Energy Fuels 2010, 24, 5109–5115 Published on Web 08/09/2010
: DOI:10.1021/ef100197k
Research on Flow Characteristics of Slag Film in a Slag Tapping Gasifier Xiaoyu Li,† Guangyu Li,*,‡ Zidong Cao,† and Shisen Xu‡ †
Xi’an Jiaotong University, 28 Xianning West Road, Xi’an, Shaanxi 710049, People’s Republic of China, and ‡Thermal Power Research Institute (TPRI), 136 Xingqing Road, Xi’an, Shaanxi 710032, People’s Republic of China Received February 21, 2010. Revised Manuscript Received July 9, 2010
A mathematical model about the flow and heat-transfer properties of a slag film inside a slag tapping gasifier is established in this work, which is based on traditional flow and heat-transfer equations. When the physical model is simplified, a solution equation derived to calculate the slag film thickness and flow velocity has been developed. In this study, the relationship of slag viscosity-temperature is investigated experimentally with 15 Chinese coal samples based on experimental observation and data. Generated slag is classified into four types: glassy, crystal, plastic, and alkalescent slags. The effects of glassy and crystal formations in slag are discussed, and flow characteristics based on their own viscosity-temperature characteristics of glassy, crystal, and plastic slags and the effects to the model are also discussed. Three coal samples are selected for modeling calculations, and the thickness of the slag film of one coal is also given through measurements. The calculation results are validated by the experimental data. The results show that the slag flowing layer is thin and the liquid slag can easily be discharged if the temperature is high enough. The thickness of the flowing layer of crystal slag is the minimum, and the thickness of glassy slag is the maximum.
have been carried out. For example, Seggiani et al.5 built a model to simulate slag behaviors in the Prenflo entrainedflow gasifier and indicated that the slag flow properties were affected by the varying time. Also, Yuan et al.6 found experimentally that the slag deposition during the gasification process required some time to reach a static state. Some efforts were made by Zhou et al.7 to establish a numerical simulation method, but they failed to describe the slag flow in detail. It was shown that the slag flow properties were related to the slag viscosity, which was affected mainly by its mineral contents and temperature.8-11 Various slag temperature-viscosity relationships were established by some researchers, according to the slag (or ash) mineral contents.12-16 However, because of the complexity of coal mineral compositions and the matrix structure, up to date, much deeper research is under way. In this work, we attempted to establish a model based on the traditional flow and heat-transfer equations, related to the slag flow properties inside an entrained-flow gasifier with a membrane wall structure. When the physical model is
1. Introduction Because of a higher energy demand and more stringent emission regulations, some novel power generation technologies have been introduced and developed fast recently. Among them, coal gasification technology and, particularly, integrated gasification combined cycle (IGCC) are thought to be ideal methods to replace conventional coal-fired thermal power generation. The O2/steam-blown entrained-flow gasification technology has been successfully used in several IGCC plants in Europe and the U.S.A.1 In China, a novel entrained-flow gasification technology, developed by Thermal Power Research Institute (TPRI, Xi’an, China), will be demonstrated in a 250 MW IGCC plant (Greengen Project, Tianjin, China). A pilot-scale gasifier based on this technology, with a capacity of 36 tons/day, was built and commissioned in 2006.2,3 In the entrained-flow gasification process, the slag tap technology is adopted, because of the high operation temperature, which is above the slag melting point. During the gasification process, melted ash (slag) flows down to the bottom of the gasifier. The slag film properties could influence the heat transfer between syngas in the furnace and membrane wall (such as Shell and TPRI gasifiers) and, hence, affect the reaction temperatures and extents of the reaction. The slag flow properties during the coal gasification process were described elsewhere.4 Recently, some relevant research studies
(6) Yuan, H. Y.; Qu, H. G.; Ren, H. P.; et al. J. East China Univ. Sci. Technol. 2005, 31, 393–397 (in Chinese). (7) Zhou, J. H.; Kuang, J. P.; Zhou, Z. J.; et al. Proc. Chin. Soc. Electr. Eng. 2007, 27, 23–28 (in Chinese). (8) Song, W. J.; Tang, L. H.; Zhu, X.; Wu, Y.; Rong, Y.; Zhu, Z.; Koyama, S. Fuel 2009, 88, 297–304. (9) Patterson, J. H.; Hurst, H. J. Fuel 2000, 79, 1671–1678. (10) Van Dyk, J. C.; Benson, S. A.; Laumb, M. L.; Waanders, B. Fuel 2009, 88, 1057–1063. (11) Oh, M. S.; Brookerb, D. D.; de Pazb, E. F.; Bradyb, J. J.; Deckerb, T. R. Fuel Process. Technol. 1995, 44, 191–199. (12) Bryers, R. W. Prog. Energy Combust. Sci. 1996, 22, 29–120. (13) Raask, E. Mineral Impurities in Coal Combustion: Behavior, Problems, and Remedial Measure; Taylor and Francis: Abingdon, U.K., 1985; pp 58-137. (14) Browning, G. J.; Bryant, G. W.; Hurst, H. J.; Lucas, J. A.; Wall, T. F. Energy Fuels 2003, 17, 731–737. (15) Xu, S. S.; Wang, B. M.; Li, G. Y.; et al. Therm. Power Gener. 2007, 7, 5–7 (in Chinese). (16) Van Dyk, J. C. Miner. Eng. 2006, 19, 280–286.
*To whom correspondence should be addressed. E-mail: liguangyu@ tpri.com.cn. (1) Beer, J. M. Science 2000, 26, 301–327. (2) Zhang, D. L.; Xu, S. S.; Ren, Y. Q. Coal Chem. Ind. 2005, 33, 23–25 (in Chinese). (3) Xu, S. S.; Li, X. Y.; Ren, Y. Q.; et al. Electr. Power 2007, 40, 42–46 (in Chinese). (4) Higman, C.; van der Burgt, M. Gasification; Elsevier: Amsterdam, The Netherlands, 2003. (5) Seggiani, M. Fuel 1998, 77, 1611–1621. r 2010 American Chemical Society
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Energy Fuels 2010, 24, 5109–5115
: DOI:10.1021/ef100197k
Li et al.
simplified, the slag film thickness as well as the flow velocity distribution could be calculated and the heat-transfer equations could be obtained. The slag viscosity could be derived by solving an equation set. The calculation results are validated by experimental work carried out on a pilot-scale gasifier, in which the temperature-viscosity relationships are obtained for different kinds of slags. 2. Modeling Section In general, the slag film on the surface of the gasifier membrane wall could be divided into three layers, as shown in Figure 1. The outer layer is called the real liquid flowing layer, in which no solid crystals exist. Below the outer layer is the plastic flow layer where some solid crystals could be found. The bottom layer is the immobile layer.4,17 There is an interface before which slag flows and after which slag does not flow. The interfacial temperature is defined as T0. Detail discussions about the slag film structure and T0 interface are carried out in the Results and Discussion. The heat flow rate inside the slag film could be calculated by Q ¼
λHf ðTzhm - T0 Þ δ
ð1Þ
where Tzhm is the known slag film surface temperature, λ is the thermal conductivity, δ is the slag thickness of the real liquid flowing layer, and Hf is the surface area. As shown in Figure 2, the force balance equation is given by dSdy1 þ γdxdy1 ¼ 0
Figure 1. Structure of the slag film.
ð2Þ
where γ is the slag specific gravity and S is the viscous drag, which could be obtained by S ¼ μ
dvy dx
ð3Þ
From eqs 2 and 3 d2 vy 1 dμ dvy γ þ þ ¼ 0 dx2 μ dx dx μ
ð4Þ
where vy is the slag flow velocity and μ is the slag viscosity, which could be derived as a function of time (t) μ ¼ f ðtÞ
ð5Þ
Equation 5 will be given by the experiments shown in the next section. In addition bzh ¼
ðt - to Þ x
ð6Þ
Figure 2. Force balance of liquid slag.
where bzh is the temperature gradient. Boundary conditions are when x ¼ 0, vy ¼ 0
The thickness of slag film δ in eq 1 can be acquired by G ¼ FLδV
ð7Þ
where G is quantity of slag running off the gasifier unit time, F is the slag density, L is the circumference of the gasifier furnace, and V is the average of vy.
and when x ¼ δ, μ
dvy ¼ Sm, y dx
ð9Þ
ð8Þ
3. Experimental Section
where Sm,y is the shearing stress, which is assumed to be zero. When eqs 4-8 are combined, the equation set could be solved.
In this section, related experimentations about the slag viscosity-temperature characteristics and the measurement of the slag film thickness in the gasifier are described. 3.1. Slag Viscosity-Temperature Characteristics. The objective of the experiments is to measure the ash viscosity-temperature characteristics and slag freezing points and to acquire
(17) Thermal Power Research Institute (TPRI). Cyclone Furnace Boiler and Its Ash and Slag Multipurpose Utilization; TPRI: Xi'an, China, 1979; pp 81-82.
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: DOI:10.1021/ef100197k
Li et al. Table 1. Tested Slag Components components (wt %)
slag type
glassy slag
plastic slag
crystal slag alkalescent slag
coal
SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
Na2O
SO3
Yungang Pingwang Binggou Babao Pingzhuang Maiming Datai Yima ShenHua Huangxian Lingzi Zaotian Yuanhua Lujiadi ShenMu Bucun Shulan HuaTing Longtan Zhaobei
40.95 56.17 59.05 59.76 58.83 55.09 56.8 57.95 51.71 46.92 47.25 48.23 47.03 57.62 47.50 53.62 57.01 54.00 15.49 17.32
14.97 20.27 19.99 20.43 22.02 24.83 19.74 22.91 12.57 10.32 25.91 21.64 30.15 26.7 17.34 29.64 30.42 24.88 8.36 9.73
30.07 13.59 8.4 8.14 7.28 10.32 6.21 8.41 10.34 5.96 13.82 15.34 12.22 8.43 10.47 7.57 4.11 5.87 5.79 11.43
5.36 3.43 3.15 5.35 3.59 1.43 1.41 3.60 11.61 24.49 4.72 6.11 2.71 2.11 10.44 3.23 3.41 3.72 45.08 31.24
1.12 1.38 2.15 1.10 1.26 1.48 1.69 1.27 4.12 1.82 1.62 1.22 1.43 1.29 1.84 1.25 1.53 3.53 2.10 2.11
0.88 1.46 2.57 2.45 2.67 2.52
0.18 0.18 2.07 0.73 0.4 0.28
2.84 1.84 1.11 1.75 1.09 1.44 2.27 1.30 1.73 1.31 1.25 0.63 0.55
0.39 0.84 0.72 0.52 0.32 0.53 0.30 0.80 0.24 0.2 1.33 0.14 0.16
5.15 2.52 2.02 2.04 2.18 0.47 2.1 2.43 5.41 5.99 2.23 4.69 2.63 1.03 8.68 1.36 1.28 3.60 21.32 27.46
analytical data were recorded as well as some other operational data, such as the thermal load. The program for the gasifier immediate shutting down was triggered, and within several seconds, the coal burners of the first and second stages as well as the steam and oxygen nozzles were turned off. The purge nitrogen pipe located in the coal and oxygen lines were switched on simultaneously, and nitrogen with a temperature of 80 °C was purged into the gasifier. At this moment, the high-temperature raw syngas inside the gasifier lower chamber was replaced by nitrogen and the local temperature dropped below the ash melting point immediately. It could be suggested that the thickness of the slag membrane is unchanged and frozen during this short period of time. The gasifier manhole was opened, and the gasifier internal chamber was vented. Then, the sampling procedure can be conducted, and the thickness of the slag membrane can be measured.
the information of ash/slag compositions and morphologies. Equipment XTPRI-2 was used during the experiments. The experimental system includes a high-temperature molybdenum wire furnace, a measurement rod, an alumina crucible, a molybdenum wire, a thermocouple as well as gas inletting, and a sample preparation system. The viscosities of the melting slag can be measured by rotating the rod, and the slag freezing points can be determined by plunging the molybdenum wire into the slag. Also, through the observation the cooled slag, some slag morphology and composition could be obtained. The main experimental procedure is described as follows. The experiments were carried out in a reduced atmosphere (with volume flow rate, H2 = 0.833 333 35 10-5 m3/s and N2=0.833 333 3510-5 m3/s). The sample was heated to 1780 °C and kept for 5 min. The viscosity can be measured by the plunged rod. Then, the viscosity was measured every 50 °C temperature drop until the value was hard to measure. The molybdenum wire was then plunged into the slag sample, and the freezing point was measured. In addition, the sample form was observed carefully. After measurement, the cool slag in the crucible was observed to find slag morphology. 3.2. Measurement of the Slag Film Thickness in the Gasifier. For the experiment carried out in the TPRI gasifier, the thickness of the slag film was measured during pilot gasifier operation. The configuration of the gasifier is shown in Figure 6. The gasifier has an erectly cylindrical vessel with a membrane wall structure, and it is divided into two reaction chambers. During the operation, approximately 80-90% of coal is injected into the lower chamber through four opposite nozzles and reacted with oxygen and steam. The reaction temperature could reach 1500-1800 °C, which is higher than the ash fusion point. The melted slag flows down along the membrane wall and forms a slag film on the chamber inside the surface. The slag is then quenched in a slag bath at the bottom of the gasifier. The generated fuel gas flows upward into the upper reaction chamber, where the remaining coal is fed together with steam. The endothermic reactions could decrease the fuel gas temperature to 900 °C. The cooling water inside the membrane wall is used to control the surface temperature of the gasification chamber. Several coal samples were tested for their gasification performance in this gasification system. The test in this study was conducted during the final stage of the system operation. The main steps of the test are described as follows. The system was operated steadily in the final stage of the plant operation. The coal sample tests had been completed, and the whole plant was ready to be shut off. During this period of time, the coal
4. Results and Discussion 4.1. Slag Viscosity-Temperature Characteristics and Slag Film Structure. Through the experimental observation and data, the slag samples could be classified into four categories, which are described as follows. 4.1.1. Glassy Slag. The glassy slag includes the slags produced from Yungang, Pingwang, Binggou, Babao, Pingzhuang, Maiming, Datai, and Yima coals. Their slag components are shown in Table 1, and their slag viscositytemperature curves are shown in Figure 3. Their viscositytemperature curve is comparatively flat. When the temperature drops to lower values, however, they are still in the melting conditions. The molybdenum wire could be used to penetrate the melting slag. When the wire is removed, some thin glassy wires appear and the slag in the crucible appears to be glassy formation. The melting slag generated under the test conditions contained some amount of crystals, which also condensed on the bottom of the crucible, forming a lay with a height of 0-5 mm. The color and appearance of the condensed crystal are totally different from the vitreous body. Because of the low concentration of the crystals (volume ratio less than 15%), this component has less influence on the slag viscosity compared to the temperature effect. Therefore, the viscosity-temperature curves have no inflection and freezing points. 5111
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Figure 5. Viscosity-temperature curve of the plastic slag. Figure 3. Viscosity-temperature curve of the glassy slag.
Figure 6. Configuration of the TPRI two-stage gasifier. Figure 4. Viscosity-temperature curve of the crystal slag.
The lithofacies contained in the crucible slag are more than that found in the glassy slag samples. Its volume could reach above 50%. Also, different lithofacies are located in the different positions in the crucible because of different densities. For example, the Lingzi coal slag is located at the bottom of the crucible, The Huangxian slag is distributed around the crucible wall, and the Lujiadi and Huainan slags are located in the middle. 4.1.4. Alkalescent Slag. This group includes the slag samples produced from Longtan and Zhaobei coals. Their slag components are shown in Table 1. It is one type of plastic slag whose viscosity is very low. From the slag-samplemaking process, it could be observed that, around a certain temperature, some large amount of mineral components in the liquid phase penetrates out of the slag sample and the mixture of liquid- and solid-phase components appear inside the crucible. Some bubbles are generated from the liquid phase, lifting the scum. The melting slag is stratified and unstable; therefore, the viscosity-temperature measurement could not be made. There is only lump scum located inside the crucible, and the slag in the liquid phase has been leaked out because of crucible corrosion. In addition, when the amount of the slag in the solid phase is above some certain value, the rotating rod slips. At this time, measuring the viscosity is difficult; therefore, in this paper, the discussion of alkalescent slag is not included.
4.1.2. Crystal Slag. The slag samples produced from Bucun and Shulan coals belong to this group. Their slag components are shown in Table 1, and their slag viscosity-temperature curves are shown in Figure 4. Their viscosity-temperature curves have a remarkable inflection point. When the temperature drops to below this temperature point, the rotating rod was halted and the molybdenum wire could not be penetrated into the sample. Once the furnace temperature increases, the sample can be unfrozen immediately. The inflection point could be regarded as the crystal slag freezing point, and the cool slag in the crucible is lithofacies. 4.1.3. Plastic Slag. This group includes the slag samples produced from Huangxian, Lingzi, Zaotian, Yuanhua, and Lujiadi coals. Their slag components are shown in Table 1, and their slag viscosity-temperature curves are shown in Figure 5. Their viscosity-temperature curve also has an inflection point. In addition, before and after the inflection point, the curve appears differently. This point should not be the slag freezing point. When the temperature of the melting slag drops to a certain value, its viscosity increases unsteadily with the time but it does not freeze immediately. Instead, the viscosity varies with the temperature according to the new rule. The molybdenum wire could be plunged into the slag sample. When the wire is taken out with a small amount of melting slag, which is thin glassy wires, some “holes” appear. 5112
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Through the observation and above description, glassy and crystal formation in slag will affect the slag viscositytemperature characteristics. If slag has a high concentration of the glassy formation, the slag viscosity-temperature characteristics are similar to those of normal industrial glass. Also, the viscosity-temperature curve is comparatively flat. The slag is still in the melting conditions when the temperature drops to lower values. That is, typical glassy slags have no freezing temperatures. If slag has a high concentration of the crystal formation, the slag viscosity-temperature characteristics are similar to those of normal crystals. There is one freezing temperature above which the crystal is in the liquid phase, and the viscosity-temperature curve is comparatively flat. When the temperature is below this point, the crystal contains some amount of frozen solid. There is one inflection point in the whole viscosity-temperature curve, and the typical crystal slags have this characteristic. If both glassy and crystal formations are considerable in slag, the slag viscosity-temperature characteristics are co-determined by both formations. When the temperature is higher than the frozen temperature of crystal formation, both formations are in the liquid phase. Below this temperature, the crystal has frozen solids and is mixed with liquid glassy formation. The slag viscosity-temperature curve has one inflection point at this temperature. However, the slag has not been frozen entirely because liquid glassy formation still exists. When the temperature drops to lower values, viscosity of glassy formation increases and the slag has frozen solids. Plastic slags typically have this characteristic. Overall, the glassy and crystal formations in slag could affect the slag viscositytemperature characteristics. In the Modeling Section, the basic model is introduced; however, it should be adjusted by different slag viscositytemperature characteristics because the interface between the flowing and quiescent layers should be determined by the viscosity-temperature characteristics of different types of slags. For crystal slags, when the temperature is lower than the inflection point, they do not flow. When the temperature is higher than the inflection point, slags are in the liquid phase and the slag viscosity is low enough to ensure its flow. Therefore, the interface temperature for the crystal is the inflection point temperature. For glassy slags, there are no freezing or inflection points. They can flow at low temperature with high viscosities if it is assumed to be Newtonian fluid. However, engineering experience shows that the slag viscosity must be greater than 1000 Pa s in the slag tapping process; therefore, plastic fluid, a type of non-Newtonian fluid, is appropriate. For plastic slag, plastic fluid is also appropriate; however, the flow characteristics are different before and after the inflection point. On the basis of the viscosity-temperature curve above, the slag viscosity-temperature function, which was established by Fulcher,14 is μ ¼ aeb=T - Tn
Figure 7. Viscosity-temperature curve of ShenHua coal.
Figure 8. Viscosity-temperature curve of HuaTing coal.
Figure 9. Viscosity-temperature curve of ShenMu coal.
ð10Þ
and discuss the effect of glassy, plastic, and crystal slag to slag film, three coal samples were used and their slag composition is given in Table 1, according to which the slag samples are glassy, crystal, and plastic slag. On the basis of the engineering data, the heat-transfer rate of the slag film is assumed to be 120 kW and the temperature of membrane wall surface is set to 260 °C. The slag flow rates were dependent upon the ash contents of the coal sample.
where T is the slag temperature and a, b, and Tn are constants. In this study, this equation is adopted to calculate the slag viscosities, and the values of a, b, and Tn are determined by the slag viscosity-temperature curves obtained by the experiments. Multiple linear regression methods are used in the calculations. 4.2. Slag Film Calculation and Experimental Data. To validate the mathematical model established in this study 5113
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: DOI:10.1021/ef100197k
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Table 2. Thickness of the Slag Film Calculated ShenHua coal flowing layer (mm) whole slag film (mm)
-5
2.81 10 6.21
HuaTing coal -5
1.01 10 9.36
Table 3. Experimental Slag Film Thickness of ShenHua Coal
ShenMu coal -5
1.35 10 6.78
sampling point
1
2
3
4
slag film thickness (mm)
6.52
6.38
6.81
6.47
Figure 12. ShenMu coal slag velocity distribution. Figure 10. ShenHua coal slag velocity distribution.
as random points on the vertical membrane wall. During the sampling process, it could be observed that a uniform slag film was formed. The calculated results are validated by the experimental data; however, deviations existed because of the following main reasons: (i) the gasifier shutting down the process, which could bring some measurement errors, (ii) different coal data used in the experiments and calculations, (iii) different thermal load between the experiments and values used in the calculations, and (iv) assumptions and simplifications in the model, which could bring some calculation errors. In general, the measurement during the gasifier operation is difficult because the gasifier is operated at high temperature and high pressure and the working environment in the gasifier reaction chamber is severe. The measurement carried out under the cold conditions is easy to achieve; however, error is inevitable. Through analysis of the calculated results, it is indicated that the slag flowing layer is thin, regardless if it is glassy, crystal, or plastic slag. It is shown that liquid slags can easily be discharged if the temperature is high enough. When the flowing layer of the three types of slags is compared, the thickness of the flowing layer of HuaTing coal is the minimum and the thickness of the flowing layer of ShenHua coal is the maximum. The reason is that the crystal slag has a high freezing temperature at which the slag viscosity is low; therefore, the viscosity of the flowing layer is low. For the glassy slags, the reverse is true. Besides, the viscositytemperature curves of glassy slags are comparatively flat. The value of HuaTing slag film thickness is the maximum, and the value of ShenHua slag film thickness is the minimum. The reason is that the thickness of slag is affected by the slag flowing temperature and that the crystal slag flowing temperature is higher than that of the glassy slag in general. The Results and Discussion show that, during the gasifier operation process, the slag thickness can be adjusted by the heat load and the slag quantity and the heat load can be changed by controlling the reaction temperature. It also shows that the thickness of the glassy slag flowing layer is easier to be adjusted than that of the plastic slag. The thickness of the crystal slag is relatively difficult to be adjusted,
Figure 11. HuaTing coal slag velocity distribution.
The derived slag viscosity-temperature curves forming three slag samples are shown in Figures 7-9. From these curves, the values of a, b, and Tn in eq 10 are obtained and the viscosity μ can be calculated as the function of the temperature T μ ¼ 0:000001e19082=ðT - 230Þ μ ¼ 0:01e3554=ðT - 1091Þ (
ðfor ShenHua coalÞ ðfor HuaTing coalÞ
μ ¼ 0:001e7026=ðT - 686Þ ðT g 1320 °CÞ μ ¼ 0:001e4506=ðT - 907Þ ðT < 1320 °CÞ
ðfor ShenMu coalÞ ð11Þ
When eqs 4, 7, 8, 9, and 11 are combined, the slag thickness and velocities can be calculated, as shown in Table 2 and Figures 10-12. In the TPRI pilot plant, the slag thickness was measured by the method described in the Experimental Section. The coal sample used was ShenHua coal. The experimental data are shown in Table 3. The four sampling points are selected 5114
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and flux can be used to change its viscosity-temperature characteristics.
groups, namely, glassy, crystal, plastic, and alkalescent slags. It is caused by the effect of glassy and crystal formations in slag on slag viscosity-temperature characteristics. (3) According to the calculations, the slag flowing layer is thin and liquid slag can be easily discharged if the temperature is high enough. (4) According to the calculation, the thickness of the flowing layer of the crystal slag is the minimum and the thickness of the flowing layer of the glassy slag is the maximum.
5. Conclusion (1) A mathematical model about the slag film flow and heat-transfer properties in a slag tapping gasifier is established in this work. The modeling results have been validated by the experimental data. (2) The slag can be classified into four
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