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Sulphur capture in circulating fluidized-bed boilers: can the efficiency be predicted?
Chemical Engineering Science 54 (1999) 5573}5584
Sulphur capture in circulating #uidized-bed boilers: can the e$ciency be predicted? Anders Lyngfelt*, Bo Leckner Department of Energy Conversion, Chalmers University of Technology, S-412 96, Go( teborg, Sweden
Abstract The present state of understanding of sulphur capture in #uidized-bed combustion is discussed with focus on the possibilities to predict sulphur capture performance and the e!ect of intermittent reducing conditions. The four key factors that determine the sulphur capture performance are: (i) particle size distribution, including the e!ect of particle size reduction, (ii) residence time as a function of particle size, (iii) reactivity as a function of particle size, and (iv) e!ect of reducing conditions. The "rst three of these can be determined and included in a model for prediction of the sulphur capture, but the problem of how to include the e!ect of reducing conditions in a model is still unresolved. The conclusion must be made that sulphur capture performance in CFBBs cannot safely be predicted at present, and more research is needed before the e!ect of reducing conditions can be incorporated in modelling. Nevertheless, existing models can be useful to approximate the e!ects of, for instance, particle size reduction or changes in particle residence time. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Sulphur; Fluidized-bed combustion; Limestone; Sorbent; Circulating #uidized-bed boiler
1. Introduction Research related to sulphur capture in #uidized-bed combustion (FBC) has been going on for more than three decades: The research is comprehensive and includes a large number of journal publications, reports and theses. As an introduction it is suitable to pinpoint the important questions relevant to sulphur capture in FBC in practice. Two such questions are: E Is it possible to increase the e$ciency of sulphur capture? E Is it possible to predict the e$ciency of sulphur capture? Other important questions are related to modi"cations of boiler design and operation. How is the sulphur capture a!ected by: E scale-up? E measures to reduce N O and NO emissions? E co-"ring with bio-fuels? * Corresponding author. Tel.: 00-46-31-772-1427; fax: 00-46-31-7723592. E-mail address: [email protected] (A. Lyngfelt)
E the fuel distribution inside the combustion chamber? E incomplete secondary air penetration? E the #uidization conditions? There are also a number of questions related to the in#uence of limestone addition on ash properties: E Are the properties of the waste material suitable for disposal or reuse? E Under what conditions may deposits and agglomerations form? In connection with more advanced FBC technology there are further questions. Pressurized FBC was introduced to increase the e$ciency of power generation. In pressurized stationary or circulating #uidized-bed boilers (FBBs) the sulphur capture process is quite di!erent from that under the atmospheric conditions. The partial pressure of CO is too high to allow calcination of the sorbent and there is a direct sulphation of calcium carbonate. This works better than could be expected, but the mechanism is not well understood at present (Iisa, 1992; Yrjas, 1996). Cycles with partial gasi"cation may further increase the e$ciency of power generation. In this case calcium sulphide is the primary product formed in the gasi"er. The problem is then to obtain su$cient
0009-2509/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 9 - 2 5 0 9 ( 9 9 ) 0 0 2 9 0 - 0
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conversion of the sulphide to sulphate in the oxidizing reactor (Ninomiya, Dong, Hashimoto & Sato, 1997; Davies, Hayhurst & Laughlin, 1994). The present paper will not try to cover all these areas. Instead, it concentrates on a few topics related to the initial questions: What is the state of modelling of sulphur capture? Is it possible to predict sulphur capture performance? Can the results obtained with various limestones and boilers be explained by available data? Size distribution, size reduction, residence time and reactivity of sorbent particles will also be discussed, since they are important for modelling. Another key issue for modelling is the e!ects of alternating oxidizing/reducing conditions in the combustion chamber. Is it possible to include these e!ects in modelling? In order to understand the conditions for sulphur capture in circulating #uidized-bed boilers (CFBBs) some experiences from stationary #uidized-bed boilers (SFBBs) are useful and will also be discussed below.
2. Areas of research The limitations of various research approaches should be observed. A schematic overview of the "eld of research is given in Fig. 1. The large bulk of published results is within the upper left box: laboratory work. This research includes chemical and physical investigations of limestone as well as methods to determine its reactivity. Much work has also been directed towards describing the conversion of the limestone by modelling, upper right-hand box (e.g. Bhatia & Perlmutter, 1981; Georgakis, Chang & Szekely, 1979; AdaH nez, GayaH n & GarcmH a-Labiano, 1996a). The models are mainly concerned with the gradual change of the physical structure as a product layer of sulphate develops in the initially porous structure. The purpose of these limestone reactivity models is to increase the understanding of the sulphation process. Another use of the models would be to provide sulphur capture models with in-put data of limestone reactivity, dotted arrow in Fig. 1. However, the limestone models have not really been used in sulphur capture models, for two reasons: (i) the limestone models are complex and therefore di$cult to incorporate in sulphur capture models and (ii) the models often do an excellent job in explaining the sulphation reaction for a certain limestone and size, but are not applicable over a wide range of limestone types and particle sizes (AdaH nez et al., 1996a). Therefore, a more simple and direct approach has been used in sulphur capture models, that is, to measure the reactivity versus conversion in the laboratory and to approximate the results by some simple function that can be included in the sulphur capture model. A number of such sulphur capture models have been proposed (e.g. Lee, Hodges & Georgakis, 1980; Schouten & Bleek van den, 1987; AdaH nez, de Diego,
Fig. 1. Research on sulphur capture.
GayaH n, Armesto & Cabanillas, 1996b). They are normally based on similar basic assumptions: a "rst-order reaction rate with respect to SO concentration and a reaction rate constant which is a function of conversion. The missing link in the study of sulphur capture is validation data for sulphur capture models from actual combustors. There are a number of reasons for the lack of detailed data from boilers: Existing commercial boilers are not suitable for research, for instance, they may not have means for adequate measurements of entering and exiting solid #ows, such as those of coal, limestone, #y ash, and bottom ash. Large-scale tests are expensive and di$cult to organize. Furthermore, the reproducibility of large-scale tests is not so good*there is normally some spread in the results. It is most likely, however, that unpublished data exist and have been used internally by boiler manufacturers for model validation. Nevertheless, a few studies with detailed data have been published, and these will be discussed below. Most of the work discussed so far was made in the 1970s and 1980s, and this work did not consider the conditions in combustion chambers. The fact that the bed material in a FBC chamber is subject to conditions which alternate between oxidizing and reducing was given minimal attention until the end of the 1980s. The e!ect of these alternating conditions is important and will be discussed in detail below, but "rst some general remarks on limestone reactivity studied under oxidizing conditions will be made.
3. Limestone reactivity under oxidizing conditions An attempt to summarize the comprehensive work on limestones will most certainly do injustice to many important results. In addition, there is an inherent di$culty in summing up the experiences; limestones have highly variable properties, with conversions that may range from 0.1 to 0.8 (Dam-Johansen & "stergaard, 1991), and do not easily conform to some general theory. One of several important pioneering research groups in the "eld
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is the one at Westinghouse. Their summary of the state of knowledge of limestone reactivity, based on experiences with 120 sorbents (Keairns, Newby & Ulerich, 1983), is still valid. Some important conclusions were: E `Grain size, porosity and impurity level of the raw sorbent appear to be key parameters but have not yet been correlated to sorbent performance.a E `Basic reaction mechanisms (for sulphation) are as yet unresolved.a E `The rate of calcination and resulting internal structure are currently not predictable.a Nonetheless the complex sulphation process with di!usion through pores, being gradually plugged by a sulphate layer, is well understood, at least under oxidizing conditions, and is described by a number of models (AdaH nez et al., 1996a). In order to reduce the amount of raw material and waste material in sulphur capture, work has also been dedicated to: E Reactivation (Julien, Brereton, Lim, Grace, Chin & Skowyra, 1995). Sorbent particles which are inactivated by a more or less impermeable sulphate layer, can be reactivated by hydration with steam. By hydration the CaO in the particles is converted to Ca(OH) , and when the particles are reintroduced into the furnace the reactivity is increased, which leads to a more e$cient use of the limestone. E Additives. Addition of salt to promote the conversion is one example (Shearer, Johnson & Turner, 1979), although the usefulness of adding chlorine to a boiler can be questioned. E Regenerable sorbents. Schemes to use a regenerable sorbent have also been proposed (Korbee, 1995). In this case SO is produced from decomposition of the sulphate in a separate reactor by means of a reducing agent.
4. Reducing conditions Boilers are operated with excess air. However, as a consequence of the combustion, zones with a de"cit of oxygen will occur locally (Makarytchev, Cen, Luo & Li, 1995). Firstly, such zones will appear in the close vicinity of burning or devolatilizing fuel particles. Secondly, these zones may be extended because of imperfect mixing. One important example is the by-pass of air through the bubbling bottom bed, resulting in inadequate exchange of the gas in the particle phase. Another example is inadequate fuel distribution in large combustion chambers (Couturier, Doucette, Stevens, Poolpol & Razbin, 1991). Insu$cient secondary air penetration may be an additional source for inadequate mixing. Air-staging, if
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used, also promotes the formation of locally reducing conditions. Measurements in the beds of circulating and stationary FBBs with a zirconia-cell oxygen probe, show that the environment can be reducing 80% of the time (Lyngfelt, Bergqvist, Johnsson, As mand & Leckner, 1993). In the case of a stationary FBB, reducing conditions predominated even under unstaged conditions at an overall air-ratio of 1.4 (Cooper & LjungstroK m, 1987). Reducing conditions in the bubbling bed of the stationary and circulating FBBs are explained by the by-pass of air according to the two-phase model and depletion of oxygen in the particle phase due to combustion (Avedesian & Davidson, 1973; Lyngfelt, As mand & Leckner, 1996). In the case of circulating FBBs, air-staging further contributes to a more reducing environment in the bottom bed. Although only the bottom bed is predominantly reducing, these reducing conditions may penetrate to some extent all the way up to the cyclone in streamers of oxygen-de"cient gas (Lyngfelt, As mand, MuK ller & Leckner, 1997). The oxygen-probe measurements not only show the fraction of time under reducing conditions, but also that the periods of reducing/oxidizing conditions may be short, typically 0.1 s. The actual length of the time periods may be even shorter, but the measurements are restricted by the response time of the probes. It should be observed that the probe measures in a "xed position, whereas the sorbent particles move between various regions in the combustion chamber, including the recycling system (Weinell, 1994). This can be stated in a general way, and the overall e!ects are described below, but little is known in detail about the changing conditions experienced by a certain lime particle in the bed.
4.1. Temperature dependence and reducing conditions Sulphur capture in FBBs depends on bed temperature with an optimum in the range of 820}8503C, and above this optimum there is a dramatic decrease in e$ciency. Fig. 2 shows that the temperature dependence varies from one boiler to another. This comparison between two circulating and a stationary FBB, where the same limestone was used, shows that the temperature dependence is smaller in the circulating FBBs. It was "rst assumed that the temperature dependence of sulphur capture was a `propertya associated with the limestone. A comparison, however, shows that the marked fall in sulphur capture performance with temperature in FBBs is not observed in laboratory experiments made under constant oxidizing conditions (Lyngfelt & Leckner, 1989a). The temperature dependence is caused by the in#uence of reducing conditions, predominantly in the bottom bed. The di!erence between the stationary and the circulating FBBs is
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Fig. 2. The sulphur retention as a function of temperature: 165 MW CFBB using KoK ping (䊐) and Ignaberga (*) limestone; 40 MW CFBB, using KoK ping (䊏) and Ignaberga (䢇); 16 MW SFBB using Ignaberga with Ca/S"1 (;) and Ca/S"2.5 (#).
Fig. 3. SO versus height in the furnace for a sand bed and an old lime bed at 8503C. The old lime bed contains CaSO which is decomposed in the bottom bed and formed again in the splash zone.
related to the extent to which the sorbent particles are exposed to reducing conditions. The mechanism behind the temperature dependence is that increased temperature promotes the reductive decomposition of CaSO by CO or H (e.g. Ghardash khani, LjungstroK m & Lindqvist, 1989): CaSO #COPCaO#SO #CO . (1) Boiler tests designed to investigate this mechanism provide evidence for this reductive decomposition (Lyngfelt & Leckner, 1989b, 1993). Tests in both circulating and stationary FBBs show that at increased temperature the sulphur retention, de"ned as the fraction of added sulphur which is captured, may become negative, indicating a net release of sulphur (captured prior to the temperature increase) from the sorbent in the bed. Such a release can be maintained for hours after a temperature increase, but it will eventually subside. Measurements inside the combustion chamber of a circulating FBB indicate that reductive decomposition of CaSO in the bottom bed occurs even at 8503C, as shown in Fig. 3 (Lyngfelt & Leckner, 1998). Here the sulphur release from an old lime bed, i.e., a bed with sulphated sorbent obtained by stopping the limestone addition for two days, is demonstrated by a comparison to a sand bed free of sulphate. The sand bed does not interact with the sulphur and the old lime bed had reached steady-state conditions so that the net release of SO from the bed material, release minus capture, is zero. The much higher concentration of SO just above the bottom bed with old lime is explained by reductive decomposition of the sulphate in the bottom bed, while the subsequent decrease is explained by a recapture of the SO by the decomposition product CaO in the splash zone where mixing is improved.
Fig. 4. Sulphation of limestone under alternating conditions (Mattisson & Lyngfelt, 1999). The inlet SO concentration is 1500 ppm (dashed line).
Laboratory experiments, where limestone was sulphated under conditions alternating between oxidizing and reducing, show that SO is released during the shifts between oxidizing and reducing conditions, as shown in Fig. 4 (Hansen, Dam-Johansen, Bank & "stergaard, 1991; Hansen, Dam-Johansen & "stergaard, 1993; Mattisson & Lyngfelt, 1999). At higher temperatures the release of SO during the shifts is so fast that in practice no net conversion to sulphate takes place. 4.2. Air-staging and reducing conditions As a consequence of the large impact of reducing conditions on the sulphur capture process, the total airratio and the extent of air-staging become important
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Fig. 5. SO emission (left axis) and sulphur retention (right axis) as function of case of air-staging and bed temperature 8503C (䢇) and 9303C (䉱). 12 MW CFBB.
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The e!ect of air-staging on sulphur capture has been studied further in connection with a method, `reversed air-staginga, for obtaining low N O emissions from FBC (Lyngfelt, As mand & Leckner, 1998).While the emission of N O decreases with raised bed temperature or lowered air-ratio, the emission of SO increases. But in contrast to the emission of SO , the emission of N O is not much a!ected by the conditions in the bottom part of the furnace. Thus, the con#ict between the desire to reduce N O emission and to maintain e$cient sulphur capture can be solved by addressing the conditions in the lower and the upper part of the combustion chamber separately. In other words, the emission of N O can be decreased by lowering the air-ratio in the upper part, while the e!ects on SO capture are neutralized if the air-ratio in the lower part is simultaneously raised.
4.3. CaS formation
Fig. 6. Fraction of time under reducing conditions at 0.65 m height (solid lines) and at 8 m (dashed lines) for the three air-staging cases. 12 MW CFBB.
(Takeshita, 1994). The e!ect of air-staging on sulphur capture has been studied in a circulating FBB (Lyngfelt & Leckner, 1993). In addition to normal air-staging with about 60% primary air, two extreme cases were studied: no air-staging where all the air was primary air, and intensixed air-staging where the primary air was lowered to about 45% and the level of secondary air addition was raised to 5.5 m. At 8503C the sulphur retention dropped to 40% under intensi"ed air-staging, compared to about 90% under normal and no air-staging, Fig. 5. The temperature dependence is clearly related to the degree of air-staging, and at a temperature of 9303C the sulphur retention was negative both for normal and intensi"ed staging, indicating a net release of sulphur from the sorbent. Zirconia-cell oxygen-probe measurements verify that the fraction of time under reducing conditions is strongly dependent on the air-staging conditions, Fig. 6 (Lyngfelt et al., 1993).
The possible formation of calcium sulphide, CaS, is also associated with reducing conditions. CaS is not stable and may decompose in the presence of moisture with release of poisonous hydrogen sulphide gas, thus causing a handling and disposal problem if the CaS content is too high. The presence of calcium sulphide in ashes has therefore been investigated (Anthony, Stephenson & de Iribarne, 1987; Anthony, Ross, Berry, Hemmings, Kissel & Doiron, 1989; Ross, Anthony, Kissel & Doiron, 1989). A study of CaS formation in a circulating FBB reveals that only small amounts of sulphide, a few percent of total sulphur, is formed, independent of the extent of air-staging (Mattisson & Lyngfelt, 1995). However, substantial amount of CaS was formed when limestone addition was stopped and the SO concentra tion increased. A similar formation of CaS after a limestone stop was also noted in one of the previously mentioned tests in the stationary FBB where as much as half of the sulphur present in the bed material was in the form of sulphide (Lyngfelt, Langer, Steenari & PuromaK ki, 1995). However, the sulphide was observed several hours after the limestone addition was stopped when the SO concentration, as a result of the limestone stop, had reached a high level. The conclusion is that signi"cant CaS formation is not expected under normal operating conditions and CaS is therefore not a problem in practice. The formation of CaS, if limestone addition is accidentally stopped, should not be a problem, since the formation is slow. Furthermore, no or little bottom ash is removed from the furnace under such conditions. Even if CaS is not present in large quantities, it is most likely an important intermediate and is therefore essential for the understanding of the sulphur chemistry in the combustion chamber (Makarytchev et al., 1995).
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5. Sorbent particle size The sorbent particle size is an important parameter which a!ects both residence time and reactivity. In the typical size range of particles in an FBB, 20}2000 lm, the residence time may vary with several orders of magnitude, and the conversion as measured in the laboratory may vary with a factor of up to three with size (Mattisson & Lyngfelt, 1998a, b). 5.1. Size reduction There may be a signi"cant reduction in the particle size from the feed size distribution to the size distribution of the particle #ow exiting from the system. Laboratory tests show that the size reduction depends on the limestone properties. It is normally rapid during the "rst minutes when calcination takes place, and much slower when the sulphation has started (Scala, Salatino, Boere"jn & Ghadiri, 1998; Couturier, Karidio & Steward, 1993; Karidio, 1994). Data on size reduction are available from one stationary and three circulating FBBs, as well as for two di!erent limestones, one soft and the other crystalline. All these cases show a signi"cant size reduction, independent of boiler design and limestone: the added sorbent particles, with an initial mass median size of about 0.6 mm, decreased to about 0.12 mm, Fig. 7 (Lyngfelt & Leckner, 1991; MjoK rnell, Leckner, Karlsson & Lyngfelt, 1991; Mattisson & Lyngfelt, 1998b). The e!ect of the added size distribution was also investigated, but the added size had a small e!ect on the resulting size distribution (MjoK rnell et al., 1991). 5.2. Residence time and conversion The sorbent particles leave the circulating FBB both as #y ash and as bottom ash. The residence time of the small #y-ash particles is mostly determined by the boiler design, cyclone e$ciency, riser height, etc. The residence time of the larger bed ash particles is given by the ratio of bed mass to the exiting bed ash #ow. The #ow of exiting bed ash depends on the entering #ows of sorbent and fuel ash and the extent of formation of smaller particles which leave the bed as #y-ash. There is probably a connection between the size reduction and the boiler design, particularly related to the cyclone e$ciency: the particles are subject to fragmentation and attrition processes which continue until the particles leave as bottom ash or until the particles are small enough to leave as #y-ash. On the other hand, the laboratory studies indicate that the attrition is slow once the sulphation has started, and part of the sorbent added also leaves the system as large particles despite a long residence time. Old sulphated sorbent particles seem to be quite resistant to attrition*attrition rates below 1%/h have been reported (Leckner, 1998).
Fig. 7. Size distribution of added limestone (Ignaberga) and sorbent particles in exiting ash for three CFBBs and an SFBB.
The residence time of sorbent particles increases with size, from a few seconds for the particles smaller than the cut-size of the cyclone, up to the point, typically 0.2}0.4 mm, where the particles are so large that they do not leave the system as #y-ash. This size is normally also the optimum size for sulphur capture (MuK nzner, Bonn & Schilling, 1985; Mattisson & Lyngfelt, 1998b). For this, and larger particle sizes, the residence time may be 15}30 h and is normally independent of size, provided that there is no active removal of larger particles by classi"ers, for instance. Below this optimum size, the conversion decreases because of reduced residence time and above this size the conversion decreases because of insu$cient penetration of sulphur to the particle's interior. Data from boilers show that the conversion may be rather independent of size as in Fig. 8 or more dependent on size as in Fig. 9. Fig. 8 shows that there is a much higher conversion of all particle sizes in the circulating FBB. The residence time for the small particles is longer in the circulating FBB, but the large particles have a much longer residence time in the stationary FBB and are therefore expected to have a higher conversion. Nevertheless, the large particles in the stationary FBB have a lower conversion, which must be attributed to the negative e!ect of reducing conditions that was also evident in Fig. 2. The higher e$ciency of sulphur capture in the circulating FBB compared to the stationary FBB, can be explained by the longer residence time for small particles in the circulating FBB and also that the particles are less exposed to reducing conditions in the circulating FBB. Although these di!erences are typical for the boiler types, they may not be inherent. For instance, a stationary FBB can be equipped with an e$cient particle recirculation system and possibly also in-bed devices to break up bubbles which would reduce the previously mentioned through-#ow.
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Fig. 8. Degree of sulphation versus particle size for a 16 MW SFBB and a 40 MW CFBB.
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Fig. 10. E!ective reaction constant versus conversion shown in dimensionless form, which enables a comparison between the approximate decay function (dashed line) and the reactivity of various particle sizes: from 1.4}2 mm (a) to 45}63 lm (k).
using a dimensionless representation, with a non-dimensional rate constant, i, and non-dimensional conversion, s: k i " G, (3) G c G s "X c . (4) G G G If these are substituted into Eq. (2) the size-dependent constants disappear and Eq. (2) is simpli"ed to i"e\Q. Fig. 9. Degree of conversion for the 12 MW CFBB compared to model results as a function of particle size.
The rate of sulphation of the sorbent is described by a "rst-order reaction dX G"k p , G K dt
6. Modelling and limestone reactivity Normally, the reactivity of limestones has been approximated by an exponential decay of the reaction rate constant with time, corresponding to a linear decrease in the rate constant versus conversion. An alternative representation (Mattisson & Lyngfelt, 1998b), shown in Fig. 10, is based on the assumption of an exponential decay of the rate constant, k , with conver sion, X: k "c e\A G6G, (2) G G where c and c are "tted functions of particle size i. G G Fig. 10 shows a comparison of Eq. (2), dashed straight line, to data of 11 particle sizes from laboratory tests of the limestone Ignaberga under oxidizing conditions. This comparison of the approximation in Eq. (2) to particles of di!erent sizes in the same diagram is made possible by
(2a)
(5)
where p is the average SO concentration to which the K sorbent is exposed. Combining Eqs. (2) and (5) and integrating yields an expression of the conversion of particle size i as a function of time. Assuming an exponential residence time distribution for this particle size, an average conversion, X , can be calculated by integrating G over all residence times:
e\R OG ln(c c p t#1) dt, (6) G G K c q G G where q is the average residence time of particles in size G fraction i. The resulting sulphur retention is then obtained by a summation of the fractional conversions of all sizes and multiplying with the molar ratio Ca/S of added limestone, c: X
" G
U g "c f X , Q G G G
(7)
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Fig. 11. Total sorbent conversion of Ignaberga limestone in the three CFBBs compared to model results.
where f is the fraction of total sorbent belonging to size G fraction i. Since the SO concentration, p , is a function K of the retention, an iterative procedure is needed. A sulphur capture model using this representation has been validated by tests with the limestone Ignaberga in three circulating FBBs, of 12, 40 and 160 MW size. In addition to detailed reactivity data in Fig. 10, the residence time and conversion were determined for the three boilers from ash analyses of 11 particle sizes in the range 0}2 mm (Mattisson & Lyngfelt, 1998b). The model results agree reasonably well with the measured conversion, as shown in Figs. 9 and 11. Although there is some underprediction of the conversion, the di!erence between the three boilers is accurately described. Similar detailed data from these boiler tests and from identical laboratory tests, were also obtained for a less reactive limestone, KoK ping, but in this case the modelling failed: In laboratory tests the "nal conversion was four times larger for Ignaberga (36%) than for KoK ping (9%), but, surprisingly enough, the di!erence between the sulphur capture performance of Ignaberga and KoK ping in the boiler tests was small (MjoK rnell et al., 1991). For KoK ping limestone the conversion in the boilers was two to three times larger than the conversion obtained in laboratory, independent of particle size, Fig. 12. Although the residence time of the larger particles is longer in the boiler, this is compensated for by a 10 times higher concentration of SO in the laboratory experiments. Even if the test period in laboratory was extended to 40 h as seen in Fig. 12, it was impossible to reproduce the much higher conversion obtained in the boilers (Mattisson & Lyngfelt, 1998c). (The di!erence between the two boilers in Fig. 12 is most likely a result of di!erent residence time; the model, when applied to Ignaberga limestone, explained the higher conversion in
Fig. 12. Degree of conversion obtained for the limestone KoK ping in two CFBBs compared to the maximum conversion obtained in laboratory versus particle size.
the 40 MW circulating FBB with the longer residence time, Fig. 11.) The obvious conclusion is that the conditions of the laboratory measurements cannot be fully relevant for what happens in a combustion chamber. Therefore, an additional study was made, where the e!ect of alternating oxidizing and reducing conditions were studied in the laboratory.
7. Limestone reactivity under alternating conditions Alternating oxidizing and reducing conditions may have a bene"cial e!ect on the sulphation (Mattisson & Lyngfelt, 1998c). The conversion of the KoK ping limestone as a function of both fraction of time under reducing conditions and complete oxidizing}reducing cycle time is shown in Fig. 13. The conversion is sensitive to the fraction of time under reducing conditions, u . Opti mum conditions with a large increase in conversion compared to constant oxidizing conditions were obtained for u "0.2}0.5, whereas the conversion at u "0.7 could be low, almost zero at shorter cycle times. X-ray mapping of limestones reacted under oxidizing conditions compared to limestones reacted under alternating conditions reveals a fundamental di!erence. While the limestone particles which were reacted under oxidizing conditions only had sulphur on the surfaces of internal cracks, as shown in Fig. 14a, the particles sulphated under alternating conditions showed a deep penetration of sulphur from the cracks, as shown in Fig. 14b. Even when the reaction time was extended with a factor of 10, to 20 h, no signi"cant penetration could be seen for the samples sulphated under oxidizing conditions, as shown in Fig. 14a. Obviously, alternating conditions promote di!usion of sulphur into the solid
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material, but the mechanism for this is not established at present. The results obtained in laboratory show a new aspect of alternating conditions, namely that they may increase the conversion of limestone signi"cantly and they also provide a likely explanation for the boiler results. The conclusion is that the high conversion obtained for KoK ping limestone in the boilers, in contrast to laboratory tests under oxidizing conditions, is most likely an e!ect of alternating conditions.
8. Pressurized conditions
Fig. 13. The conversion as a function of the fraction of time under reducing conditions after 2 h sulphation of KoK ping limestone (0.5}0.7 mm) using a reactant gas of 1500 ppm SO , 10% CO , 0}4% O , and 0}4% CO.
The work related so far has been focused on atmospheric conditions, and the question is therefore how, and if, this is applicable to pressurized conditions. It should be pointed out that the negative e!ects of alternating conditions manifested by the e!ects of increased temperature and staging are not to be expected under pressurized conditions, and consequently no temperature optimum of sulphur capture under pressurized conditions has been reported (Takeshita, 1994). Firstly, this is a simple e!ect of thermodynamics, the reductive decomposition takes place only under the combination of low oxygen partial pressure and low sulphur dioxide partial pressure (Saro"m, Goel & Morihara, 1994). Under pressurized conditions the SO partial pressure is high enough to suppress this reaction. Secondly, the #uidization pattern is di!erent in a non-circulating pressurized #uidized-bed reactor with bed internals which break up the bubbles and reduce the through-#ow. Consequently, oxygen-probe measurements indicate that reducing conditions are less prevalent in a pressurized pilot plant (Almstedt & LjungstroK m, 1987). Because the sorbent does not calcine, both the sulphur capture reaction mechanisms and the particle size reduction processes are di!erent. However, there should be no important di!erence in the application of models under pressurized conditions, except that laboratory experiments of course have to be made under pressurized conditions.
9. Discussion The four key factors that determine the sulphur capture performance are: (i) particle size distribution including the e!ect of in-bed particle size reduction, (ii) residence time as a function of particle size, (iii) reactivity as a function of particle size, and (iv) e!ect of reducing conditions. The "rst three factors can be quanti"ed and included in a model: Fig. 14. The sulphur distribution in the interior of two sulphated KoK ping limestone particles of approximate size 0.7 mm. (a) Oxidizing conditions. Total time: 20 h. (b) Alternating conditions. Total time: 2 h.
(i) The size distribution of sorbent particles leaving a given boiler can be determined from chemical
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analysis of sieved fractions of the exiting ash #ows. In the case of a planned boiler, experiences from available boilers in combination with laboratory tests of particle size reduction can be used to predict the "nal size distribution. (ii) The residence time as a function of particle size can be determined from analysis of sieved fractions of bed material and exiting ash #ows. In the case of a planned boiler, the average residence time is determined from the limestone #ow and the total amount of bed material. The actual residence times for the individual particle sizes are then derived from experiences in available boilers in combination with models describing the recirculation inside the riser and from the cyclone. (iii) The reactivity, in the form of a rate constant versus conversion, can be determined in laboratory experiments for di!erent particle sizes. The reasonably successful modelling results with Ignaberga limestone do not prove that reducing conditions can be neglected. The results obtained with KoK ping limestone reveal that there is a fundamental error in using laboratory data derived under oxidizing conditions in modelling. This leads to the question: why did the model give satisfactory results with Ignaberga but not with KoK ping limestone? Although no safe conclusions can be made, it can be speculated that this is an e!ect of the limestone properties; Ignaberga is a porous reactive limestone while KoK ping is a crystalline unreactive limestone. Alternating conditions obviously promote the mobility of sulphur in the limestone and this may have a much more signi"cant e!ect on the unreactive limestone. Available laboratory tests of Ignaberga limestone under alternating conditions, although too few for safe conclusions, indicate that this limestone is less a!ected by alternating conditions (Mattisson & Lyngfelt, 1999). Two additional questions are: (i) Are there operation conditions for which the e!ect of reducing conditions can be neglected? (ii) Can the e!ect of reducing conditions be incorporated into a model of sulphur capture? With the exception of pressurized conditions, the answer to the "rst question seems to be no. Neither increased primary air-ratio nor lower temperature is suf"cient to disregard reducing conditions: most likely a circulating FBB cannot be operated without the appearance of reducing conditions in the combustion chamber. Even if all air is added as primary air, reducing conditions are present (Lyngfelt et al., 1993). Laboratory tests show that there is a large di!erence in conversion between oxidizing and alternating conditions also at lower temperatures, e.g. 8253C (Mattisson & Lyngfelt, 1999). The second question is more di$cult to answer. One approach is to determine a comprehensive `reaction parametera versus conversion for alternating conditions
in the same way as the rate constant was obtained under oxidizing conditions. This reaction parameter would be averaged for a complete cycle of one oxidizing and one reducing period. It is di$cult, of course, to choose the proper conditions for such laboratory tests, which have to be representative for a particle in a circulating FBB. A fundamental problem with this approach, however, is that a rate constant assumes a "rst-order reaction, which is obviously incorrect in the case of this `reaction parametera: the average rate of reaction during the cycle is a complex function of two rates of capture (for the oxidizing and the reducing periods), as well as of two rates of release (during the change to oxidizing and the change to reducing conditions). Thus, a reaction parameter which averages the net of these reactions during the cycle is not a rate constant, but must be determined, not only as a function of conversion, but also as a function of gas concentration. This adds considerable experimental di$culties, since tests at low SO concentrations need corre spondingly longer test periods. Another approach is to carry out more fundamental modelling of the reactions inside the sorbent under alternating conditions, but also this modelling may prove to be di$cult in view of the complexity and the lack of detailed knowledge of the reactions. Although the overall reactions are known, the reaction mechanisms are not known. Furthermore, the reactions are heterogeneous and the conditions are transient; the compositions of both the solid phase and the gas phase change rapidly. Thus, it can be concluded that while the e!ect of reducing conditions cannot be safely neglected, there is yet no experience of how to include this e!ect in modelling.
10. Conclusion The four key factors that determine the sulphur capture performance are (i) particle size distribution including the e!ect of in-bed particle size reduction, (ii) residence time as a function of particle size, (iii) reactivity as a function of particle size, (iv) e!ect of reducing conditions. The "rst three of these items can be determined and included in a model for prediction of the sulphur capture, but the problem of how to include the e!ect of reducing conditions in a model is still unresolved. At least in the case of an unreactive limestone, it was clear that the model fails if alternating conditions are not considered. Therefore, the conclusion must be made that sulphur capture performance in circulating FBBs cannot be predicted safely at present, and that more research is needed
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before the e!ect of reducing conditions can be included in modelling. Nevertheless, existing models can be useful to approximate the e!ects of, for instance, particle size reduction or changes in particle residence time.
References AdaH nez, J., de Diego, L., GayaH n, P., Armesto, L., & Cabanillas, A. (1996b). Modelling of sulphur retention in circulating #uidized bed combustors. Fuel, 75, 262}270. AdaH nez, J., GayaH n, P., & GarcmH a-Labiano, F. (1996a). Comparison of mechanistic models for the sulphation reaction in a broad range of particle sizes of sorbents. Industrial and Engineering Chemistry Research, 35, 2190}2197. Almstedt, A., & LjungstroK m, E. (1987). Measurements of the bubble behaviour and the oxygen distribution in a pilot scale pressurized #uidized bed burning coal. Proceedings of the international conference on yuidized bed combustion, vol. 9 (pp. 575}585). Anthony, E., Ross, G., Berry, E., Hemmings, R., Kissel, R., & Doiron, C. (1989). Characterization of solid wastes from circulating #uidized bed combustion. Proceedings of the international conference on yuidized bed combustion, vol. 10 (pp. 131}137). Anthony, E., Stephenson, J. R., & de Iribarne, A. P. (1987). Calcium sulphide formation in solid wastes from circulating #uidized bed combustors. Proceedings of the international conference on yuidized bed combustion, vol. 9 (pp. 1188}1192). Avedesian, M., & Davidson, J. (1973). Combustion of carbon particles in a #uidized bed. Transactions of the Institution of Chemical Engineers, 51, 121. Bhatia, S., & Perlmutter, D. (1981). The e!ect of pore structure on #uid-solid reactions: application to the SO -lime reaction. A.I.Ch.E. Journal, 27, 226}234. Cooper, D., & LjungstroK m, E. (1987). The inyuence of bed temperature on the in-bed O2 partial pressures in a 16 MW AFBC xred with petroleum coke. Internal Report OOK A87 002. Department of Inorganic Chemistry, Chalmers University of Technology, GoK teborg. Couturier, M., Doucette, B., Stevens, D., Poolpol, S., & Razbin, V. (1991). Temperature, gas concentration and solid #ux pro"les within a large circulating #uidized bed combustor. Proceedings of the international conference on yuidized bed combustion, vol. 11 (pp. 107}114). Couturier, M., Karidio, I., & Steward, F. (1993). A study on the rate of breakage of various Canadian limestones in a circulating transport reactor. In Fourth international conference on circulating yuid beds. Somerset, PA (pp. 783}793). Dam-Johansen, K., & "stergaard, K. (1991). High-temperature reaction between sulphur dioxide and limestone*I. Comparison of limestones in two laboratory reactors and a pilot plant. Chemical Engineering Science, 46, 827}837. Davies, N., Hayhurst, A., & Laughlin, K. (1994). The oxidation of calcium sulphide at the temperatures of #uidized bed combustors. In 25th symposium (International) combustion. The Combustion Institute, Pittsburgh (pp. 211}218). Georgakis, C., Chang, C., & Szekely, J. (1979). A changing grain size model for gas}solid reactions. Chemical Engineering Science, 34, 1072}1075. Ghardashkhani, S., LjungstroK m, E., & Lindqvist, O. (1989). Release of sulfur dioxide from calcium sulfate under reducing conditions. Proceedings of the international conference on yuidized bed combustion, vol. 10 (pp. 611}615). Hansen, P., Dam-Johansen, K., Bank, L., & "stergaard, K. (1991). Sulphur retention on limestone under #uidized bed combustion conditions*an experimental study. Proceedings of the international conference on yuidized bed combustion, vol. 11 (pp. 73}82).
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Hansen, P., Dam-Johansen, K., & "stergaard, K. (1993). High-temperature reaction between sulphur dioxide and limestone*V. The e!ect of periodically changing oxidizing and reducing conditions. Chemical Engineering Science, 48, 1325}1341. Iisa, K. (1992). Sulphur capture under pressurised yuidized bed combustion conditions. Ph.D. thesis, As bo Academy University, Turkku, Finland. Julien, S., Brereton, C., Lim, C., Grace, J., Chiu, J., & Skowyra, R. (1995). Spent sorbent reactivation using steam. Proceedings of the international conference on yuidized bed combustion, vol. 13 (pp. 841}849). Karidio, I. (1994). Sulphation and breakage characteristics of various Canadian limestones. Ph.D. thesis, University of New Brunswick, Fredericton. Keairns, D., Newby, R., & Ulerich, N. (1983). Fluidized-bed combustor design. In P. Basu, Fluidized bed boilers: design and application (pp. 101}129). Toronto: Pergamon Press. Korbee, R. (1995). Regenerative desulfurization in an interconnected yuidized bed system. Ph.D. thesis, Delft University of Technology, Delft. Leckner, B. (1998). Fluidized bed combustion: mixing and pollutant limitation. Progress in Energy and Combustion Science, 24, 31}61. Lee, D. C., Hodges, J., & Georgakis, C. (1980). Modelling SO emis sions from #uidized bed coal combustors. Chemical Engineering Science, 35, 302}306. Lyngfelt, A., As mand, L.-E., & Leckner, B. (1996). Progress of combustion in the furnace of a circulating #uidized bed boiler. In Twentysixth symposium (international) on combustion. The Combustion Institute, Pittsburgh (pp. 3253}3259). Lyngfelt, A., As mand, L.-E., & Leckner, B. (1998). Reversed air staging*a method for reduction of N O emissions from #uidized bed combustion of coal. Fuel, 77, 953}959. Lyngfelt, A., As mand, L.-E., MuK ller, E., & Leckner, B. (1997). Reversed air staging*a method to reduce nitrous oxide emissions from circulating #uidized bed boilers. In Seventh international workshop on nitrous oxide emissions. KoK ln, 21}23 April. Lyngfelt, A., Bergqvist, K., Johnsson, F., As mand, L.-E., & Leckner, B. (1993). Dependence of sulphur capture on air staging in a 12 MW circulating #uidized bed boiler. In R. Clift, & J. P. K. Seville, Gas cleaning at high temperatures (pp. 470}491). Glasgow: Blackie Academic & Professional (ISBN 0 7514 0178 1). Lyngfelt, A., Langer, V., Steenari, B., & Puromi, K. (1995). Calcium sulphide formation in #uidized bed boilers. Canadian Journal of Chemical Engineering, 73, 228}233. Lyngfelt, A., & Leckner, B. (1989a). Sulphur capture in #uidized bed combustors*temperature dependence and lime conversion. Journal of the Institute of Energy, 62, 62}72. Lyngfelt, A., & Leckner, B. (1989b). SO capture in #uidized-bed boilers: re-emission of SO due to reduction of CaSO . Chemical Engineering Science, 44, 207}213. Lyngfelt, A., & Leckner, B. (1991). Sorbent size reduction and conversion versus particle size in #uidized bed boilers. In Proceedings of the institute of energy's xfth international yuidized combustion conference (pp. 179}189). London: Adam Hilger. Lyngfelt, A., & Leckner, B. (1993). SO capture and N O reduction in a circulating #uidized-bed boiler: in#uence of temperature and air staging. Fuel, 72, 1553}1561. Lyngfelt, A., & Leckner, B. (1998). Sulphur capture in circulating #uidized-bed boilers: decomposition of CaSO under local reducing conditions. Journal of the Institute of Energy, 71, 27}32. Makarytchev, S. V., Cen, K. F., Luo, Z. Y., & Li, X. T. (1995). Hightemperature sulphur removal under #uidized bed combustion conditions*a chemical interpretation. Chemical Engineering Science, 50, 1401}1407. Mattisson, T., & Lyngfelt, A. (1995). The presence of CaS in the combustion chamber of a 12 MW circulating #uidized bed boiler. Proceedings of the international conference on yuidized bed combustion, vol. 13 (pp. 819}829).
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Mattisson, T., & Lyngfelt, A. (1998a). A method of evaluating limestone reactivity with SO under #uidized bed combustion conditions. Canadian Journal of Chemical Engineering, 76, 762}770. Mattisson, T., & Lyngfelt, A. (1998b). A sulphur capture model for #uidized bed boilers. Chemical Engineering Science, 53, 1165}1175. Mattisson, T., & Lyngfelt, A. (1998c). The reaction between sulfur dioxide and limestone under periodically changing oxidizing and reducing conditions*e!ect of cycle time. Energy and Fuels, 12, 905}912. Mattisson, T., & Lyngfelt, A. (1999). The reaction between sulfur dioxide and limestone under periodically changing oxidizing and reducing conditions*e!ect of temperature and limestone type. Thermochimica Acta, 325, 59}67. MjoK rnell, M., Leckner, B., Karlsson, M., & Lyngfelt, A. (1991). Emission control with additives in CFB combustion. Proceedings of the international conference on yuidized bed combustion, vol. 11 (pp. 655}664). MuK nzner, H., Bonn, B., & Schilling, H. (1985). Reaction of sulphur dioxide emission by addition of limestone in #uidized bed combustion. German Chemical Engineering, 8, 104}112. Ninomiya, Y., Dong, Z. B., Hashimoto, K., & Sato, A. (1997). Oxidation of calcium sul"de in an advanced PFBC cycle*e!ect of O concen tration and particle size on CaS oxidation. Proceedings of the international conference on yuidized bed combustion, vol. 14 (pp. 387}396).
Ross, G., Anthony, E., Kissel, R., & Doiron, C. (1989). Field demonstration of #uidized bed combustion residue management. Proceedings of the international conference on yuidized bed combustion, vol. 10 (pp. 335}340). Saro"m, A., Goel, S., & Morihara, A. (1994). Fluidized bed combustion and emission control issues. In International clean coal technology symposium on PFBC. Kitakyushu, Japan (pp. 12}27). Scala, F., Salatino, P., Boere"jn, R., & Ghadiri, M. (1998). Interactive processes of sorbent attrition and chemical reaction during #uidized bed desulphurization. World congress on particle technology 3, Brighton, UK. Schouten, J., & Bleek van den, C. (1987). The D.U.T.*SURE-model: a simple approach in FBC sulfur retention modeling. Proceedings of the international conference on yuidized bed combustion, vol. 9 (pp. 749}761). Shearer, J., Johnson, I., & Turner, C. (1979). E!ects of sodium chloride on limestone calcination and sulfation in #uidized-bed combustion. Environmental Science and Technology, 13, 1113}1118. Takeshita, M. (1994). Environmental performance of coal-xred FBC. IEACR/75. IEA Coal Research, London. Weinell, C. (1994). Single particle behavious in circulating yuidized bed combustion. Ph.D. thesis, Department of Chemical Engineering, Technical University of Denmark, Lyngby. Yrjas, P. (1996). Sulphur capture under pressurized yuidized bed combustion and gasixcation conditions. Ph.D. thesis, As bo Academy University, Turkku, Finland.
Sulphur capture in circulating #uidized-bed boilers: can the e$ciency be predicted? Anders Lyngfelt*, Bo Leckner Department of Energy Conversion, Chalmers University of Technology, S-412 96, Go( teborg, Sweden
Abstract The present state of understanding of sulphur capture in #uidized-bed combustion is discussed with focus on the possibilities to predict sulphur capture performance and the e!ect of intermittent reducing conditions. The four key factors that determine the sulphur capture performance are: (i) particle size distribution, including the e!ect of particle size reduction, (ii) residence time as a function of particle size, (iii) reactivity as a function of particle size, and (iv) e!ect of reducing conditions. The "rst three of these can be determined and included in a model for prediction of the sulphur capture, but the problem of how to include the e!ect of reducing conditions in a model is still unresolved. The conclusion must be made that sulphur capture performance in CFBBs cannot safely be predicted at present, and more research is needed before the e!ect of reducing conditions can be incorporated in modelling. Nevertheless, existing models can be useful to approximate the e!ects of, for instance, particle size reduction or changes in particle residence time. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Sulphur; Fluidized-bed combustion; Limestone; Sorbent; Circulating #uidized-bed boiler
1. Introduction Research related to sulphur capture in #uidized-bed combustion (FBC) has been going on for more than three decades: The research is comprehensive and includes a large number of journal publications, reports and theses. As an introduction it is suitable to pinpoint the important questions relevant to sulphur capture in FBC in practice. Two such questions are: E Is it possible to increase the e$ciency of sulphur capture? E Is it possible to predict the e$ciency of sulphur capture? Other important questions are related to modi"cations of boiler design and operation. How is the sulphur capture a!ected by: E scale-up? E measures to reduce N O and NO emissions? E co-"ring with bio-fuels? * Corresponding author. Tel.: 00-46-31-772-1427; fax: 00-46-31-7723592. E-mail address: [email protected] (A. Lyngfelt)
E the fuel distribution inside the combustion chamber? E incomplete secondary air penetration? E the #uidization conditions? There are also a number of questions related to the in#uence of limestone addition on ash properties: E Are the properties of the waste material suitable for disposal or reuse? E Under what conditions may deposits and agglomerations form? In connection with more advanced FBC technology there are further questions. Pressurized FBC was introduced to increase the e$ciency of power generation. In pressurized stationary or circulating #uidized-bed boilers (FBBs) the sulphur capture process is quite di!erent from that under the atmospheric conditions. The partial pressure of CO is too high to allow calcination of the sorbent and there is a direct sulphation of calcium carbonate. This works better than could be expected, but the mechanism is not well understood at present (Iisa, 1992; Yrjas, 1996). Cycles with partial gasi"cation may further increase the e$ciency of power generation. In this case calcium sulphide is the primary product formed in the gasi"er. The problem is then to obtain su$cient
0009-2509/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 9 - 2 5 0 9 ( 9 9 ) 0 0 2 9 0 - 0
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conversion of the sulphide to sulphate in the oxidizing reactor (Ninomiya, Dong, Hashimoto & Sato, 1997; Davies, Hayhurst & Laughlin, 1994). The present paper will not try to cover all these areas. Instead, it concentrates on a few topics related to the initial questions: What is the state of modelling of sulphur capture? Is it possible to predict sulphur capture performance? Can the results obtained with various limestones and boilers be explained by available data? Size distribution, size reduction, residence time and reactivity of sorbent particles will also be discussed, since they are important for modelling. Another key issue for modelling is the e!ects of alternating oxidizing/reducing conditions in the combustion chamber. Is it possible to include these e!ects in modelling? In order to understand the conditions for sulphur capture in circulating #uidized-bed boilers (CFBBs) some experiences from stationary #uidized-bed boilers (SFBBs) are useful and will also be discussed below.
2. Areas of research The limitations of various research approaches should be observed. A schematic overview of the "eld of research is given in Fig. 1. The large bulk of published results is within the upper left box: laboratory work. This research includes chemical and physical investigations of limestone as well as methods to determine its reactivity. Much work has also been directed towards describing the conversion of the limestone by modelling, upper right-hand box (e.g. Bhatia & Perlmutter, 1981; Georgakis, Chang & Szekely, 1979; AdaH nez, GayaH n & GarcmH a-Labiano, 1996a). The models are mainly concerned with the gradual change of the physical structure as a product layer of sulphate develops in the initially porous structure. The purpose of these limestone reactivity models is to increase the understanding of the sulphation process. Another use of the models would be to provide sulphur capture models with in-put data of limestone reactivity, dotted arrow in Fig. 1. However, the limestone models have not really been used in sulphur capture models, for two reasons: (i) the limestone models are complex and therefore di$cult to incorporate in sulphur capture models and (ii) the models often do an excellent job in explaining the sulphation reaction for a certain limestone and size, but are not applicable over a wide range of limestone types and particle sizes (AdaH nez et al., 1996a). Therefore, a more simple and direct approach has been used in sulphur capture models, that is, to measure the reactivity versus conversion in the laboratory and to approximate the results by some simple function that can be included in the sulphur capture model. A number of such sulphur capture models have been proposed (e.g. Lee, Hodges & Georgakis, 1980; Schouten & Bleek van den, 1987; AdaH nez, de Diego,
Fig. 1. Research on sulphur capture.
GayaH n, Armesto & Cabanillas, 1996b). They are normally based on similar basic assumptions: a "rst-order reaction rate with respect to SO concentration and a reaction rate constant which is a function of conversion. The missing link in the study of sulphur capture is validation data for sulphur capture models from actual combustors. There are a number of reasons for the lack of detailed data from boilers: Existing commercial boilers are not suitable for research, for instance, they may not have means for adequate measurements of entering and exiting solid #ows, such as those of coal, limestone, #y ash, and bottom ash. Large-scale tests are expensive and di$cult to organize. Furthermore, the reproducibility of large-scale tests is not so good*there is normally some spread in the results. It is most likely, however, that unpublished data exist and have been used internally by boiler manufacturers for model validation. Nevertheless, a few studies with detailed data have been published, and these will be discussed below. Most of the work discussed so far was made in the 1970s and 1980s, and this work did not consider the conditions in combustion chambers. The fact that the bed material in a FBC chamber is subject to conditions which alternate between oxidizing and reducing was given minimal attention until the end of the 1980s. The e!ect of these alternating conditions is important and will be discussed in detail below, but "rst some general remarks on limestone reactivity studied under oxidizing conditions will be made.
3. Limestone reactivity under oxidizing conditions An attempt to summarize the comprehensive work on limestones will most certainly do injustice to many important results. In addition, there is an inherent di$culty in summing up the experiences; limestones have highly variable properties, with conversions that may range from 0.1 to 0.8 (Dam-Johansen & "stergaard, 1991), and do not easily conform to some general theory. One of several important pioneering research groups in the "eld
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is the one at Westinghouse. Their summary of the state of knowledge of limestone reactivity, based on experiences with 120 sorbents (Keairns, Newby & Ulerich, 1983), is still valid. Some important conclusions were: E `Grain size, porosity and impurity level of the raw sorbent appear to be key parameters but have not yet been correlated to sorbent performance.a E `Basic reaction mechanisms (for sulphation) are as yet unresolved.a E `The rate of calcination and resulting internal structure are currently not predictable.a Nonetheless the complex sulphation process with di!usion through pores, being gradually plugged by a sulphate layer, is well understood, at least under oxidizing conditions, and is described by a number of models (AdaH nez et al., 1996a). In order to reduce the amount of raw material and waste material in sulphur capture, work has also been dedicated to: E Reactivation (Julien, Brereton, Lim, Grace, Chin & Skowyra, 1995). Sorbent particles which are inactivated by a more or less impermeable sulphate layer, can be reactivated by hydration with steam. By hydration the CaO in the particles is converted to Ca(OH) , and when the particles are reintroduced into the furnace the reactivity is increased, which leads to a more e$cient use of the limestone. E Additives. Addition of salt to promote the conversion is one example (Shearer, Johnson & Turner, 1979), although the usefulness of adding chlorine to a boiler can be questioned. E Regenerable sorbents. Schemes to use a regenerable sorbent have also been proposed (Korbee, 1995). In this case SO is produced from decomposition of the sulphate in a separate reactor by means of a reducing agent.
4. Reducing conditions Boilers are operated with excess air. However, as a consequence of the combustion, zones with a de"cit of oxygen will occur locally (Makarytchev, Cen, Luo & Li, 1995). Firstly, such zones will appear in the close vicinity of burning or devolatilizing fuel particles. Secondly, these zones may be extended because of imperfect mixing. One important example is the by-pass of air through the bubbling bottom bed, resulting in inadequate exchange of the gas in the particle phase. Another example is inadequate fuel distribution in large combustion chambers (Couturier, Doucette, Stevens, Poolpol & Razbin, 1991). Insu$cient secondary air penetration may be an additional source for inadequate mixing. Air-staging, if
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used, also promotes the formation of locally reducing conditions. Measurements in the beds of circulating and stationary FBBs with a zirconia-cell oxygen probe, show that the environment can be reducing 80% of the time (Lyngfelt, Bergqvist, Johnsson, As mand & Leckner, 1993). In the case of a stationary FBB, reducing conditions predominated even under unstaged conditions at an overall air-ratio of 1.4 (Cooper & LjungstroK m, 1987). Reducing conditions in the bubbling bed of the stationary and circulating FBBs are explained by the by-pass of air according to the two-phase model and depletion of oxygen in the particle phase due to combustion (Avedesian & Davidson, 1973; Lyngfelt, As mand & Leckner, 1996). In the case of circulating FBBs, air-staging further contributes to a more reducing environment in the bottom bed. Although only the bottom bed is predominantly reducing, these reducing conditions may penetrate to some extent all the way up to the cyclone in streamers of oxygen-de"cient gas (Lyngfelt, As mand, MuK ller & Leckner, 1997). The oxygen-probe measurements not only show the fraction of time under reducing conditions, but also that the periods of reducing/oxidizing conditions may be short, typically 0.1 s. The actual length of the time periods may be even shorter, but the measurements are restricted by the response time of the probes. It should be observed that the probe measures in a "xed position, whereas the sorbent particles move between various regions in the combustion chamber, including the recycling system (Weinell, 1994). This can be stated in a general way, and the overall e!ects are described below, but little is known in detail about the changing conditions experienced by a certain lime particle in the bed.
4.1. Temperature dependence and reducing conditions Sulphur capture in FBBs depends on bed temperature with an optimum in the range of 820}8503C, and above this optimum there is a dramatic decrease in e$ciency. Fig. 2 shows that the temperature dependence varies from one boiler to another. This comparison between two circulating and a stationary FBB, where the same limestone was used, shows that the temperature dependence is smaller in the circulating FBBs. It was "rst assumed that the temperature dependence of sulphur capture was a `propertya associated with the limestone. A comparison, however, shows that the marked fall in sulphur capture performance with temperature in FBBs is not observed in laboratory experiments made under constant oxidizing conditions (Lyngfelt & Leckner, 1989a). The temperature dependence is caused by the in#uence of reducing conditions, predominantly in the bottom bed. The di!erence between the stationary and the circulating FBBs is
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Fig. 2. The sulphur retention as a function of temperature: 165 MW CFBB using KoK ping (䊐) and Ignaberga (*) limestone; 40 MW CFBB, using KoK ping (䊏) and Ignaberga (䢇); 16 MW SFBB using Ignaberga with Ca/S"1 (;) and Ca/S"2.5 (#).
Fig. 3. SO versus height in the furnace for a sand bed and an old lime bed at 8503C. The old lime bed contains CaSO which is decomposed in the bottom bed and formed again in the splash zone.
related to the extent to which the sorbent particles are exposed to reducing conditions. The mechanism behind the temperature dependence is that increased temperature promotes the reductive decomposition of CaSO by CO or H (e.g. Ghardash khani, LjungstroK m & Lindqvist, 1989): CaSO #COPCaO#SO #CO . (1) Boiler tests designed to investigate this mechanism provide evidence for this reductive decomposition (Lyngfelt & Leckner, 1989b, 1993). Tests in both circulating and stationary FBBs show that at increased temperature the sulphur retention, de"ned as the fraction of added sulphur which is captured, may become negative, indicating a net release of sulphur (captured prior to the temperature increase) from the sorbent in the bed. Such a release can be maintained for hours after a temperature increase, but it will eventually subside. Measurements inside the combustion chamber of a circulating FBB indicate that reductive decomposition of CaSO in the bottom bed occurs even at 8503C, as shown in Fig. 3 (Lyngfelt & Leckner, 1998). Here the sulphur release from an old lime bed, i.e., a bed with sulphated sorbent obtained by stopping the limestone addition for two days, is demonstrated by a comparison to a sand bed free of sulphate. The sand bed does not interact with the sulphur and the old lime bed had reached steady-state conditions so that the net release of SO from the bed material, release minus capture, is zero. The much higher concentration of SO just above the bottom bed with old lime is explained by reductive decomposition of the sulphate in the bottom bed, while the subsequent decrease is explained by a recapture of the SO by the decomposition product CaO in the splash zone where mixing is improved.
Fig. 4. Sulphation of limestone under alternating conditions (Mattisson & Lyngfelt, 1999). The inlet SO concentration is 1500 ppm (dashed line).
Laboratory experiments, where limestone was sulphated under conditions alternating between oxidizing and reducing, show that SO is released during the shifts between oxidizing and reducing conditions, as shown in Fig. 4 (Hansen, Dam-Johansen, Bank & "stergaard, 1991; Hansen, Dam-Johansen & "stergaard, 1993; Mattisson & Lyngfelt, 1999). At higher temperatures the release of SO during the shifts is so fast that in practice no net conversion to sulphate takes place. 4.2. Air-staging and reducing conditions As a consequence of the large impact of reducing conditions on the sulphur capture process, the total airratio and the extent of air-staging become important
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Fig. 5. SO emission (left axis) and sulphur retention (right axis) as function of case of air-staging and bed temperature 8503C (䢇) and 9303C (䉱). 12 MW CFBB.
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The e!ect of air-staging on sulphur capture has been studied further in connection with a method, `reversed air-staginga, for obtaining low N O emissions from FBC (Lyngfelt, As mand & Leckner, 1998).While the emission of N O decreases with raised bed temperature or lowered air-ratio, the emission of SO increases. But in contrast to the emission of SO , the emission of N O is not much a!ected by the conditions in the bottom part of the furnace. Thus, the con#ict between the desire to reduce N O emission and to maintain e$cient sulphur capture can be solved by addressing the conditions in the lower and the upper part of the combustion chamber separately. In other words, the emission of N O can be decreased by lowering the air-ratio in the upper part, while the e!ects on SO capture are neutralized if the air-ratio in the lower part is simultaneously raised.
4.3. CaS formation
Fig. 6. Fraction of time under reducing conditions at 0.65 m height (solid lines) and at 8 m (dashed lines) for the three air-staging cases. 12 MW CFBB.
(Takeshita, 1994). The e!ect of air-staging on sulphur capture has been studied in a circulating FBB (Lyngfelt & Leckner, 1993). In addition to normal air-staging with about 60% primary air, two extreme cases were studied: no air-staging where all the air was primary air, and intensixed air-staging where the primary air was lowered to about 45% and the level of secondary air addition was raised to 5.5 m. At 8503C the sulphur retention dropped to 40% under intensi"ed air-staging, compared to about 90% under normal and no air-staging, Fig. 5. The temperature dependence is clearly related to the degree of air-staging, and at a temperature of 9303C the sulphur retention was negative both for normal and intensi"ed staging, indicating a net release of sulphur from the sorbent. Zirconia-cell oxygen-probe measurements verify that the fraction of time under reducing conditions is strongly dependent on the air-staging conditions, Fig. 6 (Lyngfelt et al., 1993).
The possible formation of calcium sulphide, CaS, is also associated with reducing conditions. CaS is not stable and may decompose in the presence of moisture with release of poisonous hydrogen sulphide gas, thus causing a handling and disposal problem if the CaS content is too high. The presence of calcium sulphide in ashes has therefore been investigated (Anthony, Stephenson & de Iribarne, 1987; Anthony, Ross, Berry, Hemmings, Kissel & Doiron, 1989; Ross, Anthony, Kissel & Doiron, 1989). A study of CaS formation in a circulating FBB reveals that only small amounts of sulphide, a few percent of total sulphur, is formed, independent of the extent of air-staging (Mattisson & Lyngfelt, 1995). However, substantial amount of CaS was formed when limestone addition was stopped and the SO concentra tion increased. A similar formation of CaS after a limestone stop was also noted in one of the previously mentioned tests in the stationary FBB where as much as half of the sulphur present in the bed material was in the form of sulphide (Lyngfelt, Langer, Steenari & PuromaK ki, 1995). However, the sulphide was observed several hours after the limestone addition was stopped when the SO concentration, as a result of the limestone stop, had reached a high level. The conclusion is that signi"cant CaS formation is not expected under normal operating conditions and CaS is therefore not a problem in practice. The formation of CaS, if limestone addition is accidentally stopped, should not be a problem, since the formation is slow. Furthermore, no or little bottom ash is removed from the furnace under such conditions. Even if CaS is not present in large quantities, it is most likely an important intermediate and is therefore essential for the understanding of the sulphur chemistry in the combustion chamber (Makarytchev et al., 1995).
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5. Sorbent particle size The sorbent particle size is an important parameter which a!ects both residence time and reactivity. In the typical size range of particles in an FBB, 20}2000 lm, the residence time may vary with several orders of magnitude, and the conversion as measured in the laboratory may vary with a factor of up to three with size (Mattisson & Lyngfelt, 1998a, b). 5.1. Size reduction There may be a signi"cant reduction in the particle size from the feed size distribution to the size distribution of the particle #ow exiting from the system. Laboratory tests show that the size reduction depends on the limestone properties. It is normally rapid during the "rst minutes when calcination takes place, and much slower when the sulphation has started (Scala, Salatino, Boere"jn & Ghadiri, 1998; Couturier, Karidio & Steward, 1993; Karidio, 1994). Data on size reduction are available from one stationary and three circulating FBBs, as well as for two di!erent limestones, one soft and the other crystalline. All these cases show a signi"cant size reduction, independent of boiler design and limestone: the added sorbent particles, with an initial mass median size of about 0.6 mm, decreased to about 0.12 mm, Fig. 7 (Lyngfelt & Leckner, 1991; MjoK rnell, Leckner, Karlsson & Lyngfelt, 1991; Mattisson & Lyngfelt, 1998b). The e!ect of the added size distribution was also investigated, but the added size had a small e!ect on the resulting size distribution (MjoK rnell et al., 1991). 5.2. Residence time and conversion The sorbent particles leave the circulating FBB both as #y ash and as bottom ash. The residence time of the small #y-ash particles is mostly determined by the boiler design, cyclone e$ciency, riser height, etc. The residence time of the larger bed ash particles is given by the ratio of bed mass to the exiting bed ash #ow. The #ow of exiting bed ash depends on the entering #ows of sorbent and fuel ash and the extent of formation of smaller particles which leave the bed as #y-ash. There is probably a connection between the size reduction and the boiler design, particularly related to the cyclone e$ciency: the particles are subject to fragmentation and attrition processes which continue until the particles leave as bottom ash or until the particles are small enough to leave as #y-ash. On the other hand, the laboratory studies indicate that the attrition is slow once the sulphation has started, and part of the sorbent added also leaves the system as large particles despite a long residence time. Old sulphated sorbent particles seem to be quite resistant to attrition*attrition rates below 1%/h have been reported (Leckner, 1998).
Fig. 7. Size distribution of added limestone (Ignaberga) and sorbent particles in exiting ash for three CFBBs and an SFBB.
The residence time of sorbent particles increases with size, from a few seconds for the particles smaller than the cut-size of the cyclone, up to the point, typically 0.2}0.4 mm, where the particles are so large that they do not leave the system as #y-ash. This size is normally also the optimum size for sulphur capture (MuK nzner, Bonn & Schilling, 1985; Mattisson & Lyngfelt, 1998b). For this, and larger particle sizes, the residence time may be 15}30 h and is normally independent of size, provided that there is no active removal of larger particles by classi"ers, for instance. Below this optimum size, the conversion decreases because of reduced residence time and above this size the conversion decreases because of insu$cient penetration of sulphur to the particle's interior. Data from boilers show that the conversion may be rather independent of size as in Fig. 8 or more dependent on size as in Fig. 9. Fig. 8 shows that there is a much higher conversion of all particle sizes in the circulating FBB. The residence time for the small particles is longer in the circulating FBB, but the large particles have a much longer residence time in the stationary FBB and are therefore expected to have a higher conversion. Nevertheless, the large particles in the stationary FBB have a lower conversion, which must be attributed to the negative e!ect of reducing conditions that was also evident in Fig. 2. The higher e$ciency of sulphur capture in the circulating FBB compared to the stationary FBB, can be explained by the longer residence time for small particles in the circulating FBB and also that the particles are less exposed to reducing conditions in the circulating FBB. Although these di!erences are typical for the boiler types, they may not be inherent. For instance, a stationary FBB can be equipped with an e$cient particle recirculation system and possibly also in-bed devices to break up bubbles which would reduce the previously mentioned through-#ow.
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Fig. 8. Degree of sulphation versus particle size for a 16 MW SFBB and a 40 MW CFBB.
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Fig. 10. E!ective reaction constant versus conversion shown in dimensionless form, which enables a comparison between the approximate decay function (dashed line) and the reactivity of various particle sizes: from 1.4}2 mm (a) to 45}63 lm (k).
using a dimensionless representation, with a non-dimensional rate constant, i, and non-dimensional conversion, s: k i " G, (3) G c G s "X c . (4) G G G If these are substituted into Eq. (2) the size-dependent constants disappear and Eq. (2) is simpli"ed to i"e\Q. Fig. 9. Degree of conversion for the 12 MW CFBB compared to model results as a function of particle size.
The rate of sulphation of the sorbent is described by a "rst-order reaction dX G"k p , G K dt
6. Modelling and limestone reactivity Normally, the reactivity of limestones has been approximated by an exponential decay of the reaction rate constant with time, corresponding to a linear decrease in the rate constant versus conversion. An alternative representation (Mattisson & Lyngfelt, 1998b), shown in Fig. 10, is based on the assumption of an exponential decay of the rate constant, k , with conver sion, X: k "c e\A G6G, (2) G G where c and c are "tted functions of particle size i. G G Fig. 10 shows a comparison of Eq. (2), dashed straight line, to data of 11 particle sizes from laboratory tests of the limestone Ignaberga under oxidizing conditions. This comparison of the approximation in Eq. (2) to particles of di!erent sizes in the same diagram is made possible by
(2a)
(5)
where p is the average SO concentration to which the K sorbent is exposed. Combining Eqs. (2) and (5) and integrating yields an expression of the conversion of particle size i as a function of time. Assuming an exponential residence time distribution for this particle size, an average conversion, X , can be calculated by integrating G over all residence times:
e\R OG ln(c c p t#1) dt, (6) G G K c q G G where q is the average residence time of particles in size G fraction i. The resulting sulphur retention is then obtained by a summation of the fractional conversions of all sizes and multiplying with the molar ratio Ca/S of added limestone, c: X
" G
U g "c f X , Q G G G
(7)
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Fig. 11. Total sorbent conversion of Ignaberga limestone in the three CFBBs compared to model results.
where f is the fraction of total sorbent belonging to size G fraction i. Since the SO concentration, p , is a function K of the retention, an iterative procedure is needed. A sulphur capture model using this representation has been validated by tests with the limestone Ignaberga in three circulating FBBs, of 12, 40 and 160 MW size. In addition to detailed reactivity data in Fig. 10, the residence time and conversion were determined for the three boilers from ash analyses of 11 particle sizes in the range 0}2 mm (Mattisson & Lyngfelt, 1998b). The model results agree reasonably well with the measured conversion, as shown in Figs. 9 and 11. Although there is some underprediction of the conversion, the di!erence between the three boilers is accurately described. Similar detailed data from these boiler tests and from identical laboratory tests, were also obtained for a less reactive limestone, KoK ping, but in this case the modelling failed: In laboratory tests the "nal conversion was four times larger for Ignaberga (36%) than for KoK ping (9%), but, surprisingly enough, the di!erence between the sulphur capture performance of Ignaberga and KoK ping in the boiler tests was small (MjoK rnell et al., 1991). For KoK ping limestone the conversion in the boilers was two to three times larger than the conversion obtained in laboratory, independent of particle size, Fig. 12. Although the residence time of the larger particles is longer in the boiler, this is compensated for by a 10 times higher concentration of SO in the laboratory experiments. Even if the test period in laboratory was extended to 40 h as seen in Fig. 12, it was impossible to reproduce the much higher conversion obtained in the boilers (Mattisson & Lyngfelt, 1998c). (The di!erence between the two boilers in Fig. 12 is most likely a result of di!erent residence time; the model, when applied to Ignaberga limestone, explained the higher conversion in
Fig. 12. Degree of conversion obtained for the limestone KoK ping in two CFBBs compared to the maximum conversion obtained in laboratory versus particle size.
the 40 MW circulating FBB with the longer residence time, Fig. 11.) The obvious conclusion is that the conditions of the laboratory measurements cannot be fully relevant for what happens in a combustion chamber. Therefore, an additional study was made, where the e!ect of alternating oxidizing and reducing conditions were studied in the laboratory.
7. Limestone reactivity under alternating conditions Alternating oxidizing and reducing conditions may have a bene"cial e!ect on the sulphation (Mattisson & Lyngfelt, 1998c). The conversion of the KoK ping limestone as a function of both fraction of time under reducing conditions and complete oxidizing}reducing cycle time is shown in Fig. 13. The conversion is sensitive to the fraction of time under reducing conditions, u . Opti mum conditions with a large increase in conversion compared to constant oxidizing conditions were obtained for u "0.2}0.5, whereas the conversion at u "0.7 could be low, almost zero at shorter cycle times. X-ray mapping of limestones reacted under oxidizing conditions compared to limestones reacted under alternating conditions reveals a fundamental di!erence. While the limestone particles which were reacted under oxidizing conditions only had sulphur on the surfaces of internal cracks, as shown in Fig. 14a, the particles sulphated under alternating conditions showed a deep penetration of sulphur from the cracks, as shown in Fig. 14b. Even when the reaction time was extended with a factor of 10, to 20 h, no signi"cant penetration could be seen for the samples sulphated under oxidizing conditions, as shown in Fig. 14a. Obviously, alternating conditions promote di!usion of sulphur into the solid
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material, but the mechanism for this is not established at present. The results obtained in laboratory show a new aspect of alternating conditions, namely that they may increase the conversion of limestone signi"cantly and they also provide a likely explanation for the boiler results. The conclusion is that the high conversion obtained for KoK ping limestone in the boilers, in contrast to laboratory tests under oxidizing conditions, is most likely an e!ect of alternating conditions.
8. Pressurized conditions
Fig. 13. The conversion as a function of the fraction of time under reducing conditions after 2 h sulphation of KoK ping limestone (0.5}0.7 mm) using a reactant gas of 1500 ppm SO , 10% CO , 0}4% O , and 0}4% CO.
The work related so far has been focused on atmospheric conditions, and the question is therefore how, and if, this is applicable to pressurized conditions. It should be pointed out that the negative e!ects of alternating conditions manifested by the e!ects of increased temperature and staging are not to be expected under pressurized conditions, and consequently no temperature optimum of sulphur capture under pressurized conditions has been reported (Takeshita, 1994). Firstly, this is a simple e!ect of thermodynamics, the reductive decomposition takes place only under the combination of low oxygen partial pressure and low sulphur dioxide partial pressure (Saro"m, Goel & Morihara, 1994). Under pressurized conditions the SO partial pressure is high enough to suppress this reaction. Secondly, the #uidization pattern is di!erent in a non-circulating pressurized #uidized-bed reactor with bed internals which break up the bubbles and reduce the through-#ow. Consequently, oxygen-probe measurements indicate that reducing conditions are less prevalent in a pressurized pilot plant (Almstedt & LjungstroK m, 1987). Because the sorbent does not calcine, both the sulphur capture reaction mechanisms and the particle size reduction processes are di!erent. However, there should be no important di!erence in the application of models under pressurized conditions, except that laboratory experiments of course have to be made under pressurized conditions.
9. Discussion The four key factors that determine the sulphur capture performance are: (i) particle size distribution including the e!ect of in-bed particle size reduction, (ii) residence time as a function of particle size, (iii) reactivity as a function of particle size, and (iv) e!ect of reducing conditions. The "rst three factors can be quanti"ed and included in a model: Fig. 14. The sulphur distribution in the interior of two sulphated KoK ping limestone particles of approximate size 0.7 mm. (a) Oxidizing conditions. Total time: 20 h. (b) Alternating conditions. Total time: 2 h.
(i) The size distribution of sorbent particles leaving a given boiler can be determined from chemical
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analysis of sieved fractions of the exiting ash #ows. In the case of a planned boiler, experiences from available boilers in combination with laboratory tests of particle size reduction can be used to predict the "nal size distribution. (ii) The residence time as a function of particle size can be determined from analysis of sieved fractions of bed material and exiting ash #ows. In the case of a planned boiler, the average residence time is determined from the limestone #ow and the total amount of bed material. The actual residence times for the individual particle sizes are then derived from experiences in available boilers in combination with models describing the recirculation inside the riser and from the cyclone. (iii) The reactivity, in the form of a rate constant versus conversion, can be determined in laboratory experiments for di!erent particle sizes. The reasonably successful modelling results with Ignaberga limestone do not prove that reducing conditions can be neglected. The results obtained with KoK ping limestone reveal that there is a fundamental error in using laboratory data derived under oxidizing conditions in modelling. This leads to the question: why did the model give satisfactory results with Ignaberga but not with KoK ping limestone? Although no safe conclusions can be made, it can be speculated that this is an e!ect of the limestone properties; Ignaberga is a porous reactive limestone while KoK ping is a crystalline unreactive limestone. Alternating conditions obviously promote the mobility of sulphur in the limestone and this may have a much more signi"cant e!ect on the unreactive limestone. Available laboratory tests of Ignaberga limestone under alternating conditions, although too few for safe conclusions, indicate that this limestone is less a!ected by alternating conditions (Mattisson & Lyngfelt, 1999). Two additional questions are: (i) Are there operation conditions for which the e!ect of reducing conditions can be neglected? (ii) Can the e!ect of reducing conditions be incorporated into a model of sulphur capture? With the exception of pressurized conditions, the answer to the "rst question seems to be no. Neither increased primary air-ratio nor lower temperature is suf"cient to disregard reducing conditions: most likely a circulating FBB cannot be operated without the appearance of reducing conditions in the combustion chamber. Even if all air is added as primary air, reducing conditions are present (Lyngfelt et al., 1993). Laboratory tests show that there is a large di!erence in conversion between oxidizing and alternating conditions also at lower temperatures, e.g. 8253C (Mattisson & Lyngfelt, 1999). The second question is more di$cult to answer. One approach is to determine a comprehensive `reaction parametera versus conversion for alternating conditions
in the same way as the rate constant was obtained under oxidizing conditions. This reaction parameter would be averaged for a complete cycle of one oxidizing and one reducing period. It is di$cult, of course, to choose the proper conditions for such laboratory tests, which have to be representative for a particle in a circulating FBB. A fundamental problem with this approach, however, is that a rate constant assumes a "rst-order reaction, which is obviously incorrect in the case of this `reaction parametera: the average rate of reaction during the cycle is a complex function of two rates of capture (for the oxidizing and the reducing periods), as well as of two rates of release (during the change to oxidizing and the change to reducing conditions). Thus, a reaction parameter which averages the net of these reactions during the cycle is not a rate constant, but must be determined, not only as a function of conversion, but also as a function of gas concentration. This adds considerable experimental di$culties, since tests at low SO concentrations need corre spondingly longer test periods. Another approach is to carry out more fundamental modelling of the reactions inside the sorbent under alternating conditions, but also this modelling may prove to be di$cult in view of the complexity and the lack of detailed knowledge of the reactions. Although the overall reactions are known, the reaction mechanisms are not known. Furthermore, the reactions are heterogeneous and the conditions are transient; the compositions of both the solid phase and the gas phase change rapidly. Thus, it can be concluded that while the e!ect of reducing conditions cannot be safely neglected, there is yet no experience of how to include this e!ect in modelling.
10. Conclusion The four key factors that determine the sulphur capture performance are (i) particle size distribution including the e!ect of in-bed particle size reduction, (ii) residence time as a function of particle size, (iii) reactivity as a function of particle size, (iv) e!ect of reducing conditions. The "rst three of these items can be determined and included in a model for prediction of the sulphur capture, but the problem of how to include the e!ect of reducing conditions in a model is still unresolved. At least in the case of an unreactive limestone, it was clear that the model fails if alternating conditions are not considered. Therefore, the conclusion must be made that sulphur capture performance in circulating FBBs cannot be predicted safely at present, and that more research is needed
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before the e!ect of reducing conditions can be included in modelling. Nevertheless, existing models can be useful to approximate the e!ects of, for instance, particle size reduction or changes in particle residence time.
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