Effect of oil shale semi-coke on deposit mineralogy and morphology in the flue path of a CFB burning Zhundong lignite

Abstract

The effect of oil shale semi-coke (SC) on the mineralogy and morphology of the ash deposited on probes situated in the flue path of a circulating fluidized bed (CFB) which burns Zhundong lignite (ZD) was investigated. 10 wt% or 20 wt% SC was added to ZD, which were then combusted in the CFB furnace at 950°C. Two probes with vertical and horizontal orientations were installed in the flue duct to simulate ash deposition. Both windward and leeward ash deposits on probes (P₁W, P₁L, P₂W and P₂L) were analyzed by using a scanning electron microscopy with energy dispersive X-ray (SEM-EDX), X-ray diffraction (XRD), an inductively coupled plasma optical emission spectrometry ICP-OES, and a particle size analyzer. When ZD was burned alone, the P₁W deposit was comprised of agglomerates (<30 mm) enriched in CaSO₄ and Na₂SiO₃, incurring significant sintering. The P₁L and P₂W deposits, however, were of both discrete and agglomerated particles in similar mineral phases but with coarser sizes. The P₂L deposit was mainly fine ash particles where Na₂SiO₃ and Na₂SO₄ were absent. As SC was added, the agglomerates in both P₁W and P₁L decreased. Moreover, SiO₂ and Ca/Na aluminosilicates dominated the mineral phases whereas Na₂SiO₃ and Na₂SO₄ disappeared, showing a decrease in deposit stickiness. Likewise, the P₂W deposit was found less spread on the probe, decreasing its deposition propensity. Na-bearing minerals turned into (Na, K)(Si₃Al)O₈ and (Ca, Na)(Si, Al)₄O₈ in the P₂W deposit. Moreover, Na in the deposits decreased from 32 mg/g to less than 15 mg/g as SC presented. The addition of SC would therefore help alleviate the propensity of ash deposition in the flue path in the CFB combustion of ZD.

Keywords

ash deposition / circulating fluidized bed (CFB) / mineral transformation / oil shale semi-coke (SC) / Zhundong lignite (ZD)

Introduction

Zhundong lignite (ZD) in North-west China is believed to be a potential energy resource for power and steam generation by virtue of its abundant reserve, low ash and sulfur contents, and high reactivity [¹]. However, severe ash deposition on heat transfer surfaces has incurred inevitably in its combustion owing to its high alkali and alkali earth metals (AAEMs) contents [²]. These deposits would increase the operation and maintenance costs, and sometimes lead to unexpected plant outage [³]. Therefore, it is essential to investigate the deposition mechanisms and developing mitigation strategies for ash deposition in the CFB combustion of ZD [³].

A vast volume of literature on research into ZD ash deposition has been reported, revealing that the severe ash deposition in the combustion of ZD was incurred by its abundant AAEM (Na, Ca, and Mg) contents [⁴–⁷]. Notably, Na in ZD is mainly in the form of water-soluble and ammonium-soluble [⁸–¹⁴] and would be released to gas phase in combustion [⁶,¹⁵]. The released Na vapor would subsequently condense or deposit on the heat transfer surfaces, initiating a stickydeposition layer [⁶,¹⁶]. Ca and Mg are mainly organically bound with coal matrix and prefer to generate Ca and Mg fumes, allowing for the deposition of other ash particles [⁷,¹⁵]. The abovementioned fates of Na and Ca subsequent ash deposition and mineral phase transformation are dependent on combustion temperature. A decrease in combustion temperature from pulverized-fuel combustion of 1400°C to circulating fluidized bed (CFB) combustion of 800°C–950°C has found to decrease the amount of Na released into gaseous phase and therefore benefited deposition mitigation [³]. Despite this, obvious ash deposition remain occurring on the heat transfer surfaces particularly in the flue path in CFB combustion [¹⁷]. Na- and Ca-bearing aerosols or fumes (i.e., NaCl, Na₂SO₄, CaO, CaSO₄, MgFe₂O₄, and MgO) are still formed in the CFB combustion [¹³,¹⁸]. These Ca/Na chlorides, sulphates of low melting-points as identified in the fly ash were believed to be responsible for ash deposition on heat transfer surfaces at the flue path [¹⁸–²⁰].

Co-firing of ZD with another fuel could be an effective means for deposition mitigation, provided that sodium could be effectively captured by the incorporated inorganic constituents. These could be bituminous, oil shale SC, sewage sludge, or even biomass of high Al and Si contents [⁵,²¹]. For instance, oil shale SC is enriched in Si/Al (>80%) [²²], which would decrease the sodium volatility to 14.5% when ZD was co-fired with oil shale in a fixed bed reactor at 850°C [²³]. Therefore, co-firing of ZD and oil shale SC could be a promising means for mitigating ZD ash deposition in CFB combustion [²⁴]. However, the effect of oil shale SC on Na capture and ash deposition in the flue path in CFB combustion has less been reported. This is however essential for understanding the ash deposition characteristics on heat transfer surfaces in the flue path in practical utility boilers.

This paper presents a comprehensive investigation into the effect of oil shale SC addition on themorphology, mineralogy, and size distribution of the ash deposited on the heat transfer surfaces in the flue path in CFB combustion of ZD. ZD and oil shale SC were co-fired in a laboratory-scale CFB, and the morphology, mineralogy, ash chemistry, and particle size distribution of the ash deposited on probes were further analysed in detail. This paper would provide a reference for deposition mitigation in the combustion of ZD in CFB, and offer an effective means for SC utilization.

Methods

Material

The as received ZD was crushed and sieved into particles with a cut-size of 0–3 mm. An oil shale SC with the same size range was used for co-firing. Proximate and ultimate analysis of ZD and SC, and their ash chemistries had been well analyzed [²⁴]. ZD has a low ash content of 4.3% on a dry basis, while SC has a high ash content of 86.16%. The ZD ash was rich in Na₂O (4.9%), CaO (32.51%) and SO₃ (27.93%), indicating a high fouling propensity. The SC ash was however abundant in Al₂O₃ (22%) and SiO₂ (55.9%) but depleted in AAEM. Addition of SC of high Si/Al contents would provide availabilities for Na capture and deposition mitigation. Moreover, ZD and SC blends were prepared by physically mixing ZD with SC at a SC ratio of 0 wt%, 10 wt%, and 20 wt%, which were noted as ZD100, ZD90SC10, and ZD80SC20 for simplicity. Furthermore, pure quartz sand within a cut-size of 0.15–1 mm was used as the bed material.

CFB system

A laboratory CFB combustion system (Fig. 1) was used in experimentation, which has been well characterized previously [¹⁷,²⁵]. In brief, the dimension of the CFB furnace is 3000 mm in height, with a cross-section of 150 mm× 150 mm. The flue duct was 2000 mm in height with a cross section of 350 mm× 400 mm. Meanwhile, eight K-type thermocouples and six pressure transducers in different positions were applied to obtain the temperature and pressure distribution of the furnace. Fuel was fed into the bottom of the furnace with a screw feeder, and three electric heaters were equipped for ignition only at the early stage. Moreover, two probes (I.D. 32 mm) were inserted into the flue path to simulate ash deposition. These probes were made of 316L-stainless steel, among which a movable tube and a cover were attached to the probe by a trapezoidal groove on the top [²⁵]. Probe 1 (P₁) was vertically inserted into the flue path from the top, whereas Probe 2 (P₂) was horizontally inserted into the middle zone of the flue path. This enabled investigation into the effect of probe orientations on ash deposition in the flue path.

During experimentation, the bed temperature was maintained at 950°C for three cases. To guarantee stable combustion at the same bed temperature of 950°C, the feeding rates of the blends increased from 7 to 7.7, and 8.6 kg/h for ZD100, ZD90SC10, and ZD80SC20, whereas the fluidized air volume increased from 45.5 to 45.9 and 46.4 N·m³/h (NTP), respectively. The theoretical air volumes required for combustion of the corresponding blends was 44.1, 44.2, and 44.4 N·m³·h^–1, which slightly differed from the actual operation due to possibly incomplete combustion. Moreover, the concentration of O₂, CO, SO₂, and NO_x in the flue gas at the flue path outlet were monitored by using an “ECOM-J2KN” gas analyzer and are listed in Table 1. The O₂ concentration at tail flue outlet was 1.9%, 2.1%, and 2.4%, respectively, for ZD100, ZD90SC10, and ZD80SC20, showing a slight increase in O₂. Meanwhile, CO was decreased from 360 to 115 and 72 mg/m³, while SO₂ and NO_x increased accordingly. The variation in O₂, CO, SO₂, and NO_x concentrations around the probe might to some degree impact the ash deposition on probes, but would be minimal at low flue gas temperatures and therefore was not considered as a significant factor affecting ash deposition.

The temperature distribution of the system depicted in Fig. 2 suggests that the flue gas temperatures along the flue path were similar in each experimentation. Note that the surface temperatures of P₁ and P₂ were not cooled, whose temperature was, therefore, close to the flue gas temperature, being 550°C around P₁ and 400°C around P₂. After stable operation for 6 h, the ash deposited on both the windward and leeward surfaces of P₁ and P₂ were collected gently by using a fine brush, which were denoted as P_xW, and P_xL, where x represents the probe (1 or 2), and W and L are the windward and leeward deposits for short.

Analysis

A Bruker X-ray diffraction (XRD) with copper Kα radiation was used to identify the mineral phases in the deposits. The accelerating voltage and current were 40 kV and 40 mA, and the scanning speed was chosen as 0.0833(°)/s between (5°–85°)/2θ. The mineral phases were qualitatively analyzed by using the X'Pert High Score Plus software. Meanwhile, a TM3030 scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX) was used to observe the morphological features and analyze the elemental composition of these deposits. An inductively coupled plasma optical emission spectrometry (ICP-OES) coupled with a microwave-assisted HF/HNO₃ digestion system was also used to quantitatively determine the concentration of Ca, Na, and Mg in these deposits. In addition, a BT-9300HT laser particle analyzer was used to analyze the particles size distribution of the deposits (0.1–716 mm, ±1%). The characteristic parameter, D₅₀, representing the cumulative particle volume of 50%, was used for comparison.

Results and discussion

Ash morphology

Probe 1 windward: The morphological features and EDX elemental analysis of the ZD100, ZD90SC10, and ZD80SC20 ash deposits on P₁W are illustrated in Fig. 3. It is observed that the deposit on P₁W showed a narrow cone shape in the central region. The deposit was easily brushed off from the probe, indicating that these ash particles were discrete and less sticky. This is more evident when dealing with ZD90SC10 and ZD80SC20 deposits, suggesting that the bonding force between ash particles decreased due to SC addition. The SEM analysis suggested that the ZD100 deposit on P₁W consisted of mainly agglomerates with sizes less than 30 mm, indicating that sintering between ash particles occurred. The EDX analysis revealed that the agglomerates (e.g., particles 1–2) were rich in Ca, S, Si, Al, Mg, and Na (ca. 10%), indicating the presence of Ca/Mg/Na sulphates or aluminosilicates. Moreover, the discrete ash particles (e.g., area 3) were found enriched in Ca and S, indicating the presence of Ca sulfate. The abundance of fine AAEM sulphates and aluminosilicate shall explain the severe fouling propensity of the ash.

As 10% SC was added, the ZD90SC10 deposit on P₁W was comprised of both bulk particles with 60 mm in size and sub-microns. Compared with the ZD100 deposit, less agglomerates were observed in ZD90SC10, indicating that the sintering between ash particles lessened. Furthermore, sub-micron particles were adhered onto the surfaces of the bulk ash particles (Fig. 3(g)), implying that these fine ash particles might have been captured by the bulk ash particles. The EDX analysis revealed that these bulk particles (e.g., particle 4) were rich in Si, Al, Na and Ca, indicating the presence of Ca/Na aluminosilicate. Particles (e.g., particle 5) with a porous structure were rich in Si, Al, and Na, indicating the presence of Na aluminosilicate. This suggests that chemical reaction between Na and Si/Al from SC occurred. Meanwhile, particles with high contents of Si and Al (e.g., particle 6) also presented, which were also believed to be originated from SC. Compared with the ZD100 deposit, the Si and Al in the P₁W deposit were significantly increased, whereas the Na and S contents as identified decreased due to SC addition of high aluminosilicate.

As the SC content further increased to 20%, the ZD80SC20 deposit consisted of more coarse-grained particles in the range of 0–70 mm. The EDX analysis revealed that the agglomerates as identified (e.g., particles 8 and 9) were rich in Ca, Si, and S, indicating the presence of Ca sulfate and silicates. Meanwhile, the Si and Al contents were content in these particles (e.g., particle 7). Moreover, Na content in the ZD80SC20 deposit as identified was negligible, indicating a further decrease in Na content due to SC addition. This might be attributed to both the chemisorption of Na by SC and dilution effect of SC, which will be further discussed later. This suggests that the deposit on P₁W becomes less sticky and exhibits a less sintering and fouling tendency due to SC addition.

Probe 1 leeward: Figure 4 presents the SEM-EDX analysis of the ash deposited on P₁L. It is observed that the deposits on P₁L were thicker than those on P₁W. This might be attributed to a relatively low flue gas velocity around the probe compared with the P₁W in the center, allowing for deposition of fine ash particles on the leeward by eddy impaction [¹⁷,¹⁸,²⁶]. The SEM analysis revealed that the ZD100 deposit was composed of discrete and agglomerated particles, where sintering between ash particles had occurred. These agglomerates as analyzed (e.g., areas 1, 3 and 4) were rich in Ca, S, Al, Si, Mg, and Na, indicating the presence of Ca/Na sulphates and aluminosilicates. Moreover, particles (e.g., particles in area 2) rich in Ca, Si, Al, and Na were identified, indicating the presence of Ca/Na aluminosilicates. The high contents of Na/Ca sulphates and aluminosilicates would contribute to severe ash deposition on P₁L [²⁷].

As 10% SC was added, particles with larger sizes in the deposit evidently increased, indicating that more coarse-grained particles presented. The EDX analysis revealed that these particles (e.g., area 5) were rich in Na, Si, Al, and Ca, indicating the presence of Na/Ca aluminosilicates. Ca and S were also of high contents in certain particles (e.g., area 6), indicating the presence of Ca sulfate. Meanwhile, particles (e.g., particles 7 and 8) rich in Ca, S, Si, and Al were identified, indicating the presence of Ca sulfate or aluminosilicates. Compared with the ZD100 deposit analyzed from EDX, Na and S contents decreased, suggesting that these particles had less Na content [²⁴].

As the SC ratio further increased to 20%, the ash deposited consisted of mostly coarse-grained discrete ash particles. This indicates that ash particles were less sticky and less sintered. In terms of the ash chemistry, particles (e.g., particles in area 9) rich in Ca, S, and Mg were identified, indicating the presence of Ca/Mg sulphates. Particles (e.g., area 10) rich in Ca, Fe, and S were identified, indicating the presence of Ca/Fe sulphates. In addition, Ca, Si, Al, and Fe were of high contents in certain particles (e.g., particles 11 and 12), indicating the presence of Ca/Fe aluminosilicates. In comparison to those at ZD90SC10, Na content was negligible, indicating an absence of Na in the deposit. The increase in coarse-grained particles and the absence of Na would help mitigate ash deposition on P₁L.

Probe 2 windward: Figure 5 illustrates the SEM-EDX analysis of the ash deposited on P₂W. As P₂W was perpendicular to flue gas flow, fly ash with varied sizes could deposit on the windward surface by means of condensation, diffusion, and inertial impaction [⁶,²⁸,²⁹]. It is observed that the ZD100 deposit on P₂W was thick and consisted of a cone-shaped layer on the probe windward (Fig. 5(b)). As the 10% and 20% SC were present, this cone-shaped layer was found less spread on the surface as demonstrated in Figs. 5(f) and 5(k), indicating that the deposits are less sticky and therefore with lessened deposition propensity. The SEM revealed that the ZD100 deposit on P₂W was composed of both discrete particles and sintered agglomerates. The EDX analysis showed that the discrete sub-micron particles (e.g., areas 1 and 2) were rich in Ca, Si, Al, and Na, indicating the presence of Ca/Na aluminosilicates. Meanwhile, the contents of Ca and S were also high in the agglomerates (e.g., particles 3), indicating the presence of calcium sulfate. The high contents of Ca/Na aluminosilicates and calcium sulfate would promote ash deposition.

As 10% or 20% SC was present, more coarse-grained particles were found deposited on the surface whereas those sub-micron particles were evidently reduced when compared with the ZD100 deposit. The EDX analysis showed that the agglomerates (e.g., areas 7 and 11) were rich in Ca and S, indicating the presence of calcium sulfate. Na, Si, Al, and Ca were abundant in coarse-grained particles (e.g., particles 4–6 and 8–10), indicating the resence of Ca/Na aluminosilicates. The decrease in sub-micron particles and increase in aluminosilicates would alleviate ash deposition, and decrease the fouling propensity of fly ash.

Probe 2 leeward: Figure 6 exhibits the SEM-EDX analysis of the ash deposited on P₂L, whose visual observations could be seen in Figs. 5(a), 5(f) and 5(l)). It was found that these leeward deposits on ZD100 were much uniform than those on P₂W, due to the fact that the inertial impact on the leeward was absent. In particular, as 20% SC was added, the ZD80SC20 leeward deposit was rarer than the ZD100 in weight. This proves that the fly ash becomes less sticky to deposit on the leeward surface. The SEM analysis revealed that the ZD100 deposit on P₂L was comprised of agglomerates and discrete sub-micron particles. These particles were finer than those of windward deposits when the inertial impact on the leeward surface was not at play [¹⁸,²⁶]. The EDX analysis revealed that these discrete particles (e.g., particles in area 1 and 3) were rich in Ca and S, while the ash agglomerates (e.g., areas 2) were rich in Na and Mg but with less Si and Al, implying that Na was presented in the deposit on leeward. The abundance of Ca/Na sulfate would contribute to ash deposition on the leeward.

As 10% and 20% SC was added to ZD, the sizes of ash particles in the P₂L deposit became coarser than those of ZD100, indicating a decrease in fine particles in the deposits. The particles analyzed (e.g., particles in areas 4–6) were found enriched in Ca, Si, Al, S, and Na, and particles (e.g., particles 7–10) rich in Si, Al but with less Na and S. The increase in Ca aluminosilicates and the decrease in Na would benefit deposition mitigation on P₂L.

Ash mineralogy

Probe 1 windward: Figure 7 presents the XRD patterns of the ash deposited on P₁W in the combustion of ZD100, ZD90SC10, and ZD80SC20. It is clearly observed that the ZD100 deposit mainly consists of SiO₂, Fe₂O₃, Ca₂Al₂SiO₇, MgO, CaO, CaSO₄, MgFe₂O₄, Na₂Si₂O₅, NaAlSiO₄, Na₂SiO₃, Ca₃Mg(SiO₄)₂, and Na₂SO₄. These mineral phases were similar to those identified in the fly ash [²⁴], indicating that mineral interactions on the probe at 550°C was less likely to occur. As indicated by its peak intensity [³⁰], CaSO₄ was believed to be the dominant mineral phase. The presence of these mineral phases of low melting-points including Na₂Si₂O₅ (melting point (MP) of 874°C), Na₂SiO₃ (MP of 1089°C), and Na₂SO₄ (MP of 884°C) would thus promote ash sintering and ash deposition. For the ZD90SC10 and ZD80SC20 deposits, SiO₂ and CaSO₄ were the prevailing mineral phases. The content of CaSO₄ in P₁W, as indicated by its peak intensity, decreased as SC was added [³¹]. In addition, Na₂SiO₃ and Na₂SO₄ were not identified, whereas CaO and Ca/Na aluminosilicates increased significantly. This suggests a decrease in Na-bearing minerals of low-melting points. The increase in SiO₂, CaO, Ca aluminosilicates of high-melting points, and the absence of Na-bearing minerals suggest that the P₁W deposit are more refractory and less sintered.

Probe 1 leeward: Figure 8 presents the XRD patterns of the ash deposit on P₁L. The mineralogy of leeward deposit was largely similar to those identified in the windward deposit. CaSO₄ was the dominant mineral phase in the P₁L deposit in the combustion of ZD100, where Na₂Si₂O₅, Na₂SiO₃, and Na₂SO₄ were also identified. The presence of these Na-bearing minerals of low-melting points would aggravate ash deposition. As SC was added, SiO₂ became the dominant mineral phase, while Na₂Si₂O₅, Na₂SiO₃, and Na₂SO₄ were not identified. Instead, NaAlSi₃O₈ was present in the deposits, indicating that Na had converted to aluminosilicates due to SC addition [²³]. This is consistent with the minerals phases as identified in the fly ash [²⁴]. The abundance of Ca/Na aluminosilicates and SiO₂ with high melting points would thus help alleviate the fouling propensity of fly ash on P₁L.

Probe 2 windward: Figure 9 displays the XRD patterns of the ash deposited on P₂W. The main mineral phases in ZD100 deposit were SiO₂, Fe₂O₃, Ca₂Al₂SiO₇, MgO, CaO, CaSO₄, MgFe₂O₄, Na₂Si₂O₅, (Ca,Na)(Si,Al)₄O₈, and Ca₂Mg(Si₂O₇).The presence of Na-bearing mineral including Na₂Si₂O₅ and (Ca,Na)(Si,Al)₄O₈ would promote ash deposition on P₂W [¹⁴,¹⁹]. As SC was added, Na₂Si₂O₅ and (Ca,Na)(Si,Al)₄O₈ were not found in the deposit, while NaAlSi₃O₈ and (Na,K)(Si₃Al)O₈ were identified. This indicated that mineral reactions between Na₂O, CaO, K₂O and aluminosilicate occurred. Moreover, the contents of CaO and SiO₂ increased significantly as identified by their peak intensities. The increase in SiO₂ and Ca/Na aluminosilicates of high melting points would decrease the ash stickiness. This explains the fact that the cone-shaped layer formed on P₂W is less spread on the surface as aforementioned.

Probe 2 leeward: Figure 10 presents the XRD patterns of the ash deposited on P₂L. It is clearly seen that CaSO₄ is the dominant mineral phase in P₂L in the combustion of ZD100. The presence of Ca₃Mg(SiO₄)₂ and K_0.42Na_0.58Ca_0.03AlSi₃O₈ revealed that mineral interactions between CaO, MgO and Na aluminosilicates occurred. For the ZD90SC10 and ZD80SC20 deposits, SiO₂ was found to be the dominant mineral phase where Na₂Si₂O₅ and K_0.42Na_0.58Ca_0.03AlSi₃O₈ were not identified. The decrease in Na- and Ca-bearing minerals reveals that the viscosity of the ash deposited decreased and so did the propensity of ash fouling.

Ash chemistry

Figure 11 presents the contents of Na, Ca, and Mg in the P₁ and P₂ bulk deposit inco-firing of ZD and SC. Na content in the P₁ deposit of ZD100 was determined as 30.2 mg/g, whereas those of ZD90SC10 and ZD80SC20 were less than 15 mg/g (12.9 and 14.1 mg/g, respectively). This revealed that the Na content within the deposit decreased as SC was increased, which was ascribed to two reasons: (a) Na had been captured by SC and retained in the furnace, decreasing the amount of Na released into gas phase [²⁴,²⁹,³²]; (b) the addition of 10 wt% or 20 wt% SC incorporated more SC, having a dilution effect on Na content [³³,³⁴]. Moreover, the Ca content in the P₁ deposit of ZD100 was 131.4 mg/g, which decreased to 80.7 mg/g in ZD90SC10 and 76.4 mg/g in ZD80SC20. The Mg content was also decreased due to SC addition. Both the dilution effect and the chemisorption might be at play [¹³], consistent with those reported in Ref. [²⁹].

In comparison, the Na content in the P₂ deposit of ZD100 was 32.5 mg/g, but became 15.8 mg/g in ZD90SC10 and 15.7 mg/g in ZD80SC20, showing a 50% reduction of Na. This implies that the role of Na on ash deposition might be decreased when ZD was co-fired with SC of high Si/Al contents. Moreover, the Ca content in the P₂ deposit of ZD100 was measured as 194.2 mg/g, and as127 mg/g in ZD90SC10 and 95 mg/g in ZD80SC20.The decrease in Ca, Na and Mg contents would help alleviate ash deposition.

Particle size distribution

Figure 12 presents the particle size distribution of ZD100, ZD90SC10, and ZD80SC20 deposits. It is seen that the P₁W deposit exhibits a unimodal size distribution. The D₅₀ of the ZD100, ZD90SC10, and ZD80SC20 deposits on P₁W was determined as 20, 28, and 32 mm, respectively. This implies that the particle sizes of P₁W deposit indeed increased as the SC addition ratio increased. This is in accordance with the SEM analysis as abovementioned. Similarly, those deposits on P₁L showed a unimodal size distribution with D₅₀ being 37, 35, and 40 mm. This indicates that the effect of SC addition on the size distribution of the P₁L is insignificant. This also implies that the particles deposited on P₁L are larger than those on P₁W, indicating that larger ash particles have deposited on the leeward. This differed from the previous work among which the particles deposited on the windward were larger in size [¹⁷]. The difference in bed material might affect the ash formation and contribute to such a difference.

In addition, the ZD90SC10 and ZD80SC20 deposits showed a bimodal size distribution (Fig. 12 (c)), with D₅₀ being 120 and 97 mm. This differed from the ZD100 deposit of 140 mm and suggested that the ash particles sizes decreased as the SC addition ratio increased. As the AAEM contents decreased due to the SC addition, the ash particles tended to be discrete, instead of being sintered together [⁴], consequently decreasing the particles sizes. In contrast, the P₂L deposits of ZD100, ZD90ZS10, and ZD80SC20 were much finer and exhibited a unimodal size distribution, whose D₅₀ were 37, 31, and 40 mm, respectively, without significant differences being observed even when SC was added. This is due to the fact that most fine particles would flow along with the stream lines, and only those particles by means of eddy impaction or thermophoresis could deposit on the leeward surface as analyzed above.

Effect of probe layouts on ash deposition

Based on the above analysis, a representative schematic diagram illustrating the effect of probe layouts on ash deposition in flue path in co-firing of ZD with SC in CFB is given in Fig. 13. As P₁ was vertically inserted into the flue path and the flow gas directly eroded the windward of P₁, inertial impaction was believed to be the dominant mechanism on the windward [¹⁸,²⁶]. Moreover, as P₁ were not cooled in the experiments, their surface temperatures were close to the surrounding flue gas temperature of 550°C, direct condensation of metal vapor and thermophoresis deposition of fine ash particles might thus play a minimal role [²⁶]. At the flue gas velocity of 14.8 m/s with intensive local gas flow, only less ash particles were able to deposit on the windward. As a consequence, the P₁W deposit was of a small cone shape in the central region. On the contrary, the flue gas velocity around the probe was as low as 0.36 m/s, and a larger number of fine particles would deposit on the leeward of P₁ due to either direct impaction or eddy impaction [²⁶]. Moreover, significant sintering within the ash particles deposited was not observed at such flue gas and probe temperatures, which differed from the deposits formed within the furnace or at the cyclone outlet in the previous work [²⁴].

The P₂ probe was horizontally inserted into the middle zone of the flue path where the flue gas velocity was 0.29 m/s and flue gas temperature was 400°C. Particles with varying sizes would be able to deposit on the windward of P₂ by means of diffusion and inertial impaction, leading to a cone shape deposit on the windward. Once the ash particles deposited were refractory, for instance in the case of ZD80SC20, these particles would not spread on the surface but fell off, decreasing the propensity of ash deposition. Meanwhile, on the P₂ leeward, most fine ash particles would flow with the streamlines. The deposit was thus much thinner and uniform than those on the windward surface of this probe [¹⁸].

Conclusions

The mineralogy, morphology, chemistry, and size distributions of the ash deposited on both windward and leeward of the probes with horizontal and vertical orientations at the flue path in co-firing of oil shale SC and ZD in a laboratory-scale CFB was systematically characterized by using XRD, SEM-EDS, ICP-OES, and a particle analyzer. When ZD was burned alone, the P₁ and P₂ deposits mainly consisted of both discrete particles and agglomerates, with sintering being observed between ash particles. When SC was added to ZD, the agglomerates between ash particles decreased, and more coarse-grained discrete particles were present. In particular, the cone-shaped layer that spread on windward of P₂ lessened. These indicate a decrease in deposit viscosity and the fouling propensity. In addition, the XRD analysis showed that when ZD was burned alone, Na-bearing minerals including Na₂Si₂O₅, Na₂SiO₃, and Na₂SO₄ of low-melting points and Ca-bearing minerals (CaSO₄ and Ca₂Al₂SiO₇) were found to be deposited on the probe. As SC was incorporated into ZD, the Na-bearing minerals became NaAlSi₃O₈ in P₁ deposits, and NaAlSi₃O₈ and (Na, K)(Si₃Al)O₈ in P₂ deposits. Moreover, Ca sulfate and Na-bearing minerals in the deposits decreased, and Ca aluminosilicates, SiO₂, and CaO increased, indicating that more mineral phases with high melting points presented in the deposition due to the SC addition. Furthermore, the Na content within these deposits decreased from up to 32 mg/g to less than 15 mg/g as 10% or 20% SC was added, indicating that less Na presented in the deposits. In terms of particle size distribution, the particle sizes (D₅₀ = 20 mm) of P₁W deposits was increased as the SC addition ratio increased while the particles sizes (D₅₀ = 140 mm) of P₂W decreased as the SC addition ratio increased. These results imply that the addition of SC would help alleviate the propensity of ash sintering and deposition on heat transfer surfaces within the flue path in the combustion of ZD in CFB.

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