ARTICLE pubs.acs.org/EF
Investigation of Agglomeration and Defluidization during Spouted-Bed Gasification of High-Sodium, High-Sulfur South Australian Lignite Daniel P. McCullough, Philip J. van Eyk,* Peter J. Ashman, and Peter J. Mullinger South Australian Coal Research Laboratory, Centre for Energy Technology, School of Chemical Engineering, The University of Adelaide, Adelaide, South Australia 5005, Australia ABSTRACT: The mechanisms of agglomeration and defluidization during the fluid-bed gasification of an Australian low-rank coal are investigated. Experiments were conducted in a 77 mm inner diameter spouted-bed gasifier with a high-sodium, high-sulfur coal from the Lochiel deposit in South Australia. The effect of the bed temperature, air/fuel ratio, and superficial velocity on the stable operation of the spouted bed over a 4 h period was investigated. The results of this study indicate that stable bed operation is governed by a “high-temperature defluidization limit”, suggesting that defluidization can be delayed or avoided by operating the bed with high superficial velocity and/or low bed temperatures. In experiments that resulted in agglomeration and in some experiments that did not, the average particle size within the bed material had increased, which was mainly attributed to coating of mineral particles. These coated particles were observed to be more prevalent in runs that led to defluidization of the bed. Particle growth also coincided with the increased inorganic content of the bed compared to stable runs. Agglomeration and defluidization may hence be avoided or delayed by operating the bed below about 850 °C, increasing the superficial velocity of gas within the bed, or maintaining the ash content of the bed below approximately 80%, where possible.
1. INTRODUCTION The lignite deposits of Australia are a unique resource, capable of providing energy for many future generations. South Australian lignites typically have high salt content, rendering them unsuitable for use in high-temperature coal combustion processes, owing to volatile sodium compounds, causing severe fouling on cooler heat-transfer surfaces. This and community concerns over greenhouse gas emissions are the main limitations of these coals in conventional combustion technologies. An alternative process is fluidized-bed gasification, which offers cleaner, more efficient use of South Australian low-rank coal because gasification occurs at temperatures below the ash melting point and, thus, permits more precise control over emissions, such as by the addition of dolomite to the bed to control sulfurous gas emission. Gasification also allows for flexibility of process choice; electricity production is possible at relatively high efficiency, via processes combining steam and gas turbines, or relatively high-value products, such as synthetic transport fuels (coal-to-liquid processes) or commodity chemicals [e.g., methanol, dimethyl ether (DME), etc.] can be produced using coal-derived synthesis gas as the feedstock.1 However, agglomeration and defluidization are major inhibitors to the use of fluidized-bed technology.2 Agglomeration is generally caused when the bed temperature exceeds a critical temperature, which is sometimes referred to as the “sintering point”.3,4 Above the sintering point, bed particles enter a softened or sticky state. This reduces relative movement between particles and results in particle growth. Under worst-case conditions, the bed ceases to fluidize effectively or “defluidizes”. Controlling agglomeration and defluidization is thus critical for any commercial fluidized-bed process. Many investigations have been performed with the aim of understanding the r 2011 American Chemical Society
high-temperature agglomerationdefluidization phenomena in fluidized beds in the combustion or gasification of coal,511 combustion of petroleum coke,12,13 combustion or gasification of biomass,1422 and co-combustion of biomass and coal.23 South Australian lignite possesses high levels of sodium and sulfur. In a study by Manzoori,5 under fluidized-bed combustion conditions, it was found that these inorganic species form lowmelting-point sodiumcalcium-sulfate eutectics, which create a sticky phase in the bed. This molten material coats “inert” bed material, such as silica sand, causing the particles to cohere. The subsequent particle growth ultimately contributes to defluidization of the bed. However, very few studies have investigated inorganicmatter-related problems associated with the fluidized-bed gasification of South Australian lignite. The most extensive was performed by Kosminski24 to analyze the behavior of sodium, silicon, and aluminosilicate species when subjected to a gasification environment. Sodium disilicate was identified as a key component likely to contribute to agglomeration and defluidization during fluidizedbed gasification of high-sodium lignite. The conclusions of this study were based on experiments performed in a small-scale tubular furnace under intimately controlled conditions. The implications of Kosminski’s work for practical fluid-bed gasifier operation are yet to be fully investigated. The current paper investigates the mechanism of agglomeration and defluidization of South Australian lignite under spoutedbed gasification conditions. A spouted-bed gasifier similar to that Special Issue: Chemeca 2010 Received: February 17, 2011 Revised: May 11, 2011 Published: May 12, 2011 2772
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Figure 1. Schematic diagram of the spouted-bed gasifier system. Within the diagram, PT, pressure transducer; TC, temperature controller; T1T5, measurement thermocouples; and PC, personal computer.
used in the studies of Manzoori5,9 is used in this work to provide a basis for comparison of gasification to combustion conditions on the agglomeration and defluidization phenomena. This investigation accounts for bed behavior by both visual observations and process monitoring, assesses particle growth that occurs during gasification, and identifies the impact of typical gasifier operating parameters on agglomeration and defluidization. Analysis of these issues assists in identifying the likely physical mechanisms of agglomeration and defluidization during spouted-bed gasification for these coals.
2. MATERIALS AND METHODS 2.1. Spouted-Bed Gasification Rig. The experimental apparatus, a 77 mm diameter, atmospheric pressure, spouted-bed gasifier, is shown in Figure 1. The spouted-bed reactor includes a 65 mm high conical base, which expands from a 10 mm (inner diameter) gas inlet to a 77 mm (inner diameter) cylindrical section. The 77 mm cylindrical section is approximately 1.1 m high and includes a high-temperature glass viewport at the top of the reactor. A coal feed port is located approximately halfway between the conical base and the viewport. Gas and fine solids exit the vessel, via an exit port, to the gas-handling section consisting of a cyclone and water-cooled condenser. Compressed air or nitrogen is heated using a Leister electric hot air tool type 5000. For the experiments described here, deionized water is metered using a peristaltic pump and injected into the hot gas stream. Nitrogen is used to create an inert environment during startup or shutdown of the reactor, while air is used during experiments. External heating is used during startup and to minimize heat loss from the reactor during operation. An electrically heated furnace houses the middle section of the reactor. The outer walls of the furnace consist of thermal ceramics insulating firebricks. Two pairs of Kanthal Crusilite electrical heating elements (nominal resistance = 6.0 Ω) provide heating; the elements are situated within each corner of the furnace
and are suspended vertically from the top plate. Radiation shielding prevents hot spots on the reactor wall, which would otherwise occur because of the close proximity of the elements. The hot fluidizing gas passes through the external electrical furnace before entering a plenum chamber below the conical section. The lower portion of the chamber also contains a removable “solids canister”, which is used to collect bed material at the completion of experiments. Coal is fed above the bed, at a height of 550 mm above the entrance to the conical base, via a pair of lock-hoppers and a screw feeder. Nitrogen is used to ensure an inert atmosphere within each of the hoppers and to provide backpressure to prevent steam from condensing in the feed line and, thus, causing blockages. The coal feed line is water-cooled near the entrance to the reactor. Type-K thermocouples (3.0 mm outer diameter) measure temperature at various locations along the centerline of the reactor. Thermocouples are labeled sequentially from TC 1 to TC 5 at the following locations: TC 1 is just below the conical distributor, and TC 2, TC 3, TC 4 and TC 5 are 35, 65, 105, and 155 mm above the gas inlet, respectively. Pressure tappings are located 15 mm below the gas inlet (PT 1) and 190 mm above the gas inlet (PT 2). The absolute pressure at PT 1 is measured using a Wika pressure transmitter. The bed pressure drop is measured between PT 1 and PT 2 using an ABB Kent Deltapi K series electronic transmitter. Both pressure transmitters provide a 420 mA signal to a Mann Industries PM350 industrial process monitor. Temperatures and pressure signals were monitored using a Pico Technology eight-channel data logger (model TC-08) and logged at 1 Hz using the supplied PicoLog software. 2.2. Experimental Section. The experimental program was designed to assess the effect of the bed temperature, superficial velocity, and air/fuel (A/F) ratio on agglomeration and defluidization. The experimental settings for each gasification test are shown in Table 1. The operating conditions for the experiments were chosen to correspond to typical values reported in the literature.2529 Each experiment was conducted over a period of 4 h, regardless of whether agglomeration and defluidization were detected. Only one run 2773
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Table 1. Experimental Operating Conditions for Evaluating Agglomeration and Defluidization Behavior during Spouted-Bed Gasification of Lochiel Coal
a
run
coal rate
air rate
steam
steam/fuel
steam
run
time (h)
(kg/h, db)
(L/min)
(g/min)
Us (m/s)
A/F (w/w)
(w/w)
concentration (wt %)
Tfurnace (°C)
Tbed max (°C)
A01 A02a
4.0 4.0
0.92 0.77
35.3 32.9
6.2 5.0
0.65 0.59
2.7 3.0
0.4 0.4
13.1 11.5
825 825
926 920
A03a,b
4.0
0.77
38.2
5.2
0.68
3.5
0.4
10.3
825
939
A04a,b
4.0
0.77
35.0
5.2
0.65
3.2
0.4
11.1
825
967
A05a,b
4.0
0.77
35.0
5.2
0.62
3.2
0.4
11.1
775
915
A06a,b
4.0
0.77
35.0
5.2
0.61
3.2
0.4
11.1
725
909
B01
4.0
1.09
33.5
7.1
0.59
2.2
0.4
15.1
775
831
B02a,b
4.0
0.75
30.0
6.4
0.57
2.8
0.5
15.2
825
914
B03 B05a,b
4.0 3.7
0.88 0.75
31.2 27.0
7.4 6.5
0.57 0.50
2.5 2.5
0.5 0.5
16.7 16.7
775 775
844 861
B06
4.0
0.83
29.5
7.1
0.51
2.5
0.5
16.7
675
789
B07
4.0
1.09
32.5
6.9
0.57
2.1
0.4
15.3
775
823
B08a,b
4.0
0.88
34.4
6.7
0.62
2.8
0.5
14.2
775
887
B09
4.0
1.27
34.0
8.2
0.60
2.0
0.4
16.3
750
799
B10
4.0
1.05
32.0
6.8
0.58
2.2
0.4
15.1
825
872
B11
4.0
0.92
36.5
7.7
0.64
2.8
0.5
15.2
675
826
B12
4.0
0.88
31.4
7.4
0.61
2.5
0.5
16.7
825
910
Defluidization detected. b Agglomerates detected.
Table 2. Lochiel Coal Propertiesa coal analysis proximate analysis (wt %) moistureb (ROM, wb)
composition
60.3
moistureb (air-dried, wb)
12.7
ash yield (db)b
15.7
fixed carbon (db)c
38.1
volatile matter (db)c
46.2
ash composition (wt %)d SiO2
31.4
Al2O3 Fe2O3
8.30 4.05
MgO
8.58
CaO
9.97
Na2O
8.68
K2O
0.30
TiO2
0.48
SO3
The bed temperature was indirectly controlled by adjusting both the A/F ratio and external furnace temperature. The coal feed rate, air flow rate, and steam flow rate were varied to alter the A/F ratio while maintaining the superficial velocity at a specific value. The superficial velocity for most experiments was 0.60 m/s (double the empirically determined minimum spouting velocity), with 0.50 m/s set for runs B05 and B06. The required flow rates of steam and air were calculated prior to each run based on the expected bed temperature, while the reported flow rates were corrected using the measured average bed temperature. Although the experimental parameters A/F, bed temperature, and fluidization velocity are closely linked for the experimental reactor, the addition of external heating elements provides a degree of independent control of the bed temperature for a given A/F and fluidization velocity. The bed material collected from each experiment was sieved through sieve apertures ranging from 0.85 to 3.35 mm. The material retained on each sieve was weighed, and the particle size distribution (PSD) was calculated using the method developed by Rosin and Rammler.30 Large agglomerates and coated mineral particles, which were retained on the 3.35 mm sieve, were examined using a combination of visual inspection and scanning electron microscopy (SEM).
27.5
a
ROM, run-of-mine coal; db, dry basis; and wb, wet basis. b Analysis method: HRL method 1.6. c Analysis method: AS 2434.2. d Analysis method: AS 1038.14. was operated for less than 4 h; run B05 was stopped after 3.7 h because of equipment safety concerns as the bed temperature increased above 1000 °C. While 4 h is a relatively short period of time, it is sufficiently long to identify those operating conditions leading to severe ashrelated problems within the bed, as opposed to those that yield relatively stable operation over this period of time. Low-rank coal from the Lochiel deposit in South Australia was used for all experiments. The average proximate and ash analyses for this coal are shown in Table 2. The coal was prepared by drying in air and then sieving to þ1.003.35 mm. The maximum coal feed rate was limited to approximately 1 kg/h (dry basis).
3. RESULTS AND DISCUSSION 3.1. Monitoring of Bed Operation. The state of bed fluidization was primarily monitored using the bed temperature and pressure drop; however, visual observations using the reactor viewport were also possible in some instances. Typical operating profiles for a stable spouting run and a run in which defluidization occurred are shown in panels a and b of Figure 2, respectively. Figure 2a shows that the measured axial temperatures in the bed increased rapidly for an initial period of approximately 15 min before achieving relatively constant values for the remainder of the run. The bed pressure drop also increased initially before reaching a relatively steady value after approximately 100 min of operation. 2774
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Figure 2. (a) Bed temperature and pressure drop profiles for a stable-spouting experiment (run B12). (b) Bed temperature and pressure drop profiles for a run exhibiting defluidization (run A03). Experimental operating conditions for each run are shown in Table 1, and thermocouple positions are shown in Figure 1.
Defluidization was clearly identifiable by a rapidly increasing bed temperature and decreasing pressure drop. Figure 2b shows an operating profile displaying typical defluidization behavior. After approximately 100 min of operation, the temperatures in the upper part of the bed (TC 3TC 5) increased rapidly, reaching approximately 1000 °C. At the same time, the bed pressure drop decreased from approximately 800 to 700 Pa over a period of approximately 40 min. It is clear that defluidization occurred gradually rather than instantaneously. These observations of temperature and pressure drop during defluidization correspond to those of other authors.35,31
After the increase in the bed temperature observed at the onset of defluidization, some temperature readings subsequently decreased. The temperature at the bottom of the bed (TC 2) dropped from 1000 to 700 °C, while the temperature at the top of the conical section of the vessel (TC 3) decreased to 850 °C over a period of 100 min. However, the temperatures of the two upper sensors (TC 4 and TC 5) remained constant after the initial increase that occurred at the onset of defluidization. A possible explanation for this phenomenon is that, when the fluidized-bed slumps because of defluidization, the lack of new coal feed to the lower sections of the spout leads to no 2775
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Table 3. Maximum Bed Temperatures before and after Defluidization for Each Defluidization Experiment
Figure 3. Percentage of agglomerates exceeding 3.35 mm in diameter in each defluidized bed versus the time of operation after the onset of defluidization.
combustion of char in this section. This lack of combustion heat leads to a reduction in the bed temperature in the slumped part of the bed. The upper thermocouples remain at similar temperatures because of the fact that the combustion of char is still maintained above the slumped part of the bed. Agglomerates were found in the bed material collected after all but one of the experiments in which defluidization was identified, as indicated in Table 1. Run A02 (A/F ratio of 3.0 and maximum bed temperature of 920 °C) yielded no evidence of agglomerates in the char bed at the completion of the experiment yet demonstrated possible defluidization behavior, as indicated by the temperature and pressure drop profiles. Runs B06 and B07 also showed a decrease in the pressure drop toward the end of the run, with no agglomerates observed in the bed material. However, in these runs, a partial blockage occurred in the gas outlet (after the condenser), as indicated by the rising pressure within the vessel, which appeared to directly affect the pressure drop measurements. No such behavior was observed in run A02 however, suggesting that the rise in temperatures and fall in pressure drop were as a result of defluidization. The extent of agglomeration in each bed appeared to vary inversely with the duration of operation prior to defluidization. Figure 3 indicates that the proportion of agglomerates (i.e., particles >3.35 mm in diameter) in the bed material increases from negligible amounts where defluidization occurred at the end of the 4 h operating period to approximately 40% of the bed mass where defluidization occurred within 1 h of commencing gasification. The proportion of agglomerates within any bed mass does not exceed more than about 40% because char particles were continually fed to the spouted bed following defluidization and some breakage of agglomerates may also occur during removal. Beyond the onset of defluidization, temperatures in the bed often far exceeded the maximum bed temperatures achieved during stable spouting. Table 3 shows the maximum temperature of operation for each “defluidization” experiment, and the corresponding maximum temperature achieved after defluidization was detected. The bed temperature after defluidization is observed to be 30140 °C greater than the normal operating temperature for each run. 3.2. Particle Growth. The PSDs of bed char from each experiment are shown in Figure 4. The PSDs of the bed materials for each of the runs that was defluidized are shown in Figure 4a,
A/F ratio
Tmax before onset
Tmax after onset
run
(w/w)
defluidization (°C)
defluidization (°C)
A02 A03
3.0 3.5
920 939
954 1055
A04
3.2
967
1036
A05
3.2
915
1042
A06
3.2
909
1034
B02
2.8
914
963
B05
2.5
861
954
B08
2.8
887
1023
while those from runs that were stable for the entire 4 h period are shown in Figure 4b. The average PSD of the coal feed is indicated on each plot to show the change in size distribution that occurred during each run. The PSDs of bed materials from the stable spouting experiments (Figure 4b) are relatively consistent, regardless of the experimental conditions of each run. The PSDs of the bed materials shown in Figure 4b are relatively linear, while the PSD for the coal feed exhibits significant curvature at the finer end of the distribution. This curvature is due to the removal of particles under 1.0 mm in diameter from the feed (by sieving), thus lowering the proportion of the bed weight in the finer size fractions. All of the distributions of the bed materials from the stable spouting experiments (Figure 4b) lie to the left of the feed coal distribution, indicating that the feed coal is coarser than the bed char. This is expected because of the combined effects of char size reduction as a result of stable gasification and particle fragmentation within the moving bed material. The “defluidization” runs (Figure 4a) show a significantly more diverse range of PSDs than the stable runs. Significant curvature is also apparent at the coarse size fraction end (>3.35 mm) of each distribution, because of agglomerate formation within the beds of these experiments. Some of the runs have bed material that is significantly coarser than the feed coal, with the distributions lying appreciably to the right of the feed coal distribution in Figure 4a. The least-squares method was used to establish the slope and intercept of trend lines (not shown) through each data set in Figure 4. Particle sizes greater than 2.36 mm were neglected from this analysis to exclude any effects because of the significant curvature at the coarse end of some distributions. The slope and intercept for each of these trend lines is plotted in Figure 5, which shows that the gradient and intercept of trend lines through the RoslinRammler distributions in Figure 4 are linearly related. Data points for the stable spouting gasification runs are mostly collected in the lower right-hand corner of the plot area, while data points for defluidized runs are distributed toward the upper left-hand corner. Thus, as the particle size increases because of agglomeration, the slope of the trend line decreases. There also exists a “mixed” region in Figure 5, where both stable and defluidization runs occur with similar slopes and intercepts. Each of the stable runs in the “mixed” region in Figure 5 show characteristics that are expected to be conducive to agglomeration and defluidization. These include runs with a relatively low superficial velocity, such as run B06 (gradient = 2.0, intercept = 6.4) at 0.51 m/s and run B03 (gradient = 2.1, intercept = 6.8) 2776
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Figure 4. (a) RosinRammler PSDs for defluidization experiments, which includes PSDs for feed coal and a non-defluidization run (run A01) for comparison purposes. (b) RosinRammler PSDs for non-defluidization experiments, which includes the PSD for feed coal.
Figure 5. y intercept versus gradient of trend lines through the RosinRammler PSDs for each bed material. Stable and defluidization runs are indicated. The following groups of data are identified: (i) defluidization runs, (ii) mixed region, displaying both stable and defluidization runs, and (iii) stable runs.
at 0.57 m/s, and runs operated at bed temperatures exceeding 900 °C, including run A01 (gradient = 2.0, intercept = 6.6) at 926 °C and run B12 (gradient = 2.1, intercept = 6.9) at 910 °C. Furthermore, both of the defluidized runs in the “mixed” region exhibited defluidization toward the later stages of the run, at approximately 195 min operating time for run A05(gradient = 2.0, intercept = 6.6) and 220 min operating time for run A02 (gradient = 2.0, intercept = 6.5). These relatively short periods of operation following defluidization indicate that perhaps more severe agglomeration was not able to proceed in these cases. These findings provide evidence that, during spouted-bed operation, there exists a critical particle size limit, whereby exceeding this limit results in defluidization and rapid continued growth of agglomerates within the bed. This infers that defluidization can be avoided if particle growth can be kept in check, by physical methods, such as particle removal, and/or by avoiding operating parameters, such as high bed temperature and low superficial velocity, which may enhance the rate of particle growth. 3.3. Visual Observations of Agglomeration and Defluidization. The spouting action during all experiments could 2777
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Figure 6. Top view of the defluidized bed (run A03) at completion of the experiment.
generally be observed through the viewport at the top of the reactor. Visibility through the sight glass became limited during each run after approximately 3060 min of operation because of the deposition of elutriated fines. However, stable spouting was confirmed during each experiment (prior to defluidization, where applicable) and was characterized by a relatively even motion of particles from the center of the bed toward the vessel wall (when viewed from the top of the bed). In a limited number of cases, visibility through the sight glass was sufficient to allow for a visual account of defluidization. The onset of defluidization was observed to commence with a noticeable destabilization of the spout. This was characterized by irregular bursts of particle movement from the center of the bed, with noticeable reduction of movement in the annulus. Starting at the wall, this sluggishness spread toward the center of the bed to complete the defluidization process. Sluggish particle movement at the walls appeared to create conditions favorable for agglomerate formation in the annulus of the bed. At the completion of all of the experiments that displayed defluidization behavior, char and small agglomerates could be easily removed through the bottom of the reactor, while larger agglomerates typically remained inside the spouted-bed vessel. Inspection of the inside of the reactor showed a torusshaped agglomerated mass collected around the wall in the most severe cases, as shown in Figure 6. This high-ash material could only be removed from the reactor vessel by manual breakage. This observation indicates that, as expected, agglomeration occurs primarily in the close-packed section of the bed. At the completion of most tests, small ash particles of various sizes were observed in the char bed material, regardless of whether defluidization affected operation. Typical examples of such particles are displayed in Figure 7a. These all exceed the largest feed size of 3.35 mm, indicating that they formed during gasification rather than entering the bed with the coal. These particles were observed to fall from the reactor first when the bed char was removed at the conclusion of each experiment, indicating that they had collected at the bottom of the bed during gasification. Closer examination of these particles, using SEM, revealed that they were single-mineral particles coated in a light gray ash. Similar near-spherical ash agglomerates have been encountered previously during the spouted-bed gasification of an Australian bituminous coal.27 The average temperature at the bottom of the spouted bed in each experiment was typically 50100 °C lower than the
Figure 7. Typical ash bodies obtained from gasification experiments. (a) Coated ash particles collected from bed char of run A01 at the completion of the run. (b) Loosely bound agglomerates (run A05). (c) Sintered agglomerates, >3.35 mm in diameter (run A04). (d) Surface of a highly sintered agglomerate (run B02).
maximum temperature of the bed, and this may be the primary reason that the large coated mineral particles shown in Figure 7a did not agglomerate, even in runs that showed agglomeration and defluidization behavior. Particles such as these were collected from the bottom of the bed at the completion of many gasification tests. Under the lower temperature conditions at this location in the bed, the ash may have been below its sintering point, resulting in the particles being unable to sinter at a rate conducive to multi-particle agglomeration. The smooth nature of these coated spherical ash particles is consistent with the coating occurring as the particles were circulated within the spouted bed. Mineral particles, which were initially below the largest feed size, were presumably spouted freely but then progressively became heavier as they were coated. Eventually, the weight of these heavier particles prevented them from being further circulated, and thus, they came to rest at the bottom of the bed. The agglomerates shown in Figure 7b were loosely bound structures, clearly showing the rounded shapes (lumps) of individual mineral particles. The rounded lumps are approximately 12 mm in size and are significantly smaller than the coated particles of Figure 7a. The loosely bound nature of these agglomerates indicates that sintering had not progressed to a significant extent before the experiment was concluded. The agglomerates shown in Figure 7c were highly fused structures and were not as easily broken apart as the loosely bound agglomerates shown in Figure 7b. Such agglomerates were typical of the ash bodies found adhering to the walls inside the reactor after shutdown. A close-up view of the surface of one sintered agglomerate is shown in Figure 7d. The surface of the agglomerate is composed of small rounded lumps on its surface, much like those in the loosely bound agglomerates (Figure 7b). 3.4. Bed Temperature and A/F Ratio. In the present experiments, the bed temperature is primarily influenced by the A/F ratio and the external furnace temperature (Tf). The maximum bed temperature for each run is plotted versus the A/F ratio in 2778
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Figure 8. Maximum bed temperature as a function of the A/F ratio and external furnace temperature (Tf) for all spouted-bed gasification runs.
Figure 8 as a function of Tf. Clearly, the bed temperature increases with Tf for a given A/F ratio and with A/F ratio for any given Tf. Run A01, operated with a Tf of 825 °C and an A/F ratio of 2.7, had a maximum bed temperature of 926 °C. While this bed temperature is equivalent to the maximum temperature at which other runs in the set defluidized, no signs of agglomeration and defluidization were observed during the 4 h period of this run; although, as mentioned in the preceding section, particle growth was observed within the bed to a greater extent than for other stable spouting experiments. The other runs in the set that were defluidized were operated at an A/F ratio of g3.0, suggesting that the lower A/F ratio for run A01 may have reduced the propensity for agglomeration. This effect may be due to the higher rate of char consumption at a high A/F ratio. Kosminski24 showed that ash forms uniformly throughout char grains within a gasification environment. Hence, increasing the A/F ratio and, thus, increasing the rate of char consumption will lead to a greater level of ash within the bed and, thus, lead to a higher propensity for agglomeration. Two runs from set B, B10 and B12, showed no sign of defluidization yet were operated at temperatures that coincided with agglomeration and defluidization in other experiments in the set. Run B10 was operated at 872 °C but with an A/F ratio of 2.2, suggesting that the level of char consumption was insufficient to initiate agglomeration, at least during the 4 h experiment. Run B12 was operated at 910 °C (Tf of 825 °C and A/F ratio of 2.5) but also did not defluidize, although as mentioned above, this run did exhibit relative particle growth within the bed, suggesting that agglomeration and defluidization may well have occurred with longer operation. Furthermore, run B05 was also operated at an A/F ratio of 2.5 but with a lower Tf (775 °C) and maximum bed temperature (861 °C) than run B12, yet run B05 did exhibit defluidization behavior. The major difference between runs B05 and B12 were the superficial velocities of 0.51 and 0.61 m s1, respectively. The lower superficial velocity for run B05 would lead to reduced particle momentum and, hence, a greater propensity to agglomerate, even though the bed temperature was lower. Thus, superficial velocity appears to have influenced the rate at which defluidization occurred in these specific cases. 3.5. High-Temperature Defluidization Limit. The bed temperature and superficial gas velocity primarily determine the state
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Figure 9. Gas velocity calculated at the inlet to the spout versus maximum bed temperature. The dashed line indicates the possible “high-temperature defluidization limit”. Defluidized and stable runs from experiment sets A and B are identified. Stable runs A01 and B12 are marked separately.
of fluidization at temperatures exceeding the minimum “sintering temperature” of the bed material.3,4,31 Thus, it is often possible to identify a relationship between the minimum gas velocity required for stable fluidization and the bed temperature, which is termed the “high-temperature defluidization limit”, and this relationship depends upon the properties of the material being fluidized. Figure 9, which plots the inlet gas velocity against the maximum bed temperature for each run, appears to suggest that a high-temperature defluidization limit exists within the present spouted-bed experiments. One possible high-temperature defluidization limit is indicated in Figure 9. This line clearly divides the runs that showed defluidization behavior on the right-hand side from most of those runs that exhibited stable fluidization on the left-hand side. Two stable runs, A01 and B12, do appear in the “defluidization” region to the right of the limit shown in Figure 9. For each of these runs, the RosinRammler analysis indicates that some particle growth had occurred during operation, and therefore, perhaps defluidization may have occurred for these runs after 4 h. Further targeted experiments would be required to more precisely define the high-temperature defluidization limit. In constructing Figure 9, the inlet velocity to the spout rather than the superficial velocity within the cylindrical section was used. Superficial velocity, while the conventional method of characterizing velocity in a spouted bed, does not adequately define the velocity of gas in the spout; the inlet velocity provides a more accurate identification of spout velocity. Furthermore, agglomerating particles were found to coalesce toward the bottom of the bed, making inlet velocity more critical than superficial velocity, where significant particle growth was encountered. Inlet velocity was calculated on the basis of flow rate of gas and the gas temperature immediately prior to the gas inlet (i.e., TC 1). 3.6. Ash Content of the Bed. Carbon content or conversely ash content of a bed of low-rank coal has been shown previously32 to have a bearing on agglomeration and defluidization behavior in a gasification environment. This was attributed to the higher inorganic content of the bed, yielding a greater surface area available for inorganicinorganic interactions and increasing the probability of particle collisions resulting in 2779
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Figure 10. Inorganic content of bed material versus the maximum bed temperature. Set A and B runs are indicated and also whether defluidization occurred during the 4 h operating period. Runs that did not defluidize yet showed significant particle growth (viz., A01, B03, and B12) are labeled.
agglomeration. In these spouted-bed gasification experiments, no bed material is removed during operation; however, a commercial installation, which has this capability, would allow for better control of the amount of ash in the bed. The inorganic elemental composition of the bed material remaining at the completion of each experiment was measured using X-ray fluorescence (XRF) analysis. These analyses were used to estimate the total ash content of the sample. Agglomerates exceeding 3.35 mm were excluded from the bed material samples by sieving. However, the estimate of total ash content includes the ash contained within smaller agglomerates and spherical ash particles and the ash included within char particles. A plot of inorganic ash content of the bed at the completion of the run versus the maximum bed temperature is shown in Figure 10. Runs are distinguished by whether or not defluidization occurred during the 4 h operating period. The results show that all gasification runs displaying defluidization behavior possessed inorganic contents exceeding approximately 85 wt %. This corresponds to results published by Hsieh and Roberts32 from their work with low-rank American coals, which indicates that agglomeration only occurs to a significant extent when carbon conversion in a finite coal sample exceeds 80%. Carbon was seen to provide an inhibitory effect on ash sintering in the samples, such that ash could more readily sinter into larger agglomerates when a sufficient amount of carbon was removed. No agglomeration was observed below 860 °C, indicating that defluidization under the conditions of these experiments may depend upon both the inorganic content of the bed and the bed temperature. Runs previously identified as those showing particle growth but no observable defluidization were located close to the operating region identified above (i.e., bed temperature exceeding 860 °C and inorganic content exceeding 85 wt %). The bed material from runs A01 and B12 possessed ash contents of 81.2 and 84.4 wt %, respectively. This provides further evidence that the particle growth seen in these runs may have eventually led to defluidization if operated for a sufficient time, with ash buildup eventually creating conditions favorable for defluidization. Another run that exhibited particle growth, run B03, was seen to possess the highest inorganic content of all runs, at 97.9 wt % of
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the bed. The bed temperature of this run however, approximately 844 °C, seems to have been sufficiently low to prevent defluidization. 3.7. Physical Mechanism of Agglomeration and Defluidization. On the basis of these results, it may be concluded that the phenomena of agglomeration and defluidization during gasification of high-sodium, high-sulfur lignite occur because of the following mechanism: (1) For a given fluidizing gas composition, there appears to exist a “high-temperature defluidization limit”, which specifies a minimum superficial velocity to avoid defluidization for a given bed temperature. The exact relationship between the velocity and bed temperature is not known based on the current experiments but, nevertheless, will vary with ash and, hence, coal composition. Thus, defluidization may be avoided with high superficial velocities leading to increased particle momentum or with decreased bed temperatures leading to a decrease in the sintering propensity of the ash. (2) Under bed temperature and gas velocity conditions at which the bed is susceptible to agglomeration and defluidization, particle growth occurs during stable operation. This particle growth appears to arise mainly with the coating of mineral particles by a molten inorganic phase. For these experiments, where no bed material was removed during operation, particle growth occurs with concomitant increase in the total ash content of the bed. (3) For all experiments in which particle growth was detected, the total inorganic content of the bed material exceeded approximately 80 wt %. (4) As the particle growth reaches a critical limit, spouting becomes unstable. This leads to defluidization, which is manifested by particles in the annulus surrounding the spout becoming stationary and with gas channelling through the center of the bed. (5) The closely packed bed arising from defluidization promotes the formation of hot spots. These temperature excursions accelerate local sintering of the ash, resulting in the formation of multi-particle agglomerates. This work allows for some insight into the operation of fluidized-bed gasifiers under other regimes of fluidization. Because there exists a minimum superficial velocity to avoid defluidization for a given bed temperature, it may be proposed that, for fast fluidizing beds (e.g., circulating fluidized-bed gasifiers), the temperature at which the bed can operate and avoid defluidization will likely be higher than that for slower fluidizing beds (e.g., bubbling-bed gasifiers).
4. CONCLUSION The occurrence of agglomeration and defluidization during otherwise stable spouting of a high-sodium, high-ash lignite (i.e., Lochiel coal) was observed in a 77 mm inner diameter spoutedbed gasifier. Results indicate that bed operation is governed by a “hightemperature defluidization limit”, suggesting that defluidization can be delayed or avoided by operating the bed with high superficial velocity and/or low bed temperatures. In experiments that resulted in agglomeration and in some experiments that did not, the average particle size within the bed material increased. This particle growth was attributed mainly to the coating of mineral particles. These coated particles were observed to be more prevalent in defluidization runs. The particle growth also coincided with an increased inorganic content of the bed compared to stable runs. This suggests that defluidization occurs only when the inorganic content of the bed exceeds a 2780
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’ AUTHOR INFORMATION Corresponding Author
*Telephone: þ61-8-8303-5056. Fax: þ61-8-8303-4373. E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors gratefully acknowledge the financial and other support received for this research from the Cooperative Research Centre (CRC) for Clean Power from Lignite, which was established and supported under the Australian Government’s CRCs program. The authors also acknowledge Dr. Davide Ross for his advice and technical assistance with the experimental gasification equipment and Dr. Adam Kosminski for his advice and input into the analysis of experimental results. The authors also acknowledge the contributions of the late Dr. Peter Jackson during this work. Peter is sadly missed.
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