Energy Fuels 2010, 24, 5222–5227 Published on Web 08/23/2010
: DOI:10.1021/ef1008055
Slagging Characteristics on the Superheaters of a 12 MW Biomass-Fired Boiler Yanqing Niu,† Hongzhang Tan,*,†,‡ Lin Ma,‡ Mohamed Pourkashanian,‡ Zhengning Liu,† Yang Liu,† Xuebin Wang,† Haiyu Liu,† and Tongmo Xu† †
State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China, and ‡School of Process, Environmental and Materials Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom Received June 26, 2010. Revised Manuscript Received August 6, 2010
In an attempt to understand the problem of severe slagging in biomass-fired boilers, slags formed on the fourth-, second-, and first-stage superheaters in a 12 MW biomass-fired grate furnace have been collected, sampled, and analyzed using X-ray fluorescence (XRF) and X-ray diffraction (XRD) techniques. The slag collected on each superheater has a unique morphology and structure. The slag on the fourth-stage superheater is loose and porous, while on the secondary and the primary superheaters, the slag shows a clear layered structure of different colors and levels of hardness. Aluminosilicate is the major component in the slag on the fourth-stage superheater, which is formed by Si, Al, and alkali species. On both the secondary and primary superheaters, the sticky sylvine, potassium calcium sulfate, and aphthitalite in the bottom layer trap coarse, large particles, resulting in further slagging. In the alternating layers, the particles containing high concentrations of K, Na, and Cl form the initial deposit layer, the yellow layer in the secondary superheater. Then, it traps the coarse, large particles containing high Si, Al, Ca, Mg, and Fe to form the brown layer. Similarly, the particles in the pale-yellow layer of the primary superheater containing high concentrations of K, Na, and Cl form the initial deposit layer, and then, it traps the coarse, large particles containing high Si, Al, Ca, Mg, and Fe to form the black layer. Accompanied by the periodic blowing, the alternating layer is formed and the slag gradually becomes thicker. To prevent the deposit formation on superheaters, considerable studies have been performed. The ash deposition from biomass combustion is closely related to the concentration of protecting (Si, Al, and S) and risky (Cl, Na, and K) elements in biofuels.1,4,7 Vapor-phase alkali promotes the formation of deposits on tube surfaces at high temperatures, while Si, Al, and S can trap the alkali before it can react to form sticky deposits.4 When the concentrations of Si, Al, and S are high in the ash, it assists in the removal of the alkali from the vapor phase.7 All of these are related to the formation of vaporphase alkali in the boiler in the first place. Deposits on the fireside surfaces and superheater tubes are mainly caused by sub-micrometer particles containing mostly potassium and chlorine.8 KCl has been found in superheater deposits in biomass-fired boilers.9,10 During combustion, fuel-bound chlorine and alkali metal, as organic parts and simple inorganic salts, volatilize into gas phase as HCl and alkali chloride, mainly potassium chloride and sodium chloride that are stable at typical combustion temperatures.11,12 Potassium chloride
1. Introduction With the depletion of fossil fuels and the increasingly serious environmental problems associated with fossil-fuel combustions, the use of biomass, which is renewable and CO2-neutral, has attracted worldwide attention. In China, 50 million tons of biomass pellets will be used annually by 2020.1 In addition, approximately 1.3 billion tons of biomass will be exploited in the European Union to achieve the goal of renewable energy resources taking up to 12% of the total energy consumption in 2010.2 Unfortunately, high concentrations of chlorine and alkali in biomass result in serious deposit formation and corrosion in boilers.3 Especially, the thick deposits not only reduce heat transfer and, thus, overall boiler efficiency but also cause damage to the superheaters, leading to unscheduled shutting down and extra maintenance costs.3-6 *To whom correspondence should be addressed. E-mail: tanhouzhang@ yahoo.cn. (1) Xiong, S. J.; Burvall, J.; Orberg, H.; Kalen, G.; Thyrel, M.; Ohman, M.; Bostrom, D. Slagging characteristics during combustion of corn stovers with and without kaolin and calcite. Energy Fuels 2008, 22 (5), 3465–3470. (2) European Environment Agency (EEA). How Much Biomass Can Europe Use without Harming the Environment?; EEA: Copenhagen, Denmark, 2005; Vol. 2, p 4. (3) Aho, M.; Silvennoinen, J. Preventing chlorine deposition on heat transfer surfaces with aluminium-silicon rich biomass residue and additive. Fuel 2004, 83 (10), 1299–1305. (4) 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 (2-3), 201–212. (5) Knudsen, J. N.; Jensen, P. A.; Dam-Johansen, K. Transformation and release to the gas phase of Cl, K, and S during combustion of annual biomass. Energy Fuels 2004, 18 (5), 1385–1399. (6) Szemmelveisz, K.; Szucs, I.; Palotas, A. B.; Winkler, L.; Eddings, E. G. Examination of the combustion conditions of herbaceous biomass. Fuel Process. Technol. 2009, 90 (6), 839–847. r 2010 American Chemical Society
(7) Niu, Y. Q.; Tan, H. Z.; Wang, X. B.; Liu, Z. N.; Liu, Y.; Xu, T. M. Study on deposits on the surface, upstream, and downstream of bag filters in a 12 MW biomass-fired boiler. Energy Fuels 2010, 24 (3), 2127– 2132. (8) Johansson, L. S.; Leckner, B.; Tullin, C.; Amand, L. E.; Davidsson, K. Properties of particles in the fly ash of a biofuel-fired circulating fluidized bed (CFB) boiler. Energy Fuels 2008, 22 (5), 3005–3015. (9) Hansen, L. A.; Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Horlyck, S.; Karlsson, A. Influence of deposit formation on corrosion at a straw-fired boiler. Fuel Process. Technol. 2000, 64 (1-3), 189–209. (10) Kaufmann, H.; Nussbaumer, T.; Baxter, L.; Yang, N. Deposit formation on a single cylinder during combustion of herbaceous biomass. Fuel 2000, 79 (2), 141–151. (11) Salmenoja, K.; Makela, K.; Hupa, M.; Backman, R. Superheater corrosion in environments containing potassium and chlorine. J. Inst. Energy 1996, 69 (480), 155–162.
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then condenses on the surface of sub-micrometer particles in the flue gas or heating surfaces.13 In addition, some researchers suggested that K2SO4 nucleates when the gas temperature reduces14 and then KCl condenses on the nucleon of K2SO4 at a lower temperature.15 Several authors have proposed to use various kinds of mineral additives to solve the deposition problems, and they have been successful to some extent. Kaolin (Al2O3 3 2SiO2) addition can significantly reduce superheater deposits, corrosion, and slagging and thus enhance the operation of the biomass-fired boiler.8,16-18 The slag formation could decrease by 50 and 67% with kaolin and calcite addition, respectively.1 In addition, co-combustion of different types of biomass or biomass with other fuels, such as sewage sludge, refuse, and coal could contribute to deposit reduction.4,19-21 Adding a high concentration of aluminosilicate coal to meat and bone meal could prevent deposits on the heat-transfer surface effectively.4 Meanwhile, the efficiency is in positive correlation with the concentrations of aluminum and silicon in ash.19 Other researchers investigated the effects of leaching with water, acetate solutions, and hydrochloric acid in removing alkali from biomass fuel and, thus, reducing slagging.4,21 Although significant research on the deposition or slagging process has been performed and some precaution measures have also been attempted, previous research has been mainly conducted in the laboratory, in which there are inevitable differences as compared to the real situation in industrial boilers. Additionally, few experimental data are available with respect to the long-term deposits formed along the flue gas flow in the boiler. In fact, depositions formed at the downstream locations are significantly different from those at the upstream locations. Meanwhile, the late depositions are also significantly different from the early depositions. The main objective of the present work is therefore to determine the slagging characteristics on different superheat surfaces along the flue gas flow in the industrial boiler investigated.
Figure 1. Schematic diagram of the boiler system investigated. Table 1. Temperature of Different Heating Surfaces on the 9 MW Load (°C) outlet fourth third temperature superheater superheater flue gas steam
525 ( 13
445 ( 11
secondary superheater
primary superheater
507-645 408 ( 29
378-465 340 ( 44
Table 2. Analysis Results of the Cotton Stalk Employeda component
content (wt %)
oxides
content (wt %)
moisture ash volatiles fixed carbon Cad Had Oad Nad Sad Clad
2.63 4.22 72.61 20.54 45.86 5.53 40.96 0.61 0.19 0.44
SiO2 Al2O3 Fe2O3 CaO MgO TiO2 SO3 P2O5 K2O Na2O
13.74 4.03 1.16 25.15 10.99 0.21 5.75 6.98 25.40 6.58
a
ad is the abbreviation for air-dried basis.
2. Experimental Section 2.1. Experimental Methods. The deposits discussed in this paper are taken from a 12 MW biomass-fired grate furnace at three superheaters, i.e., the fourth, second, and first superheaters (see Figure 1). It is a typical M-type natural circulation boiler that contains four-stage superheaters. Flue gas passes through the third-stage, fourth-stage, secondary, and primary superheaters one after another, and then it goes through the economizer and air preheater. Finally, the flue gas is cleaned by bag filters before being discharged. The furnace has been described in detail elsewhere,7 and thus, only a brief description has been given here. The gas temperature along the flue gas flow has been measured under the 9 MW load (Table 1). Averaged gas temperatures at the primary and secondary superheater are 465 and 665 °C, respectively. Unfortunately, gas temperatures at the third and fourth stages have not been measured because of the lack of corresponding temperature-measuring points. The biomass fuel used in the power plant is isolated cotton stalks. The corresponding ultimate and proximate analyses, as well as ash compositions, have been listed in Table 2. As can be seen from Table 2, the concentrations of Cl, K, Na, Mg, and Ca in the ash are higher than those in typical wood ash and most straw biomass ash; however, the concentration of Al is relatively low.10,16 Slag formation in the furnace is significant, as can be seen in Figure 2, especially for the primary and secondary superheaters downstream of the fourth-stage superheater. The thickness of the slag on the secondary superheater can be as high as 90 cm.
(12) Olsson, J. G.; Jaglid, U.; Pettersson, J. B. C.; Hald, P. Alkali metal emission during pyrolysis of biomass. Energy Fuels 1997, 11 (4), 779–784. (13) Westberg, H. M.; Bystrom, M.; Leckner, B. Distribution of potassium, chlorine, and sulfur between solid and vapor phases during combustion of wood chips and coal. Energy Fuels 2003, 17 (1), 18–28. (14) Valmari, T.; Kauppinen, E. I.; Kurkela, J.; Jokiniemi, J. K.; Sfiris, G.; Revitzer, H. Fly ash formation and deposition during fluidized bed combustion of willow. J. Aerosol Sci. 1998, 29 (4), 445–459. (15) Jimenez, S.; Ballester, J. Influence of operating conditions and the role of sulfur in the formation of aerosols from biomass combustion. Combust. Flame 2005, 140 (4), 346–358. (16) Jensen, P. A.; Stenholm, M.; Hald, P. Deposition investigation in straw-fired boilers. Energy Fuels 1997, 11 (5), 1048–1055. (17) Davidsson, K. O.; Steenari, B. M.; Eskilsson, D. Kaolin addition during biomass combustion in a 35 MW circulating fluidized-bed boiler. Energy Fuels 2007, 21 (4), 1959–1966. (18) Davidsson, K. O.; Amand, L. E.; Steenari, B. M.; Elled, A. L.; Eskilsson, D.; Leckner, B. Countermeasures against alkali-related problems during combustion of biomass in a circulating fluidized bed boiler. Chem. Eng. Sci. 2008, 63 (21), 5314–5329. (19) Khan, A. A.; Aho, M.; de Jong, W.; Vainikka, P.; Jansens, P. J.; Spliethoff, H. Scale-up study on combustibility and emission formation with two biomass fuels (B quality wood and pepper plant residue) under BFB conditions. Biomass Bioenergy 2008, 32 (12), 1311–1321. (20) Pettersson, A.; Zevenhoven, M.; Steenari, B. M.; Amand, L. E. Application of chemical fractionation methods for characterisation of biofuels, waste derived fuels and CFB co-combustion fly ashes. Fuel 2008, 87 (15-16), 3183–3193. (21) Turn, S. Q.; Kinoshita, C. M.; Ishimura, D. M. Removal of inorganic constituents of biomass feedstocks by mechanical dewatering and leaching. Biomass Bioenergy 1997, 12 (4), 241–252.
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melting point and is unlikely to become liquid and stick to the surfaces of the fourth-stage superheater even when the temperature there is relatively high.17 Leucite and picrite have a similar molecular composition to the aluminosilicate; thus, they could be a result of molecular realignments of the aluminosilicate. Therefore, the slag of the fourth-stage superheater is relatively small and caducous. 3.2. Slag on the Secondary-Stage Superheater. The slag on the secondary superheater presents a clear layered structure with different hardness and colors. The thickness of the slag is above 20 cm, and it can be divided broadly into four layers, namely, layer 1 (the bottom layer), layer 2 (the transition layer), and layers 3 and 4 (alternating yellow and brown layers), as indicated in Figure 3b. The contents of all of the elements are also quantitatively analyzed using XRF, and the results are shown in Table 3. It can be seen that the concentrations of Si, Al, Ca, Mg, and Fe increase along the thickness of the slag from the bottom (near the tube wall) to the top, while the concentrations of K, Cl, and S decrease. The corresponding XRD analysis (Figure 5) shows that the major components in the slag on the secondary superheater are sylvine (2θ = 28.347, 40.528, 50.188, 58.643, and 66.393), halite (2θ = 31.692, 45.449, and 56.477), potassium calcium sulfate (2θ = 26.672, 27.232, 27.309, and 27.637), quartz (2θ = 20.728 and 26.562), monticellite (2θ= 24.465, 33.563, 50.145, etc.), and melilite [akermanite (2θ= 31.350, 36.648, 52.090, etc.) and gehlenite (2θ =31.319, 36.696, 52.025, etc.)]. As seen from Table 3 and Figure 5a, the major elements in layer 1 are Na, K, S, Cl, and Ca and the major components are sylvine, halite, and potassium calcium sulfate. The condensation temperature of sylvine is about 700 °C,22 while the flue gas temperature in the secondary superheater is 507645 °C (see Table 1), and this could lead to sylvine being condensed and adhering to the heating surfaces. In addition, the formation of the sticky potassium calcium sulfate could be explained by eqs 223 and 3.24
Figure 2. Slag formed in the boiler.
Before the experiments, the slag and deposits in the boiler are thoroughly cleaned. After 2 weeks of operation, slag on the fourth, secondary, and primary superheaters (Figure 3) are collected, sampled, and analyzed using X-ray fluorescence (XRF) and X-ray diffraction (XRD). The slag on the thirdstage superheater is very thin and, therefore, not presented here. 2.2. Analysis Technioques. In the experimental analysis of the slag samples, the XRF technique (S4-Pioneer, Bruker Co., Germany) has been used for quantitative elemental determinations. The main crystalline compounds in the slag samples are identified by XRD using a D/max2400X powder diffractometer (Japan) with the characteristic Cu KR radiation. The system is operated at 40 kV and 100 A. Peak identification is performed by a comparison to standards from the JADE5 software package.
3. Results and Discussion 3.1. Slag on the Fourth-Stage Superheater. The slag on the fourth-stage superheater is found loose and porous (Figure 3a); moreover, it is smaller than the slag on the primary and secondary superheaters and can be easily removed. The thickness of the slag is just about 1 cm. Generally speaking, it falls into the cold ash bucket under its own weight when growing up and other disturbance forces, such as periodic blowing. The contents of all of the elements have been quantitatively analyzed using XRF. The results shown in Table 3 indicate that the major elements in the slag are Si, Al, Ca, Mg, Na, K, and Fe, while the anions, such as Cl and S, are relatively low and negligible. On the basis of the XRF analysis, a further study on the compounds in the slag on the fourth-stage superheater is conducted with the XRD, and the results are shown in Figure 4. From Figure 4, it is observed that the major compounds in the slag are leucite (2θ = 16.384, 25.838, 27.168, 31.348, etc.) and picrite (diopside and augite) (2θ = 29.728, 30.190, 30.760, 35.458, 39.033, etc.). High concentrations of Si and Al (Table 3) in slag can trap alkali species via the formation of aluminosilicate and hydrogen chloride, according to eq 1 below7
K2 SO4 þ CaSO4 þ H2 O f K2 CaðSO4 Þ2 3 H2 O
ð2Þ
K2 CaðSO4 Þ2 3 H2 O f K2 CaðSO4 Þ2 þ H2 O
ð3Þ
The sticky sylvine and potassium calcium sulfate can trap the coarse large ash particles mainly containing silicon, aluminum, and calcium,8,25,26 and this further promotes the development of the slag. This is the reason why the transition layer contains a considerable amount of quartz. In the alternating yellow and brown layers (layers 3 and 4), the concentrations of Si, Al, Ca, Mg, and Fe increase (22) Christensen, K. A.; Stenholm, M.; Livbjerg, H. The formation of submicron aerosol particles, HCl and SO2 in straw-fired boilers. J. Aerosol Sci. 1998, 29 (4), 421–444. (23) Yang, X. Y.; Yu, D. G.; Wang, J. H. Research on hydration mechanism of calcium sulfate anhydrite activated by potassium sulfate. J. Shenyang Jianzhu Univ. 2008, 24 (1), 104–107. (24) Kloprogge, J. T.; Ding, Z.; Martens, W. N.; Schuiling, R. D.; Duong, L. V.; Frost, R. L. Thermal decomposition of syngenite, K2Ca(SO4)2 3 H2O. Thermochim. Acta 2004, 417, 143–155. (25) Lillieblad, L.; Szpila, A.; Strand, M.; Pagels, J.; Rupar-Gadd, K.; Gudmundsson, A.; Swietlicki, E.; Bohgard, M.; Sanati, M. Boiler operation influence on the emissions of submicrometer-sized particles and polycyclic aromatic hydrocarbons from biomass-fired grate boilers. Energy Fuels 2004, 18 (2), 410–417. (26) Strand, M.; Pagels, J.; Szpila, A.; Gudmundsson, A.; Swietlicki, E.; Bohgard, M.; Sanati, M. Fly ash penetration through electrostatic precipitator and flue gas condenser in a 6 MW biomass fired boiler. Energy Fuels 2002, 16 (6), 1499–1506.
Al2 O3 3 2SiO2 þ 2MCl þ H2 O f M2 O 3 Al2 O3 3 2SiO2 þ 2HCl
ð1Þ where M denotes K and/or Na. The formation of aluminosilicate and hydrogen chloride can reduce the level of the concentration of sub-micrometer alkali halide particles (KCl and NaCl) or vapor-phase alkali that promote the formation of deposits on tube surfaces. The aluminosilicate has a high 5224
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Figure 3. Slag sample in the boiler. Table 3. XRF-Analyzed Results of Slag on Different Heating Surfaces sample fourth-stage superheater secondary superheater
primary superheater
layer 1 layer 2 layer 3 layer 4 layer 1 layer 2 layer 3
Na2O
MgO
Al2O3
SiO2
P2O5
SO3
Cl
K2O
CaO
Fe2O3
4.12 10.0 4.94 6.32 5.63 9.21 7.02 4.93
4.61 1.60 1.93 5.17 5.68 2.62 5.71 6.63
10.1 0.86 1.77 4.66 6.35 1.86 2.30 5.78
45.9 3.14 6.49 17.1 24.1 6.74 7.72 20.5
1.21 0.38 0.48 1.92 1.54 0.90 1.59 1.85
0.7 19.2 13.7 11.5 8.59 30.0 12.9 10.7
0.35 22.9 23.0 9.9 7.02 9.78 19.9 9.71
7.52 27.2 32.0 15.1 11.5 24.8 18.4 12.2
20.0 13.3 14.0 24.9 25.6 12.4 22.8 23.9
4.01 0.92 1.19 2.70 3.23 1.34 1.45 3.10
while the monticellite that contains less Mg than Fe shows brown in color.27 Akermanite and gehlenite look either yellow or brown.28 Therefore, the alternating layers 3 and 4 are yellow and brown, respectively. In addition, particles containing high concentrations of K, Na, and Cl form the yellow layer first, and then it traps the large ash particles containing high concentrations of Si, Al, Ca, Mg, and Fe to form the brown layer. Accompanied by the periodic blowing
significantly and the sum of them is about 50-70% of the total sum, while the concentrations of K, Cl, and S go down, especially for the risky K and Cl elememts. However, the yellow layer contains more K, Na, and Cl than the brown layer. XRD (panels c and d of Figure 5) indicates that the major components in the alternating yellow and brown layers are sylvine, halite, quartz, akermanite, and gehlenite. In addition, the yellow layer also contains monticellite [Ca(MgxFey)SiO4], which represents four kinds of different monticellite compositions [MgCaSiO4, CaMgSiO4, Ca(Mg0.88Fe0.12)SiO4, and Ca(Mg0.93Fe0.07)SiO4]. The monticellite that contains more Mg than Fe shows yellow in color,
(27) http://www.hudong.com/wiki/%E6%A9%84%E6%A6%84% E7%9F%B3 (accessed on Feb 13, 2010). (28) http://baike.baidu.com/view/1193767.htm?fr=ala0_1 (accessed on Feb 1, 2010).
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Figure 4. XRD-analyzed results of slag on the fourth-stage superheater.
every 6 h, the alternating layer is formed and the slag gradually becomes thicker. 3.3. Slag on the Primary-Stage Superheater. The slag on the primary superheater presents a clear breakable layer structure also with different colors and levels of hardness, similar to what is seen on the secondary superheater, but the color changed (see Figure 3c). Layers 1, 2, and 3 correspond to the bottom and alternating pale-yellow and black layers, respectively. In comparison to the slag on the secondary superheater, the slag on the primary superheater is thinner, with a thickness of less than 10 cm. The results of XRF shown in Table 3 indicate that the concentrations of Si, Al, Ca, Mg, Fe, and P in the slag on the primary superheater increase along the thickness of the slag from the bottom to the top, while that of K, Na, and S decrease. The concentration of Cl in the pale-yellow layer is the highest. XRD results in Figure 5 show that the major components in the slag on the primary superheater are sylvine (2θ = 28.347, 40.528, 50.188, 58.643, and 66.393), halite (2θ = 31.692, 45.449, and 56.477), anhydrite (2θ = 25.376, 31.307, 52.169, etc.), quartz (2θ=20.826 and 26.593), aphthitalite (2θ = 21.711, 30.378, 31.486, etc.), and sylvine sodium (2θ=28.608, 40.902, and 50.673). As can be seen from Table 3 and Figure 6a, the major elements in the white layer (layer 1) are Na, K, S, Cl, Si, and Ca, with sylvine, halite, anhydrite, quartz, and aphthitalite as the major components. The flue gas temperature around the primary superheater is about 378-465 °C (see Table 1), and this can lead to the condensation of the sticky sylvine on the heating surfaces. In addition, the aphthitalite, which is also sticky, can be generated from the reaction between K2SO4 and Na2SO4, following eq 4 below,29 and deposited on the superheater. 3K2 SO4 þ Na2 SO4 f 2K3 NaðSO4 Þ2
ð4Þ Figure 5. XRD-analyzed results of slag on the secondary superheater.
Sticky sylvine and aphthitalite in the bottom layer close to the tube can trap ash particles in the flue gas and further promote the development of slag. XRD results in panels c and d of Figure 5 show that the major components in the alternating pale-yellow and black layers are sylvine, halite, anhydrite, and quartz. Sylvine sodium also exists in the black layer. Meanwhile, it is found that the concentrations of Si, Al, Ca, Mg, Fe, and P in the pale-yellow layer are lower than that in the black layer and the concentrations of risky K, Na, Cl, and S in the
pale-yellow layer are relatively high. The particles containing high concentrations of K, Na, and Cl form the pale-yellow layer first, and then it traps the large particles containing high concentrations of Si, Al, Ca, Mg, Fe, and P to form the black layer. Again, periodic blowing may have led to the alternating layered structure, which promotes the development of the slag on the superheater. Unfortunately, the reason for the different colors and hardness levels of the alternating layers in the secondary superheat is not yet clear, although the different characteristics of the slag on the two superheats are evident.
(29) Long, S. Z.; Chen, Y. Z.; Cheng, B.; Sun, W.; Xiong, R. J. Temperature range of eutectic mixture formation for KCl-K2SO4Na2SO4 system. J. Chin. Ceram. Soc. 2006, 34 (4), 504–506.
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showed the heaviest deposition, while the upstream tertiary superheater showed the lightest. In addition, the slag on the fourth-stage superheater is loose and porous, and the size of the slag is relatively small and caducous, with a thickness of just about 1 cm. The slag on the primary and secondary superheaters shows a clear layer structure with different colors and levels of hardness. The thickness of the slag on the secondary superheater is above 20 cm, while the thickness of the slag on the primary superheater is below 10 cm. In the slag on the fourth-stage superheater, high concentrations of Si and Al have been found that can trap alkali species to form aluminosilicate, thus reducing the level of submicrometer alkali halide particles in the flue gas that may otherwise promote the formation of deposits on tube surfaces. Therefore, the slag of the fourth-stage superheater is relatively small. The slag on the secondary superheater composes a bottom layer, transition layer, and alternating yellow and brown layers. Sticky sylvine and potassium calcium sulfate in the bottom layer are suggested to be responsible for the capture of coarse large particles in the flue gas and the development of slag on the superheater. The particles containing high concentrations of K, Na, and Cl form the yellow layer first, and then it traps the large particles containing high concentrations of Si, Al, Ca, Mg, and Fe to form the brown layer. Magnesium-rich monticellite in the yellow layer and melilite (akermanite and gehlenite) in the brown layer make the alternating layers present yellow and brown, respectively. The slag on the primary superheater consists of a bottom white layer and alternating pale-yellow and black layers. Sticky sylvine and aphthitalite in the bottom layer can trap fly ash particles, resulting in an enhanced slag formation. The particles in the pale-yellow layers containing high concentrations of K, Na, and Cl form the initial deposit on the tube, and then it traps the large particles shown in the black layers containing high concentrations of Si, Al, Ca, Mg, Fe, and P. Periodic blowing may lead to the alternating layer structure that promotes the development of slag on the superheaters. As part of the limitations of this research, the reason for the different colors and levels of hardness of the alternating layers has not been absolutely ascertained.
Figure 6. XRD-analyzed results of slag on the primary superheater.
In particular, the major components in the alternating layer (late deposits) are very different from the bottom layer (early deposits), no mater whether they are on the primary or secondary superheaters. 4. Conclusions
Acknowledgment. The work presented is supported by the Nature Science Foundation of China (50976086). The authors also thank the biomass-fired power plant of Bachu, China.
The slag formation on each superheater has its unique morphology and chemical composition. The secondary superheater
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