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3.1 Experimental Plan:
To achieve the objectives mentioned in the last chapter, the experiments were designed
to study the following characteristics:
To study the effect of coal properties of non-coking coals on the gasification
performance. For this purpose, coal samples of different ash levels have been
selected from four different areas of India viz. Coal-1 from North Karanpura
(CCL), Coal-2 from Tamnar, Chattisgarh (SECL), Coal-3 from Talcher (MCL),
and Coal-4 from Rajmahal (ECL) having ash content 27.0, 36.0, 41.3, and 48.9
% respectively.
The second series of experiments were planned to assess the effect of char
preparation temperature and gasification temperature on the gasification kinetics
of different coals. To carry out the studies according to above planning,
experiments were designed with varying one parameter at a time keeping the
other parameters constant.
A third group of experiments was planned to explore the gasification behavior of
coal in a Fluidized Bed Gasification (FBG) Pilot Plant.
The details about characterization of selected coals, including proximate, ultimate
analysis, determination of the surface properties, kinetic studies in thermogravimetric
analyzer and experimentation in the fluidized bed gasification pilot plant are discussed
in subsequent paragraphs.
3.2 Characterization of coal:
Around four tons of ROM coal was collected from each coal field and transported to
experimentation site by truck. The coal samples have been reduced in size manually and
then crushed in jaw crusher and reduced to 25 mm. Then, it crushed in double roller
crusher and screened to get -2 mm size. Further, representative sample from each coal
95
has been taken and further crushed to desired size for coal characterization (physical
and chemical) and to conduct thermo-gravimetric analysis. For FBG experimentation,
the requisite amount of coal about 100 kg having -2 mm size has been prepared as
discussed above for each experiment.
3.3 Chemical properties of coal samples
Basic coal properties like proximate, ultimate analyses, calorific value, ash analysis,
coking properties and grindability study have been carried out. Proximate and ultimate
analysis are carried out following Indian standards viz. IS: 1350 (Part-I) 1984, IS: 1350
(Part-III) 1969, IS: 1350 (Part-IV/Sec-1) 1974, IS: 1350 (Part-IV/Sec-2) 1975.
Considering fluidized bed gasification, ash analysis, fusion properties, coking properties
and hard grove index are also carried out. These properties of coal are shown in the
Table-3.1 to 3.6.
Table 3.1:
Proximate Analysis of Feed Coal Samples (air dried basis)
Coal Ash
%
Moisture, M
%
Volatile
Matter, VM
%
Fixed Carbon
FC
%
COAL-1 27.0 9.7 25.7 37.6
COAL-2 36.0 8.1 20.7 35.2
COAL-3 41.3 6.5 24.5 27.7
COAL-4 48.9 7.1 20.4 23.6
96
Table-3.2
Ultimate Analysis of Feed Coal Samples (air dried basis)
Coal C
%
H
%
N
%
S
%
O*
%
COAL-1
48.46 3.44 1.03 0.60 7.07
COAL-2
43.51 3.03 0.98 0.51 4.27
COAL-3 37.15 2.83 0.86 0.55 6.68
COAL-4
30.82 1.90 0.60 0.24 5.55
*By difference
97
Table-3.3
Calorific Value of Feed Coal Samples
Coal
Calorific Value
GCV
Kcal/kg
COAL-1
4530
COAL-2
3960
COAL-3
3520
COAL-4
2670
98
Table-3.4
Ash Analysis of Coal Samples
Coal SiO2
%
Al2O3
%
Fe2O3
%
TiO2
%
P2O5
%
SO3
%
CaO
%
MgO
%
Na2O
%
K2O
%
COAL-1
61.68 28.32 4.39 1.20 0.19 0.48 2.06 0.78 0.18 0.71
COAL-2 57.14 30.14 5.59 1.36 0.21 0.71 3.04 0.94 0.17 0.69
COAL-3
66.6 25.0 2.4 1.4 1.0 0.2 1.8 0.9 0.3 1.7
COAL-4
62.28 27.56 4.79 1.28 0.17 0.54 1.85 0.68 0.17 0.66
99
Table-3.5:
FSI, LTGK, Coke Type and HGI of Feed Coal
Coal FSI LTGK Coke Type HGI
COAL-1
0 A Pulverant mass and
perfect non caking 61
COAL-2 0 A Pulverant mass and
perfect non caking 61
COAL-3
0 A Pulverant mass and
perfect non caking 73
COAL-4
0 A Pulverant mass and
perfect non caking 136
100
Table 3.6
Ash Fusion Properties
Sample
Initial Deformation
Temperature
(IDT), oC
Hemispherical
Temperature
(HT), oC
Flow Temperature
(FT), oC
Coal-1 1220 >1400 >1400
Coal-2 1330 >1400 >1400
Coal-3 1280 >1400 >1400
Coal-4 1260 >1400 >1400
101
3.4 Thermo-gravimetric analysis:
For better understanding of the overall coal gasification phenomenon, kinetic studies or
knowledge of char gasification reactivity for char gasification are important and
necessary. The simple laboratory technique is used to determine its gasification rate at
different temperatures for each coal sample using thermo-gravimetric analyzer. Further
generated data are analyzed as discussed in result and discussion section to determine
gasification rate constant, activation energy, reactivity index.
3.4.1 Thermo-gravimetric analyzer:
Gasification consists of two steps, i.e. pyrolysis and char gasification [188]. In the
present study, both pyrolysis and char gasification were carried out by a Thermo-
gravimetric Analyzer (Figure 3.1) For the present study, NETZSCH, Germany make
Thermo-gravimetric analyzer (Model-STA 449 F3 Jupiter) was used. The TGA system
is based on a sensitive microbalance. The furnace used in the system is made up of
Silicon carbide (SiC). The high quality SiC heating elements of the furnace allow a
maximum furnace temperature of 1550oC. However, in the case of analysis above
1400oC there can happen an increased wear of the upper seal of the protective tube. The
protective tube is made up of Al2O3. The maximum heating rate in the temperature
range to 1200oC is 50K/min for this furnace. A diagram of the TGA is presented in
Figure 3.1. The sample holder is made up of Al2O3. The holder also acted as a heat
shield to protect the sensitive microbalance region from radiant heat emanating from the
reaction zone.
The temperature of the sample was measured using S-type thermocouple situated at few
millimeters below the sample holder when positioned in the reaction zone. The
measurement system is thermally stabilized via water cooling. This eliminates any
possible effect on the measurement resulting from furnace heat or fluctuating ambient
temperatures. The water supply is given to the furnace by means of a hose. The flow
rate of the cooling water was controlled via the cooling water control switch. The water
flow is controlled at the outlet in order to detect leaks in the cooling water circuit and
102
prevent damage to the furnace. Measurement can be carried out in an atmospheric
condition or under vacuum, since all the connections have a vacuum tight design.
The STA 449 F3 can analyze samples with a total weight capacity of up to 35g and a
volume of 5 ml. The extreme high resolution (0.025 µg) of the balance spans the entire
measuring range. The system has three magnetic valves and gas ports for gas inlet into
the reaction tube. Argon was used as a purge gas 1 with a flow rate of 50 ml/min. CO2
was used as purge gas 2 with 50 ml/min and argon was used as a protective gas with 20
ml/min flow rate. High purity (99.999%) of Ar and CO2 gas were used in this
instrument.
Figure 3.1: Thermo-gravimetric Analyzer
103
3.4.2 Thermo-gravimetric analysis experiments
Two sets of thermo-gravimetric experiments have been carried out for the present study:
Char preparation of coal samples under inert conditions at three different
temperatures of 800, 900 and 1000oC.
Isothermal gasification of each prepared char sample with CO2 at four
different temperatures of 900, 950,1000 and 1050oC.
3.4.2.1 Char preparation:
For carrying out isothermal coal gasification experiments conversion of coal samples to
char is essential. So, char preparation was carried out by taking around 500 mg raw coal
sample (size 72 mesh) in alumina crucible sample holder, three experiments were
performed at three different temperatures 800, 900 and 1000oC respectively for each
type of a coal sample under inert conditions, to maintain inert conditions the
experiments were performed in argon (Ar) flow. The argon flow rate was fixed at 50
cc/min. Further, the sample was heated in an inert atmosphere up to the desired
temperature at the heating rate of 10oC/min. Then the desired temperature was
maintained for 30 minutes in each case and then allowed to cool down up to the room
temperature. Char preparation was carried at different temperatures to study the effect
of char structure on the gasification reaction. It should be mentioned here, that ultra pure
argon (purity of 99.999%) was used for char preparation.
3.4.2.2 CO2-Gasification experiment in TGA:
Further, chars samples prepared at three different temperatures from each coal using
TGA as discussed above were used to conduct char-CO2 gasification kinetics.
Char gasification was carried out by using ultra pure CO2 (purity of 99.999%) as
gasifying agent following thermo-gravimetric process. 50 mg of char sample was taken
for each experiment in the alumina crucible. The flow rate of the air was fixed at 50
104
cc/min. Char gasification was carried out at 900, 950, 1000 and 1050oC. Char sample
was heated from room temperature in argon atmosphere with a constant heating rate of
10oC/min up to the desired temperature. Further, process gas was switched over to ultra
pure CO2 and then thermo gravimetric analysis was performed at that temperature for
90 minutes.
Before conducting actual experiments, the thermo-gravimetric analyzer was calibrated
and repeatability of the instrument was checked by several experiments taking calcium
oxalate as standard sample [189]. Blank run was carried out under the same
experimental conditions and to minimize the Buoyancy effect each experiment was
corrected with the blank run. S-type thermocouple was used to measure the reaction
temperature with an accuracy level of ± 2oC. Thus, weight loss with time data is
recorded with the thermo balance for each of the char sample gasification experiments
performed using TGA.
It must be emphasized that in all the experiments the particle size was kept around 72
mesh, the amount of sample loaded was kept very small (generally around 50 mg) and
particles were evenly spread on the sample holder. These precautions were necessary to
avoid internal gradients of heat and gas concentration and also to avoid problems of
particle overheating and ignition.
3.5 Surface area determination:
The specific surface area of the coal sample was measured using Tristar 3000 surface
area analyzer (Make: Micromeritics, USA), which is an automated gas adsorption
analyzer system which contains three ports, allowing to analyze up to three coal sample
simultaneously. The TriStar 3000 system is shown in Figure 3.2 and consists of the
TriStar analyzer, smart prep degasser for preparing samples, vacuum pump and control
module for entering analysis and report option. The surface area of the samples was
determined by adsorption of CO2 gas at 0oC using D-R equation. It may be mentioned
here that the surface area of coal measured by CO2 is always greater than that of coal
determined by N2 adsorption. It is due to the activated diffusion phenomenon, at such a
105
low temperature of -196oC, N2 can not access all the micropores. Surface area
determined by CO2 is considered as a micropore surface area.
The specific surface area was determined from adsorption of CO2 onto the sample
surface in a ice bath to maintain 0oC. Around 0.1gm of coal sample was taken in a
sample tube for surface area analysis. Coal size is kept as -6 +14 mesh. Before
conducting experiments, all the samples were degassed for 3 hrs at 150oC in the
degassing unit which removes adsorbed contaminants from the surface and pores of coal
sample in preparation for analysis. Then the sample tube is fitted in the instrument and
experimental analysis was performed. The specific surface area was calculated from
adsorption of CO2 in the relative pressure range 0.01 to 0.001 bar and results are shown
in Table-3.7.
The Dubinin-Radushkevich (D-R) equation is as follows
Log (V) = Log (V0) – (B x T2) /β x [log P0/P]
2 (3.1)
Where,
V - Volume adsorbed at equilibrium pressure (cm3/g STP)
V0 - The micropore capacity (cm3/g STP)
P0 - Saturation vapor pressure of gas at temperature T (mm Hg)
P - Equilibrium pressure (mm Hg)
B - Universal gas constant
β - The affinity coefficient of analysis gas relative to P0 gas (for this application
β is taken to be 1)
T - Analysis bath temperature (K)
106
Figure 3.2: TriStar-3000 Surface Area Analyzer
Table-3.7
Surface Properties of Feed Coal Samples
Coal Specific Surface Area (by CO2)
(m2/g)
COAL-1
115.2
COAL-2
108.7
COAL-3
103.6
COAL-4
86.3
107
3.6 Gasification Experiments in Fluidized Bed Gasification Pilot Plant
To utilize and validate the TGA data in actual gasifier, gasification study of same coal
samples was carried out in FBG. An air-blown Fluidized Bed Gasification (FBG) pilot
Plant (Figure 3.3) has been used to carry out the study of gasification performance.
These studies have been integrated with the chemical and physical properties of coal,
lab scale kinetic and surface structure studies of coals to study gasification performance
of coal. The major equipments of the plant have been erected on skid structure which
accommodates all the major equipment related to the gasifier, coal feeding system and
gas cleaning systems. Utilities such as coal crushing and storage, water cooling and
cleaning and flare stack has been installed at nearby area.
3.6.1 Fluidized Bed Gasification Pilot Plant Details:
The Fluidized Bed Gasifier (FBG) pilot plant used for present study has capacity around
10 - 20 kg/h coal at a gauge pressure of 3 kg/cm2 and at a temperature of up to 1000
oC.
The gasifier plant consists of the following major sub-systems:
a. Reactor
b. Coal Feeding System
c. Gaseous Reactant Supply System
d. Bottom Ash Extraction System
e. Cyclone with Ash Collection System
f. Gas Cooling and Cleaning System
g. Exhaust System and Flare Stack
Flow Diagram and SCADA Diagram of the FBG Plant have been shown in Figure 3.4
and Figure 3.5 respectively. Off-site equipments have been used to supply air, steam
and cooling, circulating and quenching water to the process. The photographs of FBG
test facilities and off site equipment have been shown on the next page. The different
modules of the FBG test facility used for this study are described below.
111
3.6.1.1 Reactor:
The reactor used have two diameter cylindrical vessel with three zone external electric
heating system with operating pressure of 3 kg/cm2 and at a temperature up to 1000
oC.
The bottom portion of the gasifier is known as Bed Section that has an I.D. of 100 mm
and heated up to 1000 oC. The air and steam mixture is introduced as small jets through
a distributor. The height of the reactor is 5 m including the air-steam distributor.
3.6.1.2 Coal Feeding System:
Coal has been fed into the Reactor through coal feeding system. Coal feeding system
has hopper and two locks. Coal feeding system is provided with panel controlled loss in
weight weighing system and capable to operate under pressure up to 4 kg/cm2. Feeders
control coal feed rate. Coal is pneumatically transported inside the gasifier by a single
pipe either using air.
3.6.1.3 Gaseous Reactant Supply System:
The pressure of Air Header is maintained about 0.5 kg/cm2 higher than that of gasifier
by a pressure control valve between the air supply compressor and air header. From air
header, air is supplied to coal and sorbent locks for pressurization, to solid transport line
for conveying coal and to the air heater for pre heating it prior to introducing it in an air-
steam mixing vessel. The electrically heated in-line air heater preheats air to the
required mixture temperature. Superheated steam is generated in a steam generator. This
air and steam mixture is used for the bed fluidization purpose.
3.6.1.4 Bottom Ash Extraction System:
The ash cooler at the bottom of the reactor is a jacketed pipe. Soft water coming from
the spray tower tank flows through the jacket for cooling the hot ash. A rotary ash
extractor located below the ash cooler extracts the ash from the reactor at a specified
rate for maintaining the bed level.
112
3.6.1.5 Gas Cooling, Cleaning and Sampling system:
During plant operation, the raw gas from the freeboard along with elutriated fines enters
cyclone. Raw gas enters from the side and exits from the top of the cyclone. The raw
gas from cyclone separator enters at the top of quench pipe and the cooled gas along
with water exits from the bottom of the seal pot. The gas cleaning section consists of a
venturi scrubber integrated with gas liquid separation vessel for cleaning the dusty gas
and a knock out drum/mist eliminator to remove moisture from the clean gas. Water
from water tank is drawn to spray in the quench pipe and scrubbers. The dust-laden
water from these equipments is discharged into the settling tank through bottom drain
pipes. The system for adding soda ash solution together with soft water in the venturi
scrubbers to trap hydrogen sulfide from the dusty fuel gas is also provided.
The clean gas coming out of the knockout drum can be sampled for gas analysis through
the sample collection station. A part of clean gas under pressure from the knockout
drum is passed through the pressure regulation valve and water sealed flare stack before
venting it to the atmosphere.
3.6.2 Experimental Procedure:
Gasification experiments with four selected coal samples were conducted at different
operating conditions. Coal at the desired mass flow rate has been fed continuously from
the coal lock to the reactor through a coal feeder and a pneumatic conveying system.
Gasifier temperature was raised by the external electric heating system operated from
the control panel. Preheated air (up to 180 oC) and superheated steam (above 50-degree
superheat) were mixed using an air-steam mixer and admitted to the reactor through the
conical distributor.
Ash in bed was extracted under controlled rate and cooled to about 400oC prior to
discharging in Ash Bin. Ash Locks sequence of operation was similar to those of Coal
Locks where Ash Lock-1 remain pressurized all the time and Ash Lock II undergoes
pressurization and depressurization.
113
The hot dusty raw fuel gases leave gasifier from Freeboard section and enter in the
cyclone where most of the elutriated particles get captured. The interconnecting pipe
between reactor and cyclone has been insulated. The captured particles were discharged
in Ash Bin from Cyclone Ash Lock-2 through a sequence of depressurization and
pressurization.
The fuel gas from the cyclone enters into the Quench Column. The interconnecting pipe
between cyclone and the quench column has been insulated. The water from the Settler
Tank after treatment in Water Softening Plant was directly sprayed onto the gas to
reduce the temperature by Spray nozzles. The Quench Column has been water jacketed
and cold water was circulated in a closed loop, drawing water from Cooling Tower. The
cooled gas along with sprayed water get settled in the Seal Pot situated at the bottom of
the quench pipe. The level of water in Seal Pot has been maintained either by
controlling the rate of water discharge to Settler Tank or by increasing the inlet water
flow rate. The cooled gas exits from the top side of the seal pot and passes into a
venturi scrubber where particulates have been further cleaned from the cold clean gas.
The clean gas from the Knockout Drum has been transferred through System Pressure
Control Valve and water sealed Flare Stack before it has been flared.
3.6.3 Fluidized Bed Gasification Pilot Plant Process Parameters:
During gasifier operation, the major parameters controlled were bed temperature,
operating velocity, and system pressure and bed height. Bed temperature control was the
major feedback for control loops. Essentially, controlling the coal feed rate controls the
bed temperature. Assuming the same quality of coal was used for the operation, the bed
temperature could vary due to the variation of air to coal mass ratio, changes in bed
height and air to steam ratio for a set operating condition. For a set pressure operation,
changing the air mass flow rate was limited since the operating velocity has to be kept
within a narrow range. The bed height was sensed by the pressure drop across the bed
and controlled by adjusting the rate of ash extraction from the bottom of the gasifier.
114
Thus, following the experimental procedure and control philosophy as discussed above,
experiments were conducted in Fluidized bed gasification pilot plant with four coals
from different coal fields of India. The results from these experiments are thoroughly
discussed in the Result and Discussion Section. The variation of process parameters in
real time is systematically shown in the Figure 3.6 in graphical form for a typical
experiment. However, process parameters for all the experiments are depicted in Table-
4.4 in Chapter-4 (Result and Discussion) for the sake of convenience to discuss and
compare gasification performance parameters with kinetic results.
116
3.7 Product Gas Analysis:
The product gas from fluidized bed gasification pilot plant is collected by water
displacement method and it is analyzed through a gas chromatograph [Model: GC 1000,
Make: Chemito, India] (Figure 3.7) to find out gas composition. The procedure for gas
sample collection and analysis is discussed below. The GC is equipped with three
sample ports, one FID detector, two TCD detectors [TCD (I) and TCD (II)] and
methanizer.
The first gas sample was injected into gas chromatopraph through automatic sampler
valve B to analyze CO, CO2 and CH4. N2 was used as carrier gas during this analysis.
CO, CO2 and CH4 were analyzed by the combination of methanizer and FID passing gas
through the spherocarb column. Then H2 and O2 were analyzed injecting a sample
through the automatic sampler valve B1 using TCDI. At this time, column used were
Porapak Q and Molecular sieve SA and N2 was used as a carrier gas. N2 was analyzed
injecting a sample through automatic sampler valve B2 and using the TCD II detector.
At this time, columns used were porapak Q and Spherocarb. It should be noted that
with this port other gases such as CO, CH4 and CO2 also could be detected. At this time,
H2 was used as a carrier gas.
During all the experiment, oven temperature, injector temperature and detector
temperature were maintained at 220oC,120
oC and 150
oC. Temperature of methanizer
was maintained at 300oC. Before measurement of unknown samples GC was calibrated
with a sample of known concentration. The observed gas composition is depicted in the
next chapter.
117
Figure 3.7: Gas Chromatograph
Thus, following the experimental procedure and control philosophy as discussed above,
four coal samples from different coal fields of India were analyzed for its various
physical and chemical properties. Experiments were conducted in Thermo-gravimetric
analyzer to study CO2 gasification rate and data generated is analyzed to determine
char-CO2 gasification performance parameters such as rate constants, activation energy
and reactivity index in the next chapter. Further, experimental data from Fluidized bed
gasification pilot plant was analyzed to study gasification performance at different
operating conditions. Further, char-CO2 gasification reactivity and kinetics parameters
and FBG gasification performance parameters are compared and also thoroughly
discussed in the next chapter.