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CHAPTER-III

EXPERIMENTAL PLAN

94

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

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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.

108

Figure 3.3: Fluidized Bed Gasification Pilot Plant

109

Figure 3.4: Flow Diagram of Fluidized Bed Gasification Pilot Plant

110

Figure 3.5: SCADA Diagrams of Fluidized Bed Gasification Pilot Plant

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.

115

Figure 3.6: Graphical representation of operating parameters for a typical experiment

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.