In this article we will discuss about:- 1. Introduction to Fluidized Bed Combustion Boilers 2. Pulverized Coal Firing System Used in FBC Boilers 3. Classification 4. Thermal Efficiency 5. Advantages.
Contents:
- Introduction to Fluidized Bed Combustion Boilers
- Pulverized Coal Firing System Used in FBC Boilers
- Classification of Fluidized Bed Combustion (FBC) Boilers
- Thermal Efficiency of FBC Boiler
- Advantages of FBC Boilers
1. Introduction to
Fluidized Bed Combustion Boilers:
The fluidized bed combustion boiler is recent development in boiler technology. In these boilers the combustion of coal occurs in form of a powder coal. The furnace part of the boiler uses pulverized fuel fired burners for combustion fuel.
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The coal is powder in to fine granulated form and is mixed with a high pressure air and discharged through the burner in high temperature flames. The powder coal utilized is a low ash and high volatile coal which highly efficient fuel for boiler.
When air is passed vertically upwards through a bed of solid particles supported on a grid, the mass will remain in suspension at a particular fluidization velocity. Initially air will tend to follow the path of least resistance and pass upwards through the bed with a pressure drop.
With increase in velocity, air starts bubbling through the bed and particles attain a state of vigorous mixing. Under such conditions, the bed expands upwards and the particles become suspended in the flowing air. The bed assumes the appearance of a fluid and hence the name fluidized bed.
The unique feature of the fluidised bed is the highly efficient air-solid contact brought about by the turbulent motion of the air and solid particles.
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If this bed of solids and air, in a fluidized state is heated to the ignition temperature of coal, and the coal is injected continuously in the bed, it burns rapidly and the bed attains a uniform temperature due to effective mixing.
The combustion is carried out at a temperature below the ash softening temperature to avoid formation of clinker which adversely affects the fluidization. This is achieved by extracting heat from the bed through immersed heat transfer tubes as well as through the walls of the bed or by maintaining very high excess air levels.
But high excess air can lead to lowered efficiency of the boiler as well as loss of bed material because of carryover of the same with high air velocity. The coal usually forms only 2-5% by weight of the bed Rest being sand, ash, limestone, dolomite, etc. While ash and sand are chemically inert materials, limestone is chemically reactive material.
Limestone or dolomite in the bed reacts with the sulphur dioxide formed during combustion of the coal and forms a solid sulphate which can be discarded as a dry solid.
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Hence, a device which employs the technique of FBC and generates steam usually in the heat exchanger tube bundles immersed in the bed, is termed as fluidized bed combustion boiler.
2. Pulverized Coal Firing System Used in FBC Boilers:
The pulverized coal firing has been widely accepted and universally used for large thermal power plants. This choice of pulverized fuel firing system in preference to the other firing methods depends upon the size of the boiler unit the type of coal available, the cost of coal; the kind of load on the power plant, the load factor and the availability of trained personnel.
The characteristics of coal are very important in the selection of coal when stroker firing is used but needs less consideration in pulverized coal firing plant. The most suitable coal that can be successfully used in pulverized form should contain about 20% of volatile matter and should not have more than medium caking power.
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In general high volatile coals are harder to grind than low volatile coals, requiring large mill for the same capacity, more power per ton and more maintenance. The disadvantage of high volatile coal are partly compensated by the fact that the gases are easily distilled and burned and ignition can be easily maintained owing to the presence of volatile gases.
The capital cost and operating cost of pulverized fuel system is more than in other methods and difference in cost must be rapid in decreased maintenance and increased thermal efficiency. The use of low cost fuel frequently justifies the increased cost of pulverizing system provided the system must be used at high rating for longer time.
The satisfactory performance of pulverized system largely depends upon the pulverizing mill performance. The pulverizer should deliver the rated tonnage of coal with minimum consumption of power and produce a pulverized fuel of satisfactory fineness over a wide range of operation. Further it must be quiet in operation, must give service with minimum output time and operate with low maintenance cost.
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The pulverized fuel firing system was used for generating the steam in the early 1920’s. The conventional fuel firing methods (stroke firing) were found to be unable to take the fluctuating loads on the plant due to limited capacity of combustion.
Further these conventional methods were found unsuitable for large capacity (100MW and above) plants and coal containing high percentage of ash due to the difficulties experienced in removing large quantities of ash and interference of the formed ash in the combustion process. The pulverized fuel system is nowadays universally used for large capacity plants and using low cost (Low grade) fuel it gives high thermal efficiency and better control as per the load demand.
In a pulverized fuel firing system, the coal is reduced to a fine powder with the help of grinding mill and then projected into the combustion chamber with the help of hot air current. The amount of secondary air required to complete the combustion is supplied separately to the combustion chamber.
The resulting air is supplied separately to the combustion chamber. The resulting turbulence in the combustion chamber helps for uniform mixing of fuel and air for proper combustion. The amount of air which is used to carry the coal and to dry it before entering into the combustion chamber is known as Primary Air and the amount of air which is supplied separately for completing the combustion is known as secondary air.
The efficiency of pulverized fuel firing system mostly depends upon the size of the powder. The fineness of the coal should be such as 70% of it would pass through a 200 mesh sieve and 98% through 50 mesh sieve. Many modern thermal power plants use pulverized fuel system when the available coal is cheap and is not suitable for stroke firing.
Advantages of Pulverized Fuels:
1. The success of the pulverized firing system lies in the fact that by breaking a given mass of coal into smaller pieces exposes more coal surface area for combustion. There is increase in surface area exposed per unit volume with the decrease in diameter of coal particle.
2. Greater surface area of coal unit mass of coal allows faster combustion as more coal surface is exposed to heat and oxygen. This reduces the excess air required to ensure complete combustion and the fan power also.
3. Wide variety and low grade coal can be burnt more easily.
4. It gives fast response to load changes as rate of combustion can be controlled easily and immediately. Automatic control applied to pulverizer fuel fired boilers is effective in maintaining at almost constant steam pressure under wide load variations.
5. The system is perfectly free from clinker and slagging troubles.
6. This system works successfully with or in combination with gas and oil.
7. It is possible to use highly preheated secondary air which helps for rapid flame propagation.
8. The pulverizing system can be repaired without calling the unit as the pulverizing equipment is located outside the furnace.
9. Large amount of heat release is possible and with such rate of heat generation, each boiler of pulverized fuel fired system can generate as large as 2000 tons, of steam per hour.
10. The banking losses are low compared with stroke firing system.
11. The boilers can be started from cold very rapidly and efficiently. This is highly important during emergency.
12. The external heating surface is free from corrosion and fouling as smokeless combustion is possible.
13. There are no moving parts in the furnace subjected to high temperature, therefore the life of system is increased and the operation is trouble free.
14. Practically no ash handling problems.
15. The furnace volume required is considerably less as the set of burners which produce turbulence in the furnace makes it possible to complete combustion with minimum travel of flame length.
Disadvantages of Stroker Firing System:
(1) The loss of coal is more through coarse sieve.
(2) There is heavy wear and tear of moving parts due to abrasive action of coal.
(3) The troubles of clinkering of combustion chamber walls are very common.
(4) Sudden variations of load cannot be met with the same degree of efficiency as in the case of pulverized fuel firing.
(5) The furnaces need fire arches which increases the construction cost and creates troubles during operation.
(6) Stand-by losses are considerably more.
3. Classification of Fluidized Bed Combustion (FBC) Boilers:
These are classified on basis of reactive gas solid motion.
Fig. 4-20 shows the classification of FBC boiler. They are classified in two categories as bubbling and circulation type.
The velocities in fluidized bed combustion vary in range of 1-10 m/s and the particle sizes which could be utilized are 350 to 1000 micron. The pressures in the fluidized bed furnaces vary in range of 1-15 ata. The following table provides the details of this data.
(a) Atmospheric Fluidized Bed Combustion Type (Bubbling):
In present industrial practice AFBC generally refers to bubbling bed only. In AFBC’s Coal is crushed to a size of 1-10 mm depending on the coal grade, type of fuel feeding etc. This fuel is fed into the combustion chamber. Air enters through distributors into the bed of sand or limestone etc.
At a velocity of 0.6 to 4.6 m/s (depending upon bed height), turbulence is created and recirculating motion of the bed fluidization air and combustion air is delivered at a pressure, and flow rate through the bed after being preheated by the exhaust flue gases.
Heat released in the bed is absorbed by cooled tubes carrying water (acting as evaporation) and heat released by the products of combustion is absorbed by convection tubes in the free board above the bed. The gaseous products of combustion pass over to other parts and then to the stack. If dolomite or limestone is used, the SO2 will be absorbed in the bed.
Fig. 4-21 shows the atmospheric fluidized bed combustion type. The operational diagram of atmospheric fluidized bed combustion type is shown in fig. 4-22.
AFBC’s can be further classified as water-tube, fire tube and combination. It can also be classified as single bed, double bed, etc.
Achieving a high carbon combustion efficiency is important for overall plant efficiency and consequently for plant economics. Carbon losses occur through the carryover of unburnt carbon particles and to a much smaller degree, the incomplete combustion of carbon to carbon monoxide and unburnt carbon withdrawn with spent bed material.
Operating variables that affect carbon combustion efficiency includes:
(1) Bed temperature
(3) Fluidizing velocity.
(b) Pressurized Fluidized Bed Combustion Type:
The PFBC is similar to AFBC, but operated at higher pressure. The forced draft fan (FD) is replaced with a compressor and the combustion chamber as a pressure vessel. The heat release rate in the bed is proportional to the bed pressure and hence a deep bed is used to extract large amount of heat. This improves combustion efficiency and sulphur dioxide absorption in the bed. This has not yet been commercialized.
(c) Atmospheric Circulating Fluidized Bed Combustion:
The atmospheric circulating fluidized bed combustion (ACFBC) system is the most recent development in fluidized bed combustion.
In a circulating system the bed parameters are so maintained as to promote solids mixing on the bed. They are lifted in a relatively dilute phase in a solids riser, and a down-comer with a cyclone provides a return path for the solids. There are no steam generation tubes immersed in the bed. Generation and super heating of steam takes place in the convection section, water walls, at the exit of the riser.
(d) Pressurised Circulating Fluidized Bed Combustion:
(PCFBC) The PCFBC is similar to ACFBC, but operates at higher pressures. The forced draft fan (FD) is replaced with a compressor and the combustion chamber as a pressure vessel. The heat release rate is proportional to the bed pressure and is used to extract large amount of heat. The circulation of air and coal creates a turbulence which improves the combustion of pulverised coal and the combustion occurs at very fast rate because of the high pressures.
4. Thermal Efficiency of FBC Boiler:
The effective generation of Steam is possible if the efficiency of the boiler using pulverised fuel fired system is high.
Efficiency is ratio of output (i.e., steam) to the input. There are two ways of knowing thermal efficiency. One is the direct method and the other one is indirect method efficiency from direct method is calculated as under.
Unburnt Fuel in Flue Gases:
Fuel contains carbon and hydrogen. Improper combustion leads to unburnt carbons which are carried away by fuel gases as carbon monoxides. This can be evaluated by knowing the loss due to combustible in Refuse (LR) as percentage.
Sensible Heat in Flue Gases:
This loss is the larger in a boiler and represents the heat carried away by the hot gases leaving through the chimney. If more air than what is required for efficient combustion is allowed to enter the boiler, additional heat will be lost in heating the excess air to the chimney temperature.
Therefore, necessary efforts have to be made not only to keep chimney temperature as low as possible but also to keep chimney temperature as low as possible but also to minimise excess air from entering the combustion chamber by proper tuning of boiler.
Furthermore, moisture present in fuel also consumes some amount of energy to evaporate which is not useful for steam generation and hence is a loss due to presence of moisture in fuel.
Radiation losses depend on the temperature of the boiler’s external surfaces. The quantity of heat lost by radiation from these surfaces is nearly independent of the load at which the boiler operates. Boilers with poor insulation and poor design characteristics tend to have higher radiation losses.
For calculating above, we have to analyze the coal and refuse besides knowing the system data like rate of coal firing, Absolute, temperature of exposed surface, % CO2, Emissivity of surface, total exposed area etc.
The direct method is more accurate method of measuring % efficiency of the boiler.
5. Advantages of FBC Boilers:
(1) High heat transfer rates (Heat transfer coefficient greater than 250 W/m2-K) and the absence of fouling by the heat exchanger placed in the bed, result in high thermal efficiency of the order of 80-85%. This reduces the fuel consumption for a given boiler.
(2) Less sulphur-di-oxide emissions, if chemically reactive solvents are used because much of the sulphur is captured by fuel bed additives. If sulphur dioxide emissions are high, then the stack height correspondingly will increase as per pollution norms. Hence, where sulphur content in coal is high, it is always viable to use chemical reactive bed material, in order to reduce the stack height and thus the cost.
(3) Due to high heat transfer rates, there is a considerable reduction in boiler size.
(4) FBC boiler can burn low grade fuels efficiently.
(5) Low fuel bed temperatures, upto about 954°C which means that there is less nitrogen oxide emissions and fewer problems caused by ash agglomeration.
(6) There are less volatilization of sodium and potassium in the coal and consequently less deposition on and corrosion of downstream components such as boiler tubes, superheaters, etc.
(7) It gives high volumetric energy release rates of 5.2 MW/m3 as compared with 0.2 MW/m3 in conventional pulverized coal fired boiler.