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Agglomeration behaviour of high ash Indian coals in fluidized bed gasification pilot plant
Accepted Manuscript Agglomeration behaviour of high ash Indian coals in fluidized bed gasification pilot plant S. Datta, P. Sarkar, P.D. Chavan, S. Saha, G. Sahu, A.K. Sinha, V.K. Saxena PII:
S1359-4311(15)00389-0
DOI:
10.1016/j.applthermaleng.2015.04.046
Reference:
ATE 6565
To appear in:
Applied Thermal Engineering
Received Date: 19 November 2014 Revised Date:
9 April 2015
Accepted Date: 12 April 2015
Please cite this article as: S. Datta, P. Sarkar, P.D. Chavan, S. Saha, G. Sahu, A.K. Sinha, V.K. Saxena, Agglomeration behaviour of high ash Indian coals in fluidized bed gasification pilot plant, Applied Thermal Engineering (2015), doi: 10.1016/j.applthermaleng.2015.04.046. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Agglomeration behaviour of high ash Indian coals in fluidized bed gasification pilot plant a
a,
a
a
a
S. Datta , P. Sarkar *, P. D. Chavan , S. Saha , G. Sahua, A. K. Sinha , V. K.
a
b
CSIR-Central Institute of Mining and Fuel Research, Digwadih Campus, P.O.-
FRI, Dhanbad-828108, Jharkhand , India
Indian School of Mines, P.O. ISM, Dhnabad-826004, Jharkhand , India
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b
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Saxena
*Corresponding author. Tel: +91-326 2388286, +91 9431512885 (mobile), fax +91 326 2381113, Email: [email protected]
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ABSTRACT Although gasification of high ash Indian coals is gaining importance, the resultant uncertainties associated with agglomerate formation are still unresolved. To address this, Stabilized
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a suitable pilot scale Fluidized Bed Gasifier was utilized in this study.
operating conditions in terms of coal feed rate, air feed rate, bed temperature, etc., already identified for maximum possible carbon conversion, were maintained in all experiments and the steam flow rate was only varied. Though the ash fusion temperature of the coals were above 1200 oC, agglomerate was formed during gasification at 950 oC
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with ‘steam to coal ratio’ less than 0.15 (kg/kg). On increasing this ratio above 0.2 local heat-concentration and agglomeration could be avoided with certainty. Chemical
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composition alone was not sufficient to explain the relative strength of ash-agglomerates. Compositional variation and state of iron within the matrix were assessed through SEMEDX and electron paramagnetic resonance (EPR) study, respectively. The probing also required the ash-loading and iron-loading factors to be freshly defined in the context of gasification. Localized heat, large compositional variation, presence of iron in Fe2+ state, ash-loading/ iron-loading factors influenced intensity of agglomerate formation. Finally,
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diagram.
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low temperature agglomerate formation was explained by SiO2-Al2O3-FeO phase
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1. Introduction Coal is one of the most important fossil fuel, and in this century it will continue to play an important role towards the world energy supply.
Energy demand in India has
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increased many folds in present days due to fast growth of rural sector. In India, about 60% of commercial energy and about 70% electrical energy are produced from coal [1]. The major disadvantage associated with the Indian coal is its high ash content and proper clean coal technology need to be adopted for the gainful utilization of coal. Coal
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gasification offers a practical means of utilizing this huge energy resource meeting the stringent environment control requirement. Considering the high ash and high reactivity
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of Indian coals fluidized bed gasification technology seems to be promising in Indian scenario. But fluidized bed operation sometimes suffers from a major operational drawback caused by the ash agglomeration, which is very much detrimental and often leads to the shutdown of the plant. Several researchers experienced agglomeration related problems in their studies with different feed coals [2-7]. Some of these studies finally led to develop remedial strategies [8, 9]. Adjustments of operational parameters to avoid such
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development of agglomerate are very important. As such Indian coals are high in ash content. Presently mined coals do have ash content in the range of 30-50% [10]. High ash content poses different operational problems in both gasification and combustion applications. In India development of gasification technologies for high ash coals are
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very much challenging [10, 11]. Fortunately enough, the Indian coal ashes are refractory in nature and their melting zone is very high as it is indicated from ash fusion test [12].
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But it has also been experienced that ash fusion test mostly fails to assess ash melting behavior in several combustors/ gasifiers. As a matter of fact, AFT was originally meant for assessing ash melting behavior in stoker fired furnace [13]. In different gasifiers the formation of agglomerates often found at a temperature lower than that is expected from AFT [3, 14]. It depends on mechanical design of the gasifier and more importantly on the factors like operating conditions, residence time, hydrodynamics, particle size, localized heat concentration, inhomogeinity of chemical constituents, etc. [15]. The present work explores the possibility of developing fluidized bed gasification technology suitable for high ash Indian coals. A fluidized bed gasification pilot plant has
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been designed and installed at CSIR- Central institute of Mining and Fuel Research to cater the present need of technology development with high ash coals. The activity was also included under clean coal technology development programme under 11th Five year plan, Govt. of India.
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This paper presents gasification studies with high ash Indian coals which are very limited in literature. A unique gasification pilot plant specially designed for high ash coals was utilized in this study. This study depicts some interesting findings on gasification of high ash coals as well as on agglomerate formation. The agglomerates formed with three
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different coals under different conditions were examined in depth. This paper describes the operational adjustments which were adopted to avoid liquid phase sintering/
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agglomeration. This paper also attempts to understand the main reason of agglomerate formation through chemical analysis, FE-SEM with EDX, SiO2-Al2O3-FeO phase diagram and using electron paramagnetic resonance (EPR). Use of EPR to probe into the cause of agglomerate formation is basically a new approach. Moreover, ash loading factor and iron loading factor have been reconstituted for its application to gasification systems and importance of these factors have also been discussed to address the problem
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of agglomerate formation.
2. Experimental methods
2.1. Coal selection and characterization
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Non coking coal samples from three different coal fields have been selected to carry out the experiments. The Proximate and ultimate analysis of samples were done using the
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following standard procedures i.e., IS: 1350 e Part-I: 1984, Part III: 1969, Part IV/1: 1974, Part IV/2: 1975 and ASTM E871, D1102, E872. The gross calorific values (GCV) of the coal samples were determined using the standard method IS 1350 (Part 2): 1970. Chemical composition of the ash was determined as per standard procedure (IS: 1355:1984). For gasification studies the coal samples were crushed to -3 mm. Then the samples were sieved to collect the fraction having size in the range of -3mm to +0.5 mm and the collected fraction was used in gasification experiments. Initial deformation temperature (IDT) of ash samples of all the three selected coals was determined using Indian standard (IS: 12891-1990).
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2.2. Gasification studies in Fluidized Bed Gasifier Gasification experiments were conducted in the existing fluidized bed gasification system having capacity of 20 kg/h coal feed rate at CIMFR, Dhanbad. Gasifier is made of high temperature alloy suitable to operate up to 1000 oC. Height of the gasifier is ~5 m.
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Schematic flow diagram and photograph of this gasifier have been shown in Fig 1 and 2 respectively. The pilot plant consists of several sections, viz., gasifier, coal feeding system, gaseous reactant supply system, bottom ash extraction system, cyclone with fly ash collection system, gas cooling-cum-cleaning system and flare stack.
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The gasifier is a cylindrical vessels fitted with three zone external electrical heating systems which allow easy and precise control of temperature. The bottom portion of the
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gasifier is known as bed section. The top section of the gasifier is known as freeboard section. The section at the bottom end of the gasifier is known as air plenum, which is integrated with a distributor plate and this portion is properly insulated to avoid any heat loss during the process. The reactant, air and steam mixture is introduced as small jets through a distributor plate. The distributor plate is removable and cleaned after few experiments. The gasifier is provided with adequate nozzles for measurement of pressure,
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differential pressure, and temperature. Inside temperature of the gasifier is measured at different levels using shielded K-type thermocouple and system pressure at different locations are measured with pressure transducers. Initially the bed of the gasifier was buildup by the coal ash. As usual practice, during the start up of the plant the set of
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external heaters were put on to raise the temperature of the gasifier at a rate of 20 oC/min up to 550 oC. At this stage, coal feeding was started in the gasifier and the coal particles caught fire. Due to coal combustion, the temperature of the gasifier further raised up to
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900 oC. At this stage, the external heaters were put off. Controlling of the temperature in the bed section of the gasifier is very crucial, which was maintained by controlling the air and steam flow rate. Further, the temperature of the gasifier was raised to the desired level very slowly to conduct the gasification reaction. After achieving the stable condition, gas samples were collected for subsequent analysis. In the experimental runs, the coal feed rate was controlled with a rotary feeder. Conveying air was introduced in the feed line to ensure unrestricted flow of feed particles
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through the feed line. Bottom ash from the system was extracted with a rotary ash extractor after cooling. 2.3. Agglomerate characterization The Field Emission Scanning Electron Microscope (FESEM) system was used for study
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of topographical microstructure of the agglomerates. The chemical analysis at several positions within the agglomerate matrix was estimated by energy dispersive X-ray spectrometer (OXFORD Instrument INCA PENTA FETX3) attached with the FESEM system (JEOL JSM 7600 F FE SEM). Initially the small pieces of agglomerate samples
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were oven dried at 110 oC for two hours to remove the traces of adsorbed moisture. Then the samples were preserved in desiccators to prevent it from re-adsorption of moisture. Small pieces of the dry samples were placed on a specific stub and then gold coating was
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done in a sputter coater at 10-2 Pa pressure. After gold coating, the stubs were placed on the sample stub holder, which was then introduced in the load lock chamber and finally fed to the specimen chamber. Then vacuum of the system was brought to 10-5 to 10-8 Pa level. The electron beam was focused over the samples after adjusting the parameters for imaging. Images were captured at different magnification with best possible resolution.
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Electron Paramagnetic Resonance (EPR) spectra was recorded with X –band Jeol (JES FA 200) spectrophotometer. Powdered agglomerate sample of around 5 mg was taken in an EPR sample tube. It was placed in the sample cavity and kept under constant nitrogen flow to prevent moisture deposition on the sample tube. Then the sample was aligned
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with the magnetic field and EPR data were captured. The same agglomerate samples were digested with dilute hydrochloric acid and
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was kept under warm condition for one hour on a hot plate. Air was blown continuously during the process of digestion to ascertain oxidation from ferrous to ferric state. The digested mass was cooled and filtered. About 2 – 3 ml of filtrate was taken in an iron free quartz sample tube and frozen in liquid nitrogen atmosphere. Then it was placed in the sample cavity of the EPR spectrophotometer. The sample tube was kept under constant nitrogen gas flow and aligned with the magnetic field as before. The data were captured for the liquid sample.
3. Results & discussion
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Table 1 shows the proximate, ultimate analysis and GCV of the feed samples. Low rank coals are selected considering the ash content in a range between approximately 27 to 49% from different active mines. From Table 2, it is clear that the silica and alumina are the major components of ash
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matrix and which is ~90% of the total mass and rest part is composed of some iron, calcium, sodium and other trace metals. The ash fusion temperature (IDT) of the coal samples are found to vary in the range of 1250- 1290 oC as shown in Table 3. Although ash fusion temperature is an important property which is utilized to assess proneness of
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formation agglomerate in combustion / gasification [16], the IDT often fails to directly assess the tendency to form agglomerates. That is why correlation of AFT results with
agglomerate formation [13].
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field experience is very much essential to enhance reliability for prediction of
Again proper correlation is difficult to establish because of variation of operating condition as well as time temperature schedule that those particles in the reactor experience. Variations of operating parameters, which are very much important to control agglomerate formation, have been reflected in Table 4. For entire set of experiments all
(kg/kg) and
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‘coal feed rate’ , ‘air/coal ratio’ and ‘bed temperature’ were fixed at 15 kg/h, 1.5 950 oC (± 10 oC) respectively. The air/coal ratio was fixed for all the
experiments but the Equivalent Ratio(ER) were 0.30 for coal A, 0.37 for coal B and 0.23 for coal C. In our previous study [17] with high ash Indian coals carbon conversion was
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recorded to be ~ 90% under similar experimental conditions as mentioned above. In the present study the only variable is steam flow rate, reflection of which may be observed
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from steam/coal ratio values in Table 4. Agglomerate formation is observed for all the three coal samples when steam/coal ratio is varied in the range of 0.05 and 0.15 but, when steam /coal ratio was raised above 0.2 smooth trouble free operation without agglomerate formation was noticed. The syn gas compositions for the conducted experiments are shown in Table-5, where no ash agglomeration eventually took place. With increased steam/coal ratio concentration of CO was found to be slightly decreased while that of H2 was found to be little bit increased. Now at the outset of discussing the agglomerate formation issues it may be noted that the experiments in fluidized bed gasifier were conducted at a temperature much below the IDT of coal ash. Hence smooth
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trouble free operations in terms of agglomeration formation were initially expected. But, ash agglomeration was repeatedly noticed in different intensity, which could be avoided by raising steam/coal ratio. Sequential observations with gradual increase in steam /coal ratio have been narrated in Table 4.
Fig. 3 shows the photographs of agglomerate
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formed at 0.05 steam /coal ratio with the three coal samples. As it appears from the nature of the agglomerates on physical and visual examination, strongest and weakest particleparticle interlocking were observed in case of coal B and C respectively, whereas the strength of particle-particle interlocking being intermediate in case of coal A. The
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highest agglomerate strength has also been accompanied by largest lump size in case of coal B (Fig.3a). Lump size in case of coal C is smallest and friable (Fig 3c). Lump size in
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case of coal A is medium and relatively less friable than in the case of coal C (Fig 3c). Design of the gasifier plays a vital role towards formation of the agglomerate. The consideration of parameters like volumetric heat loading as well as distribution of the reactants in the bed, heat dissipation inside the bed, heat loss through the wall etc. are very important to minimize agglomeration problem. As a matter of fact all the above aspects were taken into account during designing of the pilot plant for gasification of high
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ash coals. Apart from design parameters, properties of coal/ ash are very much important to assess the agglomeration phenomenon.
Conventional indices based on quality
parameters of coal/ coal ash do not work well to predict agglomeration tendencies during gasification and therefore, the parameters like differential chemical composition across
be probed .
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the ash matrix, ash loading, iron loading, oxidation state of iron, spot heating etc., need to
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As in the case of slagging fouling in PF fired boiler [13], agglomerate formation in the gasifier also depends on ash loading. Like slag formation in boiler [13], agglomeration tendency in gasifier primarily depend on the ash composition and
temperatures
experienced in the bed. Liquid forming tendency of the ash often assessed through parameters like alkali index, silica percent etc., which considers average composition of ash. But ash composition is not homogeneous over the whole matrix rather it varies a lot at different locations. Therefore, in real term, liquid forming tendency depends on locational distribution of constituent oxides. More precisely, it depends on the
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distribution of the constituent mineralogical phases on which formation of low melting eutectic composition depends. The microstructure of the agglomerates which reflect heterogeneous distributions of crystalline and glassy matrix have been shown in SEM -micrographs (Figs 4. a- f).
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Grains of different size ranges and widely distributed glassy matrix are seen in all the micrographs. In coal B ash-agglomerate the maximum bonding through glassy phase was observed (Figs. 4. c, d) which resulted maximum agglomerate strength and biggest lump size (Fig. 3a). On the contrary, in the microstructure of coal C ash agglomerate (Figures
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4. e, f) bonding through glassy matrix was found to be at the minimum. As a result agglomerate bond strength was also poor and big lump could not be formed (Fig. 3c).
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The situation in case of coal A ash- agglomerate is somewhat intermediate between the above two cases with coal B and coal C. Formation of glass phase in coal A was less than that in the case of coal B and extent of crystal- glass bonding was also less (Fig.4. a, b) which led to intermediate lump size of coal A ash-agglomerate (Fig. 3b). As a case of strong agglomerate formation in coal B (Fig. 3a), EDX analysis was done only for coal B agglomerates to investigate possible extent of compositional variations at
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different locations of the agglomerate matrix. The normalised compositions in terms of conventional oxides recalculated from elemental composition (mass %, EDX analysis) have been presented in Table 6. EDX results show that there is substantial variation of chemical composition in different location of the agglomerate matrix (Table 6). In this
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case Fe2O3, CaO, MgO and TiO2 content was found to be high as 9.2%, 13.4%, 1.7% and 4.9% respectively at certain locations of agglomerate matrix. Therefore, there are ample
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possibilities of formation of low melting eutectic of different systems. At location 5 (as indicated in Table 6), high concentration of CaO may lead to formation of anorthite (Al2O3.SiO2.2CaO) which contribute a lot to form eutectic at much lower temperature [18] particularly in presence of other assisting component like phases containing Magnesium, Iron or Titanium. The most significant component for lowering of eutectic temperature is the iron bearing phases. In the gasification condition, reducing atmosphere is maintained and therefore there is ample possibility of shifting Fe3+/ Fe2+ equilibrium towards Fe2+. On the contrary, in case of pulverised coal firing systems possibility of such shifting is much less (as it is oxidising condition) and Fe 3+ state is most likely. The
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much higher residence time in fluidised bed gasifier (residence time ~15- 20 minutes) as compared to pulverised coal combustion units (residence time is in the order of few seconds), may allow the phase transition from Fe3+ to Fe2+ to a greater extent. This Fe2+ acts as much more fluxing agent than Fe3+ [19, 20]. To probe into the actual situation
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prevailed in case of agglomerate of coal B, EPR test was done with the powdered agglomerate sample of coal B as a test case. The flat EPR spectrum of powdered dry agglomerate sample (Fig. 5) reflected absence of Fe3+ state. To confirm the same, EPR study was also conducted with oxidised filtrate of the agglomerates. Sharp peak was
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resulted in EPR spectra indicating presence of Fe3+ in this case (Fig. 5). Therefore it can be confidently commented that within the agglomerate matrix iron remained as Fe2+ state
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which contributed towards lowering of eutectic temperature. From the ternary phase diagram depicting FeO-Al2O3-SiO2 system ( Fig. 6) it may be noticed that the eutectic formation may be at a temperature even as low as 1088 oC. As per the phase diagram (Fig. 6) other eutectic may be formed at a temperature of 1148 oC and also at 1177 oC. In the process of gasification the softening temperature may be decreased further due to the formation of low melting complex compound of FeO- SiO2- Al2O3- CaO as it has also
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been found to be formed in reducing atmosphere during sponge iron production [21]. Cause of agglomerate formation is better understood when ash loading factor and liquid forming tendency is assessed together. In case of gasification ash loading may be defined as ‘ash to available carbon ratio (w/w)’ in given quantity of coal. Moreover consideration
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of iron loading is sometimes more important than ash loading. This may be defined as the ratio of weight of Fe2O3 in grams and weight of carbon in kg present in a given quantity
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of coal as shown in Table 3. It is to be noted from Fig. 3 and Table 2 that strongest and biggest lump was formed in case of coal B ash although the Fe2O3 content and CaO content are more in coal A as compared to those in coal B. It may be due to the fact that ash loading and iron loading are more in the case of coal B ash than respective values of coal A ash (Table 3). Practically in respect of agglomerate and clinker formation, the ash loading [13] and iron oxide loading [22] are more important than the respective absolute value of ash content and Fe2O3 percent in coal ash. For a high value iron oxide loading, greater rise in locational concentration of iron oxide is much more likely as compared to the case of lower value of iron loading. That is why chance of formation of bigger and
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stronger agglomerate is very high with high iron loading which actually happened in case of coal B. Another important factor that plays key role towards the formation of agglomeration is the gasifier bed temperature [23]. From literature it is clear that the mineral constituents
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present in the ash matrix may develop low melting eutectics that may be responsible for the agglomeration at low temperature in the fluidized bed combustion or gasification application [24]. To maintain uniform temperature inside the bed section of the gasifier we need to ensure certain factors like uniform particle size of the feed, proper distribution
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of the fluidizing air and steam inside the bed. Smaller is the feed particle; higher is the probability of particle temperature. Feeding the gasifier with uniform particle size is a
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challenging job. Again during fluidization gas-particle and particle –particle interaction may cause creation of smaller particle in the bed. This uneven distribution of the particles may cause several hot spots during the process. Proper dissipation of this localized heat is utmost important to avoid agglomeration formation. In actual practice average temperature inside the bed is measured. But, in certain locations inside the bed particle temperature may be quite higher than that of the average bed temperature. If proper
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dissipation of the heat from these hot spots is not done properly the local temperature may cross the IDT of some particles. These particles will produce a kind of fused sticky mass and upon interaction with the other adjacent particles it will form a bigger lump. If the temperature of the hot spots cannot be reduced at this stage, this phenomenon will
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continue and ultimately agglomerate will reach to a bigger size that may cause the defluidization of the bed. It is well known that in different chemical and petrochemical
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conversion processes steam helps to minimise the coke formation tendency through dissipation of additional heat from the hot spots. In gasification also steam plays similar role to distribute localised heat minimising tendency of agglomerate formation. Therefore the heat withdrawing property of the steam was found to be very important in controlling agglomeration phenomenon and that why beyond a certain steam/ coal ratio (> 0.2), agglomeration can be avoided in the gasifier system. However, the steam to coal ratio was not increased beyond 0.3 in this study because excessive steam flow may reduce the bed temperature, affect the carbon conversion and gas yield as well as gas
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calorific value. All these findings are also likely to be observed in case of similar fluidized bed installations with high ash coals under comparable experimental conditions.
4. Conclusions
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This paper attempted to find out cause-effect relationship in respect of agglomerate formation with high ash Indian coals. It was found that under the conditions already stabilised with respect to objective parameters (i.e., Gas composition, gas yield, carbon conversion etc.), agglomeration phenomenon may be controlled through the use of
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appropriate steam/coal ratio. It may be possible to avoid agglomeration problem by the judicious control of steam flow and role of steam on overall process is also very much
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crucial. Beyond certain steam/coal ratio no agglomerate found to take place for all the three selected coals. Flow of steam in adequate quantity prevents hot spot formation inside the bed.
As a result, the particle temperature fails to attain the temperature
required for fusion of the ash particles or ash components. Ferrous state of iron and high concentration of CaO contributes to lower down the eutectic temperatures of different phase combinations. For probing the agglomerate formation, differential chemical make
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up across the ash matrix is actually more important issue than the average chemical composition determined by chemical analysis.
Ash loading and iron loading factor
aggravates agglomerate forming tendencies and these are more crucial factors as compared to ash content and iron oxide percent in ash. The liquid forming tendency, 2+
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stickiness of the liquid phase and ash/ iron loading factor, presence of iron as Fe
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increase lump size and agglomerate strength as it is happened in case of coal B.
Acknowledgements
The authors are thankful to the Director of CSIR-Central Institute of Mining and Fuel Research for giving permission to publish the paper. Authors acknowledge the financial support of SSRC (Ministry of Coal, Govt. of India) and CSIR (Ministry of Science and Technology, Govt. of India) to establish the pilot plant and to carry out the experiments. Authors sincerely acknowledge the analytical support from the University of Calcutta and Indian Association of Cultivation of Science (IACS, Kolkata) to carry out SEM- EDX analysis and EPR study respectively.
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ACCEPTED MANUSCRIPT Table 1 Proximate analysis, ultimate analysis, GCV of feed coal Coal
Proximate Analysis
Ultimate Analysis
GCV
M
VM
FC
C
H
N
S
O
kJ/kg
A
41.3
6.5
24.5
27.7
37.15
2.83
0.86
0.55
6.68
14,737
B
48.9
7.1
20.4
23.6
30.82
1.90
0.60
0.24
5.55
11,179
C
27.0
9.7
25.7
37.6
48.46
3.44
1.03
0.60
7.07
18,966
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A
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*Proximate and Ultimate analysis(wt%, air dried basis), GCV(air dried basis), A:ash, M:moisture, VM: volatile matter, FC: fixed carbon, C: carbon, H: hydrogen, N: nitrogen, S: sulphur, O: oxygen, GCV: gross calorific value
ACCEPTED MANUSCRIPT Table 2 Ash analysis of coal samples SiO2
Al2O3
Fe2O3
TiO2
P2O5
SO3
CaO
MgO
Na2O
K2O
A
57.14
30.14
5.59
1.36
0.21
0.71
3.04
0.94
0.17
0.69
B
62.28
27.56
4.79
1.28
0.17
0.54
1.85
0.68
0.17
0.66
C
61.68
28.32
4.39
1.20
0.19
0.48
2.06
0.78
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Ash analysis (wt %)
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Coal
0.18
0.71
ACCEPTED MANUSCRIPT Table 3 Ash properties IDT ( 0C)
Ash Loading kg ash /kg carbon
Iron loading gm Fe2O3/ kg of carbon
A
1250
1.11
62.1
B
1260
1.59
76.0
C
1290
0.56
24.5
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IDT: initial deformation temperature
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Coal
ACCEPTED MANUSCRIPT Table 4 Experimental details Steam: coal ratio (kg/kg) 0.05 -0.15
Gasifier Bed Temp (oC) *
Observation
15
Air :Coal ratio (kg/kg) 1.5
A
950
Agglomerate
A
15
1.5
0.2- 0.30
950
No Agglomerate
B
15
1.5
0.05 -0.15
B
15
1.5
0.2- 0.30
C
15
1.5
0.05 -0.15
C
15
1.5
0.2- 0.30
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Coal Feed (kg/h)
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*Temperature varied within +/- 10 0C
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950
Agglomerate
950
No Agglomerate
950
Agglomerate
950
No Agglomerate
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Sample
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Volume %
CO
15-20
H2
15-20
CO2
10-12
CH4
1-2
N2
Balance
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Components
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Table 5 Syn gas composition
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Al2O3
Fe2O3
TiO2
CaO
MgO
K2O
Location 1
57.37
31.05
2.56
2.55
0.82
1.34
4.31
Location 2
53.01
32.83
9.24
0
0.27
1.36
3.29
Location 3
60.51
29.98
3.52
0
0.73
1.22
4.04
Location 4
42.46
41.59
3.07
4.13
6.52
0
2.23
Location 5
44.04
38.43
2.42
0
13.43
1.68
0
Location 6
47.51
37.6
6.54
4.04
4.31
0
0
Location 7
49.38
39.15
Location 8 43.64 Ash analysis (wt%)
46.12
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3.06
4.87
0
1.42
2.12
0
3.75
6.49
0
0
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SiO2
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Table 6. Composition at different locations of the ash agglomerate of Coal B by EDX analysis
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Fig.1. Schematic diagram of Fluidized Bed Gasification Plant
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Fig.2. Photograph of Fluidized Bed Gasification Plant
b
a
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c
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Fig.3. a) Photograph of Coal B ash agglomerate, b) Photograph of coal A ash agglomerate, c) Photograph of coal C ash agglomerate
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b
d
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a
e
f
Fig.4. a,b) SEM photograph of the ash agglomerate of coal A at different magnification, c, d) SEM photograph of the ash agglomerate of coal B at different magnification, e, f) SEM photograph of the ash agglomerate of coal C at different magnification
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Fig.5. EPR spectra of the ash agglomerate of coal B
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SiO 2 10
90
20
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80 70
30
SiO 2,wt% 40
(1083°)
50
60
(1210°)
2FeOAl 2O 35Si0 2
60
40
2FeOSiO 2
(1088°)
3Al2 O 3 2SiO 2 (1205°)
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FeO,wt%
30
80 1100° (1177°)
1300°
(1148°) 10
20
(1380°) 30
40
10
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FeO
20
1200°
FeO-Al2 O 3
80
Al 20 3
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Fig.6. Ternary phase diagram depicting FeO-Al2O3-SiO2 system [21]
ACCEPTED MANUSCRIPT Highlights: Pilot plant studies on agglomerate formation during high ash coal gasification
•
AFT , chemical analysis of coal ash could not give proper indication
•
Ash- / iron-loading factors, compositional variation, Fe2+ leads to agglomeration
•
Steam to coal ratio was controlled judiciously to avoid agglomeration
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Cause for agglomeration investigated in depth and remedial adjustment was focused
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S1359-4311(15)00389-0
DOI:
10.1016/j.applthermaleng.2015.04.046
Reference:
ATE 6565
To appear in:
Applied Thermal Engineering
Received Date: 19 November 2014 Revised Date:
9 April 2015
Accepted Date: 12 April 2015
Please cite this article as: S. Datta, P. Sarkar, P.D. Chavan, S. Saha, G. Sahu, A.K. Sinha, V.K. Saxena, Agglomeration behaviour of high ash Indian coals in fluidized bed gasification pilot plant, Applied Thermal Engineering (2015), doi: 10.1016/j.applthermaleng.2015.04.046. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Agglomeration behaviour of high ash Indian coals in fluidized bed gasification pilot plant a
a,
a
a
a
S. Datta , P. Sarkar *, P. D. Chavan , S. Saha , G. Sahua, A. K. Sinha , V. K.
a
b
CSIR-Central Institute of Mining and Fuel Research, Digwadih Campus, P.O.-
FRI, Dhanbad-828108, Jharkhand , India
Indian School of Mines, P.O. ISM, Dhnabad-826004, Jharkhand , India
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b
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Saxena
*Corresponding author. Tel: +91-326 2388286, +91 9431512885 (mobile), fax +91 326 2381113, Email: [email protected]
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ABSTRACT Although gasification of high ash Indian coals is gaining importance, the resultant uncertainties associated with agglomerate formation are still unresolved. To address this, Stabilized
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a suitable pilot scale Fluidized Bed Gasifier was utilized in this study.
operating conditions in terms of coal feed rate, air feed rate, bed temperature, etc., already identified for maximum possible carbon conversion, were maintained in all experiments and the steam flow rate was only varied. Though the ash fusion temperature of the coals were above 1200 oC, agglomerate was formed during gasification at 950 oC
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with ‘steam to coal ratio’ less than 0.15 (kg/kg). On increasing this ratio above 0.2 local heat-concentration and agglomeration could be avoided with certainty. Chemical
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composition alone was not sufficient to explain the relative strength of ash-agglomerates. Compositional variation and state of iron within the matrix were assessed through SEMEDX and electron paramagnetic resonance (EPR) study, respectively. The probing also required the ash-loading and iron-loading factors to be freshly defined in the context of gasification. Localized heat, large compositional variation, presence of iron in Fe2+ state, ash-loading/ iron-loading factors influenced intensity of agglomerate formation. Finally,
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diagram.
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low temperature agglomerate formation was explained by SiO2-Al2O3-FeO phase
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1. Introduction Coal is one of the most important fossil fuel, and in this century it will continue to play an important role towards the world energy supply.
Energy demand in India has
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increased many folds in present days due to fast growth of rural sector. In India, about 60% of commercial energy and about 70% electrical energy are produced from coal [1]. The major disadvantage associated with the Indian coal is its high ash content and proper clean coal technology need to be adopted for the gainful utilization of coal. Coal
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gasification offers a practical means of utilizing this huge energy resource meeting the stringent environment control requirement. Considering the high ash and high reactivity
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of Indian coals fluidized bed gasification technology seems to be promising in Indian scenario. But fluidized bed operation sometimes suffers from a major operational drawback caused by the ash agglomeration, which is very much detrimental and often leads to the shutdown of the plant. Several researchers experienced agglomeration related problems in their studies with different feed coals [2-7]. Some of these studies finally led to develop remedial strategies [8, 9]. Adjustments of operational parameters to avoid such
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development of agglomerate are very important. As such Indian coals are high in ash content. Presently mined coals do have ash content in the range of 30-50% [10]. High ash content poses different operational problems in both gasification and combustion applications. In India development of gasification technologies for high ash coals are
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very much challenging [10, 11]. Fortunately enough, the Indian coal ashes are refractory in nature and their melting zone is very high as it is indicated from ash fusion test [12].
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But it has also been experienced that ash fusion test mostly fails to assess ash melting behavior in several combustors/ gasifiers. As a matter of fact, AFT was originally meant for assessing ash melting behavior in stoker fired furnace [13]. In different gasifiers the formation of agglomerates often found at a temperature lower than that is expected from AFT [3, 14]. It depends on mechanical design of the gasifier and more importantly on the factors like operating conditions, residence time, hydrodynamics, particle size, localized heat concentration, inhomogeinity of chemical constituents, etc. [15]. The present work explores the possibility of developing fluidized bed gasification technology suitable for high ash Indian coals. A fluidized bed gasification pilot plant has
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been designed and installed at CSIR- Central institute of Mining and Fuel Research to cater the present need of technology development with high ash coals. The activity was also included under clean coal technology development programme under 11th Five year plan, Govt. of India.
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This paper presents gasification studies with high ash Indian coals which are very limited in literature. A unique gasification pilot plant specially designed for high ash coals was utilized in this study. This study depicts some interesting findings on gasification of high ash coals as well as on agglomerate formation. The agglomerates formed with three
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different coals under different conditions were examined in depth. This paper describes the operational adjustments which were adopted to avoid liquid phase sintering/
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agglomeration. This paper also attempts to understand the main reason of agglomerate formation through chemical analysis, FE-SEM with EDX, SiO2-Al2O3-FeO phase diagram and using electron paramagnetic resonance (EPR). Use of EPR to probe into the cause of agglomerate formation is basically a new approach. Moreover, ash loading factor and iron loading factor have been reconstituted for its application to gasification systems and importance of these factors have also been discussed to address the problem
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of agglomerate formation.
2. Experimental methods
2.1. Coal selection and characterization
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Non coking coal samples from three different coal fields have been selected to carry out the experiments. The Proximate and ultimate analysis of samples were done using the
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following standard procedures i.e., IS: 1350 e Part-I: 1984, Part III: 1969, Part IV/1: 1974, Part IV/2: 1975 and ASTM E871, D1102, E872. The gross calorific values (GCV) of the coal samples were determined using the standard method IS 1350 (Part 2): 1970. Chemical composition of the ash was determined as per standard procedure (IS: 1355:1984). For gasification studies the coal samples were crushed to -3 mm. Then the samples were sieved to collect the fraction having size in the range of -3mm to +0.5 mm and the collected fraction was used in gasification experiments. Initial deformation temperature (IDT) of ash samples of all the three selected coals was determined using Indian standard (IS: 12891-1990).
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2.2. Gasification studies in Fluidized Bed Gasifier Gasification experiments were conducted in the existing fluidized bed gasification system having capacity of 20 kg/h coal feed rate at CIMFR, Dhanbad. Gasifier is made of high temperature alloy suitable to operate up to 1000 oC. Height of the gasifier is ~5 m.
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Schematic flow diagram and photograph of this gasifier have been shown in Fig 1 and 2 respectively. The pilot plant consists of several sections, viz., gasifier, coal feeding system, gaseous reactant supply system, bottom ash extraction system, cyclone with fly ash collection system, gas cooling-cum-cleaning system and flare stack.
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The gasifier is a cylindrical vessels fitted with three zone external electrical heating systems which allow easy and precise control of temperature. The bottom portion of the
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gasifier is known as bed section. The top section of the gasifier is known as freeboard section. The section at the bottom end of the gasifier is known as air plenum, which is integrated with a distributor plate and this portion is properly insulated to avoid any heat loss during the process. The reactant, air and steam mixture is introduced as small jets through a distributor plate. The distributor plate is removable and cleaned after few experiments. The gasifier is provided with adequate nozzles for measurement of pressure,
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differential pressure, and temperature. Inside temperature of the gasifier is measured at different levels using shielded K-type thermocouple and system pressure at different locations are measured with pressure transducers. Initially the bed of the gasifier was buildup by the coal ash. As usual practice, during the start up of the plant the set of
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external heaters were put on to raise the temperature of the gasifier at a rate of 20 oC/min up to 550 oC. At this stage, coal feeding was started in the gasifier and the coal particles caught fire. Due to coal combustion, the temperature of the gasifier further raised up to
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900 oC. At this stage, the external heaters were put off. Controlling of the temperature in the bed section of the gasifier is very crucial, which was maintained by controlling the air and steam flow rate. Further, the temperature of the gasifier was raised to the desired level very slowly to conduct the gasification reaction. After achieving the stable condition, gas samples were collected for subsequent analysis. In the experimental runs, the coal feed rate was controlled with a rotary feeder. Conveying air was introduced in the feed line to ensure unrestricted flow of feed particles
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through the feed line. Bottom ash from the system was extracted with a rotary ash extractor after cooling. 2.3. Agglomerate characterization The Field Emission Scanning Electron Microscope (FESEM) system was used for study
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of topographical microstructure of the agglomerates. The chemical analysis at several positions within the agglomerate matrix was estimated by energy dispersive X-ray spectrometer (OXFORD Instrument INCA PENTA FETX3) attached with the FESEM system (JEOL JSM 7600 F FE SEM). Initially the small pieces of agglomerate samples
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were oven dried at 110 oC for two hours to remove the traces of adsorbed moisture. Then the samples were preserved in desiccators to prevent it from re-adsorption of moisture. Small pieces of the dry samples were placed on a specific stub and then gold coating was
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done in a sputter coater at 10-2 Pa pressure. After gold coating, the stubs were placed on the sample stub holder, which was then introduced in the load lock chamber and finally fed to the specimen chamber. Then vacuum of the system was brought to 10-5 to 10-8 Pa level. The electron beam was focused over the samples after adjusting the parameters for imaging. Images were captured at different magnification with best possible resolution.
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Electron Paramagnetic Resonance (EPR) spectra was recorded with X –band Jeol (JES FA 200) spectrophotometer. Powdered agglomerate sample of around 5 mg was taken in an EPR sample tube. It was placed in the sample cavity and kept under constant nitrogen flow to prevent moisture deposition on the sample tube. Then the sample was aligned
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with the magnetic field and EPR data were captured. The same agglomerate samples were digested with dilute hydrochloric acid and
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was kept under warm condition for one hour on a hot plate. Air was blown continuously during the process of digestion to ascertain oxidation from ferrous to ferric state. The digested mass was cooled and filtered. About 2 – 3 ml of filtrate was taken in an iron free quartz sample tube and frozen in liquid nitrogen atmosphere. Then it was placed in the sample cavity of the EPR spectrophotometer. The sample tube was kept under constant nitrogen gas flow and aligned with the magnetic field as before. The data were captured for the liquid sample.
3. Results & discussion
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Table 1 shows the proximate, ultimate analysis and GCV of the feed samples. Low rank coals are selected considering the ash content in a range between approximately 27 to 49% from different active mines. From Table 2, it is clear that the silica and alumina are the major components of ash
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matrix and which is ~90% of the total mass and rest part is composed of some iron, calcium, sodium and other trace metals. The ash fusion temperature (IDT) of the coal samples are found to vary in the range of 1250- 1290 oC as shown in Table 3. Although ash fusion temperature is an important property which is utilized to assess proneness of
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formation agglomerate in combustion / gasification [16], the IDT often fails to directly assess the tendency to form agglomerates. That is why correlation of AFT results with
agglomerate formation [13].
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field experience is very much essential to enhance reliability for prediction of
Again proper correlation is difficult to establish because of variation of operating condition as well as time temperature schedule that those particles in the reactor experience. Variations of operating parameters, which are very much important to control agglomerate formation, have been reflected in Table 4. For entire set of experiments all
(kg/kg) and
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‘coal feed rate’ , ‘air/coal ratio’ and ‘bed temperature’ were fixed at 15 kg/h, 1.5 950 oC (± 10 oC) respectively. The air/coal ratio was fixed for all the
experiments but the Equivalent Ratio(ER) were 0.30 for coal A, 0.37 for coal B and 0.23 for coal C. In our previous study [17] with high ash Indian coals carbon conversion was
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recorded to be ~ 90% under similar experimental conditions as mentioned above. In the present study the only variable is steam flow rate, reflection of which may be observed
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from steam/coal ratio values in Table 4. Agglomerate formation is observed for all the three coal samples when steam/coal ratio is varied in the range of 0.05 and 0.15 but, when steam /coal ratio was raised above 0.2 smooth trouble free operation without agglomerate formation was noticed. The syn gas compositions for the conducted experiments are shown in Table-5, where no ash agglomeration eventually took place. With increased steam/coal ratio concentration of CO was found to be slightly decreased while that of H2 was found to be little bit increased. Now at the outset of discussing the agglomerate formation issues it may be noted that the experiments in fluidized bed gasifier were conducted at a temperature much below the IDT of coal ash. Hence smooth
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trouble free operations in terms of agglomeration formation were initially expected. But, ash agglomeration was repeatedly noticed in different intensity, which could be avoided by raising steam/coal ratio. Sequential observations with gradual increase in steam /coal ratio have been narrated in Table 4.
Fig. 3 shows the photographs of agglomerate
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formed at 0.05 steam /coal ratio with the three coal samples. As it appears from the nature of the agglomerates on physical and visual examination, strongest and weakest particleparticle interlocking were observed in case of coal B and C respectively, whereas the strength of particle-particle interlocking being intermediate in case of coal A. The
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highest agglomerate strength has also been accompanied by largest lump size in case of coal B (Fig.3a). Lump size in case of coal C is smallest and friable (Fig 3c). Lump size in
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case of coal A is medium and relatively less friable than in the case of coal C (Fig 3c). Design of the gasifier plays a vital role towards formation of the agglomerate. The consideration of parameters like volumetric heat loading as well as distribution of the reactants in the bed, heat dissipation inside the bed, heat loss through the wall etc. are very important to minimize agglomeration problem. As a matter of fact all the above aspects were taken into account during designing of the pilot plant for gasification of high
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ash coals. Apart from design parameters, properties of coal/ ash are very much important to assess the agglomeration phenomenon.
Conventional indices based on quality
parameters of coal/ coal ash do not work well to predict agglomeration tendencies during gasification and therefore, the parameters like differential chemical composition across
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the ash matrix, ash loading, iron loading, oxidation state of iron, spot heating etc., need to
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As in the case of slagging fouling in PF fired boiler [13], agglomerate formation in the gasifier also depends on ash loading. Like slag formation in boiler [13], agglomeration tendency in gasifier primarily depend on the ash composition and
temperatures
experienced in the bed. Liquid forming tendency of the ash often assessed through parameters like alkali index, silica percent etc., which considers average composition of ash. But ash composition is not homogeneous over the whole matrix rather it varies a lot at different locations. Therefore, in real term, liquid forming tendency depends on locational distribution of constituent oxides. More precisely, it depends on the
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distribution of the constituent mineralogical phases on which formation of low melting eutectic composition depends. The microstructure of the agglomerates which reflect heterogeneous distributions of crystalline and glassy matrix have been shown in SEM -micrographs (Figs 4. a- f).
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Grains of different size ranges and widely distributed glassy matrix are seen in all the micrographs. In coal B ash-agglomerate the maximum bonding through glassy phase was observed (Figs. 4. c, d) which resulted maximum agglomerate strength and biggest lump size (Fig. 3a). On the contrary, in the microstructure of coal C ash agglomerate (Figures
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4. e, f) bonding through glassy matrix was found to be at the minimum. As a result agglomerate bond strength was also poor and big lump could not be formed (Fig. 3c).
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The situation in case of coal A ash- agglomerate is somewhat intermediate between the above two cases with coal B and coal C. Formation of glass phase in coal A was less than that in the case of coal B and extent of crystal- glass bonding was also less (Fig.4. a, b) which led to intermediate lump size of coal A ash-agglomerate (Fig. 3b). As a case of strong agglomerate formation in coal B (Fig. 3a), EDX analysis was done only for coal B agglomerates to investigate possible extent of compositional variations at
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different locations of the agglomerate matrix. The normalised compositions in terms of conventional oxides recalculated from elemental composition (mass %, EDX analysis) have been presented in Table 6. EDX results show that there is substantial variation of chemical composition in different location of the agglomerate matrix (Table 6). In this
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case Fe2O3, CaO, MgO and TiO2 content was found to be high as 9.2%, 13.4%, 1.7% and 4.9% respectively at certain locations of agglomerate matrix. Therefore, there are ample
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possibilities of formation of low melting eutectic of different systems. At location 5 (as indicated in Table 6), high concentration of CaO may lead to formation of anorthite (Al2O3.SiO2.2CaO) which contribute a lot to form eutectic at much lower temperature [18] particularly in presence of other assisting component like phases containing Magnesium, Iron or Titanium. The most significant component for lowering of eutectic temperature is the iron bearing phases. In the gasification condition, reducing atmosphere is maintained and therefore there is ample possibility of shifting Fe3+/ Fe2+ equilibrium towards Fe2+. On the contrary, in case of pulverised coal firing systems possibility of such shifting is much less (as it is oxidising condition) and Fe 3+ state is most likely. The
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much higher residence time in fluidised bed gasifier (residence time ~15- 20 minutes) as compared to pulverised coal combustion units (residence time is in the order of few seconds), may allow the phase transition from Fe3+ to Fe2+ to a greater extent. This Fe2+ acts as much more fluxing agent than Fe3+ [19, 20]. To probe into the actual situation
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prevailed in case of agglomerate of coal B, EPR test was done with the powdered agglomerate sample of coal B as a test case. The flat EPR spectrum of powdered dry agglomerate sample (Fig. 5) reflected absence of Fe3+ state. To confirm the same, EPR study was also conducted with oxidised filtrate of the agglomerates. Sharp peak was
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resulted in EPR spectra indicating presence of Fe3+ in this case (Fig. 5). Therefore it can be confidently commented that within the agglomerate matrix iron remained as Fe2+ state
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which contributed towards lowering of eutectic temperature. From the ternary phase diagram depicting FeO-Al2O3-SiO2 system ( Fig. 6) it may be noticed that the eutectic formation may be at a temperature even as low as 1088 oC. As per the phase diagram (Fig. 6) other eutectic may be formed at a temperature of 1148 oC and also at 1177 oC. In the process of gasification the softening temperature may be decreased further due to the formation of low melting complex compound of FeO- SiO2- Al2O3- CaO as it has also
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been found to be formed in reducing atmosphere during sponge iron production [21]. Cause of agglomerate formation is better understood when ash loading factor and liquid forming tendency is assessed together. In case of gasification ash loading may be defined as ‘ash to available carbon ratio (w/w)’ in given quantity of coal. Moreover consideration
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of iron loading is sometimes more important than ash loading. This may be defined as the ratio of weight of Fe2O3 in grams and weight of carbon in kg present in a given quantity
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of coal as shown in Table 3. It is to be noted from Fig. 3 and Table 2 that strongest and biggest lump was formed in case of coal B ash although the Fe2O3 content and CaO content are more in coal A as compared to those in coal B. It may be due to the fact that ash loading and iron loading are more in the case of coal B ash than respective values of coal A ash (Table 3). Practically in respect of agglomerate and clinker formation, the ash loading [13] and iron oxide loading [22] are more important than the respective absolute value of ash content and Fe2O3 percent in coal ash. For a high value iron oxide loading, greater rise in locational concentration of iron oxide is much more likely as compared to the case of lower value of iron loading. That is why chance of formation of bigger and
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stronger agglomerate is very high with high iron loading which actually happened in case of coal B. Another important factor that plays key role towards the formation of agglomeration is the gasifier bed temperature [23]. From literature it is clear that the mineral constituents
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present in the ash matrix may develop low melting eutectics that may be responsible for the agglomeration at low temperature in the fluidized bed combustion or gasification application [24]. To maintain uniform temperature inside the bed section of the gasifier we need to ensure certain factors like uniform particle size of the feed, proper distribution
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of the fluidizing air and steam inside the bed. Smaller is the feed particle; higher is the probability of particle temperature. Feeding the gasifier with uniform particle size is a
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challenging job. Again during fluidization gas-particle and particle –particle interaction may cause creation of smaller particle in the bed. This uneven distribution of the particles may cause several hot spots during the process. Proper dissipation of this localized heat is utmost important to avoid agglomeration formation. In actual practice average temperature inside the bed is measured. But, in certain locations inside the bed particle temperature may be quite higher than that of the average bed temperature. If proper
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dissipation of the heat from these hot spots is not done properly the local temperature may cross the IDT of some particles. These particles will produce a kind of fused sticky mass and upon interaction with the other adjacent particles it will form a bigger lump. If the temperature of the hot spots cannot be reduced at this stage, this phenomenon will
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continue and ultimately agglomerate will reach to a bigger size that may cause the defluidization of the bed. It is well known that in different chemical and petrochemical
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conversion processes steam helps to minimise the coke formation tendency through dissipation of additional heat from the hot spots. In gasification also steam plays similar role to distribute localised heat minimising tendency of agglomerate formation. Therefore the heat withdrawing property of the steam was found to be very important in controlling agglomeration phenomenon and that why beyond a certain steam/ coal ratio (> 0.2), agglomeration can be avoided in the gasifier system. However, the steam to coal ratio was not increased beyond 0.3 in this study because excessive steam flow may reduce the bed temperature, affect the carbon conversion and gas yield as well as gas
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calorific value. All these findings are also likely to be observed in case of similar fluidized bed installations with high ash coals under comparable experimental conditions.
4. Conclusions
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This paper attempted to find out cause-effect relationship in respect of agglomerate formation with high ash Indian coals. It was found that under the conditions already stabilised with respect to objective parameters (i.e., Gas composition, gas yield, carbon conversion etc.), agglomeration phenomenon may be controlled through the use of
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appropriate steam/coal ratio. It may be possible to avoid agglomeration problem by the judicious control of steam flow and role of steam on overall process is also very much
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crucial. Beyond certain steam/coal ratio no agglomerate found to take place for all the three selected coals. Flow of steam in adequate quantity prevents hot spot formation inside the bed.
As a result, the particle temperature fails to attain the temperature
required for fusion of the ash particles or ash components. Ferrous state of iron and high concentration of CaO contributes to lower down the eutectic temperatures of different phase combinations. For probing the agglomerate formation, differential chemical make
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up across the ash matrix is actually more important issue than the average chemical composition determined by chemical analysis.
Ash loading and iron loading factor
aggravates agglomerate forming tendencies and these are more crucial factors as compared to ash content and iron oxide percent in ash. The liquid forming tendency, 2+
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stickiness of the liquid phase and ash/ iron loading factor, presence of iron as Fe
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increase lump size and agglomerate strength as it is happened in case of coal B.
Acknowledgements
The authors are thankful to the Director of CSIR-Central Institute of Mining and Fuel Research for giving permission to publish the paper. Authors acknowledge the financial support of SSRC (Ministry of Coal, Govt. of India) and CSIR (Ministry of Science and Technology, Govt. of India) to establish the pilot plant and to carry out the experiments. Authors sincerely acknowledge the analytical support from the University of Calcutta and Indian Association of Cultivation of Science (IACS, Kolkata) to carry out SEM- EDX analysis and EPR study respectively.
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[12] Project completion report on Agglomeration formation in reducing condition in pressurised fluidized bed gasification (PFBG) of low rank high ash coal, Report no. TR/CIMFR/3.20/09-10. [13] A. Lawrence, R. Kumar, K. Nandakumar, K. Narayanan, A Novel tool for assessing slagging propensity of coals in PF boilers; Fuel 87 (2008) 946–950. [14] J. F. Vélez, F. Chejne, C. F. Valdés, E. J. Emery, C. A. Londoño, Co-gasification of
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[21] A. Sarengi, B. Sarangi, Sponge Iron Production in Rotary Kiln, PHI Learning Private Limited, New Delhi, 2011, p.214-217.
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[22] R. Hatt, Influence of Coal Quality and Boiler Operating Conditions on Slagging of Utility Boilers, http://65.163.62.71/PDF%20Files/UEFInfluenceSlag03.pdf [23 ] P. Thy, B.M. Jenkins, R.B. Williams, C.E. Lesher, R.R. Bakker, Bed agglomeration in fluidized combustor fueled by wood and rice straw blends, Fuel Processing Technology 91(2010) 1464-1485.
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ACCEPTED MANUSCRIPT Table 1 Proximate analysis, ultimate analysis, GCV of feed coal Coal
Proximate Analysis
Ultimate Analysis
GCV
M
VM
FC
C
H
N
S
O
kJ/kg
A
41.3
6.5
24.5
27.7
37.15
2.83
0.86
0.55
6.68
14,737
B
48.9
7.1
20.4
23.6
30.82
1.90
0.60
0.24
5.55
11,179
C
27.0
9.7
25.7
37.6
48.46
3.44
1.03
0.60
7.07
18,966
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A
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*Proximate and Ultimate analysis(wt%, air dried basis), GCV(air dried basis), A:ash, M:moisture, VM: volatile matter, FC: fixed carbon, C: carbon, H: hydrogen, N: nitrogen, S: sulphur, O: oxygen, GCV: gross calorific value
ACCEPTED MANUSCRIPT Table 2 Ash analysis of coal samples SiO2
Al2O3
Fe2O3
TiO2
P2O5
SO3
CaO
MgO
Na2O
K2O
A
57.14
30.14
5.59
1.36
0.21
0.71
3.04
0.94
0.17
0.69
B
62.28
27.56
4.79
1.28
0.17
0.54
1.85
0.68
0.17
0.66
C
61.68
28.32
4.39
1.20
0.19
0.48
2.06
0.78
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Ash analysis (wt %)
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Coal
0.18
0.71
ACCEPTED MANUSCRIPT Table 3 Ash properties IDT ( 0C)
Ash Loading kg ash /kg carbon
Iron loading gm Fe2O3/ kg of carbon
A
1250
1.11
62.1
B
1260
1.59
76.0
C
1290
0.56
24.5
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IDT: initial deformation temperature
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Coal
ACCEPTED MANUSCRIPT Table 4 Experimental details Steam: coal ratio (kg/kg) 0.05 -0.15
Gasifier Bed Temp (oC) *
Observation
15
Air :Coal ratio (kg/kg) 1.5
A
950
Agglomerate
A
15
1.5
0.2- 0.30
950
No Agglomerate
B
15
1.5
0.05 -0.15
B
15
1.5
0.2- 0.30
C
15
1.5
0.05 -0.15
C
15
1.5
0.2- 0.30
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Coal Feed (kg/h)
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*Temperature varied within +/- 10 0C
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950
Agglomerate
950
No Agglomerate
950
Agglomerate
950
No Agglomerate
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Sample
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Volume %
CO
15-20
H2
15-20
CO2
10-12
CH4
1-2
N2
Balance
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Components
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Table 5 Syn gas composition
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Al2O3
Fe2O3
TiO2
CaO
MgO
K2O
Location 1
57.37
31.05
2.56
2.55
0.82
1.34
4.31
Location 2
53.01
32.83
9.24
0
0.27
1.36
3.29
Location 3
60.51
29.98
3.52
0
0.73
1.22
4.04
Location 4
42.46
41.59
3.07
4.13
6.52
0
2.23
Location 5
44.04
38.43
2.42
0
13.43
1.68
0
Location 6
47.51
37.6
6.54
4.04
4.31
0
0
Location 7
49.38
39.15
Location 8 43.64 Ash analysis (wt%)
46.12
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3.06
4.87
0
1.42
2.12
0
3.75
6.49
0
0
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SiO2
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Table 6. Composition at different locations of the ash agglomerate of Coal B by EDX analysis
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Fig.1. Schematic diagram of Fluidized Bed Gasification Plant
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Fig.2. Photograph of Fluidized Bed Gasification Plant
b
a
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c
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Fig.3. a) Photograph of Coal B ash agglomerate, b) Photograph of coal A ash agglomerate, c) Photograph of coal C ash agglomerate
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b
d
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a
e
f
Fig.4. a,b) SEM photograph of the ash agglomerate of coal A at different magnification, c, d) SEM photograph of the ash agglomerate of coal B at different magnification, e, f) SEM photograph of the ash agglomerate of coal C at different magnification
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Fig.5. EPR spectra of the ash agglomerate of coal B
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SiO 2 10
90
20
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80 70
30
SiO 2,wt% 40
(1083°)
50
60
(1210°)
2FeOAl 2O 35Si0 2
60
40
2FeOSiO 2
(1088°)
3Al2 O 3 2SiO 2 (1205°)
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FeO,wt%
30
80 1100° (1177°)
1300°
(1148°) 10
20
(1380°) 30
40
10
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FeO
20
1200°
FeO-Al2 O 3
80
Al 20 3
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Fig.6. Ternary phase diagram depicting FeO-Al2O3-SiO2 system [21]
ACCEPTED MANUSCRIPT Highlights: Pilot plant studies on agglomerate formation during high ash coal gasification
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AFT , chemical analysis of coal ash could not give proper indication
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Ash- / iron-loading factors, compositional variation, Fe2+ leads to agglomeration
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Steam to coal ratio was controlled judiciously to avoid agglomeration
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Cause for agglomeration investigated in depth and remedial adjustment was focused
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•