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Sonochemical treatment of FBC ash: A study of the reaction mechanism and performance of synthetic sorbents
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Sonochemical treatment of FBC ash: A study of the reaction mechanism and performance of synthetic sorbents Arjun Rao, Edward J. Anthony *, Vasilije Manovic CETC-O, Natural Resources Canada, Ottawa, Ontario, Canada K1A 1M1 Received 12 July 2007; received in revised form 4 November 2007; accepted 12 November 2007 Available online 4 March 2008
Abstract This work explores the reaction mechanisms for the sonochemical-enhanced carbonation of fluidized bed combustion (FBC) ash. Ashes from Nova Scotia Power’s 165 MWe circulating fluidized bed combustor (CFBC) as well as synthetic ash prepared directly from limestone have been used. Acetone tests were carried out using pure acetone as well as acetone/water mixtures (4:1 ratio). Tests with acetone demonstrated that, without previous hydration of the ash, significant carbonation is not achieved. Experiments were also conducted to determine the role of hydration temperature on the carbonation of FBC ash. X-ray diffraction (XRD) analysis of synthetic ash after sonication has also been carried out. Analysis of the data obtained revealed that the process is well described by a series reaction mechanism. Initial hydration temperature does not appear to significantly impact the carbonation of FBC ash. Synthetic ash does not behave like FBC ash due apparently to its extreme susceptibility to the size reduction capability of ultrasonics. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: FBC ash; Sorbent; Carbonation; Ultrasound; Reaction mechanism
1. Introduction Fluidized bed combustion (FBC) ash produced by burning high-sulphur fuels typically contains high levels of unreacted calcium oxide, which pose a problem for ash disposal. Current disposal practices involve hydrating the ash prior to landfilling to decrease the reactivity of the unreacted calcium oxide. This method of disposing FBC ash has several drawbacks due to high water requirements, generation of dust clouds and formation of high-pH leachate once the ash has been landfilled [1]. Carbonation of FBC ash prior to landfilling would alleviate most of the above problems. The simultaneous hydration and carbonation of FBC ash is relatively slow due to the morphology of ash particles, which usually possess an unreacted CaO core surrounded by a CaSO4 shell [2]. Ultrasound can be used to simultaneously hydrate and carbonate FBC ash effectively
*
Corresponding author. Tel.: +1 613 996 2868; fax: +1 613 992 9335. E-mail address: [email protected] (E.J. Anthony).
0016-2361/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2007.11.007
and the effect of temperature, other calcium compounds (OCC) and the particle size reduction brought about by ultrasonic treatment has been discussed previously [3]. Here we examine the reaction mechanism for the simultaneous hydration and carbonation of FBC ash. Limestone sulphated to different levels (synthetic ash) has also been studied to obtain a better understanding of the reaction mechanism without the effects associated with OCC present in FBC ash.
2. Experimental Bed ash from Nova Scotia Power’s 165 MWe circulating fluidized bed combustor (CFBC), received in June 2005, and synthetic ash, prepared from Stoneport limestone, have been used for the tests. The elemental analyses of the ash and the limestone used for the experiments are shown in Tables 1 and 2. A tube furnace (TF) was used to produce partially sulphated sorbent samples (‘‘synthetic ash”). Limestone samples (12 g total) were taken in two sample holders and
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A. Rao et al. / Fuel 87 (2008) 1927–1933
Table 1 Elemental analysis of ash and limestone (oxides) Main constituents, in oxide form (mass%)
NSPI ash
Stoneport limestone
SiO2 Al2O3 Fe2O3 TiO2 P2O5 CaO MgO SO3 Na2O K2O Loss on fusion Sum
4.78 1.2 0.3 0.096 <0.03 51.99 0.62 35.47 <0.02 0.22 4.7 99.37
<0.1 <0.1 0.07 <0.03 <0.03 55.68 0.66 <0.1 <0.2 0.04 42.26 98.71
Table 2 Elemental analysis of ash and limestone (trace elements) Trace elements (ppm)
Ba
Sr
V
Ni
Mn
Cr
Cu
Zn
Sum
NSPI ash Stoneport limestone
517 <250
288 203
3087 <50
673 <50
1037 101
94 <50
69 52
59 <30
5824 786
placed in the TF. These were calcined at 850 °C for 2 h under N2. Sulphation was performed using synthetic flue gas (15% CO2, 3% O2, 2% SO2, and N2 as balance) at 850 °C. Flowrates of N2 during calcination and synthetic flue gas during sulphation were both 1 dm3/min. To produce sorbents with desired levels of sulphation, the samples were exposed to synthetic flue gas for different times: 45, 105 and 240 min. On completion of sulphation the sample was weighed. On the basis of sample mass, the degree of sulphation was calculated, assuming that the observed mass increase was due only to the formation of CaSO4. To produce sufficient quantities of sulphated sorbent samples, all experiments in the TF were repeated in triplicate and the samples obtained were merged into one sample and kept in a small, closed jar for subsequent experiments. The samples obtained had sulphation levels of 9.28%, 20.64% and 39.74%, respectively, and the free lime contents were determined to be 80%, 61% and 38%, respectively. In addition, some tests were initially carried out with calcined limestones to attempt to study the carbonation process directly. These tests used Stoneport limestone, which was sieved to a particle size <1.4 mm, and 28.91 g of the sample was calcined in a furnace at 850 °C for 2 h. Unfortunately, the limestone was so fragile under ultrasound conditions that these experiments were abandoned. The experimental setup has been described elsewhere [3]. Briefly, the ultrasonic unit used is rated for 750 W and is manufactured by Sonics and Materials. Temperature control is achieved with a digital temperature controller manufactured by Polyscience. Solid-to-water ratio of 1:10 (15 g in 150 mL) was maintained for all tests with NSPI ash. For
the experiments conducted with synthetic ash, 3.285 g of ash was added to 150 mL of water. This weight was chosen keeping in mind that the free lime content of NSPI ash is 22%. Tap water was used for all the tests. CO2 flow rate of 0.1 dm3/min was maintained throughout and was found to provide sufficient CO2 for the carbonation reaction to occur unhindered, since pH measurements after completion of the run always showed that the pH was around 6.4. The amplitude of the ultrasonic unit was adjusted such that the power delivered to the solution ranged between 90–120 W for the temperature range tested. Tests with acetone (99.5% purity, analysis grade) and acetone/water mixtures (4:1) were carried out using ultrasonics as well as stirring. The stirrer used was a Caframo BDC 2002 and a speed of 700 rpm was maintained throughout. For the tests conducted with acetone the reaction vessel was washed with pure acetone before commencement of the experiment. Tests were conducted at temperatures of 40, 60 and 80 °C for the NSPI bed ash as well as for synthetic ash. Sampling was carried out by withdrawing 10 mL of the reaction mixture using a syringe. Sampling for NSPI ash and synthetic ash (10% sulphation) was carried out at intervals of 5, 10, 15, 20 and 25 min and 2, 4, 6, and 10 min for the limestone sulphated to 20% and 40%. The samples from the syringe were filtered and dried in an oven at 110 °C. Tests where acetone and acetone/water mixtures replaced water as the liquid medium were carried out at temperatures of 20 and 40 °C for 15 min using NSPI ash. The samples were dried in a vacuum oven at 55 °C for 4 h. Tests involving complete hydration of the ash at 40 °C and 80 °C, were also carried out under stirring with the Caframo BDC 2002. The ash was hydrated for 17 h at 40 °C or 2 h at 80 °C in order to achieve complete hydration. The hydrated ash was then carbonated in solution (1:10 solidto-water ratio) at 40, 60 and 80 °C using the ultrasonic probe. The modified ASTM C-25 sugar method was used to determine the available free lime content (CaO + Ca(OH)2) of the ash prior to sonication. The degree of hydration and carbonation was determined by thermogravimetric analysis (TGA). The TGA used was a Perkin–Elmer TGA-7 thermogravimetric analyzer. Tests were carried out in a N2 atmosphere with a heating rate of 10 °C/min. Particle size analysis of the synthetic ash and NSPI ash were carried out using mesh sizes of 1.4 mm, 1.0 mm, 850 lm, 600 lm, 425 lm, 250 lm, 125 lm, 75 lm and 45 lm. Sieving was done for 20 min using a Haver–Tyler EML 200 digital7 test sieve shaker and the weight of each screen before and after sieving was noted and the fines were collected in a pan. The sample was then thoroughly mixed and sonicated for 40 min at 40 °C for the tests with NSPI ash and 20 min at 60 °C for the synthetic ash. The sonicated sample was then discharged onto the stack of screens and sieved using water to prevent agglomeration. Fines were filtered using two Whatman no. 41 ashless filter papers (20–25 lm), dried at 110 °C for one hour and then weighed.
A. Rao et al. / Fuel 87 (2008) 1927–1933
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3. Results and discussion
Table 4 Hydration/carbonation tests in acetone at 20 and 40 °C
3.1. Work with FBC ash
Treatment
Temperature (°C)
% Ca(OH)2 as CaO equivalent
% CaCO3 as CaO equivalent
When ultrasonics is used to simultaneously hydrate and carbonate FBC ash, an apparent decrease in hydration followed by a corresponding increase in carbonation occurs over time [3]. An explanation for this behaviour is that the reactions occur in series with the hydration reaction being followed by the carbonation reaction, as shown below.
Stirring Sonication Stirring Sonication
40 40 20 20
6.02 4.79 5.05 6.03
0.71 2.23 0.69 0.78
Table 5 Effect of drying samples on hydration/carbonation behaviour in acetone at 40 °C
CaO þ H2 O ! CaðOHÞ2
ð1Þ
CaðOHÞ2 þ CO2 ! CaCO3 þ H2 O
ð2Þ
Treatment
Drying in vacuum oven
% Ca(OH)2 as CaO equivalent
% CaCO3 as CaO equivalent
The use of organic solvents such as acetone and methanol to retard CaO hydration is well known in cement chemistry [4]. In order to demonstrate that the above process follows a series mechanism, tests were conducted using acetone as well as acetone/water mixtures. Table 3 displays the results of tests in which water was replaced by a mixture of acetone/water as well as by pure acetone. The tests carried out in water indicate that most of the hydroxide formed has converted to carbonate. The test with the acetone/ water mixture shows that acetone is reasonably effective in preventing hydration and subsequent carbonation of the ash. Tests with pure acetone show that acetone is effective in preventing hydration and as a result a decrease in the carbonate content of the ash can be seen. TGA tests of the stored bed ash conducted in February 2007 gave values of 5.5% Ca(OH)2 and 0.51% CaCO3 and the small increase in portlandite content could be due to experimental error. The ashed samples also contain negligible portlandite and calcite content when sonicated in pure acetone. Table 4 shows the results of the tests conducted at two different temperatures using acetone and CO2. The result for the ash sonicated at 40 °C indicates that the small amount of Ca(OH)2 present in the original ash sample has converted to CaCO3. The test carried out at 20 °C shows a higher Ca(OH)2 content with a corresponding lower CaCO3 content. These results indicate that sonication as well as temperature affects the reaction. Table 5 displays the results of the tests in which the samples were subjected to drying in a vacuum oven before testing. These results show that overnight drying in the vacuum oven has
Sonication Sonication Stirring Stirring
No Yes No Yes
4.79 4.03 6.02 5.97
2.23 2.94 0.71 0.74
Table 3 Hydration/carbonation tests using acetone and acetone/water mixture at 40 °C Ashed at 850 °C prior to testing
Reaction mixture
% Ca(OH)2 as CaO equivalent
% CaCO3 as CaO equivalent
No No
Water Acetone/water mixture (4:1) Pure acetone (99.5%) Pure acetone (99.5%)
3.9 10.00
15.95 7.24
7.62
0.7
0.22
0.09
No Yes
no effect on the hydration and carbonation of the ash. The decrease in portlandite content is due to conversion of Ca(OH)2 present in the original sample to calcite. These results indicate that sonication is effective in exposing greater surface area for the carbonation reaction. However, even with sonication, conversions to carbonate are very small in both cases (Tables 4 and 5) despite the temperature, confirming the proposed series reaction mechanism. It should be also mentioned that acetone influences CO2 solubility and, therefore, carbonation rate; however, this has not been discussed in more detail here. The effect of acetone/water mixture on hydration and carbonation can be observed from Table 6. The ash was shaken once a day and almost complete hydration of the ash occurred when it was covered with water for two days. Although the ash has hydrated significantly, very little of the CaO has reacted to form CaCO3, indicating that the ash is seeing very little CO2 from the atmosphere even over these relatively long time periods. For the same time period the acetone/water mixture shows a lower conversion to Ca(OH)2; however, by seven days the Ca(OH)2 content increased to such a level, indicating that acetone is reasonably effective only for short time periods with these ashes. The same trends were seen when the ash was subjected to ashing prior to testing. These results reflect the general trends observed with the sonication and carbonation tests carried out using acetone and acetone/water mixtures. Fig. 1 compares the particle size reduction achieved by ultrasonics at 40 oC when using water or acetone as the liquid media. Greater particle size reduction can be seen when water is used. The higher particle size reduction in water is likely due to its lower vapour pressure. It is well known from the literature that solvents with high vapour pressures are associated with less violent cavitation [5]. The vapour pressure of acetone is 56,500 Pa while that of water is 7386 Pa at 40 °C [6]; therefore, the intensity of cavitation in water must be greater than in acetone.
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A. Rao et al. / Fuel 87 (2008) 1927–1933
Table 6 Effect of acetone on hydration and carbonation under gentle agitation Reaction mixture Distilled water 5 mL D.I. water + 20 mL 5 mL D.I. water + 20 mL Distilled water 5 mL D.I. water + 20 mL 5 mL D.I. water + 20 mL
acetone acetone acetone acetone
Time period (days)
Pretreatment
% Ca(OH)2 as CaO equivalent
% CaCO3 as CaO equivalent
2 2 7 2 2 7
– – – Ashed Ashed Ashed
21 13 20.2 20.2 12.8 18.4
1.5 1 1.4 1 0.7 0.9
D.I. = deionized.
100
70 60 50 40 30
Ash
20
Ash sonicated in acetone
10
Ash sonicated in water
0 20-25
38
75
125 250 425 Mesh size (μm)
600
850
1000
Fig. 1. Particle size reduction – comparison between water and acetone at 40 oC.
Nonetheless, it is clear from Fig. 1 that ultrasound produces significant size reduction in acetone. Higher conversions to carbonate, despite decreased CO2 solubility and lower ultrasonic efficiency with increasing temperatures, have been shown previously by the authors [3]. A possible explanation for the above observation could be an increase in the fragility of the ash particle with increasing hydration temperature. To test this hypothesis samples were initially hydrated by stirring at 700 rpm at 40 and 80 °C and then carbonated. The samples were subjected to a Brunauer–Emmett–Teller (BET) analysis in order to check for an increase in surface area. The BET surface area of the sample hydrated at 40 °C was 6.68 ± 0.0091 m2/g and 7.13 ± 0.0097 m2/g for the sample hydrated at 80 °C. The surface area of the ash sample prior to hydration was 1.71 ± 0.0045 m2/g. The BET analysis suggests that higher initial hydration temperature increases the surface area of the particles. 3.2. Work with synthetic ash Figs. 2 and 3 give the conversion to hydroxide and carbonate for synthetic ash for 10% sulphation. Higher carbonate conversions are observed at 40 °C when compared to those at 60 and 80 °C, which is the reverse of what we would expect based on work with real ash [3]. Shih et al. [7] have shown that temperature has a small effect on the carbonation of pure Ca(OH)2. Their experiments were carried out between 60 and 90 °C and under varying relative
90
40oC 40°C 60°C 60oC 80°C 80oC
80 70 60 50 40 30 20 10 0 0
5
10
15 Time (min)
20
25
30
Fig. 2. Synthetic ash sulphated to 10%, conversion to hydroxide.
Conversion of CaO to CaCO 3 (%)
Cumulative weight (% )
80
Conversion of CaO to Ca(OH)2 (%)
100
90
100 90 80 70 60 50 40 30
40oC 40°C 60oC 60°C 80°C 80oC
20 10 0 0
5
10
15
20
25
30
Time (min)
Fig. 3. Synthetic ash sulphated to 10%, conversion to carbonate.
humidity conditions. They concluded that the carbonation of Ca(OH)2 was affected to a greater extent by changes in relative humidity and to a lesser extent by temperature. A free lime test of the synthetic ash (10% sulphation) indicated an available lime content of 80%. From Figs. 2 and 3, after 5 min at 40 °C, 64% of CaO has converted to Ca(OH)2 and 34% has converted to CaCO3, indicating that complete conversion of the available lime has occurred. These results indicate a high reaction rate, which is most likely due to the particle size reduction brought about by ultrasonics. The high reaction rate appears to be the reason for the relative temperature insensitivity of the reaction.
A. Rao et al. / Fuel 87 (2008) 1927–1933
high reaction rate appears to be the reason for this, and other influences take a major role in determination of observed conversions. Substantial size reduction caused by ultrasonics produces reactive suspensions that are very sensitive to the influence of different factors during the experiment, causing a non-uniformity of results that is noticeable especially in Figs. 2 and 3 (20 min points), in Fig. 6. (4 min points), as well as in Figs. 9 and 10
Conversion of CaO to CaCO3 (%)
100 90 80 70 60 50 40 40oC 40˚C 60˚C 60oC 80˚C 80oC
30 20 10 0 0
2
4
6 Time (min)
8
10
12
Fig. 5. Synthetic ash sulphated to 20%, conversion to carbonate.
Conversion of CaO to Ca(OH)2 (%)
These results are not in agreement with the results obtained with FBC ash and indicate significant differences between synthetic ash and FBC ash. The reverse influence of temperature on carbonation conversions is an unexpected result that confused the original goal of this work, which was to obtain kinetic parameters of series reactions (hydration and carbonation) using synthetic ash. The reverse influence of temperature implies that calculated values of activation energy should be negative. This means that carbonation includes a step on which temperature has a negative influence. This step may be associated with the solubility of CO2 in the reaction medium because its solubility decreases with temperature. It may be presumed that substantial size reduction caused by ultrasonics, and consequent increase of sorbent particle surface area, accelerate reaction rate to such a level that availability of CO2 becomes limiting for the reaction. In that case solubility of CO2 decreases at higher temperatures (as do concentrations of H2CO3, HCO3 and CO32 that are in equilibrium with soluble CO2) and carbonation rate decreases. Moreover, the temperature influences CO2 solubility and related chemical equilibrium via pH value, i.e., via Ca(OH)2 solubility, which changes with temperature. The concentration of Ca2+ ions also influences Ca(OH)2 solubility and pH value and, therefore, CO2 solubility and carbonation rate. More detailed discussion of these influences would be outside the scope of the work and would require further specific experimentation. Shorter durations were used for subsequent experiments and the results are shown in Figs. 4–7. These results show that hydration, as well as carbonation, decreases with increasing levels of sulphation, with the limestone sulphated to 40% displaying significantly reduced hydration and carbonation. This implies that sulphation reduces sample reactivity even if sonication is employed. Couturier et al. [8] obtained similar results for hydration of synthetic sorbents sulphated to 30% and 45% in a Calvet microcalorimeter without sonication. It is also shown in these figures that the temperature has a small effect on conversions and, more importantly, the influence is not uniform. The
1931
100 90
40oC 40˚C 60˚C 60oC 80˚C 80oC
80 70 60 50 40 30 20 10 0 0
2
4
6
8
10
12
Time (min)
Fig. 6. Synthetic ash sulphated to 40%, conversion to hydroxide.
Conversion of CaO to CaCO3 (%)
Conversion of CaO to Ca(OH)2 (%)
100 40oC 40°C 60°C 60oC 80°C 80oC
90 80 70 60 50 40 30 20 10 0 0
2
4
6 Time (min)
8
10
12
Fig. 4. Synthetic ash sulphated to 20%, conversion to hydroxide.
100 90
40oC 40°C 60°C 60oC 80°C 80oC
80 70 60 50 40 30 20 10 0 0
2
4
6 Time (min)
8
10
12
Fig. 7. Synthetic ash sulphated to 40%, conversion to carbonate.
A. Rao et al. / Fuel 87 (2008) 1927–1933
(20 min points, see below). The non-smooth shape of the result obtained, apart from its relative temperature insensitivity, additionally prevents calculation of kinetic parameters. Carbonation levels for the limestone sulphated to 40% appear to decrease after 6 min. This apparent decrease in carbonation can be attributed to the size reduction property of ultrasonics. For all tests with synthetic ash, significant loss of particles was observed visually and we believe that particles lost during filtration were predominantly CaCO3. The particles trapped on the filter paper were large particles and were thought to have undergone partial hydration and carbonation. In order to confirm this hypothesis, wet sieving was carried out with synthetic ash (20% as well as 40% sulphation) and the results are shown in Fig. 8. These results indicate that nearly 55% of the particles have sizes of 20–25 lm. It should be noted here that for these tests, two filter papers (20–25 lm) were placed one above the other in order to trap all particles. The particles trapped on the filter paper for the limestone sulphated to 20% were analyzed using TGA and were found to contain 11% Ca(OH)2 and 74% CaCO3. Particles retained on the 1 mm and the 850 lm sieves were mixed together and a TGA analysis of the samples was carried out. The results indicated that the samples contained 30% Ca(OH)2 and 5% CaCO3. The X-ray diffraction (XRD) analysis of the samples obtained after the sonication of limestone sulphated to 20% is given in Table 7. The XRD results confirm that the reaction with synthetic ash is very rapid. At 6 min only 100 Cumulative weight (%)
90 80 70 60
0.5% of unreacted CaO is present in the sample. After 15 min practically all the free lime as well as any portlandite formed has converted to carbonate. The XRD results also support the idea that the reaction occurs in series as the hydroxide formed initially converts to calcite over time. One of the original aims of the work was to obtain kinetic data from synthetic sorbents. This arises from the fact that batch tests done at different times showed sufficient variation that reliable activation energies could not be obtained [3]. On refining the experimental method to allow removal of aliquots of sample during a sonication experiment at different times, better results were obtained as shown in Figs. 9 and 10 [9], but there was still sufficient variation that it was decided to work with a model system. There are in fact a number of objections that can be made with respect to using a real ash when trying to determine fundamental kinetic parameters. First and foremost, it is possible for the CaO to form significant amounts of what we have called other calcium compounds (namely silicates, ferrites and aluminates); thus a real sorbent can be chemically different from a synthetic sorbent, even though conventional X-ray fluorescence, and sulphur determinations will fail to show this [2]. Another complication is that the chemical composition of a real ash may also vary with
Converion of CaO to Ca(OH)2 (%)
1932
100 90 80 70 60 50 40 30 20 10 0
50
40oC 40˚C 60oC 60˚C 80oC 80˚C
0
5
10
40 30
synthetic ash - 0%
20
25
30
Fig. 9. Semi-continuous sampling, conversion to hydroxide.
20
synthetic ash - 20%
10
synthetic ash - 40%
20-25 45
75
125 250 425 Mesh size (μm)
600
850 1000
Fig. 8. Particle size distribution –synthetic ash sulphated to 20% and 40%.
Table 7 XRD analysis of synthetic ash samples obtained after sonication (20% sulphation) Chemical compound
Formula
CaSO4 CaCO3 CaO Ca(OH)2
Chemical compound (%) 6 min
10 min
15 min
46.6 31.8 0.5 25.8
33.1 54.5 – 16.2
33.7 60.8 – 2.9
Converion of CaO to CaCO3 (%)
0
Anhydrite Calcite Lime Portlandite
15 Time (min)
100 90 80 70 60 50 40 30
40oC 40˚C 60oC 60˚C 80oC 80˚C
20 10 0 0
5
10
15 Time (min)
20
25
Fig. 10. Semi-continuous sampling, conversion to carbonate.
30
A. Rao et al. / Fuel 87 (2008) 1927–1933
size fraction [10], and Couturier et al. has also made a similar point that the same size particles can be expected to have various sulphation time histories as a function of their residence time in a real boiler [8]. By contrast, with a synthetic sorbent one may control particle size and sulphation levels to achieve products with consistent properties. Unfortunately, as noted above attempts to work with calcined limestones failed totally as they were simply too soft to allow examination under sonication conditions, and the results with the synthetic sorbent, even at sulphation levels as high as 40% showed no temperature effect, whereas the results from real ashes show a pronounced enhancement in carbonation at higher temperatures [3]. This work then fits with many previous observations that real ashes are different from synthetic sorbents, sometimes dramatically so. Thus, Anthony et al. [11] demonstrated that CFBC boiler ash had different geotechnical properties than ashes produced in a pilot-scale combustor even when the same parent feedstocks were employed, and Laursen et al. [12] noted that it was more difficult to hydrate real bed ashes than fully sulphated limestones; the current authors have also argued that the wider literature on sorbent reactivation demonstrates significant differences between real boiler ashes and synthetic sorbents [13]. This work appears to provide another case in point, namely that, for sonochemically enhanced carbonation, synthetic sorbents are substantially different from real boilers ashes. 4. Conclusions Tests with acetone clearly indicate that significant carbonation of FBC ash is not achieved without prior hydration. The acetone test results also show that the simultaneous hydration and carbonation of FBC ash follows a series reaction mechanism. The hydration tests reveal that higher hydration temperatures expose a greater surface area for the carbonation reaction and this may explain why higher temperatures have been observed to increase carbonation conversions with real boiler ashes. Unfortunately, work with synthetic ashes suggests that they are substantially more fragile than real boiler ashes under sonochemical activation. Thus they fail to demonstrate any significant increase in carbonation as a result
1933
of increasing temperature, which appears to be due to a high reaction rate because of the substantial size reduction caused by ultrasonics. The difficulty in obtaining the true conversions, because a large percentage of particles have sizes lower than 20–25 lm, has also been demonstrated by wet sieving. The XRD results demonstrate that the reaction takes place at a rapid rate and also show that the reaction mechanism is best described by a series reaction. An important conclusion of this work is that it appears unlikely that synthetic ashes can be used to model the behaviour of real bed ashes under sonochemical activation conditions. References [1] Merriam PR, Cousens JD. Disposal of residue from Nova Scotia Power’s 165 MWe CFB boiler. In: Proceedings of the international conference on fluidized bed combustion, vol. 2. ASME, 1993, p. 859– 65. [2] Anthony EJ, Granatstein DL. Sulfation phenomena in fluidized bed combustion systems. Prog Energy Combust Sci 2001;27:215–36. [3] Rao A, Anthony EJ, Jia L, Macchi A. Carbonation of FBC ash by sonochemical treatment. Fuel 2007;86:2603–15. [4] Taylor HFW. Cement chemistry, 128. San Diego, CA: Academic Press Inc.; 1990. [5] Thompson LH, Doraiswamy LK. Sonochemistry: science and engineering. Ind Eng Chem Res 1999;38:1215–49. [6] Perry RH, Green DW, editors. Perry’s chemical engineers’ handbook. 7th ed. New York: McGraw-Hill Inc.; 2001. [7] Shih S-M, Ho C-S, Song Y-S, Lin J-P. Kinetics of the reaction of Ca(OH)2 with CO2 at low temperature. Ind Eng Chem Res 1999;38:1316–22. [8] Couturier MF, Volmerange Y, Steward F. Hydration of partially sulphated lime particles from fluidized bed combustion. J Energy Res Tech 2001;123:173–8. [9] Rao A. Carbonation of fluidized bed combustion solids. Masters thesis, University of Ottawa, 2006. [10] Wang J, Wu Y, Anthony EJ. The hydration behavior of partially sulfated fluidized bed combustor sorbent. Ind Eng Chem Res 2005;44:8199–204. [11] Anthony EJ, Ross GG, Berry EE, Hemings RT, Kissel RK. Characterization of solid wastes from circulating fluidized bed combustion. J Energy Res Tech 1995;117:18–23. [12] Laursen K, Duo W, Grace JR, Lim CJ. Characteristics of steam reactivation mechanisms in limestones and spent calcium sorbents. Fuel 2001;80:1293–306. [13] Anthony EJ, Bulewicz EM, Jia L. Reactivation of limestone sorbents in FBC for SO2 capture. Prog Energy Combust Sci 2007;33:171–220.
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Sonochemical treatment of FBC ash: A study of the reaction mechanism and performance of synthetic sorbents Arjun Rao, Edward J. Anthony *, Vasilije Manovic CETC-O, Natural Resources Canada, Ottawa, Ontario, Canada K1A 1M1 Received 12 July 2007; received in revised form 4 November 2007; accepted 12 November 2007 Available online 4 March 2008
Abstract This work explores the reaction mechanisms for the sonochemical-enhanced carbonation of fluidized bed combustion (FBC) ash. Ashes from Nova Scotia Power’s 165 MWe circulating fluidized bed combustor (CFBC) as well as synthetic ash prepared directly from limestone have been used. Acetone tests were carried out using pure acetone as well as acetone/water mixtures (4:1 ratio). Tests with acetone demonstrated that, without previous hydration of the ash, significant carbonation is not achieved. Experiments were also conducted to determine the role of hydration temperature on the carbonation of FBC ash. X-ray diffraction (XRD) analysis of synthetic ash after sonication has also been carried out. Analysis of the data obtained revealed that the process is well described by a series reaction mechanism. Initial hydration temperature does not appear to significantly impact the carbonation of FBC ash. Synthetic ash does not behave like FBC ash due apparently to its extreme susceptibility to the size reduction capability of ultrasonics. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: FBC ash; Sorbent; Carbonation; Ultrasound; Reaction mechanism
1. Introduction Fluidized bed combustion (FBC) ash produced by burning high-sulphur fuels typically contains high levels of unreacted calcium oxide, which pose a problem for ash disposal. Current disposal practices involve hydrating the ash prior to landfilling to decrease the reactivity of the unreacted calcium oxide. This method of disposing FBC ash has several drawbacks due to high water requirements, generation of dust clouds and formation of high-pH leachate once the ash has been landfilled [1]. Carbonation of FBC ash prior to landfilling would alleviate most of the above problems. The simultaneous hydration and carbonation of FBC ash is relatively slow due to the morphology of ash particles, which usually possess an unreacted CaO core surrounded by a CaSO4 shell [2]. Ultrasound can be used to simultaneously hydrate and carbonate FBC ash effectively
*
Corresponding author. Tel.: +1 613 996 2868; fax: +1 613 992 9335. E-mail address: [email protected] (E.J. Anthony).
0016-2361/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2007.11.007
and the effect of temperature, other calcium compounds (OCC) and the particle size reduction brought about by ultrasonic treatment has been discussed previously [3]. Here we examine the reaction mechanism for the simultaneous hydration and carbonation of FBC ash. Limestone sulphated to different levels (synthetic ash) has also been studied to obtain a better understanding of the reaction mechanism without the effects associated with OCC present in FBC ash.
2. Experimental Bed ash from Nova Scotia Power’s 165 MWe circulating fluidized bed combustor (CFBC), received in June 2005, and synthetic ash, prepared from Stoneport limestone, have been used for the tests. The elemental analyses of the ash and the limestone used for the experiments are shown in Tables 1 and 2. A tube furnace (TF) was used to produce partially sulphated sorbent samples (‘‘synthetic ash”). Limestone samples (12 g total) were taken in two sample holders and
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A. Rao et al. / Fuel 87 (2008) 1927–1933
Table 1 Elemental analysis of ash and limestone (oxides) Main constituents, in oxide form (mass%)
NSPI ash
Stoneport limestone
SiO2 Al2O3 Fe2O3 TiO2 P2O5 CaO MgO SO3 Na2O K2O Loss on fusion Sum
4.78 1.2 0.3 0.096 <0.03 51.99 0.62 35.47 <0.02 0.22 4.7 99.37
<0.1 <0.1 0.07 <0.03 <0.03 55.68 0.66 <0.1 <0.2 0.04 42.26 98.71
Table 2 Elemental analysis of ash and limestone (trace elements) Trace elements (ppm)
Ba
Sr
V
Ni
Mn
Cr
Cu
Zn
Sum
NSPI ash Stoneport limestone
517 <250
288 203
3087 <50
673 <50
1037 101
94 <50
69 52
59 <30
5824 786
placed in the TF. These were calcined at 850 °C for 2 h under N2. Sulphation was performed using synthetic flue gas (15% CO2, 3% O2, 2% SO2, and N2 as balance) at 850 °C. Flowrates of N2 during calcination and synthetic flue gas during sulphation were both 1 dm3/min. To produce sorbents with desired levels of sulphation, the samples were exposed to synthetic flue gas for different times: 45, 105 and 240 min. On completion of sulphation the sample was weighed. On the basis of sample mass, the degree of sulphation was calculated, assuming that the observed mass increase was due only to the formation of CaSO4. To produce sufficient quantities of sulphated sorbent samples, all experiments in the TF were repeated in triplicate and the samples obtained were merged into one sample and kept in a small, closed jar for subsequent experiments. The samples obtained had sulphation levels of 9.28%, 20.64% and 39.74%, respectively, and the free lime contents were determined to be 80%, 61% and 38%, respectively. In addition, some tests were initially carried out with calcined limestones to attempt to study the carbonation process directly. These tests used Stoneport limestone, which was sieved to a particle size <1.4 mm, and 28.91 g of the sample was calcined in a furnace at 850 °C for 2 h. Unfortunately, the limestone was so fragile under ultrasound conditions that these experiments were abandoned. The experimental setup has been described elsewhere [3]. Briefly, the ultrasonic unit used is rated for 750 W and is manufactured by Sonics and Materials. Temperature control is achieved with a digital temperature controller manufactured by Polyscience. Solid-to-water ratio of 1:10 (15 g in 150 mL) was maintained for all tests with NSPI ash. For
the experiments conducted with synthetic ash, 3.285 g of ash was added to 150 mL of water. This weight was chosen keeping in mind that the free lime content of NSPI ash is 22%. Tap water was used for all the tests. CO2 flow rate of 0.1 dm3/min was maintained throughout and was found to provide sufficient CO2 for the carbonation reaction to occur unhindered, since pH measurements after completion of the run always showed that the pH was around 6.4. The amplitude of the ultrasonic unit was adjusted such that the power delivered to the solution ranged between 90–120 W for the temperature range tested. Tests with acetone (99.5% purity, analysis grade) and acetone/water mixtures (4:1) were carried out using ultrasonics as well as stirring. The stirrer used was a Caframo BDC 2002 and a speed of 700 rpm was maintained throughout. For the tests conducted with acetone the reaction vessel was washed with pure acetone before commencement of the experiment. Tests were conducted at temperatures of 40, 60 and 80 °C for the NSPI bed ash as well as for synthetic ash. Sampling was carried out by withdrawing 10 mL of the reaction mixture using a syringe. Sampling for NSPI ash and synthetic ash (10% sulphation) was carried out at intervals of 5, 10, 15, 20 and 25 min and 2, 4, 6, and 10 min for the limestone sulphated to 20% and 40%. The samples from the syringe were filtered and dried in an oven at 110 °C. Tests where acetone and acetone/water mixtures replaced water as the liquid medium were carried out at temperatures of 20 and 40 °C for 15 min using NSPI ash. The samples were dried in a vacuum oven at 55 °C for 4 h. Tests involving complete hydration of the ash at 40 °C and 80 °C, were also carried out under stirring with the Caframo BDC 2002. The ash was hydrated for 17 h at 40 °C or 2 h at 80 °C in order to achieve complete hydration. The hydrated ash was then carbonated in solution (1:10 solidto-water ratio) at 40, 60 and 80 °C using the ultrasonic probe. The modified ASTM C-25 sugar method was used to determine the available free lime content (CaO + Ca(OH)2) of the ash prior to sonication. The degree of hydration and carbonation was determined by thermogravimetric analysis (TGA). The TGA used was a Perkin–Elmer TGA-7 thermogravimetric analyzer. Tests were carried out in a N2 atmosphere with a heating rate of 10 °C/min. Particle size analysis of the synthetic ash and NSPI ash were carried out using mesh sizes of 1.4 mm, 1.0 mm, 850 lm, 600 lm, 425 lm, 250 lm, 125 lm, 75 lm and 45 lm. Sieving was done for 20 min using a Haver–Tyler EML 200 digital7 test sieve shaker and the weight of each screen before and after sieving was noted and the fines were collected in a pan. The sample was then thoroughly mixed and sonicated for 40 min at 40 °C for the tests with NSPI ash and 20 min at 60 °C for the synthetic ash. The sonicated sample was then discharged onto the stack of screens and sieved using water to prevent agglomeration. Fines were filtered using two Whatman no. 41 ashless filter papers (20–25 lm), dried at 110 °C for one hour and then weighed.
A. Rao et al. / Fuel 87 (2008) 1927–1933
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3. Results and discussion
Table 4 Hydration/carbonation tests in acetone at 20 and 40 °C
3.1. Work with FBC ash
Treatment
Temperature (°C)
% Ca(OH)2 as CaO equivalent
% CaCO3 as CaO equivalent
When ultrasonics is used to simultaneously hydrate and carbonate FBC ash, an apparent decrease in hydration followed by a corresponding increase in carbonation occurs over time [3]. An explanation for this behaviour is that the reactions occur in series with the hydration reaction being followed by the carbonation reaction, as shown below.
Stirring Sonication Stirring Sonication
40 40 20 20
6.02 4.79 5.05 6.03
0.71 2.23 0.69 0.78
Table 5 Effect of drying samples on hydration/carbonation behaviour in acetone at 40 °C
CaO þ H2 O ! CaðOHÞ2
ð1Þ
CaðOHÞ2 þ CO2 ! CaCO3 þ H2 O
ð2Þ
Treatment
Drying in vacuum oven
% Ca(OH)2 as CaO equivalent
% CaCO3 as CaO equivalent
The use of organic solvents such as acetone and methanol to retard CaO hydration is well known in cement chemistry [4]. In order to demonstrate that the above process follows a series mechanism, tests were conducted using acetone as well as acetone/water mixtures. Table 3 displays the results of tests in which water was replaced by a mixture of acetone/water as well as by pure acetone. The tests carried out in water indicate that most of the hydroxide formed has converted to carbonate. The test with the acetone/ water mixture shows that acetone is reasonably effective in preventing hydration and subsequent carbonation of the ash. Tests with pure acetone show that acetone is effective in preventing hydration and as a result a decrease in the carbonate content of the ash can be seen. TGA tests of the stored bed ash conducted in February 2007 gave values of 5.5% Ca(OH)2 and 0.51% CaCO3 and the small increase in portlandite content could be due to experimental error. The ashed samples also contain negligible portlandite and calcite content when sonicated in pure acetone. Table 4 shows the results of the tests conducted at two different temperatures using acetone and CO2. The result for the ash sonicated at 40 °C indicates that the small amount of Ca(OH)2 present in the original ash sample has converted to CaCO3. The test carried out at 20 °C shows a higher Ca(OH)2 content with a corresponding lower CaCO3 content. These results indicate that sonication as well as temperature affects the reaction. Table 5 displays the results of the tests in which the samples were subjected to drying in a vacuum oven before testing. These results show that overnight drying in the vacuum oven has
Sonication Sonication Stirring Stirring
No Yes No Yes
4.79 4.03 6.02 5.97
2.23 2.94 0.71 0.74
Table 3 Hydration/carbonation tests using acetone and acetone/water mixture at 40 °C Ashed at 850 °C prior to testing
Reaction mixture
% Ca(OH)2 as CaO equivalent
% CaCO3 as CaO equivalent
No No
Water Acetone/water mixture (4:1) Pure acetone (99.5%) Pure acetone (99.5%)
3.9 10.00
15.95 7.24
7.62
0.7
0.22
0.09
No Yes
no effect on the hydration and carbonation of the ash. The decrease in portlandite content is due to conversion of Ca(OH)2 present in the original sample to calcite. These results indicate that sonication is effective in exposing greater surface area for the carbonation reaction. However, even with sonication, conversions to carbonate are very small in both cases (Tables 4 and 5) despite the temperature, confirming the proposed series reaction mechanism. It should be also mentioned that acetone influences CO2 solubility and, therefore, carbonation rate; however, this has not been discussed in more detail here. The effect of acetone/water mixture on hydration and carbonation can be observed from Table 6. The ash was shaken once a day and almost complete hydration of the ash occurred when it was covered with water for two days. Although the ash has hydrated significantly, very little of the CaO has reacted to form CaCO3, indicating that the ash is seeing very little CO2 from the atmosphere even over these relatively long time periods. For the same time period the acetone/water mixture shows a lower conversion to Ca(OH)2; however, by seven days the Ca(OH)2 content increased to such a level, indicating that acetone is reasonably effective only for short time periods with these ashes. The same trends were seen when the ash was subjected to ashing prior to testing. These results reflect the general trends observed with the sonication and carbonation tests carried out using acetone and acetone/water mixtures. Fig. 1 compares the particle size reduction achieved by ultrasonics at 40 oC when using water or acetone as the liquid media. Greater particle size reduction can be seen when water is used. The higher particle size reduction in water is likely due to its lower vapour pressure. It is well known from the literature that solvents with high vapour pressures are associated with less violent cavitation [5]. The vapour pressure of acetone is 56,500 Pa while that of water is 7386 Pa at 40 °C [6]; therefore, the intensity of cavitation in water must be greater than in acetone.
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A. Rao et al. / Fuel 87 (2008) 1927–1933
Table 6 Effect of acetone on hydration and carbonation under gentle agitation Reaction mixture Distilled water 5 mL D.I. water + 20 mL 5 mL D.I. water + 20 mL Distilled water 5 mL D.I. water + 20 mL 5 mL D.I. water + 20 mL
acetone acetone acetone acetone
Time period (days)
Pretreatment
% Ca(OH)2 as CaO equivalent
% CaCO3 as CaO equivalent
2 2 7 2 2 7
– – – Ashed Ashed Ashed
21 13 20.2 20.2 12.8 18.4
1.5 1 1.4 1 0.7 0.9
D.I. = deionized.
100
70 60 50 40 30
Ash
20
Ash sonicated in acetone
10
Ash sonicated in water
0 20-25
38
75
125 250 425 Mesh size (μm)
600
850
1000
Fig. 1. Particle size reduction – comparison between water and acetone at 40 oC.
Nonetheless, it is clear from Fig. 1 that ultrasound produces significant size reduction in acetone. Higher conversions to carbonate, despite decreased CO2 solubility and lower ultrasonic efficiency with increasing temperatures, have been shown previously by the authors [3]. A possible explanation for the above observation could be an increase in the fragility of the ash particle with increasing hydration temperature. To test this hypothesis samples were initially hydrated by stirring at 700 rpm at 40 and 80 °C and then carbonated. The samples were subjected to a Brunauer–Emmett–Teller (BET) analysis in order to check for an increase in surface area. The BET surface area of the sample hydrated at 40 °C was 6.68 ± 0.0091 m2/g and 7.13 ± 0.0097 m2/g for the sample hydrated at 80 °C. The surface area of the ash sample prior to hydration was 1.71 ± 0.0045 m2/g. The BET analysis suggests that higher initial hydration temperature increases the surface area of the particles. 3.2. Work with synthetic ash Figs. 2 and 3 give the conversion to hydroxide and carbonate for synthetic ash for 10% sulphation. Higher carbonate conversions are observed at 40 °C when compared to those at 60 and 80 °C, which is the reverse of what we would expect based on work with real ash [3]. Shih et al. [7] have shown that temperature has a small effect on the carbonation of pure Ca(OH)2. Their experiments were carried out between 60 and 90 °C and under varying relative
90
40oC 40°C 60°C 60oC 80°C 80oC
80 70 60 50 40 30 20 10 0 0
5
10
15 Time (min)
20
25
30
Fig. 2. Synthetic ash sulphated to 10%, conversion to hydroxide.
Conversion of CaO to CaCO 3 (%)
Cumulative weight (% )
80
Conversion of CaO to Ca(OH)2 (%)
100
90
100 90 80 70 60 50 40 30
40oC 40°C 60oC 60°C 80°C 80oC
20 10 0 0
5
10
15
20
25
30
Time (min)
Fig. 3. Synthetic ash sulphated to 10%, conversion to carbonate.
humidity conditions. They concluded that the carbonation of Ca(OH)2 was affected to a greater extent by changes in relative humidity and to a lesser extent by temperature. A free lime test of the synthetic ash (10% sulphation) indicated an available lime content of 80%. From Figs. 2 and 3, after 5 min at 40 °C, 64% of CaO has converted to Ca(OH)2 and 34% has converted to CaCO3, indicating that complete conversion of the available lime has occurred. These results indicate a high reaction rate, which is most likely due to the particle size reduction brought about by ultrasonics. The high reaction rate appears to be the reason for the relative temperature insensitivity of the reaction.
A. Rao et al. / Fuel 87 (2008) 1927–1933
high reaction rate appears to be the reason for this, and other influences take a major role in determination of observed conversions. Substantial size reduction caused by ultrasonics produces reactive suspensions that are very sensitive to the influence of different factors during the experiment, causing a non-uniformity of results that is noticeable especially in Figs. 2 and 3 (20 min points), in Fig. 6. (4 min points), as well as in Figs. 9 and 10
Conversion of CaO to CaCO3 (%)
100 90 80 70 60 50 40 40oC 40˚C 60˚C 60oC 80˚C 80oC
30 20 10 0 0
2
4
6 Time (min)
8
10
12
Fig. 5. Synthetic ash sulphated to 20%, conversion to carbonate.
Conversion of CaO to Ca(OH)2 (%)
These results are not in agreement with the results obtained with FBC ash and indicate significant differences between synthetic ash and FBC ash. The reverse influence of temperature on carbonation conversions is an unexpected result that confused the original goal of this work, which was to obtain kinetic parameters of series reactions (hydration and carbonation) using synthetic ash. The reverse influence of temperature implies that calculated values of activation energy should be negative. This means that carbonation includes a step on which temperature has a negative influence. This step may be associated with the solubility of CO2 in the reaction medium because its solubility decreases with temperature. It may be presumed that substantial size reduction caused by ultrasonics, and consequent increase of sorbent particle surface area, accelerate reaction rate to such a level that availability of CO2 becomes limiting for the reaction. In that case solubility of CO2 decreases at higher temperatures (as do concentrations of H2CO3, HCO3 and CO32 that are in equilibrium with soluble CO2) and carbonation rate decreases. Moreover, the temperature influences CO2 solubility and related chemical equilibrium via pH value, i.e., via Ca(OH)2 solubility, which changes with temperature. The concentration of Ca2+ ions also influences Ca(OH)2 solubility and pH value and, therefore, CO2 solubility and carbonation rate. More detailed discussion of these influences would be outside the scope of the work and would require further specific experimentation. Shorter durations were used for subsequent experiments and the results are shown in Figs. 4–7. These results show that hydration, as well as carbonation, decreases with increasing levels of sulphation, with the limestone sulphated to 40% displaying significantly reduced hydration and carbonation. This implies that sulphation reduces sample reactivity even if sonication is employed. Couturier et al. [8] obtained similar results for hydration of synthetic sorbents sulphated to 30% and 45% in a Calvet microcalorimeter without sonication. It is also shown in these figures that the temperature has a small effect on conversions and, more importantly, the influence is not uniform. The
1931
100 90
40oC 40˚C 60˚C 60oC 80˚C 80oC
80 70 60 50 40 30 20 10 0 0
2
4
6
8
10
12
Time (min)
Fig. 6. Synthetic ash sulphated to 40%, conversion to hydroxide.
Conversion of CaO to CaCO3 (%)
Conversion of CaO to Ca(OH)2 (%)
100 40oC 40°C 60°C 60oC 80°C 80oC
90 80 70 60 50 40 30 20 10 0 0
2
4
6 Time (min)
8
10
12
Fig. 4. Synthetic ash sulphated to 20%, conversion to hydroxide.
100 90
40oC 40°C 60°C 60oC 80°C 80oC
80 70 60 50 40 30 20 10 0 0
2
4
6 Time (min)
8
10
12
Fig. 7. Synthetic ash sulphated to 40%, conversion to carbonate.
A. Rao et al. / Fuel 87 (2008) 1927–1933
(20 min points, see below). The non-smooth shape of the result obtained, apart from its relative temperature insensitivity, additionally prevents calculation of kinetic parameters. Carbonation levels for the limestone sulphated to 40% appear to decrease after 6 min. This apparent decrease in carbonation can be attributed to the size reduction property of ultrasonics. For all tests with synthetic ash, significant loss of particles was observed visually and we believe that particles lost during filtration were predominantly CaCO3. The particles trapped on the filter paper were large particles and were thought to have undergone partial hydration and carbonation. In order to confirm this hypothesis, wet sieving was carried out with synthetic ash (20% as well as 40% sulphation) and the results are shown in Fig. 8. These results indicate that nearly 55% of the particles have sizes of 20–25 lm. It should be noted here that for these tests, two filter papers (20–25 lm) were placed one above the other in order to trap all particles. The particles trapped on the filter paper for the limestone sulphated to 20% were analyzed using TGA and were found to contain 11% Ca(OH)2 and 74% CaCO3. Particles retained on the 1 mm and the 850 lm sieves were mixed together and a TGA analysis of the samples was carried out. The results indicated that the samples contained 30% Ca(OH)2 and 5% CaCO3. The X-ray diffraction (XRD) analysis of the samples obtained after the sonication of limestone sulphated to 20% is given in Table 7. The XRD results confirm that the reaction with synthetic ash is very rapid. At 6 min only 100 Cumulative weight (%)
90 80 70 60
0.5% of unreacted CaO is present in the sample. After 15 min practically all the free lime as well as any portlandite formed has converted to carbonate. The XRD results also support the idea that the reaction occurs in series as the hydroxide formed initially converts to calcite over time. One of the original aims of the work was to obtain kinetic data from synthetic sorbents. This arises from the fact that batch tests done at different times showed sufficient variation that reliable activation energies could not be obtained [3]. On refining the experimental method to allow removal of aliquots of sample during a sonication experiment at different times, better results were obtained as shown in Figs. 9 and 10 [9], but there was still sufficient variation that it was decided to work with a model system. There are in fact a number of objections that can be made with respect to using a real ash when trying to determine fundamental kinetic parameters. First and foremost, it is possible for the CaO to form significant amounts of what we have called other calcium compounds (namely silicates, ferrites and aluminates); thus a real sorbent can be chemically different from a synthetic sorbent, even though conventional X-ray fluorescence, and sulphur determinations will fail to show this [2]. Another complication is that the chemical composition of a real ash may also vary with
Converion of CaO to Ca(OH)2 (%)
1932
100 90 80 70 60 50 40 30 20 10 0
50
40oC 40˚C 60oC 60˚C 80oC 80˚C
0
5
10
40 30
synthetic ash - 0%
20
25
30
Fig. 9. Semi-continuous sampling, conversion to hydroxide.
20
synthetic ash - 20%
10
synthetic ash - 40%
20-25 45
75
125 250 425 Mesh size (μm)
600
850 1000
Fig. 8. Particle size distribution –synthetic ash sulphated to 20% and 40%.
Table 7 XRD analysis of synthetic ash samples obtained after sonication (20% sulphation) Chemical compound
Formula
CaSO4 CaCO3 CaO Ca(OH)2
Chemical compound (%) 6 min
10 min
15 min
46.6 31.8 0.5 25.8
33.1 54.5 – 16.2
33.7 60.8 – 2.9
Converion of CaO to CaCO3 (%)
0
Anhydrite Calcite Lime Portlandite
15 Time (min)
100 90 80 70 60 50 40 30
40oC 40˚C 60oC 60˚C 80oC 80˚C
20 10 0 0
5
10
15 Time (min)
20
25
Fig. 10. Semi-continuous sampling, conversion to carbonate.
30
A. Rao et al. / Fuel 87 (2008) 1927–1933
size fraction [10], and Couturier et al. has also made a similar point that the same size particles can be expected to have various sulphation time histories as a function of their residence time in a real boiler [8]. By contrast, with a synthetic sorbent one may control particle size and sulphation levels to achieve products with consistent properties. Unfortunately, as noted above attempts to work with calcined limestones failed totally as they were simply too soft to allow examination under sonication conditions, and the results with the synthetic sorbent, even at sulphation levels as high as 40% showed no temperature effect, whereas the results from real ashes show a pronounced enhancement in carbonation at higher temperatures [3]. This work then fits with many previous observations that real ashes are different from synthetic sorbents, sometimes dramatically so. Thus, Anthony et al. [11] demonstrated that CFBC boiler ash had different geotechnical properties than ashes produced in a pilot-scale combustor even when the same parent feedstocks were employed, and Laursen et al. [12] noted that it was more difficult to hydrate real bed ashes than fully sulphated limestones; the current authors have also argued that the wider literature on sorbent reactivation demonstrates significant differences between real boiler ashes and synthetic sorbents [13]. This work appears to provide another case in point, namely that, for sonochemically enhanced carbonation, synthetic sorbents are substantially different from real boilers ashes. 4. Conclusions Tests with acetone clearly indicate that significant carbonation of FBC ash is not achieved without prior hydration. The acetone test results also show that the simultaneous hydration and carbonation of FBC ash follows a series reaction mechanism. The hydration tests reveal that higher hydration temperatures expose a greater surface area for the carbonation reaction and this may explain why higher temperatures have been observed to increase carbonation conversions with real boiler ashes. Unfortunately, work with synthetic ashes suggests that they are substantially more fragile than real boiler ashes under sonochemical activation. Thus they fail to demonstrate any significant increase in carbonation as a result
1933
of increasing temperature, which appears to be due to a high reaction rate because of the substantial size reduction caused by ultrasonics. The difficulty in obtaining the true conversions, because a large percentage of particles have sizes lower than 20–25 lm, has also been demonstrated by wet sieving. The XRD results demonstrate that the reaction takes place at a rapid rate and also show that the reaction mechanism is best described by a series reaction. An important conclusion of this work is that it appears unlikely that synthetic ashes can be used to model the behaviour of real bed ashes under sonochemical activation conditions. References [1] Merriam PR, Cousens JD. Disposal of residue from Nova Scotia Power’s 165 MWe CFB boiler. In: Proceedings of the international conference on fluidized bed combustion, vol. 2. ASME, 1993, p. 859– 65. [2] Anthony EJ, Granatstein DL. Sulfation phenomena in fluidized bed combustion systems. Prog Energy Combust Sci 2001;27:215–36. [3] Rao A, Anthony EJ, Jia L, Macchi A. Carbonation of FBC ash by sonochemical treatment. Fuel 2007;86:2603–15. [4] Taylor HFW. Cement chemistry, 128. San Diego, CA: Academic Press Inc.; 1990. [5] Thompson LH, Doraiswamy LK. Sonochemistry: science and engineering. Ind Eng Chem Res 1999;38:1215–49. [6] Perry RH, Green DW, editors. Perry’s chemical engineers’ handbook. 7th ed. New York: McGraw-Hill Inc.; 2001. [7] Shih S-M, Ho C-S, Song Y-S, Lin J-P. Kinetics of the reaction of Ca(OH)2 with CO2 at low temperature. Ind Eng Chem Res 1999;38:1316–22. [8] Couturier MF, Volmerange Y, Steward F. Hydration of partially sulphated lime particles from fluidized bed combustion. J Energy Res Tech 2001;123:173–8. [9] Rao A. Carbonation of fluidized bed combustion solids. Masters thesis, University of Ottawa, 2006. [10] Wang J, Wu Y, Anthony EJ. The hydration behavior of partially sulfated fluidized bed combustor sorbent. Ind Eng Chem Res 2005;44:8199–204. [11] Anthony EJ, Ross GG, Berry EE, Hemings RT, Kissel RK. Characterization of solid wastes from circulating fluidized bed combustion. J Energy Res Tech 1995;117:18–23. [12] Laursen K, Duo W, Grace JR, Lim CJ. Characteristics of steam reactivation mechanisms in limestones and spent calcium sorbents. Fuel 2001;80:1293–306. [13] Anthony EJ, Bulewicz EM, Jia L. Reactivation of limestone sorbents in FBC for SO2 capture. Prog Energy Combust Sci 2007;33:171–220.