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Survey Thenmozhi Ganesan 1,

Sivakumar Lingappan 2 Asst

Professor,

Of EEE,

Coimbatore,

Description

ISSN (Print) : 2320 – 3765 ISSN (Online): 2278 – 8875

International Journal of Advanced Research in Electrical,

Electronics and Instrumentation Engineering (An ISO 3297: 2007 Certified Organization)

Issue 8,

August 2013

A Survey on Circulating Fluidized Bed Combustion Boilers Thenmozhi Ganesan 1,

Sivakumar Lingappan 2 Asst

Professor,

Of EEE,

Coimbatore,

Tamilnadu,

India1 Vice Principal,

Sri Krishna College of Engineering and Technology,

Coimbatore,

Tamilnadu,

India2 Abstract: With the growing energy demands in the power sector,

Fluidized bed combustion (FBC) technology is continuously gaining importance due to its ability to burn different low grade coals and the absence of NOx production

This survey paper is intended to comprehensively give an account of domain knowledge related to CFBC boiler

The authors touch upon the design changes which are introduced in the component levels in order to ease the operation,

enhance the performance and to meet the regulatory compliance

In addition,

salient correlations related to hydrodynamics,

heat transfer and combustion are narrated to facilitate the control and system engineers to develop mathematical models using conservation of mass,

Keywords: Circulating Fluidized Bed boilers,

INTRODUCTION Deterioration of coal quality and pollutant gases (NOx) arising out of burning coal in conventional utility boilers lead to the development of fluidized bed combustion boilers

The main advantages of the fluidized bed combustion boilers are: reduced NOx,

SOx due to relatively low combustion temperature,

better efficiency and reduction in boiler size and design

It has the ability to burn low grade coal and it is less corrosive as the combustion temperature is less when compared to that of an utility boiler

In addition to all of these,

the startup and shut down operation of FBC boilers are much easier

Fluidization is the process by which the solid particles are brought to a suspended state through gas or liquid

When air or gas is passed upward through the solid particles at low velocity,

As the velocity is increased,

the particles reach the state of „Fluidization‟

Basically the fluidized beds are classified into five types as described by Grace [1] namely Fixed bed combustor,

Atmospheric FBC / Bubbling FBC,

Turbulent FBC,

Fast bed / Circulating FBC,

Transport FBC

According to Raico [2],

they are divided into four regions except transport FBC

When the flow rate is low,

the fluid percolates through the void spaces available in between stationary solid particles [3]

This is called „fixed bed Combustor‟

Increase in velocity above the minimum fluidization velocity leads to the formation of bubbles and the solid particles behave like a boiling liquid

Such a boiler is termed as „Bubbling Fluidized Bed combustion boilers (BFBC)‟

Movement of solid particles becomes vigorous in BFBC

„Turbulent Fluidized Bed combustion (TFBC)‟ lies between the bubbling and circulating beds

Turbulent fluidization occurs after the collapse of bubbles

The mass transfer and burning rate differ for turbulent fluidization [4]

Carbon burns in a much faster rate and hence enhanced burning rate is obtained in a turbulent bed

The mass transfer rate is also higher

Some amount of solid particles which are blown out at higher velocities are circulated back to the combustor through cyclone separators

These boilers are termed as „Circulating Fluidized Bed Combustion Boilers‟ [CFBC]

Basu and Fraser [5] have defined the CFBC boiler as follows: “A circulating fluidized bed boiler is a device for generating steam by burning fossil fuels or biomass in a combustion chamber operated under special hydrodynamic condition

The solid particles are transported at velocity exceeding the terminal velocity,

yet there is a degree of refluxing of solids adequate to ensure uniformity of temperature in the combustion chamber”

If the velocity is further increased beyond the terminal velocity,

it enters the „transport bed‟ [6]

The difference between the BFBC and CFBC boilers lies with the hydrodynamics – smaller particle size,

different particle concentration,

different mixing in the beds and fuel particle circulation to the total circulation rate

The most important parameters for combustion process are combustion temperature and excess air

The bed temperature of both CFBC and BFBC are the same

As the height of the furnace is increased,

the bed temperature of CFBC boiler is constant throughout the furnace and for BFBC it is different

Gas to fuel particle velocity is the same for both the boilers

But the fluidization velocity of CFBC is more when compared to the BFBC

Copyright to IJAREEIE

ISSN (Print) : 2320 – 3765 ISSN (Online): 2278 – 8875

International Journal of Advanced Research in Electrical,

Electronics and Instrumentation Engineering (An ISO 3297: 2007 Certified Organization)

Issue 8,

August 2013 According to Bo Leckner [7],

if the fluidization velocity is increased the size of the bubbles formed increases

Hence the area of the cross section of combustor has to be increased

The heat transfer is higher for CFBC than BFB boilers and the heat transfer is mainly due to particle convection

The combustion efficiency in CFBC is increased due to the recirculation of solid particles

Limestone added to CFBC boiler reduces the Sox and NOx

It is comparatively less than BFBC boilers

There are several reasons why CFBC technology is well suited

Some of them are: fuel flexibility,

ability to burn low grade coal,

no need of fuel pulverization,

easy startup and shut down operation and is less corrosive[8]

Instead of coal,

Miccio,

Miccio [9] stated that liquid fuels can also be used for the combustion in CFBC boilers

Variables like Bed height,

excess air ratio for burning coal,

primary to secondary air ratio remain the same for liquid fuels as that of coal except the fuel feeding system

Liquid bio oil produced from biomass using fast pyrolysis process can also be used [10]

The temperature in the furnace of CFBC boilers is comparatively less with that of the conventional utility boiler which results as the outlet steam temperature in the super heater and reheater may not attain the temperature dictated by turbine inlet

Hence the solid particles and the flue gases are circulated so that the outlet temperature of the super heater and reheater can be increased

This survey paper highlights on aspects such as hydrodynamics,

heat transfer and combustion related to CFBC boilers and their important design details

CIRCULATING FLUIDIZED BED COMBUSTION BOILER AND VARIANTS IN COMPONENT DESIGN

The boiler consists of a combustion chamber,

a cyclone separator and a return leg for re-circulation of the bed particles [11]

The combustion chamber is enclosed with water-cooled tubes and a gas-tight membrane

The lower section of the combustion chamber is covered with refractory with openings for introducing fuel,

one or more gas or oil burners for start-up and bottom ash drains

Most of the combustion occurs in the lower section,

while the heat transfer to the walls is achieved mainly by particle convection and radiation in the upper section of the combustion chamber

The cyclone Copyright to IJAREEIE

ISSN (Print) : 2320 – 3765 ISSN (Online): 2278 – 8875

International Journal of Advanced Research in Electrical,

Electronics and Instrumentation Engineering (An ISO 3297: 2007 Certified Organization)

Issue 8,

August 2013 separator can be water-cooled,

steam-cooled or without cooling and is designed to separate the entrained solids from hot flue gas and return them through the return leg and possible loop seal

The gas velocity employed in a CFB is usually in the range 4

5 to 6 m/s

Air is fed to the unit as primary air,

secondary air for fuel and limestone feed,

air to the loop seal and fluidizing air to the ash classifier

The bottom ash classifier is designed to remove larger bed particles and recycle small particles back to the combustion chamber for improved heat transfer

The operating bed temperature is usually in the range of 850-900 °C,

but in the case of low grade fuels the bed temperature can even be below 800 °C

The temperature ranges around 850°C optimizing the sulphur capture efficiency of limestone,

NOx content and agglomeration of the bed material as well

The flue gases from the cyclones goes to the back-pass of the boiler and the bed particles are re-circulated to the combustion chamber through fluidised bed heat exchangers

There are four such fluidised bed heat exchangers namely Super heater I,

The combustion chamber is enclosed with water-cooled tubes and a gas-tight membrane

The lowest part of the combustion chamber is refractory-lined

The boiler has two super heaters namely final super heater (FSH) and low temperature super heater (LTSH),

The super heaters,

economizer and air pre-heater are located in the back-pass

The flue gas goes through the back-pass to the electrostatic precipitator and,

flue gases are blown to the stack

Ash is removed from the bottom of the combustion chamber by the ash-drain system

The lime feeding system is used when sulphur capture is needed

Design changes are introduced in the component levels in order to ease the operation,

to enhance the performance or meet the regulatory compliance

The following paragraphs give an account of such design modifications as appeared in the literature

Design of CFBC includes the design of riser,

The CFBC boiler has external heat exchangers and has two cyclone separators [12]

Modification in the cyclone separator is made as the temperature profile is higher

The width of the cyclone inlet duct is reduced and the vortex finder is extended [13]

Fluidizing nozzle modification (T-style) leads to pressure minimization

The performance of the CFBC boiler such as combustion efficiency,

stability etc is improved by slightly modifying the cyclone separator,

nozzle and ash reinjection system

Li Zhao,

Xiangdong Xu [14] describes a new design model called “cell model method” and split the furnace into three regions and each region has different velocity

The regions are high velocity combustion region,

low velocity heat transfer region and medium velocity suspension region

The differential velocity CFBC combustor improves the efficiency of the combustion

Circulation of bed material is by discrepancy of entrainment at differential air velocity

The velocity is in the order of 3-5m/s in the main bed and is 0

A continuous stirred tank reactor model of CFBC has been proposed [15] and in this model coal,

ashes which are collected in the furnace are mixed and blown into the furnace by the primary air

This method is stable and is preferred during startup,

shutdown operations and during abnormal conditions

Meng [16] developed a novel model of CFBC called horizontal CFBC

It consists of primary,

secondary combustion chambers,

Here the overall height of the boiler is reduced

The flow is a multi pass flow

The dilute zone comprises of upper part of primary furnace,

secondary furnace and the combustion chambers whereas the dense zone is the lower part of the furnace

The solid entrained enters into the primary,

loop seal etc and finally into the dense bed

Sung Won Kim et

al [17] has defined CFBC based on the solids flow characteristics in a loop seal

If the solid inventory is maintained a constant and if the solid circulation increases with decrease in gas velocity,

then the pressure drop across the down comer and riser increases

The flow rate of solid particles increases with the increase in aeration rate and solid inventory which results in the drop in the pressure and increase in voidage

All these are obtained with a pneumatically operated pseudo-mechanical valve for loop seal

Loop seal operation of CFBC has four sections namely the riser,

loop seal section – supply chamber and recycle chamber

In the normal CFBC,

the solid particles are sent to riser through cyclone separator

Basu and L

Cheng [18],

the solid particles which are accumulated in the cyclone separator drops into the loop seal chamber and due to the air in the chamber they are re-circulated to the riser without a pump

This is due to pressure difference between the riser and standpipe

When the riser gas velocity varies,

the operating range of the loop seal aeration also changes

Loop seal air velocity increases which lead to the increase in solids flow through this loop seal

Solids flow rate decreases as standpipe size increases at constant loop seal air rate

Slit size of loop seal has no effect on solids flow rate

For given loop seal aeration rate,

smaller particles will have a higher solids flow rate

Solids flow rate increases as solids inventory

Copyright to IJAREEIE

ISSN (Print) : 2320 – 3765 ISSN (Online): 2278 – 8875

International Journal of Advanced Research in Electrical,

Electronics and Instrumentation Engineering (An ISO 3297: 2007 Certified Organization)

Issue 8,

August 2013 Independent of the different operating conditions,

the solid particles concentration is low in the centerline of the combustor and it is more towards the wall region

This shows the presence of core-annulus type of CFBC combustor [19]

The bottom zone height of the riser is mainly due to the pressure drop in the riser

For the same operating conditions,

the pressure drop of a taller bottom zone riser is more when compared to the shorter bottom zone riser

Addition of wing walls in two different locations namely (a) the middle of the left wall and at the top of the riser (b) the middle of the left wall and 1

Basu [20]

It has been stated that the hydrodynamic conditions of water wall and wing wall are entirely different and the heat transfer coefficient of wing wall is lower than that of water wall irrespective of the position and operating condition

When the wing wall is placed at the top of the riser,

the heat transfer coefficient is more than that of the wing wall placed at the mid height of the riser

The solids flow downward when the wing walls are placed on the top of the riser but the flow is upwards when it is in the middle of the riser

higher temperature profile etc

Modification of Cyclone separator has an advantage of reduced temperature profile and SO 2 emissions

The solid hold up in the dense phase decreases and the particle circulation ratio increases

Hence the efficiency is increased

Though superficial velocity plays an important role in deciding the performance of the boiler,

bed inventory and ash circulation rate also play an important role [22]

Ash coolers are used for circulating the ash and the design of cyclone separators should be such a way that efficiency is maintained better

Laboratory results yield the result that in the freeboard section the small coal particles are easily burnt whereas the large particles are burnt in the dense bed

Further,

the particle size distribution plays an important role in determining the heat release during combustion [23],

Anusorn Chinsuwan and Animesh Dutta [25] have investigated a mechanism for the heat transfer between bed to water wall using a longitudinal finned membrane

Experiments are carried out using three different test tubes namely the membrane tube,

membrane tube with longitudinal fin at the crest and membrane tube with two longitudinal fins at 45˚ on both sides of the crest for the same hydrodynamic conditions

Experiments show that the heat transfer in the conventional membrane is improved by using a longitudinal finned membrane on the tube surface

The membrane tube has the highest HTC

Jun Su and Xiaoxing Zhao [26] have shown that power requirement and erosion rate is reduced by improving the efficiency of the cyclone

Zhang P et al

For a typical 300MWe CFBC boiler

Heat transfer coefficient is reduced along the furnace height

Similarly the heat transfer coefficient is more at the corners rather than the centre of water walls

Pneumatically operated external heat exchangers are developed to control the gas- solid flow in the CFBC

The main advantage is that the heat transfer in the external heat exchangers can be adjusted by means of height of the chamber

The air flow can also change the heat transfer rate

Also an empirical relation between mass flow rate of solids and its pressure drop has been obtained [29],

Gs = CD 2ρ(1-εmf) ∆PO

CFBC boiler bottom ash has more physical heat

This heat is reclaimed [31] by means of a Fluidized bed ash cooler called CFBAC

This is applied to 300MW CFBC boiler

Experimental set up shows that the CFBAC had good particle flow characteristics

Fluidizing velocity and height of separation are the two important parameters in this design

This has good cooling effect and energy conservation

Industrial CFBC‟s are operated at low operating pressures

Evaporation of water is more in CFBC‟s operating at low operating pressure

To avoid over heating of flue gas at furnace exit,

the evaporator tubes are submerged

But the submerged tubes get affected by erosion

In order to alleviate the erosion to the submerged tubes,

Evaporating Loop Seal (ELS) has been developed [32]

ELS work at lower fluidization velocity and hence erosion is alleviated

Internal recirculation-CFBC boilers are developed by Babcock and Wilcox and it has two stage impact solids separator namely the primary and the secondary stage

The secondary stage is multi stage dust collector

The main advantages as described by M

Maryamchik [33],

F [34] are high solid collection efficiency,

controlled furnace temperature,

high separator reliability etc

Feeding limestone leads to high sulphur retention

Fuel ash which is a combination of fly ash and bottom ash contains unburnt carbon particles and lime particles

Loffler et al

Copyright to IJAREEIE

ISSN (Print) : 2320 – 3765 ISSN (Online): 2278 – 8875

International Journal of Advanced Research in Electrical,

Electronics and Instrumentation Engineering (An ISO 3297: 2007 Certified Organization)

Issue 8,

August 2013 670t/h Solid – fuel combustion CFBC model with enriched oxygen described by J

Krzywanski et

al [37] has two different conditions

Combustion in a gas mixture based on O 2 – N2 and the other one without N2 that is O2-CO2

The temperature in the bottom dense zone increases and hence enhanced heat transfer takes place in the oxygen enriched zone

CO2 is more in the oxygen enriched CO2 based gas mixture

Increase in CO2 and decrease in CO leads to better efficiency

NOx is reduced in both the environments

This is one of the designs of CFBC boiler which yields better Heat Transfer Coefficient,

According to J

the combustion of 300MWe CFBC boilers in China are unstable

Moreover slagging outside the furnace is more and cyclone separators are overheated

CFBC boilers with once through steam cycle has better efficiency when compared to the existing boilers as the CO 2 content is reduced [39]

While using this,

the evaporation and economizer duties are reduced and the superheat duty is increased

Evaporation duty is reduced when the lower furnace refractory lining is thickened and when the evaporator wing walls are removed

Oxy-fuel combustion is oxygen fired CFBC which has major reduction in CO2

This is one of the Carbon capture and storage (CCS) technology

This has been described by Arto Hotta [40]

Different operational conditions such as excess air,

bed operational velocity and particle diameter on bed temperature and the overall CO,

NOx and SO2 emissions from the combustor are investigated [41] and are validated using 50 kW CFBC combustor and an industrial-scale 160 MW CFBC combustor which uses different types of coal

The effects of bed operational velocity and coal particle diameter on mean bed temperature and emissions of CO,

NOx and SO2 results have been investigated for three particle diameters (540,

00 and 6

Bed operational velocity has a more significant effect on CO emission than to bed temperature

Increasing excess air decreases SO 2 and NOx emissions

However,

NOx emission increases with the operational bed velocity while SO 2 emission decreases

The next important area of CFBC is controller design

Though all the fluidization types look similar,

there exists some difference between them

PID controllers,

fuzzy logic controllers are applied to CFBC by many authors

The main control loops in a CFBC boiler [42] are: Steam pressure (boiler load) control,

Flue gas O2 content control,

Combustion air distribution control,

Drum level control,

Superheated steam temperature control,

Combustion chamber pressure control,

Bed pressure control,

SO2 control

HYDRODYNAMIC BEHAVIOR AND HEAT TRANSFER OF CFBC All paragraphs must be indented

All paragraphs must be justified,

both left-justified and right-justified

The study of hydrodynamic behaviour leads to the understanding of gas- solid flow in the furnace under different operating modes

Hydrodynamics of solids are explained based on porosity or voidage of bed material,

mass flow rate of solid particles etc

Several authors have described the hydrodynamics of CFBC boilers in many ways

The hydrodynamics mainly depend on bed pressure drop

solid particles concentration,

fluidization velocity and circulation rate of solid particles

The bed pressure drop varies for circular and non circular bed [43] and packed bed [44] with fluidization height

During combustion process,

due to collisions with inert bed particles,

abration of char particles takes place and small particles of char separate from the main particles

This process is called attrition,

and it depends on the coal type [45]

An understanding of solids suspension density both in axial and radial directions gives a better flow pattern which has an impact on heat transfer [46]

The correlations for these attributes namely the bed pressure drop,

solid particles suspension density,

circulation rate of solid particles,

fluidization velocity and their impact on heat transfer are given in table 1 and 2 respectively

Yue et al

Li et al

and it is severe when a low volatile anthracite coal is burnt

The particle size distribution,

primary to secondary air ratio and fluidizing air flow rate plays a major role in the post combustion

Jun Su,

Xiaoxing Zhao et

[26] have grouped the CFBC boiler based on the bed material

The effective material is the fine particles and ineffective material is the large material

The ineffective material remains in the bed while the fine particles are entrained out of the bed

The heat transfer plays an important role in CFBC design

There exist three mechanisms for heat transfer

They are (i) fluid-to-particle heat transfer (ii) particle-to-fluid heat transfer and (iii) bed to wall heat transfer

Heat transfer between the medium and surface plays an important role in determining the efficiency of the combustion system

As far as the Copyright to IJAREEIE

ISSN (Print) : 2320 – 3765 ISSN (Online): 2278 – 8875

International Journal of Advanced Research in Electrical,

Electronics and Instrumentation Engineering (An ISO 3297: 2007 Certified Organization)

Issue 8,

August 2013 heat transfer is concerned,

much heat is involved in the case of fixed bed and hence large temperature gradients are involved

The temperature gradient remains constant or it is maintained at constant by controlling continuous feed and circulation of solid particles

Gas to particle heat transfer coefficient can be calculated [47] using the Nusselt‟s relation,

Pr are defined in the table

The heat transfer between the fluid and particles are given in two forms [48] (i) for gas-solid and (ii) for liquid-solid fluidization

For Gas- solid system,

Heat transfer from a fluidized bed to wall consists of three components namely Particle convection,

Particle radiation and Gas convection

Here the particle convection and radiation are given the prime importance whereas the gas convection is generally disregarded as the density of gas is less than that of the solids [49]

Andersson and Leckner [50] explained that the overall heat transfer co-efficient is mainly due to convective heat transfer co-efficient and radiative heat transfer co-efficient of suspended particles

Thermal conductivity is also neglected when considering heat transfer in CFBC combustion systems [51]

Werdmann and Werther [52] have extracted the following correlation for the convective heat transfer coefficient ℎ

The above correlation must be added to the radiative coefficient to obtain the overall heat transfer coefficient

By neglecting the heat radiation and convection in the dilute phase,

a simpler empirical correlation for the overall heat transfer coefficient to the water wall of a CFBC presented [53] is given by ℎ = 5

The overall heat transfer coefficient from bed to wall at the bottom dense zone is given [8] as h = 40(ρb)1/2

where ρb is given by ρb = ρ(1-ε) + C ε Heat transfer from bed material to wall tube is given by Q bw = hAw Tb − Tw

Several authors have described the structure of the riser as core and the annulus

The temperature of the core is greater than that the annulus and hence heat transfer between the thick wall and the annulus is less than the heat transfer between the thin walled annulus and the core [12]

The heat produced is more in the dense bed than the dilute bed

Radial or tangential injection of secondary air into CFBC riser also causes a change in the heat transfer coefficient

Yong Jun Cho [54] expressed that when the secondary air is injected in radial manner,

convectional heat transfer coefficient increases while the overall coefficient decreases

With the tangential injection,

the convective heat transfer coefficient increases and the particle convective heat transfer coefficient decreases

Copyright to IJAREEIE

ISSN (Print) : 2320 – 3765 ISSN (Online): 2278 – 8875

International Journal of Advanced Research in Electrical,

Electronics and Instrumentation Engineering (An ISO 3297: 2007 Certified Organization)

Issue 8,

August 2013 IV

CONCLUSION Description on a typical circulating fluidized bed combustion boiler and narration on the design changes which are introduced in the component levels in order to ease the operation,

enhance the performance and to meet the regulatory compliance are given

In addition,

salient correlations related to hydrodynamics,

heat transfer and combustion are provided

Mathematical modeling and simulation has been an effective tool in analyzing and optimizing the performance and diagnosing the faults

It is believed that this paper will be of use for control and system engineers to model CFBC boiler to analyze the plant performance during normal and abnormal situations and assess the efficacy of different control schemes to meet the performance criteria desired by the plant owners and operators

APPENDIX

ρg µg ρb ε

Surface area of tube wall surface Gas concentration Orifice discharge coefficient Bed diameter Diameter of the particle Superficial velocity Mass flow rate of solids Net heat transfer coefficient Thermal conductivity of fluid Thermal conductivity of the gas Bed temperature Wall temperature Velocity of gas Pressure drop in the orifice Voidage at minimum fluidization Fluid viscosity Suspension density of the bed Density of the gas Viscosity of gas Local bed density Voidage TABLE I HYDRODYNAMICS IN CFBC

Parameter

ωa Ka Uc ρb mc dc

ρc ϵ Gs U Umf

Circulation rate of particles fluidization velocity Minimum fluidization

average density of the bed the total mass of the bed cross-sectional area of the bed bed height

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Formulae

mass flow rate of char consumed by attrition attrition constant char velocity average bed density char mass carbon particle diameter char velocity char density voidage

ωa = k a Uc −

Gs m c'ρb dc 55

Gs (1 − ϵ)ρc

Gs = 785 U − Umf exp −6630dc where ρb =

ISSN (Print) : 2320 – 3765 ISSN (Online): 2278 – 8875

International Journal of Advanced Research in Electrical,

Electronics and Instrumentation Engineering (An ISO 3297: 2007 Certified Organization)

Issue 8,

August 2013 Ar C µ dp 2

Umf = µ [(33

0651Ar) 0

8 – 33

7] / C dp

Archimedes number Gas concentration Viscosity Mean particle diameter burning rate of char particles oxygen concentration in the free stream surface area of a char particle

1 hm + k c

char reaction rate coefficient Activation energy coefficient vary with the characteristics of coal and its chemical composition universal gas constant

carbon particle diameter molecular diffusivity of oxygen

Tb Ts Tm

bed temperature particle temperature mean temperature

Sherwood number Voidage Reynolds number Schmidt number Slip velocity Viscosity of gas

Sh = 2ε + 0

69 Re =

μg μg

ρg Dg u Gs Us = − ∈ 1−∈ ρc

diffusivity coefficient Bed temperature particle porosity Pressure Bed Pressure drop Bed height Voidage density of solid particles density of fluidizing medium acceleration due to gravity

5 where

TABLE II HEAT TRANSFER IN CFBC

Symbol ℎ

ρs ρg Tb Db dp ug µg kg

Parameter Convective heat transfer coefficient suspension density of the bed,

viscosity of gas thermal conductivity of the gas

Formulae

Overall heat transfer coefficient

53 ℎ = 5

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International Journal of Advanced Research in Electrical,

Electronics and Instrumentation Engineering (An ISO 3297: 2007 Certified Organization)

Issue 8,

August 2013 TABLE III IMPACT OF PROCESS / PHYSICAL PARAMETERS

Parameters

Authors/ Year

Impact Of Parameters

Visser and M

Porosity and heat transfer co-efficient is defined as a function of fluidizing velocity

In low velocities,

the particle convective component of heat transfer is very low or negligible

Afsin Gungor,

Operating velocity affects the efficiency of the system

Heat transfer,

Rhe-area ratio also affects the efficiency of the system

Andersson ,

Size of the Particle affects the heat transfer co-efficient

Yong Jun Cho et

The heat transfer co-efficient is high for smaller particles and low for larger particles

Yong Jun Cho et

As the circulation rate of solids increases the heat transfer co-efficient increases

The circulation rate of solid particles is a function of inventory and not the superficial velocity

Wang et

As height of the furnace is increased the concentration of solid particles is less both in the core and the annulus region

Circulation rate of solids

Leckner et

Height of the furnace L

Huilin et

Zhang H

Gupta and P

Bed Pressure

Winaya ,

Particle size distribution

Particle concentration

Solid mass inventory

Bed Temperature

CO2 concentration at high temperature

Addition of limestone

Particle suspension density

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Saastamoinen et

The heat transfer coefficient of wall to bed increases

As the height increases the HTC decreases

As the height of riser increases,

the heat transfer due to radiation decreases

Here the heat transfer due to convection is kept a constant with low circulation rate and wide particle distribution

SO2 retention is also included

Peripheral heat transfer is not uniform throughout the riser and heat distribution varies with the height

When the pressure is low,

the force which drags the solid particles is less and hence the bed voidage is also less

When the bed voidage is decreases the inventory increases

Increase in pressure leads increased bed voidage in the bottom dense region and decreased voidage in dilute region

Heat transfer coefficient increases with increase in pressure,

superficial velocity and bed temperature

With increase in pressure,

the thermal conductivity of gas particles increases which leads to further increase in heat transfer co-efficient

Particle size distribution affects heat transfer and flow dynamics in boiler

The particle concentration is more near the wall The particle concentration is more in annulus region and it is less in the core region

Wang et

Huilin et

Temperature of the gas is a function of solid mass inventory

Winaya,

Increase in bed temperature increases heat transfer coefficient

Volumetric concentration of CO2 increases the partial pressure of CO2

This leads to an increase in gas emissivity and radiative flux

Hence the HTC increases as well as the emission increases

Winaya,

the heat transfer coefficient (both convection and radiation) increases as the CO2 concentration is increased

Otherwise the O2 concentration is increased

Jim in Kim,

Guiyoung Han,

Changkeun Yi,

The axial dispersion coefficient increases with increase in suspension density and gas velocity

ISSN (Print) : 2320 – 3765 ISSN (Online): 2278 – 8875

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Electronics and Instrumentation Engineering (An ISO 3297: 2007 Certified Organization)

Issue 8,

August 2013 ACKNOWLEDGMENT The authors would like to thank the Engineers of BHEL,

Trichy and Corporate R&D,

Hyderabad for useful discussions

Further the support and encouragement given by the Managements of Sri Krishna college of Engineering and Technology and Sri Ranganathar Institute of Engineering and Technology,

Coimbatore are greatly acknowledged

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International Journal of Advanced Research in Electrical,

Electronics and Instrumentation Engineering (An ISO 3297: 2007 Certified Organization)

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BIOGRAPHY Dr

Sivakumar obtained B

E Electrical Engineering from Regional Engineering College,

Trichy in 1970,

Sc (Engg)- Applied Electronics and Servomechanism from PSG college of Technology,

Coimbatore in 1972 and PhD in Electrical Engineering from Indian Institute of Technology,

Kharagpur in the year 1980

He joined Bharat Heavy Electricals Limited (BHEL),

during October 1975 and retired as General Manager,

Corporate R&D Division during December 2007

Presently he is working as Vice Principal and Dean / Electrical Sciences,

Sri Krishna College of Engineering and Technology,

Coimbatore

His area of work include Mathematical Modelling and Simulation,

System Identification,

Training simulators,

embedded systems applications,

Development of IT related software modules for Performance Analysis Diagnosis and Optimization for power plants

He has published number of papers in International / National Journals and Conferences

He has a few software copy rights and a patent

He has been felicitated as “Eminent Electrical Engineer for the year 2007” by Institution of Engineers (India) for his contribution to power sector

He is a member of IEEE,

ASME and ISTE

Thenmozhi is an Assistant professor of Electrical and Electronics Engineering Department at Sri Ranganathar Institute of Engineering and Technology,

Coimbatore,

She received her B

E (Electrical and Electronics Engineering) and M

E (Power Electronics and Drives) in 1998 and 2007 respectively

She is pursuing her research work in Anna University,

Chennai towards doctoral programme

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