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Circulating Fluidized BedFrom Wikipedia, the free encyclopedia
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The circulating fluidized bed(CFD) is a clean process with the ability to achieve lower emission of
pollutants. By using this technology, up to 95% of pollutants[1]
will be absorbed before being emitted to the
atmosphere. At the same time, the advantages of this technology outweigh the limitations, which made it
favourable. However, the development of this technology is still at its early stages. With further R&D, this
technology is predicted to bring numerous benefits to our society.
Contents
[hide]
1 Introduction
o 1.1 Basic Fundamental
2 Range of Applications
o 2.1 Circulating Fluidized Bed Scrubber
o 2.2 Circulating Fluidized Bed Gasification System
3 Advantages and Limitations of Circulating Fluidized Bed
4 Available Design of Circulating Fluidized Bed
5 Main Process Characteristics
o 5.1 Design and Operation
6 Process Characteristics Assessments
7 Possible Heuristics to Be Used During the Design of the Process
8 New Development
o 8.1 InBed Heat Exchanger
o 8.2 U-Beam Separator Design
9 References
Introduction[edit]
Circulating fluidized bed is a relative new technology with the ability to achieve lower emission of pollutants.
Extensive research has been conducted on this technology for the past 10 years because pollution in the
world is getting more serious by the day and clean practice will be very crucial for the sustainability of the
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earth. The importance of this technology grew even more lately because of the tightened environmental
regulation for pollutant emission.
The Mercury and Air Toxic Standards (MATS) enacted in December 2011 by EPA have forced all the
countries in Europe and America to strictly adhere to this policy. This means that emissions such asmetals, acid gases, organic compound, flue gas acids and other pollutants from power plants or industrial
facilities have to meet the requirements set by EPA[2]
and upgrades have to be done for facilities that do
not meet the standards. As a result, the demand for circulating fluidized bed technology will be predicted to
go sky rocket.
Basic Fundamental[edit]
During the combustion phase, upwards jets of air will cause the solid fuels to be suspended. This is to
ensure the gas and solids will mix together turbulently for better heat transfer and chemical reactions. The
fuel will be burnt at a temperature if 1400oF to 1700oF to prevent nitrogen oxide from forming.[1]While
burning, flue gas such as sulfur dioxide will be released. At the same time, sulfur-absorbing chemical such
as limestone or dolomite will be used to mix with the fuel particles in the fluidization phase, which will
absorb almost 95% of the sulfur pollutants.
Alternatively, the sulfur absorbing chemical and fuel will be recycled to increase the efficiency of producing
a higher quality steam as well as lower the emission of pollutants. Therefore, it will be possible to use
circulating fluidized bed technology to burn fuel in a much more environmental friendly method as
compared to other conventional processes.
Range of Applications[edit]
Circulating fluidized bed technology can be implemented in many different fields ranging from oil and gas to
power stations. This technology is highly sought after due to its numerous benefits. Some of the popular
applications of circulating fluidized bed are circulating fluidized bed scrubber and circulating fluidized bed
gasification system.
Circulating Fluidized Bed Scrubber[edit]
One of the applications of a circulating fluidized bed scrubber is at power stations which utilize a dry
sorbent usually Ca(OH)2to reduce pollutants like HF, HCL, SO2and SO3in a flue gas stream.[3]
Currently,
Basin Electric Power Cooperative are the only company operating the best available circulating fluidized
bed scrubbing technology for a coal fired boiler plant near Gillette Wyoming since 2011.[4]
The three major components of the circulating fluidized bed scrubber in power plants are:
Circulating Fluidized Bed Absorber
Fabric Filter
Dry Lime Hydration System.
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In the circulating fluidized bed scrubber process, flue gas will enter the reactor from the bottom of the
vessel. Simultaneously, hydrated lime will be injected into the circulating fluidized bed absorber for reaction
to take place to convert SO2and SO3from the flue gas to calcium sulfate and calcium sulfite. Water will
also be injected at the same time to control the operation temperature for maximum absorption capacity.
The flue gas will then send to the bag house for further filtration. In the bag house, a series of air valves
across the filters, will produce compressed air bursts to ensure a more efficient solid and dust collection.
Lastly, clean flue gas will then be directed to the stack with the minimum pollutants in the flue gas
stream.[4]
The schematic diagram of the process is shown in Figure 1.
Circulating Fluidized Bed Gasification System[edit]
Gasification is the process of converting biodegradable waste materials into synthetic gas without
combustion. This process is first used at the Gussing power plant in Austria[5]
based on the steam
gasification of biomass in the internally circulating fluidized bed.
In the gasification process, fuel will be gasifies at 850oC
[5]in the presence of steam to produce a nitrogen-
free and clean synthetic gas. Charcoal with be burnt with air in the
Figure 2: Showing Gasification Process Schematic Diagram
combustion chamber to provide the heating for the gasification process as it is an endothermic process.
Thermal transfer will take place between the gasification and combustion chamber. The illustrated
gasification process is presented in Figure 2.
The chemical reaction that takes place in the gasification as shown in equation [1] and [2] whereas the
reaction in combustion chamber is represent in equation [3].
Gasification;
C + H2O = CO + H2 [1]
C + CO2= 2CO [2]
Combustion;
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C + O2= CO [3]
Dolomite lime or limestone can also be used to increase the hydrogen concentration by absorbing carbon
dioxide to increase the combustion process.
Advantages and Limitations of Circulating Fluidized Bed[edit]
Before, wet flue gas desulfurization (Wet FGD) had been used to capture the pollutants gas. However, this
machinery is expensive to be built, hard to be maintained and take a lot of space in chemical plant. Wet
FGD uses lot of water, however only marginal metals like mercury and acid gases such as HCl, HF and
SO3 can be captured.[6]
Hence, new technology of circulating fluidized bed scrubber (CFBS) was introduced circa 1984 to improve
performance. The turbulator wall design will ensure a perfect mixing and the ability to capture various
pollutants. The used of alloy metals had been replaced with a carbon steel design, reducing the installation
cost. It also comes in a compact size hence the capital costs could be reduced. The water usage also can
be reduced with the design of plug-free water spray nozzles.[7]
Besides that, the CFBS can undergo self-
cleaning process, which reduces the cost of maintenance. The operating temperature is also low thus the
production of the nitrogen oxides, which is one of the greenhouse gases, can be minimized.[8]
Despite of all the advantages, the CFBS that is higher than 400 MW will require more than one unit. The
limestone used in the CFBS is more expensive and must be kept either in a concrete or steel silo rather
than a pile[8]. Besides that, this machinery also produces a by-product, for instance CaCl that do not have
many uses due to its properties.[8]
Another type of CFB is circulating fluidized bed gasification (CFBG), which is more preferable compared to
other type of gasifiers. CFBG has a high mass and heat transfer rate as well as high efficient gas-solid
contacting. At low operating temperature of CFBG, a longer residence time of solid can be achieved
leading to a higher gasification yield.[9]
CFBG process is more energy efficient as it is an endothermic
process. Only the required heat will be generated to maintain the process at the optimum
temperature.[9]
Practically, all the heat produced will be utilized throughout all the processes, as it is an
adiabatic and isothermal process.[9]
Even though, the CBFG process is able to manage huge range of fuels but high gasification yield cannot
be achieved for the fuels that are less reactive such as anthracite and pet coke because of the low
operating temperature. The flow is also multiphase complex and every distinct particles need to be scale-
up in a different way[10]
Available Design of Circulating Fluidized Bed[edit]
Nowadays, several designs had been invented for CFBS for example the CFBS develop by Clyde
Bergemann Power Group namely Circulating Dry Scrubbers (CDS). This type of CFBS is consisted of three
distinct feedback control loops which are for temperature, pressure drop and sulphur dioxide emission.[11]
In
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order to minimized erosion, it injection was designed to be above the ventures . Not only that, the CDS
contains less moving parts compared to other type of CFBS. This design will lead to a lower maintenance
cost. Major components of the CDS are shown in Figure 3.
Similar to CFBS, there are several designs available with specific specification to fulfill various industrialdemands. One of the types is the CFBG, developed by the Phoenix BioEnergy. This type of CFBG
combines several technologies and implement the auger gasifier into one design. The large diameter of the
auger will be placed horizontally on top of the fluidized bed. This configuration will increase the gasification
efficiency, which will assist in the heat transferral over the suspended aggregate into the biofuel.[12]
Full
design of this CFBG is shown in Figure 4.
Main Process Characteristics[edit]
The circulating fluidized bed reactors have been widely used in various industrial processes such as
gasification and coal combustion.[13]
Though the circulating fluidized beds are used widely, the CFD, which
can be, describe by non-uniformity flow patterns and a thorough back mixing still possess significant radial
gradients in the particle density and a lower solid holdup inside the riser interior compared to the wall of the
reactor. These events will then result in low contact efficiency.[14]
For the case of catalytic gas-phase reaction process, gas back mixing should be avoided thus the reacted
product is the gas phase. Another characteristic of the circulating fluidized bed is, as it required promoting
the small contact time of gas and solid catalyst and plug flow, a significant high gas velocity in the riser is
needed.[14]
The significant high gas velocity in the riser is also desired to fulfill the necessity in the catalytic
gas-phase reaction.
Design and Operation[edit]
The circulating fluidized bed involves basically two balancing characteristics of the gas-solid system, which
are the design and the operation characteristics.
Design: Recirculating loop of particles occurred when entrained particles, which possess a substantial
amount of flux, are separated efficiently and externally to the reactor from a giant core reactor (riser) from
its carrying fluid and will then be circulated back to the bottommost of the riser. The carrying fluid will
circulate around this loop only once however the particle will pass through several times before finally
leaving the system[15]
The schematic diagram of a typical circulating fluidized bed can be seen in Figure 5
below.
Operational: The system is usually operated under high particle flux and high superficial gas velocity, which
are typically (101000 kg/m2s), and (212 m/s) respectively.
[15]This operational condition is chosen to
avoid a distinct interface between the dilute region and the dense bed inside the riser. Thus gas velocities
above the bubbling point is chosen for contacting.[15]
The standard operating conditions for the circulating
fluidized bed can be seen in Table 1 below.
Parameters Accepted Values
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Superficial Gas Velocity (m/s) 2 - 12
Net Solids Flux Through The Riser (kg/m2s) 10 - 1000
Temperature (C) 20 - 950
Pressure (kPa) 100 - 2000
Mean Particle Diameter (m) 50 - 500Overall Riser height (m) 15 - 40
Table 1:Typical operational condition for circulating fluidized bed in commercial use
Process Characteristics Assessments[edit]
The circulating fluidized bed (CFB) use high fluid velocity to provide better gas-solid contact by providing
more intense mixing of the fluid so that better quality of product can be obtained. However, the high gas
velocities and the recirculation of solids may make the CFB system much more expensive in term of power
requirement and investment compared to conventional f luidized bed reactors.[16]
CFBs have been widely
used in the field of solid catalyzed gas phase reactions in two situations below.[17]
1. Continuous regeneration of catalyst, which deactivates rapidly. The solid is maintained in constant
circulation where catalyst is continuously regenerated and return to the reactor.
2. Heat must be brought in or removed from a reactor. A continuous circulation of solids between
vessels can efficiently transport heat from one vessel to another since solids have relatively large
heat capacity compared to gases.
One important factor of circulating system is the ability to control the feed circulation rate. The feed
circulation rate is control by the gas velocity in the bed which determines the flow regime and density of
bed. All the circulating systems can be characterized either by the solid circulation rate, kg/s and the
transfer ratio of the suspended materials being exchanged between vessels .[17]
For circulating fluidized bed in coal combustion, the beds need to use a greater fluidizing speed, so the
particles will remained constant in the flue gases, before moving across the combustion chamber and into
the cyclone. During combustion, a dense bed is required to mix the fuel eventhough the solids are
dispersed evenly all over the unit. The bigger particles are extracted and returned to combustion chamber
for further process, which required relatively longer particle residence time. . If the total carbon conversion
efficiencies gets over 98% it shows good separation process that leaves simply a minor proportion of
unburned char in the residues.[18]
During the whole process, the operating conditions are relatively uniform
for the combustor.
Possible Heuristics to Be Used During the Design of the Process[edit]
In designing a circulating fluidized bed, it shows an excellent heat transfer. The reactors usually show
constant temperature distribution for either endothermic or exothermic reactions. In order to determine the
appropriate design for cooling or heating of the circulating fluidized bed reactors, a good approximation of
heat transfer rates are necessary for better control so that the reactors can change its performance for
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different operating conditions.[19]
For highly exothermic reactor, it is recommended to keep the conversion
of material low and recycle any possible cooled reactants. It is also recommended to separate the
components in order of decreasing percentage of material in feed. This will help in reducing the cost of
maintenance for the next separation process.
In many industrial processes that involved small, porous or light particle which have to be fluidized with
more viscous fluid in the present of gas, a Gas-Liquid-Solid circulating fluidized bed (GLSCFB) is more
preferred compared to conventional system because it can minimize dead zone and increase the
contacting efficiency among gas, liquid and solid phases by improving the shear stress between those
phases. Gas-Liquid-Solid circulating fluidized bed also can provide higher gas holdup, produce more
uniform bubble size, better interphase contact, and good heat and mass transfer capabilities. The flexibility
of using GLSCFB allow the fluidized bed to operate at much more higher liquid velocity than the minimum
fluidization velocity which in turn increase the fractional conversion as well as production efficiency per unit
cross-sectional area of the bed. Moreover, the deactivated catalyst used in the GLSCFB can be
regenerated continuously by using the circulating fluidized bed which in turn reduced the operating cost for
replacing the catalyst frequently.[20]
As for circulating fluidized bed scrubbers (CFBS), it is more preferred in industrial due to its ability to
produce higher purity product while avoiding the corrosion issue. The CFBS also preferred because it
requires low installation cost, high capture of metals, low maintenance required, wide fuel sulphur flexibility
and fast response to cope with changes in operating condition.[21]
Some modification is necessary at the
inlet in order to eliminate loss of solids materials at the bottom of bed during low-load operation. For better
quality of product, it is advisable to purify the feed stream if it is difficult to separate between the impurity
and the desired product if it is present in large amount.
This will enable the fluidized bed to operate at full capacity range in a stable manner. Every CFBS need to
have larger boilers that are connected to several cyclones in parallel as to remove the solids for
recirculation.[18]
CFBS also need to have heat recovery unit as some of the heat from the bottom ash can
be recovered as it is more economically feasible in term of lowering the operating cost. Ash coolers are
prone to foul the bed while the heat transfer tubes in fluidized bed are prone to erosion can be removed by
the use of some fluidizing air.
New Development[edit]
More new clean technology has to be implemented to maintain the sustainability of the earth. Bigger
reactors, with lower pollutants emission, have to be developed to meet the global demand. One of the best
clean technologies to be used is the circulating fluidized bed technology. However, circulating fluidized bed
technology is still at a very early stage. Currently circulating fluidized bed technology has only reached the
second generation and is more efficient in capturing pollutants with a simpler design as compared to the
first generation.[10]
The most significant development for this technology is the development of in-bed heat
exchanger and the U-beam separator design.
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InBed Heat Exchanger[edit]
Another major field that is currently being looked into is the further development of in-bed heat exchanger
used with circulating fluidized bed technology. With this design, the bed materials fill the in-bed heat
exchanger through the open top of the circulating fluidized bed furnace, which enables the control ofmaterials through the in-bed heat exchanger.
[22]By being able to control the materials throughput rate,
better control of heat absorption as well as bed temperature in the furnace is achievable. With further
development in this field, we will be able to fully utilize the energy required to drive the furnace with
minimum energy wastage.
U-Beam Separator Design[edit]
The U-beam separator design has been improved for better efficiency, reliability as well as maintainability
and it is now in the 4th generation of its design as shown in Figure 6.
With the improved design, this has contributes and bring numerous benefits to the circulating fluidized bed
technology. Some of the benefits are as follows:[22]
High solids collection efficiency
Controlled furnace temperature
Low auxiliary power
Smaller footprint
Minimal refractory use
Low maintenance
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