Download Fluidised Bed Combustion - Bureau of Energy Efficiency - Lecture Notes and more Study notes Energy Efficiency in PDF only on Docsity!
Syllabus FBC boilers: Introduction, Mechanism of fluidised bed combustion, Advantages, Types of FBC boilers, Operational features, Retrofitting FBC system to conventional boilers, Saving potential.
6.1 Introduction
The major portion of the coal available in India is of low quality, high ash content and low calorific value. The traditional grate fuel firing systems have got limitations and are techno-eco- nomically unviable to meet the challenges of future. Fluidised bed combustion has emerged as a viable alternative and has significant advantages over conventional firing system and offers multiple benefits – compact boiler design, fuel flexibility, higher combustion efficiency and reduced emission of noxious pollutants such as SOx and NO (^) x. The fuels burnt in these boilers include coal, washery rejects, rice husk, bagasse and other agricultural wastes. The fluidized bed boilers have a wide capacity range- 0.5 T/hr to over 100 T/hr.
6.2 Mechanism of Fluidised Bed Combustion
When an evenly distributed air or gas is passed upward through a finely divided bed of solid particles such as sand supported on a fine mesh, the particles are undisturbed at low velocity. As air velocity is gradually increased, a stage is reached when the individual particles are suspended in the air stream – the bed is called “fluidised”. With further increase in air velocity, there is bubble formation, vigorous turbulence, rapid mixing and formation of dense defined bed surface. The bed of solid particles exhibits the properties of a boiling liquid and assumes the appearance of a fluid – “bubbling fluidized bed”. At higher velocities, bubbles disappear, and particles are blown out of the bed. Therefore, some amounts of particles have to be recirculated to maintain a stable system - "circulating fluidised bed". This principle of fluidisation is illustrated in Figure 6.1. Fluidization depends largely on the particle size and the air velocity. The mean solids velocity increases at a slower rate than does the gas velocity, as illustrated in Figure 6.2. The difference between the mean solid velocity and mean gas velocity is called as slip velocity. Maximum slip velocity between the solids and the gas is desirable for good heat transfer and intimate contact. If sand particles in a fluidised state is heated to the ignition temperatures of coal, and coal is injected continuously into the bed, the coal will burn rapidly and bed attains a uniform temperature. The fluidised bed combustion (FBC) takes place at about 840°C to 950°C. Since this temperature is much below the ash fusion temperature, melting of ash and associated problems are avoided. The lower combustion temperature is achieved because of high coefficient of heat transfer due to rapid mixing in the fluidised bed and effective extraction of heat from the bed through in-bed heat transfer tubes and walls of the bed. The gas velocity is maintained between mini- mum fluidisation velocity and particle entrainment velocity. This ensures stable operation of the
bed and avoids particle entrainment in the gas stream. Combustion process requires the three “T”s that is Time, Temperature and Turbulence. In FBC, turbulence is promoted by fluidisation. Improved mixing generates evenly
Figure 6.1 Principle of Fluidisation
Figure 6.2 Relation between Gas Velocity and Solid Velocity
Fixing, bubbling and fast fluidized beds
As the velocity of a gas flowing through a bed of particles increases, a value is reaches when the bed fluidises and bubbles form as in a boiling liquid. At higher velocities the bubbles disappear; and the solids are rapidly blown out of the bed and must be recycled to maintain a stable system.
ton of fuel burned. Typical fluidised bed combustors of this type are shown in Figures 6.3 and 6.4. The combustion gases pass over the super heater sections of the boiler, flow past the economizer, the dust collectors and the air preheaters before being exhausted to atmosphere. The main special feature of atmospheric fluidised bed combustion is the constraint imposed by the relatively narrow temperature range within which the bed must be operated. With coal, there is risk of clinker formation in the bed if the temperature exceeds 950°C and loss of com- bustion efficiency if the temperature falls below 800°C. For efficient sulphur retention, the tem- perature should be in the range of 800°C to 850°C.
General Arrangements of AFBC Boiler
AFBC boilers comprise of following systems:
i) Fuel feeding system ii) Air Distributor iii) Bed & In-bed heat transfer surface iv) Ash handling system.
Many of these are common to all types of FBC boilers.
1. Fuel Feeding System
For feeding fuel, sorbents like limestone or dolomite, usually two methods are followed: under bed pneumatic feeding and over-bed feeding.
Under Bed Pneumatic Feeding
If the fuel is coal, it is crushed to 1–6 mm size and pneumatically transported from feed hopper to the combustor through a feed pipe piercing the distributor. Based on the capacity of the
Figure 6.4 Bubbling Bed Boiler-
boiler, the number of feed points is increased, as it is necessary to distribute the fuel into the bed uniformly.
Over-Bed Feeding
The 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. This type of fuel feeding system accepts over size fuel also and eliminates transport lines, when compared to under-bed feeding system.
2. Air Distributor
The purpose of the distributor is to introduce the fluidizing air evenly through the bed cross section thereby keeping the solid particles in constant motion, and preventing the formation of defluidization zones within the bed. The distributor, which forms the furnace floor, is normally constructed from metal plate with a number of perforations in a definite geometric pattern. The perforations may be located in simple nozzles or nozzles with bubble caps, which serve to prevent solid particles from flowing back into the space below the distributor. The distributor plate is protected from high temperature of the furnace by:
i) Refractory Lining ii) A Static Layer of the Bed Material or iii) Water Cooled Tubes.
3. Bed & In-Bed Heat Transfer Surface:
a) Bed
The bed material can be sand, ash, crushed refractory or limestone, with an average size of about 1 mm. Depending on the bed height these are of two types: shallow bed and deep bed. At the same fluidizing velocity, the two ends fluidise differently, thus affecting the heat transfer to an immersed heat transfer surfaces. A shallow bed offers a lower bed resistance and hence a lower pressure drop and lower fan power consumption. In the case of deep bed, the pressure drop is more and this increases the effective gas velocity and also the fan power.
b) In-Bed Heat Transfer Surface
In a fluidised in-bed heat transfer process, it is necessary to transfer heat between the bed material and an immersed surface, which could be that of a tube bundle, or a coil. The heat exchanger orientation can be horizontal, vertical or inclined. From a pressure drop point of view, a horizontal bundle in a shallow bed is more attractive than a vertical bundle in a deep bed. Also, the heat transfer in the bed depends on number of parameters like (i) bed pressure (ii) bed temperature (iii) superficial gas velocity (iv) particle size (v) Heat exchanger design and (vi) gas distributor plate design.
4. Ash Handling System
a) Bottom Ash Removal
In the FBC boilers, the bottom ash constitutes roughly 30 – 40 % of the total ash, the rest being the fly ash. The bed ash is removed by continuous over flow to maintain bed height
A CFBC could be good choice if the following conditions are met. Capacity of boiler is large to medium Sulphur emission and NOx control is important The boiler is required to fire low-grade fuel or fuel with highly fluctuating fuel quality. Major performance features of the circulating bed system are as follows:
a) It has a high processing capacity because of the high gas velocity through the system. b) The temperature of about 870 °C is reasonably constant throughout the process because of the high turbulence and circulation of solids. The low combustion temperature also results in minimal NOx formation. c) Sulphur present in the fuel is retained in the circulating solids in the form of calcium sulphate and removed in solid form. The use of limestone or dolomite sorbents allows a higher sulfur retention rate, and limestone requirements have been demonstrated to be substantially less than with bubbling bed combustor. d) The combustion air is supplied at 1.5 to 2 psig rather than 3–5 psig as required by bubbling bed combustors. e) It has high combustion efficiency. f) It has a better turndown ratio than bubbling bed systems. g) Erosion of the heat transfer surface in the combustion chamber is reduced, since the surface is parallel to the flow. In a bubbling bed system, the surface generally is perpendicular to the flow.
Figure 6.5 Circulating Bed Boiler Design
Circulating bed boiler
At high fluidizing gas velocities in which a fast recycling bed of fine material is superimposed on a bubbling bed of larger particles. The combustion temperature is controlled by rate of recycling of fine material. Hot fine material is separated from the flue gas by a cyclone and is partially cooled in a separate low velocity fluidized bed heat exchanger, where the heat is given up to the steam. The cooler fine material is then recycled to the dense bed.
At elevated pressure, the potential reduction in boiler size is considerable due to increased amount of combustion in pressurized mode and high heat flux through in-bed tubes. A comparison of size of a typical 250 MW PFBC boiler versus conventional pulverized fuel-fired boiler is shown in the Figure 6.7.
6.3.3 Pressurised Fluid Bed Combustion
Pressurised Fluidised Bed Combustion (PFBC) is a variation of fluid bed technology that is meant for large-scale coal burning applications. In PFBC, the bed vessel is operated at pressure upto 16 ata ( 16 kg/cm 2 ). The off-gas from the fluidised bed combustor drives the gas turbine. The steam turbine is driven by steam raised in tubes immersed in the fluidised bed. The condensate from the steam turbine is pre-heated using waste heat from gas turbine exhaust and is then taken as feed water for steam generation. The PFBC system can be used for cogeneration or combined cycle power generation. By combining the gas and steam turbines in this way, electricity is generated more efficiently than in conventional system. The overall conversion efficiency is higher by 5% to 8%.. (Refer Figure 6.6).
Figure 6.6 PFBC Boiler for Cogeneration
6.5 Advantages of Fluidised Bed Combustion Boilers
1. High Efficiency
FBC boilers can burn fuel with a combustion efficiency of over 95% irrespective of ash con- tent. FBC boilers can operate with overall efficiency of 84% (plus or minus 2%).
2. Reduction in Boiler Size
High heat transfer rate over a small heat transfer area immersed in the bed result in overall size reduction of the boiler.
3. Fuel Flexibility
FBC boilers can be operated efficiently with a variety of fuels. Even fuels like flotation slimes, washer rejects, agro waste can be burnt efficiently. These can be fed either inde- pendently or in combination with coal into the same furnace.
4. Ability to Burn Low Grade Fuel
FBC boilers would give the rated output even with inferior quality fuel. The boilers can fire coals with ash content as high as 62% and having calorific value as low as 2,500 kCal/kg. Even carbon content of only 1% by weight can sustain the fluidised bed combustion.
5. Ability to Burn Fines
Coal containing fines below 6 mm can be burnt efficiently in FBC boiler, which is very difficult to achieve in conventional firing system.
6. Pollution Control
SO 2 formation can be greatly minimised by addition of limestone or dolomite for high sul- phur coals. 3% limestone is required for every 1% sulphur in the coal feed. Low combus- tion temperature eliminates NO (^) x formation.
7. Low Corrosion and Erosion
The corrosion and erosion effects are less due to lower combustion temperature, softness of ash and low particle velocity (of the order of 1 m/sec).
8. Easier Ash Removal – No Clinker Formation
Since the temperature of the furnace is in the range of 750 – 900 °C in FBC boilers, even coal of low ash fusion temperature can be burnt without clinker formation. Ash removal is easier as the ash flows like liquid from the combustion chamber. Hence less manpower is required for ash handling.
9. Less Excess Air – Higher CO 2 in Flue Gas
The CO 2 in the flue gases will be of the order of 14 – 15% at full load. Hence, the FBC - boiler can operate at low excess air - only 20 - 25%.
10. Simple Operation, Quick Start-Up
High turbulence of the bed facilitates quick start up and shut down. Full automation of start up and operation using reliable equipment is possible.
11. Fast Response to Load Fluctuations
Inherent high thermal storage characteristics can easily absorb fluctuation in fuel feed rates. Response to changing load is comparable to that of oil fired boilers.
12. No Slagging in the Furnace–No Soot Blowing
In FBC boilers, volatilisation of alkali components in ash does not take place and the ash is non sticky. This means that there is no slagging or soot blowing.
13 Provisions of Automatic Coal and Ash Handling System
Automatic systems for coal and ash handling can be incorporated, making the plant easy to operate comparable to oil or gas fired installation.
14 Provision of Automatic Ignition System
Control systems using micro-processors and automatic ignition equipment give excellent control with minimum manual supervision.
15 High Reliability
The absence of moving parts in the combustion zone results in a high degree of reliability and low maintenance costs.
16 Reduced Maintenance
Routine overhauls are infrequent and high efficiency is maintained for long periods.
17 Quick Responses to Changing Demand
A fluidised bed combustor can respond to changing heat demands more easily than stoker fired systems. This makes it very suitable for applications such as thermal fluid heaters, which require rapid responses.
18 High Efficiency of Power Generation
By operating the fluidised bed at elevated pressure, it can be used to generate hot pressur- ized gases to power a gas turbine. This can be combined with a conventional steam turbine to improve the efficiency of electricity generation and give a potential fuel savings of at least 4%.
