Effects of Coal Bottom Ash on Deposit in a Full-Scale Biomass-Fired

Jul 31, 2016 - The efficiency of electricity production in biomass-fired circulating fluidized bed (CFB) boilers needs to be enhanced, which may incre...
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Effects of Coal Bottom Ash on Deposit in a Full-Scale Biomass-Fired Circulating Fluidized Bed Boiler Chen Chen, Zhongyang Luo,* Hengli Zhang, Hanchao Tu, and Chunjiang Yu State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China ABSTRACT: The efficiency of electricity production in biomass-fired circulating fluidized bed (CFB) boilers needs to be enhanced, which may increase the risks of high-temperature corrosion (HTC) as a side effect derived from the higher steam temperature. In this study, coal bottom ash (CBA) obtained from a pulverized coal-fired power plant was used to replace all the regular bed materials (quartz sand) in a biomass-fired CFB boiler, attempting to solve its HTC problem. Two kinds of mature deposits on the high-temperature superheater with regular bed materials and with CBA as a bed material were obtained and analyzed in detail. The deposit formation mechanisms with regular bed materials were discussed. Our results show that CBA can capture 22% of potassium during biomass combustion in the CFB boiler. However, CBA cannot effectively decrease the deposition of potassium chloride on high-temperature superheater, therefore leaving a serious HTC problem. The low effect of CBA for potassium capture may derive from its low chemical reactivity.

1. INTRODUCTION Biomass is becoming an increasingly popular alternative energy source for combustion applications because it is a carbonneutral source of energy with a limited environmental impact.1 Among the available combustion technologies, the circulating fluidized bed (CFB) system seems to be the most suitable because of its advantages in terms of fuel flexibility and low combustion temperature.2−5 Unfortunately, natural biomass is highly rich in potassium (K) and chlorine (Cl) while depleted in aluminum (Al), sulfur (S), and silicon (Si) in comparison with coal, especially when it is obtained from agroforestry residues such as straw and bark.6,7 During biomass combustion, these properties increase the risk of deposition and high-temperature corrosion (HTC) of the superheater, which becomes more serious when trying to increase the electricity production efficiency in biomass-fired CFB.5,8−10 The consequences of damaging the superheater include unscheduled shutdowns, prolonged downtime, increased maintenance costs, and more frequent replacement of superheater tubes.5,11 HTC is corrosion on high-temperature superheater (HTSH), caused by Cl deposition in a biomass-fired boiler.8−10 During biomass combustion, alkali metals (i.e., K and Na) strongly interact with Cl and play a key role in carrying Cl in deposits.12−14 The process of deposits formation has not been thoroughly understood and described yet, to the best of our knowledge. In biomass combustion, deposits on HTSH generally contain inorganic salts and oxides such as KCl, K2SO4, NaCl, Na2SO4, CaSO4, CaCO3, SiO2, Al2O3, and others.15−18 The main part of deposits is known to be formed by inertial impaction, but condensation and thermophoresis are also mechanisms of great importance for the deposition of K, Cl, and S.17 The initial deposits were found to contain either KCl17,18 or CaO/CaSO4.19,20 Alkali salts, mainly KCl or K2SO4, can act as the glue, bonding the ash particles in deposits and leading to the enhancement of the formation of deposits,15,18 which are resistant to shedding.15,21 © XXXX American Chemical Society

The employment of additives is regarded as an effective measure to destroy alkali chlorides before their deposition during biomass combustion.11,22−28 Among many kinds of additives, coal ash (both bottom ash and fly ash) obtained from a pulverized coal-fired power plant, rich in Si and Al, is considered a low-cost aluminosilicates-based additive, which has proven to be highly effective in the capture of potassium in a lab-scale experiment22 and to alleviate deposition problems in a full-scale wood suspension-firing boiler.23 However, detailed investigations on the impact of coal bottom ash (CBA) on a full-scale biomass-fired CFB plant have rarely been carried out to date. The aim of the present study was to investigate two kinds of mature deposits obtained directly from a full-scale biomass-fired CFB plant, the first obtained with quartz sand (QS) as a regular bed material and the second with CBA as an alternative bed material. We present an investigation on the deposit formation mechanisms on HTSH, the corrosivity of the deposit, and the effects of CBA on the deposition in a biomass-fired CFB boiler.

2. EXPERIMENTAL SECTION 2.1. Boiler and Sample Locations. The samples in our work were obtained from the surface of an HTSH from a biomass-fired CFB boiler. Two kinds of deposit samples (from a 4000 h long combustion with QS and CBA as bed materials, respectively) were obtained and stored under cool and dry conditions. The boiler in our work was a high-temperature and high-pressure biomass-fired CFB boiler in southern China. Figure 1 shows the schematic drawing of the boiler. The platen superheater and the HTSH are arranged in the upper side of the furnace system diagram. Point 9 in Figure 1 denotes the sampling position. Table 1 lists the parameters of the boiler. The main steam temperature was about 540 °C ranking as high temperature. The HTSH of the boiler encountered a serious HTC problem after running for 4000 h with QS as a bed material, with a sudden bursting of the Received: May 3, 2016 Revised: July 11, 2016

A

DOI: 10.1021/acs.energyfuels.6b01064 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 2. Fuel Composition eucalyptus bark ash volatile fixed carbon moisture C H N O S LHV (kJ/kg)a

Figure 1. Schematic drawing of the boiler: 1, distributor; 2, evaporation in the in-furnace; 3, in-furnace superheater; 4, hightemperature superheater; 5, low-temperature superheater; 6, economizer; 7, air preheater; 8, bag filter; and 9, sample positions.

SiO2 CaO K2O P2O5 Al2O3 MgO Fe2O3 SO3 Na2O

Table 1. Plant and Operating Parameters for Boiler bed top temperature bed bottom temperature furnace outlet temperature flue gas temperature near the platen superheater flue gas temperature near the high-temperature superheater main steam temperature main steam pressure fuel consumption (full load)

790−810 750−760 745−755 710−720 690−710

°C °C °C °C °C

530−545 °C 8.3−9.8 MPa 63 tons/h

a

sugar cane leaves

forestry residues

mixture of three fuels

Proximate Analysis (wt %, As-Recieved Basis) 8.2 5.9 7.9 51.6 52.6 50.1 12.8 12.8 15.2 27.5 28.7 26.8 Ultimate Analysis (wt %, As-Recieved Basis) 32.1 30.9 34.5 3.8 4.7 5.1 0.26 1.21 0.21 28.0 28.5 25.5 0.18 0.14 0.08 10783 10472 14250 Ash Compositions Analysis (wt %) 40.1 35.1 43.2 23.4 19.2 20.4 9.2 15.5 9.7 2.4 2.9 2.1 8.1 8.5 8.2 10.1 10.4 9.8 3.8 4.5 4.1 2.1 2.1 1.5 0.9 1.8 1.0

7.8 50.0 13.5 28.6 31.3 3.7 0.26 27.9 0.16 10752

40.7 21.5 9.6 2.3 8.2 10.1 4.2 2.1 1.0

LHV: low heating value.

plasma-atomic emission spectrometry (ICP-AES) and ion chromatograph (IC). Insoluble concentration was calculated. The regular bed material in the operating conditions of the boiler was quartz sand (QS, SiO2 content > 92 wt %). Under the operating condition with alternative bed material, CBA from the pulverized-coal boiler was used to replace all the bed materials and refreshed once every hour. The mass ratio of the fuels to the fresh bed materials for those two kinds of setups was the same, corresponding to a value of about 35:1 as-received basis. The element contents of CBA and the obtained bottom ash (OBA) from the operating condition with CBA as an alternative bed material were analyzed by energy-dispersive spectrometry (EDS). To evaluate the capability of CBA to capture K, water-soluble K, and the total K concentration of QS, the obtained bottom ash with QS (OBA-QS) as bed materials, CBA, and OBA were analyzed by ICP-AES. To analyze the mineral composition of CBA and OBA, X-ray diffraction (XRD) was performed with a scan angle of 5−80° and a step size of 0.02°. The peak was found using the software MDI JADE. The size of CBA particles was measured by Mastersizer 2000. 2.3. Deposits Analysis. A detailed analysis was performed on the obtained deposit samples on the HTSH both with QS and with CBA. The macroscopic and microscopic morphology were investigated through observation by naked eye and scanning electron microscopy (SEM). To understand the composition of deposit samples, EDS and XRD were carried out both on whole samples and locally, on four different parts along the radial direction.

tubes at the main steam outlet. Figure 2 shows the pictures of deposits and corrosion in the boiler taken at shutdowns.

Figure 2. Deposition and corrosion on HTSH: (1) deposition; (2) corrosion.

2.2. Fuels and Bed Material. In the present work, the fuels used in the boiler varied according to crop variety and seasons, the main fuels being eucalyptus bark making up 60% (energy percentage) and the rest comprising sugar cane leaves and forestry residues. Variation in composition of used fuels due to the long period of the tests would influence the experiments. The mix ratio (energy percentage), 6:3:1 eucalyptus bark, sugar cane leaves, and forestry residues, was chosen considering the typical and mean fuels during the long period of the tests. Table 2 shows the composition of the individual fuels and their mixture. The ash composition analysis was taken on the ash obtained at 600 °C. The main ash-forming elements of the mixture fuels were analyzed by chemical fractionation, which can be used to obtain the information regarding both the concentration and the associated form of each element.29 The samples were ground and sieved to less than 75 μm. The fractioning process was conducted in water, acetate (1 M ammonium acetate), and acid extraction (1 M hydrochloric acid, HCl). In addition, the samples were dissolved in an acid mixture (nitric acid, hydrogen peroxide, and hydrofluoric acid in a 8:1:1 proportion) to get the total concentration of each element. The element concentrations were determined through inductively coupled

3. RESULTS AND DISCUSSION 3.1. Fuel and Bed Material. The results of fuel analysis from chemical fractioning of the mixture fuels are shown in Figure 3. The contents of K and Cl of the fuels were 0.44 and 0.24 wt %, which are lower than those of wheat straw29 and rice straw30 and higher than those of bark3 and wood,3 and the content of Ca (0.90 wt %) was higher than that in straw29,30 and similar to that of bark.3 A considerable amount of Ca was dissolved in hydrochloric acid, suggesting that the fuel may contain calcium oxalates, which are typical authigenic minerals B

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Table 3. Water-Soluble and Total K Concentrations in CBA and OBA (wt %) element

QS

OBA-QS

CBA

OBA

whole K water-soluble K

0.11 0.00

0.49 0.24

0.27 0.01

3.02 0.17

K mainly through chemical capture. It is widely known that Si and Al in CBA can react with KCl through the following reactions:11,22−24 Al2O3 ·2SiO2 + H 2O + 2SiO2 + 2KCl(g)

Figure 3. Results of fuel analysis from chemical fraction (as-dry basis).

→ 2KAlSi 2O6 + 2HCl(g)

in plants.7 Ca has been found to play an important role in ash and deposits formation during biomass combustion,3 when calcium oxalates (CaC2O4) decompose to form calcium carbonate (CaCO3) (300−500 °C), which can decompose to form CaO (700−1000 °C).7 CaO easily reacts with SO3 to produce CaSO4 (above 600 °C).7,19,31 The results from EDS and XRD of both CBA and OBA are shown in Figures 4 and 5. EDS results show that CBA was

(1)

Al 2O3 ·2SiO2 + H 2O + 2KCl(g) → 2KAlSiO4 + 2HCl(g) (2)

The ratio of captured K to the total amount of K in the fuels was calculated according to eq 3. Potassium release from fuel during combustion32 can be captured by bad material. Captured potassium is not available for chlorine to form KCl in gas phase, which can deposit on surfaces of HTSH.32 The increase in the bed ash mass due to biomass ash joining the bed ash is negligible. R captured,K =

(C K,OBA − C K,CBA) − (C K,OBA ‐ QS − C K,QS) C K,f × αm × 100%

(3)

where CK,OBA, CK,CBA, CK,OBA‑QS, and CK,QS are the concentrations of K in OBA, CBA, OBA-QS, and QS, respectively. αm is the mass ratio of the fresh bed materials to the fuels (1:35, asreceived basis). CK,f is the K concentration in the fuels (asreceived basis) which is calculated to be 0.31 wt %, according to the sum of water and acetate soluble fraction of K (0.44 wt %, as-dry basis) from Figure 3 and the moisture content of the fuels (28.6 wt %) in Table 2. The ratio of captured K to the total amount of K in the fuels was finally calculated to be 22%. In addition, the median diameter (d50) obtained by particles size analysis in CBA was 244 μm. 3.2. Deposits Characteristics. 3.2.1. Characterization of Deposits from Morphology Analysis. The thicknesses of the deposit samples with regular bed materials and with CBA as a bed material were 5−7 mm and 4−5 mm, respectively. Both samples were dense and hard. The macroscopic morphology of deposit samples is shown in Figure 6. Naked-eye visual inspection revealed that the texture and color of the samples varied along the radial direction; therefore, the samples were divided into 4 layers (i.e., tube side 1 (adherent to the tubes), tube side 2, cross section, and flue gas side). As can be seen in Figure 6, the flue gas side of both the samples were gray and earthy yellow, uneven, and exhibited a trace possibly left by the scour of outside particles. The cross section of both samples was loose and light-colored. Tube side 2 of samples was denser and harder than the cross section. Tube side 1 was red, brown, and black, possibly as a result of corrosion products of metals in the HTSH, and it firmly adhered to the total deposits. Tube side 1 of the samples with regular bed materials contained more red matter than those obtained from the boiler with CBA. The microscopic morphology of deposit samples with regular bed materials and with CBA is shown in Figure 7. It is possible to notice that these two kinds of deposits are characterized by a

Figure 4. EDS results of CBA and OBA.

Figure 5. XRD results of CBA and OBA.

mainly composed of Si, Al, C, and O. The S content is only 0.3 wt %. The release of SO3 from S in CBA during biomass combustion can be neglected because a stabilizing process occurred during pulverized-coal combustion at high temperature. The main compounds detected in CBA were SiO2 and Al6Si2O13, with a small amount of Fe2O3 and C. The only compound with a significant presence in OBA was SiO2. The amounts of K and Ca in OBA were distinctly higher than those of CBA, which indicates that CBA can capture K. Table 3 reports the water-soluble and total K concentrations, whose values were 0.01 and 0.27 wt % in CBA. This indicates that almost all the K in CBA is inert (i.e., in the form of silicates). The water-insoluble K concentration was much higher than that in OBA, which suggests that CBA can capture C

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Figure 6. Macroscopic morphology of deposit samples with regular bed materials and with CBA as a bed material: (1−3) flue gas side, cross section, and tube side with regular bed materials; (4−6) flue gas side, cross section, and tube side with CBA as a bed material. Figure 8. EDS analysis results of the deposits. (wt %).

similar morphology. The deposits in the flue gas side are constituted of many large particles (whose size is on the order of 10 μm), which may have deposited by inertial impaction and adhered by viscous deposits. The cross-sectional images reveal the occurrence of molten phenomenon, and the corresponding particles and floccules are connected together, with the presence of interspaces. The tube side contains uniform small particles, which are likely oxides deriving from corrosion products. 3.2.2. Characterization of Deposits from Analysis of Deposits Composition. The results of element and crystal phase analysis of the deposits (whole and in the separate four layers) with regular bed materials and with CBA as a bed material are shown in Figure 8 and Table 4. The analysis revealed that the element and crystal phase of deposits without and with CBA are similar.

The element and crystal-phase analysis of the deposits with regular bed materials indicated the presence of KCl, CaSO4, SiO2, and a small amount of Fe and Ni oxides in the tube side 1. By combining this data with the morphology results, it is possible to state that KCl in the tube side 1 firmly adhered to the tube of the HTSH, causing serious corrosion. The salts (mainly KCl) and corrosion products such as metal oxides or chlorides can form compounds with a low melting point.18,21 Tube side 1 may contain molten matter or substances in the liquid phase, and corrosion products can permeate the deposits,21 thus increasing the adhesive force between salts in deposits and the surface of the tube of HTSH. Tube side 2 contained KCl, NaCl, SiO2, and CaSO4. The cross section contains mostly KCl and a little CaSO4 and K2Ca(SO4)3. These

Figure 7. Microscopic morphology of deposit samples with regular bed materials and with CBA as a bed material: (1−3) flue gas side, cross section and tube side with regular bed materials; (4−6) flue gas side, cross section and tube side with CBA as a bed material. D

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Energy & Fuels Table 4. XRD Analysis Results for the Depositsa deposits with QS KCl SiO2 CaSO4 CaCO3 NaCl Fe2O3/Fe3O4 K2Ca(SO4)3 a

deposits with OBA

tube side 1

tube side 2

cross side

flue side

tube side 1

tube side 2

cross side

flue side

* * *

* * *

* * *

* *

* * *

*

*

* * * * *

* *

* *

* * * * *

* * *

*

*

*

The presence of compounds in each region is indicated by an asterisk.

deposition will gradually tend toward dynamic equilibrium, in agreement with the literature.4,34 3.4. Discussion on the Effects of CBA. From the results of deposits characteristics with QS and with CBA in section 3.2, we found that their compositions are similar, with a Cl content on the order of 31 wt %. The deposit samples with CBA are thinner than those with QS. This indicates that CBA does not effectively decrease the Cl content of deposits and fails to solve the serious HTC problem. To discuss and find the detailed reason for the low effect of CBA, we compared the two samples in this study and the deposits on HTSH from our previous work27 on the same boiler with coal as additive. Table 5 lists the

results indicate that K2Ca(SO4)3 was formed in the deposits by a chemical reaction rather than from flue gas particles. The presence of K2Ca(SO4)3 in deposits has also been previously reported in literature,4,20 demonstrating that K2SO4 can react with CaSO4 at the temperature of 582 °C. The outmost layer (i.e., the flue gas side) contains CaCO3 and the largest amount of SiO2 and Al-based compounds, corresponding to the morphology results of deposits of the flue gas side, which indicates that SiO2 and Al2O3 from the bed materials are carried by the flue gas and deposit by inertial impaction. 3.3. Discussion of the Deposit Formation Mechanisms with Regular Bed Materials. On the basis of the detailed information on the fuels properties and the deposit characteristics on the HTSH with regular bed materials given in the above analysis, the mechanisms of deposit formation on HTSH with regular bed materials can be discussed as follows. 3.3.1. Initial Deposition. CaSO4 was found in the samples both for the entire and part of the tube side 1, in agreement with results from the literature.19,20 CaO and CaSO4 may constitute the initial deposit layer for Ca high fuels combustion,19,20 whereas KCl is generally known to constitute the initial deposit layer for K high fuels combustion.4,17,18 Thus, based on our results and on the literature,19,29 it was assumed that CaO, CaCO3, and/or CaSO4 form the initial layer of deposits by thermophoresis. The Ca-containing compounds (CaO, CaCO3, and/or CaSO4) serve as a condensation nucleus for alkali salts (KCl, NaCl, K2SO4, etc.) and enhance their deposition. 3.3.2. Main Part Deposition. A large amount of KCl was found in our samples, in agreement with literature results4,17,18 acquired under similar experimental conditions. Alkali salts (KCl, NaCl, K2SO4, etc.) can form molten salts whose melting point is lower than that of KCl, down to 460 °C in certain mix ratios.19,33 This evidence indicates that gaseous or small liquid particles of molten salts can deposit by condensation and thermophoresis. In the deposits, these alkali salts can be molten or in the liquid phase and increase the stickiness of deposits, acting as a glue between individual ash particles. These salts can adhere to larger coarse particles such as CaO, CaCO3, SiO2, and Al2O3 in flue gas, enhancing the deposit rate and increasing the compactness and soundness of the deposits. 3.3.3. Mature Deposition. The flue gas side of the samples was found to be rich in SiO2 (mainly from the bed materials) and Ca-containing matter. As the thickness of deposits increases, the temperature of the flue gas side of the deposits increases accordingly, and that of the tube side will slightly decrease. This favors the evaporation of alkali salts in the flue gas side of the deposits. A large amount of particles circulating in the CFB boiler can scour or wash the superheater surface, so

Table 5. Properties of CBA and Coal Additivea amount K + Na (wt %) Cl (wt %) Si (wt %) Al(wt %) S(wt %) Si/Al a

CBA

ref 27 coal additive

1.8 tons/h 0.4 0.1 28.9 10.9 0.3 2.6

2.9 tons/h 0.13 0.03 5.3 2.5 1.4 2.0

As-dry basis; the element ratio of Si/Al is expressed as a molar ratio.

properties of CBA in our work and the coal in our previous work. The moisture of the coal was 5.5 wt %. The amount of CBA and coal additive in Table 5 was calculated to dry mass. The reactions between coal and KCl do not only involve Si and Al through alkali metals aluminosilication (eqs 1 and 2) but also S through alkali metals sulfation, according to25−28 2MCl + SO2 + H 2O + 1/2O2 → M 2SO4 + 2HCl

(4)

where M can indicate K or Na. The main deposits characteristics are listed in Table 6. The presence of coal causes the size of the deposits to be less than 1 mm, without the presence of any Cl. The molar ratios of Cl/(K + Na) in these three deposits were 0.97, 0.96, and 0.005, respectively, indicating that most of the alkalis were primarily chlorides in the deposits with QS and with CBA, whereas they were mostly in the form of sulfates in the deposits with coal. The molar ratios of 2S/(K + Na + 2Ca) of the deposits were 0.09, 0.25, and 0.95, respectively. This increasing order corresponds to the increase of the sulfation level of coal with respect to CBA and QS. This result indicates that CBA has little effect on the deposition. The deposits obtained with CBA still contained a large amount of KCl, and they were strongly prone to corrosion. CBA failed to solve the HTC problem in the boiler; however, coal can significantly reduce the amount of deposit, significantly changing its composition. Sulfates are less E

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Energy & Fuels Table 6. Deposits Characteristics thickness main crystal phase matter Cl Cl/(K + Na) 2S/(K + Na)a 2S/(K + Na + 2Ca) a

QS

CBA

ref 27 coal additive

5−7 mm KCl, SiO2, CaSO4, CaCO3, NaCl, K2Ca(SO4)3 31 wt % 0.97 0.1 0.09

4−5 mm KCl, SiO2, CaSO4, CaCO3, NaCl, K2Ca(SO4)3 31 wt % 0.96 0.4 0.25

0.5−1 mm SiO2, CaSO4, KAlSO4, CaS, K2SO4, K2Ca(SO4)3 0 0 2.8 0.96

The element ratio of 2S/(K + Na) is expressed as a molar ratio.

corrosive than alkali chlorides,8−10 so the HTC problem can be alleviated effectively by adding coal. To explain the different properties of CBA and coal additive to capture K during biomass combustion, we calculated the molar ratio of K in the boiler fuels and the reactive elements in both CBA and the coal. Si and Al in CBA (as discussed in section 3.1) and Si, Al, and S in the coal constitute the reactive elements. We found a K/Si/Al ratio of 1:3.6:1.4 in CBA and a K/Si/Al/S ratio of 1:1.1:0.5:0.2 in coal. From eqs 1−3, we calculate the molar ratio of K/Si/Al as 1:(1−2):1 and the K/S molar ratio as 1:0.5. These results indicate that the amount of Si and Al in CBA and the amount of Si, Al, and S in coal are sufficient to capture all the K in the fuels. However, the experimental results showed that CBA captured only 22% K in the fuels, failing to solve the deposition and corrosion problems. This indicates that the low effect of CBA is due to its low chemical reactivity because the particles of CBA were sintered during combustion in the pulverized-coal boiler and their size increased accordingly (d50 = 244 μm).

biomass-fired CFB plant. The low chemical reactivity in terms of K capture was probably due to the large size of CBA particles, which additionally sintered. Furthermore, KCl in the flue gas could easily deposit on the superheater, which implies that the presence of KCl in the flue gas should be limited to a very small amount in order to eliminate the HTC problem. CBA from a coal-fired fluidized bed boiler may be a recommended choice because its reactivity in the capture of K is higher, as the combustion temperature is much lower than that of a pulverized-coal boiler.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-571-87952440. Fax: +86-571-87951616. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Natural Science Foundation of China (51336008).

4. CONCLUSIONS The present study analyzed mature deposits with the QS and with CBA (obtained from a pulverized-coal boiler) as a bed material on the surface of a HTSH obtained directly from a biomass-fired CFB plant. The fuels and the deposits were analyzed in detail. Calcium oxalates were found in the fuels. The main constituents of the deposit samples were KCl, CaSO4, SiO2, Al2O3, and CaCO3. The adhesive force between salts in deposits and the corrosion products was large, indicating that tube side 1 may contain molten substances or liquid phase matter and that corrosion products can permeate the deposits. The mechanisms on deposit formation on the HTSH with regular bed materials were investigated. Ca in fuels was found to play an important role in ash and deposits formation during biomass combustion. Our results indicate that CaO and/or CaCO3 form the initial deposit by inertial impaction; then, they react with SO3 in the flue gas to form CaSO4. CaO, CaCO3, and CaSO4 may constitute a condensation nucleus for KCl and enhance its deposition. Alkali salts (mainly KCl) can form gas and liquid phase in the flue gas and can deposit on the HTSH tubes by condensation and thermophoresis. In the deposits, these alkali salts can enhance the deposit rate and increase the stickiness, compactness, and soundness of deposits. As the thickness of deposits increases, deposition, evaporation, and shedding of the deposits will gradually tend toward dynamic equilibrium in the CFB boiler. CBA can capture K mainly through chemical capture during biomass combustion. The amount of K in the fuels captured by CBA was calculated to be 22%. CBA has a low effect in the elimination of the deposition and corrosion problems in the

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REFERENCES

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DOI: 10.1021/acs.energyfuels.6b01064 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.6b01064 Energy Fuels XXXX, XXX, XXX−XXX