Agglomeration and strength development of deposits in CFBC boilers firing high-sulfur fuels

Fuel 79 (2000) 1933–1942 www.elsevier.com/locate/fuel

Agglomeration and strength development of deposits in CFBC boilers firing high-sulfur fuels E.J. Anthony*, L. Jia CETC, Natural Resources Canada, 1 Haanel Drive, Nepean, Ont., Canada K1A 1M1 Received 2 August 1999; received in revised form 10 January 2000

Abstract Fluidized bed combustor (FBC) ashes from high-sulfur, low-ash fuels, can agglomerate if subjected to sulfating conditions for long enough (days to weeks). The degree of sulphation increases with both temperature and time under these conditions, and at a conversion equivalent to the production of 50–60% or more of CaSO4 in the deposit the ashes agglomerate. Fly ash agglomerates less readily than does bed and loop seal ash and produces weaker deposits, although all of these materials will agglomerate if sufficient time is allowed. The potential for agglomeration increases if the temperature is increased from 850 to 950⬚C. Agglomeration also occurs at lower temperatures (down to at least 750⬚C), but the mechanism may be via carbonation and then sulphation of the ash. Although experiments reported here suggest that if pure CaSO4 is compressed to the 140 kPa range it does show some tendency to agglomerate, the agglomeration of FBC ash is not produced simply by the formation of CaSO4. Finally, the agglomeration process is only weakly influenced by the partial pressure of SO2 in the flue gas. Attempts to identify physical parameters to differentiate the tendency of various bed materials to agglomerate have been only partially successful. Two bed materials with strong and weak agglomerating tendencies were studied. These were shown to have very similar particle shapes and only slightly different angles of repose, but quite different bulk densities. Residues with a greater bulk density appear to have a stronger tendency to agglomerate, and this may provide a method of ranking the agglomeration potential of different bed materials. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: Agglomeration; Limestone; Sulphation; Carbonation

1. Introduction Circulating fluidized bed combustors (CFBCs) are ideally suited to burn fuels such as high-sulfur coal or petroleum coke, as the resulting SO2 can be captured in situ by addition of limestone via a process which can be described by means of the global reactions: CaCO3 ˆ CaO ⫹ CO2

…1†

CaO ⫹ SO2 ⫹

…2†

1 2

O2 ˆ CaSO4

A particularly important example of high-sulfur fuel used in CFBCs is petroleum coke which may have 5–8 wt% sulfur content, and there are now more than a half dozen such commercial units operating worldwide [1,2]. A survey sent to commercial operators of petroleum coke-fired CFBCs [1] indicated that a number of these boilers experienced significant fouling problems. These problems were * Corresponding author. Tel.: ⫹ 1-613-996-2868; fax: ⫹ 1-613-9929335. E-mail address: [email protected] (E.J. Anthony).

usually attributed to the high V content in the petroleum coke ash causing the formation of low melting point vanadates [3]. However, a study of deposits from a 120 t/h steam capacity CFBC boiler burning 100% petroleum coke [4] suggests another explanation. The V in the deposits is present as high melting point Ca vanadates. Furthermore, the ash generated in a CFBC firing petroleum coke is almost entirely limestone derived since petroleum coke typically has less than 1% ash. The Ca in the deposits is found to be nearly quantitatively converted to CaSO4. Such levels of sulphation of limestone-derived particles must result in particle expansion, and it was suggested that this expansion was the source of the agglomeration in the FBC system. These processes were termed “molecular cramming” in that particle expansion drives the agglomeration process. This is different from chemical reaction sintering which involves the formation of sulfate links between particles. However, it is also possible that molecular cramming occurs in conjunction with chemical reaction sintering, depending on the way the reaction product, in this case sulfate, is distributed with respect to the sorbent particle. The CaSO4 distribution can occur in various forms, e.g. continuously

0016-2361/00/$ - see front matter 䉷 2000 Elsevier Science Ltd. All rights reserved. PII: S0016-236 1(00)00054-5

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E.J. Anthony, L. Jia / Fuel 79 (2000) 1933–1942

Fig. 1. Box furnace experimental setup.

through the particles, through cracks or networks, or via the formation of an outer sulfate shell with an unreacted CaO core [5–7]. The latter would seem to be the least susceptible to the formation of sulfate bridges between particles due to chemical reaction, since the outer shell is already highly sulfated (with conversions of around 75–95% [7]), but this is the sulphation pattern followed by the most susceptible bed material studied here. A possible explanation that might resolve this difficulty is that as the sorbent particle expands and CaSO4 layers “peel” off, fresh unreacted CaO is exposed and made available for the chemical reaction sintering processes. However, at this time the precise mechanism(s) for the agglomeration phenomenon whereby loose packed bed ashes become hardened deposits must be regarded as somewhat uncertain. In addition, it should be noted that this study does not deal with initiation or deposition processes, and that this is an area that requires further

Fig. 2. EDS mapping of Ca and S distribution of agglomerated CFBC bed ash samples. The samples were sulfated for 100 days at 850⬚C. (a) Ca; (b) S.

study. Finally, it should be noted that the phenomena described here are different from those in which “sticky” material (such as low melting point alkali components) first bond bed particles together and this leads to defluidization. In order to determine whether ashes from petroleum coke-fired boilers are capable of agglomeration and densification samples of ash from a number of FBC and CFBC units and a variety of limestones, including limestones used in the FBC and CFBC units studied here, were subjected to long term sulphation in crucibles placed in a temperature controlled oven (Fig. 1). The oven was operated for times up to 105 days and at typical FBC temperatures (850–950⬚C) [8]. This work showed that both the ashes and limestones achieved similar levels of sulphation as those seen in boiler deposits, and formed hard deposits with low porosities [8]. Energy-dispersive X-ray spectroscopy (EDS) showed the deposits consisted of uniformly sulfated particles (Fig. 2). Further work was done on the ashes and limestones used for the NISCO boilers, with sintering assessment based on the compressive strength tests of heat-treated cylindrical ash pellets [9]. The effects of sintering in the presence of CO2 and SO2 over a wider temperature range (750–950⬚C) were also explored. This work also demonstrated that gas-solid reactions between CO2/SO2 and calcined sorbent (CaO) can contribute significantly to sintering over the appropriate FBC temperature ranges [9]. Both studies also demonstrated the important fact that agglomeration is possible in the effective absence of Na, K or V, i.e. the elements which are often associated with the formation of low melting point eutectics, or significant ash softening. Here effective absence is taken to mean that those elements are present at levels of several hundred parts per million or less. If Na or K are present at higher levels, an earlier study makes it clear that they can contribute significantly to the agglomeration process, i.e they act synergically, causing agglomeration at lower levels of sulphation [8]. It should also be noted that the present study does not deal with any possible effects such substances might have on initiation of the agglomeration process. Both of these previous studies [8,9] can be criticized for not taking into account the possible effect of reducing

E.J. Anthony, L. Jia / Fuel 79 (2000) 1933–1942 Table 1 Analysis of bed materials and sorbents (wt%) Bed materials

Limestones

Component NSPI

NISCO

TVA

NSPI

NISCO

TVA

SiO2 Al2O3 Fe2O3 TiO2 P2O5 CaO MgO SO3 Na2O K2O BaO SrO V2O5 NiO MnO Cr2O3 LOF SUM

2.88 ⬍0.38 ⬍0.55 ⬍0.04 0.03 54.39 2.00 34.86 ⬍0.17 ⬍0.08 0.08 0.05 0.73 0.11 ⬍0.02 ⬍0.01 4.48 99.61

4.42 1.13 0.60 0.09 0.02 61.00 3.08 27.51 ⬍0.17 ⬍0.08 0.096 0.69 ⬍0.02 ⬍0.006 ⬍0.015 ⬍0.006 1.9 99.92

4.59 1.10 ⬍0.55 ⬍0.04 ⬍0.02 50.83 2.61 0.37 ⬍0.17 0.16 ⬍0.02 0.03 ⬍0.02 ⬍0.01 ⬍0.02 ⬍0.01 43.37 99.81

0.16 0.08 0.09 0.011 0.02 55.4 0.52 – ⬍0.01 ⬍0.01 ⬍0.02 0.08 ⬍0.02 ⬍0.01 ⬍0.02 ⬍0.01 43.4 100.1

2.55 0.34 ⬍0.30 ⬍0.04 ⬍0.02 51.27 2.07 ⬍0.46 ⬍0.46 0.08 – – ⬍0.02 ⬍0.01 – – 42.86 99.17

11.31 5.24 4.99 0.29 0.02 47.7 0.85 23.26 0.25 0.55 0.10 0.03 ⬍0.02 0.01 0.01 ⬍0.01 5.21 100.00

conditions, or the effect of periodic oxidizing-reducing conditions, both of which have been shown to be important in effecting the sulphation process [10,11]. However, such deposits are often formed high in the combustor (e.g. on superheater tubes) or in the back-pass where the temperatures can also be low enough to permit carbonation and sulphation to occur simultaneously (i.e. at temperatures less than about 780⬚C for a typical partial pressures of CO2 in an FBC, i.e. about 15 kPa). It is therefore, not unreasonable to suppose that overall conditions are oxidizing all or most of the time, thus, it seems reasonable to neglect such effects. Oven tests, carried out over days or weeks, may also be criticized on the grounds that the residence time for most sorbent particles is of the order of minutes to hours [12]. It should be noted, however, that in full-scale boilers there are always dead zones or quiescent regions, and previous work has clearly shown that if sorbent particles are trapped in such regions, conversions to CaSO4 can be nearly quantitative, and that agglomeration due to the sulphation process alone does take place [4,8,9]. It is also clear that once such a deposit is formed, it create its own distortions of the gas flow pattern in a boiler, allowing the deposit to grow more rapidly [13]. The oven test work [8] demonstrated that after continuous sulphation over days to weeks a wide variety of CFBC ashes develop a hard, highly sulfated agglomerated layer on the top of the test samples. The thickness of these hard agglomerates is about 4–5 mm. The ashes underneath the top layer are much less sulfated. This phenomenon suggested that there is an effective SO2 penetration depth of about 4– 5 mm. Further, it appeared that for small particles (⬍75 mm) the bonding is much weaker. The optimum particle size for this phenomenon to occur is between about 150 and 300 mm. Particles much above 600 mm failed to

1935

agglomerate, although “whole bed ashes” (which are not separated into size fractions), with particle size ranges of up to 1.4 mm did agglomerate with strengths almost comparable to the 150–300 mm fraction. Earlier studies [8,9] indicated that the tendency to agglomerate varied with different sorbent, with some limestones and bed ashes showing significantly greater agglomeration tendency than others. Finally, “inert” fuel-derived ash was shown to reduce agglomeration. However, it was suggested that “inerts” added to boilers reduce such effects by dilution rather than by chemically binding V in the ash into high melting point compounds, as had been proposed earlier, in line with experience from oil-fired boilers [14,15]. However, there are still a number of important questions related to agglomeration in FBCs including: 1. What are the effective upper and lower temperature limits for such phenomena? 2. Does temperature play an important part in the degree of conversion and strength development of such deposits? 3. What effect does SO2 partial pressure have on the CaO to CaSO4 conversion? 4. What role does the CaSO4 itself, independently of molecular cramming or chemical reaction sintering, play in the agglomeration process? 5. What factors make some limestones and bed materials more ready to agglomerate? This paper attempts to address these questions. 2. Experimental 2.1. Samples FBC ashes from the following facilities were used in this study: 1. The two 100 MWe Nelson Industrial Steam Company (NISCO) CFBC boilers in Louisiana. 2. The 183 MWe Point Aconi CFBC Boiler operated and owned by Nova Scotia Power Incorporated (NSPI), in Nova Scotia, Canada. 3. The 160 MWe Tennessee Valley Authority (TVA), FBC boiler in Paducah, Kentucky. The composition of these bed ashes and their parent limestones is given in Table 1. 2.2. Sulphation test apparatus For most of the sulphation tests, the ash samples were first placed in crucibles (4 mm high flat V4-Vitersol crucibles) and arranged on two racks, in rows of 3 or 4, in a temperature controlled oven, (Fig. 1). As independent measurements had shown that particles in the NSPI bed ash larger than 1.4 mm to be primarily shale-derived, these were deliberately sieved out. For consistency all other samples of bed or loop seal material were treated in the same way, although

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E.J. Anthony, L. Jia / Fuel 79 (2000) 1933–1942

Fig. 3. Tube furnace.

for ashes such as NISCO, particles larger than 1.4 mm are still primarily limestone-derived. Sulphation tests were conducted at 850, 900 or 950⬚C (^2⬚C). A synthetic flue gas was employed containing 1% SO2, 3% O2, and the balance N2, at a flow rate of about 130 ml/min. The 1% SO2 concentration was deemed necessary to reduce the amount of cylinder gas used during the course of a three month experiment. The SO2 concentration was not expected to greatly influence the results. It has been claimed [16] that for high levels of CaO to CaSO4 conversions, diffusion through product layer controls sulphation process for larger particles while bulk diffusion controls sulphation for smaller particles, with overall reaction orders of 0.25 and 0.2, respectively. For a few tests, a Lindberg tube furnace was used to test the effect of SO2 concentration on agglomeration and carbonation–sulphation processes. The experimental setup is shown in Fig. 3. Two ash samples were placed in the centre of the tube for each test. The gas flow rate was 50 ml/min. In

the carbonation-sulphation tests, the gas contained 1% SO2, 3% O2 and balance CO2. These tests lasted 7 days during which the temperature was maintained constant at 650, 750 or 850⬚C. Tests on the effect of SO2 concentration were also conducted in the tube furnace with synthetic flue gases containing 2000, 5000 and 10 000 ppm SO2, 3% O2 and the balance N2. The test duration was 25 days and the operational temperature was 850⬚C. All samples produced were subjected to X-ray fluorescence (XRF) analysis. 2.3. Characterization tests for bed material To determine the strength of the hardened samples, an Omega LCFA-250 tension/compression load cell was mounted on a drill press, (Fig. 4). A custom-made crusher head was screwed on to the load cell. The drill press was used to generate force. The agglomerated ash sample disk was placed on a metal block and crushed without attempting to remove the disk from the crucible. The peak force when

Fig. 4. Agglomerates crushing test.

E.J. Anthony, L. Jia / Fuel 79 (2000) 1933–1942 Table 2 Test results for extended sulphation tests Test conditions 850⬚C for 6 weeks Original % conversion Original CaSO4 % No. of agglomerated samples Mean % conversion on agglomeration Mean CaSO4 % on agglomeration No. of unagglomerated samples Mean % conversion for unagglomerated samples Mean CaSO4 % for unagglomerated samples 850⬚C for 12 weeks Original % conversion Original CaSO4 % No. of agglomerated samples Mean % conversion on agglomeration Mean CaSO4 % on agglomeration No. of unagglomerated samples Mean % conversion for unagglomerated samples Mean CaSO4 % for unagglomerated samples 900⬚C for 6 weeks Original % conversion Original CaSO4 % No. of agglomerated samples Mean % conversion on agglomeration Mean CaSO4 % on agglomeration No. of unagglomerated samples Mean % conversion for unagglomerated samples Mean CaSO4 % for unagglomerated samples 950⬚C for 6 weeks Original % conversion Original CaSO4 % No. of agglomerated samples Mean % conversion on agglomeration Mean CaSO4 % on agglomeration No. of unagglomerated samples

Fly ash

Loop seal

Bed ash

30.2 ^ 1.9 31.2 ^ 1.3 3 62.8 ^ 8.5

38.2 ^ 2.3 45.7 ^ 4.1 4 47.7 ^ 6.2

39.0 ^ 2.3 43.7 ^ 4.5 9 50.4 ^ 6.2

55.8 ^ 6.4

53.1 ^ 6.2

51.6 ^ 4.3

9 44.6 ^ 9.2

4 40.6 ^ 1.9

2 40.5 ^ 2.0

45.0 ^ 13.4

46.0 ^ 3.6

47.2 ^ 4.2

30.2 ^ 1.9 31.2 ^ 1.5 13 73.3 ^ 11.3

38.2 ^ 2.3 45.6 ^ 4.0 8 66.0 ^ 7.0

38.9 ^ 2.2 44.7 ^ 3.0 10 76.9 ^ 7.0

62.8 ^ 6.4

62.6 ^ 5.3

69.6 ^ 5.3

2 40.1 ^ 0.2

1 53.2

1 48.1

45.0 ^ 1.8

58.6

50.2

1937

obtained. If there are particles contacting each other in the image, a special software routine is run to separate them. The procedure involved first “shrinks” the particles until all of them are totally separated, and then expand the particles to its original size and shape with “boundaries” set up between them. When the images were analyzed mean particle diameter, aspect ratio and sphericity were determined. Sphericity is defined as: Sphericity ˆ

4p × Area …Perimeter†2

Another parameter of interest is the angle of repose. To determine this, the ash was dropped slowly through a funnel to form a cone on the ground. The diameter of the cone base and the height of the cone were measured. The angle of repose was then determined. Multiple measurements were made for each ash to obtain a mean angle and standard error. Finally, the bulk density was also determined. For this measurement, the ash was loaded into a beaker, tapped on top of a table ten times, and the mass and volume determined. For each ash sample, at least ten measurements were done. Tapping of the sample helped to minimize the effect of ash particle packing.

3. Results and discussion 30.2 ^ 1.9 31.2 ^1.3 13 66.6 ^ 9.7

37.9 ^ 2.9 44.8 ^ 3.9 8 54.5 ^ 4.8

39.0 ^ 2.3 43.7 ^ 4.5 9 56.8 ^ 6.7

59.3 ^ 6.2

58.8 ^ 4.0

59.0 ^ 4.9

2 53.4 ^ 4.6

0 –

0 –

50.9 ^ 1.5

–

–

30.2 ^ 1.9 31.3 ^ 1.3 15 70.4 ^ 4.5

38.2 ^ 2.3 45.5 ^3.9 9 67.4 ^ 5.1

38.9 ^ 2.2 44.8 ^ 3.3 11 69.9 ^ 7.3

62.1 ^ 3.5

68.0 ^ 5.3

67.6 ^ 6.8

0

0

0

the sample was crushed was recorded. The crushing pressure was then calculated based on the area of the crusher head. Ash particle shape was also of interest and this was analyzed with a Clemex 1024 image analysis system. The system combines hardware and software processing modules to provide users with a fast and flexible image analyzer. For these tests, a small amount of ash was dispersed on a glass plate, and multiple images were

3.1. Effect of temperature, time and composition on agglomeration In our earlier work [8] on agglomeration a subjective measurement of the strength of the agglomerated ash disks was used, i.e., whether a complete disk of the agglomerated material could be extracted from the crucible and how easy or difficult it was to break. Evidently, the deficiencies of such a method are considerable and do not allow one to make quantitative assessments of strength development. Preliminary results suggested that samples were less readily agglomerated at 900⬚C compared to at 850⬚C, but those results were not conclusive as there was some question about the SO2 distribution in the oven. In order to resolve this problem, a crush test was developed, as described earlier. The work was undertaken with NSPI CFBC ash as previous work had shown this ash agglomerates very readily in the oven test. It has been reported [8,17] that some deposits from the NSPI CFBC unit had high levels of sulfate. This boiler burns a low-ash (7–9%), high-sulfur (2.4–4%) bituminous coal. Tests were carried out with bed, loop seal and fly ash, which were subjected to sulfating conditions for up to 12 weeks. Temperatures for the oven tests ranged from 850–950⬚C, and the synthetic flue gas contained 1% SO2, 3% O2 and balance N2. Initially, three six-week tests and one twelve-week test were conducted. Approximately 98% ⫹ of the mass of both the bed and loop seal ash was between particle size ranges of

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E.J. Anthony, L. Jia / Fuel 79 (2000) 1933–1942

Table 3 Strength development tests

Fly ash sulfated at 850⬚C Bed and loop seal ash sulfated at 850⬚C Fly ash sulfated at 950⬚C Bed and loop seal ash sulfated at 950⬚C Deposits resulfated at 950⬚C

No. of samples

Strength development (MPa)

6 24

0.12 ^ 0.77 8.77 ^ 2.9

3 24

0.53 ^ 0.33 6.0 ^ 1.6

3

2.3 ^ 0.3

0.075–1.4 mm, while 100% of the mass of the fly ash was below 110 mm. The bed and loop seal ash may, therefore, be regarded as “coarse” material of an ideal size for agglomeration [8], while the fly ash material is fine and would not be expected to agglomerate readily. Sulphation levels of the original bed and loop seal samples were essentially the same for all samples, lying between 36.7 and 41.9%, while the sulphation levels of the fly ash samples varied from 28.4 to 33.0%. Test results are presented in Table 2. Here conversion is calculated assuming that all of the Ca determined by XRF is present as CaO and available to form CaSO4. Although there are some discrepancies, the results show a clear trend of an increasing degree of conversion of the CaO to CaSO4 as a function of time (6–12 weeks) and temperature (850– 950⬚C). It is evident that there is no maximum conversion associated with temperatures in the 850⬚C range, as is the case with sulfur capture at typical conversion levels under FBC conditions [18]. Also, the observation that conversion increases continuously as a function of time is not trivial, since theories exist arguing that there is an effective limit for conversion of CaO to CaSO4 [19–21]. Such a hypothesis is clearly not supported by the current results. It should be noted that the highest conversion of CaO to CaSO4 seen in this work (85% for a loop seal sample) represents a real conversion of close to 100%. This is the case, because about 10–20% of the free lime in the NSPI ash (determined through back calculation on the basis of the degree of conversion of the CaO to sulfate, carbonates and sulfides, if present) is actually chemically combined in the form of silicates, aluminates and ferrites [22]. These compounds have been termed other calcium compounds, OCCs, and earlier work [23] has shown that they sulfate significantly less readily than the parent limestone, if at all. It also appears that fly ash samples agglomerate with more difficulty than loop seal and bed ash samples. One explanation is that fly ash contains less free CaO, and given the theory that an increasing conversion of CaO to CaSO4 is related to increasing agglomeration, it is evident that the CaO in a fly ash sample must reach higher degrees of conversion than a bed or loop seal sample in order to produce a similar CaSO4 content. However, earlier work [8] also supported the idea that fine bed particles

(⬍150 mm) agglomerate less than particles in the 150–600 mm range, even when they have similar CaO contents. It is known [24] that, since the surface to volume ratio of a material markedly increases with decreasing particle size, surface forces become increasingly important allowing bridging and arching between particles, and hence producing increasing porosity as the particle size decreases. Alternatively expressed this means that the bulk density can be expected to fall as the particle size decreases. It is suggested here that decreasing bulk density reduces the particle-particle interactions that occur via the molecular cramming process. 3.2. Strength development in deposits Originally, the relative strength of agglomerates was measured simply by comparing how easily samples broke under finger pressure. While this method does give results which are for the most part in good agreement with the actual degrees of conversion or amount of CaSO4 in a sample, such a method is evidently subjective. Accordingly, an agglomerate strength test was developed. Two sets of tests were done in which samples were subjected to sulphation for a period of 4 weeks. In the first set of experiments carried out at 850⬚C, six samples of three bottom ashes, six samples of one set of loop seal ash and six samples of one set of fly ash were sulfated. In the second test series carried out at 950⬚C, the same set of samples was sulfated, but only three samples of the fly ash were tested. The remaining three spaces were replaced with three samples of crushed and sieved bed ash samples which had been previously agglomerated by being sulfated for 4 weeks at 850⬚C. As there is no significant difference between the strength of the bed ash and the loop seal ash, the averaged results of these ashes are presented together in Table 3. The strength tests clearly suggest that fly ash samples form much weaker deposits than do bed and loop seal materials. Unfortunately, these tests are not sufficiently sensitive to determine whether bed and loop seal ash agglomerates differ in strength when formed at 850 and 950⬚C. However, given the fact that sulphation increases as a function of temperature it is probably reasonable to suggest that samples become stronger as the temperature increases. An interesting result was obtained from the previously agglomerated samples that had been sulfated at 850⬚C for four weeks. These were first crushed and sieved to achieve a size distribution in the range 0.075–1.4 mm, and then resulfated for another 4 weeks at 950⬚C, whereupon they reagglomerated. However, their strength development (2.3 ^ 0.3 MPa) was lower than that achieved by the same samples earlier. These had previously achieved strengths of 10:2 ^ 1:9 and 6:2 ^ 2:0 MPa at 850 and 950⬚C, respectively. This strength was also less than that achieved by the weakest bed or loop seal sample in any crush test (which was 3 MPa for a loop seal sample sulfated at

E.J. Anthony, L. Jia / Fuel 79 (2000) 1933–1942 Table 4 Chemical composition of NSPI sulfated-carbonated ash Sample temperature

850⬚C

750⬚C

650⬚C

Agglomerated % CaSO4 from sulphation in a SO2/O2/CO2 environment % CaCO3 Free lime (as CaO) % OCCs (as CaO) % Conv. to CaSO4 % Corrected conv. to CaSO4

Yes 59.7

Yes 61.2

No 52.7

2.7 3.2 7 67.8 84

2.5 5.6 6 66 78.3

1.6 9.9 7.5 54.3 66.8

850⬚C). These results support the hypothesis that it is the process of sulphation (with consequent cramming and sintering) rather than the absolute sulfate content of the sample that gives rise to strength development, which is in line with other work reported here on attempts to agglomerate CaSO4 directly. 3.3. Sulphation at lower temperatures As the temperature of a CFBC falls below 800⬚C, one moves to a region where CaCO3 is thermodynamically stable. At a typical CO2 partial pressure of 15 kPa this will occur at 780⬚C. Given that carbonate formation is known to be much faster than sulphation [25], it is reasonable to expect that agglomeration will commence via the formation of CaCO3, with subsequent sulphation of the deposit [9]. To investigate this regime, three tests were carried out with NSPI ash, which was sulfated at 850, 750 and 650⬚C using a tube furnace. The gas composition was 1% SO2, 3% O2 and the balance CO2, which ensured that CaCO3 was thermodynamically stable over the entire temperature range examined. All samples were sulfated for 7 days. Samples agglomerated in 850⬚C and 750⬚C sulphation tests, while samples sulfated at 650⬚C did not. However, samples sulfated at 650⬚C showed some signs of clumping and may have agglomerated if sulfated for a longer time. The chemical compositions of the three sets of samples after the carbonation-sulphation treatment are given in Table 4. These results indicate that agglomeration by sulphation can occur at temperatures as low as 750⬚C and possibly below, but at around 650⬚C, the rates of conversion to sulfate and probably carbonate start to fall significantly. In practice, sulphation will be replaced by the formation of

1939

CaSO3 at temperatures much below 650⬚C [26], so this temperature can probably be regarded as a practical lower limit for agglomeration by carbonation-sulphation, although further work is needed for a more precise determination. The degree of conversion to CaSO4 also appears to be much higher than those seen in the 6 week oven test at 850⬚C (Table 2). This suggests the carbonation-sulphation process may be more rapid than direct sulphation, but more work is needed to substantiate this. In Table 4 the free lime content has been determined chemically rather than by difference [22]. As noted previously [23], it is possible for up to 10–20% of the Ca in the system to be chemically bound in the form of aluminates, silicates and ferrites, which tend to sulfate very much less readily than CaO, if at all. Table 4 gives the degree of conversion of CaO to sulfate calculated by the two methods and shows that attempts to estimate the degree of conversion of CaO can be seriously in error, if the OCCs are ignored, as is the common practice when estimating sulfur capture efficiencies in FBC boilers.

4. The effect of gas concentration on agglomeration The SO2 gas concentration (1%) used in this work is relatively high. The 1% SO2 concentration is selected to avoid the use of excessively large gas volumes during long tests. However, such a high concentration is evidently outside the range likely to be encountered even with highsulfur petroleum coke, for which values of 6000–8000 ppm are possible in the absence of limestone addition. The literature [16] on the sulphation behavior of highly sulfated limestone-derived materials suggests that sulphation shows a 0.2–0.25 order dependence on SO2 levels, when SO2 must diffuse through a preexisting CaSO4 layer. Under such circumstances one would not expect the SO2 concentration to be particularly important in determining the sulphation behavior of the CFBC ashes. In order to test this hypothesis, a limited number of runs were done in a tube furnace, in which SO2 concentration can be more precisely controlled than in an oven. The samples were placed in ceramic boats. The boats are about 60 mm long, 12 mm wide and 15 mm in depth. The samples were loaded into the tube furnace and sulfated for a preselected period. Four tests were carried out at 850⬚C and 25 days sulphation time with SO2 inlet concentrations of 2040, 4913 and

Table 5 Tube furnace tests Sample

SO2 concentration (ppm)

% CaSO4 content

Agglomerated

TVA bed ash (⬍1.4 mm) NSPI bed ash (⬍1.4 mm) NSPI bed ash (⬍1.4 mm) NSPI bed ash (⬍1.4 mm)

9860 2040 4913 9860

64 (53) a 67.2 (67.8) 66.7(67.3) 65.5 (65.9)

Yes Yes Yes Yes

a

Values within parenthesis % conversion of CaO to CaSO4.

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E.J. Anthony, L. Jia / Fuel 79 (2000) 1933–1942

Table 6 Compression tests with CaSO4 Particle size range (mm)

Pressure (kPa)

Agglomerated

⬍ 75 75–150 300–600 600–1400 150–300 300–600

7.4 4.1 11.2 4.1 147 147

No No No No Yes(?) Yes(?)

9860 ppm. Table 5 shows the conversion of CaO to CaSO4 for all four tests. The CaO conversions are determined in the standard manner, i.e., by assuming that all the CaO in the system is potentially capable of sulphation. If the data are corrected for the presence of OCCs, the conversions increase for the TVA ash to 62% and for the NSPI samples to about 80%. These results show clearly that after 25 days there is little difference between the degrees of conversion achieved with the different concentrations of SO2 employed in these tests. This result supports the idea that extended sulphation process occurs with a low fractional reaction order.

5. The influence of CaSO4 on agglomeration Although CaSO4 is known to be an important bonding agent in the formation of deposits from conventional boilers [27] the mechanism by which it contributes to strength development is not fully understood. Nor is CaSO4 normally thought of as a fouling agent in FBC boilers. However, given that deposits in a number of petroleum coke-fired boilers consist almost entirely of CaSO4, the possibility that the sulfate is contributing to the strength development process can not be ruled out. In previous oven tests, fine CaSO4 was exposed to sulphating conditions for 103 and 100 days at 900 and 950⬚C, respectively, without apparent effect. However, as the CaSO4 used was very fine (⬍45 mm), and earlier work has shown that particles in this size range do not appear to agglomerate readily under the conditions of the oven test, the involvement of CaSO4 could not be completely excluded. As the increase in molar volume due to sulphation in the 60–70% range demands particle expansion [6], it was decided to simulate this phenomenon by using a 170 g weight to compress the CaSO4 sample. This corresponds to a pressure of 1 kPa or the effect of a column of 68 mm of bed material assuming a bulk density of 1500 kg/m 3. Preliminary tests were done with two samples of CaSO4. The CaSO4 was obtained by dehydrating analytical grade gypsum in an oven overnight at 300⬚C. The CaSO4 samples were then held in an oven at 900⬚C, with flue gases containing 1% SO2, 3% O2 and the balance N2 for 1 month at a gas flow rate of 50 ml/min. Samples of NSPI bed ash were included in the experiments to confirm that agglomerating

conditions were achieved. Neither CaSO4 sample agglomerated, while the NSPI bed ashes were strongly agglomerated. A commercial grade of CaSO4 (Drierite) was then used to determine whether particle size was important in determining any tendency of CaSO4 to agglomerate. The material was first dehydrated in an oven overnight at 300⬚C and then sieved to produce size fractions of ⬍75, 75–150, 150–300, 300–600 and 600–1400 mm. Two samples for each fraction were used. One was constrained by a 170 g weight on the top and the other was not. To provide a comparison six samples of NSPI bed ash (particle size ⬍1.4 mm) were also tested. Again the temperature for the test was 900⬚C, and the test was continued for a month, with the same gas composition and flow rate as used previously. As before the CaSO4 samples were not agglomerated while the NSPI bed ashes were strongly agglomerated. This same test was then repeated with heavier steel weights (Table 6) which produced compression pressures of 4.1, 7.4, and 11.2 kPa, together with eleven CFBC bottom ash, loop seal and fly ash samples from NSPI. Again, all of the CFBC ash samples were agglomerated but none of the CaSO4 samples showed any signs of agglomeration. As the 11.2 kPa compression would correspond to a depth of 795 mm of bed ash, it appears reasonable to suggest that mechanical pressure such as might be exhibited by a column of ash resting on these samples in a CFBC boiler, would not be sufficient to cause them to agglomerate. Finally, it was decided to carry out tests with two different particle size ranges in the optimum size range for bed ash agglomeration (i.e. 150–300 and 300–600 mm), and a much greater weight equivalent to 147 kPa pressure under the same conditions as before. Upon completion of the month-long test, the weight standing on the 300–600 mm sample had toppled, and was resting against the side of the oven decreasing the compression. However, both samples had formed weak disks, capable of being lifted from the crucible, but which broke on slight finger pressure, and were, therefore, too weak to be subjected to a crush pressure test. These results suggest that, if CaSO4 is subjected to pressures of this order, it may contribute to the strength development in the agglomerated material. However, since such a pressure is equivalent to a ash layer with depth of about 10 m, it can only occur locally due to the expansion phenomenon, and not as a result of the pressures that might arise from ashes resting on surfaces in a FBC boiler.

6. Factors affecting the tendency of bed material to agglomerate Earlier work [8] showed that different bed materials exhibited different strength development, and most notably that bed ashes from the NSPI’s Point Aconi CFBC were particularly susceptible to agglomeration, while those from the two 100 MWe NISCO boilers were least

1.29 0.15 0.8 0.08 26 1019

NISCO bed ash 300–600 mm

E.J. Anthony, L. Jia / Fuel 79 (2000) 1933–1942

1941

susceptible to agglomeration of the bed materials examined. One possible explanation is that there is a difference in the way these materials pack due to their sphericity, angle of repose, or bulk density. In order to study which factor has an effect on the agglomeration process it was decided to work primarily with NSPI bed ash and NISCO bed ash, to take advantage of the differences in their agglomeration behavior.

1.35 0.2 0.84 0.1 12 591 1.33 0.21 0.86 0.08 50 1304

1.33 0.22 0.8 0.09 20 558

NISCO bed ash 150–300 mm NSPI bed ash 150–300 mm

NSPI bed ash 300–660 mm

6.1. Particle shape analysis One NSPI fly ash, and bed ashes from NSPI and NISCO in two size ranges (150–300 and 300–600 mm) were studied and the results are presented in Table 7. The aspect ratio and sphericity were almost the same for each size fraction of bed ash, with the larger particles tending to be less spherical. From the results in Table 7, it is probably reasonable to conclude that particle shape is not an important parameter in determining the tendency of a particular FBC ash to agglomerate. 6.2. Angle of repose The angle of repose of the NISCO, TVA and NSPI bed ash were examined as they represent an increasingly severe tendency to agglomerate according to previous work [8] and the results are presented in Table 8. The measured angles of repose are close to those reported for 480 mm sand (37⬚) and are significantly lower than those for CaO powder and pulverized limestone, which have angles of repose of 43⬚ and 47⬚, respectively [28]. An F test shows that there is a significant difference between the NISCO and the NSPI ash, at a 1% significance level, but not between the TVA and NISCO bed materials. Given this narrow range of variation, it seems reasonable to conclude that the angle of repose cannot be used to distinguish between the propensity of various bed materials to pack and agglomerate.

1.4 0.21 0.76 0.17 10 333 No. of fields Object count

Sphericity

Mean Std. Dev. Mean Std. Dev. Aspect ratio

Fly ash NSPI 75–106 mm

Table 7 Ash particle aspect ratio and sphericity

6.3. Bulk density Another measure of the tendency of particles to pack is bulk density (see Table 9). Again, performing an F test, the bulk densities are shown to be different at the 1% significance level. The fact that NSPI bed ash packs much more readily than does NISCO bed ash fits with the idea that agglomeration occurs most readily in those situations in which particles are more closely packed thus allowing particle expansion to provide the bonding force for agglomeration. In this connection it is worth noting that the bulk density of the NSPI fly ash is 1020 ^ 18 kg/m 3, which is significantly lower than that of the bed material [29], and this is consistent with its reduced agglomeration tendency. 7. Conclusions This study has shown that ashes from FBC boilers will continue to sulfate if exposed to sulfating conditions over

1942

E.J. Anthony, L. Jia / Fuel 79 (2000) 1933–1942

Acknowledgements

Table 8 Angle of repose of three bed ashes Sample

NISCO

TVA

NSPI

No. of measurements Angle of repose (⬚) Standard deviation (⬚)

4 34.1 1.3

6 34.4 0.6⬚

6 31.8 1.4

long periods of time, with sulphation increasing as a function of both time (up to 12 weeks) and temperature (850⬚C to 950⬚C). There appears to be no equivalent of the bed temperature maximum for extended sulphation near 850⬚C and this work provides clear evidence that sulphation is a continuous process and does not become equilibrated as suggested by some workers. For NSPI CFB ashes, agglomeration always occurs by the time the residues have achieved conversions sufficient to produce CaSO4 levels of over 60%. There is also a clear difference in the strength development of fly ash, which is much less than that for bed and loop seal materials. These experiments do not indicate that agglomeration decreases at higher temperatures. Limited tests on sulphation at lower temperatures, which can be expected to occur initially via conversion of CaO in the bed ash to CaCO3, followed by sulphation, show that those processes occur at temperatures above 650⬚C, with agglomeration being achieved for the current test regime within one week at 750⬚C. The high degree of conversion of the samples (in the range of 60–80%, when the data are corrected for the presence of OCCs) suggested that this process is faster than direct sulphation, but more work is needed to substantiate this. Further, tests with different SO2 concentrations indicate the effects of changing the partial pressure of SO2 on sulphation through dense sulfate layers is minimal, in line with earlier work. Attempts to find some significant physical parameters to differentiate the tendency of bed ashes to agglomerate have been less successful. However, there is limited evidence that bulk density may be important, with materials having a higher bulk density showing a greater tendency to agglomerate. Finally, CaSO4 itself has been demonstrated to have no tendency to agglomerate unless compressed with pressures in the 140 kPa range. Such pressures could not be achieved by the weight of a column of bed material resting on a deposit in a CFBC boiler, but might be achieved by the particle expansion phenomenon that must occur at high levels of conversion of CaO (60% ⫹ range) in FBC materials.

Table 9 The bulk density of two bed ashes Sample

NISCO

NSPI

No. of samples tested Bulk density (kg/m 3) Standard deviation

8 1217 35

13 1503 52

The authors would like to acknowledge the partial support of this work by NSPI. They would also like to thank Drs A.P. and J.V. Iribarne for analytical work on selected samples, and Prof. E.M. Bulewicz of Cracow University of Technology for some useful discussions during the course of this work. References [1] Anthony EJ. Progress in Energy and Combustion Science 1995;21:239. [2] Anthony EJ. Thirteenth annual fluidized bed conference. Council of Industrial Boiler Owners, Lake Charles, Louisana, US, 1997. p. 119. [3] Jones C. Power 1995;May:46. [4] Anthony EJ, Iribarne AP, Iribarne JV. Journal of Energy Resource Technology 1997;119:55. [5] Pickles ER, McCarthy R, Couturier M. In: Manaker A. (Ed.), Proceedings of the tenth international conference on FBC, ASME, San Francisco, CA, May 1989. p. 359–66. [6] Mattisson T. Sulfur capture during combustion of coal in circulating fluidized bed boilers, PhD thesis, Chalmers University, Sweden, 1998. [7] Laursen K, Duo W, Grace JR, Lim J. Fuel 2000;79:153. [8] Anthony EJ, Preto F, Jia L, Iribarne JV. Journal of Energy Resource Technology 1998;120:285. [9] Skrifvars B-J, Hupa M, Anthony EJ. Journal of Energy Resource Technology 1998;120:215. [10] Lyngfelt A, Leckner B. Journal of Institute of Energy 1998;77:27. [11] Mattisson T, Lyngfelt A. Journal of Institute of Energy 1998;77:190. [12] Lyngfelt A, Leckner B. Powder Technology 1992;70:285. [13] Bott TR. Fouling of heat exchangers. Amsterdam: Elsevier, 1995. [14] Gunn D, Horton R. Industrial boilers. New York: Wiley, 1989. [15] Lawn CJ, Godridge AM. Matching the combustion equipment to the boiler. In: Lawn CJ, editor. Principles of combustion engineering for boilers, New York: Academic Press, 1987. [16] De Hemptinne JBC. Sulfation of nonporous calcium oxide, PhD thesis, Massachusetts Institute of Technology, 1990. [17] Johnk C, Friedman MA, Andrews NA. In: Heinschell K. (Ed.), Proceedings of the fourteenth international conference on FBC, ASME, Orlando, FA, May 1995. p. 1105–11. [18] Anthony EJ, Preto F. Sulfur capture in FBC. In: Alvarez Cuenca M, Anthony EJ, editors. Pressurized fluidized bed combustion, London: Blackie Academic, 1995. p. 101–13. [19] Duo W, Seville JPK, Kirby NF, Clift R. Chemical Engineering Science 1994;49:4429. [20] Duo W, Kirby NF, Seville JPK, Clift R. Chemical Engineering Science 1995;50:2017. [21] Duo W. In: Seville JPK, editor. Gas cleaning in demanding applications, London: Chapman and Hall, 1997. p. 229–58 Chapter 11. [22] Iribarne AP, Iribarne JV, Anthony EJ, Blondin J. Journal of Energy Resource Technology 1994;116:278. [23] Anthony EJ, Iribarne AP, Iribarne JV, Jia L. Fuel 1997;76:603. [24] Cumberland DJ, Crawford RJ. The packing of particles. Amsterdam: Elsevier, 1987. [25] Iisa K, Tullin C, Hupa M. In: Anthony EJ. (Ed.), Proceedings of the eleventh international conference on FBC, ASME, Montreal, Quebec, April 1991. p. 83–90. [26] Allal KM, Abbessi M, Chadi H, Mansour A. Bulletin Society Chim, France 1991;128:880. [27] Laursen K, Frandsen F, Larsen OH. Energy and Fuels 1998;12:429. [28] Kunii D, Levenspiel O. Fluidization engineering. 2nd ed. Butterworth-Heinemann, 1991. [29] Anthony EJ, Jia L, Preto F, Burwell S. Journal of Waste Management 1999;19:293.