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Ash deposition of Zhundong coal in a 350 MW pulverized coal furnace: Influence of sulfation
Fuel 260 (2020) 116317
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Full Length Article
Ash deposition of Zhundong coal in a 350 MW pulverized coal furnace: Influence of sulfation
T
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Hang Shi, Yuxin Wu , Man Zhang, Yang Zhang, Junfu Lyu Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Zhundong coal Pulverized coal furnace Ash deposition Sulfation Thermodynamic calculation
The huge reserve of Zhundong (ZD) coal makes it important for energy utilization of China. However, big concerns on fouling and slagging in pulverized coal-firing boilers were risen due to its high alkali content. It’s important to investigate deposition in different heating surfaces of a boiler burning ZD coal. In this paper, XRF, XRD, ICP-OES were conducted to analyze the properties of ash deposits collected from different heating surfaces in a 350 MW boiler burning 30% Wudong (WD) and 70% Wucaiwan (WCW) coals. Thermodynamics calculations were applied to study the mineral composition of coal ash in the range of 200–1600 °C under different coal cofiring ratio. The analytical results of the ash deposits showed that in medium-temperature (800–1100 °C) flue gas zone (FR and FS), ash deposits have higher sulfate content. While in high temperature flue gas zone (WW and PS), ash deposits have higher Si/Al content and lower S content. The Na/S contents in FR (~1000 °C) are much higher than in other deposits. The thermal equilibrium calculation results showed that a large amount of liquid Na2SO4 and CaSO4 will be generated when burning WCW coal. Co-firing WD and WCW coals can reduce the amount of sulfate (l/slag) formation and the co-firing ratio greatly influence the elimination effect. It can be speculated that not all the sulfates in the ash deposits are formed in flue gas. The sulfation of the Na-containing minerals condensed on the heating surfaces will promote the ash deposition.
Abbreviations: WCW coal, Wucaiwan coal; CF coal, Co-firing coal; WD coal, Wudong coal; CF-30/10/5, co-firing 30/10/5% WD and 70/90/95% WCW coals; XRD, X-ray diffraction; XRF, X-ray fluorescence; ICP-OES, inductively coupled plasma optical emission spectrometer ⁎ Corresponding author. E-mail address: [email protected] (Y. Wu). https://doi.org/10.1016/j.fuel.2019.116317 Received 9 July 2019; Received in revised form 25 September 2019; Accepted 28 September 2019 Available online 03 October 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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1. Introduction
the ash deposits on different heating surfaces were sampled in a 350 MW boiler burning Wucaiwan (WCW) coal and Wudong (WD) coal during the plant maintenance. WCW coal is a typical Zhundong coal from Wucaiwan area with high alkali/alkaline earth metal (AAEM) content. WD coal is a coal from Wudong area in China with low AAEM content. XRF, XRD, ICP-OES were conducted to analyze the composition of AAEM materials so as to investigate the deposition of AAEM materials and conversion of sulfur on different heating surfaces. As for the effect of co-firing, it’s hard to get data at different co-firing ratio by repetitive experiments in a real boiler. Considering that the formation of deposits (especially the outside layer) in a real boiler often takes a long period, the equilibrium solver is a reliable and reasonable tool to analyze such complex cases during coal blending. Many previous studies also adopted thermodynamic equilibrium models to predict the release and conversion characteristics of AAME minerals during thermal conversion [29–31]. Thus, thermodynamics calculations based on the equilibrium model were conducted to simulate deposition of AAEM materials on the heating surface at the temperature ranging from 200 to 1600 °C under different coal co-firing ratio. And the relationships between Ca/Na/S and the coal blending ratio in real boiler operating conditions were investigated.
Zhundong (ZD) coal field in Xinjiang province have an estimated reserve of 3.9 × 1011 tons [1]. The price of ZD coal is much lower than other coals due to its low exploitation cost. The utilization of ZD coal is important to China. ZD coal is a high-quality power coal characterized by low ash yields, low-medium sulfur content and high volatile content [2]. However, ZD coal has high Na2O/CaO contents and low SiO2/ Al2O3 contents in coal ash [3,4]. Thus severe slagging and fouling occur on the heating surfaces during the combustion of ZD coal [5–7]. Understanding the ash deposition mechanism is critical for the safe, efficient and clean utilization of ZD coal. Ash deposition can be divided into slagging and fouling. There are two main research methods for the investigation on the mechanism of ash deposition. One is to conduct the morphology and composition analysis of the ash deposits collected from real boilers [8–10]. The other one is to use the ash sampling tube to collect the ash deposits either in real boilers [11–13] or in a pulverized coal firing test rig, such as drop tube furnace, one-dimensional furnace, et al [14–16]. Previous studies indicated that slagging [15,17–19] occurs on radiant heating surfaces, while fouling [20–24] takes place in convective heating surfaces of the boiler. The studies conducted on real boilers or bench/pilot scale apparatus showed similar mechanism of slagging. Slagging is dominated by molten ash [15,17–19]. Slag is mainly composed of Si/Al/Ca [15,17,25]. Some scholars [10,18,19] find that the Fe2+ containing molten particles are more viscous under the local reducing atmosphere in the high-temperature pulverized coal flame zone. It is easy to deposit and capture impacting particles. Also, Fe can react with Si/Al/Ca to form low-melting-point eutectic salt, which will promote slagging. As for the mechanism of fouling on convective surfaces [26,27]. In a real boiler, it is found that ash deposits often have a clear stratified structure. The ash deposits collected from bench/pilot scale apparatus have different morphology and can’t fully represent that in real boiler due to the different operation conditions. For the stratified ash deposits in a real boiler, the composition and particle size distribution of the inner layer are significantly different from that of the outer layer. The major elements of the ash deposits are identified as Na, Ca and S [23]. Wu et al. [19] found that Na2SO4 dominates the initial layer and CaSO4/ Na2SO4/Ca-Si-Al dominates the ash deposits. Baxter et al. [24] reported that the low-temperature sulfation of the ash layer might cause severe deposition dominated by CaSO4 in convective heating surfaces when burn high-calcium coal. Sulfate of alkali/alkaline earth metal (AAEM) plays an important role in ash deposition [15,28]. Above all, most of the studies were conducted on the mechanism of slagging and fouling in bench or pilot scale apparatus. Generally, the combustion conditions, temperatures of heating surfaces and flue gas components in bench or pilot scale experiments are different from that in real boilers. The test duration is often insufficient for steady ash deposition, which is a long-term process. Although investigations for the real boilers are desired, it’s difficult to get good data due to following challenges. Firstly, almost all of boilers (especially for the subcritical and supercritical boilers) are co-firing low AAEM content coals or additives with ZD coal to avoid slagging and fouling problems. It’s hard to dig the meaningful information of AAEM deposition from the deposit samples. Secondly, it’s difficult to get the samples and the operation data even with the help of the operation partner. Up to now, investigations on a real boiler burning ZD coal are still rare. It requires more comparison between the field test and laboratory researches so as to promote the in-depth understanding of the actual deposition in a boiler. To fill this gap, this article tries to achieve the meaningful data from a field test so that the deposition in a real boiler that burning ZD coal can be understood. Since co-firing was commonly adopted by the industrial applications to avoid serious slagging/fouling, it’s also import to investigate the influence of co-firing ratio on deposition. Therefore,
2. Experimental approaches 2.1. Experimental equipment and facilities The ash deposits were collected from a 350 MW pulverized coal furnace constantly co-firing 70% WCW coal and 30% WD coal for more than one year. The influence of coal quality change on the deposition of heating surfaces in boiler can be negligible. The ash deposits were collected from the radiant heating surfaces (Water wall, WW), the convective heating surfaces in medium flue gas temperature (Panel superheater, PS; Final reheater, FR) and in low flue gas temperature (First superheater, FS). The sampling points and the corresponding flue gas/ heating surfaces temperatures are shown in Fig. 1. Fly ash was also sampled from electrostatic precipitator. 2.2. Fuel properties The proximate and ultimate analysis of WCW coal and WD coal are shown in Table 1. It is indicated by Table 1 that WCW coal has a lower ash yield than WD coal. Also, both of WCW coal and WD coal have low sulfur content. Considering that AAEM might release from coal under high ashing temperature. XRF test was conducted for coal ash prepared under different temperatures (Low temperature, LT, 150–200 °C; Medium temperature, MT, 500 °C; High temperature, HT, 815 °C). LT/ MT/ HT ash were prepared by YAMATO PR300 low temperature plasma asher/ muffle furnace/muffle furnace respectively. The test results of metal elements and sulfur in coal ash are shown in Table 2. WCW coal ash has a higher contents of Na2O/CaO/S and lower contents of SiO2/Al2O3 than WD coal ash prepared under the same temperature. It can be indicated from Table 2 that the influence of the vaporize of Na/K (815 °C) on the overall elemental percentage is less than 1.8%. The influence of the release of Na/K on S content in coal is slight. 2.3. Experimental and simulation method The ash deposits (except FR) were grounded to a particle size smaller than 30 μm. Since inductively coupled plasma optical emission spectrometer (ICP-OES) can only be used to measure metallic elements. X-ray fluorescence (XRF) and ICP-OES were applied to get a better elemental analysis for both metallic elements and non-metallic elements such as chlorine and sulfur. For the ICP-OES test, ash deposits were digested in the Milestone Ethos microwave digestion instrument. The digestion reagent used excellent grade pure 9 ml HNO3 + 1 ml 2
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Water wall, WW
Panel superheater, PS Final reheater, FR (a) Deposition samples
First superheater, FS
(b) flue gas and heating surfaces temperatures on deposition sampling points Fig. 1. Deposition sampling points of a 350 MW boilers and temperature distributions of flue-gas/heating surfaces.
HF + 1 ml HCl. The mineral composition of the ash deposits was analyzed by X-ray diffraction (XRD). The ash deposit in FR can be divided into three layers according to the slight differences of the color and hardness. To analyze the components of different layers, the sample were peeled off by layering and then grounded to smaller than 30 μm. XRD and XRF were conducted for the elemental and component analysis. Scanning electron microscopy (SEM-EDS) was applied to obtain the microscopic morphology and growth mechanism of the deposits. Thermodynamic equilibrium models are often applied to predict the release and conversion characteristics of AAME minerals during thermal conversion. The thermodynamic equilibrium model is based on the principle of Gibbs free energy minimization. The chemical composition of fuel or ash are used as the initial condition. The presence and content of AAEM minerals in the gas phase, liquid phase and solid phase at the equilibrium state can be achieved under given conditions (temperature, pressure and atmosphere). Equilib module in the Factsage software is widely applied by scholars to study the conversion characteristics of AAME minerals in ZD coal. Oleschko et al. [29] calculated the conversion characteristics of Na and found that the main gas phase products of Na during coal combustion were NaCl and Na2SO4. Bläsing et al. [30,31] calculated the release characteristics of Na under inert atmosphere, the results indicated that the main gas phase product of Na was NaCl under gasification conditions. In this study, thermal equilibrium calculations were conducted through Factsage 6.3 to investigate the release and conversion characteristics of AAME minerals during deposition given different temperature and different ratio of WD coal to the coal mixture feeding to the boiler. The Equilib module in Factsage 6.3 was used to carried out
Table 2 XRF test results of elements in coal ash [%]. Analyte
WD Coal Ash
SiO2 Al2O3 CaO Fe2O3 Sx K2O MgO TiO2 Na2O P2O5 Cl
WCW Coal Ash
LT
MT
HT
LT
MT
HT
56.86 22.88 5.58 4.39 1.89 1.75 1.27 0.89 0.70 0.21 0.02
57.11 23.35 5.60 4.47 1.66 1.79 1.28 0.91 0.70 0.18 0.02
58.85 25.03 3.81 4.08 1.01 1.91 1.22 1.05 0.79 0.26 0.01
10.73 7.89 33.26 3.84 8.44 0.27 8.21 0.37 5.36 0.05 2.48
10.6 8.05 33.96 3.86 8.91 0.27 8.11 0.37 5.47 0.05 2.45
10.26 8.76 34.92 3.86 8.76 0.15 11.18 0.36 3.77 0.06 0.03
the calculations. The FactPS and FToxide databases were used in the calculation. Considering that the conversion of AAME is the concerned problem, a metal solution database FTsalt containing AAME Na/K/Ca/ Mg and Cl−/OH−/CO32−/SO42− was also selected. For WCW and WD coals, the C/H/N/S/O contents were obtained through ultimate analysis of Table 1. As is shown in Table 3, the Si/Al/Ca/Mg/Na/K contents in raw WCW/WD coals were identified through ICP-OES test. The Cl content was identified through XRF test. For the co-firing (CF) coal, the components contents were calculated under different co-firing ratio. In the calculation, the temperature range was 200–1600 °C with a step of 100 °C under atmospheric pressure. The normal algorithm was used for calculation. The atmosphere was 21%O2/79%N2, and the excess air coefficient was 1.2.
Table 1 The Proximate and ultimate analysis of coal.
Wudong coal Wucaiwan coal
Mad
Vad
Aad
FCa
Car
Har
Oar
Nar
Sar
3.98 11.22
29.34 29.27
14.56 3.54
52.13 55.97
61.01 65.31
4.28 4.41
11.45 14.66
0.37 0.37
0.43 0.49
3
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Table 3 Elemental test results of coal [%]. Analyte
WCW (ar, wt%)
WD (ar, wt%)
Si Al Ca Mg Na K Cl
0.23 0.20 1.12 0.23 0.19 0.01 0.11
9.72 4.50 1.46 0.28 0.19 0.54 0.01
Fig. 4. SEM-EDS results of the ash deposits at the FR.
inner layer to outer layer. At the same time, S content shows a similar trend with AAME contents. At high temperature zone, S content is very low (WW, PS and fly ash). And the S content becomes much higher in FR and FS than in WW and PS. To investigate the relationship between S and AAME metal elements, the components of the sampled deposits were acquired through XRD measurement. The XRD results are shown in Fig. 3. Based on the test results in Figs. 2 and 3, the main substances in the ash deposits are CaSO4, Na2SO4, Na3Fe(SO4)3, SiO2(Quarts), Ca2MgSi2O7, (3Al2O3·2SiO2) (Mullite), and Na(AlSi3O8) (Albite). The composition of the ash deposits on different heating surfaces are significantly different. In low-temperature flue gas zone (FR and FS), ash deposits have higher Na/S contents. Na2SO4/CaSO4 are the dominant minerals for FR-I/FRM. FR-O and FS mainly consist of CaSO4, Na2SO4, SiO2, Ca2MgSi2O7 and Na(AlSi3O8). In high temperature flue gas zone (WW and PS), ash deposits have higher Si/Al contents and lower S content. Ca2MgSi2O7 and Na(AlSi3O8) are the dominant minerals while CaSO4, Na2SO4 and Na3Fe(SO4) contents are low for WW/PS. These results are similar with the findings of previous studies conducted on test rig for pure ZD coal [32,33]. All these findings prove that Fe/Na/Ca/S in coal ash will promote ash deposition and Na deposits mainly at medium temperature range (600 °C − 800 °C). Especially for S, it promotes the ash deposition when the temperature is equal or lower than 1000 °C due to sulfation of
Fig. 2. Element contents of the ash deposits from different heating surfaces.
3. Results and discussion 3.1. Component analysis of ash deposits The elemental analysis of the ash deposits on different heating surfaces are shown in Fig. 2. The contents of Si and Al are high at WW and PS where flue gas temperature is higher than 1100 °C. FR and PS have a lower Si/Al contents and higher AAME contents than WW and PS. For FR, Na significant enriched. And the Na content gradually decreases from inner layer to outer layer. K doesn’t show a significant enrichment in the deposits. Ca content in FR gradually increases from
Fig. 3. XRD test results of the ash deposits (1-CaSO4 2-Na2SO4 3-Na3Fe(SO4)3 4-SiO2 5-Ca2MgSi2O7 6-(3Al2O3·2SiO2) 7-Na(AlSi3O8) 8-CaO 9-MgO). 4
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(a) WD coal
(b) WCW coal
(c) CF coal Fig. 5. Predicted major Na-containing components under different temperatures.
(a) WD coal
(b) WCW coal
(c) CF coal Fig. 6. Predicted major Ca-containing components under different temperatures.
sulfates. To analyze the ash deposition progress on FR, the SEM-EDS analysis of FR-I, FR-M and FR-O were carried out. As is shown in Fig. 4. The physical and chemical properties of ash deposits in three layers are
Ca. Figs. 2 and 3 indicate that temperature plays an important role on sulfation reactions and AAME deposition. When the temperature is higher than 1100 °C, sulfates tend to decompose. As the flue gas temperature drops to lower than 1100 °C, sulfation of AAME occurs to form 5
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(a) WD coal
(b) WCW coal
(c) CF coal Fig. 7. Predicted major S-containing components under different temperatures.
(1.01%) for the ash generally in WD coal. However, the S contents in FR-I/FR-M/FR-O are 9.19%/13.44%/10.67%, which are higher than that in coal ash. That’s because FR was collected at the medium temperature range of 600–1000 °C, where reaction (1) and reaction (2) occurs. Those results indicated that the sulfate reactions occurred at the medium temperature range of 600–1000 °C have a positive effect on fouling. Many scholars also reported similar results that contribution of sulfate on ash deposition at the temperature range of 500–1000 °C [3,10,19]. The original S contents in WCW/WD coals are similar, but the S content in coal ash have a big difference. Such difference may be caused by the low content of Ca in WD coal. Thus in next section, equilibrium calculations will be conducted to investigate the relationships among Ca/Na/S and the coal blending effects in real boiler operating conditions.
Fig. 8. Predicted major S-containing components under different temperatures.
different. FR-I is molten when first deposited and then quickly quench into solid and stabilize the mineral mix. Combined with the component analysis results, it can be inferred that FR-I is dominated by Na2SO4 with lower content of CaSO4, Na(AlSi3O8) and Na3Fe(SO4)3; FR-M is in a molten/semi-molten state when first deposited and then quench into solid and stabilize the mineral mix. The components are mainly CaSO4, Na2SO4 and Na(AlSi3O8); FR-O is formed by small particles captured by the molten/ semi-molten ash deposit. The particles mainly consist of CaSO4, Na2SO4, SiO2, Ca2MgSi2O7 and Na(AlSi3O8). One of the significant finding in Fig. 4 is that the S content is pretty high as well as Ca/Na content. The difference is that Na content in FR-I is dominant while Ca become more important as the deposition grows. In Fig. 2, it additionally shows that the S content concentrated on FR-I, FR-M, FR-O and FS, where the medium temperature range is from 600 °C to 1000 °C, and the following reactions occurs: CaO + SO2 + 1/2 O2 → CaSO4 (1) Na2 O+ SO2 + 1/2 O2 → Na2SO4 (2) Reviewing Table.2, it’s also found that the S content is high (8.76%) for the ash produced by pure Zhundong coal while the S content is low
3.2. Coal blending effects on Na/Ca/S-containing components Since the field samples are the productions of coal blending (WCW and WD), it’s meaningful to investigate the effect of the blended low AAME content coal on ash deposition through a theoretical analysis based on the equilibrium calculation. Fig. 5(a)-(c) present the speciation of Na-containing components under different temperatures calculated by thermal equilibrium calculation under 3 cases: the pure WD coal case, the pure WCW coal case and the real case in the boiler where 70% WCW coal and 30% WD coal were blended. It can be indicated that the Na content species for pure WCW coal and WD coal are very different. For WCW coal, A large amount of Na2SO4(s) exist at 400–800 °C. And a large amount of liquid Na2SO4 is generated when the temperature is between 800 and 1100 °C. When the temperature is higher than 1100 °C, the liquid Na2SO4 is gradually converted to NaOH (g). For WD coal, the dominate Na species below 900 °C is albite (NaAlSi3O8) due to large content of Al and Si in the ash. Then albite will convert to Na2O at 900–1100 °C. Although the S content of both two type of coals are almost the same, there is no Na2SO4 in WD coal. As the 30% of WD coal 6
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(a) Na content in coal ash and ash deposits
(b) S content in coal ash and ash deposits
(c) Ca content in coal ash and ash deposits Fig. 9. Element analysis results and Na/S contents calculated by thermal equilibrium calculation.
than 5%, sulfate (l/slag) begin to present, that is, co-firing WD and WCW coals can reduce the amount of sulfate (l/slag) formation and the co-firing ratio greatly influence the elimination effect.
and 70% of WCW coal were blended, Na is mainly released in the form of NaAlSi3O8(s), NaCl (g), NaOH (g) and a small amount of Na2O (g) When the temperature increases. Co-firing WD and WCW coals can reduce the amount of liquid sulfate formation. For CF coal, when the temperature is greater than 900 °C, Na is gradually released in the form of NaCl (g) and NaOH (g), instead of Na2SO4(l). Fig. 6(a)–(c) shows the speciation of Ca-containing components under different temperatures calculated by thermal equilibrium calculation. Compared with WD coal, pyroxene (CaAlSi3O8/Ca3MgSi2O8) which may promote melting is generated at 400–1200 °C for CF coal/ WCW coal. For WCW coal, Ca is mainly existed in the form of CaCO3 (s), CaSO4 (s). A large amount of liquid CaSO4 is generated when the temperature is between 700 °C and 1200 °C. When the temperature is higher than 1000 °C, Ca2SiO4 increases at first and then decomposes to CaO when the temperature exceeds 1200 °C. For WD coal, CaSO4 (s) and CaAl2Si2O8 are the main minerals at 200–800 °C. It can be indicated from Fig. 6(a)–(c) that co-firing WD and WCW coals can reduce the liquid CaSO4 which mill promote the ash deposition. Also, co-firing will postpone the conversion of Ca-containing component to CaO (slag). Fig. 7(a)–(c) shows the speciation of S-containing components under different temperatures calculated by thermal equilibrium calculation. For WCW coal, a large amount of liquid Na2SO4 and liquid CaSO4 are generated at 800–1100 °C. When temperature is higher than 1200 °C, Na2SO4 and CaSO4 decompose to form SO2(g). Co-firing WD coal and WCW coal can reduce the formation of liquid sulfate so as to eliminate ash deposition. Similar to previous studies, ash deposits are dominated by CaSO4/Na2SO4 at the temperature range of 600–1100 °C [34]. However, rare study was conducted on the effect of co-firing ratio on relative proportions of the key deposition elements Na, Ca and S. As liquid sulfate (Na2SO4, Na2SO4) plays an important role in the ash deposition, the proportion of liquid/slag (l/slag), solid (s) and gas (g) sulfate are presented in Fig. 8. CF-30/10/5 represent co-firing ratio of WD coal are 30%, 10% and 5%. When the co-firing ration of WD coal increases, solid sulfate content decreases slower due to large content of Al and Si in WD coal ash. When the co-firing ratio of WD coal is lower
3.3. Coal blending effects on Na/Ca/S contents in coal ash The release and conversion characteristics of AAME minerals in WCW/WD/CF-30/CF-10 coals under different temperatures show that a large amount of liquid Na2SO4 and CaSO4 will be generated when burning WCW coal. Co-firing can reduce the formation of sulfates. Fig. 9(a)–(b) shows the comparison between the elemental analysis results and Na/S contents calculated by thermal equilibrium calculation. The discrepancy between the measured elemental distributions and the calculated may be caused by several issues. Firstly, WD coal and WCW coal were injected into the furnace through different coal mills. The mixing between WD coal and WCW coal are not perfect. Secondly, the deposits we sampled on most of surface are more like rock, we have to get these samples with a hammer. These samples are only a part of the solid/liquid products during coal combustion, while the rest of them including flying ash and the loose deposits were removed through flue gas flow or soot blowing. For these reason, the discrepancy is inevitable. However, it’s still meaningful to make a comparison between the measurement and the equilibrium calculations of different co-firing ratio so that the co-firing ratio effects can be analyzed. Besides, since Na/Ca/S has close relationship with the samples that can’t not be removed by soot blowing, a comparison of these elements is useful to help us to understand the element transform under different temperature. It’s also meaningful to analyze the change of liquid phase materials under different temperatures because these materials are highly related with the fouling and deposit materials that remains on the heating surface. The Na content in all deposits are higher than calculation results of CF coal. While the sulfur content in FR is similar to the calculation results of WCW coal. Especially, the Na/S contents in FR-I/FR-M are much higher than that in fly ash. As co-firing can reduce the generation 7
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of sulfate (l/slag) and there is almost no sulfate (l/slag) for CF-30 when the temperature is higher than 800 °C. It can be speculated that not all the sulfates in the ash deposits are formed in flue gas. The sulfation of the Na-containing minerals condensed on the heating surfaces plays an important role in the deposition process.
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4. Conclusions Based on the obtained results the following conclusions can be drawn: The ash deposits of WCW coal/WD coal can be divided into hightemperature ash deposits characterised by higher Si/Al contents and lower S content and medium-temperature ash deposits characterised by higher sulfate content. There is a significant enrichment of Na/Ca/S on FR. The Na content in FR gradually decreases from inner layer to outer layer. K doesn’t show a significant enrichment in the deposits. Ca content in FR gradually increases from inner layer to outer layer. FR-I is dominated by Na2SO4 with lower content of CaSO4, Na(AlSi3O8) and Na3Fe(SO4)3; FR-M mainly consist of CaSO4, Na2SO4 and Na(AlSi3O8); FR-O is formed by small particles captured by the molten/ semi-molten ash deposit. Sulfates of AAEM promote the formation of FR. A large amount of liquid Na2SO4 and CaSO4 will be generated when burning WCW coal. Co-firing WD and WCW coals can reduce the amount of sulfate (l/slag) formation and the co-firing ratio greatly influence the elimination effect. When the co-firing ratio of WD coal is higher than 5%, there is almost no sulfate (l/slag). The sulfation of the Na-containing minerals condensed on the heating surfaces will promote the ash deposition. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement Financial support for this work by the National Natural Science Foundation of China (Grant No. 51761125011). References [1] Zhu C, Qu S, Wei B, et al. Distribution, occurrence and leaching dynamic behavior of sodium in Zhundong coal. Fuel 2016;190:189–97. [2] Jin H, Chen Y, Ge Z, et al. Hydrogen production by Zhundong coal gasification in supercritical water. Int J Hydrogen Energy 2015;40:16096–103. [3] Wang X, Xu Z, Wei B, et al. The ash deposition mechanism in boilers burning Zhundong coal with high contents of sodium and calcium: a study from ash evaporating to condensing. Appl Therm Eng 2015;80:150–9. [4] Xu JY, Yu DX, Fan B, et al. Characterization of ash particles from co-combustion with a Zhundong coal for understanding ash deposition behavior. Energy Fuels 2014;28(1):678–84. [5] Gupta RP, Wall TF, Kajigaya I, et al. Computer-controlled scanning electron microscopy of minerals in coal – implications for ash deposition. Prog Energy Combust Sci 1998;24:523–43. [6] Wall TF, Creelman RA, Gupta RP, et al. Coal ash fusion temperatures – new characterization techniques, and implications for slagging and fouling. Prog Energy Combust Sci 1998;24:345–53. [7] Bryers RW. Fireside slagging, fouling, and high-temperature corrosion of heattransfer surface due to impurities in steam-raising fuels. Prog Energy Combust Sci
8
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Full Length Article
Ash deposition of Zhundong coal in a 350 MW pulverized coal furnace: Influence of sulfation
T
⁎
Hang Shi, Yuxin Wu , Man Zhang, Yang Zhang, Junfu Lyu Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Zhundong coal Pulverized coal furnace Ash deposition Sulfation Thermodynamic calculation
The huge reserve of Zhundong (ZD) coal makes it important for energy utilization of China. However, big concerns on fouling and slagging in pulverized coal-firing boilers were risen due to its high alkali content. It’s important to investigate deposition in different heating surfaces of a boiler burning ZD coal. In this paper, XRF, XRD, ICP-OES were conducted to analyze the properties of ash deposits collected from different heating surfaces in a 350 MW boiler burning 30% Wudong (WD) and 70% Wucaiwan (WCW) coals. Thermodynamics calculations were applied to study the mineral composition of coal ash in the range of 200–1600 °C under different coal cofiring ratio. The analytical results of the ash deposits showed that in medium-temperature (800–1100 °C) flue gas zone (FR and FS), ash deposits have higher sulfate content. While in high temperature flue gas zone (WW and PS), ash deposits have higher Si/Al content and lower S content. The Na/S contents in FR (~1000 °C) are much higher than in other deposits. The thermal equilibrium calculation results showed that a large amount of liquid Na2SO4 and CaSO4 will be generated when burning WCW coal. Co-firing WD and WCW coals can reduce the amount of sulfate (l/slag) formation and the co-firing ratio greatly influence the elimination effect. It can be speculated that not all the sulfates in the ash deposits are formed in flue gas. The sulfation of the Na-containing minerals condensed on the heating surfaces will promote the ash deposition.
Abbreviations: WCW coal, Wucaiwan coal; CF coal, Co-firing coal; WD coal, Wudong coal; CF-30/10/5, co-firing 30/10/5% WD and 70/90/95% WCW coals; XRD, X-ray diffraction; XRF, X-ray fluorescence; ICP-OES, inductively coupled plasma optical emission spectrometer ⁎ Corresponding author. E-mail address: [email protected] (Y. Wu). https://doi.org/10.1016/j.fuel.2019.116317 Received 9 July 2019; Received in revised form 25 September 2019; Accepted 28 September 2019 Available online 03 October 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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1. Introduction
the ash deposits on different heating surfaces were sampled in a 350 MW boiler burning Wucaiwan (WCW) coal and Wudong (WD) coal during the plant maintenance. WCW coal is a typical Zhundong coal from Wucaiwan area with high alkali/alkaline earth metal (AAEM) content. WD coal is a coal from Wudong area in China with low AAEM content. XRF, XRD, ICP-OES were conducted to analyze the composition of AAEM materials so as to investigate the deposition of AAEM materials and conversion of sulfur on different heating surfaces. As for the effect of co-firing, it’s hard to get data at different co-firing ratio by repetitive experiments in a real boiler. Considering that the formation of deposits (especially the outside layer) in a real boiler often takes a long period, the equilibrium solver is a reliable and reasonable tool to analyze such complex cases during coal blending. Many previous studies also adopted thermodynamic equilibrium models to predict the release and conversion characteristics of AAME minerals during thermal conversion [29–31]. Thus, thermodynamics calculations based on the equilibrium model were conducted to simulate deposition of AAEM materials on the heating surface at the temperature ranging from 200 to 1600 °C under different coal co-firing ratio. And the relationships between Ca/Na/S and the coal blending ratio in real boiler operating conditions were investigated.
Zhundong (ZD) coal field in Xinjiang province have an estimated reserve of 3.9 × 1011 tons [1]. The price of ZD coal is much lower than other coals due to its low exploitation cost. The utilization of ZD coal is important to China. ZD coal is a high-quality power coal characterized by low ash yields, low-medium sulfur content and high volatile content [2]. However, ZD coal has high Na2O/CaO contents and low SiO2/ Al2O3 contents in coal ash [3,4]. Thus severe slagging and fouling occur on the heating surfaces during the combustion of ZD coal [5–7]. Understanding the ash deposition mechanism is critical for the safe, efficient and clean utilization of ZD coal. Ash deposition can be divided into slagging and fouling. There are two main research methods for the investigation on the mechanism of ash deposition. One is to conduct the morphology and composition analysis of the ash deposits collected from real boilers [8–10]. The other one is to use the ash sampling tube to collect the ash deposits either in real boilers [11–13] or in a pulverized coal firing test rig, such as drop tube furnace, one-dimensional furnace, et al [14–16]. Previous studies indicated that slagging [15,17–19] occurs on radiant heating surfaces, while fouling [20–24] takes place in convective heating surfaces of the boiler. The studies conducted on real boilers or bench/pilot scale apparatus showed similar mechanism of slagging. Slagging is dominated by molten ash [15,17–19]. Slag is mainly composed of Si/Al/Ca [15,17,25]. Some scholars [10,18,19] find that the Fe2+ containing molten particles are more viscous under the local reducing atmosphere in the high-temperature pulverized coal flame zone. It is easy to deposit and capture impacting particles. Also, Fe can react with Si/Al/Ca to form low-melting-point eutectic salt, which will promote slagging. As for the mechanism of fouling on convective surfaces [26,27]. In a real boiler, it is found that ash deposits often have a clear stratified structure. The ash deposits collected from bench/pilot scale apparatus have different morphology and can’t fully represent that in real boiler due to the different operation conditions. For the stratified ash deposits in a real boiler, the composition and particle size distribution of the inner layer are significantly different from that of the outer layer. The major elements of the ash deposits are identified as Na, Ca and S [23]. Wu et al. [19] found that Na2SO4 dominates the initial layer and CaSO4/ Na2SO4/Ca-Si-Al dominates the ash deposits. Baxter et al. [24] reported that the low-temperature sulfation of the ash layer might cause severe deposition dominated by CaSO4 in convective heating surfaces when burn high-calcium coal. Sulfate of alkali/alkaline earth metal (AAEM) plays an important role in ash deposition [15,28]. Above all, most of the studies were conducted on the mechanism of slagging and fouling in bench or pilot scale apparatus. Generally, the combustion conditions, temperatures of heating surfaces and flue gas components in bench or pilot scale experiments are different from that in real boilers. The test duration is often insufficient for steady ash deposition, which is a long-term process. Although investigations for the real boilers are desired, it’s difficult to get good data due to following challenges. Firstly, almost all of boilers (especially for the subcritical and supercritical boilers) are co-firing low AAEM content coals or additives with ZD coal to avoid slagging and fouling problems. It’s hard to dig the meaningful information of AAEM deposition from the deposit samples. Secondly, it’s difficult to get the samples and the operation data even with the help of the operation partner. Up to now, investigations on a real boiler burning ZD coal are still rare. It requires more comparison between the field test and laboratory researches so as to promote the in-depth understanding of the actual deposition in a boiler. To fill this gap, this article tries to achieve the meaningful data from a field test so that the deposition in a real boiler that burning ZD coal can be understood. Since co-firing was commonly adopted by the industrial applications to avoid serious slagging/fouling, it’s also import to investigate the influence of co-firing ratio on deposition. Therefore,
2. Experimental approaches 2.1. Experimental equipment and facilities The ash deposits were collected from a 350 MW pulverized coal furnace constantly co-firing 70% WCW coal and 30% WD coal for more than one year. The influence of coal quality change on the deposition of heating surfaces in boiler can be negligible. The ash deposits were collected from the radiant heating surfaces (Water wall, WW), the convective heating surfaces in medium flue gas temperature (Panel superheater, PS; Final reheater, FR) and in low flue gas temperature (First superheater, FS). The sampling points and the corresponding flue gas/ heating surfaces temperatures are shown in Fig. 1. Fly ash was also sampled from electrostatic precipitator. 2.2. Fuel properties The proximate and ultimate analysis of WCW coal and WD coal are shown in Table 1. It is indicated by Table 1 that WCW coal has a lower ash yield than WD coal. Also, both of WCW coal and WD coal have low sulfur content. Considering that AAEM might release from coal under high ashing temperature. XRF test was conducted for coal ash prepared under different temperatures (Low temperature, LT, 150–200 °C; Medium temperature, MT, 500 °C; High temperature, HT, 815 °C). LT/ MT/ HT ash were prepared by YAMATO PR300 low temperature plasma asher/ muffle furnace/muffle furnace respectively. The test results of metal elements and sulfur in coal ash are shown in Table 2. WCW coal ash has a higher contents of Na2O/CaO/S and lower contents of SiO2/Al2O3 than WD coal ash prepared under the same temperature. It can be indicated from Table 2 that the influence of the vaporize of Na/K (815 °C) on the overall elemental percentage is less than 1.8%. The influence of the release of Na/K on S content in coal is slight. 2.3. Experimental and simulation method The ash deposits (except FR) were grounded to a particle size smaller than 30 μm. Since inductively coupled plasma optical emission spectrometer (ICP-OES) can only be used to measure metallic elements. X-ray fluorescence (XRF) and ICP-OES were applied to get a better elemental analysis for both metallic elements and non-metallic elements such as chlorine and sulfur. For the ICP-OES test, ash deposits were digested in the Milestone Ethos microwave digestion instrument. The digestion reagent used excellent grade pure 9 ml HNO3 + 1 ml 2
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Water wall, WW
Panel superheater, PS Final reheater, FR (a) Deposition samples
First superheater, FS
(b) flue gas and heating surfaces temperatures on deposition sampling points Fig. 1. Deposition sampling points of a 350 MW boilers and temperature distributions of flue-gas/heating surfaces.
HF + 1 ml HCl. The mineral composition of the ash deposits was analyzed by X-ray diffraction (XRD). The ash deposit in FR can be divided into three layers according to the slight differences of the color and hardness. To analyze the components of different layers, the sample were peeled off by layering and then grounded to smaller than 30 μm. XRD and XRF were conducted for the elemental and component analysis. Scanning electron microscopy (SEM-EDS) was applied to obtain the microscopic morphology and growth mechanism of the deposits. Thermodynamic equilibrium models are often applied to predict the release and conversion characteristics of AAME minerals during thermal conversion. The thermodynamic equilibrium model is based on the principle of Gibbs free energy minimization. The chemical composition of fuel or ash are used as the initial condition. The presence and content of AAEM minerals in the gas phase, liquid phase and solid phase at the equilibrium state can be achieved under given conditions (temperature, pressure and atmosphere). Equilib module in the Factsage software is widely applied by scholars to study the conversion characteristics of AAME minerals in ZD coal. Oleschko et al. [29] calculated the conversion characteristics of Na and found that the main gas phase products of Na during coal combustion were NaCl and Na2SO4. Bläsing et al. [30,31] calculated the release characteristics of Na under inert atmosphere, the results indicated that the main gas phase product of Na was NaCl under gasification conditions. In this study, thermal equilibrium calculations were conducted through Factsage 6.3 to investigate the release and conversion characteristics of AAME minerals during deposition given different temperature and different ratio of WD coal to the coal mixture feeding to the boiler. The Equilib module in Factsage 6.3 was used to carried out
Table 2 XRF test results of elements in coal ash [%]. Analyte
WD Coal Ash
SiO2 Al2O3 CaO Fe2O3 Sx K2O MgO TiO2 Na2O P2O5 Cl
WCW Coal Ash
LT
MT
HT
LT
MT
HT
56.86 22.88 5.58 4.39 1.89 1.75 1.27 0.89 0.70 0.21 0.02
57.11 23.35 5.60 4.47 1.66 1.79 1.28 0.91 0.70 0.18 0.02
58.85 25.03 3.81 4.08 1.01 1.91 1.22 1.05 0.79 0.26 0.01
10.73 7.89 33.26 3.84 8.44 0.27 8.21 0.37 5.36 0.05 2.48
10.6 8.05 33.96 3.86 8.91 0.27 8.11 0.37 5.47 0.05 2.45
10.26 8.76 34.92 3.86 8.76 0.15 11.18 0.36 3.77 0.06 0.03
the calculations. The FactPS and FToxide databases were used in the calculation. Considering that the conversion of AAME is the concerned problem, a metal solution database FTsalt containing AAME Na/K/Ca/ Mg and Cl−/OH−/CO32−/SO42− was also selected. For WCW and WD coals, the C/H/N/S/O contents were obtained through ultimate analysis of Table 1. As is shown in Table 3, the Si/Al/Ca/Mg/Na/K contents in raw WCW/WD coals were identified through ICP-OES test. The Cl content was identified through XRF test. For the co-firing (CF) coal, the components contents were calculated under different co-firing ratio. In the calculation, the temperature range was 200–1600 °C with a step of 100 °C under atmospheric pressure. The normal algorithm was used for calculation. The atmosphere was 21%O2/79%N2, and the excess air coefficient was 1.2.
Table 1 The Proximate and ultimate analysis of coal.
Wudong coal Wucaiwan coal
Mad
Vad
Aad
FCa
Car
Har
Oar
Nar
Sar
3.98 11.22
29.34 29.27
14.56 3.54
52.13 55.97
61.01 65.31
4.28 4.41
11.45 14.66
0.37 0.37
0.43 0.49
3
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Table 3 Elemental test results of coal [%]. Analyte
WCW (ar, wt%)
WD (ar, wt%)
Si Al Ca Mg Na K Cl
0.23 0.20 1.12 0.23 0.19 0.01 0.11
9.72 4.50 1.46 0.28 0.19 0.54 0.01
Fig. 4. SEM-EDS results of the ash deposits at the FR.
inner layer to outer layer. At the same time, S content shows a similar trend with AAME contents. At high temperature zone, S content is very low (WW, PS and fly ash). And the S content becomes much higher in FR and FS than in WW and PS. To investigate the relationship between S and AAME metal elements, the components of the sampled deposits were acquired through XRD measurement. The XRD results are shown in Fig. 3. Based on the test results in Figs. 2 and 3, the main substances in the ash deposits are CaSO4, Na2SO4, Na3Fe(SO4)3, SiO2(Quarts), Ca2MgSi2O7, (3Al2O3·2SiO2) (Mullite), and Na(AlSi3O8) (Albite). The composition of the ash deposits on different heating surfaces are significantly different. In low-temperature flue gas zone (FR and FS), ash deposits have higher Na/S contents. Na2SO4/CaSO4 are the dominant minerals for FR-I/FRM. FR-O and FS mainly consist of CaSO4, Na2SO4, SiO2, Ca2MgSi2O7 and Na(AlSi3O8). In high temperature flue gas zone (WW and PS), ash deposits have higher Si/Al contents and lower S content. Ca2MgSi2O7 and Na(AlSi3O8) are the dominant minerals while CaSO4, Na2SO4 and Na3Fe(SO4) contents are low for WW/PS. These results are similar with the findings of previous studies conducted on test rig for pure ZD coal [32,33]. All these findings prove that Fe/Na/Ca/S in coal ash will promote ash deposition and Na deposits mainly at medium temperature range (600 °C − 800 °C). Especially for S, it promotes the ash deposition when the temperature is equal or lower than 1000 °C due to sulfation of
Fig. 2. Element contents of the ash deposits from different heating surfaces.
3. Results and discussion 3.1. Component analysis of ash deposits The elemental analysis of the ash deposits on different heating surfaces are shown in Fig. 2. The contents of Si and Al are high at WW and PS where flue gas temperature is higher than 1100 °C. FR and PS have a lower Si/Al contents and higher AAME contents than WW and PS. For FR, Na significant enriched. And the Na content gradually decreases from inner layer to outer layer. K doesn’t show a significant enrichment in the deposits. Ca content in FR gradually increases from
Fig. 3. XRD test results of the ash deposits (1-CaSO4 2-Na2SO4 3-Na3Fe(SO4)3 4-SiO2 5-Ca2MgSi2O7 6-(3Al2O3·2SiO2) 7-Na(AlSi3O8) 8-CaO 9-MgO). 4
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(a) WD coal
(b) WCW coal
(c) CF coal Fig. 5. Predicted major Na-containing components under different temperatures.
(a) WD coal
(b) WCW coal
(c) CF coal Fig. 6. Predicted major Ca-containing components under different temperatures.
sulfates. To analyze the ash deposition progress on FR, the SEM-EDS analysis of FR-I, FR-M and FR-O were carried out. As is shown in Fig. 4. The physical and chemical properties of ash deposits in three layers are
Ca. Figs. 2 and 3 indicate that temperature plays an important role on sulfation reactions and AAME deposition. When the temperature is higher than 1100 °C, sulfates tend to decompose. As the flue gas temperature drops to lower than 1100 °C, sulfation of AAME occurs to form 5
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(a) WD coal
(b) WCW coal
(c) CF coal Fig. 7. Predicted major S-containing components under different temperatures.
(1.01%) for the ash generally in WD coal. However, the S contents in FR-I/FR-M/FR-O are 9.19%/13.44%/10.67%, which are higher than that in coal ash. That’s because FR was collected at the medium temperature range of 600–1000 °C, where reaction (1) and reaction (2) occurs. Those results indicated that the sulfate reactions occurred at the medium temperature range of 600–1000 °C have a positive effect on fouling. Many scholars also reported similar results that contribution of sulfate on ash deposition at the temperature range of 500–1000 °C [3,10,19]. The original S contents in WCW/WD coals are similar, but the S content in coal ash have a big difference. Such difference may be caused by the low content of Ca in WD coal. Thus in next section, equilibrium calculations will be conducted to investigate the relationships among Ca/Na/S and the coal blending effects in real boiler operating conditions.
Fig. 8. Predicted major S-containing components under different temperatures.
different. FR-I is molten when first deposited and then quickly quench into solid and stabilize the mineral mix. Combined with the component analysis results, it can be inferred that FR-I is dominated by Na2SO4 with lower content of CaSO4, Na(AlSi3O8) and Na3Fe(SO4)3; FR-M is in a molten/semi-molten state when first deposited and then quench into solid and stabilize the mineral mix. The components are mainly CaSO4, Na2SO4 and Na(AlSi3O8); FR-O is formed by small particles captured by the molten/ semi-molten ash deposit. The particles mainly consist of CaSO4, Na2SO4, SiO2, Ca2MgSi2O7 and Na(AlSi3O8). One of the significant finding in Fig. 4 is that the S content is pretty high as well as Ca/Na content. The difference is that Na content in FR-I is dominant while Ca become more important as the deposition grows. In Fig. 2, it additionally shows that the S content concentrated on FR-I, FR-M, FR-O and FS, where the medium temperature range is from 600 °C to 1000 °C, and the following reactions occurs: CaO + SO2 + 1/2 O2 → CaSO4 (1) Na2 O+ SO2 + 1/2 O2 → Na2SO4 (2) Reviewing Table.2, it’s also found that the S content is high (8.76%) for the ash produced by pure Zhundong coal while the S content is low
3.2. Coal blending effects on Na/Ca/S-containing components Since the field samples are the productions of coal blending (WCW and WD), it’s meaningful to investigate the effect of the blended low AAME content coal on ash deposition through a theoretical analysis based on the equilibrium calculation. Fig. 5(a)-(c) present the speciation of Na-containing components under different temperatures calculated by thermal equilibrium calculation under 3 cases: the pure WD coal case, the pure WCW coal case and the real case in the boiler where 70% WCW coal and 30% WD coal were blended. It can be indicated that the Na content species for pure WCW coal and WD coal are very different. For WCW coal, A large amount of Na2SO4(s) exist at 400–800 °C. And a large amount of liquid Na2SO4 is generated when the temperature is between 800 and 1100 °C. When the temperature is higher than 1100 °C, the liquid Na2SO4 is gradually converted to NaOH (g). For WD coal, the dominate Na species below 900 °C is albite (NaAlSi3O8) due to large content of Al and Si in the ash. Then albite will convert to Na2O at 900–1100 °C. Although the S content of both two type of coals are almost the same, there is no Na2SO4 in WD coal. As the 30% of WD coal 6
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(a) Na content in coal ash and ash deposits
(b) S content in coal ash and ash deposits
(c) Ca content in coal ash and ash deposits Fig. 9. Element analysis results and Na/S contents calculated by thermal equilibrium calculation.
than 5%, sulfate (l/slag) begin to present, that is, co-firing WD and WCW coals can reduce the amount of sulfate (l/slag) formation and the co-firing ratio greatly influence the elimination effect.
and 70% of WCW coal were blended, Na is mainly released in the form of NaAlSi3O8(s), NaCl (g), NaOH (g) and a small amount of Na2O (g) When the temperature increases. Co-firing WD and WCW coals can reduce the amount of liquid sulfate formation. For CF coal, when the temperature is greater than 900 °C, Na is gradually released in the form of NaCl (g) and NaOH (g), instead of Na2SO4(l). Fig. 6(a)–(c) shows the speciation of Ca-containing components under different temperatures calculated by thermal equilibrium calculation. Compared with WD coal, pyroxene (CaAlSi3O8/Ca3MgSi2O8) which may promote melting is generated at 400–1200 °C for CF coal/ WCW coal. For WCW coal, Ca is mainly existed in the form of CaCO3 (s), CaSO4 (s). A large amount of liquid CaSO4 is generated when the temperature is between 700 °C and 1200 °C. When the temperature is higher than 1000 °C, Ca2SiO4 increases at first and then decomposes to CaO when the temperature exceeds 1200 °C. For WD coal, CaSO4 (s) and CaAl2Si2O8 are the main minerals at 200–800 °C. It can be indicated from Fig. 6(a)–(c) that co-firing WD and WCW coals can reduce the liquid CaSO4 which mill promote the ash deposition. Also, co-firing will postpone the conversion of Ca-containing component to CaO (slag). Fig. 7(a)–(c) shows the speciation of S-containing components under different temperatures calculated by thermal equilibrium calculation. For WCW coal, a large amount of liquid Na2SO4 and liquid CaSO4 are generated at 800–1100 °C. When temperature is higher than 1200 °C, Na2SO4 and CaSO4 decompose to form SO2(g). Co-firing WD coal and WCW coal can reduce the formation of liquid sulfate so as to eliminate ash deposition. Similar to previous studies, ash deposits are dominated by CaSO4/Na2SO4 at the temperature range of 600–1100 °C [34]. However, rare study was conducted on the effect of co-firing ratio on relative proportions of the key deposition elements Na, Ca and S. As liquid sulfate (Na2SO4, Na2SO4) plays an important role in the ash deposition, the proportion of liquid/slag (l/slag), solid (s) and gas (g) sulfate are presented in Fig. 8. CF-30/10/5 represent co-firing ratio of WD coal are 30%, 10% and 5%. When the co-firing ration of WD coal increases, solid sulfate content decreases slower due to large content of Al and Si in WD coal ash. When the co-firing ratio of WD coal is lower
3.3. Coal blending effects on Na/Ca/S contents in coal ash The release and conversion characteristics of AAME minerals in WCW/WD/CF-30/CF-10 coals under different temperatures show that a large amount of liquid Na2SO4 and CaSO4 will be generated when burning WCW coal. Co-firing can reduce the formation of sulfates. Fig. 9(a)–(b) shows the comparison between the elemental analysis results and Na/S contents calculated by thermal equilibrium calculation. The discrepancy between the measured elemental distributions and the calculated may be caused by several issues. Firstly, WD coal and WCW coal were injected into the furnace through different coal mills. The mixing between WD coal and WCW coal are not perfect. Secondly, the deposits we sampled on most of surface are more like rock, we have to get these samples with a hammer. These samples are only a part of the solid/liquid products during coal combustion, while the rest of them including flying ash and the loose deposits were removed through flue gas flow or soot blowing. For these reason, the discrepancy is inevitable. However, it’s still meaningful to make a comparison between the measurement and the equilibrium calculations of different co-firing ratio so that the co-firing ratio effects can be analyzed. Besides, since Na/Ca/S has close relationship with the samples that can’t not be removed by soot blowing, a comparison of these elements is useful to help us to understand the element transform under different temperature. It’s also meaningful to analyze the change of liquid phase materials under different temperatures because these materials are highly related with the fouling and deposit materials that remains on the heating surface. The Na content in all deposits are higher than calculation results of CF coal. While the sulfur content in FR is similar to the calculation results of WCW coal. Especially, the Na/S contents in FR-I/FR-M are much higher than that in fly ash. As co-firing can reduce the generation 7
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of sulfate (l/slag) and there is almost no sulfate (l/slag) for CF-30 when the temperature is higher than 800 °C. It can be speculated that not all the sulfates in the ash deposits are formed in flue gas. The sulfation of the Na-containing minerals condensed on the heating surfaces plays an important role in the deposition process.
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4. Conclusions Based on the obtained results the following conclusions can be drawn: The ash deposits of WCW coal/WD coal can be divided into hightemperature ash deposits characterised by higher Si/Al contents and lower S content and medium-temperature ash deposits characterised by higher sulfate content. There is a significant enrichment of Na/Ca/S on FR. The Na content in FR gradually decreases from inner layer to outer layer. K doesn’t show a significant enrichment in the deposits. Ca content in FR gradually increases from inner layer to outer layer. FR-I is dominated by Na2SO4 with lower content of CaSO4, Na(AlSi3O8) and Na3Fe(SO4)3; FR-M mainly consist of CaSO4, Na2SO4 and Na(AlSi3O8); FR-O is formed by small particles captured by the molten/ semi-molten ash deposit. Sulfates of AAEM promote the formation of FR. A large amount of liquid Na2SO4 and CaSO4 will be generated when burning WCW coal. Co-firing WD and WCW coals can reduce the amount of sulfate (l/slag) formation and the co-firing ratio greatly influence the elimination effect. When the co-firing ratio of WD coal is higher than 5%, there is almost no sulfate (l/slag). The sulfation of the Na-containing minerals condensed on the heating surfaces will promote the ash deposition. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement Financial support for this work by the National Natural Science Foundation of China (Grant No. 51761125011). References [1] Zhu C, Qu S, Wei B, et al. Distribution, occurrence and leaching dynamic behavior of sodium in Zhundong coal. Fuel 2016;190:189–97. [2] Jin H, Chen Y, Ge Z, et al. Hydrogen production by Zhundong coal gasification in supercritical water. Int J Hydrogen Energy 2015;40:16096–103. [3] Wang X, Xu Z, Wei B, et al. The ash deposition mechanism in boilers burning Zhundong coal with high contents of sodium and calcium: a study from ash evaporating to condensing. Appl Therm Eng 2015;80:150–9. [4] Xu JY, Yu DX, Fan B, et al. Characterization of ash particles from co-combustion with a Zhundong coal for understanding ash deposition behavior. Energy Fuels 2014;28(1):678–84. [5] Gupta RP, Wall TF, Kajigaya I, et al. Computer-controlled scanning electron microscopy of minerals in coal – implications for ash deposition. Prog Energy Combust Sci 1998;24:523–43. [6] Wall TF, Creelman RA, Gupta RP, et al. Coal ash fusion temperatures – new characterization techniques, and implications for slagging and fouling. Prog Energy Combust Sci 1998;24:345–53. [7] Bryers RW. Fireside slagging, fouling, and high-temperature corrosion of heattransfer surface due to impurities in steam-raising fuels. Prog Energy Combust Sci
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