CHAPTER-III EXPERIMENTAL PLAN

Transcription

1 93 CHAPTER-III EXPERIMENTAL PLAN

2 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

3 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) 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) Volatile Ash Moisture, M Coal Matter, VM Fixed Carbon FC COAL COAL COAL COAL

4 96 Table-3.2 Ultimate Analysis of Feed Coal Samples (air dried basis) Coal C H N S O* COAL COAL COAL COAL *By difference

5 97 Table-3.3 Calorific Value of Feed Coal Samples Coal Calorific Value GCV Kcal/kg COAL COAL COAL COAL

6 98 Table-3.4 Ash Analysis of Coal Samples Coal SiO 2 Al 2 O 3 Fe 2 O 3 TiO 2 P 2 O 5 SO 3 CaO MgO Na 2 O K 2 O COAL COAL COAL COAL

7 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

8 100 Table 3.6 Ash Fusion Properties Sample Initial Deformation Temperature (IDT), o C Hemispherical Temperature (HT), o C Flow Temperature (FT), o C Coal >1400 >1400 Coal >1400 >1400 Coal >1400 >1400 Coal >1400 >1400

9 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 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 Thermogravimetric 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 1550 o C. However, in the case of analysis above 1400 o C there can happen an increased wear of the upper seal of the protective tube. The protective tube is made up of Al 2 O 3. The maximum heating rate in the temperature range to 1200 o C is 50K/min for this furnace. A diagram of the TGA is presented in Figure 3.1. The sample holder is made up of Al 2 O 3. 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

10 102 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. CO 2 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 CO 2 gas were used in this instrument. Figure 3.1: Thermo-gravimetric Analyzer

11 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 1000 o C. Isothermal gasification of each prepared char sample with CO 2 at four different temperatures of 900, 950,1000 and 1050 o C 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 1000 o C 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 10 o C/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 ) was used for char preparation CO 2 -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-co 2 gasification kinetics. Char gasification was carried out by using ultra pure CO 2 (purity of ) 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

12 104 cc/min. Char gasification was carried out at 900, 950, 1000 and 1050 o C. Char sample was heated from room temperature in argon atmosphere with a constant heating rate of 10 o C/min up to the desired temperature. Further, process gas was switched over to ultra pure CO 2 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 ± 2 o C. 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 CO 2 gas at 0 o C using D-R equation. It may be mentioned here that the surface area of coal measured by CO 2 is always greater than that of coal determined by N 2 adsorption. It is due to the activated diffusion phenomenon, at such a

13 105 low temperature of -196 o C, N 2 can not access all the micropores. Surface area determined by CO 2 is considered as a micropore surface area. The specific surface area was determined from adsorption of CO 2 onto the sample surface in a ice bath to maintain 0 o C. Around 0.1gm of coal sample was taken in a sample tube for surface area analysis. Coal size is kept as mesh. Before conducting experiments, all the samples were degassed for 3 hrs at 150 o C 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 CO 2 in the relative pressure range 0.01 to bar and results are shown in Table-3.7. The Dubinin-Radushkevich (D-R) equation is as follows Log (V) = Log (V 0 ) (B x T 2 ) /β x [log P 0 /P] 2 (3.1) Where, V - Volume adsorbed at equilibrium pressure (cm 3 /g STP) V 0 - The micropore capacity (cm 3 /g STP) P 0 - 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 P 0 gas (for this application β is taken to be 1) T - Analysis bath temperature (K)

14 106 Figure 3.2: TriStar-3000 Surface Area Analyzer Table-3.7 Surface Properties of Feed Coal Samples Coal Specific Surface Area (by CO 2 ) (m 2 /g) COAL COAL COAL COAL

15 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 Fluidized Bed Gasification Pilot Plant Details: The Fluidized Bed Gasifier (FBG) pilot plant used for present study has capacity around kg/h coal at a gauge pressure of 3 kg/cm 2 and at a temperature of up to 1000 o C. 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.

16 Figure 3.3: Fluidized Bed Gasification Pilot Plant 108

17 Figure 3.4: Flow Diagram of Fluidized Bed Gasification Pilot Plant 109

18 Figure 3.5: SCADA Diagrams of Fluidized Bed Gasification Pilot Plant 110

19 Reactor: The reactor used have two diameter cylindrical vessel with three zone external electric heating system with operating pressure of 3 kg/cm 2 and at a temperature up to 1000 o C. The bottom portion of the gasifier is known as Bed Section that has an I.D. of 100 mm and heated up to 1000 o C. 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 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/cm 2. Feeders control coal feed rate. Coal is pneumatically transported inside the gasifier by a single pipe either using air Gaseous Reactant Supply System: The pressure of Air Header is maintained about 0.5 kg/cm 2 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 airsteam 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 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.

20 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 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 o C) 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 400 o C 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.

21 113 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 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.

22 114 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.

23 Figure 3.6: Graphical representation of operating parameters for a typical experiment 115

24 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, CO 2 and CH 4. N 2 was used as carrier gas during this analysis. CO, CO 2 and CH 4 were analyzed by the combination of methanizer and FID passing gas through the spherocarb column. Then H 2 and O 2 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 N 2 was used as a carrier gas. N 2 was analyzed injecting a sample through automatic sampler valve B 2 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, CH 4 and CO 2 also could be detected. At this time, H 2 was used as a carrier gas. During all the experiment, oven temperature, injector temperature and detector temperature were maintained at 220 o C,120 o C and 150 o C. Temperature of methanizer was maintained at 300 o C. Before measurement of unknown samples GC was calibrated with a sample of known concentration. The observed gas composition is depicted in the next chapter.

25 117 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 CO 2 gasification rate and data generated is analyzed to determine char-co 2 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-co 2 gasification reactivity and kinetics parameters and FBG gasification performance parameters are compared and also thoroughly discussed in the next chapter.