Energy Fuels 2010, 24, 1868–1876 Published on Web 02/11/2010
: DOI:10.1021/ef901480e
Ash Deposition Behavior during Char-Slag Transition under Simulated Gasification Conditions Suhui Li,*,† Yuxin Wu,‡ and Kevin J. Whitty† † Institute for Clean and Secure Energy, University of Utah, 50 South Central Campus Drive, MEB Room 3290, Salt Lake City, Utah 84112, and ‡Department of Thermal Engineering, Tsinghua University, Beijing 10084, China
Received December 4, 2009. Revised Manuscript Received January 29, 2010
Ash deposition experiments at various conversions of a bituminous coal were performed under gasification conditions using a laminar entrained-flow reactor and a deposition probe. Results showed that the particle capture efficiency (a measure of ash deposition propensity) was a function of coal conversion. In particular, the capture efficiency increased dramatically at a critical conversion, which is ascribed to the increase in particle stickiness. To clarify this phenomenon, ash formation experiments were conducted to collect char and ash particles under experimental conditions identical to those in the ash deposition experiments. Collected particles were presumed to have the same properties as the particles approaching the deposition probe in ash deposition experiments. Properties of the particles including internal surface area and morphology were determined by isothermal gas adsorption and scanning electron microscopy, respectively. The internal surface area of the particles dropped sharply at the critical conversion, which indicates a char-slag transition. This suggests that the char-slag transition is associated with a drastic increase in particle stickiness. Examination of the particle morphology revealed that physical transformation of mineral-carbon association in the particle has a major impact on particle stickiness during char-slag transition.
in deposit formation has been identified as inertial impaction.7 The other three mechanisms, which are called near-wall effects, were shown to be insignificant compared to inertial impaction.8,9 Upon impaction, the particle either sticks (trap) or rebounds (elastic reflection) depending on the particle capture efficiency, which is a function of particle surface stickiness and impaction surface stickiness.10-12 Therefore, particle surface stickiness plays a key role in determining the particle fates upon impaction on the gasifier wall. For coal ash with kinetic energy and temperature typical of coal gasifiers, the dominating factor determining particle stickiness is the viscosity. Srinivasachar13 proposed a critical viscosity range of 105-108 Pa s, below which the particle adheres to the impaction surface. This criterion has been applied when developing models11,14,15 for the prediction of ash deposition and slagging. However, this criterion was derived from properties of synthetic ash (pure inorganic minerals) and does not consider
Introduction Performance of slagging entrained-flow gasifiers is in large part dictated by the burnout behavior of char particles. If a char particle deposits on the gasifier wall, its residence time will increase substantially and its overall conversion will be affected. Furthermore, deposition of ash particles on the gasifier walls can form slag flow, which causes erosion and corrosion of the refractory wall of the gasifier.1 Computational fluid dynamics (CFD) simulations of particle trajectories in entrained-flow gasifiers indicate that a large portion of char particles strike the gasifier wall at different positions with various angles.2-5 When the fate of particles impacting the gasifier wall is determined, elastic reflection is usually assumed for low-conversion, porous char, whereas trapping is usually assumed for ultimate-conversion, molten slag. However, this assumption has not been validated for particles during the transition from porous char to molten slag (intermediate to high conversion). Baxter6 proposed four mechanisms which contribute to deposit formation: inertial impaction, thermophoresis, condensation, and chemical reaction. The dominating mechanism
(7) Strandstr€ om, K.; Mueller, C.; Hupa, M. Fuel Process. Technol. 2007, 88, 1053–1060. (8) Mueller, C.; Selenius, M.; Theis, M.; Skrifvars, B. J.; Backman, R.; Hupa, M.; Tran, H. Proc. Combust. Inst. 2005, 30, 2991–2998. (9) Selenius, M.; Theis, M.; Mueller, C.; Skrifvars, B. J.; Hupa, M. 14th IFRF Members’ Conference, Noordwijkerhout, The Netherlands, May 11-14, 2004. (10) Barroso, J.; Ballester, J.; Ferrer, L. M.; Jimenez, S. Fuel Process. Technol. 2006, 87, 737–752. (11) Ma, Z.; Iman, F.; Lu, P.; Sears, R.; Kong, L.; Rokanuzzaman, A. S.; McCollor, D. P.; Benson, S. A. Fuel Process. Technol. 2007, 88, 1035– 1043. (12) Strandstr€ om, K.; Mueller, C.; Hupa, M. Fuel Process. Technol. 2007, 88, 1053–1060. (13) Srinivasachar, S.; Helble, J. J.; Boni, A. A. Proc. Combust. Inst. 1990, 23, 1305–1312. (14) Lee, F. C. C.; Lockwood, F. C. Prog. Energy Combust. Sci. 1999, 25, 117–132. (15) Wang, X.; Zhao, D.; He, L.; Jiang, L.; He, Q.; Chen, Y. Combust. Flame 2007, 149, 249–260.
*To whom correspondence should be addressed. Telephone: þ1-801949-6986. E-mail:
[email protected]. (1) Liu, B.; Garcia, H. E.; Baxter, L. L. Proceedings of the 25th Annual International Pittsburgh Coal Conference, Pittsburgh, PA, Sept 29-Oct 2, 2008. (2) Chen, C.; Horio, M.; Kojima, T. Chem. Eng. Sci. 2000, 55, 3861– 3874. (3) Fletcher, D. F.; Haynes, B. S.; Chen, J.; Joseph, J. D. Appl. Math. Model. 1998, 22, 747–757. (4) Wu, Y.; Zhang, J.; Wang, M.; Yue, G.; Lu, J. Huagong Xuebao 2007, 58, 2369–2374. (5) Du, M.; Hao, Y. 2009 Asia-Pacific Power and Energy Engineering Conference, Wuhan, China, March 28-31, 2009. (6) Baxter, L. L. Biomass Bioenergy 1993, 4, 85–102. r 2010 American Chemical Society
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the residual carbon in the ash particle. For a coal particle in a gasifier, its carbon content decreases with conversion, but it is common to have a few percent carbon remaining in the slag.16 Koyama et al.17 found that there are some char particles that were not completely burned in the deposits formed in a two-stage entrained-bed coal gasifier. The unburned char particles served as a dispersive material which prevented complete sintering even at high temperature, and this dispersive effect decreased with decreasing residual carbon content in the particles. Lab-scale experiments were also performed by other researchers to study the deposition behavior of particles containing residual carbon, i.e., during the burnout process of coal particles. Vuthaluru and French18 investigated ash chemistry and mineralogical changes of an Indonesian coal during the combustion process. The combustion of the coal particles was conducted using a drop-tube furnace at 1200-1400 °C, and deposition studies were performed using a deposition probe at a probe temperature of 750 °C. Results showed that ash particles sparsely distributed in the deposit, suggesting a lack of deposition initiation layer. The deposit sample was found to have a higher glass content enriched in silica and iron compared to bottom ash sample, indicating that the melted glass content increased ash stickiness. Russell et al.19 studied the effect of included and excluded minerals on slagging propensity of a Spanish anthracite during combustion. They found that the included minerals produced a dense, vitreous, and iron-rich deposit, whereas the excluded minerals formed a friable and inhomogeneous deposit. This result suggests that the slagging propensity of this coal is mainly determined by the included minerals. Bool and Johnson20 studied the ash deposition behavior during coal combustion using an entrained-flow reactor. They observed that the particle stickiness increased dramatically to a maximum value at a critical char burnout and then decreased slightly throughout the burnout process, suggesting that particle stickiness is influenced by the residual carbon. It also indicates that at a critical conversion, char particles will begin to deposit, or stick, when they impact the gasifier wall. Consequently, the assumption of elastic reflection cannot be applied to particles during the char-slag transition without validation. All the ash deposition experiments mentioned above focused on combustion conditions, featuring a traditional experimental setup that utilizes a cylindrical deposition probe (rotating or nonrotating) perpendicular to the particle laden gas stream at the bottom of an entrained-flow reactor (or a drop-tube furnace). The deposition probe was usually gas cooled to a temperature much lower than that in the reactor. This kind of configuration was designed to simulate ash deposition caused by inertial impaction on cylinders in cross-flow, i.e., ash particles approaching heat exchanger tubes. In contrast, ash deposition caused by inertial impaction on walls in parallel flow is not well understood,21 especially under gasification conditions. The inertial deposition on walls in parallel flow resembles the situation of particles impacting gasifier walls, which needs to be clarified for CFD simulation of particle tracking in a
Table 1. Properties of the Illinois No. 6 Coal Used in This Work analysis Proximate Analysis (wt %, Dry) moisturea ash volatiles fixed carbon carbon hydrogen nitrogen sulfur oxygen a
3.63 10.89 36.42 52.69 Ultimate Analysis (wt %, Dry Ash Free) 74.48 4.92 1.48 4.66 14.46
As received.
Table 2. Ash Chemistry and Fusion Temperatures of the Illinois No. 6 Coal Used in This Work analysis Ash Chemistry (wt %, Oxide) Al2O3 CaO Fe2O3 K2O MgO MnO Na2O P2O5 SiO2 TiO2 SO3
17.75 5.23 18.99 2.06 0.89 0.05 1.67 0.16 46.58 0.88 4.59 Ash Fusibility (°C, Reducing)
IT ST HT FT
1104 1116 1139 1246
gasifier. Moreover, little attention has been paid to the particle deposition behavior during the char-slag transition. To the knowledge of the authors, Bool and Johnson20 performed the only study of coal ash deposition behavior at different conversions, but their work focused on inertial impaction on cylinders in cross-flow rather than on walls in parallel flow. This study was motivated by (1) the lack of understanding on particle deposition behavior in the intermediate and late stages of char conversion in a gasifier and (2) the need to establish a criterion for accurately predicting the particle fates upon impaction on gasifier walls during char-slag transition. This paper presents the results of a lab-scale experimental study on particle deposition behavior during the char-slag transition of a bituminous coal. Experimental Section Two types of experiments were performed: ash deposition experiments and ash formation experiments. All of the experiments were conducted using a lab-scale laminar entrained-flow reactor (LEFR). Details of the experiments are presented below. Coal Sample. A pulverized Illinois No. 6 coal was selected and sieved to a size fraction of 43-63 μm. This was the same coal used in a previous study22 that focused on physical changes of char particles during char-slag transition under combustion conditions. The proximate and ultimate analyses of the coal are presented in Table 1. The ash chemistry and fusion analyses are listed in Table 2. Silica, iron oxide, and alumina dominate the ash chemistry.
(16) Zhao, X.; Zeng, C.; Mao, Y.; Li, W.; Peng, Y.; Wang, T.; Eiteneer, B.; Zamansky, V.; Fletcher, T. F. Energy Fuels 2010, 24, 91–94. (17) Koyama, S.; Morimoto, T.; Ueda, A.; Matsuoka, H. Fuel 1996, 75, 459–465. (18) Vuthalurua, H. B.; Frenchb, D. Fuel Process. Technol. 2008, 89, 595–607. (19) Russell, N. V.; Mendez, L. B.; Wigley, F.; Williamson, J. Fuel 2002, 81, 657–663. (20) Bool, L. E.; Johnson, S. A. ASME Environ. Control Div. Publ. EC 1995, 1, 305–312. (21) Baxter, L. L.; Desollar, R. W. Fuel 1993, 72, 1411–1418.
(22) Li, S.; Whitty, K. J. Energy Fuels 2009, 23, 1998–2005.
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Figure 1. Schematic diagram of the LEFR used in ash deposition experiments.
Experimental Apparatus. A schematic diagram of the LEFR used in this study is shown in Figure 1. The LEFR consists of a high-temperature furnace (Carbolite, single zone, 1600 °C maximum operation temperature and 610 mm heated length), a coal feeder, a sample collector, gas supply, and cooling water circulator. Two coaxial alumina tubes (89 mm o.d. 75 mm i.d. 1500 mm long and 57 mm o.d. 50 mm i.d. 1000 mm long, respectively) are installed vertically inside the furnace and sealed with flanges. The inside tube is used as the reactor. The reaction gas (a premixed air-nitrogen mixture) is injected through three injection ports on the bottom flange and is preheated when it flows upward through the annulus between the two coaxial tubes. When the reaction gas reaches the top of the annulus, it turns and flows down into the inner tube through an alumina honeycomb flow straightener. The flow straightener has a sufficient pressure drop to generate a uniform and laminar flow so that the entrained particles can travel along the centerline of the reactor tube experiencing identical reaction conditions. Coal particles are fed into the reactor through an injection probe using a vibrating syringe pump type coal feeder with nitrogen as the carrier gas. The injection probe is water-cooled to prevent the coal particles from being pyrolyzed before reaching the reaction zone. Upon injection into the reactor, the coal particles react with the reaction gas to produce char and ash particles. After undergoing partial conversion, resulting particles exit the reactor and are collected in two different ways depending on the type of experiments being performed. Ash Deposition Procedure. A section view of the deposition probe is illustrated in Figure 2. It consists of an alumina tube and a clean alumina plate. A 60° (with respect to the cross-sectional plane of the probe) V-shaped groove was cut on top of the alumina tube for housing the alumina plate. This configuration allows simulation of inertial impaction on refractory walls because (1) the alumina plate has a ceramic surface similar to the fresh refractory wall and (2) the uncooled plate has a surface temperature of roughly the same as that of the uncooled refractory wall. A clean alumina plate was used because the present research is focused on the intrinsic particle surface stickiness and the initial stage when the slagging layer starts building up. Gasified char and ash particles struck the plate when the particle-laden gas stream approached the plate at a 45° angle. Upon impaction, particles with sufficient stickiness
Figure 2. Section view of the deposition probe with deposition plate.
adhered on the deposition plate and were designated as deposit sample. Ash particles that did not adhere onto the plate were received by a cyclone at the bottom of the deposition probe. After the ash deposition experiment, nitrogen was fed into the reactor to provide an inert environment for the deposit on the plate until the reactor cooled down. The weight of deposit was determined by the weight difference of the deposition plate before and after the deposition experiment. Each deposition experiment was run for 2 h to collect enough deposit to minimize the weighing error. The pressure inside the reactor was maintained at ambient pressure, 0.85 bar (The altitude of Salt Lake City is about 1350 m). The furnace temperature was set to either 1400 or 1500 °C, in which both are above the ash fluid temperature of the coal. The feeding rate of coal particles was 33 mg/min. The flow rate of air in the reaction gas mixture was 0.17 standard liter per minute (SLPM). These feeding rates generate a stoichiometric ratio (oxidant/fuel) of 0.7, which provided an overall reducing atmosphere in the reactor. The residence time of the coal particles in the reactor was varied in 1 s increment from 1 to 6 s. The use of a long residence time was due to the low oxygen content (0.7%4.6%) in the reaction gas in accordance with the low feeding rate of coal. Ash Formation Procedure. In order to know the properties of the particles striking the deposition plate, complementary ash formation experiments were performed. The procedure for ash formation experiments was basically the same as that for deposition experiments, except that the deposition probe was replaced by a collection probe. The collection probe was water cooled and nitrogen quenched. A detailed description of the collection probe was reported elsewhere.22 Char and ash particles were collected in a cyclone via the collection probe. The entrance of the cooled probe was positioned at the same level within the furnace as the center of the deposition plate in ash deposition experiments. Therefore, it is assumed that the properties of the collected particles are representative of those particles that approached the deposition plate in ash deposition experiments. The cyclone has a cut diameter of about 2-4 μm. 1870
Energy Fuels 2010, 24, 1868–1876
: DOI:10.1021/ef901480e
Li et al.
The particles sampled in this set of experiments are described as ash formation sample. Experimental conditions for the ash formation experiments were identical to those of the ash deposition experiments. Sample Characterization. Carbon contents of the collected char and ash particles (ash formation sample) were determined using a loss-on-ignition (LOI) instrument (FERCO, HF400). With the use of ash as a tracer, which assumes conservation of the ash in the particle before and after the reaction, the corresponding coal conversion X was calculated as ! carbon ash Cchar Ccoal X ¼ 1 100% ð1Þ carbon carbon ð1 - Cchar ÞCcoal where Ccarbon is the weight fraction of residual carbon in the char collected char and ash particle, Cash coal is the weight fraction of ash in the parent coal, and Ccarbon is the weight fraction of carbon in coal the parent coal. The particle collection efficiency η is defined as η ¼
m M
Figure 3. Burnout behavior of the Illinois No. 6 coal as a function of residence time.
ð2Þ
where m is the mass (in grams) of particles collected by the deposition plate and M is the total mass (in grams) of particles that approached the deposition plate during the deposition experiment. M is calculated by M ¼
ash ftCcoal carbon 1 -Cchar
ð3Þ
where f is the feeding rate (in grams/hour) of coal particles approaching the deposition plate, t is the elapsed time (in hours) of the deposition experiment, Cash coal is the weight fraction of ash in the parent coal, and Ccarbon is the weight fraction of carbon in the char char or ash particles that impacted the deposition plate. Ccarbon char was determined by eq 1 using samples collected in ash formation experiments. Because the reaction conditions and sampling position of collecting ash formation samples were identical to those of ash deposition experiments, it is reasonable to assume that the carbon content of the ash formation sample is the same as the particles approaching the deposition plate. The particle collection efficiency calculated by eqs 2 and 3 is an averaged value of all the particles that impacted the deposition plate. Internal surface areas of the char and ash particles (ash formation sample) were measured by isothermal gas adsorption using a surface area analyzer (Micromeritics, ASAP 2010) with N2 as the adsorptive gas at 77 K (liquid nitrogen bath). Each sample was degassed under 10 μm Hg pressure and 250 °C for 2 h in order to remove moisture before analysis. The internal surface area was calculated using the Brunauer-Emmett-Teller (BET) method. Microimages of the char and ash particles and the ash deposit were captured using a FEI Nova nanoscanning electron microscope (SEM). Particles were affixed to the sample holder using silver paste as a conductive base. This analysis shows the mineral-carbon association and the morphology of the particles and the deposit samples. Elemental composition of the ash particle was analyzed using an energy dispersive X-ray spectrometer attached on the SEM (SEM-EDS). The X-ray microanalysis has a spacial resolution of 5 μm and an accuracy of 1-2%. Sizes of the char and ash particles were measured using an Olympus optical microscope. The mean particle size was calculated with Image J software by averaging the particle sizes and assuming the particles are spherical.
Figure 4. Evolution of particle collection efficiency as a function of conversion.
Figure 3 as a function of the residence time used for preparing the sample. As expected, the residual carbon content decreased with residence time, and the conversion increased with residence time. Increasing the reaction temperature generated higher carbon conversion at the same residence time. The conversion spans from 67% to 97%, which covers the region of char-slag transition. The experiment at 1400 °C and 3 s residence time was repeated three times to test the reproducibility. The standard deviation is 4.2% for the carbon content and 2.1% for the coal conversion, respectively. The error is mainly due to the variation in controlling the flow rate of the reaction gas using rotameters and is partially introduced in determining the residual carbon content using LOI. Particle Deposition Behavior. The particle collection efficiency is a measure of ash deposition rate (dimensionless). Evolution of the collection efficiency of ash particles at 1400 and 1500 °C is shown in Figure 4. To test the reproducibility of the data, the deposition experiment at 1400 °C and 3 s residence time was repeated 5 times. The standard deviation of the collection efficiency is 1.9%, indicating that the repeatability of the experiments is acceptable. Generally, the particle collection efficiency increased with carbon conversion at both 1400 and 1500 °C. In particular, the
Results and Discussion Particle Burnout Behavior. Carbon content and conversion of samples collected in ash formation experiments is shown in 1871
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: DOI:10.1021/ef901480e
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Figure 6. Particle impaction on the deposition plate at 1 s residence time and 1400 °C. Figure 5. Principle of calculating the particle impaction efficiency.
CFD simulation of particle motion impacting the deposition plate. The principle of calculating the impaction efficiency is illustrated in Figure 5. The impaction efficiency is calculated as the mass ratio of particles (dotted lines) that impacted the plate to the total particles (dotted and dashed lines) that were carried by the flow in the projected area of the deposition plate and approached it. The simulation of particle motion was performed by using the computational program FLUENT 6.1, where the computing grid was generated using GAMBIT, which consists of a computing domain of the LEFR and a computing domain of the deposition probe. The domain for the LEFR was 61 cm long with a 5.08 cm diameter according to the geometry of the LEFR. The domain for the deposition probe was 25 cm long with a 2.54 cm diameter according to the geometry of the deposition probe. The k - ε model was used for calculating the viscous flow field as it works in both laminar and turbulent flow. The drag law was assumed to be Stokes-Cunningham with a Cunningham correction factor of 10. Particles were assumed to be inert. The particle injection type was solid-cone with a cone angle of -2°. Mean sizes of the particles collected at different conversions were used to define the particles injected into the computing domain. The mean particle sizes were assumed to represent the particle sizes just above the deposition plate. Upon impaction, the particles were assumed to be trapped by the deposition plate. The number of trapped particles divided by the number of injected particles in the flow field of the projected plate area gives the impaction efficiency. Detailed input and output parameters of the simulation for calculating the impaction efficiency at 1400 °C are listed in Table 3 in the Supporting Information. Figure 6 shows the simulated laminar streamlines and tracked particles motion near the deposition plate at 1 s residence time and 1400 °C. It shows that most of the particles impacted on the plate, whereas a small fraction of the particles did not impact on the plate. Predicted impaction efficiencies at different temperatures and conversions are presented in Figure 7. A general trend is
collection efficiency of the sample obtained at a temperature of 1400 °C remained relatively low until 85% conversion and then increased dramatically at about 88% conversion. The evolution of the collection efficiency at 1500 °C followed the same trend with a drastic increase at about 93% conversion. The dramatic rise of the particle collection efficiency at a critical conversion was also observed by Bool and Johnson19 for a Pittsburgh No. 8 coal under combustion conditions. According to Baxter,6 the particle collection efficiency in inertial impaction is the product of the particle impaction efficiency I and the particle capture efficiency G. Therefore, the particle capture efficiency can be expressed as G ¼
η I
ð4Þ
The particle impaction efficiency is defined as the mass ratio of particles that impacted the target to the particles that approached the target. The particle capture efficiency is defined as the mass ratio of particles captured by the target to the particles that impacted the target. Barroso et al.10 showed the capture efficiency is representative of the intrinsic tendency of particles to deposit on the impaction surface. Therefore, the particle capture efficiency is of particular significance for the prediction of particle fates upon impaction on the gasifier wall. In order to calculate the capture efficiency, the impaction efficiency needs to be known. The particle impaction efficiency during inertial impaction on cylinders in cross-flow has been well studied and correlated to the particle Stokes number.6,21,23,24 However, the particle impaction efficiency on walls in parallel flow has not been well understood.21 In this study, the particle impaction efficiency was calculated by (23) Baxter, L. L.; Hardesy, D. R. Task 3: Fate of Mineral Matter during Pulverized Coal Combustion. Coal Combustion Science: Quarterly Progress Report, Sandia Report SAND-8247, November 1990. (24) Israel, R.; Rosner, D. E. Aerosol Sci. Technol. 1983, 2, 45–51.
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Figure 7. Predicted particle impaction efficiency using FLUENT.
Figure 9. Surface area evolution of char and ash particles in the burnout process.
is due to the increase in particle stickiness. The ash particle stickiness is caused by the molten phase of mineral matter in the particle. At 1400 and 1500 °C, which are above the ash fluid temperature (1246 °C) of the coal, it is assumed that all the mineral matter in the particle is melted to form a molten phase regardless of conversion. That is, the ash particle stickiness might be assumed to be independent of conversion. However, data in Figure 8 suggest that the particle stickiness is also affected by conversion (or residual carbon content). In order to explain this phenomenon, Bool and Johnson20 proposed a hypothesis that the ash particle is not sticky unless the melted minerals in the particle become exposed to the particle surface and thus are able to contact the deposition surface. On the basis of this hypothesis, it can be inferred that there is a critical point at which there is sufficient melted minerals on the surface of the particle to make the particle sticky. The char-slag transition is a process during which porous char transforms into slag. The transition starts from a porous char particle in which mineral matter is encapsulated by a carbon matrix. As carbon is consumed, the encapsulated mineral matter becomes exposed on the particle surface. The transition completes when the particle surface is totally covered by the molten mineral, i.e., the remaining carbon is enclosed by the mineral and the particle transforms into slag. Therefore, the critical point at which particle stickiness rises dramatically is in the process of char-slag transition. It was found that the char-slag transition is marked by a fast drop in the internal surface area of the particle.21 In order to identify at which point the char-slag transition occurred, the evolution of internal surface areas of the char and ash particles as a function of conversion is presented in Figure 9. The surface area of particles prepared at 1400 °C dropped sharply at 88% conversion. The evolution of the surface area of particles prepared at 1500 °C follows the same trend with a more significant drop when conversion increased from 87% to 92%. These phenomena indicate that the char-slag transition occurred at about 88% conversion at 1400 °C and about 87% conversion at 1500 °C. The coincidence of the rise in ash stickiness and the drop in surface area at the same conversion suggests that the char-slag transition is associated with a change in the particle stickiness. The
Figure 8. Evolution of particle capture efficiency during the charslag transition process.
that the impaction efficiency decreases with conversion. This is due to the decrease in gas velocity and particle diameter with increasing residence time, which results in the decrease of the particle Stokes number. The deviation of the last data point might be due to the error in measuring particle diameter, which has the most significant influence on the particle Stokes number. On the basis of the predicted impaction efficiencies, shown in Figure 7, the capture efficiencies were calculated using eq 4 and the results are presented in Figure 8. A significant increase in the particle capture efficiency took place at roughly 88% conversion for the experiment conducted at a temperature of 1400 °C. Before this critical conversion, the capture efficiency remained essentially low. After the critical conversion, the capture efficiency continued to increase to up to 95%. The particle capture efficiency at 1500 °C follows the same trend. The particle capture efficiency is a function of the particle stickiness and the impaction surface stickiness.9-11 Because the deposition plates used in the deposition experiments were identical materials, the impaction surface stickiness can be assumed to be the same for all the experiments. Therefore, the increase in the particle collection efficiency 1873
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: DOI:10.1021/ef901480e
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particle stickiness is dependent on the exterior layer of the particle,20 i.e., a particle cannot be sticky unless molten minerals appear on its surface because carbon itself is not sticky. Before the char-slag transition, minerals are enclosed by the carbon in the char particle and the particle is nonsticky even if the minerals inside are melted. During the transition, the melted minerals start to become exposed to the particle surface making the particle sticky. After the transition, the particle completely becomes slag. Therefore, the particle stickiness is supposed to be zero before the transition. However, data in Figure 8 show that the particle stickiness before the critical point is not zero, although it is relatively low. This deviation can be ascribed to the presence of excluded minerals in the coal sample. The mineral matter in pulverized coal that contributes to ash formation and deposition are classified in two categories according to the association between minerals and carbon matrix: excluded minerals and included minerals.25,26 Excluded minerals are the discrete mineral grains that are not associated with the carbon structure of coal particles. Included minerals are the mineral matter that is embedded within or organically bonded with carbon matrix in the coal particle. The bituminous Illinois No. 6 coal used in this study is a high-rank coal, in which the included mineral matter is mainly present in the form of embedded minerals.27 In pulverized coal combustion and gasification, different minerals undergo different physical-chemical transformations resulting in different contributions to ash formation and deposition. Excluded minerals are melted into liquid phase at temperatures above the ash flow temperature and become sticky. Upon impacting on the deposition plate, they tend to adhere to the plate and form an ash deposit. On the other hand, included minerals can be liberated from the carbon matrix and form ash particles by char fragmentation28-30 and shedding31-33 of minerals from char particles in the burnout process. Baxter30 found that large char particles can fragment into many pieces in the initial combustion of bituminous coals. Wu and colleagues34,35 concluded that fragmentation plays a key role in the formation of large amounts of fine ash particles in the early and middle stages of pulverized bituminous coal combustion. The liberated minerals also contribute to ash deposition. Therefore, the low particle collection efficiency in initial and medium conversions (
