Study on the Deposits Derived from a Biomass Circulating Fluidized

Article pubs.acs.org/EF

Study on the Deposits Derived from a Biomass Circulating FluidizedBed Boiler Lianming Li, Chunjiang Yu,* Fang Huang, Jisong Bai, Mengxiang Fang, and Zhongyang Luo State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, People’s Republic of China ABSTRACT: Biomass tends to form deposits and tends to slag during combustion. This study focused on deposits obtained from a circulating fluidized bed (CFB) in a biomass power plant. Analyses using scanning electron microscopy, energy-dispersive spectrometry, and X-ray diffraction are also included. The results indicate that the deposits are dense with a certain hardness, which resulted from the erosion of particles in the CFB. The deposits are high in K, Na, Cl, and Ca, whereas Si, Al, Mg, and S contents are relatively lower. KCl and K2SO4 are the main components of the deposits. The particles in the flue are captured by the viscous layer that was formed by the condensation of gaseous KCl and K2Ca(SO4)2 on the surface of the tubes. Moreover, deposits are formed under the effect of the adhesion and capturing processes, in alternation.

1. INTRODUCTION Biomass is a kind of fuel that has attracted considerable attention in the power generation industry because of its wide distribution, CO2 neutrality, and renewability. The proportion of biomass power generation capacity is 70% of all generated renewable fuel power in western countries.1 In the United States, the installed capacity for biomass power generation has reached 10 GWe. The biomass power generation industry in China, especially biomass direct combustion for energy, is still in an early stage but has shown rapid advances. According to the “Renewable Energy Development Plan” of China, the total rated thermal input will reach 30 000 MW in 2020, including 8000 MW biomass direct combustion for energy by the end of 2015.2 However, most biomass resources in China are herbaceous, which has high alkaline element content. Deposits are easily induced in the boiler when this kind of fuel is employed, resulting in lower efficiency and safety issues. Studies have been conducted to mitigate the problem of deposits on the heat-transfer surface.3−8 The potassium content is known to be high in biomass, especially in the yellow stalk, and the chlorine content is higher than that in coal.9 Chlorine promotes the transfer of alkali metals to the gas phase as well as the reaction of such metals with other compounds.10 Alkaline metals can lower the melting point of ash, which results in deposits within the high-temperature part of the furnace. Chlorine can also react with alkali silicates to generate gaseous alkaline chlorides.11 The amount and viscosity of the deposits are both directly proportional to the vaporization of alkali metals.12−14 On one hand, the migration of elements under fuel combustion conditions is influenced by numerous factors: the composition of the biomass fuel, the growth environment, and the pretreatment of the fuel, among others. Several measures have been developed to deal with alkali metal issues.15,16 For example, the use of kaolin and calcite as additives significantly prevents deposits in the furnace.17−20 However, the cost is extremely high. The mix of diverse biomass fuels or coal could also cut the yield of deposits.21−24 The pretreatment of the biomass fuel is also effective in reducing the alkaline metal content.24 © 2012 American Chemical Society

On the other hand, the design of the combustion facility is an important factor that influences problems with alkaline metals during the biomass combustion process. The temperature and regime of gas−solid two-phase flow in the furnace will affect the behavior of alkaline metals and may result in different problems. The water-cooled vibrating grate boiler continues to be the main combustion technology used to burn agricultural and forest residues. The deposition issue is very serious because grate boiler combustion technology employs high-temperature combustion. Thus, the temperature of the combustion zone is quite high. The technology of biomass combustion in a circulating fluidized bed (CFB) has been developed by Zhejiang University, Hangzhou, China. Significant reduction of deposits is realized on the basis of the sufficient combustion of the biomass fuel, taking advantage of the characteristics of the lower combustion temperature and controllability of the CFB. The CFB boiler is operated using low combustion temperature and high particle flow concentration, which result in intense gas−solid reaction conditions. Thus, the deposition phenomenon in a CFB boiler differs from that of a grate boiler. The deposition phenomenon in a CFB boiler that is designed for biomass combustion requires investigation. The phenomenon of deposition that is discussed in this study is based on a biomass CFB boiler that has been in operation for over a year. The key mechanism of deposit formation is analyzed through the observation and examination of samples.

2. EXPERIMENTAL SECTION 2.1. Sampling. All of the samples used in this study were obtained from the surface of a superheater in a CFB boiler at a biomass power plant (2 × 12 MW) in mid-China. The boiler operates at medium temperature and pressure. The CFB boiler has a capacity of 75 tons/h. Quartz sand is adopted as the bed material. Flue gas flows via evaporation in the in-furnace, in-furnace superheater, high-temperature superheater, low-temperature superheater, economizer, and air Received: June 14, 2012 Revised: August 17, 2012 Published: August 22, 2012 6008

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Table 2. Experimental Conditionsa

preheater, as shown in Figure 1. The boiler has been in operation with full load for more than a year. Plant shutdown attributed to heattransfer surface deposition has not occurred during this period.

test condition

value

gas emissions particle emissions flue gas temperature near the in-furnace superheater (°C) flue gas temperature near the high-temperature superheater (°C) water temperature at the entrance of the in-furnace superheater (°C) water temperature at the entrance of the high-temperature superheater (°C) water temperature at the exit of the in-furnace superheater (°C) water temperature at the exit of the high-temperature superheater (°C) wall temperature (°C)

nd nd 715 690 272 358

a

nd 437 nd

nd = not determined.

2.2. Samples Preparation and Analysis. The morphological characterization of the samples was observed using JSM-6390A scanning electron microscopy (SEM). Energy-dispersive spectrometry (EDS) was involved in parallel aiming to detect element distribution quantitatively at different regions of the deposit samples. To understand the composition of mineralogy, X-ray diffraction (XRD) was introduced in the flue gas side, cross-section (vertical to the tube), and tube side of deposit samples, with a scan angle of 5−80° and a step size of 0.02°. The peak was found using the software MDI JADE. With alcohol as a dispersant, a Malvern Mastersizer 2000 particle size analyzer was used to measure particle size distribution.

Figure 1. Sketch of the boiler system: 1, distributor; 2, evaporation in the in-furnace; 3, in-furnace surperheater; 4, high-temperature superheater; 5, low-temperature superheater; 6, economizer; and 7, air preheater.

3. RESULT 3.1. Morphological Characterization. The boiler has been in operation with full load for more than a year. No deposits and slagging can be observed on the water-cooled wall in the furnace, which differs from that of a grate boiler, where deposits and slagging are more serious. This difference can be explained by the lower furnace temperature and the particle flow near the water-cooled wall in the CFB boiler. A certain amount of deposits has appeared on the in-furnace superheater and high-temperature superheater. The deposits present a hard character and can maintain a certain thickness during long-term operation. Thus, the heat-transfer surface is large enough that there is no need for traditional soot blowing equipment. The

The boiler is designed for agricultural and forest residues, such as straw, cotton stalk, and bark. Given the variety in crops and the difference in harvest times, the fuel used is a mixture of several kinds of biomass fuels. Table 1 shows the properties of the typical biomass fuels. The ratio of cotton stalk and bark is found to be the highest. The samples were typical deposits, which were peeled from the in-furnace superheater and high-temperature superheater during the routine maintenance of the boiler, as indicated in Figure 1. The in-furnace superheater and high-temperature superheater are both made of 12Cr1MoVG. The environmental condition of the deposit formation is listed in Table 2. The sampling process was conducted carefully to maintain the original morphology of the samples. Five samples were taken from each sample position. The obtained samples were tightly sealed and kept in a cool and dry place.

Table 1. Proximate Analysis, Ultimate Analysis, and Heat Value of Typical Biomass Fuel item

proximate analysis

ultimate analysis

heating value (kJ/kg) fuel chlorine contents

ash composition

a

symbol

straw

cotton stalk

sawdust

bamboo

bark

moisture (%, ar)a ash (%, ar) volatile matter (%, ar) fixed carbon (%, ar) C (%, ar) H (%, ar) N (%, ar) S (%, ar) O (%, ar) Q (net, ar) Cl (%) SiO2 (%) Al2O3 (%) Fe2O3 (%) CaO (%) MgO (%) K2O (%) Na2O (%)

15 13.27 58.49 13.24 36.77 4.16 0.79 0.14 29.87 13526 0.896 55.25 0.98 0.34 2.95 7.22 9.73 0.78

1.34 5.45 71.76 21.45 48.3 6.02 1.11 0.2 37.58 18378 0.473 37.87 7.47 4.52 18.61 8.78 6.06 1.22

14.04 1.54 66.78 17.64 43.01 6.19 0.61 0.05 34.56 17250 0.325 26.79 5 4.32 23.62 14.43 5.77 6.14

2.46 2.3 79.74 15.5 45.98 5.62 0.36 0.06 43.22 19234 0.22 39.78 4.14 5.67 34.13 8.49 3.44 3.01

19.29 4.12 58.52 18.07 35.88 4.56 0.25 0.01 35.89 12495 0.743 42.42 6.39 3.42 31.18 9.83 3.66 2.35

Moisture is as-received, usually fluctuating between 25 and 50% in practice. ar is the abbreviation for as-received basis. 6009

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flue gas temperature is below 750 °C around the in-furnace superheater and high-temperature superheater during operation. The status of deposits on the surface of the water-cooled wall, in-furnace superheater, and high-temperature superheater can be observed in Figure 2. The deposit on the in-furnace superheater is similar to that on the high-temperature superheater. A layer of gray deposit uniformly adheres to the surface of the tubes, whereas some small clinkers are adhered on the tubes. The average thickness of the clinkers on the surface of the high-temperature superheater is approximately 5 mm compared to that on the surface of the in-furnace

superheater at only 1−2 mm. As shown in Figure 2c, the antiwear plate is fixed on the windward side of the hightemperature superheater, on which the intensity of deposition is greater. The in-furnace superheater is placed before the hightemperature superheater in the steam-water system, although they are dimensionally close to each other. The temperature of the vapor in the in-furnace superheater is slightly lower than that in the high-temperature superheater. Preliminary analysis indicates no essential difference between deposits derived from the two superheaters. Therefore, this study only focuses on the deposits on the high-temperature superheater. 3.1.1. Macroscopic Configuration. The clinker on the surface of the high-temperature superheater is rigid. The macroscopic configuration of the flue gas side, cross-section, and tube side is shown in Figure 3. The flue gas side of the clinker is gray. The surface of the clinker is rough with white particles adhered, resulting from the molten particle. The homogeneous white matter is the main body of the crosssection, which occupies nearly the entire cross-section. The tube side is dominated by reddish brown matter, which may have resulted from corrosion, as discussed below. 3.1.2. Microscopic Configuration. The microscopic configurations of the three surfaces are presented in the SEM image in Figure 4. The flue gas side has reached the melting phase, and a large number of particles are conglutinated on the surface. Most unfused particles are quartz and can readily be recognized because of low reflectivity and polishing.25 These characteristics can be attributed to the fact that the bed material in the biomass CFB boiler is mainly quartz, which was added externally. The deposits on the tubes melt partially or completely and can thus capture bed material particles. The cross-section is made of compact gray particles, around which needle-like crystals are observed in the interstices using SEM. Given the large temperature gradients, complex chemical composition, and pore structure under the protection of the surface layer of deposits, the presence of the needle-like crystals may be explained by the fact that active salts evaporate, subliminate, and crystallize when in an airtight space under large temperature grads. The needle-like crystals may have also resulted from the formation of FexOy because of the corrosion near the tubes.26 The exact reason can be confirmed if the needle-like crystals are studied individually, which is quite difficult. A large amount of metal oxide resulting from corrosion is observed. Molten deposits in a sheet and small particles around the pore are also evident. To some extent, this finding reflects the effect of the liquid−solid reaction in the process of high-temperature metal corrosion. Although the boiler has been in operation for nearly 14 months, the deposit on the high-temperature superheater is uniform and only 5 mm thick. The antiwear plate of the boiler is 7−10 mm thick, which is 200 mm less than that of the grate boiler.27 On one hand, the low combustion temperature in the CFB boiler has numerous advantages. The possibility of the vaporization of alkaline metals and the fusion of fly ash particles is reduced under low combustion temperature. On the other hand, the deposit is scoured by the high-density particle flow in the CFB boiler, which can be proven by the S structure observed the flue gas side with the SEM. The deposits on different heat-transfer surfaces are compared. The surface of the water-cooled wall is clean. In the intermediate superheater, the surface of the in-furnace superheater is covered by a thin sheet of deposit. The case is similar to that of the high-temperature superheater. However,

Figure 2. Heating surface in the furnace. 6010

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Figure 4. XRD result of the ash.

the working medium and the capture of fly ash and bed material particles by the fused region in the tube facilities. 3.2. Analysis of Elements and Crystal Phase. The fuel chlorine content and the ash composition are presented in Table 1. The ash compositions in Table 1 do not always sum to 100%. These values may be reasonable. Because up to now, there is no special ash composition standard focused on biomass in China. The values in Table 1 were collected actually by the standard ash measurement method of coal. Some error may occur; for example, all elements in ash presented in the form of oxide are acceptable for coal but obviously not for biomass ash. Biomass ash rich in K, Cl, and chloride is a very important component, which cannot be presented correctly by oxide. Some discussion of this aspect can be found in ref 11. The corresponding XRD analysis (Figure 4) shows that CaCO3, SiO2, KCl, and NaCl are the major mineralogical components in the ash. Fuel, except that from bamboo, is found to be rich in either alkali metal elements or chlorine. This kind of fuel is difficult to burn in a boiler because of its strong slagging or fouling tendency. The ash particle size distribution is in the range of 1−60 μm, as pictured in Figure 5. Ash

Figure 3. Macroscopical configuration of clinker on the hightemperature superheater. Figure 5. Curve of the ash particle size distribution.

the in-furnace superheater, high-temperature superheater, and part of the water-cooled wall are subjected to almost the same flue gas temperature and particle flow concentration. Thus, the deposit on the heat-transfer surface is found to be affected by the temperature of the working medium in the tube or the temperature of the tube itself. The condensation of the vapor of gaseous alkaline metals on the tubes, which is washed out by the high-concentration particle flow, may not be the dominant factor in deposit formation. This finding may be the reason behind the increase in deposition under high temperatures of

fusibility and slagging or erosion potential decrease with an increasing mean diameter of the ash particles when the particle diameter is lower than 90 μm.28 Thus, the slagging or fouling probability of the fuel is strong based on the characteristics observed. The EDS and XRD analyses were conducted to understand the observed morphological configuration. The entire field of panels a−c of Figure 6 is scanned using the EDS test. A small 6011

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amount of powder particles are scraped from the flue gas side, cross-section, and tube side of the deposits for XRD analysis.

Figure 7. Major elements in the deposits.

Figure 6. SEM pictures of deposits on the high-temperature superheater.

As shown in Figure 7, the elemental compositions of the depositions in different regions are quantified using EDS analysis. Figure 6 indicates that the deposit is high in Si, S, Cl, K, and Ca contents but low in Na, Mg, Al, and P contents. When the composition of deposits of the different regions is compared, a number of observations were made: (1) Fe is mainly concentrated in the tube side of the deposit because of the high-temperature corrosion of the tube, and the elements V, Cr, and Mn are also present in small quantities. (2) Large amounts of K and Cl are found in the cross-section, implying that the aforementioned needle-like crystals are made of alkaline salts, such as chloride or sulfate, (3) The Si and Ca contents are high in the flue gas side because of the siliceous bed material and fly ash particles adhered to the surface. The results of XRD analysis are listed in Figure 8 and compared to the results of EDS analysis. The major crystal

Figure 8. XRD results of the deposit: a, KCl (sylvine); b, SiO2 (quartz); c, K2SO4 (arcanite); d, K2Ca(SO4)2 (syngenite); e, Fe2O3 (hematite); and f, SiO2 (critobalite).

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captured. During the development process of the deposit, both the ash and bed material particles with Si, Al, Ca, and alkaline metals adhere to the deposit. The oxides of sulfur also enter the deposit in the form of sulfate by reacting with alkaline metals. All of the components in the deposit will undergo complex physical and chemical transformation because of the large temperature gradient and the varying dimensional distribution. Thermal resistance increases with increasing deposit thickness, thus resulting in the higher temperature of the flue gas side. On one hand, KCl condensation from the flue gas decreases. On the other hand, the reactions with the oxides of sulfur also reduce the KCl content. As a result, the particles are less likely to be captured. Moreover, the abrasion effect on the deposits, which can be attributed to the high-concentration particle flow, should also be considered. Therefore, the thickness tends to be stable because of the combined effect of the growth and the abrasion. Deposition is a common phenomenon in boilers, regardless of the type of fuel burned. The components of the deposit derived from the coal-burning boiler have been studied by numerous researchers.36,37 The sylvinite content is found to be significantly lower than that derived from the biomass CFB boiler, which is considered as the key factor that increases the deposition and slagging potential. 9−14 Thus, a lower combustion temperature is selected during the biomass progress, which yields minerals that differ from those produced by coal.

components of the deposits are K2SO4 (arcanite), SiO2 (quartz), and SiO2 (cristobalite). The bed material is the main source of quartz and critobalite. The formation of the arcanite can be attributed to the fact that SOx in the flue gas is captured by the alkaline metals that are attached to the deposit surface. According to the SEM and EDS analyses, the main compositions of the cross-section are K2SO4 (arcanite), KCl (sylvine), SiO 2 (quartz), and SiO 2 (critobalite). The components of the tube side of the deposit are relatively complex, which include the compounds KCl (sylvine), SiO2 (quartz), K2SO4 (arcanite), K2Ca(SO4)2·H2O (syngenite), and Fe2O3 (hematite). A large amount of hematite is generated by high-temperature corrosion. However, the exact formation mechanism is unclear. Alkaline metals are currently an important factor in the process of hematite formation. The matter around the pore that is observed using SEM is identified to be amorphous KCl, which formed because of the crystallization during the cooling process that resulted from the reduction or shutdown of the boiler load.26 The previous analysis indicates that the concentrations of K, Cl, Ca, and S are high, whereas the contents of Al and Mg are low. The existence of the elements K and Cl is a precondition for deposit formation because of the low melting point of KCl. No KCl is found on the flue side of the deposit when using XRD analysis. Solid KCl is also found to be unstable under high temperature.29 On one hand, KCl begins to enter the gas phase through vaporization, which could be enhanced by the increasing temperature of the flue gas side that is induced by the growth of the deposit. On the other hand, if SO2 or SO3 exists in the flue gas, KCl will slowly transform into K2SO4, which has a higher melting point.26 The decrease of KCl in the flue gas side results in lower capture capabilities. The growth of the deposit will become slower without KCl. This pivotal factor influences the growth of the deposits. Another factor that contributes to the stable thickness of the deposits is the high-concentration particle flow in the CFB boiler. The heat-transfer surface undergoes serious abrasion because of the high-concentration particle flow. Deposits that cover the heat-transfer surface undergo abrasion. The thickness of the deposits is simultaneously affected by the growth effect and abrasion effect, which corresponds to the fact that the deposit thickness is stable after long-term operation. Given that XRD analysis is incapable of distinguishing the glassy complex silicate without a fixed crystal structure, no foundation can explain the presence of K2Ca(SO4)2 in the deposits. According to the possible chemical reaction pathways, the presence of K2SO4 may relate to FexOy formation attributed to corrosion.26 Subsequently, K2SO4 will induce the generation of viscous K2Ca(SO4)2. Reaction formulas are as follows:30,31

4. CONCLUSION In a CFB boiler, the heat-transfer surface is scoured by particle flow. The status of the deposition in industry boilers indicates that deposit thickness is directly related to the tube temperature. The temperature of the working medium in the watercooled wall or in the tube itself is low. Consequently, the deposit in this region is rigid and thin, which is unfavorable for the development of deposition under the scouring effect of the particle flow. However, the temperature of the initial deposit layer on the high-temperature superheater is relatively high. Partial fusion occurs on some regions where particles can be captured, thus resulting in the increase in deposit thickness. The temperature difference between the surface of a water-wall tube and the surface of a superheater tube explains why the water-cooled wall is clear and why a certain sheet of deposit is generated on the high-temperature superheater. Stable deposition is characterized by a typical stratified structure and component distribution and can be divided into three layers: the inner layer, the middle layer, and the outside layer, which correspond to the tube side, the cross-section, and the flue gas side, respectively. The inner layer is influenced by high-temperature corrosion. The middle layer mainly comprises concentrated alkaline metals and inert bed materials, such as quartz. The main components are K2SO4 with a high melting point and bed material particles. Given the decrease in the KCl content and the high temperature, both the partial melting phenomenon and capturing capability of the particles in the flue gas are decreased. The abrasion effect attributed to the highconcentration particle flow cannot be disregarded. The integration of these factors can explain why the thickness of the deposit remains constant even after a long-term operation.

K 2SO4 + CaSO4 + H 2O → K 2Ca(SO4 )2 · H 2O K 2Ca(SO4 )2 · H 2O → K 2Ca(SO4 )2 + H 2O

The melting matter found in the SEM analysis may relate to K2Ca(SO4)2. In summary, the process of deposit formation may be as follows: When the temperature exceeds 700 °C, KCl begins to transform into the gaseous phase.10,32−35 The initial deposit layer is formed after the condensation of gaseous KCl on the heat-transfer surface, where temperature is significantly lower than that of flue gas. With the high tube temperature and the increase in deposition thickness, the partial deposition surface begins to melt. Thus, fly ash or inert bed material particles are 6013

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-571-87952801. Fax: +86-571-87951616. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is supported by the Nature Science Foundation of China (50976102), China International Cooperation Project (2011DFA61060), and the Fundamental Research Funds for the Central Universities.

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REFERENCES

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dx.doi.org/10.1021/ef301008n | Energy Fuels 2012, 26, 6008−6014