Energy efficiency in Thermal utilities
Chapter 6: FBC Boiler
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-economically unviable to meet the challenges of future. Fluidized 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 NOx. The fuels burnt in these boilers include coal, washery rejects, rice husk, bagasse & other agricultural wastes. The fluidized bed boilers have a wide capacity range- 0.5 T/hr. to over 100 T/hr.
Mechanism of Fluidized Bed Combustion
Primary Air
The primary air is the air supplied to a combustion system that makes first contact with fuel. The primary air ensures instant combustion as the fuel enters the furnace. In case of solid fuels, primary air is introduced under the grate and passes through the bed of fuel to burn the fixed carbon content.
Secondary Air
The secondary air provides enough oxygen to complete the combustion in the furnace. In case of solid fuels the secondary air is introduced over the grate (fuel bed) to react with volatile matter released from coal during the initial burning and help in completing the combustion. It is injected into the furnace of the boiler under pressure to create turbulence above the burning fuel which helps to ensure good mixing with gases produced in the combustion process and thereby completing the combustion.
Tertiary Air
The injection of air into a furnace in addition to primary and secondary air is called tertiary air. It creates further turbulence and burns the residual volatile matter and smoke which could not be burnt completely by secondary air.
Fluidized 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 isgradually increased, a stage is reached when the individual particles are suspended in the air stream — the bed is called “fluidized”.
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 fluidized bed”.
This principle of fluidization 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 fluidized 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 fluidized
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 fluidized 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 minimum fluidization 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 fluidization. Improved mixing generates evenly distributed heat at lower
temperature. Residence time is many times greater than conventional grate firing. Thus an FBC system
releases heat more efficiently at lower temperatures.
Since limestone is used as particle bed, control of sulfur dioxide and nitrogen oxide emissions in the
combustion chamber is achieved without any additional control equipment. This is one of the major
advantages over conventional boilers.
Types of Fluidized Bed Combustion Boilers
There are three basic types of fluidized bed combustion boilers:
1.Atmospheric classic Fluidized Bed Combustion System (AFBC) 2. Atmospheric circulating (fast) Fluidized Bed Combustion system(CFBC)
3. Pressurized Fluidized Bed Combustion System (PFBC).
AFBC / Bubbling Bed
In AFBC, coal is crushed to a size of 1 — 10 mm depending on the rank of coal, type of fuel feed and
fed into the combustion chamber. The atmospheric air, which acts as both the fluidization air and
combustion air, is delivered at a pressure and flows through the bed after being preheated by the exhaust
flue gases. The velocity of fluidizing air is in the range of 1.2 to 3.7 m /sec. The rate at which air is
blown through the bed determines the amount of fuel that can be reacted.
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 0.9m to 1.5 m deep and the pressure drop averages about | inch of water per inch of bed depth. Very little
material leaves the bubbling bed — only about 2 to 4 kg of solids is recycled per ton of fuel burned.
Typical fluidized 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 pre heaters before being exhausted to atmosphere.
The main special feature of atmospheric fluidized 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 combustion efficiency if
the temperature falls below 800 °C. For efficient sulphur retention, the temperature should be in the
range of 800 °C to 850 °C.
General Arrangements of AFBC Boiler
AFBC boilers comprise of following systems:
1) 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.
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 boiler, the number
of feed points is increased, as it is necessary to distribute the fuel into the bed uniformly. In an AFBC
boiler, coal feeder height may be increased above the bed level to reduce the primary air fan power
consumption and also to reduce choking of coal feeding pipe by bed material during starting of the
boilers.
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
(Figure 6.5)
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.
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 fluidize 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
b) In-Bed Heat Transfer Surface
In a fluidized 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
(1) 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 and also by intermittent
flow from the bottom to remove over size particles, avoid accumulation and consequent defluidization.
While firing high ash coal such as washery rejects, the bed ash overflow drain quantity is considerable
so special care has to be taken.
b) Fly ash removal
The amount of fly ash to be handled in FBC boiler is relatively very high, when compared to conventional
boilers. This is due to elutriation of particles at high velocities. Fly ash carried away by the flue gas is
removed in number of stages; firstly in convection section, then from the bottom of air preheater/
economizer and finally a major portion is removed in dust collectors.
The types of dust collectors used are cyclone, bag filters, electrostatic precipitators (ESP’s) or some
combination of all of these. To increase the combustion efficiency, recycling of fly ash is practiced in
some of the units.
c) Electrostatic Precipitator
Electrostatic precipitator is a method of dust collection that uses electrostatic force and consists of
discharge wires and collecting plate. A high voltage is applied to the discharge wires to form an electric
field between the wire and the collecting plate and it also ionizes the gas around the discharge wires
to supply ions.
The flue gases containing dust flow between discharge wires and collecting plates, the dust particles
in the gas are charged by the ions. Coulomb force caused by the electric field causes the charged
particles to be collected on the collecting plate and the gas is purified. This is the principle of electrostatic
precipitation, electrostatic precipitator (Figure 6.6) which is applied on the industrial scale. The dust
collected on the collecting plates are removed by various methods such as rapping the collecting plate,
washing off with water and removing from the hopper etc. ESPs can remove from 95% to 99% of fly
ash from the flue gas.
Circulating Fluidized Bed Combustion (CFBC)
Circulating Fluidized Bed Combustion (CFBC) technology has evolved from conventional bubbling
bed combustion as a means to overcome some of the drawbacks associated with conventional bubbling
bed combustion (see Figure 6.7).
This CFBC technology utilizes the fluidized bed principle in which crushed (6 —12 mm size) fuel and
limestone are injected into the furnace or combustor. The particles are suspended in a stream of upwardly
flowing air (60-70% of the total air), which enters the bottom of the furnace through air distribution
nozzles. The fluidizing velocity in circulating beds ranges from 3.7 to 9 m/sec. The balance of
combustion air is admitted above the bottom of the furnace as secondary air. The combustion takes
place at 840-900 °C, and the fine particles (<450 microns) are elutriated out of the furnace with flue
gas velocity of 4-6 m/s. The particles are then collected by the solids separators and circulated back
into the furnace. Solid recycle is about 50 to 100 kg per kg of fuel burnt.
There are no steam generation tubes immersed in the bed. The circulating bed is designed to move a
lot more solids out of the furnace area and to achieve most of the heat transfer outside the combustion
zone - convection section, water walls, and at the exit of the riser. Some circulating bed units even
have external heat exchanges.
The particles circulation provides efficient heat transfer to the furnace walls and longer residence time
for carbon and limestone utilization. Similar to Pulverized Coal (PC) firing, the controlling parameters
in the CFB combustion process are temperature, residence time and turbulence.
For large units, the taller furnace characteristics of CFBC boiler offers better space utilization, greater
fuel particle and sorbent residence time for efficient combustion and SO, capture, and easier application
of staged combustion techniques for NOx control than AFBC generators. CFBC boilers are said to
achieve better calcium to sulphur utilization — 1.5 to 1 vs. 3.2 to 1 for the AFBC boilers, although the
furnace temperatures are almost the same.
CFBC boilers are generally claimed to be more economical than AFBC boilers for industrial application
requiring more than 75 — 100 T/hr. of steam
CFBC requires huge mechanical cyclones to capture and recycle the large amount of bed material,
which requires a tall boiler.
A CFBC could be good choice if the following conditions are met.
1.Capacity of boiler is large to medium
2.Sulphur emission and NOx control is important
3.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) Sulfur 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.
Pressurized Fluid Bed Combustion
Pressurized Fluidized 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 up to 16
ata (16 kg/cm’).
The off-gas from the fluidized bed combustor drives the gas turbine. The steam turbine is driven by
steam raised in tubes immersed in the fluidized 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 6.8).
Retrofitting fluidized bed coal fired combustion systems to conventional boilers has been carried out
successfully both in India and abroad.
The important aspects to be considered in retrofit projects are:
The important aspects to be considered in retrofit projects are:
a)Water/steam circulation design
b) Furnace bottom-grate clearance
c) Type of particulate control device
d) Fan capacity
e) Availability of space.
Retrofitting of a fluidized bed combustor to a conventional stoker fired water tube boiler may involve:
Retrofitting of a fluidized bed combustor to a conventional stoker fired water tube boiler may involve:
a) The replacement of grate by a distributor plate with short stand pipes for admitting air from
the wind box located underneath.
b) Installations of stand pipes to remove ash from the bed.
c) Provision of horizontal hairpin tubes in the bed with a pump for forced circulation from the
boiler drum.
d) Modification of crusher to size the coal/limestone mixture for pneumatic under bed injection
of the mixture.
It may be emphasized that conversion of a conventional coal fired system to a fluidized bed combustion
system can be accomplished without effecting major changes, after making a cost-benefit analysis.
Oil fired boilers can also be converted to coal fired fluidized bed combustion systems. However it has
to be examined on a case-to-case basis.
Advantages of Fluidized Bed Combustion Boilers
1. High Efficiency
FBC boilers can burn fuel with a combustion efficiency of over 95% irrespective of ash content. 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 independently 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 fluidized 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, formation can be greatly minimized by addition of limestone or dolomite for high sulphur coals.
3% limestone is required for every 1% sulphur in the coal feed. Low combustion temperature eliminates
NO, 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 in Flue Gas
The CO, 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.
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, volatilization 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 fluidized 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 fluidized bed at elevated pressure, it can be used to generate hot pressurized 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%.
Application Considerations with Biomass FBC Boilers
Biomass is a clean source of energy and is gaining huge popularity for FBC application in Industry.
Used judiciously, entrepreneurs can reduce the cost of steam generation and at the same time reduce
greenhouse gas emissions. With biomass fed boilers, the following areas need attention:
1.Uneven spreading of biomass fuel on boiler grate can lead to secondary combustion in the
super-heater zone, resulting in overheating of super heater tubes and fluctuations in steam
pressure.
2.Frequent erosion of super-heater and economizer coils can occur, due to high silica content in
the biomass, especially in rice husk.
3.High extraneous matter in biomass (sand and mud) causes tube fouling and fluidized bed to be
drained more frequently, with resultant heat loss.
4.Carbon and dust coating of boiler tubes results in lowering of steam temperatures, especially
during soot blowing.
5.Presence of Pesticides (used during farming) adds to tube failure frequencies; mainly due to
potassium constituents.
6.Corrosive constituents in biomass adversely affect boiler internals, especially the super-heater
tubes. Chloride content in certain types of biomass (like cotton stalk, 8—9%) can combine with
sodium and potassium in high temperature regime to aggravate the corrosion process.
7.Some boilers which use Red Gram husk/twigs as fuel pose corrosion problems at the cold end
(i.e., secondary super-heater and economizer tubes), due to the sulfur content.
8.The biomass fuel mix fed to the boiler, in quite a few cases, contains a combination of 6 to 7
biomass types. Each biomass has a separate air-to fuel ratio, and it is difficult to set a workable
airfuel ratio.
9.High moisture content in the biomass causes frequent jamming of the fuel in feeders, leading
to fluctuations in steam pressure and temperature.
10.High moisture content in the biomass also leads to plugging and choking of closely spaced
heating surfaces. This situation is further aggravated by the super-heater tube coil with very
close spacing, often the result of a desire to achieve a compact design.
11.Due to biomass fuel size variation, occurrence of unburnts in flue gases and bottom ash is
high, resulting in lower efficiency and also variation in steam pressure and temperature.
12.Absence of biomass feed rate measurement mechanism leaves little room for accurate
assessment of heat rate/efficiency. Providing a weighing mechanism is difficult on account of
different biomass fuel combinations being used, with different (and low) bulk densities.
13.Degradation of biomass during storage in exposed ambient wet atmosphere leads to loss of
heat value. Loss of material due to windage and carpet loss, coupled with loss of heat value on
account of decay (inherent biomass characteristics), can cause an error in assessment of input
fuel energy (as the input heat is customarily evaluated based on received biomass quantities
and GCV).
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