Environ. Sci. Technol. 2002, 36, 4447-4453
The Enhancement of Hydration of Fluidized Bed Combustion Ash by Sonication E. J. ANTHONY,* L. JIA, L. CYR, B. SMITH, AND S. BURWELL CANMET Energy Technology Centre, Natural Resources Canada, 1 Haanel Drive, Ottawa, Ontario, Canada, K1A 1M1
The use of limestone to control SO2 emissions in fluidized bed combustors (FBC) results in high CaO content in the ashes. This presents challenges for their disposal including significant exothermic behavior and uncontrolled expansion in the landfill. Hydration of the ashes is required to convert the CaO, but the current two-step hydration process is not very effective. In the present work a new technique using ultrasound to promote the hydration was examined. Initial work was done using an ultrasonic bath and subsequently an ultrasonic probe. Hydration levels greater than 80% in 20-40 min with amplitudes of 40% or more were achieved with residues containing high levels of CaO. This is about twice what can be achieved using conventional hydration technique. Similar results were obtained using FBC ashes from four utility/industrial scale and one pilot plant scale FBC units. The mechanism for the promotion of hydration by the ultrasound is also explored.
Introduction Fluidized bed combustors (FBC) burns fuel in a suspended mass of solids at temperatures between 800 °C and 950 °C. Fuels with high sulfur content require limestone to be fed to the FBC for the in-situ capture of SO2 according to the global reactions (1):
CaCO3 f CaO + CO2
(1)
CaO + 1/2O2 + SO2 f CaSO4
(2)
Relatively high Ca/S molar ratios of 2:3 are used, and as a result FBC residues consist of a heterogeneous mixture of CaO, CaSO4, fuel-derived ash, small amount of CaCO3, and char from the fuel. This mixture differs from pulverized fly ash (PFA), which is a glassy mixture of fused silicates obtained from conventional pulverized fuel (PF) boilers, in its properties and management requirements (2). In particular, the residues from FBC combustion of high-sulfur coal with limestone addition are highly exothermic in contact with water. The presence of free lime, derived from the limestone according to reaction 1, can result in temperatures in excess of 100 °C upon contact of the residues with water (3, 4). This can pose a potential safety threat to operators of the ash conditioning system used for the treatment of FBC ash prior to disposal. The high free lime content also leads to the production of an alkali leachate, with pH between 11 and 12, requiring treatment prior to discharge. Even consolidated FBC ashes * Corresponding author phone: (613)996-2868; e-mail: banthony@ nrcan.gc.ca. 10.1021/es011316p CCC: $22.00 Published on Web 09/18/2002
2002 American Chemical Society
are subject to considerable expansion as a result of freezethaw conditions that allow ingress of atmospheric moisture or groundwater via cracks in the landfill. Initial permeabilities achieved with appropriate hydration and compaction (about 10-6 mm/s) eventually increase to those more typical of sandy soil (10-3-10-2 mm/s) (5). This leads to the generation of large amounts of high pH leachate requiring treatment. It is important to note at this point that the hydration process during the conditioning stage involves only the free lime portion of the ash and not the anhydrite component, which
CaO + H2O f Ca(OH)2 ∆H ) - 65.27 kJ/mol (3) typically takes hours to days to react significantly at ambient conditions:
CaSO4 + 2H2O f CaSO4‚2H2O
∆H ) -17.20 kJ/mol (4)
Reduction of the exothermicity of FBC solids is currently carried out via hydration in a two-stage procedure. The solids are first conditioned with water in a pug mill, to hydrate unreacted quicklime, and then treated with water at the disposal site to complete the hydration process and achieve the optimum density, typically within (10% of that determined by a modified Proctor test (5, 6). This second addition of water allows cementitious reactions to go to completion in the hope that this will improve the overall strength and durability of the site. The two-step conditioning approach is recommended by Environment Canada in its guidelines for the disposal of residues from circulating fluidized bed combustion (CFBC) (7). Unfortunately, conventional conditioning requires a significant quantity of water, which can approach 40-50% of the dry weight of the ash (8). In some circumstances water use may become a constraint. Further, significant water loss occurs due to steaming caused by the exothermic hydration process. For example, the 165 MWe Point Aconi plant located at Point Aconi, Nova Scotia, has a water demand of 2.7 dm3/ s, which is ∼27 wt % of ash with only 18 wt % being required for hydration. Water losses account for 10% or more of the water used in the primary conditioning step (8). In addition, complete hydration of the CaO component in CFBC solid residues is not ensured under this program. Data from the Chatham 22 MWe CFBC demonstration project suggest that even after a two-stage conditioning process the hydration of the spent ash was not complete (at best about 70-80%) (4, 5). Effective hydration of those ashes should also reduce some of the activity in the landfill (5). It has been shown that hydration at ambient temperature may take hours to days to complete (9). A method of increasing the degree of hydration is highly desirable, as current landfill costs can be as high as $20-$30/ton due to the problems with these ashes (1). Further an effective one step hydration process might have promise for ash reactivation allowing reutilization of the hydrated ashes by reinjection into the boiler to achieve more complete sulfation than currently occurs (1). However, this aspect is outside of the scope of this paper and it will not be discussed further here. Possible methods of increasing the degree of hydration include the following: (1) increase reaction time for hydration; (2) use of a catalyst; (3) increase the concentration of the reactants; (4) increase contact between reactants; (5) increase the reaction temperature; and (6) increase pressure. Some of these possibilities appear to be impractical. For instance, increasing the reaction time is an effective strategy for conditioning but would require very large and hence, expensive vessels to allow treatment of the quantity of solids VOL. 36, NO. 20, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Ca Speciation of the Ashes (Wt %) sample
CaO
CaSO4
CaCO3
OCCa
Point Aconi CANMET NISCO Purdue TVA
14.9 17.6 20.0 15.1 31.3
38.6 18.0 59.3
2.1 1.0 2.2
6.6 2.0 8-9
21.3
0.7
6.4
a
Expressed as CaO for the calcium bound in the form silicates, ferrites, and aluminates, also as vanadates in the case of the NISCO and TVA ash.
involved in a commercial plant. There is also no obvious method of either using a catalyst to enhance the reaction rate or increasing reactant concentrations. One could also envisage using a surfactant to improve penetration of water into the pore of the spent material. The problem with this approach is that pore sizes typical of calcined limestone capable of accommodating the sulfate ion (0.1-2 µm (10)) are too small to allow surfactants to be usefully employed. By contrast increasing reaction temperature is evidently effective in promoting hydration. It was recommended to use water at 98 °C for sorbent reactivation (11). However, the practicability of this approach is questionable without considerable engineering development, as was explicitly recognized in a report to the Canadian Electrical Association (CEA) (11). The approach of using both increased reaction temperature and pressure also appears very successful (12). Unfortunately, this process is also rather expensivesthe estimated capital cost was $530 000-$560 000, about twice that of the existing ash conditioning system at Point Aconi (11, 13). Hydration of FBC residues with sonochemical activation has been conceived at CANMET. It is known that the rate of a solid/liquid reaction is enhanced by irradiation with ultrasound. For some reactions ultrasound produces enhancements of reaction by 2 orders of magnitude or more. The phenomenon of cavitation exists whereby bubbles are created in a liquid and then implode with the generation of shock waves. On the microscale, the local transient temperatures in the bubbles may reach several thousand degrees and the local transient pressure may be up to 100 MPa (1417). Ultrasound also produces pitting and strong shearing forces at the particle surface in liquids, significantly enhancing mass transfer (14-17). These phenomena can be expected to enhance the reaction between FBC solids and water thus increasing the potential for hydration of the ashes. The issue is whether sonochemical activation is effective enough to be economically attractive for this application. This work examines the application of ultrasound for hydrating FBC ashes.
Experimental Section The experimental work was carried out using FBC ashes from five different sources. The first two were bed ash and fly ash from the CANMET 0.8 MWth CFBC pilot plant and Nova Scotia Power’s Point Aconi 165 MWe CFBC unit. Both units produced ash burning Devco Prince coal and local limestone. Bed ash from two 100 MWe CFBC boilers burning petroleum coke owned and operated by the Nelson Industrial Steam company Ltd. (NISCO) in Louisiana was the third source. The remaining ash samples (bed ashes) were from the Purdue University 25 MWe CFBC boiler burning a high-sulfur coal and the Tennessee Valley Authority (TVA) 160 MWe BFBC boiler burning a 65/35 blend of coal and petroleum coke. The breakdown of the calcium speciation obtained by means of a combination of X-ray fluorescence (XRF) and thermogravimetric analysis (TGA) methods for the different ashes is presented in Table 1, where OCC stands for calcium bound 4448
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in the form of silicates, ferrites, and aluminates, also as vanadates in the case of the NISCO and TVA ash. Preliminary tests into the feasibility of using ultrasound for hydrating FBC ash were carried out using a Astrason Ultra Sonic Cleaner Model 19 ultrasonic bath made by Branson. It is a low power ultrasonic cleaning bath normally used for cleaning glassware, with a power rating calculated as 190 W following the procedure described by Mason et al. (15). The ultrasonic bath was first run for about 6-7 h to ensure a stable temperature (typically between 57° and 64 °C). A sample of about 5 g was then added to a flask and its weight was recorded. Thirty-five milliliters of deionized water was added to the samples, and they were covered to prevent loss of water. The samples were then subjected to sonication for periods of 20-180 min. The degree of hydration was determined from the loss produced by thermally decomposing any Ca(OH)2 produced: after the hydration process was completed, the sample was dried, heated at 500 °C, and then weighed to determine the weight loss owing to evolution of water
Ca(OH)2 f CaO + H2O
(5)
The remainder of the ultrasound-testing program utilized a Vibra Cell VCX 600 high-intensity ultrasonic probe manufactured by Sonics & Materials. This unit has a nominal power rating of 600 W and an operating frequency of 20 kHz. Tests were performed at specified amplitude settings and different run times to determine the effect of amplitude and time on the degree of hydration. Samples were placed in a covered flask contained in a beaker, in which the ultrasonic probe was placed and allowed to reach stable temperature prior to adding hydration water to the flask. For these runs a sample weight of 10 g was used with addition of 50 mL of deionized water. The choice of the exact amount of water to hydrate samples was somewhat arbitrary but was chosen in line with the recommendations of Pyropower to perform hydration of FBC residues with 3-5 times the sample weight of water (11). The samples were sonicated for 20, 40, 60, or 80 min with amplitudes of 20, 40, 60, or 80%. For comparison with the degree of hydration without sonication, equivalent samples were hydrated using a similar amount of water held at the same temperature and time in a control oven, and these samples are designated “control oven” in the various tables. The procedures for determination of the degree of hydration were the same as with the ultrasonic bath tests, except that for some ashes a vacuum oven was used to allow reduced drying time and higher heating temperature (550 °C), because of concerns about achieving complete decomposition of the Ca(OH)2. The results of these tests were in reasonable agreement with earlier work, suggesting that this method for determining hydration levels is adequate and that no substantial hydration or carbonation occurs during the drying period itself.
Results and Discussion The purpose of the preliminary work using the ultrasonic bath was to verify the concept that ultrasound could be used to enhance the hydration reaction (18). Once the benefits of ultrasound were demonstrated, additional work was performed using the ultrasonic probe and the full range of the five ash sources. Ultrasonic Bath. The average results from four sets of samples of this preliminary investigation using the ultrasonic bath on CANMET bed ash samples are given in Table 2. These show that early on there was an improvement in the hydration behavior by a factor of about 2 over samples hydrated in the absence of ultrasound in an oven. However, the difference
TABLE 2. Percent of CaO Converted to Ca(OH)2 for CANMET Bed Ash and Fly Ash with Ultrasonic Bath hydration time run
20 min
average conversion with sonic bath average conversion for control oven
29.0 ( 6.8 13.4 ( 3.7 67.2 ( 13.3 63.7 ( 12.5
average conversion with sonic bath average conversion for control oven
40 min
60 min
120 min
180 min
Bed Ash 55.4 ( 9.1 32.2 ( 7.5
74.0 ( 8.4 60.0 ( 7.4
85.5 ( 5.4 85.7 ( 4.4
90.4 ( 5.7
Fly Ash 70.4 ( 11.2 67.2 ( 10.0
78.7 ( 8.0 72.6 ( 8.8
86.5 ( 8.8 81.3 ( 6.9
96.5 ( 8.0 88.2 ( 10.2
decreased with time and by about 120 min the conventional hydration method produced equivalent results. Several things should be noted about the degree of conversion calculated in Table 2. First, free lime was calculated on a representative sample of the bulk material, not on each individual sample. Second, given the analytical data indicating this material has 1.09 ( 0.029% carbon content, the results may contain a contribution from any carbonaceous material oxidized. However, the errors introduced by these factors are probably small and direct determinations of the CaO in our samples gave a value of 18.1 ( 2.1%, suggesting reasonable agreement and indicating that it is not necessary to determine free lime content under nitrogen. In addition, the two TGA determinations for Ca(OH)2 were carried out on sonicated samples and these showed reasonable agreement with oven tests, thus increasing confidence in the results. Attempts to hydrate the baghouse ash following the same procedure initially gave irregular results regarding the degree of hydration. However, it was soon realized that carbon loss due to oxidation was occurring during the calcination process (reaction 5) at 500 °C and contributing to the measured weight loss used to estimate the Ca(OH)2 of the samples. In consequence, it was decided to remove the char carbon (initial content-17%) by “ashing”‘ the baghouse samples. When this was done, consistent and reproducible results were obtained with the baghouse ash samples. Three tests were performed, again with the CANMET ash, using this modified baghouse material and the results are shown in Table 2. These data show only marginal improvements in the hydration rate of fly ash for the sonicated samples over those in the control oven. It was also clear that fly ash samples behave quite differently from the bed ash samples. Initially they achieve a much higher degree of hydration regardless of the hydration method but then react much more slowly than the bed ashes, taking many hours to reach hydration at ambient conditions. Thus hydration was not complete for all samples even after 180 min. It is seen that within the first 20 min hydration levels of the fly ash are almost five times that achieved for conventionally hydrated bed materials. The CaO content in the original baghouse material was determined to be about 13.9%, but after ashing it was calculated as 16.7, which is quite similar to the bed material. Despite the relative lack of effect on the baghouse ash, these experiments were considered to provide “proof of concept”. In consequence, the work on the ultrasonic bath was suspended in favor of experiments with the more powerful ultrasonic probe. Ultrasonic Probe. The ultrasonic probe was used for a more comprehensive test program. For these runs, tests were performed at specified amplitude settings and different run times, to determine the effect of both amplitude and time on the degree of hydration. The results for the CANMET bed ash material are given in Table 3. Some results are greater than 100%, and this apparently paradoxical result is in fact perfectly possible because under some circumstances the
TABLE 3. Percent of CaO Converted to Ca(OH)2 in CANMET Bed Ash with Ultrasonic Probe
run Sonic probe 8 Control Oven 8 Sonic probe 9 Control Oven 9 Sonic probe 10 Control Oven 10 Sonic probe 11 Control Oven 11
hydration time start amplitemp tude 20 min 40 min 60 min 80 min (°C) (%) (%) (%) (%) (%) 53 53 71 71 78 78 82 82
20 40 60 80
26.7 46.0 45.5 48.3 73.9 29.5 80.7 47.7
42.6 53.4 76.7 58.0 87.5 39.8 90.9 79.5
51.7 59.7 86.9 63.6 98.9 75.6 98.9 90.9
75.0 77.8 90.9 81.3 102 88.6 96.0 92.6
other calcium compounds can supply CaO to the system (9, 18). For these runs the control oven samples were hydrated at the same steady state temperatures as those produced by the ultrasonic probe. The results are broadly equivalent to those achieved with the ultrasonic bath. As a general conclusion, the degree of hydration increases with time, and the difference between the ultrasonic and conventional hydration decreases as the reaction goes to completion at longer periods of time. As the steady-state temperatures increased with the ultrasonic amplitude, the conversions are a function of both reaction time and temperature (which ranges from 53° to 82 °C), and these results are not directly comparable with those given in Table 2. These results for the CANMET bed material show that lower amplitude produces little effect, but higher amplitude has an increasingly positive effect on hydration, offering up to twice the hydration rate at shorter times. While the results for the amplitude of 20% appear to be anomalously low, they were not repeated since the aim of the work was to determine how to achieve maximum benefit with power ultrasound. It is evident that the proportional benefit of increasing from a lower amplitude of 40-60% is much greater than that of increasing from a higher amplitude of 60-80%. It would, therefore, appear that operation at an amplitude of about 60% probably offers the greatest benefits in terms of reduced energy consumption and enhanced hydration rate for the CANMET bed materials. Again, as with the ultrasonic bath experiments, the benefits of ultrasonic hydration over conventional hydration diminish with increasing time, as the hydration approaches completion. Table 4 presents the results for the CANMET baghouse materials. As in the case of the samples examined with the ultrasonic bath, these were ashed before being hydrated. Direct determination of the CaO content following ashing at 750 °C for 2 h gave a value of 17.8 ( 0.11%, which compares well with the previously determined value of 17.9 ( 2.4%. It should be noted that for the Sonic probe 13 run, three conversion values which are larger than 100% may be due to either a systematic measurement error or the decomposition of the OCC to release more CaO into the system (18). However, the time dependence of the conversion appears to be consistent to that of the other runs. The results are similar VOL. 36, NO. 20, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 4. Percent of CaO Converted to Ca(OH)2 in CANMET Baghouse Ash with Ultrasonic Probe
run Sonic probe 12 Control Oven 12 Sonic probe 13 Control Oven 13 Sonic probe 14 Control Oven 14 Sonic probe 15 Control Oven 15
hydration time start amplitemp tude 20 min 40 min 60 min 80 min (°C) (%) (%) (%) (%) (%) 53 53 71 71 78 78 83 83
20 40 60 80
75.6 63.9 98.3 65 70 48.9 97.2 47.2
75.6 53.3 106 51.1 72.8 41.7 98.3 38.9
80 53.9 117 51.7 83.3 41.1 100 40
88.3 67.8 117 72.2 83.3 59.4 98.3 60.5
to those achieved with the ultrasonic bath, namely, the degree of early conversion is relatively high as compared with bed ash samples (Tables 2 and 3), and the degree of conversion increases only slowly as a function of time, unlike bed samples. However, it is clear that, at higher amplitudes, ultrasonic treatment provides a significant improvement in the degree of hydration by factors of 1.5-2. It seems clear that, despite a fair degree of experimental scatter in these results, significant benefits in accelerating the hydration rate are achievable for baghouse samples by using high amplitude ultrasound (corresponding to increased ultrasound energy). The implication of these data is that the best way to use power ultrasound for hydration in an industrial setting is to treat the ashes for relatively short periods of time with moderate amplitudes at ambient conditions. This is preferable to trying to obtain total conversion of the free lime as a process like the CERCHAR process does. Such an approach is compatible with the aim of achieving the maximum degree of hydration in a once through flow system of ash, prior to discharge. The shorter the period of time that can be used to achieve an acceptable degree of hydration, the greater the throughput of a flow system and the more practical the development of sonic hydration systems becomes. It should also be noted that efforts to use a surfactant gave no evidence of benefits, so this approach also appears to be unpromising (18). Point Aconi Bed Ash. The eventual aim of this work is the development of a hydration system for Point Aconi and similar plants. Comparison tests between sonicated samples and those kept at equivalent temperatures and times have demonstrated that ultrasound can produce real enhancements in the hydration rate. In practice a sonifier would likely be used on a slurry of residues, initially at ambient temperature. It was, therefore, decided to carry out tests in which samples were subjected to ultrasound starting from ambient temperature rather than first allowing the water to reach steady-state conditions, for the given amplitude. Figure 1 shows the results for Point Aconi bed ash. It is evident that hydration shows only a moderate increase with amplitude and time, with the largest increase being produced at higher amplitudes, as one would expect. However, it seems clear that one can reasonably expect conversions of about 70-80% for moderate amplitudes and a conditioning time of 20-40 min. The shorter the conditioning time, the greater the throughput of any given equipment, and these durations would represent a practical limit for commercial equipment. Point Aconi Baghouse Ash. Typically about 60 or 70% of the ash produced at Point Aconi will be baghouse ash. In consequence, it is important that baghouse ash can also be hydrated effectively using ultrasound. To investigate this, baghouse ash from the Point Aconi plant was first ashed to remove char carbon (which was determined to be 6.02 ( 0.07%). The ashed material was then combined and used for all subsequent tests on ultrasonic activation, following the 4450
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FIGURE 1. Percent CaO converted to Ca(OH)2 in Point Aconi bed ash with ultrasonic probe at ambient conditions. The dash line represents the prediction in terms of eq 7 (see later discussions).
FIGURE 2. Percent CaO converted to Ca(OH)2 in Point Aconi baghouse ash with ultrasonic probe at ambient conditions. procedures outlined above. The free lime content of the ashed product was determined to be about 21%. The results are given in Figure 2. Although there was some scatter it is clear that conversions in the range of 70-90% can be achieved within 20-40 min, at amplitudes of 40% and above. NISCO 100 MWe CFBC Ash. To examine the generality of these results, it was decided to investigate ashes from a number of other FBC boilers. One CFBC ash which is clearly of interest is from the two 100 MWe CFBC boilers operated by the Nelson Industrial Steam Company (NISCO). The ash is produced from a low ash fuel and should avoid the complexities of having large amounts of other calcium compounds (OCCs), with potentially uncertain behavior on hydration (i.e. the ability to contribute CaO to the system) (19). In the case of NISCO the free lime content was determined to be about 22%. The results in Figure 3 show that the ash achieved very high degrees of hydration in relatively short periods of time, i.e., 80% or more within 20 min of sonication. However, except for one result that appears anomalous (series 3, 80 min), the degree of hydration then decreases as a function of increased sonic energy and time. Initially, such results appear to be counterintuitive, as hydration is expected to increase with both increasing sonic energy and reaction time. However,
FIGURE 5. Percent CaO hydrated to Ca(OH)2 in Purdue bed ash and TVA bed ash with ultrasonic probe at ambient conditions.
FIGURE 3. Percent CaO converted to Ca(OH)2 in NISCO bed ash with ultrasonic probe at ambient conditions.
FIGURE 4. Percent CaCO3 in NISCO bed ash with ultrasonic probe at ambient conditions dried in a vacuum oven. the results are very regular, and, as a check, two tests were done at 120 min, to confirm that this behavior was duplicated. One explanation for such phenomena is that sonication is causing some other reaction to occur which is removing Ca(OH)2 from the system. The most logical candidate is reaction with CO2 from the atmosphere to form CaCO3. This would also fit in with other studies on NISCO bed ash that showed that it carbonated much more readily than did Point Aconi or CANMET bed ash (20). To test this possibility, the calcium carbonate content was also determined by measuring the weight loss associated with the sample for a period of 2.5 h at 800 °C after calcination for 2.8 h at 550 °C. Figure 4 gives the results for carbonate determination. It can be seen that, as a general trend, the carbonate content increases with the time of hydration. Two separate chemical determinations for the initial CaCO3 content of NISCO bed ash gave values of 2.2 and 3.3%. It is evident that at higher sonic energies and longer times, NISCO bed ash undergoes substantial carbonation. This means that hydration results can be misleading if carbonate formation is ignored. It is also possible that a “significant” amount of the portlandite or Ca(OH)2 can be converted to carbonate, giving apparently low conversions of CaO in other CFBC ashes. From the point of view of disposal, the production of carbonate over the formation of portlandite is advantageous, since CaCO3 has less deleterious effects in the landfill (a lower
pH and no ettringite ([Ca3Al(OH)6‚12H2O]2(SO4)3‚2H2O) formation is possible if it removes CaO from the system). The formation of another carbonate compound thamusite (Ca3[Si(OH)6‚12H2O](CO3)(SO4)) has not been detected in other studies using Point Aconi or CANMET ash and, therefore, was not investigated. Although determination of CaCO3 was not done for all samples, it appears that, for the CANMET and Point Aconi bed ashes, this is unnecessary given the regularity of the data which show increasing conversion to portlandite as a function of reaction time and amplitude. This conclusion was also strengthened by separate experiments on Point Aconi baghouse ash. Tests for the degree of formation of carbonates showed minimal production. These tests were carried out at an amplitude of 60% and 20 min, using deionized and boiled deionized (degassed) water. Thermogravimetric analysis (TGA) gave the carbonate content of the two samples as 1.2 and 1.4% for the deionized and boiled deionized water, respectively. Nonetheless, in future experiments it would probably be good practice to use boiled and hence “degassed” water. These considerations aside, it appears that NISCO bed ash is also readily hydrated by sonochemical activation. Purdue University 25 MW CFBC Ashes. A limited test matrix was set up for the tests using the ashes from the Purdue University 25 MW CFBC boiler burning high-sulfur coal. The amplitudes ranged from 40% to 60%, while the times ranged from 20 to 60 min. The work was carried out starting at ambient conditions. The results are given in Figure 5 and show that very high hydration levels can be achieved. Unlike the NISCO runs, there is no evidence of carbonate formation. TVA 160 MWe BFBC Ashes. A limited test matrix of four elements was again used with the ashes produced by the TVA 160 MWe BFBC boiler burning a 65/35 blend of coal and petroleum coke. The tests were carried out at intermediate values (40 and 60% amplitude, 20, 40, and 60 min runs) under ambient conditions. The results are also presented in Figure 5. Very high conversions were achieved with times between 20 and 40 min and at amplitudes of 40% or more. Interpretation of the Effect of Sonication and the Observed Hydration Behavior. One possible explanation of the effect of sonication on the hydration can be given from the viewpoint of enhanced mass transfer by the ultrasound. As has been discussed earlier, penetration of the water into the pore would be a rate limiting process. Substantial enhancement of the water penetration, possibly due to the shock waves and increased pressure, can be envisaged when ultrasound is applied. The ultrasound may also break down the sulfated shells of large size particles by the shock waves VOL. 36, NO. 20, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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and shearing forces. As a result, the hydration rate becomes more dependent on the reaction rate itself. The observed hydration behavior can be interpreted accordingly. As can be seen from Figures 1-3 and 5, the hydration rate of the different ashes showed a similar pattern: The rate was initially high and decreased rapidly with increasing time. This pattern can be explained by assuming that the hydration rate depends on the fraction of unhydrated CaO. For simplicity we assume that the rate is proportional to the unhydrated fraction, and there is enough water in the pore for the reaction because of greatly enhanced mass transfer, so that
dy ) k(1 - y) dt
(6)
where y is the hydrated fraction, t is time, and k is a proportional coefficient which may depend on the amplitude of the ultrasound. The solution of eq 6 is
y ) 1 - exp(- kt)
(7)
which describes the observed hydration pattern. An example is given in Figure 1 as the dashed line representing the result of fitting eq 7 to the data. The line shows that the equation reflects the essential feature of the time dependence of the degree of hydration. From the above discussions it can be seen that power ultrasound can produce significant improvements in the hydration of CFBC ash, with the largest gains achieved with bed ash, which appears to be more difficult to conventionally hydrate than baghouse ash. Although in the present experiments excess water was used to shorten the required hydration time, with a smaller amount of water the effect of ultrasonic activation is also expected to be significant. These results are seen to be quite general as tests with ashes from the CANMET 0.8 MWth CFBC, the 165 MWe Point Aconi CFBC, the 100 MWe NISCO CFBCs, the 160 MWe TVA FBC, and the 25 MWe Purdue University CFBC boiler all show similar benefits. The one significant difference in the behavior of these ashes was that the NISCO bed ash showed a strong tendency to carbonate, so that the apparent conversion of CaO fell as a function of increasing ultrasonic energy (amplitude) and time. However, since the aim of hydration is to reduce the CaO content of the bed ashes, this behavior, although interesting, does not affect the value of this technique for hydration of FBC ashes. It should also be noted that since it now appears that it is possible for fuel ash to influence the levels of free lime in FBC ash/water systems, the absolute values of Ca(OH)2 may not be particularly meaningful, and instead the most useful way of assessing the effect of a hydration technology is to look at the comparative effect of any such treatment (19). The most probable way of using ultrasonic enhancement would be to mix the materials at ambient conditions and pass them through a power ultrasound system, as the cavitation effect works best at lower temperatures. Other work on ash/water dense suspensions (AWDS) mixtures has shown that a 50:50 mix of ash and water can be easily made into a pumpable slurry (21). Such slurries could be ultrasonically treated, and the excess water was removed and then recycled to condition new ashes. The conditioned ash could then be landfilled. This would reduce the water demand at the second stage of addition to that required for optimum compaction only and ought to improve the performance of ash in the landfill. There are a range of possible sonic flow systems currently available that could be operated at an appropriate scale. One of the more interesting has been recently developed in Germany by Martin-Walter, which is called the “push-pull” system. The essential element of the system is a solid cylinder of titanium cut to a multiple of half-wavelengths of sound 4452
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FIGURE 6. Martin Walter push-pull sonifier. in the material with opposing piezoelectric transducers attached to each end (Figure 6) (15). These are electrically connected through a central hole in the cylinder. When operating, the bar responds to transducers operating in a push-pull mode to give a “concertina effect” down its length. Such a system can be coaxially fitted into the center of existing pipework and can be made in lengths of up to 10 m or more allowing the possibility of substantial processing period in a “once-through” mode. Typical power requirements for such systems are in the kilowatt level, making such a system quite practical with regard to cost.
Acknowledgments This work was supported in part by the Federal Program on Energy Research and Development (PERD) and was performed for the Committee on Atlantic Coal and Environment Canada. The authors would also like to thank Dr. J. Wang (CETC) for a number of useful suggestions.
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(17) Mason, T. J.; Lorimer, J. P.; Bates, D. M. Ultrasonics 1992, 30, 40-42. (18) Anthony, E. J.; Jia, L.; Preto, F.; Cyr, L.; Smith B. A New Method for Conditioning CFBC Ashes; ERL Division Report 96-08(CF); CANMET, Energy, Mines and Resources Canada: Ontario, Canada, 1996. (19) Bulewicz, E. M.; Dudek, K.; Gora, D.; Taraszka, J. Combust. Sci. Technol. 2001, 153, 141-155. (20) Anthony, E. J.; Jia, L.; Woods, J.; Roque, W.; Burwell, S. Waste Manage. 2000, 20, 1-13.
(21) Anthony, E. J.; Berry, E. E.; Blondin, J.; Bulewicz, E. M.; Burwell, S. Investigation of Advanced Ash Management Technologies of CFBC and LIFAC Residues; Canadian Electrical Association: 1997; CEA No. 9141 G 890.
Received for review September 25, 2001. Revised manuscript received August 16, 2002. Accepted August 16, 2002. ES011316P
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