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Determination of fouling characteristics of various coals under gasification condition
Journal of Industrial and Engineering Chemistry 15 (2009) 98–102
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Determination of fouling characteristics of various coals under gasification condition Li-Hua Xu a,b, Hueon Namkung a, Hyok-Bo Kwon c, Hyung-Taek Kim a,* a
Division of Energy Systems Research, Graduate School, Ajou University, Wonchon-dong, Yeongtong-gu, Suwon 443-749, Republic of Korea Institute for Advanced Engineering, Wonchon-dong, Yeongtong-gu, Suwon 443-749, Republic of Korea c Department of Material Science & Engineering, Kyungnam University, Republic of Korea b
A R T I C L E I N F O
Article history: Received 21 February 2008 Accepted 17 September 2008 Keywords: Fouling tendency Gasification condition Mineral
A B S T R A C T
In a coal gasifier and heat exchanger, adhered and deposited fouling inhibits heat transfer making the stable operation of the gasifier difficult. Since heat-transfer performance of the heat exchanger directly affects the output of Integrated Coal Gasification Combined Cycle plants, it is important to understand the effect of fouling on the characteristics of adhesion and deposition on the heat exchanger tube beforehand. But very few studies have been conducted about concerning the relationship between fouling property and adherability, which depend on the reducing condition on the gasifier system. The main purpose of the present investigation is to determine the low temperature ash deposition behavior under coal gasification condition by using drop tube furnace (DTF), in which behavior of coal particle in actual gasification condition can be simulated experimentally. Nine pulverized coal samples which are in the range of bituminous and sub-bituminous are injected into DTF under gasification condition. The ash particles are deposited onto sample collector by impacting and agglomerating actions; deposit samples of ash are collected, quantitative analyses are performed by EDX and weight measurements. Experiment results illustrated that mineral components are generally considered as a dominant parameter to determine behavior of ash deposition at low temperature among various physical and chemical properties of coal. It is also found that the alkaline earth mineral in coal ash such as MgO and CaO compounds leads to fouling deposition under the gasification condition. ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Integrated gasification combined cycle (IGCC) is recognized as the 2nd generation power plant which can provide higher efficiency of electric generation than power plant of the conventional pulverized coal combustion. But ash components of coal cause the major downtime of power plant during the operation of continuous system. The problem of ash deposition which plays a major role in hot-gas cleanup system is also a significant issue. The issues in hot-gas cleanup system generally include the deposit in the filter component, mechanism of ash removal, and vapor-phase and ash–ceramic interactions. The physical and chemical characteristics of the fly ash also influence their ability to be collected in control devices of air pollution such as selexol unit and claus unit [1]. Small amounts of the inorganic components contained in coal are converted into fly ash in the gasification process. The inorganic of fly ash, which will lead to costly shutdowns of a power plant [2–
* Corresponding author. Tel.: +82 10 2656 0770. E-mail address: [email protected] (H.T. Kim).
8], can cause problems such as deposition on refractory or heattransfer surfaces, the bed agglomeration and to cause corrosion and erosion of system parts. The type and degree of these problems are usually dependent upon ash composition of coal, operating system condition, and system design criteria. Many researchers have been conducted to determine ash formation and deposition behavior in pulverized coal boilers [1,8–14]. Most of the studies are concentrated on the development of ash formation/deposition mechanisms. And predictive models have been developed to base on the experimental data. But a limited number of studies have been done to determine the ash deposit behavior in the temperature range of fouling. In general, fouling deposits do not directly exposed to flame radiation such as more closely to spaced tubes in the convection sections of boilers. In most cases, fouling deposits do not contain the high levels of liquid phases associated with slagging-type deposits. They usually consist of a combination of silicates and sulfates that sinter the ash particles together [15,16]. In the present investigation, fouling experiments under the gasification conditions are conducted with drop tube furnace (DTF). To simulate fouling phenomenon in the outlet section of gasifier and heat exchanger section of syngas cooler, fouling
1226-086X/$ – see front matter ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2008.09.004
L.-H. Xu et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 98–102
99
Table 1 Basic analysis of coal samples during the experiment. Datong
Cyprus
Denisovsky
Shenhua
Kideco
Usibelli
Adaro
Baiduri
Roto
Proximate analysis (dry-basis) (wt%)
V.M F.C Ash
33.52 57.36 9.12
46.64 47.3 6.06
25.81 60.98 13.19
31.83 61.64 10.6
50.1 48.23 1.67
48.19 41.75 10.06
48.45 48.54 3.01
53.95 41.02 5.03
46.52 50.2 3.28
Ultimate analysis (ash-free base) (wt%)
C H O N S
73.86 4.75 20 0.73 0.66
68.22 5.25 24.55 1.19 0.79
85.6 5.16 7.75 1.13 0.36
77.67 4.4 1.07 0.29 16.57
64.12 5.2 29.91 0.24 0.53
63.56 5.32 30.17 0.75 0.2
72.73 5.38 20.89 0.81 0.19
68.16 5.04 21.74 1.27 0.79
66.99 4.49 27.44 0.97 0.11
Heating value
kcal/kg
6607
6080
7139
behavior of ash at the low temperature is experimentally performed to evaluate fouling tendency of ash by using the DTF of vertical form. Ash of coal samples are collected on the probe surface of plate type whose area of deposit plate is various. Fouling behaviors of different coal samples are analyzed with experiments data as well as approached theoretically. Commonly, fouling is initiated by the deposition of condensed vapor materials which make a thin layer. The composition of these materials is typically high in alkaline earth and alkali metals [15,17]. Fouling deposition behavior of particle can be explained by surface deposition of sticky minerals and followed by relatively high-temperature deposition [18]. For most cases, the innermost layers consist primarily of small particles and Na, K, Ca, and Mg which are transported to the surface by vapor-phase diffusion and pyrolysis. The initial deposit layers may provide a sticky surface to trap impacting particles by inertial force which is not sticky. In addition, the initial layers may provide fluxing materials that will cause larger particles to be melted. Accordingly, small particles are bonded to the heat-transfer surface, and they provide sites for continued deposition to the entire particle deposition. The liquid phase materials contribute to the thickening of bonds among particles. Because of a result of the insulating effect of the deposit layer on the surface, the outer layers of the deposit are formed at higher temperatures. These higher temperatures cause melting and interaction of the particles to a liquid phase. Once a liquid phase has formed on the outside of the deposit, it becomes an efficient collector of ash particles regardless of individual melting characteristics of the particles.
6785
6729
5304
6748
6367
6154
Fig. 1. Schematic view of the entire experimental facility and probe plate.
2. Experimental Coal samples of nine types in range of bituminous and subbituminous coal are chosen for the experiments. These coal samples represent a wide range of mineral contents. Basic analysis of coal samples is illustrated in Table 1 that indicates the results of proximate and ultimate analysis. The chemical compositions and
fusion temperatures of ash are also given in Table 2 with chemical elements of Si, Al, Fe, Ca, Mg, Na, K and Ti which are assigned as major components of the ash. Original coal samples were dried and crushed in a disk mill of fan type, and then were separated below 74 mm by electromagnetic sieve shaker. The particle size of coal should be used below 74 mm for the DTF experiments.
Table 2 Chemical components and fusion temperature of ash in coal samples used during the experiments. Datong Inorganic analysis (wt%)
SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O TiO2
Ash fusion temp. (8C)
IT ST HT FT
Cyprus 55.16 19.07 13.18 8.53 1.23 0.64 1.40 0.78 1141 1204 1243 1308
Denisovsky 62.29 16.89 7.19 8.34 2.16 1.10 1.10 0.93 1155 1165 1193 1289
Shenhua 55.36 26.74 8.41 4.45 1.92 0.41 1.71 1.00 – – – 1400
Kideco 45.51 17.18 10.89 21.49 1.55 1.50 1.10 0.78 1165 – 1242 1258
Usibelli 41.66 16.64 23.58 13.20 2.82 0.21 1.08 0.80 1265 1295 1326 1408
Adaro 45.10 19.98 6.33 22.18 3.30 0.98 1.34 0.78 1250 – 1290 1340
Baiduri 42.85 20.54 18.12 12.30 2.83 0.88 1.55 0.93 1250 – 1290 1340
Roto 34.32 19.28 11.65 21.28 6.97 4.45 1.22 0.85 1153 – 1260 1270
37.49 16.46 19.32 17.46 7.24 0.09 0.91 1.04 1153 – 1260 1270
L.-H. Xu et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 98–102
100
Fig. 2. Photograph of the attached fouling on deposit plate.
The deposition behavior of coal samples is studied by using the laminar DTF and probe plate of fouling deposit as shown in Fig. 1. That equipment simulates conditions of the coal gasifier. The DTF system consists of coal sample injector, pre-heater, main tube furnace reactor, deposit probe and the others equipments. Temperatures within the 1.5 m of DTF section are set at 1300 8C. The zone is divided to zone 1 (top), zone 2 and 3 during the experiment, because temperature can be easily controlled by proportionalintegral-derivative (PID). All of temperature sections are controlled by PID system. Pulverized coal samples are fed at the top of DTF with concentric primary air flow, where particles are injected with the primary air into the furnace by mechanical screw feeder. During each experiment, coal sample is continuously fed for 30 min to the DTF with the coal feed rate (0.5 g/min) and the O2/coal weight ratio (0.9). Both the pre-heater and reactor are set to 1300 8C and the surface temperature of probe sample was set to 700 8C. After the experiment, deposit samples removed from the DTF are measured by scratching off the loosely attached sample on probe and weighing the deposits. Once the coal samples are introduced into the furnace section, the particles are entrained by primary gas. During the flight to the bottom of main reactor, particles are heated and passed through the stages of devolatilization and char burning before it is reduced to ashes. Prior to leaving the DTF through the exhaust system, the particles enter a sampling section, in which an air-cooled deposit probe is horizontally inserted into the end of lowest furnace section. Vertical position of the probe move up and down to adjust the surface temperature of the probe by mechanical equipment which is operated by pushing the button. The reacted ash samples are collected on the funnel-shaped deposit probe at the bottom. Experiments are preceded with the change of coal sample and deposit area. The removed deposit plate is separated from the deposit probe after each experiment and the deposit plate is turned up to scrape off the attached fouling which is shown in Fig. 2. The deposit fouling can be finally analyzed. 3. Results and discussion 3.1. Result of EDX analysis The sample near the top and bottom of the fouling layer is analyzed by energy dispersive X-ray spectroscopy (EDX) equip-
Fig. 3. Images of the deposited fouling layer.
Fig. 4. EDX analysis of deposit fouling of top and bottom layer.
ment and the analysis results of the fouling are shown in Figs. 3 and 4. In Fig. 3, the top layer includes the 3 mm from the upper part and the bottom layer includes the 3 mm from the lower part. Fig. 4 shows the ratio of component contents of the major basic elements such as Mg, Ca, K and component contents of the major acid elements such as Al, Si between the top and bottom based on EDX analysis. From Fig. 4, it can be seen that the ratio of [%(MgO + CaO + K2O)/%(SiO2 + Al2O3)] in bottom layer is greater than that of top layer. This phenomenon explains that the initial deposition acts are related with the amount of basic element components. It seems that fouling including volatile alkali and alkaline earth metal is primarily responsible even though it usually exists in stable silicate such as illite [5]. It seems that volatile alkali and alkaline earth metal are primarily responsible for fouling even though those content are little. Potassium may contribute to fouling by forming low-melting K silicates on the deposit probe and providing an initial sticky surface where fly ash particles can be adhered [19]. Widely distributed potassium vapor and alkaline earth metal composition in the combustion gases contact with ash particles and metal surfaces where aluminum, silicon and other mineral components are condensed. 3.2. Deposit shape of various coal samples The plate appearance of deposit probes taken from DTF furnace is shown in Fig. 5. Major mineral content ratio of [%(MgO + CaO + K2O)/%(SiO2 + Al2O3)] from results is shown in Fig. 6. It clearly is shown in the photographs that fouling deposition of Denisovsky coal, Datong coal and Cyprus coal is more scattered on the deposit probe. But the fouling deposition of Kideco, Shenhua, and Usibelli coal are relatively concentrated on the deposit probe plate center. It indicate through comparison between Figs. 5 and 6 that the mineral ratio of [%(MgO + CaO + K2O)/%(SiO2 + Al2O3)] leads to characteristics of the deposited fouling. When mineral ratio of [%(MgO + CaO + K2O)/%(SiO2 + Al2O3)] in coal has a little amount, the fouling is dispersed on deposit plate; when mineral ratio of [%(MgO + CaO + K2O)/%(SiO2 + Al2O3)] in coal has a lot of amount, the fouling is concentrated on deposit plate center. And also the height of deposit fouling is higher. When alkali metal and alkaline earth metals of the low melting temperatures are mixed, it produces low fusibility temperatures. The low temperature contributes to deposit building. The dispersion throughout the gas stream subsequently makes other ash particles condensed together and fouling deposit on probe surfaces. The condensed alkali metal and alkaline earth metals provide a binding matrix for
L.-H. Xu et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 98–102
101
Fig. 5. Images of the fouling deposition on different coal samples.
aluminum, silicon, and ash particles to fuse together and build up on deposit surface. So the high alkali and alkaline earth mineral content make the fouling surface sticky. So particle surfaces can be bondable and fouling deposit can be rapidly formed. 3.3. Deposition prediction vs. the mineral content It is generally accepted that the chemical composition of incoming ash particles determine the deposition behavior of fly ash on surfaces. The surface composition of the ash particles is important in predicting the ash deposition, as it influences the adhesion on deposit surface [20]. And total alkali content is an alternative index, considering that fouling deposits usually contain high alkaline earth, alkali metals (Ca, K or Na), and their sulfates [15]. But the total quantity of alkalies is not necessarily an accurate index of fouling potential. Fouling phenomenon is well related to the quantity of ‘‘active alkalies’’. Organically bonded alkali metals
have the potential to form very reactive species in the combustion zone as transient elements or compounds. CaO and MgO have been found to have similar fluxing properties, and the softening temperature decreases with high values of CaO and MgO [21,22]. An empirical formula provides the estimate of total alkali. It is defined as [%(MgO + 0.719CaO)/%ash] where 0.719%CaO is the molar equivalent of MgO [16,21,23]. Based on those precedence researches, we try to predict the fouling tendency by the mineral content in feeding coal. The condensation of volatile alkali compounds on surfaces has long been thought to initiate fouling by first forming a thin sticky layer on tube surface and later to sinter the particles in the deposit [15]. So, the alkali contents have been used as a fouling index to predict fouling propensity. The mass is obtained by the following steps. After measuring of total weight of sampled fouling, weight measurements are made of the free ash and fixed ash when sampling probe is turned up. Calculation from fixed ash weight divided by total feeding coals, the propensity of fouling deposition is represented in Fig. 7. The result shows the fouling propensity, against the %(Na2O + K2O) in ash for the coals in the DTF under gasification condition. From this figure, we can see that there is no apparent trend between the fouling deposit propensity and the alkali contents. Hence, this index has trouble ranking of fouling propensity, at least for the coals tested in this study. However, it is not certain that this index is useful for other fuels or for other operation conditions, because the alkali contents for all the tested coals are relatively the lower than the other mineral contents, and the experiment has been done at high temperature and reducing condition of the DTF where volatilization easily happen. Fig. 8 shows the fouling propensity against the total alkaline earth mineral content in ash for feeding coals. total alkaline earths % ¼ %ðMgO þ 0:719CaOÞ
Fig. 6. Comparison of [%(MgO + CaO + K2O)/%(SiO2 + Al2O3)] ratio for feeding coal.
We can find from the figures that the tendency of fouling deposit is similar with tendency of total alkaline earth contents in feeding coal. Therefore the big value of %(MgO + 0.719CaO) in coal
L.-H. Xu et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 98–102
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(2) The characteristics of deposited fouling is related with the mineral contents ratio of [%(MgO + CaO + K2O)/%(SiO2 + Al2O3)] in feeding coal. When mineral ratio of [%(MgO + CaO + K2O)/ %(SiO2 + Al2O3)] in coal has a lot of amount, the fouling is concentrated on deposit plate center. And also the height of deposit fouling is higher. (3) The fouling predictions, which are based on the combined use of the extended coal ash composition, indicate that the tendency of fouling deposition is related with total alkaline earth content in feeding coal. Consequently, the big value of %(MgO + 0.719CaO) in coal has high tendency of fouling deposition under the gasification condition. References
Fig. 7. The fouling propensity against total alkali content.
Fig. 8. . The fouling propensity against total alkaline earth content.
has high tendency of fouling deposition. The reason is that the alkaline earth contents of coals are relatively higher. And, the more alkaline earth content is increasing, the less ash melting temperature is decreasing. That effect easily happens at reducing atmosphere [24]. 4. Conclusions This paper presents the experimental results on the fouling propensities of nine coals under the gasification condition by using DTF. The deposited fouling is scraped off to analyze their chemical characteristics and weights. We evaluated the low melting temperature on basic oxides CaO and MgO and major acidic oxide components. The following conclusions are made from this experiment results. (1) From result of EDX analysis, the top layer and bottom layer of the fouling from the deposit probe in the DTF is analyzed. The mineral contents ratio of [%(MgO + CaO + K2O)/%(SiO2 + Al2O3)] in bottom layer is greater than that of top layer.
[1] S.A. Benson, E.A. Sondreal, J.P. Hurley, Status of coal ash behavior research, Fuel Processing Technology 44 (1995) 1. [2] P.A. Jensen, F.J. Frandsen, J. Hansen, K. Dam-Johansen, N. Henriksen, S. Ho¨rlyck, SEM investigation of superheater deposits from biomassfired boilers, Energy Fuels 18 (2004) 378. [3] B.M. Jenkins, L.L. Baxter, T.R. Miles Jr., T.R. Miles, Combustion properties of biomass, Fuel Process Technology 54 (1998) 17. [4] L.L. Baxter, T.R. Miles, T.R. Miles Jr., B.M. Jenkins, T. Milne, D. Dayton, et al., The behavior of inorganic material in biomass-fired power boilers: field and laboratory experiences, Fuel Process Technology 54 (1998) 47. [5] R.W. Bryers, Fireside slagging, fouling and high-temperature corrosion of heattransfer surfaces due to impurities in steam-raising fuels, Progress Energy Combustion Science 22 (1996) 29. [6] J. Werther, M. Saenger, E.U. Hartge, T. Ogada, Z. Siagi, Combustion of agricultural residues, Progress Energy Combustion Science 26 (2000) 1. [7] B.J. Skrifvars, R. Backman, M. Hupa, G.A. Sfiris, T. byhammar, A. Lyngfelt, Ash behaviour in a CFB boiler during combustion of coal, peat or wood, Fuel 77 (1998) 65. [8] M. Theis, B.J. Skrifvars, M. Zevenhoven, M. Hupa, H. Tran, Fouling tendency of ash resulting from burning mixtures of biofuels, Fuel 85 (2006) 2002. [9] S.A. Benson, M.L. Jones, J.N. Harb, Ash formation and deposition, in: L.D. Smoot (Ed.), Fundamentals of Coal Combustion for Clean and Efficient Use, Elsevier, Amsterdam, 1993, p. 299. [10] S.A. Benson, J.P. Hurlery, C.J. Zygarlicke, E.N. Steadman, T.A. Erickson, Predicting ash behavior in utility boilers, Energy and Fuels 7 (6) (1983) 746. [11] G. Fariborz, Characteristics and composition of fly ash from Canadian coal-fired power plants, Fuel 85 (2006) 1418. [12] S. Khirani, R.B. Aim, M.H. Manero, Improving the measurement of the Modified Fouling Index using nanofiltration membranes (NF–MFI), Desalination 191 (2006) 1. [13] H.B. Vuthaluru, Remediation of ash problems in pulverised coal-fired boilers, Fuel 78 (1999) 1789. [14] B.C. Choi, H.T. Kim, Comparison of theoretically and experimentally determined simulated coal syngas turbulent jet flame lengths, Journal of Korean Industrial & Engineering Chemistry 8 (2002) 578. [15] G. Couch, Understanding Slagging and Fouling in pf Combustion, IEA Coal Research, London, 1994. [16] S. Su, J.H. Pohl, D. Holcombe, Fouling propensities of blended coals in pulverized coal-fired power station boilers, Fuel 82 (2003) 1653. [17] L.L. Baxter, T.R. Miles, T.R. Miles Jr., B.M. Jenkins, D.G. Dayton, R.W. Bryers, L.L. Oden, The behaviour of inorganic material in biomass-fired power boilers-field and laboratory experiences, Alkali Deposits Found in Biomass Power Plants, vol. II, National Technical Information Service US Department of Commerce, Springfield, 1996. [18] L. Douglas Smoot, Fundamentals of Coal Combustion for Clean and Efficient Use, 1st edn., Elsevier, New York, 1993, p. 339–368. [19] S. Su, Combustion behavior and ash deposition of blended coals, Ph.D. Thesis, The University of Queensland, Brisbane, 1999. [20] H.B. Vuthaluru, Remediation of ash problems in pulverized coal-fired boilers, Fuel 78 (1999) 1789. [21] J.G. Singer, Combustion fossil power systems, in: Combustion Engineering inc, 3rd edn., Rand McNally, New York, 1981, pp. 3–15. [22] A. Alastuey, A. Jime´nez, F. Plana, X. Querol, I. Sua´rez-Ruiz, Geochemistry, mineralogy, and technological properties of the main Stephanian_Carboniferous/coal seams from the Puertollano Basin, Spain, Coal Geology 45 (2001) 247. [23] T. Reg Bott, The assessment of fouling and slagging propensity in combustion systems, Inorganic Transformations and Ash Deposition during Combustion 511 (1992). [24] C.S. Park, S.H. Lee, S.I. Choi, H.S. Yang, Studies of the fusibility of coal ashes in oxidizing and reducing conditions, Journal of Korean Industrial & Engineering Chemistry 8 (1997) 179.
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Determination of fouling characteristics of various coals under gasification condition Li-Hua Xu a,b, Hueon Namkung a, Hyok-Bo Kwon c, Hyung-Taek Kim a,* a
Division of Energy Systems Research, Graduate School, Ajou University, Wonchon-dong, Yeongtong-gu, Suwon 443-749, Republic of Korea Institute for Advanced Engineering, Wonchon-dong, Yeongtong-gu, Suwon 443-749, Republic of Korea c Department of Material Science & Engineering, Kyungnam University, Republic of Korea b
A R T I C L E I N F O
Article history: Received 21 February 2008 Accepted 17 September 2008 Keywords: Fouling tendency Gasification condition Mineral
A B S T R A C T
In a coal gasifier and heat exchanger, adhered and deposited fouling inhibits heat transfer making the stable operation of the gasifier difficult. Since heat-transfer performance of the heat exchanger directly affects the output of Integrated Coal Gasification Combined Cycle plants, it is important to understand the effect of fouling on the characteristics of adhesion and deposition on the heat exchanger tube beforehand. But very few studies have been conducted about concerning the relationship between fouling property and adherability, which depend on the reducing condition on the gasifier system. The main purpose of the present investigation is to determine the low temperature ash deposition behavior under coal gasification condition by using drop tube furnace (DTF), in which behavior of coal particle in actual gasification condition can be simulated experimentally. Nine pulverized coal samples which are in the range of bituminous and sub-bituminous are injected into DTF under gasification condition. The ash particles are deposited onto sample collector by impacting and agglomerating actions; deposit samples of ash are collected, quantitative analyses are performed by EDX and weight measurements. Experiment results illustrated that mineral components are generally considered as a dominant parameter to determine behavior of ash deposition at low temperature among various physical and chemical properties of coal. It is also found that the alkaline earth mineral in coal ash such as MgO and CaO compounds leads to fouling deposition under the gasification condition. ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Integrated gasification combined cycle (IGCC) is recognized as the 2nd generation power plant which can provide higher efficiency of electric generation than power plant of the conventional pulverized coal combustion. But ash components of coal cause the major downtime of power plant during the operation of continuous system. The problem of ash deposition which plays a major role in hot-gas cleanup system is also a significant issue. The issues in hot-gas cleanup system generally include the deposit in the filter component, mechanism of ash removal, and vapor-phase and ash–ceramic interactions. The physical and chemical characteristics of the fly ash also influence their ability to be collected in control devices of air pollution such as selexol unit and claus unit [1]. Small amounts of the inorganic components contained in coal are converted into fly ash in the gasification process. The inorganic of fly ash, which will lead to costly shutdowns of a power plant [2–
* Corresponding author. Tel.: +82 10 2656 0770. E-mail address: [email protected] (H.T. Kim).
8], can cause problems such as deposition on refractory or heattransfer surfaces, the bed agglomeration and to cause corrosion and erosion of system parts. The type and degree of these problems are usually dependent upon ash composition of coal, operating system condition, and system design criteria. Many researchers have been conducted to determine ash formation and deposition behavior in pulverized coal boilers [1,8–14]. Most of the studies are concentrated on the development of ash formation/deposition mechanisms. And predictive models have been developed to base on the experimental data. But a limited number of studies have been done to determine the ash deposit behavior in the temperature range of fouling. In general, fouling deposits do not directly exposed to flame radiation such as more closely to spaced tubes in the convection sections of boilers. In most cases, fouling deposits do not contain the high levels of liquid phases associated with slagging-type deposits. They usually consist of a combination of silicates and sulfates that sinter the ash particles together [15,16]. In the present investigation, fouling experiments under the gasification conditions are conducted with drop tube furnace (DTF). To simulate fouling phenomenon in the outlet section of gasifier and heat exchanger section of syngas cooler, fouling
1226-086X/$ – see front matter ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2008.09.004
L.-H. Xu et al. / Journal of Industrial and Engineering Chemistry 15 (2009) 98–102
99
Table 1 Basic analysis of coal samples during the experiment. Datong
Cyprus
Denisovsky
Shenhua
Kideco
Usibelli
Adaro
Baiduri
Roto
Proximate analysis (dry-basis) (wt%)
V.M F.C Ash
33.52 57.36 9.12
46.64 47.3 6.06
25.81 60.98 13.19
31.83 61.64 10.6
50.1 48.23 1.67
48.19 41.75 10.06
48.45 48.54 3.01
53.95 41.02 5.03
46.52 50.2 3.28
Ultimate analysis (ash-free base) (wt%)
C H O N S
73.86 4.75 20 0.73 0.66
68.22 5.25 24.55 1.19 0.79
85.6 5.16 7.75 1.13 0.36
77.67 4.4 1.07 0.29 16.57
64.12 5.2 29.91 0.24 0.53
63.56 5.32 30.17 0.75 0.2
72.73 5.38 20.89 0.81 0.19
68.16 5.04 21.74 1.27 0.79
66.99 4.49 27.44 0.97 0.11
Heating value
kcal/kg
6607
6080
7139
behavior of ash at the low temperature is experimentally performed to evaluate fouling tendency of ash by using the DTF of vertical form. Ash of coal samples are collected on the probe surface of plate type whose area of deposit plate is various. Fouling behaviors of different coal samples are analyzed with experiments data as well as approached theoretically. Commonly, fouling is initiated by the deposition of condensed vapor materials which make a thin layer. The composition of these materials is typically high in alkaline earth and alkali metals [15,17]. Fouling deposition behavior of particle can be explained by surface deposition of sticky minerals and followed by relatively high-temperature deposition [18]. For most cases, the innermost layers consist primarily of small particles and Na, K, Ca, and Mg which are transported to the surface by vapor-phase diffusion and pyrolysis. The initial deposit layers may provide a sticky surface to trap impacting particles by inertial force which is not sticky. In addition, the initial layers may provide fluxing materials that will cause larger particles to be melted. Accordingly, small particles are bonded to the heat-transfer surface, and they provide sites for continued deposition to the entire particle deposition. The liquid phase materials contribute to the thickening of bonds among particles. Because of a result of the insulating effect of the deposit layer on the surface, the outer layers of the deposit are formed at higher temperatures. These higher temperatures cause melting and interaction of the particles to a liquid phase. Once a liquid phase has formed on the outside of the deposit, it becomes an efficient collector of ash particles regardless of individual melting characteristics of the particles.
6785
6729
5304
6748
6367
6154
Fig. 1. Schematic view of the entire experimental facility and probe plate.
2. Experimental Coal samples of nine types in range of bituminous and subbituminous coal are chosen for the experiments. These coal samples represent a wide range of mineral contents. Basic analysis of coal samples is illustrated in Table 1 that indicates the results of proximate and ultimate analysis. The chemical compositions and
fusion temperatures of ash are also given in Table 2 with chemical elements of Si, Al, Fe, Ca, Mg, Na, K and Ti which are assigned as major components of the ash. Original coal samples were dried and crushed in a disk mill of fan type, and then were separated below 74 mm by electromagnetic sieve shaker. The particle size of coal should be used below 74 mm for the DTF experiments.
Table 2 Chemical components and fusion temperature of ash in coal samples used during the experiments. Datong Inorganic analysis (wt%)
SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O TiO2
Ash fusion temp. (8C)
IT ST HT FT
Cyprus 55.16 19.07 13.18 8.53 1.23 0.64 1.40 0.78 1141 1204 1243 1308
Denisovsky 62.29 16.89 7.19 8.34 2.16 1.10 1.10 0.93 1155 1165 1193 1289
Shenhua 55.36 26.74 8.41 4.45 1.92 0.41 1.71 1.00 – – – 1400
Kideco 45.51 17.18 10.89 21.49 1.55 1.50 1.10 0.78 1165 – 1242 1258
Usibelli 41.66 16.64 23.58 13.20 2.82 0.21 1.08 0.80 1265 1295 1326 1408
Adaro 45.10 19.98 6.33 22.18 3.30 0.98 1.34 0.78 1250 – 1290 1340
Baiduri 42.85 20.54 18.12 12.30 2.83 0.88 1.55 0.93 1250 – 1290 1340
Roto 34.32 19.28 11.65 21.28 6.97 4.45 1.22 0.85 1153 – 1260 1270
37.49 16.46 19.32 17.46 7.24 0.09 0.91 1.04 1153 – 1260 1270
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Fig. 2. Photograph of the attached fouling on deposit plate.
The deposition behavior of coal samples is studied by using the laminar DTF and probe plate of fouling deposit as shown in Fig. 1. That equipment simulates conditions of the coal gasifier. The DTF system consists of coal sample injector, pre-heater, main tube furnace reactor, deposit probe and the others equipments. Temperatures within the 1.5 m of DTF section are set at 1300 8C. The zone is divided to zone 1 (top), zone 2 and 3 during the experiment, because temperature can be easily controlled by proportionalintegral-derivative (PID). All of temperature sections are controlled by PID system. Pulverized coal samples are fed at the top of DTF with concentric primary air flow, where particles are injected with the primary air into the furnace by mechanical screw feeder. During each experiment, coal sample is continuously fed for 30 min to the DTF with the coal feed rate (0.5 g/min) and the O2/coal weight ratio (0.9). Both the pre-heater and reactor are set to 1300 8C and the surface temperature of probe sample was set to 700 8C. After the experiment, deposit samples removed from the DTF are measured by scratching off the loosely attached sample on probe and weighing the deposits. Once the coal samples are introduced into the furnace section, the particles are entrained by primary gas. During the flight to the bottom of main reactor, particles are heated and passed through the stages of devolatilization and char burning before it is reduced to ashes. Prior to leaving the DTF through the exhaust system, the particles enter a sampling section, in which an air-cooled deposit probe is horizontally inserted into the end of lowest furnace section. Vertical position of the probe move up and down to adjust the surface temperature of the probe by mechanical equipment which is operated by pushing the button. The reacted ash samples are collected on the funnel-shaped deposit probe at the bottom. Experiments are preceded with the change of coal sample and deposit area. The removed deposit plate is separated from the deposit probe after each experiment and the deposit plate is turned up to scrape off the attached fouling which is shown in Fig. 2. The deposit fouling can be finally analyzed. 3. Results and discussion 3.1. Result of EDX analysis The sample near the top and bottom of the fouling layer is analyzed by energy dispersive X-ray spectroscopy (EDX) equip-
Fig. 3. Images of the deposited fouling layer.
Fig. 4. EDX analysis of deposit fouling of top and bottom layer.
ment and the analysis results of the fouling are shown in Figs. 3 and 4. In Fig. 3, the top layer includes the 3 mm from the upper part and the bottom layer includes the 3 mm from the lower part. Fig. 4 shows the ratio of component contents of the major basic elements such as Mg, Ca, K and component contents of the major acid elements such as Al, Si between the top and bottom based on EDX analysis. From Fig. 4, it can be seen that the ratio of [%(MgO + CaO + K2O)/%(SiO2 + Al2O3)] in bottom layer is greater than that of top layer. This phenomenon explains that the initial deposition acts are related with the amount of basic element components. It seems that fouling including volatile alkali and alkaline earth metal is primarily responsible even though it usually exists in stable silicate such as illite [5]. It seems that volatile alkali and alkaline earth metal are primarily responsible for fouling even though those content are little. Potassium may contribute to fouling by forming low-melting K silicates on the deposit probe and providing an initial sticky surface where fly ash particles can be adhered [19]. Widely distributed potassium vapor and alkaline earth metal composition in the combustion gases contact with ash particles and metal surfaces where aluminum, silicon and other mineral components are condensed. 3.2. Deposit shape of various coal samples The plate appearance of deposit probes taken from DTF furnace is shown in Fig. 5. Major mineral content ratio of [%(MgO + CaO + K2O)/%(SiO2 + Al2O3)] from results is shown in Fig. 6. It clearly is shown in the photographs that fouling deposition of Denisovsky coal, Datong coal and Cyprus coal is more scattered on the deposit probe. But the fouling deposition of Kideco, Shenhua, and Usibelli coal are relatively concentrated on the deposit probe plate center. It indicate through comparison between Figs. 5 and 6 that the mineral ratio of [%(MgO + CaO + K2O)/%(SiO2 + Al2O3)] leads to characteristics of the deposited fouling. When mineral ratio of [%(MgO + CaO + K2O)/%(SiO2 + Al2O3)] in coal has a little amount, the fouling is dispersed on deposit plate; when mineral ratio of [%(MgO + CaO + K2O)/%(SiO2 + Al2O3)] in coal has a lot of amount, the fouling is concentrated on deposit plate center. And also the height of deposit fouling is higher. When alkali metal and alkaline earth metals of the low melting temperatures are mixed, it produces low fusibility temperatures. The low temperature contributes to deposit building. The dispersion throughout the gas stream subsequently makes other ash particles condensed together and fouling deposit on probe surfaces. The condensed alkali metal and alkaline earth metals provide a binding matrix for
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Fig. 5. Images of the fouling deposition on different coal samples.
aluminum, silicon, and ash particles to fuse together and build up on deposit surface. So the high alkali and alkaline earth mineral content make the fouling surface sticky. So particle surfaces can be bondable and fouling deposit can be rapidly formed. 3.3. Deposition prediction vs. the mineral content It is generally accepted that the chemical composition of incoming ash particles determine the deposition behavior of fly ash on surfaces. The surface composition of the ash particles is important in predicting the ash deposition, as it influences the adhesion on deposit surface [20]. And total alkali content is an alternative index, considering that fouling deposits usually contain high alkaline earth, alkali metals (Ca, K or Na), and their sulfates [15]. But the total quantity of alkalies is not necessarily an accurate index of fouling potential. Fouling phenomenon is well related to the quantity of ‘‘active alkalies’’. Organically bonded alkali metals
have the potential to form very reactive species in the combustion zone as transient elements or compounds. CaO and MgO have been found to have similar fluxing properties, and the softening temperature decreases with high values of CaO and MgO [21,22]. An empirical formula provides the estimate of total alkali. It is defined as [%(MgO + 0.719CaO)/%ash] where 0.719%CaO is the molar equivalent of MgO [16,21,23]. Based on those precedence researches, we try to predict the fouling tendency by the mineral content in feeding coal. The condensation of volatile alkali compounds on surfaces has long been thought to initiate fouling by first forming a thin sticky layer on tube surface and later to sinter the particles in the deposit [15]. So, the alkali contents have been used as a fouling index to predict fouling propensity. The mass is obtained by the following steps. After measuring of total weight of sampled fouling, weight measurements are made of the free ash and fixed ash when sampling probe is turned up. Calculation from fixed ash weight divided by total feeding coals, the propensity of fouling deposition is represented in Fig. 7. The result shows the fouling propensity, against the %(Na2O + K2O) in ash for the coals in the DTF under gasification condition. From this figure, we can see that there is no apparent trend between the fouling deposit propensity and the alkali contents. Hence, this index has trouble ranking of fouling propensity, at least for the coals tested in this study. However, it is not certain that this index is useful for other fuels or for other operation conditions, because the alkali contents for all the tested coals are relatively the lower than the other mineral contents, and the experiment has been done at high temperature and reducing condition of the DTF where volatilization easily happen. Fig. 8 shows the fouling propensity against the total alkaline earth mineral content in ash for feeding coals. total alkaline earths % ¼ %ðMgO þ 0:719CaOÞ
Fig. 6. Comparison of [%(MgO + CaO + K2O)/%(SiO2 + Al2O3)] ratio for feeding coal.
We can find from the figures that the tendency of fouling deposit is similar with tendency of total alkaline earth contents in feeding coal. Therefore the big value of %(MgO + 0.719CaO) in coal
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(2) The characteristics of deposited fouling is related with the mineral contents ratio of [%(MgO + CaO + K2O)/%(SiO2 + Al2O3)] in feeding coal. When mineral ratio of [%(MgO + CaO + K2O)/ %(SiO2 + Al2O3)] in coal has a lot of amount, the fouling is concentrated on deposit plate center. And also the height of deposit fouling is higher. (3) The fouling predictions, which are based on the combined use of the extended coal ash composition, indicate that the tendency of fouling deposition is related with total alkaline earth content in feeding coal. Consequently, the big value of %(MgO + 0.719CaO) in coal has high tendency of fouling deposition under the gasification condition. References
Fig. 7. The fouling propensity against total alkali content.
Fig. 8. . The fouling propensity against total alkaline earth content.
has high tendency of fouling deposition. The reason is that the alkaline earth contents of coals are relatively higher. And, the more alkaline earth content is increasing, the less ash melting temperature is decreasing. That effect easily happens at reducing atmosphere [24]. 4. Conclusions This paper presents the experimental results on the fouling propensities of nine coals under the gasification condition by using DTF. The deposited fouling is scraped off to analyze their chemical characteristics and weights. We evaluated the low melting temperature on basic oxides CaO and MgO and major acidic oxide components. The following conclusions are made from this experiment results. (1) From result of EDX analysis, the top layer and bottom layer of the fouling from the deposit probe in the DTF is analyzed. The mineral contents ratio of [%(MgO + CaO + K2O)/%(SiO2 + Al2O3)] in bottom layer is greater than that of top layer.
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