Fragmentation of dolomite bed material at elevated temperature in the presence of H2O & CO2: Implications for fluidized bed gasification

Fragmentation of dolomite bed material at elevated temperature in the presence of H2O & CO2: Implications for fluidized bed gasification

Fuel 260 (2020) 116340 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Full Length Article Fragmenta...

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Fuel 260 (2020) 116340

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Full Length Article

Fragmentation of dolomite bed material at elevated temperature in the presence of H2O & CO2: Implications for fluidized bed gasification

T

Chunguang Zhou , Christer Rosén, Klas Engvall ⁎

Department of Chemical Engineering, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden

ARTICLE INFO

ABSTRACT

Keywords: Dolomite Fragmentation mechanism Crystalline Calcination Fluidized bed

With the aims of improving the understanding of the use of dolomite bed materials in a fluidized bed (FB) gasifier, dolomite primary fragmentation behaviors and mechanisms at elevated temperatures were thoroughly investigated by means of quantitative analysis of particles production, size distribution, and particles shape (in terms of HS circularity and convexity). Crystalline Glanshammar and amorphous Sala dolomites of different original particle sizes (300–350 μm, 350–500 μm, and 500–1000 μm) were treated under various conditions, such as temperatures of 650, 750, and 950 °C and gas conditions of N2, N2 & steam, and H2O & CO2. Crystalline Glanshammar dolomite exhibited severe fragmentation, while amorphous Sala dolomite only fragmented slightly. Fragmentation mechanisms were proposed for amorphous and crystalline dolomite. In case of the Glanshammar dolomite, the release of H2O trapped in the crystal lattices and trace level of CO2 at structure defects, contributed to 10% and 55% of the primary fragmentation, respectively. The presence of CO2 can significantly mitigate the fragmentation of crystalline Glanshammar dolomite, which is probably either due to the prevention of CO2− dissociation at the defects or filling of cations by interchanging CO2− with the dolomite, consequently, allowing for a stabilization of dolomite structure. The analysis shows that elutriation in fluidized bed gasifier can be reduced significantly when either using amorphous Sala dolomite as bed material or treatment of the Glanshammar dolomite in the presence of CO2 before its utilization in a fluidized bed.

1. Introduction Fluidized bed (FB) gasification allows for in bed catalytic tar cracking using bed materials, such as dolomite, CaMg(CO3)2, a cheap and abundant material with adequate tar cracking and anti-agglomeration properties, compared to conventional bed materials [1–4]. A suitable dolomite bed material has to be catalytically active (depends on its chemical state and pore structure and sizes) and simultaneously mechanical stable (depends on macro- and microstructure). Conditions influencing the performance of a dolomite bed material are the temperature, the pressure and the chemical conditions present in the gasifier. A major obstacle feeding raw dolomite to a gasifier is the fragmentation at elevated temperatures, generating a considerable amount of smaller fragmented particles [5,6], and thereby increasing fine elutriation, affecting hydrodynamic behaviors. This leads to a higher tendency of bed segregation with substantial amounts of fines concentrated at the bed surface, which consequently increases the amount of elutriated fines [7]. A number of research works has been carried out to investigate mixing/segregation/fluidization behaviors of binary mixture of



particles differing in sizes [8–13]. Larger particles (denoted jetsam) are apt to stay near the bed bottom, while relatively smaller particles (denoted flotsam) would stay at the bed top [8,9]. Better particle mixing can be obtained by increasing the fluidizing velocity [8], however, the amount of elutriated fines may then increase accordingly. For a segregated bed differing in sizes, even though most of flotsam particles are fluidized, a dead region where particles are extraordinarily difficult to fluidized, can still be found at the bottom of the bed [10]. By inspecting the bubbling and segregation patterns by increasing the width for all particle size distributions (PSDs) investigated, Chew and Hrenya [11] observed a bubble-less bottom region for a segregated system despite a fully fluidized bed. The segregation results in an accumulation of large particles in the center and bottom, inhibiting the transportation of solid particles along axial and radial directions [12,13]. The mechanism for fragmentation of a dolomite at elevated temperatures may be complex [14–16], due to the complex calcination process occurring either in a single step or in sequence with an initial partial calcination, followed by full calcination in sequence, depending on the conditions [17]. To date, there is a general lack of experimental

Corresponding author. E-mail address: [email protected] (C. Zhou).

https://doi.org/10.1016/j.fuel.2019.116340 Received 26 June 2019; Received in revised form 18 August 2019; Accepted 2 October 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

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Notation

Ni p P Pe PeC vi Vi Vfresh Vt1

Cross section area of particles, m2 Probability density function of particle convexity distribution C Normalized cumulative convexity distribution Convexity An indicator to show the roughness of a particle F Fracture ratio h Probability density function of particle HSC distribution H Normalized cumulative HSC distribution HSC High sensitivity circularity mfresh Mass of original dolomite sample, mg ni Particles number of i size fraction per unit of dolomite sample, mg−1

A c

Vt2

Y

data and corresponding fragmentation mechanisms, enabling a selection of a suitable dolomite to be used as bed material in FB gasification. So far, most fragmentation studies were carried out under isothermal conditions, as a consequence of the complexity of the dolomite material. Therefore, only the final stage of the fragmentation outcome was determined [18–20]. A sequential fragmentation mechanism, including initiation, evolution, and termination, is a consequence of a number of factors, such as, thermal shock, pressure build-up due to vaporization of internally trapped H2O, and release of calcination gases, intertwined during the rapid heat-up. As a consequence, there is very little or noneuseful information on measures to mitigate fragmentation and attrition in FB gasifiers in the literature. A review on previous works investigating calcium-based materials fragmentation is presented in the following theory section below. This study presents an experimental investigation of primary fragmentation of a crystalline (Glanshammar) and an amorphous (Sala) dolomite at various conditions, previously applied as bed materials in a pressurized biomass FB gasifier [4]. The objective of the study was to clarify dolomite fragmentation behaviors at different stages, i.e. before half calcination, during half/full calcination, as well as the corresponding mechanism, by controlling the final temperature and varying the gas composition. The different factors, such as, thermal shock, pressure build-up due to vaporization of trapped H2O, and release of calcination gases, may intertwine during primary fragmentation under the real conditions present in a fluidized bed, where the heating rate is very large with a heat up of the particle in less than a second. To possibly study the factors separately, tests to produce fragmented dolomite particles for analysis were conducted with a well-controlled constant heating-rate until a selected final temperature in an atmospheric thermogravimetric analyzer (TGA). Tests at rapid heating rates are not performed in the present study and saved for the future. Although not fully representative for dolomite fragmentation under real FB conditions, including also secondary fragmentation and attrition except for the primary fragmentation, the results still provide a deeper understanding of the effect of different factors causing the primary fragmentation. The effects of calcination temperature, original particle size, and gas atmosphere on fragmentation were evaluated. All particles produced in the TGA were collected and scanned for analyzing particles production, size distribution, and particles shape. Fragmentation mechanisms for the two dolomites were investigated and evaluated in terms of measures for improving fluidized bed gasifier performance, e.g. mixing/segregation, and elutriation reduction.

Number of collected particles in i size fraction Probability density function of particle size distribution Particle size, mm Perimeter of particles, m Perimeter of the convex hull, m Volume fraction of particles in i size fraction Volume of collected particles in i size fraction, mm3 Volume of original dolomite sample, mm3 Volume of particles above the lower boundary of the size range of fresh dolomite samples, mm3 Volume of particles below the lower boundary of the size range of fresh dolomite samples, mm3 Normalized cumulative size distribution

process is the partial pressure of CO2 [21]. The general view is that dolomite decomposes in a single step R(1) and in two steps R(2) at low (< 26.7 kPa) and high (> 26.7 kPa) partial pressure of CO2, respectively [22]. In the two steps decomposition process, the double carbonate is first decomposed to MgO at a temperature of 780–800 °C and CaCO3, and thereafter the CaCO3 is decomposed to CaO in a second step at a temperature of 880–900 °C. The single step reaction generally takes place at a temperature around 700 °C. CaMg(CO3)2(s) → CaO(s) + MgO(s) + 2CO2(g)

R(1)

CaMg(CO3)2(s) → CaCO3(s) + MgO(s) + CO2(g) → MgO(s) + CaO(s) + 2CO2(s)

R(2)

There is, however, no consensus on the exact mechanism for this mechanism in the literature. A more comprehensive description is presented by Samtani et al. [23], where the first stage decomposition occurs by formation of individual carbonates. The magnesium carbonate is in an unstable state and decomposes immediately. They also proposed a net reaction for the first step, nCaMg(CO3)2 → (1−n)CaCO3+(1−n)MgO+(1−n)CO2

R(3)

based on the results with both dolomite and half-burnt dolomite. n goes from 1 to 0 with time. The second step decomposition of the calcium carbonate is proposed to be a reversible process, CaCO3 ↔ CaO + CO2

R(4)

in contrast to the second step in reaction equation R(2). The reaction rate for the decomposition of a carbonate rock is most likely determined by the interrelationship between rate controlling processes such as: 1) heat transfer in the rock, 2) mass transfer of gas components formed and influencing the decomposition and 3) chemical reaction kinetics [24]. For instance, Gallagher and Johnson [25] showed that the decomposition reaction rate was limited by heat transfer. Other examples are the study performed by Borgwardt [26], concluding that the decomposition reaction of limestone is controlled by chemical reaction kinetics, except for the final stage of decomposition, where the diffusion of CO2 through the oxide layer product was the limiting rate. There are many other examples, displaying how the complex picture of can be during the decomposition. Gallucci et al. [27] reported the wide deviation of CO2 evolution temperature in different gas atmospheres that the decomposition of the two carbonates occurred at 741 °C and 830 °C in N2, at 773 °C and 909 °C in air, and at 760 °C and 930 °C in CO2 atmosphere, respectively. The reason was attributed to a delayed CO2 release in the sample holders, subsequently changing the partial pressure surrounding the particles, affecting the calcination temperature of the second endothermic reaction.

2. Theory 2.1. Thermal decomposition and recarbonization of dolomite The calcination of dolomites is a complex process and factors, such as heating rate, grain size, sample quantity and atmospheric conditions, influence the result. Of considerable importance to the calcination

2.2. Fines production in FB Production of fines in FB reactors may be due to a variety of 2

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different physical and chemical processes, roughly divided into primary and secondary fragmentation, and attrition. Calcium-based carbonate rock in general first exhibits a primary fragmentation process during the thermal heat-up and decomposition, resulting in a more fragile product of lower density (depending on its mechanical structure). The fragile product can usually not sustain the severe secondary fragmentation and attrition in the FB. The further fragmentation of the produced friable samples from primary fragmentation is called secondary fragmentation. Due to collisions, coarser particles produced from primary fragmentation fragment into pieces, releasing its mechanical stresses inside, producing smaller coarse particles and fines. Attrition is another contributor of fine production, following the secondary fragmentation. It is highly dependent on operating conditions in the reactor and the particle shape. Since primary and secondary fragmentation produce coarse particles apt to round off due to frequent high- and lowenergy collisions, the attrition rate is high in the beginning, but decreases gradually in time [28]. It can be reasonably concluded that both the particle breakage during secondary fragmentation and the attrition at the very beginning are highly dependent on the impact velocity in the fluidized bed and the particle mechanical integrity that is formed and depends on calcination conditions at the primary fragmentation stage [29,30].

exceeded the mechanical strength of the particle. Presence of bound water, e.g. hydrated compounds like silicate and aluminate hydrates, has been observed by Gunasekaran and Anbalagan [37], illustrated by a detection of a strong band around 3400 cm−1 in dolomite using infrared spectroscopy. With the sample heated to 500 °C, the intensity of the broad band in the O–H region decreases, while other bands were not affected. The findings confirm the existence of water vapor during the dolomite decrepitation process. Nevertheless, the fragmentation mechanism at different conditions i.e. temperature, gas atmosphere, dolomite property, and particle size, and the quantification of contributing factors, such as thermal shock, water vapor, and calcination are still lacking. 2.4. Materials Glanshammar and Sala dolomite, previously applied as bed material in a pressurized biomass FB gasifier [4], were used for investigating dolomite primary fragmentation behaviors. The main components of both dolomites are presented elsewhere [4]. Raw dolomite with a wide size distribution was initially sieved by using a stainless steel mesh sieve to fractions of 500–1000 μm, 350–500 μm, and 300–350 μm. 2.5. Characterization of gas release during dolomite calcination

2.3. Dolomite fragmentation

During dolomite calcination, release of gaseous products (CO2, H2O, etc.), considered as a possible reason for fragmentation [19], is expected to occur with the observed mass loss profile during a thermogravimetric (TG) test. However, it is hard to determine the initial release of the trace amount of gas in the beginning, since at this stage there is no obvious mass change to be observed in the mass loss curve. In the present study, water vapor and CO2, emitted during dolomite primary fragmentation with the increase of temperature, were detected using a TG-FTIR (thermogravimetric-fourier transform infrared spectrometer). Their absorption wave numbers are given in Table 1. Wave number ranges of 4000 to 3400 cm−1 and 2000 to 1300 cm−1 are valid for the absorption of H2O [38,39]. The characteristic absorption band of CO2 locates near 2350 cm−1 and 670 cm−1. Blank tests without dolomite samples in the TG crucible were performed for comparison.

Primary fragmentation of limestones (CaCO3-based materials) has been widely investigated [16,18,19,31–36]. Scala et al. [31] reported calcination fragmentation to be one of the main reasons of primary fragmentation, due to the release of CO2. This implies that prevention of calcination in gasification or combustion conditions could hinder fragmentation significantly. One example illustrating this is a study at oxy-firing condition, where the primary fragmentation was significantly reduced compared to air-fired operation, due to high CO2 atmosphere [19]. Thermal shock has also been reported as a reason for fragmentation [16,34], caused by a high heating rate, occurring even before the initiation of calcination. Silcox et al. [35] reported on more moderate size reduction for injected pulverized limestone particles at 1360 °C, compared to1830 °C, indicating less limestone fracture at lower temperature and slower heating rate. Non-significant fragmentation after injection at 1200 °C is also observed [19]. Saastamoinen et al. [36] investigated limestone fragmentation in a fluidized bed and reported thermal shock to cause much less fragmentation compared to calcination. Yao et al. [20], after analyzing the influence on primary fragmentation of several parameters as heating rate, particle size, different limestones, and calcination/sulfatation, clarified primary fragmentation of limestone was dominated by CO2 release instead of thermal stress. Dolomite has different chemical and physical properties, compared to limestone, certainly influencing its fragmentation behavior. Hu et al. [16] investigated the fragmentation behavior of calcium-based sorbents at high heating rates and short residence time, and observed a more severe fragmentation for dolostone particles, compared to limestone at similar conditions [16]. This was explained as a rapid decomposition of magnesium carbonate building up a sufficiently high pressure of CO2 inside the particles exceeding the particle inherent mechanical strength. The effect of calcination on dolomite fragmentation was also more pronounced compared to limestones. Significant fragmentation of dolostones was observed already before 10% of calcination, followed by a further reduction in sizes during the period 10 to 30% of calcination. During the remaining calcination process a relatively constant size distribution was observed. McCauley and Johnson [15] used TGA to assess decrepitation of dolomite and found that dolomite decrepitation is a phenomenon related to particle size rather than a normal decomposition reaction. The cause of the decrepitation was attributed to a pressure built up by water trapped in the dolomite structure at the decrepitation temperature, causing a mini explosion when the pressure

2.6. Experimental procedure of fragmentation tests The Glanshammar and Sala dolomite were treated under various conditions, aiming at investigating the effect of temperature, type of gas, dolomite crystalline structure, and original particle size on primary fragmentation. A Netzsch STA 449 F3 Jupiter TG apparatus, equipped with a Bronkhorst High-Tech unit for steam generation, was used to treat dolomite samples at a controlled heating rate and atmosphere, including steam, CO2, and N2 (carrier gas), as well as a mixtures in different proportions [40]. Primary fragmentation is presented in form of comparison of particle number production, probability density function of size, particle size distribution (PSD), and particle shape of treated dolomite with that of fresh dolomite. The TG operating conditions are provided in Table 2. Samples of Glanshammar dolomite, case #1, #2, and #3, was treated in the reactor with a heating rate of 10 °C/min until final temperatures of 650, 750, Table 1 FTIR absorption wavenumbers of water vapor and carbon dioxide. Name

Wave numbers (cm−1)

Corresponding species and functional groups

Reference

CO2

2400–2250 600–770 1700–1600 4000–3500 2000–1300

C]O, carbon dioxide C]O C]O stretching vibration OeH, water multiplet OeH, water multiplet

[38,39]

H2O

3

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Table 2 Operating conditions for TGA experiments at different temperatures and gas conditions. Case

#1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14 #15 #16

Dolomite

Glan. Glan. Glan. Glan. Glan. Glan. Glan. Glan. Glan. Sala Sala Sala Glan. Glan. Sala Sala

Size* (μm)

300–350 300–350 300–350 300–350 300–350 300–350 300–350 300–350 300–350 300–350 300–350 300–350 350–500 500–1000 350–500 500–1000

Heating rate (°C/min)

10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10

Tfinal (°C)

Time at Tfinal (min)

650 750 950 650 750 950 650 750 950 950 820 950 950 950 950 950

0 10 10 0 10 10 0 10 10 10 10 10 10 10 10 10

Pressure (bar)

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Carrier gas N2

H2O

CO2

100% 100% 100% 70% 70% 70% 20% 20% 20% 100% 20% 20% 20% 20% 20% 20%

0% 0% 0% 30% 30% 30% 30% 30% 30% 0% 30% 30% 30% 30% 30% 30%

0% 0% 0% 0% 0% 0% 50% 50% 50% 0% 50% 50% 50% 50% 50% 50%

*Measured using a mesh sieve.

and 950 °C, respectively. Dolomite has in general a low calcination temperature at approximately 700 °C, where MgCa(CO3)2 forms MgO and CaCO3, and a subsequent high calcination temperature of around 800–950 °C where CaCO3 decomposition, depending on the CO2 partial pressure in the carrier gas [41–43]. The calcination occurring at the lower temperature and high temperature is also called half calcination and full calcination, respectively. The test at 650 °C was conducted to evaluate primary fragmentation induced by thermal shock and water evaporation, excluding possible effects of calcination, occurring at a relatively high temperature. Experiments #2 and #3 were aimed at producing half-calcined and full-calcined dolomite for comparing effects of each calcination stage on fragmentation. To investigate effects of steam on primary fragmentation, 30% steam, similar to the steam level in a typical air/steam blown FB gasifier, was added to the carrier gas. Case #4, #5, and #6 were tested to evaluate the effect of steam addition in the carrier gas. During biomass gasification, there is a significant partial pressure of CO2, expected to play a decisive role of CaCO3 calcination. Approximately 50% CO2 is effective in rising the calcination temperature to above 850 °C, according to a theoretical calculation [44]. In experiments #7 – #9, a mixture of 30% steam and 50% CO2 was used. Sala dolomite exhibits a different crystalline structure compared to Glanshammar. For comparison, effects related to the crystalline structure was tested in experiments #10 – #12. Since the Sala dolomite showed a relatively higher end temperature for half calcination in the presence of 50% CO2, a higher final temperature of 820 °C was used in this case. Dolomite particles with sizes of 350–500 μm and 500–1000 μm (#13–14 for the Glanshammar dolomite and #15–16 for the Sala dolomite) were also tested and compared with tests performed using sizes of 300–350 μm.

contained multi-aspect information of a large number of particles. Several indicators, as described below, are proposed for characterizing the dolomite fragmentation. 2.7.1. Circle equivalent diameter (CE diameter) CE diameter is the diameter of a circle with the same area as the 2D image of the particle captured by the Morphologi G3 for each particle. It is used for the characterization of particle size before and after the treatment in the TG tests. 2.7.2. Particles production Particles number in i size fraction per unit of dolomite sample is expressed as

ni =

Ni mfresh

(1)

where Ni is number of collected particles in i size fraction, mfresh is mass of original dolomite sample. 2.7.3. Particle size distribution Probability density function of particle size distribution is defined as

p=

dY dP

(2)

where Y is normalized cumulative size distribution, P is particle size. 2.7.4. Volume fraction distribution Volume fraction of particles in i size fraction is expressed as

vi = 2.7. Characterization of particle size and shape

Vi Vfresh

(3)

where Vi is volume of collected particles in i size fraction, Vfresh is volume of orginal dolomite sample.

The particle size and shape were measured using a Malvern Morphologi G3 characterization system (Malvern Instruments, Malvern, UK). The samples were first loaded in a holder, and then accelerated and dispersed over a glass plate by air injection using a Sample Dispersion Unit (SDU). The instrument, equipped with a motorized slide, acquired subsequent high-resolution images over the whole glass plate to be analyzed in detail later. To avoid errors, resulting from sampling a small not representative part of particles, all particles in each sample from TG tests were loaded into the Morphologi G3 instrument and scanned. After identifying and analyzing all particles’ images, the particle size and corresponding shape parameters were calculated and obtained. The exported data

2.7.5. Particle shapes distribution Several particle shape parameters, including high sensitivity circularity (HSC) and convexity, can be obtained by the Malvern Morphologi G3, as shown in a Fig. 1 [45]. HSC, which is similar to roundness, is an indicator, expressing the similarity of a particle to a circle. It is defined as

HSC =

4 ×A Pe 2

(4)

where A is the cross-section area of the particle, and Pe is the perimeter. 4

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425 °C to 550 °C. The FTIR spectrum can determine gas evolutions by tracing their characteristic absorbances when heating up dolomite at elevated temperatures. Examples, detected at temperatures of 236 °C (before fragmentation), 495 °C (during fragmentation), and 753 °C (during calcination), are presented in Fig. 3(a). Broad bands of the wave number 1300–2000 cm−1 and 4000–3500 cm−1, attributed to the stretching vibration of the O–H bond and indicate H2O release, are observed. The spectrum intensity vs. temperature at the selected wavenumbers for H2O after subtracting the baseline value (to exclude the effect of the shift) are presented in Fig. 3(b-c). The intensity of H2O characteristic absorption bands increases with increasing of temperature below 400 °C. Fig. 3(b) and (c) also display a deviation of Glanshammar and Sala curves, representing H2O characteristic absorption bands, starting at around 250 °C and becoming more significant after 380 °C. 3.2. Particle shape and morphology

Fig. 1. Characterization parameters of particle shape [45].

The morphologies in Fig. 4 indicate an angular shape of both Glanshammar and Sala dolomite particles before and after test. It is worth noting that the particle CE diameter is bigger than the size characterized using a mesh sieve. Taking 300–350 μm fresh Glanshammar dolomite as an example, the lower and upper boundary of size range is 300 μm and 350 μm, respectively, while a CE diameter of larger than 500 μm can still be observed, as illustrated in Fig. 4(a), where only some large particles are presented. The mesh sieve screened out particles that could not pass through the 0.35 × 0.35 mm2 orifice; however, the imaging technique of Malvern Morphologi G3 detected the perimeter of irregular dolomite particles and calculated the CE diameter.

Convexity is an indicator to show the roughness of a particle. Its expression is as

Convexity =

Pe PeC

(5)

where PeC is the perimeter of the convex hull. Probability density function of particle HSC distribution is defined as

h=

dH dHSC

(6)

where H is normalized cumulative HSC distribution. Probability density function of particle convexity distribution is defined as

c=

dC dconvexity

(7)

where C is normalized cumulative convexity distribution. 2.7.6. Particle fracture ratio The total volume, Vfresh, of fresh dolomite samples, is calculated by summing up the volume of each particle size that are detected by the Malvern Morphologi G3. The volume of treated dolomite particles consists of two parts: Vt1, the volume of the particles above the lower boundary of the size range of fresh dolomite samples; Vt2, is the volume of produced particles below the lower boundary of the size range. The fracture ratio is defined as

F=

Vfresh

Vt1

Vfresh

(8)

3. Results 3.1. Fragmentation initiation and gas evolution The dolomite primary fragmentation is detected from thermogravimetric and differential thermogravimetric (DTG) curves of Glanshammar and Sala dolomite calcination at a heating rate of 5 °C min−1 as shown in Fig. 2. Since the sample was detained in an alumina crucible without a lid, part of fragmented pieces flew out, resulting in sudden weight losses in the weight loss curves, also displayed as peaks in the derived weight loss curves. In Fig. 2(a), the primary fragmentation of Glanshammar dolomite initiates at around 390 °C, while the fierce and frequent explosive cracking falls in the range from 425 °C to 550 °C, at a significantly lower temperature, compared to the calcination, starting at approximately 650 °C and ending at around 750 °C. In Fig. 2(b), the Sala dolomite only shows two peaks from

Fig. 2. Weight change of dolomite in the TG-FTIR experiments: (a) Glans., 5 °C/ min; (b) Sala, 5 °C/min. 5

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Fig. 3. FTIR spectrum during dolomite fragmentation: (a) Glans., 5 °C/min; (b) wavenumber: 1507.16 cm−1; (c) wavenumber: 3587.10 cm−1.

Since the Sala dolomite is bar-shaped and thinner, its CE diameter could be even larger compared to the Glanshammar dolomite.

level, has a significant effect on the particle production in the N2 atmosphere, as shown in Fig. 5(a). For case #1, treated in N2 up to 650 °C, a significant reduction of the particles with sizes above 300 μm (the fraction with sizes equivalent to the original particle distribution) and a high production of particles below 300 μm, can be observed. This indicates severe fragmentation already at 650 °C, consistent with the results observed from Fig. 2. When increasing the temperature to 750 °C, case #2, at which the calcination is not yet finished, the fraction of particles below 300 μm increases dramatically. With a further increase up to 950 °C, case #3, which is adequate for a full calcination, the number of particles below 300 μm even decreases. The PSD of the corresponding size fraction, showed in Fig. 5(b), displays a similar trend with particles production, when the temperature increases from 650 to 750 °C. The PSD of case #3 at 950 °C, has a higher PSD compared to case #2 at 750 °C, indicating a smaller total number of particles above 30 μm and a larger production of fines below 30 μm for case #3.

3.3. Particle production and size probability distribution After fragmentation, the fragmented particles are characterized by the number of produced particles vs. particle size with an interval size of 25 μm, and the probability density function (PSD) of the corresponding particles, as shown in Fig. 5. For 300–350 μm fresh Glanshammar dolomite, Fig. 5(a), the number of particles in the size range from 100 μm to 300 μm is very low, while the number of particles increases sharply below 50 μm with decreasing size. The possible reason could be attributed to fines either adhered to the large particles or produced from particles attrition. The friable structure is also evidenced by the fines and dust adhered to the hands after touching the sieved fresh dolomite particles. Thus, in order to exclude the effect of the fines, only particles above 30 μm were considered for the particles production and size distribution analysis. The range of fresh dolomite sample given in Table 2 and the legend in Fig. 5 is the size obtained from a steel mesh, allowing particles with different shapes, such as bar shaped to pass through. While the range exhibited by the curves is the size measured using Malvern Morphologi G3, which measure a 2D (the width and length) image of each particle for CE diameter calculation. Therefore, the steel mesh most likely underestimates the size of bar shaped particles compared to the Malvern instrument using the CE diameter calculations.

3.3.2. Effect of gas conditions Fig. 5(a) shows the particle production of Glanshammar dolomite treated at different gas conditions. The presence of 30% H2O in the carrier gas at 650 °C inhibits the fragmentation of particles with sizes equivalent to the original particles. This is illustrated by e.g. the number of large particles for case #5 and #6 with the final temperature of 750 °C and 950 °C, respectively, compared to the corresponding test with N2. With an addition of 50% CO2 in the carrier gas, the fragmentation is significantly prevented for all temperatures of 650 °C, 750 °C, and 950 °C, as shown for case #7 to #9 in Fig. 5(a). The PDF curves in Fig. 5(b) do not deviate from each other as distinct as in case of particle number curves in Fig. 5(a). In Fig. 5(b), case #4 has a lower

3.3.1. Effect of temperature The final temperature, a decisive factor of the dolomite calcination 6

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Fig. 4. Dolomite morphologies imaged by Malvern Morphologi G3: (a) fresh Glanshammar, 300–350 μm; (b) after treated (Glans., 300–350 μm) in N2 up to 950 °C at 10 °C/min; (c) fresh Sala, 300–350 μm; (d) after treated (Sala, 300–350 μm) in N2 up to 950 °C at 10 °C/min.

PDF in the zone from 100 μm to 300 μm than #1, while its PDF of large particles is higher. Case #7 shows the highest PDF for both 100–300 μm and large particles, except for the part at the peak, attributed to the largest production of all particles above 30 μm, compared to cases #1 and #4.

large peak for case #9, shown in Fig. 5(a). A similar result is also observed before full calcination of Sala and Glanshammar, comparing case #11 in Fig. 5(e) and case #8 in Fig. 5(a). The PDF curves in Fig. 5(f) clearly shows a much higher PDF level of Sala, compared to the corresponding cases of Glanshammar, shown in Fig. 5(b) and (d). Fig. 5(c), case #15, Sala particles with an original size of 350–500 μm, only a slight change in the particles production curve, compared to its fresh original particle distribution is observed. With an increase in the fresh original particle size used, as for case #12 and #15 in Fig. 5(e), the valley for the low-level particles production gets wider on the right side, but do not deviate much on the left side. Both cases also exhibit a similar slope rate in the 100–300 μm size range of PDF curves in Fig. 5(f), the same findings as observed for Glanshammar in Fig. 5(d).

3.3.3. Effect of original particle sizes The production and PDF of fragmented particles with larger original sizes are presented in Fig. 5(c-d). The particle number at the peak of case #13 and #14 changes slightly compared to the original particle sizes, shown in Fig. 5(c), while the production at the peak of case #9 in Fig. 5(a) decreases significantly. The PDF curves in Fig. 5(d) have profiles similar to that of case #7 in Fig. 5(b). It is interesting to note that both curves for treated cases in Fig. 5(d), below the size where the valley falls, have a very similar slope rate, indicating a similar fragmentation mechanism in the same gas atmosphere.

3.4. Volume fraction The volume fraction of treated dolomite samples is calculated using Eq. (3).

3.3.4. Effect of dolomite Dolomite property is a decisive factor for the primary fragmentation behavior. A considerablely smaller fragmentation level can be observed for the Sala dolomite, compared to the Glanshammar, as illustrated by case #10 and #3, both treated at 950 °C in N2, and shown in Fig. 5(e) and (a), respectively. In case of the Sala dolomite, most of the remaining particles have larger sizes in comparison to the original particles after primary fragmentation. In the presence of CO2, the primary fragmentation of the Sala dolomite is also effectively inhibited, and its particle production differs from that of the Glanshammar dolomite. In Fig. 5(e), for case #12, the number of large particles is comparable to or only slightly lower than the fresh original particle distribution. Moreover, the number of particles in the range from 100 μm to 300 μm is very low, compared to the Glanshammar dolomite, as illustrated by the

3.4.1. Effect of temperature From Fig. 6(a) it is obvious that there is a decrease of the volume fraction at the peak of the curves for all three temperature cases in N2 atmosphere. This indicates a large production of fines below 30 μm, which is consistent with the production of particles observed in Fig. 5(a). 3.4.2. Effect of gas conditions The effect of steam and carbon dioxide on the volume fraction at different temperatures is displayed in Fig. 6(b). The addition of 30% H2O in the surrounding gas does not effectively prevent the primary 7

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Fig. 5. Effect of temperature and gas condition on dolomite particles production and PDF: (a) particles production, Glans.; (b) PDF, Glans.; (c) particles production, Glans.; (d) PDF, Glans.; (e) particles production, Sala; (f) PDF, Sala.

fragmentation, promoting a volume fraction of large particles. Case #4 and #5 have comparable volume fraction levels with case # 1 and #2, as shown in Fig. 6(a) and (b), respectively. The presence of additional 50% CO2, shown in Fig. 6(b) has a significant effect on retaining the volume fraction of large particles in case of Glanshammar. For instance, for case #7, treated in 50% CO2 and 30% H2O, a much larger volume fraction of both 30–300 μm particles and large particles of similar size as the original fresh particles distribution is observed, compared to case #4 with only 30% H2O. Therefore, a large production of fines would be expected when Glanshammar dolomite is exposed in N2 and H2O. Although, the fines cannot be accurately detected by the imaging technique, the loss of volume due to fines are shown in Fig. 6(a) and (b).

3.4.4. Effect of dolomite Fig. 6(d) shows the volume fraction of Sala dolomite exposed at different conditions. Consistent with the observed results for the particles production in Fig. 5(e), the volume fraction after primary fragmentation only changes to some extent. This is completely different from the observed results for the Glanshammar dolomite in Fig. 6(a-c). Case #10 in Fig. 6(d), displaying the exposure of Sala in N2 to a final temperature of 950 °C, has the lowest volume fraction at the peak among the curves, but is still much larger than for Glanshammar at the same conditions, case #3, shown in Fig. 6(a). Only a slight increase of the volume fraction in the range of 30–300 μm can be observed in Fig. 6(d). For case #11, exposed to 50% CO2 and 30% H2O, a similar curve as for the fresh 300–350 μm dolomite is obtained. In case of the Glanshammar dolomite, the difference is much larger, as illustrated by case #8 in Fig. 6(b). Comparing the curves for samples exposed to a final temperature of 950 °C, 50% CO2 and 30% H2O, shown in Fig. 6(c) and (d), it is evident that the volume fractions of particles in the range of 50–300 μm is much smaller in case of the Sala dolomite, compared to the Glanshammar dolomite.

3.4.3. Effect of original particles With an increase of the original size of fresh dolomite, as shown in Fig. 6(c), the shoulder representing the volume fraction of small daughter particles gets wider and more distinct. A bimodal distribution of volume fraction along the whole size range is observed for case#14, indicating a high production of particles with a specific size for different sizes of original dolomite particles. For case #13 and #14, a transition point at around 200 μm is observed. 8

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Fig. 6. Effect of primary fragmentation on volume fraction (ratio of volume each size fraction over the total volume of fresh dolomite samples): (a) in N2, 300–350 μm Glans.; (b) in H2O, and H2O & CO2, 300–350 μm Glans.; (c) in H2O & CO2, 500–1000 μm Glans.; (d) Sala.

3.5. Medium and average particle sizes

dolomite in N2, are further compared using the PDF of particle HS circularity and convexity. This provides with information about the circular resemblance and roughness of particles. The quantitative analysis of convexity is important to enhance the understanding of the peeling-off mechanism, providing with information about indentations and pits on a particle, possibly formed after peeling off of fine particles.

To facilitate an overall evaluation of the effect of the different factors, the medium and average sizes are investigated. As shown in Fig. 7(a), the medium size number for the Glanshammar dolomite is around 100 μm for all cases, independent of its original medium size and surrounding gas atmosphere. In case of the Sala dolomite, only the cases exposed to N2 and with a large original size fall into the same area as the Glanshammar dolomite. All the cases within the black circle have a significantly large particle number, mainly related to fines below 100 μm. Fig. 7(b) and (c) shows the average particle sizes of both Sala and Glanshammar samples, treated at different conditions. In Fig. 7(b), the final average size (in the detected range above 30 μm) of all cases is smaller than the initial average diameter, mainly due to primary fragmentation. The final average diameter of Glanshammar dolomite advances almost along a line parallel to the diagonal line in Fig. 7(b). The final average diameter of small Sala dolomite particles is close to the initial average diameter, which is consistent with the observation in Fig. 5(e) and (f), while that of large Sala dolomite particles (500–1000 μm) deviates significantly from the corresponding initial average diameter. The gas conditions do not show any distinct effect on the final average diameter in Fig. 7(b). However, in Fig. 7(c), where the final average diameter at 50% of initial sample mass is presented, the distinct effect of gas conditions can be observed, which is consistent with the volume fraction profiles in Fig. 6.

3.6.1. HS circularity In Fig. 8, the dolomite particles of cases #1, #2, and #3, are divided into two groups with the sizes of 50–300 μm and 300–350 μm (corresponding to the size of the original particles), allowing a comparison with the original particle distributions. In case of the particle fraction 300–350 μm, the probability density function of the HS circularity is similar as the original fresh Glanshammar dolomite for all three cases, as shown in Fig. 8(a). This indicates minor changes in roundness after primary fragmentation. However, for the 50–300 μm particles, a much lower HS circularity is observed. This may be due to a production of chip- or bar-shaped pieces during the primary fragmentation of the Glanshammar dolomite, displayed as a low HS circularity. In case of Sala dolomite, Fig. 8(b), after primary fragmentation, the probability density function at both the peak summit and the shoulder of 300–350 μm particles deviate from the original fresh Sala size distribution, indicating a more angular shape of the larger particles. The PDF of the 50–300 μm particles shifts to the right side, opposite to that of the 50–300 μm particles from Glanshammar dolomite in Fig. 8(a), inferring different fragmentation mechanisms for Glanshammar and Sala dolomite.

3.6. PDF of particle circularity and convexity The shape of particles, produced from Glanshammar and Sala 9

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Fig. 7. Initial and final size of Glanshammar and Sala dolomite: (a) medium particle size (only sizes above 50 μm considered); (b) average diameter in the detected size range; (c) average diameter at 50% of initial sample mass.

3.6.2. Convexity Fig. 8(c) shows a slight increase in the convexity of 300–350 μm Glanshammar particles exposed to N2, compared to the original fresh dolomite, indicating a smoother surface of the larger particles after fragmentation. In case of Sala dolomite, Fig. 8(d), the convexity of 300–350 μm particles decreases significantly, which could be attributed to the formation of a large number of pits on the surface after peeling off of small fragmented pieces. The convexity of the 50–500 μm Sala dolomite, Fig. 8(d), shows a higher PDF, compared to the corresponding Glanshammar particles, case #1, in Fig. 8(c), inferring a high production of round fines for the Sala dolomite.

calcination, also a pressure build-up, caused by CO2 formation, during calcination may be discarded as a plausible explanation for the fragmentation at low temperatures. McCauley and Johnson [15] reported a similar temperature range of 380 to 420 °C for dolomite decrepitation to be attributed to water trapped in the dolomite structure. Moreover, Gunasekaran and Anbalagan [37] observed a gradual decrease in the O–H intensity, starting from fresh dolomite, when heated up to 500 °C , while other bands remained unaffected. This indicates that water vapor trapped in the structure could be a reason for dolomite decrepitation below 650 °C. However, the fragmentation occurs in a wide temperature range from 420 to 600 °C, and it is therefore not likely this is only due to internal pressure, created by evaporated water in lattices. Valverde et al [46], observed a significant weight loss in a TGA test in N2 at around 550 °C, even though no obvious dolomite phase transformation was detected by in-situ XRD analysis. This weight loss is either attributed to release of H2O or CO2. In the present study, it is shown that fragmentation of Glanshammar dolomites is significantly reduced in the presence of 50% CO2 with a final temperature of 650 °C. Since the presence of CO2 is more likely to affect CO2 release rather than H2O, it is reasonable to conclude that CO2 may be released before 650 °C, although at a very low level (without triggering obvious phase transformation of dolomite) in the absence of CO2. This results in a significant effect on the structure of the dolomite also before the dolomite phase transformation starts. Therefore, water evaporation can be discarded as the solely cause of the fragmentation. This is supported by the fragmentation behaviors of different sorbents observed by Hu and Scaroni [16] and Zhou et al. [4,47]. Nevertheless, the occurrence of fragmentation before the dolomite phase transformation suggests that the structure may be unstable at a much lower temperature than the temperature where a phase transformation occurs [46]. This indicates that

4. Discussion 4.1. Mechanism for primary fragmentation Three mechanisms of dolomite primary fragmentation are commonly proposed in the literature: thermal shock; internal pressure builtup due to water trapped in lattices; high pressure of CO2 induced by calcination. In the present study, via TG-FTIR tests with a low heating rate of 5 °C/min, the different stages of fragmentation were characterized separately without overlapping. From the results, the primary fragmentation of both Glanshammar and Sala dolomite initiate at a temperature around 390 °C, but with completely different behaviors in the subsequent temperature zone before calcination, displaying as expected that thermal-shock fragmentation is not the dominating mechanism in the TG. The major part of the Glanshammar dolomite fragmentation occurs in the temperature range of 400 to 550 °C, while only minor fragmentation is observed between 650 and 950 °C. Since fragmentation mainly occurs at temperatures below the onset of 10

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Fig. 8. Effect of primary fragmentation on particle shape: (a) PDF vs. HS Circularity, Glans., #1, #2, and #3; (b) PDF vs. HS Circularity, Sala, #10; (c) PDF vs. Convexity, Glans., #1, #2, and #3; (d) PDF vs. Convexity, Sala, #10.

of dolomite, leaving oxygen ions O2−, resulting in an opposite migration of Ca2+ and Mg2+. Carbon dioxide release at an early stage, which may distort the structure and results in structure instability, is the main contributor of the severe fragmentation. This conclusion is supported by the inhibited fragmentation of dolomite in the presence of 50% CO2, which may prevent the CO2 release at the defects and therefore prevent the dolomite structure instabilities. Due to the presence of CO2 the Glanshammar dolomite possibly could sustain its structure without fragmentation until the dolomite calcination temperature is reached. The very small increase of fragmentation for case #8 and #9, compared to case #7 at 650 °C, indicates that the dolomite structure is stabilized during the dolomite heat-up, either via preventing CO2− dissociation at the defects or a fill of cations by interchanging CO2 with the dolomite. The role of CO2 during this stage is further confirmed by the much lower cumulative pore volume of samples calcined in CO2 compared to those calcined in a N2 [46], indicating the ability of CO2 to change the dolomite structure via interchanging CO2 with the dolomite rather than CO2 release outward the dolomite particles. In case of Sala dolomite, minor fragmentation may occur during heating or calcination, but the mean particle size and size distribution remain almost the same throughout the calcination process. In the presence of CO2, the fragmentation increases slightly with increasing temperature and particle size. This indicates that amorphous structure can be decisive to the fragmentation of dolomite. Due to its amorphous structure, its decomposition in N2 does not come with the rapid structure collapse, therefore no severe fragmentation. It is worth to note that the two peaks, observed in Fig. 2(b), could still be caused by the minor crystalline

dolomite phase transformation and calcination level are unreliable indicators for characterizing dolomite fragmentation. Based on the discussion above, we conclude that the dolomite calcination mechanism, especially during the initial calcination at low temperatures, is crucial for understanding the fragmentation. A large number of studies on dolomite calcination, mostly using TGA tests or final product characterization, have been carried out [46,48–51]. However, only a few of them tried to monitor the corresponding phase transition along the dolomite conversion from a low temperature to its final calcination stage. Valverde et al. [46] suggested that dolomite decomposes directly at a temperature around 700 °C into MgO and CaO, followed by an immediate carbonation of nascent CaO crystals. There was no phase transformation observed before dolomite calcination under both CO2 and N2 atmosphere from their study. Galai et al. [49] also reported that, within the first 5% of dolomite conversion (treated to temperature up to 680 °C with 0.025 atm CO2), neither MgO nor any intermediate compound could be detected using XRD. A new diffraction peak turned out to be visible only when the conversion of dolomite is over 20% and gradually shifted towards that of CaCO3 with the increase of conversion. Gunasekaran and Anbalagan [37] found the CO3− bands of dolomite became weak at 650 °C and shifted from 1446 cm−1 (typical for dolomite) to 1420 cm−1 (typical for calcite), indicating a direct formation of calcite from dolomite. It seems that in the very beginning of the decomposition, a loss in the initial CO3 stoichiometry of the dolomite structure may occur, before a nucleation and growth process of MgO and calcite outwards in the particles. Galai et al. [49] proposed a model where CO2 may release at defects first on the surface 11

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dolomite contained in Sala, determined in a previous XRD test [4]. To summarize, as also supported by the evidence from the shape results mentioned above, Glanshammar dolomite fragments by peeling of barshape or chip-shape particles, while Sala dolomite generally lose very small amount by formation of cavities on the surface, and therefore producing a number of round fines. However, the presence of CO2 is effective in inhibiting fragmentation of Glanshammar dolomite at low temperatures before calcination. Fig. 9 illustrates the fracture ratio together with a corresponding fragmentation mechanism at different conditions for both Glanshammar and Sala dolomite. In Fig. 9(a), for the cases with 30% H2O and 50% CO2, the fracture ratio of Glanshammar particles increases when increasing the initial size from 450 μm to 600 μm, while a plateau of the fracture ratio can be observed when further increasing the size to 800 μm (a slight decrease for the case with the end temperature of 950 °C), indicating the degree of fragmentation reaches the maximum level at the initial particles size of around 600 μm. This is much higher compared to previously reported results [14,15], investigating dolomite decrepitation in N2 atmosphere. High pressure fragmentation induced by water trapped in crystal lattices is considered to be the main contributor of fragmentation in the presence of CO2. Also, larger particles have too much mass to be influenced by the released pressure and smaller particles do not have sufficient strength to allow the pressure to build up. One interesting finding is the production of 100 μm particles for case #13–14, shown in Fig. 6(c), starting from different original particle sizes. This implies that the fragmentation has a high probability to be initiated, exhibiting similar fragmentation behavior at the similar size grains, even in particles with different sizes. Therefore, fragmentation is not just a result of the pressure built-up within the lattice,

exceeding the mechanical strength, but also an effect of atmosphere and dolomite properties, as exemplified for the Sala dolomite in N2 atmosphere in Fig. 9(a). As mentioned above, dolomite fragmentation advances via several mechanisms at elevated temperatures. A proposed fragmentation mechanism schemes for crystal and amorphous dolomite are illustrated in Fig. 9(c) and (d), respectively. By comparing particles shape in terms of HS circularity and convexity, the Sala dolomite fragments by peeling of small and round fines from the outer surface, while Glanshammar dolomite fragmentation occurs by peeling of bar-shape or chip-shape particles. The large macoscopic crystalline grain size of Glanshammar promotes break up along the grain boundaries, supported by the observed well-developed network of macro-pores of used Glanshammar in pressurized FB gasification [47]. It has been concluded that the primary fragmentation of crystalline dolomite is induced by a structure instability, initiated due to the release of H2O trapped in crystal lattices as well as release of trace level of CO2 from structure defects, occurring before dolomite phase transformation. Furthermore, the presence of CO2 effectively prevents the fragmentation by either averting CO2− dissociation at defects or filling the cations by interchanging CO2 with the dolomite, which is crucial for stabilizing dolomite structure. Based on these findings, we propose two mechanisms, one for crystalline and another one for amorphous dolomite. In case of an amorphous dolomite, Fig. 9(c); Fragmentation slightly changes from low temperature to a full calcination temperature, independent of the gas conditions, if the effect of minor crystal component is excluded. For a crystalline dolomite, Fig. 9(d), fragmentation initiates by water trapped in crystal pattern from 390 to 550 °C (Stage 1), and intensifies due to a trace level release of CO2 at defects at low temperatures before 650 °C (Stage 2),

Fig. 9. Effect of primary fragmentation on particles shape: (a) Fracture ratio; (b) fracture contributed by each factor; (c) amorphous dolomite fragmentation mechanism; (d) Crystalline dolomite fragmentation mechanism. 12

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and then further advances with the half calcination (Stage 3) and full calcination (Stage 4) (in one step or in sequence, depends on calcination conditions). Stage 1 and 2 contribute to 10% and 55% of the fragmentation, respectively, while both half calcination and full calcination contribute to around 5%, as shown in Fig. 9(b).

was widely used to characterize the required height for fluidized bed design. A correlation from [59] was used to calculated the required height for particles with different sizes. As shown in Fig. 10(b), the required TDH at the critical size (110 μm, where the terminal velocity equals to the superficial velocity) is 2.8 m, higher than that for 300 μm particles. Entrainment from both bursting bubbles at the bed surface and direct entrainment of critical fines may be calculated using a correlation proposed from Choil et al [60]. Calculated elutriation flux of both fines (with the terminal velocity below the superficial velocity) and bed surface flux increases significantly with the decrease in particles size, as displayed in Fig. 10(c). The total elutriates for dolomite particles of cases #3, #6, #9, #10, and #12 were calculated and are presented in Fig. 10(d). In case of the Glanshammar dolomite, the elutriation rate of dolomite particles of case #9, exposed to 50% CO2 and 30% H2O, is only half of that of case #3, exposed to N2, indicating that the elutriation in a FB gasifier may be reduced significantly, if treated in the presence of CO2 in advance. For the Sala dolomite, the elutriation only changes slightly for particles exposed to different atmospheres. The low elutriation rate for Sala is due to its much smaller primary fragmentation, compared to Glanshammar. Nevertheless, the Sala dolomite exhibited a more friable structure in previous studies compared to the Glanshammar dolomite [4,47], implying it may still be unable to sustain secondary fragmentation, caused by velocity impaction and a subsequent attrition in a FB, resulting in a large number of elutriable fines.

4.2. Elutriation in fluidized bed gasifier The size distribution of dolomite particles fragmented in N2 may become considerable wider, compared with the initial narrow size, complicating the fluidization pattern and exerting profound influences on mixing and segregation behaviors [52]. Dolomite fines whose terminal velocity is below the superficial velocity can entrain with the fluid and be elutriated [53,54]. Taking an atmospheric fluidized bed gasifier as an example, a considerable quantity of fines may be produced after severe fragmentation when using Glanshammar dolomite as bed material, resulting in a large amount of elutriated fines whose terminal velocity is below the superficial velocity, as shown in Fig. 10(a). The elutriate may originate either from the bed surface layers where a segregated layer of fine particles exists, or from the wakes of rising bubbles [55]. The wake, which can be a quarter of the bubble volume, may eject half of the particles it carries [55,56]. Moreover, the bursting bubbles, which always has a higher velocity compared to the superficial velocity due to bubble coalescences along the bed height, can also eject large particles into the freeboard [57,58]. These large particles will deaccelerate due to its gravity force after ejection and fall back to the bed. Transport Disengaging Height (TDH)

Fig. 10. Dolomite fluidization and elutriation in a fluidized bed gasifier (original Glanshammar dolomite particle size: 300–350 μm; bed diameter: 1 m; superficial velocity: 0.4 m/s, bed temperature: 850 °C; bed pressure: 1 bar): (a) fluidization velocity; (b) TDH; (c) elutriation flux; (d) dolomite elutriation rate. 13

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5. Conclusion [8]

Dolomite primary fragmentation behaviors and mechanism at elevated temperatures, including its full calcination were thoroughly investigated by quantitatively analyzing particles production, size distribution, and shape of fragmented particles. The primary fragmentation mechanism of both crystalline and amorphous dolomite was presented and discussed. Measures to inhibit particle fragmentation were identified. The general conclusions are summarized as follows:

[9] [10] [11]

• The primary fragmentation of crystalline Glanshammar dolomite







[12]

initiated at 390 °C and significantly increased at temperatures from 400 to 550 °C before final calcination in N2 atmosphere. By introducing 30% H2O, during the exposure, no effectiveness in inhibiting fragmentation was identified, while 50% CO2 effectively prevented fragmentation even at lower temperatures below 650 °C. Amorphous Sala dolomite fragmented, but retained its shape with very small mass losses, caused by peeling off of a number of relatively roundly shaped fines from the original particles, resulting in a minor size reduction. Crystalline Glanshammar, on the other hand, started breaking up from the interior of the particles, producing a large number of small and fine particles during primary fragmentation. Different mechanisms were proposed for amorphous and crystalline dolomite. The primary fragmentation of crystalline dolomite is induced by a structure instability at low temperatures, initiated due to a release of H2O trapped in crystal lattices and a trace level of CO2 at structure defects, which may contribute 10% and 55% of the fragmentation, respectively. The presence of CO2 prevents the fragmentation, occurring during the release of a trace level of CO2 at low temperatures by either preventing CO2− dissociation at the defects or filling the cations by interchanging CO2 with the dolomite, which is crucial for stabilizing the dolomite structure. In case of the amorphous dolomite, fragmentation mainly occurred due to thermal shock and the presence of minor crystal dolomite domains in the particles, independent of the gas conditions. The elutriation in a FB gasifier may be significantly reduced when treating the crystalline dolomite in the presence of CO2 before use.

[13] [14] [15] [16] [17]

[18] [19] [20] [21] [22] [23]

Acknowledgements

[24] [25]

This work was carried out within the Swedish Gasification Center consortium. Funding from the Swedish Energy Agency (34721-2), academic and industrial partners (E.ON and ANDRITZ) is gratefully acknowledged.

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