Experimental evaluation of additives and K2O–SiO2–Al2O3 diagrams on hightemperature silicate meltinduced slagging during biomass combustion

Fuel 179 (2016) 52–59 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Experimental evaluation of additives and K2O–SiO2–Al2O3 diagrams on high-temperature silicate melt-induced slagging during biomass combustion Yanqing Niu, Zhizhou Wang, Yiming Zhu, Xiaolu Zhang, Houzhang Tan, Shi’en Hui ⇑ Key Laboratory of Thermo-Fluid Science and Engineering of MOE, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China h i g h l i g h t s  Propose FT as an evaluation index of high-temperature silicate melt-induced slagging potential  Propose evaluation criteria on FT and high-temperature silicate melt-induced slagging  K2O–SiO2–Al2O3 diagram built on biomass ash undervalues FT of doped biomass 140–190 K  K2O–SiO2–Al2O3 diagram built on doped biomass over-predicts FT of pure biomass 200 K  FTs show ‘V’ shaped distributions with increased SiO2, Al2O3, and K2O, respectively a r t i c l e i n f o Article history: Received 22 February 2016 Received in revised form 17 March 2016 Accepted 19 March 2016 Available online 28 March 2016 Keywords: Biomass Ash Slagging Silicate Combustion K2O–SiO2–Al2O3 a b s t r a c t As one major barrier for biomass combustion, the high-temperature silicate melt-induced slagging is studied by additions of SiO2, kaolin, and soil and two types of K2O–SiO2–Al2O3 ternary phase diagrams constructed on basis of the real biomass ash properties and biomass by addition of K2O (in the form of KOH), SiO2, and Al2O3, respectively Results show that FT can be as the evaluate index for hightemperature silicate melt-induced slagging which increases with decreased FT Meanwhile, a set of qualitative criteria on high-temperature silicate melt-induced slagging are proposed However, because of the refractory minerals originated from additives directly or alumina-silication reactions indirectly when biomass blended with additives, the quantitative prediction of pure biomass and the biomass added additives should be based on the K2O–SiO2–Al2O3 ternary phase diagrams build by pure biomass ash properties and the biomass added Si/Al/K additives, respectively Overall, FTs show ‘V’ shaped distributions with increased SiO2, Al2O3, and K2O in ash, respectively Unlike SiO2 which exacerbates lowtemperature silicate melt-induced slagging, soil can substitute for expensive kaolin served as additives during biomass combustion The whole research provides useful guidelines for biomass selection, improvement, and slagging prevention during combustion Ó 2016 Elsevier Ltd All rights reserved Introduction Biomass combustion has been developed around the world because of the worsening environment and increasing energy crisis In China, the biomass power generation capacity will reach 30 GW in 2020 and accounts for 3% of the total power installed capacity, meanwhile, more than 130 dedicated biomass fired power plants have been operated in the country In Europe, biomass power generation capacity has taken up 70% of all generated ⇑ Corresponding author Tel.: +86 13709181734 E-mail address: sehui@mail.xjtu.edu.cn (S Hui) http://dx.doi.org/10.1016/j.fuel.2016.03.077 0016-2361/Ó 2016 Elsevier Ltd All rights reserved renewable fuel power, and in USA the biomass power installed capacity has reached 10 GW [1] Unfortunately, severe slagging happened in both biomass-fired fluidized bed (FB) and grate furnace [2,3] not only reduce heat transfer efficiency but also damage super-heaters and eventually lead to unscheduled shutdown frequently [3,4] Consequently, alkali metals (especially K) that have inescapable responsibilities on slagging has been investigated widely, including the migration and transportation behaviors during combustion [5–11], the existing forms in biomass [12,13], and the influence on ash fusion characteristics [5,6,14–16] During combustion, alkali mainly released as gaseous hydroxides, chlorides, and sulfates [7], but biomass species [7], cultivated fields [17], combustion temperature and 53 Y Niu et al / Fuel 179 (2016) 52–59 atmosphere [6,7,18], and the concentrations of K/Cl/SO2 [8,9,11,19] has significant effects on the concentration distribution It goes without saying that the widely studies on additives [20–23], leaching [24], and cofiring [2,18,25] that change the fuel properties directly also have remarkably effects on the alkali migration and transportation behaviors Recently, on basis of the different slagging formation mechanisms alkali-induced slagging and silicate melt-induced slagging (further classified into low-temperature and high-temperature silicate melt-induced slagging) have been proposed and investigated, respectively [5,17] Simply, the alkali metals in biomass are mainly existed in the forms of volatile, non-volatile water-soluble, and water insoluble [26] The volatile alkaline metal compounds such as KCl and K2SO4 can serve as adhesive and bond fly ash themselves and fly ash and heating surfaces together resulting in alkali-induced slagging [5,27] Meanwhile, both the volatile alkaline metal compounds and the non-volatile but water-soluble alkaline metal compound such as carbonates can react with SiO2 into lowmelting silicates causing low-temperature silicates melt-induced slagging [5,28] A global reaction of KCl and SiO2 resulting in the generation of low-melting silicates can be expressed by (R1) where when n equals 1, and 4, the corresponding melting points of K2OÁnSiO2 are below 1073 K [29,30] This is also a reason that agglomeration formation in FB furnace (typical combustion temperature 1123–1223 K) where KCl released from biomass reacts with quartz bed material into low-melting silicates 2KCl þ nSiO2 þ H2 O ! K2 O Á nSiO2 þ 2HCl ðR1Þ The insoluble alkali silicates, alkali aluminum silicates, and alkali calcium/magnesium silicates as refractory skeleton structure in biomass ash dominate the high-temperature silicate meltinduced slagging [6,14] When the temperature is above the melting points of abovementioned substances or the fluidized temperature (FT) of the ash, the ash will melt, bind other compounds, and adhere on heating surface resulting in slagging This is the reason why kaolin and calcite have been served as effective additives used to mitigate slagging during biomass combustion [21–23] Through Rs (2)–(5), kaolin not only suppresses the release of alkali chlorides and sulfates consequently eliminating alkali-induced slagging but also generates high-melting alkali aluminum silicates preventing the occurrence of high-temperature silicate melt-induced slagging [5], such as KAlSiO4 (kalsilite) and KAlSi2O6 (leucite) with the melting temperatures of greater than 1873 K and 1773 K, respectively [20,31], which is obviously higher than the combustion bath temperatures either in FB or in grate furnace Al2 O3 Á 2SiO2 Á 2H2 O þ 2MCl ! 2MAlSiO4 þ 2HCl þ H2 O ðR2Þ ðR3Þ Al2 O3 Á 2SiO2 Á 2H2 O þ M2 SO4 ! 2MAlSiO4 þ SO3 þ 2H2 O Al2 O3 Á 2SiO2 Á 2H2 O þ 2MCl þ 2SiO2 ! 2MAlSi2 O6 þ 2HCl þ H2 O ðR4Þ Al2 O3 Á 2SiO2 Á 2H2 O þ M2 SO4 þ 2SiO2 ! 2MAlSi2 O6 þ SO3 þ 2H2 O ðR5Þ where M represents K and Na In comparison with experiments such as abovementioned which provide detailed identification on special conditions, quantitative criterion numbers and evaluation index or simply qualitative trendline based on statistic analysis of numbers of experiment data can provide more general reference For alkali-induced slagging formed by the re-enrichment of fine particles primarily contained high concentrations of K, Na, Cl, and S in the forms of KCl and K3Na(SO4)2 and the re-capture of coarse large particles primarily contained higher Si, Al and so on [17], gaseous alkali salts promote the formation and development of the deposits, while Si and Al play opposite functions by trapping the alkali salts before they forms sticky deposits [32] High Si and Al in the ash assist in the alkali removal from the vapor phase and therefore reduce alkaliinduced slagging [5] Furthermore, considering the formation mechanisms of alkali-induce slagging, by means of a detailed analysis on the effects of S, Cl, Si, Al, and K on the distinct slagging characteristics of two cotton stalks burned in utility grate furnaces and a series of statistical data from references, a quantitative criterion number of alkali-induced slagging has been proposed recently as follow[17] While Cl ratio ðCl þ K2 O þ Na2 OÞ=ðSiO2 þ Al2 O3 Þ P 2:4 S ratio ðSVolatile þ K2 O þ Na2 OÞ=ðSiO2 þ Al2 O3 Þ P 1:9  While Cl ratio 1:0 slight slagging S ratio 0:5  serious slagging ðExp 1Þ Due to the remarkably formation of vapor alkali chlorides and sulfates, severe alkali-induced slagging occurs when the Cl ratio and S ratio are greater than 2.4 and 1.9, respectively By contrary, slagging is slight when the both ratios are lower than 1.0 and 0.5, respectively Slagging potential increases with increased Cl ratio and S ratio in the ranges of 1.0–2.4 and 0.5–1.9, respectively This criterion number clearly certifies the exciting function of kaolin which mitigates slagging effectively [20–23,31] For low-temperature silicate melt-induced slagging, Si2O and Al2O3 can raise the initial deformation temperature (IDT) [16], while the introduction of Al into potassium silicate melt lowers the melting temperature for high K/Al and low (K2 + Al2)/Si ratio [15] Recently, the authors studied the low-temperature silicate melt-induced slagging by additives and 30 biomasses, and found that IDT can be used as the evaluation index for low-temperature silicate melt-induced slagging, and high IDT reduces the potential for low-temperature silicate melt-induced slagging occurrence IDT increases with increase in Al2O3 and SiO2/K2O, while decreases with increase in K2O, SiO2, SiO2/Al2O3, and (SiO2 + K2O)/Al2O3 The significant effects of the compounds on IDT follow: Al2O3 > K2O > SiO2 > SiO2/K2O > SiO2/Al2O3 > (SiO2 + K2O)/Al2O3 Based on the significant effects a set of criteria to evaluate the potential of low-temperature silicate melt-induced slagging is proposed in detailed [26] In aspect of high-temperature silicate melt-induced slagging, that is more depended on refractory minerals [33] The refractory minerals, which has been identified as quartz, potassium iron oxide, and potassium magnesium silicate, potassium aluminum silicate, potassium calcium silicate, calcium silicate, mullite, diopside, pyrope, and monticellite, etc [6], provide structural support for the skeleton-like structure in biomass ash [14] Once the combustion temperature was above the melting point of the refractory minerals or access the FT, they will melt and adhere and result in high-temperature silicate melt-induced slagging, especially on water wall in furnace where the combustion temperature is high Generally, it is commonly accepted that SiO2 can inhibit the slagging, whereas the high SiO2 in biomass may exacerbate slagging [34] attributed to the unquestioning addition that just causes the generation of low-melting K2OÁnSiO2 [29,30] In addition, Xiong et al [35] pointed out that high K/(Ca + Mg) can inhibit slagging; but the opposed trends that the slagging is strengthened with increased K2O and decreased CaO and Al2O3 were found [14] Although various research on high-temperature silicate meltinduced slagging have been conducted, a detailed criteria like the proposed for alkali-induced slagging (Exp 1) [17] and lowtemperature silicate melt-induced slagging [26] and originated from real biomass ash properties rather than simulated ash has not be reported Drawing lessons from the previous research on lowtemperature silicate melt-induced slagging [26], this paper therefore focused on the effect of ash compounds on hightemperature silicate melt-induced slagging aims to provide a 54 Y Niu et al / Fuel 179 (2016) 52–59 Table The distribution of inorganic elements in additives and biomass ashes at different temperatures, wt.% a b Material Si Al Fe Mg Ca Na K S Cl Others Si/Al Biomass Kaolin Soil SiO2 16.9 22.9 23.4 46.6 2.1 24.7 13.5 1.9 0.4 4.6 4.3 0.3 7.9 0.2 0.1 0.3 0.1 0.1 12.8 0.2 1.7 0.1 1.7 0 51.3 51.4 56.2 53.4 7.8:1a 0.9:1a(2.6:1b) 1.7:1a(3.9:1b) 1a(13.4:1b) The mole ratios of Si/Al for 100 wt.% materials The mole ratios of Si/Al for 97 wt.% biomass + wt.% additives detailed method to evaluate the slagging potential, and the effects of the concentrations of Si, Al, and K in ash, and SiO2, kaolin, and soil additives on high-temperature silicate melt-induced slagging are studied simultaneously Firstly, the effects of SiO2, kaolin, and soil on ash fusion characteristics are tested; secondly, through the statistic analysis of the effects of ash compounds (Si, Al, and K) from 30 biomass, a set of detailed evaluation criteria that can be used to qualitatively guild biomass selection and improvement by additives, leaching, and cofiring and consequently mitigating or eliminating high-temperature silicate melt-induced slagging is proposed; thirdly, two types of K2O–SiO2–Al2O3 ternary phase diagrams constructed on basis of the real biomass ash properties and Fig Effects of SiO2, kaolin, and soil on ash fusion characteristics [26,36] biomass by addition of K2O (in the form of KOH), SiO2, and Al2O3, respectively, are compared in order to provide practice guideline on biomass selection, improvement, and subsequent slagging predication and research Experiments 2.1 Experiment materials In experiment, the wheat straw, an abundant agricultural residue in China, is selected as biomass representative In order to study the effects of different Si/Al compounds on the hightemperature silicate melt-induced slagging, pure biomass, biomass blended with wt.% SiO2, wt.% kaolin and wt.% soil additives, respectively, are selected comparably The corresponding inorganic element compositions of the materials are listed in Table Seen from Table 1, the biomass contains high Si and K, and low Al content, as well as high Si/Al mole ratio being around 7.8:1; while it is around 0.9:1, 1.7:1, and infinite in kaolin, soil, and SiO2 additive, respectively In addition, the content of Si in SiO2 is approximate two times of that in kaolin and soil; and the content of Al in kaolin is about times of that in soil So the significant differences in the additives facilitate the efficiency comparisons on the high-temperature silicate melt-induced slagging In order to systematically identify the effects of the concentrations of Si, Al, and K in ash on high-temperature silicate meltinduced slagging, the ash compositions and ash fusion characteristics of thirty biomasses fired in operating biomass power plants are tested, and a K2O–SiO2–Al2O3 ternary phase diagram is built on basis of the ash properties Meanwhile, three K2O–SiO2–Al2O3 ternary phase diagrams (total K2O–SiO2–Al2O3, water insoluble Fig SEM–EDS for the materials incinerated at 1088 K Y Niu et al / Fuel 179 (2016) 52–59 K2O–SiO2–Al2O3, and water soluble K2O–SiO2–Al2O3) are constructed by addition of K2O, SiO2, and Al2O3 into biomass In comparison with K2O–SiO2–Al2O3 ternary phase diagrams constructed by additions of K2O, SiO2 and Al2O3 oxides, the K2O–SiO2–Al2O3 ternary phase diagram built on basis of the thirty pure biomasses are more comparable with the real biomass components, whereas the K2O–SiO2–Al2O3 ternary phase diagrams constructed by additions of K2O, SiO2 and Al2O3 oxides may be more appropriate for the prediction of improved biomass by additives and leaching 2.2 Experiment apparatus The fusion temperature testing on biomass ash is conducted in a sintering instrument, which mainly consists of a temperature controllable electric heating furnace by program and a high-precision digital read-out and photographic record camera (SJY, Xiangtan Instrument Co., Ltd., China) Elements determination and main compounds identification are accomplished with XRF (X-ray fluorescence, S4-Pioneer, Bruker Co., Germany) and XRD (X-ray powder diffractometry, Xpert pro, PANalytical B.V Netherland) respectively Detailed descriptions on the instruments can be seen in previous papers [3,14] Meanwhile, the element distribution outside the ash particle and the ash morphology analysis are performed by using SEM–EDS (scanning electron microscopy–energy dispersive spectrometer, JSM-6390A, Japan), and ICP-AES (Inductively coupled plasma atomic emission spectroscopy, ICPE-9000, Japan) is used to test the concentration of water soluble-K in biomass ash consistent with each other due to the silication reactions at 1088 K, and the distribution of Al does not match the distributions of Si and K; while for addition of kaolin and soil, except few certain zones which may be originated from the additives directly or generated by the insufficient silication and alumina-silication reactions at 1088 K, the distributions of Si, Al and K are not consistent with each other Thus, the sufficient silicate reactions lead to considerable formation of low melting silicates which result in lower IDT and ST of the ash of biomass adding SiO2 (especially IDT), and the alumina-silication reactions of potassium chlorides and sulfates (Rs (2)–(5)) and the generation of more high-melting substances at elevated temperature result in increased FT Therefore, It can be deduced that the IDT affected by the significant formation of low melting silicates through the silication of alkali chlorides and sulfates can be as an evaluate index for biomass low-temperature silicate melt-induced slagging [26]; while the FT, which is mainly affected by the high temperature refractory substances in biomass ash, can be as an evaluation index for hightemperature silicate melt-induced slagging mainly affected by high Results and discussion 3.1 Effects of SiO2, kaolin and soil Commonly, the high-temperature silicate melt-induced slagging is dominantly affected by the high-temperature refractory materials which provide a supporting skeleton structure in the biomass ash [14] Therefore, the effects of SiO2, kaolin, and soil on ash fusion characteristics are performed It can be seen from Fig that except the IDT and soften temperature (ST) gained by addition of SiO2, in comparison with pure biomass the additions of SiO2, kaolin, and soil increase the ash fusion temperatures as a whole, particular in FT, which considerably increase because of the formation of more high-temperature refractory materials from the additives directly and/or reactant products indirectly Meanwhile, the additions of kaolin and soil present the almost same level increasing in the ash fusion temperatures Therefore, it seems that the soil can substitute for kaolin served as additives during biomass combustion Whereas, further study on soil need conducted because of the various compounds of the different soil sources Both IDT and ST increase with the addition of kaolin and soil, while decrease with the addition of SiO2 as singular points The decreased IDT and ST of biomass with the addition of SiO2 should be caused by the significant silication of KCl (R1) [29,30] Consequently, the considerable formation of K-silicates with melting temperature below 1073 K results in the decreasing IDT and ST, especially IDT And later, along with the occurrence of alumina-silication reactions of potassium chlorides and sulfates (Rs (2)–(5)) and the generation of more high-melting substances, FT increase This guesses that the decreasing IDT and ST of biomass with the addition of SiO2 are really attributed to the silication reactions (R1) can be verified from the SEM–EDS analysis on the ash generated by incineration at 1088 K as shown in Fig The distributions of Si and K in the ash of biomass with the addition of SiO2 are highly 55 Fig Statistic analysis on the effects of various components on FT 56 Y Niu et al / Fuel 179 (2016) 52–59 temperature refractory skeleton structure constructed by alkali calcium/magnesium silicates and alkali aluminum silicates originated from biomass contaminants directly and/or the reaction produces of the alumina-silication of potassium chlorides and sulfates indirectly Detailed investigation on low-temperature silicate melt-induced slagging can be seen from previous reference [26] In comparison with kaolin, soil which presents the almost same effect on ash fusion temperatures can substitute for expensive kaolin served as additives during biomass combustion However, SiO2 which exacerbates low-temperature silicate melt-induced slagging by reacting with KCl into low melting silicates is not suitable for additive 3.2 Evaluation criteria Once the temperature is above the melting points of the refractory ash compounds or the FT of the ash, the ash will undergo deformation and melting, and then adhere on the heating-surface by inertial impaction That is the typically high-temperature silicate melt-induced slagging happened in furnace [5] It can be seen from Fig 3a that FT dramatically increases with increase in K2O, and decreases with increase in Al2O3 and SiO2 Al2O3 shows the highest effect on FT seen from the highest slope (À462.8, 23.1 for 20Al2O3), followed by K2O (187.3) and SiO2 (À8.8) in turn From Fig 3b, it can be seen that FT decreases with the increase in SiO2/Al2O3 (À9.9) and (SiO2 + K2O)/Al2O3 (À9.6), and increases with the increase in SiO2/K2O (14.8) The slightly larger slope of (SiO2 + K2O)/Al2O3 than SiO2/Al2O3 indicates that K2O has certain positive effect on FT Moreover, from the slopes, it can be concluded that the effects of above parameters on FT are ordered as follow: Al2O3 > K2O > SiO2/K2O > SiO2/Al2O3 > (SiO2 + K2O)/Al2O3 > SiO2, and the positive effect orders are K2O > SiO2/ K2O, and the negative effect orders are Al2O3 > SiO2/Al2O3 > (SiO2 + K2O)/Al2O3 > SiO2 Therefore, a detailed evaluation criterion on high-temperature silicate melt-induced slagging based on FT is described as follow and also illustrated in Fig clearly Route 1: If the biomass contains higher Al2O3, SiO2, and lower K2O, it presents lower FT and higher high-temperature silicate melt-induced slagging potential Route II: If the biomass contains higher Al2O3 and lower SiO2, it needs to consider the combined parameter SiO2/Al2O3 due to the negative effects of both Al2O3 and SiO2 Higher Al2O3 and lower SiO2 lead to lower SiO2/Al2O3, so if the biomass possesses higher K2O at the same time, then it presents higher FT and lower high-temperature silicate melt-induced slagging potential; while if the biomass has lower K2O, (SiO2 + K2O)/Al2O3 must be considered because of the opposite effects of SiO2/ Al2O3 and K2O When the biomass holds lower (SiO2 + K2O)/ Al2O3, it shows higher FT and lower low-temperature silicate melt-induced slagging potential; Conversely, it shows lower FT and higher high-temperature silicate melt-induced slagging potential with higher (SiO2 + K2O)/Al2O3 Route III: If the biomass contains higher Al2O3, SiO2 and K2O, the combined parameter SiO2/K2O must be considered due to the opposite effect of K2O relative to Al2O3 and SiO2 If the biomass has lower SiO2/K2O, it possesses lower FT and higher hightemperature silicate melt-induced slagging potential Similarly, once the biomass contains higher SiO2/K2O, (SiO2 + K2O)/Al2O3 becomes the sole option because of the collision caused by the opposite trends of higher Al2O3 and higher SiO2/K2O The higher the (SiO2 + K2O)/Al2O3 is, the lower the FT is, and the easier the low-temperature silicate melt-induced slagging becomes, and vice versa “■”Biomass+Kaolin; “●”Biomass+Soil; “▼”Biomass; “ƾ”Biomass+SiO2 ; “·”Experimental value; unit: weight ratio Fig K2O–SiO2–Al2O3 ternary phase diagrams based on 30 biomass ash properties Fig Evaluation criteria on FT and high-temperature silicate melt-induced slagging potential Y Niu et al / Fuel 179 (2016) 52–59 Fig K2O–SiO2–Al2O3 ternary phase diagrams based on biomass by additions of K2O, SiO2 and Al2O3 oxides 57 58 Y Niu et al / Fuel 179 (2016) 52–59 3.3 K2O–SiO2–Al2O3 ternary phase diagrams Although abovementioned statistic analysis provides useful qualitative guidelines for high-temperature silicate melt-induced slagging and it is user-friendly by remembering the effect orders, it unavoidably omits some key-points as shown in Fig Therefore, in view of the limitation, and to compare the high-temperature silicate melt-induced slagging quantitatively, conveniently, and directly, two sets of K2O–SiO2–Al2O3 ternary phase diagrams of FT are constructed on basis of the thirty pure biomass ash properties (Fig 5) and biomass by additions of K2O, SiO2 and Al2O3 oxides (Fig 6), respectively Fig shows the K2O–SiO2–Al2O3 (actually should be K2O–SiO2– 20Al2O3) ternary phase diagrams built on the ash properties of the thirty pure biomasses It can be seen that the predicted temperatures of the pure biomass is highly consistent with the measuring value; while the measured FTs of the doped biomass are about 140–190 K higher than the predicted values Therefore, it can be concluded that even both pure biomass and the doped biomass possess the same K2O–SiO2–Al2O3 constructions, in comparison with pure biomass the doped biomass present higher FT due to the newly generated high-temperature refractory silicates through Rs (2)–(5) and/or the extra and un-reacted Si/Al compounds which mainly exist in oxides or original refractory minerals increasing the FT Thus, the K2O–SiO2–Al2O3 ternary diagram built on pure biomass ash properties is improper for the FT predication of doped biomass because of the existence of the excess oxide monomers and/or refractory minerals originated from additives directly or the reaction products through Rs (2)–(5) in the doped biomass Also, it can be seen from Fig that there exist some singular zones where FT holds the relatively highest and lowest temperatures, i.e where the occurrence of high-temperature silicate melt-induced slagging is the hardest or easiest One typical low temperature zones is around where K2O:SiO2:20Al2O3 equals 0.3:0.55:0.15 more or less; and two high temperature zones are around where K2O:SiO2:20Al2O3 is approximately (0.15–0.75):(0 05–0.1):(0.25–0.75) and where K2O:SiO2:20Al2O3 equals 0.05:0.75:0.2 more or less Moreover, the FTs show ‘V’ shapes with increased SiO2, Al2O3, and K2O, respectively This should be the reason why some conflicting results were reported when the research were located on the two sides of the ‘V’ shapes, such as the reports on SiO2 [34] and K/(Ca + Mg) [14,35] that have been described in introduction Fig shows the three K2O–SiO2–Al2O3 ternary phase diagrams constructed on basis of biomass by additions of K2O, SiO2, and Al2O3 It can been seen that in either one of the three diagrams (total K2O–SiO2–Al2O3, water insoluble K2O–SiO2–Al2O3, and water soluble K2O–SiO2–Al2O3) the measured FT of pure biomass is 230– 245 K lower that the prediction value, while the measured FTs of the doped biomass are well consistent with the predicted values Thus, it can be concluded that the FT prediction and comparison of pure biomass should be according to the K2O–SiO2–Al2O3 ternary phase diagrams built on pure biomass ash properties (i.e., Fig 5); whereas, the prediction and comparison of biomass blended with Si/Al/K additives should be based on the K2O–SiO2– Al2O3 ternary phase diagrams constructed on basis of biomass by additions of K2O, SiO2 and Al2O3 oxides (i.e., Fig 6), and anyone of the three K2O–SiO2–Al2O3 ternary phase diagrams (either total K2O–SiO2–Al2O3, or water insoluble K2O–SiO2–Al2O3, or water soluble K2O–SiO2–Al2O3) can provide high precision prediction Similarly, it can be seen from Fig that there also exist some singular zones where FT holds the maximum or minimum temperature On the whole, it is a low temperature zone when K2O: SiO2:20Al2O3 is around (0.4–0.7):(0.3–0.6):(0–0.1), and there are two high temperature zones where K2O:SiO2:20Al2O3 is around (0–0.2):(0.7–1.0):(0–0.2) and where the normalized ratio of SiO2 is lower than 0.25 The distribution is similar with that presented in Fig 5, and the FTs show ‘V’ shapes with increased SiO2, Al2O3, and K2O, respectively As a continuous research on biomass triple slagging (i.e., alkaliinduced slagging, low-temperature silicate melt-induced slagging, and high-temperature silicate melt-induced slagging) [5,26], this research focused on high-temperature silicate melt-induced slagging provides useful guidelines for biomass selection, improvement, and slagging prevention during combustion Further study will be focused on the acquisition of the quantitative criterion number that can provide integration guidelines on the triple slagging Conclusions The high-temperature silicate melt-induced slagging during biomass combustion is studied by additions of SiO2, kaolin, and soil additives, statistic analysis on the ash properties of thirty biomass fired in operating power plants, and K2O–SiO2–Al2O3 ternary phase diagrams of FT constructed on basis of the thirty biomass ash properties and biomass by additions of K2O, SiO2, and Al2O3 oxides, respectively Results indicate that: (1) For high-temperature silicate melt-induced slagging, FT can be as the evaluate index The higher the FT is, the lower the high-temperature silicate melt-induced slagging potential is FT increases with increase in K2O and SiO2/K2O, and decreases with increase in Al2O3, SiO2, SiO2/Al2O3, and (SiO2 + K2O)/Al2O3 The significances are ordered as: Al2O3 > K2O > SiO2/K2O > SiO2/Al2O3 > (SiO2 + K2O)/Al2O3 > SiO2 Meanwhile, on basis of the significance order a set of evaluation criteria which can provide qualitative comparison on the potential of high-temperature silicate melt-induced slagging is proposed as illustrated in Fig (2) The K2O–SiO2–Al2O3 ternary phase diagrams built on pure biomass ash properties (former, for short) and biomass added Si/Al/K additives (latter, for short) provide high precision prediction on themselves respectively However, because of the oxide monomers and/or refractory minerals originated from additives directly or generated from alumina-silication reactions indirectly when biomass blended with additives, the former K2O–SiO2–Al2O3 ternary phase diagram underestimates the FT of doped biomass about 140–190 K, and the latter over-predicts the FT of pure biomass above 200 K (3) The FTs show ‘‘V” shapes with increased SiO2, Al2O3, and K2O content in biomass ash, respectively And in the K2O–SiO2– Al2O3 ternary phase diagrams, there exist some singular zones where FT is the relatively highest and lowest, i.e where the occurrence of high-temperature silicate meltinduced slagging is the hardest or easiest (4) In comparison with kaolin, soil which presents the almost same effect on ash fusion temperatures can substitute for expensive kaolin served as additives during biomass combustion However, SiO2 which exacerbates lowtemperature silicate melt-induced slagging by reacting with KCl into low melting silicates is not suitable for additive Acknowledgements The present work was supported by the National Nature Science Foundation of China (Grant No 51406149) and the Fundamental Research Funds for the Central Universities (Grant No 2014gjhz08) Y Niu et al / Fuel 179 (2016) 52–59 References [1] Li L, Yu C, Huang F, Bai J, Fang M, Luo Z Study on the deposits derived from a biomass circulating fluidized-bed boiler Energy Fuels 2012;26:6008–14 http://dx.doi.org/10.1021/ef301008n [2] Elled AL, Davidsson KO, Amand LE Sewage sludge as a deposit inhibitor when co-fired with high potassium fuels Biomass Bioenergy 2010;34:1546–54 http://dx.doi.org/10.1016/j.biombioe.2010.05.003 [3] Niu YQ, Tan HZ, Ma L, Pourkashanian M, Liu ZN, Liu Y, et al Slagging characteristics on the superheaters of a 12 MW biomass-fired boiler Energy Fuels 2010;24:5222–7 http://dx.doi.org/10.1021/ef1008055 [4] Reichelt J, Pfrang-Stotz G, Bergfeldt B, Seifert H, Knapp P Formation of deposits on the surfaces of superheaters and economisers of MSW incinerator plants Waste Manage 2013;33:43–51 http://dx.doi.org/10.1016/j wasman.2012.08.011 [5] Niu Y, Tan H, Hui Se Ash-related issues during biomass combustion: alkaliinduced slagging, silicate melt-induced slagging (ash fusion), agglomeration, corrosion, ash utilization, and related countermeasures Prog Energy Combust Sci 2016;52:1–61 http://dx.doi.org/10.1016/j.pecs.2015.09.003 [6] Niu Y, Du W, Tan H, Xu W, Liu Y, Xiong Y, et al Further study on biomass ash characteristics at elevated ashing temperatures: the evolution of K, Cl, S and the ash fusion characteristics Bioresour Technol 2013;129:642–5 http://dx doi.org/10.1016/j.biortech.2012.12.065 [7] Wei XL, Schnell U, Hein KRG Behaviour of gaseous chlorine and alkali metals during biomass thermal utilisation Fuel 2005;84:841–8 http://dx.doi.org/ 10.1016/j.fuel.2004.11.022 [8] Blomberg T A thermodynamic study of the gaseous potassium chemistry in the convection sections of biomass fired boilers Mater Corros 2011;62:635–41 http://dx.doi.org/10.1002/maco.201005880 [9] Li B, Sun Z, Li Z, Alden M, Jakobsen JG, Hansen S, et al Post-flame gas-phase sulfation of potassium chloride Combust Flame 2013;160:959–69 http://dx doi.org/10.1016/j.combustflame.2013.01.010 [10] Kokko L, Tolvanen H, Hamalainen K, Raiko R Comparing the energy required for fine grinding torrefied and fast heat treated pine Biomass Bioenergy 2012;42:219–23 http://dx.doi.org/10.1016/j.biombioe.2012.03.008 [11] Yang TH, Kai XP, Sun Y, He YG, Li RD The effect of coal sulfur on the behavior of alkali metals during co-firing biomass and coal Fuel 2011;90:2454–60 http:// dx.doi.org/10.1016/j.fuel.2011.02.031 [12] Nutalapati D, Gupta R, Moghtaderi B, Wall TF Assessing slagging and fouling during biomass combustion: a thermodynamic approach allowing for alkali/ ash reactions Fuel Process Technol 2007;88:1044–52 http://dx.doi.org/ 10.1016/j.fuproc.2007.06.022 [13] Liaw SB, Wu H Leaching characteristics of organic and inorganic matter from biomass by water: differences between batch and semi-continuous operations Ind Eng Chem Res 2013;52:4280–9 http://dx.doi.org/10.1021/ ie3031168 [14] Niu YQ, Tan HZ, Wang XB, Liu ZN, Liu HY, Liu Y, et al Study on fusion characteristics of biomass ash Bioresour Technol 2010;101:9373–81 http:// dx.doi.org/10.1016/j.biortech.2010.06.144 [15] De Geyter S, Ohman M, Bostrom D, Eriksson M, Nordin A Effects of non-quartz minerals in natural bed sand on agglomeration characteristics during fluidized bed combustion of biomass fuels Energy Fuels 2007;21:2663–8 http://dx.doi org/10.1021/ef070162h [16] Li QH, Zhang YG, Meng AH, Li L, Li GX Study on ash fusion temperature using original and simulated biomass ashes Fuel Process Technol 2013;107:107–12 http://dx.doi.org/10.1016/j.fuproc.2012.08.012 [17] Niu Y, Zhu Y, Tan H, Hui S, Jing Z, Xu W Investigations on biomass slagging in utility boiler: criterion numbers and slagging growth mechanisms Fuel Process Technol 2014;128:499–508 http://dx.doi.org/10.1016/ j.fuproc.2014.07.03 [18] Fryda L, Sobrino C, Cieplik M, van de Kamp WL Study on ash deposition under oxyfuel combustion of coal/biomass blends Fuel 2010;89:1889–902 http:// dx.doi.org/10.1016/j.fuel.2009.11.022 59 [19] Kassman H, Bafver L, Amand LE The importance of SO2 and SO3 for sulphation of gaseous KCl – an experimental investigation in a biomass fired CFB boiler Combust Flame 2010;157:1649–57 http://dx.doi.org/10.1016/ j.combustflame.2010.05.012 [20] Steenari BM, Lundberg A, Pettersson H, Wilewska-Bien M, Andersson D Investigation of ash sintering during combustion of agricultural residues and the effect of additives Energy Fuels 2009;23:5655–62 http://dx.doi.org/ 10.1021/ef900471u [21] Bostrom D, Grimm A, Boman C, Bjornbom E, Ohman M Influence of kaolin and calcite additives on ash transformations in small-scale combustion of oat Energy Fuels 2009;23:5184–90 http://dx.doi.org/10.1021/ef900429f [22] Davidsson KO, Amand LE, Steenari BM, Elled AL, Eskilsson D, Leckner B Countermeasures against alkali-related problems during combustion of biomass in a circulating fluidized bed boiler Chem Eng Sci 2008;63:5314–29 http://dx.doi.org/10.1016/j.ces.2008.07.012 [23] Xiong SJ, Burvall J, Orberg H, Kalen G, Thyrel M, Ohman M, et al Slagging characteristics during combustion of corn stovers with and without kaolin and calcite Energy Fuels 2008;22:3465–70 http://dx.doi.org/10.1021/ef700718j [24] Dayton DC, Jenkins BM, Turn SQ, Bakker RR, Williams RB, Belle-Oudry D, et al Release of inorganic constituents from leached biomass during thermal conversion Energy Fuels 1999;13:860–70 http://dx.doi.org/10.1021/ ef980256e [25] Wang L, Skjevrak G, Hustad JE, Gronli MG Effects of sewage sludge and marble sludge addition on slag characteristics during wood waste pellets combustion Energy Fuels 2011;25:5775–85 http://dx.doi.org/10.1021/ef2007722 [26] Niu Y, Zhu Y, Tan H, Wang X, Hui Se, Du W Experimental study on the coexistent dual slagging in biomass-fired furnaces: alkali- and silicate meltinduced slagging Proc Combust Inst 2015;35:2405–13 http://dx.doi.org/ 10.1016/j.proci.2014.06.120 [27] Mu L, Zhao L, Liu L, Yin H Elemental distribution and mineralogical composition of ash deposits in a large-scale wastewater incineration plant: a case study Ind Eng Chem Res 2012;51:8684–94 http://dx.doi.org/10.1021/ ie301074m [28] Brus E, Ohman M, Nordin A Mechanisms of bed agglomeration during fluidized-bed combustion of biomass fuels Energy Fuels 2005;19:825–32 http://dx.doi.org/10.1021/ef0400868 [29] Pettersson A, Amand L-E, Steenari B-M Chemical fractionation for the characterisation of fly ashes from co-combustion of biofuels using different methods for alkali reduction Fuel 2009;88:1758–72 http://dx.doi.org/ 10.1016/j.fuel.2009.03.038 [30] Kai X The influence of sulfur on the migration characteristics of alkali during biomass and coal co-combustion In: Safety Technology-Engineering Shen Yang Institute of Aeronautical Engineering; 2009 [31] Wang L, Becidan M, Skreiberg O Sintering behavior of agricultural residues ashes and effects of additives Energy Fuels 2012;26(59):17–29 http://dx.doi org/10.1021/ef3004366 [32] Aho M, Ferrer E Importance of coal ash composition in protecting the boiler against chlorine deposition during combustion of chlorine-rich biomass Fuel 2005;84:201–12 http://dx.doi.org/10.1016/j.fuel.2004.08.022 [33] Vassilev SV, Kitano K, Takeda S, Tsurue T Influence of mineral and chemicalcomposition of coal ashes on their fusibility Fuel Process Technol 1995;45:27–51 http://dx.doi.org/10.1016/0378-3820(95)00032-3 [34] Lindstrom E, Larsson SH, Bostrom D, Ohman M Slagging characteristics during combustion of woody biomass pellets made from a range of different forestry assortments Energy Fuels 2010;24:3456–61 http://dx.doi.org/10.1021/ ef901571c [35] Xiong SJ, Ohman M, Zhang YF, Lestander T Corn stalk ash composition and its melting (slagging) behavior during combustion Energy Fuels 2010;24:4866–71 http://dx.doi.org/10.1021/ef1005995 [36] Niu Y, Zhu Y, Tan H, Hui Se, Du W Experimental study on the synthetic effects of kaolin and soil on alkali-induced slagging and molten slagging Energy Procedia 2014;61:756–9 http://dx.doi.org/10.1016/j.egypro.2014.11.959