Sulfation phenomena in fluidized bed combustion systems

PERGAMON

Progress in Energy and Combustion Science 27 (2001) 215–236 www.elsevier.com/locate/pecs

Sulfation phenomena in fluidized bed combustion systems E.J. Anthony*, D.L. Granatstein CETC, Natural Resources Canada, 1 Haanel Drive, Nepean, Ont., Canada K1A 1M1 Received 3 November 1999; accepted 15 May 2000

Abstract Fluidized bed combustors (FBCs) are noted for their ability to capture SO2 in situ via direct reaction with Ca-based sorbents. However, despite more than 30 years of intensive study of sulfation processes in atmospheric FBC boilers and numerous laboratory studies, there are still many uncertainties and disagreements on the subject. In particular, the mechanisms of the sulfation reaction are still not properly understood, and there is dispute over the explanation of the well-known temperature maximum for optimum sulfur capture found in FBC boilers. This paper discusses these points of contention and suggests the most probable mechanisms and explanations for the various phenomena seen with sulfur capture, based on current literature and personal experimentation. 䉷 2001 Elsevier Science Ltd. All rights reserved. Keywords: Sulfation; Fluidized bed combustors; Limestone; Sorbents

Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. The sulfation reaction and mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Limestone and the sulfation process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Limestones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Limestone performance in a FBC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. The calcination reaction and sulfation process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Oxidizing and reducing conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. The sulfur capture efficiency temperature maximum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Limits to sulfation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Fragmentation and attrition of sorbent particles in FBC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. Agglomeration due to sulfation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9. CaO–ash reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10. Reactivation methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

215 216 218 218 218 219 221 223 225 226 227 230 231 232 232 232

1. Introduction

* Corresponding author. Tel.: ⫹1-613-996-2868; fax: ⫹1-613992-9335. E-mail address: [email protected] (E.J. Anthony).

Fluidized bed combustors (FBCs), whether circulating, bubbling, pressurized or at atmospheric pressure, have the ability to capture SO2 in situ by use of a sulfur sorbent which is typically either a calcitic limestone or dolomite. Other

0360-1285/01/$ - see front matter 䉷 2001 Elsevier Science Ltd. All rights reserved. PII: S0360-128 5(00)00021-6

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Table 1 Ca–S speciation as a function of temperature (here “⫹” represents “greater than”) Temperature range (⬚C)

Oxidizing conditions

Reducing conditions

⬍ 450 ⬍ 650–700

CaSO3 CaSO3, CaSO4 CaSO4

CaSO3, CaSO4, CaS

700⫹ 830⫹

CaS, CaSO4

sorbents have been considered, but price and availability have favored Ca-based sorbents, especially where the sorbent is not to be regenerated [1]. Work on sorbent regeneration and regenerable sorbents is beyond the scope of this review, and readers interested in the subject in the FBC context are referred elsewhere [2–7]. Here regenerable sorbents are defined as sorbents specifically designed and used with the aim of sorbent regeneration in mind, and sorbent regeneration is a process in which the reacted sorbent is converted back to its original state. The economic barrier to alternative sorbents is important, as FBC competes with other technologies such as pulverized coal (PC) combustion with backend flue gas desulfurization (FGD), or integrated gasification combined cycle (IGCC). Any significant increase in the cost of the FBC sulfur capture process itself is likely to favor one of the other technologies. In atmospheric FBC boilers sulfur capture occurs via the following overall reactions:

The present discussion provided here is devoted mainly to atmospheric FBC (currently the more commercially significant of FBC systems) except where consideration of sulfation at pressure may elucidate some feature. Readers interested in pressurized FBC (PFBC) can find valuable discussion of the issues surrounding sulfur capture elsewhere [8–11]. The sulfation reaction is far from quantitative; typically 30–40% CaO conversion is obtained, but for atmospheric systems, is effectively enthalpy-neutral in that on a molar basis 2–3 times as much CaCO3 must be calcined as is converted to CaSO4. This relatively low utilization of limestone is one of the major limitations of the technology, and will be discussed later.

2. The sulfation reaction and mechanisms

…4b†

Depending on the temperature and partial pressure of SO2, various chemical species can be produced at steady state conditions by reaction of SO2 with CaO, as indicated in Table 1. Temperature ranges in Table 1 should be regarded as approximate since different workers have quoted various temperature limits for the formation and oxidation of CaSO3, and CaS. They have also proposed multiple reaction schemes, involving various gaseous intermediates including elemental S under reducing conditions. The absolute proportion of any particular species will also change as a function of temperature; for instance, CaSO3 oxidation to CaSO4 becomes increasingly important above 560⬚C [12–14]. It should also be noted that many of the solid–solid and solid–gas reactions might not be equilibrated because of insufficient particle residence time. For example, high levels of unreacted dolomite (41%) were found in the cyclone ash from the Tidd 70 MWe PFBC plant [15]. In practice FBCs are usually operated in the 800–950⬚C range, under overall oxidizing conditions; however, localized reducing conditions are known to exist. These occur as a result of variations in gas concentrations in the dense bed that arise from bubble phenomena, air staging and the existence of volatile plumes. 2 Thus, while the main product of the sulfation process is always CaSO4, some (usually much less than 1%) CaS is normally found in FBC residues. Some sulfide may also result from the disproportionation of CaSO3 [12,13,16–18]. In limited experiments involving the addition of CaSO3 waste to a bubbling FBC, there were indications of a small increase in the CaS production and a near-quantitative conversion of the added CaSO3 to CaSO4 [19]. The sulfation process can follow a number of routes. One

1 Dolomites decompose endothermically over a wide temperature range up to about 620⬚C to form a mixture of calcium carbonate and magnesium oxide or calcium and magnesium oxides depending on the temperature and partial pressure of CO2.

2 It should be noted that even in circulating FBC, a dense bed will typically exist at the bottom of the riser, and strong reducing conditions will prevail either lower in the bed, below the point where secondary air is introduced or at the walls.

CaCO3 ˆ CaO ⫹ CO2

DH ˆ 182:1 kJ=gmol

CaO ⫹ SO2 ⫹1=2O2 ˆ CaSO4

…1†

DH ˆ ⫺481:4 kJ=gmol …2†

In pressurized systems, operating at pressures of 1– 2 MPa, calcination of CaCO3 does not occur. Here the reactions can be described, depending on whether either calcitic or dolomitic stones are used, 1 by the following equations, respectively: CaCO3 ⫹SO2 ⫹1=2O2 ˆ CaSO4 ⫹CO2

…3†

DH ˆ ⫺303 kJ=gmol CaMg…CO3 †2 ˆ CaCO3 ·MgO ⫹ CO2

DH ˆ 132 kJ=gmol …4a†

CaCO3 ·MgO ⫹ SO2 ⫹1=2O2 ˆ CaSO4 ·MgO ⫹ CO2

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of the earliest discussions of the possibilities was by Moss [20,21] who examined mechanisms wherein reaction proceeded via either the formation of CaSO3 or SO2 first converting to SO3 and directly reacting with CaO to form CaSO4. These routes can be represented as CaO ⫹ SO2 ˆ CaSO3

…5†

CaSO3 ⫹1=2O2 ˆ CaSO4

…6†

or SO2 ⫹ 1=2O2 ˆ SO3

…7†

CaO ⫹ SO3 ˆ CaSO4

…8†

He suggested that reactions (5) and (6) are important at lower temperatures where CaSO3 is stable, while processes (7) and (8) dominate at higher temperatures (850⬚C⫹) because of the thermal instability of CaSO3 [20,21]. Such an argument is dubious, however, since examples of reactions proceeding via short-lived intermediate species abound. The second route was supported by Burdett [22] and more recently by workers at the Technical University of Delft [23–25], although Lin [24] suggested that both mechanisms might operate, based on work by Hansen [26]. Burdett [27] even went as far as to suggest that SO2 must react to form SO3 before reacting with CaO; however, most other workers have preferred the idea that reaction of SO3 with CaO is simply faster than that with SO2. The importance of the second route is, however, highly questionable for a number of reasons. First, SO3 is normally present in FBC at levels well below the equilibrium levels (typically an order of magnitude less than equilibrium levels [26,28]) and work carried out in a sand bed suggests it is mainly formed catalytically over surfaces [28]. Second, even if SO3 were to be formed at equilibrium levels (e.g. via a catalyst) this is only equivalent to about 10–5% conversion of the SO2 at typical FBC temperatures employed to achieve optimum sulfur capture (850–900⬚C). More importantly, while any SO3 formed in a FBC can react directly with CaO to form CaSO4, a study carried out by Dennis and Hayhurst [29] indicated that the rate of reaction between SO2 and CaO was independent of the concentration of O2. This is evidently not what one would expect if SO3 was a necessary intermediate for the sulfation reaction. They also found the rate of reaction of CaO with a SO2/O2 mixture to be similar to that of direct reaction with SO3. This was confirmed in a more recent and detailed study on the reactions of SO3 with CaO by Allen and Hayhurst [30,31]. They therefore concluded that sulfation occurred via the formation of CaSO3, but also included the following reactions: 4CaSO3 ! CaS ⫹ 3CaSO4 CaS ⫹ 2O2 ! CaSO4

…9† …10†

217

Their work further suggested that ionic species such were involved in the sulfation process at as Sy On⫺ x higher levels of conversion. It is interesting to note that earlier workers studying the rate of reaction of SO3 and SO2, with Zn, Mg and Ca oxides found the rate of reaction with CaO to be only slightly greater for SO3 than for SO2, over the temperature range 600– 1100 K [32]. Nonetheless, the idea that SO3 is especially reactive has more recently received support from some thermogravimetric analysis (TGA) work by Wieczorek-Ciurowa using calcined limestone sulfated in the presence of a platinumtreated asbestos catalyst [33]. She noted a large increase in the initial conversion of CaO to CaSO4, and an overall increase in the final sulfation levels by a factor of two, in experiments carried out at 830⬚C in which the sulfation behavior of a limestone was compared with and without the presence of a Pt catalyst. She suggested that sulfation normally proceeds via reactions (5), (6), (9) and (10), but in the presence of enhanced SO3 levels, reaction (8) can be important. The explanation for the higher overall level of conversion is based on the concept that SO3 reacts more readily with CaO than SO2 at FBC conditions, and arises from earlier work [34–37]. However, as noted earlier this idea is explicitly rejected by Dennis and Hayhurst [29]. Further they note that Fieldes and co-workers earlier found a decrease in the overall conversion of limestone with increasing SO3/ SO2 ratios [35,38], which is diametrically opposed to the findings of Wieczorek-Ciurowa of increased overall conversion [33]. There is no clear explanation for the findings of Wieczorek-Ciurowa [33], who worked with chemically pure limestone of similar particle size to the materials studied by Fieldes et al. [35]. There are also various ways one might criticize this finding. For instance, simple calculations show that even at equilibrium levels the SO3 concentration under her conditions can only be of the order of 11% or so of the SO2 concentration [26]. However, the higher conversion was presumably not an effect of mass transfer since the Pt asbestos catalyst for SO3 production was not placed in physical contact with the limestone-derived particles. Nor could the catalyst produce sintering, as might be the case if the samples were coated directly with a Pt salt. Ultimately, the work of Dennis and Hayhurst [29] seems to be definitive that SO3 is not a necessary precursor for sulfation at FBC conditions. Nonetheless, studies such as Wieczorek-Ciurowa’s [33] show that without a clear consensus on mechanisms, different researchers working with different limestones can produce contradictory results. In the absence of detailed knowledge of the way any given study was carried out, or of the characteristics of a particular limestone, it is extremely difficult to resolve issues relating to mechanisms for sulfation in a FBC and the lack of agreement on mechanisms exacerbates this problem.

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Table 2 Limestone classification based on average grain size Description

Name

Size (mm)

Micrograined Fine-grained Medium-grained Coarse-grained

Calcilutite Calcilutite Calcarenite Calcirudite

Boynton [39] ⬍4 4–50 50–250 ⬎250 (to about 1000)

3. Limestone and the sulfation process 3.1. Limestones Limestones vary greatly in properties, and the interested reader is referred to the books of Boynton [39] and Oates [40]. There are two types of limestone, based on two crystal forms of CaCO3, namely calcite (rhombohedral) and aragonite (orthorhombic), although aragonite is metastable in the presence of water, and slowly recrystallizes to the calcite. Most limestone occurs in the form of calcite, which is typically the sorbent used in FBC. Another Ca-based sorbent is dolomite. When chemically pure, it has the formula CaMg(CO3)2 and the crystals are rhombohedral, but Ca:Mg molar ratios normally range from 1 to 1.2, and many limestones have some dolomitic content. Limestones also vary considerably as to impurities and other properties. Oates [40] for instance lists six different methods of classifying limestones. One of the more interesting criteria for classification, from the point of view of sulfation is probably the average grain size, although there is no exact agreement as to the cutoff in grain size for a given category (Table 2). Most limestone has low porosity, and Boynton [39] quotes values ranging from ⬍1% to about 9%, while Oates [40] suggests a range of 0.1–30% for limestone, and 1–10% for dolomitic stones. It is probably fair to say that no powerful method of classifying limestones as sulfur sorbents exists, to compare with, for example, the concept of “coal rank” in coal combustion science, nor is there a general method of classifying limestones as sulfur sorbents. 3.2. Limestone performance in a FBC In a study carried out by the Coal Research Establishment (UK), Ford and Sage [41] concluded the following regarding limestone performance: 1. Limestone performance bears no relationship to stone geological type or chemical properties, except for cretaceous type stone or chalk, which has very high sulfation capacities. 2. Stones obtained from different locations within the same quarry can exhibit very different SO2 absorption properties, despite similar chemical properties. 3. Due to the significant contribution of transportation cost

Oates [40] 4–60 60–200 ⬎200 (to about 1000)

to the overall cost, particularly for coarse grades (whose price will be the lowest because they have been subject to minimal processing), the most cost-effective sorbent is likely the one that is locally available. However, it has been suggested that younger limestones are significantly more reactive than older ones, and this point of view is normally presented in the literature in apparent contradiction to conclusion one of the Ford and Sage study [41]. See for instance Hamer [42] and Dam-Johansen and Østergaard, [43] who examined 25 Canadian and 23 European limestones, respectively. It has been noted by Hansen [26] that geologically older limestones tend to be more compact and “almost” crystalline and this is an indicator of both low reactivity and overall conversions. This conclusion is based on work of Dam-Johansen and Østergaard [43]. In fact these workers do not use the word crystallinity. Instead, they discuss compactness and physical texture of the limestone, and they also find a negative correlation between sulfation capacity and calcination times, and note that older limestones tend to be more compact and, therefore, calcine more slowly. Hamer [42] expresses his view on the effect of geological age on sulfation capacity as follows: “As a general rule of thumb, the younger the geological age of the rock, the greater its porosity, and the sulfur capture”. In a study on the sulfur capacity of limestones using a temperature-controlled oven, Bulewicz [44] noted that marble achieved the lowest overall conversions of the sorbents she examined, and chalk the greatest. This provides further support for the view that geologically younger limestones tend to be better sulfur sorbents and they also tend to be less crystalline. The consensus view is, therefore, that older limestones tend to be less reactive, but it is evident that different workers explain this in somewhat different ways. Conclusions two and three of Ford and Sage’s study [41] have received important support from a study carried out by Pennsylvania State University [45]. In this study 24 local sorbents (49.6–99.4% CaCO3 content) were tested at the bench-scale and in the 30 MWe CFBC boiler operated by the Westwood Generating Station. These workers found no simple chemical or physical indicator from the bench-scale tests that could be used to reliably rank the performance of limestones in the full-scale unit. In the case of bench-scale reactors or TGAs in the study

E.J. Anthony, D.L. Granatstein / Progress in Energy and Combustion Science 27 (2001) 215–236 Table 3 Molar volumes (cm 3/gmol) Species

CaCO3

CaO

CaSO4

CaS

Molar volume

36.9

16.9

46.0

28.9

of sulfur capture, the Pennsylvania State University workers note that it is difficult to get such equipment to operate in the same regime as a real CFBC boiler. Expressed another way, these workers point out that it is difficult for bench-scale reactor to mimic actual field conditions (e.g. combustor temperature, gas composition, continual reduction in particle size due to sorbent attrition and particle residence time. Further, the ability to correlate bench-scale data with fullscale data is compromised by the accuracy and precision of the full-scale data (e.g. ability to accurately measure the fuel and sorbent feed rate and the amount of sulfur fed to the comubstor for example). They also point out that chemical composition and limestone purity provided no indication of the performance of any limestone as sulfur sorbent. Interestingly, they failed to find any correlation between sulfur capture performance and calcination times in contrast to the work of Dam-Johansen and Østergaard [43]. A final caveat on comparing limestone performance in a bench- or pilot-scale unit to that in a commercial unit is to ensure chemical steady state, as sorbent may only be removed slowly from the bed. A simple formula to calculate the bed replacement time or mean sorbent residence time, is as follows: % Replacement ˆ 100{1 ⫺ exp…⫺t=t†}

…11†

where t is the duration of the run at thermal steady state, and t the mean sorbent residence time or bed residence time, which can be approximated by using the bed mass or hold-up divided by the bed withdrawal rate. Schouten and van

219

den Bleek [25], using this type of approach, note that t can vary from 1 to over 100 h, and explicitly warn against validating models with data from large-scale FBC combustors, because the results may not have been obtained at true steady state. They recommend a minimum of five bed turnovers before data collection. However, this is arbitrary. Workers at Queen’s University for example use 3 bed turnovers (95% bed replacement) [46,47]. It should be noted that formulae like Eq. (11) are considerable oversimplifications. In reality, it is likely that particle residence times may adopt a bimodal distribution, with fines (i.e. particles with diameter of less than 100 mm) spending only seconds in the combustor, while larger particles may be held there for significantly greater times. Furthermore the size distribution of the sorbent particles may change significantly due to attrition and fragmentation, although there is evidence, which will be discussed later in the section on attrition, that sulfation dramatically reduces limestone attrition. Lyngfelt and Leckner [48] carried out work in a 40 MW CFB. They determined that the major fraction of the sorbent (80–85%) had a residence time of one hour or more (with a peak of 32 h), and mean residence times for commercial boilers were found to approach 10 h [49]. Another consequence of the long residence time of limestone-derived particles in FBC boilers is that sulfur capture is not significantly influenced by position at which the limestone has been injected, although other emissions may be [50]. 3.3. The calcination reaction and sulfation process Table 3 presents molar volume data for solid reactants, intermediates and products in the calcination and sulfation processes. On calcination limestone changes from a relatively nonporous solid to a porous calcine, which will then sulfate. The molar volume of the sulfate being greater than

Fig. 1. Typical sulfation pattern for limestone particle.

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Fig. 2. Sulfation patterns according to Laursen [61].

that of either CaCO3 or CaO leads to the process of pore plugging, which prevents further conversion. A nonporous calcitic limestone might be expected to give a calcine with a porosity of about 36%, providing the limestone particles do not shrink on calcination. In practice figures of up to 55% are found for the porosity of commercial quicklimes [40]. Couturier [51] notes that the assumption that limestone particles do not shrink generally holds in the absence of significant impurities in the limestone, for the temperature range of 750–1000⬚C, and short calcination periods (⬍2 h). In consequence, typical non-porous limestone particles will, in a matter of minutes, become highly porous at atmospheric FBC conditions (800–950⬚C). This occurs because the equilibrium pressure of CO2 above the limestone will exceed the usual value of 15 kPa found in a combustor operating at temperatures in excess of 780⬚C. It should be noted that in the matter of shrinkage, as in many other respects, limestones can be expected to be very variable in their behavior. Murray et al. [52], in a study of 43 limestones, noted that some particles can show significant signs of shrinkage within an hour down to about 925⬚C. They also noted that the particles of other limestones even showed expansion at 950⬚C and that attempts to correlate the variation of shrinkage behavior with chemical composition, spectrographic analysis, geological characteristics and insoluble residues were not successful. The sulfation process is normally viewed as continuing until significant blocking of external pores occurs, leading to the formation of an impenetrable CaSO4 shell which leaves a significant amount of unreacted CaO core (Fig. 1). Based

on the data in Table 3, the maximum possible conversion for nonporous limestone ought to be 69%, although in practice much lower conversion figures are typical for FBC. Studies using 32SO2 and 34SO2 convincingly demonstrate that coupled diffusion of Ca 2⫹ and O 2⫺ ions occurs through the CaSO4 product layer to the CaSO4/gas interface [53,54]. However, this work has been carried out at a temperature of 1300⬚C, and it is difficult to ascertain its relevance to typical FBC conditions. It is, therefore, assumed in this paper that the classical picture of SO2 diffusing through the CaSO4 product layer to reach unreacted CaO is the correct one. Dolomites are generally more reactive on a Ca molar basis than calcitic limestones. 3 This is presumably due to the first step of the calcination process in which the dolomite calcines to form MgO·CaCO3 and, as MgO does not sulfate at FBC conditions (MgSO4 is unstable above 760⬚C), it enhances sorbent porosity and assists the sulfation process [56]. In practice, dolomitic stones have not found wide use in atmospheric FBC because, on a mass basis, they are not particularly effective due to the inability of MgO to react with SO2. Interestingly, industrial experience [8] suggests that calcitic limestones perform fairly well in PFBC situations. This has been explained on the basis that back diffusion of CO2 via reaction (3) helps to maintain an open pore structure upon sulfation and such an effect has been detected at the bench scale [57,58]. However, these studies were 3

However, this is not always the case; for instance, Salatino and his co-workers carried out TGA work on a dolomite which achieved a conversion of only 25% at 850⬚C [55].

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221

Fig. 3. Phase diagram for CaSO4 stability over typical FBC conditions.

carried out on micron-sized limestone particles, rather than on particles more typical of FBC systems. It should be noted that the simple core shell picture of sulfation is an over-simplification. Thus Pickles et al. [59] noted that for smaller particles (500 mm and lower), sulfation was continuous throughout the particle. More recently, work at Chalmers University has shown that there is a significant difference in the sulfation patterns seen in limestone particles under alternating oxidizing and reducing conditions [60]. This work indicated that particles sulfated under oxidizing conditions showed relatively little indication of sulfation internally, and what did occur was concentrated primarily around edges and cracks. In contrast, under alternating oxidizing/reducing conditions, much greater penetration of sulfur into the internal structure of the limestone particles was apparent. Possible explanations offered by Mattisson [60] included:

4. Or that CaS formation in the product layer may create cracks or disruptions in the crystal structure ensuring a higher rate of gaseous or ionic transport through the product layer.

1. Cyclic release and uptake of sulfur promoted greater penetration of sulfur into the particle than did sulfation on oxidizing conditions. 2. The direct conversion of CaO to CaS followed by oxidation to CaSO4 was more effective than reaction (2). 3. The heat release and temperature rise (measured as an increase of 1–15⬚C) associated with the oxidation of CaS to CaSO4 may have effected the transport of gases through the product layer, and perhaps influenced the average pore size of the sorbent ensuring enhanced sulfur capture.

4. Oxidizing and reducing conditions

However, as Mattisson himself noted [60], such suggestions must only be regarded as speculations at this point. Finally, work done at the University of British Columbia on sorbent reactivation showed three distinct sulfation patterns [61]: the classic core shell pattern; uniform sulfation; and sulfation through networks of cracks in the calcined limestone structure (Fig. 2). The full implications of this for sulfation studies is not yet clear, but at the very least, any attempt to model sulfation behavior should account for the possibility that a particular limestone may be affected by sulfation patterns.

At typical FBC temperatures (800–950⬚C) and overall oxidizing conditions, CaSO4 is the favored final product of the sulfation reaction and is thermodynamically stable, although its stability decreases with increasing temperatures (Fig. 3) [51]. Much of the early understanding of the FBC process has been obtained from TGA, differential reactors under oxidizing conditions. This implies that overall oxidizing conditions (in which a particle may be subject to periods under reducing conditions) are equivalent to continuously

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˚ mand [73]). Fig. 4. Sulfur capture efficiency temperature maxima (modified from Leckner and A

oxidizing conditions for the sulfation process such as would be seen in a TGA. While the existence of localized reducing conditions due to volatile plumes or poor fuel distribution in fluidized beds has long been known [62,63], their significance was not at first fully recognized. Realization that recurring reducing conditions could prevail for substantial periods, even under overall oxidizing conditions in the dense bed in both bubbling and circulating FBC systems, developed with the use of electrochemical sensors. Results from such sensors indicated that partial pressures of O2 could drop to levels of 10 mPa or less for up to 80– 90% of the time, despite overall oxidizing conditions prevailing. Initially, much of the work focussed on the potential for in-bed corrosion due to such conditions [64,65]. However, Lyngfelt and Leckner at Chalmers University suggested that significant reduction of CaSO4 could occur even at temperatures of the order of 850⬚C

[49,66–69]. In one experiment, for example, they showed that if the limestone feed was stopped for several days and the bed temperature raised to 920⬚C, the spent sorbent might actually become a source of SO2. In this context it is worth noting that it has been long known that above 900⬚C CaS can react with CaSO4 to release SO2 [70]. CaS ⫹ 3CaSO4 ˆ 4CaO ⫹ 4SO2

…12†

Another conclusion of their most recent work is that significant recapture of SO2 from the bottom bed due to reductive decomposition might occur in the more oxidizing splash zone, again highlighting the importance of reducing processes in determining the overall performance of a FBC boiler [69]. Hansen [26] examined the influence of periodic oxidizing/reducing conditions on sulfation in a FBC, finding no effect on overall sorbent performance, in disagreement with

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the earlier preliminary findings of Jonke et al. [62]. Mattisson [60] recently revisited this subject, concluding in contrast to Hansen [26], that periodic oxidizing/reducing conditions can influence sulfation capacity of some sorbents. In particular, Mattisson’s work has been used to explain anomalous sulfur capture results first obtained by Mjo¨rnell et al. [71] for a 40 MWth CFBC, then later confirmed on a 165 MWth CFBC boiler [60]. In these trials two limestones (Ignaberga and Ko¨ping) achieved similar sulfation levels. This was significant because one of the limestones was very porous and reactive, while the other, a highly crystalline limestone (Ko¨ping), performed very poorly in a TGA (10–27% utilization depending on particle size). Previously, this result had been tentatively explained on the basis that the crystalline limestone had decrepitated and attrited, in consequence, displaying superior performance in the CFBC boiler. However, as a caveat, Mjo¨rnell and her coworkers [71] also noted the actual performance of the Ko¨ping limestone in the CFBC boiler was better than the best behavior of “fines” in the TGA tests. Mattisson [60] confirmed that overall conversion levels of Ko¨ping limestone doubled in tests carried out under alternating oxidizing/reducing conditions. He was also able to rule out any effect of boiler residence time by carrying out long sulfation trials. These showed that, for 0.5–0.7 mm Ko¨ping limestone, sulfation increased from levels of 9% only to 12% for a change in sulfation duration from 2 to 40 h. While Mattisson’s findings are in complete contradiction to Hansen’s work, it can be pointed out that Hansen [26] did in fact obtain evidence of increased sulfation levels due to oxidizing/reducing conditions, using old crystalline limestone, and this result was rejected for reasons that are unclear. One can hypothesize that more crystalline limestones may be more susceptible to the effects of oxidizing/reducing conditions. However, the significance of Mattisson’s work is that, if TGA or similar evaluations of limestone sulfation capacity are to be generally useful, one must clarify when the effects of oxidizing/reducing conditions are likely to have an important effect on sulfur capture.

5. The sulfur capture efficiency temperature maximum It has been known for over 20 years that sulfur capture in atmospheric FBC systems is strongly influenced by temperature, typically with a maximum sulfur capture efficiency at about 850⬚C (Fig. 4) [72,73]. However, variation in the actual temperature at which this occurs is considerable depending on the limestone and the unit [74]. Yates [63] noted that the presence of a temperature maximum could not be explained on the basis of thermodynamic instability of CaSO4 under oxidizing conditions over typical FBC operating parameters. Instead, he speculated that the reverse sulfa-

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tion reaction might provide an explanation: CaSO4 ⫹CO ˆ CaO ⫹ SO2 ⫹CO2

…13†

It should be noted that reaction (13) is functionally equivalent to: CaSO4 ⫹4CO ˆ CaS ⫹ 4CO2

…14†

3CaSO4 ⫹CaS ˆ 4CaO ⫹ 4SO2

…12†

The concept that reducing reactions might be involved in causing the sulfur capture efficiency temperature maximum is supported by Chalmers University [66,67,69] and others [26,74]. Lin [24] has summarized other explanations for the AFBC sulfur capture efficiency temperature maximum: 1. Sintering of sorbent particles is enhanced at higher temperatures resulting in lower porosity and surface area, hence reducing the overall conversion of limestone. 2. SO2/SO3 equilibrium determines the maximum, with higher temperatures reducing the availability of SO3 for reaction with CaO. 3. High temperatures result in an enhanced sulfation rate which causes small pores to become blocked, hence preventing entry of SO2/SO3 to the interior of the calcined limestone particle. 4. Oxygen depletion in the dense phase of the bed, due to increasing volatile combustion at higher temperatures, inhibits sulfation above 900⬚C. In this theory it is supposed that coal volatiles and oxygen can coexist below 850⬚C, allowing sulfation to proceed, while at temperatures above 900⬚C, oxygen is depleted thus diminishing the production of CaSO4 and allowing regeneration of CaO through reduction of CaSO4 by CO. That sintering determines the capture efficiency temperature maximum can probably be rejected immediately. The shape of the curve for the temperature maximum varies greatly in FBC systems (Fig. 4) [73], and it is difficult to believe that sintering is so powerfully affected by a 20 or 50⬚C change for example. An alternative formulation is that more porous calcines are formed at lower temperatures and this idea was supported by Mulligan et al. [75]. However, the balance of the literature seems to support the idea that sintering is typically not pronounced much below 1000⬚C and with residence times under 2 h [51], and this view received early support from Hartman and Trinka [76]. Further, strong evidence against such structural impairment theories also comes from work of Lyngfelt and Leckner [66]. They showed that limestone sulfated at 850⬚C in a 16 MWth FBC released SO2 when bed temperature was increased to 930⬚C; returning to 850⬚C, the limestone recovered its sulfation capacity. Similar observations were made by Dennis and Hayhurst [77] in experiments in which a sorbent in a FBC burning coal failed to undergo significant sulfur capture at 1000⬚C. However, the recovered sorbent

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particles performed similarly to fresh sorbent sulfated at 850⬚C when they were re-sulfated in a bed fluidized by gases containing SO2 at 850⬚C. It thus seems reasonable to reject such theories as not generally valid. The ability of sorbents to recover their sulfation capacity also argues against early ideas that the sulfation efficiency temperature maximum was due to a competition between SO2 diffusion into the particle, and reaction in narrow-mouthed surface pores [78]. That is, it should not be possible for the “limestone” to recover if all of its smaller surface pores are filled with a CaSO4 product layer, thus blocking access to the core of the sorbent particle. That SO3 is involved in determining the sulfur capture efficiency temperature maximum can also be rejected because, as discussed earlier, evidence in the literature is unequivocal that formation of SO3 is nonessential to the sulfation process. Interestingly, the idea was rejected by Mulligan et al. [75] on the basis that, if it were the case, the temperature maximum would be the same for all limestones, since it would be controlled by a gas phase reaction, not by limestone properties. There is more support for the view that an accelerated sulfation reaction resulting from increasing bed temperature blocks the surface pores and decreases the overall rate of conversion. The idea received early support by Ulerich et al. [78], as noted earlier, and later by Burdett [27] who further assumed that SO3 was the critical intermediate and that SO2 oxidation to SO3 was faster at higher temperatures. This hypothesis has also received more recent support by workers at Pennsylvania State University, who carried out sulfation tests in a quartz fluidized bed reactor using synthetic flue gases [79]. In criticizing this concept, Mattisson [60] noted that a number of studies carried out under oxidizing conditions have failed to find as dramatic a fall-off in the SO2 capture efficiency with rising temperature as is normally seen in fluidized beds burning fuel. He suggests that this is evidence for some other more important mechanism determining the temperature maximum. Dennis and Hayhurst [77] had made a similar observation, running experiments on bench-scale fluidized bed reactors, noticing a much more dramatic falloff in the sulfur capture maximum when coal was fired than when sulfation was achieved with synthetic flue gas. Finally, Hansen et al. [74] report evidence that the sulfur capture maximum is strongly influenced by the design of the combustor, which would seem to rule out theories based on limestone particle modification. The particulate phase oxygen depletion theory of Dennis and Hayhurst [77] can explain the decrease of sulfation at higher temperatures. However, it was developed for the case of a BFBC, and it is not clear exactly how to apply it to CFBC conditions, where sorbent particles spend considerable time in nominally oxidizing conditions. It is also interesting to note that Allen [80] remarks that the oxygen depletion theory implies that the rate of sulfation would be

dependent upon the O2 concentration in the bed, a hypothesis that was disputed by the same authors in a latter publication [77]. 4 In consequence, Lyngfelt and Leckner [66,67] and Hansen et al. [74] offer the suggestion preferred here. That is the temperature maximum can best be regarded as competition between sulfation and reduction, with reduction becoming more important at higher temperatures. Recently, Allen and Hayhurst [30,31] have offered an alternative explanation of the temperature maximum. They have suggested that, at up to 850⬚C, sulfation proceeds via CaSO3, reaction (5); above this temperature sulfation is directed toward CaSO4 and CaS, at a rate that is only 1/5 that of reaction (5). Unfortunately, it is quite difficult to reconcile these observations with the earlier ones of Lyngfelt and Leckner [66,69]. It is also difficult to understand how such a finding would account for significant differences in the sensitivity of the sulfur capture temperature maximum curve to change either as a function of limestone or of the combustion system. In a study of the reduction of CaSO4 by CO, Hayhurst and Tucker, noticed an acceleration of the rate of decomposition above 830⬚C, which they suggested might be caused by a melt [5]. Similarly, Kamhuis et al. [82] explained their results on the decomposition of CaSO4 in terms of the existence of a CaSO4 –CaS melt. More recently, Davies and co-workers [83,84] carried out work on CaS destruction. They suggest reaction (12) might become important, perhaps with the formation of a liquid melt of CaS and CaSO4, at some threshold temperature above 830⬚C. This process might serve as a new explanation for the sulfur capture maximum near 850⬚C. 5 They also note that the formation of a melt might cause pore plugging in the sorbent, thus further reducing mass transfer rates for SO2 and O2 into the core of the sorbent particle. They have further supported these claims by finding evidence of a melt from SEM and EDAX measurements on pellets made of equal quantities of CaS and CaSO4, heated to 1000⬚C in a TGA [84]. It should, however, be noted that Steenari [85] has observed that while the literature on eutectic melting in the CaS–CaSO4 system is not in complete agreement, melting has generally been observed to occur at temperatures higher than 1000⬚C. In addition, her own work on the 4

There is a referencing problem here. Both Allen [80] (page 13) and Lin [24] (page 23) attribute this theory to Dennis and Hayhurst, 1984 [77]. In Allen’s thesis this is a paper in Instn Chem Engnrs Symp Ser [81], which is in fact on the high-pressure sulfation behavior of limestone, and in Lin’s thesis the reference does not exist. Therefore, the first public domain reference to this theory appears to be the Twentieth International Combustion Symposium paper quoted by Allen as a 1985 publication, which in fact provides a contradiction of this theory. 5 It could also be used to explain the difference between the findings of Wieczorek-Ciurowa [33] who carried out her tests on Pt catalysis at 830⬚C and the earlier studies of Fieldes et al. [35] discussed earlier in this paper.

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Fig. 5. Percent conversion of CaO to CaSO4 in three bed ashes. Sulfated in a temperature-controlled oven.

CaS–CaSO4 system, carried out over a temperature range of 400–1100⬚C under both reducing and oxidizing conditions employing high temperature X-ray diffractometry, failed to find any evidence of eutectic melting in the absence of alkali metals. Similarly, two groups that have recently examined the sulfidation of Ca-based sorbents, failed to find any evidence of a melt caused by reaction (12) [86,87]. Therefore, while the work of Davies and co-workers [83,84] suggesting a liquid melt of CaS and CaSO4 does provide a possible explanation for the sulfur capture efficiency temperature maximum, their explanation must be treated with some caution at this time. The above explanations for the sulfur capture efficiency temperature maximum are by no means exclusive, and Lyngfelt and Leckner [66] discuss some other less likely theories. However, at this time the consensus view is that the maximum seen in FBC is due to a competition between sulfation and reduction reactions. The present discussion should, however, clearly indicate that there is still considerable dispute on this subject. Finally, the existence of a temperature maximum for sulfur capture efficiency in experiments carried out under oxidizing conditions is also somewhat of a concern, as theories that invoke reducing conditions do not apply to such situations. However, it has been noted such temperature maxima are less pronounced, although this is based on a fairly limited data set [26,60]. It seems, therefore, that there are two distinct temperature maxima. One occurs under oxidizing conditions, due to chemical processes associated with the sulfation reaction and for which the explanations of

Allen and Hayhurst [30,31] may well be correct; and another more pronounced maximum occurs under actual combustion conditions, and this must be superimposed on the “oxidizing” temperature maximum.

6. Limits to sulfation It is well known from batch experiments carried out in TGA, differential reactors, etc. that limestone sulfated under oxidizing conditions will attain a maximum conversion in typically under 90 min and thereafter fail to react further [26,42,51]. The concept of a maximum conversion level for limestone has also been employed in many modeling efforts as noted by Mattisson and Lyngfelt [88]. Nonetheless, a common view has been that a maximum conversion limit is an artifact of the system and reaction conditions, and that the final fate of a limestone particle is quantitative conversion via solid state diffusion through the product layer [89]. Moreover, in studies carried out by De Hemptinne [90] on nonporous CaO, it was found that a low fractional order (0.2–0.25) could be used to fit their results for SO2 diffusing through a dense CaSO4 product layer. With nonporous limestone that does not undergo significant sintering on calcination, 69% conversion corresponds to total pore blockage by CaSO4 product [60]. However, Duo et al. [91,92] suggested that there is a certain maximum conversion, Xm for typical gas–solid reactions, which is frequently less than 100%. They have postulated both a maximum product layer thickness for any given set of

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Fig. 6. Typical TGA sulfation curve for NSPI limestone.

conditions, and a competition between the chemical driving force for a reaction and the mechanical work that must be done to disrupt or displace the product layer. This idea suggests there is an effective conversion limit, well below the limit demanded by complete pore plugging, which cannot be exceeded for any given conditions for the sulfation reaction. However, new experimental evidence indicates there is no upper limit to the sulfation reaction until complete conversion has been achieved, and that conversions greater than 69% can occur with long sulfation times (days to weeks). These results come from two sources: studies on deposits obtained from FBC boilers, which have achieved near quantitative conversions to CaSO4; and extended sulfation tests in which bed materials and limestones from FBC boilers have been subjected to sulfation conditions for similar times [93–96]. Fig. 5 shows the percent conversion to CaSO4 of bed ashes obtained from the Nova Scotia Power Inc. (NSPI) 165 MWe CFBC boiler, the 100 MWe Nelson Industrial Steam Company (NISCO) boiler, and a 0.8 MW CFBC pilot plant, sulfated in a temperature controlled oven [94]. More recent work by Anthony and Jia [96] has also demonstrated that sulfation levels under such conditions show fall-off at temperatures above 900⬚C. It should be noted that conversions well above 69% imply that if sulfation occurs over a long enough period, particle expansion must occur [60]. Anthony and Jia [95,96] have also examined the process of long-term sulfation under conditions for which CaCO3 is stable, and found that carbonation appears to reduce the overall degree of conversion to CaSO4. It is of interest that this near-quantitative conversion has been noticed for CaO in hot gas filters from PFBC units using dolomitic limestones. Here, sulfation over filter cake can be associated with a significant amount of the sulfur capture [89]. An important difference, however, between this situation and that of the AFBC case is the formation

of the salt Mg2Ca(SO4)3 [97] which has not been detected in deposits from atmospheric CFBC boilers or in the deposits produced in long-term sulfation tests. Given that typical residence times in boilers are of the order of hours, it might be assumed that the long-term sulfation process described above has no implications for sulfur capture. However, this view has been recently disputed by Abanades et al. [98]. These workers have looked at sulfation for periods of up to 24–60 h using a TGA. They have designated the sulfation that occurs over this period as residual sulfation to distinguish it from the short term and rapid sulfation process that typically occurs within the first 1– 2 h after calcination (Fig. 6). These workers have modeled the sulfation process over the first 24 h, noting that particles over 200 mm can have residence times of the order of 10– 20 h in an industrial CFBC boiler. Using their TGA data for residual sulfation they conclude that if residual conversion is included in a typical sulfation model for such periods, substantial increases in sulfation may be expected. As an example based on their data, they calculate that increases in the overall conversions of the order of 10% are possible for sorbents over those which would be calculated based on typical TGA test data for short-term sulfation over the first hour or so. These workers have also concluded that shortterm conversion seen in a typical TGA test has no relationship to the long-term conversion, and the same conclusion has been reached by Anthony and Jia [96].

7. Fragmentation and attrition of sorbent particles in FBC Fragmentation and attrition are potentially important phenomena in determining sulfur capture performance of limestones. There are in fact some limestones which decrepitate so severely on calcination that they are likely to

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perform very poorly as sulfur sorbents [40]. Generally, a primary difference between limestone and fuel with respect to combustion performance in FBC is that, whereas fragmentation and attrition of fuel particles usually lead to combustion inefficiency, this is not necessarily the case with limestone, although such phenomena can cause significant loss of unreacted sorbent. In an early study on attrition of seven European limestones, Spitsbergen et al. [99] noted an inverse relationship between time for complete calcination and the degree of entrainment of fines. For this work, which employed a 40 mm diameter bench-scale quartz reactor operated at 850⬚C, calcination times ranged from 9 to 17 min. In a later study of eight Canadian limestones by Karidio [100], using a circulating transport reactor built to simulate a CFB combustor, it was noted that at normal bed temperatures (850–950⬚C), calcination is normally complete in 20 min. This work also suggests that the mechanical action of the FBC is normally necessary to promote fragmentation. To demonstrate this point, experiments were carried out in which limestone samples were heated in a muffle furnace. Under these circumstances, there is relatively little size reduction of limestone samples, although it is evident that the heating rate in such an apparatus must be considerably less than in a FBC. Karidio further noted that the primary breakage of the limestone particles studied occurred within the first 5 min, and that whereas calcination weakened the limestones, sulfation increased their resistance to attrition [100]. The fact that sulfation decreased the attrition rate of sorbents was noted much earlier [101], although the explanation for the effect differed. Chandran and Duqum suggested that sulfation makes the surface harder, while Karidio suggests that sulfation fills mechanical cracks in the surface, thus increasing the strength of the sorbent particle [100,101]. Additional findings of this work were that increased impurities from about 4 to about 18% by weight, led to more attrition resistance in the limestones studied, and that the primary fragmentation occurring in the first 5 min was independent of operating conditions. From this latter result Karidio concluded that fragmentation depended essentially on the limestone properties, and always occurred as long as “a minimum amount of energy” is available to split the particles [100]. Salatino and his co-workers have revisited the subject of fragmentation and attrition of sorbents most recently [102– 104]. Their work has included both the bubbling and circulating FBC case, and they have also investigated the behavior of dolomite as well as calcitic limestones [105]. Their basic conclusions are as follows: 1. Dolomitic limestones behave very similarly to calcitic limestones from the point of view of fragmentation and attrition (although for the dolomitic stone examined there was no evidence of primary fragmentation). This is in contrast to work by Hu and Scaroni [106] using a laser

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heating technique, which suggested that dolomitic limestones are particularly susceptible to fragmentation due primarily to the overpressures generated by calcination. Possible criticisms of the use of lasers are that the heating rate can be expected to be different, and the type of heating will be different (i.e. directional, as opposed to uniform external heating). 2. Primary fragmentation occurs immediately after the injection of the particles into the bed, as a consequence of thermal shock and the internal overpressure caused by CO2 generation, and this may produce both coarse and fine particles. (However, primary fragmentation tends to be minimal in BFBC in the absence of the mechanical shocks that are experienced in CFBC or transport reactors such as used by Karidio [100]). This is followed by continued but decreasing loss of material, due to the removal of surface irregularities, which reaches a steady-state value when the calcined particle is “rounded off”. 3. Sulfation brings about a dramatic reduction in fines produced by surface abrasion, by an order of magnitude or more. In a sense this behavior parallels that of coal fragmentation and attrition, where devolatilization is the major cause of fragmentation, but unlike coal, where combustionassisted attrition dramatically enhances attrition rates and hence enhances combustion inefficiency, here sulfation dramatically reduces sorbent attrition. These workers also note that early studies of limestone particle attrition in “inert” or non-sulfating conditions dramatically overestimated attrition rates. They also point out that the texture and particle size of the fresh sorbent crucially affects primary fragmentation, and also the subsequent comminution of the sorbent. Consequently they suggest the characterization of any sorbent from the point of view of attrition be carried out at appropriate temperatures and with suitable heating rates. While it is clear that fragmentation producing fines is likely to have a deleterious effect on sulfation, a number of workers have suggested that fragmentation may contribute significantly to improving sorbent performance by reducing the effects of pore plugging [100,107]. Pisupati et al. [107] have gone as far as to suggest that limestones which are particularly susceptible to thermally induced fractures (TIF), may be regarded as particularly good sorbents.

8. Agglomeration due to sulfation A particularly important example of high-sulfur fuel used in CFBCs is petroleum coke. This fuel may have 5–8 wt% sulfur content, and there are now more than a half dozen commercial FBC units operating worldwide burning this fuel [108,109]. A survey sent to commercial operators of petroleum coke-fired CFBCs [108] indicated that a number

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Fig. 7. Temperature-controlled oven for long-term sulfation work.

of these boilers experienced significant fouling problems. These problems were usually attributed to the high V content in the petroleum coke ash causing the formation of low melting point vanadates [110]. However, a study of deposits from a 120 t/h steam capacity CFBC boiler burning 100% petroleum coke [93] suggests another explanation. The V in the deposits is present as high melting point Ca

vanadates and, therefore, unlikely to form low melting point eutectic mixtures. Furthermore, the ash generated in a CFBC firing petroleum coke is almost entirely limestonederived since petroleum coke typically has less than 1% ash. The Ca in the deposits is nearly quantitatively converted to CaSO4. Such levels of sulfation of limestone-derived particles must result in particle expansion, and it was suggested

Fig. 8. EDS mapping of Ca and S distribution of agglomerated CFBC bed ash samples sulfated for 100 days at 850⬚C: (a) Ca; (b) S.

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that this expansion was the source of the agglomeration in the FBC system [93–95]. 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 and would provide an alternative explanation for the agglomeration process [111]. 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 through the particles, through cracks or networks, or via the formation of an outer sulfate shell with an unreacted CaO core [59–61]. 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% [61]). However, this is the sulfation pattern followed by the most susceptible bed material studied by Anthony and his co-workers [94,95]. A possible explanation to 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. 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 currently available information does not deal with initiation or deposition processes, an area requiring further study. Nonetheless, 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, leading to defluidization. In order to study the agglomeration and densification process, samples of ash from a number of FBC and CFBC units and a variety of limestones, including those used in the FBC and CFBC units were selected. These were subjected to long-term sulfation in crucibles placed in a temperaturecontrolled oven (Fig. 7) [94–96]. The oven was operated for times up to 105 days and at typical FBC temperatures (850–950⬚C). This work showed that both the ashes and limestones achieved similar levels of sulfation as those seen in boiler deposits, and formed hard deposits with low porosities. Energy-dispersive X-ray spectroscopy (EDS) showed the deposits consisted of uniformly sulfated particles (Fig. 8). Further work was done on the ashes from and limestones used in the NISCO boilers, with sintering assessment based on the compressive strength tests of heattreated cylindrical ash pellets [111]. 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 reaction between CO2/SO2 and calcined sorbent (CaO) can contribute significantly to sintering over the appropriate FBC temperature ranges.

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Both studies also demonstrated the important fact that agglomeration is possible in the presence of very low levels of Na, K or V; i.e. the elements that are often associated with the formation of low melting point eutectics, or significant ash softening. Here low levels are taken to mean that those elements are present at levels of several hundred parts per million or less. If Na and/or K are present at higher levels (i.e. the percent level), they can contribute significantly to the agglomeration process, i.e. they act synergistically, causing agglomeration to occur at lower levels of sulfation than otherwise would have this effect [94]. It should also be noted that the work described here does not deal with any possible effects such substances might have on initiation of the agglomeration process. All of these studies [93–96,111] can be criticized for not taking into account the possible effect of reducing conditions, or the effect of periodic oxidizing-reducing conditions. However, such deposits are often formed high in the combustor (e.g. on superheater tubes) or in the backpass where such effects ought to be minimal. It is, therefore, reasonable to suppose that overall conditions are oxidizing all or most of the time in such regions and thus conclude that such effects are not necessary to explain the agglomeration observed in real boilers. 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. However, in full-scale boilers there are always dead zones or quiescent regions. The long-term sulfation studies of Anthony et al. [94–96] have clearly shown that, if sorbent particles are held under such conditions, agglomeration due to the sulfation process alone takes place. Therefore, when agglomerated deposits of almost pure CaSO4 are found in a boiler, it appears unnecessary to invoke any other explanations. Finally, it is also well known that when a deposit is formed, it creates its own distortions and dead zones in the gas flow patterns of the boiler, allowing the deposit to grow more rapidly [112]. This test work has also demonstrated that after continuous sulfation 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 suggests that there is an effective SO2 penetration depth of about 4–5 mm. It also appears 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 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 [94–96]. The work by Anthony and his co-workers [94–96] indicated that the tendency to agglomerate varied with different sorbents, with some limestones and bed ashes showing significantly greater agglomeration tendency than others.

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Fig. 9. Free lime and OCC contents from the NSPI CFBC boiler taken at monthly intervals over a one-year period.

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 [113,114]. The long-term oven test work has also shown that agglomeration is possible over a considerable temperature range (650–950⬚C), and that the strength of deposits increases as a function of temperature up to 950⬚C. 6 In addition, strength development and degree of sulfation appeared to be only weakly influenced by the SO2 partial pressures. Finally, the CaSO4 itself appears not to cause agglomeration, although if compressed by external pressures of the order of 150 kPa CaSO4 could be made to form weak deposits over the temperature range of 850–950⬚C [95,96].

9. CaO–ash reactions It was suggested early on that CaO might react with ash components [72,115] at FBC conditions, and this was the basis for an early explanation of the sulfur capture maximum. It is also clear from the cement literature that FBC boilers operate in the temperature range where CaO and fuel ash might be expected to form belite (Ca2SiO4) and other compounds [116]. More recently, ignition experiments carried out at 927⬚C with mixtures of silica, alumina, CaSO4 and coal have shown it is possible to release 30% 6

Recent unpublished work by the authors of this article has found one limestone with some dolomitic character for which the strength falls with increasing temperature (from 850 to 950⬚C).

of the sulfur due to the formation of Ca silicates and aluminosilicates [117]. Nevertheless, such results have largely been ignored and it is common for instance when calculating the Ca conversion to use the absolute value of the Ca or CaO content in a solid residue. In bench- and pilot-scale units, this may well be a good approximation. Work carried out on samples from a pilot-scale CFBC found only very low levels of conversion (2%) of CaO to Ca silicates, aluminates, aluminosilicates, and ferrites which are termed here other calcium compounds (OCC) [118]. In contrast, work carried out on the 165 MWe CFBC boiler owned and operated by NSPI has consistently shown very high levels of these compounds [119]. This unit burns a high-sulfur bituminous coal and uses a highcalcitic limestone (95% CaCO3). Results obtained over several years show that 30–50% of the apparent free lime content that would be determined by difference is present as OCC. Alternatively expressed, the free lime content determined by a chemical method (e.g. the sucrose method) is about 30–50% less than obtained by calculating it by difference as shown in Eq. (15): True ash free lime content ⬍ ‰CaOŠT ⫺{‰CaOŠCaSO4 ⫹ ‰CaOŠCaS ⫹ ‰CaOŠCaCO3 }

…15†

where [CaO]T is the total amount of Ca in the ash, expressed as CaO, and ‰CaOŠCaSO4 ; [CaO]CaS and ‰CaOŠCaCO3 are CaSO4, CaS and CaCO3 content of the ash expressed in terms of CaO equivalent. Thus, up to 10–15% of the CaO in the system is not readily available for sulfation (other work has shown that some of the OCC sulfates much less readily, if at all, than does the parent limestone [120]). Fig. 9 presents some

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Table 4 Potential sorbent enhancement methods Method

Originator

Limitations

Precalcination of sorbent under controlled CO2 atmosphere Addition of halides or alkali metal salts, such as carbonates Hydration by means of steam or water

First observed by Westinghouse Electric Corporation [121] First suggested by Argonne National Laboratories [122] First suggested by Argonne National Laboratories [123–125]

4.

Fine grinding and pelletization of spent sorbent

Proposed and tested by CANMET [42]

5.

Grinding and reinjection of spent sorbent

6.

Pelletization of fly ash

7.

Using steam in situ in an external heat exchanger or similar structure Complete hydration of ashes to deliver a dry product using the CERCHAR hydrator Use of landfilled ashes

Not known, but first used by Combustion Power Company on a commercial scale [127] Proposed by Community Energy Alternatives Incorporated [128] Suggested by the University of British Columbia, under contract to ABB [129] Suggested by CANMET [127] and explored by Ahlstrom Pyropower [130] Suggested by CANMET [120]

Development of specialty apparatus, which may be prohibitively expensive. A strong potential for both corrosion and fouling, due to the presence of alkalis or halides. A requirement to develop a specialty hydrator. The suggested route of Ahlstrom Pyropower, in a report to the CEA [126]. This approach has been shown to be effective, but costly, as this work was done prior to the widespread use of CFBC; there is also some question of how robust the pellets of reactivated material need to be. Produces only limited reactivation, some of which is probably due to the effects of thermal stresses on the particles due to cooling. Relatively effective, but requiring the use of a pelletization technique. Probably easily applicable only to Lurgi or similar CFBC designs, but having considerable potential. Possible control problems. Very effective but expensive, and justifiable only if the CERCHAR hydrator is being used to condition the ashes at the site. Positive results expected, due in part to ettringite formation in the ashes, which has been shown to be a very effective sorbent in FBC. However, tested only at bench-scale.

1. 2. 3.

8.

9.

results for samples of bed and fly ash collected over a 12month period from this unit. Similar results indicating high levels of conversion of CaO in bed ashes to OCC have been obtained from the 160 MWe TVA bubbling bed [95].

10. Reactivation methods The low utilization of limestone (typically 30–40%) in a FBC is a limitation for the technology. Usually ashes must be landfilled, at relatively high cost in some cases (from $10–20/t [108]). Thus, a considerable incentive to improve the overall utilization exists. Numerous methods have been considered, ranging from: improving the pore size distribution of the sorbent by treatment with alkali metal salts; controlling the calcination process to modify pore size distribution by the use of additives; and hydration of spent sorbent to crack the sulfate shell around the unreacted CaO core. Table 4 gives a list of the various processes considered by the authors as most interesting, and a comparative assessment of them. This list does not include methods that involve, for instance, decomposing and regenerating the CaO, or modifying the combustor or cyclone substantially, as this paper concentrates on reactivation technologies which could be

used with existing CFBC boilers. It should also be noted that techniques such as cool-side humidification [131], or the use of spent sorbent in flue gas desulfurization systems [132], which also appear to be feasible, are similarly excluded from the present discussion. In practice, only reactivation by hydrating the ashes using either water or steam has shown promise. Water treatment leads to the formation of Ca(OH)2, whose larger molar volume causes expansion within the particle, cracking the sulfate shell [123]. Typically, after hydration with water, the Brunauer–Emmett–Teller (BET) surface area increases from values of around 1–2 to 10 m 2/g or more and workers at the University of New Brunswick [133,134] found an almost linear improvement in the sulfur capture performance of spent sorbent with increasing hydration levels. Reactivation methods usually concentrate on treating bed material which may represent about 30% of the solid residues or more when firing a high-sulfur fuel in a CFBC (for petroleum coke-fired units levels of up to 50% are not uncommon). The choice of reactivating only bed material is made for two reasons: first, the bed materials will usually have a higher CaO content; second, even if the finer fly ash is hydrated its short residence time may preclude it as an effective sulfur sorbent. Despite its technical merit, sorbent reactivation has not yet been commercialized for essentially

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economic reasons and most of the current effort is directed at overcoming this limitation. There are, however, a number of new insights on this subject that merit attention. First, as noted above, significant amounts of the apparent analytical CaO in the bed material may actually be present as OCC. Further, CaO itself is capable of undergoing reaction with ash components which can modify its behavior as a sulfur sorbent [120]. Thus, before reactivation methods can be employed effectively on a commercial basis, it is necessary to study the hydration chemistry of these materials. This is borne out by new results from Cracow University of Technology. These show that the amount of free lime found in a reactivated spent sorbent can be dependent on the hydration method [135]. Finally, some recent work carried out by Laursen et al. [61] has shown that, for steam reactivated samples which were previously sulfated in a packed bed reactor, there is a significant difference in sulfation performance depending on the manner in which the limestone originally sulfated. Limestones which sulfate producing an unreacted CaO core and sulfate shell show the highest ability to reactivate following steam treatment. Limestones that sulfate via a network of pores are intermediate in character, whereas limestones which sulfate in a uniform fashion show little or no improvement in sulfur capture performance after steam reactivation.

11. Conclusions This paper has discussed various aspects of the sulfur capture process in FBC. Despite over 30 years of serious study of sulfation phenomena, there are still major uncertainties in this area. At this time, the mechanisms of the sulfation process are not fully understood. Nor is there agreement on the origin of the well-known sulfur capture efficiency temperature maximum seen near 850⬚C. However, the idea that this arises from a competition between the sulfation reaction and reductive decomposition superimposed on a more gradual maximum produced by a change in reaction mechanism around 850⬚C, seems most likely. New results from Chalmers University on the influence of periodic oxidizing and reducing conditions on sorbent performance have also shown that this may be potentially important, in contradiction to earlier findings. Similarly, findings on the existence of OCC, and a variation in resulfation performance depending on the “sulfation pattern” of the limestone also suggest the sulfation behavior in FBC is more complicated than previously thought. This paper has also suggested that better methods for limestone characterization are needed, as are more in-depth studies of the fundamentals of the sulfation process. Despite the considerable attention this subject has received, it deserves further study.

Acknowledgements The authors would like to thank Dr Jia Lufei of Natural Resources Canada, and Mr Qiu Kuanrong of Chalmers University, Sweden for their assistance during the preparation of this paper, and Professors E.M. Bulewicz (Cracow University of Technology, Poland) and A. Lyngfelt (Chalmers University) for many helpful suggestions during the course of writing this paper.

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