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Impact of oxy-fuel combustion on ash properties and sintering strength development Jianqun Wu, Dunxi Yu, Fangqi Liu, Xin Yu, Xianpeng Zeng, Jingkun Han, Ge Yu, and Minghou Xu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03161 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on February 4, 2018
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Impact of oxy-fuel combustion on ash properties and sintering strength development Jianqun Wu, Dunxi Yu*, Fangqi Liu, Xin Yu, Xianpeng Zeng, Jingkun Han, Ge Yu and Minghou Xu* State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China
*Corresponding Author. E-mail:
[email protected]; Tel: +86-27-87545526; Fax: +86-27-87545526 *Corresponding Author. E-mail:
[email protected]; Tel: +86-27-87546631; Fax: +86-27-87545526 Postal address: State Key Laboratory of Coal Combustion, Huazhong University of Science & Technology, 1037 Luoyu Road, Wuhan 430074, PR China.
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Abstract For a successful implementation of the oxy-fuel combustion technology, a good understanding of ashrelated issues has to be achieved. Knowledge is still lacking on how oxy-fuel combustion can affect ash properties and its sintering behavior. These are the focus of the present work. Combustion experiments of a sub-bituminous coal were carried out in a drop tube furnace at 1573 K and under air and different oxy-fuel conditions (i.e.OXY21: 21 vol%O2/79 vol%CO2, OXY27: 27 vol%O2/73 vol%CO2 and OXY32: 32 vol%O2/68 vol%CO2). The bulk ash samples were collected and subjected to analyses by techniques such as computer-controlled scanning electron microscopy (CCSEM), X-Ray Diffraction (XRD) and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS). Sintering tests of the bulk ashes were performed at temperatures ranging from 1023 to 1223 K. The compressive strength of sintered ash was characterized. The results show that oxy-fuel combustion has significant impacts on ash speciation rather than its elemental composition. Ash particle size distributions are also affected, which is mainly attributed to changes in char burning temperatures. Oxy-fuel combustion seems to have insignificant effects on the temperature at which the ash pellet starts to sinter, but apparently affects ash sintering strength. Compared with the ash from air combustion (AIR ash), the OXY21ash has much higher sintering strength. Nevertheless, a decrease of sintering strength is observed for the OXY27 and OXY32 ashes, compared with the OXY21 ash. Such differences are attributed to changes in ash chemical properties. Keywords Coal combustion, Oxy-fuel combustion, Ash deposition, Ash properties, Ash sintering
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Introduction Oxy-fuel combustion (O2/CO2 combustion) has been identified as a promising technology for CO2 reduction in coal-fired power plants.1 The use of recycled CO2-rich flue gas instead of nitrogen as the diluent in oxy-fuel combustion has been found to affect various aspects of the combustion process, such as coal devolatilization, ignition, combustion, pollutant formation, ash formation and deposition.1-3Therefore, fundamental understanding of these impacts is critical to the successful implementation of the oxy-fuel combustion technology in either existing or newly-built power plants. Ash deposition may result in slagging in the radiative region and fouling in the convective region of the combustion system.4 The deposition of ash on heat exchanger surfaces can cause undesirable consequences such as deterioration of heat transfer, damage of boiler parts and even shutdown of the boiler.4, 5Therefore, ash deposition is a troublesome issue that has always beset operation engineers. Investigations conducted under air combustion conditions have shown that, deposit build-up is closely related to ash physicochemical properties while its strength development largely depends on ash sintering characteristics.5, 6 In this regard, to understand ash deposition in oxy-fuel combustion, one must know how oxy-fuel combustion would affect ash properties and its sintering characteristics. So far, properties of ash formed in oxy-coal combustion have been widely investigated7-10. However, consistent conclusions have not been obtained. For example, some researchers concluded that oxy-fuel combustion did not affect ash formation and hence ash properties11, 12, which is only based on ash elemental composition. In contrast, other work observed apparent differences in the mineralogy of ashes from air and oxy-fuel combustion8, 9, though the ash elemental composition is similar. Therefore, further investigations on this subject are quite necessary. As for ash sintering behavior in oxy-fuel combustion, the related work is extremely lacking. To the authors’ knowledge, only Zhou et al.13 investigated the effect of temperature on the sintering behavior of a lignite ash in oxy-fuel combustion. They found that the height shrinkage rates of sintered samples were slightly greater in oxy-fuel combustion than in air combustion (except for 1300 °C case). In contrast, the area shrinkage rates of the sintered samples were less in oxy-fuel combustion. In addition, differences in the contents of some elements (e.g. Si, Ca, Al and Fe) and the mineralogy were also observed between sintered samples treated under air and oxy-fuel 3 ACS Paragon Plus Environment
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combustion conditions. However, ash sintering strength, a vital indicator of deposit strength, was not evaluated in that work. This work aims to enrich the knowledge of the impacts of oxy-fuel combustion on ash physicochemical properties (e.g. particle size distribution, elemental composition and mineralogy) and ash sintering strength development. In addition, the correlation between them is also explored. 2 Experimental 2.1 Fuel properties A Xinjiang sub-bituminous coal (denoted as XJ) was tested in this work. The fuel was pulverized and sieved into the size range of smaller than 100 µm. The proximate, ultimate analysis, ash composition and fusibility data are presented in Table 1. It is evident that the XJ coal ash is highly enriched in oxides of alkali and alkaline earth metals (AAEMs), especially Na2O(6.2 wt%) and CaO(21.6 wt%). The modes of occurrence of AAEMs in coal are compared in Figure 1. It shows that most of the AAEMs are present in the forms of water soluble(WS), hydrochloric acid soluble (HS) or ammonium acetate soluble (AS). The different content of WS, HS, AS and IS AAEMs mainly related to the mode of their occurrence in the coal. Usually, WS AAEMs mainly present as chloride or oxides. HS and AS AAEMs mainly present as organicbound. IS AAEMs mainly present as silicates. Both water soluble and organic-bound AAEMs are very volatile during combustion. A relatively high content of Fe2O3 (8.8 wt%) is observed in the low temperature ash. The data in Figure 2 shows that iron in the coal is mainly present as siderite and pyrite. In contrast, the contents of SiO2 and Al2O3 are lower than most bituminous coals. Due to these features, the XJ coal ash has low ash fusion temperatures similar with typical international low rank coals14-16 (Indonesian UBC coal, American PRB coal and Pittsburgh coal). 2.2 Combustion experiments and sampling Three oxy-fuel combustion cases (i.e., OXY21: O2/CO2=21 vol%/ 79 vol%, OXY27: O2/CO2=27 vol%/ 73 vol%, OXY32: O2/CO2=32 vol%/ 68 vol%,) were investigated. For comparison, air combustion simulated by a mixture of 21 vol% O2 and 79 vol% N2 was also tested. Combustion experiments were carried out on a well-controlled drop tube furnace (DTF). The DTF is electrically heated and consists of a 4 ACS Paragon Plus Environment
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sample feeding system, corundum reactor and sampling system. The inner diameter of the corundum reactor is 56 mm and the length is about 2000 mm. The details of the DTF can be found elsewhere17. The furnace temperature was kept at 1573 K for all tests. The gas flow rate was maintained at about 10 L/min (under the conditions of 25 °C and 101.325 kPa) and the fuel feeding rate was about 0.3 g/ min. Under such conditions, complete combustion is almost achieved and the residence time of fuel particles in the furnace is estimated to be about 2 s18. For bulk ash sampling, the ash-laden flue gas stream was first directed into a water-cooled collection probe at the outlet of the DTF. And then, the bulk ash was collected by glass fiber filters for further analysis. The bulk ash sampling probe consists of four concentric stainless-steel tubes. The outer three tubes are designed for water cooling, while the inner two are for the introduction of nitrogen quench gas. The pure nitrogen is injected near the tip of the probe to quench particle reactions in the probe. The inner diameter and length of the probe are 12mm and 700mm, respectively. More details on the sampling probe can be found elsewhere 19. 2.3 Ash sintering test and compressive strength measurements Ash sintering test was similar to that in the study by Jung et al.20 The ash sample was compressed into a small pellet with density comparable to that of the deposits in the boiler (about 1.2g/cm3). The prepared ash pellet was heated to the desired temperature at a heating rate of about 6 K/min in a well-controlled sintering furnace, and then held at that temperature for about 1h. After that, the treated ash pellet was taken out of the furnace and cooled to room temperature. The compressive strength was tested at a compression rate of about 1mm/min. Sintering temperature was from 1023 -1223K with an interval of 50 K. The sintering tests of all ash samples were repeated four times for obtaining accurate results. 2.4Analyses The bulk ash mineralogy and particle size distribution were characterized by computer-controlled scanning electron microscopy (CCSEM). For that purpose, the bulk ash was first prepared into a pellet. The particle size and elemental composition of individual ash particles were analyzed by a CCSEM (SEM: JSM6510, EDS: EDAX126 GENESIS). Ash mineral species were classified according to the previous research2123
. The details of CCSEM analysis procedures can be found in our previous work24. 5 ACS Paragon Plus Environment
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The crystalline species in the bulk ash was characterized by X-Ray Diffraction (XRD, Model: X'Pert PRO). To quantify the amounts of amorphous phases in the ash, about 15% corundum was added as an internal standard, which was also adopted by Hosseini et al.25 Peak identification and crystalline mineral quantification were accomplished with the software SIROQUANT. When all the crystalline minerals were quantified, the amorphous phases in the bulk ash could be estimated by difference26. All ash pellets before and after sintering tests were embedded in Epoxy. The hardened ash pellet was then carefully cut using a precision saw (IsoMet 4000) along the radial direction. The cross-section of the pellet was polished by diamond powder in ethanol and then coated with carbon for further characterization. Ash particle morphology and composition were analyzed by a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) under back-scattered electron (BSE) modes. Minerals in the ash pellet appear bright while Epoxy matrix appears dark in the BSE images. 3 Results 3.1Bulk ash characterization 3.1.1 Bulk ash particle size distribution The bulk ash particle size distributions(PSD) were presented in Figure 3. It can be seen, compared with AIR combustion, OXY21 combustion shifts the bulk ash to larger particle sizes. Nevertheless, increasing the O2 concentration in oxy-fuel combustion decreases the bulk ash particle size distribution. Different ash PSDs observed in Figure 3 are mainly due to changes in ash fragmentation and coalescence9, which largely depend on combustion temperature and ash properties27. On the one hand, higher char combustion temperature will enhance ash melting and promote ash coalescence. On the other hand, higher char combustion temperature will promote char fragmentation and suppress ash coalescence. Char combustion temperatures in different atmospheres were estimated with the model used by Jia et al.28 and the results are presented in Table 2. It shows that char combustion temperature is much higher than ash fusion temperatures (Table 1), and most ash is expected to melt and prone to coalescence under the experimental conditions investigated. Therefore, char fragmentation would play a dominant role in determining final ash PSDs. As OXY21 combustion has a lower char combustion temperature than AIR 6 ACS Paragon Plus Environment
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combustion mainly due to the lower mass diffusion rate of O2 in CO2 than in N229, char fragmentation in OXY21 is expected to be less violent. As a result, coalescence of included minerals will be enhanced in OXY21 case, which accounts for the coarser PSD of OXY21 ash than that of AIR ash. In contrast, increasing O2 concentration in oxy-fuel combustion(i.e. OXY27 and OXY32) leads to higher char combustion temperatures due to higher overall reaction rates28. This will promote char fragmentation and thus suppress mineral coalescence, which results in finer ash PSDs in higher O2 cases (Figure 3). It is not surprising to see that AIR ash has a similar PSD to that of OXY27 ash, because similar char combustion temperatures are achieved for both cases. These results suggest that XJ coal ash PSDs in different combustion cases mainly depend on char combustion temperatures. As demonstrated by the particle size distribution data (Figure 3), variations of char combustion temperature in different atmospheres change particle fragmentation and mineral coalescence behavior. It implies that ash chemical properties will be affected as well. This is discussed in the following section. 3.1.2 Bulk ash elemental composition and mineralogy The bulk ash elemental composition was analyzed by SEM/EDS while its mineralogy was characterized by CCSEM and XRD. As the mineralogy determined by CCSEM is classified according to particle elemental composition, it has the advantage to analyze mineralogy of both crystalline and amorphous minerals24. As to the XRD, it can give complementary information on the crystalline mineral species. The elemental composition data of four ashes are presented in Figure 4. Similar to Jones et al.’s research12, oxy-fuel combustion has insignificant effects on bulk ash elemental composition when uncertainties are considered. In general, bulk ash elemental compositions are similar to the low temperature ash composition in Table 1, except the much lower S content in the bulk ash. This is mainly because that the bulk ashes were generated at much higher temperatures, and the S tended to emit as sulfur oxides rather than form AAEM sulfates. The result seems somewhat different from the observed higher S retention in oxy-fuel ash in Yu’s research11 when burning a PRB sub-bituminous coal. Such inconsistency warrants further research into S chemistry in oxy-fuel combustion. Mineral species of the bulk ash determined by CCSEM are presented in Figure 5. It is found that similar 7 ACS Paragon Plus Environment
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mineral species were present in different ashes. The dominant phases in the bulk ash are complicated aluminosilicates, followed by quartz, iron oxide and mullite. Little AAEM oxides or sulfates are detected in the ash, though the XJ coal contains considerable amounts of AAEMs as shown in Table 1. This may be due to that AAEMs are mainly present in the forms of water-soluble or organically-bound in typical Xinjiang sub-bituminous coals, and they can readily interact with clay to form aluminosilicates30. Similar mineral species generated in different combustion cases suggest oxy-fuel combustion has insignificant effects on ash transformation mechanisms9. Quartz is the direct carryover of quartz and formed from kaolin in the raw coal. Iron oxides are mainly from the decomposition and oxidization of pyrite and siderite. Mullite is from the transformation of kaolin in the coal. These three minerals are all from the direct transformation of individual minerals in the raw coal rather than from mineral interactions. Ca aluminosilicate is most likely from the interaction of kaolin with Ca-bearing minerals. The detailed elemental composition of complicated aluminosilicates is presented in the Si+Al-Ca+Mg+Fe-Na+K ternary phase diagram in Figure 6. The ash particles mainly contain Si+Al in the range of 20% to 70%, Ca+Mg+Fe in the range of 10% to 80% and Na+K in the range of 0 to 20%. It suggests that the complicated aluminosilicates are mainly from the interactions of quartz, kaolin or illite with Na/K/Ca/Fe/Mg-containing minerals. Although combustion atmospheres have insignificant influence on the mineral species generated, they do affect minerals’ relative amounts as shown in Figure 5. Switching from AIR combustion to OXY21 combustion, mullite, iron oxide and quartz decreases. In contrast, the amounts of complicated aluminosilicates that are mainly from mineral interactions increase. It suggests that OXY21 combustion promotes iron melting into aluminosilicates to form iron glass. Similar results were reported in Sheng et al.’s work9 when burning four Chinese thermal coals in the DTF. This mainly lies in two folds. On the one hand, less char fragmentation during OXY21 combustion (as suggested by Figure 3) will promote mineral interactions. On the other hand, the lower char combustion temperature in OXY21 combustion (Table 2) is expected to slow the transformation of siderite and pyrite to iron oxides but favor the formation of aluminosiliactes9. 8 ACS Paragon Plus Environment
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Figure 5 also shows that, increasing O2 in oxy-fuel combustion tends to increase the amounts of mullite, iron oxide and quartz while decrease the amounts of complicated aluminosilicates. This result suggests that increasing O2 suppresses the formation of iron aluminosilicates but favors the formation of iron oxides. This is mainly because increasing O2 results in higher char combustion temperature(Table 2). It enhances char fragmentation and suppresses mineral interactions, which favor the transformation of siderite and pyrite to iron oxides. Crystalline minerals in the bulk ash detected by XRD are shown in Figure 7. It provides complementary information to the CCSEM data. Besides the corundum added in the bulk ash, quartz, hematite, gehlenite, calcite and magnetite are observed in Figure 7 for all the four ashes. The extremely weak peak of calcite indicates its trace amount in the bulk ash. Comparing the mineralogy characterized by CCSEM and XRD, we can determine that iron oxides detected by CCSEM are mainly present as magnetite and hematite. Both iron species have very high melting points. Ca aluminosilicate is mainly present as gehlenite. Mullite is not detected by XRD, though it was identified by CCSEM(Figure 5). It is reasonable, since kaolin may transform into amorphous mullite due to the introduction of impurities(eg. AAEMs) and rapid cooling 31. A small hump around 20~40° in Figure 7 suggests the formation of amorphous phases in the bulk ash. It indicates that the complicated aluminosilicates detected by CCSEM are mainly present as amorphous phases in the bulk ash. It also confirms that interactions of Si and Al with basic elements will enhance ash melting and lead to the formation of amorphous phases. The combination of the CCSEM and XRD data suggests that combustion atmospheres have insignificant impacts on ash transformation mechanisms. However, they do affect the degree of mineral interaction. Compared with AIR combustion, OXY21 combustion tends to enhance mineral interaction mainly due to its lower combustion temperature. It promotes the formation of complicated aluminosilicates, which are mainly present as amorphous phases in the ash. Increasing O2 in oxy-fuel combustion tends to inhibit mineral interaction due to higher combustion temperature. It increases the amounts of high melting minerals such as iron oxides, quartz and mullite in the ash, while decreases the amount of complicated aluminosilicate that has much lower ash melting temperature. All these results imply that combustion atmosphere may have an 9 ACS Paragon Plus Environment
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appreciable effect on ash sintering behavior. 3.2 Ash sintering strength characterization Bulk ash characterization shows that combustion atmospheres have significant effects on bulk ash PSD and mineralogy. Since both properties are known to have significant influence on ash sintering6,13, the results suggest that combustion atmospheres will affect ash sintering behavior due to changes of ash properties. Sintering strength, an important parameter determining how easily deposits can be cleaned in the furnace, is characterized in this work to understand its correlation with the changes of ash properties. As usually done in previous research20,32, compressive strength is used to estimate the sintering strength of an ash pellet. According to this method, a higher compressive strength suggests higher sintering strength of the ash pellet. The compressive strength of the ash pellets after sintering at 1173 K is presented in Figure 8. It generally shows that the XJ ash has very high compressive strength after sintering at 1173 K, which is comparable to those of brown coal ashes (about 10-30 MPa) after sintering under similar conditions20. The compressive strength of AIR ash pellet is about 13MPa, while that of OXY21 ash pellet is about 29 MPa. It is evident that OXY21 ash pellet has much higher compressive strength than AIR ash pellet. It suggests that switching from AIR combustion to OXY21 combustion will greatly enhance the ash sintering strength. Comparing the compressive strength of ash generated under different O2 conditions, it is interesting to find that ash generated under higher O2(OXY27 and OXY32) conditions has lower compressive strength. It suggests that sintering strength decreases in the order OXY21>OXY27>OXY32. To explore the mechanisms of sintering strength development after sintering, the cross-section of ash pellets before and after sintering tests was analyzed by SEM. Here, we take the OXY21 ash for example, as shown in Figure 9. In general, ash particles are mainly present as individual grains that are separate from each other before sintering test (Figure 9(a)). After sintering in the furnace, sintering neck growth between ash particles is observed in the ash pellet (Figure 9(b)). Sintering neck growth resulting from viscous flow is believed to account for the sintering strength development of ash pellet, which has also been confirmed by the previous researches5, 33. To better understand the correlation between ash particle composition and sintering strength development, 10 ACS Paragon Plus Environment
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a higher magnification SEM image of sintered pellet is presented in Figure 10, and the EDS results are also given in tabular forms. The results show that sintering necks (spot 1 and 2) between particles contain relative high contents of basic elements, in addition to Si and Al. This is expected, since the interactions between basic elements and Si/Al tend to decrease ash viscosity and promote ash sintering strength by enhancing ash viscous flow29. Quartz(spot 3) and mullite(spot 4) remain their irregular shapes after sintering at 1173 K. Iron oxides in the pellet(spot 5) are mainly present as individual spherical particles. Quartz, mullite and iron oxides do not seem to sinter with other minerals. This is mainly because these minerals have very high melting points and viscosity. Consequently, viscous flow is not expected to take place in these minerals and they usually act as dispersed components to decrease the ash sintering strength. As shown in Figure 8, ashes generated indifferent atmospheres show different sintering strength at 1173 K. How the sintering temperature would affect ash sintering strength is further examined. Figure 11 compares the compressive strength of ash pellets after sintering at different temperatures. For the four ashes examined, their sintering strength development unexceptionally starts at about 1073 K. It suggests that combustion atmosphere does not have significant influence on the initial ash sintering temperature. Nevertheless, increasing sintering temperature evidently increases ash sintering strength. This is because that increasing sintering temperature will greatly decrease ash viscosity and promote ash viscous flow33. Comparing sintering strength of ashes from different atmospheres, it is interesting to find that combustion atmosphere has similar influence on ash sintering strength when sintered at the same temperature. OXY21 ash has higher sintering strength than AIR ash, and ashes generated at higher O2(OXY27 and OXY32) have lower sintering strength than OXY21 ash. These findings imply that, in oxy-fuel combustion boilers it may be more difficult to clean ash deposits because of the higher ash sintering strength. 4 Discussion The data presented above show that oxy-fuel combustion has significant effects on the physicochemical properties of bulk ash. Specifically, OXY21 combustion tends to shift the ash PSD to larger size and promotes mineral interactions when compared with AIR combustion. Increasing O2 concentration in oxyfuel combustion decreases the ash PSD and inhibits ash coalescence. As a result, ash composition is altered. 11 ACS Paragon Plus Environment
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Therefore, the variations of ash sintering strength (Figures 8 and 11) are most likely due to the changes of ash physicochemical properties (eg. PSD, mineralogy and crystallization)5, 16. These are discussed in this section. Previous work5,
29
and Figure 9 suggest that ash sintering strength development mainly results from
sintering neck growth between ash particles by viscous flow. It has been well established that the effects of ash PSD and mineralogy on sintering neck growth can be described by Frenkel model34, as shown in Eq(1):
γ 1 1 x = 1.225( ) 2 t 2 r ηr (1) Where, x stands for the radius of the interface, r stands for the radius of the spherical particles, γ stands for surface tension, η stands for particle viscosity and t stands for the sintering time. According to Rassk’s research5, ash particles with x / r > 0.1 will greatly enhance sintering strength. Therefore, the amount of ash particles contributing to sintering strength development (with x / r > 0.1 ) can be estimated with Eq(1). As particle surface tension is a linear function of composition surface tension35 and particle viscosity can be calculated according to its composition with the modified Urbainequation36, parameters on the right side of Eq(1) can be obtained on the basis of the CCSEM results and experimental conditions. Weight percent of ash particles with calculated x/r>0.1 under different conditions is shown in Figure 12. Compared with AIR ash, OXY21 ash has a higher percentage of ash with calculated x/r>0.1(contributing to sintering strength development). For ash generated in higher O2 combustion cases (OXY27 and OXY32), it has a lower percentage of ash with x/r>0.1. It is found that, in general, the amount of ash contributing to sintering strength development (Figure 12) positively correlates with ash sintering strength (Figure 11). This result confirms that differences in ash sintering strength are due to different patterns of sintering neck growth that are affected by ash properties. As indicated by Eq(1), smaller particle size, lower ash viscosity or higher surface tension will lead to higher x / r ratios and thus higher ash sintering strength. However, comparisons between Figure 3 and Figure 11 show that smaller particles do not necessarily result in higher sintering strength. This result 12 ACS Paragon Plus Environment
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indicates that changes of ash PSD are not the reason for different sintering strength as observed in Figure 11. In addition, surface tension of typical oxides does not differ much, as shown in Table 3. Therefore, the differences in sintering neck growth between different ashes are mainly attributed to the changes of ash particle viscosity, which is closely related to its composition (mineralogy). Figure 3 suggests that switching from AIR combustion to OXY21 combustion promotes mineral interaction. Consequently, OXY21 combustion decreases the amounts of minerals (e.g. iron oxides, mullite and quartz) that have high viscosity, but increases the amounts of complicated aluminosilicates(Figure 5) that have lower viscosity. This is believed to account for the higher fraction of ash particles contributing to sintering strength development in the case of OXY21 ash. Increasing O2 concentration in oxy-fuel combustion tends to promote char fragmentation and suppress mineral interaction (as suggested by Figure 3). As a result, the amounts of high melting minerals such as iron oxide, mullite and quartz increase while those of complicated aluminosilicates decrease with increasing the O2 concentration. This is consistent with the observations that, compared with the OXY21 ash, the OXY27 and OXY32 ashes contain less amounts of particles contributing to sintering strength development (Figure 12) and have lower compressive strength (Figure 11). Mineral crystallization also affects ash sintering strength development. It has been reported that amorphous ash has higher sintering strength than crystalline ash with similar composition and under similar sintering conditions32. The amounts of amorphous phases in different ashes were quantified with the method used by Hosseini et al.25 The results are presented in Figure 13. It shows that all ashes contain large amounts of amorphous phases (78~89 wt%). This is a typical feature of ashes from high AAEM Xinjiang subbituminous coals. For example, the amounts of amorphous phases in Xinjiang sub-bituminous coal fly ashes could range from 73 wt% to 91 wt%37 because of the formation of low melting eutectics. Figure 13 shows that the OXY21 ash has more amorphous phases than the AIR ash. Therefore, the OXY21 ash has higher sintering strength (Figure 11). Increasing O2 concentration in oxy-fuel combustion is observed to decrease the amounts of amorphous phases in the ash (Figure 13). This is consistent with the lower sintering strength of the OXY27 and OXY32 ashes, compared with the OXY21 ash(Figure 11). 13 ACS Paragon Plus Environment
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Another reason for the differences in ash sintering strength may be related to the presence of high melting minerals in the ash. As shown in Figure 10, high melting point minerals (e. g. iron oxide, mullite and quartz) are not expected to result in ash sintering. Their presence tends to decrease ash sintering strength by reducing the contacts between particles that are prone to sintering. As shown in Figure 5, the OXY21 ash has less high melting minerals than the AIR ash. Therefore, it shows higher sintering strength, as observed in Figure 11. 5 Conclusions A typical Xinjiang sub-bituminous coal was subjected to combustion in the DTF under air and different oxy-fuel combustion conditions, respectively. Bulk ash properties were characterized by SEM/EDS, CCSEM and XRD. Ash sintering tests were carried out under air conditions and the compressive strength of ash pellets was determined. The impacts of oxy-fuel combustion on ash physicochemical properties and its sintering strength development were investigated. The following results are obtained. 1) Compared with air combustion, oxy-fuel combustion with 21 vol% O2 shifts the bulk ash to a coarser PSD. Increasing O2 concentration in oxy-fuel combustion tends to decrease the bulk ash PSD. 2) Combustion atmospheres have insignificant influence on bulk ash elemental composition, but affect the amounts of minerals and amorphous phases in the ash. 3) Oxy-fuel combustion does not seem to affect the temperature at which the ash pellet starts to sinter, but has evident impacts on ash sintering strength. The OXY21 ash shows higher sintering strength than the AIR ash. Increasing O2 concentration in oxy-fuel combustion tends to decrease ash sintering strength. 4) Under the conditions investigated, differences in ash sintering strength are attributed to changes in ash chemical properties. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant Nos. 51520105008, 51676075 and 51661125011). The support from the Analytical and Testing Center at Huazhong University of Science & Technology is also acknowledged. References 14 ACS Paragon Plus Environment
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Energy & Fuels
List of Table Captions Table 1 Fuel properties. Table 2 Char temperature for particle with diameter 60 µm. Table 3. Surface tension of some oxides at 1173K.
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Table 1 Fuel properties. Proximate analysis (wt%, air dry basis) Fixed Volatile carbon matter XJ
50.4
33.6
Ultimate analysis (wt%, air dry basis)
Ash
Moisture
C
H
S
N
O*
10.0
5.9
59.1
4.3
0.4
0.7
19.5
Low temperature ash analysis (wt%)
XJ
Na2O
MgO
Al2O3
SiO2
P2O5
SO3
K2O
CaO
Fe2O3
6.2
8.3
17.4
28.0
0.8
7.9
1
21.6
8.8
Ash fusibility (K) XJ
UBC14
PRB15 Pittsburgh16
Initial deformation temperature
1371
1340
1415
1407
Hemisphere temperature
1395
1503
1433
1483
Flow temperature
1413
1610
1464
1598
* By difference
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Table 2 Char temperature for particle with diameter 60 µm.
Char temperature (K)
AIR 1922
OXY21 1861
OXY27 1929
OXY32 1998
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Table 3. Surface tension of some oxides at 1173K. Oxide SiO2 Al2O3 CaO Fe2O3 MgO K2O Na2O
Surface tension (mN m-1) 285 295 292 293 296 268 280
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List of Figure Captions Figure 1 Mode of AAEMs occurrence in XJ coal (IS:acid insoluble; HS:hydrochloric acid soluble; AS: ammonium acetate soluble; WS: water soluble). Figure 2 Mineralogy of XJ coal detected by CCSEM. Figure 3 Bulk ash particle size distribution detected by CCSEM. Figure 4 Elemental composition of bulk ash generated in different atmospheres. Figure 5 Mineralogy of raw coal and bulk ash detected by CCSEM. Figure 6 Elemental composition of complicated aluminosilicate detected by CCSEM. Figure 7 Mineralogy of bulk ash detected by XRD (1: corundum, 2: quartz, 3: hematite, 4: gehlenite, 5: calcite, 6: magnetite). Figure 8 Compressive strength of ash pellet sintering at 1173 K. Figure 9 Cross section morphologies of ash pellets detected by SEM. Figure 10 Morphology and composition of ash particles in pellet after sintering. Figure 11 Compressive strength of ash pellet sintering at different temperature. Figure 12 Weight percent of ash particles with calculated x/r>0.1. Figure 13 Amorphous phase in the bulk ash.
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Figure 1 Mode of AAEMs occurrence in XJ coal (IS:acid insoluble; HS:hydrochloric acid soluble; AS: ammonium acetate soluble; WS: water soluble).
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Figure 2 Mineralogy of XJ coal detected by CCSEM.
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Figure 3 Bulk ash particle size distribution detected by CCSEM.
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Figure 4 Elemental composition of bulk ash generated in different atmospheres.
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Figure 5 Mineralogy of raw coal and bulk ash detected by CCSEM.
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Figure 6 Elemental composition of complicated aluminosilicate detected by CCSEM.
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Figure 7 Mineralogy of bulk ash detected by XRD (1: corundum, 2: quartz, 3: hematite, 4: gehlenite, 5: calcite, 6: magnetite).
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Figure 8 Compressive strength of ash pellet sintering at 1173 K.
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(a) OXY21 ash pellet before sintering
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(b) OXY21 ash pellet after sintering
Figure 9 Cross section morphologies of ash pellets detected by SEM.
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Figure 10 Morphology and composition of ash particles in pellet after sintering.
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Figure 11 Compressive strength of ash pellet sintering at different temperature.
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Figure 12 Weight percent of ash particles with calculated x/r>0.1.
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Figure 13 Amorphous phase in the bulk ash.
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Figure 1 Mode of AAEMs occurrence in XJ coal (IS:acid insoluble; HS:hydrochloric acid soluble; AS: ammonium acetate soluble; WS: water soluble). 58x43mm (300 x 300 DPI)
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Figure 2 Mineralogy of XJ coal detected by CCSEM. 57x40mm (300 x 300 DPI)
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Figure 3 Bulk ash particle size distribution detected by CCSEM. 58x43mm (300 x 300 DPI)
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Figure 4 Elemental composition of bulk ash generated in different atmospheres. 58x42mm (300 x 300 DPI)
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Figure 5 Mineralogy of raw coal and bulk ash detected by CCSEM. 55x37mm (300 x 300 DPI)
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Figure 6 Elemental composition of complicated aluminosilicate detected by CCSEM. 70x62mm (300 x 300 DPI)
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Figure 7 Mineralogy of bulk ash detected by XRD (1: corundum, 2: quartz, 3: hematite, 4: gehlenite, 5: calcite, 6: magnetite). 62x48mm (300 x 300 DPI)
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Figure 8 Compressive strength of ash pellet sintering at 1173 K. 59x44mm (300 x 300 DPI)
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Figure 9(a) Cross section morphologies of ash pellets detected by SEM. 60x45mm (300 x 300 DPI)
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Figure 9(b) Cross section morphologies of ash pellets detected by SEM. 60x45mm (300 x 300 DPI)
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Figure 10 Morphology and composition of ash particles in pellet after sintering. 93x108mm (300 x 300 DPI)
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Figure 11 Compressive strength of ash pellet sintering at different temperature. 61x47mm (300 x 300 DPI)
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Figure 12 Weight percent of ash particles with calculated x/r>0.1. 60x45mm (300 x 300 DPI)
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Figure 13 Amorphous phase in the bulk ash.
57x42mm (300 x 300 DPI)
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