as Indonesia and South Africa which contains sulphur contents 0.6–0.9 per cent. Thus, it is mandatory to install the FGD plant in order to maintain the ambient air-quality standards.

Sulphur dioxide is extracted from flue gases in wet scrubber, slurry of alkaline sorbent; lime stone reacts with the sulphur dioxide. More than 90 per cent of the limestone particles in the limestone powder (85% purity) are made to pass through a screen (325 mesh) to ensure that maximum amount of limestone particles come in contact with the sulfur dioxide molecules in the flue gas.

To have a chemical reaction, the limestone powder is mixed into 15–30 per cent slurry ­introduced into the FGD vessel, re-circulated, and sprayed into the flue gas stream. The following chemical reactions take place in the FGD system:

1. Flue gases containing SO₂ enter the absorber and come in contact with limestone slurry (CaCO₃) in the wet scrubber thereby producing calcium sulfite.

   CaCO₃ + SO₂ → CaSO₃ + CO₂

2. Further, Ca(OH)₂ in lime, when combined with SO₂ gas produces calcium sulfite and water

   Ca(OH)₂ + SO₂ → CaSO₃ + H₂O

3. Calcium sulfite is further oxidized by forced oxidation utilizing blowers to produce marketable CaSO₄ 2H₂O, popularly known as gypsum.

   CaSO₃ + H₂O + ½O₂ → CaSO₄.2H₂O

6.7 QUESTIONS

6.7.1 Objective Questions

1. Combustion temperature in FBC boilers is
   1. 1,200^oC
   2. 900^oC
   3. 1,400^oC
   4. 1,800^oC
2. Working pressure of an PFBC boilers
   1. 12 bar
   2. 14 bar
   3. 16 bar
   4. 18 bar
3. The efficiency of a typical FBC boiler would be
   1. 84%
   2. 48%
   3. 44%
   4. 34%
4. The efficiency of a typical combined cycle system increases by
   1. 4%–5%
   2. 5%–8%
   3. 14%–24%
   4. 34%
5. Low combustion temperature in the furnace of FBC is responsible for
   1. SO_x
   2. NO_x
   3. CO₂
   4. CO
6. In AFBC boilers, coal size used is
   1. 1–10 mm
   2. 10–15 mm
   3. 5 cm
   4. 3 cm
7. The limestone or dolomite used in FBC plant removes
   1. SO_x
   2. NO_x
   3. ash
   4. sulphur

Answers

1. 1. b 2. c 3. a 4. b 5. b 6. a 7. d

6.7.2 Review Questions

1. Explain the different regimes of combustion of an FBC boiler.
2. Explain the principle of working of an FBC.
3. How do you classify the FBC boilers? Explain.
4. With a neat sketch, explain the working of an AFBC boiler.
5. Sketch and explain the working of a CFBC boiler.
6. What are the benefits a CFBC boiler offers? Discuss.
7. List and discuss the advantages of an FBC boiler.
8. Write a short note on PFBC boiler.
9. Explain briefly the methods used to control SO_x and NO_x in FBC boilers.

6.7.3 References

1. Energy Technology Handbook. Considine, D. M.; New York: McGraw-Hill, Inc., 1977.
2. Pressurized FBC Technology. Podolski, W. F.; US: Noyes Data Corporation, 1983.
3. Fluidised Bed Coal Fired Boilers – Department of Coal Publications, Government of India Fluidised Combustion of Coal – A National Coal Board Report, London.
4. Bureau of Energy Efficiency, Government of India