ARTICLE pubs.acs.org/EF
Char Wall Interaction and Properties of Slag Waste in Entrained-Flow Gasification of Coal Fabio Montagnaro,*,† Paola Brachi,† and Piero Salatino‡ †
Dipartimento di Chimica, Universita degli Studi di Napoli Federico II, Complesso Universitario del Monte di Sant’Angelo, 80126 Napoli, Italy ‡ Dipartimento di Ingegneria Chimica, Universita degli Studi di Napoli Federico II, Istituto di Ricerche sulla Combustione, Consiglio Nazionale delle Ricerche, Piazzale Vincenzo Tecchio 80, 80125 Napoli, Italy ABSTRACT: The properties of solid wastes (fly and bottom ashes) generated from an industrial-scale pressurized entrained-flow gasifier were investigated. Bottom ashes consist of coarse slag granules and fine slag particles, both retrieved from the quench bath but characterized by distinctively different morphological and physicochemical properties. Characterization of fly ash, coarse slag, and fine slag has been accomplished by a combination of experimental techniques: elemental, granulometric, and X-ray diffractometric analyses, scanning electron microscopy, and energy-dispersive X-ray analysis. Analysis of results gives useful information on the properties and partitioning of carbon among the three main ash streams (coarse slag, slag fines, and fly ash). Residual carbon in slag granules is present in a segregated embedded form, while slag fines are composed of both porous (highcarbon) and compact (low-carbon) material. Carbon partitioning and properties of the different phases of which bottom ashes are composed are further analyzed in light of the flow patterns that establish the gasification chamber, with a focus on char slag micromechanical interaction and char segregation. The properties of the three ash residues are consistent with a mechanistic framework, developed in a recent study (Montagnaro, F.; Salatino, P. Combust. Flame 2010, 157, 874 883), according to which extensive bulk-to-wall transfer of char is followed by the establishment of a segregated dense-dispersed char phase in the near-wall region of the gasification chamber.
1. INTRODUCTION Entrained-flow coal gasifiers of the new generation are characterized by operating conditions (high operating temperatures and multi-stage feedings of coal and gaseous reactants, swirled/ tangential flow patterns) to favor ash migration/deposition onto the reactor walls, where the molten ash (slag) flows and is eventually drained at the bottom of the gasifier.2,3 These phenomena are also regulated by the physicochemical characteristics of both the parent fuel and ash.4,5 Detailed studies concerning the fate of char particles as they impinge the wall slag layer have only recently been developed.6 9 In particular, the relationship between the extent of carbon conversion and the sticky or fluid behavior of char particles, relevant to the char wall interaction, has been largely disclosed. Along a different path, Montagnaro and Salatino1 addressed the relative importance of the parallel pathways of coal conversion associated with entrained flow of carbon particles in a lean-dispersed gas phase versus segregated flow of char particles in a dense-dispersed phase in the near-wall region of the gasifier. Char segregation in the dense-dispersed phase is promoted by bulkto-wall particle migration and inelastic interaction of char particles with the molten slag wall layer. By taking into account characteristics such as char density, particle diameter and impact velocity, slag viscosity, and interfacial particle slag tension, Montagnaro and Salatino developed theoretical criteria for either char particle entrapment inside the molten slag or carbon coverage of the wall ash layer. Figure 1 outlines the possible alternative regimes of the char slag micromechanical interaction: regime E (entrapment), in which char particles reaching the slag surface become permanently embedded into the molten layer and a further course of r 2011 American Chemical Society
Figure 1. Regimes of the char slag micromechanical interaction (E, entrapment; S, segregation; SC, segregation and coverage).
heterogeneous char gasification by O2, CO2, and H2O is hindered; regime S (segregation), in which char particles Received: June 1, 2011 Revised: July 21, 2011 Published: July 25, 2011 3671
dx.doi.org/10.1021/ef200815w | Energy Fuels 2011, 25, 3671–3677
Energy & Fuels
ARTICLE
establishment of a segregated dense-dispersed phase in the nearwall region of the gasifier (Figure 2).
Figure 2. Outline of flow patterns in the entrained-flow gasification chamber (the symbol “W” stands for mass flow rates).
reaching the wall adhere to the surface of the slag layer without being fully engulfed, so that the progress of combustion/gasification is permitted; and regime SC (segregation and coverage), in which the coverage of the slag layer with carbon particles is extensive. In this last regime, a dense-dispersed annular phase is established in the close proximity of the wall ash layer, formed by accumulation of the excess impinging char particles, which cannot be accommodated on the slag surface. The establishment of a segregated dense-dispersed phase close to the reactor wall is beneficial to carbon conversion because char particles belonging to this phase are likely to experience residence times much longer than the hydraulic residence time based on gas flow. In this context, it is useful to remind that ashes collected at the bottom of the gasifier are usually quenched in a water bath generating, besides the slag (sometimes referred to as coarse slag), a black water that, when filtered, gives rise to the slag fines.10 The different thermal/conversion history of these two solid wastes is likely to strongly influence their properties, in particular, as far as carbon content, morphology, and further reactivity are concerned. Only recently did the question concerning the differences between coarse and fine slag receive deeper consideration; the reader is referred, for example, to the studies by Wu et al.,10 Xu et al.,11 and Zhao et al.12 The complex phenomenological framework within which the fate of char/ash particles in the gasification chamber can be analyzed is outlined in Figure 2. In particular, the presence of three different sources of solid wastes is underlined, that is the slag phase, yielding coarse slag granules upon the interaction with the quench bath at the bottom of the gasifier; the dense-dispersed phase, giving rise to slag fines upon the interaction with the quench bath; and the lean-dispersed phase, ultimately escaping the gasifier as fly ash in the gas effluents. In the present study, the partitioning of ash among the different streams (fly and bottom ashes and coarse and fine slag) issuing from an industrial-scale gasifier and the properties of each phase are investigated. Analysis of results is directed to gain a better understanding of the flow and conversion patterns of char in the gasification chamber, with specific reference to the char slag micromechanical interaction (Figure 1) and to the
2. PROPERTIES OF ASH SAMPLES Coarse slag and slag fine samples were supplied by ELCOGAS, Puertollano, Ciudad Real, Spain. Samples were collected during operation of the industrial-scale pressurized (25 bar),13,14 entrained-flow gasifier, operated in the slagging regime (at temperatures around 1700 1900 K).14,15 This material was provided in the summer of 2009, and because of the considerable amount of wastes produced by the plant and the variability of operating conditions of the gasifier, it could also be not fully representative of the ash generated by the industrial gasifier during normal operation. The gasification chamber has an internal diameter of 3.8 m and a length of 13 m. Mass feeding ratios are WOX/WF = 0.8 and WS/ WF = 0.1 (see Figure 2). The solid fuel feed rate is WF = 30 kg s 1, and typically, the solid fuel is a 50:50 coal/pet coke mixture. Practical operation of the gasifier revealed a value of the mass ratio WFLY/(WSLAG + WSF) smaller than expected at the design stage (about 0.1 versus 0.4).14,16 This was primarily ascribed to an unexpectedly large value of the mass flow rate of slag fines (WSF) leaving the quench bath. Moreover, the C content in fly ash was rather limited (about 5%),16 while that in slag fines was reported to be quite high. Finally, a non-negligible organic fraction was detected in the slag waste,17 and this appears consistent with findings reported for similar systems by other authors.10 12 3. EXPERIMENTAL TECHNIQUES Coarse slag and slag fines were characterized by carbon elemental analysis, performed by a LECO CHN-2000 instrument; granulometric analysis, performed by either a Malvern Instruments Master Sizer 2000 laser granulometer (operated down to a minimum particle size of 0.02 μm) or mechanical sieving (in 10 size ranges between 0 and 9.5 mm); X-ray diffraction (XRD) analysis, performed by a Bruker D2 Phaser diffractometer (operated at diffraction angles ranging between 10° and 60° 2θ, with a scan velocity equal to 0.02° 2θ s 1); and scanning electron microscopy (SEM), performed by a FEI Inspect microscope equipped with an energy-dispersive X-ray (EDX) probe (operated up to magnifications of 3000).
4. RESULTS A preliminary elemental analysis on coarse slag and slag fines revealed no appreciable carbon content (for the former) and a carbon content as high as 57.4% (for the latter). Figures 3 and 4 show absolute and cumulative particle size distributions for coarse slag and slag fines, respectively. Because of the much bigger size of the coarse slag material, the particle size analysis for this waste was carried out by mechanical sieving instead of laser granulometry. Figure 3 indicates that the size of slag granules extended over a broad range, with the maximum size being nearly 9 mm.17 Nonetheless, a very distinct peak for the absolute distribution could be appreciated at 1.7 mm; the mean Sauter diameter for this distribution was equal to 1.18 mm. Finally, a median value (d50) of 1.27 mm was obtained from the cumulative distribution. Slag fines were characterized by much smaller values of the particle size (Figure 4). Particles coarser than 700 μm were not observed. The peak and the Sauter diameter for this distribution were equal to 100 and 20 μm, respectively, and the d50 value for the cumulative distribution was 72 μm. 3672
dx.doi.org/10.1021/ef200815w |Energy Fuels 2011, 25, 3671–3677
Energy & Fuels
ARTICLE
Figure 3. Absolute and cumulative particle size distributions of coarse slag.
Figure 4. Absolute and cumulative particle size distributions of slag fines.
Figure 5. XRD analysis for coarse slag and slag fines (A = anhydrite, CaSO4, CPDS 228; P = periclase, MgO, CPDS 2693).
Figure 5 reports XRD patterns for the wastes under investigation. The results for coarse slag revealed no distinct peaks, because of the amorphous and vitreous nature of this material related to its very peculiar thermal history in the gasifier.5,15,17 Although deriving from the same main stream, slag fines separated from the slag after the quench bath showed a different crystalline microstructure, with the peaks of anhydrite and periclase being clearly recognizable. While elemental analysis on coarse slag did not indicate the presence of carbon in this material, interestingly, different results were obtained when cross-sections of coarse slag granules were analyzed by SEM EDX (Figure 6a and Table 1). The crosssection of the whole slag particle shown in Figure 6a appeared to be mostly vitreous and dense, in agreement with XRD results. The inorganic fraction appeared to be primarily constituted by Si + Al (47.5%) together with Ca (7.4%, related to the feeding of limestone to the gasifier as the fluxing agent), Fe (4.7%), and K (2.2%). A remarkable carbon content (9.3%) was also detected. The possibility that such a large amount of carbon could be 3673
dx.doi.org/10.1021/ef200815w |Energy Fuels 2011, 25, 3671–3677
Energy & Fuels
ARTICLE
Figure 6. SEM micrographs of cross-sections of (a) a whole coarse slag particle (magnification of 50) and (b and c) selected zones of slag particles where the existence of carbon-rich patches is highlighted (magnification of 1600).
Table 1. Results of EDX Elemental Analysis (wt %) Referred to SEM Micrographs Reported in Figures 6 and 7 Figure 6a
Figure 6b
Figure 6c
Figure 7a
Figure 7b
Figure 7c
C
9.27
48.77
54.23
86.47
82.27
O Na
26.95 0.14
11.50 0.14
8.67 0.13
6.71 nd
8.18 nd
Mg
0.79
0.46
0.44
nd
0.18
nd
0.64
0.64
Al
14.91
9.20
8.31
0.31
0.35
0.26
11.00
10.92
Si
32.62
19.10
17.82
0.51
0.38
0.29
22.08
24.13
S
nd
1.24
1.70
3.03
3.39
2.91
0.69
nd
K
2.25
1.41
1.44
0.20
0.17
nd
2.86
1.51
Ca
7.38
4.75
3.96
nd
nd
nd
2.52
4.80
Ti V
0.59 0.38
0.38 0.19
0.40 0.23
nd 0.34
nd 0.28
nd 0.31
0.70 0.46
0.44 0.30
Fe
4.72
2.86
2.67
2.15
4.32
3.96
1.14
2.17
Ni
nd
nd
nd
0.28
0.47
0.41
nd
nd
associated with the presence of CaCO3 was ruled out; even if all of the calcium were present as carbonate, the “free” carbon would anyway be as large as 7.1%. Analyses performed on the crosssections of other whole particles led to comparable observations. Panels b and c of Figure 6 (and Table 1) report SEM EDX results obtained carrying out the analysis at a greater magnification on the cross-section of two selected zones of slag particles. In both cases, the occurrence of darker patches was observed. Pointwise quantitative EDX results referred to these patches,
Figure 7d
Figure 7e
84.89
18.36
13.90
6.98 nd
39.21 0.33
40.94 0.25
yielding local carbon contents as high as 48.8 54.2%. This finding contributes to the assessment of the relevance of carbon entrapment (regime E in Figure 1) in slag particles. It is worth noting that elemental analysis did not show the presence of any organic fraction, and this was essentially because C was permanently entrapped in the slag matrix in a way that could not be disclosed during thermal analysis. Only the cutting procedure associated with the SEM EDX analysis of granule cross-sections was able to disclose the unreacted carbon, which appeared to be 3674
dx.doi.org/10.1021/ef200815w |Energy Fuels 2011, 25, 3671–3677
Energy & Fuels
ARTICLE
Figure 7. SEM micrographs of slag fines at magnification of (a c) 1600 and (d and e) 3000.
segregated (in the form of patches) with respect to the inorganic slag matrix. When we took into account that quantitative results relevant to Figure 6a referred to the cross-section of a slag particle, an average carbon content of about 3 4% was estimated for this waste. Figure 7 and Table 1 report the results of the SEM EDX analysis performed on whole slag fine particles. In particular, particles having prevailing either porous (panels a c of Figure 7) or compact (panels d and e of Figure 7) structures were observed. In any case, the carbon content was larger than the value obtained from the inspection of the coarse slag particles, in line with results of elemental analysis. This is particularly evident for porous particles (panels a c of Figure 7); carbon content ranged between 82.3 and 86.5%. Fe (2.2 4.3%) and S (2.9 3.4%)
could also be appreciated. Thus, these particles should be mainly associated with unreacted char present in the dense-dispersed phase (Figure 2), giving rise to slag fines upon impingement on the quench bath. On the other hand, dense particles (panels d and e of Figure 7) display morphological and chemical features that are closer to those of coarse slag particles, at least as far as SEM EDX results are concerned. The carbon content ranged between 13.9 and 18.4%. The Si + Al fraction was as high as 33.1 35.0%, and Ca (2.5 4.8%), K (1.5 2.9%), and Fe (1.1 2.2%) could also be detected. It is also highlighted that elements such as Na, Mg, Al, Si, K, Ca, and Ti, while revealed in smaller amounts (or not at all) in high-carbon porous slag fines, were present in larger percentages in both coarse slag and low-carbon dense slag fines. The opposite is true for S and Ni, 3675
dx.doi.org/10.1021/ef200815w |Energy Fuels 2011, 25, 3671–3677
Energy & Fuels
ARTICLE
Figure 8. SEM micrographs of slag fines at magnification of (left) 100 and (right) 200.
and this is in line with coarse slag characterization reported in the literature.15,17 The presented results are consistent with the previously reported carbon content of slag fines (about 57%), obtained by standard elemental analysis carried out on waste containing both high-carbon porous and low-carbon dense materials. Moreover, the more compact slag fines should be regarded as having intermediate properties between porous slag fines and coarse slag; this observation, jointly with XRD results, highlights again the establishment of a dense-dispersed phase that, together with the slag phase, generates both streams: coarse slag and slag fines (Figure 2).
5. DISCUSSION Results can be analyzed in the frame of the char slag interaction and char segregation phenomena proposed by Montagnaro and Salatino.1 It has been observed that the slag fines, generated in large quantities in the gasifier, are very rich in carbon (see elemental and SEM EDX analysis results). This is believed to be a clue in favor of the existence of a dense-dispersed phase (consistent with the establishment of regime SC; Figures 1 and 2), which would be the source of high-carbon slag fines upon the interaction with the quench bath. The poor degree of carbon burnoff, which characterizes slag fines, might be the consequence of a combination of factors: (a) selective accumulation of the less reactive carbon particles (e.g., residual petcoke) in the dense-dispersed phase, once the more reactive ones have been gasified; (b) loss of gasification reactivity of carbon because of the severe heat treatment (thermal annealing) experienced by char particles over their lifetime in the gasifier;18,19 and (c) occurrence of char agglomeration phenomena during bulk-to-wall transfer and in the near-wall region, possibly promoted by the molten or semimolten status of char particles. Indeed, some evidence of partial agglomeration of char particles is given in Figure 8, where SEM micrographs of whole slag fine particles are reported at smaller magnifications (100 200). On the other hand, the coarse slag waste presented a non-negligible content of carbon, mostly entrapped (in a segregated fashion) into the slag matrix. This would be consistent with the occasional establishment of regime E. It is interesting to analyze these findings in light of results that have recently been published by Li et al.8,9 These authors investigated the char slag transition during entrained-flow oxidation
of coal particles, observing a distinct transition from porous/nonsticky char to fluid/sticky slag occurring at temperatures above the ash flow temperature only when carbon conversion exceeded a threshold, typically set at about 90%. These findings further reinforce the conclusion that regimes SC and E coexist. If one assumes based on the findings by Montagnaro and Salatino1 that regime SC is the dominant regime under typical operating conditions of entrained-flow gasifiers, then some char particles belonging to the dense-dispersed phase could well be permanently embodied into the slag layer in the late burnoff stage, consistent with the findings by Li et al.8,9 Although the carbon content of these particles is likely to be modest, it is anyway not negligible and could possibly justify the estimated carbon content of 3 4%. As far as fly ash is concerned, its carbon content (5%) was unexpectedly small. This finding can be interpreted by considering that bulk-to-wall transfer of char/ash particles is dominated by the inertial mechanism associated with turbophoresis and centrifugal forces because of swirl/tangential flow. This mechanism would make wall transfer of coarser particles more effective than the transfer of fines.1 Considering, as reported by Wu et al.,10 that the coarser the char particle, the larger its carbon content, this mechanism would imply that transfer of carbon to the wall would be more effective than transfer of ash. These two aspects (coexistence of regimes and preferential mass transfer) could explain why fly ash was selectively carbon-depleted and why slag fines did show such a large mass flow rate and carbon content, contrary to the expectations. Moreover, a significant fraction of the fly ash is likely to derive by nucleation and growth of fine (carbon-free) inorganic particles from the gas phase under the extremely high-temperature conditions experienced by the fuel in the flame region of the oxidizer. This would further justify the comparatively small carbon content of the fly ash.
6. CONCLUSION The properties and partitioning of carbon among the three main sources (coarse slag, slag fines, and fly ash) of solid residues generated from an industrial-scale entrained-flow coal gasifier have been characterized by a combination of experimental techniques. The carbon content of slag fines is very large, on the order of 60%. The carbon content of fly ash is around 5%. The carbon content of slag granules as assessed by standard elemental analysis techniques is negligible, but combined microscopy and 3676
dx.doi.org/10.1021/ef200815w |Energy Fuels 2011, 25, 3671–3677
Energy & Fuels
ARTICLE
EDX analysis of the cross-sections of granules indicates that residual carbon is present in slag granules as segregated embedded carbon-rich patches, amounting to about 3% by mass of the sample. The properties of these three types of residues are consistent with a mechanistic framework of entrained-flow gasification of coal developed by the authors, which considers the bulk-to-wall transfer of solids and the establishment of segregated phases in the near-wall region of the gasifier.
’ AUTHOR INFORMATION Corresponding Author
*Telephone: +39-081-674029. Fax: +39-081-674090. E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors express their gratitude to Mr. F. García Pe~na, Dr. A. M. Mozos, and Dr. P. Coca (ELCOGAS, Spain) for supplying raw materials and for useful discussion. Dr. M. Urciuolo and Mr. S. Russo (IRC CNR, Italy) are gratefully acknowledged for their support in solid characterization. ’ REFERENCES (1) Montagnaro, F.; Salatino, P. Combust. Flame 2010, 157, 874–883. (2) Walsh, P. M.; Sarofim, A. F.; Beer, J. M. Energy Fuels 1992, 6, 709–715. (3) Shimizu, T.; Tominaga, H. Fuel 2006, 85, 170–178. (4) Walsh, P. M.; Sayre, A. N.; Loehden, D. O.; Monroe, L. S.; Beer, J. M.; Sarofim, A. F. Prog. Energy Combust. 1990, 16, 327–346. (5) Song, W.; Tang, L.; Zhu, X.; Wu, Y.; Rong, Y.; Zhu, Z.; Koyama, S. Fuel 2009, 88, 297–304. (6) Wang, X. H.; Zhao, D. Q.; He, L. B.; Jiang, L. Q.; He, Q.; Chen, Y. Combust. Flame 2007, 149, 249–260. (7) Shannon, G. N.; Rozelle, P. L.; Pisupati, S. V.; Sridhar, S. Fuel Process. Technol. 2008, 89, 1379–1385. (8) Li, S.; Whitty, K. J. Energy Fuels 2009, 23, 1998–2005. (9) Li, S.; Wu, Y.; Whitty, K. J. Energy Fuels 2010, 24, 1868–1876. (10) Wu, T.; Gong, M.; Lester, E.; Wang, F.; Zhou, Z.; Yu, Z. Fuel 2007, 86, 972–982. (11) Xu, S.; Zhou, Z.; Gao, X.; Yu, G.; Gong, X. Fuel Process. Technol. 2009, 90, 1062–1070. (12) Zhao, X.; Zeng, C.; Mao, Y.; Li, W.; Peng, Y.; Wang, T.; Eiteneer, B.; Zamansky, V.; Fletcher, T. Energy Fuels 2010, 24, 91–94. (13) Seggiani, M. Fuel 1998, 77, 1611–1621. lvarez-Rodríguez, R.; Clemente-Jul, C.; Martín-Rubí, J. A. Fuel (14) A 2007, 86, 2081–2089. (15) Aineto, M.; Acosta, A.; Rincon, J. M.; Romero, M. Fuel 2006, 85, 2352–2358. (16) Font, O.; Moreno, N.; Díez, S.; Querol, X.; Lopez-Soler, A.; Coca, P.; García Pe~na, F. J. Hazard. Mater. 2009, 166, 94–102. (17) Acosta, A.; Aineto, M.; Iglesias, I.; Romero, M.; Rincon, J. M. Mater. Lett. 2001, 50, 246–250. (18) Senneca, O.; Salatino, P.; Masi, S. Fuel 1998, 77, 1483–1493. (19) Salatino, P.; Senneca, O.; Masi, S. Energy Fuels 1999, 13, 1154–1159.
3677
dx.doi.org/10.1021/ef200815w |Energy Fuels 2011, 25, 3671–3677
