Vol:5 Issue:12 December 2012 ISSN:0974-6846 Indian Journal of Science and Technology

Measurement of Residence Time Distributions Of Coal Particles in a Pressurized

Fluidized Bed Gasifier (PFBG) using Radio Tracer Technique

J.S.Rao1*, N.V.S.Ramani 1 , H.J.Pant2 , D.N.Reddy3

1
Corporate R&D Divn, Bharat Heavy Electrical Limited, Vikasnagar, Hyderabad, India.
2
Isotope Applications Division, Bhabha Atomic Research Centre, Mumbai, India.
3
Ex Vice Chancellor, Jawaharlal Nehru Technological University, Hyderabad, India.
jsrsystem@yahoo.com, jsrao@mpotgroup.com

Abstract
A pressurized fluidized bed gasifier (PFBG) system of an integrated coal gasification and combined cycle (ICGCC) plant is designed to
behave as a well-mixed flow system for coal; and any deviation from the well-mixed flow condition will deteriorate the performance
and efficiency of the gasification system. This paper describes a radiotracer investigation carried out to measure RTDs of coal particles
in a pilot-scale PFBG with objectives to determine mean residence time (MRT) of coal/ash particles in the gasifier and estimate degree
of mixing at different operating and process conditions. Lanthanum-140 labeled coal (100 gm) was used as a radiotracer. The tracer
was instantaneously injected into the coal feed line and monitored at ash and gas outlets of the gasifier using collimated scintillation
detectors. The measured RTD was used to determine mean residence time (MRT) of coal particles within the system and simulated us-
ing fractional tank-in-series model. The results of simulation indicated that the system behaved as a well-mixed system with undesired
bypassing of a small fraction coal particle from the system. The results of the study were used to improve the design of the gasifier and
optimize the system.
Keywords: Pressurized fluidized bed gasifier, residence time distribution, radiotracer, Lanthanum-140, tanks-in-series
model, bypassing
1. Introduction investigations in this system to evaluate feasibility of using
radiotracer technique for measurement of RTD of the coal
Integrated coal gasification and combined cycle (ICGCC) is
particles in the PFBG at cold as well as hot conditions [8]. They
one of the most promising advanced clean coal technologies
reported use of two different radiotracers i.e. gold-198 and
wherein coal is converted into low calorific value (CV) gaseous
lanthanum-140 and concluded that both are equally suitable to
fuel in a pressurized fluidized bed gasifier (PFBG) and
trace the coal particles in gasifiers. They used fractional tanks-in-
combusted in a gas turbine combustor of combined cycle plant.
series model to simulate the measured RTD and found that the
The gasifier involves flow of two different phases i.e. solid (coal)
system behaved as a well-mixed system with bypassing of some
and gas (mixture of air and steam) and knowledge of dynamics of
of the coal particles from the system. The paper describes some
these two phases is important to assess the performance of the
of the preliminary results of the RTD measurement carried out in
system as well as for scale up of the process. The concept of
the same PFBG system as described by Pant et al. (2009).
measurement and analysis of residence time distribution (RTD) is
often used to investigate dynamics of flowing phases in industrial 2. Experimental
process system [1]. The analysis provides vital information such
as mean residence time (MRT) of process fluid, degree of axial 2.1. Pressurized fluidized bed gasifier (PFBG)
mixing and abnormality in flow behavior, if any. Radiotracer The schematic diagram of the pilot-scale PFBG system is
techniques are widely used to measure RTD of process material shown in Fig.1 [8]. The system consists of various subsystems
in pilot-scale as well as the full scale industrial systems because such as gasifier, coal feeding system, combustor, air compressor,
of their high detection sensitivity, online detection, steam supply system, gravity recycle system, gas cleaning and
physicochemical compatibility, availability of wide range of cooling system and ash extraction system. The gasifer is designed
suitable tracers, limited memory effect and utility in harsh for gasifying 50 kg/h of sub bituminous coal has an internal
industrial environment [2,3,4,5,6,7]. diameter of 200 mm and consists of air plenum, distributor
The Research and Development Division of M/s Bharat assembly and freeboard section. An air compressor supplies the
Heavy Electricals Limited, Hyderabad, India has designed, fluidizing air required for the process. The steam required for the
fabricated and commissioned a pilot-scale of the PFBG system to process is supplied by a steam generating system and passed to
study the feasibility of coal gasification/combustion process for the fluidizing bed through fluidizing air supply line. The
power generation. It was required to investigate flow dynamics of combustor assembly is directly coupled with the air plenum of
coal particles in the system to evaluate its performance and scale the gasifier. The gasifier is designed to operate at 3 atmospheric
up of the process. Pant et al. [8] carried out a series of radiotracer pressure and 1000oC temperature. The air-plenum acts as a
www.indjst.org 3746 Research article
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Indian Journal of Science and Technology Vol:5 Issue:12 December 2012 ISSN:0974-6846

header for the fluidizing media i.e. air/steam and also distribute Fig.1. Schematic diagram of PFBG facility and
the same uniformly into the gasifier by means of a conical experimental set-up
distributor attached to it. The freeboard section is slightly conical
with 200 mm diameter at the bottom and 250 mm diameter at the D3 D4 Gas to cleaning and
top. Gasifier and free board sections were provided with a cooling system
number of view ports and tappings for temperature and pressure Cyclone
measurements. Coal feed
Initially the gasifier is filled with a known quantity of coal Fines to
particles (50 microns-4 mm). Subsequently, coal and fluidizing Freeboard combustor
air/steam are fed to the gasifier through the respective feeding
Tracer
systems. During the fluidization process, the gasification and Injecti
combustion of coal occurs and various gases such as carbon on
dioxide, carbon monoxide, hydrogen, methane etc. are produced.
During the combustion process, the temperature of the fluidized
DAS
bed ranges from 900-1000 oC. The mixture of gases flows Gasifier and D1
upward in the freeboard section of the gasifier and passes through Combustor
Air/Steam in D2 D1 D2 D3 D4
the cyclone system, where the fine coal particles are separated.
Distributor
The separated fines are fed back to the gasifier using recycle and
system while the gaseous mixture is fed to the wet gas cleaning Air plenum Ash extraction hopper
system. The cleaned gaseous mixture is used as a fuel gas for
power generation and various other applications. Valve To ash collection tray
The burnt coal i.e ash is generated in the process is extracted
from the bottom of the gasifier at regular intervals. 2.2. Residence time distribution measurements
Properties of the coal used as fluidizing material are given in
Radiotracer technique was employed to measure the RTD of
Table 1. The fluidization phenomenon of gas-solid systems
the coal in the gasifier. The technique involves the instantaneous
depends very much on the particle characteristics. Geldart [9]
injection of a suitable radiotracer into the process stream and
was the first to classify the behavior of the solids fluidized by
monitoring its movement at strategically important locations,
gases into four distict groups, namely A, B, C and D,
using scintillation detectors. A series of four different RTD runs
characterized by the density difference between the particle and
were carried out at different process and operating conditions as
the fluidizing medium and mean particle size. According to the
shown in Table 2. All the experiments were carried out at
Geldart classification of fluidizing particles, coarser coal particles
atmospheric pressure. The temperature of the bed during the hot
are classified as Type D particles, whereas the fine coal particle
run ranged from 900-1000 0C. For the present study, lanthanum-
can be classified as Group B particles.
140 radioisotope (gamma energies: 1.16 (95%), 0.92 (10%), 0.82
Table 1. Properties of coal particles used in experiments
(27%), 2.54 (4%), half-life: 40 hours) as lanthanum chloride was
selected to be used as a tracer, as it has strong affinity to get
Property Value
adsorbed on solid particles. Lanthanum-140 was produced by
Bulk Density 815 Kg/m3
irradiating lanthanum oxide powder (La2O3) in DHURVA reactor
Particle density 1680 Kg/m3
at Bhabha Atomic Research Centre, Trombay Mumbai. The
Average particle size 1.86 mm
irradiated target was processed to produce lanthanum chloride
Composition of coal Content Caron (C): 38.3% wt, (LaCl3). About 1 mCi (37 MBq) activity of lanthanum-140 was
Hydrogen (H2): 2.4 % wt, taken from mother solution and diluted in about 300 ml of
Sulphur (S): 0.3 % wt distilled water. About 100 gm of coal was poured into the diluted
Nitrogen (N2): 0.71 % wt, solution and stirred for about 5 minutes using a glass rod. The
Oxygen (O2): 9.6% wt. coal soaked in the radioactive solution was left for about half an
Type of particle according to Group D hour. After half an hour, the radioactive solution was decanned
Geldart Classification and the coal particles were dried using an electrical heater. Based
on the previous studies, it was observed that more than 60-70%
of the initial lanthanum activity gets adsorbed on the coal
particles. So it was assumed that about 600-700 micro curie (220-
260 MBq) activity might have got adsorbed on the coal particles

Research article 3747 www.indjst.org

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Vol:5 Issue:12 December 2012 ISSN:0974-6846 Indian Journal of Science and Technology

(100 gms) and was used as tracer in each run. The lanthanum-140 t
labeled coal was instantaneously injected into the coal feed line ∫ t .c( t ).dt
using a specially fabricated injection arrangement. The injection
tE = 0 (3)
point is shown in Fig. 1. The tracer was injected after the system t
was stabilized and attained a steady state condition. The ∫ c( t ).dt
movement of the tracer in the gasifier was monitored using seven 0
different collimated scintillation detectors (D1 to D7) mounted at The first moment of the RTD curve gives MRT of the process
four different locations as shown in the Fig.1. The detectors were material in the system. The MRTs determined from measured
connected to corresponding channels of a common computer RTD curves for four different runs are shown in Table 3. The
controlled data acquisition system (DAS) set to record tracer theoretical MRT ( t T . ) of the material in a closed system is given
concentration data at an interval of every one minute. The tracer as:
concentration versus time data acquired was saved in the M
computer for further analysis. tT = (4)
Q
Table 2. Operating and process conditions during experiments Where M: weight of bed material and Q: flow rate. The above
equation holds good for systems with closed-closed boundaries.
The freeboard section of the present experimental setup is a
QExtraction (kg/hr)

system with 'open-open' boundaries and thus above equation can

QSteam (kg/hr)
QFeed (kg/hr)

be applied. Whereas, the fluidized bed section of the system can

Q Air (kg/hr)

be approximated as 'closed-closed system' and the above equation

WBed (kg)
HBed (m)

tT (min)
Run No.

holds good for it and was used to estimate the approximate

T(oC)

theoretical mean residence time of coarser coal particles flowing

out from bottom of the system. However, the exact mean
1 15.2 13.3 0.17 6.3 115 0 Ambient 30
residence time is determined from residence time distribution
2 7.6 7.2 0.2 7.6 115 0 Ambient 60
measurement using radiotracer technique. The values of the t T
3 10.2 8.7 0.2 7.6 115 0 Ambient 45
were calculated based on bed-weight and corresponding flow
4 23.3 7.2 0.2 12.2 55 2 900-1000 30
rates and are given in Table 2.
Table 3. Results of model simulation of residence time
3. Data Analysis
distribution data
The radiotracer concentration is measured in terms of counts
Run QFeed tT tE Fractional tanks-in-
per unit time and measured acer concentration versus time data
No. (kg/hr) (Min) series model
were treated and analyzed using Residence Time Distribution (Min) τ
m N RMS
software provided by International Atomic Energy Agency
(Min)
Vienna, Austria [10]. The normalized RTD curve was obtained
1 15.2 30.0 31.7 30.0 0.74 0.00149
using the following relation:
2 7.6 60.0 57.0 57.0 0.92 0.00053
C( t )
E( t ) = ∞
(1) 3 10.2 45.0 42.0 38.0 0.85 0.00073
4 23.3 30.0 34.0 34.0 0.78 0.00096
∫ C( t )dt
0 In order to quantify the degree of mixing and investigate the
flow behavior of the gasifier, suitable mathematical models are to
The area under the normalized RTD function (E(t)) is equal to
be used to simulate the experimentally measured RTD data.
unity. Thus:
Based on the shape of the RTD curve, configuration of the
∞ gasifier and prior information available about the system, tanks-

∫ E(t )dt = 1 (2)

in-series was selected to simulate the experimentally obtained
RTD data [11]. This model assumes that the system under
0
investigation consists of a series of well-mixed tirred tanks each
The treated and normalized RTD curves measured at the bottom
of volume V. The total volume of the system will be NV, where
of the gasifier during four different runs are shown in Fig. 2.
N is number of well-mixed tanks connected in series. The
The first moment of the RTD curve was determined using the
physical representation of the model is shown in Fig. 2.
following relation:

www.indjst.org 3748 Research article

50
 Nt
N N t N −1e m
τ (8)
Indian Journal of Science and Technology E(t)=Vol:5 Issue:12 December 2012 ISSN:0974-6846
N
τ m Γ(N)
Fig.2. Typical Tanks in Series Model where x=Vf/V and N is positive and need not to be an
where, Γ(N) is called gamma function and is defined as:
integer. The above model is an extension of tanks-in-series model
V V V V V
Q Q , where N need not to be an integer ∞ -x and is treated as an adjustable
Γ(N)= ∫ eThe x Nmain
- 1dx (9)
1 2 3 (N-1) N index of mixing performance. 0
use of this model is to fit
small deviations from the exponential distribution of a single
where x=Vf/V and N is positive and need not to be an integer. The above model is an
stirred tank. If N < 1, then this implies that the system behaves as
extension of tanks-in-series model , where N need not to be an integer and is treated as an
a well-mixed system with some amount of bypassing or short-
If the system is operating at steady state flow condition adjustable index of mixing
and circuiting performance.
of the The main
process fluid. Theusetanks-in-series
of this model is model
to fit small
hasdeviations
the
an impulse of tracer of amount (A) is injected at the inlet of the advantage of mathematical simplicity but the parameter lacksthat
from the exponential distribution of a single stirred tank. If N < 1, then this implies a the
th
first tank, then the tracer balance equation for N tank can be behaves
system physical interpretation,
as a well-mixed systemexcept
with somewhenamountit isof an integeror .short-circuiting
bypassing The RTD of
written as: the processfunctions of tanks-in-series
fluid. The tanks-in-series model
model has for different
the advantage values of
of mathematical tank but
simplicity
numbers are shown in Fig. 3.
dC N dC
the parameter lacks a physical interpretation, except when it is an integer . The RTD
V = Q.(CV N =−QC .(C N −)
Ndt−11 − CN ) N (5) (5) Fig.3. Impulse responses of gamma function model for different tank no.
functions of tanks-in-series model for different values of tank numbers are shown in Fig. 3.
dt Normalised tracer concentration, E(t)
where, Q is fluid flow rate, CN and CN-1 are tracer concentrations at the outlet of Nth and (N-0.09
where, Q is fluid flow rate, C of the
1)th tanks, respectively. If an impulse N
andtracer
CN-1 are tracer concentrations
is injected at the inlet of the first tank, then0.08
N=0.25

at the outlet of Nth and (N-1)th tanks, respectively. If an impulse N=0.5

the tracer concentration at the outlet of the system is obtained by solving the above equation0.07 N=0.75
of the tracer is injected at the inlet of the first tank, then the tracer
and is given as: 0.06 N=1
concentration at the outlet of the system is obtained by solving
N=2
the above equation and is given as: N (N -1) 0.05

AN t N=5
C (t)= e
- Nt/τ m (6) 0.04
N N
N (N - 1) Qτ m (N - 1)! N=15

AN t - Nt/τ (6)
0.03

e (6)mcan be written as:

C (t)= form the above equation
In normalized
N N
Qτ m (N - 1)!
0.02

N N - 1 - Nt/τ m
N t e
0.01

E(t) = (7)
In normalized form the above equation
τ mN (N - 1)! (6) can be written as: 0.00
0 20 40 60 80 100 120 140 160 180 200
Time (Min.)
Where, E(t) is called dimensionless residence time distribution function. t and τm are time
Fig.4. Experimentally measured residence time distributions
- Nt/τ
variable and model predicted mean residence time, respectively. The value of N equal to
N N t N - 1e m
unity for well-mixed system or continuously stirred tank reactor and tends to be infinity for a Fig. 3 : Impulse responses of gamma function model for different tank number
E(t) =
plug flow system. The main drawback of the model is that the value of N should (7) be an Normalised tracer concentration, E(t)
τ (N - 1)!
N
m 0.05 "Experimental RTD measured by
detector D2 (MRT=31.7 Min.)
Gamma distribution model simulated RTD
Where, E(t) is called dimensionless residence time distribution 0.04 (N=0.74, MRT =30 Min., RMS=0.00149)

function. t and τm are time variable and model predicted mean 0.03

residence time, respectively. The value of N equal to unity for

well-mixed system or continuously stirred tank reactor and tends 0.02

to be infinity for a plug flow system. The main drawback of the 0.01

model is that the value of N should be an integer. This implies

that the model fails to describe the flows in system with high 0.00
0 20 40 60 80 100 120 140 160

degree of mixing with bypassing of some of the process material Time (Min.)

without adequate mixing within the system. This drawback can

be overcome by fractional tanks-in-series model which assumes The4: Experimentally
Fig. model simulated RTD residence
measured curves were fitted to the
time distributions
that a flow system consists of N equal sized tanks of volume V experimental data using least square curve-fitting method and
and a fractional compartment of volume Vf. Thus the above optimum The model simulated RTD curves were fitted to the experimental data using least
values of the model parameters corresponding to best fit
equation (7) can be written as [11]: square
were obtained method
curve-fitting and optimum
[12]. The quality ofvalues
theoffittheismodel
judgedparameters corresponding to
by choosing
Nt best
thefitmodel
were obtained [12]. Thetoquality
parameters of the the
minimize fit is sum
judgedofbythe
choosing
squaresthe model
of theparameters
τ (8)
N N t N −1e m to differences
minimize the between
sum of thethe experimental,
squares E(t) and
of the differences model
between simulated E(t) and
the experimental,
E(t)=
τ mN Γ(N) or predicted curves, Em(t,N, τm,). Thus root mean square (RMS)
model simulated or predicted curves, Em(t,N, τm,). Thus root mean square (RMS) vale is
where, Γ(N) is called gamma function and is defined as: vale is given as:
given as:
∞ 0.5
Γ(N)= ∫ e - x x N -1dx (9)  ∞
1


∫

0 RMS =  [E(t) − E m (t, N,τ m )]2 dt  = Minimum
 n  (10)
 0 
Research article 3749 www.indjst.org
where, RMS is root mean square n is number of data points. The comparison of experimental
51
Vol:5 Issue:12 December 2012 ISSN:0974-6846 Indian Journal of Science and Technology

0.5 Fig.5. Experimental and model simulated RTDs (Run 1)

 ∞ 
1
RMS = 
n ∫ 2 
[E(t) − Em (t,N,τ m )] dt 

= Minimum 0.06
Normalised tracer concentration, E(t)
D2 (Run 1)
D2 (Run 2)
 0  D2 (Run 3)
0.05
D2 (Run 4)
(10)
where, RMS is root mean square n is number of data points. The 0.04

comparison of experimental and model simulated RTD curves

obtained from experimental run 1 to run 4 are shown in Fig. 5 to 0.03

Fig 8.
0.02

4. Results and Discussion 0.01

In tracer test carried out at cold condition it was observed

0.00
that, the tracer concentration curves were recorded by detectors 0 20 40 60 80 100 120 140 160

D3 and D4 mounted at the inlet and outlet of the cyclone during Time (Min.)

cold conditions and it indicates an instantaneous increase in

tracer concentration soon after injection and subsequently become Normalised tracer concentration, E(t)
Experimental RTD (MRT=57 min.) detector D2
constant. The increase in background levels at inlet and outlet of 0.03 Gamma distribution model simulated RTD
the cyclone could be due to residual tracer particles in the Fig.6. Experimental and model simulated RTDs N=0.92, (Run 2MRT=57
) min, RMS=0.00053)
injection system and getting detected by detector D3 and D4. 0.02
Normalised tracer concentration, E(t)
Experimental RTD (MRT=57 min.) detector D2
However, at hot condition i.e. temperature > 900 0C (run 4), the 0.03 Gamma distribution model simulated RTD
increase in background levels recorded by detector D3 and D4 0.02
N=0.92, MRT=57 min, RMS=0.00053)
were more than that of the cold runs. This could be due to 0.02
0.01
generation of fine coal particles at hot conditions and are
accumulated at the bottom outlet of the cyclone thus further 0.02 0.01
increasing the background at detector D3 and D4.
0.01
0.00
The tracer concentration curves monitored by detector D1 at 0 50 100 150 200 250 30

the coal feed inlet and by detector D2 at the distributor outlet 0.01 Time (Min.)

were considered for detailed flow analysis. The tracer

concentration curve monitored by detector D1 is a sharp pulse of 0.00
0 50 100 150 200 250 300
narrow width in comparison to the curve monitored at distributor Time (Min.)
outlet by detector D2 and could be considered as an impulse for
analysis. The normalized tracer concentration curves measured at
the distributor outlet (D2) during the four different runs are Fig.7. Experimental and model simulated RTDs (Run 3)
shown in Fig. 4. Each measured RTD curve was simulated using Normalised tracer concentration, E(t)
tanks-in-series model and it was observed that the model 0.04
Experimentally measured RT at detector D2
predicted response does not fit well to the experimental data. This (MRT=42 Min.)
0.04
Gamma distribution simulated RTD
indicated that the simple tanks-in-series model with N=1 is not a Normalised tracer concentration, E(t)
0.03 (N=0.85, MRT=38 Min., RMS=0.00073)
suitable model to describe the flow behavior of the coal in the
0.04
Experimentally measured RT at detector D2
gasifier. Thus an extension of tanks-in-series model as described 0.03
(MRT=42 Min.)
0.04
above was used to simulate the measured RTD data and the 0.02 Gamma distribution simulated RTD
(N=0.85, MRT=38 Min., RMS=0.00073)
model was found suitable to describe the dynamics of the coal 0.03
0.02

particles in the gasifier. The model predicted RTD corresponding 0.03

0.01

to the minimum RMS and experimentally measured RTD curves 0.02

0.01

are shown in Fig. 5 to Fig. 8. The model parameters i.e. tanks 0.02
0.00
number (N) and mean residence time (τm) corresponding to best 0.01
0 50 100 150 200 250

Time (Min.)
fit i.e. minimum RMS value are tabulated in Table 3 0.01
www.indjst.org 3750 Research article
52 0.00
0 50 100 150 200 250
 Indian Journal of Science and Technology Vol:5 Issue:12 December 2012 ISSN:0974-6846

Fig.8. Experimental and model simulated RTDs (Run 4) 6. References

Normalised tracer concentration, E(t)
0.05
1. Danckwerts, P.V., “Continuous flow systems, distribution
Experimentally measured RTD (MRT=34 min.) of residence times”, Chem. Eng. Sci. 2, pp.1-13 (1953).
0.04
Gamma distribution model simulated RTD 2. Charlton, J.S. (Eds)., Radioisotope Tracer Techniques for
(N=0.78, MRT=34 min., RMS=0.00096)
0.04 Problem solving in Industrial Plants, Leonard Hill (1986).
0.03 3. Pant, H.J., Kundu, A. and Nigam, K.D.P., “Radiotracer
Applications in Chemical Process Industry”, Reviews in
0.03
Chemical Engineering, 17, pp.165-252 (2001).
0.02
4. Thyn, J., Zitny, R., Jaroslav, K. and Cechak, T., Analysis
0.02 and Diagnostics of Industrial Processes by Radiotracers
0.01 and Radioisotope Sealed Sources, 1 & 2, CVUT, Praha,
(2000)
0.01
5. Pant, H.J., Saroha, A.K. and Nigam, K.D.P.,
0.00
0 20 40 60 80 100 120 140 160
“Measurement of Liquid Holdup and Axial Dispersion in
Time (Min.) Trickle Bed Reactors Using Radiotracer Technique”,
Nukleonika 45, pp. 235-241 (2000).
TheFig. 8. Experimental
values of theoretical andMRTs,
model experimental
simulated RTDs MRTs(Runand 4) 6. Pant, H.J., Thyn, J., Zitny, R. and Bhatt, B.C.,
model predicted MRTs were in good agreement with one other. “Radioisotope Tracer Study in a Sludge Hygienization
The values of tank number (N) estimated for all the four runs Research Irradiator (SHRI)”, Applied Radiation and
were Thefound
valuestoofbetheoretical
less than MRTs, experimental
one. This indicatedMRTs and model
bypassing predicted MRTsIsotopes 54, pp.1-10 (2000).
or short-
circuiting of the process material. This indicates that a small 7. Pant, H.J., “Flow Rate Measurement in a Draft Tube
were in good agreement with each other. The values of tank number (N) estimated for all theCrystallizer By Means of a Neutrally Buoyant Sealed
fraction of coal immediately leaves the combustor/gasifier soon
Radioactive Flow Follower Technique”, Applied
fourafter entering
runs were foundinto
to betheless
system. The
than one. bypassing
This indicated of processormaterial
bypassing short-circuiting of the
is a kind of malfunctioning and reduces the efficiency of the Radiation and Isotopes 53, pp. 999-1004 (2000).
process
system. material. This indicates
Therefore, that a smallof fraction
the bypassing of coal immediately
the material is highly leaves 8. theH.J. Pant, V.K. Sharma, M. Vidya Kamudu, S.G. Prakash,
undesired in the S. Krishanamoorthy, G. Anandam, P. Seshubabu Rao,
combustor/gasifier sooncoal
aftergasification/combustor
entering into the system. The process.
bypassing of process material is a
N.V.S. Ramani, Gursharan Singh, R.R. Sonde,
5. Conclusions
kind of malfunctioning and reduces the efficiency of the system. Therefore, the bypassing of“Investigation of flow behaviour of coal particles in a
pilot-scale fluidized bed gasifier (FBG) using radiotracer
the material Radiotracer technique
is highly undesired in thewas
coal successfully applied process.
gasification/combustor to measure
technique”, Applied Radiation and Isotopes 67 (9),
MRTs and investigate flow dynamics of coal particles in a pilot-
pp.1609-1615 (2009).
scale PFBG. The values of the measured MRTs are close to the
5. Conclusions 9. Geldart, D., "Types of Gas Fluidization", Powder
theoretical and model simulated MRTs.
Technology, 7 (5), pp. 285-292 (1973).
An extension of tanks-in-series model (gamma
10. International Atomic Energy Agency, Residence Time
distribution model) was used to simulate the measured RTD
Distribution Software Analysis User’s Manual”,
curves and was found suitable to describe the dynamics of coal in
Computer Manual Series No.11, IAEA, Vienna, Austria,
the gasifier. The values of model parameter i.e. tank number
pp-218 (1996).
were found to be less than 1 (N<1) indicating bypassing or short
11. Buffham, B.A. and Gibilaro, L.G., “A Generalization of
circulating of coal in the gasifier at all the operating and process
the Tanks in Series Mixing Model”, AIChE J., 14 (5),
conditions used in the experiments. The gasifier behaved as a
pp.805-806 (1968).
well-mixed system with a small fraction of coal particles
12. Michelsen, M.L., “A Least-Squares Method for Residence
bypassing the system, which is highly undesired. The results of
Time Distribution Analysis”, Chem. Eng. J., 4, pp.171-
the study were used to improve the design of the system; and
179 (1972).
optimize and scale up of the process.
Acknowledgements
The authors would like to thank the members of Group members
at M/s Bharat Heavy Electricals Limited, Hyderabad, India for
their help and support during the experiments and suggestions in
preparation of the paper.

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Vol:5 Issue:12 December 2012 ISSN:0974-6846 Indian Journal of Science and Technology

7. Nomenclature
A: Amount of radiotracer (MBq)
t: time variable (second)
C(t): Radiotracer concentration (Counts/unit time)
E(t): Experimental RTD curve (second-1)
Em(t,τm, N): Theoretical RTD curve (second-1)
t E : Experimental mean residence time (second-1)
τm: Model predicted mean residence time (Min)
tT : Theoretical mean residence time (second-1)
T: Temperature (0C)
M: Weight of bed material (kg)
N: Number of tanks
N: Data points
V: Volume of tank (m3)
Vf: volume of fractional tank (m3)
x: Fraction of volume (m3)
Q: Flow rate (kg/hr)
QFeed : Feed flow rate of coal particles (kg/hr)
QExtraction: Extraction flow rate of coal particles (kg/hr)
Q Air : Flow rate of air (kg/hr)
QSteam: Flow rate of air steam (kg/hr)
HBed: Bed height (m)
WBed : Bed weight (kg)

8. Greek Symbols
τm: Model predicted mean residence time (Min)
Γ : Gamma function

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